Smart Materials for Smart Living

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MATERIALS SCIENCE AND TECHNOLOGIES

SMART MATERIALS FOR SMART LIVING

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MATERIALS SCIENCE AND TECHNOLOGIES

SMART MATERIALS FOR SMART LIVING

RADHESHYAM RAI EDITOR

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CONTENTS

Preface vii Chapter 1 Smart Material Nanofibers for Day to Day Life 1 Madan Lal, Mamta Shandilya, Seema Sharma and Radheshyam Rai Chapter 2 Possible Applications of Zinc and Titanium in Modern Life 67 Anjali Sharma, Madan Lal, Naheed Ahmad and Radheshyam Rai Chapter 3 High Dielectric Materials for Supercapacitors 95 Poonam Kumari, Mamta Shandilya, Madan Lal and Radheshyam Rai Chapter 4 Development of Double Perovskite Electroceramics 137 Shweta Thakur, Mamta Shandilya and Radheshyam Rai Chapter 5 Magnetic Properties, Magnetoresistance and Functionality of Perovskite Manganese Oxides 173 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin, S. A. Garvilov, S. V. Dubkov and V. V. Sikolenko Chapter 6 Piezoresponse Force Microscopy of P(VDF-TrFE)- Graphene Oxide Films 191 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky, N. Nasani, G. Goncalves, A. V. Sysa, A. V. Solnyshkin, P. A. A. P. Marques, Budhendra Singh and I. K. Bdikin Chapter 7 Piezoelectric Electroceramic Perovskites and Their Applications 205 Poonam Kumari, Madan Lal, Shashi Prakash Rai and Radheshyam Rai Chapter 8 Biodiversity and Sustainable Development 257 Naheed Ahmad, Anjali Sharma and Radheshyam Rai Chapter 9 Fabrication of KNN Thin-Films via a Spin Coating Technique 277 Rashmi Rani, Seema Sharma, Marzia Quaglio, Simelys Hernandez, Stefano Bianco, Angelica Chiodoni and Candido Fabrizio Pirri vi Contents

Chapter 10 Ferrroelectric and Ferromagnetic Properties of Bi1-x-yDyxCyFe1-yTiyO3 Solid Solution 291 Radheshyam Rai, Anjali Sharma, Igor Bdikin, M. A. Valente and Seema Sharma Chapter 11 M-Type Barium Nanohexaferrite Material: A Novel EntRant for Storage Enrichment and High Frequency Applications 303 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi, K. M. Battoo, R. K. Kotnala and M. Singh Chapter 12 Hyperspectral Imaging: A Brief Introduction for Beginners 337 Ankit Gupta

Chapter 13 Dielectric Relaxation in BaTiO3-Based Perovskite 345 Mamta Shandilya, Shweta Thakur, Radheshyam Rai and Jagtar Singh Chapter 14 Biosynthesis of Nanoparticles Using Plant Extracts 365 Sapna Thakur, Shweta Thakur, Mamta Shandilya, Madan Lal and Radheshyam Rai About the Editor 383 Index 385

PREFACE

In view of the potential importance of Smart Materials and its diversified applications, the book provides adequate material to researchers in the field of Material Science. The concept of development in present world can be understood by the infrastructure, social development and health. Now in these days, basic needs are centered on “Materials”. How judiciously one uses the materials and resources is the point of concern. The purpose of this book is to synthesize materials that prove to be beneficial for the society in different respect. Smart material like (Ferroelectric, Piezoelectric ceramics and nanomaterials) are presently one of focused point in materials research. Smart materials are becoming more interested subject as materials scientists. They are focus on materials chemistry, physics and application in real world because it induces conformational changes in complex structures and properties which are useful for devise and control them. The smart materials with the most promising ability to alter the mode via an external stimuli including stress, electric or magnetic fields, temperature, moisture or etc. These types of materials have been found to be very useful and interesting for different solid state devices in real life. Changing structures, advances in technologies and the greater reliance of researchers on materials poses a number of challenges for maintaining good manufacturing practices. Also, the theoretical and experimental aspects have been dealt with in detail. Now these days, everything in human surrounding are moves towards materials. The focus of the chapters is too dealt with different type of environment friendly materials. The main challenge faced while compiling the matter for this book was identification and prioritization of the various materials that help in making our lives better. I would love to bring about changes for a better living through the research material included in this book. I am thankful to the students and all other team members for encouraging me to write this book. I express my gratitude to “Nova publisher” for bringing out this book in its present form. The work has been described in fourteen chapters. The book Smart Materials for Smart Living has a wide range of collection of materials that are involved in the things that govern our day to day lives. The research work presented in the book begins with Chapter 1 on General Introduction on Smart Material Nanofibers giving a general idea of nanofibers and their application. Applications of SMNFs are used in different areas such as Filtration media, Gas sensing and a great deal of biomedical applications too. The treatment of heart blockage, brain hemorrhage has been significantly affected by these materials, due to their properties of extremely small pore size, electrical and viii Radheshyam Rai magnetic properties, large surface to volume ratio and being light in weight. These are also useful to be better materials in solar cells, batteries and many other applications. Chapter 2 deals with detailed description of possible application on Zinc and Titanium in our life. Zinc is a material that has been used since ancient times, especially in India. Its antibacterial and biocompatible properties make it one of the best materials to be used in medical industry. Applications of TiO2 are used for reducing air and water pollution and separation of cadmium. Also ZnO has worldwide applications in rubber industry, paints, cosmetics, photocopying, fire retardants etc. They are also used in pharmaceutical industry in curing Diarrhea (children), bad breath, Down’s syndrome, Immune function etc. The applications of such materials are obviously possible only and only if they are synthesized in the best manner. Nano fibers are used in wound dressing for absorption of exudates from the wound, it maintains a moist environment. It also maintains flexibility in the dressing mat. Chapter 3 illustrates the high dielectric materials for supercapacitor and their advantages, disadvantages, and performance in supercapacitors are also discussed through extensive analysis of the literature, and new trends in material development. Chapter 4 deals with detailed description of double perovskite with their magnetic properties, synthesis process and applications. Double perovskites are simply two different ternary perovskites ABO3 and ABʹO3 that are arranged alternately on a three dimensional checkerboard lattice. If A'=A" and B'=B" the system reduces to the simple perovskite. Chapter 5 describe the magnetic properties, magnetoresistance and functionality of perovskite manganese oxides. The obtained results testify that the chemical substitution leads to an increase in the average oxidation state of Ni and Co ions from 2+ into 3+ one while manganese ions remain 4+ oxidation state. The character and stability of the superexchange interaction between pairs of Co, Mn and Ni ions via oxygen ion which govern magnetic properties of the compounds are discussed depending on the oxidation state and electron configuration of the respective transition metal ions and structural peculiarities of the compounds. Chapter 6 describe the Piezoresponse force microscopy of p(vdf-trfe)- graphene oxide films. A detailed computational molecular modeling for all the three compositions was undertaken to trace the polarization behavior for these composites and to understand the underlying phenomena. Molecular modeling results were found to correspond to the results of nanoscale PFM measurements. Chapter 7 deals with detailed description of development piezoelectric perovskites electroceramic and their Applications. In this review, an attempt is made to review recent developments on lead-free piezo materials emphasizing their preparation, piezoelectric property relations, and consequent physical properties. Piezoelectric properties of the most promising lead-free compositions/families including titanates, alkaline niobates and bismuth perovskites and their solid solutions, along with perovskites such as KNN and BaTiO3 ferroelectrics are reviewed in detail Chapter 8 illustrates the biodiversity and sustainable development. The rapid industrial growth has made water pollution, air pollution, hazardous wastes and other pressing environmental problems in many areas of the developing world. Transboundary air pollution, water shortages, drinking water contamination, freshwater and marine pollution, deforestation, climatic disasters, and other environmental problems are posing serious threats to the well-being of people. It is well established that human societies and cultures co-evolve with their environment. Preface ix

Chapter 9 describe the fabrication of knn thin-films by spin coating technique. Nowadays the requirements for the miniaturization of micromechanical and microelectronic components cause an increasing demand of thin films, whose dimensions in the nanometer range give rise to new physical phenomena and properties which greatly differ from those of homogeneous bulk materials of the same composition and which need to be understood in order to develop ferroelectric and piezoelectric devices Chapter 10 deals with ferrroelectric and ferromagnetic properties of Bi1-x-yDyxCyFe1- yTiyO3 solid solution. In this study, the crystal structure, dielectric, ferroelectric, piezoelectric and ferromagnetic properties in the ternary perovskite Bi1-x-yDyxBayFe1-yTiyO3 (x = 0.1 and y = 0.1, 0.2, 0.3, 0.4 and 0.5) are studied in order to improve the properties of multiferroic ceramics. The origin of the weak ferromagnetism will be discussed in terms of magnetic structure of this perovskite system. Chapter 11 deals with M-type bariumn nanohexaferrite materials: a novel entant or storage enrichment for high frequency application. Chapter 12 describes hyperspectral imaging: a brief introduction for beginners. hyperspectral images provides plenty of information to identify and discriminate unique materials with well defiend spectral signature. HSI provides precise and thorough information than possible with any other type of technique for collecting data from remote location. Chapter 13 will discuss on dielectric relaxation in BaTiO3 based perovskite. Chapter 14 describes the Biosynthesis of nano particles using plant extracts for different application.

Chapter 1

SMART MATERIAL NANOFIBERS FOR DAY TO DAY LIFE

Madan Lal1, Mamta Shandilya1, Seema Sharma2 and Radheshyam Rai1,* 1School of Physics and Materials Science, Shoolini University, Solan (H.P.) India 2Ferroelectric Research Laboratory, Department of Physics, A. N. College, Patna, India

ABSTRACT

Smart materials are a common name for a wide group of different substances. The general feature of all of them is the fact that one or more properties might be significantly altered under controlled condition. The present age is considered to be the smart materials era. Earlier, the smart material was defined as the material, which response to its environments in a timely manner. However, the definition of smart materials has been expanded to the materials that receive, transmit, or process a stimulus and respond by producing a useful effect that may include a signal that the materials are acting upon it. In this chapter, we have focuses on the introduction of smart materials, their properties, methods of fabrication, piezoelectricity and their applications including filtration media, biomedical, transducer etc. Advanced approaches in electrospinning have also discussed and the possible application of smart materials nanofibers.

Keywords: electrospinning, piezoelectricity, nanofibers, energy harvesting, polymer, filtrations

1. INTRODUCTION

Environmental pollution has become a major global concern due to the rapid growth of industrialization, urbanization, and modern agricultural development. Technological innovations and advancements in products and processes in industries have given rise to new

* Corresponding Author address, Email: [email protected]. 2 Madan Lal, Mamta Shandilya, Seema Sharma et al. products and new pollutants in abundant level which is above the self-cleaning capacity of the environment (Wheeler and Elkington 2001, Hoffman, Hotze, and Wiesner 2007, Webster et al. 2005). Pollution is the introduction of contaminants into an environment that causes instability, disorder, harm, and discomfort to the ecosystem, that is, physical systems or living organisms. Water and air represent two major environmental systems where the most pressing environmental pollution issues persist (Herzog 2005, GLEICK et al., Gray and Stephen 2001). As we know that water is essential for human life, development, and environment, but it is a finite and vulnerable resource which has quantitative limitations and qualitative vulnerability (McIntyre 2009, Vörösmarty et al. 2010). It is estimated that more than 50% of nations in the world will face freshwater stresses or shortages by 2025 and will increase to 75% by 2075. Thus, water pollution and dwindling freshwater supplies are becoming a critical global issue due to increasing population, economic growth, and climate change (Hoffman, Hotze, and Wiesner 2007, Thavasi, Singh, and Ramakrishna 2008). Similarly, air is another environmental system that is of concern. Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. The major air pollutants are particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide (Pope III and Dockery 2006, Nel 2005). Motor vehicles, household combustion devices, and industrial activity are the main man-made sources of outdoor air pollution, causing respiratory and other diseases. The World Health Organization states that 2.4 million people die each year from causes directly attributable to air pollution, with 1.5 million of these deaths attributable to indoor air pollution (Krewski et al. 2009, Marshall, Brauer, and Frank 2015, Anderson 2009). Given the recognized threats to the world’s collective energy security and environment, the focus must be redirected, as quickly as possible, toward addressing these critical challenges and driving global research to develop technology and devices for clean energy conversion, storage and conservation, and a clean environment—water and air pollution abatement (Ramakrishna et al. 2006, Thavasi, Singh, and Ramakrishna 2008). These demands for such high-performance materials have led to increasing attention in advanced functional nano-sized materials. Among those existing nanostructures, One-dimensional (1D) nanostructured materials such as nanofibers (NFs), nanowires (NWs) (Chan et al. 2010), nanotubes (NTs) (Yoo et al. 2012), and nanorods (NRs) (Lee et al. 2009) have attracted extensive attention due to their unique physical and chemical characteristics. Continuous NFs have been the focus of studies because of their unique and tremendous properties compared to other nanostructured materials. The physical and chemical properties of nanostructured materials (such as optical absorption and fluorescence, catalytic activity, magnetism, melting point, electric and thermal conductivity, etc.) typically differ significantly from the corresponding bulk material (Liu, Goebl, and Yin 2013). These 1D nanostructure materials can be prepared by many methods such as template- directed methods, vapor-phase methods, interface synthesis techniques, solvothermal synthesis, solution-phase growth controlled by capping reagents, nanolithography and self- assembly (Imaizumi et al. 2011, Braghirolli, Steffens, and Pranke 2014, Shrestha et al. 2014, Choi et al. 2014, Nirmala, Navamathavan, et al. 2014) However, each of these methods has limitations, such as material restrictions, high cost, and high process complexity. Recently, electrospinning, a simple, inexpensive technique, has attracted significant attention in the preparation of nanostructured materials (Ding et al. 2010, Nirmala, Jeon, et al. 2014, Wang, Ding, et al. 2011, Cao et al. 2013). Among various methods of fabricating polymeric fibers, electrospinning has become a popular method to generate continuous, ultrathin fibers with Smart Material Nanofibers for Day to Day Life 3 diameters on the order of microns and sub-microns from a variety of polymeric materials. It has been used to prepare nanofibers, nanotubes, nanobelts and porous membranes. These electrospun nanomaterials have unique properties applicable to a wide range of fields, including the fabrication of nanomaterials for use in energy conversion devices. Currently, almost all kinds of materials including inorganic and organic polymers can be conveniently electrospun into nanofibrous morphology and utilized for many technological applications (Persano et al. 2013, Huang et al. 2003, Raghavan et al. 2012). In the present scenario, most of the research activities are mainly focused on controlling the experimental parameters, as these represent the most important factors for determining the physical properties of the electrospun nanofibers. On the other hand, electrospun fibers and their corresponding membranes also have their own unique properties, such as average fiber diameters in the nanometer range, high porosities, large surface areas, fully interconnected pore structures, and sufficient mechanical strengths. These outstanding properties make electrospun fibers attractive for a wide range of applications, including tissue engineering, wound dressing, military protective clothing, filter media, as well as nanosensor and electronics applications (Hou and Reneker 2004, Lai et al. 2008, Mitra, Shukla, and Sampath 2001, Kim and Yang 2003). Formhals Anton (Anton 1934) was the first who have patented a detailed method to produce the polymer filaments using electrostatic force in more practical terms in 1935. Recently, electrospinning has gained momentum due to its controlled parameters which help in creating fibers from sub-micrometer to nanoscale through an electrically charged jet of a polymer solution/melt (Feng, Khulbe, and Matsuura 2010). In electrospinning, a solid fiber is generated from a suitable viscous polymeric solution that is continuously stretched due to the electrostatic repulsions between surface charges and the evaporation of the solvent (Zhuang). Nanofibers have great potential to test fundamental quantum mechanics concepts and to play important role in various applications such as photonics, nanoelectronics and data storage. In addition, many special and fascinating properties have been proposed and demonstrated for this class of materials including higher luminescence efficiency and superior mechanical toughness, enhancement of the thermoelectric (Ramaseshan et al. 2007).

1.1. Smart Material Nanofibers (SMNFs)

Nanofiber consists of two terms “Nano” and “fiber.” Nano means a material having of size in the range of nanometers and fiber means a thread or filament-like structure of material. Nanofibers are the fibers with diameters less than 100 nanometers (Kumar 2012). Smart material nanofibers are that material in nanofiber form which has one or more properties that can be significantly changed in a controlled manner by different fields, such as temperature, pressure, electric flow, magnetic flow, light, mechanical, etc., originating internally or externally (Shandilya, Rai, and Singh 2016, Kamila 2013). Here are the properties (Nayak et al. 2011, Khan et al. 2013) which making Nanofibers – Smart Materials Nanofibers.

1.1.1. Larger Surface Area to Volume Ratio The high surface area-to-volume ratio (Huang et al. 2003, Nayak et al. 2011) is ideal for cell attachment and drug loading (Pham, Sharma, and Mikos 2006, Yoo, Kim, and Park 2009). Compared to macro scale surfaces, nanofibers have shown higher rates of protein 4 Madan Lal, Mamta Shandilya, Seema Sharma et al. adsorption, a key mediator in cell attachment to a biomaterial surface. For example, poly(l- lactic acid) (PLLA) fibers with diameters ranging from 50 to 500 nm were shown to have four times higher rates of protein adsorption than porous PLLA constructs with macro-scale features. Additionally, the nanofibrous constructs were found to selectively enhance the adsorption of specific proteins, such as fibronectin and vitronectin (Woo et al. 2007), which is significant as fibronectin is one protein known to mediate cell adhesion and to bind many growth factors (Tayalia and Mooney 2009).

1.1.2. Extremely Small Pore Size Fibers that have porous cross sections also have increased the specific surface area, which enhances the ability of the fibers to absorb oil (Lin, Ding, et al. 2012). Using a BET instrument, (Lin, Ding, et al. 2012, Lin et al. 2013) have found that the pore size distribution curves by nitrogen adsorption– desorption isotherms of the fibers could be categorized as type II. In which the pores of the fibers are mesopores (2–50 nm pore width) and macropores (N50 nm pore width). Fibers with small diameters and high porosity have a high capacity for the absorption of oil due to the significant number of interconnected voids (Zhu, Qiu, et al. 2011). The small diameter of a fiber also favors the adhesion of high-viscosity oil (Lee, An, et al. 2013) (Wu, Wang, et al. 2012). Fibers that have larger diameters have larger void spaces between the fibers, and they have comparatively low capacities for the absorption of oil (Lin, Shang, et al. 2012).

1.1.3. Light Weight and Flexibility Electrospun carbon nanofibers (CNFs) paper has attractive features, such as lightweight (Afghahi et al. 2016, Wang et al. 2016, Zhang et al. 2016), flexibility (Dalmas et al. 2007, Alcoutlabi et al. 2016, Huang et al. 2016), high conductivity, and large accessible specific surface (Inagaki, Yang, and Kang 2012). Besides the direct use as electrode materials for the electric double-layer capacitor (EDLC), (Xu et al. 2015) CNF paper can also serve as the porous backbone to support active materials (Wang et al. 2013). The flexible CNF paper not only enables full utilization of active materials and fast electronic/ionic transfer but also avoids using a binder. In contrast to the commercial carbon, cloth-composed wove fibers with diameters usually in the micrometer size, the fabrication of CNF paper can be easily and directly controlled by electrospinning-nanosized fibers. As a result, the CNF paper has a much larger accessible specific surface than carbon cloth, thus leading to a much higher mass loading of active materials (Xu et al. 2015).

1.1.4. Superior Mechanical Properties Further, nanofibers have been shown to display unique mechanical properties. Specifically, the tensile modulus (Tan and Lim 2005, 2004, Chew et al. 2006, Shin et al. 2006), tensile strength (Chew et al. 2006) and shear modulus (Yang et al. 2008) have been shown to increase as fiber diameter decreases. The decrease in fiber diameter leads to an increase in macromolecular chain alignment within the fibers (Yang et al. 2008, Tan, Ng, and Lim 2005, Tan and Lim 2006), with nanofibers of a smaller diameter having a higher degree of crystallinity (Lim, Tan, and Ng 2008). This might especially be true of electrospun fibers, where flow-induced crystallization is thought to occur during spinning (Zhmayev, Cho, and Joo 2010). These unique mechanical properties are useful for modulating cell behavior as Smart Material Nanofibers for Day to Day Life 5 well as providing adequate tension and strength to resist the forces from the cell cytoskeleton (Chew et al. 2006).

2. METHODS OF FABRICATION OF SMNFS

 Drawing  Template synthesis  Phase separation  Self-assembly  Electrospinning

2.1. Drawing

In this process, a glass micropipette having a diameter of a few micrometers was dipped into the droplet near the contact line using a micromanipulator. The micropipette was withdrawn from the liquid and moving with a speed of approximately 1x10-4 m/s, resulting in the formation of nanofibers (Kumar 2012). The fibers are fabricated by contacting a previously deposited polymer solution droplet with a sharp tip and drawing it as a liquid fiber which is then solidified by rapid evaporation of the solvent due to the high surface area (Bajakova et al. 2011). This process was repeated several times on every droplet to form nanofiber (Joachim 1998). The viscosity of the droplet continuously increases with time due to solvent evaporation from the deposited droplet. The frequent shrinkage in the volume of the polymer solution droplet affects the diameter of the fiber drawn and limits the continuous drawing of fibers (Nain et al. 2006). The solution was concentrated at the edge of the droplet and broke in a cohesive manner. Hence, drawing process requires a viscoelastic material that can undergo strong deformations while being cohesive enough to support the stresses developed during pulling (Kumar 2012).

2.2. Template Synthesis

Metals deposition can be done within the pores of the template membranes by either electrochemical or chemical (“electroless”) reduction of the appropriate metal ion. Electrochemical deposition is accomplished by simply coating one face of the membrane with a metal film. This metal film is used as a cathode for electroplating (Brumlik and Martin 1991, Martin 1995, Chakarvarti and Vetter 1991) this method has been used to prepare copper, platinum, gold, silver, and nickel fibrils. Fibrils lengths can be controlled by varying the amount of metal deposited. By depositing a small amount of metal, short, squat fibrils can be obtained. Alternatively, long, needlelike fibrils can be prepared by depositing large quantities of metal. This ability to control the aspect ratio (length to diameter) of the metal fibril is especially important in our optical investigations because the optical properties of nanometals are dependent on the aspect ratio (Foss Jr et al. 1994). To conduct an electroless deposition of metal within the pores of the template membrane, a catalyst must be applied to 6 Madan Lal, Mamta Shandilya, Seema Sharma et al. the pore walls (Menon and Martin 1995). As a result, we have a “molecular anchor,” and metal tubules are obtained after brief deposition times. These tubules close up to form solid metal fiber at longer deposition times. Unlike the electrochemical method, where the length of the metal fiber can be controlled at will, the electroless method yields fibrils or tubules that run the complete width of the template membrane.

2.3. Phase-Separation

In phase-separation, it is difficult to control the porous morphology of nanofibers because it involves five basic steps: polymer dissolution, solvent extraction, gelation, freezing, and freeze-drying (Kumar 2012). The polymer solution was rapidly transferred into a refrigerator at the gelation temperature by maintaining the homogeneity of the solution at required concentration (Ma and Zhang 1999). Polymer concentration and the gelation temperature help in varying the duration of gel formation. The nano-scale fiber network is formed at low gelation temperature, whereas, high gelation temperature led to the formation of bead-like structure. The uniform nanofibers formation can be achieved by controlling the cooling rate. Nanofibers properties can be tuned by polymer concentration. As the polymer concentration is increased, the porosity of fiber decreased which leads to increase mechanical properties of fiber (Kumar 2012). Then gel is dissolved in distilled water for solvent exchange. The distilled water is removed with the help of filter paper and finally, the residue is transferred to freeze-drying vessel resulting in the formation of nanofiber matrix (Ma and Zhang 1999).

2.4. Self-Assembly

Self-assembly is a ‘bottom-up’ manufacturing technique. In this manufacturing method, small molecules are used as basic building blocks. The small molecules are aligned along a normal plane which produces the longitudinal axes of the fibers. These molecules combine with each other for self-assembly by a chemical process called convergent synthesis method. In self-assembly, the final desired product is ‘encoded’ in the shape of the small blocks, as compared to lithography where the desired structure must be carved out from a large block of matter. Because lithography is a ‘top-down’ technique. In this method, the shape, size, and properties of nanofibers depend upon the nature of molecules and the intermolecular forces which binding the molecules (Hartgerink, Beniash, and Stupp 2001, Nayak et al. 2011).

2.5. Electrospinning

2.5.1. History of Electrospinning Electrospinning is very old technique. It was first observed by Rayleigh in 1897 (Keulder 2013), studied in detail by Zeleny in 1914 (Bhattarai et al. 2014). Formhals Anton (Anton 1934) published a series of patents from 1934 to 1944, describing an experimental setup for the production of polymer filaments using an electrostatic force. In this technique, the electric field was applied to the polymer solution. The polymer solution was passed through the Smart Material Nanofibers for Day to Day Life 7 electric field between two electrodes resulting in a plurality of threads. One electrode was connected to the spinneret and other to the collector. When electric field overcome the surface tension of polymer solution, then solution spills out of the spinneret and form extremely fine continuous fibers. These fibers were collected on the collector. The fibers production was modified by reversing the electric field. The potential difference (about 5 to 10 kV) employed depends upon the properties of the spinning solution. In 1952, Vonnegut and Neubauer (Vonnegut and Neubauer 1952) were able to produce streams of highly electrified uniform droplets of diameter about 0.1 mm. A glass tube was drawn down to a capillary having a diameter in the order of a few millimeters. The tube was filled with water or some other liquid and an electric wire connected to a source of variable high voltage (about 5–10 kV) was introduced into the liquid (Huang et al. 2003, Anton 1934, Burger, Hsiao, and Chu 2006, Li and Xia 2004). In 1955, Drozin (Drozin 1955) investigated the dispersion of a series of liquids into aerosols under very high electric potentials. He used a glass tube ending in a fine capillary similar to employed by Vonnegut and Neubauer in their experiment. He found that for certain liquids and under proper conditions, the liquid was supplied from the capillary as a highly dispersed aerosol consisting of droplets of a relatively uniform size. He also captured different stages of the dispersion (Drozin 1955). In 1964 Taylor (Taylor 1964) gave a theory of charged droplets states that if an electric field is applied to any droplet, the electric charge generates an electrostatic force inside the droplet, called as the Coulomb force, which competes with the cohesive force intrinsic to the droplet. When the applied Coulomb force overcomes the cohesive force of the droplet manifested in the surface tension, the droplet will result into smaller droplets in the micro to nano-scales. This phenomenon begins at the Taylor Cone, referring to the progressive shrinkage of the unstable, charged macro-droplet into a cone from which the smaller charged droplets will be ejected as soon as the surface tension is overcome by the Coulomb force (Bock et al. 2011). In 1971, Baumgarten (Baumgarten 1971) made an apparatus to electrospun acrylic fibers with diameters in the range of 0.05–1.1 microns. The spinning drop was suspended from a stainless-steel capillary tube and maintained constant in size by adjusting the feed rate of an infusion pump. A high-voltage dc current was connected to the capillary tube whereas the fibers were collected on a grounded metal screen (Drozin 1955, Burger, Hsiao, and Chu 2006). Typically, there are two types of electrospinning setups; horizontal and vertical which are shown in Figure 1. (a and b). In the horizontal status, the syringe pump is located parallel to the ground and the collector is located vertical to it, through the needle of the syringe. Since the resulting electrical field vector is parallel to the ground, this setup is named horizontal. In the vertical status, the collector is located on the ground and the syringe pump is placed above the collector and the resultant electrical field vector is vertical to the ground. The electrospinning process starts at the center of the collector plate which the syringe pump is located across its center for collecting the fibers. Among various chemical or physical synthetic approaches, electrospinning appears to be the most straightforward and versatile technique for the fabrication of nanofibers. Electrospinning is currently the only technique that allows the fabrication of continuous fibers with diameters down to a few nanometers (Wu, Pan, et al. 2012). It is not only employed in university laboratories but is also increasingly being applied in industry. The scope of applications, in fields as diverse as filtration, sensor technology, optoelectronics, catalysis, biomedical and medicine, is very broad (Li, Wang, and Xia 2003, Li et al. 2006). 8 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Figure 1. Typical schematic diagram of electrospinning setup (a) horizontal setup and (b) vertical setup (Bhardwaj and Kundu 2010).

2.5.2. Fundamentals of Electrospinning Comparing various method of fabrication of nanofibers, electrospinning is one of the most versatile and efficient fabrication methods in which molten material can be turned into fibers with diameters ranging from nanometers to few micrometers (Anton 1934, Shafii 2014). Electrospinning is the process using electrostatic forces to form a fine filament of the polymer solution. The range of “electro spinnable” materials includes organic or synthetic polymers, polymer alloys, metals and ceramic materials (Greiner and Wendorff 2007). Electrospinning is based on the electrostatic repulsion of charges which causes the liquid stretching into the fiber in the presence of electric field. There is no breakage in the stretched Smart Material Nanofibers for Day to Day Life 9 solution, results in a single continuous fiber formation upon solvent evaporation (Li and Xia 2004). The electrospinning setup mainly consisting of three components as shown in Figure 1. (b), (i) A High voltage power supply, (ii) The Collector and iii) The Syringe (which is filled with polymer solution). These three components are playing very important role in the formation diameter and the morphology of nanofibers. When a high voltage is applied to the metal syringe needle, electrical charge builds up on the surface on the solution. As the solution is ejected by the needle tip, transit to a predominantly convective current. The charges are transferred from the needle to target through the deposition of the fiber. The current stops oscillating when the deposition becomes stable (Li, Wang, and Xia 2003). This can be used to control the spinning process. Finally, the nanofibers are collected and aligned on the collector plate (Khan et al. 2013). Although most reported electrospinning experiments were carried out by using positive potential as compared to the negative potential. The reason is that electrons can be dispersed more rapidly and uniformly than the much heavier protons. It also has been reported that negative potential produces nanofibers with small diameter (Huang et al. 2003, Pham, Sharma, and Mikos 2006). As nanofibers result from evaporation or solidification of polymer fluid jets, the size of the fiber depends upon jet as well as on the polymer contents in the jet. It is reported that during the transformation of solution jet from the needle to collector plate, the solution jet may be or not may be split into multiple jets, resulting in a fiber with different diameters (Bergshoef and Vancso 1999, Deitzel et al. 2001, Koombhongse, Liu, and Reneker 2001).

2.6. Electrospinning Parameters

Figure 2. A flow chart representing the different electrospinning parameters. 10 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Different parameters are tested to achieve different sizes and shapes of nanofibers for the industrial applications. System parameters include solvent types and polymer and structures, while process parameters include electric potential, polymer concentration and flow rate; distance between the capillary and collection screen; temperature, humidity, and air velocity effects in the chamber; and additions conductive nanoparticle (Fe, Au, Ag, and Pt) to improve polymer charge capacity during the electrospinning process. By controlling these parameters, we can modify the nanofiber size and shape nanofibers according to their applications. These various parameters are divided into three categories i.e., instrumental parameter, solution parameter and environmental parameter respectively (Sigmund et al. 2006, Dersch et al. 2007, Sharma et al.) shown in Figure 2.

2.6.1. Instrumental Parameters

2.6.1.1. Voltage At high voltage, a large amount of charge causes the faster acceleration of jet, which leading to smaller and unstable Taylor. High voltage provides greater stretching in the solution due to the small diameter fiber formation (Kumar 2012). At low voltage, the time of flight of fiber to collector plate increases, which leads to the formation of fine fiber. Thus at high voltage have a greater tendency for bead formation (Deitzel et al. 2001, Buchko et al. 1999) with better crystallinity in the fibers. But polymer molecules don’t have much time to align themselves and fibers are formed with less crystallinity. Higher voltages can increase the electrostatic repulsive force, favoring the narrowing of fiber diameter (Kumar 2012, Reneker and Chun 1996). Thus, we can found that voltage does influence fiber diameter, but the level of significances varies with the polymer solution concentration and on the distance between the tip and the collector (Yördem, Papila, and Menceloğlu 2008).

2.6.1.2. Spinneret

Figure 3. Sketches of the possible geometries of the electrospinning spinnerets (Persano et al. 2013). Smart Material Nanofibers for Day to Day Life 11

The traditional “needle-type” electrospinning method is, in principle, subject to problems related to polymer clogging at the spinneret nozzle, which may limit the achievable throughput of continuous production processes. Such clogging events can occur because of the fineness of the needle, and become more frequent for high solution concentrations or when spinning composite blends embedding nanoparticles (Persano et al. 2013). In this respect, electrospinning techniques possess greater up-scaling potentialities, because of availability of different types of spinnerets (as shown in Figure 3.). Thus, due to this availability the electrospinning equipment has overcome most of the issues and to meet industrial requirements for functional nanofibers in the different application fields.

2.6.1.3. Distance between the Electrodes In electrospinning distance between the needle and the collector ‘d’ affecting the electric field strength ‘E’ by relation (E = V/d), where ‘V’ is voltage (Wang and Kumar 2006). A Larger distance between the electrodes provides the more time for the drop stretching before deposition on the collector in the presence of electric field. Moreover, the solvent has more time for evaporation (Jia et al. 2008). On increasing distance between the electrodes (i.e., needle and the collector), electric field strength gets decreasing because of the dependence of E on d (Pena 2009, Matthews et al. 2002). Some literature reported that the thicker and thinner fiber formation can be controlled by the distance between the electrodes. By optimizing the distances between the electrodes we can produce the fine diameter and good morphology fiber (Thompson et al. 2007, Ki et al. 2005).

2.6.1.4. Collector Generally, aluminum (Al) is used as a collector to collect both random and aligned fibers (Wang et al. 2005, Park et al. 2007). Al foil is a very good conductive substrate to collect the nanofibers (Li and Wang 2013). To achieve the stable potential difference between the electrodes, the collector is grounded. The collector is always made up of conductive material which helps to improve the fiber deposition with high packing density. A good conductive collector material helps to collect the fibers with thickness and thinner diameters with appropriate size distribution (Yang et al. 2009). Fibers can dry by rotating the collector as it provides more time for the evaporation of the solvent (Jiri and Marek 2011). Thus, to enhance the properties of the fibers there are various collectors can be used which are shown in Table 1.

Table 1. Schematic diagram of various electrospinning setups for multiple spinnerets and to obtain various fibrous assemblies

1. Rotating drum collector Advantage (Matthews et Simple set-up al. 2002) Large area of aligned fibers can be fabricated Disadvantage Highly aligned fibrous assemblies are difficult to fabricate Fiber breakage may occur if rotating speed is too high 12 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Table 1. (Continued)

2. Rotating wire drum collector Advantage (Katta et al. Simple set-up 2004) Highly aligned fibers are possible

Disadvantage Thicker layer of aligned fibers are not possible Fibers may not be aligned throughout the whole assembly

3. Drum collector with wire wound on it Advantage (Bhattarai et Simple set-up al. 2005) Highly aligned fibers are possible Area of aligned fibers on the wire is adjustable by varying wire thickness

Disadvantage Aligned fibers are concentrated on the wire instead of the whole drum

4. Rotating tube collector with knife-edge Advantage (Ramakrishna electrodes below Highly aligned fibers possible et al. 2005) Aligned fibers covered the whole tube Thicker layer of aligned fibers deposition is possible

Disadvantage Set-up requires a negative electrode to be effective Only possible for small diameter tube

Smart Material Nanofibers for Day to Day Life 13

5. Controlling electrospinning Advantage jet Advantage (Ramakrishna using knife-edge electrode jet using knife-edge electrodes Highly et al. 2005) aligned fibers possible Able to control the direction of fibers alignment on the tube Thicker layer of aligned fibers deposition is possible

Disadvantage Set-up requires a negative electrode to be effective Only possible for small diameter tube

6. Disc collector Advantage (Theron, Simple set-up Zussman, and Highly aligned fibers are possible Yarin 2001), Able to fabricate arrayed fibers by (Ma et al. attaching a rotatable table on the edge 2005) of the disc

Disadvantage Unable to retain high fibers alignment at the same rotating speed when the deposited fibers are thicker Small area of fibers alignment

7. Rotating drum with sharp pin inside Advantage (Sundaray et Large area of arrayed fibers can be al. 2004) fabricated

Disadvantage Set-up is complicated Thicker area of arrayed fibers assembly may not be possible

14 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Table 1. (Continued)

8. Yarn collection using water bath Advantage (Khil et al. Simple set-up 2005) Long continuous yarn can be fabricated Fibers in the yarn are generally well aligned

Disadvantage Yarn collection speed is relatively slow

2.6.1.5. Flow Rate

Figure 4. SEM micrographs of electrospun nanofibers (TCD=10 cm, Voltage=12 kV). (a) flow rate of 10 ml/h; fiber diameter 30-60 nm, bead diameters 300 nm, (b) flow rate of 6ml/h; fiber diameters 50- 100 nm, bead diameters 440 nm, (c) flow rate of 1.6 ml/h; fiber diameters 170-220 nm, non-beaded structure (d) flow rate of 1.1 ml/h; fiber diameters 230-476 nm, non-beaded structure (Rodoplu and Mutlu 2012). Smart Material Nanofibers for Day to Day Life 15

Another important factor for electrospinning process is the flow rate of the polymer solution from the syringe. In general, a lower feed rate is more recommended because the solvents have enough time for evaporation. Thus, uniform and smooth fibers tend to form whereas at high flow rate beaded structures will be obtained due to inaccessibility of suitable drying time to reaching the collector (Greiner and Wendorff 2007). By decreasing the flow rate there is a decrease in the bead size and increase the average diameters of the nanofibers (as shown in Figure 4.) (Tripatanasuwan, Zhong, and Reneker 2007). Because increasing the flow rate decreases the shear force on the surface per unit volume solution, which results in the formation of thicker fibers. In the electrospinning process, a lower flow rate is normally utilized to ensure that the solvents completely evaporate from the nanofibrous scaffolds during the process (Tripatanasuwan, Zhong, and Reneker 2007, Hasan et al. 2014).

2.6.2. Solution Parameter

2.6.2.1. Viscosity Solution viscosity has direct dependence with solution concentration. The increase in the solution concentration always provides an increase in solution viscosity and vice-versa. These two factors play a crucial role for fine fiber morphology. As the increase in the solution concentration and viscosity, lowering the surface tension which favors the formation of uniform fibers (Larrondo and St John Manley 1981). The high viscosity means a stronger stretching in liquid drop at high voltage (Thompson et al. 2007), but with very high viscosity difficult to eject the solution from the needle tip. This may cause the drying of the polymer solution in the tip of the needle. Whereas very low viscosity leads to bead formation with fiber formation (Kumar 2012). Various papers have reported that viscosity is a critical key to form continuous and uniform fiber with better morphology (Huang et al. 2008, Ding et al. 2002, Ki et al. 2005, Lee et al. 2004).

2.6.2.2. Surface Tension Different solvents have different surface tension. Surface tension can be tuned by viscosity (Li and Wang 2013). Lower viscosity leads to decrease in the surface tension which resulting in the beads formation along the length of fiber. Surface tension gets neutralize at a high viscosity of solution due to the uniform distribution of solution over the twisted polymer molecules (Kumar 2012). So keeping the concentration fixed, reducing the surface tension of solution resulting into smooth fiber formation (Yang et al. 2004). Surface tension can be lowered by adding surfactants in the polymer solution (Kumar 2012).

2.6.2.3. Molecular Weight The molecular weight (MW) is the reflection of the entanglement of polymer chains in solutions. Morphology of fibers can be monitored by the molecular weight. Low molecular weight beads formation whereas high molecular weight favors for the formation of smooth fiber (Koski, Yim, and Shivkumar 2004). A polymer solution with too high MW having low concentration result into the formation of micro-ribbons.

16 Madan Lal, Mamta Shandilya, Seema Sharma et al.

2.6.2.4. Solution Concentration Drastic changes in the morphology were observed when the concentration of the polymer solution was changed (Zong et al. 2002). Thus, solution concentration plays an important role in the fiber formation. Solution concentration has a significant effect on the final size and distribution of particles. Solution concentration range can be determined by the surface tension and viscosity of the solution (Deitzel et al. 2001). The increase of solution concentration leads to the formation of uniform fiber with a larger diameter. At very low concentration the beads nanofibers can be obtained with rough morphology. The morphology strongly depends upon the nature of polymer solution (Yang et al. 2004). The high volatility of solvent allows adequate evaporation of the solvent before the deposition of fiber in collector plate leading to the formation of highly aligned fibrous structure. While non-volatile solvent reduces the evaporation and leading to deposition of wet fiber on the collector plate (Haroosh, Chaudhary, and Dong 2011).

2.6.2.5. Solution Conductivity Solution conductivity is mainly determined by the nature of polymers, the solvent used and the presence of ionizable salts. The jet formation is highly influenced by the charged ion present in the polymer solution (Bhardwaj and Kundu 2010). Number of the researcher has reported that jet radius is varied inversely with the cube root of the electrical conductivity of the solution (Baumgarten 1971, Fong et al. 1999, Zuo et al. 2005, Haghi and Akbari 2007, Sharma 2013). Electrospun nanofibers with the smallest fiber diameter can be obtained with the highest electrical conductivity and it has been found that there is a drop in the size of the fibers is due to the increased electrical conductivity (Bhardwaj and Kundu 2010). Electric Conductivity of the polymer solution further can be enhanced by the addition of salts (like NaCl, KH2PO4, NaH2PO4, etc.). The addition of the salts results into the higher charge density on the surface of the solution. The size of ions has an important impact on the diameter of fiber. Small size ions have high charge density and hence have high mobility under the application of an external electric field (Zong et al. 2002). Many researchers have reported that addition of ionic salts favors the beadless fiber with better morphology and smaller diameters (Mandal, Yoon, and Kim 2011, Huang et al. 2008, Costa, Bretas, and Gregorio 2010). Some organic acid is also used to achieve the highly conductive solution. Pyridine has been used to eliminate the beads and increasing the conductivity of the solution (Li and Wang 2013).

2.6.3. Environmental Parameter

2.6.3.1. Temperature Temperature is an ambient parameter which has a direct dependence of the fiber diameter. Temperature has an effect on the average diameter of the nanofibers because it is related to the evaporation rate of the solvent and rigidity of the polymer chain (Fang, Wang, and Lin 2011). The fiber diameter decreases with increasing temperature. The increase in temperature caused the decrease of solution viscosity, surface tension, conductivity, and resulting fiber diameter. In the case of polyamide-6 fibers ranging from 25 to 60°C temperature, it found that with an increase in temperature, there is a yield of fibers with decreased fiber diameter (Mit‐uppatham, Nithitanakul, and Supaphol 2004). Smart Material Nanofibers for Day to Day Life 17

2.6.3.2. Humidity The variation in humidity while spinning of polymer solutions has been studied and shows that with an increase in the humidity leads to an appearance of small circular pores on the surface of the fibers (Casper et al. 2004). The increase in the relative humidity leads to decrease in evaporation rate which favoring fiber formation with good morphology. The increase in water concentration in the atmosphere also slowing the solidification process which provides more time of flight and finer fiber formation takes place (Hardick, Stevens, and Bracewell 2011). It has also been reported that the high humidity helpful in the discharging of the fibers during the electrospinning, because of the presence of gas or water concentration in the surrounding (Sharma 2013). Bognitzki et al. (Bognitzki, Czado, et al. 2001) reported that a lower vapor pressure solvent was used to reduce the formation of pores on polylactic acid porous nanofibers. Casper et al. (Casper et al. 2004) observed that no. of pores on the surface goes on increasing as the humidity goes on increasing (i.e., the pore diameter and the pore size distribution). High molecular weight solutions cause fibers to contain larger pores that are less uniform in shape and size.

3. ADVANTAGES OF ELECTROSPINNING

3.1. Control on Fibers Dimension and Morphology

The fibers morphology and dimensions can be adjusted during the electrospinning and calcination process. For example, the addition of Zn(Ac)2·2H2O to the precursor solution changes its response to the electric field, such that the resulting fibers are of smaller diameter than those produced from a polymer solution. The fibers obtained by electrospinning a precursor solution containing 7.5 wt.% Zn(Ac)2·2H2O are smooth, continuous and exhibit an average diameter of about 500 nm (Figure 5. a and a’). The thermal treatment further reduces the fiber diameter, to around 250 nm, but also modifies the overall aspect of the non-woven mats, which became discontinuous, most likely due to the contraction caused by the burning of the polymer matrix. Moreover, in the case of using a 2 or 5 wt.% Zn(Ac)2·2H2O precursor solution, the calcined fibers have the tendency to break and form disparate particles (Figure 5. (b and b’) and (c and c’)) (Busuioc et al. 2014), a phenomenon that is caused by the Rayleigh instability (Fan et al. 2012). It can be concluded that an insufficient content of Zn2+ in the electrospinning solution leads to the formation of discontinuous ZnO fibers or even ZnO particles. As well, the crystallites size and the apparent quality of individual fibers increase with the calcination temperature. The samples treated at 450°C present fibers interconnected by membranes as aggregates of ZnO crystallites (Figure 6. (a and a’)). At 600 and 800°C (Figure 6. (b and b’) and (c and c’)) the membranes disappear and the crystallites join in larger grains which represent the segments of individual ZnO fibers (Busuioc et al. 2014).

18 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Figure 5. SEM images of the precursor fibers (a and a’) ZnO fibers prepared from 2 wt.% (b and b’) and 5 wt.% (c and c’) Zn(Ac)2·2H2O solution and calcined at 800°C (Busuioc et al. 2014). Smart Material Nanofibers for Day to Day Life 19

Figure 6. SEM images of ZnO fibers prepared from 7.5 wt.% Zn(Ac)2·2H2O solution and calcined at 450°C (a and a’), 600°C (b and b’) and 800°C (c and c’) (Busuioc et al. 2014).

3.2. Incorporation of Other Materials into SMNFs

Electrospun nanofibers are widely used in drug delivery due to the flexibility in material selection a number of drugs can be delivered including antibiotics, anticancer drugs, proteins, and DNA. Using the various electrospinning techniques, a number of different drug loading methods can also be utilized: coatings, embedded drug, and encapsulated drug (coaxial and 20 Madan Lal, Mamta Shandilya, Seema Sharma et al. emulsion electrospinning). These techniques can be used to give finer control over drug release kinetics (Sill and von Recum 2008). The basic mechanism of drug loading into the nanofiber:

 Dissolve the drug in any solvent like distilled water or cellulose or acidic or basic medium in which the drug can be dissolved easily.  Prepare a solution of the polymer in which drug have to loaded.  Mixed both the solution together and stirred it for acquiring homogeneity in the solution.  The electrospun the solution by optimizing the electrospinning parameters.  The drug-loaded nanofibers were collected and dried overnight under vacuum at room temperature (Kataria et al. 2014, Taepaiboon, Rungsardthong, and Supaphol 2006, Sasikala et al. 2016).

In this way, we can incorporate any type of drug into the nanofibers. These nanofibers have a broad range of biomedical applications such as in biocompatible valves in the body that could be opened and closed magnetically. The initial applications could in vitro where magnetically-addressable, blood-compatible fibers with conjugated antibodies could be added to blood (ex vivo in an extracorporeal circuit) without the requirement of anticoagulation to bind specific proteins or toxins removal of from the blood. These fibers could also be used to prepare non-woven membranes for blood filtration circuits that could be moved into the blood flow for filtration and out of the blood flow for cleaning/de-fouling. In vivo applications are much further in the future but may take the form of a magnetically actuated non-woven heart valve that can be seeded with cells and implanted. Alternatively, they could be used in the purification of heparin binding proteins or in blood collection and ex vivo blood diagnostics. An interesting direction for future research would be the use of heparin to selectively bind proteins in blood with membranes modified with heparin to obtain the retention of desired proteins. Considering that membranes are heavily used in the downstream separations train of biologics, heparin modified membranes would be interesting to investigate (Hou et al. 2016).

3.3. Fabrication of Different Types of SMNFs

There are numerous types of materials that could be used for electrospinning, and individual material properties must be considered depending on its applications. The electrospinning process may be modified so as to yield electrospun fiber with the desired morphology and properties. In the production of ceramic fibers, post processes are required after the fibers are electrospun. Up to now, more than 200 types of electrospun nanofibers have been fabricated, including natural polymer, synthetic polymer, ceramic, and carbon. Thus, it is important to have a basic understanding of the different groups of materials before selecting the most appropriate electrospun fibers for specific applications. Nanofibers are made by the electrospinning of oxide/acetate/chloride/hydroxide precursors in the presence of polymer followed by calcination at higher temperatures. In order to generate well-controlled and high-quality ceramic nanofibers by electrospinning, one typically process has to use the following procedure: Smart Material Nanofibers for Day to Day Life 21

 Preparation of an electrospinning solution containing a polymer and sol-gel precursor to the ceramic material.  Electrospinning of the solution under appropriate conditions generate precursor nanofibers containing inorganic precursor and polymer assistant materials.  Calcination of the precursor nanofibers at high temperature to remove polymers and obtain the ceramic phase.

Various nanofibers have been being synthesized by this approach. Notable examples includes ZnO (Xiaolu et al. 2009), CuO (Sharma et al. 2013), NiO, TiO2, SiO2, Co3O4, Al2O3, SnO2, Fe2O3, LiCoO2, BaTiO3, LaMnO3, NiFe2O4, LiFePO4, PbMgNbO3-PbTiO3 (Xu, Poirier, and Yao 2012), BiNaKTiO3 (Chen, Zheng, et al. 2010), KNaNbSbO3-BiNaKZrO3 (Zhu et al. 2016), BiCeTi3O12 (Jiang et al. 2012), BaCaTiSnO3 (Sahoo and Panda 2015), BaCaTiO3-BaZrTiO3 (Fu et al. 2015). Electrospinning technique can also be used for fabricating nanofibers composed of non-oxide ceramics including carbide, boride, nitride, silicide and sulphide (Lin et al. 2007, Wu et al. 2009, Rose et al. 2010, Kim, Yun, and Lee 2010, Cui et al. 2008). Wu et al. (Wu et al. 2009) synthesized non-oxide nanofibers of nitride via electrospinning for the first time.

3.4. Use of Various Types of Polymers

Typical spinnable precursor solution should contain a salt precursor, a polymer and a relatively volatile solvent such as ethanol, water, isopropanol, chloroform, and dimethylformamide (DMF) (Ramaseshan et al. 2007, Pham, Sharma, and Mikos 2006, Li, Herricks, and Xia 2003, Park et al. 2008). Poly(vinyl alcohol) (PVA) is one of the most popular polymers employed as a matrix due to its high solubility in water and its good compatibility with many salts, including zinc acetate and copper nitrate (Xiaolu et al. 2009). In addition, other polymers, such as poly(vinyl pyrrolidone) (PVP), poly(vinyl acetate) (PVAc), poly-acrylonitrile (PAN), poly(methyl methacrylate) (PMMA), poly(acrylic acid) (PAA) (Ramaseshan et al. 2007, Dersch et al. 2007, Sharma et al. 2013, Wu, Lin, and Pan 2006, Shao et al. 2004, Bhatnagar et al. 2016, Bao et al. 2016), poly(ethylene glycol) (PEG) (Bhatnagar et al. 2016), poly(D,L-lactide-co-trimethylene carbonate) (PLMC) (Bao et al. 2016), poly(ethylene oxide) (PEO) (Bao, Clarke, and Gorga 2015), poly(ethylene terephthalate) (PET) (Lee and Cho 2015), polystyrene (PS), poly(vinyl chloride) (PVC), Nylon-6, poly(caprolactone) (PCL), poly(p-phenylene-terephthalamide) (PPTA), poly(vinylidene fluoride) (PVDF), polybenzimidazole (PBI), polyurethanes (PUs), polycarbonates, polysulfones (Burger, Hsiao, and Chu 2006), Cellulose (Hou et al. 2016) have also been widely used.

4. PIEZOELECTRICITY IN SMNF

Piezoelectricity, a property possessed by a select group of materials, was discovered in 1880 by Jacques and Pierre Curie during their systematic study of the effect of pressure on the generation of electrical charge by crystals, such as quartz, zinc-blende, and tourmaline. The 22 Madan Lal, Mamta Shandilya, Seema Sharma et al. name “piezo” is derived from the Greek, meaning “to press;” hence, piezoelectricity is the generation of electricity as a result of a mechanical pressure (Haertling 1999). The piezoelectricity of any material is measured by its piezoelectric coefficient or piezoelectric modulus (D). ‘D’ can be defined as the change in volume that it undergoes when subjected to an electric field or as the polarization that it undergoes when mechanical stress is applied. This is mathematically represented as D = P/σ, with ‘P’ denoting the polarization and ‘σ’ the stress. There can be many piezoelectric coefficients, depending on the crystal orientation (Damjanovic 1998). The concept of piezoelectricity in solids begins with an understanding of the internal structure of the material. For example, here consider a single crystallite. This crystallite has a definite chemical composition and, hence, is made up of ions (atoms with positive or negative charge) that are constrained to occupy positions in a specific repeating relationship to each other, thus building up the structure or lattice of the crystal. The smallest repeating unit of the lattice is called the unit cell, and the specific symmetry possessed by the unit cell determines whether it is possible for piezoelectricity to exist in the crystal. Furthermore, the symmetry of a crystal’s internal structure is reflected in the symmetry of its external properties (Neumann’s principle) (Afghahi et al. 2016, Li and Mataga 1996). The elements of symmetry that are utilized by crystallographers to define symmetry about a point in space, e.g., the central point of a unit cell, are (1) a center of symmetry, (2) axes of rotation, (3) mirror planes, and (4) combinations of these. All crystals can be divided into 32 different classes or point groups utilizing these symmetry elements, as shown in Figure 7. These 32 point groups are subdivisions of seven basic crystal systems that are, in order of ascending symmetry, triclinic, monoclinic, orthorhombic, tetragonal, rhombohedral (trigonal), hexagonal, and cubic. Out of the 32 point groups, 21 classes are non-centrosymmetric (a necessary condition for piezoelectricity to exist) and 20 of these are piezoelectric. One class, although lacking a center of symmetry, is not piezoelectric because of other combined symmetry elements. A lack of a center of symmetry is all-important for the presence of piezoelectricity when one considers that a homogeneous stress is centrosymmetric and cannot produce an unsymmetric result, such as a vector-quantity-like polarization, unless the material lacks a center of symmetry, whereby a net movement of the positive and negative ions with respect to each other (as a result of the stress) produces electric dipoles, i.e., polarization. Furthermore, for those materials that are piezoelectric but not ferroelectric (i.e., they do not possess spontaneous polarization), the stress itself is the only means by which the dipoles are generated. For piezoelectricity, the effect is linear and reversible, and the magnitude of the polarization is dependent on the magnitude of the stress and the sign of the charge produced is dependent on the type of stress (tensile or compressive) (Haertling 1999). Piezoelectricity is the property that possessed by certain materials, becoming electrically charged when mechanical stress is applied to these materials. Such materials also show the converse effect i.e., there is mechanical deformation on the action of an electric field. There are certain compounds which can be made piezoelectric on the application of a high electric field (polarization), these materials are called ferroelectric materials. The materials with only non-centrosymmetric crystallographic exhibit piezoelectricity. Aluminum nitride (AlN) and quartz are examples of non-centrosymmetric structures that are used in piezoelectric devices. In many of piezoelectric materials, a spontaneous polarization also exists due to the separation of positive and negative charge centers in the crystallographic unit cell. Perovskite structure (i.e., ABO3) is the example which exhibits a spontaneous polarization. The Smart Material Nanofibers for Day to Day Life 23 perovskite structures can undergo several different phase transitions like tetragonal, rhombohedral, monoclinic or orthorhombic when it is cooled from the high-temperature cubic phase (centrosymmetric). In ABO3 structure, B is central atom and oxygen octahedron (O3) displaces non-uniformly relative to the corner atom (A), resulting in a non-centrosymmetric structure. A spontaneous polarization exhibits by the structure at a temperature, when there is phase transition from the high temperature to the first structure and this transformation coincides with the Curie temperature. When a material is cooled to the Curie temperature, different regions of the material goes different types of crystallographic orientations of the lower symmetry crystal structure. These different regions are called as domains and the regions that separate different domains are named as domain walls. Ferroelectricity is the ability of a material to change its direction of spontaneous polarization by applying an electric field. The reverse electric field needed for this reorientation to occur is referred as the coercive field and this involves the motion of ferroelectric domain walls (Matthias and Von Hippel 1948, Haertling 1999). Often there is a distribution of local coercive fields in a polycrystalline material because of the compositional variations across a grain or different stress states of different grains. The electric field required to obtain zero macroscopic polarization is called a coercive field (Ec), which is observed macroscopically this is due to compensating negative and positive local polarization states. On cooling from the high processing temperatures required for ceramic materials, polycrystalline ceramics exhibit neither a net macroscopic spontaneous polarization nor piezoelectricity at the macroscopic scale. The reason behind this is that there is an equal distribution of domains in all directions, the local area of spontaneous polarization cancel each other and hence materials do not exhibit a net macroscopic polarization. In polycrystalline material, the piezoelectricity is exhibiting by polling process at macroscopic scale. When an electric field is applied all the domains are aligned parallel to the electric field direction. After polling, the material has a net macroscopic polarization parallel to the direction of applied field and thus will exhibit piezoelectricity at the macroscopic scale. The converse piezoelectric effect is describing the strain developed in a piezoelectric material due to the external applied electric field. This effect is expressed as:

Si=dijEj (1)

Where Si is the strain developed in the material due to the applied electric field, Ej is the applied electric field and dij is the piezoelectric coefficient. The piezoelectric coefficient is a third rank tensor, although is written in Equation (1) in reduced matrix notation by representing the mechanical strain as a 1-dimensional matrix with elements i = 1, 2…6. The converse piezoelectric effect is used in actuator devices. For sensing applications, the direct piezoelectric effect describing a change in polarization due to an applied mechanical stress and it is expressed as:

Di = dijσj (2)

Where Di is the dielectric displacement and σj is the applied mechanical stress (Sirohi and Chopra 1999, 2000).

24 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Figure 7. The interrelationship of piezoelectric and subgroups on the basis of symmetry (Haertling 1999).

In Equation (1) and (2), the coordinate axes are defined by the polarization of the sample in 1-dimensional. Now, these two equations can be expressed for 3-dimensional. When an electric field is applied parallel to the 3-direction and strain is also measured in the 3- direction, the piezoelectric coefficient of relevance is the longitudinal piezoelectric coefficient, d33:

S3 = d33E3 (3)

Mathematically the piezoelectric coefficients described by the direct and converse piezoelectric effects are equivalent. Therefore, the longitudinal piezoelectric coefficient described by the converse piezoelectric effect (in Equation (3)) is equivalent to the longitudinal piezoelectric coefficient described by the direct piezoelectric effect:

D3 = d33σ3 (4)

One more another important property of a piezoelectric material is the permittivity. This means how much electrical potential energy can be stored in a given volume of the material Smart Material Nanofibers for Day to Day Life 25 under the influence of an electric field. This is often maximized near phase transitions and its distribution with temperature is broad in relaxor ferroelectric materials. Often, the permittivity is reported relative to the permittivity of free space (ε33 T/ε0), also called the relative permittivity (εr) or often the dielectric constant. Finally, electromechanical coupling factor (kp) relates the electrical energy output to the total input mechanical energy or vice versa (Kaur et al. 2012). Piezoelectric materials in the fiber form are attractive due to their anisotropic properties and increased flexibility (Bhattarai et al. 2014). Piezoelectric materials have been widely used as high voltage power sources, sensors and actuators. All of the commercially available piezoelectric ceramic fibers or rods are based on lead zirconate titanate (PZT), because it has high piezoelectric coefficient (300–1000 pCN-1) (Haertling 1999, Zhan et al. 2007). Recently, more and more attentions have been attracted to scavenging energy by piezoelectric nanostructures, which has shown the potentials application to power integrated nanosystems (Lee, Song, and Yoon 2010, Qin, Wang, and Wang 2008). ZnO nanowires were the first demonstrated piezoelectric nanostructure for energy harvesting, which produced a voltage of 8mV using Atomic Force Microscope (AFM) to bend nanowire array (Choi et al. 2004). Piezoelectric BaTiO3 nanowires (Wernike et al. 2010), PZT nanofibers and nanotubes and PVDF nanofibers have also shown the excellent performance for energy harvesting. Piezoelectric nanostructures are experiencing as urge interesting for sensor applications as well. ZnO single nano-wire-based strain sensor and PZT nanofiber composites-based acoustic emission sensor have been developed. Although these nanostructures have presented their good performances, novel piezoelectric nanostructures are still needed to both show better piezoelectric properties and match special requirements for various novel devices. Single crystal PMN-PT has exhibited a piezoelectric effect of ten times larger (2500pm/V) (Fu and Cohen 2000) than that of conventional ceramics. It has also been theoretically predicted that PMN-PT nanowires could generate higher output power with higher efficiency than ZnO nanowires (Lee, Song, and Yoon 2010). However, in the last decade, use of lead-containing materials even in electronic equipment are gradually being restricted through national laws or directives of international bodies due to the toxicity of lead (Mandal, Yoon, and Kim 2011). The fact behind the toxicity is that PbO’s evaporates during the sintering process (Rørvik, Grande, and Einarsrud 2011). So there is need of developing lead-free materials which are eco-friendly and have high piezoelectric coefficient (Maurya et al. 2013). Recently many kind of lead free morphotropic phase boundary (MPB) based piezoelectric ceramics have been explored and studies, such as, KNN (Cheng et al. 2014, Sun et al. 2003), KNN–LT (Liu and Ren 2009), KBT–BT (Zhu et al. 2016), NBT–KBT–BT (Zhu et al. 2016), BZT–BCT (Liu and Ren 2009), BT (Dalton, Klee, and Möller 2005), BCZT (Bognitzki, Frese, et al. 2001), NBKT (Chen, Zheng, et al. 2010), ZnO etc. Among these materials, BZT–BCT material with MPB (having composition 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3) and has attracted much research interest due to its high piezoelectric coefficient (620 pCN-1). Piezoelectric materials, especially in the form of low-dimensional nanostructures, own a number of advantages in micro-electro-mechanical systems (MEMS), such as low hysteresis, high available energy density, high sensitivity with wide dynamic range, and low power requirement, therefore it is worth considering the impetus for integrating piezoelectric nanostructures into MEMS devices (Bai et al. 2012). Piezoelectric ceramics have been used as sensor and actuator materials in smart material and structural systems, non-volatile ferroelectric memory devices, micro-electromechanical 26 Madan Lal, Mamta Shandilya, Seema Sharma et al. systems (MEMS), because they have the ability to transform energy from electrical to mechanical and vice versa (Gu, Zhou, and Cao 2016, Phillip 2016). Low-dimensional nanostructures have attracted extensive interest due to their unique physical and chemical properties different from their bulk counterparts (Drozin 1955, Anton 1934). One- dimensional (1D) nanostructures, such as nanowires, nanotubes, and nanofibers, represent the smallest dimension for efficient transport of electrons and excitons and thus are ideal building blocks for hierarchical assembly of functional nanoscale electronic and photonic structures. Several previous studies have been focused on the syntheses and properties of various nanofibers by electrospinning technique, such as Nd-substituted bismuth titanate ferroelectric nanofibers (Keulder 2013), multiferroic CoFe2O4–PbZr0.52Ti0.48O3 nanofibers (Vonnegut and Neubauer 1952), PMN-PT nanofibers (Fu and Cohen 2000), BT nanofibers (Ávila et al. 2013), BNKT nanofibers (Chen, Zheng, et al. 2010), ZnO nanofibers (Di Mauro et al. 2016) NaNbO3 nanofiber (Gu, Zhou, and Cao 2016). The flexibility and robustness of piezoelectric devices are great significant for them to be adapted to wearable applications. However, the brittleness of piezoelectric bulk ceramics has seriously limited their application in the flexible systems. Piezoelectric nanofibers have the unique combination of flexibility, lightweight, and availability in ultra-long lengths, making them good candidates for applications in flexible devices (Wu et al. 2013, Wu, Bai, et al. 2012, Tyagi et al. 2015). To prepare cost efficient piezoelectric nanofibers, electrospinning technique has been proved to be a promising approach. Piezoelectric nanofibers, such as PZT (Chen, Xu, et al. 2010), BaTiO3 (Wang et al. 2015), (K, Na)NbO3 (Kang et al. 2014) and (Ba,Ca)(Zr, Ti)O3 (Wu et al. 2013), which were prepared by electrospinning method, have been used to fabricate flexible nanogenerator. Some of the piezoelectric nanofibers are listed in Table 2.

Table 2. Piezoelectric Coefficient (d33) of piezoelectric nanofibers with their diameter range

Materials Diameter range Piezoelectric References of Nanofibers Coefficient (nm) (d33) (pm/V) 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 148 – 216 52 (Xu, Poirier, and Yao (PMN-PT) 2012) (Na0.82K0.18)0.5Bi0.5TiO3 150–600 96 (Chen, Zheng, et al. 2010) 0.96(K0.48 Na0.52) (Nb0.95Sb 0.05)O3– 200–300 338 (Zhu et al. 2016) 0.04Bi 0.5(Na 0.82K0.18)0.5ZrO3 Bi3.4Ce 0.6Ti3O12 100-200 158 (Jiang et al. 2012) (Ba0.95Ca0.05)(Ti0.92Sn0.08)O3 80–275 398 (Sahoo and Panda 2015) Ba(Ti0.80Zr0.20)O3-0.5(Ba0.7Ca0.3)TiO3 100 236.54 (Fu et al. 2015)

Ceramic PZT and polymeric PVDF are two piezoelectric materials which have been demonstrated as viable nanofiber nanogenerator materials (Li, Gao, et al. 2009, Rørvik, Grande, and Einarsrud 2011, Meyer, Shrout, and Yoshikawa 1998). In these efforts, either the conventional far-field electrospinning (FFES) or near-field electrospinning (NFES) (Sun et al. 2006) process has been the key manufacturing tool to produce nanofibers for such Smart Material Nanofibers for Day to Day Life 27 applications (Huang et al. 2003). In NFES process, a continuous single nanofiber can be deposited in a controllable manner, whereas in FFES process producing dense nanofibers networks on large areas for the nanogenerator demonstrations. In general, a polling process, consisting of both electrical poling and mechanical stretching, is required for the fabrication of materials with piezoelectric properties at moderate temperature. At the high electrostatic field and polymer jet characteristics of the electrospinning process, electrospinning is ideally suited for producing piezoelectric nanofibers through in-situ electric poling and mechanical stretching (Chang et al. 2012). PVDF has superior piezoelectric properties because it’s polar crystalline structure as compared to other types of polymeric materials. In nature, PVDF polymer consists of at least five different structural forms depending on the chain conformation of trans (T) and gauche (G) linkages. Figure 8. (a) shows the crystalline structure of α and β-phase of PVDF, respectively. While the α -phase is known as the most abundant form in nature, β-phase is responsible for most of PVDF’s piezoelectric response due to its polar structure with oriented hydrogen and fluoride (CH2–CF2) unit cells along with the carbon backbone. In order to obtain the β-phase PVDF, electrical poling and mechanical stretching processes are required during the manufacturing process to align the dipoles in the crystalline PVDF structures as illustrated in Figure 8. (a).

Table 3. Fabrication parameters of PVDF-based nanofibers

Material Molecular Solvent Method Typical Tip-to- Mean Piezo- Ref. weight bias substrate field electricity? (wt%) (kV) (cm) strength (V/m) PVDF 172,000 (16) DMF FFES 15 15 105 Yes (Fang, Wang, and Lin 2011) PVDF 534,000 (20) DMSO (50%) + NFES 1 1 106 Yes (Chang et al. acetone (50%) 2010) PVDF 534,000 (20) DMF (60%) + FFES 12 10 1.2x105 No (Farrar et al. acetone (40%) 2011) PVDF 534,000 (12) DMF (40%) + FFES 12 15 0.8x105 Yes (Wang, acetone (60%) Zheng, et al. 2011) PVDF 275,000 DMF (60%) + FFES 13 15 0.87x105 Yes (Zheng et al. (various) acetone (40%) 2007)

PVDF 1100 (20) DMF (60%) + FFES 15 15 0.75x105 Yes (Chang et al. acetone (40%) 2012) P(VDFT 77:23 (mol%) Butan-2-one FFES 20 10 2x 105 Yes (Mandal, rFE) Yoon, and Kim 2011) PVDF 687,000 (10– DMF FFES 20 15 1.3x 105 Yes {Chang, 20) 2009 #114} PVDF + 115,000 (20%) DMF (60%) + FFES 15 N/A 105 Yes (Huang et al. CNT acetone (40%) 2008) 4000HD DMF + acetone FFES 10 3 3.3x105 Yes (Costa, PVDF (various) various ratio Bretas, and Gregorio 2010) 28 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Most of literature reports (Mandal, Yoon, and Kim 2011)(Huang et al. 2008), (Costa, Bretas, and Gregorio 2010)(Fang, Wang, and Lin 2011)(Chang et al. 2010)-(Chang et al. 2012)(Wang, Zheng, et al. 2011) on piezoelectricity or high contents of β-phase structure in electrospun PVDF nanofibers shown in Table 3. For example, the near-field electrospinning process has an average electrical field about one order of magnitude larger than the far-field electrospinning process, and this could be enhanced piezoelectricity for nano-generator applications (Chang et al. 2010). The rest of the nanofibers (shown in Table 3) are from the far-field electrospinning process and only one report stated that electrospun PVDF nanofibers have shown poor dipole alignments by using the second harmonic generation (SHG). SHG method is used to detect the polarity of the nanofiber. PZT is another good piezoelectric material shown in Figure 8. (b) with its crystalline structure. After applying and removing an external electric field in PZT, Zr/Ti atom can shift up/down about their relative positions for the piezoelectric property. In their bulk or thin film format, PZT can generate a high voltage as compared with other piezoelectric materials for sensing, and actuation (Jung and Kim 1994) and energy harvesting applications (Shen et al. 2008). As a ceramic material, bulk PZT is more fragile in comparison to organic PVDF but has very good mechanical strength in nanowire form (Chen et al. 2009). During the commercial piezoelectric PVDF thin-film production process, a high electrical potential land mechanical stretching is applied at a raised temperature for enhanced piezoelectricity. PVDF nanofibers fabricated by the conventional electrospinning process are under a high bias voltage (410 kV) which could transform some non-polar α- phase structures to polar β-phase structures for piezoelectricity. The near-field electrospinning (NFES) process is shown in Figure 9.

Figure 8. (a) Schematic diagrams showing crystalline structures of PVDF: (top) non-polar α-phase, and (bottom) polar β-phase. (b)Schematic diagrams showing crystalline structures of PZT. An electric polarization of PZT can shift up/down of Zr/Ti atom and remain their positions after applying and removing an external electric field for the piezoelectric property (Drozin 1955).

The PVDF nanofiber network was fabricated using a modified far-field electrospinning process to align individual nanofibers. Experimentally, a single PVDF nanofiber based nanogenerator was able to generate 0.5–3 nA of current and 5–30 mV of voltage under Smart Material Nanofibers for Day to Day Life 29 repeated long term reliability tests without noticeable performance degradation. The nanoscale fibers preserve and even enhance piezoelectric properties and the potential wide- range of applications. Sensors built upon the PZT nanofiber technology can be used to monitor aircraft or bridges for structural fatigue in real time. Because the sensors can also harvest and store energy, they could power themselves indefinitely, eliminating the need for external power or a battery. Mechanical energy could potentially be gathered from heartbeats or body motion to create a self-powering pacemaker. In the future, clothes woven with a combination of thermoelectric and piezoelectric fibers could harvest energy from the movement for athletic or military applications (Meyer, Shrout, and Yoshikawa 1998).

Figure 9. Schematic diagram of the near-field electrospinning process showing possible dipole directions (black arrows) and electrical filed direction (red arrows). The PVDF polymer solution experiences mechanical stretching and in-situ electrical poling during the formation of nanofibers due to the high electrostatic field toward the substrate (Bhattarai et al. 2014).

Zinc oxide is a II-VI metal oxide semiconductor material, is known for its versatility. ZnO nanostructures exhibit anisotropic piezoelectric properties due to its structural non- central symmetry (Ávila et al. 2013, Chen, Zheng, et al. 2010). High aspect ratio ZnO nanostructures can be easily synthesized using hydrothermal methods (Di Mauro et al. 2016), and these nanowires or nanorods exhibit piezoelectric properties (Gu, Zhou, and Cao 2016). In single crystal solids like ZnO, the piezoelectric property of the material originates with its atoms and is repeated throughout the solid due to high crystallinity. The non-symmetric distribution of positive and negative charges starts at a unit cell and repeats through the whole material. Strained material results in net polarization on the surface. ZnO nanostructures are very versatile, like their bulk counterparts, and can endure huge deformations (Phillip 2016). This characteristic attribute of the semiconducting piezoelectricity of ZnO nanorods/ nanowires is being studied intently for the creation of novel devices. This is paving the way for size-attuned sources of power for wireless devices which have themselves been 30 Madan Lal, Mamta Shandilya, Seema Sharma et al. undergoing a continuous size reduction over the years (Drozin 1955, Anton 1934). A few of the important material properties of ZnO are given in Table 4. Electrospinning method has used for the fabrication of PVP-ZnO nanofibers (as shown in Figure 9). The formation of nanofibers was conforming by the FE-SEM having a diameter range from 150-450 nm (Figure 10).

Table 4. Important material properties of ZnO

Crystal Structure Hexagonal Wurtzite Piezoelectric Coefficient 12 pC/N Molecular Weight 81.38 Lattice Constants (27°C) a = 0.32469 A°, c = 0.52069 A° Melting Point 1975°C Density 5.606 g/cm3 Thermal Conductivity 25 W/mK at 20°C Fusion Heat 4,470 cal/mole Band Gap 3.37 eV at room temperature Thermal Expansion Coefficient 4.3x10-6/°K at 20°C and 7.7x10-6/°K at 600°C Refractive Index 2.008 Lattice Energy 964 kcal/mole Exciton Binding Energy 60 meV Electron and Hole Effective Mass me * = 0.24 eV, mh * = 0.59 eV Dielectric Constant ε0 = 8.75, ε∞ = 3.75 Intrinsic Carrier Concentration < 106 cm-3 2 2 Electron and Hole Mobility (300K) μe = 200 cm /(V.s), μh = 5-50 cm /(V.s)

Figure 9. Flow chart for the synthesis of Electrospun PVP-ZnO composite nanofibers (SAHOO 2014). Smart Material Nanofibers for Day to Day Life 31

Figure 10. FE-SEM image of PVP-ZnO nanofibers (SAHOO 2014).

5. APPLICATION OF SMNFS IN DIFFERENT AREAS

Figure 11. Block diagram for application of nanofibers.

32 Madan Lal, Mamta Shandilya, Seema Sharma et al.

5.1. Filtration Media

Taking advantages of the highly porous fibrous nanostructures, ceramic nanofibers membrane can be used for filter and separator applications (Ke et al. 2011). Filtration has been widely used in both households and industries for removing the solid substance from air or liquid. The filtration media work on the principal that the pore size of filtration is smaller than the pollutants size (Homaeigohar, Dai, and Elbahri 2013). On the basis of pore size and filtration application, the water purification process can be defined as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). According to the Baker’s definition: Microfiltration refers to filtration processes that use porous membranes to separate suspended particles with diameters between 0.1 and 10 μm. Pre-filtration of waste water streams is a known application of the MF membrane. Pre-filters are used to remove coarse particles thereby maintaining the performance of the downstream filter unit such as RO and NF membranes for a much longer time before cleaning and/or replacement. The MF membranes help in rejection of micron scale unwanted species e.g., particles, flocs, bacteria etc (Kaur et al. 2012, Richard 2004, Homaeigohar, Buhr, and Ebert 2010). Ultrafiltration (UF) is another liquid filtration process discriminating a diverse range of pollutants, such as emulsions, viruses, proteins and colloids (Richard 2004). It enables the process to remove dissolved and colloidal materials. The pore size of the UF membrane between 5 and 20 nm, which limits the filtration to macromolecules and suspended solids (Fu and Cohen 2000). Whereas the pore size in the case of RO is 10-4 μm, dimensions that have been developed to correspond with the environmental legislation (Picq). RO has been proved more effective than NF and UF for heavy metal removal from wastewater in the case of metal concentrations between 21 to 200 mg/L. Also, RO shows the best results for a pH between 3 and 11 and a pressure between 4, 5 and 15 bars, the pressure being the major parameter affecting the efficiency of heavy metal removal. RO presents a lot of advantages such mechanical strength, chemical and thermal stability, high water flux rate, high salt rejection and resistance to biological attack. Nanofiltration (NF) has unique properties and Nanofiltration is the intermediate process between UF and RO (Kurniawan et al. 2006). NF technology is used for the rejection of heavy metal ions such as nickel, chromium, copper and arsenic from wastewater (Fu and Cohen 2000). Figoli et al. (Figoli et al. 2010) have reported that two commercial NF membrane (NF90 and N30F) can be used to remove pentavalent arsenic from synthetic water. The significance of NF membrane lies in its small pore and membrane surface charge, which allows charged solutes smaller than the membrane pores to be rejected along with the bigger neutral solutes and salts (Ahn et al. 1999). H.A. Qdais (Qdais and Moussa 2004) have done a comparative study on the removal of Cu(II) and Cd(II) ions from synthetic wastewater using nanofiltration (NF) and reverse osmosis (RO) was conducted. At the same initial metal concentration of 200 mg/L, 98% of Cu (II) removal and 99% of Cd(II) removal could be attained by using RO, while NF was capable of removing more than 90% of Cu(II) and 97% of Cd(II). These results indicate that both types of the membrane filtration are effective for metal removal from contaminated wastewater. However, NF requires a lower pressure than RO, making NF more preferable due to its lower treatment costs (Mohammad, Othaman, and Hilal 2004). Nanomaterial's played a beneficial role in environmental remediation and membranes for filtration including their high surface area-to-volume ratio, low environmental impact, high strength, functional ability, and sustainability (Carpenter, de Lannoy, and Wiesner 2015). Smart Material Nanofibers for Day to Day Life 33

5.2. Biomedical Applications

In biological viewpoint, almost all of the human tissues and organs are deposited in nanofibrous forms or structures. Examples include bone, dentin, cartilage, collagen, and skin (as shown Figure 12). All of them are characterized by well-organized hierarchical fibrous structures realigning in nanometer scale. As such, current research in electrospun polymer nanofibers has focused one of their major applications on bioengineering. We can easily find their promising potential in various biomedical areas.

Figure 12. A schematic applications of tissue engineering in various fields such as Skin and Wound dressing, Cartilage, Bone, Neural tissue, Heart valves, Muscle fibers, SEM picture of nanofibers, the nanofibers are also used in cosmetics (facial masks, cleansing agents etc.) and pharmaceutical industries for localized drug delivery applications (Hiremath and Bhat 2015). 34 Madan Lal, Mamta Shandilya, Seema Sharma et al.

5.2.1. Drug Delivery System Nanofibers for application as drug delivery Protect the drugs in the case of a systemic application from decomposition, e.g., in the blood circuit. They should allow controlled the release of the drug at a release rate as constant as possible over a longer period of time, adjusted depending on the field of application. They are supposed to concentrate the drug release only on the targeted body. Nanofibers have potential medical applications, which include, drug and gene delivery, artificial organs, artificial blood vessels, and medical facemasks. For examples carbon nanofibers, hallow Nanofibers are smaller than blood cells, have potential to carry drugs into blood cells (Kim et al. 2004). Drug delivery with polymer nanofibers is based on the principle that dissolution rate of drug particulate increases with increased surface area of both the drug and the corresponding carrier if necessary. For controlled drug delivery, in addition to their large surface area to volume ratio, polymer nanofibers also have other additional advantages. For example, unlike common encapsulation involving, Controlled delivery systems are used to improve the therapeutic efficacy and safety of drugs by delivering them to the site of action at a rate dictated by the need of the physiological environment. A wide variety of polymeric materials have been used as delivery matrices, and the choice of the delivery vehicle polymer is determined by the requirements of the specific application (Ratner et al. 2004). Polymeric nanofibers have recently been explored for their ability to encapsulate and deliver bioactive molecules for therapeutic applications (Kenawy et al. 2002). In electrospinning method, a loading efficiency of 90% into PDLA nanofibers was reported for the antibiotic drug Mefoxin. Covalent conjugation to polymers represents another method to modulate drug release. It has also been suggested that the high porosity of nanofibers allows for rapid diffusion of degradation byproducts (Thanou and Duncan 2003). However, the burst release may also be indicative of the drug being attached only on the surface. As the drug and carrier materials can be mixed together for electrospinning of nanofibers, the likely modes of the drug in the resulting nanostructured products are (Pham, Sharma, and Mikos 2006):

1. Drug as particles attached to the surface of the nanofibers, where nanofibers act as a carrier. 2. Both drug and carrier are nanofibers form; hence the end product will be the two types of nanofibers interlaced together. 3. The blend of drug and carrier materials integrated into one type of fibers containing both components, and 4. The carrier material is electrospun into a tubular form in which the drug particles are encapsulated.

5.2.2. Tissue Template For the replacement of tissues or organs in a malfunction in a human body, one of the challenges to the field of biomaterials/tissue engineering is the design of ideal scaffolds/synthetic matrices that can mimic the structure and biological functions of the natural extracellular matrix (ECM). Human cells can attach as well as organize around fibers with diameters smaller than those of the cells (Fertala, Han, and Ko 2001). In this regard, nanoscale fibrous scaffolds can provide an optimal template for cells to seed, migrate, and grow. A successful regeneration of biological tissues and organs calls for the development of Smart Material Nanofibers for Day to Day Life 35 fibrous structures with fiber architectures beneficial for cell deposition and cell proliferation (Jin et al. 2002).

5.2.3. Wound Dressing Polymer nanofibers can be used for the treatment of wounds or burns of a human skin, as well as designed for Hemostasis devices with some unique characteristics. With the aid of electric field, fine fibers of biodegradable polymers can be directly sprayed/spun onto the injured location of skin to form a fibrous mat dressing, which can let wounds heal by encouraging the formation of normal skin growth and eliminate the formation of scar tissue which would occur in a traditional treatment (Saha, Butola, and Joshi 2014). Non-woven nanofibrous membrane mats for wound dressing usually have pore sizes ranging from 500 nm to 1 mm, small enough to protect the wound from bacterial penetration via aerosol particle capturing mechanisms. High surface area of 5–100 m2/g is extremely efficient for fluid absorption and transdermal delivery.

5.2.4. Cosmetics The current skin care masks applied as lotions or ointments, topical creams, may include dust or liquid sprays which may be more likely than fibrous materials to migrate into sensitive areas of the body such as the nose and eyes where the skin mask is being applied on the face. Electrospun polymer nanofibers have been used as a cosmetic skincare mask for the skin cleaning, for the treatment of skin healing, or other therapeutical or medical properties with or without various additives. This nanofibrous skin mask with high surface area and very small interstices can facilitate far greater utilization and speed up the rate of transfer of the additives to the skin for the fullest potential of the additive. The cosmetic skin mask made from the electrospun nanofibers can be applied gently and painlessly. These nanofibers can also be applied directly to the three-dimensional topography of the skin to provide healing and for the skin care (Huang et al. 2003, Khan et al. 2013).

5.3. Protective Clothing Application

The protective clothes in the military are mostly expected to help maximize the survivability, sustainability, and combat effectiveness of every individual soldier system against extreme weather conditions, ballistics, and NBC (nuclear, biological, and chemical) warfare. In peace ages, breathing apparatus and protective clothing with the particular function of against chemical warfare agents and mustard gas from inhalation and absorption through the skin become a special concern for fighters in conflicts and civilian populations in terrorist attacks. Current protective clothing containing charcoal absorbents has limited water permeability, extra weight imposed to the article of clothing. A light-weighted and breathable fabric, which is permeable to both air as well as for water vapors, insoluble in all solvents and highly reactive with nerve gasses and other deadly chemical agents, is desirable. Because of their great surface area, nanofibers fabrics are capable of the neutralization of chemical agents and without impedance of the air and water vapor permeability to the clothing. Electrospinning results in nanofibers laid down in a layer that has high porosity but very small pore size, providing resistance to the penetration of chemical harm agents in aerosol 36 Madan Lal, Mamta Shandilya, Seema Sharma et al. form (Gibson, Schreuder‐Gibson, and Rivin 1999). Initial investigations have indicated that compared to conventional textiles the electrospun nanofibers present both minimal impedance to moisture vapor diffusion and efficient in trapping aerosol particles (Gibson, Schreuder‐Gibson, and Rivin 1999, Gibson, Schreuder-Gibson, and Rivin 2001, Schreuder- Gibson et al. 2002), as well as show strong promises as ideal protective clothing.

5.4. Gas Sensing

Conducting polymers are playing importance in the development of smart sensors due to their room temperature operation, low-cost fabrication, ease of deposition onto a wide variety of (Thust et al. 1996, Li et al. 1994) and their rich structural modification chemistry (Janata and Josowicz 2003). Recently, nanostructured materials, in the form of nanowires, nanobelts, nanofibers or nanotubes have received much attention. These one-dimensional materials process extremely high surface area without increasing the dimension of the device. Therefore, they should have improved performance in applications where a high surface contact area is needed between the device and its environment, such as in sensors (Huang et al. 2004, Virji et al. 2004). Nanofibers which have cylindrical morphology form porous structures when deposited as thin films. Polymeric nanofibers, with diameters in the nanometer range, possess larger surface areas per unit mass and allows easier addition of surface functionalities compared (Huang and Kaner 2006). The three-dimensional porous structure of a nanofiber film allows easy diffusion of gas molecules into and out of the film and the nano-scale fiber diameters lead to the rapid diffusion of gas molecules on the surface of nanofiber (Kang and Wise 2000). At present, metal oxide semiconductors (Giang et al. 2011), palladium gate field effect transistors (Braga and Horowitz 2009), catalytic combustion sensors (Liu et al. 2011), fuel cell sensors (Xu et al. 2010, Pijolat, Tournier, and Viricelle 2009), and optical sensors (Yun et al. 2008, Chen, Liu, and Huang 2011) are the mainly methods applied to detect these characteristic fault gases. Simple fabrication process have remarkable advantages like rapid response and recovery time, low maintenance cost and long life service, metal oxide semiconductors such as ZnO (Ang et al. 2011), SnO2 (Zeng et al. 2011), TiO2 (Zeng et al. 2009, Gong et al. 2010), Fe2O3(Sun et al. 2005), NiO (Dirksen, Duval, and Ring 2001), WO3 (Cao et al. 2009), In2O3 (Waitz et al. 2009) etc., have been widely used for gas sensors. Among all of these sensing materials, ZnO has gain attention and been highly useful in sensing material for detecting both oxidizing as well as for reducing gasses (Kim and Yong 2011). In recent years, great efforts have been made to fabricate low-dimensional ZnO nanostructure (Ahn et al. 2009, Zhang et al. 2009). Taking advantage of their small and uniform particle size, specific pore structure, high surface-to-volume ratio, anti-aggregation properties and so on, these low-dimensional nanostructures exhibiting better sensing properties than those of traditional nanoparticles and thin films. Three types of gas sensors were fabricated with the prepared ZnO nano bulks, nanorods and nanofibers. Gas sensing experiments of these sensors towards CH4 were first performed at different operating temperatures to find out the optimum working temperature. Q. Zhou et a. (Zhou et al. 2013) reported that the gas response of the sensor to 200 μL/L of CH4 as a function of operating temperature ranging from 100 to 425°C. The optimum operating temperature of the prepared nanorods and nanofibers sensors to CH4 gas are both about Smart Material Nanofibers for Day to Day Life 37

275°C, and it is 300°C for nano bulks sensor, where the sensor exhibits the maximum gas response. The response and recovery times for the nanorods, nanofibers, and nano bulks are evaluated to be about 13–6 s, 14–8 s and 18–9 s, respectively. The long-time stability of the three kinds of sensors has been also measured, as shown in Figure 13. It can be clearly seen that the rod-shaped and fiber-shaped ZnO sensors exhibit nearly a constant voltage values during the long experimental cycles, whereas higher fluctuations are observed for the bulk- shaped ZnO sensors. These results are evidence which confirming the long-term stability and good repeatability for the flower-like ZnO nanorods and net-like ZnO nanofibers gas sensors.

Figure 13. Stability and repeatability of the sensors to 100 and 30 μL/L of CH4 (Zhou et al. 2013).

On comparison, traditional ZnO nano bulks, ZnO nanorods, and nanofibers exhibit much higher gas responses at a low temperature with rapid response-recovery times, good repeatability and long-term stability. These can be explained by flower-like nanorods and the hierarchical ZnO nanostructures and net-like nanofibers. The specific surface area and pore structure are the main factors that influence the gas sensing behaviors of the materials.

5.5. Transducer Applications

Since its invention in 2006 (Wang and Song 2006) NGs have become a potential power source for portable, personal and wearable self-powered nano/microsystems. On Comparing with traditional piezoelectric cantilevers, NGs can harvest irregular weak mechanical movement over a wide frequency range, which requires piezoelectric materials to work under different conditions (Bai et al. 2012, Agrawal, Peng, and Espinosa 2009, Qin, Wang, and Wang 2008). For high-performance NGs, piezoelectric NWs are desirable due to their high flexibility and strain tolerance (Qin, Wang, and Wang 2008, Gu et al. 2012). Upon further considering the efficiency of electromechanical conversion and environmental friendliness, lead-free NWs with high piezoelectric coefficients should be most desirable. So BZT–BCT 38 Madan Lal, Mamta Shandilya, Seema Sharma et al.

NWs are perfect candidates for NGs. But due to their complicated composition and symmetry structure, they are difficult to synthesize via conventional approaches such as CVD and the hydrothermal method (Rørvik, Grande, and Einarsrud 2011). Currently, the synthesis of BZT–BCT NWs and the corresponding textiles consisting of well-aligned NWs still presents challenges. Thus, exploring a simple, low cost and highly efficient method to prepare BZT– BCT NWs is of great value for the development of NGs and energy harvesting. Recently in 2016 piezoelectric nanofiber sensors have reported that nanogenerator can generate an output voltage up to 10 V in 45 sec by hand-pressing (Zhu et al. 2016).

5.5.1. Dye-Sensitized Solar Cells (DSSCs) TiO2 has been a material of choice and most widely used in the field of energy and solar harvesting. This TiO2 when electrospun into vertical nanofibers have been reported to behave as a photoelectrode (Krishnamoorthy, Thavasi, and Ramakrishna 2011, Yu et al. 2012). The first step to prepare such vertical nanofibers of TiO2 is to electronspin, followed by post- treatment of nanofibrous TiO2 ribbons and cutting of such vertically aligned nanofibers so as to give a 3D structure. These as-prepared vertical nanofibers can have a height of around 27 μm but can be easily raised from 10 to 100 μm with an average area of 0.2 cm2 and 90±30 nm diameter. The conversion efficiency, short-circuit current, and open circuit voltage of such TiO2 nanofibers was measured as 2.87% and 5.71 mA/cm2, 0.782 V, respectively. Optimizing the varying parameters of the nanofibers e.g., porosity, diameter, or height can likely enhance the photo-conversion efficiency of such TiO2 nanofibers.

5.5.2. Supercapacitors Supercapacitors (SCs) are used widely as one of the most promising new energy storage devices applications due to their high power performance, long cycle life, and low maintenance cost. SCs can be divided into two categories depending upon different energy storage mechanisms i) pseudo-capacitors (PCs) and ii) electrical double layer capacitors (EDLCs). PCs store energy based on fast reversible surface redox reactions, whereas EDLCs store energy using ion adsorption and desorption at the electrode and electrolyte interface. Recently, novel carbon-based materials with rational design of material composition, size, and morphology have been explored for high-performance EDLCs (Shi et al. 2015). Electrospinning is perhaps the most facile route to prepare the highly porous nanofibers. Thus, electrospun carbon NFs from polymer precursors such as polybenzimidazole (PBI), Polyacrylonitrile (PAN), and Polyimide (PI) have triggered widespread investigations. And these electrospun CNFs can be utilized as an electrode for EDLCs after undergoing the process of stabilization, carbonization, and activation, in which the surface area and porosity of the NFs can be improved. Some additives like ZnCl2 (Kim et al. 2007), silver (Han et al. 2012), and nickel (Li, Liu, et al. 2009) have been used in the precursor solution to enhance the capacitance of electrospun nanofiber-based EDLCs. It has been found that the addition of ZnCl2 has a great influence on fiber morphology and the specific capacitance of carbon NFs containing 5 wt% ZnCl2 reaches to the highest value of 140 F/g when compared to those containing 1 wt% and 3 wt% ZnCl2. Furthermore, the coaxial electrospinning technique has been the research hotspot due to its advantage of material preparation (Kim et al. 2007). Coaxial NFs with different morphologies using PVP-doped Sn and a PVP/PAN mixture as an inner and outer solution, respectively, by the method of electrospinning combined with the reduction of H2. The electrochemical test has shown that the capacitance of capacitor reaches Smart Material Nanofibers for Day to Day Life 39 maximum on the condition that the mass fraction of Sn is 8% in the PVP solution. And it is believed that this enhanced electrochemical performance is ascribed to the synergistic effect of active sites on the surface of fibers and pore structure after the reduction of H2 (An and Ahn 2013). RuO2 and V2O5 have also attracted much attention as PCs electrode materials because of their electrochemical stability, high electrical conductivity, and capacitances. For instance, Choi et al. (Choi, Hyun, et al. 2010) prepared electrospun Pt NFs to support the deposited hydrous RuO2 over-layers and found that the resulting composite electrode showed good performance with a capacity loss of only 21.4% passing from 10 to 1000 mV/s. Lee et al. (Lee et al. 2011) reported that the RuO2–Ag2O composite NWs electrode produced from electrospinning exhibited a high capacitance of 173.25 F/g at 10 mV/s and excellent retention of capacitance up to 97% after 300 cycles. However, considering the high cost and toxicity of Ru, researchers recently have focused on conducting polymers such as PANI, polypyrrole (PPy), and poly-p-phenylene (PPP) as electrode materials for PCs. PANI is currently the most promising candidate due to its low cost and high electrical conductivity. Chaudhari et al. (Chaudhari et al. 2013) have prepared electrospun PANI NF and found that the specific capacitance and rate performance of PANI NFs are much higher than that of PANI powder.

5.5.3. Rechargeable Lithium-Ion Batteries Lithium-ion batteries (LIBs) have become a cornerstone for the development of energy storage technologies and have been applied in many areas including mobile phones, laptop computers, camcorders, digital cameras, and many other commercial and military applications due to their high energy density and long cycle life. LIBs are mainly composed of an anode (generally graphite), a carbonate-based organic electrolyte, and a cathode (generally LiCoO2). Li-ions are intercalated and deintercalated between graphite and LiCoO2 through the electrolyte during discharging and charging. Nanofibers are promising materials for LIBs because of their good electrochemical activity, high mechanical strength, and high specific surface area. In this section, recent advances in the areas of electrospun nanofibrous cathode, anode, and separator materials for LIBs are briefly summarized.

5.5.3.1. Cathode Materials LiCoO2 is a typical cathode material used in LIBs due to its high specific capacity, high voltage, and long cycle life. Nanostructured LiCoO2 fiber electrode prepared by electrospinning exhibited enhanced Li-ion and electron conductivity due to the short diffusion distance. As-prepared electrode could display good rate capability and high power density. However, such nanofibrous electrodes have shown poor cycling stability. Regarding this, Cavaliere et al. (Cavaliere et al. 2011) synthesized the core-shell LiCoO2/MgO composite nanofiber by coaxial electrospinning. As-prepared LiCoO2/MgO could retain 90% of its initial charge capacity after 40 cycles compared with 52% for the fiber electrode without MgO. The enhanced cyclability is attributed to the coating of MgO which could protect the surface from passive surface film formation during cycling. Besides LiCoO2, V2O5, and LiFePO4 have also been extensively studied as cathode materials because of their high theoretical specific capacity (400 mAh/g and 170 mAh/g, resp.), good cycling stability, and batter safety. However, both of them suffer low electronic conductivity, leading to low rate capability. The utilization of electrospinning to prepare nanofibrous V2O5 (Mai et al. 2010, Cheah et al. 2011, Toprakci et al. 2011, Zhu, Yu, et al. 2011), LiFePO4 (Zhu, Yu, et al. 2011, 40 Madan Lal, Mamta Shandilya, Seema Sharma et al.

Toprakci et al. 2011), or their respective composite materials cathode electrodes is regarded as the better way to solve these problems.

5.5.3.2. Anode Materials Carbon NFs have been the popular anode materials for LIBs due to several advantages such as low cost, easy availability, and long cycle life. However, there are also some drawbacks about carbon NFs such as relatively low specific capacity and rate capability. Regarding this, C/PAN NFs prepared from electrospinning combined with thermal treatments exhibited higher reversible capacity compared with that of carbon NFs without PAN, which was attributed to their highly disordered structure and defects (Kim et al. 2006). Several studies have been also reported on composite C/Si NFs (Choi, Lee, et al. 2010), C/Sn NFs (Yu et al. 2010), and C/MnOx NFs (Lin et al. 2010) obtained by electrospinning as anode materials for LIBs in terms of cycling stability and electrical conductivity. Recently a simple coaxial electrospinning approach was adopted for the synthesis of hollow lD anatase TiO2NFs as an anode material for lithium-ion batteries by Zhang et al. (Zhang et al. 2013). When LiFePO4 is used as a cathode, the battery showed a high reversible capacity of 103 mAh/g at a current density of 100 mA/g and retained 88% of its reversible capacity after 300 cycles. Such excellent battery performance was mainly ascribed to the unique structure which provided a large surface area and shortened diffusion pathways of Li ion and electron. In addition, Li et al. (Li et al. 2013) reported hierarchical CaCo2O4 (CCO) NFs prepared by electrospinning as anode materials for LIBs. It was found that the hierarchical CCO-NFs exhibited excellent cycling performance and high rate capability.

5.5.3.3. Separator Materials The separator is a critical component in LIBs and plays an important role in determining the battery performance. However, the commercial separators based on microporous membranes have several disadvantages, such as low porosity, unsatisfactory thermal stability, and poor wettability in liquid electrolytes (Lee, Alcoutlabi, et al. 2013). In this regard, electrospun polymer nanofiber membranes provide hope for researchers because of their high porosities, large specific surface areas, and better-interconnected conduction pathway for ions within the membrane. Liang et al. (Liang et al. 2013) reported that PVDF fiber membranes, used as separators for LIBs, prepared by electrospinning combined with heat treatments, exhibited high ionic conductivity at room temperature, a good electrochemical stability, and stable cycle performance. These results indicate that heat-treated PVDF fiber membranes are promising separator candidate for high-performance LIBs. In addition, nanofiber composite membranes including polyvinylidenefluoride-co-chlorotrifluoroethylene (PVDF-CTFE) and PVDF-CTFE/PVDF-HFP were also prepared by electrospinning used as separator membranes for LIBs (Lee, Alcoutlabi, et al. 2013). Similarly, Jayaraman et al. reported a gelled electrospun PVDF-HFP NFs membrane. The separator exhibited the liquid-like conductivity of ~2.9 mS cm−1 at ambient conditions when it was used as a separator in LIBs where full-cell (electrospun TiNb2O7 and LiMn2O4 acted as anode and cathode, respectively). Excellent cyclability and high rate capability, which is due to faster ionic transport property (Jayaraman et al. 2014). Recently electrospun SiO2/nylon 6, 6 nanofiber membranes were fabricated and their electrochemical. The performance was evaluated for use as separators in LIBs by Yanilmaz et al. (Yanilmaz, Dirican, and Zhang 2014). It was found that SiO2/nylon 6,6 nanofiber membranes showed superior thermal stability, mechanical Smart Material Nanofibers for Day to Day Life 41 strength, and high porosity. When this SiO2/nylon 6, 6 nanofiber membrane was used as a separator, the batteries exhibited good cycling performance and superior rate performance, compared to those using commercial microporous polyolefin membrane.

6. ADVANCED APPROACHES IN ELECTROSPINNING

Electrospinning process is known for its ability to construct nanofibrous structures in different forms, such as core-shell, hollow, porous, honeycomb, etc. The scaffolds made by one of these methods are extremely useful due to their structural similarities to the natural ECM which makes them suitable for a wide range of tissue engineering applications.

6.1. Core-Shell Nanofibers

Figure 14. Cross-section of coaxial and triaxial fibers: (a) coaxial fiber with hydrophobic sheath; (b) coaxial fiber with hygroscopic sheath; (c) triaxial fiber with hygroscopic sheath; (d) triaxial fiber loaded with dual drugs (Han and Steckl 2013).

Core-shell structures for the fabrication of nanofibrous scaffolds are one of the most recent useful approaches. This strategy is able to include bioactive factors such as bacteria, enzymes, drugs, viruses and proteins within the core, by which their release kinetics can be controlled (Li, Song, and Zhao 2010). One of the most frequently used setups for making 42 Madan Lal, Mamta Shandilya, Seema Sharma et al. such structures is a coaxial core-shell electrospinning setup. This technique contains two separate solutions (generally a hydrophobic polymer and a solution of water soluble bioactive factors) that can be co-electrospun with no direct mixing, by using two aligned syringes (Konno et al. 2014). Several factors can affect the coaxial electrospinning process, which is generally affected in an aqueous phase, in the core part (Sun et al. 2003). Since the process of coaxial electrospinning is similar to the conventional electrospinning, all factors that control the quality of the process and the morphology of the fibers in the conventional technique also affect the behavior of the coaxial electrospinning method. A novel dual drug delivery system is presented using triaxial structured nanofibers, which provides different release profiles for model drugs separately loaded in either the sheath or the core of the fiber. Homogenous, coaxial and triaxial fibers containing a combination of materials (PCL, polycaprolactone; PVP, polyvinylpyrrolidone) were fabricated (as shown in Figure 14.). The drug release profiles were simulated using two color dyes (KAB, keyacid blue; KAU, keyacid uranine), whose release in physiological solution was measured using optical absorption as a function of time. To reach the level of 80% release of encapsulated dye from the core, triaxial fibers with a PCL intermediate layer exhibited a ∼24× slower release than that from coaxial fibers. At the same time, the hygroscopic sheath layer of the triaxial fibers provided an initial burst release (∼ 80% within an hour) of a second dye as high as that of conventional single and coaxial fibers. The triaxial fiber membrane provides both a quick release from the outer sheath layer for short-term treatment and a sustained release from the fiber core for long-term treatment. The intermediate layer between the inner core and outer sheath acts as a barrier to prevent leaching from the core, which can be especially important when the membranes are used in the wet application. The formation of tri/multiaxially electrospun nanofibrous membranes will be greatly beneficial for biomedical applications by enabling different release profiles of two different drugs from a membrane (Han and Steckl 2013).

6.2. Hollow Nanofibrous Scaffolds

Hollow nanofibrous scaffolds are appropriate for special applications such as hydrogen storage and nano-fluidic systems. In general, there are two methods for fabricating hollow nanofibrous scaffolds, including the direct coaxial electrospinning method and chemical vapor deposition (CVD) (Dalton, Klee, and Möller 2005, Bognitzki, Frese, et al. 2001). However, for constructing hollow nanofibrous scaffolds coaxial electrospinning is more considered by researchers. In this method, nanofibers are fabricated by the same technique which is produced core-shell structures but there is a difference in the process, that the core material must be dissolved in the appointive solvent at the end of the electrospinning process (Gu and Jian 2008). Also, by a coaxial electrospinning technique, it is possible to fabricate highly porous hollow nanofiber scaffolds used in very specific applications such as optoelectronics, catalysis, nano-fluidics, biosensor systems, drug delivery and tissue engineering. Lee et al. (Lee, Song, and Yoon 2010), constructed hollow carbon nanofibers (HCNFs) by coaxial electrospinning method of poly (styrene-co-acrylonitrile) (SAN) and poly (acrylonitrile) (PAN) solutions. In their study, SAN was discovered to be a very appropriate material for the core and PAN was used as the shell. According to the nature of Smart Material Nanofibers for Day to Day Life 43

SAN, it has a good thermal sustainability that can prevent the PAN shell from shrinking through the carbonization and stabilization process. In another study, Kang et al. (Kang et al. 2015), used PCL/PEO core–shell hollow structured fibers to load and deliver dual growth factors, which objects bone restoration therapy. This design using novel MBN (Mesoporous bioactive glass nanospheres) nanocarriers with a core–shell structure aims to release two types of growth factors, FGF2 and FGF18, simultaneously, which is considered to provide a promising therapeutic scaffold that is efficacious for bone tissue reconstruction. The results proved that FGF2-FGF18-loaded fibrous scaffolds have noticeably higher bone formation ability, in terms of bone density and volume.

6.3. Highly Porous Nanofibers

Comparing with hollow nanofibers, highly porous nanofibers are more comprehensive to utilize in several types of biomedical applications. According to their high aspect ratio, highly porous nanofibers are appropriate candidates for using in various applications such as membrane (Agrawal, Peng, and Espinosa 2009), filtration, fuel cells (Choi et al. 2004), catalysis, drug delivery (Wernike et al. 2010) and tissue engineering (Zhan et al. 2007). They can be fabricated with specific topology by choosing specific solvents or mixtures of them, or polymer composites. One method to obtain highly porous nanofibers is that immiscible polymers are electrospun by a common solvent which leads to dissolving one of the polymers in the electrospun materials, so porous nanofibers can be obtained as scaffolds (Malda et al. 2004). This method is based on phase separation method, which is dependent on the evaporation rates. In another method, porous structures obtain by phase separation of the polymer and the solvent is produced by eliminating the solvent under vacuum (Garcia et al. 2012). Moreover, larger pores are essential to allow cell penetration through entire scaffold, and also allow ingrowth of blood vessels, and guide tissue construction. One of the critical factors for bone tissue engineering is controlling the pore size as many studies have shown that pore size has an impressive effect on cell behavior and bone formation. Through this effect, controlling pore size becomes another method to control tissue restoration and also appropriate pore size could enhance osteoblast adhesion, proliferation, differentiation, and mineralization (Kang et al. 2015).

7. POSSIBILITIES OF SMART MATERIALS NANOFIBERS IN APPLICATIONS

7.1. Catheter Application

The catheter is a flexible tube inserted through a narrow opening into a body cavity, particularly the bladder, for removing fluid (Geng et al. 2012, Che et al. 2007). Issues that should be considered when choosing a catheter are

1. Catheter materials should have ease of use (Geng et al. 2012). 2. Catheter Material should be having strength and flexibility (Smith 2003). 44 Madan Lal, Mamta Shandilya, Seema Sharma et al.

3. It should be tissue compatibility, smooth and frictionless which help to prevent encryption and irritation (Tenke et al. 2008, Cottenden et al. 2009, Hooton et al. 2010, Schumm and Lam 2008). 4. Catheter materials should have short-term and long-term indwelling, which depend upon the need of requirement (Tenke et al. 2008, Gould et al. 2010, Cottenden et al. 2009).

As know that smart materials nanofibers have features like higher surface area to volume ratio (Huang et al. 2003, Nayak et al. 2011), Superior mechanical properties (Chang et al. 2010), flexibility (Dalmas et al. 2007, Alcoutlabi et al. 2016, Huang et al. 2016), ultra long length (Che et al. 2007). On these properties basis, we can say that some of the piezoelectric materials (like ZnO) with biocompatible polymers (like cellulose) can be used for catheter application.

7.2. Heart Blockage

The coronary artery’s function is to deliver blood and oxygen to the heart muscle and is divided into two main branches. The left coronary artery supplies oxygen-rich blood into the heart ventricles and the left atrium (as shown in Figure 15.). The right coronary artery is split between the right posterior artery and a larger marginal branch, which delivers blood to the ventricles, right atrium and sinoatrial node. The sinoatrial node is a cluster of cells located in the right atrial wall which regulates the rate at which the heart pumps. A significant decrease in oxygen leads to an increase in blood pressure and heart rate as the heart begins to work harder to supply the body with the necessary blood and oxygen supply (Kumar and Kumar).

Figure 15. Shows the layout of the coronary arteries in the heart muscle (Kumar and Kumar).

Regardless of the insertion point, it is imperative to monitor the artery and blockage during the procedure. In order to produce images of the artery, an iodine-based dye is injected into the blood stream, so that doctors can clearly view the progression of blood flow during Smart Material Nanofibers for Day to Day Life 45 the operation. Through these arteries, differently shaped catheters can be passed towards the heart. Through the catheter, an ultra-thin wire is threaded across the blockage region in the coronary artery. Over this wire, a thin, expandable balloon is passed to the blockage, where it is then inflated to several atmospheres of pressure. As the balloon expands, the plaque is forced up against the walls of the artery and broken up to allow for a restoration of blood flow (Goosen, French, and Sarro 2000, Tanase et al. 2002). In this process, SMNFs can be used as the wire as well as catheter material respectively due to their unique properties.

7.3. Brain Hemorrhage

A localized bleeding inside the cranium caused due to the bursting of the blood vessels is referred as brain hemorrhage as in Figure16. Brain hemorrhages are also called intracranial hemorrhages (ICH), cerebral hemorrhages or, intra-cerebral hemorrhages which constitute to about 13 percent of strokes (Levy, Haltia, et al. 1990, Levy, Carman, et al. 1990). A bleeding can occur within the brain, between membranes that cover it and the brain, or the covering of the brain between and the skull. With bleeding, the pooled blood inside the skull can cause irritation leading to the swelling of the brain cells. A mass is formed by this pooled blood which is called as a hematoma. This causes an increase in the intracranial pressure leading to a reduction in the blood flow to the brain cells. The brain cells then get killed due to lack of blood supply causing a hemorrhage (http://www.desimd.com). SMNFs can be used as a vehicle to deliver the drug to the targeted area in the brain.

Figure 16. Bursting of the blood vessels is causing bleeding in the brain (http://www.desimd.com/).

46 Madan Lal, Mamta Shandilya, Seema Sharma et al.

CONCLUSION

Smart material nanofibers (SMNFs) are those materials in nanofiber form which have one or more properties that can be significantly changed in a controlled manner by different fields, such as temperature, pressure, electric flow, magnetic flow, light, mechanical, etc., originating internally or externally. Electrospinning technique is the best method to fabricate these SMNFs easily, because of various controllable parameters which help for the modification of properties of these SMNFs. These controllable parameters including Instrumental, Solution and Environmental parameters.

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Chapter 2

POSSIBLE APPLICATIONS OF ZINC AND TITANIUM IN MODERN LIFE

Anjali Sharma*, Madan Lal, Naheed Ahmad and Radheshyam Rai School of Physics and Materials Science Shoolini University, Solan (H.P.) India Department of Botany, Patna University, Patna, Bihar, India

ABSTRACT

Titanium dioxide, also known as Titania, is the naturally occurring oxide of titanium, chemical formula TiO2. Generally, it is sourced from ilmenite, rutile, and anatase. It has a wide range of applications, from paint to sunscreen to food coloring and even wound healing. TiO2 is an Earth mineral which is chemically stable. It is known for having low reactivity. Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, ceramics, ointments, adhesives, food and first- aid tapes. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically. Antibacterial activity of zinc oxide nanoparticles (ZnO-NPs) has received significant interest worldwide particularly by the implementation of nanotechnology to synthesize particles in the nanometer region. ZnO-NPs exhibit attractive antibacterial properties due to increased specific surface area. The reduced particle size leads to enhanced particle surface reactivity. ZnO is a bio-safe material that possesses photo-oxidizing and photocatalysis impacts on chemical and biological species. Both titanium dioxide and zinc oxide are derived from chalky, reflective materials. While this is beneficial in reflecting the sun’s rays, the application of these materials can be unappealing on the skin, because it results in the white, chalky appearance, according to the Environmental Working Group. Titanium dioxide is scaled down to range between 10 and 100 nanometers (nm), while zinc oxide is scaled down to 30 to 200 nm. The nano range of the above makes them less noticeable on the skin. But even after being so small it does not penetrate the skin easily. Titanium dioxide is noncomedogenic, (meaning it

* Corresponding Author Email: [email protected]. 68 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

does not contain oil that can clog the skin’s pores), according to “The New York Times.” In addition to its sun-fighting properties, zinc oxide is known for its noncomedogenic and antimicrobial properties. This can help to enhance wound healing. In this chapter, we will discuss the synthesis of these nanoparticles by sol-gel method. Also, we will discuss their present applications and future work based on nanofibers.

Keywords: TiO2, ZnO, nanoparticles, wound dressing, applications

1. INTRODUCTION

Nanotechnology has attracted global attention because nanoparticles have properties that are unique from their bulk equivalent. This technology is capable of providing miscellaneous novel applications that range from innovative fabric compounds, food processing, and agricultural production to sophisticated medicinal techniques (Sahoo 2010). The intrinsic properties of metal nanoparticles (NPs) such as ZnO, TiO2 are mostly characterized by their size, composition, crystallinity, and morphology. Reducing the size to nanoscale can modify their chemical, mechanical, electrical, structural, morphological, and optical properties. These modified features allow the NPs to interact in a unique manner with cell biomolecules and thus facilitate the physical transfer of NPs into the inner cellular structures (Rasmussen et al. 2010). Nano-sized particles of ZnO and TiO2 have more pronounced antimicrobial activities than large particles. This is due to the small size (less than 100 nm) and the high surface-to- volume ratio of nanoparticles that allow better interaction with bacteria. Interestingly, ZnO-NPs are reported by several studies as non-toxic to human cells (Colon, Ward, and Webster 2006), this aspect necessitated their usage as antibacterial agents, noxious (harmful) to microorganisms, and hold good biocompatibility to human cells (Padmavathy and Vijayaraghavan 2008). Because of new clinical evidence presented at the World Union of Wound Healing Societies’ meeting in Paris 2004, it is timely to re-evaluate the potential benefits offered by zinc therapy in wound management. Although numerous clinical trials claim to show the benefits of using oral or topical zinc therapy in wound management, variations in treatment regimen and zinc formulations used have obscured the true efficacy of the protocols. In keeping with the early studies on supplementary zinc therapy in pilonidal sinus management (Pories et al. 1967), the evidence is now available to show that not only is zinc beneficial in the healing profile but that it provides an effective level of anti-infective action (Ågren et al. 2006).

1.1. Properties of ZnO

 Zinc oxide is an inorganic compound.  ZnO particles are effective for inhibiting both Gram-positive and Gram-negative bacteria. They even have antibacterial activity against spores that are high- temperature and high-pressure resistant (Yamamoto 2001; Sawai et al. 1996). Possible Applications of Zinc and Titanium in Modern Life 69

 Smaller ZnO particles have a better antibacterial activity (Yamamoto 2001; Makhluf et al. 2005).  The antibacterial activity depends on the surface area and concentration, while the crystalline structure and particle shape have little effect. The higher the concentration and larger the surface area, the better is the antibacterial activity (Yamamoto et al. 1998)  High-temperature treatment of ZnO particles has a significant effect on their antibacterial activity. Treatment at a higher temperature leads to a lower activity (Sawai et al. 1996).  Antibacterial agents are broadly of two types, organic and inorganic. At high temperatures/pressures, organic antibacterial materials are found to be less stable compared to inorganic antibacterial agents. Thus Zinc Oxide has proved to be a powerful antibacterial agent in the formulation of the microscale and nanoscale systems for therapeutic applications. ZnO nanoparticles show greater antibacterial activity than microparticles. The exact mechanisms of the antibacterial action have not yet been clearly identified.  In the field of research and development, there are a lot of results obtained related to the ZnO nanoparticles, synthesis and its applications. Preparation and utilization of ZnO nanoparticles were explored for the anti-microbial effect in the fourth era.  Many of the biochemical and molecular events in wound repair can be expedited by the addition of supplementary zinc ion through up-regulation of MTs (Lansdown 2002) and zinc metalloenzymes (Ravanti and Kahari 2000). Furthermore, any defect in the expression of zinc finger transcription factors in mRNA coding of growth factors is consistent with impaired wound healing (Zhu et al. 2004). The quantitative and qualitative distribution of zinc in skin wounds is determined by atomic absorption spectrometry and immune histochemical techniques for demonstrating zinc-binding proteins.

Titania (TiO2) has attracted noticeable attention and has been assessed widespread in recent years. Reports of TiO2 with different shapes such as nanoparticle thin films, nanorods, nanowires, and nanotubes have spurred a great interest in studies on TiO2 nanostructure synthesis and their application (Kavitha, Gopinathan, and Pandi 2013). Nanomaterial with different shape and structure usually has varied chemical, optical and electrical properties. Shape control has been a significant concern in nanotechnology. Properties also vary as the shapes of the shrinking nanomaterials change (Vetrivel, Rajendran, and Kalaiselvi 2015). Many excellent reviews and reports on the preparation and properties of nanomaterial have been published recently. The specific surface area and surface-to-volume ratio increase dramatically as the size of material decreases. The performance of TiO2 based devices is largely influenced by the sizes of the TiO2 building units, apparently at the nanometer scale. Bulk TiO2 is known to be very beneficial non-toxic, environment-friendly, exceptionally biocompatible. The TiO2 nanoparticles provide a slow release of titanium ions that have wound healing and antimicrobial properties. The titanium ions released from nanoparticle inhibits microbial proliferation and hence it accelerates wound healing. It is considered non- toxic and has been approved by the American Food and Drug Administration (FDA) for use in human food, drugs, cosmetics and food contact materials (Lansdown 1996). 70 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

Titanium dioxide is frequently used in the pharmaceutical and cosmetic industries. It is an important product for nanotechnology because of its photocatalysis, anticorrosion and high stability. TiO2 is considered very safe. We must take into consideration that the maximum amount and concentration of TiO2 that can be added is restricted since high levels of TiO2 in the skin surface can cause irritation of the skin and the viscosity becomes excessive. The use of nanoparticles of TiO2 should be avoided after UV irradiation since these nanoparticles may pass through the skin and penetrate into the tissue and may be a risk to human health (Tamaddon, Behnia, and Behnia 2014; Wu et al. 2009). Both compounds are of low toxicity and are constituents of many commercial preparations. They are listed as sunscreen scattering agents and physical screens to solar radiation (Hassiba et al. 2016). Zinc oxide (as calamine) has been recognized for its healing properties for many years and its value in the therapy of nappy (diaper) rash, bed sores, venous ulcers and skin wounds is well recognized (Fischer 1986). Normal growth, maturation, and homeostasis in the skin and its appendages depend on the sufficiency of certain trace metals and their incorporation into metalloenzymes, structural components and physiological systems (Lansdown 1995). Most of this trace metal requirement is provided by a balanced diet but many trace metals can be absorbed percutaneously (Hostýnek et al. 1993). In the skin, at least zinc, calcium, copper, magnesium, and iron have well-defined roles. Nanofiber structure provides unique properties making them highly tissue compatible to enhance wound repair in comparison to other conventional dressings. An ideal wound dressing should have a structure that resembles the natural skin’s structure, and such nanofibers, due to their physical features, are able to mimic the healthy extracellular matrix network found in the skin. Therefore, nanofiber mats improve cell repair mechanisms (Hostýnek et al. 1993).

2. BACKGROUND AND MOTIVATION

With the increased levels of toxic gases and particles in the atmosphere, there is a dire need for us to protect ourselves so that these harmful particles do not enter our body. The field of nanotechnology is one of the most active research areas in modern materials science. It turns out that beauty is more than skin-deep: The average person slathers, lathers, rubs, and sprays, 10 different skin care products on his or her body every day--and since our skin acts more like a sponge than a barrier, we absorb the nearly 130 chemicals we regularly expose ourselves to (http://www.treehugger.com/htgg/how-to-go-green-natural-skin-care.html). Zinc is a transitional metallic element known from ancient times. It is widely distributed in the human environment, being found in water, air, and virtually all foodstuffs. The medicinal properties of zinc in the form of calamine were documented more than 3,000 years ago in the Ebers Papyrus and in ancient Ayurveda manuscripts in early Indian medicine (Lansdown et al. 2007). Zinc is second only to iron in being the most abundant trace element in the human body, but its nutritional significance came to light only in the 1960s following reports of zinc-responsive growth failure in infants in rural Egypt and Iran. Titanium dioxide white pigments are developments of the 20th century, and because of their high hiding power, low toxicity and reasonable cost, they have eclipsed other traditional white pigments. It has the highest refractive index of any material known to man, greater even than diamond. To Possible Applications of Zinc and Titanium in Modern Life 71 take advantage of this property, titanium dioxide must be mined, refined and ground to a fine, uniform particle size. The motivation behind using Titania is that it is antimicrobial, antibacterial and also antioxidant (http://www.naturalpigments.com/titanium-dioxide-pigment.html). Titanium is corrosion resistant, provides a surface conducive to cell proliferation, differentiation and survival and is structurally stable under function. These properties make this metal biocompatible and thus so universally popular (Larjava 2012).

3. SCOPE AND SIGNIFICANCE

Skin is a multilayer organ that acts as an interface between the internal organs and the external environment, forming a barrier that prevents the body dehydration and the penetration of external microorganisms. As the skin is permanently exposed to the external atmosphere, it is extremely vulnerable to the appearance of different types of lesions, such as burns, ulcers, and wounds (Pereira and Bartolo 2016). Wound-healing therapies can be broadly classified into traditional and modern therapies, which have distinct levels of efficacy, clinical acceptance, and side effects. Traditional therapies have been used for many centuries mainly by the rural populations in developing countries. Usually, these therapies involve the use of herbal- and animal-derived compounds, living organisms, silver and traditional dressings. On the other hand, modern therapies comprise the use of grafts, modern dressings, bioengineered skin substitutes, and cell/growth factor therapies. Zinc and titanium oxides both compounds are of low toxicity and are constituents of many commercial preparations. They are listed as sunscreen scattering agents and physical screens to solar radiation (Lansdown and Taylor 1997). Zinc is an essential trace element in the human body and its importance in health and disease is appreciated. It serves as a cofactor in numerous transcription factors and enzyme systems during wound repair. Zinc deficiency of hereditary or dietary cause can lead to pathological changes and delayed wound healing. Zinc oxide in paste bandages (Unna boot-An Unna boot is a compression dressing made by wrapping layers of gauze around your leg and foot. It is often used to protect an ulcer or open wound. The compression of the dressing helps improve blood flow in your lower leg. Compression also helps decrease swelling and pain. Protects and soothes the inflamed peri- ulcer skin (tissue surrounding the wound) (Lansdown et al. 2007). Titanium dioxide is frequently used in the pharmaceutical and cosmetic industries. It is an important product for nanotechnology because of its photocatalysis, anticorrosion and high stability. TiO2 is considered very safe (Naves and Almeida 2015). Titania explored as adhesion and growth support platforms for bone and stem cells for the prevention of bacterial adhesion, drug delivery and enhancing blood clotting for control of hemorrhage (an escape of blood from a ruptured blood vessel). It is considered non-toxic and has been approved by the American Food and Drug Administration (FDA) for use in human food, drugs, cosmetics, and food contact materials. The nanoparticles provide a slow release of titanium ions that have wound healing and antimicrobial properties. The titanium ions released from nanoparticle inhibits microbial proliferation and hence it accelerates wound healing (Archana et al. 2013). Such factors increase the significance and hence the scope is further established. 72 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

4. HISTORIC BACKGROUND OF TIO2 AND ZNO

The history of nanomaterials is quite long; nevertheless, major developments within nanoscience have taken place during the last two decades. The idea of Nanotechnology was first highlighted by Noble laureate Richard Feynman, in his famous lecture at the California Institute of Technology, 29th December 1959. Richard Feynman in one of his articles published in 1960 titled, “There is plenty of room at the bottom” discussed the idea of nanomaterials. He pointed out that if a bit of information required only 100 atoms, then all the books ever written could be stored in a cube with sides 0.02 inch long. In 1970 Norio Taniguchi first defined the term Nanotechnology. According to him, “Nanotechnology mainly consists of the processing of, separation, deformation, and consolidation of material by one atom or by one molecule.” And in 1980 another technologist; K. Eric Drexler promoted technological significance in nanoscale. The main important thing in nano dimension is the properties of particles are far different than bulk-scale properties. Zinc oxide (ZnO) is no stranger to scientific study. Zinc is a transitional metallic element known from ancient times. It is widely distributed in the human environment, being found in water, air, and virtually all foodstuffs. The medicinal properties of zinc in the form of calamine were documented more than 3,000 years ago in the Ebers Papyrus and in ancient Ayurvedic manuscripts in early Indian medicine (Prasad 1994; Jones and Williams 2004) but the observation by Raulin in 1869 that the mold Aspergillus niger would not grow on a zinc- deficient medium was fundamental in establishing the importance of zinc in biological systems. Subsequent research has shown that zinc is present, albeit in minute concentrations, in all living plant and animal cells, mainly in the form of cofactors or structural components in key enzyme systems in cell replication, protein synthesis, and repair systems following injury. In the past 100 years, it has featured as the subject of thousands of research papers, dating back as early as 1935 (Look et al. 1998). Valued for its ultraviolet absorbance, wide chemistry, piezoelectricity and luminescence at high temperatures, ZnO has penetrated far into industry, and is one of the critical building blocks in today’s modern society (Segawa et al. 1997). It can be found in paints, cosmetics, plastic and rubber manufacturing, electronics and pharmaceuticals, to name just a few. Zinc was identified as an essential micronutrient by the Wisconsin group of biochemists in 1934. The nutritional value of zinc was widely researched by McCance and Widdowson in the 1940s (McCance and Widdowson 1942) but the true clinical significance of zinc was not appreciated until much later (Lansdown et al. 2007). The body requirements for zinc in humans are normally satisfied by a well-balanced diet leading to an average daily intake of 10–15 mg per day in concordance with the recommended daily allowance for zinc in healthy adults of 8–15mg per day (Osis et al. 1972; Group 2002). Diets rich in protein are usually high in zinc (Osis et al. 1972) whereas vegetable diets containing high plant fiber are low in absorbable zinc (Prasad 1994). The surface-related process is primarily governed by the adsorption and desorption of the chemisorbed oxygen at the surface of the ZnO, which is exploited for gas sensing applications (Van de Walle 2000). This process becomes prominent in nanocrystalline films, where the surface area is large. It usually appears as a white powder and is nearly insoluble in water. ZnO is present in the Earth crust as a mineral zincite; however, most ZnO used commercially is produced synthetically. ZnO is non-toxic and is compatible with human skin making it a suitable additive for textiles and surfaces that come Possible Applications of Zinc and Titanium in Modern Life 73 in contact with human body. The increase in surface area of nanoscale ZnO compared to bulk has the potential to improve the efficiency of the material function. Zinc is second only to iron in being the most abundant trace element in the human body (Scrimshaw and Young 1976), but its nutritional significance came to light only in the 1960s following reports of zinc-responsive growth failure in infants in rural Egypt and Iran (Prasad et al. 1990; Reinhold 1971). Because of new clinical evidence presented at the World Union of Wound Healing Societies’ meeting in Paris 2004, it is timely to re-evaluate the potential benefits offered by zinc therapy in wound management. Although numerous clinical trials claim to show the benefits of using oral or topical zinc therapy in wound management, variations in treatment regimen and zinc formulations used have obscured the true efficacy of the protocols. In keeping with the early studies on supplementary zinc therapy in pilonidal sinus management (Pories et al. 1967), the evidence is now available to show that not only is zinc beneficial in the healing profile but that it provides an effective level of anti-infective action (Ågren et al. 2006). Furthermore, a young boy with Hirschsprung’s disease with symptoms of zinc deficiency successfully treated with zinc following gastrointestinal surgery provides further irrefutable evidence for the value of zinc in wound healing (Patel and Harding 2004).

Figure 1. Block diagram of various Applications of TiO2 (Macwan, Dave, and Chaturvedi 2011).

74 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

5. APPLICATIONS OF TIO2

5.1. Medical Applications

5.1.1. Dental Implant Application When a biomaterial is implanted into the human body, it is unavoidable that blood will contact the implant surface and on contact, the implant surface could be covered with a layer of plasma proteins that mediate the next cellular responses. This leads to a better biocompatibility for the implant surface. As titanium (Ti) and its alloys have satisfactory mechanical properties, corrosion resistance, and biocompatibility, they have been widely used for orthopedic and dental implant materials. The Ti metal spontaneously forms a protective TiO2 layer in the atmosphere. When the Ti implant is inserted into the human body, the surrounding tissues directly contact the TiO2 layer on the implant surface. The surface characteristics of the TiO2 layer determine the biocompatibility of Ti implant. Therefore, it is important to use appropriate surface modifications to increase the biocompatibility of the Ti implant for long-term clinical applications (Yang et al. 2009).

5.1.2. Polylactide Nanofibers/nano-TiO2 Blends on Bio Recognition of Anticancer Drug Daunorubicin The biocompatibility and biodegradability of polylactide (PLA) polymers make them well adopted in drug delivery, tissue engineering, and temporary therapeutic applications in pharmacology and surgery. Metallic nanoparticles (e.g., Au or Ag) or semiconductor nanoparticles have valuable applications in electronics, optics, proteomics (it is the set of expressed proteins in a given type of cell or organism, at a given time, under defined conditions), and bioanalytical fields, due to their unique properties such as large surface area, pore structure, embedded effect, and size effect, unique property and high reactivity of nano titanium dioxide (TiO2) makes it possible to be utilized in the fields of biomedical and bioengineering. The PLA nanofibers have been synthesized by electrospinning and the blending of nano-TiO2–PLA nanofibers have been utilized to remarkably enhance the performance and detection sensitivity of the bio-recognition as well as the binding affinity of anticancer drug daunorubicin to DNA. Results of the electrochemical and atomic force microscopy (AFM) studies demonstrate that the new nano-TiO2–PLA polymer nanocomposites could facilitate the binding of daunorubicin to DNA and remarkably enhance the detection sensitivity of the relative bio-molecular recognition (Song et al. 2008).

5.2 Applications for Reducing Air/Water Pollution

Antimicrobial nanomaterials for water disinfection and microbial control: potential applications and implications TiO2 can kill both Gram-positive and Gram-negative bacteria (thick mesh-like cell wall made of peptidoglycan and thinner wall), although Gram-positive bacteria are less sensitive due to their ability to form spores. More recently, nano-sized TiO2 was also reported to kill viruses including poliovirus 1, hepatitis B virus, Herpes simplex virus, and MS2 bacteriophage. The concentration of TiO2 usually required to kill bacteria varies between 100 and 1,000 ppm, depending on the size of the particles and the intensity Possible Applications of Zinc and Titanium in Modern Life 75

and wavelength of the light used. The antibacterial activity of TiO2 is related to hydroxyl free radicals and peroxide formed under UV-A irradiation via oxidative and reductive pathways, respectively. The Strong absorbance of UV-A renders activation of TiO2 under solar irradiation, significantly enhancing solar disinfection. It is found that the bacteria exposed to TiO2 photocatalytic disinfection do not self-repair. However, bacterial death also occurred in the dark indicating that other unknown mechanisms may be involved. TiO2 is suitable for applications in water treatment because it is stable in water, non-toxic by ingestion and low cost and suitable for drinking water disinfection. The photoactivity in the UV-A range and the potentially visible light activity, when doped with metals, makes TiO2 photocatalytic disinfectant. Especially useful in developing countries where electricity is not viable (capable of working successfully). However, TiO2-based solar disinfection is, in general, a very slow process due to the small fraction of UV-A in solar radiation. Therefore, success in research on metal or nitrogen doping to improve visible light absorbance of TiO2 or UVA activity is critical to the application of TiO2 solar disinfection. Recently, it was demonstrated that doping TiO2 with silver greatly improved photocatalytic bacterial inactivation by UV-A activated TiO2 (Li et al. 2008).

5.3. Pre-Concentration and Separation of Cadmium

Recently, determination of ultra-trace metals such as cadmium in environmental and food samples has become more serious due to increasingly lower limits imposed on trace metal content of such samples. Cadmium is one of the most dangerous trace metals in the environment of human, not only because of its high-level toxicity but also because of its wide distribution and its many important applications. The FAO/WHO Joint Expert Committee on Food Additives recommended provisional maximum tolerable daily intake of cadmium from all sources (water, food, and air) of 1.0–1.2 g/kg body mass. The maximum permissible level of cadmium in drinking water is 5.0 g/L. The direct determination of extremely low concentrations of the required trace metals by modern spectroscopic methods is still difficult due to insufficient sensitivity of the techniques and matrix interferences. In order to overcome this problem, preliminary separation and/or pre-concentration techniques for the separation of trace metals from complex matrices into a known matrix is widely used. Among the preconcentration techniques, solid phase extraction (SPE) has been used increasingly in compared with other classical methods. Numerous substances have been synthesized and used as solid phase extractor (Kalfa, Yalçınkaya, and Türker 2009).

5.4. Nano-gold Supported on TiO2 Coated Glass Fiber for Removing Toxic CO Gas from Air

Outstanding catalytic activities of nano-gold for oxidizing CO at low temperature, various reactions over nano-gold catalysts have been studied. These include CO oxidation, preferential oxidation of CO in the presence of excess hydrogen (PROX), water gas shift reaction (WGSR), hydrogenation, and oxidation. However, in most investigations, the nano- gold particles (some in the form of gold nanotubes) are supported on metal oxide powders, such as TiO2, Fe2O3, Al2O3 and MgAl2O4, or on porous materials, such as zeolite, on the other 76 Anjali Sharma, Madan Lal, Naheed Ahmad et al. hand, the powder form of these catalysts may limit their application, because the drop in pressure becomes an important problem when the packing density of the powder is too high. Hence, nano-gold was prepared on glass fiber is expected to have various applications, one of which is as a packing material in safety gas masks for removing toxic CO gas from the air because nano gold catalysts have advantages over other catalysts for removing CO, including higher reaction activity at room temperature and higher moisture resistant. The catalytic activity of nano-gold supported on TiO2-coated glass fibers depends on the TiO2 crystal size. Au/TiO2-glass fiber catalysts can continuously convert CO to CO2 for more than 15 min, meeting the European standard (Kuo et al. 2007).

5.5. Application of Nano-TiO2 toward Polluted Water Treatment Combined with Electro-Photochemical Method

With the rapid development of industry and fast increase of population density in the city, a variety of poisonous and harmful substances, especially organic pollutants, were discharged into natural waters without appropriate treatment, which has caused serious pollution. Up to now, great attention has been paid to water pollution and its treatment, photocatalysis because it can completely degrade the organic pollutants into harmless inorganic substances (such asCO2, H2O, etc.) under moderate conditions, and would not bring with any serious secondary pollution. Nano-TiO2 is one of the suitable semiconductors for photocatalyst but its properties, not only the photo-efficiency or activity but also the photoresponse are not sufficient, for the moment the high recombination ratio of photoinduced hole – electron pairs also reduces its catalytic efficiency. Therefore, various modifications have been performed on Nano -TiO2 to promote its catalytic ability and develop new Photocatalytic functions (Chen et al. 2003).

5.6. Photo Catalytic Oxidation of VOCs Using Nano-TiO2 Photo Catalyst

Indoor air pollution is becoming a serious problem recently. Pollutants in the indoor air such as volatile organic compounds (VOCs), NO2, and SO2 can cause adverse health impacts on occupants. VOCs are the main components of the indoor air pollutants, which are from indoor furnishing, tobacco smoke, vehicular emissions, and the use of liquefied petroleum gas. Conventionally, VOCs pollutants are removed by air purifiers that employ filters to remove particulate matters or use sorption materials (e.g., granular-activated carbon) to adsorb the pollutants. Improper maintenance of these purifiers may even become a source of the pollutant. The photocatalytic process is emerging as a promising alternative technology for the degradation of VOCs. The thin film nano-TiO2/UV process is an advanced oxidation process. TiO2 is superior to other photocatalysts because of its interesting characteristics (Yu, Zhang, and Rossi 2007)

(a) It is low cost, safe, and very stable showing high photocatalytic efficiency. (b) It promotes ambient temperature oxidation of the major classes of indoor air pollutants. Possible Applications of Zinc and Titanium in Modern Life 77

(c) Complete degradation of a broad range of pollutants can be achieved under certain operating conditions; (d) No chemical additives are needed. It is known that the thin film nano-TiO2 photocatalytic degradation efficiency of VOCs can be potentially limited by several parameters. (a) The degradation efficiency may be very low inside a thick catalyst due to the UV light intensity attenuation by the catalyst. (b) The VOCs molecules diffusion in the catalyst can be very slow so that there are not enough molecules available for degradation inside the catalyst.

5.7. Aluminum-Doped TiO2 Nanopowders for Gas Sensors

Transition metal oxides are the most widely used ceramics for gas sensor applications. A change in their electrical conductivity due to the presence of a target gas is used for sensing measurement. Among different transition metal oxides, SnO2-based sensors are the most widely used. As gas-sensitive resistors, these sensors show good sensitivity and selectivity 0 0 below 250 C However, at temperatures [250 C], SnO2-based sensors show poor sensitivity due to lack of stability. It has been reported that titanium dioxide (TiO2)-based gas sensors show good sensitivity and stability in adverse environments and have the potential to become the material of choice for high-temperature gas sensors. It has been shown that sensitivity of TiO2 sensors can be improved by the addition of dopants such as Nb, Cr, Sn, Pt, Zn, Al, La, and Y. The most important effects of dopant addition in TiO2 are increasing the conductivity, slowing down anatase-to-rutile transformation, and reducing grain growth. Among those dopants, Al-doping in TiO2 shows retardation of phase transformation from anatase to rutile by stabilizing the surface state of TiO2 particles and also inhibits grain growth. It has been reported that the conductivity of the Al-doped TiO2 is higher than that of pure TiO2, also enhances sensitivity and selectivity toward certain gasses, also been shown to have higher sensitivity to humidity in thick and thin films. With dopant addition, the higher crystallinity was also needed to improve the sensitivity of these gas sensors (Bozzi et al. 2005).

5.8. Self-Cleaning of Modified Cotton Textiles by TiO2 at Low Temperatures under Daylight Irradiation

Acrylic emulsions of TiO2 have been coated on textiles with the purpose of inducing self- cleaning effects under daylight irradiation, but the coating showed some degradation after being exposed to the action of daylight. The commercial potential of the TiO2 semiconductor on cotton textiles prepared at relatively low temperatures for self-cleaning purposes aims at the discoloration of organic materials including dyes, pigments, and grease on the modified cotton textile loaded with TiO2. The use of TiO2 loaded flexible substrates will possibly allow their application during the photodegradation of micelles, oils, solvents, smooth, aromatic, and aliphatic hydrocarbons under daylight. The fabric is coated with a thin layer of titanium dioxide nanoparticles. When this semi-conductive layer is exposed to light, photons with energy equal to or greater than the band gap of the titanium dioxide excite electrons up to the

78 Anjali Sharma, Madan Lal, Naheed Ahmad et al. conduction band. The excited electrons within the crystal structure react with oxygen atoms in the air, creating free radical oxygen. These oxygen atoms are powerful oxidizing agents, which can break down most carbon-based compounds through oxidation–reduction reactions. In these reactions, the organic compounds (i.e., dirt, pollutants, and microorganisms) are broken down into substances such as carbon dioxide and water. Since the titanium dioxide only acts as a catalyst to the reactions, it is never used up. This allows the coating to continue breaking down stains over and over (Choi et al. 2007).

6. APPLICATIONS OF ZNO

6.1. World Wide Use

Over the years zinc oxide applications have changed. Some major uses such as photocopy paper and linoleum (floor covering sheets) have disappeared. Paint, once the dominant use is important now only in niche applications. The most visible applications for zinc oxide are in skin care, for example, sunscreens, ointments, creams, and powders. In product volume, these are not so important. The most recent major new use for zinc oxide is in varistors, ceramic zinc oxide components that protect circuits from voltage spikes. Zinc oxide has typically been able to generate new uses to take the place of older applications. There is an unprecedented level of research work being directed toward electronic and photonic uses. These include LED’s, transparent transistors, solar cells and memory devices. Many of these applications, for example, LED’s for solid state lighting, even if widely adopted, would not be a high volume zinc oxide application, due to the minimal quantities used in such devices (http://www.zinc.org.in/zinc-oxide-applications).

Far East - 38% Africa - 2% Near East - 4% South America -7% Europe - 31% North America - 18%

Figure 2. Worldwide consumption of zinc oxide.

Possible Applications of Zinc and Titanium in Modern Life 79

6.2. Rubber Industry

Rubber is the most important application of zinc oxide. It has been used in this capacity for over a century. Zinc oxide along with stearic acid activates sulfur crosslinking of rubber. In addition, it provides pigmentary properties which enhance the ability to absorb frictional heat an important property in tire performance. The level of zinc oxide use is expressed in terms of ‘parts per hundred of rubber (phr). The usual level is 3-5 phr. Essentially all rubber goods contain zinc oxide.

Figure 3. (a) A rubber tire (http://www.zinc.org.in/zinc-oxide-applications), and (b) an electronic strip with Zinc content.

French process zinc oxide, made from metallic zinc, is preferred for rubber uses. The major reason for this is that the purity and physical characteristics can be controlled within close limits. The important properties of zinc oxide that are relevant to rubber are:

 Low oversize: To prevent point defects in the compound.  Particle size: To provide the required level of reactivity. Note particle size is usually expressed as the inverse measurement, surface area.  Purity: Some elements, for example, manganese, are detrimental to rubber curing at very low levels.  Soluble salts: Soluble salts would be expected to reduce the resistance of rubber to degradation.

Latex rubber products are often formulated with very fine zinc oxide known as Active grades which have low levels of opacity (http://www.zinc.org.in/zinc-oxide-applications).

80 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

6.3. Ceramic and Glass Compositions

Zinc oxide is a standard ingredient of ceramic glazes, for example in tiles. The zinc oxide is part of the frit formulation from which the glaze is derived. Zinc oxide has two important properties:

 It reduces the melt temperature of the glaze, thereby reducing the energy and equipment requirements.  It allows other pigments to develop stronger intrinsic colors.

Zinc oxide is also present in heat resistant glass and cookware as well as specialty glass applications such as photochromic lenses (http://www.zinc.org.in/zinc-oxide-applications).

Figure 4. Colored ceramic glaze tile (http://www.zinc.org.in/zinc-oxide-applications).

6.4. Animal Feed

Animal feed is a major use of zinc oxide. All living things need zinc to function because zinc compounds form major pathways in the body’s metabolism. Our animals usually have better diets than we do. The product requirements for animal feed applications are significantly different to most other uses. As the zinc oxide is formulated with many other ingredients into custom mixes, it is essential that it can be accurately metered. Regular zinc oxides do not flow very readily and also tend to stick to surfaces. Consequently, the zinc oxides used in this application are the product of kiln (a furnace or oven for burning, baking, or drying) operations and typically have a very high bulk density and flow very readily (Park et al. 2004) and http://www.piraiki.gr/index.php/en/products-zn/30-feed-grade-72-zno-en).

Possible Applications of Zinc and Titanium in Modern Life 81

Figure 5. Zinc oxide used as a diet for pigs (http://www.piraiki.gr/index.php/en/products-zn/30-feed- grade-72-zno-en).

6.5. Varistors

Varistors were developed around 30 years ago, and still, represent the most recent significant new use for zinc oxide. Varistors are zinc oxide compositions that have the ability to change their electrical resistance depending on the applied voltage. Commercially this is of great value because it is the basis of protection devices for electrical components. Varistors consist of ceramic blocks of zinc oxide that have been sintered along with small quantities of other metals such that there are defined grain boundaries between crystals. Depending on the formulation, the varistor’s electrical characteristics can be controlled for certain applications.

Figure 6. Radial Leaded (LDD) Metal Oxide Varistors Offer Transient Voltage Protection (https://www.pddnet.com).

82 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

The varistor is connected from the circuit to an earth connection. If a voltage spike occurs, the varistor conducts the excess voltage to earth. This device has an extremely rapid response time. Voltage spikes are very common for example through switching and induced currents by lighting. Small varistors are used extensively in electrical circuitry, much larger ones are used to protect the high power grid systems from both terrestrial and solar storms. The most important property of the ZnO varistor is its nonlinear I-V characteristic. Functionally, the varistor acts as a near insulator (resistor) prior to reaching a voltage known as the breakdown or threshold voltage, and it acts as a conductor thereafter. The electrical features’-15 that make the ZnO varistor attractive to the designer are the nonlinear or non- ohmic characteristic in the conductive mode and the low leakage (power loss) at the steady operating voltage in the resistive mode. These features can be highlighted by referring to three important regions of the curve (Gupta 1990).

6.6. Lubricating Oil Additive

Engine lubricating oils contain a package of ingredients that are essential in providing the performance, protection, and serviceability that we all take for granted. One of these additives is zinc dialkyl dithiophosphate, ZDDP. These groups of additives were developed with the growth of the Automobile Industry. These products are formed by the reaction of zinc oxide with the dialkyl dithio phosphoric acid intermediate. Properties can be modified by changing of the alkyl groups.

Figure 7. Zinc is used as one of the additives in Engine lubricating oils (https://www.hemmings.com/ blog/2012/10/18/tech-101-zinc-in-oil-and-its-effects-on-older-engines).

6.7. Paints

Zinc-rich paints have long been recognized for their excellent paint adherence to both new and weathered galvanized surfaces. Zinc-rich paints have been used in the U.S. for more than 75 years and in Europe for well over a century. One of the key reasons for the success of zinc-rich paints is their barrier and cathodic protection. In a 1960s study by the American Iron and Steel Institute and the Steel Structures Painting Council, zinc-rich paints outperformed all Possible Applications of Zinc and Titanium in Modern Life 83 other classes of paint. Direct Process zinc oxide is preferred due to its lower reactivity, the primary function of the zinc oxide is a fungistat. Zinc oxide is also used in ship’s antifouling paint compositions (https://www.briteproducts.com/zincpaint.php).

6.8. Sulfur Absorbent

Zinc oxide and carbonate compositions are made into absorbent pellets which can be used to desulfurize liquid and gas streams. Zinc oxide is able to absorb SO2 and H2S.

6.9. Fire Retardants

Zinc oxide is an ingredient of fire retardant compounds. Zinc borate, formed by the reaction of zinc oxide and boric acid, is used extensively in plastics.

6.10. Dielectric Strength

In high-voltage wire and cable insulation, Zinc Oxide improves the resistance to corona effects by its dielectric strength, and at elevated operating temperatures, it contributes to the maintenance of the physical properties of the rubber compound by neutralization of Acidic decomposition product.

6.11. Photocopying

Some of the unique electronic properties of Zinc Oxide are distinctively utilized in the photocopying process. For use in that process, Zinc Oxide is increased in photoconductivity and semiconductor properties by special heat and/or doping treatments (addition of foreign elements). Also, Zinc Oxide is greatly modified in optical properties to increase its absorption of light rays in the visible region. This process known as sensitization is generally carried out by the addition of certain dyes, which are absorbed on the surface of the Zinc Oxide. Commercial Zinc Oxide for photocopying is generally produced from metallic zinc, rather than ore, to obtain a product of higher purity.

6.12. Cosmetics

Zinc Oxide (ZnO) is powdered, oxidized zinc derived from the naturally occurring mineral, zincite, and routinely used in a wide range of consumer products. Zinc Oxide is well recognized as a key sunscreen ingredient that is effective in providing protection from UV light. It plays an important role in safeguarding public health (http://gluzin.com/zinc- uses.php-pharmacy).

84 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

Historical Facts Zinc compounds were likely used by early humans as a paint or medicinal ointment, but their composition is uncertain. The use of pushpanjan, probably zinc oxide, as a salve for eyes and open wounds, is mentioned in the Indian medical text the Charaka Samhita, thought to date from 500 BC or before. Zinc oxide ointment is also mentioned by the Greek physician Dioscorides (1st century AD.) Avicenna mentions zinc oxide in The Canon of Medicine (1025 AD), which identifies it as a preferred treatment for a variety of skin conditions, including skin cancer, though it is no longer used for treating skin cancer.

Why Is It Used in Cosmetics and Personal Care Products? Zinc oxide is used in a wide range of cosmetics and personal care products including makeup, baby lotions, bath soaps and foot powders. It is used as a bulking agent, a colorant a skin protectant in over-the-counter (OTC) drug products and as a sunscreen. Zinc oxide also has antibacterial and deodorizing properties. When used in sunscreen products, ZnO acts as a physical block to the sun’s ultraviolet (UV) radiation, helping to reduce or prevent sunburn and premature aging of the skin. Preventing sunburn is an important factor in reducing skin cancer risk. Zinc oxide is the broadest spectrum UVA and UVB reflector approved for use as a sunscreen by the FDA and is allowed in concentrations up to 25%. When used as an ingredient in sunscreen, zinc oxide screens both UVA (320–400 nm) and UVB (280–320 nm) rays of ultraviolet light. ZnO is widely used to treat a variety of skin conditions in products such as calamine cream and antiseptic ointments and as a skin protectant ingredient in products including in concentrations up to 40% in diaper rash ointments.

6.13. Pharmaceutical Industry

6.13.1. Diarrhea (children) Multiple studies in developing countries found that zinc supplementation in malnourished children with acute diarrhea may reduce the severity and duration of diarrhea, especially in children with low zinc levels.

6.13.2. Gastric Ulcers The healing process of gastric ulcers may be enhanced through treatment with zinc, although further studies will be needed to determine to what extent zinc may be beneficial for patients with this condition. Most studies report no or few adverse effects associated with its use.

6.13.3. Sickle Cell Anemia (Management) There is strong scientific evidence to suggest that zinc may help manage or reduce symptoms of sickle cell anemia. Most of these studies reported increased height, weight, immune system function, and testosterone levels and decreased numbers of crises and sickled cells following zinc treatment. Possible Applications of Zinc and Titanium in Modern Life 85

6.13.4. Zinc Deficiency Causes: Zinc deficiency is caused by inadequate intake or absorption, increased zinc excretion, or increased bodily need for zinc. Symptoms: Zinc deficiency symptoms include growth retardation, hair loss, diarrhea, delayed sexual maturation, impotence, eye and skin conditions, and loss of appetite. Additional symptoms may include weight loss, delayed wound healing, taste changes, and mental lethargy. Diagnosis: Zinc can be measured in plasma, red blood cells, white blood cells, and hair.

6.13.5. Down’s Syndrome In several studies, zinc supplements seemed to counteract hypothyroidism and slightly reduce the number of infections in children with Down syndrome. However, zinc did not seem to improve depressed immune systems. Additional human research is needed before a firm conclusion can be made.

6.13.6. Fungal Infections (Scalp) Evidence from human trials suggests that zinc pyrithione shampoo may be an effective treatment for tinea vesicular fungal infections of the scalp. No side effects were noted. Additional research is needed before a strong recommendation can be made.

6.13.7. High Cholesterol Zinc may improve blood cholesterol levels in hemodialysis patients. There is some evidence that zinc may improve cholesterol ratio of HDL “good cholesterol” versus LDL “bad cholesterol,” which would be considered a positive effect. Well-designed clinical trials are needed before a strong recommendation can be made.

6.13.8. Immune Function Zinc appears to be an essential trace element for the immune system, but research on the effect of zinc supplementation on immune function is scant and mostly focuses on patients with specific diseases. Zinc gluconate appears to have beneficial effects on immune cells. There are relatively few studies that examine zinc levels and the effects of zinc supplementation on the health of the elderly population. Further research is needed before a recommendation can be made.

6.13.9. Plaque/ Gingivitis A few studies have reported a significant reduction in plaque accumulation following treatment with zinc rinses and dentifrices. Preliminary research suggests that zinc citrate dentifrice may reduce the severity and occurrence of supragingival calculus formation. However, more well-designed studies are needed to confirm such benefits. More research might help to determine zinc’s potential efficacy in other dental applications.

6.13.10. Bad Breath Chewing gum containing zinc or rinsing out the mouth with a solution containing zinc seemed to reduce bad breath (halitosis) in early studies.

86 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

6.13.11. Boils In one study, patients with recurrent boils (furunculosis) treated with zinc found their furuncles did not reappear. Well-designed clinical trials are needed to confirm this potential benefit.

6.13.12. Dandruff Shampoo containing 1% of zinc pyrithione has been shown to reduce dandruff in some people.

6.13.13. Diaper Rash Zinc may reduce the incidence of diaper rash and have a preventative effect. (http://gluzin.com/zinc-uses.php)

7. PROCESSING METHODS

The synthesis of the nanomaterial can be well accomplished by two approaches. Firstly, by “Bottom Up” method where small building blocks are produced and assembled into larger structures. Here the main controlling parameters are morphology, crystallinity, particle size, and chemical composition. Examples: chemical synthesis, laser trapping, self-assembly, colloidal aggregation, etc. Secondly, by “Top Down” method where large objects are modified to give smaller features. For example film deposition and growth, nanoimprint /lithography, etching technology, mechanical polishing etc. The main reason of alteration in different mechanical, thermal and other property is due to increase in surface to volume ratio. Synthesis of nanomaterial is most commonly done based on three strategies i.e.:

 Liquid-phase synthesis.  Gas-phase synthesis.  Vapour-phase synthesis.

7.1. Liquid-Phase Synthesis

Under liquid phase synthesis the techniques used for synthesis are:

 Co-precipitation.  Sol-gel Processing.  Micro-emulsions.  Hydrothermal/Solvothermal Synthesis.  Microwave Synthesis.  Sono-chemical Synthesis.  Template Synthesis.

Possible Applications of Zinc and Titanium in Modern Life 87

7.2. Gas-Phase Synthesis

 Supersaturation achieved by vaporizing material into a background gas, then cooling the gas.

7.3. Methods Using Solid Precursors

 Inert Gas Condensation  Pulsed Laser Ablation  Spark Discharge Generation  Ion Sputtering

7.4. Methods Using Liquid or Vapor Precursors

 Chemical Vapour Synthesis  Spray Pyrolysis  Laser Pyrolysis/ Photochemical Synthesis  Thermal Plasma Synthesis  Flame Synthesis  Flame Spray Pyrolysis  Low-Temperature Reactive Synthesis

Nanostructured materials can have significantly different properties, depending on the chosen fabrication route. Each method offers some advantages over other techniques while suffering limitation from the others.

7.4.1. SOL-GEL Method The sol-gel method was developed in the 1960s mainly due to the need of new synthesis methods in the nuclear industry. A method was needed where dust was reduced (compared to the ceramic method) and which needed a lower sintering temperature. In addition, it should be possible to do the synthesis by remote control. The sol-gel process may be described as: “Formation of an oxide network through polycondensation reactions of a molecular precursor in a liquid.” A sol is a stable dispersion of colloidal (particles ranging between 1 and 1000 nanometers in diameter, yet are still able to remain evenly distributed throughout the solution) particles or polymers in a solvent. The particles may be amorphous or crystalline. An aerosol (a substance enclosed under pressure and released as a fine spray by means of a propellant gas) is particles in a gas phase, while a sol is particles in a liquid, A gel consists of a three-dimensional continuous network, which encloses a liquid phase. In a colloidal gel, the network is built from the agglomeration of colloidal particles. In a polymer gel, the particles have a polymeric sub-structure made by aggregates of sub-colloidal particles. Generally, the sol particles may

88 Anjali Sharma, Madan Lal, Naheed Ahmad et al. interact by van der Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains. In most gel systems used for materials synthesis, the interactions are of a covalent nature and the gel process is irreversible. The gelation process may be reversible if other interactions are involved.

 The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner.  Multi-component compounds may be prepared with a controlled stoichiometry by mixing sols of different compounds.  The sol-gel method prevents the problems with co-precipitation, which may be inhomogeneous, be a gelation reaction.  Enables mixing at an atomic level.  Results in small particles, which are easily sinterable.

8. NANOFIBERS IN WOUND DRESSING

Wound healing, as a normal biological process in the human body. The main aim of wound healing is a speedy recovery with minimal scarring and maximal function. The selection of materials is very important from a wound healing application point of view. Nanosized titanium dioxide (TiO2) particles occupy a special place due to its favorable biological effects and high corrosion resistance. Nanofiber mats have drawn the attention of numerous researchers in the wound dressing development field since they can mimic a natural nanometer dimension of the tissue they are healing (Zhang et al. 2005). Nanofiber mats can provide fine polymeric fibers at the scale of nanometers. Their structure provides mats with different unique properties, making them highly tissue compatible to enhance wound repair in comparison to other conventional dressings. An ideal wound dressing should have a structure that resembles the natural skin’s structure, and such nanofibers, due to their physical features, are able to mimic the healthy extracellular matrix network found in the skin (Kanani and Bahrami 2010). As mentioned earlier, the extracellular matrix is important in the repair process as it provides structural support for cell attachment and proliferation. Therefore, nanofiber mats could enhance proliferation and improve cell repair mechanisms (Hassiba et al. 2016). In fact, there are several advantages of using nanofibers as next-generation wound dressings as discussed. below. An ideal wound dressing should have the ability to: first, minimize scarring of the tissue after healing; second, support and strengthen newly formed tissue; third, deliver nutrients needed for wound healing such as proteins; fourth, absorb excess exudates from inflammation; fifth, minimize pain from the wound by blocking the endings to reduce pain; sixth, stimulate clot formation; seventh, protect the wound from microorganisms and foreign particles and eighth, provide a road map for tissue regeneration.

Possible Applications of Zinc and Titanium in Modern Life 89

Figure 8. Required properties of an ideal wound dressing.

8.1. Absorption of Exudates from the Wound

Absorption of exudates is one of the main objectives for an ideal wound dressing material. Due to the same physical properties of having small interstices and a high surface area to volume ratio, nanofibers can absorb between 18 and 213% more water than films made from the same polymers. This would also help prevent wound desiccation.

8.2. Maintaining a Moist Environment and Permeability

Electrospun mats consist of nanofibers, which resemble a mesh and provide a moist environment for cell respiration and, eventually, for cell proliferation and viability. The porous nature of such mats not only works against bacterial infection due to a small pores size but it also maintains a high gas permeation, which protects against tissue dehydration.

8.3. Drug-Controlled Release

Different fabrication techniques for making nanofibers allow for the addition of drugs to be released for topical applications. The type of drug loaded can be chosen according to the wound need for improved healing. Examples include using one or more antibacterial, antifungal, growth factors, and vitamins. 90 Anjali Sharma, Madan Lal, Naheed Ahmad et al.

8.4. Flexibility in the Dressing Mat

Wound dressings should provide flexibility to the patients upon applying and comfort after placement, and these nanofiber dressings can provide the adaptability and compliance needed (Hassiba et al. 2016).

8.5. Scar-Free

Although not easy to achieve, researchers have been conducting studies to create nanofiber dressings to cause minimal scarring.

CONCLUSION

The present chapter provided insights into the recent study in wound dressing materials namely zinc oxide and titanium dioxide. The background of these has been studied. It shows us that zinc has been mentioned in Ayurveda also. Much other research works also show us that these have antibacterial properties and are also biocompatible. Of the methods used to fabricate TiO2 and ZnO, sol-gel is the most frequently chosen one due to its simplicity and cost–effectiveness. It has been discussed that the nanoparticles synthesized can be drawn into nanofibres for future work. Electrospun nanofiber mats have morphologies of high surface area to volume ratio and porous structure. Hence this chapter has thrown light on the variety of applications in our day to day lives that are composed of zinc and titanium.

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Chapter 3

HIGH DIELECTRIC MATERIALS FOR SUPERCAPACITORS

Poonam Kumari*, Mamta Shandilya1, Madan Lal and Radheshyam Rai School of Physics and Materials Science Shoolini University, Solan (H.P.), India

ABSTRACT

Electrochemical capacitors, also called supercapacitors, due to storage by using either ion adsorption or fast surface redox reactions. They can complement or replace the batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. BaTiO3 based ceramics attracted extensive interest in the field of supercapacitor research due to its high dielectric and ferroelectric properties such as supercapacitor. This ceramics sintered using advanced processes provides a high control of grain boundaries in bulk composites. As a result, supercapacitor behavior was evidenced which came from the balance between inner grain conductivity and grain boundary dielectric barrier. Modern semiconductor integrated circuits require low-voltage capacitors which can store high electric energy in a minimum volume. In the fast development of the power electronics, dielectric materials with high energy-storage density, low loss, and good temperature stability are eagerly desired for the potential application in advanced pulsed capacitors. In this paper, the properties of BaTiO3 based dielectric and ferroelectric ceramics for the use of energy storage devices and supercapacitors are reviewed in detail. Their advantages, disadvantages, and performance in supercapacitors are also discussed through extensive analysis of the literature, and new trends in material development.

Keywords: capacitor, battery, ceramics, supercapacitor, dielectrics

* Corresponding Author address. Email: [email protected]. 96 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

1. INTRODUCTION

Since the discovery of barium titanate in the 1940s (Devonshire 1949), the tetragonal perovskite structure becomes versatile composition which exhibits high dielectric permittivity (ɛ’) around 10,000 and these materials used as a dielectric material in energy storage devices (Huang et al. 2006), PTCR thermistors, piezoelectric sensors, optoelectronic devices, transducers and actuators ceramic capacitors:-multilayer ceramic capacitors and supercapacitor. Supercapacitors are governed by the same fundamental equations as conventional capacitors, but utilize higher surface area electrodes and thinner dielectrics to achieve greater capacitances. Supercapacitors have energy densities greater than conventional capacitors and power densities greater than batteries (Grbovic et al. 2011). As a result of supercapacitors may become an attractive power solution for an increasing number of applications. Supercapacitor has large capacitance and excellent charge-discharge performance compared to Li-batteries. It is well known that the supercapacitor have a higher order of magnitude compare to Li-batteries, when they are fully discharged (Musolino and Tironi 2010). Generally, the capacitance obtained with conventional capacitors discovers its origin in the electronic, ionic, and dipolar polarization arising in the bulk (Gonon and El Kamel 2007), whereas for supercapacitors, the capacitance is linearly proportional to the ionic conductivity instead of the dielectric relaxation. Although solid electrolytes have a lower ionic conductivity compared to liquids, the low ionic conductivity of solid state supercapacitors may be compensated by decreasing the thickness of solid electrolytes (Lim et al. 2001) to reduce the diffusion path of charged ionic defects or by increasing the electrode surface area by using porous electrodes with an extremely large internal effective surface. In such capacitors the double layer capacitance can reach values up to several F/cm2 depending on the nature of both dielectric and electrode creating the structure. The dielectric material not only separates the electrodes but also has electrical properties that affect the performance of a capacitor. Because of the high losses of such giant permittivity materials, their use for supercapacitor devices is rather limited. This is why ferroelectric compounds of intrinsically high dielectric permittivity have recently proposed as potential candidates. However, to stand the high impulse currents, such ferroelectric ceramics have to be strongly improved as to overcome the grain boundary losses. Since 2000, giant dielectric permittivity oxides (Ti and Li substituted NiO, LuFe2O4 and CaCu3Ti4O) are under deep investigations. Their main dielectric features are the following: (i) unprecedented dielectric constant (ε > 105) stable over a large temperature range (ii) sharp decrease of this permittivity at low temperatures (T = 200K) which strongly shifts with the operating frequency (iii) related maximum of the dielectric losses which largely exceeds the requested limits (tan (δ) = 5%) at high frequencies (f > 10MHz) and low temperatures. Supercapacitors not only have a traditional dielectric material like ceramic, polymer films or aluminum oxide to separate the electrodes but instead have a physical barrier made from activated carbon that when an electrical charge is applied to the material a double electric field is generated which acts like a dielectric. The thickness of the electric double layer is as thin as a molecule. The surface area of the activated carbon layer is extremely large yielding several thousands of square meters per gram. This large surface area allows for the absorption of a large amount of ions (Zhou and Liu 2008). This review focuses on the different types of high dielectric materials used in the supercapacitors. Commercial productions of High Dielectric Materials for Supercapacitors 97 supercapacitors in nowadays markets are basically from the high surface area porous carbon materials as well as noble metal oxides systems. For instance, Matsushita Electric Industrial (Panansonic, Japan) developed Gold capacitors, as high performance supercapacitors for military applications that were produced by Pinnacle Research (USA). The commercial supercapacitors are widely used as power sources for activators, elements for long time constant circuits, standby power for random access memory devices, and hand phone equipment’s, etc. (Kiamahalleh et al. 2012). In recent years, ES or ultracapacitors have attracted significant attention, mainly due to their high power density, long lifecycle, and bridging function for the power/energy gap between traditional dielectric capacitors (which have high power output) and batteries/fuel cells (which have high energy storage) (Largeot et al. 2008). The earliest ES patent was filed in 1957. The number of reported literature related to electrochemical supercapacitors from 1997 to 2014 shown in Figure 1.

2. DEFINITION OF SUPERCAPACITOR

A supercapacitor is a high-energy form of a conventional capacitor, holding hundreds of times more energy per unit volume or mass (Cultura II and Salameh 2015). A supercapacitor is an electrochemical device consisting of two porous electrodes immersed in an electrolyte solution that stores charge electrostatically. The capacitance is determined by the effective- area of the plates, the separation distance of the plates and the dielectric constant of the separating medium which is the same in the conventional capacitor. A conventional capacitor gets its area from plates of a flat, conductive material while supercapacitor gets its area from a porous carbon-based electrode material. The porous structure gives it a very high effective surface area compared to a conventional plate structure. It also insures a minimal distance between the “plates”. These two factors lead to a very high capacitance compared to a conventional electrolytic capacitor.” Supercapacitors have more than 100 to 1000 times the capacitance per unit volume compared to a conventional electrolytic capacitor (Cultura II and Salameh 2015).

2.1. Principle of Energy Storage or Electrochemical or Supercapacitor

These devices have an operating principle similar to a traditional capacitor. However, their capacity and discharge current are much higher. The main difference compared to conventional capacitors is based on two aspects: Energy is stored at the interface between a porous conductive electrode and a liquid electrolyte ionic conductor. The surface is greatly increased due to the very high porosity of the electrode. Use of high surface-area electrodes result in extremely large capacitance. Single cell voltage of ECs is typically limited to 1–3 V depending on the electrolyte used. Supercapacitors are used for voltage drop compensation in weak networks, allowing a very intense peak power. 90% efficiencies can be achieved in the complete cycle of charging and discharging. To analyze the behavior of these devices, supercapacitors can be represented by an equivalent electric circuit. Figure 2 shows a super capacitor model consisting of RC elements. 98 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Figure 1. Total number of reported literature based on electrochemical supercapacitors (ES) from 1997 to 2014 (Wang, Zhang, and Zhang 2012).

The parameter analyzed in this model is the super capacitor capacity, which defines its behavior and the energy stored. This capacity is not constant and depends on the voltage across its terminals. For that reason, capacity is modeled as a constant value, C0, in parallel with a conventional capacitor, Cu, showing a linear dependence on voltage, u, as shown in expressions (1) and (2).

Figure 2. Equivalent electric circuit of a supercapacitor (San Martín et al. 2011). High Dielectric Materials for Supercapacitors 99

C = C0 + K.U (1)

Cu = K.u (2)

In addition, resistance Rs represents voltage drops during charge and discharge and resistance R1 loss of charge when the device is in stand-by. Finally, the ‘n’ parallel RC circuits represent the relaxation phenomenon produced inside the super capacitor due to diffusion of charges. Capacitors which store the energy within the electrochemical double-layer at the electrode/electrolyte interface are known under various names which are trademarks or established colloquial names such as ‘double-layer capacitors’, ‘supercapacitors’, ‘ultracapacitors’, ‘power capacitors’, ‘gold capacitors’ or ‘power cache’. ‘Electrochemical double-layer capacitor’ describes the fundamental charge storage principle of such capacitor (Kötz 2000).

2.2. The Most Relevant Parameters that Define These Storage Devices

2.2.1. Storage Capacity Defined as the amount of energy available in the storage device after completing the charging cycle. The discharge is often incomplete and therefore, storage capacity is defined based on the total energy stored, Wst, which is higher than the useful energy at a particular point of operation, Wut.

2.2.2. Discharge Time Defined by following expression (3)

W τ (s) = st (3) Pmax

Where: τ (s) = Discharge time (s), Wst = Total energy stored (Wh), Pmax = Maximum or peak power (W).

2.2.3. Efficiency Evaluated from the ratio between the energy released and the stored energy. Its value is given by expression (4).

W η = ut (4) Wst

Where: η = Efficiency of storage technology, Wut = Useful or recoverable energy for a given point of operation, (Wh).

100 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Table 1. Comparison of different parameters of Capacitor, Supercapacitor and Battery

Factors Conventional Supercapacitor Conventional Capacitor (Ultracapacitors) Batteries Charge Time 10-6 ̴ 10-3 s 1 ̴ 30 s 0.3 ̴ 3 h Discharge Time 10-6 ̴ 10-3 s 1 ̴ 30 s 1 ̴ 5 h Specific Power (W/kg) 10,000 1000 ̴ 2000 50 ̴ 200 Specific Energy (Wh/kg) = 0.1 1 ̴ 10 20 ̴ 100 Cycle Life (cycle) >500,000 >100,000 500 ̴ 2000 Charge/Discharge Efficiency (%) -100% 90% - 95% 70% ̴ 85%

2.2.4. Durability It is given by the number of times that the storage device can release energy, from the level for which it was designed. It is expressed as the maximum number of cycles, N, each one corresponding to a charge and discharge processes.

2.2.5. Autonomy This refers to the maximum time that the system can continuously release energy. It is defined by equation (5).

W a = ut (5) Pdt

Where: a = Autonomy, in s, Pdt = Maximum power discharge, in W. A comparison of the properties and performance between battery, capacitor, and supercapacitor is given in Table 1.

3. THEORY OF DIELECTRICS

A capacitor typically consists of two conductor plates filled with certain dielectric materials, and is commonly in the parallel-plate form, as shown in Figure 3. The electric energy storing is the function bases of capacitors in the electronics devices. The energy-stored ability of a capacitor is the so called capacitance, which is only determined by the physical dimension (geometry) of the conductors and the dielectric constant of the dielectrics. It is independent of the potential difference between the conductors and the total charge on them. For example, the capacitance of a parallel-plate capacitor constructed of two parallel plates fulfilled with certain dielectrics is approximately equal to the following Eqn.(6):

C = εrε0A/d (6)

Where C is the capacitance, A is the area of overlap of the two plates, εr is the relative –12 –1 permittivity, “ε0 is the electric constant (ε0 = 8.854 × 10 F m ), and d is the distance between the plates. Obviously, the capacitance is directly proportional to the overlap area of the conductor plates and the dielectric constant of the dielectrics, while inversely proportional to the separation distance between the plates. As shown in Figure 3, if an external voltage V is applied on the conductor plates, the electric polarization is happened. This will result in High Dielectric Materials for Supercapacitors 101 positives and negative charges with equal content accumulating on the two plates, respectively, which is the so-called charge process of the capacitor. The charge process will be finished when electrical potential caused by the accumulated charge (Q) on both plates is equal to the external applied voltage V. Q = V is equal to the capacitance (C) of the capacitor. Sometimes, the dielectric constant of the dielectrics is changed by the external bias, causing the capacitance to vary. Energy-storage efficiency  (Hao 2013) can be defined as Eqn. (7):

J  = reco × 100% Jstore

Where the electrical energy is stored in the capacitor as Jstore, the recoverable electrical energy density as Jreco.

Figure 3. The diagram of charge separation in parallel-plate capacitor under the function of electric field (Hao 2013).

3.1. Potential Dielectrics for High Energy-Storage

According to above analysis, to design a proper dielectric material with high recoverable energy-storage density and high efficiency (small energy loss) for practical application, three requirements have to be satisfied simultaneously at least: high electric breakdown field, large saturated polarization, and small remnant polarization (Fletcher, Hilton, and Ricketts 1996). Figure 4 shows the typical P-E loops and the energy-storage illustration of four kinds of dielectrics: (a) linear dielectric with constant permittivity (e.g., Al2O3, glass), (b) ferroelectric with spontaneous polarization (e.g., BaTiO3, PbTiO3, (c) relaxor ferroelectric with nanosized domains (e.g., (Pb,La)(Zr,Ti)O3, and (d) antiferroelectric with zero net remnant polarization (e.g., PbZrO3). Although, linear dielectrics usually possess higher breakdown field and lower energy loss, their smaller polarization value (permittivity) makes them not suitable for high 102 Poonam Kumari, Mamta Shandilya, Madan Lal et al. energy-storage application. Ferroelectrics often have larger saturated polarization and moderate electric-field endurance, but their larger remnant polarization lead to a smaller energy-storage density and lower efficiency. Comparatively, as shown in Figure 4c, relaxor ferroelectrics and antiferroelectrics are more likely to be used for high energy storage because of their larger saturated polarization, smaller remnant polarization and moderate breakdown field. The development of new manufacturing processes of materials, such as glass- crystallization technique and composite technology, another two kinds of materials, glass- ceramic and polymer-based ferroelectrics, are also be found to have the potential for application in this area, which combine with the higher breakdown field of linear dielectric and larger polarization of ferroelectrics. Thus, in general, above mentioned four kinds of dielectrics: antiferroelectrics, dielectric glass-ceramics, relaxor ferroelectric and polymer based ferroelectrics, are believed to be the most promising candidates for high energy-storage application.

Figure 4. (Color online) Diagram of hysteresis and energy storage density for (a) linear dielectrics, (b) ferroelectrics, (c) relaxor ferroelectrics, and (d) antiferroelectrics. The green area in the first quadrant is the recoverable energy density Jreco, and the red area is the energy loss Jloss (Hao 2013).

3.2. Energy Storage in Antiferroelectric

The definition of antiferroelectrics, it is necessary to mention ferroelectrics because they have a close relationship in terms of polarization process. In ferroelectric materials, the adjacent dipoles in one domain share the same polarization orientation, and the orientation of dipoles can be changed to by the applied external dc electric field. Differently, in true antiferroelectric materials, the adjacent dipoles are aligned in opposite orientation, and under sufficient high dc electric-field the orientation of dipoles along direction of dc field and be changed into ferroelectric state, because of the smaller free energy between antiferroelectric and ferroelectric phase. Accordingly, antiferroelectric could be defined as: spontaneous polarization direction of adjacent dipoles are opposite, and could be induced to same orientation under the function of electric field (Zhai, Li, and Chen 2004). High Dielectric Materials for Supercapacitors 103

Thus, as compared with ferroelectrics, antiferroelectrics possess two distinct features. One is that the net macroscopic remnant polarization is zero. Another is that P-E curves under sufficient high electric field display double hysteresis loops. In the group of antiferroelectric materials, generally there are several subcategories, such as perovskite group, pyrochlore group (Subbarao 1973, Bernard, Pannetier, and Lucas 1978), liquid crystal (Nishiyama 1994), and so on. Among all of these antiferroelectric, materials with perovskite structure are the most important ones, which is usually expressed as ABO3. Antiferroelectrics are more likely to be used for high-energy storage because of their larger Ps, smaller Pr and moderate breakdown field strength (BDS). While most of antiferroelectrics contain environmentally harmful element Pb, such as in PbZrO3 (Zhang, Zhu, et al. 2015) and (Pb,La)(Zr,Ti)O3 (Wang, Jin, et al. 2015, Shi, Fan, et al. 2014) systems. However, most of antiferroelectrics are lead-based materials, such as Pb(Zr,Sn,Ti)O3 and (Pb,La)ZrO3 (Shen, Luo, and Li 2015, Peng et al. 2015, Lan and Green 2015, Zhang, Jiang, et al. 2015), which are not environmentally friendly. Obviously, relaxor ferroelectrics stand out and serve as promising candidate materials for energy storage ceramic capacitors. In recent years due to restriction of the lead- based ferroelectric materials, researchers frequently achievement of the environment friendly lead-free systems. However, the energy storage density of lead-free antiferroelectric ceramics seems not as high as these of lead-based systems. Recently, (Na0.5Bi0.5TiO3-BaTiO3-based composites (Samara 1970, Zhelnova and Fesenko 1987) also showed antiferroelectric-like behavior in certain condition, such as, elevated temperature and/or special sintering atmosphere. the energy storage density of bismuth-based 0.89Bi0.5Na0.5TiO3-0.06BaTiO3- 3 0.05K0.5Na0.5NbO3 antiferroelectric ceramics was rarely larger than 1 J/cm (Gao, Dong, Mao, Cao, et al. 2011, Gao, Dong, Mao, Liu, et al. 2011).

3.3. Energy Storage in Relaxor Ferroelectrics

First reported in 1954 (Reynolds and Buchanan 2004), relaxor dielectrics are characterized by a very high dielectric constant and a diffuse phase transition (Randall and Bhalla 1990, Cross 1987). In the past 60 years, quite a few relaxors have been confirmed, such as Pb(Mg1/3Nb2/3)O3-PbTiO3, (Pb,La)(Zr,Ti)O3, and Ba(Ti,Sn)O3, and so on (Cross 1994, Bokov and Ye 2007). The P- E loops of relaxors also show a decay of polarization with temperature because of their diffused phase switching. These characteristics make relaxors possess the possibility for application in high energy storage (Narayanan et al. 2012). Relaxor ferroelectrics exhibit high Ps, lower Pr and slim hysteresis loop, which makes them promising candidate materials used for the energy storage ceramic capacitors (Jin, Li, and Zhang 2014). The dielectric constant of a typical relaxor such as Pb(Mg1/3Nb2/3)O3 can exceed 20,000 at the peak dielectric constant. This dielectric behavior is attributed to a microstructure of numerous polar nano regions surrounded by a nonpolar matrix on the scale of nanometers (Randall and Bhalla 1990). As a result, relaxors also exhibit frequency dependence of permittivity (and hence the name relaxor). However, some traditional relaxor ferroelectrics, such as Pb(Mg1/3Nb2/3)O3 (PMN) (Cross 1987, De Mathan et al. 1991) and Pb(Zn1/3Nb2/3)O3 (PZN) (De Mathan et al. 1991, Li et al. 2014) are not suitable for energy storage, because of their poor temperature stability. It is urgent to develop new material system for energy storage. Recently, a perovskite-type lead free relaxor ferroelectric (1-x)BaTiO3-xBiScO3 (BT- BS) (Ogihara, Randall, and Trolier‐McKinstry 2009) system has induced much interest. 104 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

3.4. Ceramic Dielectric Materials

A wide variety of ceramic materials with a broad spectrum of dielectric properties can be used to fabricate capacitors. Commercially available ceramic dielectrics are categorized into three classes:

i) Dielectrics are low K (5 to a few hundred) ceramics with low dissipation factor ( = = 0.01). They usually have a linear temperature coefficient of permittivity from zero to several thousand ppm/°C with a prescribed tolerance. ii) Dielectrics are high K materials (1,000 to >20,000) based on ferroelectric ceramics with dissipation factor usually in the range of 0.01 to 0.03. An important feature of this class of dielectrics is the moderate-to-high temperature dependence of dielectric constant. iii) Dielectrics are the basis for barrier layer capacitors. When voltage is applied to the dielectric, the electric field concentrates in the thin barrier layer, which results in extremely high capacitance but low operating voltage (usually = 25V). Because the operating principle and processing of barrier layer capacitors differ greatly from typical ceramic dielectrics, this class of materials will not be discussed in the present article.

Table 2. Important High-k Dielectric Materials

Sample Dielectric Constant Application PbMgNbO3+PbTiO3 22,600 Capacitor dielectrics PbLaZrTiO3 1000 Capacitor dielectrics BaSrTiO3 300 Capacitor dielectrics TiO2 50 Gate dielectrics, Ta2O5 25 Gate dielectrics CeO2 20 Gate dielectrics BaZrTiO3 19 Gate dielectrics for organic transistors on plastic Al2O3 9 Capacitor dielectrics (Bz,Ca,Sr)F2 8 Epitaxial dielectrics

As shown in Table 2, a large variety of materials have been used for a number of applications. High-k dielectric materials have recently become important mainly in three areas: memory cell dielectrics, gate dielectrics, and passive components Memory chips use the presence or absence of charge in a capacitor to represent a “1” or “0”. Until recently the approach to this problem has been to employ thinner layers of traditional silicon dioxide and silicon nitride dielectrics. But, as memory chips surpass about 64 Mb, this approach is no longer adequate because, to achieve the required cell capacitance, these materials must become so thin (less than 1 nm), that they would suffer from unacceptable levels of leakage current or would suffer catastrophic breakdown even with the reduced operating voltage of modern-day chips. Clearly, if one looks at the capacitor equation, the only factor left to adjust is the dielectric constant. Then the solution is to develop higher-k materials to enable the capacitor dielectric to maintain a robust thickness while still providing sufficient charge storage with a continuously shrinking area and storage voltage.

High Dielectric Materials for Supercapacitors 105

4. FUNDAMENTALS OF ELECTROCHEMICAL SUPERCAPACITOR

The fundamentals of electrochemical supercapacitors have been reviewed and discussed by many excellent reviews (Augustyn, Simon, and Dunn 2014, Shi, Li, et al. 2014, Lee et al. 2011) and books (Conway 2013, Yu, Chabot, and Zhang 2013), which will not be discussed in detail in the present review. Basically, the electrochemical supercapacitor is a special type of capacitor, which is different from the classical electrostatic capacitors (Figure 5A). ESs can be distinguished in several ways such as the charge storage mechanism, the electrolyte, the electrode material and the cell structure. Depending on the charge storage mechanism, ESs can be classified into three categories:

4.1. Electric Double-Layer Capacitors (EDLCs):

Electric double-layer capacitors (EDLCs), where the capacitance is produced by the electrostatic charge separation at the interface between the electrode and the electrolyte (Figure 5B). To maximize the charge storage capacity, the electrode materials are usually made from highly porous carbon materials.

4.2. Pseudo Capacitors

Pseudo capacitors which rely on fast and reversible faradaic redox reactions to store the charges (Figure 5C).

Figure 5. Schematic diagram of (A) an electrostatic capacitor, (B) an electric double-layer capacitor, (C) a pseudo-capacitor, and (D) a hybrid-capacitor (Halper and Ellenbogen 2006). 106 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

4.3. Hybrid Electrochemical Supercapacitors

Hybrid electrochemical Supercapacitors, which refer to ones using both the electrical double-layer (EDL) and faradaic mechanisms to store charges. With the latest developments in this area, some new battery-type hybrid devices have been developed. The hybrid electrochemical supercapacitors based on (i) the composite electrodes made from both EDL capacitive materials and pseudo capacitive materials (ii) the asymmetric design with one EDL electrode and the other pseudo-capacitive or battery-type electrode; as well as (iii) the of the asymmetric structure with one pseudo-capacitive electrode and the other rechargeable battery- type electrode. The typical energy storage and conversion devices are presented in the so called ‘Ragone plot’ in terms of their specific energy and specific power. Electrochemical capacitors fill in the gap between batteries and conventional capacitors such as electrolytic capacitors or metallized film capacitors. In terms of specific energy as well as in terms of specific power this gap covers several orders of magnitude.

4.4. Electrolytes and Electrodes

Electrolytes as shown in Figure 6, the electrolyte, which resides inside the separator as well as inside the active material layers, is also one of the most important ES components. The requirements for an electrolyte in ES include: wide voltage window, high electrochemical stability, high ionic concentration and low solvated ionic radius, low resistivity, low viscosity, low volatility, low toxicity, low cost as well as availability at high purity. The electrolyte used in an ES can be classified into three types: (i) aqueous electrolyte, (ii) organic electrolyte, and (iii) ionic liquids (ILs).

i. Aqueous electrolyte: Compared with organic electrolytes, aqueous electrolytes (such as H2SO4, KOH, Na2SO4 and NH4Cl aqueous solution and so on) can provide a higher ionic concentration and lower resistance.

ii. Organic electrolyte: Compared to aqueous electrolytes, organic electrolytes can provide a voltage window as high as 3.5 V. This is a large advantage of organic over aqueous electrolytes.

iii. Ionic liquids (ILs): A number of electrodes and devices have been prepared using Ionic liquids. For example, ethyl-methyl-imidazolium-bis (trifluoromethane- sulfony l) imide was used to investigate the relationship between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance.

High Dielectric Materials for Supercapacitors 107

Figure 6. Principle of a single-cell double-layer capacitor and illustration of the potential drop at the electrode/electrolyte interface (Zhang et al. 2009, Zhang and Zhao 2009).

4.5. Electrode Materials

As shown in Figure 7, the capacitance and charge storage of ES intimately depend on the electrode materials used. Therefore, further developing new materials with high capacitance and improved performance relative to existing electrode materials is the most important method to overcome these challenges (Arico et al. 2005).

Figure 7. Effects of the electrolyte on the supercapacitors performance (Mirvakili et al. 2015). 108 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Apparently, the capacitance of electrochemical supercapacitors heavily depends on the specific surface area of the electrode materials. Since not all the specific surface area is electrochemically accessible when the material is in contact with an electrolyte, the measured capacitance of various materials does not linearly increase with increasing specific surface area. A definition called the electrochemical active surface area may be more accurate in describing the electrode capacitance behavior. The pore size of the electrode material plays an important role in the electrochemical active surface area. According to Largeot et al., (Largeot et al. 2008) the pore size of electrode materials that produced maximum double-layer capacitance was very close to the ion size of the electrolyte (with respect to an ionic liquid electrolyte), and both larger and smaller pores led to a significant drop in capacitance. The porosity relevant to the development of high capacitance is itself not a simple parameter, involving both pore sizes -1 and pore-size distribution for a given overall specific area (m2g ) of the material. Therefore, electrochemical supercapacitors capacitance strongly depends on the surface area of the electrode accessible to the electrolyte. In general, the electrode materials of electrochemical supercapacitors can be categorized into three types: (Lee et al. 2010, Choi and Kumta 2006) (i) carbon materials with high specific surface area (Ruiz et al. 2007, Frackowiak et al. 2002), (ii) conducting polymers, and (iii) metal oxides, such as RuO2 (Ahn et al. 2006, Patake, Lokhande, and Joo 2009), IrO2 (Hu, Huang, and Chang 2002) MnO2 (Yan et al. 2010, Jiang and Kucernak 2002), NiO (Jiang and Kucernak 2002, Patil et al. 2008) Co2O3 (Kandalkar, Gunjakar, and Lokhande 2008), SnO2 (Babakhani and Ivey 2010) V2O5, (Hu, Huang, and Chang 2008, Zhou et al. 2009) and MoO (Nakayama et al. 2005) etc.

Figure 8. The percentage of different electrolytes from 1997 to 2014. Search formulation for ESs: (Electrochemical capacitor or supercapacitor or ultra-capacitor or pseudo-capacitor or double layer capacitor) (Andreas and Conway 2006). High Dielectric Materials for Supercapacitors 109

Besides the property of the electrolyte potential window, the interaction between the electrolyte and the electrode materials also plays an important role in the electrochemical supercapacitors performance. The pseudo-capacitances from the carbon-based materials and transition metal oxides are also strongly dependent on the nature of the electrolytes (Andreas and Conway 2006). The percentage of different electrolytes from 1997 to 2014 that are used for supercapacitors shown in Figure 8. An “electric double-layer capacitor” is a capacitor that utilizes the phenomenon called electric double layer. Concretely, the structure of an electric double-layer capacitor involves two electrodes made by forming active carbon into thin sheets that are separated by a semi- permeable membrane called a separator and placed in an electrolyte solution such as diluted sulfuric acid. The electrodes are subjected to a low voltage (about 0.8 volts) that does not cause electrolysis of electrolyte, this therefore results in ions rapidly being stored on the surface of the active carbon as shown in Figure 9. The active carbon has an extremely large surface area of between 1,000 and 2,000 square meters per gram, which allows the absorption of a large number of ions. In fact, the capacity reaches 1 farad, that is, 100 thousand times the capacity of normal capacitors. This type of super capacitor was currently used as the backup electric power source for clocks in VCRs and other appliances (http://www.nec-tokin.com/english/product/supercapacitor/pdf.) The problem with “Super capacitor” was that the internal resistance was too high. In other words, if this internal resistance could be reduced, the amount of electric current would increase and the scope of applications would also significantly grow. Through consequent research, it was found that the reason the internal resistance of “Super capacitor” was high had to do with the fact that the active carbon electrode was manufactured in a crimping process. This method was simple and advantageous as a manufacturing process, but detailed analysis revealed that there were numerous gaps between the active carbon particles and that this was the main reason of the resistance.

5. CERAMIC CAPACITOR

The ceramic capacitor is the most widely used passive component in modern electronics. In 2008, it accounted for ~90% of the capacitor market in part volume and ~40% in value (Pan and Randall 2010). The multilayer ceramic capacitor (MLCC), characterized by its high capacitance and compactness, is the dominant form of ceramic capacitor. With hundreds of MLCCs used in typical electronic devices such as cell phones and computers, approximately 1.5 trillion pieces of MLCC were manufactured in 2009, and in excess of 2 trillion pieces will be manufactured in 2011. Specialty ceramic capacitors with distinctive capabilities cover performance gaps in other capacitor technologies, e.g., high temperature capacitors up to several hundred degrees Celsius and single-element high voltage capacitors that withstand 50 to 100 kV. A multilayer ceramic capacitor formed by alternately stacking dielectric layers constituted by dielectric sintered grains, and electrode layers, wherein: a thickness of each dielectric layer is 0.5 μm or less; and a primary component of the dielectric layer is an ABO3 compound (A contains Ba (barium) and B contains Ti (titanium)) and a volume ratio to total dielectric sintered grains of those whose grain size is in a range of 0.02μm to 0.15μm is 1% to 10%. 110 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

The majority of (MLCC) ceramic capacitors are used in two applications. The first includes resonant circuits and filters, which require high stability, low loss (high Q), and a linear temperature coefficient of capacitance. The second major application is power supply bypass and decoupling, which requires high capacitance but can tolerate moderate loss, temperature dependence, and voltage dependence of capacitance. Ceramic capacitors are also used extensively in high temperature and high voltage applications. The abundance of ceramic compositions and their diverse dielectric behavior make ceramic capacitors ubiquitous in many extreme environments.

5.1. Important Perovskite Materials and Their Dielectric Properties

The known dielectric ceramics offering good dielectric constant vs. temperature characteristics are those whose ceramic crystal has a core-shell structure. For example, it is known that, by adding to the primary component of BaTiO3 (barium titanate) a component that contains rare earth metal, etc., and then sintering the component mixture while suppressing grain growth, dielectric ceramics of core-shell structure whose dielectric constant is subject to minimal temperature-dependent change can be obtained. ABO3 compound (A represents Ba, Ba-Ca or Ba-Ca-Sr, while B represents Ti or Ti-Zr) is used as the dielectric ceramic component, where the average grain size of the material powder thereof is 0.1μm to 0.3μm. Perovskite ferroelectric nanomaterials are of great attraction and show potential applications in different fields including solar energy conversion, storage memory, etc., since they have remarkable properties such as ferroelectric, piezoelectric, and nonlinear optical properties (Frey and Payne 1993, Scott 2007). The most widely investigated perovskite materials include BaTiO3, CaTiO3, PbTiO3, PbZrTiO3, KaTaO3, and SrTiO3 along with suitable dopants and multiferroic oxides, BiFeO3, with unique properties of both ferromagnetism and ferroelectricity. The multiferroic materials are BiFeO3, CoFeO3, BiMnO3, TbMnO3, TbMn2O5, YMnO3, LuFeO4 and Ni3B7O13, etc. However, BiFeO3 (BFO) is the only material which gives ferroelectricity and antiferromagnetism at room temperature. One of the promising applications of ferroelectric nanomaterials is a ferroelectric bypass capacitor for 2.3GHz operation in mobile digital telephones (Scott and De Araujo 1989). In this area, some important applications of perovskite ferroelectric materials are introduced. Particularly, this review covers the promising applications of perovskite ferroelectric ceramic materials.

5.2. BaTiO3

5.2.1. Phase Transitions of BaTiO3 Barium titanate (BaTiO3), discovered simultaneously in several countries during WWII(Reynolds and Buchanan 2004), is the first simple metal oxide compound in which ferroelectric behavior was observed. The ferroelectricity in BaTiO3 gives rise to high dielectric constant (maximum K > 10,000) that was orders of magnitude greater than any existing dielectric at that time. More than 60 years later, barium titanate is still the base High Dielectric Materials for Supercapacitors 111 material of choice for ceramic dielectrics. A detailed description of the evolution of barium titanate–based dielectrics will be presented in a future article in this capacitor series. Barium titanate has a perovskite crystal structure that undergoes three phase transitions when cooled from high temperature: cubic to tetragonal at ~1200C, tetragonal to orthorhombic at 00C, and orthorhombic to rhombohedral at −900C. As shown in Figure 10, each phase transition is associated with a sharp dielectric anomaly, which is far from ideal behavior for a capacitor dielectric. To reduce the temperature dependence to a tolerable level, the composition of barium titanate can be modified by forming a solid solution with other perovskite compounds or by adding dopants. For example, strontium titanate (SrTiO3) forms a complete solid solution with barium titanate and is used to shift the dielectric peaks while broadening the dielectric peaks. The effect of dopants such as Ca, Zr, Sn, and rare earth elements in shifting and suppressing dielectric peaks is also well documented. Modification of the dielectric response of BaTiO3 can also be achieved through the so-called core-shell grain structure (Moulson and Herbert 1990, Kim et al. 2008). In this approach, each ceramic grain consists of a pure BaTiO3 core and an outer region with diffuse phase transition characteristics that is created by adding small amounts of dopants (donor, acceptor, and amphoretic) to the BaTiO3 powder and controlling the diffusion of dopants during sintering. The combined dielectric behavior of the two regions results in relatively little temperature dependence of the dielectric constant. A detailed understanding of core-shell formation mechanisms has enabled precise engineering of grain microstructures and dielectric properties by capacitor manufacturers. The presence of these dopants, mostly segregated in the shell region, also causes a small lattice mismatch with the core, which induces mechanical stress to stabilize the tetragonal phase of the core (Tian et al. 2010).

Figure 10. (a) Barium titanate dielectric constant as a function of temperature (b) The ion displacements due to the cubic-tetragonal distortion in BaTiO3. 112 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Figure 11. Dielectric constant versus temperature of coarse-grained BaTiO3 (Hennings 1987).

Equally important, the dopant-rich grain boundaries result in low conductivity and high reliability dielectrics by limiting ionic migration of oxygen vacancies. The continuing trend of decreasing by forming a solid solution with other perovskite compounds or by adding dopants. For example, strontium titanate (SrTiO3) forms a complete solid solution with barium titanate and is used to shift the permittivity peaks while broadening the dielectric peaks. The effect of dopants such as Ca, Zr, Sn, and rare earth elements in shifting and suppressing dielectric peaks is also well documented. Detlev Hennings (Hennings 1987) reported that the maximum dielectric constant of the classical BaTiO3 based materials shown in Figure 11. The dielectric constant maximum at the Curie point is often used for materials with very high capacitances (‘high dielectric materials’) which, however, show the inferior temperature stability of specification Y5V. The ferroelectric region below the Curie point is used for materials with lower capacitances, which then have the higher temperature stability of specification X7R. Pure BaTiO3 ceramics obviously are not qualified for dielectric use. In ‘high-K materials’ the Curie maximum has to be broadened and shifted to room temperature. In the temperature- stable X7R materials, the permittivity of the ferroelectric region has be increased, and in particular the temperature characteristic has to be improved at the low temperature end.

5.2.2. Dielectric Constant of BaTiO3 Materials For many years the BaTiO3 based dielectrics could only be improved by making simple changes to their chemical composition or by the formation of mixed crystals. By incorporation of so-called ‘shifters’, like Sn or Zr for Ti, or Sr for Ba, the Curie point of BaTiO3 can be reduced to room temperature. The permittivity maximum then rises by a factor of 3 or more (Kniekamp and Heywang 1954). This observed rise of the permittivity maximum with increasing Zr content was explained in terms of a gradual change of the ferroelectric phase transition from first to second order, theoretically, the permittivity High Dielectric Materials for Supercapacitors 113 maximum at a second-order phase transition should be infinitely high. The maximum dielectric constant at Curie point of (Ba0.87Ca0.13)(Ti0.88Zr0.12)O3 ceramics shown in Figure 12(a). However, due to the more or less pronounced broadening of the peak, the maximum hardly ever exceeds a value of r,max~35000. The broadening of the peak is strongly dependent on the microstructure (Hennings, Schnell, and Simon 1982).

5.2.3. Grain Size Effects in Ferroelectric BaTiO3 Below the Curie Point

Figure 12. (a). Maximum permittivity at Curie point of (Ba0.87Ca0.13)(Ti0.88Zr0.12)O3 ceramics with 0 different grain sizes. (b) Grain size dependence of the permittivity of BaTiO3 ceramics at 25 C and 700C (Hennings 1987).

The ferroelectric property of BaTiO3 was below the Curie point show a strong dependence on the grain size. Whereas in normal coarse-grained (>10µm) BaTiO3, dielectric 0 constant of r ~ 1500-2000 are found in the temperature range of 20-80 C, in fine-grained materials (= 1µm) much higher dielectric constant of r  4000-5000 are observed in the same temperature range. At very small grain sizes (= 0.5µm), however, the dielectric constant is rather low again. The measured dielectric constants plotted against the average grain size show a pronounced maximum of r,max  5000-6000 at 0.7-0.8µm as shown in Figure 12(b). This maximum value seems to be the highest dielectric constant attainable in ferroelectric BaTiO3 below the Curie point. The grain size dependence of the permittivity observed in the ferroelectric region of BaTiO3 again illustrates the importance of careful grain size control.

5.2.4. Doped BaTiO3 Based Ceramics and Their Dielectric Constant As one of lead-free materials, BaTiO3 (BT) has been extensively studied and applied to capacitors for many years due to its good dielectric properties (Wang, Hao, et al. 2015, Xiong et al. 2011, Singh and Tiwari 2012, Dittmer et al. 2011, Eitel et al. 2001, Paunovic et al. 2014, Xu et al. 2014). However, the strong temperature dependence of the dielectric constant around the cubic to tetragonal phase transition restricts the application of pure BaTiO3 (BT).

114 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Figure 13. Dielectric properties of BT@3(0.7BT–0.3BZT)@yNb ceramics: (a) y = 0.10; (b) y = 0.15; (c) y = 0.20; (d) the temperature variation in capacitance of different samples at 1 kHz (Wang, Hao, et al. 2015).

In our previous studies, 0.7BaTiO3-0.3Bi(Zn1/2Ti1/2)O3(0.7BT–0.3BZT)(Wang et al. 2014) ceramics exhibited a better dielectric-temperature stability in high-temperature end 0 0 (~200 C). Thus, an improved dielectric characteristic for X9R (-55 to 200 C, C/C25°C ≤ 15%) capacitors can be expected for a multilayer core–shell structure BT-based ceramics using Nb oxide and 0.7BT–0.3BZT compositions produced by polymer-network method. Ting Wang et al.(Wang, Hao, et al. 2015) reported the X9R BaTiO3-Based Dielectric Ceramics and the dielectric properties as shown in Figure 13. Nitish Kumar et al. (Kumar et al. 2015) reported the multilayer ceramic capacitors based on relaxor BaTiO3-Bi(Zn1/2Ti1/2)O3 for temperature stable and high energy density capacitor applications. In bulk ceramic embodiments, BT-BZT has been shown to exhibit relative dielectric constant greater than 1000, high resistivity’s ( > 1 G-cm at 3000C), and negligible saturation up to fields as high as 150 kV/cm. The energy density for the multilayer ceramics reached values of 2.8 J/cm3 at room temperature at an applied electric field of 330 kV/cm. This represents a significant improvement compared to commercially available multilayer capacitors. SH Yoon et al. (Yoon et al. 2013) found the effect of excess Ba concentration on the dielectric nonlinearity was investigated in Mn and V-doped BaTiO3 multilayer ceramic capacitors (MLCC) under the same grain size condition. They also reported (Yoon et al. 2014) that the difference in dielectric non-linearity was investigated by contrasting BaTiO3 and Ba0.925Ca0.075TiO3 multilayer capacitors utilizing the first order reversal curve (FORC) distribution based on the preisach model. Ca incorporation caused a High Dielectric Materials for Supercapacitors 115 decrease of the dielectric constants in the low field but little differences in the high field region resulting in a steep ac field dependence, which became more significant with the decrease in temperature. Such behavior can be correlated with a decrease of the reversible and an increase of the irreversible FORC distribution near origin by Ca incorporation. Their results suggest that Ca incorporation, which is known to cause the asymmetric off-center displacement of Ba-site, has the role of weakly pinning centers that increase the portion of irreversible domain walls that are immobile at low field but can contribute to polarization beyond a threshold field. Preeti Sharma et al. (Sharma et al. 2015) fabricated the Iron doped barium calcium titanate (BCT) ceramics with compositions Ba0.90Ca0.10Ti1-3x/4FexO3 were by solid state reaction method for capacitor applications. They observed that with increase in Iron content, the room temperature dielectric constant (εr) is observed to increase from 1450 to 2199 whereas, room temperature tanδ decreases from 0.024 to 0.009 as shown in Figure 14. L. Zhang et al. (Zhang, Jiang, et al. 2015) reported anti-ferroelectric composite ceramics of (Pb0.858Ba0.1La0.02Y0.008)(Zr0.65Sn0.3Ti0.05)O3-(Pb0.97La0.02)(Zr0.9Sn0.05Ti0.05)O3 (PBLYZST- PLZST) have been fabricated by Spark Plasma Sintering (SPS) method. The effect of SPS process on phase structure, anti-ferroelectric and energy storage properties of the composites has been investigated in detail. L. Zhang et al. (Zhang, Jiang, et al. 2015) also reported same anti-ferroelectric composite PBLYZSTPLZST have been fabricated by the conventional solid-state reaction process. The maximum value of energy storage density is obtained to be 4.65 J/cm3. The effect of PBLYZST/PLZST ratio on phase dielectric, anti-ferroelectric and energy storage properties has shown in Figure 15.

Figure 14. Variation of ε′ with temperature at different frequencies for Ba0.90Ca0.10Ti1-3x/4FexO3 ceramics with x = 0.015 (Sharma et al. 2015). 116 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Figure 15. (a)The variation in the permittivity of various samples as a function of temperature and PLZST content measured at 10 kHz. (b) Dielectric constant and dielectric loss of the composites with 50% PLZST content in wide frequency range (from 1 kHz to100 kHz) as a function of temperature. (c) Recoverable energy density, energy loss and energy efficiency of various samples.

Figure 16. Temperature dependence of dielectric constant and loss of BaxSr1-xTiO3 ceramics at 1 kHz upon heating process. The inset shows an expanded viewed of dielectric constant from 20 to 1800C (Wang, Shen, et al. 2015).

Yu Wang et al. (Wang, Shen, et al. 2015) reported the Ba0.3Sr0.7TiO3 ceramics possessed an optimized energy storage density (γ = 0.23 J/cm3) and efficiency (η=95.7%) under the applied electric field of 90 kV/cm. Together with relatively high dielectric constant (εr=650 -4 at1 kHz) and very low dielectric loss (tanδ = 7.6 × 10 at1 kHz), the Ba0.3Sr0.7TiO3 ceramics should be more suitable for solid state compact portable pulse power electronic applications. 0 As detected from Figure 16, the Curie temperature of BST ceramics with x = 0.4 (Tc = -60 C) High Dielectric Materials for Supercapacitors 117 is far below room temperature, which means paraelectric cubic phase at room temperature. The Tc decreases with the decrease of x value, and beyond the measuring temperature range in this work when x≤0.2. Ba1-xSrxTiO3 (BST) ceramics are widely studied ferroelectric materials with tunable ferroelectric to paraelectric transition temperature by varying of Ba/Sr ratios. BST is a continuous solid solution of BT and ST over the entire range of concentrations (Zhang, Zhai, and Yao 2009). Due to its high dielectric constant (K), low dielectric loss and high tunability, BST has been extensively investigated for its potential applications including capacitors, sensors, phase shifters, and dynamic random access memories (Guo, Gao, and Yoo 2004, Tsai, Sun, and Tseng 1999). Hu, Wanbiao, et al. (Hu et al. 2015) synthesized the (Nb+ Al) co-doped rutile TiO2 4+ 5+ 3+ ceramics with nominal composition Ti 0.995Nb 0.005yAl 0.005zO2 z = (4-5y)/3 and y = 0.4, 0.5, 4+ 5+ 3+ 0.6, 0.7, and Ti 0.90Nb 0.05Al 0.05O2 ceramic for energy storage devices. Colossal permittivity 4 -1 (εr) of over 10 with dielectric losses (tanδ) at the 10 level was obtained (Figure 17a) for ceramic pellets prepared with sintering temperatures between 1300-15000C. With increasing sintering temperature from 1300 to 1350 to 14000C, both the dielectric permittivity and loss tangent change in a systematic way. From 1400 to 15000C, however, this trend is reversed with the dielectric permittivity increasing significantly (rather than continuing to decrease) and the loss tangent also reducing significantly thus giving rise to superior dielectric properties. Pellets synthesized at 15000C gave colossal dielectric constant (>2 × 104) and low tanδ ( = 0.1) over the 1 Hz to 1 MHz measured frequency range. Indeed, tan δ is lower than 0.05 over a broad frequency span within this range.

Figure 17. The frequency-dependent dielectric properties of (Nb+Al) co-doped rutile ceramics with the nominal composition Ti0.995(0.5Nb+0.5Al)0.005O2. (a) The dielectric properties of the ceramic pellets prepared at different sintering temperatures. (b) The dielectric properties of polished ceramic pellets prepared at 15000C with sample thickness varying from 1.6 mm to 0.8 mm. Au and Ag electrodes were used for the dielectric property measurements in (a) and (b), respectively (Hu et al. 2015). 118 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

Peter Hansen reported the multilayer capacitor (Hansen 2000) comprising the barium- titanate doped with silver and rare-earth metal in 1998. They found that a capacitor including a ceramic dielectric of a dielectric ceramic composition including a doping barium-calcium- zirconium-titanate of the composition (Ba1-α-μ-νAμDνCaα) [Ti1-x-δ-μ’-ν’MnδA’μ’Zrx]zO3, where A = Ag, A’ = Dy, Er, Ho, Y, Yb and Ga; D = Nd, Pr, Sm, Gd; D’ = Nb, Mo, 0.10 = x = 0.25, 0 = μ = 0.01, 0 = μ’ = 0.01, 0 = ν = 0.01, 0 ν’0.01, 0 = δ = 0.01 and 0.995 = z = 1 and 0 = α = 0.05, and including at least two electrodes, is characterized by a high dielectric constant K and a long service life, a low loss factor, a high insulation resistance and a capacitance with a low voltage dependence. Wen-His Lee reported a dielectric ceramic preparation of a barium- calcium-manganese-zirconium-titanate as a basic material having the general formula (Ba1- xCax)y(Ti1-a-b-c-dZraMnbNbcDyd)O3 with 0 = a = 0.25, 0 = b = 0.015, 0.001 = c = 0.01, 0.005 = d = 0.02, 0 = x = 0.20, 1.001 = y = 1.014, 0.0005 = z = 0.03 (Lee, Hennings, and Schreinemacher 2002). In 2002, J. Schenk, et al. (Wohlfahrt-Mehrens et al. 2002) Ruthenates based on SrRuO3 perovskite exhibit pseudo-capacitance behavior. Replacing up to 20 mol% Sr by La leads to an increase of the specific capacity. The stability window of ruthenates is strongly dependent on B-site cations. Fe or Co substitution reduces the stability of the electrolyte whereas 20 mol% doping by Mn on the B-site increases the capacity without reducing the potential window. A capacitance up to 270 F/g was obtained by optimizing composition and preparation route. Comparison of dielectric properties for BT based with high-temperature capacitor materials (Chen et al. 2015).

System Dielectric tan δ Operating Ref. constant temperature range + Δɛr/ɛrmid ≤−15%) BCT–0.6BMT  2100 ≤ 0.025 170-3000C (Zeb and Milne 2013) 50BZT-50BCT  5529 = 0.014 1080C (Mishra and Kumar 2015) 0.6BT–0.4BMT  2000 ≤ 0.025 200-400 0C (Zhang et al. 2011) 0.75KBT–0.25BS  2150 ≤ 0.025 200-300 0C (Kruea-In et al. 2012) BT-0.06BMZ  7000 ≤0.05 200-350 0C (Chen et al. 2015) BT-0.08BMZ  5000 ≤0.04 200-370 0C (Chen et al. 2015) BT-0.1BMZ  3000 ≤0.09 200-400 0C (Chen et al. 2015) 1-x(BT)-x(BMW)  5000 (Chen et al. 2014) (0.07≤x≤0.24)

BCT–BMT (1-x)Ba0.8Ca0.2TiO3-xBi(Mg0.5Ti0.5)O3; BT-BMT, (1-x)BaTiO3-xBi(Mg0.5Ti0.5)O3; KBT– BS, K0.5Bi0.5NbO3-BiScO3; BT–BMZ, (1-x)BaTiO3-xBi(Mg0.5Zr0.5)O3. 1-x(BT)-x(BMW); (1- x)BaTiO3–xBi(Mg0.75W0.25)O3. 1-xKNN-xBZW; (K 0.5Na0.5)NbO3-Bi(Zn0.75W0.25)O3.

6. APPLICATIONS OF PEROVSKITE MATERIALS

6.1. Multilayer Ceramic Capacitors (MLCs)

This paper described the perovskite structure, based on barium titanate composition, and interpret the theory of dielectric properties first. Then discuss how different compositions, additives, substitution, fabrication processes and defects that would influence BaTiO3 based structure’s dielectric properties. Last, briefly introduce the applications related to barium High Dielectric Materials for Supercapacitors 119 titanate dielectric properties such as multiplayer ceramic capacitors (MLCs) and positive temperature coefficient (PTC) thermistors. Multilayer capacitors structure, as shown in Figure 18, enables the maximum capacitance available from a thin dielectric to be packed into the minimum space in a mechanically robust form (Chen et al. 2002). Recently, multilayer ceramic capacitors (MLCs) with Ni electrodes have been increasingly produced to meet growing requirements for miniaturization, large capacitors and cost reduction. Firing the dielectric materials in a low-oxygen partial pressure to prevent Ni from oxidizing is one method to miniaturize MLCs. It was suggested that the microstructures and the electrical properties were influenced by the change of substitution modes of Mg and rare-earth oxide in perovskite (Kishi et al. 1999). For larger ion (La, Sm)-doped samples larger amount of MgO is necessary to suppress the grain growth and form the core-shell structure than smaller one (Dy, Ho, Er)-doped samples. Also, the solubility of rare-earth ions in BaTiO3 has a linear relationship with the ionic radius. It is confirmed that the larger ion acting as donors mainly dissolves Ba site, while the smaller ion acting as both donors and acceptors dissolves both Ba- and Ti-sites. In recent years, MLCs with Ni internal electrodes composed of about 400 dielectric layers of below 2μm thickness have been developed (Kishi et al. 2001).

Figure 18. Schematic diagram of a multilayer ceramic capacitor construction (Saito et al. 1991).

6.2. Ferroelectric Storage Memory

The ferroelectric random-access memories (FeRAM) (Chung et al. 2010) are one of the main applications of perovskite materials to replace magnetic core memory, magnetic bubble memory systems, and electrically erasable read-only memory for many applications (Scott and De Araujo 1989). The intrinsic non-volatility (retention of memory when power is out) and radiation hardness of ferroelectric RAMs provide a new pathway to develop novel computer memories. Furthermore, the dynamic random access memory (DRAM) is today’s memory core in most electronic systems. Due to leakage of charge, it needs to be renewed thousands of times per second. This process requires more power and therefore drains the battery. Also, it cannot store information while the power is off. For example, in computers (Chung et al. 2010), the startup of the computer needs longer boot-up times since its operating system must be copied from the hard drive. Non-volatile memories provide the solution for 120 Poonam Kumari, Mamta Shandilya, Madan Lal et al. this issue. One of the future options for storage memory is ferroelectric RAM (FeRAM) which stores data in the polarization state on the ferroelectric material. The ferroelectric material for this memory was PbZr0.5Ti0.5O3 (PZT). Starting from 1992, there have been significant developments in increase of the capacity and reduction of the size of FRAM. The current capacity of FeRAM is 64 Mbytes and the length of the FeRAM is reduced to 0.13 mm in the integrated CMOS transistor gate. Ferroelectric oxides used in FeRAM devices so far are of thin film types as follows: perovskite structures such as PbZrxTi1-xO3 (PZT) or BiFeO3; bilayer structures such as SrBi2Ta2O9 (SBT), (BiLa)4Ti3O12 (BLT) or Bi4Ti3O12 (BTO). To construct FeRAM in memory devices, a few properties of the materials are considered (Chung et al. 2010, Wouters et al. 2006). The coercive field Ec is required to be small, which relies on the operating voltage of the FeRAM.

6.2.1. Ferroelectric Varactors and Devices Although both ferroelectric and paraelectric phases of perovskite materials have been applied in tunable microwave devices (Gevorgian and Kollberg 2001), the paraelectric phase is often favored due to the absence of hysteresis. Therefore, paraelectric (SrTiO3, KTaO3, the Curie temperature of the materials can be easily tuned by simply varying the Ba composition. The ferroelectric devices rely on parameters including the composition of the film, the strain, defects, electrode/ferroelectric interface chemistry, fabrication method, design, etc.

6.2.2. Solar Energy Conversion Perovskite materials have been actively explored for solar energy conversion (Nuraje, Asmatulu, and Kudaibergenov 2012, Chen et al. 2010, Kudo and Miseki 2009) since they can have high conduction band gap position, stability, and solar absorption band gap. Among the perovskite materials for solar energy storage, the following perovskite materials have been mostly investigated: PbTiO3 (Kim et al. 2006), 310 SrTiO3 (Wrighton et al. 1976, Domen et al. 1980, Domen et al. 1982), KTaO3 (Reverón et al. 2005), NaTaO3 (Kato and Kudo 2001), BaTiO3(Nuraje, Asmatulu, and Kudaibergenov 2012), CaTiO3 (Mizoguchi et al. 2002), La2TiO5 (Kim et al. 2002), BiFeO3 (Luo and Maggard 2006, Gao et al. 2006) etc. BiFeO3 was investigated for production of oxygen under UV179 conditions. However, Luo and Maggard (Luo and Maggard 2006) reported that BiFeO3 nanoparticles coated with SrTiO3 can produce H2 under visible light irradiation while pure BiFeO3 failed to generate hydrogen evolution. In this system, a small-bandgap metal oxide, BiFeO3, was utilized as a visiblelight sensitizer in a SrTiO3 photocatalytic material. Recently however, the results on hydrogen generation of BiFeO3 are contradictory to a previous report (Luo and Maggard 2006). The highly crystalline structure avoids electron trapping inside the particles. Kudo and coworkers (Kudo and Miseki 2009), investigated the photocatalytic water splitting of perovskite nanomaterials such as NaTaO3, and KTaO3 and reported higher activities under UV irradiation.

High Dielectric Materials for Supercapacitors 121

7. BENEFITS, CHALLENGES AND APPLICATION OF SUPERCAPACITORS

7.1. High Power Density

The comparison of specific power and specific energy of modern storage devices shown in Figure 19. It is clear that ES display a much higher power delivery (1-10kWkg-1) when compared to lithium ion batteries (150Wkg-1). Since an ES stores electrical charges both at the electrode surface and in the bulk near the surface of the solid electrode, rather than within the entire electrode, the charge-discharge reaction will not necessarily be limited by ionic conduction into the electrode bulk, so the charging and discharging rates are much faster than the electrochemical redox reactions inside batteries.

7.2. Long Life Expectancy

In contrast, when energy is stored in an ES, no or negligibly small chemical charge transfer reactions and phase changes are involved in charging and discharging, so an ES can have almost unlimited cyclability. ES do not need any maintenance during their lifetimes and can withstand a huge number of charge-discharge cycles, up to 1000000. Moreover, ES can run deeply at high rates for 500000-1000000 cycles with only small changes in their characteristics. Such longevity is impossible for batteries even if the depth of discharge is as small as 10-20% of the overall energy. The life expectancy for ES is estimated to be up to 30 years, which is much longer than for lithium ion batteries (1000-10000 cycles and a life expectancy of only 5–10 years). Even for FS, although fast redox reactions are involved during charging and recharging, their life expectancy is also much longer than that of batteries (Inagaki, Konno, and Tanaike 2010, Burke 2000, Yoo et al. 2011, Zhang et al. 2009).

7.3. Long Self-Life

Another advantage of supercapacitors is their long shelf life. Most rechargeable batteries if left on the shelf unused for months will degrade and become useless due to self-discharge and corrosion. In contrast, electrochemical supercapacitor maintain their capacitance and thus are capable of being recharged to their original condition, although self-discharge over a period of time can lead to a lower voltage. It is reported that ES can sit unused for several years but still remain close to their original condition (Burke 2000).

122 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

7.4. High Efficiency

Electrochemical supercapacitor are reversible with respect to charging and discharging throughout their complete operating range of voltage, and the energy loss to heat during each cycle is relatively small and readily removed (i.e., heat management is easy). This means that the cycle efficiency of electrochemical supercapacitor is high (95%) even when operating at rates above 1 kW kg-1 (Miller and Burke 2008).

7.5. Wide Range of Operating Temperatures

Electrochemical supercapacitor can function effectively at extremely high and low temperatures. The typical operating temperature for electrochemical supercapacitor ranges from -40 to 700C. This is advantageous for military applications, where reliable energy storage is required to run proprietary electronic devices under all temperature conditions during war.

7.6. Environmental Friendliness

Electrochemical supercapacitor, in general, do not contain hazardous or toxic materials, and their waste materials are easily disposed.

7.7. Safety

In normal circumstances, electrochemical supercapacitor (ES) are much safer than batteries, in particular lithium-ion batteries. In summary, compared to other energy storage systems such as batteries and fuel cells, ES are superior in the areas of life expectancy (without any noticeable performance degradation after long-term operation), reversibility, power density, shelf life, efficiency, operating temperatures, environmental friendliness, and safety (Kötz and Carlen 2000, Burke 2000, Sarangapani, Tilak, and Chen 1996, Zhang et al. 2009, Chen et al. 2009, Lee et al. 2005).

7.8. Challenges for Supercapacitors

Although ES have many advantages over batteries and fuel cells, they also face some challenges at the current stage of technology.

7.9. Low Energy Density

ES suffer from limited energy density (about 5Whkg-1) when compared with batteries (>50Whkg-1), as shown in Figure 19. Commercially available ES can provide energy High Dielectric Materials for Supercapacitors 123 densities of only 3-4Whkg-1. If a large energy capacity is required for an application, a larger supercapacitor must be constructed, driving up the cost. Low energy density is the major challenge for ES applications in the short and medium terms.

7.10. High Cost

The costs of raw materials and manufacturing continue to be major challenges for ES commercialization. The main cost of an ES arises from its electrode materials. At present, for practical purposes carbon and RuO2 are the most common electrode materials used in commercial ES.

7.11. High Self-Discharging Rate

ES have a low duration and a high self-discharging rate of 10-40% per day. In some applications, this has been considered a major obstacle to their practical use.

7.12. Industrial Standards for Commercialization

Currently, carbon/carbon ES with capacitance of 50-5000F have become commercially available, the electrolyte used in such kind of ES is acetonitrile, which can give a cell voltage of 2.7V with a typical specific energy of 4Whkg-1. This performance was achieved when charging the device at 400Wkg-1 from 100% to 50% of the rated voltage. Although this kind of ES is commercially available, it is necessary to establish some general industrial standards such as performance, electrode structure, electrode layer thickness and porosity and so on.

Figure 19. Specific power versus specific energy of modern storage devices (Burke 2000). 124 Poonam Kumari, Mamta Shandilya, Madan Lal et al.

For example, for energy sensitive applications, the coating may be around 150 microns, while for power sensitive applications, the thickness is probably about 100 microns. Therefore, it is necessary to put some effort on ES standard establishment for different applications.

7.13. Applications of Supercapacitors

With their many advantages, ES have become very competitive choices for applications such as electric vehicles, electric hybrid vehicles, digital communication devices, digital cameras, mobile phones, electrical tools, pulse laser techniques, uninterruptible power supplies, and storage of the energy generated by solar cells (Chuang et al. 2010, Miller 2006, Reddy and Reddy 2003). For example, in memory back-up, batteries’ poorer cycle life makes their frequent replacement expensive (adding 20% onto the price of battery operated appliances) (Lai, Levy, and Rose 1992). In addition, in battery powered electric vehicles, batteries with lower power density cannot meet peak load requirements, originating from accelerating or climbing. With advantages such as long lifetime and high power density, ES can solve these problems (Zhang et al. 2009). Combining ES with batteries can yield improved performance in hybrid electric vehicles, including powerful acceleration, braking energy recovery, excellent cold weather starting, and increased battery life. Thus, ES has the potential to play an important role in complementing or replacing batteries in the energy conversion and storage fields. The main market targeted by ES manufacturers in the coming decades may be transportation, including hybrid electric vehicles and metro trains (Reddy and Reddy 2003, Huang, Wang, and Li 2006). Due to their relatively low energy density and high cost, the market development of ES is still in the early commercialization stage. Currently, ES occupy less than 1% of the world market for electrical energy storage (batteries and supercapacitors). In 2007, the total supercapacitors market generated revenues of about $99.6 million. At present, NEC (Japan), Elna (Japan), and Panasonic (Japan) occupy a large share of the ES market (Zhang et al. 2009). Other manufacturers of ES include SAFT (France), Cap-XX (Australia), NESS (South Korea), Korchip (South Korea), Econd (Russia), ESMA (Russia), ELIT (Russia), Maxwell (USA), AVX (USA), Copper (USA), EPCOS (USA), EVANS (USA), Kold Ban (USA), Power System Co. (Japan), and Chubu Electric Power (Japan) (Zhang et al. 2009, Sharma and Bhatti 2010). The ES market is moving ahead steadily and changing significantly every year.

8. DEVELOPMENTS IN ELECTROCHEMICAL SUPER CAPACITORS (ES)

With increasing demands for clean, sustainable energy, the advantages of high power density, high efficiency, and long life expectancy have made electrochemical supercapacitors one of the major emerging devices for energy storage and power supply. In particular, their feasibility for practical applications in hybrid power sources, backup power sources, starting power for fuel cells, and burst-power generation in electronic devices has been demonstrated. High Dielectric Materials for Supercapacitors 125

To develop new materials with optimal performance, two important research directions in ES electrode exploration are:

8.1. Composite Materials

Regardless of the materials for ES electrodes, combining different materials to form composites should be an important approach because the individual substances in the composites can have a synergistic effect through minimizing particle size, enhancing specific surface area, inducing porosity, preventing particles from agglomerating, facilitating electron and proton conduction, expanding active sites, extending the potential window, protecting active materials from mechanical degradation, improving cycling stability, and providing extra pseudocapacitance. As a result, the obtained composites can overcome the drawbacks of the individual substances and embody the advantages of all constituents. High capacities over 1700Fg-1 have been reported on the basis of composite materials (Liang, Bao, and Li 2007, Hu et al. 2009, Zhang et al. 2010, Dong, Rolison, and Dunna 2000). But it is worth to point out that the reverse effects may also take place in the process of making composites. Consequently, there should be a compromise among the composition of individual substances and an optimized molar ratio of constituents for every composite material.

8.2. Nanomaterials

Recent trends in ES also involve the development of nanostructured materials, such as nanoaerogels, nanotubes/rods, nanoplates, nanospheres, and so on. Nanostructured materials possess high specific surface area. They can provide short transport/diffusion path lengths for ions and electrons, leading to faster kinetics, more efficient contact of electrolyte ions, and more electroactive sites for faradaic energy storage, resulting in high charge/discharge capacities even at high current densities. Material morphology is closely related to the specific surface area and the diffusion of ions in the electrode, and one-dimensional nanostructure materials seem to be very promising for ES application due to their reduced diffusion paths and larger specific surface areas.

CONCLUSION

Until recently, supercapacitors were referred to fairly ordinary applications such as memory protection and internal battery backup, but in the last few years the application space has broadened significantly into hybrid vehicles, smartphones, and energy harvesting. New technologies on the horizon promise to bring supercapacitors into full competition with rechargeable batteries. Supercapacitors offer a promising alternative approach to meeting the increasing power demands of energy storage systems and electronic devices. With their high power density, ability to perform in extreme temperatures, and millions of charge-recharge cycle capabilities, supercapacitors can increase circuit performance and extend the life of batteries. This can add value to the end-product and ultimately reduce the costs to the 126 Poonam Kumari, Mamta Shandilya, Madan Lal et al. customer by reducing the amount of batteries needed and the frequency of the replacement of the batteries, which adds greatly to the environmental friendliness of the end-product as well. High-dielectric ceramics have become an international research frontier in the fields of high technology and new materials. In this review, we systematically reviewed the developments of high dielectric materials as well as the dielectric properties of BaTiO3-based materials, and some suggestions for the future development of A-site and B-site doping on BaTiO3 materials for storage applications. As a result, we believe that BaTiO3-based high dielectric ceramics will gain wide practical application in energy storage devices.

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Chapter 4

DEVELOPMENT OF DOUBLE PEROVSKITE ELECTROCERAMICS

Shweta Thakur*, Mamta Shandilya and Radheshyam Rai School of Physics and Materials Science Shoolini University, Solan (H.P.), India

ABSTRACT

Double perovskites (DPs) are a class of transition metal oxides with the general chemical formula of AʹAʹʹBʹBʹʹO6. Where A is an alkaline earth, metal and B, Bʹ are two transition metals. These transition metals are surrounded by an octahedral cage of Oxygens. Double perovskites are simply two different ternary perovskites ABO3 and ABʹO3 that are arranged alternately on a three dimensional checkerboard lattice. If A' = A" and B' = B" the system reduces to the simple perovskite as previously described. Generally, one species are chosen for the A cations based on size, usually a large species such as Sr2+, Ba2+ or La3+. A greater variation of the properties of the double perovskites can be observed by varying the B cations and studies of virtually every transition metal and 4f element have been performed. Hence, in double perovskite systems it is usually only the B cations that are chosen to have two or more different species. The B cations are six-coordinate and are located in the center of the octahedra. Multiferroics are currently in the focus of the material science and the list of multiferroics is constantly updated due to its various uses in different applications (Ivanov et al. 2012; Schmid 2008; Spaldin 2007). Combining the two different properties like ferroelectricity and magnetism in a single-phase compound would obviously be of tremendous interest not only for practical applications, but also for fundamental physics (Eerenstein, Mathur, and Scott 2006; Fiebig 2005; Ivanov et al. 2014). Perovskites have also played a leading role in multiferroic materials research (Khomskii 2009). Although magnetism and ferroelectricity usually exclude each other (Filippetti and Hill 2002), it has been known since the early 1960s that they can exist in a few materials known as multiferroic perovskites and double perovskite (Venevtsev and Gagulin 1994; Singh et al. 2009). These rare compounds exhibit magnetic and electric orders and thus provide a unique opportunity to exploit several functions in a single material. The strategy of

* Corresponding Author address. Email: [email protected]. 138 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

introducing magnetic behavior into ferroelectric compounds was connected with an incorporation of sufficient concentration of uncompensated spins in the B-sublattice. However, limited progress has been made during the last several decades for the following reasons. First, the coupling in existing single-phase compounds was too weak to be of practical use. Secondly, there were very few compounds displaying a coexistence of dipole and spin orders and a magnetoelectric coupling. Furthermore, the Curie or Neel temperature of most of the compounds was far below room temperature. Therefore, it remains a critical need to find new single-phase materials with strong magnetoelectric coupling at room temperature for practical applications (Ivanov et al. 2010). Generally the perovskite compound behaves like antiferromagnetic insulator, but when magnetic metal is doped in pervoskite suddenly it behaves like a ferromagnetic semiconductor by forming a double perovskite compound. Hence the perovskite compounds are antiferromagnetic insulator and the double perovskite compounds are ferromagnetic semiconductor. Double perovskite systems are multifunctional magnetic materials are attractive due to their functional properties and high Curie temperature. The fact that the double perovskites seem to be ferromagnetic metals and apparently have highly spin polarized conduction band makes these materials interesting for the applications in spintronic devices such as magnetic tunnel junctions or low field magnetoresistive sensors (Philipp et al. 2003), potential technological applications and such as nonvolatile memories (Shimakawa, Azuma, and Ichikawa 2011; Spaldin, Cheong, and Ramesh 2010), multiple-state memory elements and electric-field controlled magnetic sensors (Singh et al. 2009), spintronics (Kleemann 2013; Tsymbal 2012). Thus Double perovskites are used for the development (of spin polarized conduction and ferromagnetic semiconductors.

Keywords: ferrites, perovskite, multiferroic

1. INTRODUCTION

A perovskite oxide is a compound which has the general molecular formula of ABO3 and similar crystal structure as that of calcium titanium oxide (CaTiO3), known as the perovskite structure, where A and B denote a large cation and small cation respectively and O is an anion respectively. Calcium titanium oxide (CaTiO3) was first discovered by Gustav Rose in Russia in 1839. The perovskite oxide structure is considered as one of the most common structures of the complex oxides materials on the earth. A cation can have monovalent to trivalent state, i.e., Ax+; x = 1 – 3 and B cation can have trivalent to hexavalent state, i.e., By+; y = 3 – 6. In the structure of an ideal cubic unit cell, A-site cations occupy the tetrahedral site at the corners of the cube (co-ordinated no.-12), O ions occupy the octahedral site at the face center position (co-ordination no.-6), and B ion occupy the octahedral site at the center of the unit cell position (co-ordination no.-6) as shown in Figure 1(a). Due to the flexibility of the corner-sharing octahedra, the perovskite structure can be easily distorted to accommodate a wide range of valence states on both the A- and B- sites, by contracting the lattice or by rotating the bond angles. The resulting symmetry of distorted perovskite could be tetragonal, orthorhombic, rhombohedral or monoclinic. At room temperature, BFO has a rhombohedrally distorted perovskite structure with R3c symmetry. A schematic hexagonal structure with a rhombohedral unit cell; consisting of two distorted perovskite unit cells connected along their body diagonal as shown in Figure 1(b). Development of Double Perovskite Electroceramics 139

Perovskite with useful properties such as ferroelectricity and piezoelectricity are the backbone of the electroceramic industries; i.e., barium titanate and lead zirconium titanate etc. (Mitchell 2002; Kulik 1999). Perovskite-type materials also exhibit other properties, such as ferromagnetism, colossal magneto-resistance, ionic conductivity and super-conductivity, which are useful for significant technologies (Mitchell 2002). In the last 50-60 years many double perovskites have also been synthesised and studied for potential application. The members of the double perovskite family with this structure leads to exhibiting a wide range of fascinating properties. The interesting properties of perovskites are known to be subtle structural variations. In perovskite structure A ions reside on the corner of the crystal structure and B ion in the center of the crystal structure, i.e., at body center position, and usually A ion have less direct influence on the properties of the perovskite than the [B06] octahedral ion. A good example of this is that varying the degree of octahedral tilting in a perovskite structure changes the extent of orbital overlap through the BO6 octahedral network, which affecting the electronic properties such as conductivity, certain dielectric properties and magnetic properties also (Cox 2010). Therefore, by controlling the degree of tilting in the perovskite structure can be used to optimize these properties. As a result of the relation between the structure and properties of these compounds it is expected that further understanding of the factors responsible for stabilizing the various perovskite structures; which will play a crucial role in designing the materials with improved properties. In particular, there is significant interest in understanding the structure and properties of perovskites containing oxygen vacancies and/or cations with mixed valencies. Mixed valence cations and oxygen vacancies are known to be related to electronic and ionic conductivity, so understanding the relative stability of these in the perovskite structure is important.

Figure 1. (a) Cubic perovskite unit cell. Blue spheres represent the A cations, yellow spheres represent the B cations, and white spheres represent oxygen anions forming an octahedral and (b) BiFeO3 (BFO) perovskite structure. 140 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

Figure 2. Schematic of a double perovskite. Double perovskites have the general formula of A2BʹBʹʹO6.

The double perovskite structure has been studied for almost as long as the perovskite structure, since the early 1950's (Snow 1951; Galasso, Katz, and Ward 1959; Fresia, Katz, and Ward 1959). Double perovskites (DPs) are a class of transition metal oxides with the general chemical formula of AʹAʹʹBʹBʹʹO6. A schematic of DP is given in Figure 2 is shown where A is an alkaline earth metal that sits the center of the pseudocubic structure. Bʹ and Bʹʹ are two transition metals. These transition metals are surrounded by an octahedral cage of Oxygens. Double perovskites are simply two different ternary perovskites ABO3 and ABʹO3 that are arranged alternately on a three dimensional checkerboard lattice. If A' = A" and B' = B" the system reduces to the simple perovskite as previously described. Generally, one specie is chosen for the A cations based on size, usually a large species such as Sr2+, Ba2+ or La3+. A greater variation of the properties of the double perovskites can be observed by varying the B cations and studies of virtually every transition metal and 4f element have been performed. Hence, in double perovskite systems it is usually only the B cations that are chosen to have two or more different species. The B cations are six-coordinate and are located in the center of the octahedral. There are three possible arrangements of B cations in the double perovskites.

1) The random arrangement of B' and B" cations amongst the octahedral site. 2) The second type is an alternating arrangement of B' and B" cations at the center of the oxygen octahedral site and is known as the rock salt or 1:1 ordered structure. 3) The third possible arrangement is a layered structure which has alternate layers of only B ', then only B" at the center of the octahedral site.

The last type is extremely rare and was first synthesised in 1990 in La2CuSn06 (Anderson and Poeppelmeier 1991), followed by Sr doped La2-xSrxCuSn06 (Anderson, Poeppelmeier, et al. 1993) and finally Ln2CuM06 (Ln = La, Pr, Nd, and Sm; M = Sn and Zr) (Azuma, Kaimori, and Takano 1998). Anderson et al. have reported the synthesis of the layered compound compiled all of the data on over 300 double perovskites studied since the 1950's (Anderson, Greenwood, et al. 1993) to suggest the likely structure that would be adopted by a new compound. They reported that the most important factor in determining the B cation arrangement was the charge difference, followed by the difference in ionic radii between the two B cations. As the difference in the valence state of the two B cations was increased, then Development of Double Perovskite Electroceramics 141 the 1: 1 ordered (rock salt) arrangement would be favored. Also, that as the difference in ionic radii of the two B cations was increased this would also favor the 1:1 ordering. That means as the B cations become more similar to each other it matters less on which site they are located, hence tending towards a random arrangement. In fact, for charge differences greater than 2, the rock salt arrangement is preferred, whereas if this is less than 2, the random arrangement is more common. The layered structure appears to be energetically favored at the fine boundary between the two other arrangements. The B cations are the most important ions in the material in terms of both structure and properties. Once the B cations have been chosen, the A cation is selected largely based on its size and hence ability to stabilize the perovskite structure. Multiferroics are currently in the focus of the material science and the list of multiferroics is constantly updated due to its various uses in different applications (Ivanov et al. 2012; Schmid 2008; Spaldin 2007). Combining the two different properties like ferroelectricity and magnetism in a single-phase compound would obviously be of tremendous interest not only for practical applications, but also for fundamental physics (Eerenstein, Mathur, and Scott 2006; Fiebig 2005; Ivanov et al. 2014). The intrinsic ability to couple the electrical polarization to the magnetization allows an additional degree of freedom in the design of conventional devices. Recent observations of magnetoelectric effects in materials with complex magnetic structures, in which the spontaneous electric polarization is induced by the magnetic order, opened the additional prospects in the research for the new multiferroics (Tokura 2006). From the materials chemistry point of view, ceramics with the perovskite structure present an incredible wide array of structures and phases with totally different properties, which are directly related to nature of its constituent cations, as well as on its structural features, including distortion and ordering. Perovskites have also played a leading role in multiferroic materials research (Khomskii 2009). Although magnetism and ferroelectricity usually exclude each other (Filippetti and Hill 2002), it has been known since the early 1960s that they can exist in a few materials known as multiferroic perovskites and double perovskite (Venevtsev and Gagulin 1994; Singh et al. 2009). These rare compounds exhibit magnetic and electric orders and thus provide a unique opportunity to exploit several functions in a single material. The strategy of introducing magnetic behavior into ferroelectric compounds was connected with an incorporation of sufficient concentration of uncompensated spins in the B-sublattice. However, limited progress has been made during the last several decades for the following reasons. First, the coupling in existing single-phase compounds was too weak to be of practical use. Secondly, there were very few compounds displaying a coexistence of dipole and spin orders and a magnetoelectric coupling. Furthermore, the Curie or Neel temperature of most of the compounds was far below room temperature. Therefore, it remains a critical need to find new single-phase materials with strong magnetoelectric coupling at room temperature for practical applications (Ivanov et al. 2010). Generally the perovskite compound behaves like antiferromagnetic insulator, but when magnetic metal is doped in pervoskite suddenly it behaves like a ferromagnetic semiconductor by forming a double perovskite compound. Hence the perovskite compounds are antiferromagnetic insulator and the double perovskite compounds are ferromagnetic semiconductor. Double perovskite systems are multifunctional magnetic materials are attractive due to their functional properties and high Curie temperature.

142 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

2. FUNDAMENTAL OF MAGNETISM

Magnetism has been a human curiosity for the last four thousand years and has evolved as one of the most interesting branch of science and technology. The discovery of magnets has done about 4,000 years ago by Cretan shepherd named Magnes. Loadstones contain magnetite, a natural magnetic material Fe3O4. This type of rock was named magnetite, after either Magnesia or Magnes himself. The magnetite (Fe3O4) was the first magnetic material to come into the existence. For many years the discovery of loadstone magnetism was just a curious natural phenomenon. The very first attempt to separate the real fact from superstition was done in 1269 by Gilbert Peter Peregrines (Cvejic et al. 2009). Gilbert was the first person who realized that the Earth was a giant magnet and that magnets could be made by beating wrought iron. Magnetism can be divided into two parts, intrinsic magnetism and technical magnetism. Intrinsic magnetism is the correlation between fundamental magnetic properties and electronic structure where as technical magnetism is concerned with the properties of magnetic domains, anisotropy, magnetostriction and related phenomena (Gama et al. 2007; Dionne and West 1987; Ahmed et al. 2012). The magnetic properties of magnetic materials are completely due to the electrons of the atoms; as electrons have a magnetic moment because of their motion. The nucleus of an atom also has a small magnetic moment, but its magnetic moment is insignificant as compared to that of the electrons and it does not affect the overall magnetic properties of the atom. Motion of an electron is further divided into two types, orbital motion and spin motion and each has a magnetic moment associated with it. The orbital and spin magnetic moment in the first Bohr orbit (Bohr magnetron) is given by

eh orbital / spin   0.9271020erg / Oe  B.M 4mc

2.1. Magnetic Dipole

Magnetic forces are generated by moving electrically charged particles; these magnetic forces are in addition to any electrostatic forces that may prevail. Many times it is convenient to think of magnetic forces in terms of fields. Imaginary lines of force may be drawn to indicate the direction of the force at positions in the vicinity of the field source. The magnetic field distributions as indicated by lines of force are shown in a current loop and also a bar magnet in Figure 3. Magnetic dipoles are found to exist in magnetic materials, which, in some respects, are analogous to electric dipoles. Magnetic dipoles may be thought of as small bar magnets composed of north and south poles instead of positive and negative electric charges. Magnetic dipoles are influenced by magnetic fields in a manner similar to the way in which electric dipoles are affected by electric fields. Within a magnetic field, the force of the field itself exerts a torque that tends to orient the dipoles with the field. A familiar example is the way in which a magnetic compass needle lines up with the earth’s magnetic field.

Development of Double Perovskite Electroceramics 143

Figure 3. Magnetic field lines of force around a current loop and a bar magnet.

2.2. Magnetic Dipole Moment

The magnetic dipole moment can be defined in a manner analogous to that for the electric dipole moment. The magnetic quantity that corresponds to an electric charge is the magnetic monopole. Although a magnetic monopole has never been observed, it nevertheless is used to define the general expression for the magnetic dipole moment. Consider a collection of N magnetic monopoles, m1, m2, . . . mN. Relative to a fixed coordinate system, each pole is located at a point given by a vector 푟⃗; 푟⃗1 for m1, 푟⃗2 for m2, etc. as shown in Figure 4). The magnetic dipole moment µ is a vector quantity is defined as:

푁 휇⃗ = 푚1푟⃗⃗⃗1⃗ + 푚2푟⃗⃗⃗2⃗ … … … . 푚푁푟⃗⃗⃗푁⃗⃗ = ∑푖=1 푚푖 푟⃗⃗⃗푖 (1)

The most common dipole moment vector is the magnetic dipole: two equal but opposite magnetic poles separated by a distance l as shown in Figure 4(b). For example, most of the external effects of a small bar magnet, such as a compass needle, can be viewed as resulting from equal but opposite poles at its two ends. With the strength of the poles taken as +m and −m, the dipole moment of this magnet is found from the general definition, Eq.1given by;

휇⃗ = 푚푟⃗⃗⃗1⃗ − 푚푟⃗⃗⃗2⃗ = 푚(푟⃗⃗⃗1⃗ − 푟⃗⃗⃗2⃗) (2)

⃗ Where 푟⃗⃗⃗1⃗ − 푟⃗⃗⃗2⃗ = 푙 is the vector separation of the two poles. The magnetic dipole moment can thus be written as

휇⃗ = 푚푙⃗ (3)

Above eq. is analogous to the expression 푝⃗ = 푞푙⃗ for the electric dipole moment. As in that case, the magnetic dipole moment of two equal but opposite poles is independent of the coordinate system used to describe it; it is a property of the system. 144 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

Figure 4. (a) Magnetic monopoles mi located at positions ri and (b) Magnetic dipole with dipole moment µ = ml.

2.3. Permeability

Permeability is one of the most important parameters used in evaluating magnetic materials. Not only is it a function of the chemical composition and crystal structure, but it is also strongly dependent on microstructure, temperature, stress, time taken for complete demagnetization and several other factors. The permeability of the magnetic material depends on microstructural conditions and is found to be true during excitation by alternating currents and becomes extremely important at high frequencies. This is because the ac excitations produce rapid a movement of the magnetic domain wall and the permeability is related to the ease of movement of these walls. The presence of any imperfections in the material structure will reduce the permeability. Fewer the grain boundaries, larger will be the grains and higher the permeability, as grain boundaries act as impediments to the domain wall motion. Van den Berg, H. pointed out that in very high permeability metallic materials, domain walls appear to move across grain boundaries easily (Van den Berg 1984). In ferrites as the grain boundaries are thicker, the same kind of unhindered movement does not occur as in the case of metals. Lack of purification, presence of pores and inclusions prevent the attainment of very high permeability, which extends up to 100,000 (in the case of metals).

2.3.1. Dependence of Permeability

(A) Microstructure Dependence Microstructure plays a very important role in deciding the magnetic properties of materials because of fine grains where there is a reversal of magnetization of rotation, since the domain walls are energetically favorable in the fine grains. The permeability dependents on grain size, density and porosity. Porosity causes hindrance to domain wall motion and it decreases initial permeability [30]. Also the coercive force increases with porosity because of the internal demagnetizing field where the decrease in permeability is caused by the same reason. The larger grain size causes reduction in permeability due to the presence of voids that induce demagnetizing field and creates impediments for domain wall motion. Therefore the grain size is a very important parameter in the permeability. Development of Double Perovskite Electroceramics 145

(B) Temperature Dependence Temperature dependence of initial permeability is one of the important parameter in a magnetic component. From the graph of variation of initial permeability with temperature, the slope at a specific temperature can be expressed as a material parameter called the temperature factor (T.F.) which is defined as

∆휇

푇. 퐹. = ⁄휇2 ∆푇

Where, ∆휇 is the difference in permeability (μ2–μ1) between the two temperatures, T2 and T1respectively, while ∆푇 is the difference in temperature. The temperature factor can be used to predict the variation in magnetic properties of a magnetic component. A thermal hysteresis is observed when the temperature is cycled from higher temperature above Tc to lower temperature.

(C) Frequency Dependence At low frequencies, permeability is almost independent of frequency, as the domain wall motion is dominant mechanism. Also at low frequencies, the applied field causes domain wall shift and this motion results in change in net magnetization. The domain structure is responsible for high frequency permeability. At high frequencies, the domain wall inertia precludes any appreciable wall motion, but the mechanism can rotate within each domain. This mechanism is same as ferromagnetic resonance. Smit and Wijn have explained the extension of the loss over a relatively broad frequency region in terms of additional effects upon the resonance condition due to demagnetizing field in the domain structure (Smit and Wijn 1959).

2.4. Magnetisation

When an external magnetic field (H) is applied to any magnetic material, the randomly oriented magnetic moments in the material get oriented more or less with applied field so that the net magnetic moment is non zero. To describe the state of magnetic polarization of the material, a vector quantity magnetization (M) is introduced. It plays a similar role of polarisation in electronics. Magnetisation is defined as the magnetic dipole moment per unit volume developed inside the material (Frenkel and Dorfman 1930). The sensitivity of the material to the applied magnetic field is defined in terms of susceptibility (χ):

푀 휒 = ⁄퐻

This relation gives the susceptibility of only linear materials. The magnetic induction (B) produced in the material due to the applied magnetic field (H) is given by:

퐵 = 휇0 (퐻 + 푀) 146 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

2.5. Hysteresis Loop

A plot between the variation of magnetization with applied magnetic field is called a hysteresis loop as shown in Figure 4. The hysteresis loop is used to characterize magnetic materials. Various parameters like coercivity, remanence, and saturation magnetization can be estimated with the help of the hysteresis loop. As the magnetic field is applied to a sample, the growth of magnetic domains occurs along easy direction of the crystal. After a certain applied magnetic field, domains start rotating away from the easy axis to align the domain along the direction of applied magnetic field. When all of the domains are completely aligned, a saturation magnetization (Ms) is achieved. When the applied magnetic field is reduced to zero, the curve does not retrace its original path. At zero magnetic field, a residual magnetization remains in the system called remanence or remanent magnetization Mr is retained, indicating that the material remains magnetized even in the absence of an applied magnetic field. To make this available residual magnetization to zero an external magnetic field is applied, which is known as coercivity. The ratio of remanence to magnetization saturation (Mr/Ms) is indicative of square area of the M-H loop. Properties of ferromagnetic materials are analogous to those of ferroelectrics; the magnetization M corresponding to the electric polarization P, the magnetic field H corresponding to the electric field E, and the magnetic flux density B corresponding to electric displacement D. Ferromagnetic materials have domains and normally do not show magnetizations because the magnetizations of domains in the sample are oriented in different directions. When a magnetic field H, is applied to the sample; the reorientation of domains will result in the net magnetization and flux density B. Similar to a ferroelectric material, a hysteresis loop can be obtained with a ferromagnet by applying an external magnetic field. A hysteresis loop starts with the unmagnetised state of the ferromagnetic material, and with the applied magnetic field, increasing in the positive direction, due to the motion and the growth of the magnetic domains; the magnetization increases from zero to a saturation value, Ms. When this saturation point is reached, the magnetization curve no longer retraces, because of the irreversibility of the domain wall displacements. When the applied field H decreases to zero, due to the existence of some domains still aligned in the original direction of the applied field the sample still retains some magnetization, known as remnant magnetization, Mr. The reverse field required to reduce the corresponding magnetic induction, Br; to zero is termed as the coercivity, Hc. As the field increases in the negative direction, the material will again become magnetically saturated, but in the opposite direction, thus switching the magnetization. A ferromagnetic material has a phase transition from paramagnetic phase, which have a net magnetic moment to ferromagnetic phase that has a spontaneous magnetization even in the absence of an applied magnetic field with temperature changes from high to low. The transition temperature point is called Curie temperature TC. If the phase transition is from paramagnetic to antiferromagnetic, the corresponding temperature point is called Neel temperature, TN.

Development of Double Perovskite Electroceramics 147

Figure 5. Ferromagnetic (MH) hysteresis loop.

2.6. Different Parameters of Hysteresis Loop

(A) Retentivity Retentivity is the measure of magnetization left behind in a ferromagnetic material after the removal of an external magnetic field (H). In other words, it is a material's ability to retain a certain amount of residual magnetization when the magnetizing force is removed after achieving saturation. (The value of B at point E on the hysteresis curve.)

(B) Coercivity The coercivity is the measure of the ability of the ferromagnetic material of reverse driving field required to demagnetize the material to make the magnetic flux return to zero. In ferromagnetic material the coercivity is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually measured in Oersted or ampere/meter units and is denoted HC. (The value of Hc at point c on the hysteresis curve.)

(C) Saturated Magnetization The saturation magnetization (MS) is a measure of the maximum amount of field that can be generated by a material. It will depend on the strength of the dipole moments on the atoms that make up the material and how densely they are packed together. The atomic dipole moment will be affected by the nature of the atom and the overall electronic structure within the compound. The packing density of the atomic moments will be determined by the crystal structure (i.e., the spacing of the moments) and the presence of some non-magnetic elements 148 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

within the structure. For ferromagnetic materials, at finite temperatures, MS will also depend on how well these moments are aligned, as thermal vibration of the atoms causes misalignment of the moments and a reduction in MS. For ferrimagnetic materials, not all of the moments align parallel, even at zero Kelvin and hence MS will depend on the relative alignment of the moments as well as the temperature.

2.7. Types of Magnetic Order

The magnetic properties of a material are classified by the electronic structure of the atoms within the material. They vary from weakly magnetic to permanent magnetic property. The most important property of a magnetic material is the magnetic susceptibility (χ) which is defined by:

푀 휒 = 퐻

Here M is the magnetization and H is the magnetic field, both of these having units of A/m. The magnetic susceptibility is different for each material and is temperature dependent (except diamagnetic materials) and is given by:

퐶 휒 = 푇 ± 휃

Here C and  are constants that differ for each material (Jiles 1998). Figure 6 shows the alignment of magnetic moments at room temperature for the different types of magnetic materials.

2.7.1. Diamagnetism Diamagnetism is a basic property of all matter, although it is usually very weak. Diamagnetic materials have a relative magnetic permeability which is less than or equal to 1, as susceptibility is defined as χv = μv – 1, and therefore a magnetic susceptibility which is less than 0. That means susceptibility () is negative and the order of about 10-5 and the susceptibility is independent of temperature. This means that diamagnetic materials are repelled by magnetic fields. Diamagnetism is a weak property and it is not observable in everyday life. For example, the magnetic susceptibility of diamagnets such as water is χv = −6 −4 −9.05×10 . The most strongly diamagnetic material is bismuth, χv = −1.66×10 . The origin of diamagnetism is due to the orbiting electrons which are not co-operative with an applied external magnetic field. Diamagnetic materials are composed of atoms having no net magnetic moments when a small field is applied; a small negative magnetization is produced, which is proportional to the applied field strength.

Development of Double Perovskite Electroceramics 149

Figure 6. Schematic of magnetic moment alignments in zero applied-field at room temperature for: a) paramagnetic or superparamagnetic material, b) ferromagnetic material, c) antiferromagnetic material and d) ferrimagnetic material.

2.7.2. Paramagnetism Paramagnetic properties are due to the presence of some unpaired electrons and from the re-alignment of the electron paths caused by the external magnetic field. However, when the applied magnetic field is removed, the thermal fluctuations would make the magnetic moment of the paramagnetic atom to move randomly. Paramagnetic materials have a relative magnetic permeability greater than or equal to unity, i.e., the susceptibility is positive and in the order of 10-3 to 10-5, therefore paramagnetic materials are attracted to magnetic fields. In the presence of relatively low magnetic field, this effect can be described by Curie’s law,

푀 퐶 휒 = = 퐻 푇

Here C is the Curie constant. The expected behavior of the paramagnetic materials in the presence of applied external magnetic field and temperature (Figure 6 a).

2.7.3. Ferromagnetism Ferromagnetism is the basic mechanism by which certain materials form permanent magnets. Ferromagnetic materials have very high magnetic susceptibilities. These materials are made of atoms with permanent dipole moments. Ferromagnetic materials exhibit a parallel alignment of the magnetic moments of one another, resulting in large net magnetization even in the zero magnetic fields known as spontaneous magnetization. Ferromagnetism arises from the spontaneous alignment of magnetic dipole moments which gives a net magnetization, Similar to ferroelectricity. The driving force for ferromagnetism is 150 Shweta Thakur, Mamta Shandilya and Radheshyam Rai the quantum mechanical exchange energy. According to Stoner theory if all the electrons have the same spin orientation then the driving force is minimum (Sujatha et al. 2012). Ferromagnetic materials have two different features are magnetic ordering temperature and spontaneous magnetization. Ferromagnetism is strongly temperature dependent and the magnetization of a ferromagnetic material is inversely proportional to temperature by Curie- Weiss law, as given below

퐶 휒 = 푇 − 휃

Here C is Curie temperature; it is the temperature above which the exchange forces to be present. i.e., above this temperature a ferromagnetic material randomizes due to the thermal energy as in paramagnetic systems (Figure 6 b).

2.7.4. Antiferromagnetism Antiferromagnetism is a type of magnetism in which adjacent ions behave as tiny magnets, spontaneously align themselves at relatively low temperatures into opposite or antiparallel arrangements throughout the material, so that it exhibits almost no external magnetism. Antiferromagnetic materials are characterized by having weak magnetic susceptibility of the order of paramagnetic materials. This spontaneous antiparallel coupling of atomic magnets is disrupted by heating and disappears entirely above a certain temperature, called the Néel temperature, i.e., above the Néel temperature antiferromagnetic materials becomes paramagnetic (Figure 6 c).

2.7.5. Ferrimagnetism Ferrimagnetism is a type of permanent magnetism that occurs in solids in which the magnetic fields associated with individual atoms spontaneously align themselves, some in parallel (as in ferromagnetism), and others in antiparallel direction (as in antiferromagnetism). Ferrimagnetic materials have a spontaneous magnetization below a critical temperature called the Curie temperature (TC) like ferromagnetic materials. Ferrimagnetic materials spontaneously hold magnetization below the Curie point and exhibit paramagnetic properties above the Curie temperature. The magnitude of magnetic susceptibility () for ferrimagnetic materials is similar to the ferromagnetic materials. Ferrimagnetic substances are similar to antiferromagnetic substances; the only difference is that the spins of the atoms are completely opposite. Magnetization of ferromagnetic material is inversely proportional to temperature, as the temperature increases, the magnetization decreases as the alignment of the spins is disturbed by thermal energy (Figure 6 d).

2.7.6. Superparamagnetism Superparamagnetism is a phenomenon by which magnetic materials may exhibit a behavior similar to paramagnetism even at temperatures below the Curie or the Néel temperature. This is a small length-scale phenomenon, where the energy required to change the direction of the magnetic moment of a particle is comparable to the ambient thermal energy. At this point, the rate at which the particles will randomly reverse direction becomes significant. Superparamagnetism occurs when the material is composed of very small Development of Double Perovskite Electroceramics 151 crystallites of few nm. In this case even when the temperature is below the Curie or Néel temperature (and hence the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms), the thermal energy is sufficient to change the direction of magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field average to zero. The concept of superparamagnetism was originally developed and proposed by Néel to explain the possibility of thermal fluctuations in single-domain ferromagnetic clusters. In general, the magnetic anisotropy energy of a particle is proportional to its volume. In a given crystal of volume V, the magnetic anisotropy energy is given by

2 퐸퐴 = 퐾푉푠푖푛 휃

Where K is the anisotropy energy constant and  is the angle between the magnetization vector and the easy axis of nanoparticles. When the volume of a single domain cluster is small enough the magnetic anisotropy energy of the cluster approaches to its thermal energy, hence causes the magnetization to flip between easy axis through an anisotropy barrier in the same way as in a classical paramagnetic system, but with a giant magnetic moment that of a single atom.

2.8. Comparison of Different Types of Magnetism

Figure 7. Comparison of different types of magnetism. 152 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

Figure 8. Schematic of a perovskite structure (ABO3). Blue spheres, Orange spheres and gray spheres represent the oxygen cations, B cations and A anions respectively.

3. STRUCTURAL DIFFERENCE BETWEEN THE DIFFERENT MATERIALS

3.1. ABO3 Perovskite Structure

The most commonly studied ferroelectrics have the cubic perovskite structure (in the paraelectric state). As conventionally drawn, A-site cations occupy the corners of a cube, while B-site cations sit in the body center. Three oxygen atoms per unit cell rest of the faces. The structure of perovskite compound is ABO3, where A and B are cations and O is anions respectively, where A is an alkaline earth metal located at the corners of the cubic unit cell and B is a transition metal that sits at the center surrounded by an octahedral cage of oxygens. The lattice constant of these perovskites has been always close to 4Å due to the rigidity of the oxygen octahedra network and the well-defined oxygen ionic radius of 1.35Å. All materials in this study have structures based on the perovskite structure, with displacements which are important because they result in polarization and can also lead to crystal symmetry changes. Another common ferroelectric structure is the layered perovskite structure which consists of a varying number of perovskite unit cells separated by an oxide layer. This structure is also found in many high Curie temperature (TC) superconductors. A practical advantage of the perovskite structure is that many different cations can be substituted on both the A and B sites without drastically changing the overall structure. Complete solid solutions are easily formed between many cations, often across the entire range of composition. Even though two cations are compatible in solution, their behavior can be radically different when apart from each other. Thus, it is possible to manipulate a material’s properties such as Curie temperature or piezoelectric constant with only a small substitution of a given cation. Also, it is possible to combine materials that would otherwise have different symmetries; this can have the effect of radically altering the piezoelectric behavior. More recently, this compatibility within the perovskite system has made it possible to tailor a material’s lattice constant for matching with a substrate in thin film applications. Development of Double Perovskite Electroceramics 153

Perovskite ABO3 structure with the A and B cations on the corner and body center positions, respectively. Three oxygen anions per unit cell occupy the faces and form octahedra surrounding the B-site as shown in Figure 8 (B.S. 1995). In a structural point of view the perovskite lattice composed of tiny B cations within oxygen octahedra and larger cations are eleven fold coordinates of oxygen. The compound BaTiO3 having an orthorhombic structure with space group Pnma was first named then the structural family of the perovskite compound was named. For example a perovskite structure 3+ 3+ of type A B O3 has a rhombohedral structure with space group R3c (e.g., LaAlO3), when it’s associated with a rotation of the BO6 octahedra with respect to the cubic structure. However, this distortion has slightly changes from the cubic symmetry. The structure of an ideal cubic perovskite is shown in Figure 8.

3.2. AB2O4 Compounds

Chemical composition of ferrites may be written as Me²⁺Fe2 ³⁺ O₄²⁻, where Me²⁺ represents a variety of divalent metallic ions such as Fe²⁺, Co²⁺, Mn²⁺, Zn²⁺, Cd²⁺, Mg²⁺ etc. The spinel ferrite structure MFe2O4 can be described as a cubic close-packed arrangement of oxygen ions, with M2+ and Fe3+ ions at two different crystallographic sites. These sites have tetrahedral and octahedral coordination (A- and B-sites respectively), so that the local symmetries of both the sites are different. An A-site ion is surrounded by four oxygen ions at the four corners of a tetrahedron and the B-site ion is surrounded by six oxygen ions at the six corners of the octahedral, the B-site being at the center of the octahedral. A unit cell of the 2+ spinel structure ferrite contains eight molecules of MFe2O4 i.e., a unit cell contains 8 M , 16Fe3+ and 32 O2- ions. The smallest cubic unit cell of a spinel structure contains 64 tetrahedral and 32 octahedral sites (Figure 9).

Figure 9. (a) Spinel structure with octahedral (blue) and tetrahedral units (yellow). Red balls represented Oxygen atoms. (b) tetrahedral (above) and octahedral (lower) sites of spinel structure.

154 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

The cations occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites. There are total 24 cations, 16 trivalent iron ions and 8 divalent metal ions, ranging radius between 0.4 Å to 1 Å, are distributed among 8 tetrahedral interstices. The ionic distribution of this kind of structure may be represented by the equation

2 3 2 3 2 M Fe1 M1 Fe1 O4

Where the cations enclosed by parenthesis occupy A-sites comprising the tetrahedral sublattice, while the cations enclosed by the square bracket occupy B-sites comprising the octahedral sublattice and  is the inversion parameter;  = 0 and 1 stand for the inverse and normal spinel and for mixed spinel, 0    1. The factors, which can influence the distribution of the metal ions over A and B-sites: the electronic configuration of the metal ions, the ionic radii and the electrostatic energy of the spinel lattice and more recently the preparation conditions (Hill, Craig, and Gibbs 1979). In spinels, A and B can be divalent, trivalent, or tetravalent cations, including magnesium, zinc, iron, manganese, aluminum, chromium, gallium, titanium, and silicon among other elements of the Periodic Table. Some of the principal members of the oxide spinel family are: spinel (MgAl2O4), gahnite (ZnAl2O4), hercynite (FeAl2O4), cuprospinel (CuFe2O4), magnetite (Fe3O4), ulvöspinel (TiFe2O4), chromite (FeCr2O4), magnesiochromite (MgCr2O4), galaxite (MnAl2O4), magnesioferrite (MgFe2O4), franklinite (ZnFe2O4), trevorite (NiFe2O4). Spinels are usually ordered (A and B in tetrahedral and octahedral sites, respectively) and they are referred as “normal” spinels, but mutual substitution of A and B cations has been reported in many materials.

3.3. A2B′B″O6 Double Perovskites

Figure 10. Structure of A2B′B″O6 structure. The ternary perovskite structure can be described as a network of corner sharing BO6 octahedra with the larger A-site cation occupying the cubo-octahedral (12 corner) cavities. Development of Double Perovskite Electroceramics 155

The ternary perovskite structure with ABO3 stoichiometry, where A is generally an alkaline-earth or a lanthanoid ion and B is generally a transition-metal ion, forms a very large class of compounds (Mitchell 2002). They have received immense attention due to their numerous functional properties such as ferroelectricity, ferromagnetism, multiferroicity, giant magnetoresistance and even high Curie temperature (Tc) superconductivity. Most often, these functionalities come from the transition-metal octahedral (BO6) units. Expanding the complexity of the ternary perovskites by including two transition-metal ions arranged in a rocksalt fashion at the B-site gives rise to a sub-class of compounds commonly known as double perovskites with A2B′B″O6 stoichiometry (Anderson, Poeppelmeier, et al. 1993). Introducing two different transition-metal ions within the structure also imparts multifunctionalities that cannot be achieved with their compositionally simpler counterparts. Double perovskites are however much less studied than the ternary perovskites. Hence, many of their interesting properties remain unexplored. The structure of double perovskite compound is of type A2BʹBʹʹO6. There are two types of B-site patterning in B-site ordering. One type of pattern is that more than 160 perovskite oxides, which is represented by rock-salt ordering. The cations between Bʹ and Bʹʹ order into alternate octahedral. In double perovskite structure where Bʹ has partially filled and Bʹʹ has empty orbitals or vise-versa. It gives a unique opportunity that provides the multiferroic behaviours due to their ferromagnetic insulator behaviours which has 180o-superexchange interactions between Bʹ and Bʹʹ cations via the oxygen ions. The ordering between B and B’ in A2BʹBʹʹO6 gives important functional properties. Now the most important aspect is that how to achieve and control the structural ordering between Bʹ and Bʹʹ in double perovskite compound and the correlation between their functional properties (Jena).

3.4. Comparison between the Different Magnetic Parameters of Ferrites, Perovskite and Double Perovskite

Table 1. Comparison of Different magnetic properties of Ferrites, Perovskites and Double Perovskites

Sr.no. Composition MS Coercive Bohr TN Reference (emu/g) field magneton (0C) Hc (Oe) (μB) Ferrites MgFe2O4 22-28 1935 1.8 320 (Glass 1988; Chen et al. 2006) MnFe2O4 57.1 196 3.3 340 (Kolhatkar et al. 2013) Mn0.1Co0.9 Fe2O4 68.97 44 (Msomi et al. 2011) CoFe2O4 84.95 1202.22 4.46 520 (Mane et al. 2013) Co0.5Zn0.5Fe2O4 80.99 75.01 3.45 538 (Varshney, Verma, and Kumar 2011) Co0.5Mg0.5Fe2O4 51.09 849.98 1.98 560 (Varshney, Verma, and Kumar 2011) Ni0.5Mg0.5Fe2O4 29.78 198.71 2 (Hashim, Kumar, Shirsath, et al. 2012)

156 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

Table 1. (Continued)

Sr.no. Composition MS Coercive Bohr TN Reference (emu/g) field magneton (0C) Hc (Oe) (μB) Ni0.5Mg0.5Fe1Cr1O4 5.88 60.01 1.4 (Hashim, Kumar, Shirsath, et al. 2012) Ni0.5Zn0.5Fe2O4 61.97 84.67 395 (Gama et al. 2007; Dionne and West 1987) Ni0.5Zn0.5Fe1.9Cr0.1O4 40.56 19.62 (Gama et al. 2007) Ni0.7Zn0.3Al0.5Fe1.5O4 16.4 43.1 (Hashim, Kumar, Ali, et al. 2012) Ni0.2Zn0.4Cu0.2Fe2 O4 52.12 320 (Xia et al. 2011) (Ni0.25Cu0.20Zn0.55) 65 2.22 (Roy, Nayak, and Bera La0.025Fe1.975O4 2008) Ni0.5Zn0.5Pr0.05Fe1.95O4 74.1 37.6 543 (Yan et al. 2015) Zn0.4Fe2.6 O4 165 60 1.73 320 (Noh et al. 2012) Mn1.5 Fe1Ti0.5O4 6.28 2.6454 (Farea et al. 2009) Mg0.5Cu0.05Zn0.45 47.68 14.36 132 (Bhosale et al. 1999; Fe2O4 Sujatha et al. 2012) Mg0.95Mn0.05 Fe2O4 12.5 8.2 240 (Kumar et al. 2009) CuFe2O4 49 0.3 9 710 (Ahmed et al. 2012) Cu0.5Mg0.5Fe2O4 41.4 6.8 5.5 660 (Ahmed et al. 2012) Cu0.5Zn0.5Fe2O4 17.53 156.32 0.75 (Hankare et al. 2010) BaFe12O19 72 594 20 450 (Smit and Wijn 1959) Fe2.55In0.45O4 63 700 (Cvejic et al. 2009) Fe2.85Tm0.15O4 57 1550 (Cvejic et al. 2009) Fe2.85Gd0.15O4 77 900 (Cvejic et al. 2009) Fe2.85Dy0.15O4 58 1213 (Cvejic et al. 2009) Fe2.85Ho0.15O4 68 800 (Cvejic et al. 2009) Fe2.85Tm0.15O4 66 880 (Cvejic et al. 2009) Fe2.85Yb0.15O4 60 1100 (Cvejic et al. 2009) Li0.5Fe2.5 O4 52 151 244 (Shirsath et al. 2011) Perovskite 0.5(PbFe0.5Nb0.5O3)- 0.085 1560.29 (Amonpattaratkit, 0.5(PbZr0.44Ti0.56 O3) Jantaratana, and Ananta 2015) 0.80(PbZr0.52Ti0.48O3)–0.20 8.18 763 0.60 405 (Peng et al. 2015) (CoFe2O4) 0.65(PbZr0.52Ti0.48O3)–0.35 15.7 664 1.05 409 (Peng et al. 2015) (CoFe2O4) 0.50(PbZr0.52Ti0.48O3)–0.50 22.8 722 1.50 411 (Peng et al. 2015) (CoFe2O4) 0.80(PbZr0.52Ti0.48O3)- 10.23 55 430 (Gupta and Chatterjee 0.20(Mn0.3Co0.6Zn0.4 Fe1.7O4) 2010) 0.5(Ni0.93Co0.02Mn0.05Fe1.95O 31 12 (Mudinepalli et al. 2015) 4)–0.5(Na0.5Bi0.5TiO3) Pr0.50Ca0.50MnO3 0.6 2.5 168 (Tikkanen, Huhtinen, and Paturi 2015) BiFeO3 25.1 93 1 647 (Catalan and Scott 2009) Bi0.9Pr0.1Fe0.9Cr0.1O3 48.7 103 (Das, Gopal Khan, and Development of Double Perovskite Electroceramics 157

Sr.no. Composition MS Coercive Bohr TN Reference (emu/g) field magneton (0C) Hc (Oe) (μB) Mandal 2012) Bi0.97Ce0.03FeO3 3.03 110 (Bhushan et al. 2012) Bi0.85Ba0.15FeO3 1.2 3600 0.06 373 (Das and Mandal 2012) Bi0.85Tb0.15FeO3 2.8 1000 73 (Lotey and Verma 2013) Bi0.6Nd0.4FeO3 0.9 5000 620 (Zhang et al. 2010) Bi0⋅8La0⋅1Er0.1FeO3 1.75 8000 (Pandit et al. 2011) 0.9(BiFeO3)– 3.0 500 300 (Khelifi et al. 2015) 0.1(Ba0.8Sr0.2TiO3) BaZr0.07Ti0.93O3 3.5 4500 (Jarupoom et al. 2012) + 3vol% NiO 70(BiFeO3)–30(BaTiO3) 0.05 5000 (Kumar et al. 2013) DyFeO3 165 115 55 (Zhu et al. 2012) Bi0.9Gd0.1Fe0.8Mn0.2O3 3.19 38.67 (Tang et al. 2015) Tb0.67Ho0.33MnO3 90 380 10.6 (Staruch et al. 2015) Bi0.80Tb0.20Fe0.80Mn0.20O3 0.50 533 (Kumar, Aswini, and Venkateswaran 2014) La0.5Ca0.5Mn0.95Bi0.05O3 79 5.94 192 (Krichene et al. 2015) La0.65Sr0.35Mn0.90Fe0.10O3 35 280 (Hu et al. 2002) La0.65Sr0.35Mn0.90Ni0.10O3 20 278 (Hu et al. 2002) La0.8Ca0.05Pb0.15FeO3 5 478.1 670 (Benali et al. 2015) La0.65Eu0.05Sr0.3Mn0.85Cr0.15 35 278 (Bellouz et al. 2015) O3 (Nd0.93Y0.07)0.7Sr0.3MnO3 85 175 (Phan et al. 2014) La0.6Pr0.1Ba0.3Mn0.90Ni0.1O3 38 162 (Oumezzine et al. 2014) La0.57Y0.1Ba0.23Ca0.1MnO3 80 300 (Abassi et al. 2014) Pr0.6La0.1Ca0.3Mn0.90Fe0.1O3 47 65 (Zouari et al. 2014) Double Perovskite La2CoMnO6 73.3 204 (Kim et al. 2015; Penchal Reddy et al. 2014) Sr2FeMoO6 13 0.92 350 (Hu et al. 2015) Sr2Fe0.9Mo1.1O6 15.65 (Markandeya, Suresh, and Bhikshamaiah 2011) Sr1.85La0.15FeMoO6 35 427 (Zhang et al. 2006) Sr1.94K0.06FeMoO6 18 0.83 346 (Hu et al. 2015) Sr2Fe(W0.85Mo0.15)O6 2 200 (Kobayashi et al. 2000) Sr1.5Nd0.5 FeMoO6 1.7 400 (Habib et al. 2005) Sr1.6Ba0.4FeMoO6 3.5 413 (Feng et al. 2004) Sr2CrMoO6 0.22 60 (Blasco et al. 2010) Sr1.9La0.1CrMoO6 0.16 100 (Blasco et al. 2010) Sr2CuIrO6 0.055 15 (Vasala, Yamauchi, and Karppinen 2014) Ca1.5La0.5FeMoO6 1.17 2.80 401 (Burzo et al. 2015) Ca1.5La0.5FeMo0.7W0.3O6 2.25 1.75 441 (Burzo et al. 2015) Pb2CrMoO6 0.095 33 (Zhao et al. 2015) Sr2GdMoO6 5.7 100 7 (Pinacca et al. 2015) Sr2DyMoO6 4.9 12 (Pinacca et al. 2015) Sr2HoMoO6 5.8 13 (Pinacca et al. 2015)

158 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

Table 1. (Continued)

Sr.no. Composition MS Coercive Bohr TN Reference (emu/g) field magneton (0C) Hc (Oe) (μB) Sr2ErMoO6 4.6 7 (Pinacca et al. 2015) Sr2YbMoO6 0.35 3.81 23 (Pinacca et al. 2015) Sr2HoTaO6 0.45 45 (Dutta, Sinha, and Das 2014) Ba2UCuO6 0.35 10 (Murasik, Fischer, and Zygmunt 1994) Ba2CuOsO6 0.02 70 (Feng et al. 2014) Ca2HoTaO6 0.63 58 (Dutta, Sinha, and Das 2014) Ca2InOsO6 0.08 14 (Feng et al. 2013) LaPbCoSbO6 0.33 10 (Bai et al. 2014) LaPbNiSbO6 0.11 32 (Bai et al. 2014) Nd2CoMnO6 2 145 (Yang et al. 2014) Sm2CoMnO6 1 125 (Yang et al. 2014) SrBiFeTiO6 0.12 300 40 (Yao et al. 2014) SrBiMnTiO6 1.728 440 25 (Yao et al. 2014)

3.5. Different Techniques Used to Synthesis the Double Perovskite Materials

3.5.1. Hydrothermal Synthesis The hydrothermal synthesis of ceramic powders provides two major advantages: (i) the elimination or at least minimization of any high temperature calcination or conditioning steps and (ii) the utilization of relatively-inexpensive precursor materials. While generally not limited to oxide compounds, the technique is particularly suitable for preparing not only advanced functional ceramic powders such as PZT or BNT as well as a wide range of magnetic ferrite oxides with magneto plumbite or spinel structures but also advanced structural oxide ceramics such as alumina and zirconia. Consequently, hydrothermal synthesis will find its commercial niche precisely in the area of producing advanced electronic ceramics if process-related and economic obstacles can be successfully overcome in the future. Some advantages are:

(1) The process utilizes comparatively inexpensive precursor chemicals such as oxides, hydroxides, chlorides, acetates and nitrates rather than the expensive alkoxide required for sol-gel processing. (2) Reactants that are normally volatile at the required reaction temperature tend to condense during the hydrothermal process and thus maintain the reaction stoichiometry. Consequently, highly pure, multi-component anhydrous ferroelectric powders can be obtained. (3) Hydrothermal synthesis is a low-temperature process with many effects achievable even below 300°C. The relatively low temperature can break down stable precursors under pressure, thus avoiding the extensive agglomeration that solid state reactions usually cause at high sintering temperature. The low reaction temperatures also avoid Development of Double Perovskite Electroceramics 159

other problems encountered with high temperature processes, for example, poor stoichiometry control due to volatilization of components (e.g., Pb volatilization in Pb-based ceramics). (4) The process is amenable to produce solid solution particles with controlled size distribution, shape, and complex chemical composition. Multi-doped perovskite ABO3 ceramic powders, for example, can be grown down to sub micrometer or even nanometer size by close control of the nucleation and growth steps. (5) Hydrothermal synthesis is that the purity of hydrothermally synthesized powders significantly exceeds the purity of the starting materials. It is because the hydrothermal crystallization is a self-purifying process, during which the growing crystals/crystallites tend to reject impurities present in the growth environment. The impurities are subsequently removed from the system together with the crystallizing solution, which does not take place during other synthesis routes, such as high temperature calcination. (6) Powders grown by the hydrothermal process rarely require presintering or calcination steps. This is particularly important for synthesizing high quality PZT powders since lead oxide is quite volatile at conventional calcination or sintering temperatures (Su, Button, and Ponton 2004; Wu, Vilarinho, and González 2010). (7) Synthesis is accomplished in a closed system from which different chemicals can be recovered and recycled, thus making it an environmentally-benign technology. (8) The process can be easily scaled up to industrial demand since hydrothermal synthesis in principle lends the opportunity for cost effective and reproducible production of high quality ceramic powders on a large industrial scale. Hydrothermal methods are more environmentally benign than many other synthesis methods, which can be attributed in part to energy conserving low processing temperatures, absence of milling, ability to recycle waste, and safe and convenient disposal of waste that cannot be recycled (Suchanek and Riman 2006).

Materials synthesized under hydrothermal conditions often exhibit differences in point defects when compared to materials prepared by high temperature synthesis methods. For instance, in barium titanate, hydroxyapatite, or α-quartz, water-related lattice defects are among the most common impurities and their concentration determined essential properties of these materials. The problem of water incorporation can be overcome by either properly adjusting the synthesis conditions or by use of non-aqueous solvents (solvothermal processing). Another important technological advantage of the hydrothermal technique is its capability for continuous materials production, which can be particularly useful in continuous fabrication of ceramic powders (Cousin and Ross 1990). These decisive advantages have to be judged against the following disadvantages of the hydrothermal processing technology:

(1) The comparatively high initial cost of the equipment such as autoclaves, liners, valves, pressure tubing, control equipment and other ancillary tools. (2) Requirement of a stringent safety regime caused by the high pressure applied, i.e., appropriate shielding and utilization of busters disks etc. (3) Potential high temperature corrosion problems arising from the presence of alkaline or acidic mineralizers that require inert noble metal or polymeric autoclave liners. 160 Shweta Thakur, Mamta Shandilya and Radheshyam Rai

(4) Closed autoclaves do not normally allow to visually observe the progress of the reaction. Application of transparent windows of corrosion-resistant materials such as alumina, magnesia or spinel is possible, but adds to the cost since they have to be frequently replaced.

Thus far, we have investigated numerous unary, binary, and ternary oxide syntheses using the M-H process, which was shown to (a) lead to rapid heating to temperature of heat treatment, (b) increase the kinetics of reaction by one to two orders of magnitude, (c) lead to the formation of some novel phases, and (d) lead to selective crystallization of phases in the chemical system used. Materials synthesized under hydrothermal conditions often exhibit differences in point defects when compared to materials prepared by high temperature synthesis methods. For instance, tungstates of Ca, Ba, and Sr synthesized at room temperature by an hydrothermal- electrochemical method do not contain Schottky defects usually present in similar materials prepared at high temperatures (Sen and Pramanik 2001), which results in improved luminescent properties. Other types of defects, such as hydroxyl ions substituted for oxygen ions in barium titanate generate barium ion vacancies, which are believed to degrade the dielectric properties (Suchanek, Lencka, and Riman 2004).

3.5.2. Solid State Reaction (SSR) This is the most widely used method for the synthesis of polycrystalline bulk ceramics. Solid State Reaction (SSR) method provides a large range of selection of starting materials like, oxides, carbonates, etc. Since, solids do not react with each other at room temperature (RT), it is necessary to heat them at elevated temperatures as high as up to 1500oC for the proper reaction to take place at an appreciable rate. Thus, both, thermodynamic and kinetic factors are important in SSR. In SSR method, the solid reactants react chemically without the presence of any solvent at high temperatures yielding a product which is stable. The major advantage of SSR method is, the final product in solid form is structurally pure with the desired properties depending on the final sintering temperatures. Large single crystals are not usually obtained by this method. Steps Involved-

 Take appropriate high purity starting materials, fine grain powders, in stoichiometry proportions.  Weigh all of them, as per the calculations performed  Mix them together, thoroughly using agate mortar and pestle or ball milling (for large quantity sample)  Heat the solid powder mixture (calcination) at elevated temperatures in air using muffle furnace.  Repeat the calcination process twice with intermittent grinding  The powder is palletized to form a uniform and compact pellets which are sintered at more elevated temperatures for prolonged duration.

Development of Double Perovskite Electroceramics 161

3.5.3. Co-Precipitation By using the co-precipitation route, it is possible to achieve a high degree of homogenization together with a small particle size and faster reaction rates. The extent to which a component can be separated from solution can be determined from the solubility- product constant obtained by determining the quantity of dissolved substance present in a known amount of saturated solution. This value is known as the solubility. The solubility can be drastically altered merely by adding to the solution any of the ions which make up the precipitate. Although, solubility can be altered over a wide range, the solubility product itself remains practically constant over the same range. The process is designed to avoid any sequential precipitation of metal ions in order to achieve desired stoichiometry. Solution based methods are generally used to improve homogeneity and purity and also to reduce particle size. Soluble metallic salts (typically nitrates) are dissolved in aqueous solvent and then precipitated from the liquid phase. The anion solution is used to simultaneously precipitate the cation species as carbonates and/or hydroxides. The low processing temperatures used in this method results in powders with small particle size at nanoscale. Further, the homogeneously mixed precursors increase the reaction rates during calcination. Few commonly used steps involved in the synthesis of mixed oxide material are

 Taking appropriate stoichiometry amounts of starting materials,  Making the appropriate amount of basic solution having very high Phachieving the reducing state (acidic state) of a mixture of all the starting materials by centrifugation maintaining high pH ~ 14 of the precipitation solution performing decantation to remove excess water, undesired ions and impurities.  Co-precipitation method for mixed oxide compounds results into small sized particles, homogeneously and evenly distributed having single phasic nature.  It requires very low heating treatment, sometimes no need to sinter the product, only calcination is sufficient.  It is preferable when large quantity of product (powder) is required.

It offers variety of precursor selections to choose as starting materials starting from simple salts of complicated organic-inorganic materials, cost effective and easy to set-up also. In this method, the pH is a very sensitive parameter which should be carefully controlled to achieve better product (Solanki et al. 2009; Song Y.J 2001). The coprecipitation method does not work well in cases where:

(a) The two reactants have different solubilities in water, (b) The reactants do not precipitate at the same rate or supersaturated solutions commonly occur.

3.5.4. Sol-Gel Process In materials science, the sol-gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon and titanium. The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides. In this chemical 162 Shweta Thakur, Mamta Shandilya and Radheshyam Rai procedure, the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. In the case of the colloid (Constantin et al. 2001), the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid (Model and Thickener 1989). Centrifugation can also be used to accelerate the process of phase separation.

3.5.5. Pyrophoric Process It is the popular wet chemical method route for fabrication of materials, favorably metal oxides. In this method aqueous solution of the requisite amount of ingredient materials are taken in stoichiometric proportion. The individual solution is heated up and concentric HNO3 is added drop wise until it becomes clear, then the fuel is added with these solutions in such a way that metal ions to fuel ratio is maintained fraction 1:1:2 to make viscous solution. The clear solution of fuel complexes, metal nitrates was evaporated around 200 oC with constant stirring. The continuous heating of these solutions causes foaming and puffing. During evaporation the nitrate ions provide a situ oxidation environment for fuel, which partially converts the hydroxyl group of fuel into carboxylic acid. When complete dehydration occurs, the nitrate themselves is decomposed with the voluminous. Organic based black fluffy powder, i.e., precursor powder. The precursor powder after proper grinding are calcined at various temperature to get desired compound with variable particle size. In this method the TEA (Tri Ethalo Amine) is used as a fuel (Jena).

3.6. Applications of Double Perovskite

The double perovskites have transition-metal ions in them, which impart a variety of magnetic and electronic properties to them. The transition−metal ions also have strong electron−electron correlations which have a significant impact on the properties. In the case of Sr2FeMoO6, which is probably the most widely studied double perovskite offering tremendous prospects for spintronics devices due to its half-metallic nature, we find FeMo and MoFe antisites to be thermodynamically the most stable defects. Within stoichiometric Sr2FeMoO6, we find that antisite disorder leads to a significant decrease in spin-polarization and saturation magnetization. We also find that it is energetically favorable for pairs of FeMo and MoFe antisites to remain close to each-other. This suggests short-range ordering, even in the presence of long-range disorder. In the case of nonstoichiometric Sr2FeMoO6, we find that the saturation magnetization decreases linearly for both Mo-rich and Fe-rich stoichiometries. However the spin polarization decreases only for Mo-rich Sr2FeMoO6. These results suggest that whenever it is difficult to achieve perfect stoichiometry, for instance, while growing thin- films, it would be preferable for spintronics applications to remain in the Fe-rich region. We also find oxygen vacancies to be thermodynamically possible, however, spin-polarization of Sr2FeMoO6 is not found to be affected by them. The simultaneous presence of ferromagnetism and ferroelectricity in a single phase, and the possible spin-phonon and spin- polar couplings in these systems offer the appealing opportunity to design various Development of Double Perovskite Electroceramics 163 unconventional devices, such as multiple-state memory elements and electric-field controlled magnetic sensors. However, their ultimate uses depend upon the nature and net amplitude of the coupling around 300 K. The coexistence of various electronic order parameters that can simultaneously interact in a given system also poses new challenges for physics, chemistry and technology (Singh et al. 2009). Double perovskite systems are multifunctional magnetic materials are attractive due to their functional properties and high Curie temperature. The fact that the double perovskites seem to be ferromagnetic metals and apparently have highly spin polarized conduction band makes these materials interesting for applications in spintronic devices such as magnetic tunnel junctions or low field magnetoresistive sensors (Philipp et al. 2003), potential technological applications and such as nonvolatile memories (Shimakawa, Azuma, and Ichikawa 2011; Spaldin, Cheong, and Ramesh 2010), multiple-state memory elements and electric-field controlled magnetic sensors (Singh et al. 2009), spintronics (Kleemann 2013; Tsymbal 2012). Thus Double perovskites are used for the development of spin polarized conduction and ferromagnetic semiconductors. The double perovskites 2+ 2+ 2+ 2+ LaPbMSbO6 (M = Mn , Co , and Ni ) were obtained by an aqueous synthesis route as pure polycrystals. Order between M2+ and Sb5+ among the two octahedral sites is higher for Mn2+ and Co2+ than for Ni2+, as obtained from the Rietveld analysis of the X-ray powder diffraction patterns. The magnetization curves show an antiferromagnetic order at temperatures below 20 K. However, for LaPbMnSbO6 an additional feature is observed at higher temperatures, 45 K, probably due to a small disorder which originates shorter super 2+ 2 exchange Mn -O2-Mn + paths and uncompensated isolated regions of ferromagnetic nature are generated. This kind of observations are very useful for applications in spintronics (Franco, Carbonio, and Nieva 2013).

CONCLUSION

Expanding the complexity of the ternary perovskites by including two transition-metal ions arranged in a rock salt fashion at the B-site gives rise to a sub-class of compounds known as double perovskites with A2B′B″O6 stoichiometry. Introducing two different transition-metal ions within the structure also impart multifunctionalities that cannot be achieved with their compositional simpler counterparts. In double perovskite structure, Bʹ has partially filled and Bʹʹ has an empty orbital or vice-versa. It gives a unique opportunity that provides the multiferroic behaviors due to their ferromagnetic insulator behavior, which has 180o- superexchange interactions between Bʹ and Bʹʹ cations via the oxygen ions. The double perovskites have transition-metal ions in them, which impart a variety of magnetic and electronic properties to them. The transition−metal ions also have strong electron−electron correlations which have a significant impact on the properties. Sr2FeMoO6 is the most widely studied double perovskite offering tremendous prospects for spintronics devices due to its half-metallic nature, we find FeMo and MoFe antisites to be thermodynamically the most stable defects. Within stoichiometric Sr2FeMoO6, we find that antisite disorder leads to a significant decrease in spin-polarization and saturation magnetization. Double perovskite systems are multifunctional magnetic materials are attractive due to their functional properties and high Curie temperature. The fact that the double perovskite seems to be ferromagnetic metals and have highly spin polarized conduction band. This property make these materials 164 Shweta Thakur, Mamta Shandilya and Radheshyam Rai interesting for application in spintronic devices like magnetic tunnel junctions or low field magnetoresistive sensors, potential technological applications like nonvolatile memories, spintronics, multiple-state memory elements and electric-field controlled magnetic sensors.

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Chapter 5

MAGNETIC PROPERTIES, MAGNETORESISTANCE AND FUNCTIONALITY OF PEROVSKITE MANGANESE OXIDES

I. O. Troyanchuk1, D. V. Karpinsky1,2,*, M. V. Silibin1, S. A. Garvilov1, S. V. Dubkov1 and V. V. Sikolenko3,4 1National Research University of Electronic Technology “MIET”, Zelenograd, Moscow, Russia 2Scientific-Practical Materials Research Centre NAS of Belarus, Minsk, Belarus 3Joint Institute for Nuclear Research, Dubna, Russia 4REC “Functional nanomaterials” Immanuel Kant Baltic Federal University, Kaliningrad, Russia

ABSTRACT

Ceramic samples of La1-xSrxMn0.5Ni(Co)0.5O3 with perovskite structure are considered to be promising functional materials due to their intriguing magnetic and magnetoresistive properties. Physical properties of these compounds dependent on their structural parameters and electron configuration of the related transition metal ions. These compounds have been thoroughly studied by neutron diffraction technique, magnetometry and magnetoresistance measurements. The obtained results have allowed us to determine the evolution of the structural parameters of the compounds upon chemical substitution while their crystal structure remains to be rhombohedral up to the dopant concentration of 20%, in the case of the Co-doped compounds further increase of Sr content favors a stabilization of cubic structure. The substitution by strontium ions leads to a degradation of long-range ferromagnetic order attributed to both initial compounds towards either antiferromagnet and spin glass state in the case of Ni and Co containing samples respectively. All the compounds are semiconductors and exhibit large magnetoresistance which gradually increases with temperature decreasing, an increase of strontium concentration leads to a decrease of magnetoresistance effect. The obtained results testify that the chemical substitution leads to an increase in the average oxidation

* Corresponding Author Email: [email protected]. 174 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

state of Ni and Co ions from 2+ into 3+ one while manganese ions remain 4+ oxidation state. The character and stability of the superexchange interaction between pairs of Co, Mn and Ni ions via oxygen ion which govern magnetic properties of the compounds are discussed depending on the oxidation state and electron configuration of the respective transition metal ions and structural peculiarities of the compounds.

Keywords: magnetic properties, magnetoresistance, perovskite manganese oxides

1. INTRODUCTION

Solid solutions of manganese, nickel and cobalt with perovskite structure are the model objects to study magnetic properties of transition metal oxides demonstrating intriguing properties which often remain to be unexplained. An existence of a number of the phase transitions, promising magnetic and magnetoresistive properties permit to consider these compounds as perspective functional oxides with promising physical properties. Ferromagnetism of manganites and cobaltites materials as well as their metallic type conductivity can be induced by chemical substitution of La3+ ions by Sr2+ ones resulted in 3+ 2+ heterovalent Mn and/or Co ions in related solid solutions as La1-x Srx Mn(Co)O3 (Raccah and Goodenough 1967; Señarı́s-Rodrı́guez and Goodenough 1995; Zhou and Goodenough 1999; Tokura 2006). There is another way to get dielectric ferromagnetic state of manganites – via chemical substitution of Mn3+ ions by nickel or cobalt ones (Troyanchuk et al. 2000a; Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008). Maximal values of the Curie points have been observed for the compounds with 50% substitution of the Mn ions which is primarily associated with structural order of Mn and Co(Ni) ions (Troyanchuk et al. 2000a; Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008; Guo et al. 2013; Yuqiao et al. 2013). It is known that the synthesis conditions determine the values of the Curie points which can vary significantly, e.g., 280 K for LaMn0.5Ni0.5O3 compound (Guo et al. 2013; Yuqiao et al. 2013) and 230 K for LaMn0.5Сo0.5O3 one (Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008) depending on the structural order of the related ions. It is considered in Ref (Joseph Joly, Joy, and Date 2002) that transition metal ions are structurally disordered in the compounds having l ow temperature magnetic transition, structural order of these ions favors higher transition temperature (Subramanian et al. 1996). The recent structural data obtained by XMCD method (Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008) testify that preferred oxidation states of the transition metal ions are Mn4+ and Ni2+ (Co2+) ones which highlights important role of the structural order of the heterovalent ions. It should be noted that different methods to study structural order show significantly different results, e.g., neutron diffraction study of the compounds with relatively high Curie points testify a well ordered structure formed by heterovalent ions Co, Ni and Mn. While the results of the electron diffraction measurements testify an incommensurate crystal structure only in a few crystallines (Mahendiran et al. 2003). In the cobalt doped system the most pronounced ferromagnetic state is observed in the composition LaCo0.5Mn0.5O3 (Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Magnetic Properties, Magnetoresistance and Functionality … 175

Chen, et al. 2008; Barón-González et al. 2011), where the magnetization is maximal and the Curie point reaches 237 K (I.O. Troyanchuk et al. 2016). In LnCo0.5Mn0.5O3 (Ln – lanthanide) compositions the Co ions are considered to be mainly in 2+ oxidation state whereas the Mn ones adopt 4+ oxidation state in accordance with the data obtained by the X-ray spectroscopy and NPD measurements (Troyanchuk et al. 2000a; Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008; Barón-González et al. 2011). The value of the 2+ 4+ magnetization and TC depend on the structural ordering of Co and Mn ions reaching a maximum in well-ordered compounds (Barón-González et al. 2011). LnCo0.5Mn0.5O3 (Ln = Nd, Sm, Eu, Gd, Tb) compounds show metamagnetic transitions below the Curie point (Troyanchuk et al. 2000a; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008). The origin of the metamagnetic transition in LnCo0.5Mn0.5O3 compounds is still a matter of discussion (Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008). Manganites and cobaltites doped with alkaline-earth ions can form solid solutions with intriguing magnetic properties (Karpinsky et al. 2016; Troyanchuk et al. 2017; I.O. Troyanchuk 2016). Relatively small substitutions of Mn ions for Co ones in the Pr0.5Sr0.5Co1-xMnxO3 system are accompanied by a significant decrease of the magnetization and of the Curie point values. However, as the content of Mn ions exceeds 35% the Curie temperature increases and the transition into the paramagnetic state becomes well pronounced (Troyanchuk et al. 2011). The origin of such behavior is still unknown. We have performed the structural, magnetic and magnetoresistance study of the La1- xSrxMn0.5Ni0.5O3 and La1-xSrxMn0.5Сo0.5O3 ceramics in a wide range of the La/Sr ratio. Chemical substitution of the La3+ ion by Sr2+ ones should change the electronic configuration of Ni and Co ions from 2+ towards to 3+ oxidation state assuming constant oxygen stoichiometry of the compounds. It is considered that an appearance of the Ni3+/Co3+ ions should disrupt the charge order. The mentioned approach to substitute Mn ions by other transition metal ions can clarify the effect of the charge order of the Mn, Co and Ni ions on the magnetic properties of the respective compounds; the sign and the magnitude of the superexchange interactions between transition metal ions can also be estimated.

2. EXPERIMENTAL METHOD

Polycrystalline samples of La1-xSrx(Mn0.5Ni0.5)O3 (0 ≤ x ≤ 0.2) and La1-xSrxMn0.5Сo0.5O3 (x ≤ 0.75) have been obtained by standard ceramic technology using oxides La2O3, Mn2O3, NiO, Co3O4 and carbonate SrCO3 of high purity. The oxide La2O3 was annealed at 1050°C to remove humidity. The oxides taken in stoichiometric ratio were thoroughly mixed in planetary mill RETSCH PM 100. Preliminary annealing was performed at 1100°C for 5 hours and the obtained product was thoroughly ground. A final synthesis was performed in air at temperature of 1420°C for 7 hours followed by gradual temperature decrease down to 300°C during 12 hours. Phase purity has been estimated using step-scanned X-ray diffraction data collected at room temperature using a DRON-3M diffractometer with Cu-Kα radiation. Neutron diffraction data were collected using the high resolution powder diffractometer Е9 at Helmholtz–Zentrum for Materials and Energy (Berlin) with the wavelength of 1.7982 Å. The 176 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al. experimental data were analysed by the Rietveld method using the FullProf software package (T. Roisnel 2001). Magnetization and magnetoresistance measurements by four-probe method were made using physical properties measurement system (PPMS, Cryogenic Ltd) in magnetic fields up to 14 Т. Ultrasonically deposited indium was used for formation of contacts.

3. RESULTS AND DISCUSSION

3.1. Crystal Structure, Magnetic and Magnetoresistive Properties of the Ni Doped Compounds

Analysis of the XRD patterns recorded for the La1-xSrx(Mn0.5Ni0.5)O3 compounds with 0 ≤ x ≤ 0.15 did not reveal any impurity phases while the XRD data of the compound with x = 0.2 testify an existence of the admixture oxide NiO (~3%). The unit cell parameters of the compounds with x ≤ 0.2 have been calculated assuming the rhombohedral crystal structure (space group R-3c). The structural data testify a decrease of the unit cell volume with increase of the Sr2+ content. This modification of the crystal structure proves an increase in the oxidation state of the nickel ions as the ionic radius of the La3+ ions is significantly less than that of Sr2+ one and the ionic radius of the Ni3+ is notably smaller than that attributed to Ni2+ ion.

Figure 1. Temperature dependencies of magnetization obtained in FC and ZFC modes for La1- xSrx(Mn0.5Ni0.5)O3 compounds with x = 0 and x = 0.1.

Figure 1 shows the temperature dependencies of the magnetization measured in FC and ZFC modes for the compounds with x = 0, 0.1 in magnetic fields less than 100 Oe. The magnetization data testify a ferromagnetic behavior of the compound LaMn0.5Ni0.5O3 with Curie temperature of about 270 K. Surprisingly, the critical temperature of the magnetic transition remains nearly stable with an increase in strontium content (Figure 1). The observed behavior contradicts to the decrease of the spontaneous magnetization observed upon an increase of the Sr2+ content. Spontaneous magnetization is nearly nullified in the Magnetic Properties, Magnetoresistance and Functionality … 177 compounds with x ≥ 0.1 (Figure 2). The unsaturated magnetization denoted in the strong magnetic fields points at the competition between superexchange interactions of different signs which becomes dominantly antiferromagnetic upon chemical substitution of the La3+ ions by the Sr2+ ones. The conductivity measurements testify a semiconductor behavior of all the compounds under study with resistivity of about ~107-108 Ohm·cm at low temperatures. We didn’t observe any anomalies on the conductivity dependencies near the temperature transition to the magnetically ordered state. It should be noted that the magnitude of the magnetoresistance increases upon temperature decrease and diminishes with an increase of strontium content above x ≥ 0.1 (Figure 3). There is no magnetoresistance effect at temperatures near the Curie point and we didn’t observe a tunnel magnetoresistance effect in the whole measured temperature range unlike the results presented in the Ref (Guo et al. 2013). The observed magnetoresistance behavior can be explained assuming a weak spin polarization of the charge carriers and magnetoresistance effect is most probably associated with the magnetic and structural disorder and strong 3d-2p hybridization of the related orbitals of the manganese and oxygen ions which is strengthened in magnetic field.

Table 1. Structural and magnetic parameters (lattice parameters, fractional atomic coordinates and magnetic moments) and reliability factors calculated for La0.9Sr0.1Mn0.5Ni0.5O3 sample

Temperature (K) 4 300 Space group R-3 R-3 a (Å) 5.4911(1) 5.4950(1) c (Å) 13.2014(2) 13.2437(2) V (Å3) 344.728(6) 346.317(7) La/Sr: x 0 0 y 0 0 z 0.2509(3) 0.2519(4) Biso (Å2) 0.12(4) 0.55(4) *Mn: (x y z) (0 0 ½) (0 0 ½) Biso (Å2) 0.43(7) 0.53(7) *Ni: (x y z) (0 0 0) (0 0 0) Biso (Å2) 0.43(7) 0.53(7) O: x 0.5501(5) 0.5468(5) y -0.0018(9) -0.0049(8) z 0.2506(3) 0.2508(3) Biso (Å2) 0.44(3) 0.81(3)

μF (μB) 0.9 –

μAF (μB) 0.4 – 2 χ /Rwp(%) 2.30/4.12 2.65/4.37

RBragg(%)/Rmag(%) 2.68/47.4 5.47/ – (∗) The refined ionic occupation gives an amount of antisite defects to be around of 34%.

178 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

Figure 2. Field dependencies of magnetization obtained for the compounds La1-xSrx(Mn0.5Ni0.5)O3 with x < 0.2.

Figure 3. Magnetoresistance vs. magnetic field at different temperatures for La1-xSrxMn0.5Ni0.5O3 (x = 0; 0.15) samples.

Figure 4. The neutron powder diffraction patterns recorded at 300 K and 4K for La0.9Sr0.1Mn0.5Ni0.5O3 sample. The black line and red points denote the calculated and observed profiles, respectively. The bottom blue line represents their difference. The row of vertical ticks marks the Bragg reflections of the structural and magnetic phases. Magnetic Properties, Magnetoresistance and Functionality … 179

Neutron diffraction measurements for the compounds with x = 0.1 and 0.2 have been performed in the temperature range 2 – 300 K (Figure 4). The diffraction patterns recorded at 300 K have been successfully refined considering a monoclinic type of the unit cell distortion. The structural refinement has been performed with space group R-3 which assumes a chess- like ionic order of the Mn and Ni cations. At temperatures lower than 25 K one can denote a significant increase in the intensity of the diffraction peak indexed as (101) which involves both structural and magnetic components of neutron scattering. An increase in the intensity of the mentioned peak upon temperature decrease points at a formation of the antiferromagnetic order of G-type which onset nearly coincides with the anomaly on the magnetization dependence at high temperature and an appearance of the spontaneous magnetization. It should be noted that the discussed reflection is significantly broader than other reflections thus indicating the inhomogeneous distribution of the Ni and Mn ions itself as well as their magnetic moments. The main parameters of crystal and magnetic structures are denoted in the Table 1. Low values of the reliability factors calculated during the refinement (see Table 1) testify a good correlation between the observed and the calculated parameters which confirms an oxygen stoichiometry and in turn the oxidation states of the Ni and Mn ions to be thus 3+ and 4+ respectively. The calculated value of the magnetic moment at 2 K is about 1.5 µВ which is lower than that estimated in the case of the homogeneous magnetic structure. The structural parameters calculated for the compound with x = 0.1 are quite close to those calculated for the compound x = 0.2 while the unit cell volume of the latter compound is slightly smaller while the magnetic moment is slightly larger: 1.7 µВ. In order to clarify an evolution of the magnetic properties of the compounds as a function of the chemical substitution of La3+ ions by the Sr2+ ions one should know the sign and the strength of the superexchange interactions between 3d ions. According to the Goodenough- Kanamori rule the interaction between Mn4+-O-Mn4+ ions always has negative sign as well as that between Ni2+-O-Ni2+ ions (Goodenough 1955). Therefore, it is considered that ferromagnetism of the compound LaMn0.5Ni0.5O3 is governed by positive exchange interaction between Ni2+ and Mn4+ ions. While the magnetic moment estimated for the present compound with x = 0 is significantly lower than that calculated assuming pure ionic model. The most reasonable argumentation of the obtained experimental data assumes that the compound contains the structural regions of well-ordered Ni2+ and Mn4+ ions. These regions are characterized by a maximal number of positive exchange interactions which governs high Curie point and magnetization. Other structural regions are considered to possess mainly Ni3+ and Mn3 ions which are disordered in the lattice and antiferromagnetic interactions become to be dominant thus decreasing the magnetization and a transition temperature. The chemical substitution by Sr2+ ions transforms all manganese ions into the 4+ oxidation state while the structural order remains stable in spite of the presence of Ni3+ ions and most probably negative interaction between Ni3+ and Mn4+ ions. In the case than the positive component of the superexchange interaction is nearly equal to the negative one it is possible than the critical temperatures of the transitions to the paramagnetic state of these magnetic phases is nearly coincide and the structural order remains to be stable.

180 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

3.2. Magnetic and Magnetotransport Properties of La1-xSrxMn0.5Сo0.5O3 Perovskites

The X-ray diffraction peaks of the x = 0 sample at room temperature can be indexed within the orthorhombic space group Pnma or monoclinic P21/n. The monoclinic space group P21/n has been used in the previous NPD studies (Troyanchuk et al. 2000a; Barón-González et al. 2011) for the compounds with ordered Co2+ and Mn4+ ions whereas space group Pnma is associated with structural disorder of the Co and Mn ions. The crystal structure of the compositions 0.05 ≤ x ≤ 0.6 have been refined within the rhombohedral space group R-3c, whereas the x = 0.65 and x = 0.75 compositions have a cubic crystal structure (space group Pm3m ). The unit cell volume gradually decreases with increasing of Sr content thus indicating an increase in the oxidation state of the cobalt ions as the ionic radius of the Sr2+ ions is significantly larger than that of La3+ one and the oxidative state 5+ attributed to the Mn ions is not stable.

Figure 5. Temperature dependencies of magnetization for La1-xSrxMn0.5Сo0.5O3 samples. Magnetic Properties, Magnetoresistance and Functionality … 181

Figure 6. Field dependencies of magnetization for La1-xSrxMn0.5Сo0.5O3 samples measured at 5 K. Arrows indicate a direction of magnetic field.

Figure 5 shows the temperature dependencies of the FC (field cooling) and ZFC (zero field cooling, for x = 0.75 compound) magnetizations recorded in a small magnetic field for 0 ≤ x ≤ 0.75 compositions. The parent compound LaMn0.5Co0.5O3 is ferromagnetic with TC = 215 K (Figure 5). The arrow indicates a magnetic anomaly on the M(T) dependence at ~150 K (Figure 1), apparently this composition consists of domains with disordered Co and Mn ions (TC = 150 K) and partially ordered Co/Mn domains (TC = 215 K). The Curie point drops down to 147 K for the x = 0.05 composition. Further increasing of the Sr content in La1-xSrxMn0.5Сo0.5O3 system gradually increases the onset of the transition into the paramagnetic state up to 260 K for the x = 0.75 sample (Figure 5). At the same time an increase in the Sr content gradually lowers the spontaneous magnetization (Figure 6).

Figure 7. Hysteresis loops of x = 0.3 sample at 5, 100 and 200 К (left panel) and x = 0.4 and x = 0.5 samples at 5K (right panel). 182 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

Figure 8. Temperature dependencies of resistivity for La1-xSrxMn0.5Сo0.5O3 (x = 0; 0.05; 0.1; 0.4) samples.

The transition into the paramagnetic state remains sharp up to the concentration x = 0.6 whereas the x = 0.75 sample shows a diffuse ferromagnet-paramagnet transition thus indicating magnetically inhomogeneous state. The magnetic hysteresis loops recorded for the compositions with x = 0.3, x = 0.4 and x = 0.5 reveal a very large coercive field at low temperature (Figure 7). Magnetic moments of the sample with x = 0.3 start to reorient in a field of 3.3 T at 5 K (Figure 7). Increasing the temperature leads to a strong decrease of the coercive field. The observed coercive field at low temperature is associated with a large magnetic anisotropy due to a non-frozen orbital moment of the cobalt ions. Resistivity vs. temperature dependencies presented in the Figure 8 show strongly insulating state of the compounds 0 ≤ x ≤ 0.4. The resistivity values of 108 – 109 Ohm*cm registered below 100 K are close to the upper measuring limit of the PPMS set-up (Figure 8). No anomalous behavior of the resistivity has been observed around the Curie point. The chemical substitution by Sr ions leads to a slight decrease of the resistivity. Apparently the resistivity is mainly determined by the cobalt atoms adopting different oxidation states whereas the oxidation state of Mn4+ ions is stable and does not play an important role. The magnetoresistance gradually decreases with decreasing temperature without any anomalies. The magnetoresistance values defined as MR = [ρ(H)-ρ(0)]/ρ(0)·100% are plotted as a function of temperature and magnetic field in the Figure 9. Large values of the magnetoresistance are found for the compounds with x ≤ 0.15 below the Curie point, whereas the x = 0.4 compound which possesses only a small magnetization and a relatively high Curie point demonstrates a magnetoresistance at low temperature typical for superparamagnetic manganites above Curie points. It should be noted that the decrease in the magnetoresistance value of about one order of magnitude in a field of 14 T (x ≤ 0.15 compositions) points at a strong correlation between magnetic order and transport properties. Note that colossal magnetoresistance has been observed in homovalent manganese oxides with pyrochlore structure (Subramanian et al. 1996).

Magnetic Properties, Magnetoresistance and Functionality … 183

Figure 9. Magnetoresistance vs. magnetic field at different temperatures for La1-xSrxMn0.5Сo0.5O3 (x = 0; 0.05; 0.1; 0.4) samples.

The NPD patterns of the compositions x = 0.15, 0.5, 0.6 and 0.75 were obtained in the temperature range 2 – 300 K. The NPD patterns of the x = 0.15 and x = 0.5 samples recorded at 2 K and 300 K are shown in the Figure 10. The NPD peaks can be indexed within the rhombohedral space group R3c in the temperature range 2K – 300 K. The results of the crystal and magnetic structure refinement at low temperature are shown in the Table 2. There is no visible evidence for a structural ordering of Co and Mn ions. The composition with x = 0.15 exhibits a ferromagnetic component with magnetic moment of 0.9 μB at 2 K (Table 2). We could not reveal any coherent magnetic contribution in the NPD patterns of the x = 0.5 and x = 0.6 samples at 2 K. This indicates on the paramagnetic or a spin glass state of these compounds but not a long-range magnetically ordered one. The refined oxygen content of La0.5Sr0.5Co0.5Mn0.5O3 corresponds to the stoichiometric value. The neutron diffraction data show that the crystal structure of the x = 0.75 sample is cubic (space group Pm3m ) in the temperature range 10 - 300 K. In contrast to the x = 0.5 composition this compound shows significant oxygen deficiency, viz. the refined oxygen content is 2.85. Apparently the oxygen deficiency leads to a diffuse transition into the paramagnetic state (Figure 5c). No ferromagnetic contribution to the patterns has been detected, however, a very weak contribution from a G-type antiferromagnetic state was observed (Figure 11). The antiferromagnetic contribution was observed below 225 K and appears apparently simultaneously with a spontaneous magnetization. The refined magnetic moment at 10 K is about 0.5 μB per formula unit. One can suggest that this antiferromagnetic component is associated with the oxygen deficiency.

184 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

Table 2. Structural parameters of La1-xSrxCo0.5Mn0.5O3 obtained from the NPD data at low temperature

x 0.15 0.5 0.6 0.75 T (K) 2 2 2 10 Space group R-3c R-3c R-3c Pm-3m a, b (Å) 5.483(1) 5.441(1) 5.427(7) 3.834(1) c (Å) 13.209(7) 13.238(6) 13.242(5) 3.834(1) V (Å3) 344.03(1) 339.41(6) 337.85(4) 56.34(1) La/Sr: (0;0;0,25) (0.5;0.5;0.5) Biso (Å2) 0.068(2) 0.025(6) 0.001(5) 0.092(2) Co/Mn: (0;0;0) Biso (Å2) 0.173(1) 0.111(3) 0.058(1) 0.165(1) O x 0.451(1) 0.469(1) 0.475(1) 0 y 0 0 0 z 0.25 0.25 0.25 0.5 Biso (Å2) 0.279(2) 0.236(4) 0.376(3) 0.451(2) Oxygen content 3.00(4) 3.00(6) 2.94(6) 2.83(5) μy, μB 0.9 ±0.5 Rp/Rwp (%) 3.08/4.18 3.05/3.71 3.69/4.46 4.90/6.69 RBragg (%) 2.21 2.85 4.74 4.07 Magnetic R-factor 15.4 17.3 χ2 2.61 1.11 0.809 6.21

Figure 10. The neutron powder diffraction patterns recorded at 300 K and 2K for La0.85Sr0.15Co0.5Mn0.5O3 (left panel) and La0.5Sr0.5Co0.5Mn0.5O3 samples (E9 instrument). The black line and red points denote the calculated and observed profiles, respectively. The bottom blue line represents their difference. The row of vertical ticks marks the Bragg reflections of the structural phase. Inset shows regions of neutron diffraction pattern recorded at 2K (circles) and 300K (squares). Magnetic Properties, Magnetoresistance and Functionality … 185

Figure 11. Temperature evolution of (1/2; 1/2; 1/2) antiferromagnetic peak for x = 0.75 composition (D20 instrument).

In order to estimate the sign and the magnitude of the magnetic interactions it is necessary to know the oxidation and spin states of the Co ions in the Co-doped manganites. It 2+ 4+ is known that in LnCo0.5Mn0.5O3 the oxidation states of Co and Mn are the dominant ones (Troyanchuk et al. 2000a; Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, Chen, et al. 2008; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, Tristan, et al. 2008; Barón-González et al. 2011). So in the manganites doped with cobalt ions the heterovalent states Co2+ and Mn4+ should be more stable than the homovalent state of the pair Co3+ and Mn3+. In fact, the results of X-ray spectroscopy measurements have revealed 2+ the presence of Co ions in the strongly doped manganite Pr0.5Ca0.5(Mn1-xCox)O3 having metallic type conductivity at quite small levels of the Co doping (Toulemonde, Studer, and Raveau 2001). The magnetic interactions between Co2+ and Mn4+ ions are ferromagnetic with a magnitude comparable to the magnetic interaction between Mn ions in the mixed valence manganites (Troyanchuk et al. 2000b; Burnus, Hu, Hsieh, Joly, Joy, Haverkort, Wu, Tanaka, Lin, and Chen 2008; Vasiliev, Volkova, Lobanovskii, Troyanchuk, Hu, Tjeng, Khomskii, Lin, Chen, and Tristan 2008). According to the Goodenough-Kanamori rules (Goodenough 1955) the superexchange interaction between Co2+ and Mn3+ ions as well as that between Co2+ – Co2+ ions or Mn4+ – 4+ Mn should be antiferromagnetic. The LaCo0.5Mn0.5O3 parent composition studied in the present work has a Curie point at around 215 K (Figure 5). This indicates that Co2+ and Mn4+ ions are partially ordered in the lattice because the disordered compound LaCo0.5Mn0.5O3 has a much lower Curie point of around 150 K (Barón-González et al. 2011). The neutron diffraction studies have shown the composition La0.5Sr0.5Co0.5Mn0.5O3 to be oxygen stoichiometric (Table 2). Therefore the Co ions have to be in 3+ oxidation state and the Mn ones in the 4+ state as the 5+ oxidation state of manganese ions is not stable. This means that 3+ 2+ 3+ the substitution of La ions by Sr ones in La1-xSrxCo0.5Mn0.5O3 creates Co ions while the manganese oxidation state remains practically unchanged. The Curie points of the x = 0.05, 0.12 and 0.15 compositions are around 150 K (Figure 5). 186 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al.

The decrease in the Curie point relative to that of the parent compound can be associated with cobalt and manganese ions being disordered. In fact the value of the Curie point is comparable to that attributed to the LaCo0.5Mn0.5O3 compound where the Co and Mn ions are disordered. Further increase of the Sr content (x = 0.25) sharply decreases the spontaneous magnetization (Figure 5). This indicates that some clusters with the Co and Mn are in spin glass, antiferromagnetic or paramagnetic states. The paramagnetic state can appear when a 3+ 6 major part of the Co ions adopts a low spin diamagnetic trivalent state (S = 0; t2g ) and the paramagnetic state is associated with the Mn4+ ions. In this case the value of the Curie point should decrease sharply due to diamagnetic dilution. However, starting from the composition х = 0.25 the Curie point suddenly increases in spite of the decrease of the spontaneous magnetization (Figure 5). The super-exchange interactions between pairs of Mn4+ or pairs of Co3+ ions are definitely antiferromagnetic therefore the spontaneous magnetization should be strengthened due to the super-exchange interactions between Co3+ and Mn4+ ions. Hence, one must conclude that a fraction of the Co ions (Co3+) is not in a diamagnetic low-spin state but probably (assuming a covalency of the 1 5 2 4 chemical bonds) in the intermediate (S = 1; egt2g ) or high-spin one (S = 2; egt2g ). The following arguments favor the high-spin state scenario to explain the ferromagnetic interactions:

3+ 2 4 2+ 1. The electron configurations of the high-spin ions Co (S = 2; egt2g ) and Co (S = 2 5 3/2; egt2g ) are very similar: both ions contain two unpaired eg-electrons which are strongly bonded with oxygen 2p orbitals. This indicates that the superexchange interaction between high-spin Co3+ and Mn4+ ions can be ferromagnetic as in the case 2+ 4+ of the high spin Co and Mn ions attributed to the parent LaMn0.5Co0.5O3. 2. According to the Goodenough-Kanamori rules the electron configuration of Co3+ 1 5 ions in the intermediate spin state (S = 1; egt2g ) allows a positive sign of the exchange interaction between Co3+ and Mn4+. However, a stabilization of the intermediate spin state of Co ion assumes a significant component of covalent bonds which is considered to be a collective phenomenon (Korotin et al. 1996). At the same time a strong enhancement of the Curie point can only be reached in the case of a chess-like structural ordering of Co3+ and Mn4+ ions similar to the parent compound, which is incompatible with the model assuming collective character of a stabilization of the intermediate spin state of the Co3+ ions because every Co3+ ion located in the ordered domain would be predominantly surrounded by Mn4+ ions. 3. The Co3+ ions being in the different spin states have significantly different ionic radii and therefore the high spin state can be stabilized by a relaxation of the stresses within the crystallines or near their surface as it was observed in the epitaxial thin films of LaCoO3 (Kwon et al. 2014) where magnetically ordered domains are associated with oxygen stoichiometric stripes induced by strains. The Co3+ ions in the high spin state have an ionic radius much larger than that of the Mn4+ ion. The large difference in ionic radii between Co3+ (in the high spin state) and Mn4+ favors to the ionic ordering. Based on the magnetization results obtained for the La1- Magnetic Properties, Magnetoresistance and Functionality … 187

xSrxMn0.5Сo0.5O3 (x ≤ 0.75) (Figures 6, 7) one can suggest that the majority of the Co3+ ions are in the diamagnetic low-spin state while a minority of Co3+ ions adopt the high-spin state. This suggestion is in a good accordance with the present results of the neutron diffraction studies of x = 0.5 and x = 0.6 samples for which no coherent magnetic scattering was revealed.

The magnetic phase diagram for La1-xSrxCo0.5Mn0.5O3 is presented in the Figure 12. There are four different regions between which the magnetic behavior changes strongly. The region with x < 0.05 is associated with the domains containing ordered or disordered Co2+ and Mn4+ ions. The concentration range 0.05 ≤ x ≤ 0.2 is associated with structural disordering and a coexistence of ferromagnetism and a spin glass state. Above the concentration level of 20% the Curie point increases strongly in spite of the magnetization decreasing. We believe that such behavior is associated with the emergence of Co3+ ions and the tendency to ionic ordering of Co3+ (in the high spin state) – Mn4+ in some strained domains. An oxygen deficiency estimated for the compounds with x > 0.5 leads to the appearance of both antiferromagnetic and ferromagnetic long-range order in separated domains while the antiferromagnetic component is dominating as the ferromagnetic component has not be registered in the NPD patterns of the compounds with x = 0.75.

Figure 12. Preliminary magnetic phase diagram proposed for La1-xSrxMn0.5Сo0.5O3 system.

CONCLUSION

The structural studies of the system La1-xSrxMn0.5Сo0.5O3 performed by neutron powder diffraction method have showed shown that up to the concentration of x = 0.5 the compounds remain to be stoichiometric which indicates that the chemical substitution by Sr2+ ions leads to a change of the average oxidation state of cobalt ions from 2+ up to 3+ one. The Curie point decreases from 215 K for the compound with x = 0 down to 147 K for the x = 0.05 then it 188 I. O. Troyanchuk, D. V. Karpinsky, M. V. Silibin et al. increases again up to 260 K (x = 0.75). In spite of the high Curie point no magnetic contribution is found into the diffraction patterns of the x = 0.5 and 0.6 samples. The magnetoresistance is large at low temperature and decreases with an increase of temperature and strontium content above 15%. The Co3+ ions are suggested to be mainly in the diamagnetic low-spin state and only a small part of them is in the high-spin state. A small ferromagnetic component observed in the compounds with x ≥ 0.25 with high Curie point is most probably caused by positive superexchange interactions between Co3+ (being in the high-spin state) and Mn4+ species which are partially ordered in some domains. The strength of these interactions should be close to that attributed to the positive superexchange 2+ 4+ interactions between Co and Mn ions in the compound LaCo0.5Mn0.5O3 as the number of 2+ 3+ unpaired eg-electrons is equal in the Co and Co ions being in the high spin state. The contribution from the eg-electrons determines both the sign and the value of the superexchange interactions. It is suggested that the high spin state of Co3+ ions is stabilized by stresses within the crystallines and in the surface layer. An increase of strontium content in the Ni containing system leads to a modification of dominant magnetic state from ferromagnetic to antiferromagnetic one. The Curie temperature of the pristine compound nearly coincides with that estimated for the doped compounds. Magnetoresistance effect decreases upon temperature increase and Sr2+ content. The obtained results are discussed in the model which assumes that positive and negative components of the superexchange interaction Ni2+-O-Mn4+ are nearly equal to each other, the interaction between Ni3+- Mn4+ ions is negative. Large negative magnetoresistance effect is associated with structural and magnetic disorder as well as relatively large 3d-О2p hybridization which strengthens in a magnetic field. The obtained results describe the magnetic and magnetoresistive properties of the manganites as a function of temperature, structural peculiarities and electron configuration of the transition metal ions; it is shown that chemical substitution by strontium ions in the selected manganites can be used to control their magnetic and magnetoresistive properties and thus to manage their functionality.

ACKNOWLEDGMENTS

This work was supported by the Russian Science Foundation (project #15-19-20038). Neutron diffraction experiments have been supported by the European Commission under the 7th Framework Programme through the ‘Research Infrastructure’ action of the ‘Capacities’ Programme, NMI3-II Grant number 283883.

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Chapter 6

PIEZORESPONSE FORCE MICROSCOPY OF P(VDF-TRFE)- GRAPHENE OXIDE FILMS

M. V. Silibin1, V. S. Bystrov1,2, D. V. Karpinsky1,3, N. Nasani4, G. Goncalves4, A. V. Sysa1, A. V. Solnyshkin1,5, P. A. A. P. Marques4, Budhendra Singh4 and I. K. Bdikin1,4,* 1National Research University of Electronic Technology “MIET,” Moscow, Russia 2Inst. Mathematical Problems of Biology, Keldysh Inst. Appl. Math. RAS, Pushchino, Moscow region, Russia 3Scientific-Practical Materials Research Centre of NAS of Belarus, Minsk, Belarus 4TEMA-NRD, Mechanical Engineering Department and Aveiro Institute of Nanotechnology (AIN), University of Aveiro, Aveiro, Portugal 5Tver State University, Tver, Russia

ABSTRACT

Spin coating technique was employed to obtain thin films of Poly(vinylidene fluoride-trifluoroethylene)-Graphene oxide (P(VDF-TrFE) - GO) composite with different concentration of GO (0, 1 and 2 wt%) with a nominal thickness in the range of 0.50-0.55 m. Experimental and theoretical investigation of the piezoelectric/ ferroelectric behavior of P(VDF-TrFE)-GO composite were done. Piezoresponse force microscopy (PFM) study showed a significant change in the switching behavior and piezoelectric response of the films with increasing concentration of GO in the composite films. A detailed computational molecular modeling for all the three compositions was undertaken to trace the polarization behavior for these composites and to understand the underlying phenomena. Molecular modeling results were found to correspond to the results of nanoscale PFM measurements.

Keywords: PVDF-TrFE, piezoresponse, graphene oxide (GO), piezocofficient

* Corresponding Author address: Email: [email protected]. 192 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky et al.

INTRODUCTION

In the recent past much attention among the researchers are drawn for piezo active organic materials and/or polymers (Tayi et al. 2012; Zhang et al. 2015; Zhao, Katsouras, et al. 2014; Chen and Zeng 2014; Heredia et al. 2012; Kholkin et al. 2010). In particular, copolymers of poly (vinylidene fluoride) and poly trifluoroethylene wiz. P(VDF-TrFE) are promising materials, which find applications as sensors, transducers, actuators, etc. These polymers are readily available in the form of fibers or films (Bae et al. 2013; Md Ataur et al. 2013; Heredia et al. 2010; Sencadas et al. 2012). Using standard equipment’s, it’s possible to produce various microstructures with predefined shapes. Recently, many organic materials including carbon materials such as carbon nanotubes and single-layer carbon sheet structure (graphene) have been investigated to improve their mechanical and electrical properties (Geim 2009; Wang et al. 2012). Furthermore, a comparison of mechanical properties of pure PVDF membranes and PVDF-GO composite membranes indicated a significant improvement in the tensile strength and Young's modulus, which reached a value of 10.33 and 148.47 MPa, respectively. This corresponds to a 55.11% and 67.14% increase in its tensile strength and Young’s modulus, respectively (Zhao, Xu, Chen, and Yang 2014). In addition, the PVDF-GO composite membrane has also demonstrated sustained permeability, lower cleaning frequency, and filtration time than that of pure PVDF membrane (Zhao, Xu, Chen, Wang, et al. 2014). Furthermore, it is suggested that for PVDF-GO composite with 0.5 wt% of GO, an enhanced surface hydrophilicity and anti-fouling performance by the synergistic effects due to the formation of hydrogen bonds between GO and PVDF can be obtained (Chang et al. 2014). In addition, the atomic layer of sp2-hybridized carbon in two-dimensional hexagonal lattice of graphene exhibits a unique electrical and mechanical property and is a key element for terahertz and microwave applications (Talbot, Konn, and Brosseau 2002; Nan et al. 2008), because of the mechanism of activation of wave processes in these materials (Chen et al. 2014). The combination of the unique electrical properties of graphene (Castro Neto et al. 2009) with abnormal polarizability and pronounced anisotropy of piezoelectric properties of PVDF can lead to a drastic change in the properties of PVDF- graphene nanocomposites (Layek et al. 2010). The addition of graphene in a ferroelectric polymer matrix induces changes in its ferroelectric and mechanical properties (Kuilla et al. 2010; Ataur Rahman and Chung 2013; Adohi et al. 2014; Jiang et al. 2014). Furthermore, with a uniform dispersion of graphene layer in polymer matrix gives a high dielectric permittivity of the PVDF-graphene composite when compared to pure PVDF (Shang et al. 2012). In addition, the charge/discharge results for PVDF-graphene composite showed an enhanced electrochemical behavior along with a higher specific electric capacitance (311 F/g versus 249 F/g) (Kim and Park 2012). Furthermore, PVDF-graphene nanocomposites have shown enhanced mechanical behavior as well as higher thermal stability when compared to pure PVDF (Jaleh and Jabbari 2014; El Achaby et al. 2012; Silibin et al. 2017). Polymer/graphene composites and polymer/carbon nanotube (CNT) composites have been studied by molecular mechanics (MM) and molecular dynamics (MD) methods (Lv et al. 2010). However, there are only a few reports available on computational works which focus on the interfacial information and properties of polymer-GO composites (Ding et al. 2012). In this chapter we have presented the results of polarization measurements and electrical characteristics of P(VDF-TrFE))-GO composites as a function of external electric field and Piezoresponse Force Microscopy of P(VDF-TrFE)- Graphene Oxide Films 193

GO doping concentration, which allowed to estimate dipole ordering of the polymer matrix in the presence of graphene oxide inclusions.

EXPERIMENTAL AND COMPUTATIONAL DETAILS

Graphene oxide (GO) was prepared by the chemical exfoliation of graphite (Graphite powder, < 45 μm, ≥ 99.99%, Sigma-Aldrich) (Goncalves et al. 2009). Briefly, it consists on the reaction of graphite flakes with concentrated H2SO4 and KMnO4 in order to obtain individual sheets in an oxidized state. The resultant suspension was extensively washed with distilled water by ﬁltration and centrifugation and finally subjected to dialyses to remove ionic contaminants. The resulting GO was dried by lyophilization to avoid aggregation. For the composite sample preparation, poly (vinylidene fluoride-trifluoroethylene) copolymer with a TrFE content of 30% was used. The P(VDF-TrFE) copolymer was chosen because it favours crystallization from a solution or melt directly to the ferroelectric -phase without additional stretching in comparison with the pure PVDF polymer. 4 g weight of the copolymer powder was added to 100 ml of a dimethyl sulfoxide (DMSO) and acetone mixture in 80/20 ratio. The powder was dissolved within 2 hours at a temperature of 100 C. The solution was then carefully filtered to remove impurities. Dried graphene oxide was added to the solution at various concentrations of GO and then stirred magnetically for 30 min at 50C. For a homogeneous distribution of GO in the solution, this mixture was subjected to ultrasonic vibration for 1 h and stirred again for 3 h at 50C. The resulting composition was used to produce the thin composite films on the base of copolymer P(VDF- TrFE) as the matrix. Composite films with different concentration of GO (0-2 wt%) were prepared by spin coating technique using P(VDF-TrFE)-GO solutions with film thickness of 500-550 nm. Glass with conductive coating of InSbO4 was chosen as a substrate for the films deposition. Before the sample fabrication, the substrates were carefully cleaned with acetone and then dried. In this chapter, several versions of molecular models for PVDF (β−phase) - graphene oxide ferroelectrics systems were developed and investigated using HyperChem 7.52 as well as 8.0. Various computational methods were used, including molecular mechanics (MM) methods (such as BIO CHARM), quantum mechanical (QM) self-consistent field (SCF) Hartree-Fock (HF) calculations based on density functional theory (DFT), as well as semi- empirical methods (such as PM3), in both restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) approximations. The main approach of both the MM and QM methods used for molecular modeling is to obtain the minimum of the total, or potential energy surface (PES), of a studied molecular system. The optimization of molecular geometry is executed using the Polak–Ribere (conjugate gradient) algorithm, which determines an optimized geometry at the minimum energy point (using PES) (HyperChem(TM) Professional 7.51 2002). Local piezoelectric properties of the films were visualized simultaneously by using Atomic Force Microscopy (AFM) in contact mode and piezoresponse force microscopy (PFM) methods (Roelofs et al. 2002; Balke et al. 2009). AFM and PFM measurements were carried out using a Veeco AFM Multimode Nanoscope (IV) MMAFM-2, Veeco microscopy. The PFM technique is based on the converse piezoelectric effect, which is a linear coupling 194 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky et al. between the electrical and mechanical properties of a material. Since all ferroelectrics exhibit piezoelectricity, an electric field applied to a ferroelectric sample results in changes of its dimensions. To detect the polarization orientation the AFM tip is used as a top electrode, which is moved over the sample surface (Figure 1). Piezoresponse force microscopy relies on the linear strain response as a function of the external electric field. Regardless of this linear response that is inherent to materials without center of inversion symmetry, all materials dispose of a quadratic strain response in the electric field, known as electrostriction. Electrostriction therefore provides a local electromechanical response at the second harmonic of the AC voltage in piezoresponse force microscopy. This electrostrictive response provides valuable insight into the material properties in itself. It becomes particularly interesting in systems where the symmetry of the crystal does not allow for particular modes of deflection of the AFM, especially for lateral deflections. In this scenario, this type of microscopy provides a powerful imaging tool for local material defects like misfit dislocations. Special attention devoted to the role of lateral symmetry in ferroelectric thin films as this turns out to be the key to a deconvolution of many image features equally at first and second harmonic. For the arbitrary oriented sample the components of the piezoelectric tensor in the laboratory coordinate system, dij, are linear combinations of the tensor components in the * coordinate system related to the principal crystal axes d ijk (,, )  ail a jmakn d lmn (Nye 1985), where , , and  are the Euler angles (Bader and Nguyen-Dang 1981) and aij are the elements of the Euler matrix that describes the rotation defined by the Euler angles. In principle (for electroded unclamped grain), the measured piezoresponse should be determined by the relevant piezoelectric coefficient (d33, d31 and d15) and by the orientation of the individual grain relative to the substrate plane. For c-oriented grains the piezoelectric strain  is simply related to the longitudinal (d33 coefficient) and differs by the phase (0 or 180 degrees between the applied filed and measured displacement): 3 = d33E3 for (hkl) grain and

-3= d33(-E3) for (-h-k-l) grain. 0 degree of the phase corresponds to “positive” orientation of the grains and positive piezoresponse signal when electrical field is positive. Same 180 degree of the phase corresponds to “negative” orientation of the grains (Figure 1).

Figure 1. Schematic diagram of experiments for piezoelectric measurements.

Piezoresponse Force Microscopy of P(VDF-TrFE)- Graphene Oxide Films 195

EXPERIMENTAL RESULTS

To evaluate the polarization switching process of the P(VDF-TrFE)-graphene oxide thin films, piezoresponse hysteresis loops were locally acquired when the PFM tip was stopped near selected location (graphene oxide grain) and the voltage pulses of both polarities were sequentially applied between the tip and the bottom electrode. The loops are characteristic of a local switching process, where the contrast change is due to the integrated piezoresponse of nascent domain and background (unswitched) polarization. The measurements were done in the so-called pulse mode. The loop acquisition consists of applying a dc voltage, Vdc, for a short time, 1s, and measuring the PFM signal in between the pulses. The voltage was swept by 1 V increments in the range of -75V  Vdc  +75 V or -90V  Vdc  +90 V (Figure 2, 4).

Figure 2. Piezoresponse hysteresis loops of P(VDF-TrFE)-graphene oxide thin films with different concentration of GO. Uac = 3 V, 50 kHz.

Figure 3. AFM images of the PVDF/Graphene oxide films. (a, b) - 0% GO, (c, d) - 1% GO and (e, f) - 2% GO. (a), (c) and (e) images denote topography of the films. (b), (d) and (f) PFM images after poling -90V during 100s in the center point, marked by the arrow. 196 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky et al.

Figure 4. Piezoresponse hysteresis loops of P(VDF-TrFE)-graphene oxide thin films with different concentration of GO. Uac = 3 V, 50 kHz. (a) - 0% GO, (b) - 1% GO and (c) - 2% GO.

The switching PFM spectroscopy technique was used to measure nanoscale hysteresis loops for different concentration of GO. The results are shown in Figure 2. The loops look similar for all the films, demonstrating initial small piezoresonse and significant offset relative to the basic level. The maximum value of the piezoelectric response signal corresponds to an estimated value of piezoelectric coefficient d33eff. The obtained results have permitted to estimate the value depending on the concentration of the graphene oxide. It is seen that hysteresis loops of films with 2% GO have smaller coercivity which indicates easier polarization switching throughout the film. Figure 3 shows topography and piezoresponse the self-polarization (Pi) behavior as a function of GO concentration. Figure 3 (a), (c) and (d) demonstrate the morphology of P(VDF-TrFE)/GO as observed with AFM. In this chapter GO sheets were used as scaffolds for P(VDF-TrFE) grew on the surface. From AFM images, it is observed that GO particles supported on the surface of film have a size of about several microns. The topography of the films was checked for each composition. Roughness and average grain size increase with concentration of graphene oxide RMS: 11.8nm, 23.7nm 83.3nm for 0%, 1% and 2% GO, correspondently. High roughness of the films containing graphene oxide can be explained by the presence of relatively large grain size of graphene oxide and creating irregularities stress in the crystallization of the films. Considering that piezoelectric response can be described as d33 = 2Q11ε0εP, where Q11 is the electrostriction coefficient and ε0 is the dielectric permittivity of vacuum, the polarization P in our films can be qualitatively described as P ∝ d33/ε, where d33 is the maximum piezoresponse distribution. As the domain structure of the studied films does not change appreciably it is believed that the effective Q11 remains almost constant. The hysteresis loops show that all studied films may result in a polarized state. Piezoresponse images after point poling in the selected area show the emergence of the polar domain. It can be seen that the domain size is different for different films. Effective diameters of domains are 1.56, 0.96 and 1.70 m for 0%, 1% and 2% GO, correspondently. It should be noted that the domain for 2% GO film very often was not circle, and reflect border sharp of graphene oxide particles. Figure 3(f) shows that the domain is split onto two domains, although there was one point of polarization and it located in the center of the larger domain. The hysteresis loops for 0 and 1% GO are notably changed (Figure 4(a) and (b)). For 2% GO film the hysteresis loops vary greatly depending on the measured area (Figure 4(c)) due local distribution graphene oxide particles. For estimate mean piezocoefficient is necessary to analyze large area of the film or averaged over many hysteresis loops. The result of this Piezoresponse Force Microscopy of P(VDF-TrFE)- Graphene Oxide Films 197 statistical analysis shows that for 1% GO film piezoelectric coefficient decreases and for 2% of the film it increases again (Figure 5). For understanding the mechanism of piezoresponse evolution of the composite we have used our previous models of PVDF chain (Figure 6(a)) (Bystrov et al. 2007), its behavior in electrical field (Figure 6(b)) and computed the data for piezoelectric coefficients (Bystrov et al. 2013; Bystrov et al. 2017). The symmetrized models of graphene oxide based on graphene layer consisting of 96 carbon atoms: with oxygen and OH groups (Figure 7(a)) and with COOH groups (Figure 7(b)) arranged by hydrogen were used here for first trials.

Figure 5. GO concentration dependencies of the piezoelectric coefficient after poling.

Figure 6. Model of PVDF chain: (a) – PVDF and (b) - its central part.

It is possible to assume several simplest models for PVDF/Graphene oxide complex (Figure 8) and compute its piezoelectric coefficients by the same calculations algorithms as declared in Ref (Bystrov et al. 2013). We have constructed the models of PVDF/Graphene oxide composites: viz. three variants: 1) with H-side (hydrogen atom) connected from PVDF to graphene oxide, 2) with F-side (fluorine atom) connected from PVDF to graphene oxide (these both first variants show approximately the same values of piezoelectric coefficients) and 3) Graphene Oxide/ PVDF with both sides (sandwich type) as show in Figure 8. For 198 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky et al. deeper understanding, we also considered two versions of the mutual rotation of the graphene layers in relation with PVDF chain. The results are presented in the Table 1.

Figure 7. Graphene oxide layers models: (a) with OH group, (b) with COOH group.

Figure 8. Models of PVDF /graphene oxide structures: (a) with H-side, PVDF / graphene oxide, (b) a sandwich model for the graphene oxide/ PVDF/ graphene oxide.

An optimization strategy similar to that used in paper (Bystrov et al. 2013) was used: firstly the model without external applied electric field has been used, to find the initial optimal position of modeled composite structure (the distance between graphene oxide layer and PVDF chain was approximately d ~ 4 Å) and to determine the initial optimal parameters of PVDF chain heights in its central part (h1, and h2). After that we have applied electric field along Z direction (along main PVDF polarization vector) and tried to search optimal geometry for new atomic configuration under action of electric field. Then we have compared the changes of the main parameters (h1 and h2, Figure 6(b)) from initial optimal parameters, to determine the deformation Δh1 and Δh2, to calculate the corresponding values of voltage U and, finally, to compute the piezoelectric coefficient d33, using dielectric permittivity value of ε= 10 (for simplest case comparable to other data). Comparison with the initial data known for d33 testifies that under the influence of graphene oxide layer the piezoelectric coefficient d33 is decreased and now have three times lower value: d33 = - 14.6 pm/V (or pC/N) for the first models (Figure 7(a)) of GO eight OH groups as compared with the average value of the pure PVDF d33= - 38.5 pm/V (pC/N). It is important to note, that the sign of d33 is negative in Piezoresponse Force Microscopy of P(VDF-TrFE)- Graphene Oxide Films 199 all cases as in the initial pure PVDF case. In the case of double sides graphene oxide model (sandwich structure) the piezoelectric coefficient d33 is increased to the value of d33 = -29.8 pm/V (pC/N) for this model (see Table 1). The mentioned value is calculated for the case of the simplest graphene oxide structure and its corresponding orientation. For other graphene oxide compositions and various mutual orientations, the piezoelectric coefficient has somewhat different values, but the main tendency remains the same (e.g., the values noted in Table 1 which are computed for the models of GO with COOH groups (Figure 7(b)). One interesting peculiarity here is that the mutual orientation influences on the resulted values of the piezoelectric coefficients. The main obtained data and comparison with the data presented in the paper (Bystrov et al. 2013) are presented in the Table l. These are the first averaged modeling data, which are in line with above mentioned experimental PFM data, and it needs further more detailed investigations for different mutual orientations, number of layers and the order of the GO and PVDF. Experimental results qualitatively correlate with those obtained in the calculations (Figure 5). We can assume that experimental data obtained for the composite with 1% concentration of graphene oxide can be associated with the model constructed for the case of PVDF with graphene oxide from one side only. This leads to a reduction in the piezoresponse coefficient. Increasing of GO content most probably corresponds to the model assuming sandwich clusters in the composite. Experimentally, piezo signal is reduced for 1% of GO content due to the statistical disorientation of graphene oxide and PVDF layers, and uncontrolled thickness of the individual layers of graphene oxide and PVDF. Statistical disorder does not yield an exact match with the simulation. Even at low concentrations, there is the effect of molecular order, but for 2% GO composite the probability that the sandwich structures are formed is quite significant. In the case of the controlled hetero-structures one can assume much greater effect.

Table 1. The piezoelectric coefficients d33 calculated for different types of structures of Graphene Oxide with OH and COOH groups and PVDF (electric field Ez ~ 500 GV/m, for comparison the data from our paper (Bystrov et al. 2013) were taken)

200 M. V. Silibin, V. S. Bystrov, D. V. Karpinsky et al.

CONCLUSION

The P(VDF-TrFE)/GO ferroelectric films prepared by the spin coating technique were investigated by the PFM technique. The writing of polarization domain and local hysteresis loops was done by biasing the AFM tip. Molecular modeling results were found to compliment the experimental PFM measurements. The switching behavior of the obtained films was analyzed from the distribution of local piezoresponse signal after poling. The molecular modelling for different ordering of P(VDF-TrFE) and GO suggested mainly two types of molecular ordering in present composite wiz. 1) two-layer simple architecture of P(VDF-TrFE)/GO and 2) three layer sandwiched architecture GO/P(VDF-TrFE)/GO. Two layers P(VDF-TrFE)/GO structures are associated with decreased d33 value whereas sandwich structure GO/P(VDF-TrFE)/GO favors to increased magnitude of the piezoresponse. Mixed and statistic distribution of these possible order types in the P(VDF-TrFE)/GO ferroelectric films with small (~1-2%) GO concentration may be the cause of the ferroelectric anomalies observed for these composites.

ACKNOWLEDGMENT

The authors wish to acknowledge the Russian Science Foundation (Grant 16-19-10112).

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Chapter 7

PIEZOELECTRIC ELECTROCERAMIC PEROVSKITES AND THEIR APPLICATIONS

Poonam Kumari1,*, Madan Lal1, Shashi Prakash Rai2 and Radheshyam Rai1 1School of Physics and Materials Science, Shoolini University, Solan (H.P.), India 2University Department of Physics, V.K.S. University, Ara, India

ABSTRACT

Starting with the history of the fundamental science of the relation of structure to composition delineated completely by Goldschmidt, we use the perovskite structure to illustrate the enormous power of crystal chemistry-based intelligent synthesis in creating new materials. The general formula of perovskite-type oxides is ABO3. The classical ceramic solid-solid reaction, hydrothermal and sol-gel methods, commonly used for the synthesis of perovskite-type oxide, involve high reaction temperature and hence yield perovskite-type oxides with a low surface area due to their sintering. A method based on hydrothermal treatments is described for increasing the surface area of sintered ABO3- type perovskite oxides. In a perovskite solid solution, the Curie temperature can be tuned by varying the compositions of the end members. For example, for the Pb(Zr,Ti)O3 (PZT) binary system, the Curie temperature of the MPB composition can be considered as an average (386°C) of the Curie temperature of two end members (PbZrO3: 230°C, PbTiO3: 490°C). This chapter is concerned with piezoelectric perovskite materials and their properties. Perovskite compositions are the best known and the largest family of ferroelectric and piezoelectric materials. In this review, an attempt is made to review recent developments on lead-free piezo materials emphasizing their preparation, piezoelectric property relations, and consequent physical properties. Piezoelectric properties of the most promising lead-free compositions/families including titanates, alkaline niobates and bismuth perovskites and their solid solutions, along with perovskites such as KNN and BaTiO3 ferroelectrics are reviewed in detail. Piezoelectric perovskite type ceramics seem to be suitable for actuator and high power applications that are required a large piezoelectric constant, d33 and a high Curie temperature. By appropriate changes in composition one can modify the most

* Corresponding Author address. Email: [email protected]. 206 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

significant electroceramic piezoelectrics (BaTiO3 and KNN) phase in industry, into metallic conductors, superconductors or the highest pressure phases in the earth. A brief coverage of the recent developments in the area of piezoelectric energy harvesting is also encompassed.

Keywords: piezoelectric, electroceramic, perovoskite ceramics, lead free perovskite ceramics

1. INTRODUCTION

Perovskite ceramics are current primary piezoelectric materials widely commercialized and applied to various devices such as sensors and actuators (Uchino 2015). Since all pyroelectric, ferroelectrics and dielectrics materials are piezoelectrics. These ferroelectric, piezoelectric and pyroelectric materials, belong to the family called smart materials and have been extensively studied for several years, not only because they have interesting physical phenomena but also because it can be used in wide variety of applications in the most diverse fields, ranging from the automotive industry as temperature or pressure sensors, even in scientific circles as polarizers or light waveguide modulators. Piezoelectric perovskites solid solutions with high piezoelectric response are used in piezoelectric devices. The general formula of perovskite-type oxides is ABO3 and can crystallize in cubic structure with space group Pmm or in distorted orthorhombic, rhombohedral, tetragonal, monoclinic symmetry, where A and B are cations and O (oxides or halides) is an anion. The stability of the perovskite phases is largely governed by the Goldschmidt tolerance factor (Goldschmidt (r퐴 +r푂) 1926), t = where r퐴, r퐵 and r푂 are the respective radii of A, B, and oxygen ions. √2(r퐵 +r푂) For an ideal perovskite, t is unity; however, the perovskite structure is also found for lower t- values (0.75≤ t ≤ 1). When t ≥ 1, the crystal usually has a high symmetry with cubic or tetragonal structure; when t < 1, it usually leads to a low symmetry structure, such as rhombohedral, monoclinic, or orthorhombic. Several combinations of A- and B-site cations can form a stable perovskite-like structure. This paper reviews the history of piezoelectric perovskites and predictions the future development directions from the author’s view. In this review, the piezoelectric ceramics were described as well as the concept of piezoelectric effects, piezoelectric parameters, hysteresis loop and why lead free materials are now in the top of the interest in piezoelectric materials and electro ceramics. We also discuss about the different synthesis processes of perovskite piezoelectric ceramics.

1.1. Fundamentals of Piezoelectricity

Piezoelectrics ceramics are used as piezoelectric devices. Piezoelectric devices into four general categories: generators, sensors, actuators, transducers, ceramic filter, resonator and high power applications. Generators and sensors make use of the direct piezoelectric effect, meaning that mechanical energy transformed into a dielectric displacement. Knock sensors are placed near the engine in order to detect irregular combustions. The piezo element itself converts the vibrations into an electric charge proportional to the applied force. Usually, Piezoelectric Electroceramic Perovskites and Their Applications 207 piezoelectric ceramics with specially tailored properties are used. Also, the piezoelectric coefficient of the material must be almost independent of the temperature and remain stable over the vehicle’s lifetime. Distance sensors are ultrasonic transducers used in vehicles as so called parking pilots. During backing, the transducer emits ultrasonic waves reflected by the obstacle and then, in turn, changed into an electrical system by the same transducer now acting as an ultrasonic sensor. Piezoelectric transducers are also used as sensors and actuators in vibration control systems. The transducers applications divide into two general areas: i) conversion of electrical energy into mechanical energy for hydraulic or motive power, and ii) converting mechanical energy into electrical energy for communications and electronics. Compared to strain gauges, piezoelectric sensors offer superior signal to noise ratio, and better high-frequency noise rejection. Sonar as a successful application of a piezoelectric material stimulated others to discover new piezoelectric materials and to develop new devices. Lead zirconate titanate is used for actuation because of its favorable piezoelectric properties and electromechanical coupling coefficients. Piezoelectric actuator that is crimped with an elastic vibrator (stator) generates high-order bending vibration on the surface of the stator. Piezoelectric-based MEMS are generally attractive due to their high sensitivity and low electrical noise in sensing applications and high-force output in actuation applications. The multimillion dollar markets for multilayer capacitors, piezoelectric transducers, and PTC thermistors are based on ferroelectric ceramics made from oxide perovskites. A number of engineering applications of perovskite ceramics and crystals are listed in Table 1, but only the first three are profitable. Multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors all make use of ferroelectric ceramics with the perovskite structure.

Table 1. Electrical applications of various perovskite systems

Multilayer Capacitor BaTiO3 Piezoelectric Transducer Pb(Zr,Ti)O3 P.T.C. Thermistor BaTiO3 Electro-optic Modulator (Pb,La)(Zr,Ti)O3 Dielectric Resonator BaZrO3 Thick Film Resistor BaRuO3 Electrostrictive Actuator Pb(Mg,Nb)O3 Superconductor Ba(Pb,Bi)O3 Magnetic Bubble Memory GdFeO3 Laser Host YA1O3 Ferro-magnet (Ca,La)MnO3 Refractory Electrode LaCoO3 Second Harmonic Generator KNbO3

The several of perovskite materials are reviewed in this paper. Among lead-free candidates, (K,Na)NbO3 (KNN) has become one of the most extensively investigated piezoelectric systems in the past 10 years due to its large d33 and high TC (Saito et al. 2004; Rödel, Jo, et al. 2009; DAMJANOVIC et al. 2010; Zhang, Xia, and Shrout 2007a; Guo, Kakimoto, and Osato 2004; Guo, Kakimoto, and Ohsato 2005; Xiao et al. 2009; Wang, Wu, et al. 2014; Li et al. 2013). Recently, a number of countries have invested considerable manpower and financial resources into the study of KNN-based piezoelectrics, and some 208 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. significant breakthroughs have been achieved (Saito et al. 2004; Guo, Kakimoto, and Ohsato 2004; Du, Luo, et al. 2007; Wu, Wang, Xiao, et al. 2007a; Wang and Li 2010; Zhang et al. 2013; Cheng, Wu, Wang, Zhang, Zhu, et al. 2013; Wang, Xiao, et al. 2014).

Figure 1. Publications on lead-free piezoceramics in refereed journals for the time range from 2004 to 2014 (Wu, Xiao, and Zhu 2015).

Piezoelectricity is a fundamental process linking electrical and mechanical properties (Ballato 1995; Rajapakse 1997). This process has been put in use to convert mechanical energy to electricity and vice-versa. Intriguing piezoelectric effects have been reported in these perovskite (ABO3) compounds. For instance, the Pb(Zr1-xTix)O3 (PZT) solid solutions have been known for more than 30 years to exhibit a very high piezoelectric response, but only for a very narrow range of composition x (Jaffe 2012). One of the major drawbacks of these materials, however, is their high lead content: the ceramics contain about 60 wt.% lead, and therefore represent a possible ecological hazard. As a consequence much of the current research is oriented towards more environmentally friendly lead-free materials. An alternative group of lead-free ferroelectric materials are those based on the sodium potassium niobate (K,Na)NbO3 solid solution, with the composition K/Na (50/50) being close to the morphotropic phase boundary and exhibiting a moderate dielectric constant and an optimum piezoelectric response. Similarly, the class of Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) (Park and Shrout 1997) materials were found to exhibit remarkably large piezoelectric constants, when synthesized in single-crystal form. The MPB can be obtained from the compositional boundary between two different phases which can be achieved by controlling the ratio of each perovskite component. The most famous example for MPB phase is lead zirconate titanate [Pb(Zr,Ti)O3]. The origin of high piezoelectric response in MPB can be explained by high degree of freedom of electric dipoles within the boundary which can be usually oriented via an electric field. Recent structural studies also indicate an intermediate phase at a composition at the MPB with possesses monoclinic distortion which is responsible for the enhancement piezoelectric properties. Piezoelectric Piezoelectric Electroceramic Perovskites and Their Applications 209

materials are characterized by several coefficients (i) Strain coefficients (dij) [m/V]-Strain developed (m/m) per electric field applied (V/m). (ii) Charge output coefficients [C/N] charge density developed (C/m²) per given stress (N/m²). (iii) Voltage coefficients (gij) or field output coefficients [Vm/N] - Open circuit electric field developed (V/m) per applied mechanical stress (N/m²) or strain developed (m/m) per applied charge density (C/m²) (iv). Piezoelectric coupling coefficients (Kij) - The coefficients are energy ratios describing the conversion from mechanical to electrical energy or vice versa. K² is the ratio of energy stored (mechanical or electrical) to energy (mechanical or electrical) applied. In recent years, the authors’ group concentrates the researches on the composition design and the properties study of perovskite lead-free piezoelectric ceramics, especially on the (Bi1/2Na1/2)TiO3 (BNT) and K1/2Na1/2NbO3 (KNN)-based ceramics. For example, Satio et al. observed a large d33 value (∼416 pC/N) in (K,Na,Li)(Nb,Ta,Sb)O3-textured ceramics fabricated using reactive templated grain growth (RTGG) methods (Saito et al. 2004). In 2014, we developed new KNN-based material systems with large piezoelectricity (d33 ≈ 490 pC/N) by designing new phase boundaries consisting of rhombohedral and tetragonal (R−T) phases;(Wang, Wu, et al. 2014) moreover, this material was able to demonstrate both a large d33 and a high TC through refining of its composition,(Zhang et al. 2013; Cheng, Wu, Wang, Zhang, Zhu, et al. 2013) further increasing research interest in KNN-based piezoelectrics. Figure. 1 displays statistics related to refereed publications on lead-free piezoceramics from 2004 to 2014 using the keywords “lead-free” and “piezoelectric”. We classified the referenced publications according to the keywords “lead-free piezoelectric”, “alkali niobium- based”, and “construction of alkali niobium-based materials”, as shown in Figure 1. There is a gradually increasing trend in the number of publications concerning alkali niobium-based materials, among which the construction of phase boundaries has become a prominent tool for the improvement of their electrical properties.

1.1.1. Piezoelectric Effect Piezoelectric effect arises from crystal structure is exhibited by 20 out of 32 crystal classes and is always associated with noncentro-symmetric crystals. Naturally occurring materials, such as quartz, exhibit this effect as a result of their crystalline structure. Engineered materials, like lead zirconate titanate (PZT) for example, are subjected to a process called poling to impart the piezoelectric behavior. Let’s find out what happens at the microscopic scale that helps in creating the piezoelectric effect. A typical noncentro- symmetric crystal structure such as a perovskite (ABO3) has a net non-zero charge in each unit cell of the crystal. A mechanical stress on the crystal further shifts the position of the B- site ion, thus changing the polarization strength of the crystal. This is the source of the direct effect. When the crystal is subjected to an electric field, it also results in a relative shift in the position of the B-site ion, leading to the distortion of the unit cell and making it more (or less) tetragonal. This is the source of the inverse effect. Piezoelectric perovskite materials exposed to a fairly constant electric field tend to vibrate at a precise frequency with very little variation, making them useful as time-keeping devices in electronic clocks, as used in wristwatches and computers. The piezoelectric effect also arises due to the coupling of electrical behavior and Hooke’s law in a dielectric material. All ferroelectric materials show piezoelectric effects. This phenomenon is generally of two types namely; direct and converse piezoelectric effect. In direct piezoelectric effect, on application of stress to the material, potential difference 210 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. develops across the material whereas in case of converse piezoelectric effect, stress or/and strain develops in the material as a result of application of an electric field across the material (Zhuang et al. 1987) as shown in Figure 2.

1.1.1.1. Direct Piezoelectric Effect When mechanical pressure is applied to a piezoelectric material, the crystalline structure produces a voltage proportional to the pressure. For sensing applications, the direct piezoelectric effect describes a change in polarization due to an applied stress and is written as: Di = dijσj where Di is the dielectric displacement and σj is the applied stress.

1.1.1.2. Converse Piezoelectric Effect Conversely, when a piezoelectric material subjected to an electric field the structure changes in shape, producing dimensional changes in the material. Piezoelectric materials are anisotropic, that is their mechanical, electrical and electromechanical properties depend strongly on the crystal orientation. The converse piezoelectric effect describes the strain generated in a piezoelectric material in response to an applied electric field. This effect is written as: Si=dijEj. Where Si is the electric field induced strain, Ej is the applied electric field and dij is the piezoelectric coefficient. The converse piezoelectric effect is exploited in actuator devices.

1.1.2. Principle of Piezoelectric Effect Piezoelectric crystal consists of multiple interlocking domains which have positive and negative charges. These domains are symmetrical within the crystal, causing the crystal as a whole to be electrically neutral. The piezoelectric effect occurs when the charge balance within the crystal lattice of a material is disturbed. When there is no applied stress on the material, the positive and negative charges are evenly distributed and so there is no potential difference. When the lattice is slightly altered (when stress is applied on the crystal), the charge imbalance creates a potential difference. Even a tiny bit of piezoelectric crystal can generate voltages in the thousands. However, the current is extremely small and only causes a small electric shock. The converse piezoelectric effect occurs when the electrostatic field created by electric current causes the atoms in the material to move slightly. Piezoelectricity is found in useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultrafine focusing of optical assemblies.

Figure 2. Schematic diagram for piezoelectric effects. Piezoelectric Electroceramic Perovskites and Their Applications 211

1.2. Piezoelectric Parameters

1.2.1. Piezoelectric Coefficients The piezoelectric coefficients and the dielectric permittivity’s generally increased with temperature for hard and soft type of PZT perovskite ceramics (Sabat et al. 2007). The converse piezoelectric effect describes the strain generated in a piezoelectric material in response to an applied electric field. This effect is written as:

Si = dij Ej

Where Si is the electric field induced strain, Ej is the applied electric field and dij is the piezoelectric coefficient.

1.2.2. Piezoelectric Modulus or Charge Coefficient (Dij) The piezo modulus is the ratio of induced electric charge to mechanical stress (T = constant). Example, d33 mechanical strain induced per unit of electric field applied in V/m or charge density in C/m2 per unit pressure in N/m2, both in polarization direction. “d” is piezoelectric charge coefficient and is measured in Coulomb/Newton (C/N). The piezoelectric coefficient (d33), related to the direction of the applied electric field to the direction of the faces on which charges induced. d33 is the piezoelectric constant when the direction of the applied stress is along the direction of the faces of the ceramic on which charges are developed (Cross 1987).

1.2.3. Piezoelectric Voltage Coefficient (Gij) The piezoelectric voltage coefficient (g) is the ratio of electric field (E) to the effective mechanical stress (T). Dividing the respective piezoelectric charge coefficient (dij) by the corresponding permittivity gives the corresponding gij coefficient.

1.2.4. Elastic Compliance (Sij) The elastic compliance coefficient s is the ratio of the relative deformation (S) to the mechanical stress (T). Mechanical and electrical energy are mutually dependent, the electrical boundary conditions such as the electric flux density (D) and electric field (E) must therefore E be taken into consideration. E.g., s33 : The ratio of the mechanical strain in direction 3 to the D mechanical stress in the direction 3, at constant electric field (for E = 0: short circuit). s55 : The ratio of a shear strain to the effective shear stress at constant dielectric displacement (for D = 0: open electrodes). The often used elasticity or Young’s modulus Yij corresponds in a first approximation to the reciprocal value of the corresponding elasticity coefficient.

1.2.5. Frequency Coefficient (Ni) The frequency coefficient N describes the relationship between the geometrical dimension A of a body and the corresponding (series) resonance frequency. The indices designate the corresponding direction of oscillation N = fs A. E.g., N3: describes the frequency coefficient for the longitudinal oscillation of a slim rod polarized in the longitudinal direction. N1: is the frequency coefficient for the transverse oscillation of a slim rod polarized in the 3- direction. N5: is the frequency coefficient of the thick - ness shear oscillation of a thin disk. 212 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

NP: is the frequency coefficient of the planar oscillation of a round disk. Nt: is the frequency coefficient of the thickness oscillation of a thin disk polarized in the thickness direction.

1.2.6. Mechanical Quality Factor Quality factors of a piezoceramic resonator at the resonance and ant- resonance frequencies in “weak field” have been investigated. Their interrelation depending on the electroelastic piezoceramic constants, type of vibration, harmonic number, relative size of electrode and degree of polarization in view of the mechanical, dielectric and piezoelectric energy losses. The mechanical loss of the material is generally described by its reciprocal quantity, i.e., the mechanical quality factor 푄푚, which is proportional to the ratio of the total stored mechanical energy over the energy loss within one complete vibration cycle,

푊푠푚 푄푚 = 2휋. 푊푙푚

Where 푊푠푚 = the total is stored mechanical energy and 푊푙푚 = is the loss of the mechanical energy. In general, 푄푚 is associated with the spatial amplitude attenuation coefficient α of the mechanical waves (San Emeterio et al. 2000).

휋 휋. 푓 푄 = = 푚 훼.  훼. 푣

Where λ, v, f are the wavelength, wave velocity and frequency of mechanical wave, respectively. For a mechanical vibrator, 푄푚 can be characterized by the bandwidth of the resonance peak, i.e., (La-Orauttapong et al. 2002).

푓 푄 = 0 푚 ∆푓

Where f0 is the center frequency of the vibrator, ∆푓 is the 3 dB or half power bandwidth. Thus, the mechanical quality factor 푄푚 of a piezoelectric material can be derived either from the measurement of the spatial amplitude attenuation coefficient α of the propagating mechanical waves of the piezoelectric vibrator. There are two methods which are used to measure mechanical loss in high power resonance conditions: the impedance method and the transient method. The impedance method requires the measurement of impedance data from a frequency sweep between resonance to anti-resonance using a constant excitation condition, be it either voltage, current, power, or vibration velocity (Uchino, Zhuang, and Ural 2011; Seyit et al. 2009; Hideki et al. 2011).

1.2.7. Electromechanical Coupling Factor (K) The coupling factor k is a measure of how the magnitude of the piezoelectric effect is (n o t an efficiency factor!). It describes the ability of a piezoelectric material to convert electrical energy into mechanical energy and vice versa. The coupling factor is determined by Piezoelectric Electroceramic Perovskites and Their Applications 213 the square root of the ratio of stored mechanical energy to the total energy absorbed. At resonance, k is a function of the corresponding form of oscillation of the piezoelectric body. E.g., k33: the coupling factor for the longitudinal oscillation, k31: the coupling factor for the transverse oscillation, kP; the coupling factor for the planar radial oscillation of a round disk, kt: the coupling factor for the thickness oscillation of a plate, k15: the coupling factor for the thickness shear oscillation of a plate.

1.2.8. Dynamic Behavior of Piezoelectric Ceramics The electromechanical behavior of a piezoelectric body excited to oscillations can be represented by an electrical equivalent circuit diagram. C0 is the capacitance of the dielectric. The series circuit, consisting of C1, L1, and R1, describes the change in the mechanical properties, such as elastic deformation, effective mass (inertia) and mechanical losses resulting from internal friction. This description of the oscillatory circuit can, however, only be used for frequencies in the vicinity of the mechanical intrinsic resonance. Most piezoelectric material parameters are determined by means of impedance measurements on special test bodies at resonance. The series and parallel resonances are used to determine the piezoelectric parameters. These correspond to a good approximation of the impedance minimum fm and maximum fn.

1.2.9. Acoustic Impedance (Z) Acoustic Impedance Z The acoustic impedance Z is a parameter used for evaluating the acoustic energy transfer between two materials. It is defined, in general, by

Z2 = (pressure/volume velocity)

In a solid material, Z = √ρc where ρ is the density and c is the elastic stiffness of the material. In more advanced discussions, there are three kinds of impedances; specific acoustic impedance (pressure/particle speed), acoustic impedance (pressure/volume speed) and radiation impedance (force/speed).

1.2.10. Pyroelectric Coefficient (Pi) It was found by the used of Byer and Roundy method, the ceramic was placed into a programmable oven and temperature was increased gradually, the electrodes were connected to a resistance and voltage drop on the resistance was measured with a high sensibility multimeter (Byer and Roundy 1972); for pyroelectric detectors the relationship between 푝 pyroelectric coefficient and permittivity is calculated with the expression as 푖 where pi is the √휀 pyroelectric coefficient and ε is the permittivity, for many pyroelectric materials, where pi is the pyroelectric coefficient and ε is the permittivity, for many pyroelectric materials, the ratio 푝 푖 tends to be constant and has a value of (3+1.0) × 10−9CK−1cm−2 (Liu, Zook, and Long √휀 − 1975) and is an indicator of how good pyroelectric detector is the material.

214 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

2. PIEZOELECTRIC PEROVSKITE CERAMICS

Much attention has been paid to perovskite-based oxides due to their promising superconducting, ferroelectric, ferromagnetic and optical properties. Ferroelectric BaTiO3 is one of the most important perovskite oxide materials in electronics owing to its various device applications (Cohen 1992; Sayer and Sreenivas 1990). An interesting feature of BaTiO3 is that its electrical properties can be effectively tuned by incorporation of dopants. For example, in pure BaTiO3, there is no mobile carrier, which makes it an insulator. When doped with Nb5+, some free electrons are introduced into the material with Nb5+ substituted 4+ for Ti and therefore the doped BaTiO3 becomes an n-type semiconductor. This chapter reviews recent advances in the studies of the piezoelectric properties of perovskite materials. Figure 3 represents the ABO3 crystal structure that is cubic (Chiang, Kingery, and Birnie 1997). The perovskite structure is common in many ternary oxides in which the A and B cations have significantly different sizes. The larger cations, A and the oxygen’s form a face- centered cubic (FCC) lattice. The smaller cations, B, occupy octahedral sites in this FCC lattice and are only surrounded by oxygen nearest neighbors. These B cation occupied octahedra share corners with each other, but share faces with the dodecahedra surrounding the A cations. The A and B cations are shielded from one another by the oxygen ions (Chiang, Kingery, and Birnie 1997). Perovskite compositions are the best known and the largest family of ferroelectric and piezoelectric materials. The dependence of the longitudinal, d33, piezoelectric coefficient on crystallographic orientation was examined in single crystals of BaTiO3, PbTiO3, Pb(Zr,Ti)O3, KNbO3, relaxor-ferroelectric solid solutions [e.g., Pb(Mg1/3Nb2/3)O3-PbTiO3 and Pb(Zn1/3Nb2/3)O3-PbTiO3] and Na1/2Bi1/2TiO3. In most cases the largest piezoelectric coefficient was found in a direction which does not coincide with the polarization direction. In relaxor ferroelectrics, some compositions near the morphotropic phase boundary (MPB) exhibit very large piezoelectric (2000 pC/N) and coupling coefficients (k33 > 90%). These results are discussed in terms of so-called engineered domain structure and crystal anisotropy. The article also discusses evidence for the presence of a monoclinic phase between the rhombohedral and tetragonal phases at the MPB in Pb(Zr, Ti)O3.

Figure 3. ABO3 perovskite structure. Piezoelectric Electroceramic Perovskites and Their Applications 215

Figure 4. Morphotropic phase boundary in PZT ceramic (Zheng, Hou, et al. 2014).

But, due to the toxicity in the lead-containing device there has been compositional development of lead-free piezoelectric materials. Among all lead-free piezoelectric materials NBT is one of them about which is discuss in this chapter. Figure. 5 shows the perovskite materials related to BaTiO3 with different ions substitutions.

Figure 5. Crystal chemistry of multiple ions substitution in BaTiO3 perovskite structure. 216 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

Polycrystalline ferroelectric ceramics such as Barium Titanate (BaTiO3) and Lead Zirconate Titanate (PZT) exhibit larger displacements or induce larger electric voltages. PZT piezoceramic materials are available in many modifications and are most widely used for actuator or sensor applications. Special doping of the PZT ceramics with, e.g., Ni, Bi, Sb, Nb ions make it possible to specifically optimize piezoelectric and dielectric parameters. At temperatures below the Curie temperature, the lattice structure of the PZT crystallites becomes deformed and asymmetric. This brings about the formation of dipoles and the rhombohedral and tetragonal crystallite phases which are of interest for piezo technology. The ceramic exhibits spontaneous polarization as shown in Figure 4.

2.1. Soft and Hard PZT Piezoelectric

The piezoelectric properties of PZT materials can be improved by the formation of a morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases in solid solutions of perovskite-type PbTiO3 and PbZrO3 (Jaffe, Roth, and Marzullo 1955; Zhang, Randall, and Shrout 2003; Jaffe, Cook, and Jaffe 1955). The doping effects on PZT system has been conducted as early as the late 1950s (Kulcsar 1959a, 1959b; Gerson 1960). It is shown that the dielectric and piezoelectric properties of PZT ferroelectric ceramics could be readily tailored by means of low level doping, especially through the aliovalent dopants. In general, these aliovalent dopants fall into two distinct groups. The soft donor dopants where the charge on the cation is larger than that which it replaces in the PZT structure and hard acceptor dopants where the charge on the cation is smaller than that of the ion which it replaces. The donor dopants enhance both the dielectric and piezoelectric properties at room temperature, and show symmetric hysteresis loops with good “squareness” and lower coercivity with respect to the high fields. The acceptor dopants in general reduce both dielectric and piezoelectric properties, they give rise to highly asymmetric hysteresis response, larger coercivity and higher electrical and mechanical quality factor (Q). It is known that in hard PZT ceramics properties decrease with time. This behavior is termed as “aging”. In contrast, this aging effect could not be detected in soft materials (Jaffe 2012). Understanding origin of aging in hard and its absence in soft materials may thus help identifying details of hardening and softening mechanisms. It is believed that the hardening mechanismis associated with the domain wall stabilization. However, the exact mechanism dominating the stabilization of the domain walls is still disputable (Jonker 1972; Carl and Hardtl 1977; Lambeck and Jonker 1986; Robels and Arlt 1993; Tan, Li, and Viehland 1997). Three main models are usually discussed in the literatures: (i) the bulk effect (caused by alignment of charged defects with polarization within ferroelectric domains) (Carl and Hardtl 1977; Ren 2004), (ii) the domain wall effect (diffusion of charges toward domain walls thus creating pinning centers) (Postnikov, Pavlov, and Turkov 1970; Warren et al. 1994) and (iii) surface effect (drift and gathering of charges near grain boundaries and other interfaces). For soft PZT samples, the structure of defects induced by donor doping is still not clear. What is more important is to clarify the relationship between those defects or defect dipoles and the domain walls. There are only “hand waving” arguments as to how or why the domain walls should become more mobile in soft materials. Much more work is needed to determine the physics of the softening in these materials. Hard Piezoelectric Materials ferroelectrically soft piezoelectric ceramic materials can be polarized fairly easily even at relatively low field Piezoelectric Electroceramic Perovskites and Their Applications 217 strengths. This is due to the comparably high domain mobility typical for them. The advantages of soft PZT materials are their large piezoelectric charge coefficient, moderate permittivities and high coupling factors. The various properties of Pb-based ceramics are given in Table 2.

Table 2. Piezoelectric properties of Lead based perovskite samples

ퟑퟑ 33 3 Composition d33 훆퐓 /훆ퟎ g (10 d33 X g33 References (pC/N) Vm/N) (1015 m2/N) PbTiO3 51 170 34 1734 (Ikegami, Ueda, and Nagata 1971) Pb(Zr,Ti)O3 220 800 31 6820 (Messing et al. 2004) (Na,Pb)(Zr,Ti)O3 157 725 24 3768 (Mahato, Chaudhary, and Srivastava 2003) (La,Pb)(Zr,Ti)O3 406 3480 13 5278 (Yadav and Choudhary 2005) (La,Ba,Sr,Pb) 538 2377 26 13,988 (Roeder et al. 2001) (Zr,Ti,Nb)O3 0.22PbZrO3- 10 4200 16 10,006 (Augustine and 0.37PbTiO3- Ramachandra Rao 0.41Pb(Ni1/3Nb2/3)O 2015) 0.23PbZrO3- 740 4400 19 14,056 (Yadav and Choudhary 0.36PbTiO3- 2005) 0.41Pb(Ni1/3Nb2/3)O3 0.24PbZrO3- 460 2900 18 8241 (Futakuchi, Matsui, and 0.35PbTiO3- Adachi 1999) 0.41Pb(Ni1/3Nb2/3)O 0.6Pb(Mg1/3Nb2/3)O3- 690 5000 16 11,040 (Messing et al. 2004) 0.4PbTiO3

2.2. Lead-Free Piezoceramics and Their Properties

Lead (Pb) is one of the most toxic materials known, and continuous exposure to an environment contaminated by this element has potential hazards. Being in contact with Pb can cause serious damages to vital human organs such as the kidney, heart, and brain. Since most commercial piezoelectric materials are based on PZT containing about 60 wt% lead, intensive efforts have been undertaken to eliminate the Pb and find good replacement candidates. There has been a growing interest in developing alternative lead free piezoelectric materials that can eventually replace the current lead based ones. Intensive research efforts have been spent on related studies all around the world for over two decades. In the following sections, current lead free piezoelectric materials will be reviewed. For the development of new, lead-free piezoelectric materials, we designed a compositional formation of MPB between a different pair of crystal structures-namely, the pseudo-ilmenite-type and perovskite-type structures, having rather different lattice forms and unit sizes from each other. The general need for stable piezoelectric characteristics over a wide temperature range made us select high Curie- o temperature (TC > 250 C) end members: orthorhombic perovskite-type (K0.5Na0.5)NbO3 (TC = o o 415 C) and hexagonal pseudo-ilmenite-type LiTaO3 (TC = 615 C). Besides the formation of a MPB, we also exploited the hybridization of covalency onto ionic bonding for further 218 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. improvement in piezoelectricity, on the basis of Cohen’s calculation for the titanate perovskite system. In addition to LiTaO3, we also used LiSbO3 as an end member for the compositional study with (K0.5Na0.5)NbO3, because the higher electro-negativities of Sb and Ta compared to Nb were expected to make the alkaline niobate-based perovskite more covalent (Cohen 1992). Table 3 displays the influences of different additives on the phase transition temperatures of KNN. In the past, innovations in electromechanical devices have been the driving force for new development in piezoelectric ceramics. Currently, the driving force also comes from environmental regulations, such as the draft directives on waste from electrical and electronic equipment (WEEE) and restriction of hazardous substances (RoHS), which require the replacement of lead contained ferroelectric materials because of their toxicity, hence, an enormous surge in the research on lead free ferroelectric ceramics occurred in the past decade. Up to now, perovskite-type lead free ferroelectrics were believed to be promising materials to replace PZTs due to their relatively high dielectric and piezoelectric properties, in which potassium sodium niobate (KNN) and bismuth sodium titanate (Na0.5Bi0.5TiO3, NBT for short) based materials are two lead-free piezoelectric ceramics received a lot of attentions (Saito et al. 2004; Wang, Wu, et al. 2014). This review highlights recent developments in several lead-free piezoelectric materials including BaTiO3, K0.5Bi0.5TiO3, Na0.5K0.5NbO3 etc. and their solid solutions. The factors that contribute to strong piezoelectric behavior are described and a summary of the properties for the various systems (Jaeger and Egerton 1962; Smolenskii et al. 1961; Ranjan and Dviwedi 2005; Sasaki et al. 1999; Jaffe 2012; Forsbergh Jr 1949; Berlincourt and Jaffe 1958; Merz 1949; Takenaka, Nagata, and Hiruma 2008; Guillemet‐Fritsch et al. 2008; Flores-Cuautle, Cruz-Orea, and Suaste-Gomez 2008; Rodríguez-Ruiz et al. 2008; Suchanicz and Ptak 1990; Suchanicz, Jeżowski, and Poprawski 1998; Tu, Siny, and Schmidt 1994; Suchanicz and Kwapulinski 1995; Jones and Thomas 2002; Davis 2007; Von Hippel 1950; Hiruma et al. 2005) is provided. Useful lead-free materials are often binary or ternary solid solutions.

Table 3. Different additives on the phase transition temperature and piezoelectric coefficient of lead free KNN ceramics

Samples Tc d33 References (°C) (pC/N) K0.47Na0.47Li0.06NbO3 475 235 (Guo, Kakimoto, and Ohsato 2004)

0.94K0.49Na0.51NbO3-0.06LiNbO3 246 (Shen et al. 2011; Wang, Li, and Liu 2008) 0.92Na0.535K0.48NbO3-0.08LiNbO3 467 280 (Shen et al. 2011) 0.93K0.5Na0.5NbO3-0.07LiNbO3 475 274 (Wongsaenmai, Ananta, and Yimnirun 2012) (K0.5Na0.5)0.935Li0.065NbO3 188 (Kakimoto et al. 2005) (K0.5Na0.5)0.94Li0.06NbO3 471 235 (Du, Tang, et al. 2007) 0.94K0.5Na0.5NbO3-0.06LiNbO3 450 215 (Song et al. 2007) 0.948(K0.5Na0.5)NbO3-0.052LiSbO3 373 265 (Zhang et al. 2006) 0.95(K0.48Na0.52)NbO3-0.05LiSbO3 358 262 (Wu, Wang, Xiao, Zhu, Yu, et al. 2007) 0.94(K0.5Na0.5)NbO3-0.06LiSbO3 340 212 (Lin et al. 2007) 0.95K0.49Na0.51NbO3-0.05LiSbO3 364 256 (Li, Shen, et al. 2011) Piezoelectric Electroceramic Perovskites and Their Applications 219

Samples Tc d33 References (°C) (pC/N) 0.95(K0.40Na0.60)NbO3-0.05LiSbO3 364 280 (Jiagang et al. 2008) 0.95(K0.42Na0.58)NbO3-0.05LiSbO3 390 270 (Wu, Xiao, Wang, Zhu, Yu, et al. 2007) (Na0.5K0.5)1-x(LiSb)xNb1-xO3 386 260 (Zang et al. 2006a) 0.95K0.5Na0.5NbO3-0.05LiSLbO3 215 (Palei and Kumar 2012) (Na0.474K0.474Li0.052)(Nb0.948Sb0.052)O3 242 (Zhao et al. 2013)

(Na0.5K0.5)0.945Li0.055Nb0.96Sb0.04O3 240 (Li, Shih, and Shih 2007) (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 253 416 (Cross 2004) (K0.38Na0.58Li0.04)(Nb0.91,Ta0.05,Sb0.04)O3 306 (Wu, Xiao, Wang, Zhu, Wu, et al. 2007) (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 ∼249 345 (Akdogan et al. 2008) (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 ∼225 413 (Akdogan et al. 2008) (K0.52Na0.48-x)(Nb1-x-ySby)O3-xLiTaO3 ∼230 400 (Gao et al. 2011) (Na0.52K0.44Li0.04)Nb0.87Sb0.08Ta0.05O3 ∼270 376 (Zuo, Fu, and Lv 2009) (Na0.5K0.5)0.975Li0.025Nb0.76Sb0.06Ta0.18O3 ∼210 352 (Du et al. 2012) (Na0.52K0.4375)(Nb0.9075Sb0.05)O3- 315 321 (Du et al. 2011) 0.0425LiTaO3 (Na0.52K0.48-xLix)Nb1-x-ySbxTayO3 339 308 (Fu et al. 2010) (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375 271 304 (Ming et al. 2007) )O3 Li0.04(Na0.54K0.46)0.96Nb0.81Ta0.15Sb0.04O3 340 293 (Pang et al. 2011) 0.96(K0.48Na0.52)(Nb0.95Ta0.05)O3- 348 250 (Yoo 2012) 0.04LiSbO3 0.9825K0.5Na0.5NbO3-0.0175BiScO3 351 253 (Du et al. 2008) 0.99(Na0.5K0.5)NbO3-0.01BiAlO3 372 202 (Zuo et al. 2009)

0.99K0.5Na0.5NbO3-0.01Bi0.8La0.2FeO3 370 144 (Zhang et al. 2011) (1-x)(Na0.5K0.5)NbO3-xBiCoO3(x=0.01) 370 165 (Wu et al. 2011) (1 − x)(Na0.4725K0.4725Li0.055NbO3) − 402 (Kumari, Rai, and Kholkin 2015) x(BiFe0.5Ta0.5O3) (x =0.007)

Figure 6. Intrinsic difference between (a) polymorphic phase transition (PPT) and (b) morphotropic phase boundary (MPB) (Rödel et al. 2015; Dai, Zhang, and Zhou 2007). 220 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

2.3. Phase Boundaries of Lead Free KNN Ceramics

The modification of phase boundaries is a powerful tool for the promotion of electrical properties of piezoelectric materials, i.e., the involved phase boundaries and their corresponding types are largely responsible for the enhancement in piezoelectric activity, regardless of whether a material is lead based or lead free (Takenaka and Nagata 2005; Rödel, Kounga, et al. 2009; Shrout and Zhang 2007; Jo et al. 2011; Liu and Ren 2009; Cross 2004; Guo, Kakimoto, and Ohsato 2005; Xiao et al. 2009; Cheng et al. 2014a). A number of experimental (Fu and Cohen 2000; Noheda et al. 2002; Seo et al. 2013; Noheda et al. 2000; Cox et al. 2001; Damjanovic 2005; Frantti et al. 2002; Bellaiche and Vanderbilt 1999) and theoretical methods (Bellaiche, García, and Vanderbilt 2000; Damjanovic 2010; Baettig et al. 2005; Grinberg et al. 2005; Íñiguez and Bellaiche 2001; Chen 2008) have been used to investigate the relationships between phase boundaries and the piezoelectric activity of a material. It is generally accepted that a high degree of alignments of ferroelectric dipoles can be driven by a large amount of thermodynamically equivalent states under a driving electric field during the poling process, easily generating enhanced electrical properties (Zheng, Wu, Cheng, Wang, Zhang, Xiao, Zhu, Wang, et al. 2014; Noheda et al. 2002; Seo et al. 2013; Noheda et al. 2000; La-Orauttapong et al. 2002; Cox et al. 2001; Kornev et al. 2006; Frantti et al. 2002; Han and Cao 2003). The construction of morphotropic phase boundaries (MPB) in PZT is considered a classic case in the field of piezoelectric materials, (Seo et al. 2004; Berlincourt and Jaffe 1958; Noheda et al. 2000) wherein the crystal structure changes abruptly and the piezoelectricity is maximized for compositions at MPBs (Seo et al. 2004; Berlincourt and Jaffe 1958; Noheda et al. 2000). For example, enhanced piezoelectric activity was observed in PZT materials with a Zr/Ti ratio of 52:48 owing to the structural changes going from tetragonal to rhombohedral via an intermediary monoclinic phase (Du et al. 1998); this kind of phase boundary is thought to be an MPB, which is only dependent on the compositions and independent of the measurement temperatures, as shown in Figure. 6b. However, the phase boundaries (e.g., R-O and O-T) in KNN-based materials were considered as the intrinsic characteristics of “polymorphic phase transition (PPT)”(Rödel et al. 2015), and their electrical properties are very sensitive to not only the compositions but also the temperatures as shown in Figure 6 (a). According to advances in KNN materials, there are three kinds of existing phase transitions (Figure 6), corresponding to the rhombohedral−orthorhombic phase transition temperature (TR-O), the orthorhombic−tetragonal phase transition temperature (TO-T), and the rhombohedral−tetragonal phase transition temperature (TR-T). Figure 7 shows the phase transition behavior of K0.48Na0.52Nb0.98Sb0.02O3 ceramics, measured at -150-550°C and f = 10 kHz. One can see that TR-O, TO-T, and TC are clearly shown according to the temperature dependence of the corresponding dielectric peaks. At room temperature, the R-O or O-T phase boundary could be formed in KNN by shifting TR-O or TO-T to room temperature using additives such as LiNbO3(Guo, Kakimoto, and Ohsato 2004; Higashide, Kakimoto, and Ohsato 2007; Hollenstein, Damjanovic, and Setter 2007; Wang et al. 2007; Wang and Li 2010), Li(Ta/Sb)O3 (Bomlai et al. 2008), Li(Ta-Sb)-O3 (Rubio-Marcos, Ochoa, and Fernandez + + + 2007; Juhyun et al. 2006; Akdogan et al. 2008), (Bi0.5A0.5)TiO3 (A = Na , K , Li ), BiBO3 3+ 3+ 3+ (B = Al , Fe , Sc ) (Zuo et al. 2009; Jiang et al. 2009; Zuo, Ye, and Fang 2007), SrZrO3 5+ 5+ (Rödel, Jo, et al. 2009), Sb (Zuo et al. 2010), BaZrO3(Wang, Bando, et al. 2009), Ta (Ishibashi and Iwata 1999) etc. In contrast, the phase boundary corresponding to R-O or O-T Piezoelectric Electroceramic Perovskites and Their Applications 221 possesses the PPT characteristic (Zhang, Xia, and Shrout 2007; Zhang, Xia, and Shrout 2007), depending on not only the compositions but also the temperatures, as shown in Figure 6 (a). Such a phase boundary is obviously different from that of PZT as shown in Figure 6 (b). Regardless of the presence of MPB or PPT characteristics, the polarization of a piezo- material can be more easily rotated among different symmetries when the compositions are located at the phase boundaries, inducing an enhancement in dielectric and piezoelectric properties. However, recent advances indicate that there is a limited improvement in the d33 values for KNN materials with O-T or R-O phase boundaries, which is also inferior to most PZT ceramics or even textured KNN (Cross 2004). As a result, the formation of R-T boundaries that are similar to PZT may become necessary to further enhance the piezoelectric activity of KNN, because the easy rotations of polarization axes can be induced in compositions near such a phase boundary (Guo et al. 2000; Liu and Ren 2009; Zheng, Wu, Cheng, Wang, Zhang, Xiao, Zhu, and Lou 2014). In addition, we recently confirmed this assumption in potassium-sodium niobate piezoceramics by experimental methods (Wang, Wu, et al. 2014; Cheng, Wu, Wang, Zhang, Lou, et al. 2013; Cheng et al. 2014b), and a larger d33 value has been attained by constructing R-T phase boundaries (Cheng et al. 2014b; Wu et al. 2014). In particular, we achieved a breakthrough in the piezoelectricity of KNN-based ceramics by forming new R-T phase boundaries (Wu et al. 2014; Cheng, Wu, Wang, Zhang, Lou, et al. 2013; Wu, Wang, Xiao, et al. 2007b). As a result, the construction of new phase boundaries has become an efficient way to further enhance the piezoelectricity of KNN materials, narrowing the gap between lead-free and lead-based materials.

2.4. Phase Boundaries and Piezoelectricity

Generally speaking, the piezoelectric activity of KNN-based ceramics can be enhanced in different degrees by constructing phase boundaries and modifying the microstructure of a material, while its piezoelectric activity is strongly sensitive to the phase boundary types. In this section, we further clarify the relationships between phase boundary and piezoelectric activity of KNN-based materials, addressing the weakness and the corresponding solution methods. Figure 7(a) shows the piezoelectric constant, phase boundary types, and Curie temperature of KNN-based ceramics. The types of phase boundaries strongly affect the d33 value of KNN materials: R-O ceramics possess a poorer d33 with respect to those with R-T and O-T, the O-T ceramics have a wide d33 distribution from 200 to 350 pC/N, and a large d33 of >330 pC/N is usually induced in R-T ceramics. In addition, it was also found that the piezoelectric activity of R-T KNN- based ceramics can be comparable to part-PZT ceramics. However, during the development of KNN-based materials a general trend has emerged whereby the d33 value is enhanced by sacrificing a material’s TC, and the ceramics with a large d33 (>330pC/N) always have a low TC (<300°C). As a result, the application temperature range of KNN-based ceramics should be considered when the phase boundaries are constructed, and it is necessary to choose additives that increase or at least do not significantly reduce-TC (Cheng et al. 2014b). We also compared KNN with PZT in terms of their d33 and TC, as shown in Figure 7 (b). For TC >200°C, the d33 of KNN-based ceramics has been comparable or even superior to PZT- 222 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. based ceramics. As a result, we believe that KNN-based lead-free piezoceramics can replace PZT in some applications. The excellent piezoelectric properties of KNN-based materials result in some practical applications (e.g., piezoelectric energy harvesting, piezoelectric actuators, etc.), similar to PZT ceramics.

Figure 7. (a) Relationship between phase boundary type, piezoelectric constant, and Curie temperature, and (b) comparison of the piezoelectric properties of PZT and KNN (Hungrıa, Galy, and Castro 2009; Zhang, Xia, and Shrout 2007a; Safari and Abazari 2010; Saito et al. 2004).

Table 4. Comparison table of lead free perovskite piezoelectric ceramics and their piezoelectric properties

퐓 Compositions d33 훆ퟑퟑ/훆ퟎ kp (%) Pr Ec References (pC/N) (μC/ (kV/m cm2) m) BaTiO3 single crystal 85.6 168 - - - (Davis 2007) BaTiO3 ceramic 191 1,680 - - - (Davis 2007) (Merz 1949) (Bi1/2Na1/2)0.94Ba0.06TiO3 125 580 (10 - 20 - (Takenaka, kHz) Maruyama, and Sakata 1991) (0.90)(Bi1/2Na1/2)TiO3– 148 700 (1 34 35.9 - (Wang, Tang, and 0.05(Bi1/2K1/2) TiO3– kHz) Chan 2004) 0.05BaTiO3 0.852(Bi1/2Na1/2) TiO3– 191 1,141 (1 33 - - (Nagata et al. 2003) 0.028BaTiO3-0.12(Bi1/2K1/2) kHz) TiO3 (1 - x)Bi0.5Na0.5TiO3- 70 to 164 (Kang and Koh 2015) xBaTiO3 at x=0.04 0.88NBT-0.08KBT-0.02BT 181 - - - - (Takenaka et al. (MPB) 2007) 0.78NBT-0.146KBT- 128 - - - - (Takenaka et al. 0.074BT 2007) 0.935Bi1/2Na1/2TiO3- 229 31 31 (Cheng et al. 2015b) 0.065BaTi O3- xmol⋅Bi2NiMnO6(x = 0- 0.008) Piezoelectric Electroceramic Perovskites and Their Applications 223

퐓 Compositions d33 훆ퟑퟑ/훆ퟎ kp (%) Pr Ec References (pC/N) (μC/ (kV/m cm2) m) (1-x-y) Bi1/2Na1/2TiO3- 219 29.5 38.05 21.42 (Cheng et al. 2015a) xBaTiO3-yY2NiMnO6 0.705BiFeO3 − 0.275BaTiO3 140 31.4 19 (Li et al. 2015) − 0.02Bi0.5Na0.5TiO3 + 1 mol% MnO2 (Ba1−xAx) (Ti0.98Zr0.02) O3 330 4.69 (Chandraiah and (x=0.015) (A=Mg2+, Ca2+ Panda 2015) and Sr2+) (1−x)Ba(Zr0.15Ti0.85)O3- 450 (Liang et al. 2015) x(Ba0.8Sr0.2)TiO3 with x=0.70 (Ba0.93Ca0.07)(Ti0.95Zr0.05)O3 387 44.2 (Li, Xu, et al. 2011) Li-doped 512 (Chen et al. 2015) Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) Ga2O3-doped 440 56 15.3 0.15 (Ma, Liu, and Li (Ba0.99Ca0.01)(Zr0.02Ti0.98)O3 2013) (BCZT-xGa) Bi0.5K0.5TiO3 69.8 517 (1 - 22.2 5.25 (Hiruma et al. 2005) MHz) KBT + 0.6wt% Bi2O3 101 764 (1 - 27.6 5.30 (Hiruma, Nagata, MHz) and Takenaka 2007) Bi0.5Na0.5TiO3 72.9 343 16.8 - - (Hiruma, Nagata, (1MHz) and Takenaka 2009) [Na0.5Bi0.49La0.01] TiO3 68 - 13.8 - - (Yi, Lee, and Hong 2004) [Bi0.5(Na 164-231 1,190 36.3- 38.8- 2.47- (Lin et al. 2006) 0.775K0.15Li0.075)0.5]TiO3 41.0 40.2 3.73 [Bi0.5(Na1-xKx)0.5] TiO3 x = 192 1,007 32.5 19.5 - (Zhang, Li, and 0.22 Zhang 2008) [Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 210 35.78 22.42 (Bhupaijit et al.) ] doped with La2O3 at 0.1 wt% (BNKLLT) ceramics Bi0.5(Na0.84K0.16)0.5TiO3 185 34.3 (Yoo et al. 2004) 0.995(Bi1/2Na1/2) TiO3- - 530-700 - 33.6 6 (Nagata, Koizumi, 0.005BiFeO3 (1 MHz) and Takenaka 1999) (Bi1/2Na1/2)0.78 167 - 35.5 27.6 2.79 (Yao et al. 2009) (Bi1/2K1/2)0.22TiO3- 0.03(Na0.5K0.5) NbO3 Na0.5K0.5NbO3 148 559 38.9 - - (Li et al. 2006) (100kHz) Mn-substituted KNN 178 15.48 (Dahiya et al. 2014) (Na0.5K0.5)1-x (LiSb)xNb1−xO3 286 1,372 51 - - (Zang et al. 2006b) (x = 0.052) (Na0.5K0.5)1-x (LiSb)x 286 1,372 51 - - (Zang et al. 2006b) Nb1−xO3 (x = 0.052) [Lix(Na0.5K0.5)1−x] NbO3 235 - 42 - - (Guo, Kakimoto, and x = 0.06 Ohsato 2004) 0.995(Na0.465K0.465Li0.07)Nb 258 (Bomlai, Muensit, O3 - 0.005 CaTiO3 ceramics and Milne 2012)

224 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

Table 4. (Continued)

퐓 Compositions d33 훆ퟑퟑ/훆ퟎ kp (%) Pr Ec Ref. (pC/N) (μC/ (kV/m cm2) m) 0.995(Na0.465K0.465Li0.07)NbO3 - 258 (Bomlai, Muensit, 0.005 CaTiO3 ceramics and Milne 2012) (1 − 176 (Wang, Wu, et al. x)[(K0.50Na0.50)0.955Li0.055]NbO3- 2009) x(Ag0.5K0.5)NbO3 with x = 0.02 (1 − 220 37 22.4 11.3 (Wang et al. 2008) x)[(Na0.4725K0.4725)Li0.055]NbO3– x(Ag0.5Li0.5)NbO3 (K0.5Na0.5)0.97Li0.03(Nb0.9 Ta0.1)O3 245 890 (Wu et al. 2009) (1-x) (0.95K0.5Na0.5NbO3- 265- 45–54 (Minhong et al. 2011) 0.05LiSbO3)-xBiScO3 0.002 ≤ x 305 ≤ 0.008 BiScO3-modified 280 49 (Li et al. 2010) (K0.475Na0.475Li0.05)(Nb0.95Sb0.05) O3 SrZrO3-modified Bi0.5Na0.5TiO3 102 32 (Maqbool et al. 2014) with x=0–0.15 0.99(K0.45Na0.52Li0.03)(Nb1−xSbx) 341 52.1 (Liu et al. 2014) O3-0.01BiScO3 (K0.40Na0.60)0.94Li0.06Nb0.94SbO3 272 43.5 (Chen et al. 2014) K0.5Na0.5NbO3-xLiTaO3 187 37 19.85 (Hao et al. 2014) Na0.47K0.47Li0.06NbO3 450 (Ray et al. 2015) (1−x)(0.948 K0.5Na0.5NbO3– 233 35 27.3 (Liu et al. 2011) 0.052LiSbO3)–xBiAlO3 CuO on Li/Ta-modified 180 36 > 20 40 (Shen, Xu, and Li (Na,K)NbO3 2012) CuO-doped Ba(Zr0.05Ti0.95)O3 320 44 (Liang et al. 2014) (BZT) ceramics Bi1/2 Na1/2)1−x Bax TiO3 TiO3 18 10 (Oh and Kim 2006) BiFeO3 15 - 60 ∼30 - - - (Catalan and Scott (GHz) 2009) BiFeO3 thin film 70 - - 50 - - (Wang et al. 2003) 60 BiFeO3 single crystal - - - 100 1.2 (Lebeugle et al. 2007) BiFeO3 ceramic 50 - 60 - - 40 - (Shvartsman et al. 2007) Bi0.86Sm0.14FeO3 thin film 110 - - 70 - (Fujino et al. 2008)

2.5. Comparison of Piezoelectric Properties of Lead Free Perovskite Ceramics

There are the various lead free piezoceramics which are used in piezoelectric energy harvesting applications. These compositions show relatively high piezoelectric constant (d33), and are suitable for actuator and high power applications. The most studied perovskite-based lead-free piezo-ceramics are listed in Table 4.

Piezoelectric Electroceramic Perovskites and Their Applications 225

3. SYNTHESIS TECHNIQUES FOR PIEZOELECTRIC PEROVSKITE CERAMICS

3.1. Solid State Reaction

Piezoelectric ceramics continue to be prepared from the most economical, Mixed Oxide (MO) processing, (Haertling 1986). This method is extensively used in the preparation of conventional ceramic products because of its simplicity and dependence on solid state reaction (Kingery 1960). In this method the oxides and carbonates are weighed and mixed thoroughly in dry or wet condition and then calcined at very high temperature (but the temperature should be less than the melting point of the different constituents) for certain hours. The reacted powder is remixed and calcined. This process is repeated till a homogeneous powder of the compound is achieved. A flow chart describing the essential steps for the MO process is given in Figure 8. After the calcination, powder is again mixed with an organic binder (Polyvinyl acetate) and then pressed by hydraulic press under the pressure of 6 to 8 × 107 kg/m2 in the desire shape and dimensions. The pallets are sintered at an appropriate temperature in an appropriate atmosphere. During the sintering process binder is burnt out at 3000C temperature. The density, porosity and X- ray structure are subsequently determined, after which the materials are cut and lapped to the final dimensions. Ceramic processing has profound effects upon the microstructure and electrical properties of given materials. These effects are

Figure 8. Flow chart of the processing route by high temperature solid state reaction method. 226 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

(1) The purity, crystal form and particle size of the raw materials, (2) The reactivity of the mixed materials and the minimization of the loss of volatile components. (3) The grinding method and control against impurity pick up and particle size.

A great deal of effort has been made to prepare the BZT ferroelectric compounds, in their polycrystalline form because of difficulties in achieving their perovskites single-phase form (Swartz and Shrout 1982). Depending on the processing conditions, a second phase (stable pyrochlore structure) detrimental to the dielectric properties of the materials may be present in the structure (Swartz et al. 1984) it has been found quite difficult to obtain the perovskite structure of the ferroelectric compounds using the conventional high temperature solid state reaction methods. Jang (S.J. Jang 1979) have reported that these compounds can be successfully fabricated using conventional methods which require repeated calcinations at very high temperature (>10000C) and long time (period 12hrs). This approach has found to be time consuming with problems in reproducibility and the control of BaO stoichiometry. In order to achieve the single-phase perovskite compound, a better understanding of the perovskite – pyrochlore problem is required.

3.2. Hydrothermal Method

Hydrothermal processing of materials encompasses processes like synthesis, crystal growth, treatment, fabrication, alteration, hot pressing, recycling, sintering, etc., under hydrothermal conditions. The materials processing under hydrothermal conditions dealing with the preparation of ultrafine particles, ceramics, whiskers, composites, thin films, and reinforcement. The hydrothermal preparation of very fine powders is an excellent approach to ideal powders. The ideal powder should have the following parameters: fine powder less than 1μm; soft or no agglomeration; narrow particle size distribution; morphology sphere or equiaxed; chemical composition controllable; microstructure controllable; uniformity; free flowing; and process control etc. The technique is especially handy for the preparation of perovskite ceramics, composites, and ultrafine particles with a desired shape. With the aim to obtain piezoelectric ceramic powders (e.g., KNN) at low temperature and to avoid the loss of sodium and potassium, the hydrothermal method has been used recently (Sun et al. 2007; Maeda et al. 2010). This method involves placing the reagents into a pressurized reactor or autoclave, the reaction is carried out at low temperature (< 300°C) where the pressure generated depends on the temperature at which the reactor is heated. The studies reported until now suggest a processing time of 12-48 hours at the desired temperature. Nevertheless, these studies also indicate that the resultant products are composed of two phases, a sodium rich phase and another with greater quantity of potassium. The reagents that have been used in these experiments are potassium and sodium hydroxides, whit a KOH/NaOH molar ratio between 3/1 and 4/1, and the total concentration around 6 M of hydroxides. Alternatively, the synthesis of KNN has also been reported by means of the microwave-hydrothermal method at 160°C for 7 hours with an alkalinity of 6 M (Zhou et al. 2010) the authors underline that improved piezoelectric constant d33 was obtained (126 pC/N), compared with other reports (80 and 90 pC/N), but important parameters like kp and tan δ were not reported. As a final comment for this synthesis section, it is important to Piezoelectric Electroceramic Perovskites and Their Applications 227 mention that the powder characteristics obtained by any synthesis method may aid the sintering stage, therefore the powders should be chemically pure i.e., without secondary phases, the calcination temperature (except in hydrothermal synthesis) must be as low as possible to avoid the considerable loss of alkaline compounds, and the nanometric powders are more suitable since these contribute to an additional driving force for sintering.

3.3. Sol-Gel Method

The use of sol-gel techniques to prepare ceramic materials ranks high among those areas in ceramic science and technology which are changing most rapidly and which offer the greatest promise for outstanding improvements in both understanding and applications. Taking into account the characteristics of the powders obtained by means of the conventional method, the so-called chemical routes have been investigated for the synthesis of lead-free ferroelectric ceramic powders. Among them, the sol-gel method (Shiratori et al. 2005) has been reported to produce KNN nanometric powders. The technique consists of mixing metal- organic compounds (mainly alkoxides) in an organic solvent, the subsequent addition of water generates two reactions, hydrolysis and polymerization, producing the gel which is dried and calcined for obtaining crystalline ceramics. The method has some advantages, such as the nanometric and chemical homogeneity of the powders and the low crystallization temperature (Shiratori et al. 2005). The disadvantages of this procedure are the utilization of metal-organic chemicals, which are expensive. Besides, they need of a strict control of the conditions for the reaction since they generally possess a different hydrolysis rate and must be handled under free moisture atmosphere for avoiding the rapid decomposition of alkoxides. The addition of organic compounds is necessary to improve the dispersion and to obtain fine powders.

4. MEASUREMENT TECHNIQUES

Microstructure studies able to investigate the internal structure of the material as a powder and in form pallets was carried out by using the technique reported in the following paragraph.

4.1. Scanning Electron Microscope (SEM)

Scanning electron microscopy (SEM) is the part of electron microscopy. The scanning electron microscopy (SEM) is one of the powerful techniques in determining the intrinsic properties of the materials based on size, porosity and inclusions. SEM possesses high magnification and resolving power with 3D images. There is an intimate relation between microstructure and properties of the ceramics (Park et al. 2015). SEM is used to determine the average crystallite size and to understand the surface morphology. It gives information about the grain evolution and grain size. It also gives the information about the intergranular and the intragranular pores and the distribution of grains in the bulk samples. The average grain size 228 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. was determined by the linear intercept technique (Wurst and Nelson 1972). Several lines were drawn on a photograph and the number of intercepts between the test lines and grain 1.56퐶 boundaries were counted. The average grain size was obtained using the relation 퐷 = , 푀푁 Where 'D' is the average grain size, 'N' is the number of intercepts and 'M' is the magnification of the photograph. Proportionality constant (c) 1.56 is a factor which was derived by Mendelson for random slices through a model system consisting of space filling tetrakaidecadrally shaped grains, with a long normal size distribution (Mendelson 1969). The following phenomena take place on the interaction of the electron and specimen.

Figure 9. Schematic illustration of how a SEM works.

The fundamental principle of scanning electron microscopy (SEM) is mainly based on a beam of electrons which are fired by electron gun down towards a sample, and is accelerated and focused though apertures and electromagnetic lenses, as illustrated in Figure 9. High resolutions can be obtained in SEM due to the small wavelength of electrons. The limitations of using an electron beam include the level of vacuum (and absorption by air molecules) and charging of the sample. SEM analysis is considered to be “non-destructive”; that is, X-Rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyse the same materials repeatedly (Suzuki 2002; Lawes 1987). The specimen under SEM investigation must be electrically conductive at least at the surface and electrically grounded to prevent the accumulation of electrostatic charge at the surface. The nonconductive specimen tends to charge accumulation which generally causes scanning faults and other imaging artefacts. Therefore it is usually coated with an ultrathin coating of electrically Piezoelectric Electroceramic Perovskites and Their Applications 229 conducting material, commonly gold deposited on the sample either by low vacuum sputter coating or by high vacuum evaporation (Lawes 1987; Jeffree and Read 1991). Essential components of all SEMs include the following:

 Electron Source (“Gun”), Electron Lenses, Sample Stage, Detectors for all signals of Interest, Display/Data output devices  Infrastructure Requirements: Power Supply, Vacuum System, Cooling system, Vibration-free floor, Room free of ambient magnetic and electric fields.

SEMs always have at least one detector (usually a secondary electron detector), and most have additional detectors. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates.

4.2. Transmission Electron Microscopy

In 1931, Max Knoll and Ernst Ruska invented the first transmission electron microscope (TEM), allowing the structural information on ferroelectric materials to be directly characterized (Fu, Zuo, and Xu 2011; Randall et al. 2003; Kling et al. 2010). For example, both orthorhombic-orthorhombic and paraelectric−ferroelectric phase transitions in SrBi4Ti4O15 have been directly observed by TEM (Reaney and Damjanovic 1996). Randall et al. (Randall et al. 2003) identified the differences between domain structure and symmetry of (1-x)BiScO3-xPbTiO3 materials using TEM, clearly showing the domain structures with R and T mixed phases. In addition, the domain structure evolutions under an applied electric field were characterized in situ in (Bi1/2Na1/2)-TiO3-BaTiO3-(K0.5Na0.5)NbO3 ceramics using TEM (Kling et al. 2010). This result indicates that the domain structure cannot be clearly observed when an electric field is retrieved, while a lamellar domain structure can be driven by an alternating electric field, graphically showing the involvement of electric field-induced phase transitions from a nonpolar to a ferroelectric state (Kling et al. 2010). Yao et al. (Yao et al. 2012) reported that the ferroelectric domains and local structures of Bi0.5Na0.5TiO3-BaTiO3 single crystals could be identified by TEM, the size of polar nano-regions were refined, and the tetragonal phase volume fraction was shown to increase as the BT content increased, indicating that both polarization rotation and polarization extension lead to large electric-field induced strains in materials with MPBs (Yao et al. 2012). Recently, Zuo et al.(Fu, Zuo, and Xu 2011) discussed the origin of the high piezoelectric activity in (Na0.52K0.48-x)(Nb0.92- xSb0.08)O3-xLiTaO3 ceramics by characterization of the domain evolutions using TEM, indicating that the nano-domain morphologies are responsible for their enhanced piezoelectricity. In the past, the relationships between grain size and domain of ferroelectric materials were extensively investigated using TEM in order to understand the size effect on their electrical properties (Zheng et al. 2015; Wurfel, Batra, and Jacobs 1973; Kwak et al. 1992; Scott, Araujo, and McMillan 1993; Udayakumar et al. 1995). It is well known that the domain size of ferroelectric materials is closely related to the corresponding grain size, (Udayakumar et al. 1995) i.e., Domain Size ≈ (Grain Size)m. According to this relation, the grain size of the samples used for TEM observation can strongly affect their domain size (Cao and Randall 1996), that is, the prepared samples are in 230 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al. agreement with the bulk domain structures. As a result, it becomes critical to accurately characterize the domains to evaluate the underlying physical mechanisms. To attain useful information about material structure and properties, TEM becomes an indispensable tool. However, the size effects of ferroelectric materials presumably play a part in the TEM observations of domains, since the TEM samples are quite thin. Therefore, the sample preparation technique is very important for TEM analysis in order to gain real insight into a material’s structure. It is usually very challenging to prepare a suitable sample of polycrystalline ceramics for TEM since some additional cracks and artifacts may be introduced during the sample preparation (Ma 2004). In addition to the TEM observation of domains, other tools (scanning electron microscopy (López-Juárez et al. 2011), Piezo- response force microscopy (Yao et al. 2014; Du 2014) should also be used to further confirm the domain structure and to avoid artificial information. As a result, TEM can directly characterize domain structure and morphologies, and the size effects should be given special attention in the observation of domains.

4.3. Piezoresponse force Microscopy (PFM)

Piezoresponse force microscopy (PFM) was first demonstrated by Guthner and Dransfeld in 1992 investigating a ferroelectric polymer film where they locally poled domains with the help of the tip and subsequently imaged the generated domain pattern (Güthner and Dransfeld 1992). Ferroelectrics are a subclass of piezoelectrics, namely, materials that experience mechanical deformation under applied voltage or charging under mechanical force. Ferroelectrics exhibit a wide range of functional properties, including high and switchable electric polarization, strong piezoelectricity, high nonlinear optical activity, outstanding pyroelectricity, and notable nonlinear dielectric behavior (Lines and Glass 1977). These properties are indispensable for the applications in numerous electronic devices such as sensors, actuators, infrared detectors, microwave filters, and recently, nonvolatile memories. Owing to this unique combination of properties, researchers and engineers have been focusing on visualization of ferroelectric domains (areas with unique polarization direction) at different scales. Recent advances in synthesis and fabrication of micro- and nanoscale ferroelectrics brought to life new physical phenomena and devices that need to be studied and understood at this scale (Gruverman and Kholkin 2006). As structure dimensions are getting smaller, ferroelectrics exhibit a pronounced size effect manifesting itself in a significant deviation of the properties of low-dimensional structures from their bulk analogs. In this sense, ferroelectrics are similar to magnetic materials because surface energy cannot be neglected in small volumes and long-range dipole interaction is significantly modified in reduced geometries. It also depends on whether a ferroelectric is confined in one-, two-, or all three-dimensional structures. In this sense, ferroelectrics are similar to magnetic materials because surface energy cannot be neglected in small volumes and long-range dipole interaction is significantly modified in reduced geometries. It also depends on whether a ferroelectric is confined in one-, two-, or all three-dimensional structures. Following the miniaturization challenge, novel techniques are required for the evaluation of ferroelectric and piezoelectric properties with the high, ultimately nanoscale resolution. Many fundamental issues have nowadays to be addressed such as effect of the geometry confinement on Piezoelectric Electroceramic Perovskites and Their Applications 231 ferroelectric and piezoelectric properties, relationship between local piezoresponse and macroscopic properties, as well as microscopic mechanisms of polarization switching, domain stability, and degradation, including polarization phenomena at the interface. Beyond the novel nanoscale applications, functionality of ferroelectric films, polycrystalline ceramics, and even single crystals is often dominated by defects (Levanyuk and Sigov 1988) that act as nucleation and pinning centers for moving domain walls and thus determine the piezoresponse. In addition, the unique electromechanical properties of relaxor ferroelectrics (Cross 1987) (materials with giant strain and dielectric constant) originate from the interplay of polarization (Rabe, Janser, and Arnold 1996; Yamanaka and Nakano 1998; Kester et al. 2000).

4.4. Ferroelectric (P-E) Hysteresis Measurements

Ferroelectric (P-E) hysteresis was measured by using Radiant Technologies, Precision High Voltage Interface Trek MODEL 609B at room temperature. A conventional Sawyer- Tower circuit was used to measure the polarization hysteresis (P-E) loop at 250C, 500C, 1000C and 1500C. A simple Sawyer-Tower circuit allows the measurement of all facets of ferroelectric capacitor characteristics. Sawyer-Tower circuit shown in Figure 10. With the Sawyer-Tower circuit one can measure

 Polarization Hysteresis  Switching and Non-switching Half-Loops  Fatigue  Retention  Imprint

The piezoelectric charge coefficient, d33, was directly recorded by a Berlincourt piezometer (Channel products, Inc., Chesterland, OH). Poisson’s ratio was measured from the (2) ratio of the first overtone to the fundamental resonance frequency 푓푠 /푓 of the planar mode in accordance with IEEE standards (Meeker 1996). Piezoelectric planar coupling coefficient

(푘푝) was calculated from the resonance and anti-resonance frequencies of the impedance 2 2 퐾푝 푝 2  −1 traces, based on the following relations 2 = ∆푓(휎 ) + 푝 , Where the fs and fp are 1−퐾푝 푓푠(1+휎 ) the measured fundamental resonance and anti-resonances, σp is the Poisson’s ratio and  is the frequency constant of a disk resonator. The planar mechanical quality factor (Qm) was calculated from relation

1 푄푚 = 퐿 푅√ 퐶푎

Where the R, L and Ca are the resistance, inductance and capacitance in the equivalent electrical circuit of the piezoelectric resonators, respectively.

232 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

Figure 10. Sawyer-Tower method implemented with an instrumentation amplifier.

The most often quoted method of hysteresis loop measurement is based on a paper by Sawyer and Tower. A variation of the Sawyer-Tower method, shown in Figure 10, replaces the oscilloscope with an instrumentation amplifier for measuring the shunt capacitor voltage. This has the advantage of a much higher input impedance than an oscilloscope which reduces drift-particularly important for low frequency measurements. It is also much easier to interface with a PC controlled data acquisition system for automation of experiments. Drift still occurs primarily due to input bias currents of the instrumentation amplifier. If a low input bias current amplifier is used, then this drift will be quite small, but can cause problems at very low frequencies (<1 Hz) or where measurements are made over a period of time. The drift can be measured by disconnecting the sample and compensation made in the analysis. Eventually the drift will move beyond the measuring range of the amplifier, so the circuit shown in Figure 10 is usually equipped with a reset switch to discharge the capacitor.

4.5. Raman Spectroscopy

Spectroscopy is used to investigate the interactions between a material and radiated energy; (Noheda et al. 1999; Souza Filho, Lima, Ayala, Guedes, Freire, Melo, Mendes Filho, Araújo, et al. 2002) thereby, the phase transitions of a material can be analyzed by spectroscopic techniques including Raman and infrared spectroscopies (Wylie-van Eerd et al. 2010). Raman spectroscopy is used to characterize vibrational, rotational, and other low- frequency modes in a material and can also identify the phase structure of a material (Noheda, Cox et al. 1999). The Raman spectra are usually plotted in intensity versus the difference in wave number between the incident beam and the scattered beam and the peaks are in correspondence to the phonon frequency. Due to the small wave vector of the optical photons, the phonons involved in the Raman scattering of crystalline solids have (from the wave vector conservation law) a very small momentum compared with the Brillouin zone. So only the zone-centered phonons participate to the Raman scattering. Light is treated as an electromagnetic wave and the molecules are modeled as small spheres connected by the spring as shown in the Figure 11. Recently, confocal Raman microscopy (CRM) coupled with atomic force microscopy has been used to observe the domains of KNN based materials (Rubio-Marcos et al. 2012). For example, Rubio-Marcos et al. identified the spatially resolved structure of the ferroelectric domains in (K,Na)NbO3-based ceramics by confocal Raman In crystalline solids, the Raman Effect deals with phonons, instead of molecular vibrations. The fundamental requirement of a phonon to be Raman active is that the first derivative of the polarisability with respect to the vibrational normal coordinate has a non-zero value. A phonon can be active only in the crystals with no center of inversion. Piezoelectric Electroceramic Perovskites and Their Applications 233

Figure 11. Model for the molecular vibrations in molecules.

4.6. Infrared Spectroscopy

Infrared spectroscopy uses radiation from the infrared region of the electromagnetic spectrum, covering a range of techniques based on absorption spectroscopy. It is well known that the onset of ferroelectric states can cause a change of infrared vibrational frequencies owing to its temperature-dependent phase transitions and thus can be used to characterize their phase transitions (Du 2014; Souza Filho, Lima, Ayala, Guedes, Freire, Melo, Mendes Filho, Araujo, et al. 2002). For example, Araujo et al. (Araújo et al. 2002) showed that monoclinic → tetragonal phase transitions in PbZr0.51Ti0.49O3 occur at 237 K by infrared spectroscopy, confirming this technique as a useful tool to analyze phase transitions. The monoclinic → tetragonal phase transitions in PbZr0.50Ti0.50O3 can be confirmed by infrared spectroscopy with different measurement temperatures, and the obvious differences in higher frequency ν1-(Ti-O) and ν1-(Zr-O) modes can show

5. APPLICATIONS OF PIEZOELECTRIC PEROVSKITE CERAMICS

Piezoelectric devices divided into four general categories, depending upon physical effect: generators, sensors, actuators, and transducers. Generators and sensors make use of the direct piezoelectric effect, meaning that mechanical energy is transformed into a dielectric displacement. This is measurable as a charge or voltage signal between the metallized surfaces of the piezoelectric material. Actuators make use of the inverse piezoelectric effect, meaning that transforming electrical energy into mechanical energy. Finally, in transducers both effects are used within one and the same device (Janocha 2004). Piezoelectric perovskites with high piezoelectric response are also used in current and next-generation military SONAR devices. When deformed by the external underwater sound vibrations, a piezoelectric perovskites material generates an electric field. This is then understood to gain information about depth, distance, and the identity of the source of the sound. The piezoelectric effect and its converse are the primary means used in biomedical ultrasound for converting acoustical energy into electrical energy and vice versa. Piezoelectricity has found many bioengineering applications ranging from ultrasound imaging and therapeutics, to piezoelectric surgery and micro-electro-mechanical systems, and to biomedical implants with associated energy harvesting (Manbachi and Cobbold 2011). 234 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

Important fields of application for soft piezo ceramics are: Actuators for micropositioning and nanopositioning, sensors, such as conventional vibration detectors, ultrasonic transmitters and receivers, e.g., for flow or level measurement, object identification or monitoring, as well as for electro-acoustic applications as sound transducers and microphones, and also as sound pickups on musical instruments. The advantages of these materials are their moderate permittivity, large piezoelectric coupling factors, high mechanical qualities and very good stability under high mechanical loads and operating field strengths. Low dielectric losses facilitate their continuous use in resonance mode with only low intrinsic heating of the component. Especially high-power acoustic applications benefit from the properties of hard piezo materials. Examples of their fields of application include ultrasonic cleaning (typically in the kHz frequency range), the machining of materials (ultrasonic welding, bonding, drilling, etc.), ultrasonic processors (e. g. to disperse liquid media), the medical sector (ultrasonic tartar removal, surgical instruments, etc.) and sonar technology.

5.1. Piezoelectric Energy Harvesting

The term “energy harvesting” refers to the generation of energy from sources such as ambient temperature, vibration or air flow. Converting the available energy from the environment allows a self-sufficient energy supply for small electric loads such as sensors or radio transmitters. Kinetic energy can be converted into electrical energy by means of the piezoelectric effect: Piezo elements convert the kinetic energy from vibrations or shocks into electrical energy. Using suitable electronics, this effect can be used for creating a self- sufficient energy supply system. This is of particular interest whenever a power supply via cable is not possible and the use of batteries and the associated maintenance expenditure are not desired.

The design of an energy harvesting solution depends on the characteristics and requirements of the environmental parameters. For the energy source, for example, one needs to distinguish between continuous and pulsed motions. Moreover, the conditions of the electric user must also be taken into consideration: The important parameters include the required voltage, the power and the input impedance, i.e., capacitive or resistive. With this data it is possible to design and dimension the transducer and storage electronics, including the mechanical system.

Energy Piezoelectric transducer Mechanics Consumer source

Piezoelectric materials can produce electrical charges when they are subject to external mechanical loads. Figure 12 shows working principle of a piece of piezoelectric material. The magnitude and direction of the electrical current are determined by the magnitude and direction of the external mechanical stress/strain applied to the materials. There have been various modes of vibration that can be used to construct piezoelectric harvesting devices.

Piezoelectric Electroceramic Perovskites and Their Applications 235

Figure 12. Schematic showing the response of a piece of piezoelectric ceramics to external mechanical stimulation (Kong et al. 2014).

5.2. Piezoelectric Microphones

A piezoelectric microphone consists of a thin diaphragm that is either provided with a piezoelectric material or mechanically connected to a bimorph bender, which is a cantilever beam of two layers of piezoelectric material having opposite polarizations. The movement of the diaphragm causes stress in the piezoelectric material, which generates an electric voltage.

5.3. PZT Thin-Film Microactuators

The measured d33 can now be used to design the PZT thin-film membrane microactuator shown in Figure 9. The microactuator consists of four parts: a membrane, a bulk silicon substrate, a PZT thin-film layer, and a pair of electrodes. (Note that the parts in Figure. 13 are not drawn in proportion.) The membrane is a moving component of the actuator anchored to the silicon substrate. As a result of its small thickness, the silicon membrane has low structural stiffness compared with the substrate. Often, the membrane can be fabricated by releasing part of the bulk silicon substrate, for example, using deep reactive ion etch (Oxford Instruments ICP 380). On top of the membrane is a layer of PZT thin film with a pair of electrodes. When a driving voltage is applied to the electrodes, the PZT thin film extends or contracts in the plane of the membrane; thus creating a bending moment to flex the membrane out of its plane Lee et al. (Lee, Cao, and Shen 2009) has studied the PZT thin-film membrane actuator extensively. Experimentally, they measure the actuator displacement using a laser Doppler vibrometer. They also measured actuator dimensions using SEM. Numerically, they predict the actuator displacement using piezoelectric constants from bulk PZT. The numerical predictions, however, disagree with the experimental measurements by an order of magnitude

5.4. Monitoring and Structural Health Monitoring (SHM)

A typical application example is the monitoring of components in complex systems such as airplane wings. Piezo transducers perform a series of functions, they measure deformations, but also supply transmitters with energy for wireless data transmission. 236 Poonam Kumari, Madan Lal, Shashi Prakash Rai et al.

Figure 13. Schematic drawing of PZT thin-film membrane actuator (Guo, Cao, and Shen 2013)

5.5. Product Monitoring During Transport

If vehicle vibrations are used for generating energy, products can be continuously monitored during transport without the corresponding sensors having to be connected to a power supply or equipped with batteries. This is useful if temperatures have to be recorded inside closed containers, for example.

5.6. Data Monitoring and Data Transmission

Power supply for sensors and radio transmitters, for example in heating and air conditioning technology for monitoring the temperature or the air flow in pipes.

CONCLUSION

It is very important to understand the physical origin of high piezoelectricity in order to investigate the relationships between the phase boundaries, phase compositions, and piezoelectric properties of perovskite electroceramics. The different piezoelectric parameters are discussed in this review. The different phase boundaries can be constructed by doping additives, then the phase compositions concerning phase boundaries can be optimized by refining the types and concentration of the additives, and finally the piezoelectric activity can be enhanced by modifying the phase compositions of perovskites. The greatest challenge for KNN-based materials is how to improve the stability of piezoelectricity in case of lead free perovskite electroceramics. It was found that the dopants at A or B sites of perovskite structure have improved the dielectric and piezoelectric properties; and has more significant influence on the stability of the crystal structures of KNN-based ceramics, respectively. As a result, phase boundaries of perovskites with MPB characteristics may be an effective way to solve this issue. Potassium−sodium niobate lead-free piezoceramics will be a promising lead- free materials for actuator applications. It was wondered whether a large strain can be induced by phase transitions. As a result, it may be very interesting to investigate the relationships between phase transitions and strains of potassium-sodium niobate materials. High- performance lead-free piezoceramics have become an international research frontier in the Piezoelectric Electroceramic Perovskites and Their Applications 237 fields of high technology and new materials. In this review, we systematically reviewed the developments of piezoelectric perovskite electroceramic with their phase boundaries as well as the piezoelectric properties of lead free-based materials, and some suggestions for the future development of KNN and BaTiO3 materials were also addressed. In the past decade, piezoelectric properties have been greatly increased by constructing phase boundaries, and piezoelectricity comparable to those of PZT-based materials has been achieved. It is believed that lead-free piezoelectric ceramics will be put into practical applications in various devices in the very near future.

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Zuo, Ruzhong, Danya Lv, Jian Fu, Yi Liu, and Longtu Li. 2009. “Phase transition and electrical properties of lead free (Na 0.5 K 0.5) NbO 3–BiAlO 3 ceramics.” Journal of Alloys and Compounds no. 476 (1):836-839. Zuo, Ruzhong, Chun Ye, and Xusheng Fang. 2007. “Dielectric and piezoelectric properties of lead free Na0. 5K0. 5NbO3–BiScO3 ceramics.” Japanese Journal of Applied Physics no. 46 (10R):6733.

Chapter 8

BIODIVERSITY AND SUSTAINABLE DEVELOPMENT

Naheed Ahmad1,*, Anjali Sharma2 and Radheshyam Rai1,2 1Department of Botany, Patna University, Patna, Bihar, India 2School of Physics and Materials Science, Shoolini University, Solan, India

ABSTRACT

Biodiversity is a modern term which simply means “the variety of life on earth.” Biologists most often define biodiversity as the “totality of genes, species, and ecosystems of a region.” Biodiversity is the result of 3.5 billion years of evolution and indicates that the living diversity is dynamic. It increases when new genetic variation is produced, a new species is created or a novel ecosystem is formed. It decreases when the genetic variation within the species decreases, or when a species becomes extinct or an ecosystem complex is lost. There is inter relationship between the living world and the processes. The survival of each is linked to the health of the two and together they comprise of the wealth of the ecosystem with huge number of species in them. Biodiversity is not evenly distributed. Diversity consistently measures higher in the tropics and lower in Polar Regions generally. Flora and fauna diversity depends on climate, altitude, soils and the presence of other species. Diversity is the reservoir of genetic traits which is present in wild varieties and in the traditionally grownland races which are extremely important in improving crop performance. However human activities have resulted in climate change leading to the mass rapid extinction. This is likely to precipitate the collapse of the ecosystems at a global scale. Increase in life span and greater number of neo-natals result in growth of populations. These along with consumerisms is leading to unsustainable life styles. The rapid industrial growth has made water pollution, air pollution, hazardous wastes and other pressing environmental problems specially in the developing world. Transboundary air pollution, water shortages, drinking water contamination, freshwater and marine pollution, deforestation, climatic disasters, and other environmental problems are posing serious threats to the well-being of people. It has been well established that human societies and cultures co-evolve with their environment.

* Corresponding Author address Email: [email protected]. 258 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

Biological resources including agro-biodiversity are the basis of life in the tropical world. What is weed for one is food for the other. In this type of scenario there should be all types of species of animals, plant and even microorganisms that are valuable to the different people of the world. So, we need to conserve at all level of diversity, that is- all ecosystems of the world (Terrestrial, Aquatic, Mangrove, Coral reefs etc.), diversity at the species level and genetic levels (all types of higher and lower plants and animals all types of viruses, bacteria and fungi) known or unknown to us, irrespective of its commercial importance, known to us. Thus conservation of biological diversity is important. The conservation of the environment includes the conservation of all the natural resources Thus there is need to readdress environmental challenges and an equal urgency for adopting integrated development which ensures environmental sustainability so that we humans have access to pure drinking water, food, clean air – healthy life.

Keywords: biodiversity, conservation, ecosystem, millennium development goals (MDGs)

1. INTRODUCTION

The earth and its environment is necessary for our survival, development and prosperity. Nature and environment provide essential goods and services for human development. Human well-being and quality of life depend crucially on the quantity and quality of food, water, energy and biodiversity available to man. In our haste to for development we have neglected and degraded the environment. The enormous environmental challenges we face are both global and local in origin (Milner-Gulland 2012; Hunter 2007). The evidences for the impact of global warming are mounting, and are becoming progressively more extreme in the past 20 years. Temperatures are rising. The oceans have absorbed most of the increased heat from global warming. This warming has caused the oceans to expand and, combined with melting glaciers and ice sheets, sea levels are rapidly increasing. Warmer sea temperatures lead to stronger storms. Storms are stronger. The past two years have seen records broken in the number and intensity of typhoons and hurricanes. Rain and floods are more intense. Globally, the frequency of major flooding events has increased in Europe, Americas and Asia. Tsunami is no longer an unheard word, 2005 and 2011 have seen some of the worst cases of mass destruction by nature. Drought and desertification is on the rise. Increased temperatures are changing disease patterns. For example, the land area affected by malaria is increasing. Rising temperatures mean that the malaria mosquito’s habitat is increasing in elevation. High altitude cities could become malaria endemic if temperatures continue to rise. Thus there is need to readdress environmental challenges and an equal urgency for adopting integrated development which ensures environmental sustainability so that we humans have access to pure drinking water, food, clean air –healthy life.

1.1. History of Development Predicaments

Increased productivity of food lead to greater number of births and with the discovery of antibiotics the mortality rates went down. The industrial revolution brought about greater comforts in life and the life expectancy increased. Food surpluses needed to be marked and Biodiversity and Sustainable Development 259 hence roadways and bazaars developed. Societies evolved from agrarian to industrial type. Material comforts and availability of goods made roads to urban developments and cities were formed. The big cities attained inordinately large population size due to better access of facilities like electricity, roads, transport, medical facilities, job opportunities and markets. Human technological advancements make it possible to sustain larger and larger population by exploiting more and more natural resources (Rotherham 2015). Industrialization is central to economic development and improved prospects for human well- being. However the industries depended on the natural resources for functioning which lead to deforestation, Massive deforestation leads to a vicious cycle of environmental degradation. As population pressure increases, particularly in the highlands, farmers intensively exercise deforestation for firewood, building materials and animal fodder. The loss of forest cover leads to massive erosion. The water that runs freely down the denuded slopes adds to flooding conditions and blocks natural springs with a heavy cap of mud. The loss of trees means that nature’s natural cycle is broken and soil is not fertilized through the breakdown of vegetation. The water that runs freely down the denuded slopes adds to flooding conditions and blocks natural springs with a heavy cap of mud. Deforestation means that the forest habitat is lost. Many developing counties like India are home to a rich bounty of diverse plants and animals. But of all species evaluated in many are threatened to various degrees, including the Bengal lion, the elephant, the rhino, various types of antelope and gazelle, the cheetah, eagles and falcons, Biodiversity, crucial for delivering ecosystem services (Ibisch, Vega, and Herrmann 2010), is deteriorating at an unprecedented rate. Africa and Asia face the highest loss rates currently. The most important pressure factor is agricultural expansion. Nature and environment provide essential goods and services for human development. Human well-being and quality of life depend crucially on the quantity and quality of food, water, energy and biodiversity available to man.

Figure 1. Deforestation and cooking fuel (chain of effects through ecosystem and social system.

260 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

1.2. Consequence of Lopsided Development

The three revolutions of human history, agricultural, industrial and green have all been responsible for overpopulation. As a consequence, industrialization leads to environmental degradation. Degradation of the air, defiling of the soil, deterioration in water quality and deforestation all these lead to soil erosion, desertification, trapping of green-house gases which challenged the entire earth. Today, we are experiencing the floods, droughts, storms hurricanes and tsunamis -all cumulative effects of Global warming caused by defiling the environment.

1.3. Factors Responsible for the Degradation of Environment:

A Number of Factors Are Responsible for the Environmental Problems of Today. They are as:

1.3.1. Population Population growth is the primary source of environmental damage. Yet, the human population challenge has really occurred only recently. More people have been added to the Earth’s population in the 20th century than at any other time in human history. In 1900, just 100 years ago, the world’s human population numbered two billion people. Today, the total human population has grown three times as large and is now over six billion people.

The number of people who use the industrial and agricultural products. A small population can enjoy high levels of consumption without placing excessive demands on the environment. The level of consumption of wealthy nations is enormously greater than that of poor nations. The significance of the population in wealthy nations lies not only in the large numbers of people that they already have but also the fact that their heavy demands extend to ecosystems beyond their own boundaries. Developing world nations aspire to economic development with higher levels of industrial production and consumption, aspirations that are thwarted by rapid population growth now typical in that part of the world.

Biodiversity and Sustainable Development 261

Figure 2. Showing interrelationship between social system and ecosystem.

Humans are part of the ecosystem. The human - environment interaction affects the social system and the rest of the ecosystem. The social system is a central concept in human ecology. This is because human activities have impact on ecosystems. These activities are strongly influenced by the society in which people live. Values and knowledge - which together form our perception as individuals and as a society translate it into action. These belief and practices involve the ecosystem services. These ecosystem services include water, fuel, food, materials for clothing, construction materials and recreation. Material, energy and information move from social system to ecosystem as a consequence of human activities that impact the ecosystem:

 People affect ecosystems when they use resources such as water, fish, timber and livestock grazing land.  After using materials from ecosystems, people return the materials to ecosystems as waste.  People intentionally modify or reorganize existing ecosystems, or create new ones, to better serve their needs.

Transfer of information, energy and material all result in modification reorganization of the ecosystem. As for example the crop that a farmer plants, the spacing of plants in the field, results in alteration of the field’s biological community by weeding. It also modifies the soil chemistry with fertilizer applications. Thus they are not only material transfers but also information transfers as the farmer restructures the organization of his farm ecosystem Deforestation is another chain reaction which affects the ecosystem. Fuel is an issue with major part of underdeveloped rural population. For thousands of years people in have cut branches from trees and bushes to provide fuel for cooking their food. This was not a problem as long as there were not too many people; but the situation has changed with the radical increase in population during the past 50 years (see Figure 1). Many forests have disappeared in recent years because people have cut so many trees and bushes for cooking fuel. Now there are not enough trees and bushes to provide all the fuel that people need. People have 262 Naheed Ahmad, Anjali Sharma and Radheshyam Rai responded to this ‘energy crisis’ by having their children search for anything that can be burned, such as twigs, crop residues (bits of plants left in farm fields after the harvest) and cow dung. Fuel collection makes children even more valuable to their families, so parents have more children. The resulting increase in population leads to more demand for fuel. This chain of effects involving human population growth, deforestation, fuel shortage and lower food production is a vicious cycle that is difficult to escape. Increasing growth of populations due to increase in life span and greater number of neo natals along with increasing consumerisms leads to unsustainable ecosystems. Intense use of non-renewable resources exhausts the supply more quickly. Sustainable interaction with ecosystems is only possible if demands are kept within bounds. Intensity of demands on ecosystems:

 The total quantity of material and energy resources required for industrial and agricultural production; plus  Pollution generated by industrial and agricultural production.

1.3.2. Industrialization The Industrial Revolution began in England in the middle of the 18th century. A variety of ingenious inventions paved the way for machines to be used in production. The industrial societies replaced the human labor and deeply altered human and environmental history. Populations in industrialized countries moved from rural areas to urban areas and development of cities and towns. While the Industrial Revolution meant that more goods could be produced for human consumption, it also meant that more pollution would be emitted into the sky and more natural resources would have to be exploited in the production process. Harmful waste produced as a byproduct resulted in pollution of water, air and soil. All of these problems pose significant threats to both the environment and to human life. This rapid industrial growth has made water pollution, air pollution, and hazardous wastes pressing environmental problems in many areas of the developing world. Trans boundary air pollution, water shortages, drinking water contamination, freshwater and marine pollution, deforestation, climatic disasters, and other environmental problems are becoming serious threats to the well-being of people in this densely populated region.

1.3.3. Agriculture Development Agriculture was also affected by industrialization. More advanced machines and techniques for farming became available. These new methods like use of fertilizers, use of synthetic pesticide, selective breeding- practicing monocultures etc. caused the soil to degenerate. Mechanized farming also increased the amount of land used in farming - ruining animal and plant habitats. Over-working of the land has led to soil depleted of nutrients. The ever increasing population leads to an upward pressure on the demand for land. There is greater need for housing and arable land for cultivation. This demand leads to deforestation, filling up of wetlands and reclaiming sea etc. The result of these activities is habitat loss and fragmentations accompanied by loss of keystone species. This demand may even further increase if the international market for biofuels further develops. Moving to more intense practices may offset, at least in part, the need for expansion Biodiversity and Sustainable Development 263 of agricultural land. However, higher yields are often associated with higher emissions to air, water and soil. Increasing inputs of nutrients in agriculture results in eutrophication of inland water bodies and coastal waters pose risks to health and fresh water and marine ecosystems (algae blooms, “dead zones”). Over fishing of the world’s marine stocks has an important impact on ecosystems and biodiversity. A large coastal population depends on fisheries for food and employment. Reducing destructive fishing practices asks for reducing fishing effort, transforming market and governance. Solutions have to be location specific to the different marine ecosystems.

2. PROBLEMS OF THE WORLD TODAY

2.1. Introduction

With the understanding that global economies, ecosystems, and human needs are inextricably linked, the global society is now concerned. The impact of climate change has started effecting the world there is an increased interest and concern about on loss of biodiversity along with other environmental issues. Sustainable development has become an important issue for international dialogue. Population growth is the primary source of environmental damage. Yet, the human population challenge has really occurred only recently. More people have been added to the Earth’s population in the 20th century than at any other time in human history. In 1900, just 100 years ago, the world’s human population numbered two billion people. Today, the total human population has grown three times as large and is now over six billion people. Before 1900, when antibiotics had not been discovered, young children died of many diseases that we now immunize against such as diphtheria, tetanus, measles, pneumonia and whooping cough. In the 20th century, as these diseases became less common, more children lived to adulthood. The result was that more children than ever before were born and lived and had their own children, all of which increased the size of the world’s population. At the same time, people are also living longer. The average life expectancy in 1950 was 40 years. Now people, on average, can expect to live 77 years. People living longer increase the population size, and this means that more people are living together on Earth at the same time. Water stress will increase substantially in large parts of the world with growing population and expanding economies. Climate change enhances this development even more in several already vulnerable regions (e.g., Africa and parts of Asia). Competition for water between users (including the natural environment) requires strong water management preferably at river basin level. Promoting water supply and sanitation seems to be the most effective direct environmental measure to be taken to enhance human health.

264 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

Energy is crucial for poverty alleviation and economic development. Conventional development in energy is not realizing the necessary improvements in access to energy these conventional sources are expected to lead to a serious increase of urban and regional air pollution in developing countries, along with a further increase of greenhouse gas concentrations in the atmosphere. Limiting the risks of climate change for sustainable development requires global greenhouse gas emissions to be significantly reduced during this century. Solution to these problems are of local level and require low-technology but high awareness along with an integrated approach to social and economic development and environment issues.

2.2. Causes of Loss of Biodiversity

 Pollution resulting in desecration of the environment both terrestrial and aquatic.  Habitat destruction due to developmental or human intervention.  Overexploitation due to rising population and economic demand.  Genetic drift and inbreeding.

2.3. Concept and Importance of Conservation

Extinction is a fact of life. Species have been evolving and dying out ever since the origin of life. However, species are now becoming extinct at an alarming rate, almost entirely as a direct result of human activities (Mawdsley, O’Malley, and Ojima 2009; Brickhill 2015; Mace, Norris, and Fitter 2012). The loss of species in tropical ecosystems such as the rain forests and the loss of habitat is of great concern. This mass extinction greater in quantity than that which removed dinosaurs from the earth. Biodiversities are product of process of evolution and indicates that the living diversity is dynamic. It increases when new genetic variation is produced, a new species is created or a novel ecosystem is formed. It decreases when the genetic variation within the species Biodiversity and Sustainable Development 265 decreases or when a species becomes extinct or an ecosystem complex is lost. There is interrelationship between the living world and the processes. The survival of each is linked to the health of the two and together they comprise of the wealth of the ecosystem with huge number of species in them. The mass rapid extinction which is taking place, is thus likely to precipitate the collapse of the ecosystems at a global scale. India is among the mega diversity centres of the world. India possesses considerable biodiversity and is home to a number of endemic plants and animal species. Human societies and cultures coevolve with their environment. Thus conservation of biological diversity is important Thus, conservation of the environment includes the conservation of all the natural resources.

2.4. Why to Conserve

Conservation is the protection of biodiversity for sustainable utilization. Individual species and ecosystems have evolved over millions of years into a complex interdependence. This can be viewed as being akin to a vast jigsaw puzzle of inter-locking pieces. If one removes enough of the key pieces on which the framework is based then the whole picture may be in danger of collapsing. We have no idea how many key ‘pieces’ we can afford to lose before this might happen, nor even in many cases, which are the key pieces. The ecological arguments for conserving biodiversity are therefore based on the premise that we need to preserve biodiversity in order to maintain our own life support systems (Donohoe 2003). Two linked issues which are currently of great ecological concern include world-wide deforestation and global climate change. Forests not only harbour untold numbers of different species, but also play a critical role in regulating climate. Forests also affect rainfall patterns through transpiration losses and protect the watershed of vast areas. Deforestation therefore results in local changes in the amount and distribution of rainfall. It often also results in erosion and loss of soil and often to flooding. The destruction of forest, particularly by burning, results in great increases in the amount of carbon in the atmosphere. This is significant because carbon dioxide is one of the main greenhouse gases implicated in the current global warming trend. Associated with the rising temperatures is the rising sea levels which could drown many of our major cities, extreme weather conditions resulting in drought, flooding and hurricanes, together with changes in the distribution of disease-bearing organisms are all predicted effects of climate change. The effects described translate directly into economic effects on human populations. As for example environmental disasters such as floods, forest fires and hurricanes indirectly or directly caused by human activities, all have dire economic consequences for the regions afflicted. Further, erosion and desertification, often as a result of deforestation, reduce the ability of people to grow crops and to feed themselves. This leads to economic dependence on other nations. Large-scale habitat and biodiversity losses mean that species with potentially great economic importance may become extinct before they are even discovered. The vast, largely untapped resource of medicines and useful chemicals contained in wild species may disappear forever. The wealth of species contained in tropical rain forests may harbour untold numbers 266 Naheed Ahmad, Anjali Sharma and Radheshyam Rai of chemically or medically useful species. Many marine species defend themselves chemically and this also represents a rich potential source of new economically important medicines. Additionally, the wild relatives of our cultivated crop plants provide an invaluable reservoir of genetic material to aid in the production of new varieties of crops. If all these are lost, then our crop plants also become more vulnerable to extinction.

3. BIODIVERSITY

Biodiversity is a modern term which simply means “The variety of life on earth.” Diversity is a resource for the species own survival and future evolution (Dale 2009). This variety can be measured on several different levels such as –

3.1. Genetic

Variation between individuals of the same species e.g., plants population with different flower colour in humans genetic diversity is observable in the variation in eye size, eye color etc. This includes genetic variation between individuals in a single population, as well as variations between different populations of the same species (Vellend and Geber 2005; Van Dyke 2008). These differences are the raw material of evolution. The genetic constitution of an organism - the arrangement of the DNA into genes on the chromosomes - is also referred to as its genotype. Hence, variation that exists within the genetic constitution of an organism is often referred to as genotypic variation. In any species genetic diversity is the variation of heritable characteristics present. Genetic diversity refers to variation in the nucleotides, genes, chromosomes, or whole genomes of organisms. Different species can have different numbers of genes within the entire DNA or genome of the organism. Besides having distinct combinations of genes, species may also have variation in the shape and composition of the chromosomes carrying the genes in the total number of chromosomes present. The DNA is contained in the chromosomes present within the cell can be the chromosomes of mitochondria and chloroplast). Nucleotide variation is measured for sections of the chromosomes, called genes. Thus, each gene compromises a hereditary section of DNA that occupies a specific place of the chromosome, and controls a particular characteristic of an organism. At its most elementary level it is represented by differences in the sequences of nucleotides (adenine, cytosine, guanine, and thymine) that are the units of the DNA (deoxyribonucleic acid) present within the cells of the organism. Within any single organism, there may be variation between the two (or more) alleles for each gene. As each allele codes for the production of amino acids that express together to form proteins. Thus the differences in the nucleotide sequences of alleles result in the production of slightly different amino acids or variant forms of the proteins. These proteins are responsible for characteristics of the organism both at the developmental levels of morphological, anatomical and physiological features of the organism. This variation is introduced either through mutation of one of the alleles, or as a result of recombination of genes during sexual reproduction. It can also occur Biodiversity and Sustainable Development 267 when there has been migration or hybridization of organisms, specially so when the parents come from different populations and gene pools. Each population of organism is distinct from the other due to the presence of unique genetic characteristics. Greater the population greater is the diversity of alleles. This diversity of alleles indicates a greater potential for the evolution of new combinations of genes and, subsequently, a greater capacity for evolutionary adaptation to different environmental conditions. In small populations, the individuals are likely to be genetically, anatomically, and physiologically more homogeneous than in larger populations and less able to adapt to different environmental conditions Genetic diversity is usually mentioned with reference to agriculture and maintaining food security. This is because genetic erosion of several crops has already occurred leading to the world’s dependence for food on just a few species. Currently, a mere 100-odd species account for 90% of the supply of food crops, and three crops – rice, maize, and wheat – account for 69% of the calories and 56% of the proteins that people derive from plants. The analysis of genetic diversity is a key element for the study of biodiversity, ecosystem functioning, and the consequences of man-made impact on natural systems such as climate change, habitat fragmentation, and biological invasions. Genetic diversity is also central for the understanding of epidemics, the impact of diseases, or the effects of medical intervention. To measure genetic diversity within and among populations using a variety of modern molecular technologies. These include:

 DNA sequences  DNA fingerprints  Hypervariable microsatellite DNA loci  Targeted analysis of fitness-related loci

3.2. Species

Species diversity is one component of the concept of biodiversity. The species can be a microbes, plant or animal, small or large. The total number of known and identified species of Earth is more than 1.7 million which includes bacteria, flowering plants, insects, birds, mammals. This is usually referred to as the taxonomic or phylogenetic diversity. The species diversity is the variety of species in a given region or area. Species diversity is a measure of the diversity within an ecological community that incorporates both species richness (the number of species in a community) and the evenness of species’ abundances. This can either be determined by counting the number of different species present, or by determining taxonomic diversity. Species diversity is influenced by species richness. The diversity of species present in an ecosystem can be used as one gauge of the health of an ecosystem. Species richness is a measure of the number of different species present in an ecosystem, while species evenness measures the relative abundance of the various populations present in an ecosystem. In an ecological survey designed to measure species diversity, a wildlife biologist might determine the number of individuals of each species present in an area, then calculate a “diversity index” for the area. Comparison of the diversity 268 Naheed Ahmad, Anjali Sharma and Radheshyam Rai index with that of other areas provides insights into the species diversity and the health of the ecosystem. In general the greater the species diversity better is the health of the ecosystem. A rich ecosystem with high species diversity has a large value for the Shannon Diversity Index (H’), while an ecosystem with little diversity has a low H.’

3.3. Ecosystem Diversity

Ecosystem diversity refers to the variety of habitats and climates on Earth. It also incorporates community diversity. Communities of plants and animals, together with the physical characteristics of their environment (e.g., geology, soil and climate) interlink together as an ecological system, or ‘ecosystem. An ecosystem is a unique combination of plant, animal and microorganism communities and their non-living physical characteristics interacting as a functional unit. Inherent in ecosystem diversity are thus both biotic (living) and abiotic (non-living) components, which makes it different from both genetic and species diversity. Ecosystems can be terrestrial, Aquatic, Mangrove, Coral reefs or can be manmade cities, parks, agricultural farms etc. There are interactions between these ecosystems and the species that live in them. This biodiversity helps keep our air and water clean, regulates our climate, and provides us (and other plants and animals) with food, shelter, clothing, medicine, and other useful products. In brief the ecosystem diversity ensures the health and sustenance of the entire planet. The global biosphere is a complex system. Ecosystems are structured -- can be viewed as a series of biotic components that are linked together and thus interact with one another like a cybernetic web. The fact that ecosystem components are linked has an important ramification: disturbances to one component impact on all other components of the ecosystem to varying degrees. Nutrient cycling. Energy Flow is a one-way process in ecosystems -- in order to persist, ecosystems require a constant input of energy. The sun is the ultimate source of energy for most ecosystems. Primary producers capture a fraction of energy in sunlight striking the earth and convert it into chemical energy (carbohydrate) that is stored in tissues of the primary. Energy in tissues of primary producers transferred to consumers as each consumes tissue of other organisms -- about 90% - 95% of energy present in one component is lost as heat at each transfer -- very inefficient process -- very little energy left when decomposers get to it. The energy of an ecosystem occurs on an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic level of a food web. All energy transfers in ecosystems follow the laws of thermodynamics. No life can exist without water. Requirement of water varies from organism to organism and their distribution varies according to their needs e.g., xerophytes, Hydrophytes, Mesophytes. The movement of mineral ions and molecules in and out of ecosystems occurs through biogeochemical cycles. The composition of the atmosphere is due to biochemical processes in soil and biota as well as chemical and photochemical processes in the atmosphere. The atmospheric concentration of carbon dioxide is markedly influenced by the terrestrial and marine ecosystems and, in the long term perspective, also by geological processes. The water or hydrologic cycle intersect and connects with most of the other element cycles, including the cycles of carbon, nitrogen, sulfur, and phosphorus, as well as the sedimentary cycle. The processes involving each one of these elements may be strongly coupled with that of other elements, and ultimately, with Biodiversity and Sustainable Development 269 regional and global climate. The atmospheric concentration of carbon dioxide is markedly influenced by the terrestrial and marine ecosystems and, in the long term perspective, also by geological processes.

Biogeochemical cycles always involve equilibrium states: a balance in the cycling of the element between compartments of life. Life and climate interact. As climates change and human impacts are being felt the biogeochemical cycles are being studied to conserve the balance of these cycles. Hence healthy biosphere is critical for the survival and health of human beings. Life on Earth as we know it would not exist if its rich biodiversity were severely altered. All living organisms, including humans and the ecosystems in which they live are linked together though the flows of energy and materials. Harming the world’s biodiversity could have serious repercussions. One change in the life of one species could be the start of extinction for many other species including humans. It is thus obvious that all the three levels of diversity are interconnected or have a cybernetic web. Hence there is need to conserve biodiversity at the ecosystem level, species level and also at the genetic level for a sustainable a healthy existence on earth (Rotherham 2015).

3.4. Technologies Used for Conservation of Biodiversity

 All green technologies for production of various industrialized products which would lessen toxic products released into environment  Initiatives for the use of renewable energy –solar, wind and water  Initiatives for development and use of biofuels. 270 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

 Recycling and purification of fresh water.  Biological aspects would include:

In Situ conservation Ex- Situ conservation

1. In Vitro – like Seed banks, Pollen banks, synseeds etc. 2. In Vivo – Botanical garden, Arborata, Zoological garden etc.

Ecosystem conservation or habitat restoration. It refers to the task of “fixing” damaged ecosystems Species level conservation practices like:

 Germplasm preservation, particularly in case of heterozygous hybrids, plants where seeds are recalcitrant or not produced, or if the plant is diseased and/or plant material is very limited.  Recombinant DNA technology e.g., transferring Bacillus thuringiensis (Bt) gene, for production of Bt cotton Bt Brinjal, Agrobacterium-mediated transformation of lettuce: Agrobacterium rhizogenes transformed hairy root culture of Hyoscyamus muticus using a plant bioreactor, scaling up of the desired plant metabolite through DNA technology. Genetic engineering of polyamine metabolism with a view to studying plant growth and development as well as to inducing stress tolerance in rice, eggplant and tobacco.  Tissue culture for on mass propagation of economically important and threatened species.  Pollen banks  DNA banks  Field gene Banks

Who Should Conserve? It was realized that conservation of biological diversity is common concern of human kind and it is thus imperative to anticipate, prevent and tackle loss of biodiversity. Human culture co- evolve with environment and the conservation of biological diversity can be important to for cultural identity, especially in India. Thus diversity has to be conserved by all individuals, all societies, all nations and all governments. It was with this realization that that the convention on biological diversity (CBD) came into force on 29th Dec 1992. The CBD provides the global mechanism to ensure the conservation, sharing of genetical resources technology and sustainable use of biodiversity for the present and future generations. India signed the biological diversity Act and became a signatory to CBD in Dec 2002. the Act passed directed all Central and state governments, industries and institutions “ to provide means and ways for conservation of biological diversity, sustainable use of its components and equitable sharing of benefits of biological resources and for maters concerned herewith” Farmer’s especially small-scale, resource-poor farmers could be excellent conservators of biodiversity. They breed local crop varieties for improved production using informal innovation systems based on indigenous knowledge. They often employ their own taxonomy, encourage introgression, select, hybridize, field test, record data and name their varieties. Biodiversity and Sustainable Development 271

Researchers found that farmers evaluate cultivars using a wide variety of criteria that can be of immense interest and value to crop breeders. For example, the farmers’ evaluation of a high-yielding hybrid maize variety and description of the positive and negative characteristics of locally-adapted open-pollinated varieties led to a more effective national maize breeding program. In India local communities had the practiced the conservation of nature by having “sacred grooves “which ensured conservation and preservation of local vegetation for posterity. Further temple trees and different coloured flowers and fruits, animals (insects, reptile’s mammals) to be presented to deities (Gods) or as incarnations of gods was yet another way Indians practiced conservation of diversity.

4. TECHNOLOGY AND DEVELOPMENT

It has now been well established that the positive economic and social results of industrial growth has been accompanied by serious environmental degradation, as well as growing threats to health from occupational hazards. This demand is driven largely by population, economic activity, and income and continues to set the broad context for resource management. Whether technology is used to protect or merely exploit the natural environment will be one of the most challenging policy choices of the 21st century. Recognizing this distinction is the key to the appropriate development and application of technology. The demand for renewable resources is being regarded as the best achieve a balance of environmental and economic goals. Thus there is need to adopt green methods of development, which generate clean renewable energy and also technologies that will reduce greenhouse emissions. Green technologies include all those methods which can be adopted for a clean environment. As for example renewable sources for energy generation like solar, hydro or wind. Recycling of water, paper, wood etc. Reduction in toxic wastes produced by various industries.

Role of Governments and Various Agencies

Indigenous knowledge and biodiversity are complementary phenomena essential to human development. Global awareness of the crisis concerning the conservation of biodiversity is assured following the United Nations Conference on Environment and Development held in June 1992 in Rio de Janeiro. The importance of the indigenous knowledge which reflected many generations of experience and problem-solving by thousands of ethnic groups across the globe was emphasized. Concerns raised about this unrecorded immensely valuable information, which could serve as data base for existing floral and faunal resources. Indigenous knowledge, has long been ignored and maligned by outsiders. Today, however, a growing number of Indian governments and international development agencies are recognizing that local-level knowledge and organizations provide the foundation for participatory approaches to development that are both cost-effective and sustainable. 272 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

A vast heritage of knowledge about species, ecosystems, and their use exists, but it does not appear in the world literature, being either insufficiently ‘scientific’ or not ‘developmental.’ Much of this information can be interpreted only by local scientists. Hence there is a research priority for documentation and compilation of indigenous knowledge. Thus an active role has to be played by rural communities in India and other parts of the world for (a) generating knowledge based on a sophisticated understanding of their environment, (b) devising mechanisms to conserve and sustain their natural resources, and (c) establishing community-based organizations that serve as forums for identifying problems and dealing with them through local-level experimentation, innovation, and exchange of information with other societies.

What Are Sustainable Approaches?

Sustainable development is a pattern of resource use that aims to meet human needs while preserving the environment so that needs can be met not only in the present but also be available for the future generations. Development should be done along these lines so that there is no adverse impact on the global or local environment, community or the society. During the past decade a rapidly growing set of evidence indicates a strong relationship between indigenous knowledge and sustainable development. Serious investigation of indigenous knowledge of ecological zones, natural resources, agriculture, aquaculture, forest and game management, to be far more sophisticated than previously assumed. And they offer new models for development that are both ecologically and socially sound.” Ancient farming practices recognized the advantages of poly crops where the farmer used to plant a number different crops at the same time called polyculturing. As for example for growing rice with maize was intermixed for cultivation. The nodules present in the maize provide nitrogen to the soil, thus enriching it. Local farmers also “recognize” several dozen different potato, rice, wheat, pulse varieties, which they distinguish according to plant and tuber traits, as well as agronomic and culinary characteristic, important differences in taste, texture, storability, marketability, disease and pest resistance, and response to moisture stress. These varieties are actually land races in the complex agricultural systems and constitute an important part of the world crop genetic heritage. These ecologically complex agricultural systems are often associated with centers of crop genetic diversity which include not only the traditional cultivars or ‘landraces’ but also wild plant and animal species that serve humanity as biological resources. For each crop the traditional farmer knows at least nine possible end uses, many of them simultaneously relevant on a single farm.

Why Bother for Sustainable Approach?

Environmental degradation in intricately linked with problems of poverty, hunger, gender inequality and health. Protecting and managing natural resource for economic and social development and changing consumption and production patterns are fundamental requirements for poverty eradication. Integrating the principles and practices of Biodiversity and Sustainable Development 273 environmental sustainability into countries policies and training programs is key to successful poverty reduction strategy.

Ancient Ways and Sustainable Approach

Achieving sustainable environment first requires understanding the drivers of environmental changes. Sustainable agriculture in all nations will require greater scientific respect for, and more effective collaboration with, those who possess the wisdom of generations of ‘nonscientific’ farming.” The recording of the indigenous agricultural knowledge for a given ethnic group can provide important guidance for the research agenda for both national and international agricultural research centers. Thus the use of farm manure and bio fertilizers which was ignored is now again being thought as an attractive option. Polyculture is the norm in farming systems in India and other parts of the world, “a traditional strategy to promote diet diversity, income generation, production stability, minimization of risk, reduced insect and disease incidence, efficient use of labor, intensification of production with limited resources and maximization of returns under low levels of technology.”

Rain water harvesting is another area in which the traditional people used to conserve fresh water.

Steps by Various Global Bodies for Conservation

Two influential policy documents have recently been prepared by the National Research Council,  One focused on the conservation of biodiversity. 274 Naheed Ahmad, Anjali Sharma and Radheshyam Rai

 The other on sustainability issues in agriculture and natural resource management which included safeguarding Indigenous knowledge.

The International Society of Ethno-biology has played a key role in formulating the inextricable link between cultural and biological diversity. The most cost-effective way in which indigenous knowledge can be systematically recorded and stored so it can be used to facilitate national development efforts may be through the growing global network of indigenous knowledge resource centers. There are now ten formally established centers, three with global functions (CIKARD, LEAD, and CIRAN), two with regional roles (ARCIK, REPPIKA), and five with national roles (Gha RCIK, INRIK, RIDSCA, KENRIK, PhiR CIKSD). According to Mooney1992 “The formal sector is only starting to open its eyes to the fact that farmers innovate and that local communities do and can contribute to conservation and breeding. If the world is properly to conserve and use genetic resources for both present and future generations, the informal sector of the Third World, that is, the farmers, herbalists, gardeners and pastoralists, must lead us into the next agricultural revolution.” Reaching the Millennium Development Goals (MDGs) will be highly dependent on environmental conditions. However, this crucial role of environment is only partly reflected in the MDG-framework. Often there will be direct conflicts between human development and environment. Further elaborating MDG 7, including quantified objectives would help ensuring environmental sustainability.

Biodiversity and Sustainable Development 275

CONCLUSION

Biodiversity, traditional knowledge, poverty reduction all are linked. The poor and vulnerable suffer most severely from biodiversity degradation. The link between ecosystem services and the fate of poor people implies that biodiversity should be a priority in national and international efforts to address poverty reduction. Their empowerment in the management of biodiversity is key to improving their well-being. Traditional knowledge which is with various communities specially the poor have time and again proved that their knowledge and practices outweigh the risks associated with technology-based interventions to achieve sustainable development both in macroeconomic component or as a micro-enterprise led developments. Hence there is the realization that there is need to learn from the traditional knowledge and collate the experiences of communities on issues of managing natural resources for sustained development which includes all sections of the society.

REFERENCES

Brickhill, Daisy. 2015. “Ecosystem services and the environment. In-depth report 11 produced for the European Commission, DG Environment.” Dale, Ann. 2009. “Biodiversity and sustainable development.” Area Studies (Regional Sustainable Development Review): Canada and US-Volume I:254. Donohoe, Martin. 2003. “Causes and health consequences of environmental degradation and social injustice.” Social Science & Medicine no. 56 (3):573-587. Hunter, Philip. 2007. “The human impact on biological diversity.” EMBO reports no. 8 (4):316-318. Ibisch, PL, E Vega, and TM Herrmann. 2010. “Interdependence of biodiversity and development under global change.” CBD Technical Series (54). Mace, Georgina M, Ken Norris, and Alastair H Fitter. 2012. “Biodiversity and ecosystem services: a multilayered relationship.” Trends in ecology & evolution no. 27 (1):19-26. Mawdsley, Jonathan R, Robin O’Malley, and Dennis S Ojima. 2009. “A review of climate‐change adaptation strategies for wildlife management and biodiversity conservation.” Conservation Biology no. 23 (5):1080-1089. Milner-Gulland, EJ. 2012. “Interactions between human behaviour and ecological systems.” Phil. Trans. R. Soc. B no. 367 (1586):270-278. Rotherham, Ian D. 2015. “Eco-history: an introduction to biodiversity and conservation.” ECOS no. 36:1. Van Dyke, Fred. 2008. “Genetic Diversity–Understanding Conservation at Genetic Levels.” Conservation Biology: Foundations, Concepts, Applications:153-184. Vellend, Mark, and Monica A Geber. 2005. “Connections between species diversity and genetic diversity.” Ecology letters no. 8 (7):767-781.

Chapter 9

FABRICATION OF KNN THIN-FILMS VIA A SPIN COATING TECHNIQUE

Rashmi Rani1,2,3,*, Seema Sharma1, Marzia Quaglio2, Simelys Hernandez3, Stefano Bianco3, Angelica Chiodoni2 and Candido Fabrizio Pirri2,3 1Ferroelectric Research Laboratory, Department of Physics, A. N. College, Patna, India 2Center for Space Human Robotics (CSHR), Instituto Italiano di Tecnologia (IIT), Torino, Italy 3Department of Applied Science and Technology (DISAT), Politecnico di Torino, Torino, Italy

ABSTRACT

Lead-free (K0.5 Na0.5)NbO3 (KNN) thin films were fabricated on FTO substrate using spin coating technique. Two types of additives were used to fabricate KNN thin films. The effect of different calcination temperatures on the structural and morphological properties of the KNN films was investigated by X-ray diffraction, scanning electron microscopy respectively. Low calcination temperatures led to decreasing the crystallite size. All films showed perovskite structure. The addition of additives plays an important role in the successful fabrication of KNN thin films.

Keywords: KNN, sol-gel, spin-coating

1. INTRODUCTION

Nowadays the requirements for the miniaturization of micromechanical and microelectronic components cause an increasing demand of thin films, whose dimensions in

* Corresponding Author Email: [email protected]. 278 Rashmi Rani, Seema Sharma, Marzia Quaglio et al. the nanometer range give rise to new physical phenomena and properties which greatly differ from those of homogeneous bulk materials of the same composition and which need to be understood in order to develop ferroelectric and piezoelectric devices (Setter et al. 2006; Petzelt 2010). KNN thin film is a promising candidate for lead-free piezoelectric materials used in FeRAMs and microelectro-mechanical systems (MEMS) for the environmental protection (Shibata et al. 2008, Lee et al. 2009, Rödel et al. 2009, Saito et al. 2004). There are several methods commonly employed to synthesize KNN films, like sputtering (Wang et al. 1998), pulsed laser deposition (PLD) (Cho and Grishin 1999) and the sol-gel process (Söderlind, Käll, and Helmersson 2005; Tanaka, Kakimoto, and Ohsato 2006). Between all of them, only the sol-gel process is not vacuum-based and therefore is cost-effective for large area coatings. Other benefits of the sol-gel process are the possibility to be easily scaled up towards mass production and the chance to realize a homogeneous doping (Sakamoto et al. 2005, 2006). However, process parameters like the selection of the starting materials or of the thermal treatment temperature play a critical role for the microstructure and the electrical properties of the resulting films (Yan et al. 2010). In most of the sol-gel solutions employed to produce KNN films, methoxyethanol was utilized as solvent. Goh and Yao (Goh, Yao, and Chen 2009) used sodium, potassium and niobium ethoxide as metal precursors with methoxyethanol as solvent. In contrast, Wang et al. (Wang et al. 2010) and Ahn et al. (Ahn et al. 2010) used sodium and potassium acetate and niobium ethoxide as metal precursors with methoxyethanol as solvent. Methoxyethanol can also act as a stabilizer in sol-gel solutions, but it is known to be toxic (Kwak, Kingon, and Kim 2012) and should therefore be replaced by other solvents. To get rid of this problem, Wiegand et al. (Wiegand et al. 2013b) used to fabricate KNN films by acetic acid-based sol-gel process, but the high cost and the easiness to hydrolysis of ethoxide and acetate precursors make this process difficult to be carried out. To the best of our knowledge, very limited study was performed towards the fabrication of lead-free KNN thin films by a simple and cost-effective method. With the aim of fulfill this lack, in this present work KNN thin-films were prepared by exploiting the spin-coating method and the effect of the use of two different additives (i.e., polyethylene oxide (PEO) and Tween 20 (T20)), on the final structural and morphological properties of the KNN films was investigated.

2. EXPERIMENTAL PROCEDURES

2.1. Sol Synthesis

A solution based on niobium oxide (Nb2O5, 99.9%), potassium hydroxide (KOH, 97%), sodium carbonate (Na2CO3, 99.8%), potassium carbonate (K2CO3, 99%), acetic acid (CH3COOH, 99.5%), oxalic acid ((COOH)2·2H2O, 99.5%), citric acid (C6H8O7·H2O, 99.5%), nitric acid (HNO3, 65.0-68.0%), polyethylene oxide (PEO, 99%) and Tween 20 (T20, 99%) was used for the synthesis of KNN thin films. In a first stage, Nb2O5 and KOH were firstly mixed and calcined at 350°C for 2 h to obtain a soluble potassium niobate (K3NbO4). The latter was subsequently solved into distilled water and titrated by nitric acid to form a precipitate of niobium hydroxide Fabrication of KNN Thin-Films via a Spin Coating Technique 279

(Nb(OH)5). After washing several times to remove potassium ion, a soluble niobium precursor was gained in virtue of the freshly precipitated Nb(OH)5 chelated with oxalic acid. The chemical reactions that take place in the preparation of niobium precursor solution can be written as follows:

Nb2O5 + 6KOH → 2K3NbO4 + 3H2O↑ + 8- - 6K3NbO4 + 5H2O → 18K + Nb6O 19 + 10OH 8- + Nb6O 19 + 11H2O + 8H → 6 Nb(OH)5↓ 5+ 2- 2Nb(OH)5 + 5HOOC-COOH → 2Nb + 10H2O + 5(COO) 2

According to the chemical stoichiometry of KNN, the as-prepared niobium precursor solution was mixed with Na2CO3 and K2CO3 into a diluted solution of citric acid, and subsequently acetic acid was added to control the pH value. For obtaining the homogeneous solution, vigorous stirring for 2 h was required. Polyethylene oxide and Tween 20, both polymer and surfactant, were used as solvents for two different solutions. In a second stage, 10 wt% solution of KNN was dissolved in two different solvents (PEO and T20) and then resulting solutions were stirred for 8 h at room temperature.

2.2. Coating and Thermal Treatment

FTO substrates were used for the deposition of the KNN films by spin coating. A drop of the sol was dispensed onto the rotating substrate (3000 rpm/min). After a rotation of 60 s, the films were dried for 10 min at 150oC on a hot plate. The process was repeated three times in view of realizing a multilayer structure. Finally the samples were calcined in a furnace at two different temperatures, namely 500oC and 600oC, for 2 h.

(a) (b)

Figure 1. Thin-film of KNN-PEO calcined at (a) 500oC and (b) 600oC.

2.3. Characterization

The crystallographic phases of the KNN thin films were examined by X-Ray Diffraction (Panalytical X’pert pro X-ray Diffractometer, CuKα = 1.54 Å) technique. The surface 280 Rashmi Rani, Seema Sharma, Marzia Quaglio et al. morphology of the KNN films was investigated by Field Emission Scanning Electron Microscopy (FESEM, Dual Beam Auriga from Carl Zeiss, operating at 5 keV).

3. RESULTS AND DISCUSSION

3.1. XRD Analysis for Pure KNN, KNN-PEO and KNN-Tween 20 Thin-Films Calcined at 500°C

Figure 2 shows the XRD patterns of the three different KNN films calcined at 500oC for 2 h. All the three films exhibit a crystalline perovskite structure with a small amount of o secondary phases such as K2Nb4O11 at around 20 = 29 possessing a tetragonal tungsten bronze structure, which was caused by the high volatility of K and Na ions during the heat treatment (Tanaka, Kakimoto et al. 2007; Tanaka, Hayashi et al. 2007; Nakashima et al. 2007). However, in case of the KNN-PEO and KNN-T20 thin-films, the K2Nb4O11 peak was suppressed and the intensity peaks proved to be sharper, thus indicating a more intense crystallinity with respect to the pure KNN thin-films. The XRD patterns were indexed as pseudo cubic structure, since the peak split typical of the orthorhombic structure was not present. The lattice parameter of the KNN films calculated from the XRD data results to be a = 0.3973 nm, 0.3988 nm and 0.3987 nm for the pure KNN, KNN-PEO and KNN-T20, respectively. These values are in good agreement with a = 0.397 nm which was reported by Wiegand et al. (Wiegand et al. 2013a). The intensity of the (110) reflex is the highest within each diffractogram for the pure KNN, KNN-PEO and KNN-T20 films. Taking into account that the intensity ratio of (100)/(110) for KNN powder is about 0.6 (Wang, Yao, and Ren 2008), each KNN film shows nearly powder-like orientation with intensity ratios ranging from 0.76 to 0.99 (see Table 1). The crystallite size of the KNN films was calculated from the XRD data by means of the Scherrer equation. The crystallite size of the pure KNN film was 60 nm and it decreased down to 52 nm for KNN-PEO and increased up to 58 nm for KNN- T20, as evidenced by Table 1. These values are in agreement with those reported by Kupec et al. (Kupec et al. 2012) (50-90 nm range), but are in disagreement with the ones obtained by Wang et al. (Wang et al. 2010) (9.5-12.5 nm range). Figure 3 shows the diffractograms of the KNN films calcined at 600oC. All films presented a crystalline perovskite structure as before, but secondary phases almost disappeared in KNN-PEO and KNN-T20 films. The XRD patterns were also indexed as pseudo cubic structure since, as before, no peak splitting characteristic for the orthorhombic phase appeared. The intensity of the peaks shown by each film was reduced with respect to the ones of the films annealed at 500oC. The lattice parameter of the KNN films calculated from the XRD data proves to be a = 0.3971 nm nm, 0.3982 nm and 0.3987 nm for the pure KNN, KNN-PEO and KNN-T20, respectively, which is very similar to the previous one.

Fabrication of KNN Thin-Films via a Spin Coating Technique 281

(a)

(b)

(c)

Figure 2. XRD patterns of (a) pure KNN, (b) KNN-PEO and (c) KNN-T20 at 500oC. 282 Rashmi Rani, Seema Sharma, Marzia Quaglio et al.

(a)

(b)

(c)

Figure 3. XRD patterns of (a) pure KNN, (b) KNN-PEO and (c) KNN-T20 at 600oC. Fabrication of KNN Thin-Films via a Spin Coating Technique 283

For the KNN-PEO films the intensity of the (110) reflex was the highest within each diffractogram, as before, while for the pure KNN and KNN-T20 thin films the intensity of the (100) and (110) reflexes was nearly equal in height. This phenomenon was attributed to the lowest surface energy of (Azmi et al.) faces in the simple perovskite cell (Cho and Grishin 2000) and is in agreement with the other reports on alkali niobate thin films (Shibata et al. 2008; Cho and Moon 2002; Ryu et al. 2007; Tanaka, Hayashi, et al. 2007; Cakare- Samardzija, Malic, and Kosec 2008). The intensity ratio (100)/(110) for the films calcined at 600oC ranges from 0.76 and 1.03, thus results to be higher than the one reported for thin films annealed at 500oC (see Table 1). The crystallite size of the KNN films was calculated from the XRD data by using the Scherrer Equation. The crystallite size was 62 nm, 56 nm and 59 nm for pure KNN, KNN-PEO and KNN-T20 films, respectively. These values are nearly in the same range of those obtained in KNN films annealed at 500oC. No influence of the calcination temperature on the crystallite size was determinable. The intensity ratio (100)/(110) for the films calcined at 600oC was comparable to the one estimated for the films annealed at 500oC. The calcination temperature of 600oC has apparently only a minor influence on the lattice parameter and orientation.

3.2. FESEM Analysis for KNN-PEO and KNN-Tween 20 Thin-Films Calcined at 500 and 600oC

The surface microstructure of the KNN-PEO and KNN-Tween 20 thin films calcined at 500oC is shown in Figure 4. In particular, a wooly fiber shaped morphology of the sol-gel derived films can be easily recognized by looking panels (c) and (d) of the above mentioned figure. For both the thin films, many distinct pores, cracks and roughness, attributable to the shrinkage and outgassing of volatile species, were present inside the surface (Jaffe 1971). It is difficult to eliminate the pores in pure KNN bulk since the morphology of the grain is quadrate (Du et al. 2007). The grain size of both films ranges from 175 up to 300, and so it is slightly greater than the 160 up to 250 nm range reported by Tanaka et al. (Tanaka, Kakimoto, and Ohsato 2006). In both films the grains are aggregated structures of small crystallites, as it can be evidenced by the comparison of the grain size with the crystallite size (see Table 1). Thus, every grain in average consists of about 5 crystallites.

Table 1. Lattice parameter a, crystallite size d and intensity ratio of the reflexes (100) and (110) for the pure KNN, KNN-PEO and KNN-T20 thin films calcined at 500oC and 600oC calculated from XRD measurements

Sample a/nm d/nm (100)/(110) Temperature Pure KNN 0.3973 59 0.83 500oC KNN-PEO 0.3988 52 0.76 500oC KNN-T20 0.3985 57 0.80 500oC Pure KNN 0.3971 62 0.99 600oC KNN-PEO 0.3982 56 0.76 600oC KNN-T20 0.3987 61 0.98 600oC

284 Rashmi Rani, Seema Sharma, Marzia Quaglio et al.

(a)

(b)

(c)

Figure 4. (Continued). Fabrication of KNN Thin-Films via a Spin Coating Technique 285

(d)

Figure 4. FESEM images of the surface of the KNN-PEO and KNN-Tween 20 thin-films calcined at 500°C acquired with two different magnifications.

(a)

(b)

Figure 5. (Continued). 286 Rashmi Rani, Seema Sharma, Marzia Quaglio et al.

(c)

Figure 5. FESEM images of the surface of the (a) KNN-PEO, (b) KNN-Tween 20 with inset and (c) magnified KNN-PEO thin films calcined at 600oC.

Figure 5 reports FESEM photographs of KNN-PEO and KNN-Tween 20 thin films annealed at 600oC. Both thin-films show a smaller amount of porosity and a dimension of the grains reduced of about 100 nm with respect to the thin-films calcined at 500oC. Nucleation, with the formation of smaller grains at the boundaries between large grains, is promoted not only by a higher calcinations temperature, but can be also ascribable to the volatilization of alkaline elements from the grain boundary of large grains. No cracks were observed in both films, but some roughness was still present in KNN-T20 thin-films. In KNN-T20 the grains were randomly shaped as shown in the inset of Figure 5 (b). In contrast, the morphology of KNN-PEO films reported in Figure 5 (c) is characterized by the presence of bigger clusters of perovskite particles (representing agglomerates of smaller spherical particles of ≈ 60 nm in size) surrounded by fine cuboidal particles of about 200 nm. The comparison of the crystallite (56 and 59 nm for PEO and T20-based KNN, respectively) and average grain (~110 nm) sizes of both the film reveals that each grain contains in average about two crystallites.

CONCLUSION

The KNN, KNN-PEO and KNN-T20 thin-film calcined at 600oC was more crystallizes in the perovskite structure than calcined at 500oC. FESEM images showed that the porosity and the grain size of KNN-PEO and KNN-T20 were decreased with increasing the temperature. It has been also verified by FESEM that Pure KNN thin-film was not successfully synthesized because it was cracked.

Fabrication of KNN Thin-Films via a Spin Coating Technique 287

REFERENCES

Ahn, Chang Won, Hak-In Hwang, Kwang Sei Lee, Byung Moon Jin, Sungmin Park, Gwangseo Park, Doohee Yoon, Hyeonsik Cheong, Hai Joon Lee, and Ill Won Kim. 2010. Raman spectra study of K0. 5Na0. 5NbO3 ferroelectric thin films. Japanese Journal of Applied Physics no. 49 (9R):095801. Azmi, Nurul Azuwa, Umar Al-Amani Azlan, Mohd Asyadi, Azam Mohd Abid, Mohd Warikh Abd Rashid, and Maziati Akmal Mohd Hatta. 2016. Effect of samarium concentration on the structural and electrical properties of (K, Na) NbO3 thin films. Proceedings of Mechanical Engineering Research Day 2016 no. 2016:100-101. Cakare-Samardzija, L, B Malic, and M Kosec. 2008. K0. 5Na0. 5NbO3 thin films prepared by chemical solution deposition. Ferroelectrics no. 370 (1):113-118. Cho, Choong-Rae, and Alex Grishin. 1999. Self-assembling ferroelectric Na 0.5 K 0.5 NbO 3 thin films by pulsed-laser deposition. Applied Physics Letters no. 75 (2):268-270. 2000. Background oxygen effects on pulsed laser deposited Na 0.5 K 0.5 NbO 3 films: From superparaelectric state to ferroelectricity. Journal of Applied Physics no. 87 (9):4439- 4448. Cho, Choong-Rae, and Byung-Moo Moon. 2002. (Na, K) NbO 3 Thin Films Using Metal- Organic Chemical Vapor Deposition. Integrated Ferroelectrics no. 45 (1):39-48. Du, Hongliang, Fusheng Tang, Daijun Liu, Dongmei Zhu, Wancheng Zhou, and Shaobo Qu. 2007. The microstructure and ferroelectric properties of (K 0.5 Na 0.5) NbO 3-LiNbO 3 lead-free piezoelectric ceramics. Materials Science and Engineering: B no. 136 (2):165- 169. Goh, Phoi Chin, Kui Yao, and Zhong Chen. 2009. Titanium diffusion into (K0. 5Na0. 5) NbO3 thin films deposited on Pt/Ti/SiO2/Si substrates and corresponding effects. Journal of the American Ceramic Society no. 92 (6):1322-1327. Jaffe, WR. 1971. Cook, and H. Jaffe. Piezoelectric Ceramics:271-80. Kupec, Alja, Barbara Malic, Jenny Tellier, Elena Tchernychova, Sebastjan Glinsek, and Marija Kosec. 2012. Lead‐ Free Ferroelectric Potassium Sodium Niobate Thin Films from Solution: Composition and Structure. Journal of the American Ceramic Society no. 95 (2):515-523. Kwak, Jiyeon, Angus I Kingon, and Seung-Hyun Kim. 2012. Lead-free (Na 0.5, K 0.5) NbO 3 thin films for the implantable piezoelectric medical sensor applications. Materials Letters no. 82:130-132. Lee, Hai Joon, Ill Won Kim, Jin Soo Kim, Chang Won Ahn, and Bae Ho Park. 2009. Ferroelectric and piezoelectric properties of Na 0.52 K 0.48 NbO 3 thin films prepared by radio frequency magnetron sputtering. Applied Physics Letters no. 94 (9):092902. Nakashima, Yoshifumi, Wataru Sakamoto, Hiroshi Maiwa, Tetsuo Shimura, and Toshinobu Yogo. 2007. Lead-free piezoelectric (K, Na) NbO3 thin films derived from metal alkoxide precursors. Japanese Journal of Applied Physics no. 46 (4L):L311. Petzelt, Jan. 2010. Dielectric grain-size effect in high-permittivity ceramics. Ferroelectrics no. 400 (1):117-134. Rödel, Jürgen, Wook Jo, Klaus TP Seifert, Eva‐ Maria Anton, Torsten Granzow, and Dragan Damjanovic. 2009. Perspective on the Development of Lead‐ free Piezoceramics. Journal of the American Ceramic Society no. 92 (6):1153-1177. 288 Rashmi Rani, Seema Sharma, Marzia Quaglio et al.

Ryu, Jungho, Jong-Jin Choi, Byung-Dong Hahn, Dong-Soo Park, Woon-Ha Yoon, and Ki- Hoon Kim. 2007. Fabrication and ferroelectric properties of highly dense lead-free piezoelectric (K 0.5 Na 0.5) Nb O 3 thick films by aerosol deposition. Applied Physics Letters no. 90 (15):152901. Saito, Yasuyoshi, Hisaaki Takao, Toshihiko Tani, Tatsuhiko Nonoyama, Kazumasa Takatori, Takahiko Homma, Toshiatsu Nagaya, and Masaya Nakamura. 2004. Lead-free piezoceramics. Nature no. 432 (7013):84-87. Sakamoto, Wataru, Yu-ki Mizutani, Naoya Iizawa, Toshinobu Yogo, Takashi Hayashi, and Shin-ichi Hirano. 2005. Processing and properties of ferroelectric (Bi, La) 4 (Ti, Ge) 3 O 12 thin films by chemical solution deposition. Journal of the European Ceramic Society no. 25 (12):2305-2308. Sakamoto, Wataru, Yu-ki Mizutani, Naoya Iizawa, Toshinobu Yogo, Takashi Hayashi, and Shin-ichi Hirano. 2006. Synthesis and properties of ferroelectric Si-doped (Bi, Nd) 4Ti3O12 thin films by chemical solution deposition. Journal of electroceramics no. 17 (2-4):293-297. Setter, N, D Damjanovic, L Eng, G Fox, Spartak Gevorgian, S Hong, A Kingon, H Kohlstedt, NY Park, and GB Stephenson. 2006. Ferroelectric thin films: Review of materials, properties, and applications. Journal of Applied Physics no. 100 (5):051606. Shibata, Kenji, Fumihito Oka, Akio Ohishi, Tomoyoshi Mishima, and Isaku Kanno. 2008. Piezoelectric properties of (K, Na) NbO3 films deposited by RF magnetron sputtering. Applied physics express no. 1 (1):011501. Söderlind, Fredrik, Per-Olov Käll, and Ulf Helmersson. 2005. Sol-gel synthesis and characterization of Na 0.5 K 0.5 NbO 3 thin films. Journal of Crystal Growth no. 281 (2):468-474. Tanaka, Kiyotaka, Hiromi Hayashi, Ken-ichi Kakimoto, Hitoshi Ohsato, and Takashi Iijima. 2007. Effect of (Na, K)-excess precursor solutions on alkoxy-derived (Na, K) NbO3 powders and thin films. Japanese Journal of Applied Physics no. 46 (10S):6964. Tanaka, Kiyotaka, Ken-ichi Kakimoto, and Hitoshi Ohsato. 2006. Fabrication of highly oriented lead-free (Na, K) NbO 3 thin films at low temperature by sol-gel process. Journal of Crystal Growth no. 294 (2):209-213. Tanaka, Kiyotaka, Ken-ichi Kakimoto, Hitoshi Ohsato, and Takashi Iijima. 2007. Effects of Pt bottom electrode layers and thermal process on crystallinity of alkoxy-derived (Na, K) NbO3 thin films. Japanese Journal of Applied Physics no. 46 (3R):1094. Wang, Lingyan, Wei Ren, Kui Yao, Phoi Chin Goh, Peng Shi, Xiaoqing Wu, and Xi Yao. 2010. Effect of Pyrolysis Temperature on K0. 5Na0. 5NbO3 Thick Films Derived from Polyvinylpyrrolidone‐ Modified Chemical Solution. Journal of the American Ceramic Society no. 93 (11):3686-3690. Wang, Lingyan, Kui Yao, and Wei Ren. 2008. Piezoelectric K 0.5 Na 0.5 NbO 3 thick films derived from polyvinylpyrrolidone-modified chemical solution deposition. Applied Physics Letters no. 93 (9):092903. Wang, Xin, Ulf Helmersson, Sveinn Olafsson, Staffan Rudner, Lars-David Wernlund, and Spartak Gevorgian. 1998. Growth and field dependent dielectric properties of epitaxial Na 0.5 K 0.5 NbO 3 thin films. Applied Physics Letters no. 73 (7):927-929. Wiegand, Sebastian, Stefan Flege, Olaf Baake, and Wolfgang Ensinger. 2013a. Effect of different calcination temperatures and post annealing on the properties of 1, 3 Fabrication of KNN Thin-Films via a Spin Coating Technique 289

propanediol based Sol-Gel (Na 0.5 K 0.5) NbO 3 (NKN) thin films. Journal of Alloys and Compounds no. 548:38-45. Wiegand, Sebastian, Stefan Flege, Olaf Baake, and Wolfgang Ensinger. Properties of Acetic Acid Based Sol-Gel (Na 0.5 K 0.5) NbO 3 (NKN) Thin Films. Journal of Materials Science & Technology no. 29 (2):142-148. Yan, Xin, Wei Ren, Xiaoqing Wu, Peng Shi, and Xi Yao. 2010. Lead-free (K, Na) NbO 3 ferroelectric thin films: preparation, structure and electrical properties. Journal of Alloys and Compounds no. 508 (1):129-132.

Chapter 10

FERRROELECTRIC AND FERROMAGNETIC

PROPERTIES OF BI1-X-YDYXCYFE1-YTIYO3 SOLID SOLUTION

Radheshyam Rai1,*, Anjali Sharma1, Igor Bdikin2, M. A. Valente3 and Seema Sharma4 1School of Physics and Materials Science, Shoolini University, Solan, India 2TEMA-NRD, Mechanical Engineering Department and Aveiro Institute of Nanotechnology (AIN), University of Aveiro, Aveiro, Portugal 3Departamento de Fisica, I3N, Universidade de Aveiro, Campus Universitario de Santiago, Aveiro, Portugal 4Ferroelectric Research Laboratory, Department of Physics, A. N. College, Patna, India

ABSTRACT

The solid solutions of Bi1-x-yDyxCyFe1-yTiyO3 (C = Ba, x = 0.1, and y = 0.1, 0.2, 0.3, 0.4 and 0.5) ceramics have been prepared by solid state reaction method. Dielectric properties of these ceramics have been characterized in the temperature range between room temperature and 3000C and magnetic properties between 5 and 300 K. For the understanding of the multiferroic property, the relation between the crystal structures, magnetic transition and ferroelectric transitions with increasing temperature have been analyzed. All ceramic samples show single perovskite phase. The calculated and observed d values of all diffraction lines (reflections) of above compounds suggest that there is a no change in the basic crystal structure (tetragonal) as y varies from 0.1 to 0.5. The maximum ferroelectric transition temperature (Tc) of this system was in the range 433-443K at 100 kHz. Well saturated piezoresponse hysteresis loops were observed for all compositions indicating room temperature ferroelectricity. With increasing Ba content (up to 0.3) the remanent magnetization Mr increased and the coercive magnetic field decreased.

* Corresponding Author address Email: [email protected] (Radheshyam Rai). 292 Radheshyam Rai, Anjali Sharma, Igor Bdikin et al.

Keywords: ceramics, dielectric properties, phase transitions, ferroelectricity, magnetic properties

INTRODUCTION

The materials exhibiting multiple ferroic properties, such as ferroelectricity, ferroelasticity, and ferromagnetism (or antiferromagnetism) in the single phase are referred to as multiferroics. Recently, ferroelectromagnetic materials have become widely known due to their potential applications in the memory devices, sensors, and spintronics. Nickel iodine boracite, Ni3B7O13I, and its derivatives were the first and indisputable ferromagnetic ferroelectrics evidenced by both the ferroelectric P–E and ferromagnetic M–H hysteresis measurements. During the last decade, there has been a great interest in the area of multiferroics, which is motivated by the coupling of charge, lattice and spin order parameters (i.e., by the fundamental underlying physics) as well as by the potential applications in information storage. Due to the absence of true room-temperature multiferroic materials, the current research activities on multiferroics are limited to few representative perovskite oxides, based on BiFeO3, BiMnO3, etc., which combine several useful properties in the same material such as room temperature ferroelectricity and antiferromagnetic order (Catalan 2006; Hill 2000, 2002; Nan 1994; Wang et al. 2003). The large difference between magnetic and ferroelectric transition temperatures is one of the obstacles for the exploitation of multiferroics for real applications at room temperature. The multiferroic BiFeO3 is a perovskite intensively studied in recent years, due to the expectations of magnetoelectric properties at room temperature. BiFeO3 has long been known to be ferroelectric with a Curie temperature of about 1103 K and antiferromagnetic with a Neel temperature of 643 K. Preparation of pure BiFeO3 in the bulk ceramic form without traces of impurities has been a very difficult task (Rai et al. 2010). However, the magnetoelectric interactions in the bulk samples of pure BiFeO3 are weak because the staggered antiferromagnetic G-type structure is modulated by a cycloid with a large period of 620 Å (Lin et al. 2007). In this phase, the linear magnetoelectric effect is forbidden, while much weaker quadratic magnetoelectric effect is only allowed (Cheng et al. 2008). To implement the linear magnetoelectric interaction, the incommensurate magnetic structure should be destroyed. Chemical substitution in both A-and B-sites could tune the incommensurate magnetic structure (Yuan et al. 2006; Uniyal and Yadav 2008; Uniyal and Yadav 2009; Wang et al. 2006; Khomchenko et al. 2007; Khomchenko et al. 2008). Sosnowska et al. (Sosnowska, Neumaier, and Steichele 1982) have prepared BiFeO3 in the bulk form with the procedure recommended by Achenbach et al. (Achenbach, James, and Gerson 1967), but ended up with a few traces of Bi2Fe4O9. Tabares-Munoz et al. (Tabares- Mun̄ oz et al. 1985) have also attempted to prepare pure BiFeO3 in the bulk form but found small traces of Bi46Fe2O72 in the sintered ceramics. Another major problem found in the early studies of BiFeO3-based materials is the low resistivity which has prevented practical applications as piezoelectric or magnetoelectric functional components. Relatively high 3+ 2+ conductivity of BiFeO3 is believed to originate from the reduction of Fe ions to Fe , creating oxygen vacancies for charge compensation (Cheng, Li, and Cross 2003). At present, there are two ways used to solve the problems, one is doping at B-Site with other perovskites Ferrroelectric and Ferromagnetic Properties … 293

(e.g., ferroelectric PbTiO3, BaTiO3, and SrTiO3.) (Kumar, Srinivas, and Suryanarayana 2000; Ivanova and Gagulin 2002) into solid solution with BiFeO3 in order to stabilize a perovskite structure formation and to enhance the electric insulation resistance. Another way is to add other dopants, e.g., gallium (Cheng, Li, and Cross 2003), tantalum, neodymium (Mathe et al. 2004; Kim et al. 2003) or other rare earth materials to improve the magnetic properties. Therefore, BiFeO3-ABO3 solid solution systems have attracted great attention as a means to increase structural stability and sintering ability. Hence, the combination of these perovskite members could open various routes for achieving and improving multiferroic properties in a single phase. Recently, several groups have reported on the synthesis and characterization of multiferroic perovskite systems such as antiferromagnetic–ferroelectric [BiFeO3 (BF)–BaTiO3 (BT)], weak ferromagnetic/antiferromagnetic–ferroelectric [PrFeO3 (PF) +PbTiO3 (PT)], and antiferromagnetic/weak ferromagnetic–ferroelectric [(BF)–PrFeO3 (PF)–PbTiO3 (PT), and BFDyFeO3 (DF)–BaTiO3 (BT)] systems (Kim et al. 2001; Kim et al. 2004). Rai et al. (Rai et al. 2009; Geller and Wood 1956) reported that the Bi(MgTiO3)–PT and Gd-doped BiFeO3-BaTiO3 (x = 0.1) ceramics exhibit many interesting features, such as structural transformation and shift of ferroelectric transition temperature in different compositions. The orthoferrites with the formula RFeO3 (R is a rare earth element) have been reported by a number of authors (Dzyaloshinsky 1958; Nikolov et al. 1996; Niu, Du, and Du 2004). These orthoferrites are the G-type antiferromagnets with weak ferromagnetism and good electrical insulators (Wolf and White 1969). Their space group is Pbnm, which is a distorted perovskite. It is well known that the perovskite structure has the ability to stabilize cations in unusually high oxidation states, and the anion sublattice can accommodate a high concentration of vacant sites. The overall magnetic behaviors of bulk REFeO3 (RE = Gd, Tb, Dy etc) is described as the result of two contributing magnetic ‘sub lattices’: (i) an antiferromagnetic iron oxide lattice in which the spins are coupled via an Fe3+–O2-−Fe3+ superexchange mechanism; and (ii) a paramagnetic contribution from essentially non-coupled ions. Due to spin-canting in the iron containing sublattice, a small ferromagnetic moment is observed as a result of the distorted perovskite structure in one particular crystallographic direction (Nikolov et al. 1996). The materials exhibiting multiple ferroic properties, such as ferroelectricity, ferroelasticity, and ferromagnetism (or antiferromagnetism) in the single phase are referred to as multiferroics. Perovskite-type materials provide a broad range of magnetic and electrical properties covering antiferroelectric, antiferromagnetic, metallic, semiconductor, and insulator behaviors. Recently, ferroelectromagnetic materials have become widely known due to their potential applications in the memory devices, sensors, and spintronics. Hence, the combination of these perovskite members could open various routes for achieving the multiferroic properties in a single phase. Bi1-x-yDyxBayFe1-yTiyO3 are studied in order to improve the properties of multiferroic ceramics. In this study, the crystal structure, dielectric, ferroelectric, piezoelectric and ferromagnetic properties in the ternary perovskite Bi1-x-yDyxBayFe1-yTiyO3 (x = 0.1 and y = 0.1, 0.2, 0.3, 0.4 and 0.5) are studied in order to improve the properties of multiferroic ceramics. The origin of the weak ferromagnetism will be discussed in terms of magnetic structure of this perovskite system.

294 Radheshyam Rai, Anjali Sharma, Igor Bdikin et al.

EXPERIMENTAL METHODS

Polycrystalline samples of Bi1-x-yDyxBayFe1-yTiyO3 (x = 0.1 and y = 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized from high purity oxides (Bi2O3, Fe2O3, Dy2O3, BaCO3 and TiO2, 99.9% pure) using high temperature solid-state reaction technique. The constituent compounds in appropriate stoichiometries were thoroughly mixed in a ball milling unit for 12 hr. Then the powder was dried at 1000C and calcined at 9500C for 3 h in alumina crucibles. The calcined fine powder was cold pressed into cylindrical pellets of 10 mm in diameter and 1-2 mm in thickness using a hydraulic press with a pressure of 50 MPa. These pellets were sintered at 10000C for 3 hr. The formation and quality of compounds were verified with x-ray diffraction (XRD) technique. The XRD patterns of the compounds were recorded at room temperature using x-ray powder diffractometer (Rigaku Minifiex, Japan) with CuKα radiation (λ = 1.5405 Å) in a wide range of Bragg angles 2θ (20° ≤ 2θ ≤ 60°) at a scanning rate of 2° min−1. The dielectric constant () and loss tangent (tan) of the compounds were measured using a HP 4284A Precision LCR meter as a function of temperature (RT to 573K) at 100 kHz and frequency at room temperature (RT) with a home-made furnace. A commercial AFM (Multimode, Nanoscope IIIA, Veeco) was used for the ferroelectric and magnetic domain imaging. The microscope was equipped with an external lock-in amplifier (SR-830, Stanford Research) and a function generator (FG120, Yokogawa). The amplitude and frequency of the ac voltage were 1–2 V and 50 kHz, respectively. For piezoresponse hysteresis acquisition, the dc voltage varied from zero to ±30 V and was applied to the tip kept at a fixed position. Stiff conducting Si cantilevers (42 Nm−1, PPP- NCHR, Nanosensors) were used for the measurements performed in ambient environment. The tip has the shape of a polygon-based pyramid with the height of 10-15 µm and the radius rtip 10 nm at the apex. The magnetic data were recorded with the help of vibrating sample magnetometer (VSM) (Cryogenic). SEM micrographs were obtained with a JEM-2000FX (JEOL Ltd.) scanning electron microscope operated at 20 KeV.

RESULT AND DISCUSSION

Room temperature XRD patterns of the Bi1-x-yDyxBayFe1-yTiyO3 perovskite ceramics are shown in Figure 1 (a). All the reflection peaks were indexed using observed inter-planar spacing d, and lattice parameters of Bi1-x-yDyxBayFe1-yTiyO3 were determined by using least- squares refinement method. The calculated and observed d values of all diffraction lines (reflections) of above compounds suggest that there is a no change in the basic crystal structure (tetragonal) as y varies from 0.1 to 0.5. The tetragonality of samples varies with the composition (Table 1). Progressive broadening and splitting of XRD peaks (200) and (201) is characteristic of the tetragonal phase. These ceramics showed an increase of the relative density with doping Figure 1 (b). As such, the sintering of pure phase of could be controlled by the sintering temperature and doping concentration. With increasing the doping concentration some impurity peaks exist in samples. We found also weak traces of Bi2Fe4O9, which disappear with increasing BaTiO3. As for BiFeO3, it had always been a difficult task to preparing a pure compound and the available literature indicates that a small impurity concentration is always present. Ferrroelectric and Ferromagnetic Properties … 295

Figure 1. (a) Room temperature XRD patterns of Bi1-x-yDyxBayFe1-yTiyO3 samples (x = 0.1) with different concentration y.

Figure 1. (b) Relative density and tetragonality of samples with different concentrations y.

Table 1. Structural data for different compositions of Bi1-x-yDyxBayFe1-yTiyO3 compounds

Samples (y) Structure a (Å) c (Å) c/a Relative density (%) y = 0.1 Tetragonal 4.5658 4.8429 1.06 74 y = 0.2 Tetragonal 4.2234 4.6256 1.09 74 y = 0.3 Tetragonal 4.1568 4.7249 1.13 76 y = 0.4 Tetragonal 3.5089 4.0302 1.14 78 y = 0.5 Tetragonal 3.4652 4.0542 1.16 79 296 Radheshyam Rai, Anjali Sharma, Igor Bdikin et al.

Figure 2. SEM micrographs of fresh fractured surfaces for y = 0.1-0.5 concentrations.

The microstructure of ceramics was investigated by SEM. The influence of the thermal treatment conditions is very important in the formation of high-quality ceramics. SEM images of samples for all y concentrations were taken from freshly fractured surfaces. SEM images of samples with different y contents are presented in Figure 2. The fine-grained material with an average grain size of 3–10 μm was obtained. The microstructure is quite uniform with moderate porosity. Figure 2 demonstrates that the grain size increases with increasing y. The explanation is that when the concentration of y is small, these ions (Ba and Ti) will enter crystal lattices substituting for Fe3+, leading to crystal lattice distortion and to improving the sintering ability of the ceramic. However, when the concentration exceeds 0.3 wt% (perhaps above the solubility level), the sintering ability of the ceramic and the properties of the material are deteriorated accordingly. Ferrroelectric and Ferromagnetic Properties … 297

Figure 3. (a) Dielectric constant and tan (b) vs. temperature for different y at 100 kHz.

The dielectric vs. temperature dependences were obtained on silver electroded samples in plane capacitor configuration. The dielectric constant measured at f = 100 kHz shows a broad maximum at the temperature of about 433-443K for all samples (Figure 3 (a, b)) accompanied with a maximum of the dielectric losses tanδ shifted to lower temperatures as common in ferroelectrics. The observed maximum corresponds to the ferroelectric-paraelectric phase transition. In addition, tanδ increases (Figure 3 (b)) at high temperature and shows anomalies associated with mechanisms other than the phase transition. High dc conductivity (increasing at high temperature) was the main difficulty in producing functional BiFeO3-based ceramics components, where even very small level impurity (less than 0.5%) concentrations segregated at the grain boundaries can drastically contribute to leakage (Wang et al. 2004). Therefore, the problem persists even in a very pure BiFeO3 ceramics and it was explained as originating from the spontaneous change of the oxidation state of Fe3+ to Fe2+ (Xing et al. 2008). This results in the formation of oxygen vacancies for preserving the local electrical neutrality and causes thermally activated hopping conduction. In addition, an important contribution to the losses may occur in bulk ceramics by defects associated with the grain boundaries (Kalinin et al. 2002). All these phenomena often make difficult detection of the ferro-paraelectric phase transition by polarization hysteresis measurements. According to the present data, our ceramics may exhibit ferroelectric behavior at temperatures below 450K. The diffuse character of the phase transition can be understood in terms of the inhomogeneous distribution of ions in A and B sites of the ABO3 perovskite cell. Below Tmax Bi1-x- yDyxBayFe1-yTiyO3 could exhibit a magnetoelectric effect if the ferromagnetic property is found. In our samples ε increases gradually to its maximum value (εmax) with increasing temperature up to transition temperature and then decreases smoothly for all the compositions with a new increase at higher temperature (Mahajan et al. 2002; Smolenskii 1970). In addition, the value of dielectric peak (max), dielectric constant (RT), and loss factor at room temperature (tanRT) together with Tmax all increase with increasing y. The role of Ba, Ti and Dy ions is to increase the Tmax (roughly corresponding to the phase transition temperature) in this ferroelectric system. Tmax variation is small, for all compounds. It is clear that doping provides is necessary for controlling the properties of this type of ceramics. 298 Radheshyam Rai, Anjali Sharma, Igor Bdikin et al.

Figure 4. Variation of magnetization (ZFC-FC)) with temperature for Bi1-x-yDyxBayFe1-yTiyO3 (y = 0.1- 0.5) samples.

The magnetization as a function of temperature between 5 and 300 K under zero-field cooled (ZFC) and field cooled (FC) conditions with an applied magnetic field of 0.1 Tesla is given in Figure 4. It is a clear overlap of the ZFC and FC curves for all samples, with the exception of the sample with y = 0.3. The magnetization (M) vs. temperature (T) curves of the Bi1-x-yDyxBayFe1-yTiyO3 samples under the ZFC and FC conditions present a systematic change. The increase in the magnetization value at lower temperature suggests the presence of paramagnetic non-coupled Dy3+ ions and uncompensated antiferromagnetic spins (Fe3+) at the nanoscale (Sorop et al. 2004). As the structure of samples with y = 0.1 and y = 0.2 is similar thus the decrease of magnetization for the sample with y = 0.2 at low fields (0.1 Tesla) can be ascribed to the decrease in the number of magnetic ions (Fe). The difference and the decrease in the number of Fe ions can justify the decrease of magnetization for the samples. The variation of M–T curves with varying y composition may be attributed to different factors like (i) variation in the oxygen stoichiometry and doping A and B sites, (ii) reduction in particle size and (iii) change in the magnetic anisotropy (Mishra et al. 2008). The saturation of magnetization is not reached for all studied samples even with an applied magnetic field up to 5 T (Figure 5). Weak hysteresis was observed for the samples with y = 0.1 - 0.3 at 5 and 300 K. The effect of concentration (y) on the magnetization is clearly perceptible in the evolution of hysteresis loop indicating that Dy doped BF-BT has introduced ferromagnetic contribution. Kumar et al. (Mahesh Kumar et al. 1998) has also reported that in BF-PT bulk ceramics, a weak ferromagnetism is induced which was attributed to the continuing collapse of space- modulated spin structure in these samples. The origin of improved magnetic properties may be traced to increased canting effect by a structural distortion or uncompensated anti-parallel sub-lattice magnetization on A and B site doping (Ederer and Spaldin 2005). The observation of a narrow loop coupled with a very small value of magnetization also confirms that the magnetization is an induced one and has its origin due to the canting of the

Ferrroelectric and Ferromagnetic Properties … 299 anti-ferromagnetically ordered spins even at low temperatures. However, a minor loop traced for all samples indicates an anti-ferromagnetic nature. The appearance of hysteresis loops may be due to the canting of anti-ferromagnetically ordered Fe-O-Fe chain of spins resulting in the weak spontaneous moment. The low values of susceptibility and absence of saturation may be due to the uncompensated antiferromagnetic nature persisting in the samples. Figure 6 (a) shows a representative topography image of the polished sample x = 0.1 and y = 0.1 ceramic surface which was smooth enough for the PFM measurements. RMS roughness estimated from the image is around 100 nm. Polishing scratches with depths down to several hundred nm, holes with depth about 1 µm and some grains characterized by notable height differences are visible on the image. From both topography and piezoresponse images we could easily distinguish sample grains with the lateral size about 5 µm. Comparison of the topography and PFM images attests that the polishing scratches and other corrugations of the sample surface are not reflected on the corresponding PFM images thus giving an evidence of the piezoelectric nature of the contrast. The observed PFM signal is roughly uniform within the grains and but differs from grain to grain by magnitude (value of the corresponding effective piezoelectric coefficient) and by polarization direction (phase of the signal). These images are almost identical to typical PFM contrast in polycrystalline materials with single domain grains, being much smaller in amplitude. The effective vertical piezoelectric coefficient never exceeded 20-27 pm/V as judged by comparison with commercial thick PZT films. The dark and bright contrast corresponds to the polarization head directed to the sample surface. This polarization component apparently varies in both sign (polarization direction) and magnitude (amplitude of the effective piezoresponse) depending on crystallographic orientation. The linearity of the observed contrast with applied ac voltage was verified down to the resolution limit of PFM and also confirmed the piezoelectric nature of the contrast. Contrast is roughly proportional to the effective d33 coefficient and determined by the projection of the polarization vector P on the normal N to the ceramic surface. The dark and bright contrasts correspond to the polarization head directed to the sample surface.

Figure 5. M-H loops of Bi1-x-yDyxBayFe1-yTiyO3 samples at 5 K (a) and 300 K (b).

300 Radheshyam Rai, Anjali Sharma, Igor Bdikin et al.

2 Figure 6. Topography (a), piezoresponse (b, c), (c) - after poling +30 V, 15x15 µm , images of the Bi1-x- yDyxBayFe1-yTiyO3 (x=0.1, y = 0.1) sample.

Since the sign of the piezo signal is referred to the direction of the polarization projection onto the sample’s normal, it can be said that all investigated samples have average non-zero piezoresponse in the virgin (unpoled) state. In our definition, it corresponds to the polarization direction directed from the free surface into sample (self-polarization effect). If we apply a dc voltage between the tip and the bottom electrode higher than the corrective one, local switching occurs with polarization reversing in accordance with applied voltage (Figure 6 (c)).

CONCLUSION

The ternary solid solutions of Bi1-x-yDyxBayFe1-yTiyO3 ceramics were prepared by using solid-state reaction method. The samples exhibited tetragonal phase transition at room temperature accompanied with better sintering of the compounds. Sintered samples demonstrated diffuse phase transition shifting to higher temperature with increasing y, and appreciable electromechanical properties. We also observed that these samples have increased dielectric constant values. The magnetization increases with the increase of y content up to 0.3%. A minor loop traced for all these samples indicates an antiferromagnetic nature with weak ferromagnetism. We also observed distinct piezoresponse hystersis varying with the compositions and fine magnetic domains for low y concentrations.

REFERENCES

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Mishra, RK, Dillip K Pradhan, RNP Choudhary, and A Banerjee. 2008. “Effect of yttrium on improvement of dielectric properties and magnetic switching behavior in BiFeO3.” Journal of Physics: Condensed Matter no. 20 (4):045218. Nan, Ce-Wen. 1994. “Magnetoelectric effect in composites of piezoelectric and piezomagnetic phases.” Physical Review B no. 50 (9):6082. Nikolov, O, I Hall, SN Barilo, and AA Mukhin. 1996. “Field-induced spin reorientations in TbFeO3 at 4.2 K.” Journal of Magnetism and Magnetic Materials no. 152 (1-2):75-85. Niu, Xinshu, Weimin Du, and Weiping Du. 2004. “Preparation, characterization and gas- sensing properties of rare earth mixed oxides.” Sensors and Actuators B: Chemical no. 99 (2):399-404. Rai, Radheshyam, Igor Bdikin, Manuel Almeida Valente, and Andrei L Kholkin. 2010. “Ferroelectric and ferromagnetic properties of Gd-doped BiFeO 3–BaTiO 3 solid solution.” Materials Chemistry and Physics no. 119 (3):539-545. Rai, Radheshyam, Abinhav Sinha, Seema Sharmac, and NKP Sinha. 2009. “Investigation of structural and electrical properties of (1− x) Bi 0.5 Mg 0.5 TiO 3–(x) PbTiO 3 ceramic system.” Journal of Alloys and Compounds no. 486 (1):273-277. Smolenskii, Georgii Anatol’evich. 1970. “Physical phenomena in ferroelectrics with diffused phase transition.” J. Phys. Soc. Jpn no. 28 (1):26-37. Sorop, TG, M Evangelisti, M Haase, and LJ De Jongh. 2004. “Superparamagnetic behaviour of antiferromagnetic DyPO 4 nanoparticles.” Journal of Magnetism and Magnetic Materials no. 272:1573-1574. Sosnowska, I, T Peterlin Neumaier, and E Steichele. 1982. “Spiral magnetic ordering in bismuth ferrite.” Journal of Physics C: Solid State Physics no. 15 (23):4835. Tabares-Mun̄ oz, Cristobal, J-P Rivera, A Bezinges, Alain Monnier, and Hans Schmid. 1985. “Measurement of the quadratic magnetoelectric effect on single crystalline BiFeO3.” Japanese Journal of Applied Physics no. 24 (S2):1051. Uniyal, P, and KL Yadav. 2009. “Pr doped bismuth ferrite ceramics with enhanced multiferroic properties.” Journal of Physics: Condensed Matter no. 21 (40):405901. Uniyal, Poonam, and KL Yadav. 2008. “Study of dielectric, magnetic and ferroelectric properties in Bi 1− x Gd x FeO 3.” Materials Letters no. 62 (17):2858-2861. Wang, DH, WC Goh, M Ning, and CK Ong. 2006. “Effect of Ba doping on magnetic, ferroelectric, and magnetoelectric properties in mutiferroic Bi Fe O 3 at room temperature.” Applied Physics Letters no. 88 (21):212907. Wang, J, JB Neaton, H Zheng, V Nagarajan, SB Ogale, B Liu, D Viehland, V Vaithyanathan, DG Schlom, and UV Waghmare. 2003. “M, Wuttig, and R. Ramesh.” Science no. 299:1719. Wang, YP, L Zhou, MF Zhang, XY Chen, J-M Liu, and ZG Liu. 2004. “Room-temperature saturated ferroelectric polarization in BiFeO 3 ceramics synthesized by rapid liquid phase sintering.” Applied Physics Letters no. 84 (10):1731-1733. Wolf, WP, and RL White. 1969. “Rare-earth compounds.” J. Appl. Phys no. 40 (3):1061-69. Xing, XJ, YP Yu, LM Xu, YL Zhang, and SW Li. 2008. “Preparation and magnetic properties of BiFeO 3 films in trilayered Bi 3.25 La 0.75 Ti 3 O 12/BiFeO 3/Bi 3.25 La 0.75 Ti 3 O 12 structures.” Materials Science and Engineering: B no. 147 (1):95-99. Yuan, GL, Siu Wing Or, JM Liu, and ZG Liu. 2006. “Structural transformation and ferroelectromagnetic behavior in single-phase Bi 1− x Nd x Fe O 3 multiferroic ceramics.” Applied Physics Letters no. 89 (5):052905. In: Smart Materials for Smart Living ISBN: 978-1-53612-269-5 Editor: Radheshyam Rai © 2017 Nova Science Publishers, Inc.

Chapter 11

M-TYPE BARIUM NANOHEXAFERRITE MATERIAL: A NOVEL ENTRANT FOR STORAGE ENRICHMENT AND HIGH FREQUENCY APPLICATIONS

Virender Pratap Singh1,2,*, Gagan Kumar3, Ramprakash Dwivedi1, K. M. Battoo4, R. K. Kotnala5 and M. Singh1 1Department of Applied Sciences, Shoolini University, Bajhol, Solan, India 2Department of Physics, Himachal Pradesh University, Shimla, India 3Department of Physics, IEC, University, Baddi, India 4King Abdullah Institute for Nanotechnology, King Saud University, Riyadh, Saudi Arabia 5 National Physics Lab, New Delhi, India

ABSTRACT

The landmark lecture in the history of nanoscience was first of all delivered by Feynman entitled as “There is a plenty of rooms at the bottom.” Basically nanoscience refers to the ability to manipulate individual atoms and molecules, making it possible to build machines on the scale of dimensions 1 to 100 nanometres. At the nanoscale, the physical, chemical, and biological properties of materials differ in fundamental and valuable ways from the properties of individual atoms and molecules or bulk matter. Nanotechnology (R&D) is directed toward understanding and creating improved materials, devices, and systems that exploit these new properties.” Keeping in view these targets, material science research is focused on the invention of new materials with enhanced properties and novel synthesis techniques. Nannocrystalline materials are in focus to recent scientific research because of their prospective applications and fascinating physics involved in them. Bulk materials have constant physical properties regardless of its size, but at the nano-scale size-dependent properties are often observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. Ferrites are the ceramic compounds which much high electrical resistivity than the metallic

* Corresponding Author address Email: [email protected] (V. P. Singh). 304 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

ferromagnetic materials. Ferrites reduce the eddy current losses and also absorb the electromagnetic field penetration. Ferrites can be divided into three categories namely spinel, hexagonal and garnets according to their crystal lattice structure. The ferrites, having formula MFe12O19 (M = Ba, Sr, Pb), are those ferrites which have magnetoplumbite structure commonly called as hexagonal structure. The hexagonal ferrites containing Barium as divalent cations are the best ferrites. The unit cell of BaFe12O19 is combination of two structural blocks which are aligned in the direction of c- axis: RSR*S* where * indicate the 1800 rotation of the structural block with respect to c- axis, where S-block has a spinel structure with closed packed ‘O’ ions and Fe ions on its tetrahedral and octahedral sites. The R block is formed of hexagonally closed-packed ‘O’ ions and one ‘Ba’ ion and ‘Fe’ ions occupy the interstitial, tetrahedral, octahedral and bipyramidal sites, respectively. Now a day due to the changing nature of day to day technologies like RADAR, microwave, communication etc. there is a need of materials which must have a high saturation magnetization, high coercivity, high magnetic anisotropy, excellent chemical stability, high natural resonant frequency and good capability of absorbing the unwanted electromagnetic signals. All these properties stated above are possessed by BaFe12O19 hexaferrite so M-type ferrites has been investigated during the last few years intensively and become one of the most high-tech materials. Due to larger intrinsic magnetocrystalline anisotropy M-type Barium hexaferrite can be used at much higher frequency than the ferrites with spinel and garnet structure. It can be used as a permanent magnet, high density magnetic and magneto-optic recording media and filters for microwave devices. Since synthesizing technique plays a very vital role for the modifications of properties of BaFe12O19 hexaferrite thus many workers have synthesized the same by different methods. M type Barium hexaferrites are quite heavy as in granular materials, which restrict their practical application in microwave absorbing and shielding materials. This can be reduced by producing these materials with low dimensional and large specific surface area, which will enhance the absorbing ability per unit mass. Since BaFe12O19 hexaferrite is quite versatile because of its practical applications and the dc resistivity of a ferrite is an important property, since it determines its performance at high frequencies, where eddy currents losses may be high, resulting in a significant loss of energy. Further, frequency dependent dielectric and magnetic properties is a very important factor to be considered in designing any component for practical applications.

Keywords: M-type nanohexaferrites, permanent magnetism, high saturation magnetization, high coericivity

INTRODUCTION

The first truly scientific study of magnetism was made by the Englishman William Gilbert (1540–1603), who published his classic book “On the Magnet” in 1600. He experimented with lodestones and iron magnets, formed a clear picture of the Earth’s magnetic field, and cleared away many superstitions that had clouded the subject. Later on, in the eighteenth century, compound steel magnets were made, composed of many magnetized steel strips fastened together, which could lift 28 times their own weight of iron. This is all the more remarkable when we realize that there was only one way of making magnets at that time: the iron or steel had to be rubbed with a lodestone, or with another magnet which in turn had been rubbed with a lodestone. There was no other way until the first electromagnet was made in 1825, following the great discovery made in 1820 by Hans Christian Oersted (1775– M-Type Bariumn Nanohexaferrite Materials 305

1851) that an electric current produces a magnetic field. Research on magnetic materials can be said to date from the invention of the electromagnet, which made available much more powerful fields than those produced by lodestones, or magnets made from them (Cullity and Graham 2009).From the Neolithic age, the magnetic materials were used in various kinds viz. a supended lodestone to navigate. The lodestone is an iron ore, magnetite, which is one of a wide range of magnetic ceramics based on iron (III) oxide, called the ferrites. Further, ferrites can be divided into two categories. Firstly, in ferrites, magnetite (Fe3O4), is in a structural class of compounds known as the spinel’s with the composition MeFe2O4, where Me is a divalent cation and have a cubic structure, and secondly, the ferrites with hexagonal structure known as hexaferrites. At presently, from commercial and technological point of view, these hexaferrites have become massively important materials. Specially, barium hexaferrites (BaM) hexaferrite alone accounting for 50% of the total magnetic materials manufactured globally, at over 300,000 tons per year, and they have a multitude of uses and applications. The various applications (Pullar 2012) are shown in Figure 1.

1. THE DISCOVERY OF HEXAGONAL FERRITES

The hexaferrite as a magnetoplumbite magnetic materials was first of all described in 1925 (Cochardt 1963) and in 1938, the composition PbFe7.5Mn3.5Al0.5Ti0.5O19 clarifies its crystal structure as being hexagonal (Theophilo 2011). PbFe12O19 or pure PbM was found to be the synthetic form of magnetoplumbite magnetic material, also give new direction to many compounds including BaFe12O19 (BaM). Until after the Second World War the structure of BaM was not found. Initially, under the direction of Snoek in Phillips Laboratries, BaFe12O19 (BaM) is recognized by different names like ferroxdure, barium ferrite, hexaferrites, M-ferrite and BaM. Also, Went et al. (Went et al. 1952) found that BaM has hexagonal structure. Further investigation by Wijn and Braun, then produced more hexagonal compounds having both trivalent and divalent form of iron (BaFe2O4 and BaFe18O27) (Wijn 1952; Braun 1952; Jonker 1957). In 1950s, the exclusive report on different types of hexaferrites was published by Phillips Laboratories and culminated in the well-known book “Ferrites” by Smit and Wijn in 1959 (Smit and Wijn 1959; Snoek 1946). The important member and their physical properties of the various hexaferrites where Me = divalent ions such as Co2+, Ni2+ or Zn2+, and Ba2+ can be replaced by Sr2+ ions, are shown in Table 1, where M-type ferrites are such as BaFe12O19(BaM), SrFe12O19(SrM) or BaFe12-2xCoxTixO19 (CoTiM), Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41 or Co2Z, Y-type ferrites (Ba2Me2Fe12O22) such as Ba2Co2Fe12O22 or Co2Y, W-type ferrites (BaMe2Fe16O27) such as BaCo2Fe16O27 or Co2W, X- type ferrites (Ba2Me2Fe28O46) such as Ba2Co2Fe28O46 or Co2X and U-type ferrites(Ba4Me2Fe36O60) such as Ba4Co2Fe36O60 or Co2U (Pullar 2012). To describe the crystal structures of all the above stated hexagonal ferrites, two lattice parameters ‘a’ and ‘c’ are used unlike the spinel ferrites which have only one lattice parameter ‘a.’ The value of parameter ‘c,’ density (ρ) and basic building blocks which leads to the formation of different hexagonal structure are also shown in Table 1.

306 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

Figure 1. Applications of Barium hexaferrites.

Table 1. Various hexaferrites with physical properties

S. Hexaferrites Molecular Formula Mol.wt Density c(Ȧ) Magnetization Building Blocks No. combinations (g) ρ (g cm-3) 1. BaM M BaFe12O19 1112 5.28 23.18 Uniaxial SRS*R* 2. Co2Y Y Ba2Co2Fe12O22 1410 5.40 43.56 In plane STST 3. Co2Z Z = M+Y Ba3Co2Fe24O41 2522 5.35 52.30 In plane RSTSR*S*T*S* 4. Co2W W = M+2S BaCo2Fe16O27 1577 5.31 32.84 In cone SSRS*S*R* 5. Co2X X = W+M Ba2Co2Fe28O46 2688 5.29 84.11 In cone SRS*S*R* or 2M+2S 6. Co2U U = Z+M or Ba4Co2Fe36O60 3624 5.31 38.16 In plane RSR*S*TS* 2M+Y

All the hexagonal ferrites had a preferred direction of magnetization when placed in a magnetic field, giving them magnetocrystalline anisotropy which was often parallel to the c- axis, coming out of the basal plane of hexagonal crystal. This uniaxial anisotropy in effect fixes the magnetization in the direction of the c-axis, and magnetization can only be moved out of this direction at the expense of the high anisotropic energy (Pullar 2012).

1.1. M-Ferrites and Their Structures

The hexagonal BaM, BaFe12O19, belong to the categories of hard ferrites which have very high magnetic saturation value, high retentivity and high coericivity (160-255 kAm-1). The melting point of BaM hexaferrites is 1117K (Pies and Weiss 1977). BaM was initially named ferroxdure, to distinguish it from the spinel ferrite which was named ferroxcube (Snoek M-Type Bariumn Nanohexaferrite Materials 307

1946). It was first studied and characterized magnetically in the early 1950s by Philips (Went et al. 1952). It was much cheaper to produce, had a high electrical resistivity of 108 Ωcm and the high magnetic uniaxial anisotropy along the c-axis (Smit and Wijn 1959). The molecular mass of BaM is 1112 g and the maximum density is 5.295 g cm-3 (File 2007).

Figure 2. A hexagonal crystal with two lattice parameters “a” and “c.”

Figure 3. A close-packed layers of spheres occupying positions A, B or C.

Figure 4. (a) S-block and (b) R-block structures in BaM. 308 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

2. BUILDING BLOCKS OF M-TYPE HEXAGONAL FERRITES

In solids, the atoms give the impression of attracting hard spheres with close packing as possible (Nakamura et al. 2009; Kittel 1966). Figure 2. Shows a close-packed layer of identical spheres which occupy positions A. This layer is formed by placing each sphere in contact with six others in a plane. A second and identical layer of spheres can be placed on top of this layer and occupy positions B. Each sphere in the second layer is in contact with three spheres in the first layer. A third layer of spheres may be added in two ways: they can occupy either positions A or positions C. In principle, there are an infinite number of ways of stacking the close-packed layers. Two very common stacking sequences are “ABAB…” and “ABCABC…” The first one gives a hexagonal close-packed (hcp) structure. The second one gives a structure known as face centered cubic (fcc). Hexagonal ferrites encompasses, close-packed layers of oxygen ions O2- (Wu 2012) which leads to formation of six fundamental blocks, S, S*, R, R*, T, and T* (S*, R*, and T* blocks can be obtained simply through the rotation of the S, R, and T blocks, respectively, by 1800about the c axis). Table 1 lists the chemical compositions and building blocks of five types of hexagonal ferrites. Basically, the M-type hexagonal ferrites are built from S, R, S*, and R* blocks. Figure 3 (a) shows the stacking of close-packed oxygen layers in an “ABCABC…” sequence for S-block with both oxygen ions and cations. The small solid circles show the cations on tetrahedral sites, while the small open circles show the cations on octahedral sites. The S block contains eight oxygen ions, with four from each layer. Between the top and middle oxygen layers, there are one cation at an octahedral site and five cations at tetrahedral sites. Among five tetrahedral cations, one occupies the tetrahedral site formed by three oxygen ions in the top layer and another in the middle layer; the other four occupy the corners of a 600 rhombus, with each shared by four 600 rhombuses. In general, between the top and middle layers, there are one cation at an octahedral site and two cations at tetrahedral sites. Between the middle and bottom oxygen layers, there are five cations at the octahedral sites, all of which are within a 600 rhombus halfway between the oxygen layers. Among these five cations, four are at the middle points of four rhombus sides, with each shared by two rhombuses; and the other is at the center of the rhombus and is not shared by any other rhombuses. Overall, there are three octahedral cations between the middle and bottom oxygen layers. In total, each S block contains eight oxygen ions in close-packed plans, four cations at octahedral sites, and two cations at tetrahedral sites. If the cations are iron ions, the block contains two formula units of Fe3O4. The vertical axis of the structure is referred to as the c axis. Among the three types of fundamental blocks which make up hexagonal ferrites, the S block is the smallest one and is the only one containing no barium ions and also, the S block is often referred to as a spinel block. Figure 3 (b) shows the structure of an R block which consists of three close-packed oxygen layers in an “ABAB…” sequence, with one oxygen ion in the middle layer replaced by a barium ion Ba2+. The top, middle, and bottom layers contain four, three, and four oxygen ions, respectively. Overall, each R block contains eleven oxygen ions and one barium ion. The small solid circles show four cations which occupy the trigonal bipyramedal sites in the middle layer. As each trigonal site is shared by four 600 rhombuses, only one of these four cations belongs to the structure unit shown. The small open circles show five cations at octahedral sites. Among the five sites, one lies halfway between the top M-Type Bariumn Nanohexaferrite Materials 309 and middle oxygen layers, one lies halfway between the middle and bottom oxygen layers, and three are underneath the bottom oxygen layer. Both 2Fe3O4 (S block) and BaFe6O11 (R block) are not electrically neutral if the iron ions are trivalent. The combination of one S block and one R block yields BaFe12O19 which is indeed electrically neutral when the iron ions are Fe3+.

2.1. Y Ferrites Y ferrites have a preferred plane of magnetisation perpendicular to the c-axis at room temperature (Jonker 1957). The molecular mass of Co2Y is 1410 g, and it has a density of -3 5.40 g cm (Smit and Wijn 1959). Co2Y has a planar magnetic anisotropy at room temperature, but these changes to a cone of magnetisation below -580C. From this temperature to the Curie point the anisotropy remains in the preferred plane (Jonker 1957). Cu2Y is the only Y ferrite that has been found to have a preferred uniaxial direction of magnetisation (Castelliz 1969). The molecular unit of Y ferrite is one S and one T unit, with a total of six layers, the unit cell consists of three of these units, with the length of the c-axis being 43.56 Å, and is a member of the space group R3m (Braun 1956). The T block does not have a mirror plane, and therefore a series of three T blocks is required to accommodate the overlap of hexagonal and cubic close packed layers, with the relative positions of the barium atoms repeating every three T blocks. This gives the unit cell formula as simply 3(ST)

2.2. Z Ferrites The Z ferrites have formula Ba3Me2Fe24O41 or Co2Z, discovered at the same time as the ferroxplana Y ferrites (Jonker 1957), a molecular mass = 2522 g and a maximum density of 5.35 g cm-3 (Smit and Wijn 1959). These ferrites have a uniaxial anisotropy parallel to the c- axis, except for Co2Z, which is planar at room temperature but has a complex magnetic anisotropy, with at least four different anisotropic states. At low temperatures Co2Z has an easy cone of magnetization, at an angle of 650 to the c-axis, and this remains constant up to - 1030C. Between this temperature and-530C the angle increases to 900, and the preferred magnetization remains in the basal plane until it switches to the c-axis at some temperature between 207 and 2420C (Pullar and Bhattacharya 2001). The Z unit is composed of Y + M, and therefore consists of ST + SR, with a mirror plane in the R block and a repeat distance of 11 oxygen layers. Therefore, two molecular units are required to form a single unit cell of Z ferrite, one rotated 1800 around the c-axis relative to the other, to give STSRS*T*S*R*, with a c axis length of 52.30 Å and is a member of the space group P63/mmc (Braun 1956).

2.3. W Ferrites W ferrites have the compositional formula BaMe2Fe16O27, where Me belongs to first row transition metal or some other divalent cation, and the barium can be substituted by another group two metal. The Fe2W (BaFe2Fe16O27) (Went et al. 1952) was the first reported hexaferrites. Single phase Fe2W was found to have an easy axis of magnetization in the c-axis of the hexagonal crystal structure, but had a much higher electrical conductivity than BaM 2+ due to the Fe ions. All of the W ferrites have uniaxial anisotropy, except Co2W ferrite (BaCo2Fe16O27) (Jonker 1957), which has a molecular mass of 1581 g and a density of 5.31 g cm_3 (Smit and Wijn 1959). This has a cone of easy magnetization at a constant angle of 700 to the c-axis from -2730C to 1800C, at which point this anisotropy rotates towards the c- axis with increasing temperature until it becomes uniaxial at 280 0C, and the magnetization 310 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al. remains in the c-axis with a further rise in temperature (Samaras et al. 1989). The molecular unit of W ferrite is composed of two S blocks and one R block, so it is similar to the M structure but not identical. There are now two S blocks above and below the R block, but again there is a mirror plane in the R block and the unit cell consists of two molecular W units to give SSRS*S*R*. The cell length of Fe2W is 32.84 Å, it is a member of the space group P63/mmc (Braun 1956).

2.4. X Ferrites The X ferrites have the chemical formula Ba2Me2Fe28O46, where Me is a divalent first row transition metal or some other divalent cation. The first reported X ferrite was Fe2X, in which Me = Fe2+ (Braun 1956), and this was also found to have a uniaxial magnetic anisotropy along the c-axis and density of 5.29 g cm-3 (Smit and Wijn 1959). All the X ferrites have this uniaxial anisotropy at room temperature, except for Co2X (molecular mass = 2688 g), which has a cone of magnetisation at an angle of 740 to the c-axis (Jones et al. 1990). The X structure is very similar to that of W, being composed of one M and one W molecular units, to give the structure SRS*S*R*, with the blocks of the W section rotated through 1800 relative to the M section. The unit cell is constructed from three identical units to give the crystal structure 3(SRS*S*R*), c = 84.11 Å, and it is a member of the R3M space group (Braun 1956).

2.5. U Ferrites The U ferrites, Ba4Me2Fe36O60, although identified at the same time as the other hexagonal ferrites mentioned here, were not characterized much either structurally or magnetically until recently. The densities of Co2U and Zn2U have been calculated as 5.44 and -3 5.318 g cm . They all have uniaxial anisotropy except Co2U, which has planar anisotropy at room temperature (Braun 1956), and a molecular mass of 3622 g. The U ferrite structure has been found to consist of the molecular units Z + M, or M + Y + M, to give the block structure SRS*R*S*T (Fig. 12). Unusually the unit cell was originally stated to consist of only one molecular unit, where c = 38.16 Å (Hibst 1982), in which the R and S blocks of the Z section are rotated through 1800 relative to the M section. These days it is more usually stated as consisting of three molecular units, with a = 5.88 Å and c = 113 Å (Okumura et al. 2011).

Literature Review Hexaferrites belong to the categories of hard ferrites, and are of different types depending upon the crystal structure and their chemical formula (Nabiyouni et al. 2014). Out of various categories of hard ferrites; especially M-type hexaferrites are very intensively studied, because of their excellent behaviour of magnetism. The keen interest of many material scientists on these hard ferrites is due to their potential applications at ultra-high frequency regions, like microwave antennas, wireless communications, radar technologies, and high magnetic storage devices (Pawar et al. 2015; Kotnala et al. 2012). The BaFe12O19 is M-type hexaferrite and belong to magnetoplumbite group. The unit cell of hexaferrites contains 19 O2- ions and 1 Ba2+ ion and 12 Fe3+ ions. The Fe3+ ions are distributed among octahedral (12k, 2a, 4f2), tetrahedral (4f1) and trigonal bipyramidal site (2b). In hexaferrites, the electrical and magnetic properties mainly depend on the synthesizing technique, type of metal ions and their M-Type Bariumn Nanohexaferrite Materials 311 distribution between the different interstitial sites. So, because of the ability to distribute the metal ions over different interstitial sites, material scientists have worked on the synthesis and characterization of barium hexaferrites substituted with different metal ions (Sharma et al. 2013; Alam et al. 2015; Chawla et al. 2014; Lee et al. 2012; Li, Wang, and Wang 2012). Now a days due to the changing nature of day to day technologies, there is a need of materials which must have a high saturation magnetization, high coercivity, high magnetic anisotropy, excellent chemical stability, high natural resonant frequency and good capability of absorbing the unwanted electromagnetic signals. All these properties stated above are possessed by BaFe12O19 nano-hexaferrite so M-type ferrites has been investigated during the last few years intensively and become one of the most high-tech materials. Due to larger intrinsic magnetocrystalline anisotropy M-type Barium nanoheaxferrite can be used at much higher frequency than the ferrites with spinel and garnet structure (Dhage et al. 2011). Since synthesizing technique plays a very vital role for the modifications of properties of BaFe12O19 nanohexaferrite thus many workers have synthesized the same by different methods (Yang and Wang 2014; Meng et al. 2014; Mikheykin et al. 2014; Šepelák et al. 2014; Pullar, Bdikin, and Bhattacharya 2012; Aen et al. 2011; Bsoul and Mahmood 2010; Ting and Wu 2010; Rai, Iqbal, and Kubra 2010; Iqbal and Farooq 2009; Ahmed, Okasha, and El-Sayed 2007; Litsardakis et al. 2007; Kumar, Shah, and Kotnala 2015). M type Barium hexaferrites are quite heavy as in granular materials, which restrict their practical application in microwave absorbing and shielding materials. This can be reduced by producing these materials with low dimensional and large specific surface area, which will enhance the absorbing ability per unit mass (Lee et al. 2012). Since BaFe12O19 nanohexaferrite is quite versatile because of its practical applications and the dc resistivity of a ferrite is an important property, since it determines its performance at high frequencies, where eddy currents losses may be high, resulting in a significant loss of energy. Further, frequency dependent dielectric and magnetic properties is a very important factor to be considered in designing any component for practical applications. Many research worker have investigated substituted M-Type hard ferrites in the bulk form viz. Alam et al. (Alam et al. 2015) reported the decrease in magnetization (84.53-52.81 emu/g), and coercivity (3750-440 Oe) for the simultaneous 2+ 2+ 4+ substitution of Co , Zn , and Zr ions in BaFe12O19 nanohexaferite processed via co- precipitation method, Chawla et al. (Chawla et al. 2014) reported the decrease in magnetization (62.45-56.94 emu/g), and coercivity (5428.32-630.21 Oe) for the simultaneous 2+ 4+ substitution of Co , and Zr ions in BaFe12O19 nanohexaferite synthesized via sol-gel technique, Lee et al. (Lee et al. 2012) reported very low values of coercivity for BaFe9.6Co1.2Ti1.2O19 hexaferite synthesized via ball-milling and two step sintering processes. Li et al. (Li, Wang, and Wang 2012) reported Ms = 77.188 emu/gm and Hc = 4324 Oe for 3+ BaFe12O19 naohexaferrite, and for pure as well as La substitution they have not obtained quality hysteresis curves. Dhage et al. (Dhage et al. 2011) reported the decrease in magnetization (40.443-5.12 emu/g), and coercivity (5689.28-5396.41Oe) for the Cr3+ substituted BaFe12O19 nanohexaferites synthesized via sol-gel auto combustion method. Although, the structural and magnetic properties of substituted BaFe12O19 hexaferrites studied extensively (Lee et al. 2012; Alam et al. 2015)(Dhage et al. 2011)(Li, Wang, and Wang 2012), the available literature on gadolinium and holonium doped M-type nanohexaferrites is very scarce. To best of our knowledge only Rai et al. (Ting and Wu 2010) reported the effects 3+ of Ho ions on structural and magnetic properties of BaFe12O19 hexaferrites processed by 312 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al. solid state reaction technique and obtained very small values of saturation magnetization and coercivity.

3. ORIGIN OF MAGNETISM IN HEXAFERRITES

In an atom, when an electron revolve around the nucleus with angular velocity “ω,” charge is displaced, therefore current (I = eω/2π) is developed in the orbit, which results in orbital magnetic momentum (μL). Secondly, the electron also spins about its own axis with half rotation and produce spin angular momentum (μS). The total magnetic momentum is the sum of orbital and spin magnetic momentum of the electron in the first Bohr’s orbit measured in magnetic unit, called Bohr magneton (μB). The third factor which causes the magnetic moment is nucleus spin (Zeb et al. 2016).

μN =eh/4πmc (1)

The nucleus magnetic moment is 1/2000 times less than that of the total magnetic moment produced by the electron’s motion, so it is neglected. According to Pauli exclusion principle, in a large number of magnetic materials specially transition metal atoms, the orbital magnetic moments of electrons cancels out each other, therefore resultant magnetic momentum due orbital motion of electrons is zero, but magnetic moment arises only due to spin motion of the electrons. In our periodic table, the rare earth elements are the only magnetic materials posses both orbital angular momentums as well as spin magnetic momentum; therefore both motions of the electrons contribute to resultant magnetism. Depending upon the behavior of interaction of different magnetic materials with the externally applied magnetic fields, the magnetism is divided into various following types:- (a) Diamagnetism, (b) Paramagnetism, (c) Ferromagnetism, (d) Antiferromagnetism, and (e) Ferrimagnetism.

3.1. Diamagnetism

When the contribution to resultant magnetic moments by all electronic states is zero, the diamagnetic phase of the material is arises. Basically, it is a fundamental property of all the materials having the tendency of electrical charges partially to shield the interior of a body from an applied magnetic field. According to Lengevein, diamagnetism is the occurrence of a negative magnetic susceptibility (Kittel 1996) and having magnetization in the direction opposite to the applied magnetic field. Diamagnetism is an inherent property of the orbital motion of individual electrons in a field. The orbital motion even though compensated sets up a field opposite to the applied field in a manner similar to the back emf of Lenz’s Law.

3.2. Paramagnetic

If the atom has a net magnetic moment, the material is paramagnetic; this moment may be partially aligned in the direction of an applied magnetic field. Each of the atoms therefore M-Type Bariumn Nanohexaferrite Materials 313 acts as an individual magnet in a field. The process of alignments of these dipoles in the direction of applied magnetic field is congested by the thermal motion of the electron. Therefore, with the increase in the temperature the magnetic susceptibility of paramagnetic substance decreases. The value of magnetic susceptibility is small positive. Those material which are single domain materials and exhibit the properties of both ferromagnetic and paramagnetic substances, are called superparamagnetic materials.

3.3. Ferromagnetism

Ferromagnetism arises due to the spontaneously magnetized atomic domains. The enhanced magnetization comes out from the cooperative interaction of large number of atomic domains, where all atomic spins within it are aligned parallel (positive exchange interaction). These materials are called ferromagnetic. In 1907, P. Weiss proposed the existence of these regions called “magnetic domains” and also postulated the existence of “molecular field” which produces the interaction between the neighboring atoms. These interactions by molecular fields are called “quantum mechanical exchange forces.” In ferromagnetic materials, the alignments of magnetic domains in an applied external magnetic field at high temperature are decreased. The temperature at which the ferromagnetism vanishes (ferromagnetic material changes to paramagnetic material), is known as Curie temperature. The magnetic susceptibility has very large positive value and it decreases with the increase in temperature above Curie point (Tc). This variation of magnetic susceptibility is in accordance with Curie-Weiss law;

C   T  (2) where C is the Curie- Weiss constant, and  is the ferromagnetic Curie temperature.

3.3.1. Nature of Domain In a domain, there is a parallel alignment of the atomic moments and each domain becomes a magnet composed of smaller magnets i.e., ferromagnetic moments. Each domain contained 1012 to 1015 atoms and their dimensions are of the order of 10-4 to 10-6cm. The main reason of domains formation is to reduce the magnetostatic energy which is the magnetic potential energy contained in the field lines connected north and south poles outside the material. The length of the flux path is reduced continuously when a domain splits into a number of small domains. Due to this splitting process the energy of the system get lowered until the point that more energy is required to form the domain boundary than is decreased by magnetostatic energy change. When a large domain is split into n domains, the energy of new structure is about 1/nth of the single domain sturture. The moments in the adjacent domains are oriented at angle of 1800 to each other. The size and shape of a domain may be determined by minimization of five types of energies Firstly, the Magnetostatic Energy; is the work needed to put magnetic poles in special geometric configurations. It is also the energy of demagnetization and calculated as

314 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

2 3 Ep = 2πMs per cm . (3) where, Ms is satration magnetization of the ferrite, this energy decreases with decrease in the width of the domain, secondly, Magneto crystalline Anisotropy Energy; most matter is crystalline in nature i.e., it composed of repeating units of definite symmetry. In magnetic materials the domain magnetization tends to align itself along one of the main crystal directions. This crystal direction is known as easy direction of magnetization (may be at the edge of the cube or at a body diagonal). The difference in energy of a state where the magnetization is along an easy direction and one where it is along a hard direction is called the magnetocrystelline anisotropy energy. The energy of the domain can be lowered by this amount by having the spins or moments align themselves in the direction of easy magnetization, thirdly, Magnetostrictive energy; when a magnetic material is magnetized, a small change in the dimensions occurs. The relative change is on the order of several parts per million and is called magnetostrion. Also, if the magnetic material is stressed the direction of the magnetization will be align parallel to the direction of stress. The energy of magnetostriction depends on the amount of stress and on a costant chracterstics of the materials called magnetostriction constant (Zeb et al. 2016)(Zeb et al. 2016)(Zeb et al. 2016)(Zeb et al. 2016)(Zeb et al. 2016);

E = 3/2λσ (4) where λ = magnetostriction constant, σ = Applied stress, Fourth type is Domain Wall Energy; in the domain structure of bulk materials, the domain wall or boundary is that region where the magnetization direction in one domain is gradually changed to the direction of the neighboring domain. If δ is the thickness of the domain wall which is proportional to the atomic layers through which the magnetization is to change from the initial direction to final direction, the exchange energy stored in the transition layer due to the spin interaction is:

Ee = kTc/a (5) where kTc = thermal energy at the Curie point and a = distance between atoms. ie exchange energy is reduced by an increase in the width of the wall or with the number of atomic layers in that wall. Also, in the presence of an isotropiy energy, rotation of the magnetization from an easy direction increases the energy so that wall energy due to anisotropy is Ek = k δ, in this case the energy is increased as the domain width or number of atomic layers is increased. The two effects oppose each other and the minimum energy of the wall per unit area of the wall is:

1/2 Ew = 2(KaTc/a) (6) where Ka is the anisotropic constant. If magnetstriction is taken into consideration, the modified equation is:

1/2 1/2 Ew = 2(KaTc/a) (Ka +3λsσ/2) (7)

M-Type Bariumn Nanohexaferrite Materials 315

where λs is the magnetostriction constant. The domain wall thickness for the condition of minimum energy is (Zeb et al. 2016):

δ = Constant X a (E/K)1/2. (8)

For soft magnetic materials the value of δ is 10-6cm while for hard magnetic materials the value of δ is 10-4cm and fifth type is Exchange energy; The quantum origin of exchange interaction derives from the combination of electrostatic coupling between electron orbitals and the necessity to satisfy the Pauli’s Exclusion principle, leading to spin –spin interaction that favours long range spin ordering over a macroscopic range. If there exist a direct coupling between the two spins therefore exchange interaction between electrons with spin th th angular momentum Si and Sj at i and j sites respectively of a particular lattice can be expressed as follow

E  2JS  S i j (9) where ‘ J ’ is the exchange integral which represents the strength of the exchange coupling. Also, exchange integral can be calculated as The values of exchange integral J were calculated by using the following relation (Pullar and Bhattacharya 2001):

3푘 푇 퐽 = 퐵 퐶 (10) 2푧×푠(푠+1)

−23 −1 where z = 2, s = 1/2 and kB = 1.38×10 JK . In addition to this, certain micro structural imperfection such as voids, non-magnetic inclusions and grain boundaries may also affect the local variations in domain structure.

3.4. Antiferromagnetic

In convinced materials, the exchange forces produce anti-parallel alignments of spins of neighboring atoms. This phenomenon is known as antiferromagnetism. According to Neel, those magnetic materials which obey the Curie-Weiss law at high temperature are antiferromagnetic materials;

C   (11) T  where “ “ is the experimentally determined constant. Neel noticed that at very low temperature, the negative exchange forces prevented the normal paramagnetic alignment in a field, so that susceptibility was very low. As the temperature increased further, the susceptibility starts on increasing, the temperature at which there is maximum susceptibility, the temperature is called Neel Temperature follow the equation: 316 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

C   T T N (12)

where, TN is the Neel Temperature.

For antiferromagnetic materials the exchange integral Je is –ve. In these materials no net magnetization is possible. At Neel temperature, magnetization becomes maximum. Above Neel temperature, the magnetization goes on decreasing which indicated that a phase transition from ferromagnetic to paramagnetic. The materials exhibiting antiferromagnetism are comprised of two sub lattices generally known as A and B site. The spin moments of atoms in one sub lattice have an opposite sense from the spin moments of atoms in other sub lattice.

3.5. Ferrimagnetism

Ferrimagnetism is a special case of antiferromagnetism where spins of two sub lattices have different magnitudes so that the spins do not cancel each other. Ferrites exhibits ferrimagnetism phenomenon. Ferrite magnets have largely displaced Alnico magnets in these applications. Ferrites crystallize in three different types, namely spinel, garnet and magnetoplumbite. Depending on the nature of magnetic behavior of the ferrite materials, they are classified into two broad categories. (a) Soft ferrites and (b) Hard ferrites. Soft ferrites are easy to magnetize and demagnetize and have low coercive field Secondly, hard ferrites retain their magnetization once they are magnetized and have high coercive field. The characterization of soft and hard ferrites in general is based upon some important factors

 The residual magnetism (remanence / coercivity) MR that the material retains when the external magnetic field is removed.  The demagnetization field or the value of the external magnetic field applied in the negative direction that removes the residual magnetic field i.e., coercive force HC (Kladnig and Zenger 1992).  The saturation flux or the maximum magnetic field that can be induced in the materials i.e., saturation magnetization Ms.

As ferrites have got their direct origin from magnetite which has chemical formula 2+ 3+ 2+ (Fe O.Fe2 O3) and are obtained by substituting divalent ion in place of Fe , trivalent ions in place of Fe3+.The other valances (+1, +4, +5, +6) can be incorporated into lattice by charge compensation for appropriate change in Fe2+/Fe3+ ratio. The divalent ions are from first transition elements like Co, Mg, Zn, Cd and Ge. Trivalent ions are Al, Cr, Ga and Mn etc. the monovalent is Li and tetravalent are Ti and Sn. In all the cases the ionic radii of substituting ion should be between 0.5 to 1.0, discussed by Gorter (Gorter and Schulkes 1953). Taking into the crystal structure and magnetic ordering ferrites can be grouped into four different categories namely spinel, garnet, magnetoplumbite and orthoferrite.

M-Type Bariumn Nanohexaferrite Materials 317

Table 2. Basic types of ferrites

Sr No. Types Structure General Formula Examples

1. Spinel Cubic M Fe2O4 M = Mn, Zn, Ni, Mg, Cu, Co, Li.

2. Garnet Cubic Ln3 Fe5O12 Ln = Y, Sm, Gd, Ib, Dy, Ho, Er, Tm and Lu

3. Magnetoplumbite Hexagonal M Fe12 O19 M = Ba, Sr and Pb

4. Ortho-Ferrite Perovskites Ln Fe O3 Ln = same as in Garnet

This section presents relevant theoretical background like formulae pertaining to study of structural, electrical and magnetic properties of ferrites. The extrinsic properties of ferrites such as d.c. resistivity, dielectric constant, initial permeability and magnetic properties are significantly affected by the change in structure of the material viz. particle size, grain size distribution, presence of different phases, impurities or imperfections etc. It is therefore very important to investigate various properties of the ferrites in order to understand the role of these in modifying some of the electrical and magnetic properties of the nanohexaferrites. In addition to this, for all nanohexaferrites, few electrical properties at GHz range and Mossbauer properties were also studied.

4. STRUCTURAL PROPERTIES

X- ray diffraction patterns are obtained for all the samples for structural analysis and detection of phases present in the ferrite samples. From the X-ray diffraction data the lattice constant “a” of each sample is calculated from the observed dhkl values using well known procedure given by Culity (Wang, Clark, and Oyama 2002) and MacEwan (Singh 1977). Using the calculated lattice constants “a” and “c,” the theoretical X-ray density of the samples is calculated by using formula

ZM 3 ; (13) Th.  gcm NVCell where M is the molecular weight of the ferrite sample, Z is the number of molecules present in a unit cell. In case of hexaferrite Z= 2 because hexagonal structure containing two formula units, N is Avogadro number and V is theoretical value of unit cell. The formula for calculating density in hexagonal structure becomes;

2M 2M   gcm 3or 103 Kgm3 Th. NV NV Cell Cell (14) and volume of the cell is calculated by using formula

318 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

3 V  a2c (15) Cell 2 where “a” and “c” are the lattice parameters. For our present work, in structural properties, we also calculate the porosity by using formula

(  ) Porosity  Th. Obs. 100%  Th. (16)

5. ELECTRICAL PROPERTIES

5.1. DC Resistivity

The dc resistivity of ferrites is that property which determines performance of ferrites at high frequencies, where the eddy current losses may be high, resulting in a significant loss of energy. The range of dc resistivity in ferrites has been found to be 10-2 to 1011 ohm-cm. Its value also depends upon the technique or method of preparation and could be more than a million times greater than those of magnetic alloys. The origin of electrical conductivity in hexagonal ferrites has been investigated extensively by various workers (Verwey and De Boer 1936; Van Uitert 1955, 1956; Jonker 1959; Yamashita and Kurosawa 1958; Tuller and Nowick 1977; Srinivasan and Srivastava 1981; Mott 1968; Chorley and Lednor 1991; Krupička and Novak 1982; Austin and Mott 2001). Like normal semiconductors, the ferrites semiconductor, the ferrite semiconductors also exhibits-n and p-type conductor behavior depending on type of ions, causes the p-type conduction (Karche, Khasbardar, and Vaingankar 1997). The conductivity in ferrites arises due to the mobility of extra electron (from Fe2+) or the hole (from Me3+) through the crystal lattice. Also, the mobilities of carriers in ferrites are found to be quite low and the carrier concentration does not increase with the temperature. Therefore, the localization of charge carriers in ferrites is responsible for conduction mechanism unlike in normal semiconductor. As each metal ions is surrounded by oxygen ion either in tetrahedral or octahedral configuration and there is a little overlap between the wave functions of cations on the adjacent sites. To explain the conduction mechanism, the band theory is not applicable and various following models proposed. The most recognized model are discussed below:

5.1.1. Verwey’s Hoping Mechanism This model is proposed by Verwey (Verwey and De Boer 1936), According to this mechanism, the electrical conduction in ferrites takes place due to hopping of electrons between ions of same element having different valence state, distributed randomly over crystallographically equivalent lattice sites. Thus it is a thermally activated process and takes place in the presence of lattice vibrations. The hopping probability depends upon the distance between the ions and the activation energy. Since the distance between the neighboring B sites is less than the distance between neighboring A sites, which is still smaller than the M-Type Bariumn Nanohexaferrite Materials 319 distance between the neighboring A and B sites, the hopping probability is greatest amongst the ions occupying B sites and negligible between ions occupying A sites and those occupying B sites. The temperature dependence of the electrical conductivity is given by the relation

E    0 exp( ) KT (17)

where K is the Boltzman constant, E is the activation energy for conduction and  0 is a constant. The activation is not associated with band gap energy as in normal semiconductor but with the crystal around the site of the electrons. It is in fact, the energy needed for producing the required lattice deformation. In case of conduction through both the electrons and holes, the total conductivity is given by;

  e(n  p ) n p where “e” is the electronic charge, n and p represent concentration of electrons and holes respectively. n and  p are corresponding mobility’s. In ferrites the carrier concentrations do not change much with temperature. Thus the temperature dependence of conductivity arises only due to the temperature dependence of mobility. For hopping of electrons and holes, the mobility’s are given by:

 E ed 2 f exp( n ) n KT n  KT (18)

 E ed 2 f exp( p ) p KT  p  KT (19)

where “ d “ is the jump length on octahedral sites, fn and f p are the lattice frequencies active in the jumping process, and En and E p are the activation energies in eV.

5.1.2. Small Polaron Model A small polaron is a defect created when an electronic carrier becomes trapped at a given site as a consequence of the displacement of adjacent atoms or ions. The entire defect (carrier + distortion) then migrates by an activated hopping mechanism small polaron formation takes place in incomplete d or f subshells which due to small electron overlap, tend to form extremely narrow band. The migration of small polaron requires the hopping of both the electron and polarized atomic configuration from one site to an adjacent one (Blue and Gaines 1992; Dearnaley, Morgan, and Stoneham 1970; Snow et al. 1965; Walden 1972). 320 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

(1 m)ea 2R   KT (20) where ‘ e ’ is the electronic charge, a is the lattice parameter, m is the fraction of site which contain an electron ( m  n , n being the number of electrons and N the number of N available sites per unit volume) and R is the jump rate of polaron from one site to a specific neighbouring site and is given by;

  E  R  Po exp   KT  (21)

 is the appropriate optical mode phonon frequency, E is the activation energy and P is the o factor which gives the probability that electron will transfer after the polarized configuration has moved to the adjacent site. Therefore equation becomes

2  P(1 m)ea o    E     exp   KT   KT  (22)

Considering only electronic conduction, the conductivity is given as:

 2 2    ne NPm(1 m)e a o   E    exp   KT   KT  (23)

A   E    exp  T  KT  (24)

 where, A  o NPm(1 m)e2a2 K (25)

5.1.3. Phonon Induced Tunneling Srinivasan and Srivastava (Srinivasan and Srivastava 1981) have developed a phonon induced tunneling mechanism to explain the electrical conductivity in ferrites. It has been proposed that the electron participating in Fe2+ =Fe3++e- exchange process are strongly coupled to the lattice and tunnel from site to other. According to this model, the conductivity is given by expression;

 u     o KT      u   u  exp   exp   KT   KT  (26) M-Type Bariumn Nanohexaferrite Materials 321 whereu , represents the change in lattice energy when oxygen ions are moving towards Fe3+ 2+ compared to the case when they are moving towards Fe and o is the factor which is related to the nearest neighbor Fe2+-Fe2+ distance. The replacement of one ion by another one in ferrites matrix affect the resistivity. The substitution of a cation of low resistivity for another one of high resistivity element is found to be enhance conduction in ferrites. Also the resistivity in ferrites is also affected by sintering atmosphere, temperature, pressure, applied field, duration of applied field etc.

5.2. Dielectric Constant, Loss factor and AC Conductivity

Ferrite posses high dielectric constants of the order of a few thousand at low frequencies, falling to a very low values of 10-20 at microwave frequencies (Koops 1951; Brockman, Dowling, and Steneck 1950). Brockmann et al. (Koops 1951; Brockman, Dowling, and Steneck 1950) explained the low frequency behavior of dielectric constant on the basis of dimensional resonance effect. Dielectric materials posses one or more of four basic types of electric polarization viz. electronic polarization, ionic polarization, dipolar polarization and interfacial polarization. The degree of the overall polarization depends on the time of variation of the electric field. The high values of dielectric constants are due to interfacial polarization (Tareev 1979), which causes charges to accumulate on the interfaces of different heterogeneous dielectric materials. The origin of interfacial polarization has been attributed to the distribution of electrical resistivity caused by the non-uniform distribution of oxygen ions produced during the sintering process (Verma et al. 1999). Since an oxygen ions has got two extra electrons which are loosely bound to the nucleus, a small electric field applied to the material, suffices to distort it and produce polarization. The change in dielectric constant of a ferrite with frequency is known as dispersion. To explain dispersion in dielectric constant, Koops (Koops 1951) proposed a general model for hetrogeneuous dielectric consisting of a well conducting grains separated by layers of low conductivity as found in polycrystalline ferrites. Koops found that at high temperature some of the Fe3+ ions get reduced to Fe2+ ions which increase the conductivity of the grains. On cooling the ferrites in a slightly oxidizing atmosphere, Fe2+ ions present in the outer layer of grains again gets oxidized to Fe3+ producing layer of lower conductivity. The presence of air gaps also contribute to the formation of heterogeneous structure in ferrrites (Koops 1951; Moltgen 1952). The dielectric constant or permittivity  is a complex quantity and is written as (Cochardt 1963);

' ''     j (27) where ' represents the real or dispersive part of the dielectric constant and  '' represents its imaginary parts or dissipative part, which accounts for losses. The dielectric loss factor is given by

' tan  '' (28) 322 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

The change of dielectric loss factor (Ta n ) with frequency is given by the equation;

4 ta n  '  (29) where,  is the conductivity of the dielectric. The AC conductivity is related to dielectric loss and frequency by following relation:

   '  tan AC o (30)

6. MAGNETIC PROPERTIES

6.1. Saturation Magnetization

When the external magnetic field is applied to the magnetic material, the material gets magnetized. The degree up to which we can magnetized the material is called intensity of magnetization and the dipole moments associated with all molecules are aligned in the direction of the applied magnetic field, resultant moment per unit volume is called saturation magnetization. It is an intrinsic property of the material and depends on the chemical composition, electronic structure of the constituent ions and the crystal structure of the lattice. It is not affected by the microstructure of the material and mathematically it can be expressed as;

M  N s m (31)

where N is the number of dipoles per unit volume and m is the net dipole moment of each molecules. In ferromagnetic materials like ferrites, the contribution to net magnetic moments arises due to the orbital motion and the parallel uncompensated spins of the electrons of the individual ions. The major contribution to the magnetic moment comes from the spins motion of the electrons, while the contribution due to orbital motion is negligible small due to quenching effect of the internal crystalline field (Smit and Wijn 1959). If we consider the contribution of the individual ions present in the ferrites to the magnetic moment, the free oxygen atoms having unfilled 2p sub shell, become O2-, by acquiring two electrons from the neighboring metal atoms. The oxygen ion have therefore, got zero magnetic moment and do not contribute to the net magnetic moment. Hence the magnetic moment in ferrites arises due to the uncompensated electrons spins of the metal ions, in the two sub lattices. According to Neel’s theory (Neel 1948) of ferrimagnetism, there exist a exchange interaction among the neighboring dipoles of the cations present in a molecule. These interactions are of two types (i) direct exchange interaction and (ii) super-exchange interaction. The direct exchange interaction takes place due to overlap of the orbital of the two interacting magnetic ions. This interaction is important only when the separation between the interacting magnetic ions is M-Type Bariumn Nanohexaferrite Materials 323 small enough to allow an overlap. In spinels, the separation between the cations is large and there is no overlap of the orbital of the magnetic ions therefore in these ferrites only super exchange takes place through the participation of oxygen ‘anions.’ Since, in ferrites magnetic ions occupy (A) and (B) sites, the interaction takes place by three ways between the magnetic ions located at A site and B site: A-A interaction, B-B interaction and A-B interactions. Among three interaction AB interaction being the strongest and AA interaction is weakest; but the BB interaction is the intermediate between the two. Since the AB interaction are strongest, it will align all the magnetic spins at A site in one direction and those at the B- site in the opposite direction; thus constitute two saturated and oppositely magnetized sublattices at 0K. The resultant magnetization is therefore, the difference between the magnetization of the B and A sublattices, where in former generally has a large values. Thus, the net saturation magnetization is given by;

MS= MB -MA (32)

To understand the behavior in magnetization in the present work it is essential to 3+ understand the distribution of Fe ions in the BaFe12O19 nanohexaferrite and the same is shown in Figure 5. In M-type nanohexaferrites, the Fe3+ ions are distributed over three interstitial sites, tetrahedral, octahedral and trigonal bipyramidal, which are further divided 3+ into different sub-lattices. Tetrahedral site has only 4f1 sub-lattice which has two Fe ions in spin down state. Octahedral site is divided into three sub-lattices (12k, 2a and 4f2). Out of 3+ 3+ three sub-lattices, 12k has six Fe ions and 4f2 sub-lattice has two Fe ions in spin up state while 2a sub-lattice has one Fe3+ ion in spin down state. Trigonal bipyramidal site has only 2b sub-lattice which has one Fe3+ ion in spin down state. As Ho3+ ions prefer to occupy octahedral site and replaced Fe3+ ions at 12k sub-lattice due to which total number of spins in the downward direction increased in accordance with the above reported papers (Li et al. 2011; Iqbal et al. 2013; Nellis and Legvold 1969). In hexaferrites the net magnetization at different sub lattices is given as

M (T)  6M (T) 1M (T) 1M (T) 2M (T)  2M (T)  total 12k 2b 2a 4 f 1 4 f2 (33)

3+ 3+ where Mn describes the magnetic moment of Fe ion in the n-th sublattice. Since, each Fe ion has a magnetic moment of 5 µB; the pure barium nanohexaferrite has a total magnetic moment of 20 µB per formula unit. For a unit having two formula units, the total magnetic moment is 40 µB per formula unit.

6.2. Initial Permeability

Initial permeability is a property of magnetic material which describe the effect of applied magnetic field on the state of magnetization of the material or it is a quantity which gives how much magnetic lines of force can pass through the material. Mathematically it can be expressed as

324 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

1  B  i  lim H 0  (34) o  H 

7 where, B is magnetic induction, and H is the magnetizing field and o  4 10 henry/meter is the permeability of free space. On the application of alternating applied magnetic field, the magnetization does not remain in phase with the applied field due to damping and results in dissipation of energy. The initial permeability is therefore generally expressed as

   '  j '' i i i (35)

' '' where i , is the dispersive part of the initial permeability and i is the dissipative part. Both these parameters vary with the frequency. Weiss (Weiss 1907) predicted that presence of ferromagnetic.

3+ Figure 5. Distribution of Fe ions in BaFe12O19 nanohexaferrites.

Domains having parallel spins are separated by layers called “domain wall” or “Bloch wall.” The direction of magnetization gradually changes from one orientation to another and this mechanism that contribute to the initial permeability are attributed to the simultaneously rotation of the spins in each Weiss domain and the reversible displacement or bulging of domain walls (Bloch 1932; Jiles and Atherton 1986). The former is referred to as domain rotation and the later as domain wall motion. The contribution of various processes to the initial permeability was explain by Globus (Bellad et al. 2000; Globus 1962). This model explain both reversible and irreversible processes of magnetization. According to Globus, the M-Type Bariumn Nanohexaferrite Materials 325 rotational permeability is isotropic in ferromagnetic material and is given by relation (van Vleck 1937);

M  18 s ir 3H a (36)

where ir is the contribution of domain rotations to initial permeability, M s is the saturation magnetization and H a represents the field due to various types of anisotropies present in the material and it is given by

2K H  1 a M s (37)

where K1 is the magnetocrystalline anisotropy constant. Thus equation (31) becomes

M 2  1 4 s ir 3K 1 (38)

The contribution due to reversible bulging walls to initial permeability is given by (Cochardt 1963)

4M 2 D2   s iw  d w (39)

where iw represents the domain wall contribution to initial permeability, D is the span of the domain, w is the domain wall energy per unit area and d represents the domain wall motion. According to this equation, the initial permeability due to domain wall motion depends not only on the anisotropies but also depends significantly upon the span of the wall D. The resonance generally takes place when frequency of the applied field is equal to the larmor frequency and the resonance frequency can be expressed as;

 K  w  2  1   M   s  (40) where  is the magneto-mechanical ratio. From above stated equations we can conclude that the initial permeability is directly proportional to saturation magnetization and also inversely proportional to anisotropic constant. It also depends upon methods of preparation, grain size and its distribution and porosity (Slick 1980; IGARASHI and OKAZAKI 1977). 326 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

7. MICROWAVE SPECTROSCOPY

The nanohexaferrites were found their technical suitability for design of smart miniature and reconfigurable antennas. In the present research work, the complex permeability or complex permittivity and magnetic loss and electric loss were calculated by using the following formulae reported by J. Sheen et al. (Sheen 2007).

푉 (푓 −푓 ) 휇′표푟 휀′ = 0.539 푐 푐 푠 + 1 (41) 푉푠푓푠

푉 1 1 휇′′표푟휀′′ = 0.269 푐 ( − ) (42) 푉푠 푄푠 푄푐 where ε′ represents the real part of the permittivity, µ′ represents the real part of the permeability, ε′′ represents the dielectric loss, µ′′ represents the magnetic loss, Vc is volume of the cavity, Vs is volume of the sample, fs is resonant frequency of the cavity with sample, fc is resonant frequency of the cavity without sample, Qs is Quality factor with sample and Qc is Quality factor without sample. The compositional dependence of real permittivity can be explained on the basis of Verway’s model (Verwey and De Boer 1936). According to that model, the dielectric properties of the ferrites originate from their physical and chemical properties through various mechanisms of polarization (Haijun et al. 2003; Kuanr and Srivastava 1994). The dielectric dispersion can be explained on the basis of different electric dipole formation by cations Ba2+, Fe3+ of the structure with their surroundings O2- ions. Secondly, the main source of polarization in ferrites is the conversion of Fe3+ to Fe2+ ions on octahedral 2a sites. This electron hopping that occurs between adjacent Fe3+ and Fe2+ ions lead to local displacement of electric charge carriers, and then finally they contribute to dielectric polarization and relaxation (Stergiou et al. 2010). However, the permittivity of nanoferrites at higher frequencies is due to the atomic and electronic polarization, and the same can be explained by Maxwell-Wagner model (Wagner 1913) while the variations in real permeability and magnetic loss can be correlated to spin rotation and domain wall motion.

8. MOSSBAUER SPECTROSCOPY

The discovery of Mössbauer effect or The Mössbauer spectroscopy, also known as the recoilless nuclear gamma resonance fluorescence (NRF) spectroscopy, in 1957 by the German physicist Rudolf Mossbauer was hailed as a breakthrough in nuclear and solid state physics (Mössbauer 1958). This spectroscopy is, one of the important techniques which can measure the comparatively weak interactions between the nucleus and the surrounding electrons. Mossbauer shared the 1961 Physics Noble prize with the American nuclear Physicist Robert Hofstadter, who was honored for electron-nuclear scattering concerning the structure of the nucleons. Mossbauer was investigating nuclear resonance scattering of 129 keV gamma rays from 191Ir. The basic principle of NRF spectroscopy involves, 57Co decays to the excited state of iron 57Fe* by electron capture (EC) which further decays to the stable 57 Fe state by the emission of delayed gamma ray as shown in Figure 6, this phenomena is called γ – ray fluorescence. In the present target nucleus 57Fe, this gamma ray can be resonantly M-Type Bariumn Nanohexaferrite Materials 327 absorbed. Since the excited state 57Fe* has a finite life-time (τ), the uncertainty in the energy of the emmited γ – ray is governed by the Heisenberg uncertainty principle;

∆E ∆t ≈ ћ (43) which can be rewritten as

Γ τ ≈ ћ, (44) where Γ is the line width (10-4 < Γ< 10-11eV) and τ is the life time (10-11 < τ < 10-4s) of the excited state. The momentum of the emitted gamma ray is given, according to the de- Broglie relation, by

p = h/λ (45) where λ is the γ – ray wavelength. Since momentum must be conserved, the nucleus must recoil in the opposite direction, with recoil energy R given by

R = p2/2M (46) where M is the mass of the recoiling nucleus. The target nucleus, too small, must recoil with energy R on receiving the γ – ray, with the result that some of the energy of γ – ray transition, Eγ, is converted into the recoil energy. Thus, for the emiiting nucleus

E = Eγ – R (47)

And for absorbing nucleus (absorber)

E = Eγ + R (48)

From above equation, we conclude that the emission and absorption lines are centered 2R apart.

Since Eγ = hν = hc/λ = pc (49)

2 2 2R = Eγ /Mc (50)

For resonance absorbtion to occur, Γ must be greater than or equal to loss in γ – ray energy due to recoil, i.e.,

Γ ≈ 2R (51)

For the Mossbauer nuclide 57Fe, it is found that Eγ = 14.4 keV; and M = 1.67 X 10-27 kg. Hence R = 2 x 10-3 eV. The life time of the lowest excited state of 57Fe is 10-7s, which 328 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al. correspons to Γ- value 5 x 10-9 eV. In other words, R>> Γ. Obviously, the resonance absorption condition is not obtained. Mossbauer took the sample containing the emmiter nuclei in the form of solid at low temperature, and therefore no energy is lost to recoil for the absorber. Thus according to this theory, the emission and absorption spectra contain very strong lines of natural width. Thus, Mossbauer was first person to demonstrate the feasibility of observing gamma ray resonance fluorescence by embedding the emitting and observing nuclei in a well bound solid. Under these condition, he showed that a fraction of gamma rays have natural width and are not broadened by the phonon spectrum of the lattice. Mossbauer in one single experiment eliminated the recoil and the Doppler broadening. Consequently, Mossbauer effect is sometime called nuclear gamma ray resonance or zero phonon emission and absorption of gamma rays or recoilless emission and absorption of gamma rays. The following parameters (Rochow and Abel 2014) which we can obtain from the Mossbauer spectrum are important to draw meaningful conclusions about the structure of the samples yielding useful information about their ionic distribution, type of ordering, valency state and deviation from stoichiometry etc;

a) Isomer Shift b) Electric Quadrupole Splitting and c) Magnetic Hyperfine interactions.

8.1. Isomer Shift (δ)

In atomic spectroscopy, lines from an isotopic mixture show a splitting which is not present in the spectrum of an isotopically pure element. In heavy elements, this isotopic splitting is due to the fact that the addition of one or more neutron changes the nuclear radius. This change in turn shifts the atomic energy level (Kopfermann 1958). A change in nuclear radius can occur even without a change in nucleon number when the nucleon goes from one state to other, for instance, when it decays from an isomeric state to the ground state. The corresponding shift in energy is called an isomeric shift (δ). The isomer shift arises from the difference between the electrostatic interactions between the positively charged nucleus and the s-electron density at the nucleus in the source and the observer. Only s-electron has a finite probability of overlapping with the nuclear charge density, so the isomer shift can be evacuated by considering this interaction. Although isomer shift (δ) are due to variation in s- electron density, difference in isomer shift (δ) are observed also on addition or removal of p or d-electrons, which do not interact with nuclear charge density directly. The total s-electron density at the atomic nucleus in a compound may be considered to be composed of contributions from filled s-orbitals, where valence electrons contribution to the s-electron at the nucleolus is very sensitively affected by changes in the electronic structure of the valance shell by chemical influences such as oxidation state, spin state and bound properties by electron-delocalization etc. Such changes in the valance shell influence the s-electrons population in the valance shell influences the s-electron density in two ways. First directly by altering the s-electron population in the valance shell and secondly by shielding s-electrons by electrons of non zero angular momentum. Hence, isomer shift is a useful parameter which provides information regarding valency states, ligand bonding states, electron shielding. The isomer shift in Fe3+ Mossbauer transition is shown in Figure 6. M-Type Bariumn Nanohexaferrite Materials 329

Figure 6. Schematic diagram showing the decay of the radioactive 57Co to 57 Fe.

Figure 7. Isomer shift and electric quadrupole splitting in 57Fe energy levels.

Figure 8. Isomer shift and nuclear Zeeman splitting in 57 Fe energy levels. 330 Virender Pratap Singh, Gagan Kumar, Ramprakash Dwivedi et al.

8.2. Electric Quadrupole Splitting (QS)

The existence of electric quadruple interaction is a useful feature of Mossbauer spectroscopy. The theory is closely related to that used in nuclear quadruple resonance spectroscopy (Meyer and Scott 1959). Any nucleus with a spin I = 1/2, has a non spherical charge distribution. An asymmetric charge distribution around the nucleus causes an asymmetrical electric field at the nucleus characterized by a tensor quantity called electric field gradient (EFG) which when interact with the nuclear quadrupole moment (q) at the site of nucleus give rise to the quadrupole splitting. The EFG can arise due to atomic electrons as well as ions, which surrounds the nucleus. For 57Fe nucleus the excited state (I = 3/2) split into two substrates (I = 3/2 and ±1/2). This leads to a two line spectrum separated by the quadruple splitting. This parameter is of a great importance in application to investigate the local symmetry around the Mossbauer atom and the configuration of its valance electrons. The quadruple splitting in 57 Fe Mossbauer transition shown in Figure 7.

8.3. Magnetic Hyperfine Interaction (MHF)

The splitting in the Mossbauer spectrum which originates because of the coupling between the nuclear magnetic moment and the magnetic field at the nucleus is called hyperfine interaction or Zeeman splitting. In general, the magnetic field originate either within the atom itself, within the crystal via exchange interactions and from applied external field (B). The Hamiltonian for the interaction can be expressed as

H  .H  g I.H m N (52) Hm  gN HIZ

where N is the nuclear Bohr magneton,  is nuclear magnetic moment, I is the nuclear spin and g is the gyromagnetic ratio, the eign values of Hamiltonian can be expressed as;

Em  gN Hml (53)

where ml ( = +I, I-2, I………..-I) is the magnetic quantum number representing the z- component of I. The magnetic field is therefore splits a nuclear spin into 2I+1 sublevels. A Mossbauer transition can takes place if the change in value is 0 or ±1. So in case of 57Fe, the excited level I=3/2 splits into 4 sublevels and I=1/2 ground state splits into 2 sublevels, giving six Mossbauer transitions between these sublevels i.e., sextet as shown in Figure 8.

CONCLUSION

The present chapter work has been aimed to investigating the structural, electrical, magnetic and Mössbuer properties of rare earth ions substituted M-type barium M-Type Bariumn Nanohexaferrite Materials 331 nanohexaferrites. In addition to this, we have made attempt to investigate the microwave properties at very high frequency (GHz) range, for above mentioned nanohexaferrites. The particle size was estimated for the (114) and (107) peaks. The existence of extra phase is due to the large ionic radius of metal ions as compared to Fe3+ ions (0.067 nm). The lattice parameters ‘a’ and ‘c’ for all the gadolinium doped nanohexaferrites can be calculated by using the relations reported by I. Bsoul et. al. For rare earth metal ion doped M-type nanohexaferrites, the Lattice parameter “a” is found to be increasing, whereas Lattice parameter “c” is found to be X-ray density is increasing (5.23-5.43g/cm3) with the incorporation of metal ions. At room temperature, the dc resistivity was observed to be increasing with increasing the concentration of the rare-earth metal contents, whereas the value of dc resistivity decreases with increase in temperature. The drift mobility was observed to decreasing at room temperature and it shows the opposite trend with increasing the temperature. Dielectric constant was found to be decrease with increase in the frequency over MHz range and it show decrease with increase in the content. Dielectric loss tangent showed the similar behavior with the frequency and the values of dielectric loss tangent so obtained is very small and suggests the utility of these nanohexaferrites over wide frequency range. The electric and magnetic loss over microwave frequency region have quite low values which shows the utility of these nano hexaferrites for anteena applications up to S band as well as C- band. The ac conductivity was observed to increase with increase in frequency. The Mössbauer spectroscopy analysis depicted that nanohexaferrites series have defined sextets and all the Mossbauer hyperfine parameters collectively should supports our M-H and M-T studies.

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Chapter 12

HYPERSPECTRAL IMAGING: A BRIEF INTRODUCTION FOR BEGINNERS

Ankit Gupta* School of Electrical and Computer Science Engineering, Shoolini University, (H. P.) Solan, India

ABSTRACT

Hyperspectral images provide adequate information to identify and discriminate distinctive materials with well-defined spectral signature. HSI provides precise and detailed information than possible with any other type of technique for collecting data from remote location. Data collected by this technology provides continuous band spectrum ranging from the wavelength of 0.35 - 12.5 micrometers (µm). A typical hyperspectral imaging system consists of an imaging spectrometer, and flight computer coupled to the global positioning system. It can be used in numerous applications like detection of oil spill in marine environments, military detection targets etc. With the advent of platforms like Push Broom, LCTF etc it can be used in many areas such as food safety and processing following its usage as real-time system, micro-organism detection at cellular level and high throughput analysis in future. Although it can provide detailed spectral signatures of the spectrally unique materials but challenges associated with this technology generates a need for the development of noise filtering algorithms without changing or disturbing its spectro-spatial components.

Keywords: hyperspectral imaging, imaging spectrometer, remote sensing, target detection

* Corresponding Author address: Email: [email protected]. 338 Ankit Gupta

INTRODUCTION

Hyperspectral Imaging

Most vital step forward in remote sensing is the development of hyperspectral imaging systems and programs to analyze the resulting image data. The “hyper” in hyperspectral means “more” and refers to the bands with a relatively large number of varying wavelengths. Hyperspectral images are highly spectrally resolute, which means that they provide sufficient information to identify and discriminate exclusive materials with distinct spectral signatures. Hyperspectral imagers measure reflected radiation at a succession of narrow and contiguous bands of variable wavelength. Most of the hyperspectral sensors are airborne while few of them are satellite sensors. HSI provides precise and detailed information than possible with any other type of technique for collecting data from a remote location (Shippert 2003).

Figure 1. Sample Hyperspectral image data cube its spectral and spatial dimensions (http://www.tankonyvtar.hu/en/tartalom/tamop425/0032_terinformatika/ch04s04.html).

Hyperspectral remote sensing combines the concept of remote imaging with spectroscopy to a solo system which often embraces big data sets and requires novel dispensation methodologies. It deals with apprehending images using a wide range of spectrum from ultraviolet to infrared region. The hyperspectral image is an incessant spectrum of bands of relatively small wavelengths ranging from 0.35 to 12.5 (µm) micrometer. It comprises of about 100 to 300 spectral bands of relatively narrow bandwidths (5-10 nm). Hyperspectral Hyperspectral Imaging: A Brief Introduction for Beginners 339 imagery is typically poised as a data cube which consists of spatial information in the X-Y plane, and spectral information in the Z-direction respectively.

Table 1. Satellite Hyperspectral imaging sensors (Shippert 2003)

Satellite Sensors Manufacturer # Bands Spectral Range (µm) FTHSI on MightySat II Air Force Research Lab 256 0.35 to 1.05 Hyperion on EO-1 NASA Goddard Space Flight 220 0.4 to 2.5 Center

Table 2. Airborne Hyperspectral Imaging Systems (Shippert 2003)

Airborne Sensors Manufacturer # Bands Spectral Range (µm) AVIRIS (Airborne Visible NASA Jet Propulsion 224 0.4 to 2.5 Infrared Imaging Lab Spectrometer) HYDICE (Hyperspectral Naval Research Lab 210 0.4 to 2.5 Digital Imagery Collection Exp.) PROBE-1 Earth Search Sciences 128 0.4 to 2.5 Inc. CASI (Compact Airborne ITRES Research up to 228 0.4 to 1.0 Spectrographic Imager) Limited HyMap Integrated Spectronics 100 to 200 Visible to thermal infrared VIS/NIR (76), VIS/NIR (0.43 to 1.05), EPS-H (Environmental SWIR1 (32), SWIR1 (1.5 to 1.8), GER Corporation Protection System) SWIR2 (32), SWIR2 (2.0 to 2.5), TIR (12) TIR (0.8 to 12.5) VIS/NIR (32), VIS/NIR (0.43 to 1.05), SWIR1 (8), SWIR1 (1.5 to 1.8), DAIS 7915 (Digital Airborne GER Corporation SWIR2 (32), SWIR2 (2.0 to 2.5), Imaging Spectrometer) MIR (1), MIR (3.0 to 5.0), TIR (6) and TIR (8.7 to 12.3) VIS/NIR (76), VIS/NIR (0.40 to 1.0), DAIS 21115 (Digital SWIR1 (64), SWIR1 (1.0 to 1.8), Airborne Imaging GER Corporation SWIR2 (64), SWIR2 (2.0 to 2.5), Spectrometer) MIR (1), MIR (3.0 to 5.0), TIR (6) and TIR (8.0 to 12.0) AISA (Airborne Imaging Spectral Imaging up to 288 0.43 to 1.0 Spectrometer)

Hyperspectral Imaging (HSI) System

A typical hyperspectral imaging (HSI) system entails following components (Imaging Spectrometer, Flight Computer, Global Positioning System) (Rand Swanson, Hyperspectral 340 Ankit Gupta

Imaging, http://www.vision-systems.com/articles/print/volume-15/issue-7/Features/Hyper spectral_Imaging.html).

Imaging Spectrometer The imaging spectrometer is an instrument used for capturing hyperspectral images which use two distinct technologies: spectroscopy and remote imaging of Earth. Spectroscopy deals with studying deviations in the energy of light and its wavelength emitted or reflected by innumerable materials. In the context of remote sensing, spectroscopy is used to study the spectrum of diffused or emitted light radiations from the earth surface. The spectrometer is provided with a dispersing element (prism or diffraction grating) that splits the light into narrow bands of different adjoining wavelengths. The energy of every band is measured by a separate detector. Remote Imagers focus and measure light reflected from the target zone on Earth (Smith). Figure 2 shows the basic constituents of the spectrometer.

Figure 2. Schematic Diagram of basic constituents of the spectrometer (Smith).

Flight Computer It is basically used in flight training to store the information related to flight counts, and calculating items like consumption of fuel, wind correction etc. In hyperspectral imaging, it is used for the same purpose. A flight computer with 32 GB hard drive can measure up to 1.5 h of data which equals to seizing hyperspectral data in an area of 10 square miles.

Global Positioning System (GPS) GPS is used to categorize the target area for which measurements needs to be carried out. In HSI system, Flight computer’s CPU clock is harmonized with GPS clock. Time stamps were positioned on both GPS and hyperspectral data and warehoused in flight computer. An example of the hyperspectral imaging system is shown in Figure 3. HSI systems can be airborne e.g., AVIRIS by NASA jet propulsion laboratory or satellite based like FTHSI on MightySat II manufactured by Airforce Research lab. Some of the satellite based hyperspectral sensors along with their spectral range and band number are listed in Table1 and airborne in Table 2. Hyperspectral Imaging: A Brief Introduction for Beginners 341

Figure 3. HSI system consisting of imaging spectrometer coupled to GPS and PC-104 formal flight computer (Rand Swanson, Hyperspectral Imaging, http://www.vision- systems.com/articles/print/ volume-15/issue-7/Features/Hyperspectral_Imaging.html).

APPLICATIONS OF HSI SYSTEM

Hyperspectral imaging can be engaged in diverse regions with an objective of detecting materials with distinguished reflectance spectra. It can be used for mapping of minerals in a particular area of earth (Clark, Swayze, and Gallagher 1992; Clark and Swayze 1995), sensing moisture, organic content in the soil (Ben-Dor, 2000), identification of vegetation species (Clark and Swayze 1995), armed target recognition goals, oil spill detection.

Oil Spill Detection

Oil spill pollution is one of the most critical environmental pollutions in the marine environments. Hyperspectral imaging can be employed to sense oil spills in an aquatic system. This study includes a collecting broadband images of a long-wave infrared spectrum. 342 Ankit Gupta

These images were then smoothed using Non-local means (NLM) filtering algorithm followed by extraction of spectral features using the most appropriate bands. Finally, the data was classified using a model differentiating oil from aquatic environment (Clark and Swayze 1995).

CHALLENGES

Data collected as hyperspectral image grieves from spectral-spatial blurring due to the immersion of light at different altitudes. One of the main subjects for processing hyperspectral data is a reduction of its dimensions. Pixels at the high dimensional space will have high computational complexity and degrading accuracy which makes it highly challenging for target detection, image segmentation, pixel classification and spectral unmixing in HSI. Identification of relatively small objects necessitates good spatial resolution which is challenging to achieve. If the spatial domain of the image is not used, the resulting image map looks noisy. Spectral/spatial classification is used to allocate each image pixel to one class using a feature vector based on the spatial and spectral information. ECHO (Extraction and Classification of Homogenous Objects) is one such classifier used for above classification of multivariate images (Velasco-Forero and Angulo 2013). Hyperspectral data processing requires ample amount of time and hardware due to its bulky size. There are total six types of disturbances, which can distress the image cube. This includes water vapor, carbon monoxide, the ozone layer, nitrogen etc. Figure 4 shows a graph of spectral irradiance caused by different noise sources.

Figure 4. Spectrum with absorption at different levels (http://www.csr.utexas.edu/projects/ rs/hrs/process.html). Hyperspectral Imaging: A Brief Introduction for Beginners 343

This opens up an challenge for the expert from different domains to develop universal noise filtering algorithms that can cope up with eradicating noise created by different issues discussed above. Although a range of algorithms such as ECHO (Extraction and Classiﬁcation of Homogeneous Object) (Tarabalka et al., 2010), Empirical Line Method (Smith), have been developed to exploit the extensive information confined in hyperspectral imagery. Most of these algorithms also offer accurate, although more limited, analysis of hyperspectral data. Spectral analysis methods usually compare pixel spectra with a reference spectrum (often called a target) for material identification present in the image. Reference spectrum can be taken from different resources, including spectral libraries, spectral images entailing region of interest, or using its pixels. Some of predominantly used HSI analysis methods are whole pixel methods, sub-pixel methods etc. Former determines the abundance of target materials within each pixel on the basis of the spectral similarity between the pixel and target spectra. Later calculates a number of target materials in each pixel of the image. Besides these methods, there are some more strategies like identification of a small set of parameters which explains the underlying structure (Li et al., 2010), use of multi-arrays which are also called tensors. The multi-way arrays have an advantage of integrating spatial and spectral domains simultaneously for hyperspectral image analysis unlike other methods (Smith) which follows the principle of dimensionality reduction.

CURRENT ASPECTS

Earlier hyperspectral imaging systems were highly expensive due to which they were restricted to very few industrial applications. Nowadays, development of highly compact and economical systems showed its significance in numerous disciplines including food safety, animal protein detection in compound feeds etc. Current HSI systems also offer an immense computation power, as a result, larger cubes can be processed in a short span of time. Various platforms are available for HSI measurements depending on its implementation in different fields. Some of them are Push broom, Liquid Crystal Tunable Filter (LCTF) etc. Push broom techniques to have capricious switching speed with high transmission and spectral resolution whereas LCTF has random access image capture with prior spectral resolution.

FUTURE PROSPECTS

With the advent of cost-effective fabrication, hyperspectral imaging technology has been engaged in wide-ranging fields. HSI system using push broom platform with suitable imaging algorithm could be used as a real-time application for food industry Future usage of HSI technology includes its usage as a real-time application, high throughput HSI for microscale sample detection, or microscopic imaging technology for detection of microorganisms at the cellular level (Park 2016). Figure 5 shows a real-time HSI system used for inspection of broilers.

344 Ankit Gupta

Figure 5. Real-time Hyperspectral Imaging technology for inspection of broilers (Park 2016; Smith).

Figure 5 shows a real-time system comprising a line-scan hyperspectral camera, an objective lens, power supplies, two pairs of light sources, imaging spectrograph with the camera sensor. Future usage for HSI technology is not limited to examples discussed in this section. With the pace of evolution of technology and enormous potential of HSI technology to provide highly detailed data of the target, it can be applied to many fields.

REFERENCES

Clark, Roger N., and Gregg A. Swayze. 1995. “Mapping minerals, amorphous materials, environmental materials, vegetation, water, ice and snow, and other materials: the USGS Tricorder algorithm.” Clark, Roger N., Gregg A. Swayze, and Andrea Gallagher. 1992. “Mapping the mineralogy and lithology of Canyonlands, Utah with imaging spectrometer data and the multiple spectral feature mapping algorithm.” Li, Fang, Michael K. Ng, Robert Plemmons, Sudhakar Prasad, and Qiang Zhang. 2010. Hyperspectral image segmentation, deblurring, and spectral analysis for material identification. Paper read at SPIE Defense, Security, and Sensing. Park, Bosoon. 2016. “Future trends in hyperspectral imaging.” NIR news no. 27 (1):35-38. Shippert, Peg. 2003. “Introduction to hyperspectral image analysis.” Online Journal of Space Communication no. 3. Smith, Randall B. “Introduction to hyperspectral imaging, 2012.” Available on line.(Cited on page 1.). Tarabalka, Yuliya, Mathieu Fauvel, Jocelyn Chanussot, and Jón Atli Benediktsson. 2010. “SVM-and MRF-based method for accurate classification of hyperspectral images.” IEEE Geoscience and Remote Sensing Letters no. 7 (4):736-740. Velasco-Forero, Santiago, and Jesus Angulo. 2013. “Classification of hyperspectral images by tensor modeling and additive morphological decomposition.” Pattern Recognition no. 46 (2):566-577. In: Smart Materials for Smart Living ISBN: 978-1-53612-269-5 Editor: Radheshyam Rai © 2017 Nova Science Publishers, Inc.

Chapter 13

DIELECTRIC RELAXATION IN BATIO3-BASED PEROVSKITE

Mamta Shandilya1,*, Shweta Thakur1, Radheshyam Rai1 and Jagtar Singh2 1School of Physics, Shoolini University, Solan, HP, India 2XRD Lab, SAIF, Panjab University, Chandigarh, India

ABSTRACT

The study of relaxors is most interesting because of their curious properties that are not yet understood and because they are useful materials from a technological point of view. In this chapter, we only draw a general picture of dielectric relaxation in perovskite ceramic to underlying concepts and pay attention to the common behavior of relaxor rather than to the features observed in specific materials. We hope that this chapter approach is beneficial to those physicists, chemists, material scientists and device engineers who deal with relaxor. The analysis of dielectric properties from many research papers, a comprehensive definition of relaxor is proposed. Relaxors are defined as those ferroelectrics in which the maximum temperature dependence of static susceptibility occurs within the temperature range of dielectric relaxation and it does not coincide with the temperature of the singularity of relaxation time or soft mode frequency.

Keywords: relaxor ferroelectrics, dielectric permittivity, polar nanoregions (PNRs)

INTRODUCTION

In the past 60 years, quite a few relaxors have been confirmed, such as Pb(Mg1/3Nb2/3)O3 (PMN), (Pb,La)(Zr,Ti)O3, and Ba(Ti,Sn)O3, and so on. The BaTiO3–BaSnO3 was the first system in which relaxor properties were discovered (Smolenskii and Isupov 1954). A number

* Corresponding Author, Email: [email protected]. 346 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al.

of relaxor compounds based on BaTiO3 (BT) have been found since A or B or on both A and B-sites of the perovskite lattice ABO3, for example, in Ba1-x(Sm0.5Na0.5)xTiO3 (Abdelmoula et al. 2006), and BaTiO3–BiScO3 (Bharadwaja et al. 2011), BaTiO3–BaSnO3 or BaTiO3– La(Mg0.5Ti0.5)O3 (Salak, Seabra, and Ferreira 2004) respectively. In the second case, the relaxor behavior appears in compounds with substituted B-site cations, such as BaTiO3– BaSnO3 (Smolenskii and Isupov 1954) Ba(ZrxTi1-x)O3 (Wang et al.; Maiti, Guo, and Bhalla 2008). Barium titanate, the ferro-piezoelectric oxide with perovskite structure (BaTiO3) has been of practical interest for more than 60 years because of its attractive properties. Therefore, since the discovery of BT system in the early 1940‟s, great deals of efforts have been devoted to studying the dielectric properties of barium titanium based materials. It has a perovskite structure that possesses a high dielectric constant (Purwanto et al. 2007) and widely used in multilayer ceramic capacitors (MLCCs), dynamic random access ferroelectric memories (DRAMs) (Ghosh et al. 2007; Zhao et al. 2004) because very high permittivity, stable phase transition and low temperature coefficient of the dielectric constant with high quality factor. BaTiO3 (BT) is a ferroelectric and has a tetragonal perovskite structure at room temperature (Aggarwal et al. 2010). Because BT is a ferroelectric, this polarization is reversible and can be switched by applying an electric field to the crystal. Many efforts have been made to further modify the dielectric properties of these ceramics by substitution of lattice atoms by other isovalent cations (Ba, Mg, Ca, Sr, Zr, Hf) which modify the dielectric properties and the ferroelectric transition, and also significantly broadens the phase transition. Additives change the microstructure of the ceramic which can also modify the dielectric properties of BaTiO3 ceramics. Substitution of isovalent/off-valent ions for the host lattice cations in BaTiO3 perovskite lattice plays a significant role in these modifications. These materials form solid solutions with BaTiO3 and alter its structural features, resulting in a shift in phase transition temperature along with modified dielectric properties. Different substitution types and levels have been shown to promote a continuous crossover from the ferroelectric state to the relaxor state (Cross 1994). Some relaxor features have been revealed also in nominally pure BT crystals (Wang et al. 2014) they may be related to the incorporation of uncontrolled impurities and the associated emergence of local stresses. The relaxor state differs qualitatively in several features of its dielectric response. In relaxor ferroelectrics or relaxor, the temperature dependences of real and imaginary parts of the complex dielectric permittivity show high and broad maxima (Tm) with a dispersive shift to higher temperatures with increasing the measurement frequency (Petzelt et al. 2014; Nuzhnyy et al. 2012). Relaxor properties exist in a number of crystal structures, including tungsten bronzes and disordered perovskites relaxor ferroelectrics are a class of disordered crystals possessing unusual structure and properties (Shvartsman and Lupascu 2012). Relaxation means a system's monotonous approach to the equilibrium state after some excitation. In the case of dielectric relaxation, one considers the response of polarization to an external (usually small) electric field. Since their discovery by Smolensky and co-authors in 1954, the relaxor ferroelectrics or relaxors as a subclass of disordered crystals attracted continued interest because of their peculiar properties, such as large permittivity, high piezoelectric coefficient d33, and large field-induced strain, which make them a candidate for application in advanced microelectronic devices (Harmer et al. 1989; Jonscher 1999).The properties of relaxors are believed to be originated from their nanometer-size polar regions and their response to external stimulus. These polarization-related regions appear at burns temperature (TB), which Dielectric Relaxation in BaTiO3-Based Perovskite 347

is usually far above the maximum point of permittivity Tm. A general feature of relaxors is the existence of quite wide peak in the temperature dependent permittivity in which Tm is shifted to a higher temperature as the measured frequency increases. Because of the global air pollution, energy deficiency and climate change, various new energy generation technologies, such as solar, wind and thermal energy, are developed to replace the fossil fuel energy resources with cleaner renewable sources. It, in turn, leads to the high demand of the devices for effectively storing, absorbing, and supplying the electricity (Kleemann 1993). Ferroelectrics, with the nature of spontaneous polarization in a certain temperature range, show competitive edges in many technological applications such as capacitors, actuators, transducers, sensors, memories, and other functional devices (Uchino 2009; Wilson et al. 2007). Among these materials, barium titanate (BaTiO3, BT), with ABO3 perovskite structure, has been both widely investigated in the research community and applied in the electronic, especially the capacitor industry. The dopants such as Mg, Nb, Zr, Sn, Sr and rare-earth elements can be integrated, which not only successfully lead to improved permittivity-temperature stability, but also enhanced other properties, such as resistivity, anti-fatigue, and dielectric properties, etc. A high percentage of isovalent or heterovalent doping concentrations, a ferroelectric relaxor transition may occur in the ceramics that the sharp peak at the Curie point transforms into a broad and diffuse dielectric- temperature plateau with frequency dispersion, which has enabled a number of modern applications.

DIELECTRIC RELAXATION

Relaxor ferroelectrics or relaxors are a class of disordered crystals (Shvartsman and Lupascu 2012; Suchaneck and Gerlach 2015; Samara and Venturini 2006). At T (transition temperature) they exist in a non-polar paraelectric (PE) phase, which is similar in many respects to the PE phase of normal ferroelectrics. Upon cooling they transform into the ergodic relaxor (ER) state in which polar regions of nanometer scale with randomly distributed directions of dipole moments appear. This transformation, which occurs in the so- called Burns temperature (TB) cannot be considered a structural phase transition because it is not accompanied by any change of crystal structure on the macroscopic or mesoscopic scale. However, the polar nanoregions (PNRs) affect the behavior of the crystal dramatically and giving rise to unique physical properties. For this reason the state of crystal at T TB) and the range of dielectric dispersion is located at very high 348 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al. frequency (>1010 Hz) (Poplavko et al. 1985; Bovtoun et al. 1984), inaccessible by routine experiments. Recent infrared investigations revealed that χFE, in the paraelectric phase of perovskite relaxors is essentially the ionic susceptibility. Its temperature dependence is determined by the ferroelectric-type phonon softening. Therefore, not only phenomenologically, but also microscopically, the dielectric permittivity in the paraelectric phase of relaxors follows the behavior characteristic of normal displace ferroelectrics. The complication that one encounters while trying to fit experimental data to the CW law in relaxors is that TB is often not known a priori, and the deviation from the CW law below TB is quite subtle at the beginning. Therefore, careful fitting is needed and the analysis of residuals (differences between theoretical and measured data), which is a desirable step in any fitting procedure, becomes mandatory in this case (Lei, Bokov, and Ye 2007).

Figure 1. Schematic of the dipolar structure in (a) ordinary spin or dipole glass, and (b) relaxor ferroelectric in ergodic relaxor phase. Small arrows represent the dipole moments of unit cells (or spins); large arrows are the total dipole moments of PNRs (BOKOV and YE 2012).

Being absent in the paraelectric phase, the dielectric relaxation appears upon cooling into the (ergodic relaxor) ER phase (below TB). Similar to many other materials, the dielectric response in the ER phase is related to thermally activated reorientations of microscopic dipoles (Samara 2003; Burns and Dacol 1983). However, the polar structure of relaxors is very complex and can be characterized by several combined prominent features, making these materials unique. In normal ferroelectrics, permanent electric dipoles contributing to relaxation can be typically represented by ions or electrons hopping among several allowed positions in the crystal unit cell or by reorienting (flipping) single molecules. In relaxors, however, giant dipoles exist which are composed of many unit cells whose dipole moments are permanently correlated. Figure 1 shows the schematically growth of PNRs which is different from polarization fluctuations (precursor polar clusters) which appear in normal ferroelectrics around the Curie temperature. These fluctuations are non-permanent, i.e., they can appear and disappear in the particular micro volume of the crystal. While the very existence of PNRs seems to be indisputable, their dynamics are not unambiguously verified. It is commonly believed that in the ER phase the PNRs are dynamic, i.e., due to thermal motion the total dipole moment of these giant elementary dipoles can vary in direction, giving rise to dielectric relaxation. The concept of dynamic PNRs helps explain the relaxation Dielectric Relaxation in BaTiO3-Based Perovskite 349 behavior not only in dielectric experiments (Bovtun et al. 2001), but also in light scattering (De Mathan et al. 1991; Smirnova et al. 2014) neutron diffuse scattering (Phelan et al. 2014), piezoresponse force microscopy, (Shvartsman et al. 2010, 2011)MNR,(Blinc et al. 2006; Blinc 2007) etc. Recent atomic-resolution electron microscopy investigations confirmed directly the existence of static PNRs in the ER phase (Liu et al. 2007; Wang et al. 2014).

Figure 2. Fitting to the Curie Wisse law (Solid line) and the quadratic law (dotted line) of the dielectric permittivity of Ba0.90Ca0.10TiO3 single crystal. Crosses are the experimental data measured at 100 kHz.

In the material, massive dipoles (PNRs) and normal size dipoles are also present. In the nonpolar matrix outside PNRs, the cations are typically shifted from the symmetric positions of perovskite unit cell, forming noncorrelated, presumably dynamic dipoles. Besides, according to soft nanoregion model, the dipole moments of some free unit cells inside PNRs can fluctuate independently, i.e., change their directions with respect to the PNR's total moment (Bokov and Ye 2002; Ye and BOKOV 2004). First of all most important feature of relaxors is the considerable strength of interactions among elementary dipoles, which can lead to the freezing of their reorientation motion, forming a new phase below a certain transition temperature. These interactions can be aggravated, in which case a glassy NR phase appears below the freezing temperature (Tf). Secondly, the directions of PNRs' and other elementary dipole moments in the NR phase (nonergodic relaxor) remain random. It can say that if there is the growth of PNRs size without reorientation ferroelectric phase is developed (Bing, Bokov, and Ye 2011; Leschhorn and Kliem 2016a). Another feature that distinguishes relaxors from other dielectrics, dipolar ferroelectrics is the characteristic temperature evolution of the dipole subsystem. PNRs do not exist in the paraelectric phase, and both their number and size increase upon cooling in the ER phase. Besides the dynamic dipoles contributing to the relaxation directly, static (quenched) microscopic dipoles also exist in the ER phase. These are static PNRs and the dipoles created by compositionally disordered cations and other structural defects. They produce static random local fields which influence the dipole dynamics and play an important role in the dielectric and many other properties of relaxors (Leschhorn and Kliem 2016b). The permittivity observed in proper ferroelectrics (i.e., those for which polarization is the order parameter) can be represented as: 350 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al.

ɛ(ω,T) = ɛ(ω,T) + ɛ(ω,T) + ɛ∞ ɛ(ω,T) (1)

The static value of in the paraelectric phase follows the Curie Wisse law as:

(2)

In the case of continuous (second-order) FE transitions T0 coincides with the phase transition (Curie) point, TC, and relation (2) is applicable also at T < TC, but with different CCW (see Figure 2 for illustration). First-order transitions occur at TC > T0 and T0 (which is theoretically the stability limit of the paraelectric phase) can be determined experimentally only by extrapolation. where χFE is the FE order parameter-related susceptibility and the two remaining terms represent all other possible contributions. Where as in relaxor ferroelectric total permittivity is represented by

ɛ(ω,T) = χLFɛ(ω,T) + χR(ω,T) + χFEɛ(ω,T) + ɛ∞ ɛ(ω,T) (3)

All terms of this relationship except χR have the same meaning as in normal ferroelectrics (and can originate from the same microscopic mechanisms), while χR represents the polarization mechanisms inherent in relaxors. In perovskite relaxors, X-ray and neutron diffraction and polarized light microscopy experiments indicate a cubic symmetry for the paraelectric, ER and NR phases which implies a macroscopic isotropy for the dielectric properties (Ye 1998). Many researchers reported that the permittivity in the FE phase of relaxors (including field-induced FE phase in canonical relaxors) depends on directions as expected for noncubic crystals (Zhao et al. 2007; Kutnjak, Blinc, and Ishibashi 2007). The dielectric behavior in the paraelectric phase of relaxors is similar to that of normal ferroelectrics, i.e., the permittivity can be expressed by Eq. (4) with χR = 0 and the dominating intrinsic contribution to the dielectric response, χFE, obeys, in the static limit, the Curie Wisse law. Therefore, the total permittivity measured at frequencies high enough to ensure vanishing supplementary contributions also follows the Curie Wisse law as:

(4)

and relaxors can be well characterized by two empirical formulae:

(5)

: 1 ≤ 2 (6) where is the value of at T = Tm, η and B are the adjustable parameters. This expression was suggested to describe the permittivity at T Tm and initially, only the value of η = 2 was considered. Later the extension of 1 was introduced (i.e., one more Dielectric Relaxation in BaTiO3-Based Perovskite 351 adjustable parameter was added) to fit the data in those relaxors whose dielectric peak is only slightly or moderately diffuse (Uchino and Nomura 1982). For the sake of convenience, the parameter B can be written in the form B = (Santos and Eiras 2001).The parameter defined in this way characterizes the width (diffuseness) of the permittivity peak. In normal order-disorder ferroelectrics around TC and in relaxors around TA, significant dispersion of the main susceptibility ( and in the former and latter case, respectively) can be observed at all measurement frequencies, leading to the frequency dependent permittivity maxima at Tm(ω) (Figure 3). Fitting of experimental data to Eqs. (2), (3) and (4) should be fulfilled in the dispersion free temperature range, i.e., at T > Tm +ΔTr (Bokov et al. 2003; Bokov and Ye 2000). In all cases, measurements at different frequencies are highly desirable. The absence of dispersion not only confirms the condition T > Tm +ΔTr, but also signifies (but not guarantees) the absence of secondary relaxation contributions. Two remarks concerning the exponent should be made. First, in the case of FE phase transition, it corresponds to the critical exponent for susceptibility (which is conventionally designated as γ). However, in the ER phase of relaxors, it would be misleading to call a critical exponent because there is no critical point in this case (susceptibility does not diverge). The second remark is that there are no intermediate values of in all materials where this parameter was reliably verified it adopted the value of two (around the permittivity peak of relaxors) or unity (in the paraelectric phase of relaxors and normal ferroelectrics).

Figure 3. Schematic of the temperature dependence of susceptibility in normal ferroelectrics with (a) first (b) second order phase transition at the Curie point. Static and high frequency susceptibilities are shown by solid and dashed lines, respectively. (c) in canonical relaxor ferroelectrics and (b, c) in relaxors that exhibit a relaxor-to-FE phase transition. Static and high-frequency susceptibilities are shown by solid and dashed lines, respectively. Arrows indicate increasing frequency (BOKOV and YE 2012). 352 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al.

COMPARISON OF LEAD-FREE PEROVSKITE

Figure 4 shows the temperature dependences of relative dielectric permittivity in the canonical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN), the solid solution of PMN and PbTiO3, and BaTiO3–BaMO3 (M = Sn) Solid Solutions for comparison. However PbMg1/3Nb2/3O3 (PMN) is relaxor and having high piezoelectric properties, However, these compositions have the obvious drawbacks associated with the volatility and toxicity of PbO. On the other hand, lead-free compositions of (BTSn) show diffuse phase transition behavior and relaxor like behavior (Shvartsman et al. 2008). Some researchers attribute it to the relaxor state with the distinction between a frequency- independent diffuse phase transition and a frequency-dependent relaxor like behavior due to different sizes of the PNRs yielding different relaxation times. When these relaxation times are close to the experimental observation rates, relaxor behavior is observed; when they differ much, only DPT behavior is expected.

(a) (b)

Figure 4. (a) The temperature dependence of the real and imaginary part of the dielectric permittivity measured at different frequencies for a single crystal of the canonical relaxor compound Pb(Mg1/3Nb2/3)O3 (BOKOV and YE 2012) (b) Temperature dependences of the real part of the dielectric permittivity measured at different frequencies for isovalent substituted (1-x)BaTiO3-xBaSnO3 compound (Shvartsman and Lupascu 2012).

The BT crystallizes in the cubic perovskite structure (point group m3m) above its Curie temperature, TC = 393 K (Figure 5). Below TC, it successively transforms to three ferroelectric phases: first to the tetragonal (4mm), then to orthorhombic (mm) at about 278 K, and finally, to a rhombohedral (3m) phase at 183 K. The spontaneous polarization in the three ferroelectric phases lies along [001], [011], and [111] direction of the parent cubic structure, respectively (Figure 5 (a)). Above TC, the dielectric permittivity follows a Curie–Weiss law. ɛ = C/(T-θ) with the Curie–Weiss temperature, θ < TC, indicating a first-order phase transition. When solid solutions with other perovskite type compounds are formed, BaTiO3 yields relaxor compositions with a number of interesting features, like relaxor behavior in compositions without nominal charge disorder. In addition diffuse phase transitions as an intermediate state between relaxor and ferroelectric states. Coexistence of static and dynamic Dielectric Relaxation in BaTiO3-Based Perovskite 353

PNRs in a broad temperature range and existence of ferroelectric domains above the transition temperature (Samara 2003).

Figure 5. (a) Phase transitions in BaTiO3 according to the displacive scenario with Ti ions shifted from the center. (b) Phase transitions in BaTiO3 according to the order–disorder scenario with Ti ions exhibiting jumps between eight off-center positions. Shades of gray from bright to dark illustrate occupancy probabilities from most to less probable (Shvartsman and Lupascu 2012).

RELAXOR PROPERTIES OF BT CRYSTALS

D. Wang et al. (Wang et al. 2016) reported the simulations further relate such features to the decomposed dielectric responses, each associated with its own polarization mechanism, therefore, enhancing the current understanding of relaxor behavior and find positive results. From Figure 6 the strong dispersion and shift of Tm raise an interesting question, that is whether the BZO/BTO superlattice can be categorized as true relaxor. To explain it another (Wang et al.) made the quantitative relation between Tm and , which is shown in Figure 6 (b). Since a reliable fitting of susceptibility versus temperature is unknown yet, we simply choose the position of maximum susceptibility as Tm, therefore causing some rough edges in Figure 6 (b). However, we verified that Tm versus can be fitted well with the relation Tm =

T0+ where ‘an’ is a fitting parameter. This result clearly distinguishes BZO/BTO superlattice from disordered BZT, where the Tm versus v follows the Vogel–Fulcher law and shows that BZO/BTO superlattice cannot be categorized as relaxor. As a matter of fact, a generally accepted definition of relaxor is hard to find. Bokov and Ye provided an empirical definition of relaxors, specifying that the Vogel-Fulcher law shall be followed for ferroelectrics to be taken as relaxors. While it may not be the ultimate answer about what 354 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al. relaxor is, this definition does exclude BZO/BTO superlattice from being classified as relaxor, although its susceptibility shows a strong dispersion.

Figure 6. (a) The Tm variation with respect to frequency is shown in Panel (b), where the solid blue dots are the positions of maximum susceptibility obtained from curves in Panel (a) and the red curve is the best fit line (Wang et al. 2016).

Figure 7. Temperature dependencies of the real (solid lines) and imaginary (dashed lines) parts of the dielectric permittivity (left scale) and AE count rate (right scale) of a KF-BT crystal at 0.1, 1.0 and 10.0 kHz frequencies (Roth et al. 2016).

Michel Roth et al. (Roth et al. 2016), reported relaxor properties of barium titanate crystals grown by Remeika method. The ferroelectric properties of the FE-PE phase transition in butterfly twin BT crystals have been studied using a combination of the complex dielectric permittivity (ɛ = ɛ' - ɛ'') measurements and the AE method. The cumulative results obtained with BT crystals grown from KF fluxes. Temperature dependences of both the real and imaginary parts of the permittivity, ɛ' (T) and ɛ'' (T) have been measured at 0.1, 1.0 and 10.0 kHz frequencies on heating from room temperature. The temperature interval, 100-130°C, Dielectric Relaxation in BaTiO3-Based Perovskite 355 associated with the tetragonal to cubic (FE-PE) phase transitions is shown in Figure 7. The s s 0.1 kHz ɛ' (T) curve contains two quite well-resolved peaks, Tc and Tb , at about 116 and 122°C respectively. As the measurement frequency is increased to 1.0 KHz, the low- temperature peak moves to higher temperatures and diminishes in size, and it turns into a hardly distinguishable shoulder under the high-temperature peak at 10.0 KHz. This constitutes a typical relaxor-type behavior in ferroelectrics (Ge et al. 2013; Kalinin et al. s 2010), and we assign the Tm dispersion to the surface layer relatively rich in KF impurities. In b contrast, the high-temperature peak practically does not shift from the Tc = 122°C value typical for the classical BaTiO3 ferroelectric, and we attribute it to be the diffuse ferroelectric Curie point in the crystal bulk. Indeed, the bulk contains almost an order of magnitude lower concentration of KF, which is too small to produce an RFE. However, the presence of a certain amount of randomly distributed K+ and F- ions in the crystal matrix broadens the Tc peak without dispersion (Dul’kin, Kojima, and Roth 2011). The temperature dependence of the imaginary part of the dielectric permittivity is quite remarkable. The ɛ'' (T) peaks shift to the right as the measurement frequency increases, like ɛ' (T), showing clear dispersion, or an RFE behavior. The ɛ'' (T) bands are broad reflecting the broad distribution of relaxation times associated with the presence of a large number of small ferroelectric domains, the intersecting groups of multiple laminar domains forming the “Forsberg net” during the cool down through the Curie point. However, the shoulder at about 120°C related to the bulk ferroelectric phase transition and appearing at the right sides of all three ɛ'' (T) curves does not show any sign of dispersion, as expected. Most importantly, the peak temperatures of the ɛ'' (T) curves can be readily determined, unlike the peaks of the ɛ' (T) curves that are obscured by the more intense and b broad Tc band. Concurrently, the AE activity has been monitored on heating the sample in course of dielectric measurements at all three frequencies. Sharp acoustic responses have been detected exactly at temperatures corresponding to the ɛ'' (T) curves peaks, or maximal losses within the “Forsberg net” layer, and a less intense count rate at the bulk Tc. Indeed, strong AE is induced by the movement of ferroelectric domain walls (Dul'kin et al. 2011). The maximum velocity of the ferroelectric domain growth is also known to be of the order of the velocity of sound (Ducharme et al. 2000). We can now compare the ɛ'' and AE peak temperature of 110°C taken at 0.1 kHz with the phase diagram of KF-doped BT crystals (Akishige 2008; Akishige et al. 2010) based on dielectric measurements carried out at a close to our frequency of 0.3 kHz. We deduce from the phase diagram that the 110°C tetragonal to cubic phase transition corresponds roughly to a KF concentration of 1.5 at%, which is excellent agreements with our experimental EDS measurements indicating that the K and F concentrations on the impurity-rich surfaces of our samples are in the 1-2%. Relaxor properties are related to structural properties of the material hence Shahid Anwar et al. (Anwar, Sagdeo, and Lalla 2006) describe the TEM micrographs of BaTiO3 and Ba(Ti0.7Hf0.3)O3 in Figure 8. The occurrence of large grains of sizes 3–5 m of BaTiO3 and

Ba(Ti0.7Hf0.3)O3 phases can be seen in Figure 8 (a) and (b), respectively. A relatively high- magnification micrograph of BaTiO3 is shown in Figure 8 (b), which depicts the occurrence of ferroelectric twin-domains of the tetragonal BaTiO3 phase. Figure 8(d) shows a high- magnification micrograph of Ba(Ti0.7Hf0.3)O3 phase. One can see that unlike BaTiO3 phase no contrast showing the twin-domains are present in its microstructure but it is rich in nano contrast regions of 4–10 nm apparent sizes. The detailed features of these nano contrasts are 356 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al. typical to the presence of strained regions (Panigrahi and Panigrahi 2011; Haguenau et al. 2003). To confirm that these strains are not due to some secondary phase inclusions, the selected area diffraction (SAD) from these areas as shown in the inset of Figure 8.

Figure 8. Transmission electron micrographs corresponding to BaTiO3 (a, b) and Ba(Ti0.7Hf0.3)O3 (c, d) respectively.

It is obvious from this SAD pattern that no extra spots are present other than those from the basic perovskite structure. This confirms that these contrasts are not due to any secondary phase inclusions. Since these regions were found to change contrast simultaneously during tilt, these might originate from strain fields due to some defect, like tiny dislocation loops extending just to few lattice sites with their burger vectors parallel. The occurrence of these defect features in Ba(Ti0.7Hf0.3)O3 will lower its structural correlation length as compared to BaTiO3 in which no such features were found. The ‘structural correlation length’ is basically a measure of the effective extent to which the long-range order of an atomic arrangement gets limited as a result of The cumulative effect of all types of defect features, which some how either interrupt the chemical order or produce strain in the lattice. Beyond this extent, the atoms don’t scatter coherently and contribute to the width of the diffraction maxima. Thus the structural correlation length may be used as a measure of comparison of the defects, presence in two similar types of structures. It is concluded that the defect feature observed through TEM is a characteristic of the whole bulk and is not only limited to few grains. The temperature dependence of real (ɛ′) and imaginary (ɛ′′) parts of the dielectric permittivity of Ba(Ti0.7Hf0.3)O3 ceramic are shown in Figure 8. Unlike BaTiO3 the transition is quite diffuse. The paraelectric to ferroelectric phase transition temperature (Tc) as compared to that of the BaTiO3, have decreased. The three phase transitions which are observed in BaTiO3 have got pinched and merged into one round peak in ɛ′-T variation. The results obtained can be Dielectric Relaxation in BaTiO3-Based Perovskite 357

described as a broad peak around Tm = 200 K in the ɛ′-T curve. With increasing frequency Tm increases, while the magnitude of the peak decreases. Additionally strong dielectric dispersion in radio frequency region around and below Tm in the ɛ′-T and the value of T′m (dielectric absorption maxima temperature), is much less than Tm and around and above T′m the dielectric absorption (ɛ′′) exhibits a strong frequency dependence. With increasing frequency T′m shifts to higher temperature with increasing dielectric absorption. The described features of (ɛ′-T) and (ɛ′′-T) variations shown in Figure 9 are very much similar to the observations by Cross, Lu, Cheng, Chen and other workers (Lu and Calvarin 1995; Cross 1994; Anwar, Sagdeo, and Lalla 2006; Yu et al. 2002; Maiti, Guo, and Bhalla 2006; Tang and Chan 2005) for various lead based and lead free ferroelectric relaxor materials. In order to further confirm the relaxor behavior, the quantitative characterizations as described in the following have been done.

Figure 9. Temperature dependence of ɛ′ and ɛ′′ of Ba(Ti0.7Hf0.3)O3 ceramic at 0.1,1, 10 and 100 kHz.

Longwen Wu et al. (Wu et al. 2017) reported the systematic investigation of the (1- x)BaTiO3-xBi(Zn2/3Nb1/3)O3 ceramics with in a wide range of x = 0.01–0.30 to unravel the structure-property relationships and x = 0.06–0.08 show relaxor properties. (1-x)BaTiO3- xBi(Zn2/3Nb1/3)O3 [(1-x)BT–xBZN, BTBZN] ceramics were fabricated via conventional solid-state reaction method. From Figure 10. (1-x)BaTiO3- xBi(Zn2/3Nb1/3)O3 for x = 0.06 and x = 0.08 which become frequency dispersive both in the dielectric constant and loss tangent data. The temperature of the Curie point (Tc) or the permittivity maximum (Tm) at 1 MHz for both compositions first decreases and then increases. In addition, the temperature difference (ΔTm) of the dielectric constant maxima at 1 MHz and 100 Hz, a simple quantifier of relaxor-like behavior(Lei, Bokov, and Ye 2007). For the compositions belonging to the ferroelectric zone, ΔTm is equal to zero due to no frequency dispersion; nevertheless, for those with relaxor-like behavior, ΔTm increases monotonically.

358 Mamta Shandilya, Shweta Thakur, Radheshyam Rai et al.

Figure 10. Temperature dependence of the dielectric constant and loss tangent for the (1-x)BT-xBZN ceramics: (a) x = 0.06; (b) x = 0.08.

Figure 11. Typical (a) (−1,2,0) and (b) (−1,4,0) zone axis EDPs of BTZ, (c) a [−1,2,] zone axis EDP of BTSn, (d) a [−1,4,0] zone axis EDP of BSrT, and (e)~ [−1,2,0] and (f) [−1,4,0] zone axis EDPs of BTN. Dielectric Relaxation in BaTiO3-Based Perovskite 359

Y. Liu, et al. reported that The typical BaTiO3-doped relaxor ferroelectrics BaTi0.7Zr0.3O3 (BTZ), BaTi0.7Sn0.3O3 (BTSn), Ba0.925Ti0.85Nb0.15O3 (BTN), and (Ba0.75Sr0.25TiO3 (BSrT) exhibit characteristic relaxor ferroelectric behavior and Electron diffraction, on the other hand, is known to be sensitive to dynamical displacive disorder (Figure 11) and was thus used to look for evidence of the dynamic PNRs (Withers 2005; Liu et al. 2007). Very similar, relatively sharp, transverse polarized, [001]* sheets of diffuse intensity were observed for each of the four BZT, BNT, BSrT, and BTSn samples. Essentially identical diffuse scattering was long ago reported to be characteristic of the undoped end-member compound BaTiO3 and also shown to be closely associated with the paraelectric to tetragonal ferroelectric and subsequent phase transitions thereof. This strongly suggests that the observed diffuse distributions and hence the 1D PNRs in the doped BaTiO3 relaxor ferroelectrics are not induced by the dopant ions but rather are an inherent characteristic of the end-member BaTiO3 compound itself.

CONCLUSION

A large scale research data of BaTiO3 based relaxor ferroelectric ceramics conclude that relaxor dielectric property may be useful for different applications. Ordering studies have shown that compositional heterogeneity leading to a breakdown of translational symmetry is responsible for relaxor character. BaTiO3-doped relaxor ferroelectrics are characterized by broad frequency dispersive dielectric constant maxima at a “diffuse phase transition” temperature Tm, a slim P-E ferroelectric hysteresis loop, and ideal, cubic perovskite, average structures both above and below Tm. They continue to be widely studied for their potential applications as lead-free, electrostrictive, and/or piezoelectric sensors and actuators as well as for the electrical field tunability of their dielectric properties. The relaxor ferroelectric behavior of the above type has long been attributed to the existence of so-called polar nanoregions PNRs. The precise nature of these PNRs and their relationship to the compositional heterogeneity sometimes called chemical nanoregions CNRs characteristic of all relaxor ferroelectrics, however, is still far from well understood. It is often assumed that PNRs are not an inherent feature of such materials but rather are somehow induced by, and directly coupled to, the compositional heterogeneity, i.e., to the CNRs.

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Chapter 14

BIOSYNTHESIS OF NANOPARTICLES USING PLANT EXTRACTS

Sapna Thakur1,*, Shweta Thakur2, Mamta Shandilya2, Madan Lal2 and Radheshyam Rai2 1Akal College of Agriculture, Eternal University, Sirmour (H.P.), India 2School of Physics and Material science, Shoolini University, Solan (H.P.), India

ABSTRACT

Biosynthesis of metallic nanoparticles is easily scaled up, low cost, rapid conducted at room temperature and pressure and environmentally benign. Extracts of several plants have been successfully used for making nanoparticles that must be free of toxic contaminants as required in therapeutic applications. The plant extract based synthesis can provide nanoparticles of a controlled size, shape, and morphology. Here we discussed various methods of metal nanoparticle synthesis and its applications in different aspects are developing.

Keywords: nanoparticles, plant extracts, top-down approach, bottom-up approach, antibacterial, cancer cell

1. INTRODUCTION

This review is concerned with the Biosynthesis of metallic nanoparticles using extracts of plants. In recent years nanotechnology has provided extensive research with emergence in Physics, Chemistry, Engineering, Biotechnology, Food technology and Medical sciences and forming impact on all forms of life with an important aspect due to its innumerable applications (Baker & Satish, 2012; Iravani, 2011; Shankar, Rai, Ahmad, & Sastry, 2004; Song & Kim, 2009). Methods used for biosynthesis of metallic nanoparticles and its characteristics in multiple aspects are discussed (Huang, Jain, El-Sayed, & El-Sayed, 2007;

* Corresponding Author Email: [email protected]. 366 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al.

Jagtap & Bapat, 2013; Kouvaris et al., 2012; Mallikarjuna et al., 2011; Vijayakumar, Priya, Nancy, Noorlidah, & Ahmed, 2013). The field of nanotechnology is simply too vast, too interdisciplinary, and too rapidly changing to cover exhaustively. The concept of nanotechnology was first begun in 1959 when Richard Feynman delivered a visionary and prophetic lecture at a meeting of the American Physical Society, where he speculated on the possibility and potential of Nano-sized materials (Feynman, 1960). In 1965 he was awarded Nobel Prize in physics for his contribution to quantum electrodynamics, a subject far removed from nanotechnology. The capability of plant extracts to reduce metal ions into nano size due to the involvement of different reducing agents and stabilizing agents present in plants has been known for several decades. It has attracted considerable attention due to cost-free, rapid, natural and safe way of nanomaterial synthesis (Ahmed, Ahmad, Swami, & Ikram, 2016; Ankamwar, 2010; Armendariz et al., 2004; Sadowski, 2010). When a bulk material is divided into small size particles with one or more dimensions (length, width, or thickness) in the nanometer range or even smaller, the individual particles exhibit unexpected properties which are different from those of the bulk material. It is known that atoms and molecules possess totally different behaviors than those of bulk materials; while the properties of the former are described by quantum mechanics, the properties of the latter are governed by classic mechanics. The behavior of material changes between these two distinct domains and the nanometer range is considered as the threshold for the transition of the material's behavior. The fundamentals of nanotechnology lie in the fact that properties of substances dramatically change when their size is reduced to the nanometer range (Amil Usmani et al., 2017; Chattopadhyay & Patel). Methods employed for the synthesis of nanoparticles are broadly classified under two processes such as “Top-down” process and “Bottom-up” process (Hobbs, Petkov, & Holmes, 2012; Roy, 2010; Wang & Xia, 2004). These terms were first applied to the field of nanotechnology by the Foresight Institute in 1989 in order to distinguish between molecular manufacturing (to mass-produce large atomically precise objects) and conventional manufacturing (which can mass-produce large objects that are not atomically precise).

1.1. Bottom-Up Approach

Seeks to have smaller (usually molecular) components built up into more complex assemblies. It uses the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly, e.g., DNA nanotechnology utilizes the specificity of Watson-Crick base pairing to construct well-defined structures out of DNA and other nucleic acids (Mukherjee et al., 2001).

Biosynthesis of Nanoparticles Using Plant Extracts 367

Figure 1. Bottom-up approach.

1.2. Top-Down Approach

Seeks to create nanoscale devices by using larger, externally-controlled ones to direct their assembly. It often uses the traditional workshop or micro-fabrication methods where externally-controlled tools are used to cut, mill, and shape materials into the desired shape and order. Micropatterning techniques, such as photolithography and inkjet printing belong to this category (e.g., Atomic force microscope tips can be used as a nanoscale “write head” to deposit a chemical upon a surface in a desired pattern in a process called dip-pen nanolithography (Morachevskii & Beloglazov, 2006; Thakkar, Mhatre, & Parikh, 2010) (Figure 2).

Figure 2. Methods employed for the synthesis of nanoparticles.

The classic laws of science are different at the nanoscale. Nanoparticles possess large surface areas and essentially no inner mass, i.e., their surface-to-mass ratio is extremely high. This new “science” is based on the knowledge that particles in the nanometer range, and nanostructures or Nanomachines that are developed from these nanoparticles, possess special properties and exhibit unique behavior. These special properties, in conjunction with their 368 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al. unique behavior, can significantly impact physical, chemical, electrical, biological, mechanical, and functional qualities. These new characteristics can be harnessed and exploited by applied scientists to engineer “Industrial Revolution II” processes. Present-day and future applications include chemical products, including plastics, specialty meals, powders, computer chips, computer systems, and miscellaneous parts, pollution prevention areas that can include energy conservation, environmental control, and health/safety issues, plus addressing crime and terrorism concerns (Roco, 2001, 2004). In effect, the sky’s the limit regarding efforts in this area, and as far as the environment is concerned, this new technology can terminate pollution as it is known today.

2. GREEN SYNTHESES

Nanoparticles are synthesized by physical and chemical methods these are suffering from various limitations such as expensive reagents, generation of hazardous toxic chemicals, longer time, tedious process to isolate nanoparticles etc. Hence, upsurge the researchers to develop new methods for the synthesis of nanoparticles which should be required inexpensive reagent, develop safe as well as less drastic reaction condition and eco-friendly alternative approaches. Nature has devised various processes for the synthesis of nano and micro- length scaled inorganic materials which have contributed to the development of relatively new and largely unexplored area of research based on the biosynthesis of nanomaterials. Synthesis using bio-organisms is compatible with the green chemistry principles. “Green synthesis” of nanoparticles makes use of environmentally friendly, non-toxic and safe reagents (Bankura et al., 2014; Ibrahim, 2015; Sivaraman, Elango, Kumar, & Santhanam, 2009). If the exact mechanism of biological synthesis is explained, it could offer an extra advantage over the chemical methods by means of higher productivity at lower cost. Biosynthesis of nanoparticles is a kind of bottom-up approach where the main reaction occurring is reduction/oxidation. The microbial enzymes or the plant phytochemicals with antioxidant or reducing properties are usually responsible for the reduction of metal compounds into their respective nanoparticles (Ahmed et al., 2016; Mittal, Chisti, & Banerjee, 2013; Mohanpuria, Rana, & Yadav, 2008). Biologically synthesized gold and silver nanoparticles could be of immense use in medical and biomedical textiles for their efficient antibacterial, antimicrobial properties and also in other applications like spectrally selective coatings for solar energy absorption and intercalation material for electrical batteries; they also find use as optical receptors and as catalysts in chemical reactions (Ankamwar, Damle, Ahmad, & Sastry, 2005; Isaac, Sakthivel, & Murthy, 2013; Krishnaraj et al., 2010; V. Kumar & Yadav, 2009). Green synthesis of nanoparticles can be done by using five methods (Sharma, Yngard, & Lin, 2009). They were a) Polysaccharide method, b) Tollens method, c) Irradiation method, d) biological methods, and e) Polyoxometalates method. These methods are briefly described here below.

Biosynthesis of Nanoparticles Using Plant Extracts 369

2.1. Polysaccharide Method

In Polysaccharide method, Ag NPs are prepared using water and polysaccharides as a capping agent, or in some cases, polysaccharides serve as both reducing and capping agent. For instance, synthesis of starch-Ag NPs was carried out with starch as capping agent and β-D-glucose as reducing agent in a gently heated system. Additionally, the binding interactions between starch and Ag NPs are weak which is reversible at higher temperatures, allowing separation of the synthesized particles. Importantly, starch-protected nanoparticles can be easily integrated into systems for biological and pharmaceutical applications.

2.2. Tollens Method

The Tollens synthesis method gives Ag NPs with a controlled size in a one-step process. In the modified Tollens procedure, Ag+ ions are reduced by saccharides in the presence of ammonia, yielding Ag NP films with particle sizes from 50–200 nm, Ag hydrosols with particles in the order of 20–50 nm, and Ag NPs of different shapes.

2.3. Irradiation Method

Ag NPs can be successfully synthesized by using a variety of irradiation methods. For example, laser irradiation of an aqueous solution of Ag salt and surfactant can fabricate Ag NPs with a well-defined shape and size distribution. No reducing agent is required in this method.

2.4. Biological Method

Extracts from bio-organisms may act both as reducing and capping agents in Ag NPs synthesis. The reduction of Ag+ ions by combinations of biomolecules found in these extracts such as enzymes/proteins, amino acids, polysaccharides, and vitamins is environmentally benign, yet chemically complex. An extensive volume of literature reports successful Ag NPs using bioorganic compounds.

2.5. Polyoxometalates Method

Polyoxometalates, the POMs, have the potential of synthesizing Ag NPs because they are soluble in water and have the capability of undergoing stepwise, multi-electron redox reactions without disturbing their structure. Bio-Nanotechnology combines biological principles with physical and chemical approaches to produce nano-sized particles with specific functions. It also represents an economic substitute for chemical and physical methods of nanoparticles formation. this method of synthesis can be divided into intracellular and extracellular (Ahmad, Senapati, 370 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al.

Khan, Kumar, & Sastry, 2005) with three main steps, which must be evaluated based on green chemistry perspectives, including (1) selection of solvent medium, (2) selection of environmentally benign reducing agent, and (3) selection of nontoxic substance for the NPs stability. Metallic nanoparticles have a high definite surface area and a high fraction of the surface atom; have studied extensively because of their exceptional physicochemical characteristics including catalytic, optical properties, electron properties etc. Silver, Aluminum, Gold, Zinc, Carbon, Titanium, Palladium, Iron, Copper etc. have been rottenly used for the synthesis of nanoparticles. However, former three metals are most popular metals as a biomaterial. Nanoscience will leave no field untouched by its ground-breaking technical innovations; the agricultural sector is no exception (Sahayaraj & Rajesh, 2011). So far, the use of nanoscience in agriculture has been predominantly theoretical, but it has begun and will continue to have a significant effect in the main areas of plant disease management. The advantage of using plants for the synthesis of nanoparticles is that they are easily available, safe to handle and possess a broad variability of metabolites that may aid in reduction. A number of plants are being currently investigated for their role in the synthesis of nanoparticles. Recently much work has been done with regard to plant assisted the reduction of metal nanoparticles and the respective role of phytochemicals. The main phytochemicals responsible have been identified as terpenoids, flavones, ketones, aldehydes, amides and carboxylic acids in the light of FTIR spectroscopic studies. The main water-soluble phytochemicals are flavones, organic acids, and quinones which are responsible for the immediate reduction.

3. SYNTHESIS OF DIFFERENT NOBEL METAL NANOPARTICLES USING PLANTS

Metallic nanoparticles are of great interest due to their excellent physical and chemical properties, such as high surface-to-volume ratio and high heat transfer (thermal conductivity). Amongst them, copper-based nanoparticles are of great interest due to their low cost and easy availability and because they possess properties similar to that of other metallic nanoparticles. Silver nanoparticles ranging from 55 to 80 nm in size be developed, as well as triangular or spherical shaped gold nanoparticles, could be easily fabricated using leaf extract of Cinnamomum camphora. The reduction into nanoparticles was formed due to the phenolics, terpenoids, polysaccharides and flavones compounds present in the extract. These nanoparticles were found an antibacterial activity at a concentration of 45 μg/mL (J. Huang et al., 2007). Aqueous leaves extract of Moringa oleifera was used for bioreduction of silver nanoparticles with an average size of 57 nm. Characterization was performed using UV-Vis spectrophotometry, Transmission Electron Microscopy (TEM) (T. Prasad & Elumalai, 2011). Silver nanoparticle with average particle size of 8 nm synthesized using tobacco leaf extract. UV-Vis absorption spectroscopy showed absorption maxima at 418 nm. Excitation maximum and emission maximum were obtained at 414 and 576 nm, respectively. Nanoparticle showed the highest sensitivity towards Pseudomonas aeruginosa and Escherichia coli DH5α (K. S. Prasad et al., 2011). Aloe Vera leaf extract used as a reducing agent to synthesized biogenic gold nanotriangles and spherical silver nanoparticles. The slow rate of the reaction along with the Biosynthesis of Nanoparticles Using Plant Extracts 371 shape directing effect of reducing agent of the extract was accountable for the formation of single crystalline gold nanotriangles, as well as spherical silver nanoparticles of 15.2 nm (Chandran, Chaudhary, Pasricha, Ahmad, & Sastry, 2006). Uniformly dispersed silver nanoparticles with a uniform size and shape in the range of 1 to 10 nm with an average size of 6 nm using geraniol were synthesized. Having concentration of 1 µg/ml, silver nanoparticles was able to inhibit the cancer cell line’s growth by less than 30%. On the other hand, the presence of 5 µg/ml of silver nanoparticles significantly inhibited the cell line’s growth (>60%). The concentration necessary to produce 50% cell death was 2.6 µg/ml for these silver nanoparticles (Safaepour, Shahverdi, Shahverdi, Khorramizadeh, & Gohari, 2009). Stable silver nanoparticles with an average size of 450 nm were synthesized using Annona squamosa leaf broth, which was tested as mosquito larvicides against Aedes aegypti, Anopheles stephensi, and Culex quinquefasciatus. The median lethal concentrations (LC50) of silver nanoparticles that killed fourth instars of Ae. aegypti, Cx. quinquefasciatus, and An. stephensi were 0.30, 0.41, and 2.12 ppm, respectively. Adult longevity (days) in male and female mosquitoes exposed as larvae to 0.1 ppm silver nanoparticles was reduced by *30% (p < 0.05), whereas the number of eggs laid by females exposed as larvae to 0.1 ppm silver nanoparticles decreased by 36% (Arjunan, Murugan, Rejeeth, Madhiyazhagan, & Barnard, 2012). For the bio-reduction behavior for the synthesis of silver nanoparticles five plant leaf extracts (Malva parviflora, Beta vulgaris subsp. Vulgaris, Anethum graveolens, Allium kurrat and Capsicum frutescens) were screened. From these five plants, M. parviflora (Malvaceae) was reported to exhibit the best reducing with the diameters in the range of 19–25 nm of silver nanoparticles. FTIR analysis proved that particles are reduced and stabilized in solution by the capping agent that is likely to be proteins secreted by the biomass. The study reported as a simple, easy to perform, pollutant free and inexpensive for the synthesis of silver nanoparticles (Zayed, Eisa, & Shabaka, 2012). The study reported the green synthesis of silver nanoparticles from an extract of Foeniculum vulgare (fennel, saunf). NTA revealed the polydispersed nanoparticles in the range of 18-83 nm. Phytosynthesized silver nanoparticles showed antibacterial activity against the Staphylococcus aureus (ATCC-25923) and Escherichia coli (ATCC-39403). Silver nanoparticles in combination with vancomycin showed maximum activity against E. coli (increase in fold area 5.76. and followed by S. aureus (1.08) and Gentamicin showed the maximum activity S. aureus (2.6) while E. coli (0.96) (Bonde, 2011). Coleus aromaticus leaf extract was used to the bioreduction of silver ions to nanoparticles. Size and shape of nanoparticles were analyzed by Scanning electron microscope (SEM) shows the polydispersed and mostly spherical shape of nanoparticles with aggregation. This green synthesis method has many advantages over the chemical method because it reduces the use of toxic metals in the synthesis process and it is a single step eco-friendly method (Vanaja et al., 2013). The ethanolic extract of leaves of Pisonia grandis was implemented for the synthesis of silver nanoparticles with particle size was found to be less than 150 nm, possessing spherical shape. The UV-visible spectrophotometer, XRD, Scherrer’s equation and SEM analysis was employed for the characterization of synthesized silver nanoparticles (Firdhouse, Lalitha, & Sripathi, 2012). Mulberry leaves extract used for the development of the silver nanoparticles (AgNPs) at room temperature. Silver nanoparticles were characterized using UV-visible absorption spectroscopy, scanning electron microscopy (SEM) andX-ray diffraction (XRD). Such nanoparticles showed effective antibacterial activity against Staphylococcusaureus and 372 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al.

Shigella sp (Awwad & Salem, 2012). Rhizophora mucronata leaf extract used for the synthesis of silver nanoparticles with the average size of 60-95 nm to identify the larvicidal activities against the larvae of Aedes aegypti (Ae. aegypti) and Culex quinquefasciatus (Cx. quinquefasciatus). The LC50 value of the synthesized silver nanoparticle was identified as 0.585 and 0.891 mg/L for Ae. aegypti and Cx. quinquefasciatus larvae respectively (Gnanadesigan et al., 2011). Silver nanoparticles synthesized using the aqueous extract of Origanum vulgare (Oregano). The nanoparticles were found to be spherical with an average particle size distribution of 136 ± 10.09 nm. The green synthesized silver nanoparticles showed dose- dependent response against human lung cancer A549 cell line (LD50 – 100 g/ml) (Sankar et al., 2013). A novel switchgrass (Panicum virgatum) extract mediated green process was observed for the synthesis of silver nanoparticles at ambient temperature. Synthesized silver nanoparticles were subjected to x-ray diffraction (XRD) for structural characterization, which confirms the FCC symmetry of silver nanoparticles with the lattice parameter of 4.0962 Å. The particle size of biosynthesized silver nanoparticles was identified through transmission electron microscopic (TEM) analysis and found to be in the range of 20 - 40 nm (Mason, Vivekanandhan, Misra, & Mohanty, 2012). Aqueous extract of Caltropis procera fruit or leaves used to synthesize silver or zinc nanoparticles with particles sizes of approx. 90–100 nm. Antibacterial activity against two pathogens Vibrio cholera and Escherichia coli exhibited similar resistance profiles with minimal inhibitory concentrations ranging between 5 × 105 and 107 particles/ml. Interestingly, zinc nanoparticles showed a slightly higher efficacy, but sublethal concentrations caused adverse effects and resulted in increased biofilm formation of V. cholera (Salem et al., 2015). Zingiber officinale extract acts both as reducing and stabilizing agent for the synthesis of gold nanoparticles, with a particle size ranging from 5 to 15 nm. Gold nanoparticles synthesized using citrate and Z. Officinale extract demonstrated very low protein adsorption as well as highly stable at physiological condition compared to citrate-capped nanoparticles, which aggregated. Thus the usage of nanoparticles, synthesized with Z. officinale extract, as vectors for the applications in drug delivery, gene delivery or as biosensors, where a direct contact with blood occurs is justified (K. P. Kumar, Paul, & Sharma, 2011). Biosynthesis of gold nanoparticles (AuNPs) using Cassia auriculata aqueous leaf extract has been carried out within 10 min at room temperature (28◦C), suggesting a higher reaction rate than chemical methods involved in the synthesis. Stable, triangular and spherical crystalline AuNPs with well-defined dimensions of the average size of 15–25 nm were synthesized using C. Auriculata (V. G. Kumar et al., 2011). The reducing and capping potentials of ethanolic flower extract of the plant Nyctanthes arbortristis for the synthesis of gold nanoparticles was explored. Transmission electron microscope (TEM) revealed the dominant spherical morphology of the gold nanoparticles with an average diameter of 19.8 ± 5.0 nm. X-ray diffraction (XRD) study confirmed crystalline nature of the synthesized particles. Low reaction temperature (250C) favored anisotropy (Das, Gogoi, & Bora, 2011). Extracellular biological synthesis of gold nanoparticles in the size range from 6.75–57.91 nm was reported using coriander extract as the reducing agent. The gold nanoparticles were characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), Fourier transform infra-red spectroscopy (FT-IR) and transmission electron microscopy (TEM). This eco-friendly approach for the synthesis of nanoparticles is simple, amenable for large scale commercial production and technical applications (Narayanan & Biosynthesis of Nanoparticles Using Plant Extracts 373

Sakthivel, 2008). Cassia fistula stem bark aqueous extract was used for the synthesis of gold nanoparticles to evaluate the hypoglycemic effects of the plant. The investigation reported the efficacy of the gold nanoparticles as promising in the treatment of hyperglycemia. A significantly larger decrease in serum biochemistry parameters and an increase in body weight, total protein levels, and high-density lipoprotein were observed in rats with streptozotocin-induced diabetes treated with gold nanoparticles than in the ones treated with the aqueous extract. The results of this study confirm that C. fistula gold nanoparticles have promising antidiabetic properties (Daisy & Saipriya, 2012). Gold nanoparticles with average size of Au-NPs were 50 nm were synthesized at room temperature using Morus alba (mulberry) leaf extract as reducing and stabilizing agent. TEM studies showed the particles to be nearly spherical with few irregular shapes and particle size ranges 15−53 nm. Biosynthesized Au-NPs show strong zone of inhibition against Vibrio cholera (gram- negative) and Staphylococcus aureus (gram-positive) whereas, chemically synthesized Au-NPs and mulberry leaf extract exhibit a fair zone of inhibition (Adavallan & Krishnakumar, 2014). Gold nanowires were synthesized using sugar beet pulp as a capping agent. Polysaccharides and proteins are involved in the bio-reduction and synthesis of nanoparticles. The formation of nanowires was induced by both basic mediums, due to the competence between biomolecules and hydroxide ions, and high concentrations of gold ions, because of the lack of capping agent to stabilize the preliminary nanoparticles formed that stick together producing wire-like nanostructures instead of nanospheres. The synthesis of other metallic nanostructures such as silver and platinum could be achieved following a similar procedure (Castro et al., 2011). Triangular nanoparticles have been synthesized by using a seeded growth process. The extract from the lemongrass plant, act as a reducing agent, yields a high percentage of thin, flat, single-crystalline gold nanotriangles. The anisotropy in nanoparticle shape results in large near-infrared absorption by the particles, and highly anisotropic electron transport in films of the nanotriangles (Shankar, Rai, Ankamwar, et al., 2004). Green tea (Camellia sinensis) extract as reducing and stabilizing agent produced gold nanoparticles and silver nanostructures. Colloidal systems of silver and gold nanoparticles exhibit highly efficient single photon-induced luminescence. This optical response can be manipulated by changing concentrations of metal ions and the quantity of reducing agent plays a crucial role information, growth and luminescence response of these noble-metal nanostructures (Vilchis- Nestor et al., 2008). Biosynthesis of nanoparticles from plants seems to be a very effective method in developing a rapid, clean, non-toxic, and eco-friendly technology. The use of plant biomass or extracts for the biosynthesis of novel metal nanoparticles (silver, gold, platinum, and palladium) would be more significant if the nanoparticles are synthesized extracellularly and in a controlled manner according to their dispersity of shape and size (Akhtar, Panwar, & Yun, 2013). Palm oil mill effluent (POME) used in the synthesis of gold nanoparticles (AuNps) without adding an external surfactant, capping agent or template. The obtained AuNps are predominantly spherical with an average size of 18.75 ± 5.96 nm. In addition, some triangular and hexagonal nanoparticles were also observed. This study reported the feasibility of using the agro waste material for the biosynthesis of AuNps which is potentially more scalable and economic due to its lower cost (Gan, Ng, Huang, & Li, 2012).

374 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al.

Syntheses of gold and silver nanoparticles by various chemical and physical methods as well as the biosynthetic approach mediated by numerous microorganisms have been actively researched. A more scalable and economic route to produce these metallic nanoparticles would be through the plant-mediated synthetic approach (Gan & Li, 2012). The stem latex of, Euphorbia nivulia was successfully used to induce room temperature/microwave synthesis of silver and copper nanoparticles even at high concentrations, which leads to the formation of nanoparticles with 5–10 nm diameter. The non-cytotoxic metal–latex aqueous solution offers a rational approach towards antimicrobial application and for integration to biomedical devices (Valodkar et al., 2011). Leaf extract of Catharanthus roseus used to synthesize nanoparticles of titanium dioxide. The particles were of irregular shape and ranged in size from 25 to 110 nm. Suspensions of these nanoparticles were adulticidal and larvicidal against the hematophagous fly Hippobosca maculate and the sheep louse Bovicola ovis (Velayutham et al., 2012). Cacumen Platycladi leaf extract used for biogenic fabrication of gold nanoparticles (AuNPs. Several analytic methods such as UV–Vis spectrophotometry, X-ray diffraction, transmission electron microscopy, and thermogravimetric study were adopted to characterize the AuNPs. The results showed that flavonoid and reducing sugar were the main reductive and protective components in the extract vital in the biosynthesis of the AuNPs (Zhan et al., 2011). Copper nanoparticles were synthesized using an extract of clove. UV-V in absorption shows characteristic absorption peak at 500-600 nm range of Cu nanoparticles. X-Ray diffraction (XRD) pattern reveals the formation of Cu nanoparticles, which shows crystallinity. Transmission electron microscopy (TEM) suggested particles size and shape in the range of 14-50 nm. Scanning electron microscopy (SEM) image reveals that the particles are of spherical and granular nature (Nayak, 2013). Biosynthesis of stable copper nanoparticles was done using Ocimum sanctum leaf extract. These biosynthesized Cu nanoparticles were characterized with the help of X-ray diffraction (XRD) Fourier transform infrared spectroscopy (FTIR)., The XRD pattern shows a high crystallinity of Cu sample level with diffraction angles of 22.3°, 25.9°, 28.3° and 44.8. FTIR spectrum of Cu nanoparticles suggested that Cu nanoparticles were surrounded by different organic molecules such as terpenoids, alcohols, ketones, aldehydes and carboxylic acid (Vasudev D. Kulkarni 2013). Biosynthesis of copper nanoparticles using a rapid time scales and low-cost procedures using environmentally benign natural resources as an alternative to chemical synthesis protocols. Reduction of copper ions by leaf extracts resulted in the formation of stable copper nanoparticles which are spherical in shape and at a core is capable of rendering antimicrobial efficacy and proved to be active against the pathogenic bacterias (Thakur, Rai, & Sharma). Copper nanoparticles were biologically synthesized using plant leaf extract Magnolia as reducing agent. UV-vis spectroscopy was used to monitor the quantitative formation of copper nanoparticles. The synthesized nanoparticles were characterized with ICP, EDS, XPS, TEM and SEM. Electron microscopy analysis of copper nanoparticles indicated that they ranged in size from 40 to 100 nm. Antibacterial tests were carried out by counting viable Escherichia coli cells and foams coated with biologically synthesized copper nanoparticles showed higher antibacterial activity compared with foams untreated (Gopalakrishnan, Ramesh, Ragunathan, & Thamilselvan, 2012). A novel way to synthesize carbon nanotubes and Cu/ZnO nanoparticles using metal hyperaccumulator plants was reported. Brassica juncea L.was used to produce carbon nanotubes and Cu/ZnO nanoparticles. The chlorophyllin reacted with Cu/Zn(NO3)2 to form Cu/Zn chlorophyllin. Cu/ZnO nanoparticles were Biosynthesis of Nanoparticles Using Plant Extracts 375 synthesized by direct precipitation of Cu/Zn chlorophyllin with NaOH and ethanol. The outer diameter of carbon nanotubes was around 80 nm. Cu/ZnO nanoparticles have a Cu0.05Zn0.95O composition and had a diameter of about 97 nm (Qu, Luo, Cong, & Yuan, 2012). Platinum nanoparticles were synthesized using leaf extract of Diopyros kaki. The synthesized platinum nanoparticles were characterized resulting in 2 to 12 nm in size FTIR analysis revealed that the platinum nanoparticles synthesized with D. kaki extract are surrounded by some metabolites like terpenoids that have functional groups of amines, alcohols, ketones, aldehydes, and carboxylic acids (Song, Kwon, & Kim, 2010). A greater conversion of platinum ions to nanoparticles was achieved by employing a tulsi leaf broth with a reaction temperature of 100°C. Fourier-transform infrared spectroscopy revealed that the compounds such as ascorbic acid, gallic acid, terpenoids, certain proteins and amino acids act as reducing agents for platinum ions reduction. X-ray diffraction spectroscopy suggested the associated forms of platinum with other molecules and the average particle size of platinum nanoparticle was 23 nm. This bio-friendly method of platinum nanoparticles production increases the rates of synthesis faster which can use in water electrolysis applications (Soundarrajan et al., 2012).

4. APPLICATION OF NANO-PARTICLES IN DIFFERENT AREAS

Nanotechnology is a field of science which deals with production and manipulation of nanoparticles have expressed significant advances owing to wide range of applications in the field of electronics, optical fibers, sensors, semiconductors, automobiles, nano-fabrics, biomedical, catalysts, agriculture, cosmetics, packaging, bio-engineering, medicines, drug delivery, and other areas (Figure 3). Metallic nanoparticles synthesized by the several methods have been used in varied in vitro diagnostic applications. The adaptability and broad applicability of nanotechnology replicate the spectra of composite materials (e.g., metals, semiconductors or polymers), geometries (e.g., sphere, prism or rod), and structures (e.g., solid, core or shell or dendrimers) (Fortina et al., 2007; Madl & Pinkerton, 2009; Veigas, Doria, & Baptista, 2012). Silver and gold nanoparticles have been found to be active against plasmodial pathogens and cancer cells as well as gold nanorods as novel contrast agents for both molecular imaging and photothermal cancer therapy (El-Sayed, Huang, & El-Sayed, 2005; Huang, El-Sayed, Qian, & El-Sayed, 2006; Kathiravan, Ravi, & Ashokkumar, 2014; Ling, Lee, & Hyeon, 2015). The ever increasing diversity of nanoparticles has included immunoassays and targeted delivery of drugs and antigens, and optical bioimaging of cells and tissues with state-of-the- art nanophotonic detection systems (Dykman & Khlebtsov, 2012; Saha, Agasti, Kim, Li, & Rotello, 2012). The potential of nanomaterials in sustainable agriculture management as modern approaches of nanotechnology. Nanomaterials (NM) used in plant protection or fertilizer products have exponentially increased since the several decades (Gogos, Knauer, & Bucheli, 2012; R. Prasad, Kumar, & Prasad, 2014). Metallic nanoparticles have been commonly found to have broad spectrum antimicrobial activity against gram positive and gram negative pathogens (Gurunathan, Han, Kwon, & Kim, 2014; Thakur et al.). Silver and gold nanoparticles are by now widely used as antimicrobial agents in commercial medical and 376 Sapna Thakur, Shweta Thakur, Mamta Shandilya et al. consumer products due to its toxicity against most microorganisms. Nanoparticles are known to affect the permeability of membranes of microbial and other cells (Adavallan & Krishnakumar, 2014; Chandran et al., 2006; Krishnaraj et al., 2010). Nanoparticles have been incorporated in polymeric matrices in order to provide antimicrobial activity to the packaging material and improve packaging properties (Avella et al., 2005; Espitia et al., 2012). Metal nanoparticles have elicited lots of interest for important biomedical applications because of their ease of synthesis, characterization and surface functionalization. Current and potential applications of nanoparticles in biology and medicine are further discussed in the literature for several applications (Couvreur, 2013; Rai et al., 2014).

Figure 3. Applications of nanoparticles in different areas.

CONCLUSION

The biomedical nanoparticle is a developing field having enormous potential to positively impact the health care system, that may have potential clinical applications include targeted drug delivery, detection, and imaging. Basic understanding of how nanomaterials, the building blocks of nanotechnology, interact with the cells and their biological consequences are beginning to evolve with lots of scopes and hopes. Biosynthesis of Nanoparticles Using Plant Extracts 377

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ABOUT THE EDITOR

Dr. Radheshyam Rai Assistant Professor School of Physics and Materials Science, Shoolini University, Solan (H.P.) 173229, India Email: [email protected]

Dr. Radheshyam Rai had joined the National Physical Laboratory in 2003 during the PhD. He did his PhD from Magadh University Bodh Gaya in 2004 in physics. During his PhD he worked on PLZT ferroelectric materials with different dopants and also worked on LPG and CNG gas sensor devices in National Physical Laboratory and Indian Institute of Technology, New Delhi. He has quite significant list of publications and research activities. After that he joined as Young Scientist in Department of Physics, Indian Institute of Technology, Delhi. During this period he worked on ferroelectromagnetic materials for devices application. He worked as Post Doctoral Fellowship at Universidade de Aveiro, under the Fundacão para a Ciênciaea Tecnologia, Lisboa, Portugal. During this period he was working on non-lead based piezoelectric materials for energy harvesting and ferromagnetic materials. Recently he is working as an Assistance Professor in Shoolini University, Solan and working on the piezoelectric energy harvesting and nano fibers.

INDEX

ceramics, vii, ix, 21, 23, 25, 26, 46, 49, 50, 52, 57, A 58, 59, 60, 63, 67, 77, 95, 96, 102, 103, 104, 110, 112, 113, 114, 115, 116, 117, 126, 127, 128, 130, ABO3, viii, 22, 103, 109, 110, 137, 138, 140, 152, 131, 132, 133, 134, 141, 158, 159, 160, 164, 168, 153, 155, 159, 205, 206, 208, 209, 214, 293, 297, 169, 170, 171, 175, 205, 206, 208, 209, 211, 213, 346, 347, 362 214, 216, 218, 220, 221, 222, 223, 224, 225, 226, Ag NPs, 369 227, 229, 230, 231, 232, 233, 234, 235, 236, 237, anticorrosion, 70, 71 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, antiferromagnetic, 138, 141, 146, 149, 150, 163, 177, 248, 249, 250, 251, 252, 253, 254, 255, 287, 291, 179, 183, 185, 186, 187, 188, 292, 293, 298, 299, 292, 293, 294, 296, 297, 298, 300, 301, 302, 305, 300, 302, 315, 316, 361 331, 334, 346, 347, 357, 358, 359, 360, 362, 363 anti-infective action, 68, 73 cerebral hemorrhage, 45, 56 antimicrobial, 62, 68, 69, 71, 74, 92, 368, 374, 375, coercivity, 146, 147, 196, 216, 304, 311, 316 376, 377, 378, 379, 380, 381 conductivity, 2, 4, 16, 30, 39, 40, 77, 95, 96, 112, antioxidant, 71, 368 139, 174, 177, 185, 292, 297, 309, 318, 319, 320, Au-NPs, 373 321, 322, 331, 335, 370 conservation, 2, 232, 258, 264, 265, 269, 270, 271, B 273, 274, 275, 368 co-precipitation, 86, 88, 161, 311, 331

BaTiO3, vi, viii, ix, 21, 25, 26, 46, 95, 101, 103, 110, crystallinity, 4, 10, 29, 68, 77, 86, 280, 288, 374 111, 112, 113, 114, 118, 119, 120, 126, 128, 130, curie point, 112, 113, 150, 174, 175, 177, 179, 181, 131, 132, 133, 134, 153, 157, 205, 207, 214, 215, 182, 185, 186, 187, 309, 313, 314, 347, 351, 355, 216, 218, 222, 229, 237, 238, 241, 243, 244, 250, 357 253, 254, 293, 294, 301, 345, 347, 352, 353, 355, 356, 357, 359, 360, 361, 362, 363 D biocompatibility, 68, 74 biodiversity, v, viii, 63, 257, 258, 259, 263, 264, 265, DC resistivity, 304, 311, 318, 331 266, 267, 268, 269, 270, 271, 273, 275 deficiency, 71, 73, 85, 92, 93, 183, 187, 347 biosynthesis, vi, ix, 365, 368, 372, 373, 374, 377, diamagnetism, 148, 312 378, 379, 380, 381, 382 dielectric constant, 25, 96, 97, 100, 103, 104, 110, 111, 112, 113, 114, 115, 116, 117, 118, 208, 231, C 294, 297, 300, 317, 321, 331, 332, 333, 346, 357, 358, 359, 363 capping agent, 369, 371, 373 dielectric properties, 104, 111, 113, 114, 117, 118, cell proliferation, 35, 71, 89 126, 129, 131, 133, 134, 139, 160, 164, 165, 202, ceramic capacitor, 96, 103, 109, 110, 114, 119, 127, 226, 246, 247, 288, 291, 292, 300, 301, 302, 326, 130, 131, 132, 134, 346 335, 345, 346, 347, 350, 359, 361 386 Index dielectrics, 64, 95, 96, 100, 101, 102, 103, 104, 111, frequency, vi, ix, 37, 96, 103, 116, 117, 126, 145, 112, 127, 128, 206, 250, 349, 360, 361 189, 192, 207, 209, 210, 211, 212, 231, 232, 233, DNA, 19, 74, 266, 267, 270, 366 234, 237, 246, 248, 252, 258, 287, 294, 303, 304, domains, 23, 101, 142, 146, 181, 186, 187, 188, 196, 310, 320, 321, 322, 324, 325, 326, 331, 333, 335, 210, 216, 229, 230, 232, 248, 300, 313, 324, 343, 345, 346, 347, 348, 351, 352, 354, 355, 357, 359, 353, 355, 366 361 double perovskite, viii, 137, 139, 140, 141, 155, 162, 163, 164, 166, 168, 169, 170, 171, 189, 190 dye-sensitized solar cells, 38, 56 G

genetic drift, 264 E gram-positive and gram-negative bacteria, 68, 74 graphene oxide, viii, 191, 193, 195, 196, 197, 198, ecosystem, 2, 257, 258, 259, 261, 264, 267, 268, 199, 201, 202, 203 269, 270, 275 green synthesis, 368, 371, 377, 378, 379, 380, 381, electrochemical deposition, 5 382 electrospinning, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 20, 21, 26, 27, 28, 30, 34, 35, 38, 39, 40, 41, 42, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, H 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 74 hard ferrites, 306, 310, 316 energy density, 25, 39, 101, 102, 114, 116, 122, 124, hexagonal ferrites, 304, 305, 306, 308, 310, 318, 130, 360 331, 332, 334 ergodic relaxor, 347, 348 hollow nanofibrous scaffolds, 42 extracellular matrix (ECM), 34, 41, 57, 70, 88 hyperspectral images, ix, 337, 338, 340, 344 extraction and classification of homogenous objects hyperspectral imaging system, 337, 338, 340, 343 (ECHO), 342, 343

F I imaging spectrograph, 344 fabrication, v, ix, 1, 3, 4, 5, 7, 8, 20, 27, 30, 36, 41, imaging spectrometer, 337, 340, 341, 344 46, 47, 49, 52, 55, 57, 58, 59, 61, 62, 63, 64, 65, inhibition, 373 87, 89, 118, 120, 159, 161, 162, 193, 202, 226, isomer shift, 328, 329 230, 248, 250, 277, 278, 288, 343, 367, 374, 379 far-field electrospinning (FFES), 26, 27, 28 ferrimagnetism, 150, 312, 316, 322, 334 K ferrites, 138, 144, 153, 155, 165, 170, 171, 243, 304, 305, 306, 308, 309, 310, 316, 317, 318, 319, 320, KNN, v, viii, 25, 205, 207, 209, 218, 220, 221, 222, 321, 322, 326, 331, 332, 333, 334, 335 223, 226, 227, 232, 236, 240, 242, 277, 278, 279, ferroelectricity, 23, 110, 126, 127, 129, 137, 139, 280, 281, 282, 283, 285, 286 141, 149, 155, 162, 166, 201, 202, 203, 239, 251, KNN-PEO, 279, 280, 281, 282, 283, 285, 286 287, 291, 292, 293, 300, 360 ferroelectrics, viii, 102, 103, 126, 127, 128, 130, 132, 133, 135, 146, 152, 170, 171, 193, 194, 201, 205, L 206, 214, 218, 230, 237, 238, 239, 241, 242, 245, 246, 247, 248, 249, 250, 253, 254, 287, 292, 297, larger surface area to volume ratio, 3 301, 302, 345, 346, 347, 348, 349, 350, 351, 353, lead-free, viii, 25, 37, 49, 58, 64, 65, 103, 113, 127, 355, 359, 360, 361, 362, 363 168, 205, 207, 208, 209, 215, 217, 218, 221, 222, ferromagnetic, vi, ix, 138, 141, 145, 146, 147, 148, 224, 227, 236, 237, 238, 239, 240, 241, 242, 243, 149, 150, 151, 155, 163, 164, 166, 170, 173, 174, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 176, 181, 183, 185, 186, 187, 188, 189, 214, 291, 254, 277, 278, 287, 288, 289, 352, 359, 361, 363 292, 293, 297, 298, 301, 302, 304, 313, 316, 322, light weight and flexibility, 4 324, 325, 332, 333, 335, 336, 383 line-scan hyperspectral camera, 344 FESEM, 280, 283, 285, 286 Lithium-ion battery, 39, 40, 121, 122 Index 387

near-field electrospinning (NFES), 26, 27, 28, 29, 61 M noncomedogenic, 67 non-toxic, 63, 68, 69, 71, 72, 75, 368, 373 magnetic moment, 142, 145, 146, 148, 149, 150, 151, 177, 179, 182, 183, 312, 322, 323, 330 magnetic properties, viii, 139, 142, 144, 145, 148, O 155, 164, 165, 166, 167, 168, 169, 170, 171, 172, 174, 175, 179, 188, 189, 291, 292, 293, 298, 302, orthoferrites, 293, 301 304, 310, 317, 332, 333, 334, 335 magnetic susceptibility, 148, 150, 312, 313 magnetization, 141, 144, 145, 146, 147, 148, 149, P 150, 151, 162, 163, 175, 176, 178, 179, 180, 181, 182, 183, 186, 187, 291, 298, 300, 304, 306, 309, paramagnetic, 146, 149, 150, 151, 164, 175, 179, 311, 312, 313, 314, 316, 322, 323, 324, 325, 332, 181, 182, 183, 186, 190, 293, 298, 312, 313, 315, 335 316 magnetoresistance, v, viii, 155, 166, 167, 168, 173, parts per hundred of rubber (PHR), 79 174, 175, 176, 177, 178, 182, 183, 188, 189 peri-ulcer skin, 71 metallic nanoparticles, 74, 365, 370, 374, 375, 377, permeability, 35, 89, 144, 145, 148, 149, 192, 317, 380, 381 323, 324, 325, 326, 331, 376 metalloenzymes, 69, 70 permittivity, 24, 96, 100, 101, 103, 104, 112, 113, micromanipulator, 5 116, 117, 127, 128, 134, 192, 196, 198, 200, 211, microscopic dipoles, 348, 349 213, 234, 241, 287, 321, 326, 345, 346, 347, 348, microwave spectroscopy, 326 349, 350, 352, 354, 355, 356, 357, 360, 361 Moringa oleifera, 370, 380 Perovskite, v, vi, viii, ix, 23, 51, 57, 96, 103, 110, morphology, 3, 6, 9, 11, 15, 16, 17, 20, 36, 38, 42, 111, 112, 118, 119, 120, 126, 129, 132, 133, 135, 48, 50, 53, 57, 58, 60, 68, 86, 125, 170, 196, 226, 137, 138, 139, 140, 141, 152, 153, 154, 155, 156, 227, 253, 280, 283, 286, 365, 372 157, 158, 159, 162, 163, 164, 166, 167, 169, 172, Mossbauer spectroscopy, 330 173, 174, 189, 190, 205, 206, 207, 208, 209, 211, 214, 215, 216, 217, 218, 222, 224, 225, 226, 233, 236, 237, 239, 240, 241, 242, 243, 247, 250, 252, N 253, 254, 277, 280, 283, 286, 291, 292, 293, 294, 297, 301, 345, 346, 347, 348, 349, 350, 352, 356, nanofibers, v, vii, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 359 15, 16, 17, 19, 20, 21, 25, 26, 27, 28, 29, 30, 31, phase transitions, 23, 25, 111, 135, 174, 220, 229, 32, 33, 34, 35, 36, 37, 38, 39, 41, 42, 43, 44, 46, 232, 233, 236, 240, 241, 243, 249, 250, 251, 292, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 352, 353, 355, 356, 359, 360, 363 60, 61, 62, 63, 64, 65, 66, 68, 70, 74, 88, 89, phonon induced tunneling, 320 91,93, 333 photocatalysis, 54, 67, 70, 71, 76 nanometer, ix, 3, 33, 36, 67, 69, 88, 159, 278, 346, piezocoefficient, 196 347, 366, 367 piezoelectricity, 1, 21, 22, 28, 29, 50, 58, 72, 130, nanoparticles, vi, 11, 36, 49, 52, 55, 59, 67, 68, 69, 139, 194, 202, 206, 208, 209, 210, 218, 220, 221, 70, 71, 74, 77, 90, 92, 93, 120, 132, 151, 167, 229, 230, 233, 236, 237, 240, 246, 251, 253, 254 168, 169, 170, 171, 172, 201, 302, 331, 332, 334, piezoresponse, v, viii, 133, 191, 193, 194, 195, 196, 335, 336, 362, 365, 366, 367, 368, 369, 370, 371, 197, 199, 200, 202, 230, 231, 291, 294, 299, 300, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 349, 362 382 piezoresponse force microscopy, viii, 191, 193, 202, nanoregions (PNRs), 345, 347, 348, 349, 352, 353, 230, 349, 362 359, 361 plant extracts, ix, 365, 366, 377, 380 nanoscience, 72, 170, 303, 370, 377 polarization, viii, 22, 23, 24, 28, 29, 51, 96, 101, 102, nanostructures, 2, 25, 26, 29, 32, 36, 37, 55, 57, 58, 103, 115, 120, 141, 145, 146, 152, 162, 163, 177, 60, 65, 367, 373 191, 192, 194, 195, 196, 198, 200, 201, 203, 209, nanotechnology, 47, 49, 52, 55, 60, 61, 62, 67, 68, 210, 211, 212, 214, 216, 221, 229, 230, 231, 239, 69, 70, 71, 72, 93, 126, 127, 132, 170, 191, 291, 240, 244, 249, 252, 297, 299, 300, 302, 321, 326, 303, 365, 366, 369, 375, 376, 377, 379, 380, 381 346, 347, 348, 349, 350, 352, 353, 360, 361 388 Index polycondensation, 87 spectroscopy, 131, 167, 175, 185, 189, 196, 200, porosity, 4, 6, 34, 35, 38, 40, 41, 97, 108, 123, 125, 232, 233, 237, 244, 252, 326, 328, 330, 331, 338, 144, 225, 227, 286, 296, 318, 325 340, 362, 370, 371, 372, 374, 375, 379, 382 Push Broom, LCTF, 337 spin coating, ix, 191, 193, 200, 277, 279 PVDF, 21, 25, 26, 27, 28, 29, 40, 49, 60, 63, 191, spinel, 153, 154, 158, 160, 304, 305, 306, 308, 311, 192, 193, 195, 197, 198, 199, 200, 201, 202, 203 316, 317, 322 PVDF-TRFE, 191 spinneret, 7, 10, 11 PZT, 25, 26, 28, 29, 46, 49, 53, 61, 120, 158, 159, stimulus, 1, 346 164, 169, 205, 208, 209, 211, 215, 216, 217, 220, supercapacitors, v, viii, 38, 48, 54, 56, 64, 95, 96, 97, 221, 222, 235, 236, 237, 244, 246, 248, 249, 253, 98, 99, 105, 106, 107, 108, 109, 121, 122, 124, 254, 299 125, 127, 128, 129, 130, 131, 132, 133, 134 surface tension, 7, 15, 16 surface-to-volume ratio, 36, 68, 69, 370 R sustainable development, viii, 263, 264, 272, 275 reference spectrum, 343 refractive index, 70 T relaxor, 25, 101, 102, 103, 114, 126, 127, 130, 131, 132, 133, 214, 231, 239, 244, 246, 247, 345, 346, target recognition, 341 347, 348, 349, 350, 351, 352, 353, 354, 355, 357, tensors, 202, 343 359, 360, 361, 362, 363 thin film, ix, 28, 36, 48, 50, 69, 76, 77, 93, 120, 129, remote imaging, 338, 340 130, 131, 132, 133, 134, 152, 168, 186, 191, 194, retentivity, 147, 306 195, 196, 203, 224, 226, 235, 244, 248, 251, 277, 278, 279, 283, 286, 287, 288, 289 Titania, 67, 69, 71 S transducer, 1, 37, 207, 234, 252 semiconductor, 29, 50, 59, 74, 77, 83, 95, 126, 138, 141, 177, 214, 293, 318, 319, 379 V SMNFs, vii, 3, 19, 20, 45, 46 soft ferrites, 316 viscoelastic material, 5 sol gel, 21, 68, 86, 87, 88, 90, 158, 161, 205, 227, viscosity, 4, 5, 15, 16, 70, 106 278, 283, 311 solid state reaction, 115, 158, 225, 226, 291, 312 species, 32, 67, 137, 140, 161, 188, 257, 258, 259, X 262, 264, 265, 266, 267, 268, 269, 270, 272, 275, x-ray density, 317, 331 283, 341 spectral libraries, 343