SODIUM POTASSIUM NIOBATE BASED PIEZOELECTRIC CERAMICS
A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences
Year of Submission: 2012 Margaret Węgrzyn
School of Materials CONTENTS
Contents ...... 2 Abbreviations ...... 5 Abstract ...... 6 Declaration ...... 7 Copyright Statement ...... 7 Acknowledgements ...... 8 1 Introduction ...... 9 1.1 General Introduction ...... 9 1.2 Aims and Objectives of this Investigation...... 11 2 Literature Review ...... 12 2.1 Electroceramic Properties ...... 12 2.1.1 Dielectric Properties ...... 12 2.1.2 Piezoelectric Properties ...... 16 2.1.3 Ferroelectric Properties ...... 19 2.1.4 Impedance Spectroscopy ...... 24 2.2 Piezoelectric Materials and their Properties ...... 26 2.2.1 Lead Zirconium Titanate (PZT) ...... 27 2.2.2 Barium Titanate (BT) ...... 28
2.2.3 Potassium Niobate, KNbO3 [KN] ...... 31
2.2.4 NaNbO3 [NN] ...... 32
2.2.5 Lithium Niobate, LiNbO3 [LN] and Lithium Tantalate [LT] ...... 34 2.2.6 Strontium Barium Niobate (SBN) ...... 34 2.3 Sodium Potassium Niobate (NKN) ...... 35 2.3.1 Recent Pure NKN investigations ...... 35 2.3.2 NKN Doping for Enhanced Properties...... 39 2.4 Texturing ...... 48 2.4.1 Introduction to Texturing ...... 48 2.4.2 Texturing of Polycrystalline Ceramics ...... 50 2.4.3 Texturing of NKN...... 54 2.4.4 Summary of Texturing Piezoelectric Ceramics...... 58 3 Experimental Procedures...... 60 3.1 Raw Powders ...... 60 3.2 Sample Preparation ...... 60 3.2.1 Mixed Oxide Processing Route ...... 60 3.2.2 Orientation Casting ...... 62 3.3 Characterisation Techniques ...... 68 3.3.1 Density Measurement ...... 68 3.3.2 X-Ray Diffraction (XRD) Procedures ...... 69 3.3.3 Synchrotron Energy Dispersive Powder Diffraction ...... 71
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 2 3.3.4 Rietveld Refinement ...... 75 3.3.5 Scanning Electron Microscopy (SEM) and EDAX ...... 77 3.3.6 Electrical Property Measurements ...... 78 4 The Effect of Temperature on NKN-based Formulations ...... 81 4.1 Introduction ...... 81 4.1.1 Background and Objectives ...... 81 4.1.2 Methodology ...... 81 4.1.3 Materials Selection ...... 82 4.2 Niobium and Copper Doped NKN ...... 82 4.2.1 Diffraction data for the sintered pellet ...... 82 4.2.2 Powder diffraction analysis for Copper and Niobium Doped NKN ...... 88 4.2.3 Electrical Properties of Copper and Niobium Doped NKN ...... 95 4.3 Synchrotron XRD Study of Lithium Tantalate Doped NKN...... 95 4.4 Summary ...... 102 4.4.1 Copper and Niobium Doped NKN (Sintered Pellet) ...... 102 4.4.2 The Copper and Niobium Doped NKN (Crushed Powder) ...... 103 4.4.3 Lithium and Tantalum Doped NKN (Crushed Powder) ...... 103 5 Doping NKN with Piezoelectric SBN ...... 104 5.1 Introduction ...... 104 5.2 Characterisation of Starting Powders ...... 104 5.2.1 Sodium Carbonate ...... 104 5.2.2 Potassium Carbonate ...... 105 5.2.3 Niobium Oxide ...... 106 5.2.4 Strontium Carbonate ...... 106 5.2.5 Barium Carbonate ...... 107 5.2.6 Iron Oxide ...... 107 5.3 Densification of Ceramics in the NKN-SBN System ...... 108 5.4 X-ray Diffraction Analysis ...... 111 5.4.1 Undoped xSBN ...... 111 5.4.2 xSBNF Samples ...... 112 5.5 SEM Investigation ...... 113 5.6 Electrical Properties ...... 115 5.6.1 Dielectric Properties ...... 115 5.6.2 Field Response Hysteresis Loops ...... 118 5.6.3 Impedance Spectroscopy ...... 123 5.7 Chapter Summary ...... 129 6 Synchrotron Analysis of xSBNF Formulations ...... 131 6.1 Introduction ...... 131 6.2 Synchrotron Data Analysis ...... 132 6.2.1 NKNF Refinement ...... 133
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 3 6.2.2 0.5 and 1SBNF Refinement ...... 134 6.2.3 2-3 SBNF Refinements ...... 134 6.2.4 4 SBNF Refinement ...... 135 6.3 Summary of Data from Refinements ...... 136 6.3.1 Phase Content ...... 136 6.3.2 Lattice Parameters ...... 137 6.4 Chapter Summary and Conclusions ...... 140 7 Development of a Route to Produce Oriented NKN Thick Films ...... 141 7.1 Introduction ...... 141 7.2 The Production of BNN Particles...... 142 7.2.1 Crucible Selection...... 142 7.2.2 Preparation of BNN particles ...... 143 7.2.3 The Effect of Salt to Oxide Ratio ...... 145 7.3 The Production of NN Particles ...... 146 7.4 Development of Tape Casting Procedure ...... 148 7.4.1 Tape Casting Method Development...... 148 7.4.2 Slurry Development ...... 150 7.4.3 Cast Tape Quality Control ...... 151
7.5 Doping NKN with LiNbO3 (LN) ...... 152 7.6 Tape Casting of Copper-Doped 5LN and 6LN Compositions ...... 153 7.7 Film Thickness ...... 155 7.8 Tape Casting Oriented 94NKN-6LN Tapes using NN particles ...... 157 7.8.1 Preparation of Tapes with Addition of 10% NN Particles ...... 157 7.8.2 Adding 15% NN Particles...... 166 7.9 Summary and Chapter Conclusions ...... 169 8 Conclusions ...... 170 8.1 SBN doping of NKN ...... 170 8.2 Synchrotron Analysis of NKN-based Formulations ...... 171 8.2.1 Synchrotron Analysis for Cu and Nb doped NKN ...... 171 8.2.2 Synchrotron Analysis of 94NKN-6LT...... 172 8.3 Tape Casting of LN-doped NKN...... 172 8.3.1 Tape Casting Procedure Development...... 172 8.3.2 Orientation using NN Template Particles ...... 172 8.3.3 CuO doped NKN-LN Tape Casting Development ...... 173 9 Recommendations for Further Study ...... 174 9.1 NKN-SBN System ...... 174 9.2 NKN-LN System ...... 175 References ...... 176 Appendix 1 - Figures ...... 191 Appendix 2 - Tables ...... 200
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 4
ABBREVIATIONS
BiT Bismuth Titanate, Bi4Ti3O12 BKNT Bismuth Sodium Potassium Niobate (Bi, K, Na)TiO3 BKT Bismuth Potassium Titanate, Bi0.5K0.5TiO3 BLSF Bismuth Layered-Structured Ferroelectric BNN Bismuth Sodium Niobate, Bi2.5Na3.5Nb5O18 BNT Bismuth Sodium Titanate, Bi0.5Na0.5TiO3 BT Barium Titanate, BaTiO3 CAS Chemical Abstracts Service d33 piezoelectric constant EC coercive field EC Enzyme Commission EDAX Energy Dispersive X-ray Spectroscopy ELV End of Life Vehicles EU European Union FCC Face Centred Cubic GOF Goodness of Fit HATSAXS High Automated Throughput Small Angle X-ray Scattering ICDD International Centre for Diffraction Data ICSD Inorganic Crystal Structure Database KN Potassium Niobate, KNbO3 kp planar electromechanical coupling coefficient kt thickness coupling coefficient LBT Lithium Bismuth Titantate, Li0.5Bi0.5TiO3 LN Lithium Niobate, LiNbO3 LT Lithium Titanate, LiTaO3 MPB Morphotropic Phase Boundary MRI Materials Research Instruments MSS Molten Salt Synthesis NKN Sodium Potassium Niobate, (Na0.5K0.5)NbO3 NKNF NKN + 0.45 wt% Fe2O3 NN Sodium Niobate, NaNbO3 OPS Oxide Polishing Suspension ortho orthorhombic Pr remnant polarisation Psat saturated polarisation PVB Polyvinyl Butyral PZT Lead Zirconium Titanate, (Pb, Zr) TiO3 RoHS Restriction of Hazardous Substances RTGG Reactive Templated Grain Growth SBN Strontium Barium Niobate, (Sr0.5Ba0.5)Nb2O6 SBNF SBN + 0.45 wt% Fe2O3 SEM Scanning Electron Microscopy SiC Silicon Carbide SRS Synchrotron Radiation Source
ST Strontium Titanate, SrTiO3 TBT Tungsten Bronze Type TC Curie temperature TEM Transmission Electron Microscopy tet tetragonal TO-T orthorhombic to tetragonal transition temperature WEEE Waste from Electrical and Electronic Equipment XRD X-Ray Diffraction
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 5 ABSTRACT
NKN doped samples, (100-x)NKN-xSBN (0 ≤ x ≤ 10) were produced using the conventional mixed oxide route with 0.45 wt% Fe2O3 sintering aid (xSBNF). After 20-24 hours mixing, samples were calcined at 850°C and sintered at 1100–1140°C (± 180°C/hour) for 4 hours. By XRD 4 mol% SBN was found to be the solubility limit for single phase structure. By SEM, second phases were visible when 2 ≤ x ≤ 4; their structure was subsequently shown to be tungsten bronze type (TBT). 2-4 SBNF samples were high density, over 96% theoretical. For x = 0, TC = 2 457°C, TO-T = 234°C, Pr = 22 μC/cm and EC = 16.5 kV/cm. TC was found to decrease by 14.7°C and TO-T by 9.0°C per 1 mol% addition SBN. 2SBNF was the optimal formulation in terms of microstructure and electrical properties, with 2 average grain size 3 μm, Pr = 25 μC/cm and EC = 8.8 kV/cm, ρ = 4.7 kΩm and Q = 1.16 eV. This material comprised approximately 90% orthorhombic and 10% tetragonal phases coexisting. Pseudo-cubic lattice parameters are a’ = c’ = 3.947180 Å, and b’ = 3.999996 Å for orthorhombic phase; the tetragonal has a’ = c’ = 3.989798 Å, and b’ = 3.975777 Å.
Synchrotron XRD studies were undertaken as a function of temperature on 99.5NKN-0.5CuO + 0.6 wt% Nb2O5 solid and powder samples. The data were Rietveld refined. The solid sample underwent two polymorphic phase transitions at 300°C and 515°C; the latter was between two tetragonal phases: lattice parameters for the tetragonal phase (300-520°C) were a’ = c’ = 4.99557 Å, and b’ = 4.0363 Å; high temperature tetragonal (>500°C) exhibited a’ = c’ = 4.9519 Å, and b’ = 4.4941 Å The powder sample of the same formulation exhibited more, smaller transformations. It was only orthorhombic at temperatures <140°C with a’ = c’ = 4.10680 Å, and b’ = 4.02620 Å. Above 140°C both orthorhombic and tetragonal phases were present. Another significant transformation occurred at 360°C where the structural unit cell parameters changed significantly. Parameter lengths are 2 provided. P-E data was characterised by Pr = 19.9 μC/cm and EC = 13.5 kV/cm.
Synchrotron X-ray diffraction analysis of 94 NKN-6LiTaO3 showed that tetragonal phase was present at 20-390°C, although an orthorhombic phase was identified at 20-200°C and again at 340-390°C just before the cubic transition temperature at 390°C. This is a new observation for NKN.
A new and simple method for tape casting was developed to reduce powder wastage, enabling thick films of 50 μm to be cast. The reactive templated grain growth (RTGG) method was employed to orient 95NKN-5LiNbO3 and 94NKN- 6LiNbO3 samples; CuO was utilised as a sintering aid. Pre-cursor BNN and NN template particles were produced using the molten salt synthesis (MSS) method, using a salt to oxide ratio of 1:1. Resulting NN particles were 15 μm wide and 0.5 μm thick. Eight layered 6LN + 0.4 wt% tapes produced using 10 wt% template particles resulted in 210 μm thick tapes with 67% orientation when sintered at 1150°C. Resulting properties include TC = 440ºC and TO-T = 70ºC, 25 kΩ resistance and capacitance 21.6 pF.
Margaret Węgrzyn for Doctor of Philosophy at The University of Manchester “Sodium Potassium Niobate Based Piezoelectric Ceramics” March 2012
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 6 DECLARATION
No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.
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Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 7 ACKNOWLEDGEMENTS
I would like to first and foremost like to thank my supervisor Professor Robert Freer without whom this PhD thesis would not exist. Without his guidance, support and faith I would not have reached the end of this educational journey. I am truly grateful for his patience and help with my scientific writing. Alongside him, I must also thank Dr. Feridoon Azough for his support throughout my four years during my PhD, particularly with practical work, who can find positive outcomes from any situation. I would also like to thank the Engineering and Physical Sciences Research Council and the School of Materials for giving me the funds and facilities to undertake the PhD.
I am indebted to the Functional Ceramics research group for their support and camaraderie, in particular Dr. Michael Thrall and Colin Norman, whose knowledge and assistance I could not have done without. Also to Professor R.J. Cernik, whose continuing support, guidance, exceptional knowledge and witty humour always helped; and to Dr D.A. Hall for his assistance and direction which aided the success of this research project.
I would like to convey thanks to all those who helped with practical work by training me on specialist equipment and aiding me in times of complex situations. In particular I have deepest gratitude to Mike Faulkner whose expertise on SEM machinery makes it look so simple, who made complex imaging manageable and most importantly had a great sense of humour that I will never forget. Also to Judith Shackleton and Gary Harrison for their extensive knowledge of XRD, in aiding my many experimental requirements and the friendly manner they maintained throughout. An honourable mention to the administrative staff who were always willing to help, in particular, Sue Brandreth, Olwen Richert and Sandra Kershaw. Also to the Outreach and Widening Participation staff who aided me to impart my love of science and engineering to the public, and helped fund the PhD once the EPSRC funding finished, in particular Emma Lewis and Karen Donnelly. Huge thanks to the two women in Materials Science Centre that I could not have succeeded without: Philippa “Pip” Lee and Dr. Rachel Saunders.
Many thanks to my friends and colleagues in the Materials Science Centre and in the Widening Participation team, too many to list, but each in their own way an important part of my postgraduate life in Manchester. Also to all the Polish dancers around the world who helped keep me going, in particular “Orlęta” and Basia Klimas-Sawyer, who allowed me to continue to dance with the group even though I was living in a distant city, and “Polonez Manchester” who allowed me to participate in their events, performances and with whom I have made lasting friendships. Most importantly thanks to my friends in “Team GB” who were a constant support and provided valuable friendship, in particularly Tom Pawluczyk.
Finally, my deepest gratitude goes to my parents Robert and Jola Węgrzyn, and my brother Daniel Vinci, whose undying encouragement, counselling and love made it possible for me to make it to the end of this experience. From the bottom of my heart I am thankful for every little thing they have done for me.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 8 1 INTRODUCTION
1.1 General Introduction Piezoelectric materials are widely used in electronics due to their electronic versatility. They are mainly utilised as strain gauges, sonar devices, microphones, phonograph pickups, accelerometer, fuel jet valves, control of oscillator frequencies in communication equipment, sonic delay lines, and transducers in ultrasonic devices.[1-4] There were 3,500,000,000 piezoelectric devices produced in 2006.[5] In 2008, it was reported that there were 1012 ferroelectric capacitors produced, and sophisticated piezoelectric systems were a growing market of €6 billion annually.[6]
The most popular material used for piezoelectric materials around the world is that of PZT (lead zirconium titanate). However, PZT components contain over 60% lead,[7] which has, particularly its oxide compound PbO, been found to be toxic and can cause problems with health and learning abilities, and can even cause death.[8] Most significantly is that the EU has produced legislation whereby the manufacture of any new electronic equipment produced after July 1st 2006 do not contain any components of lead (along with other hazardous substances such as mercury and cadmium).[9] There have also been directives from the EU for environmental protection against the waste of wasted electrical components, such as the Restriction of Hazardous Substances (RoHS), Waste from Electrical and Electronic Equipment (WEEE)[10-11] and End of Live Vehicles (ELV), legislation introduced to the UK in 2003.[12-13] Due to these health claims and EU directives, the amount of research in lead-free piezoelectric ceramics has grown exponentially since 1997, as shown by Figure 1.1.
Figure 1.1. Number of publications cited in refereed journals based on lead-free piezoceramics (between 1050 and November 2008) (from Rödel et al).[3]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 9 The subject of lead-free piezoelectrics is one of high interest, and was even reported on by L.E. Cross in Nature.[14] In this report he mentioned the high usage of PZT, and how Saito et al. had successfully produced a lead-free NKN-based material with properties similar to
PZT (and a d33 piezoelectric coefficient value that was higher) in the same publication.[15] Additionally, this research group, based at Toyota (Japan) oriented the NKN-based material which resulted in properties comparable to doped and optimised PZT (this is further discussed later).[15-16] These publications have caused much interest in NKN- based piezoelectric ceramics and so is a relevant and currently topical material under investigation. The characteristic properties of lead-free properties compared to those of PZT are shown in Figure 1.2. The European LEAF project program (2001 - 2004), involved the investigation of piezoelectric alternatives for PZT, mainly focussing on Sodium
Potassium Niobate, Na0.5K0.5NbO3 hereafter named NKN, and its dopants.[17]
Figure 1.2. Piezoelectric coefficient – Curie temperature comparison of PZT materials to
BaTiO3 (BT), BNT and NKN based materials in the literature (from Shrout and Zhang).[18]
Other lead-free piezoelectric material systems include BNT (Bi0.5Na0.5TiO3) and BKT
(Bi0.5K0.5TiO3). These were also investigated in the LEAF program.[17] BNT is a ferroelectric perovskite with rhombohedral structure at room temperature with a high Curie temperature of 320°C and large remnant polarisation of 38 μC/cm2.[19-22] It is, however, known for its poor resistivity, high coercive field (EC = 73 kV/cm) and difficulty to pole (saturation is difficult to achieve).[3, 19] Highly dense samples need a conventional sintering temperature of over 1200 °C or superior processing methods such as hot pressing.[3] BKT has a slightly higher Curie temperature of 380°C with tetragonal structure at room temperature,[22] though like BNT, it is difficult to sinter without using hot pressing methods.[23] By combining the two systems (BKNT) superior properties have been
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 10 found,[24-25] When BKNT has been combined into a tertiary system with BaTiO3 (BT) enhanced properties have also been found, and this is a material under high interest also.[26-28]
NKN is a highly researched material as a lead-free piezoelectric ceramic. It is a mixture between ferroelectric KNbO3 and antiferroelectric NaNbO3 where there is a morphotropic phase boundary (MPB) for the (Na0.52K0.48)NbO3 formulation;[29-30] most research, however, is conducted at an equal ratio of sodium to potassium. Like BNT, NKN is known for its sintering difficulties, and suffers from deliquescence (degradation in contact with moisture). Typical properties of NKN include Pr and EC of 15 μC/cm2 and 13 kV/cm.[31] Doping NKN significantly improves the density, microstructure and piezoelectric properties. Many variations have been reported, however the most significant and effective finding has been the addition of lithium to the system. By substituting lithium into the A site of the perovskite structure, the properties are significantly enhanced.
Typical lithium doped NKN properties include Pr and EC of 22-23 μC/cm2.[32-34]
1.2 Aims and Objectives of this Investigation This investigation is aimed at reporting novel and innovative research that has been undertaken in order to further the field of research in NKN-based materials. A major objective for this investigation is to examine how NKN behaves under doping. In this investigation, perovskite NKN is doped with copper, lithium and tantalum additions, as well as with strontium barium niobate (Sr0.5Ba0.5)Nb2O6 (a piezoelectric material with tungsten bronze structure) which has not been previously attempted in the literature. High resolution X-ray diffraction methods have been utilised in order to obtain structural information for the doping effects. This involves utilising this technique in-situ with doped NKN as a function of temperature.
Another objective of this investigation is to develop NKN-based thick films in order to investigate the effect of orientation to the formulations. Template particles are produced in order to orient the thick films as an RTGG (reactive templated grain growth) process. A method of thick film processing is developed in order to utilise small amounts of powder, to produce tapes 25-200 μm thick. The sintering conditions are investigated in order to find the optimal processing conditions for the optimal formulation of NKN-based oriented thick films.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 11 2 LITERATURE REVIEW This chapter is devoted to the processing and properties of piezoelectric materials being investigated in this study. Based upon information in literature, this review is intended to show a foundation for the novel, original research carried out in this study.
2.1 Electroceramic Properties Throughout the twentieth century, the use of ceramics in electronics has evolved. In the early 1900s, the use of ceramics was for the properties of high resistivity and chemical stability. However throughout the last century, the widespread properties of electroceramics have been exploited for a wide range of applications.[35] Many different properties of electroceramics can be observed and manipulated for various applications.
2.1.1 Dielectric Properties Dielectric materials are good electrical insulators, meaning that they have high electrical resistivities, however they are affected by an applied electric field, E. An applied field causes the positive and negative charges in the material to slightly rearrange or displace and form electric dipoles, each having a moment, p. The dipole moment per unit volume is the polarisation, P.[36] There are four types of polarisation mechanism, and which are present depend upon the material. These mechanisms are illustrated in Figure 2.1. 1. In all dielectric materials, atomic, or electronic, polarisation can occur where the negatively charged electrons in an atom will displace slightly to align themselves with the positive electrode of the applied field. The positively charged nucleus shifts slightly towards the negative electrode. The electrons and nuclei return to their original positions once the field is removed. The overall polarisation effect is relatively small compared to the other types; however it occurs in all dielectric materials. It is not affected by the temperature of the material. 2. Where the material has an ionic lattice, the anions and cations will adjust themselves so as to align with the field. There are many different combinations of polarisation effect, depending upon factors such as the crystal structure. This creates a higher amount of charge carriers and ion mobility, thus is a prevalent mechanism and is affected significantly by the temperature of the material. It is this polarisation mechanism that creates ferroelectric and piezoelectric effects. 3. In dipolar materials, such as polymers, the applied field will orientate the molecules to align with the field. This is also known as orientation polarisation, and its net effect is larger than atomic polarisation because molecular dipoles are larger than atomic
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 12 dipoles. This effect causes the dielectric constant to be inversely proportional to the applied temperature. 4. Space charge polarisation is slightly different, in that electrons will travel across the grain until they hit a defect such as a grain boundary. As the defect presents a potential barrier, space charge is created across the grain. These charges can occur due to the manufacturing process (charges become trapped) though only occur in low frequencies (up to 1000 Hz).[2, 35-36]
Figure 2.1. Schematic diagram showing different polarisation mechanisms in a material (from Moulson and Herbert).[35]
Polarisation or charge storage, allows for the miniaturisation of technology in the modern world. A higher degree of polarisability of a material allows miniaturisation. An example is capacitors. When an electric field, E, is applied across two parallel capacitor plates of area A and distance h from each other (as illustrated in Figure 2.2a), one plate becomes positive and the other negative. This means that a surface charge density, σp, is stored on each plate. The permittivity of a vacuum is a constant of ε0 = 8.854 X 10-12 F/m[37] and so the total capacitance, C0, of the plates in vacuum is given by Equation 2:1.[38]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 13
Equation 2 :1
When a dielectric material is placed in between the parallel plates, the applied field causes the material to become polarised. This is illustrated in Figure 2.2b. The susceptibility of the material, χe, results in the capacitance of the plates to increase by a factor of (1+ χe) and thus the permittivity, ε, of the material is defined by Equation 2:2. This creates the concept of the relative permittivity of the material, εr, otherwise known as the dielectric constant, which is an intrinsic property. This is defined in Equation 2:3.
The amount of charge that can be stored in a capacitor, Q, is dependent upon the voltage applied and the capacitance, C. This relationship is shown in Equation 2:4. The capacitance itself is dependent upon the permittivity, εr, the area of the parallel plates, A, and the thickness of the dielectric material, t. The capacitance is defined by Equation 2:5.
Equation 2 :2
Equation 2 :3
Equation 2 :4
Equation 2 :5
Figure 2.2. Schematic diagrams showing how charge can store on capacitor plates in vacuum (a) and how a dielectric material (grey) in between them can play a role (b) (from Moulson and Herbert).[35]
The relative permittivity, εr, is a dimensionless property found in materials and is defined as the ratio between the permittivity of the material and the permittivity of free space. There are two components that make up the relative permittivity of a material – real, ε’,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 14 and imaginary, ε”, parts. Their relation to the dielectric constant is given as Equation 2:6. ε’ is the permittivity, already explained. In electronic physics, the “out of phase” voltage of an ac electric field is always 90° out of phase behind the “in-phase” current. In reality, this difference is not exactly 90° as the current lags behind slightly in dielectric materials. If the angle of lag is defined as δ, then the amount of lag is defined as tan δ, otherwise known as the loss tangent.[2] This is defined by Equation 2:7. It is the amount of capacitive current that is lost, or dissipated, as heat.[35] Jaffe also explains this as loss from resistive leakage or dielectric absorption.[39]
Equation 2 :6
Equation 2 :7
The permittivity, or dielectric constant, is affected by heat and applied frequency; these alter the dipoles within the material and also increase the mobility of charge carriers.[2, 35, 40] The four polarisation mechanisms that were previously described are shown in Figure 2.3 as a function of frequency.
Figure 2.3. Polarisability of dielectric constant (ε’r) and loss (ε”r) as a function of frequency (after Moulson and Herbert).[35]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 15 2.1.2 Piezoelectric Properties Piezoelectricity was discovered by Jacques and Pierre Curie in 1880.[41] It translates as “pressure electricity”.[2] Piezoelectric ceramics are ferroelectric by definition. They must have a Curie temperature, TC, below which the structure of the material cannot be symmetrical from the centre, due to the formation of dipoles within the material (as explained in Section 2.1.3). Of the 32 crystal classes that characterise the symmetry of a unit cell, 21 do not have a centre of symmetry. Those which are centrosymmetric are unable to polarise, as the symmetry of the atoms compensate for any small changes in the ions under an applied field. Of the 21 non-centric classes, twenty exhibit the piezoelectric effect. The one that does not exhibit this effect is the cubic class 432, which have elements of symmetry, though not through the centre.[42]
The piezoelectric phenomenon has two interchangeable stimuli, depending upon the application of the ceramic. Under an applied stress, forcing compression of the material, it undergoes a strain whereby it induces electric polarisation. The relationship between the two is proportional, and so conversely, under an applied electric field, it will also exhibit induced strain. This strain causes a slight shrinkage of the piezoelectric. This only occurs when the material is below TC, when the unit cell structure is non-centrosymmetric (ie not cubic).
Figure 2.4. Structure of PZT cubic perovskite (left) and how it is affected in its tetragonal form to create a dipole under applied electric field (right) (from Callister).[43]
Figure 2.4 shows schematically how an electric field can influence the material to shrink and expand by creating dipoles in the unit cell structure. As explained, the application of an electric field, or a mechanical stress, induces a dipole to be created in the unit cell, which in turn creates a mechanical stress or electric field respectively. This causes a shape change in the material, where usually it is known for piezoelectric ceramics to shrink. How much the material shrinks by is known as the piezoelectric coefficient of the material, and each is unique to a particular composition as an intrinsic property.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 16 The direct piezoelectric effect is when an electric charge, Q, is produced from a material with the application of a stress, T. This allows the dielectric displacement, D, to be defined in Equation 2:8, where d is a constant. When an applied field, E, is subjected to a piezoelectric material, it generates a strain, S, which is defined as Equation 2:9. Again, there is a constant showing the proportionality of the two parameters, also d. d is the same for both instances, and so is known as the piezoelectric constant, which is defined in Equation 2:10. Due to two differences in measurement parameters, d has two units of measurement – that of Coulombs per Newton (C/N – a small number, so usually shown in pC/N) for the direct piezoelectric effect, and meters per Volt (m/V) for the converse effect.[39]
Equation 2 :8
Equation 2 :9
Equation 2 :10
Piezoelectric materials need to undergo a poling process in order to allow the materials to exhibit their piezoelectric characteristics, allowing the direction of polarisation to change within the material. This is done by subjecting the material to a static electric field with varying temperature with respect to time. The amount of polarisation with respect to a polar axis depends upon the structure of the material. Although a single crystal can completely polarise, polycrystalline ceramics cannot due to the pinning of domains by grain boundaries and defects. Tetragonal structures can reach up to 83% polarisation, whereas orthorhombic structures can polarise significantly better to 91% due to the difference in the packing of atoms in the structure.[35] However in tetragonal Barium Titanate, the polarisation can only reach approximately 50% of a single crystal due to the internal strains preventing 90 domain switching.[35]
There are many different piezoelectric constants that can be measured in a ceramic material. They are generically denoted as dij, where i denotes the electric field or displacement, and j denotes the direction of the mechanical stress or strain. For example, the transverse piezoelectric coefficient is denoted as –d31.[44] This relates to the field along the polar axis and the strain perpendicular to it. However the most common piezoelectric coefficient used in publications and comparisons is that of d33, where the strain and the applied field are measured along the polar axis.[35] It can be defined by
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 17 Equation 2:11, where Q is charge applied, C is capacitance, V is voltage applied and F is force applied.[45]
Equation 2 :11
Figure 2.5. Schematic of k parameter (after van Randeraat and Setterington).[41]
Another piezoelectric parameter that must be discussed is the electromechanical coupling coefficient, k. k2 measures the conversion ratio of electrical and mechanical energies, depending upon whether the converse or direct piezoelectric methods are applied for measurement. Like the relative permittivity, it is also a dimensionless parameter, as it is a proportion being calculated. It is defined by Equation 2:12 (for direct piezoelectric effect) and Equation 2:13 (for converse piezoelectric effect).[4, 39] It must be noted that k2, and thus k, are both < 1 due to the energy conversions never being fully complete.[39]
Equation 2 :12
Equation 2 :13
There are many types of k factor, however the most important are kp and kt. In a thin piezoelectric disc, the coupling between the electric field in the z-axis (or 3-direction – ie through the thickness of the disc) and the simultaneous mechanical actions in the x- and y- axis directions is known as the radial coupling or radial vibration, known as the planar coupling coefficient, kp. A schematic presentation of this is shown in Figure 2.5. The formula defining kp is given in Equation 2:14,[46] where fr is the resonant frequency and fa is the anti-resonant frequency. The coupling between the electric field in the z- (or 3-) direction and the mechanical vibration in the same direction is known as the thickness coupling coefficient, kt.[41, 47]
Equation 2 :14
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 18 2.1.3 Ferroelectric Properties A ferroelectric material is one which exhibits spontaneous polarisation, where the direction of electric moments within can be changed by applying a field, and the relationship between field and polarisation which takes the form of a hysteresis loop.[35, 39, 42] This phenomenon was first discovered by Valasek in 1921 with Rochelle salt.[48] This is the reason why ferroelectricity is sometimes termed “Rochelle-electricity” in
France and Germany.[49] This behaviour is only observed until its Curie temperature, TC, where the material is no longer ferroelectric. Although given the prefix of “ferro”, a ferroelectric is not observed in iron-based materials, but was given the name due to the similarities to ferromagnetism.[49] Ferroelectric materials are a subset of piezoelectric materials, where some exhibit ferroelectric properties and some do not.[50]
There are three main structures that exhibit ferroelectric properties, that of Perovskites, Tungsten Bronze Type (TBT) and Bismuth Layered-Structured Ferroelectrics (BLSFs). Perovskite materials are known to have properties most useful for actuator and high power applications, whereas BLSFs have better characteristics for resonator and ceramic filter applications.[13] In this report, Perovskite materials are predominant, being the structure of the lead-free material NKN, and the traditional PZT material.
Figure 2.6. Variation of εr of BaTiO3 as a function of temperature. Corresponding phase structures are also provided (after Richerson).[2]
A ferroelectric material undergoes the polarisation described in section 2.1.1, however this significantly varies according to the applied temperature, as a material can undergo phase transformations in order to keep the crystal structure at its most energetically favourable. As the polarisation reaches the upper limit of its stable structure, the relative dielectric constant, or permittivity, εr, increases significantly. This is evident in the example of Barium Titanate in Figure 2.6. Eventually, a certain temperature is reached
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 19 where the permittivity reaches a considerable maximum at the Curie point, TC, and then experiences a considerable loss. This limiting point is where a ferroelectric material’s properties behave as a paralectric material (a standard dielectric, and no longer ferroelectric). By how much the permittivity decreases after this point is defined by the Curie-Weiss law, defined in Equation 2:15, where A is a material constant and T is a given temperature above TC (it must be noted, that Equation 2:15 is given in different forms in different sources, though the general principle is the same – the one provided here is that provided by Jaffe et al[39]). There is a difference between θC and TC, which is that TC is the temperature at which the structure changes and constant drops, as described, whereas
θC is a point near TC but is an extrapolation of the experimental data in order to follow the formula.[35, 39, 48] Although the Curie-Weiss law is well-renowned, Uchino and Nomura modified the equation in 1980 to allow a better fit to observed behaviour shown as
Equation 2:16,[51] and it is this that is employed in current literature.[52- 53] [m denotes the property at the transition. C and γ are constants, 1 ≤ γ ≤ 2 where γ = 1 is for a normal ferroelectric material and γ = 2 is for a ferroelectric relaxor]
Equation 2 :15
Equation 2 :16
Figure 2.7. The displacements of ions in BaTiO3 that occur in the cubic-tetragonal transition (from Moulson and Herbert).[35]
An example of how εr changes with respect to temperature is illustrated in Figure 2.6, with the example of BaTiO3 (BT). As illustrated, the increase of applied temperature transforms BT through three different phases of ferroelectric behaviour (that of rhombohedral, orthorhombic and tetragonal), before reaching a maximum permittivity at TC ~ 120ºC. These three structures are non-centro-symmetric, meaning that the central ion (titanium
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 20 in this case) is not exactly central, and conversely the barium ions also shift slightly in order to compensate for this, and thus a dipole is created in the unit cell. The central atom in perovskites is usually highly polarisable (for example, Ti ions have a charge of 4+) which helps the dipole to form easily. How much the ions distort in BT is illustrated in
Figure 2.7. At TC, however, the unit cell becomes cubic, thus symmetrical, no longer allowing spontaneous polarisation to occur in the material. This means that the material is then paraelectric and so the permittivity significantly drops, as described earlier.
When a ferroelectric material undergoes spontaneous polarisation, PS, a surface charge is created from the dipoles in the material. A depolarising field, ED, is then also created in the material (Figure 2.8); to minimise the large opposing energies, the material undergoes twinning. This means that the material divides into regions of opposing polarity, called domains. In this case, they are called 180° domains as they are directly opposing direction, as illustrated in Figure 2.8. The boundaries between the domains are called domain walls, and these are displaced when the domains alter their direction in order to align with the applied electric field.[49] The size of the domains is determined by energy and grain size conservation when transforming between the paraelectric cubic to the ferroelectric state.[54]
Figure 2.8. The initial surface charges in a material when spontaneously polarised (right) and how energy is preserved through 180° domains (left) (from Moulson and Herbert).[35]
(a) (b)
Figure 2.9. Schematic diagrams showing (a) how a polycrystalline ferroelectric material splits into 90º and 180º domains (from Moulson and Herbert).[35] and (b) domain walls in tetragonal BT (A-A’ are 90º walls, B-B’ are 180º walls) (from Jona and Shirane).[42]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 21 In polycrystalline materials, each grain will have a different direction of domains, so an unpoled ceramic will have no overall polarity as the domains will cancel each other out.
However, as the material is taken below its paraelectric phase at TC, the grains need to adjust their structure to remain stable and remain in an energetically favourable state. In order to minimise the strain within the structure, 90° domains can also be formed, as these are more energetically favourable.[38] Both these types of domain are found in tetragonal structures,[48] and are schematically depicted in Figure 2.9. Arlt reported in 1987 that there are two main configurations of domain, with the example of tetragonal BT.[54] In grains smaller than 10μm, lamellar structures are visible where 90º domains are formed across the grain. If the grain size is larger than 10 μm, a herringbone structure is observed, where 90º domains (Figure 2.9b) are separated by one direction and then split by 180º domains, illustrated in Figure 2.9a. This, however, is the case for BT, whose visible domains were reported by Matthias and Von Hippel in 1948.[55-56] In 1990, Arlt published a more generic theorem for all ferroelectric materials.[57] This is where there needs to be a critical grain size in order to see any lamellar domains form in a grain, which sees the stress energy in the grain decrease, as shown in Figure 2.10. As the stress energy decreases further, the grain size reaches a larger critical grain size needed to produce more complex herringbone-type domains within it. Eriksson et al. confirmed the theory that as the grain size increases, so does the domain size (in nano- sized grains).[58]
Figure 2.10. The stress energy in grains in terms of grain size, accompanied by the domain structure visible in the corresponding grains (from Arlt).[57]
How domains move within their boundaries depends upon vacancies present in the material. Vacancies on the A-site in perovskites allow for easy domain wall motion as they reduce local stresses in the domain, thus allowing domain switching to occur easily. This results in a “soft” material. O-site vacancies, however, act as pinning points along with the
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 22 grain boundaries, reducing the wall mobility (especially where grain size is relatively small). This is “hardening” of a ceramic, and leads to a reduced tan δ. [35, 59]
Figure 2.11. Schematic of typical ferroelectric P-E hysteresis loop, including diagrams showing the domain orientation at various points (from Damjanovic).[60]
A ferroelectric material is where dielectric materials under an external electric field can change the spontaneous polarisation direction.[40] The grains in a ceramic will all have random orientation of domains in a polycrystalline ceramic, thus creating a net polarisation of zero, shown by point A on Figure 2.11. With the addition of an applied dc field to the material, as long as the material temperature remains below the Curie point,
TC, the domains are forced to align with the direction of the electric field. This increases with applied field until a saturation point, D, where all domains that are not pinned or under internal stresses have aligned with the field. When point D is extrapolated back to zero applied field, this is the saturation point of domains, or saturation polarisation, shown as Ps in Figure 2.11. The material has now undergone poling. This means that when the applied field is taken away, there will still be a net alignment of domains, known as the remnant polarisation, Pr. Once the field is reversed, the domains change direction, taking slightly longer to switch to align in the opposite direction, again to a point of saturation, known as –PS shown at Point G. As the field is alternating between negative and positive field, the pattern exhibited is shown as a hysteresis loop. This means that as the field is increasing in a positive direction, the loop goes through point EC, the coercive field (the field at which the polarization is zero); conversely when the field is decreasing to the negative, the loop goes through –EC. This is the strength of the field applied where the net polarisation of the material is zero. As these points are not where the field is zero, it is here where domain switching occurs. The hysteresis loop is most pronounced when it is for a single crystal,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 23 and thus polycrystalline materials show a smaller loop due to the defects in the grains, such as grain boundary pinning, vacancies and impurities. The shape of the hysteresis loop is dependent upon factors such as the temperature, test sample dimensions, humidity of the sample before testing (it can be detrimental to some materials), the texturing of the specimen, grain size and the thermal and electrical testing the sample was previously subjected to.
Above TC, the loop no longer exhibits hysteresis and the behaviour resembles a diagonal straight line, as now it is no longer ferroelectric.[2] The highest and most desired piezoelectric properties of ferroelectric materials are exhibited when there is a high degree of hysteresis.[61]
2.1.4 Impedance Spectroscopy Impedance Spectroscopy is a relatively new non-destructive method of electrical characterisation of materials.
When impedance points are plotted on a complex plane (that is a Z”-Z’ Nyquist plot) with respect to the applied angular frequency, the resulting plot should show in the form of a semicircle. These semicircular arcs change with increasing temperature.[62-63]
Figure 2.12. Exemplar impedance plot for yttria-stablised zirconia, showing the three possible contributions for polarisation processes (from Moulson and Herbert).[35]
There are three electrical phenomena that can be represented in impedance spectra, those due to the bulk material, grain boundary effects and interfacial polarisation. This is shown by Figure 2.12. These three interface contributions, along with their equivalent circuit, is illustrated in Figure 2.15. When there is a single arc seen on the Nyquist plot, starting very close to the origin, for dielectric materials (such as NKN), the electrical contribution is thought to be from the bulk or grain. If there is a smaller second semicircular arc interfering on the right hand side (that is, from lower applied frequencies, usually seen at lower temperatures), this is the beginning of grain boundary effects.[63]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 24
Figure 2.13. Illustration showing the electrode-ceramic interface cross-section, with respective equivalent circuits (from Waser).[64] [el is the electrode-ceramic interface, b is the bulk and gb is the grain boundary contribution]
It is from these contributions that an equivalent circuit of the material can be derived, using electrical components for each of the phenomena for an accurate model of the how the material behaves. An exemplar diagram is shown in Figure 2.13 and Figure 2.14. It is necessary for an appropriate equivalent circuit to be modelled on a material in order to understand it and its electrical properties.
Figure 2.14. An example of (a) a series-parallel circuit which could produce the impedance plot in part (b) (after Moulson and Herbert.).[35]
Impedance spectroscopy can determine the type of conduction the sample uses can be found, such as ionic or electronic conduction. Computational simulations of ionic transport in ABO3 perovskites have been modelled by Saiful Islam in 2000 and show the activation energy for the migration of oxygen vacancies is ~ 1 eV, for A-site cation transport is ~ 4 eV and B-site cation migration is ~ 14 eV.[65]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 25 2.2 Piezoelectric Materials and their Properties The majority of piezoelectric materials considered in this investigation are perovskite,
ABO3, which in its most basic cubic form is illustrated in Figure 2.17. In the case of barium titanate [BT], the Ba2+ ions are situated in the A-site corners of the standard unit cell, each surrounded by 12 oxygen ions, creating a face-centred cubic (FCC) lattice. The Ti4+ ions sit on the B-site central octahedral interstice.
Figure 2.15. The cubic unit cell of perovskite BaTiO3 (after Richerson).[2]
Perovskites are known for their high degrees of porosity and thus are difficult to achieve a high density. They are also known to have low Curie temperatures and can be difficult to pole.[13] Despite this, there is much interest in perovskites to replace PZT.
There are two types of ferroelectric material – “soft” ferroelectrics are based upon hydrogen-bonding where under TC, the ferroelectric ordered structure that forms provides specific positions for the hydrogen atoms to pair up and these form the reversible dipoles. Such materials include Rochelle salt, sulphates, sulphites, nitrides and nitrites. “Hard” ferroelectrics are typically oxides that exhibit similar properties to BT; they are not water-soluble and are mechanically hard and polarise due to a single central cation of high charge within an octahedron of oxygen anions.[2] It is these “hard” ferroelectrics that are addressed in this investigation. It must also be noted that the “soft” and “hard” terms are applied to oxide ferroelectrics to describe donor and acceptor doped materials, respectively.
In many of the solid solutions discussed in this section, there is an optimum composition at which enhanced electrical properties are exhibited. At this composition two structural phases coexist in the solution, and there is a structural transformation. It is called the morphotropic phase boundary (MPB).[35] This region is further discussed in each section.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 26 2.2.1 Lead Zirconium Titanate (PZT)
Although the first piezoelectric ceramic was BaTiO3 (BT), the PbTiO3-PbZrO3 (PZT) system is now the most popular ferroelectric and piezoelectric material in industrial applications. It has been developed and optimised for a wide range of uses, and is now recognised as a smart material.[66]
The Pb(Zr1-xTix)O3 phase diagram is shown in Figure 2.16. It is a solid solution of ferroelectric PbTiO3, which has TC = 495ºC, which is tetragonal at room temperature, with antiferroelectric PbZrO3 which is orthorhombic at room temperature and has TC = 234ºC.[35]
Figure 2.16. Phase diagram for PZT, showing various properties of PZT at different compositions and the PZT MPB (in green) (after Lee and Bell).[67]
The morphotropic phase boundary (MPB) indicated on Figure 2.16 is of major interest as superior piezo- and dielectric properties are found there. Jaffe, Roth and Marzullo discovered this MPB effect in PZT in 1954.[1] For PZT, it is the state of structural change from rhombohedral to tetragonal where the two structures coexist. This in the range of
0.455 x 0.48 for PbTiO3 and seems to be temperature independent.[35]
As with other ceramics, the doping of other ions into the PZT structure can also increase its properties. For example, the MPB can shift with doping; isovalent Hf4+ replacing Zr4+ in the B-site increases the MPB to 52 mol% PbTiO3, though conversely adding Sn4+ to the B- site reduces the MPB to 42 mol% PbTiO3 (and lowers TC from 370 to 250 °C).[35]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 27 In terms of electrical properties, undoped PZT has a Pr of 50-75 μC/cm2.[68] With increasing density, the piezoelectric and dielectric constants rise from 60 pC/N and 330 to 240 pC/N and 680 respectively.[69] The reason for the poor initial electrical properties in undoped PZT is due to the difficulty in orienting domains in the tetragonal region, and domain clamping by oxygen vacancies.[69] Barium titanate, like PZT, is also tetragonal and exhibits high dielectric and piezoelectric properties without doping. Domain clamping is synonymous with the higher density and electrical properties, as with higher densities there are fewer oxygen vacancies in the system.
Berlincourt discussed the differences between acceptor and donor doping in PZT.[68] Acceptor doping is where a substitution occurs in the lattice and there is a free electron present, such as adding Fe3+ to a Pb4+ B-site. Acceptor doping leads to a lower permittivity and piezoelectric coupling, and a higher mechanical Q value. Conversely, donor doping is the site substitution where an electron hole is left over, such as doping Bi3+ onto the Pb2+ A-site in PZT. Donor doping leads to higher permittivity, piezoelectric coupling and resitivity (can be 1000 times higher) whilst losing mechanical Q value properties.[68]
PZT is used for transducer applications and load-bearing applications, for example in actuators used in applications below 400°C.[34, 69] The fact that PZT contains a significant fraction of PbO, which is deemed toxic by the US Health Department, and EU legislation has limited the manufacture of new components containing Pb means that there is a growing interest in Pb-free piezoelectric materials. These alternatives include
BaTiO3, Bi1/2Na1/2TiO3 and NKN-based ceramics.[3, 70-71]
2.2.2 Barium Titanate (BT)
Barium Titanate, BaTiO3, is the classic ferroelectric perovskite material. Its cubic unit cell is illustrated in Figure 2.15, showing barium (2+) cations in the corner A-site positions, with the oxygen (2-) anions in the centre of the cell faces, creating a face-centred-cubic (FCC) structure. The octahedral interstice, the B-site in the centre, is filled with a titanium (4+) cation, which due to its high polarisability (4+ charge) can easily form a permanent dipole below 130ºC, its Curie temperature, TC.[39] At room temperature, however, the tetragonal phase of BT is stable. This is where one of the cubic edges has elongated, whilst the other two have decreased in length slightly, though staying at the same ratio to each other.[42] This is depicted in Figure 2.17.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 28 There is also another structure barium titanate [BT] can be metastable at room temperature in, which is the hexagonal unit cell, however this is non-ferroelectric and is less common, so is not discussed here.[39]
Figure 2.17. The phase structures, polarisation temperatures and transition temperatures of Barium Titanate (after Moulson and Herbert).[35] a and c are parameter lengths of the unit cell, and P is the polarisation direction
Barium titanate is an intermediate compoud found at a 1:1 ratio in the BaO-TiO2 phase diagram, thus defining BaTiO3 as the optimal stoichiometric material that exhibits superior dielectric and piezoelectric properties. This structure is indicated by a green dashed line in Figure 2.18.
In 1953, Merz reported that above 107ºC, the ferroelectric P-E hysteresis loops become pinched in the middle, creating two smaller hysteresis loops.[72] This antiferroelectric behaviour is visible until 128ºC, where a straight line is exhibited in replacement of hysteresis. This is where paraelectric behaviour begins, and so it is very close to the 130ºC found by Jaffe, Cook and Jaffe.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 29
Figure 2.18. The phase diagram of the BaO-TiO2 system, with BaTiO3 shown in green (after Jaffe).[39]
As the temperature continues to decrease to below freezing, the edges between the two sides of equal length distort their 90º edges to create an orthorhombic unit cell. Jona and Shirane, however, report that this transition occurs at 5ºC and not zero, which is in accordance with the report by Matthias and von Hippel in 1948.[42, 56] The final phase transformation occurs at -90ºC, where the lengthened edge, c, shortens to the same length as the other two parameters, a. However at this stage there are no right angles in the unit cell as all sides are now under shear to create a rhombohedral structure. The polarisation direction of the dipoles within each cell is parallel to the direction of elongation in each transforming instance. This, in its simplest form, is due to the central B-site Ti4+ cation shifts from the centre towards the oxygen anions (towards one in tetragonal, two in orthorhombic and three and rhombohedral).[39] This is also illustrated in Figure 2.17 as P, without ions being shown. Although these are well-known properties for BT, the exact transition temperatures can vary due to impurities, doping, grain size, rate of cooling, and stress conditions of the sample.[39]
By doping BT with other perovskite materials, the TC characteristics will move from
120ºC. Whether TC goes higher or lower depends upon the dopant. SrTiO3, for example,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 30 causes the temperature to decrease from 130ºC to below zero with over 30% doping.[2] Conversely, when the Ba2+ions are replaced by isovalent Pb2+ and Ca2+, the Curie temperature increases with increasing doping additions. Figure 2.19 illustrates how TC can be altered by the addition of Pb, Ca, Sr, Zr and Sn dopants on the Ba2+ A-site.
Figure 2.19. Graph depicting the change in Curie temperature of BT with x mol% of each dopant on the Ba site (from Yanagida).[40]
In tetragonal BT, 90º and 180º domains are visible in the grains. 90º domains are usually seen to be lamellar, herringbone or dagger-like, however 180º domains are significantly different in that their boundaries are curved and seems to be like a watermark.[73]
2.2.3 Potassium Niobate, KNbO3 [KN]
KN is a perovskite material, whose TC is found at 425ºC.[48] Below this, the tetragonal phase is most favourable until the temperature falls below 220ºC where it transforms to an orthorhombic structure. The final phase transition, to rhombohedral, occurs at -140ºC. All of these phase transformation temperatures are much higher than those of BT.[48] However these transition temperatures are reported to be 435, 225 and -10ºC by Jona and Shirane.[42] Due to the relatively low melting point of KN (1050ºC), well sintered KN samples are difficult to produce. This is unlike BT, whose phase symmetries and phase transition sequences are identical to KNbO3 (unlike any other ferroelectric material).[42].
The ferroelectric behaviour of KN was first reported by Matthias and Remieka in 1951.[74] It exhibits spontaneous polarisation of 0.9 μC/cm2 at room temperature, which increases to 26μC/cm2 near TC. Like with BT, there is hysteresis of the dielectric constant of KN with respect to temperature, illustrated in Figure 2.20.[48]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 31 Figure 2.20. The dielectric constant hysteresis behaviour of KN with respect to temperature (after Megaw).[48]
There are problems processing KN as K2O is volatile and evaporates at high temperatures. Potassium oxide is also known for deliquescence, and so must be carefully handled.[75]
2.2.4 NaNbO3 [NN]
Kakimoto reported a TC for NN at approximately 365°C, which is similar to that of PZT.[75] It is known to be antiferroelectric – meaning it has a remnant polarisation of zero,[42] and when subjected to an electric field produces two hysteresis loops, one each in the positive and negative applied field directions (Figure 2.21). The structure of NN is orthorhombic at room temperature up to 360ºC (±15ºC), where it undergoes a phase transition (it still remains orthorhombic, but in reality becomes pseudo-tetragonal as the c/a ratio is 1.0023) at 375ºC. This ratio decreases until the structure becomes paraelectric cubic from 640ºC. It must be noted, however, that there are transitions at 450-470ºC and 518ºC as there are changes in electrical properties.[48]
Figure 2.21. Typical antiferroelectric hysteresis behaviour (after Uchino).[4]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 32 In 1951, the Curie temperature for NN was reported to be 480°C by Matthias and Remeika.[74] According to Kakimoto, NN undergoes seven phase transitions as temperature increases,[75] showing it is highly sensitive to temperature. Jona and Shirane classed the phase transitions into the major transitions.[42] NN has a cubic phase until 640ºC, below which the phase becomes non-polar tetragonal (or pseudo-tetragonal) until 562ºC. At this temperature, the structure becomes non-polar orthorhombic, or a pseudo-tetragonal phase until it is cooled to below 354ºC, where it remains orthorhombic but becomes anti-ferroelectric. The material finally becomes truly ferroelectric in its final monoclinic phase, which occurs at -200ºC. It was found by Reisman that these phase transitions are highly sensitive to internal strains, and so the exact transitions were only found after much annealing of the sample.[42, 76] The lattice parameters for NN are provided in Figure 2.22. The large differences in between the rhohombohedral-orthorhombic phase transitions reported by Megaw[48] (and by Reisman et al.[76]) are explained by Jona and Shirane in terms of the hysteresis behaviour.[42] During cooling, the phase transition is at -200º; however when heating the material, it occurs at a much higher temperature of - 10ºC.
Figure 2.22. Diagram showing the lattice prarmeters of NN (after Jona and Shirane).[42] Volume cube root indicated in green
Although NN was shown to be ferroelectric in 1951, with a spontaneous polarisation value, Cross and Nicholson reported that a double hysteresis loop developed, similar to that exhibited in the paraelectric phase of BT. Like in BT, if the field is large enough, then the double loop behaviour is like that expected for a ferroelectric, thus the ferroelectric characteristics are only slightly larger than those for an antiferroelectric. This has resulted in the material being known as anti-ferroelectric.[48]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 33 Dungan and Golding that showed that at 270°C there is a significant drop in the ferroelectric properties of NN, but not a structural change.[77] There is a significant loss in polarisation at this temperature, which is irreversible, exhibiting a ferroelectric to antiferroelectric transition. Once this temperature has been reached, no hysteresis loops are observed in the material, even at room temperature. However, (Na0.98K0.02)NbO3 does exhibit hysteresis once the antiferroelectric transition is reached.
2.2.5 Lithium Niobate, LiNbO3 [LN] and Lithium Tantalate [LT] LN and LT have a rhombohedral unit cell at room temperature. It is not known whether it is possible for the LN ions to sit in the correct perovskite positions, but the closest comparison is to rhombohedral BT. The actual structure for these two piezoelectric materials is trigonal ilmenite (similar to FeTiO3) at room temperature.[35, 75] The space group for both LN and LT is R3c, where parameter a is 5.4920 and 5.4703 Å, and angle β is 55° and 56°, for LN and LT respectively.[48]
LN and LT are reported to be piezoelectric at room temperature, but only exhibit ferroelectric properties in single crystals above 200°C (Figure 2.23).[42, 48]
Due to poor densification during sintering, and high TC, both LN and LT tend to be utilised in single crystal form.[78] Unlike the other ferroelectric materials reported here, their coercive field strength and spontaneous polarisations rise with temperature, and exhibit no Curie temperature.[42].
Figure 2.23. Dielectric Constant, ε, Saturation Polarisation, Ps, and Ec values for LN as a function of temperature (from Jona and Shirane).[42]
2.2.6 Strontium Barium Niobate (SBN)
Strontium barium niobate (Sr1-xBax)Nb2O6 (0.25 ≤ x ≤ 0.75), SBN, has a tetragonal bronze- type structure, composed of a network of NbO6 octahedra. Due to this network, the structure is comparable to a Perovskite with different octahedra rotations, and a longer unit cell.[79]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 34 In 2009, Lili et al. produced SBN with a density of 79.4% of the theoretical, when sintered at 1320ºC for 2h.[80] This exhibited a low dielectric constant of <600 at temperatures up to 140ºC, along with losses below 0.012 at 10kHz. When adding 1.5wt% SiO2 the material properties significantly improved, the dielectric constant reaching a maximum of 4322 at 85ºC, and losses below 0.085. It is thought that an amorphous silica layer developed at the grain boundaries, restraining abnormal grain growth, creating a more equiaxed microstructure.[80-81]
2.3 Sodium Potassium Niobate (NKN) 2.3.1 Recent Pure NKN investigations NKN was first reported as a potential piezoelectric material by Egerton and Dillon in
1959.[29] Feng et al. explained that as NaNbO3 is added to KNbO3 the stability of the perovskite structure decreases.[82] Unless the KN-NN solution is very close to the NN side (>98%), the entire solid solution is ferroelectric. However enhanced dielectric and piezoelectric properties are observed along the MPB, which is indicated on the NKN phase diagram (Figure 2.24). The labels F and P show which phases are ferroelectric (F) and paraelectric (P) whilst the O, T, M and C subscripts for the phase structures indicate whether they are orthorhombic, tetragonal, monoclinic or cubic, respectively. The MPB is usually found in between two different phases, such as orthorhombic and tetragonal, but in this case, the MPB is different as it separates two different orthorhombic phases (at lower temperatures) and two tetragonal phases at higher temperatures. The MPB materials are generally known to exhibit enhanced and superior electrical properties,[1] and this case is no exception.
In order to form an ordered ferroelectric perovskite structure, it is more likely when the two cations sharing the A or B sites have ionic radii that are significantly different in size.[83] NKN follows this rule, as the ionic radii of Na+ and K+ are 0.97 and 1.33 Å respectively[83-84] (Chang et al. report these to be 1.39 and 1.64 Å[85]).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 35 Figure 2.24. The phase diagram of the NN-KN solid solution system (after Jaffe et al).[39]
In 2009, Baker et al. published the phase diagram for NKN at temperatures up to 500°C (until the paraelectric cubic phase) (Figure 2.25).[86] The figure shows the polymorphic phase transitions in the NKN system, as well as slight changes in oxygen tilting. Jaffe et al. did not specify the actual phase structures. It must be noted that this phase diagram is a function of KN addition to NN, whereas Figure 2.25 is a function of NN addition to KN (ie reversed). The space groups are given, and underneath these there are the parameters in terms of magnitude (for example, cubic structure is defined as aaa as all parameters are the same size) and oxygen tilting (superscripts of 0, + and – denote the direction of tilting in the x, y and z axes), a notation defined by Glazer.[87] The subscripts define the B-site cation displacement directions (also as 0, + and – in the same way) a notation developed by Stokes et al.[88]
It is noted that there are discrepancies in the reported structure theory. In 2010, Inagaki et al.[89] reported NKN to have a Bmm2 structure after Katz defined the parameters of
KNbO3 (an NKN end member) as such.[90] However, Zhang et al in 2011 indicated NKN to have pseudo-cubic Amm2 structure.[91] This Amm2 structure concurs with that of Baker et al.[86] however both forms have been utilised in publications.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 36
Figure 2.25. Revised phase diagram for NKN including phase structures (after Baker et al).[86] Dotted lines indicate change in tilting.
The main focus of NKN in this investigation is (Na0.5K0.5)NbO3, a composition very close to
the MPB (Na0.48K0.52)NbO3. Its transition temperatures are TO-T ≈ 190-200ºC and TC = 420ºC.[17-18, 29] This material is well known for its deliquescence and difficulties in high densification using the conventional mixed oxide processing route.
(a) (b)
Figure 2.26. NKN P-E hysteresis behaviour according to (a) Ichiki et al.[92] and (b) Zhang et al.[31]
Typical piezoelectric properties for undoped NKN are d33 = 80 pC/N,[29] but this has been found as high as 105 pC/N.[93-94]
Ichiki et al. in 2004 found hysteresis in NKN to have Pr = 8.9 μC/cm2 and Ec = 9.0 kV/cm (shown in Figure 2.26a) though only in a field of 1.5 kV/cm (though the publication says up to 4 kV was applied),[92] but higher remnant polarisations have been found.[31,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 37 93, 95-97] Zhang et al., however, produced an NKN sample with significantly better properties.[31] They showed that a sintering temperature of 1060ºC produced fully saturated hysteresis loops with Pr and Ec being 15 μC/cm2 and 13 kV/cm respectively in a
4 kV/mm field (Figure 2.26b), and d33 a significantly higher 122 pC/N. No other publication finds such high piezoelectric constant results for pure NKN. Chang et al. reported the highest hysteresis properties for undoped NKN.[95] Whilst investigating the doping of alkaline earth titanates to NKN, the control sample for pure NKN gave hysteresis values of Pr = 18.8 μC/cm2 and Ec = 9.65 kV/cm. These are high values compared to others in the literature. Table 2–1 shows a exemplar summary of undoped NKN properties from the literature. These increases of NKN properties from the original findings by Egerton and Dillion[29] are due to the further investigation and understanding of the NKN system, and optimisation of processing procedures.
Table 2–1. Hysteresis Properties of pure NKN (in order of field threshold, E).
Reference Authors Year Pr (μC/cm2) EC (kV/cm) E (kV/cm) [92] Ichiki et al. 2004 8.9 9.0 1.5 [95] Y. Chang et al. 2006 18.8 9.65 3 [93] Zuo et al. 2008 13.6 9.9 3 [96] Zuo et al. 2006 15.4 11.5 4 [31] Zhang et al. 2007 15 13 4 [85] R-C. Chang et al. 2007 15 14.8 4 [97] Fukada et al. 2008 9.0 26 4
The processing conditions for NKN must be taken into consideration, as NKN is notoriously difficult to sinter and densify. Zhang et al. investigated the hysteresis behaviour as a function of sintering temperature.[31] They showed that a sintering temperature of 1020ºC led to a density of 69.4% theoretical, and hysteresis behaviour was not observed, which could be due to the insufficient densification. When the same formulation was sintered at 1080ºC, densification reached a significantly higher 94.4% theoretical, and produced fully saturated hysteresis loops. This shows how important the sintering temperature is to the processing of NKN ceramics.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 38
Figure 2.27. P-E Hysteresis for (Na0.5K0.5)(Nb0.995Mn0.005)O3 with different cooling rates (see text) (from Inagaki et al).[89]
The cooling rate, as well as the sintering temperature must also be taken into consideration. Inagaki, Kakimoto and Kagimiya tested two 0.5 mol% Mn-doped NKN samples, sintered at 1050°C, but upon cooling between 1050 and 950°C (1.25 and 0.25 °C/min).[89] The resulting hysteresis data for the two samples is significantly different; the sample having a higher cooling rate, shows “leaky” hysteresis loop with Pr = 26
μC/cm2 and EC = 20.4 kV/cm (which are normal results for NKN) shown as “Sample A” in Figure 2.27. The sample cooled at the slower rate shows enhanced properties in terms of hysteresis loops and significantly higher remnant polarisation of 52 μC/cm2 and coercive field of 13.0 kV/cm (“Sample B” in Figure 2.27). These properties are highly desirable for NKN, and shows how sensitive NKN is to processing conditions.
2.3.2 NKN Doping for Enhanced Properties There are five main effects that doping can have on a ceramic material: 1. Modifying the transition temperatures of a material in order to adjust the temperature range of a material for a particular application. 2. Restrict domain wall motion so that the dissipation factor of the material can be reduced. 3. Cause the formation of second phases and thus a heterogeneous composition which can also change the Curie temperatures and widen the range of temperatures exhibiting a large permittivity. 4. Control the size of crystallites by introducing vacancies and electron holes which will inhibit crystal growth due to pinning and increase the dielectric constant below the Curie temperature. 5. Control the valency and the oxygen content of the B-site ion so that the material can be sintered in oxygen deprived atmospheres and yet still keep its dielectric properties of high resistivity. This is due to lower valency substitutions of the B-sites which act as acceptors.[35]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 39
Figure 2.28. XRD diffractograms of (a) orthorhombic and (b) tetragonal NKN, with peak identification (after Skidmore et al.).[98]
As sintering temperature increases, the A-site Na+ and K+ are lost due to volatilisation, leading to increasing amounts of A-site vacancies in the lattice.[99] When NKN is doped, its characteristic orthorhombic structure can change to tetragonal. This is where doping NKN brings the polymorphic phase transition (in this case, orthorhombic to tetragonal) down to room temperature (from ~200°C).[99] Due to similarities of the XRD spectra for these two phases (where orthorhombic and tetragonal peaks are located at the same 2θ values, shown in Figure 2.29), there are two main ways of distinguishing the phases. The first is that the orthorhombic phase has split peaks at 22° and 32° 2θ (the (100)/(010) and (110)/(011)peaks respectively). The tetragonal phase shows a characteristic peak splitting at 45° 2θ for the (002)/(200) peaks.[100] This, however, is not always the case.
(a) (b)
Figure 2.29. Diffraction profiles of <100> and <200> peaks for (1-x)NKN-xLi0.5Bi1.5TiO3 ceramics, at areas of interest (a) 20 ≤ 2θ ≤ 25° and (b) 44 ≤ 2θ ≤ 48° (from Jiang et al).[101]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 40 The second method focuses on the intensity ratio of the double peaks. Upon closer inspection of the (001) / (100) double peaks at 20-25° 2θ (Figure 2.29a) and the (002) / (200) double peaks at 44-49° 2θ (Figure 2.29b), it can be seen how the orthorhombic and tetragonal peaks are defined. In the case of Figure 2.29, NKN has been doped with x mol%
Li0.5Bi0.5TiO3 (LBT), and specifically shows how the orthorhombic structure of NKN can transform into tetragonal with LBT addition. Pure NKN has the left peak (h00) higher than the right (0k0), as expected for orthorhombic structures. At x = 3 and 5 mol% LBT, the structure is tetragonal, which is shown by the left peaks (now (00l)) in tetragonal structures) being significantly smaller than the right (now (h00) in tetragonal).[101] As the ratios of the two peaks have significantly changed, this is a good indication of whether the NKN structure is orthorhombic or tetragonal at first glance, and is commonly used as the argument for the presence or transition of the two structures.[83, 100-102] Skidmore and Milne suggested a peak intensity ratio (α) formulation in order to investigate the presence of orthorhombic and tetragonal phases.[103] These are shown as Equation 2:17 and Equation 2:18, where I is the intensity of the peak. They proposed that α = 1.85 for orthorhombic NKN and α = 0.53 for tetragonal NKN.[98]
Equation 2 :17
Equation 2 :18
2.3.2.1 Excess K and Na Doping Potassium and sodium are both A-site cations that sit in the NKN structure. Having an excess or depletion of one of these changes the properties of NKN. It is useful to dope NKN with excess sodium due to the well-known volatility of K and Na during sintering.
Kim et al. investigated the addition of Na2O to 95NKN-5LiTaO3 (5LT) and found that excess sodium caused increased abnormal grain growth.[104] The addition of sodium also resulted in a lowering of optimal sintering temperature. The optimal addition was 1 mol%
Na2O (sintered at 1050°C) where the material exhibited properties of d33 = 230 pC/N, kp =
0.43, εr = 470, Pr = 11.7 μC/cm2 and EC = 11.8 kV/cm.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 41
Figure 2.30. Density and relative density of (Na0.52+xK0.48)0.942Li0.058NbO3 with 0 ≤ x ≤ 0.035 (from Li et al).[99]
Li et al. doped (Na0.52K0.48)0.942Li0.058NbO3 with excess Na and the effect on density was significant, increasing from 77.4% with no excess sodium up to 94.5% at x = 0.015 (Figure 2.30). [99] They showed that excess Na promoted liquid phase sintering; this is desired as it promotes densification, accelerates grain growth and thus enhances the electrical properties. The liquid phase is thought to be present due to the excess of A-site ions (as lithium is also substituted into the A-site as it has a 1+ charge) and so creates a liquid phase as the solid solution saturation limit has been reached. Electrical properties for this formulation are given in Section 2.3.2.3 due to the discussion regarding Lithium doping in NKN.
2.3.2.2 Alkaline Earth Doping
Malic et al. reported that the addition of 0.5 mol% AETiO3 (AE = Sr, Ba) to NKN promoted densification and decreased the NKN transition temperatures.[7] The addition of MgTiO3 is detrimental to the densification process, whereas Barium on the A-site causes minimal changes, and generates a second phase. All of this was confirmed by Chang et al. in 2006 and 2007;[95, 105] density, kp and d33 values significantly decreased with MgTiO3 addition
(though the dielectric εr value rose from 421 to 480). Ca and Sr additives to the A-site increased the density from 96.4 to 97.8 and 97.1 g/cm3 respectively, but the electrical properties diminished slightly. [95]
The addition of 3 mol% strontium to NKN (not in the form of a titanate) promoted superior piezoelectric properties, and increased the grain size and thus resistivity.[106] As this is undesirable, MnO was also added to the system; the addition of over 0.5 mol% MnO resulted in enhanced properties, although exact figures are not reported.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 42 (a) (b)
Figure 2.31. (a) Piezoelectric d33 and Qm values, and (b) P-E hysteresis behaviour for (1-x)NKN-xBT + 1 mol% CuO (from Lin, Kwok and Chan.[83]
Lin, Kwok and Chan doped NKN with BaTiO3.[107] Their optimal formulation is 94NKN- 6BT + 1 mol% CuO (copper oxide is added as a sintering aid) which exhibits enhanced properties, such as a Curie temperature lowered from 421ºC for NKN down to 314ºC for 6 mol% BT doping (both with 1 mol% CuO added). The Pr = 14.0 μC/cm2 is higher than that found for undoped NKN, though the Ec of 11.6 kV/cm is higher, which is undesirable.[83, 93] Although the remnant polarisation value was not the largest (Figure 2.31a), other values were optimal, such as the d33 value was 193 pC/N (Figure 2.31b), considerably higher than for NKN. Chang et al. investigated the addition of 0.5 mol% BT and found properties including ρr = 95.8%, d33 = 85 pC/N, kp = 0.26 and εr = 393.[95] Park et al. investigated a much higher addition of 5 mol% BT to NKN, and as a function of sintering temperature.[108] They reported that with an optimum sintering temperature of 1060°C, enhanced piezoelectric properties of d33 = 225 pC/N and ε = 1058 were exhibited; much higher than those reported by Chang et al.[95] This shows that BT is a evidently successful dopant to enhance the properties of NKN.
CuO doping is often added to perovskites as a sintering aid. It promotes a liquid phase during sintering and promotes densification (which in turn promotes enhanced piezoelectric properties).[46, 109] It yields NKN ceramics that are denser than those without Cu-doping.[83, 109] As Cu2+ is introduced into the NKN lattice, it occupies the Nb5+ B-site (similar ionic radii at 0.73 and 0.64Å, respectively) which causes the unit cell to expand slightly.[46, 109] It also produces oxygen vacancies in the lattice, as the doping is not charge balanced. This replacement is shown in Equation 2:19 in Kroger-Vink notation.[46] If oxygen vacancies are introduced to an electroceramic, they become “harder”.[35, 59] Yang et al. found the optimal addition of CuO to be 0.25 mol% where the maximum relative density and kp values were ~93% and 41.7%, respectively.[46]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 43
Equation 2 :19
Park et al. also investigated the addition of CuO as a sintering aid to 95NKN-5BT.[109] They found the optimal addition to be 1 mol% CuO. In this case, the optimal properties included ρr = 94.5%, d33 = 230 pC/N, kp = 0.37 and εr = 1150.[109]
Mn, like Cu, also sits on the B-site in NKN and so is also used as a sintering aid.[89] Again in 95NKN-5BT, along with 2.0 mol% CuO and 0.5 mol% MnO (as sintering aids), enhanced properties were reported by Ahn et al. including d33 = 248 pC/N, kp = 0.41 and εr = 1258.[110] These are both higher than reported by Yang et al.[46] and Park et al.[109], and also includes a higher copper oxide addition.
2.3.2.3 Li, Ta, Sb and Ag Doping These four oxides are successful dopants for NKN, leading to enhanced electrical properties. The addition of lithium to NKN brings about an MPB (morphotropic phase boundary) where enhanced electrical properties are observed at the transition from orthorhombic to the tetragonal phase. The addition of Li2+ to the A-site of NKN brings about an MPB with a wide transition, as literature reports suggest it to be between 0.3 -
0.7 mol% lithium additions. Lithium is added in various forms, such as LiNbO3, LiTaO3 and
LiSbO3.[103, 111-113]
Niu et al. studied the addition of lithium to NKN.[114] Figure 2.32 shows XRD profiles for x mol% Li addition to NKN. For up to 4 mol% Li doping there is an orthorhombic structure that is expected for NKN, but additions above 7 mol% lead to a tetragonal structure. Hence the 5 and 6 mol% doping gives rise to a phase boundary (shown as PB on Figure 2.32a) where the two phases coexist. This is more so in the 5 mol% formulation, as 6% shows both phases but has a higher degree of tetragonality. Similarly, Zhang et al reported the addition of 5.2 mol% Li addition to NKN gave mixed orthorhombic and tetragonal phases at room temperature.[115] Thus the 5-6 mol% Lithium addition to NKN is of particular interest because of the enhanced electrical properties that are exhibited at the MPB in the system. Song et al. reported the solubility limit for Lithium addition to be 7 mol%.[116]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 44
(a) (b)
Figure 2.32. XRD profiles of (a) (Na0.5K0.5)1-xLixNbO3 (after Niu et al)[114] and (b)
Li0.058(Na0.521+xK0.48)0.942NbO3 (after Li et al).[99]
Li et al. showed that there must be an MPB (as opposed to just a polymorphic phase transition) with lithium addition when investigating the addition of excess sodium to 5.8 mol% Li-doped NKN.[99] By the addition of excess Na, there is a significant discontinuity in the peaks between 0.015 and 0.020 doping for Na (Figure 2.32b). As all the processing conditions remain the same, the orthorhombic peaks should not have this anomaly; this is seen to be a difference in orthorhombic parameters and so an MPB. This is confirmed as the MPB is known to show enhanced electrical properties, which is clearly seen in this doping region (0.015-0.020) in Figure 2.33. The optimal excess Na-doping addition for this 94.2NKN-5.8LN formulation is x = 0.015, where properties include d33 = 279 pC/N, Pr
= 27.1 μC/cm2, kp = 0.46 and TC = 465°C (higher than for pure NKN).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 45
Figure 2.33. Piezoelectric and dielectric properties for Li0.058(Na0.521+xK0.48)0.942NbO3 (from Li et al).[99]
Shen et al. reported the lattice parameters for (Na0.535K0.485)1-xLix(Nb0.08Ta0.02)O3 for 0.02 ≤ x ≤ 0.07.[102] It is clearly shown the MPB region (indicated on Figure 2.34) is between the two distinguishable orthorhombic and tetragonal phases. It should be noted that the orthorhombic phase has relatively similar parameter lengths, and angle β does not stray far from 90° (Figure 2.34); this concurs with the concept of considering orthorhombic NKN as pseudo-cubic.[89, 117-118] [β is the angle between a and b parameters in the pseudo-cubic unit cell. This is further explained in Section 4.2.1.2 and Figure 4.6]
Figure 2.34. Lattice parameters and β (angle) value for structures of (Na0.535K0.485)1-
xLix(Nb0.8Ta0.2)O3 (0.02 ≤ x ≤ 0.07) (from Shen et al).[102] Shaded area indicates MPB region.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 46 In terms of transition temperatures, 6 mol% lithium is the optimal addition for NKN. This addition causes the Curie temperature to increase, from 420°C[29] to 450°C (although Niu et al. reported a high value of 474°C).[111, 114, 119]
The TO-T phase transition for NKN is at 200°C,[70, 120] Niu et al. reported this to be significantly lower by 67°C.[114] Higashade et al. report this to be even lower, at
15°C.[121] These findings show that the 94NKN-6LN (LiNbO3) composition subjects NKN to a much larger temperature range in the tetragonal region (a range of up to 240°C larger), although this is still a formulation with mixed orthorhombic and tetragonal phase.
6 mol% Li doping compositions exhibit d33 = 175 pC/N, which is significantly higher than for pure NKN (110 pC/N)[114] and Pr = 20 μC/cm2 and Ec = 22 kV/cm.[121] In comparison with undoped NKN, the Pr value is significantly higher than for NKN (13.6
μC/cm2) but the Ec value is also significantly higher (undesirable) as it is 9.9 kV/cm for NKN.[93]
These results are significant, and show lithium to be a successful sintering aid for NKN. The Li+ sits in the A-site of the perovskite lattice, allowing the single phase to remain, and the grain size to significantly increase (from 1.3 μm for undoped NKN to 3.3 μm for 6 mol% addition (and an even higher value of 4.2 μm for 6.5 mol% addition).[114] This is also reported by Song et al. where the addition of Li increases the average grain size as a direct relationship to the amount of liquid phase found in the ceramic.[116] Similarly
Zhang et al. showed the addition of 5.2 mol% LiSbO3 (another Perovskite in the form
ABO3) enhances the P-E hysteresis properties up to Pr = 25 μC/cm2 and Ec = 18.5 kV/cm.[115]
The most promising material was reported by Saito et al. from the Toyota research group in Japan.[15] They produced a material doped with Li, Ta and Sb in the formulation
(K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 that had properties comparable to standard non- doped PZT [Pb(Zr0.52Ti0.48)O3]. Although limited information is given, this ceramic has a TC of approximately 260°C and a d33 ~ 300 pC/N. This formulation was further enhanced through texturing, which is discussed in Section 2.4.
The highest remnant polarisation of lithium doped NKN was published by Wu et al, where
5 mol% LiSbO3 addition to K0.4Na0.6NbO3 produced results of Pr = 30.8 μC/cm2 and Ec =
14.0 kV/cm.[122] The d33 value is also significantly higher at 280 pC/N.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 47 2.4 Texturing
2.4.1 Introduction to Texturing Texturing is the alignment of grains in the microstructure. An oriented material exhibits enhanced electrical properties as it behaves more like a single crystal. The orientation of a material is measured by the Lotgering factor, f, using Equations 2:20-22 from the sum of the main {h00} peaks and the sum all of the peak intensities for the oriented material against the same in the randomly-oriented material (P and P0, respectively).[123-124]
Equation 2 :20
Equation 2 :21
Equation 2 :22
2.4.1.1 Tape Casting The tape casting process has been developed for electronics requiring thin sheets of dielectrics to be produced at low cost and high quantity. There are three main tape casting processes: 1. The doctor blade process. This involves the ceramic powder to be converted to a slurry using binders and solvents, and then casting it onto a moving surface. A blade is positioned at a particular height in order to achieve a specific thickness. A flexible tape is produced, which can then be utilised differently depending upon the application. The general process is illustrated in Figure 2.35. This is the most utilised process for tape casting. 2. The “waterfall” technique. The slurry is dropped into a trough at a rate at which there is a perpetual “curtain” falling into a trough underneath (which then recirculates around up to the top again) and a substrate is slid across this slurry “curtain” to obtain a film of uniform thickness and is quickly dried once on the other side. This is illustrated in Figure 2.36. 3. The paper-casting process. Low-ash paper (the type results in different thicknesses of film) is used as a substrate and is rolled through a slurry, to which it gets wet and the slurry adheres to it, and is then passed through a heater to dry and is rolled up. The paper is later removed through firing.[2]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 48
Figure 2.35. Schematic of the general doctor blade tape casting process (from Richerson).[2]
Figure 2.36. Schematic Diagram for the waterfall casting process (from Richerson).[2]
The slurries created are usually created by adding binder, solvent and plasticiser to the ceramic powder. The binder is usually a long chain polymer to provide the film with internal flexibility and holds the ceramic slip together once dried; the solvent is added for the slip, so that it can be cast easily onto the tape, and the plasticiser is to provide flexibility of the tape (for easier handling).[125-126] The amounts added are dependent upon the desired thickness, the fabrication process and the nature of the ceramic powder. Deflocculants and homogenisers may also be introduced to the slurry, if needed for the slurry properties. All the additives must be able to be “burnt out” easily during sintering.
Resulting tapes are usually divided up, electroded (if necessary), stacked on top of each other, laminated through pressing, fired to burn out the binder and other additives, and finally electroded to attach to the relevant circuit.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 49 2.4.2 Texturing of Polycrystalline Ceramics There has been much interest in the texturing of electroceramics, due to the possibility of achieving enhanced electrical properties as the material approaches the characteristics of a single crystal.
A popular method of texturing ceramics is to incorporate oriented seed particles, which can promote the growth of grains in a particular orientation.[127-128] There are two common forms of oriented template particles: (i) needle-shaped acicular particles and (ii) plate-like particles (Figure 2.37).
Figure 2.37. SEM micrographs of (a) TiO2 acicular particles, and (b) plate-like BiT particles (after Jing et al.).[129]
In order to incorporate these particles into the ceramic, they are often mixed with the calcined ceramic powder to produce a slurry that can be tape cast (discussed in section 2.4.1.1) and sintered. Examples of texturing ceramics are outlined below.
2.4.2.1 Using BiT Particles for RTGG
Plate-like BiT (Bi4Ti3O12) particles are utilised in RTGG texturing of Bi0.5Na0.5TiO3 (BNT) and Bi0.5(Na,K)0.5TiO3 (BKNT)-based formulations.[129-135]
An example of BiT particle morphology is shown in Figure 2.37b. Hong et al. report BiT plate-like particles 3-20μm in diameter and 0.5 μm thick.[131] Very similar sizes (5-20 μm diameter and 0.5-0.7 μm thick) were reported by Motohashi and Kimura.[132] Jing et al. reported slightly smaller BiT particles of approx. 5 μm diameter and a thickness of less than 0.5 μm.[129] Fuse and Kimura, by contrast, produced three different sizes of BiT particles by sintering at temperatures of 950, 1050 and 1150ºC, yielding particle sizes in the ranges of 1-3, 5-15 and 20-30 μm, (and thickness 0.14, 0.3 and 0.6 μm), respectively.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 50 Fuse and Kimura do not specify the concentration of BiT particles added to
Bi0.5(Na0.5K0.5)TiO3 but kept the addition within the stoichiometric ratio of the product.[130] They reported that TiO2 was added as a sintering aid. Samples with BiT particle sizes of 5-15 μm and 70 nm sized TiO2 particles resulted in the highest Lotgering orientation factor, F, of approximately 90%. (The Lotgering factor is fully explained and defined using Equations 7:1 – 7:3 in Section 7.8.1.1). Additionally, the Lotgering factor increased with the sintering temperature, though using larger TiO2 particles of 150 nm did not achieve higher F factor than 60%. This shows that powder particle size is also an important factor in RTGG processing. This is agreed by Hong et al, who report that a fine particle size promotes densification, and at lower temperatures.[131]
Figure 2.38. SEM micrographs of BiT particles (a) and the resulting BNKT microstructure (after Jing et al,).[129]
Seni and Tani reported the successful orientation of using Bi4Ti3O12 (BiT) plate-like particles.[136] They report a 92% Lotgering factor using the RTGG method, however do not provide any evidence (such as XRD patterns) to prove this. A previous report by Tani does, however, include the XRD spectra demonstrating the orientation.[137]
In terms of electrical properties, the texturing of BNKT using BiT particles is advantageous. In terms of P-E hysteresis behaviour, Hong et al. reported a significant increase in the remnant polarisation of textured Bi4Ti2.996Nb0.004O12 (ie Nb-doped BNKT) tape in comparison to one produced using no template particles (and thus randomly oriented), where Pr = 24.5 and 12.4 μC/cm2, respectively.[131] The result is shown in Figure 2.39. Here, only 5% BiT particles were introduced to the matrix powder, and produced a Lotgering factor of over 96%. Although this is evident from Figure 2.39, Yilmaz et al. (the same research group) give contrasting statements in a publication three years afterwards.[138] Here they claim that randomly oriented ceramics are expected to give a higher Pr and lower EC value than TGG textured ceramics (near to the MPB composition). Although this was for a different
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 51 material (NBT-5.5BT), this is the opposite of what they reported for BNKT. Furthermore, in the same investigation, they report that BiT particles cannot orientate NBT-5.5BT
(94.5% (Na0.5Bi0.5)TiO3 - 5.5% BaTiO3) successfully.[131]
Figure 2.39. Hysteresis behaviour of Nb-doped BNKT thick films, showing results for randomly oriented and textured (using BiT particles) (after Hong et al.).[131]
2.4.2.2 Using SrTiO3 Particles Yilmaz et al. report that the Lotgering factor of BNT-BT materials increases with sintering time.[139] Having added 5 vol% SrTiO3 (ST) platelets to BNT-5.5BT, a Lotgering factor of 70% was seen after 1 hour, which significantly increased to 94% after 12 hours sintering at the same temperature (illustrated in Figure 2.40a). This shows that the sintering time enhances good texturing when the main driving force is the small matrix grains reacting with the oriented template particles in the initial stages. This rapid rate from the first hour decelerates significantly as the sintering time increases, although the orientation continues to steadily increase to 96% after 12 hours. The rate is slower due to grain coarsening and grain impingement.
(a) (b)
Figure 2.40. (a) The orientation of NBTBT as a function of sintering time, and (b) The
diffraction patterns for randomly oriented (blue) and 5% added SrTiO3 template particles (pink)in NBTBT (after Yilmaz et al).[139]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 52 After 12 hours, the texture in the NBTBT sample is clearly evident (Figure 2.40). The XRD spectra are significantly different for randomly orientated and for the textured samples. The two are shown in Figure 2.40b. The significant rise in the {200} peak and decrease of the {100} peak is a clear indication of successful orientation in the sample. This is a desired result, and so shows that ST template particles are viable for orienting NBTBT.
2.4.2.3 Using TiO2 Particles
TiO2 particles are different to the BiT and SrTiO3 particles described previously. In contrast to their plate-like morphology and orientation, TiO2 particles are acicular (needle-shaped) and fibrous; their morphology is shown in Figure 2.41a. Jing et al. reported these to be 2-15 μm long with a diameter of 0.2-2 μm.[129] These are similar dimensions to those reported by Sato, Yoshida and Kimura who reported length and diameter to be 10 and 0.3 μm, respectively.[140]
Sato et al. utilised acicular TiO2 particles which are aligned in the <001> directions.[140] A sintering temperature of 1200°C was needed in order to produce equiaxed grains (as opposed to needle shaped grains) in RTGG of BaTiO3. However these grains are very small (1-2 μm) and did not lead to high density. Cold Isostatic Pressing (CIP) was required in order to reach 96% theoretical density and a desireable microstructure with grains of size over 50 μm. This was due to the disintegration of the whisker particles into equiaxed grains at sintering temperatures above 1200ºC. The process of CIP, however is undesirable as it increases the time and cost of production, especially when other methods have been proven to be successful without this extra step. The resulting electrical properties of this material are not reported, so cannot be compared to other RTGG processes.
Figure 2.41. TiO2 template particles, and the resulting BNKT microstructure (after Jing et al).[129]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 53 Jing et al. utilised TiO2 particles for the RTGG production of BNKT.[129] In this case, they reported the whiskers to produce very low orientation, of maximum 26.4% with a 40% template addition (up to 80% template particles were added in this investigation). The optimal resulting microstructure using TiO2 particles is shown in Figure 2.41b. In comparison with the microstructure for the identical BNKT material utilising BiT particles
(Figure 2.38b), it is clear that utilising TiO2 particles was not successful for BNKT production. This is due to the diffusion of TiO2 into the matrix grains, whose stability is questioned in the work of Sato et al.[140] By investigating this in parallel with the same methodology using BiT particles (previously discussed in section 2.4.2.1), Jing et al. came to the conclusion that the utilisation of template particles in RTGG must meet the following three criteria: Anisotropy – there must be a large amount of anisotropy in the particle, and especially in comparison to the raw powders Low reactivity – there must be a low reactivity of the particles in order to avoid dissolution of the particles into the matrix grain (and thus forming undesired secondary and/or intermediate phases) Crystallographic similarity – the particles must have some order of similar crystal structure with the final ceramic product to ensure a homogenous and complete transformation during sintering.[129]
2.4.3 Texturing of NKN
2.4.3.1 Using BNN and NN particles The orientation of NKN is a relatively new approach, and few research groups have investigated this method. The first research group to publish any data regarding textured NKN was Toyota in Japan in 2006.[16] Takao et al. reported a textured NKN sample with a Lotgering factor, F, of 96% (though the graphic shows 93%) with 1mol% CuO doping.
This was done by using 5 at% NaNbO3 template particles in the tape casting slurry. It is reported that during sintering at temperatures above 1000ºC, the template particles react with the formed NKN matrix powder at the oriented particle’s surface, and so orientation of the tape occurs, as the temperature continues to increase. They found that between 1050 and 1075ºC orientation increases from 22 to 75%. The optimal results were found using a sintering temperature of 1100ºC for one hour in an oxygen atmosphere, giving a density and orientation of over 95%.
The production of NN particles is a two-stage process, and all research groups employing NN particles as a template for NKN orientation all report this process (with slightly
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 54 different conditions). First, oriented Bi2.5Na3.5Nb5O18, BNN, particles are produced using the molten salt synthesis (MSS) approach (where powders react and sinter in a molten saline environment). This is done using Bi2O3, Na2CO3 and Nb2O5 raw powders, in the stoichiometric ratios as shown in Equation 2:20[141] and synthesised at temperatures above 850ºC in a salt flux. The synthesised BNN particles are, after multiple hot washing, then reacted with Na2CO3 in order to produce a topochemical conversion process, as shown in Equation 2:21[141] which results in {100} oriented NN particles to be produced, again using the MSS approach.
5 Bi2O3 + 7 Na2CO3 + 10 Nb2O5 4 Bi2.5Na3.5Nb5O18 + 7 CO2 ↑ Equation 2 :23
4 Bi2.5Na3.5Nb5O18 + 3 Na2CO3 10 NaNbO3 + 5 Bi2O3 + 3 CO2 ↑ Equation 2 :24
Bi2O3 + 6 HCl 2 BiCl3 + 3 H2O Equation 2 :25
The topochemical reaction involves a weak covalent network of [Bi2O2]2+ in between large layers of Perovskite [(Bi0.5Na3.5)Nb5O16] – which are based around Nb2O6 octohedra. As the
[Bi2O2]2+ layers are broken down, and converted to Bi2O3, the Bi3+ A site vacancies are replaced by Na+ ions from the Na2CO3. This results in a strong homogenous NaNbO3 morphology to be formed.[118] This conversion reaction is illustrated in Figure 2.42.
Figure 2.42. Schematic diagram of the BNN topochemical conversion reaction to NN (from Yan et al).[118]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 55 In 2007, a group led by Yan et al. in China reported the production of NN template particles.[118] This publication is significant, as it is the first one to report a full experimental procedure of the two-stage process; however some minor details are omitted. The BNN and following NN particles were produced in a similar method to the Toyota group, though there are more details of the process. They also explain that the final hot washing procedure is done by hot washing with alternative hot de-ionised water and HCl. This not only washes away the salt from the MSS procedure but also the Bismuth
Bi2O3 bi-product by washing with HCl (shown in Equation 2:22).
Yan et al.[118] also reported that there are other BNN compounds that can be produced, and so the production of the desired BNN depends upon the synthesis conditions and must be monitored. Other possible formulations that have been detected include
Bi2O2[(Bi0.5Na0.5)Nb2O7] and Bi2O2[(Bi0.5Na2.5)Nb4O13]. It is further reported, however, that the presence of small amounts of these two compounds is not detrimental to the second stage of NN particle production process, as these both also easily transform into NN.
Another group from China led by Feng looked at BNN and NN particle production for the orientation of BT and NBT (Na0.5Bi0.5TiO3).[127] Their production of BNN used the same conditions, although the NN conversion conditions are not reported. The resulting template particles were significantly different than those reported by Takao and Saito;[16] they produced particles 20μm wide (slightly larger) however with ten times the thickness of 5μm. This is possibly due to using an alumina crucible as opposed to the platinum crucible used by the other two groups, however full production conditions were not specified.
Feng et al also reported an optimum of 30 wt% addition of template particles in order to orient BT).[127] They found 10 wt% addition led to an orientation of 0.32; this increased to 0.62 with 30 wt% particles. Although these figures cannot be directly compared to behaviour in NKN due to the possible differences in orientation process in the two Perovskite materials, it can be assumed that the difference are affected in some way by the different morphology of the template particles.
Table 2–2 shows a summary of the optimal processing conditions and results for BNN particle formation. This amalgamates all the above research for a quick reference.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 56 Table 2–2. Summary of properties and processing of BNN particles (optimal results) with references.
Particle size / Tsint (ºC) / Salt Salt : oxide Ref. Authors Year thickness (μm) time (h) medium ratio
[16] Takao et al. 10-15 / 0.5 1100 / - NaCl - 2006
[142] 2008 Chang et al. - 1125 / 6 NaCl 3:2 [123] 2009
[141] Zhang et al. 15-30 / 0.6-1.0 1130 / 4 NaCl 1:1 2009
NaCl + [127] Feng et al. 20 / 5 1100 / 2 1:1 2008 KCl [124, NaCl + 2008 Gao et al. 5 / 1 1100 / 2 1:1 143] KCl 2010 850 / 1, [118] Yan et al. 15 / 0.5 NaCl 1:2 2006 1100 / 2
2.4.3.2 Using KN Particles
KNbO3 (KN) is one of the end members of NKN, and so is a very good candidate for a precursor for oriented NKN. Saito and Takao first published this new processing route in
2007, reporting the synthesis of KN particles by forming K4Nb6O17 initially, and subsequently forcing a topochemical microcrystal conversion with K2CO3 to form KNbO3 (in a similar process to the formation of NN from BNN particles).[144]
Figure 2.43. XRD diffraction profiles for 1 mol% CuO doped NKN (a) randomly oriented (Tsint
= 1050ºC) and (b) with 10% KN particles added (Tsint = 1175ºC) (from Saito and Takao).[144]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 57 Having added 10% KN in a stoichometric ratio, Figure 2.43 shows how the orientation in the ceramic changes the ratio of the {110} and {111} peaks. This change brings about a Lotgering factor of 39.7% orientation along the {001} direction. The 1 mol% CuO-doped NKN containing the precursor particles requires a higher sintering temperature (1175°C as opposed to 1050ºC) though does significantly modify the electrical properties. Texturing increases the dielectric constant from 220 to 306, even though the density decreases from 99.2 to 90.5% theoretical.
2.4.4 Summary of Texturing Piezoelectric Ceramics In order to promote orientation of a ceramic, tape casting should involve template particles, where the templates are already oriented in a particular direction. These are then used as seeds to promote growth in a particular crystallographic direction of the matrix grain. The following points must be considered in determining a suitable template and method in order to achieve advantageous results: 1. Template particle material. Kimura et al. report that it is vital that the precursor is anisotropic and contains elements that are in the final product composition.[133] Without this, diffusion between the matrix and templates during sintering will not take place easily, thus destroying effective sintering and texturing. Also, the particle material must have low reactivity, as Jing et al. and Sato et al. reported the
diffusion of TiO2 into the matrix lattice, thus destroying the morphology of the template particles.[129, 140] 2. Template morphology. The shape of the template particle will affect the growth of the matrix grain; hence necessary to make a choice between acicular (needle- shaped) and plate-like particles. Jing et al. showed that the production of oriented BNKT was significantly more successful using plate-like BiT particles than using
acicular TiO2 whiskers.[129] 3. Template particle size. There is a correlation between the powder particle size and the template particle size for successful texturing. Fuse and Kimura investigated
three different sizes of BiT particles and two sizes of TiO2 powder particle size.[130] They reported that the optimal orientation and microstructure depended upon the template grain surface area and matrix grain size; these are the factors controlling adhesion between the two, and thus determine the growth rate of the template grains. 4. Sintering Temperature. Fuse and Kimura reported that the Lotgering factor increases with increasing sintering temperature.[130] 5. Sintering Time. Yilmaz et al. report that in order to reach a Lotgering factor of over 90%, a sintering time of 12 hours is needed for ST templated NBTBT. However
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 58 after only one hour, a factor of 70% can be achieved.[139] After this initial accelerated orientation, further texture enhancement slows due to grain impingement and coarsening. This is in concurrence with Zhao, Zhou and Yan in their investigation with BiT particles in BNKT.[134] This, then, is a factor that is time and resource dependent. 6. Amount of Particle Addition. The amount of particles to be added has been highly investigated. Hong et al. report excellent results for BNKT using only 5% BiT particles (and a resulting Lotgering factor of >96%).[131] In the case of NBTBT, the optimal addition of BNN particles is 30 wt%, though this only produces a Lotgering factor of 62%.[127] 7. Slurry dispersion. Kimura et al. report that the powder particles must be well dispersed within the slurry; this can be ensured through the use of a dispersant.[133] If there is not sufficient homogeneity in the mixing of the slurry, heterocoagulation (agglomeration of particles) can occur.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 59 3 EXPERIMENTAL PROCEDURES
3.1 Raw Powders The starting powders, including the manufacturers and the purity, are presented in Table 3.1 below. EC (Enzyme Comission) and CAS (Chemical Abstracts Service) registry numbers are also provided as these are a commonly accepted standard used for chemicals. The sodium and potassium carbonate powders were stored in covered glass beakers in an oven at 100C at all times, due to their susceptibility to deliquescence in moisture. All other starting powders were stored in the manufacturer’s plastic bottles in a cool, dry cupboard until use. Table 3–1. Details of raw starting powders Compound EC No. CAS No. Manufacturer Purity
Na2CO3 207-838-8 497-19-8 Sigma Aldrich (Fluka) 99.8 %
K2CO3 209-529-3 584-08-7 Sigma Aldrich (Fluka) 99.0 %
SrCO3 216-643-7 1633-05-2 Sigma Aldrich 99.9+ %
BaCO3 208-167-3 513-77-9 Sigma Aldrich 99.98 %
Nb2O5 215-213-6 1313-96-8 Sigma Aldrich 99.9 %
Fe2O3 215-168-2 1309-37-1 Alfa Aesar 99.945 %
Bi2O3 215-134-7 1304-76-3 Sigma Aldrich (Fluka) 98.0 % NaCl 231-598-3 7647-14-5 Sigma Aldrich 99.5%
3.2 Sample Preparation
3.2.1 Mixed Oxide Processing Route
The starting powders of Na2CO3, K2CO3 and Nb2O5 were used to produce (Na0.5K0.5)NbO3, which will from here be called by its acronym of pure NKN. They were mixed with SrCO3 and BaCO3 in order to make (100-x)NKN-xSBN compositions, where SBN is an acronym for the composition (Sr0.5Ba0.5)Nb2O6.
The amounts of the oxides of Na2O, K20, SrO, BaO and Nb2O5 required to produce stoichiometric NKN and (100-x)NKN-xSBN composition were calculated on the basis of their molecular weights to produce 50g and 100g batches. These amounts were weighed using a Ohaus Adventurer Pro AV213 electronic balance (0.0005g), before being put into a plastic bottles. For milling, each batch of powder had an equal ratio of powder mass to weight of zirconia balls (of diameter 6mm) and the same amount in cubic centilitres of propan-2-ol as a reagent. These batches were vibratory milled for 20-24 hours, then dried in an oven at 90C for 12 hours and the zirconia milling balls subsequently removed.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 60 The resulting powder was then transferred into an alumina crucible, covered with an alumina tile, and calcined in a Carbolite Chamber Furnace at 850-1000C for 4 hours, with a heating and cooling rate of 180C per hour. The calcined powder was subsequently mixed with 0.0-0.6wt% additive, such as Fe2O3 powder, as a sintering aid, and re-milled in a plastic bottle, again with the 1:1:1 ratio of powder to zirconia balls to propan-2-ol. After drying, the resulting powder was stored in a glass bottle in an oven at 100C until required.
A flow chart of the processing route is illustrated in Figure 3.1. Table 3.2 shows the formulations that were prepared in this study.
Figure 3.1. Flow chart illustrating the processing route and characterisation techniques used for this mixed oxide route study
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 61 Table 3–2. Samples prepared in this study. % NKN in composition % SBN in composition wt% addition of additive 100 0 0 -
100 0 0.2 Fe2O3
100 0 0.3 Fe2O3
100 0 0.45 Fe2O3
99 1 0.3 Fe2O3
99 1 0.45 Fe2O3
98 2 0.45 Fe2O3
97 3 0.45 Fe2O3
96 4 0.45 Fe2O3 96 4 0.3 ZnO 96 4 0.3 NiO 90 10 0 -
An Ohaus Adventurer Pro AV213 electronic balance was used to weigh out 5g batches of the composition powders. They were then transferred into a steel die and pressed uniaxially into compacts 20mm in diameter and approximately 16mm in height, using a load of 0.8-0.9MPa. The compacts were placed on platinum foil (to ensure there were no reactions between the ceramic powder and the furnace substrate), covered by an aluminium crucible, and sintered in air using a Vecstar Chamber Furnace at 1050-1160C for 4-72 hours (with a heating and cooling rate of 180C/hr).
The (100-x)NKN-xSBN compositions are denoted as xSBN; for example 98NKN-2SBN will be denoted as 2SBN. The 0.45wt% iron-doped samples will be denoted as xSBNF, such as
97NKN-3SBN + 0.45wt% Fe2O3 will be denoted as 3SBNF.
3.2.2 Orientation Casting Tape casting is a processing technique used to produce ceramics with a preferred orientation with respect to one particular direction. In order to achieve oriented grains in the NKN material, pre-oriented template particles needed to be produced. The experimental procedure is based upon a patent by Toyota in Japan[145] that has already been used by other research groups in Asia.[15, 118, 123, 127, 141-143] This is a two- stage process. First Bi2.5Na3.5Nb5O18 particles are produced, from which NaNbO3 particles can be formed by a topochemical reaction. The experimental procedure has been altered from those in other publications in order to achieve the optimum template particle results. A flow chart of the following procedure is given as Figure 3.2.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 62
Figure 3.2. Flow Chart illustrating the processing and characterisation techniques used for this tape casting study
3.2.2.1 BNN Particle Production
Bi2.5Na3.5Nb5O18 (hereafter named BNN) particles are produced by mixing Bi2O3, Na2CO3 and Nb2O5 to their stoichiometric BNN formula; the chemical equation is below as Equation 3:1.[141]
5 Bi2O3 + 7 Na2CO3 + 10 Nb2O5 4 Bi2.5Na3.5Nb5O18 + 7 CO2 ↑ Equation 3 :1
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 63 The powders were mixed together with a 1:1 ratio of powder mass to zirconia balls (of diameter 6mm) and an equal amount in cubic centilitres of propan-2-ol as a reagent, and vibratory milled for 18-20 hours. Once the mixture was left overnight to dry in an oven set at 90°C, a weight for weight ratio of NaCl was added to the resulting powder and further vibratory milled for 1 hour, again in propan-2-ol. This mixture was then left in the oven set at 90°C to dry overnight. Once dry, the resulting powder was taken in 2g batches and put into a platinum crucible sealed with an alumina lid and heat treated in a Vecstar Chamber Furnace for 1050-1100°C for 2 hours (heating rate 300°C/hour, cooling rate 180°C/hour).
3.2.2.2 Hot Washing Once complete, the platinum crucible was placed into a beaker of deionised water, and sonicated for 30 minutes at approximately 65°C. Light stirring was applied from a glass rod to ensure complete dissolution of the resultant powder. The resulting hot solution was then used for hot washing. The set up for this is shown in Figure 3.3.
Figure 3.3. Schematic showing set-up of Hot Washing Procedure.
The hot solution was poured into a Buchner funnel with a 24mm filter base, which has a Whatman 25mm diameter GF/C glass microfibre circular filter (thickness 1.2μm) placed on it. The conical flask under the Buchner funnel was under vacuum so that the filtration is accelerated. The water of the solution , along with the NaCl salt that was dissolved in it was filtered through, leaving the BNN particles behind. These were lightly scraped off the filter paper using a microspatula and placed into a beaker. These again were dissolved in deionised water, lightly stirred with a glass rod and again filtered using a new filter if necessary. Filters were usually changed every 1-2 filtrations. After the 8th filtration, the resulting filtered water product was tested for salt using diluted silver nitrate (AgNO3). If Cl- ions are present, then the water turns cloudy.[143] Eight hot washes were sufficient to show all the NaCl had been filtered away. The resultant particles were placed in a small glass vial and placed in the oven set at 100°C in order to dry for further use.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 64 3.2.2.3 NN Particle Production The BNN particles were taken from their storage oven and mixed with the formulaic ratios of Na2CO3 and NaCl as a flux according to the formula in Equation 3.2.2.[141] The powders were lightly mixed together in a platinum crucible using a glass rod, in order to minimise aggravation of the BNN particles. The crucible was covered with alumina and put in a Vecstar Chamber Furnace at 950°C with a soaking time of 4-8 hours (heating rate 300°C/hour, cooling rate 180°C/hour).
4 Bi2.5Na3.5Nb5O18 + 3 Na2CO3 10 NaNbO3 + 5 Bi2O3 + 3 CO2 ↑ Equation 3 :2
The resultant mixture then underwent alternate hot washing, which is described in section 3.2.2.2, and washing in HNO3 (1N) in order to remove the BiCl3 bi-product of the topchemical conversion, as per Equation 3.3.[141] After successful subsequent hot washing, the resultant NN particles were left in a glass beaker in an oven set at 100ºC in order for the water to evaporate and the NN particles remained.
Bi2O3 + 6 HCl 2 BiCl3 + 3 H2O Equation 3 :3
In order to avoid particle agglomeration in samples (discussed in Chapter 7) NN particles were mixed with dispersant Dispex A40 by Siba. The particles were held in a beaker, having been dried in an oven, and added 2 drops along with approximately 50ml de- ionised water. They were then sonicated for 20 minutes and then left in the same oven set at 100°C in order for the water and solvent to dry.
3.2.2.4 Oriented NKN Production The flow chart for the process is shown as Figure 3.4.
Powders of composition 95NKN-5LN and 94NKN-6LN, with varying additives of 0, 0.4 and 0.8wt% CuO added as a sintering aid, were prepared using the mixed oxide route described in Section 3.2.1. This is to fulfil the formulation (100-x)NKN-xLN + y wt% CuO, which will now be denoted as xLN + y wt% CuO (x = 5, 6 and y = 0, 0.4, 0.8).
Small batches of powder were utilised as the techniques and volumes of binder, plasticiser and solvent were being refined throughout the investigation, which is discussed in Chapter 7. The optimal route is described here.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 65
Figure 3.4. Flow chart for the production of templated NKN-based oriented tapes
In order to produce a successful tape for casting, a binder, solvent and plasticizer is required. The manufacturer’s data are presented below in Table 3–3. The binder is Polyvinyl Butyral (PVB) and the solvent is a combination of 45% Ethanol and 55% Toluene in solution. The plasticizer is Dibutyl Phthalate, which is used to prevent brittleness in the tape for easy removal from the substrate.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 66 Table 3–3. Manufacturer’s data for slurry additives
Compound EC No. CAS No. Manufacturer Purity PVB (Butvar B-98) - 63148-65-2 Sigma Aldrich (Fluka) - Toluene 203-629-5 108-88-3 Fisher Scientific - Ethanol 200-578-6 64-17-5 Fisher Scientific - Dibutyl Phthalate 201-557-4 84-74-2 Sigma Aldrich ≥ 99%
A batch of 0.6g pre-calcined copper-doped xLN powder was placed into a MAX 100 plastic jar (made for the Hauschild SpeedMixer DAC 150FVZ-K). When necessary 10 or 15 wt% NN particles were added to the powder and mixed gently with a glass rod in order to prevent damage to the particles. The slurry additives were then added to this powder mixture, in order to be mixed. The optimal ratio of powder to slurry additives was 60% powder to 40% slurry additives. This led to the addition of 0.03g binder, 0.35ml solvent and 0.056g plasticiser to every 0.6g pre-calcined powder. The mixture was mixed with a glass rod for 10 seconds and then the jar was sealed with a lid and placed in the SpeedMixer at 2200 rpm for 30 seconds. The resulting slurry was then ready to be cast.
3.2.2.5 Tape Casting Techniques Thick film tapes (50-300μm thickness) were cast using two methods. The first method was using a steel die. The die measured 100 x 50 x 15mm, with a removable 20 x 55 x 14.9mm steel block that sits inside. This created a 100μm track within the steel block. In the same way as the glass slide casting method, the slurry was poured onto one side of the track, and a slide was used to drag the slurry across and create a 100μm tape. This is illustrated in Figure 3.6. As the block being cast onto was removable, the tape was easy to remove using a razor blade to slide under the tape.
Figure 3.5. Schematic illustrating the Mechanism of Metal Die Casting.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 67 The second method of casting was using a glass slide with masking tape attached to either side (lengthwise) in order to create a track approximately 100μm thick for casting the slurry. The slurry was poured onto one side of the track, and using another glass slide, pushed along the track to form a tape. This method is illustrated in Figure 3.5. The tape was easily removed by removing the masking tape from the glass slide, and then running a razor blade under the tape to remove it from the glass slide.
Figure 3.6. Schematic Diagram of Glass Slide Casting.
In order to investigate thicker films, once one tape was cast and had dried, another layer of masking tape was added on top of that already present, and then another drop of slurry was added on top of the dried cast tape, and cast again. This tape measured approximately 200μm thickness and was also investigated.
Once the tapes were dry on the glass and metal slides, each tape was sliced into approximately 1cm2 sections and peeled off using a stainless steel razor blade. These sections were placed onto platinum foil ready to put into the furnace. As the investigation continued, the cut tapes were placed on top of each other using tweezers, then placed in between two highly ground, polished and surface ground (using a Jones & Shipman 1400X Surface Grinder) 50 x 50 mm stainless steel plates that were made specially for this procedure. The tapes were pressed at 10kg/cm2, peeled off the bottom plate carefully using a razor blade, and then placed onto platinum foil ready for sintering. The sintering was undertaken in the same way as in the mixed oxide route, explained in Section 3.2.1, at temperatures 1030-1150°C (±180°C/hour) on a platinum foil substrate, and covered with an alumina crucible.
3.3 Characterisation Techniques The starting powders were characterised in terms of structure and grain size.
3.3.1 Density Measurement The diameter, and thus radius, r, and height, h, dimensions of the centre of each sample were measured using a micrometer screw gauge (0.0005mm). These dimensions are
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 68 used to find the volume, V, of each sample. The sample mass, m, was measured using an Ohaus Adventurer Pro AV213 electronic balance (0.0005g). The density of the sample was calculated using Equation 3:4.[2]
m m V r2 h Equation 3 :4
In order to find the theoretical density, T, of each material, Equation 3:5 was used, using the number of formula units per cell, Z, the molecular mass, M, the unit cell volume, V, and
Avogadro’s constant, NA = 6.023 x 1023 molecules/mol).
Z.M Equation 3 :5 T N AV
The relative density, r, is defined as the ratio of calculated density to the theoretical density in order to give a percentage of sample densification, given in Equation 3:6.
c r Equation 3 :6 T
______
For the following characterisation techniques, the sample was cut into discs of 1-2mm thickness using a circular diamond wheel. This created sample discs ready for analysis.
3.3.2 X-Ray Diffraction (XRD) Procedures X-ray diffraction techniques are used to identify the phases present in powders and sintered ceramics, and to determine lattice parameters. X-rays have high energy and short wavelengths, λ, on the scale of the atomic spacing. The operational principle of XRD is that a beam of X-rays are directed at the surface of a sample. In the case of a simple crystalline solid, the beam can be considered impacting on a series of atomic lattice planes (Figure 3.7). Here the parallel X-rays that are in phase with each other (1 and 2) are directed from a beam at an angle, θ, onto a material with regular crystalline atomic spacings and lattice plane spacing depth, dhkl, as illustrated in Figure 3.7. The X-rays are both diffracted and absorbed, however it is the diffracted ones that are considered in this instance. Atoms P and Q (on Figure 3.7) diffract the X-rays, both at angle θ to the atomic plane. The path length of incident beam 2 is longer than that of beam 1 due to the depth of the second layer of atoms (row B). This means that the beam has a longer path length through S-Q-T on the diagram. If S-Q and Q-T both have a distance dsinθ, the extra path length of beam 2 is defined as 2dsinθ.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 69 n 2dsin Equation 3 :7
If this extra path length is the same length as one, or a complete amount of wavelengths, nλ, then the diffracted beams will be in phase with each other and thus constructively interfere creating superposition of the peaks, given in Equation 3:7, known as Bragg’s Law.[146] This means that there is a high intensity of diffracted beams at the detector at that point. A detector is positioned on the opposite side of the sample, in order to record the intensity of the diffracted beams. The angle between the X-ray source producing the incident beams and the plane of the sample surface is θ (as illustrated in Figure 3.7) and this is the same angle on the other side between the surface of the sample and the detector. This results in the detector recording all data in terms of 2θ. As 2θ increases, the constructive and destructive interference of the diffracted beams, depending upon the atomic structure of the sample, will record a particular intensity at the detector for each 2θ step, resulting in a peak profile. It is through this peak profile that the atomic structure of a material can be calculated.
Figure 3.7. Theoretical Illustration of X-Ray Diffraction (from Callister).[43]
During this research study, all XRD undertaken was in the form of powder diffraction. All samples were examined this way in order to determine the composition without internal strains in the sample. Raw oxide and calcined powders were used in their original state, slightly ground to ensure an even fine powder distribution. For sintered samples, a section of the sample bulk was crushed to a fine powder using a pestle and mortar. A thin layer of silica paste was applied to an amorphous silicon disc for adhesion, onto which the powder was applied to achieve a thin even layer of powder for XRD characterisation. Any excess powder was removed from the disc. A Philips X’Pert-MPD θ-θ Diffractometer was used for phase determination, using a Cu-Kα X-ray radiation tube. It had a beam of wavelength λ = 1.54060Å, a current of 40 mA and a 45kV voltage. The diffraction scanned in 0.050º steps between scanning angles of 10° and 80° 2θ. The beam and detector were held for 15 seconds at each step in order to achieve a high resolution.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 70 The resulting XRD data was analysed using several methods. Peak profiles were matched on the PANalytical X’Pert HighScore Plus software using existing stick pattern data from the International Centre for Diffraction Data (ICDD) database. This confirms a single material phase, or can identify the possible various phases present in a sample. This is done by inputting the possible elements in the sample, and searching for all inorganic compounds with these elements with similar peak profiles. These are then scrutinised by eye, in order to match the most suitable and realistic published stick patterns to the observed data. Another method of XRD analysis was using the TOPAS program to undergo Rietveld refinement. This procedure is explained in Section 3.3.5.
3.3.3 Synchrotron Energy Dispersive Powder Diffraction Synchrotron data was collected at two different Synchrotron radiation sources. The technique involved in analysing synchrotron data works on the same principle as XRD. However, as a synchrotron source generates a much higher intensity of X-rays, this higher intensity allows a better resolution and thus makes Rietveld Refinement (see Section 3.3.5) more accurate.
An electron gun in high vacuum and electric field emits a beam of electrons through a linear accelerator, shown as section 1 in Figure 3.8. The beam then enters a booster ring (section 2), where the beam is accelerated by a radio frequency voltage source. Electromagnets provide the necessary forces to allow the accelerating beam to orbit around the ring repeatedly until the beam is at a high enough speed (near to that of the speed of light) and is subsequently released into the main synchrotron storage ring (section 3). Here the beam orbits in a circle, again its path bent by high field electromagnets. As the direction of the beam changes, the beam is accelerated and thus emits high energy photons, emitted as X-ray beams released at a tangent to the direction of travel. This allows many beamline stations (section 4), each with a high energy X-ray beam, to be used simultaneously for experimentation. The high intensity of the X-ray beam can be filtered down to a very small wavelength (at Diamond this is done in the Optics Hutch (section 7)). This small wavelength allows for a larger penetration depth in a sample, in comparison with laboratory XRD (as explained in Section 3.3.3).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 71
Figure 3.8. Schematic Diagram of Diamond Synchrotron (Oxford, UK), with its main components highlighted.[12] (1)Electron gun and linear accelerator; (2) Booster Synchrotron; (3) Storage Ring; (4) Beamlines; (5) Front End; (6) Optics Hutch; (7) Experimental Hutch; (8) Control Cabin; (9) Radiofrequency (RF) Cabin; (10) Diamond House
3.3.3.1 Daresbury Synchrotron Data Collection Initial data collection was collected at Station 6.2 at Daresbury Synchrotron Radiation Source (SRS), UK. This was a Powder Diffractometry Station, which for this study was especially set up for in situ data collection. The sample was placed in an alumina crucible on a stage inside an enclosed MRI (Materials Research Instruments) TC-Basic furnace,[147] shown in Figure 3.9. The furnace was tilted to 11° 2θ and sample height adjusted (using a integrated micrometer underneath the stage) to ensure the beam reached the sample surface exactly. The sample was then heated via a filament underneath the stage whilst simultaneously recording diffraction profiles for the material.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 72 sample
filament thermocouple
Figure 3.9. Diagram of inside MRI TC-Basic furnace utilized for experiment at Daresbury SRS, station 6.2 (after MRI).[147]
Each sample was run with the radiation beam at wavelength λ = 1.40000 Å, with the furnace set to increase temperature at a rate of 2ºC/minute between 20 and 550°C, with
38 sample scans per minute, for the first sample, 99.5NKN-0.5CuO + 0.6wt% Nb2O5 and subsequently 3°C/minute for the remainder of samples. The samples investigated are given in Table 3–4.
Table 3–4. Samples under Investigation at Daresbury SRS, Station 6.2.
Sample Formulation Sample State
99.5NKN-0.5CuO + 0.6wt% Nb2O5 Solid Pellet
99.5NKN-0.5CuO + 0.6wt% Nb2O5 Powder 96NKN-6LT Powder Data was extracted and normalized to find the d-spacing for each channel on the detector, the capillary scan was subsequently subtracted. The beam intensity slowly decreased throughout the day at Daresbury SRS, and thus this had to be normalized as well. This was done by determining the base ion reading for each scan, finding the average intensity and dividing the base ion reading by the average intensity. This yields a correction for the change in background intensity. The corrected data was then analysed using Microsoft Excel (2003) and TOPAS (v.4.2) for refinement (see Section 3.3.4).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 73 In order to directly compare the results, data for every four scans (that is, covering every 12ºC for Section 4.2.1 and 8ºC for the subsequent investigations) were collated and the intensity values had a multiple of 2,000 added; they were then plotted against 2θ (in order to view the profiles one above another). By taking the initial values of every four scans, and creating a .xy file of 2θ and intensity, the file could be opened in TOPAS (v4.2) ready for refinement. As the calculated 2θ values were in error by a small amount, an hkl phase was obtained using the double peak at 22°C. This hkl phase was determined to have an orthorhombic structure, which was expected. By running an initial refinement, the calculated model gave a very good fit with the observed data. By applying this hkl phase to other refinements with similar peak shape, an accurate and close fit was obtained. When the peak shape changed significantly, a new hkl phase was determined in the same way, until a close fit was again found, and refinements were obtained as before.
3.3.3.2 Diamond Synchrotron Data Collection The second Synchrotron source was Diamond Light Source in Oxford, UK. Here Beamline I11 (BL-I11) was used for powder diffraction. A schematic representation of this beamline is illustrated in Figure 3.10.
Figure 3.10. Schematic Representation of Beamline I11, showing approximate distances of main components from source (from Thompson et al.).[13]
The finely crushed powder samples were fed into a borosilicate glass capillary and mounted onto a brass holder provided by BL-I11 set up to be rotating in the path of the beam in the diffractometer. The wavelength of the beam was 0.826988 Å; it was set to scan for one hour at room temperature, between the angles of 5 and 70º two-theta in 0.01 millidegree 2θ steps. Resulting data was converted to .xls and .xye files, opened in Microsoft Excel (2003) for initial peak comparison and then TOPAS (v4.2) for refinement, respectively. The samples that were investigated at this station are shown in Table 3.4. All samples were in powder form, where a section taken from the bulk of the sample was finely crushed using a pestle and mortar.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 74 Table 3.4 Samples investigated at Diamond Light Source, BL-I11.
Sample Formulation of (100-x)NKN-xSBN + 0.45wt% Fe2O3 x Sintering Temperature 0 1100°C 0.5 1100°C 1 1100°C 2 1140°C 3 1140°C 4 1140°C
3.3.4 Rietveld Refinement The program used for Rietveld refining XRD and Synchrotron profiles in this investigation was TOPAS, initially version 3.0 and subsequently version 4.2. It can be used alongside the ICSD (Inorganic Crystal Structure Database) and is widely used for structure analysis and refinement.
The initial structure file for NKN was created using the structural information for KNbO3 published by Kumada et al in 2007.[148] No Na0.5K0.5NbO3 structural data was available until 2010. By creating this structure file and applying it to a synchrotron diffraction profile of undoped NKN; the slightly adjusted parameters were saved as a new structure file for NKN. This new .str file was applied to synchrotron and laboratory XRD data of all other formulations in this investigation for refinement.
The theory of the refinement of crystallographic profiles was conceived and developed by Rietveld in the 1960s. There was a difficulty in the refinement of overlapping reflections, and so he employed a least mean squares refinement program, adapted to recognise and obtain intensity values from overlapping peaks. Rietveld, after two less successful publications,[149-150] published a paper in 1969 successfully describing the parameters that are needed for successful refinement.[151] It is defined as the least mean squares rule, given as Equation 3:8, where wi is the weight of each observation point, yi (obs) is the observed intensity of each step, and yi (calc) is the calculated model intensity at each step.
Equation 3 :8
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 75
Figure 3.11. An original Gaussian peak fitting of observed data (dotted) and calculated profile (linear) by Rietveld’s refinement method (material unknown) (from Rietveld).[151]
This method worked very well, as illustrated in Figure 3.10, and is still used in computer programs used today. There are five areas that are important in peak refinement of diffraction data: 1. Peak shape – for neutron diffraction patterns, peak shapes are Gaussian (Figure 3.10) and can be fitted easily. For other diffraction methods, however, peaks can rarely be fitted so easily in this way. The other possible peak shape is Lorentzian, which is narrower and sharper, with a longer ‘tail’ at the base. In synchrotron radiation profiles, the peaks are more complex as they are a combination of the two types. This is solved by using a pseudo-voigt function, where the addition of the Lorentzian and Gaussian functions can be adjusted using the η parameter. 2. Peak width – particle size can affect the peak by broadening it. To take this into consideration, a peak’s width is measured at half the height of the maximum, a term known as FWHM (Full Width at Half Maximum). It is a function of 2θ, and so gives approximate starting values for each peak to initialise refinement. 3. Preferred orientation correction – in polycrystalline materials, there can be a tendency for a particular orientation to be favourable in grains. This affects the peak profiles, which is considered in the refinement. 4. Method of calculation – the minimal different in generic terms is given as Equation 3:8. 5. Background – although not a major factor, it must be considered in refinements. Some materials naturally have high backgrounds, and most sample holders or substrates are amorphous. If the background is taken into consideration, the refinement can lead to a better fit and a lower R-value.[151-152]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 76 The R-value is an important indicator of a successful refinement. The lower the R-value
(most importantly, the weight profile, Rwp), the lower the deviation between the observed data and the calculated model. A difference plot is usually given, showing any maxima of differences between the two data sets, and a successful refinement shows no significant maxima. The Rexp value is the expected R-value for the refinement taking into consideration the number of data points in the data and the number of parameters used for refinement. The ratio of Rwp to Rexp is known as the Goodness of Fit (GOF) value. This is a commonly used value to determine a good fit between the observed data and calculated model; the lower the value the better the fit.[153]
3.3.5 Scanning Electron Microscopy (SEM) and EDAX Electron microimaging was undertaken using a Zeiss EVO60 Scanning Electron Microscope (SEM), equipped with EDAX for chemical analysis. It was used to examine both powder and solid ceramic samples.
In SEM, an electron gun fires a beam of electrons down towards a sample, and is accelerated and focussed though apertures and electromagnetic lenses, as Figure 3.11 illustrates. 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 absorbtion by air molecules) and charging of the sample.
Figure 3.12. Schematic Illustration of how an SEM works (from University of South Denmark).[14]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 77 In order to avoid sample charging in SEM imaging, they are first carbon-coated. Sintered solid samples from both the as-fired top and the bulk were analysed. The bulk sample was ground using 400 to 1200-grade SiC, followed by polishing on 6 m then 1 m diamond paste, followed by polishing with dilute OPS (Oxide Polishing Suspension) Colloidal Silica solution for 20 minutes. The sample was washed with soap and ethanol in between each stage. One half of the polished bulk sample was subsequently thermally etched in hot dilute sulphuric acid for 10-30 seconds. This sample, along with a as-sintered top sample, was then stuck onto an aluminium SEM stub using a conductive carbon adhesive, and carbon-coated using an Edwards Sputter Coater (E306A). Silver DAG was then applied between the sample and the stub, in order to aid electrons dissipation from the sample during SEM imaging.
3.3.6 Electrical Property Measurements Samples were cut into discs, as previously described, approximately 1 mm in thickness, and lightly ground using 1200-grade SiC. One cylindrical face of the sample was coated with 71% wt/wt silver paste (C2000107P3 Gwent Electronic Materials), dried in an oven set at 100C for 15-20 minutes, the other face coated and dried in the same way. It was then placed diagonally onto an alumina crucible and fired in a Carbolite ESF 12/10 Chamber Furnace for 2 hours at 550C (with a ramping rate of 180C). The sample was then ground around the edges using low grade SiC in order to avoid short-circuiting and conductance of the sample disc. A new diameter was measured, around various points of the sample, where the average diameter was taken when needed.
3.3.6.1 Field Response Measurements (P-E Hysteresis Loops) High field measurements were conducted on silver-coated sample disc under silicone oil at room temperature (22 ±2ºC) using a computer-controlled function generator (Hewlett Packard 33120A) and a high voltage amplifier (Trek 609D-6). A specially constructed current amplifier was used to measure the induced current, which was then integrated numerically to yield the charge, Q, and then the polarisation P. The applied field and induced current waveforms were downloaded to the PC using a 16-bit National Instruments A/D card.[154] Field-Polarisation response loops were obtained whilst exposed to fields up to 6kV/mm, or until breakdown was observed.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 78 3.3.6.2 Dielectric Measurements Another coated sample, with a surface area A and thickness t, was placed in between two electrode wires, as illustrated in Figure 3.12. This set-up was clamped in between two alumina plates, and placed into a glass tube in a Carbolite MTF 9/15/130 Tube Furnace.
Figure 3.13. Representation of Sample prepared to be clamped in between two alumina plates, in preparation for dielectric and impedance analysis.
The furnace was set to rise between 25ºC (room temperature) to 555ºC, which is known to be above the Curie temperature of NKN, TC = 418ºC for pure NKN,[97, 155] in 10ºC steps with 20 minutes dwelling time at each temperature. A Hewlett Packard 4192A Impedance Analyser was used to determine the capacitance, C, and loss tangent, tan δ, for the material at each temperature, at frequencies of 10 Hz to 10 MHz. The capacitance can then be converted to relative permittivity, r, by use of Equation 3:9. [ε0 is the permittivity of free space, ε0 = 8.854187 x 10-12 F/m] [37]
0r A C Equation 3 :9 t
3.3.6.3 Impedance Spectroscopy Two electrode wires were placed on either side of the disc and with the same set up in the glass tube as described in Section 3.3.8.2. Angular frequencies (ω) of 5 Hz to 12.5 MHz were applied to the sample at temperatures in the range of 25 to 645ºC, in steps of 20ºC with 20 minutes at each temperature. The resulting impedance data (Z’ and Z”) readings for each frequency are then Z*A Z*r2 1 * converted into resistivity (ρ’ and ρ” respectively) by using t t Equation 3:10, and then plotted on a ρ”-ρ’ graph to produce a Cole-Cole semicircular plot. [Here ε0 is the permittivity of free space, ε0 = 8.854187 x 10-12 F/m,[37] A is the surface area of the capacitor plates (the surface area of the disc top), and t is the distance between them.]
Equation 3 :10
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 79 From a semicircular plot, the point at which the semicircle reaches the x-axis (real resistivity component) on the right hand side defines ρ’max. As resistivity is inversely 1 'min 'max proportional to conductivity σ, the σ’min was found for each temperature by Equation 3:11.
Equation 3 :11
Ea kBT 0e Equation 3 :12
Using Equation 3:12, the Arrhenius relationship, where Ea is the activation energy, kB is Boltzmann’s constant (kB = 1.38065 × 10−23 J/K) [37] and T is an absolute temperature, a ln σ’-1000/T graph was plotted. From this, a linear relationship is normally observed, where the gradient of the line is -Ea/kB. From this, the activation energy was obtained.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 80 4 THE EFFECT OF TEMPERATURE ON NKN-BASED FORMULATIONS
4.1 Introduction
4.1.1 Background and Objectives The aim of collecting synchrotron data was twofold. As well as collecting original data for doped NKN, a synchrotron investigation of the structure of a powder as a function of temperature had not been attempted before at the Daresbury SRS (Warrington, UK). The station (Station 6.2) was set up to enable the experiment to be undertaken. Not only was it a first for the station, but the experimental work aided the design of a new station for Diamond in Oxfordshire, UK. High resolution diffraction spectra could be collected on the station in approximately 30 seconds, so the limiting factor of the experiment was the heating rate in the furnace. Initially this was 3 °C/minute but was subsequently adjusted to 2 °C/minute).
A problem with the RAPID2 detector system,[156] on the SRS station only became apparent after the data had been collected. Essentially due to incorrect adjustment, x-rays falling in between two detectors were recorded by one or the other detector, not both. This gave rise to irregular peak shapes in some of the data reported in this chapter. Had the system been working correctly, the GOF values would have been significantly lower and the final parameters more accurate.
The primary aim of the study was to investigate the change in structures of the NKN materials as a function of temperature.
4.1.2 Methodology A significant time after collection of the data, it was found that there was misalignment of the detector in the synchrotron station. This caused the measured 2θ values (and their corresponding intensity counts) to be in the wrong positions, such that they were stretched over a wider range of 2θ than they should have been. This led to refinement problems, as the standard NKN model could not be applied for refinement in the normal way. In order to address this, the characteristic of one set of peaks was investigated as a function of temperature. In many XRD investigations of NKN, the 200 and 002 double peak is used in most literature as the primary basis for phase determination.[119] Due to the overlap of orthorhombic and tetragonal diffraction peaks for NKN, this double peak can reveal the dominant phase by comparing the ratio of the two peak heights.[98, 113, 157] Therefore, in the present study, this double peak was investigated and modelled at each temperature.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 81 4.1.3 Materials Selection Prior studies indicated the benefit of doping NKN with Cu, Li and Ta. Lithium and Tantalum are also highly researched dopants to NKN, where there is a significant increase in properties at the 5-6 mol%[5, 103-104, 112, 158-162] In the present study, samples of
99.5NKN-0.5CuO + 0.6wt% Nb2O5 and 94NKN-6LiTaO3 (6LT) were prepared using the mixed oxide route.
4.2 Niobium and Copper Doped NKN
The 99.5NKN-0.5CuO + 0.6wt% Nb2O5 samples (density 95% theoretical) were investigated in two forms: as a sintered pellet (of 8mm diameter and 1mm thickness) and as a crushed powder. The ceramic sample was seen to deform under high temperatures, most probably due to internal strain, and so there were problems with the resulting diffraction data. For this reason, the same material was also investigated as a crushed powder, over an identical temperature range.
4.2.1 Diffraction data for the sintered pellet The collected diffraction spectra for the sintered pellet were converted into workable files for Microsoft Excel (see Experimental section 3.3.3). Due to the large amount of data, every fourth scan was normalised and plotted above one another for comparison (Figure 4.1).
Figure 4.1. Diffraction spectra of 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample over 20 – 60° 2θ range, as a function of temperature (°C).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 82 The diffraction spectra for the sintered pellet show two significant phase transitions at
~300ºC and ~515ºC (Figure 4.2). The transition temperatures, TO-T and TC, for NKN are known to occur at ~210ºC and 418ºC;[17, 29] therefore the observed transitions are up to 100ºC higher than expected. Furthermore, the higher temperature transition clearly shows peak splitting, indicating that the final cubic phase is not yet fully developed. This behaviour is not expected for NKN at such high temperatures.
Figure 4.2. Diffraction spectra of 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample in the double peak area under investigation with respect to temperature (ºC).
By refining the diffraction data, the modelled peaks fitted very closely to the observed profile, for example Figure 4.3. This is for data collected at 31ºC (one of the initial refinements) and shows the close fit between the modelled (red) and the observed profile (blue). The grey line beneath the data shows the differences between the experimental data and the model, indicating a good fit.
Figure 4.3. Peak modelling for data collected at 31ºC for orthorhombic 202 and 020 peaks. The blue line shows the observed data, and the red line shows the calculated model.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 83 4.2.1.1 Lattice Parameters The resulting lattice parameters are plotted in Figure 4.4. Below the transition at 300ºC, NKN exhibits an orthorhombic structure and all three lattice parameters are clear and distinct. There is however, no systematic relationship between the lattice parameters and temperature (Figure 4.4). The correlation may be stronger if the whole diffraction data were refined, rather than working with just two peaks in the spectra. It must also be noted that refinement is different for every profile. The modelling of data for identical parameters and conditions can produce significantly different results. This is shown by the Goodness of Fit (GOF) function (indicating the similarity of the calculated and observed profiles). The GOF value for the data points at 175ºC (which looks anomalous in Figure 4.4) is actually the lowest (having a very low GOF of 2.5 in comparison to 6.9 - 9.8 for data collected at lower temperatures). Thus the 175 ºC data are not anomalous. The data do, however, confirm an orthorhombic phase, which is expected for NKN, but up to 300ºC, approximately 100ºC higher than expected.
6.5
6.0
5.5 a (ortho) a (tet) a (new tet) 5.0 c (ortho) b (ortho) b (tet) 4.5 parameter length (A) length parameter b (new tet)
4.0
3.5 0 50 100 150 200 250 300 350 400 450 500 550 Temperature ( C)
Figure 4.4. Lattice parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample. Symbols: Green for orthorhombic phase, red for tetragonal phase, and blue for new tetragonal phase; circle for a, square for b and triangle for c parameters.
Above the phase transition of 300ºC, there was very good refinement in fitting the data to a tetragonal structure. The fitting to an orthorhombic phase was impossible after 307°C, and so the transition is clear and defined. Figure 4.5 shows the refinement for data collected at 307°C where the majority of peaks are associated with the tetragonal phase. There is, however, a small peak at 40.2° 2θ, which is associated with the orthorhombic phase, showing that the two phases coexist at this temperature. By the next temperature
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 84 scan (315°C) this small peak has and disappeared. The coexistence of two structures at a phase transition in NKN was reported by Tellier et al.[163] The b lattice parameter has merged into the c parameter (tetragonal structure has b = c) and does not vary significantly with incrasing temperature. Again, due to limitations of the data, a direct relationship between the two lattice parameters and temperature cannot be drawn. However, it is clear that a tetragonal phase is present after the transition.
Figure 4.5. Profile Refinement for data collected at 307°C. Thick blue line denotes the tetragonal phase component of the refinement.
At temperatures above 500ºC, a new phase starts to develop. Although this is also a tetragonal phase, the a and c parameters are very close together, and by 520 ºC increasingly becomes the dominant phase; for example the dominant split peak at 523ºC in Figure 4.2. It is clear that the two lattice parameters are converging to represent the cubic phase in NKN. The transition at 500ºC is much higher than the 418 ºC expected for undoped NKN.[29] Whilst the doping may well have modified the transition temperatures slightly, additional problems were caused by the uncalibrated furnace on the station, and so the inferred transition temperatures must be treated with caution.
4.2.1.2 Pseudo-cubic Parameters The parameters reported in much of the literature are given as pseudo-cubic cell parameters, as there is a relationship between the orthorhombic parameters known for NKN and a monoclinic phase, with similar parameters of (a ≈ b ≈ c ≈ 4Å, β ≈ 90.3º).[163] This cell choice is becoming increasingly popular for NKN, although was initially reported by Katz and Megaw in 1967.[90] A schematic diagram showing the relationship between the structures is shown in Figure 4.6.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 85
Figure 4.6. Relationship between orthorhombic Bmm2 unit cell (a0 b0 c0), tetragonal unit
cell (at bt ct) and pseudocubic unit cell (a1 a2 a3 β) with atomic positions (from Ishizawa et al).[117]
In 2001, Singh et al.[164] reported the orthorhombic parameters of NKN, with no mention of pseudo-cubic parameters. However in later papers by other workers,[117, 163, 165] similar dimensions are reported as pseudo-cubic, and with higher precision. This is shown in Table 4–1. It must be noted, however, that using these parameters or the original parameters for Rietveld refinement will lead to the same conclusion, and so it is understandable for Singh et al.[164] to report these parameters as orthorhombic.
Table 4–1. Summary of reported NKN pseudo-cubic parameters (at room temperature).
Author Year a (Å) b (Å) c (Å) β (º) Singh et al.[164] 2001 3.994 4.016 3.935 - Wu et al. [165] 2008 4.000 3.940 4.000 90.34 Tellier et al.[163] 2009 4.0046(4) 3.94464(3) 4.00200(5) 90.3327(5) Ishizawa et al.[166] 2010 4.0054(1) 3.9551(1) 4.0054(1) 89.854(3)
Figure 4.7 shows the pseudo-cubic parameters from this study for the copper-doped NKN, along with corresponding schematics of the unit cell shape using these parameters. The solid figures are provided to show an exaggerated (not to scale) example of the dimensions of the unit cell for each phase. It is clear that as the temperature increases, the parameters increase. The initially relatively small orthorhombic unit cell lengthens in the a’ and c’ lengths at 300°C before b’ also increases significantly to form a near-cubic unit cell above 500°C. Ishizawa et al. reported the cubic unit cell to have a length of 3.9924(1) Å above the cubic transition (at 401°C, which is lower than the transformation temperature recorded in this investigation); the new tetragonal (near-cubic) phase has dimensions a and b of 4.9393(4) and 4.96568(9) Å, respectively. Even though these two dimensions are very close to each other, they are not close to the cubic unit cell
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 86 dimensions found by Ishizawa et al.[117] and Tellier et al.[163] This shows that when there is a major transition from tetragonal to cubic, the unit cell could significantly change to a smaller volume. Such behaviour, however, can only be speculated in the present investigation.
5.2 a' = c' (ortho) 5.0 a' = c' (tet) a' = c' (new tet)
4.8 b' (ortho) 4.6 b' (tet) b' (new tet) 4.4 b’ c’
a’ parameter length (A) length parameter 4.2
4.0
3.8 0 50 100 150 200 250 300 350 400 450 500 550 Temperature ( C)
Figure 4.7. Pseudo-cubic parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample. Symbols: Green for orthorhombic phase, red for tetragonal phase, blue for new tetragonal phase; square for a’ (and c’) and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison.
Table 4–2 shows the average lattice parameters for each phase present (in the pseudo- cubic monoclinic format). The orthorhombic phase has a slightly larger unit cell than those reported in Table 4–1, although lattice parameters are similar. This shows that copper does affect the unit cell by increasing lengths of the parameters slightly.
Table 4–2. Average pseudo-cubic lattice parameters of phases present in Cu doped NKN (sintered pellet) (accurate to 0.008 Å)
Pseudo-cubic Phase a (Å) b (Å) Orthorhombic 4.062 3.976 Tetragonal 4.995 4.036 New Tetragonal 4.952 4.494
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 87 4.2.2 Powder diffraction analysis for Copper and Niobium Doped NKN Similar to the data set for the sintered pellet, every four diffraction profiles (that is, every 12°C) was normalised and plotted (Figure 4.8).
Figure 4.8. The diffraction profiles for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 (crushed powder) (°C), as a function of temperature, at 20-60° 2θ.
Although there were problems with the analysis of the diffraction data for the sintered pellet, the data was relatively simple to refine. In contrast, the powder diffraction data is more complex, showing every phase present, not just those present at the surface of the specimen. The detailed powder diffraction data is presented in Figure 4.9. Whilst the structural transitions are not as clearly defined as those seen in the data for the sintered pellet (Figure 4.2), the main transitions for powder sample data are seen at ~140ºC, ~308ºC and ~364ºC.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 88
Figure 4.9. Diffraction profiles for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 (as a crushed powder) as a function of temperature (ºC) at 38-43° 2θ.
At temperatures below 140ºC, the refinement of the double peak is relatively simple; the orthorhombic phase is dominant and easily refined. This is shown in Figure 4.10, representing the refinement for data collected at 72ºC. This refinement had a Goodness of Fit (GOF) value of 3.9, which is a very low value for synchrotron data. This shows that the phase is purely orthorhombic. It is a good representation of refinements below 140ºC, where all refinements had a GOF value between 3.2 and 5.5.
Figure 4.10. The Rietveld refinement of the double peak at in data collected at 72ºC (using orthorhombic parameters)
At temperatures around 140ºC, the data in Figure 4.9 shows a significant transition to a new phase. The data collected at 144ºC was particularly difficult to refine, although a good fit was found using both orthorhombic and tetragonal structures (with a GOF value of 9.8). Nevertheless, at 152ºC, the modelling also showed a mixed phase system, although the data exhibited a much better fit with a GOF value of 5.7. Data for the two components are shown in Figure 4.11 indicating that two phases coexist; this is well documented.[83, 167]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 89 Due to the complexity of the orthorhombic and tetragonal peaks and their overlapping peak profiles, Rietveld refinement tends to result in lower precision data. Nevertheless, in this region a GOF value of less than 10 is highly desirable.
At temperatures above 152ºC, mixed phases were detected in all the refinements. Although the orthorhombic phase is a rather small component it cannot be ignored in refinements as its presence does significantly affect the final outcome. It must be noted, then, that the orthorhombic data in the final summary shows that the phase is present but it is not the major phase in the system. Detecting the two phases in this region reflects the benefit of high resolution synchrotron diffraction of a powder sample, in comparison with a solid sample.
(a)
(b)
Figure 4.11. Profile refinement for data collected at 152ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase.
The orthorhombic phase remains a small component with increasing temperature until 280ºC. At this temperature, the orthorhombic phase becomes more prominent as the second peak becomes more distinct. This can be seen clearly in Figure 4.9 where the 40.4º 2θ peak increases with temperature up to 304ºC due to development of the orthorhombic phase component (Figure 4.12). This is an interesting finding since the well-documented analysis suggests that the second peak being higher than the first in this region indicates a
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 90 tetragonal structure.[100, 113, 119] From the present investigation, however, it is the orthorhombic phase that causes the second peak to fit better so that the tetragonal phase is identified. The GOF values for data in the 168-296ºC temperature range are reasonable at 5.3-6.1.
(a)
(b)
Figure 4.12. Profile refinement for data collected at 296ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase.
After 304ºC, the sharp second peak reduces in size and from 344ºC the two peaks integrate and become one large, broad peak. At 344ºC, only one peak is visible in the diffraction profiles, but it is clear that there are two peaks superimposed on each other, as can be seen in Figure 4.13. It shows how both orthorhombic and tetragonal phases are present in the structure, which is not expected for NKN at this temperature. The quantities of each component cannot be calculated, due to the nature of the peak refinements, where only a small section of the diffraction profile is being investigated.
(a)
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 91 (b)
Figure 4.13. Profile refinement for data collected at 344ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase.
With increasing temperature from 344ºC the superimposed peak develops to form one single peak at 360ºC, which is narrow and clearly distinct. As this occurs, the orthorhombic phase is lost and the material is purely tetragonal, which is expected for NKN.[163] The pure tetragonal phase is present with increasing temperature until 432ºC, although the peak becomes increasingly broader and shallower. At 432ºC, there is a reintroduction of the orthorhombic phase to the system. There should be no orthorhombicity in NKN at this stage (nor tetragonal, as NKN is reported to be cubic above 418°C).[117, 163] However there is an extra peak that cannot be defined as tetragonal and fits when an orthorhombic phase is introduced to the refinement model.
4.2.2.1 Lattice Parameters The collected parameters are shown in Figure 4.14. It clearly shows an orthorhombic phase below 144°C, which does not change as temperature rises, until the tetragonal phase begins to form after the transition. When reflecting upon Figure 4.9, there is clear peak broadening in the orthorhombic phase before the transition. Broadening can be the result of strain which is relieved at the transition, transferring the system to a lower energy state and changes in the unit cell (in this case, to tetragonal). As the temperature rises towards the transition, the peak broadening shows that there is strain in the sample, and the tetragonal phase is detected at 144°C. As the orthorhombic phase is still detected throughout the temperature range, it shows that not all of the specimen transforms to tetragonal phase. This is possibly due to the inhomogeneous sample heating, where the edges of the sample will have been hotter than the middle. However, it is clear that the orthorhombic phase that is still present in the sample has a smaller unit cell, with all three of its parameters shrinking in size, in order to accommodate a larger tetragonal unit cell (whose symbols are denoted by squares). This remains so until 360°C, where the tetragonal parameters reduce significantly. As
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 92 discussed previously, at this temperature, the double peak becomes a superimposed double peak (Figure 4.9). Although this still represents tetragonal, it has a new set of lattice parameters from this stage.
6.0
5.5
a (ortho) 5.0 b (ortho) c (ortho) a (tet) 4.5 parameter length (A) length parameter b (tet)
4.0
3.5 0 100 200 300 400 500 Temperature ( C)
Figure 4.14. Phase parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 crushed powder sample. Markers: Square for tetragonal phase and triangle for orthorhombic phase; red for a parameter, green for b parameter and blue for c parameter.
There is an issue with the sample, where it does not fully transform into cubic, nor does it exhibit lattice parameters suggesting that such a transition is likely to take place (as observed in Section 4.2.1 with the solid pellet). Undoped NKN has a Curie temperature of 418°C; doping should not affect the transition temperature significantly. In the present case, the experimental set-up (the position of the thermocouple) could have resulted in a discrepancy between the actual temperature of the sample and the temperature recorded by the thermocouple. This does call into question data for the initial phase transition at
~140°C, which is lower than that of the literature data for TO-T for NKN at approximately 200°C.[17, 29]
4.2.2.2 Pseudocubic Parameters Figure 4.15 shows the parameters calculated for the powder sample after pseudo-cubic conversion. The schematic solids (not to scale) show how the orthorhombic unit cell decreases in size once the tetragonal phase is introduced to the system at 140°C. From this temperature, the tetragonal unit cell has a smaller b’ parameter wheras the a’ and c’ parameters are significantly larger. At 360°C there is a change in this tetragonal unit cell,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 93 where the b’ parameter increases in length and the a’ and c’ parameters decrease slightly. At this time, the tetragonal phase’s unit cell is slowly growing in size up to 500°C. 5.2
a' = c' (tet) 5.0 a' = c' (ortho)
4.8 b' (tet) b' (ortho) 4.6
4.4 b’
parameter length (A) length parameter c’ a’ 4.2
4.0
3.8 0 100 200 300 400 500 Temperature ( C)
Figure 4.15. Pseudo-cubic parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 crushed powder sample. Markers: Red for tetragonal phase and blue for orthorhombic phase; square for a’ (and c’) and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison.
The final pseudo-cubic parameters were divided into three stages: 0 to 140ºC, 140 to 360ºC and 360 to 500ºC as these were where the significant changes in parameters occurred. The average lattice parameters are presented in Table 4–3.
Table 4–3. Average pseudo-cubic lattice parameters for phases present in Cu doped NKN (powder sample) (accurate to 0.006 Å)
Temperature Orthorhombic Tetragonal range (ºC) a’ (Å) b’ (Å) a’ (Å) b’ (Å) 0 – 140 4.107 4.026 - - 140 – 360 4.059 3.939 5.027 4.007 360 - 500 4.103 3.974 4.928 4.068
When comparing the initial monoclinic (pseudo-cubic orthorhombic) phase parameters with published data (Table 4–1) the results are similar to the ~4.005 and ~3.950 Å a’ and b’ parameters, although here they are both slightly higher. This shows that the copper doping increases the unit cell slightly for NKN below 140ºC and above 360ºC; however they are very similar in the intermediate 140-360ºC stage.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 94 4.2.3 Electrical Properties of Copper and Niobium Doped NKN
Figure 4.16 shows the hysteresis behaviour of for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 with
Pr = 19.9 μC/cm2 and EC =13.5 kV/cm. In this investigation, undoped NKN saturated hysteresis could not be achieved. The highest Pr value for NKN found in the literature is from Chang et al. with values of Pr = 15 μC/cm2 and EC =14.8 kV/cm.[95] The present values for copper doped NKN, are desirable, indicating that doping has a positive effect on the remnant polarisation (higher) and coercive field (lower).
30
20
10
0 -40 -20 0 20 40
-10 p (uC/cm2) p
-20
-30
E (kV/cm)
Figure 4.16. P-E hysteresis behaviour for 99.5NKN-0.5CuO + 0.6wt% Nb2O5.
4.3 Synchrotron XRD Study of Lithium Tantalate Doped NKN Figure 4.17 shows the full diffraction profiles of every four scans (every 12°C) as a function of the temperature.
Figure 4.17. Diffraction profiles for 94NKN-6LT as a function of temperature (°C).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 95 The NKN-LiTaO3 formulation has been studied extensively, exhibiting superior properties to undoped NKN.[33, 103-104, 112, 160, 162] The objective of this study was to understand how the structure of the material changes with temperature. The analysis of the data in terms of the double peak was again undertaken, due to problems associated with alignment of the synchrotron station (Section 4.1). The change in peak profile with temperature can be seen in Figure 4.18. The addition of 6 mol% LT to the starting formulation gives rise to a tetragonal structure, as opposed to orthorhombic for the previous cases. This is instantly recognisable as the second peak (002) is higher than the first (200), indicating a tetragonal structure (in contrast for orthorhombic structures, the first peak (200) is higher than the second (002)).[103]
Figure 4.18. Diffraction profiles for 200 and 002 peaks for 94NKN-6LT as a function of temperature (ºC).
As the temperature increases from room temperature (22ºC), there is a clear transition from tetragonal, as the double peaks transform into a single peak at 390ºC in Figure 4.18. (a)
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 96 (b)
Figure 4.19. The refinement of 94NKN-6LT for data collected at 30ºC. The top blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase.
The room temperature data initially showed a tetragonal phase, but the refinement improved when an orthorhombic phase was introduced (Figure 4.19). The two phases were identified in all refinements until 130ºC. In contrast published data for solid samples[162, 168] do not report an orthorhombic phase. As the temperature increased above 130ºC, the data could be refined on the basis of a single tetragonal phase. As the temperature increased, the c parameter decreased whilst the a parameter remained constant. Although the precision of the refinements is still lower than desirable (due to the double peak refinement procedure), the lack of orthorhombic-tetragonal overlap gives a clearer impression of how the structure changes with temperature. It must be noted, however, that the first peak (200) begins to show signs of broadening at 270ºC (Figure 4.20). Although many different refinement parameters were explored, the broadening could not be modelled satisfactorily. The Goodness of Fit (GOF) value remained relatively low, between 5 and 6.6 due to the broadening, but remained low due to the simplicity of the one phase model. In Figure 4.20, for example, the data for 270ºC has a GOF value of 6.6. The peak broadening, however, is a minor issue. The refinement confirms a fully tetragonal phase.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 97
Figure 4.20. The diffraction profile for data collected at 270ºC, with peak broadening on first peak.
At temperatures above 334ºC, the orthorhombic phase had to be reintroduced to the refinement, as the main 200 peak (on the right in Figure 4.21) was asymmetric. The orthorhombic phase is a very small component (Figure 4.21), although it does increase in volume at higher temperatures, as shown in Figure 4.23. It must be noted that due to the broadening of the first peak (200) and the introduction of a second phase, the GOF value increased to 10-12, reflecting the lower precision of the refinement (for example, the data in Figure 4.21 has a GOF value of 11.2).
Figure 4.21. The Rietveld refinement of 94NKN-6LT for data collected at 334ºC, the thick black line showing the orthorhombic phase component.
As mentioned above, with increasing temperature the orthorhombic phase becomes more important for the refinement model. A good example is shown in Figure 4.22b, indicating that the orthorhombic phase is present, and the model would not refine without it. Although the phase would be defined as tetragonal from laboratory XRD, it is clear under higher resolution investigation that this is not simply the case. The GOF value for the data in Figure 4.22 is 6.3, quite respectable. There is a small undefined peak visible at 40.35º 2θ; this is insignificant as the peak disappears with increasing temperature and did not warrant further attention.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 98
(a)
(b)
Figure 4.22. The refinement at 336ºC, the thick blue line in (a) showing the tetragonal phase component, and the thick black line in (b) showing the orthorhombic phase component.
As the temperature increased from 336ºC, the two peaks in Figure 4.22 slowly converged into one broad peak leading to a significant transition at 390ºC. At 390ºC, this peak can be modelled using both tetragonal and cubic parameters. Both refinements are similar but the GOF value for the cubic refinement is marginally lower (11.5) than for the tetragonal refinement (12.0), indicating that the cubic region has been reached. With further temperature increase, the tetragonal phase is no longer compatible with the refinement and so transformation to the cubic phase is complete. The final parameters for the refinements for 94NKN-6LT are shown in Figure 4.23.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 99 4.3.1.1 Resulting Parameters 6.2 6.0 5.8 5.6
5.4 5.2 c (tet) 5.0 c (ortho) 4.8 b (ortho) a (tet)
parameter length (A) length parameter 4.6 a (ortho) 4.4 a (cubic) 4.2 4.0 3.8 0 50 100 150 200 250 300 350 400 450 500 Temperature ( C) Figure 4.23. Resulting lattice parameters for 94NKN-6LT powder as a function of temperature. Markers: Green for orthorhombic phase, red for tetragonal phase and orange for cubic phase; square for a parameters, triangle for b parameters and circle for c parameters.
Figure 4.23 shows the four stages of the structural development. The most unambiguous section is the second stage (200-330 ºC), where no orthorhombic phase was detected. There is a clear reduction in the length of the c lattice parameter as the tetragonal structure is adjusting towards the cubic transition at 390ºC.
Lin et al. reported the properties of (Na0.5K0.5)0.96Li0.06 (Nb0.8Ta0.2)O3 and indicated the tetragonal – cubic transition to be at 350°C.[107] This is a little lower than the value determined by the present study (390ºC), where there was slightly less Ta in the formulation (Ta0.06) and the difference between the crushed powder and solid sample[107] may have been relevant. As discussed previously, it is also possible that the temperatures recorded for the experiment are not reliable.
The detection of an orthorhombic phase before the cubic transition is also of interest. This is possibly due to stresses within the material as the unit cell is adjusting in size. This has not been reported previously, although it is rare for samples to be studied in powder form for diffraction investigation for this kind. Tellier et al. found that there can be a mixture of phases at a transition,[163] although only between the two phases on either side of the transition. For an orthorhombic structure to be present near the TC is a new finding.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 100 4.3.1.2 Pseudocubic Parameters The resulting pseudo-cubic parameters are plotted in Figure 4.24, along with corresponding schematic diagrams of the unit cell dimensions. The lattice parameters of the orthorhombic phase (green) decrease in size as temperature increases from 50 to 200 ºC, and it is this smaller unit cell size that is present when the orthorhombic phase reappears at 340ºC. The tetragonal pseudocubic cell has significantly larger a’ and c’ parameters, showing a wider and deeper unit cell, whilst the b’ parameter is relatively unchanged. 5.2 a' = c' (ortho) 5.0 b' (ortho) a' = c' (tet) 4.8 b' (tet) a (cubic) 4.6
4.4 b’ parameter length (A) length parameter c’ 4.2 a’
4.0
3.8 0 50 100 150 200 250 300 350 400 450 500 Temperature ( C) Figure 4.24. Pseudocubic parameters for 94NKN-6LT. Symbols: Green for orthorhombic phase, red for tetragonal phase, and orange for cubic phase; square for a’ and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison.
The average lattice parameters are given in Table 4–4. The orthorhombic parameters are very similar to those found for the orthorhombic phase of the Cu-doped NKN (sintered pellet) of a = 4.06245 and b = 3.97616 Å. The copper doped crushed powder, however, has a larger orthorhombic phase (a = 4.40680 and b = 4.02620 Å) than that found for the 6LT sample.
Table 4–4. Average parameters of phases present in 94NKN-6LT (accurate to 0.008 Å)
Pseudocubic Phase a’ (Å) b’ (Å) Orthorhombic 4.079 3.948 Tetragonal 4.98 4.0172 Cubic 4.058 -
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 101
Saito and Takao[112] reported lattice parameters for (Na0.5K0.5)0.94Li0.06(Nb0.8Ts0.2)O3 having a similar composition to 6LT (different tantalum addition), but a single tetragonal phase (using laboratory XRD) with a and c parameters of 3.948 and 4.015 Å respectively. They do not report whether they are pseudocubic parameters, though it is clear that the c parameter found in this study is significantly larger than that found by Saito and Takao.[112]
4.4 Chapter Summary NKN was doped in two different ways: (i) with 0.5 mol% copper doping and (ii) with 6 mol% LiTaO3. The copper-doped NKN was investigated in two forms: as a solid pellet and as a crushed powder. All samples were investigated by x-ray diffraction in situ as a function of temperature. Detailed analysis of the data revealed calibration problems associated with the synchrotron station.
The experiments did prove useful, however, as the HATSAXS (High Automated Throughput Small Angle X-ray Scattering,) beamline B21 at Diamond Light Source was developed on the basis of the findings from this experimental work.
4.4.1 Copper and Niobium Doped NKN (Sintered Pellet) For the sintered pellet, the x-ray spectra clearly showed two phase transitions at ~300ºC and ~515ºC, somewhat higher than the expected values for TO-T and TC for NKN at ~190- 200ºC and 418ºC.[17, 29] Some of the difference may have been due to inadequate furnace calibration and the set up of the thermocouple (between the sample and the element, see Section 3.3.3.1). Up to 500°C the two phase transitions were observed for the sintered pellet orthorhombic followed by a tetragonal phase. The orthorhombic phase exists to temperatures up to 300°C with average pseudo-cubic parameters of a’ ~ 4.06245 Å, and c’ ~ 3.97616 Å. From 510-525°C two tetragonal phases coexist in the material. The first is the tetragonal phase that was observed throughout (pseudo-cubic parameters of a’ ~ 4.99557 Å and b’ ~ 4.0363 Å); the second phase is initially detected and determined at 510°C and soom becomes the dominant phase as the temperature continues to increase above 525°C. This new tetragonal phase has near-cubic parameters (pseudo-cubic parameters where a’ ~ 4.9519 Å and b’ ~ 4.4941 Å).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 102
P-E hysteresis data was collected this formulation; critical properties were Pr = 19.9
μC/cm2 and EC = 13.5 kV/cm, which is a significant improvement compared to the properties for undoped NKN.[95]
4.4.2 The Copper and Niobium Doped NKN (Crushed Powder) At temperatures < 140 ºC there is only orthorhombic phase present, with pseudo-cubic parameters of a’ ~ 4.10680 Å and b’ ~ 4.02620 Å. Above 140ºC orthorhombic and tetragonal phases coexist up to the limiting temperature of 500ºC. Up to 360°C, the orthorhombic cell parameters decrease in size (average pseudo-cubic parameters a’ = 4.05845 Å and b’ = 3.93863 Å) in order to accommodate the tetragonal phase. The tetragonal phase in this region has pseudo-cubic parameter sizes of a’ = 5.02694 Å and b’ = 4.00747 Å. From 360ºC up to 500°C, in both the orthorhombic and tetragonal unit cells the b parameter increases in size; the tetragonal a parameter also increases, but the orthorhombic a parameter decreases in size. New average orthorhombic phase pseudo- cubic cell parameters are a’ = 4.10285 Å and b’ = 3.97435 Å; for the tetragonal unit cell they are a’ = 4.92752 Å and b’ = 4.06810 Å. Copper doping increased the unit cell size compared to that for published data for undoped NKN.[117, 163]
4.4.3 Lithium and Tantalum Doped NKN (Crushed Powder) There is a transformation to a cubic structure at 390°C, where the unit cell parameter is 4.05795Å. Below this temperature, both orthorhombic and tetragonal phases coexist. The tetragonal phase was detected at all temperatures below 390°C, as is expected for 6 mol%
LiTiO3 doping,[162] with average pseudo-cubic parameters a’ = 4.98349 Å and b’ = 4.01715 Å. The a’ parameter found here is significantly larger than that reported by Saito and Takao.[112] An orthorhombic phase (with average parameters of a’ = 4.0916 and b’ = 4.0172 Å) was detected at low temperatures (20 - 200°C). The orthorhombic phase was again detected just before the tetragonal to cubic transition (340 – 390°C), with a slightly smaller unit cell volume. The existence of an orthorhombic phase just before the transition to a cubic structure has not been detected previously in synchrotron studies of NKN.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 103 5 DOPING NKN WITH PIEZOELECTRIC SBN
5.1 Introduction The focus of the study was to investigate the effect of doping piezoelectric NKN with piezoelectric strontium barium niobate, (Sr0.5Ba0.5)Nb2O6, hereafter written as SBN. NKN is an intensively researched material with much potential (after optimisation) as a replacement for lead-based piezoelectrics. NKN can be doped by many different elements and compounds,[95, 101, 169-172] however always in a perovskite (ABO3) form. Here the perovskite (Na0.5K0.5)NbO3 (NKN) was doped with Sr0.5Ba0.5Nb2O6 (SBN), which is a tetragonal tungsten bronze (TTB) type material, having a different crystal structure to perovskite. This investigation is novel and innovative as there has been no evidence of such research in the literature.
5.2 Characterisation of Starting Powders The starting powders were characterised in terms of morphology, structure and chemistry prior to use. The International Centre for Diffraction Database (ICDD) version PDF4+ was utilised to determine raw powder phases.
5.2.1 Sodium Carbonate
(b) (a)
Figure 5.1. (a) SEM micrograph of Na2CO3 particle and (b) X-ray diffraction spectrum for
Na2CO3 with fitting to reference data shown by “stick patterns”.
Sodium carbonate, Na2CO3, particles are elongated, typically 2mm x 0.5mm in size. An exemplar particle from the powder is shown in Figure 5.1a. It is white in colour and the purity is reported by the manufacturer to be 99.8%. EDAX examination showed the Na:O ratio to be 45:55, which is expected for this powder (carbon content could not be determined due to the use of carbon coating process for SEM analysis).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 104 X-ray diffraction spectra for sodium carbonate were matched to two different forms of
Na2CO3 (ICDD 00-019-1130 [green lines] and 00-037-0451 [blue lines]) as shown in Figure 5.1b. No extra phases were detected. As sodium carbonate is known to be deliquescent, that is it reacts with moisture, the powder was stored at all times (in a covered beaker) in an oven held at 200°C at all times.
5.2.2 Potassium Carbonate
Potassium Carbonate, K2CO3 (KC), was in the form of white particles approximately 10μm in diameter. An SEM micrograph of the powder is shown in Figure 5.2.
Figure 5.2. SEM micrograph of Potassium Carbonate powder.
When examining KC powder by X-ray diffraction, using identical conditions to those for other powders in this section, it was clear that the diffraction pattern changed by the end of the scan. It is thought that this was due to the potassium reacting with moisture in the air causing its structure to change. For this reason, it was not possible to fit the data to a reference stick pattern from the ICDD database, and so the data is not presented here. This highlights the problem of the deliquescence of KC. In order to combat this reaction with air moisture, the powder was kept in a covered glass beaker in an oven held at 200°C at all times.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 105 5.2.3 Niobium Oxide
Niobium pentoxide, Nb2O5, is a much finer powder than the two discussed above. Nb2O5 is white in colour with a particle diameter of approximately 200nm (Figure 5.3a). The X-ray diffraction data for niobium oxide is shown in Figure 5.3b, where there was one phase detected. This was a match to ICDD 04-008-0301 for niobium pentoxide.
(b) (a)
Figure 5.3. SEM micrograph of Niobium Oxide powder and (b) X-ray diffraction spectrum for
Nb2O5 with fitting to reference data shown by “stick patterns”.
5.2.4 Strontium Carbonate
SrCO3 is a white powder of acicular shape and has a particle size approximately 3μm; a typical SEM micrograph is shown in Figure 5.4a.
(a) (b)
Figure 5.4. (a) SEM micrograph of SrCO3 particles and (b) X-ray diffraction spectrum for
SrCO3 with fitting to reference data shown by “stick patterns”..
An X-ray diffraction spectrum of SrCO3 is shown in Figure 5.4b. The data showed two phases were present in the sample (represented by green and blue lines in Figure 5.4b). These were strontium carbonate (ICDD 00-005-0418) which is represented by the blue lines, and so is the main phase present; also strontium oxide was also found (ICDD 00-001- 1113) denoted by the green lines on the stick pattern. As the second phase was only a small percentage of the total, it will not affect the resulting formulations, as only small amounts of strontium carbonate are included.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 106 5.2.5 Barium Carbonate Barium carbonate is a white powder with an elongated particle form, as shown in Figure 5.5a. The average particle length is 1.5μm. EDAX analysis showed an exact ratio of 25:75 Ba:O which is expected for this composition (again, carbon could not be detected due to the carbon coating of samples in preparation for SEM analysis). The stick pattern fitting from XRD analysis resulted in one phase being detected, that of BaCO3 (ICDD 01-074-2663).
(b) (a)
Figure 5.5. (a) SEM micrograph of BaCO3 particles and (b) X-ray diffraction spectrum for
Na2CO3 with fitting to reference data shown by “stick patterns”.
5.2.6 Iron Oxide
Iron Oxide (Fe2O3) is a fine dark red powder. Figure 5.6a shows an SEM micrograph of the powder morphology. The average particle size was found to be 200 nm, which at the same scale as for niobium pentoxide.
(a) (b)
Figure 5.6. (a) SEM micrograph of Fe2O3 powder and (b) X-ray diffraction spectrum for
Na2CO3 with fitting to reference data shown by “stick patterns”..
The X-ray diffraction spectrum for Fe2O3 (Figure 5.6b) closely matches the reference data (ICDD 00-033-0664). This shows that there are no other phases present.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 107 5.3 Densification of Ceramics in the NKN-SBN System In order to investigate the effect of SBN addition, 10 mol% SBN was initially mixed with 90 mol% NKN using the conventional mixed oxide route. After sintering for 4 hours at 1100ºC, the material had a very low density and was visibly unchanged. XRD analysis showed that there was significant second (non-perovskite) phase in the material; it was clear that the amount of SBN needed to be reduced. In general, NKN exhibits superior properties when it is a single phase product. Subsequently samples were prepared with 8, 6 and 4 mol% SBN; they were mixed, calcined and sintered, under the same conditions; it was clear that the 4 mol% doping was the limit for this system. There was no second phase present in the XRD spectra for the 4 mol% SBN samples, although the density was still low (~50% theoretical); the sample remained white in colour and had the texture of chalk. The single phase that resulted for 96NKN-4SBN, however, showed potential and so this formulation was initially investigated in some detail. It was clear that a sintering aid would be needed in order to increase the density.
From this point, the x mol% SBN addition to NKN will be denoted as xSBN, such as the 96NKN-4SBN composition will be denoted as 4SBN.
It was first necessary to increase the sintering temperature of the 96NKN-4SBN from the optimal 1100ºC for pure NKN up to 1140ºC in order to achieve significant densification. Although the samples were of higher density, they still needed a sintering aid to achieve a density of over 90% theoretical (this is 4.51 g/cm3 for NKN[173] and 4.50 g/cm3 for 4SBN. As the difference between the two is so minimal, the theoretical density for NKN was used throughout for all xSBN compositions (0 ≤ x ≤ 4).
There are many successful sintering aids for NKN in the literature,[32, 93-94, 174-175] and so various available powders, such as nickel, manganese, zinc, calcium, cobalt oxides, were added to the 4SBN system, all at 0.2 wt% levels, in order to investigate which had the most potential. The most successful was CaO yielding over 4.3 g/cm3, which is over 95% of the theoretical. The density of 4SBN processed in the same way without additive was 2.37 g/cm3, showing doping is needed to achieve higher densities and good microstructure development.
In a preliminary study of pure NKN the effect of iron doping on densification was investigated.[176] All samples were sintered at 1090ºC for 4 hours. It was found that 0.3 wt% Fe2O3 addition was the optimal amount increasing the relative density from 90.7%
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 108 up to over 94%, as shown in Figure 5.7. Such an increase in density is highly desirable in view of the correlation between high density and the resulting electrical properties of NKN.[92, 177] This is consistent with the findings of Zuo et al. who reported that the sintering of NKN ceramics can be significantly improved by the addition of a small amount of iron oxide.[70] Zuo et al. also found that the addition of iron oxide creates oxygen vacancies in the system, which could lead to a slight decrease in the size of the lattice. However the material remained single phase and the diffraction peaks were unchanged.[178] This proposal of Zuo et al. was tested for all compositions investigated in this study. The data are entirely consistent with the theory and findings of Zuo et al.[70]
96%
95%
94%
93%
92%
91%
Relative Density (%) Density Relative 90%
89%
88% 0 0.1 0.2 0.3 0.4 x% Added Fe O 2 3
Figure 5.7. Relative density of NKN as a function of x% Fe2O3 additions (after Wegrzyn).[176]
In view of the previous work on iron oxide doping of NKN, a range of Fe2O3 doping levels were employed on the 4SBN composition. The use of ZnO additions was also investigated. The results are shown in Figure 5.8. It is clear that high levels of ZnO do not enhance the density, but the higher levels of Fe2O3 significantly increased the density to 96.7%, which is the highest density achieved. The addition of 0.6 wt% iron proved to be too excessive, density fell to 85%. From this preliminary investigation, 0.45 wt% Fe2O3 was identified as the optimum additions for the SBN system.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 109 100%
90%
80%
70% Fe2O3
60% ZnO Relative Density (%) Density Relative
50% 0.00 0.10 0.20 0.30 0.40 0.50 0.60 x% Dopant Addition
Figure 5.8. Densities of 96NKN-4SBN (Tsint = 1140ºC) as a function of x wt% Fe2O3 and ZnO addition.
From this point, 0.45 wt% Fe2O3-doped samples in the (100-x)NKN-xNKN system will be given an “F” suffix, and so will be denoted as xSBNF, in the same way as the xSBN notation.
For example, 96NKN-4SBN + 0.45 wt% Fe2O3 will be referred to as 4SBNF. In the same way, the NKN + 0.45 wt% Fe2O3 formulation will be denoted as NKNF. 100%
98%
96% 94% 92% 90% 88%
Relative Density (%) Density Relative 86% 84% 0 1 2 3 4 x wt% SBN
Figure 5.9. Relative densities of (1-x)NKN – xSBN + 0.45 wt% Fe2O3 (0 ≤ x ≤ 4), all sintered at 1140°C.
As 0.45 wt% Fe2O3 was found to be the optimum doping level found for 4 mol% SBN- doped NKN, the same addition was used with 0, 1, 2 and 3 mol% SBN samples. The results are plotted in Figure 5.9. In order to keep the results comparable, all samples were sintered at 1140°C. This is in part the reason for the low densities; for other compositions (other than 4 wt% SBN) it is believed that lower additions and sintering temperatures are nearer the optimum. There is, though, a clear and distinct area for investigation from these results, which show a high relative density of over 96% to the theoretical in the iron- doped formulations of 2, 3 and 4 mol% SBN doping of NKN. Consequently, these three compositions were chosen for more detailed investigation.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 110 5.4 X-ray Diffraction Analysis
5.4.1 Undoped xSBN The (1-x)NKN – xSBN (0 ≤ x ≤ 4) formulations were analysed with laboratory XRD in order to assess how the structure changed with SBN addition. Figure 5.10 shows the diffraction profiles for the formulations under investigation. The pure NKN sample shows a clear orthorhombic phase, as expected.[96, 179] This was also the case, for all formulations up to 3SBN. This is shown by the ratio of the first peak to the second at approximately 45° 2θ (ratio of (202) to (020) peaks); the first peak is higher than the second in all cases. This is an accepted method of differentiating between orthorhombic / tetragonal Perovskite structures in diffraction data. [101, 109, 180-181] The 4SBNF composition, however, shows this doublet of peaks having approximately the same magnitude. This shows a transition between orthorhombic and tetragonal in the system, where the two phases are coexisting (with the possibility of a morphotropic phase boundary, MPB). For this reason, the investigation concentrated on SBN addition up to 4 mol%. For compositions above
4SBNF, there was a clear second phase in the XRD data with additional peaks from SBN.
002 200
101 010
113
11 , 11
022 , 220 , 022 3
202 020
103 212 121 111
4 SBN
3 SBN
2 SBN intensity (a.u.) intensity 1 SBN
0.5 SBN pure NKN
10 15 20 25 30 35 40 45 50 55 60 two theta Figure 5.10. Diffraction spectra for (1-x)NKN – xSBN (0 ≤ x ≤ 4). Indexing of orthorhombic peaks based on the work of Wu et al.[165]
It must be noted that these SBN compositions (except pure NKN) did not sinter fully. Their texture remained chalk-like and the samples were easily broken, even when the sintering temperature was raised from 1090 to 1140°C. This is undesirable and so iron oxide was employed as a sintering aid.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 111 It must be noted that the 101/010 peaks of 0.5SBNF and 1SBNF samples are significantly smaller than for the other samples. It is unclear whether this is related to texture of the material. If a material has orientation, some peaks in the diffraction profile disappear whilst others grow (favouring the direction of orientation).[182-183] In this case, however, all the other peaks are present, and so texture in the material is unlikely. It is also difficult to achieve orientation using the mixed oxide route. As the reason for the peak to disappear is not likely to be orientation, it is unclear for the reason behind this. As the predominant materials being investigated are 2-4 SBNF formulations, further attention to this anomaly was not given.
5.4.2 xSBNF Samples The addition of Iron Oxide significantly enhanced the sintering process, as described in section 5.3. As the optimal doping level was 0.45wt% Fe2O3, this amount was added to all formulations for direct comparison.
The resulting XRD spectra for xSBNF are shown in Figure 5.11. All the samples in Figure 5.11 were fully dense, however they were still crushed to a powder prior to XRD examination to ensure direct comparability with the data for undoped samples (Figure 5.10). It is clear from the diffractograms that all samples (Figure 5.11) are orthorhombic with no evidence of a second phase, although the 4SBNF sample is the solubility limit for SBN addition; larger amounts result in a second phase of SBN (as discussed earlier). This resulted in 4SBNF being the compositional limit of the investigation.
4 SBNF
3 SBNF
2 SBNF
1 SBNF intensity (a.u.) intensity
0.5 SBNF
NKNF
20 25 30 35 40 45 50 55 60 65 70 75 80 two theta
Figure 5.11. Diffraction spectra for (1-x)NKN – xSBN + 0.45 wt% Fe2O3 (0 ≤ x ≤ 4).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 112 5.5 SEM Investigation In much of the published literature for NKN-based systems, micrographs are presented for as-fired surfaces. This is an unreliable method for comparing specimen microstructures. The interiors of the samples tend to yield more representative microstructures, when grain growth has not been enhanced by free surfaces and liquid phases or evaporating components. Micrographs in this chapter are therefore based on polished internal sample sections, not as-fired surfaces.
NKNF samples had a grain size of 3 μm, which is the same as reported by Zuo et al. for
99NKN-1Fe2O3.[70] Samples prepared with 2-4 mol% SBN addition had a very similar density of over 96% theoretical. However the grain morphologies varied. The micrograph of 2SBNF (Figure 5.12) shows a relatively uniform grain size of average 3 μm with lamellar domains. However, as the amount of SBN is increased to 3 and 4 mol% (Figure 5.13) the distribution of grain sizes is more uneven, with small grains for 3SBNF approximately 2 μm in diameter and large grains up to 15 μm in size (Figure 5.13); 4SBNF grains had the same microstructure, though the larger grains were up to 20 μm in size. The large grains reveal more complex domain formations, including herringbone domain structures (indicated by arrows in Figure 5.13). These complex herringbone domains are only visible in the larger sized grains.[54, 184] Abnormal grain growth stems from the presence of a liquid phase during sintering, and the driving force is from the surface energy of the small fine grains.[106, 109] Abnormally large grains are commonly found in NKN, where a bimodal grain size is expected in samples with optimal electrical properties.[185-186] Jenko et al. reported NKN had larger grains measured 20 μm surrounded by a matrix of smaller grain size of 2 μm.[185]
Figure 5.12. SEM micrograph of chemically etched 2SBNF.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 113
Figure 5.13. SEM micrograph of chemically etched 3SBNF. Arrows indicate herringbone domain structures.
(a) (b)
Figure 5.14. SEM backscattered imaging micrographs of second phases found in polished (a) 2SBNF and (b) 3SBNF. Yellow arrows indicate light grey second phase.
Figure 5.15. SEM backscattered imaging micrograph of second phases found in polished 4SBNF. Yellow arrow indicates light grey second phase, and red arrow indicates white second phase.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 114 In 2008, Jenko et al. reported that a second phase was present in NKN when sintered at 1100ºC.[185] This phase is found to be sodium deficient, however was not determined. Such a second phase is also visible in the 2, 3, and 4 SBNF samples. In the 2SBNF and 3SBNF samples, a light grey second phase is visible (Figure 5.14); it is sparse in 2SBNF and slightly more prominent in 3SBNF. EDAX analysis confirms that the second phase is potassium rich, though contains all elements included in the formulation (Na, K, Sr, Ba, Nb, Fe). In 4SBNF, however, there are two second phases detected in backscattered mode. The SEM micrograph for a polished sample (Figure 5.15) shows the matrix grains in dark grey, and the second phases are shown as light grey and white (both indicated by yellow and red arrows, respectively). This white second phase was not visible in the other samples, suggesting that 3SBNF is the solubility limit for SBN addition. EDAX analysis shows that this phase, in comparison to the one found in other SBNF-doped formulations, is potassium deficient and barium rich. Laboratory XRD, however, shows no evidence of a second phase in the system. Consequently, higher resolution XRD was undertaken by synchrotron XRD. This is reported in Chapter 6).
5.6 Electrical Properties
5.6.1 Dielectric Properties There were two threads of investigation. The first was investigating the relative permittivities and transition temperatures of xSBNF (x = 2, 3, 4) in comparison with NKNF. The second focussed on one formulation (2SBNF) and examined how the permittivities and transition temperatures varied with sintering time.
5.6.1.1 Relative Permittivity as a Function of SBN Doping By converting the capacitance for each sample into relative permittivity (for experimental procedure, see section 3.3.6.2), which takes the sample dimensions into account, the relative permittivity of each formulation can be directly compared to those for other samples. As the formulations of interest are 2-4 SBNF, only these are being discussed in this section. The NKNF sample was also analysed, in order to directly compare the effect of SBN addition to NKN.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 115 7000 1400 NKNF 1200 6000 2SBN 1000 3 SBN 5000 800 4 SBN
600
4000 400
200 3000 0
20 60 100 140 180 220 260 Permittivity 2000
1000
0 0 50 100 150 200 250 300 350 400 450 500 550 Temperature (C)
Figure 5.16. Permittivities of (100-x)NKN-xSBN + 0.45wt% Fe2O3 (4h) (at 100 kHz), with lower temperatures magnified, inset.
It is clear in Figure 5.16 that as SBN is added to NKN, the transition temperatures, both TO-
T and TC decrease. The NKNF sample has a Curie transition temperature of 457°C and TO-T of 234°C. This sample was sintered at 1100°C, giving the optimal density and thus sintering temperature for NKN. The SBN-doped samples were sintered at the higher temperature of 1140°C to yield optimal electrical results. When plotted together (Figure 5.17) the transition temperatures significantly reduce with increasing SBN. However there is an interesting effect at the Curie transition. The 2SBNF sample has a significantly lower relative permittivity value of 4500 at TC (427°C), which is relatively small in comparison to NKNF which was 6500 at 457°C. So the 2SBNF sample has a smaller relative permittivity value at the transition temperature than NKN (where TC is 30°C lower).
500 450
400 350 Tc 300 To-t 250
Temperature (ºC) Temperature 200 150 0 1 2 3 4 x SBN (%)
Figure 5.17. The transition temperatures of (100-x)NKN-xSBN + 0.45wt% Fe2O3 (4h)
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 116 As the SBN addition increases, the transition temperatures continue to decrease, but the relative permittivity at TC increases again, to 6000 (at 416°C) for 3SBNF. Similarly for the 4SBNF formulation there is a decrease again to 3800 (at a lower transition temperature of 397°C). Although there is no direct relationship between the relative permittivity value and SBN doping at TC, there is the significant decrease in the transition temperature, as shown in Figure 5.17. This is the same for the orthorhombic to tetragonal transitions, TO-T, where SBN addition to iron-doped NKN decreases from 234 to 196°C. Again, there is no direct relationship between the values of permittivity, as shown in the inset of Figure 5.16, although there is a significantly higher permittivity in the 3SBNF sample.
Tashiro, Ishii and Wada reported that the Curie temperature of NKN decreases as the addition of strontium to the A-site increases.[106] This is also the case for the doping of NKN by barium titanate (BT); Lin, Kwok and Chan reported a linear decrease of approximately 22ºC per 1 mol% addition of BT.[83] In the present study, the addition of SBN decreased the Curie temperature by 14.7ºC per 1 mol% addition, which is at a lower rate than with the addition of BT. The decrease in the orthorhombic to tetragonal phase transition temperature is at a rate of 9.0ºC per 1 mol% SBN addition, whereas the BT rate of decrease was by 25ºC per 1 mol%. The rates for SBN doping are significantly lower for both transition temperatures, although significantly lower than the BT rate for TO-T. This significant difference could be due to the Ti4+ atomic substitution into the Nb5+ B-site in BT (where Nb5+ is in the B-site in both NKN and SBN).
5.6.1.2 Relative Permittivity as a Function of Sintering Time To continue the investigation of SBN doping of NKN, one formulation was chosen (2SBNF) and samples were sintered at various sintering times. These were 4, 8, 16, 24 and 72 hours. All samples were over 94% dense. The resulting relative permittivities and transition temperatures were determined (Figure 5.18). The data shows that the sintering time is not a significant factor. The 4 hour sintered sample has a TC of 427°C, whereas the 72 hour sintered sample has 436°C. There was even less difference for the orthorhombic- tetragonal phase transition temperatures. These results show that it is not necessary to examine (in any subsequent studies) the effect of sintering time in SBN doped NKN samples.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 117 500
450
400
350 Tc 300 To-t
250 Temperature (ºC) Temperature 200
150 0 10 20 30 40 50 60 70 sintering time (hours) Figure 5.18. The effect of sintering time on the transition temperatures of 2SBNF.
5.6.2 Field Response Hysteresis Loops
5.6.2.1 Effect of SBN doping The hysteresis behaviour of xSBNF (2 ≤ x ≤ 4) formulations were also investigated. Field response measurements take into account the size of the sample, and so the results are directly comparable to each other. In 2006, Chang et al reported the P-E hysteresis behaviour of NKN along with 0.5 mol% AETiO3 (AE = Mg, Ca, Sr, Ba) alkaline earth dopants.[95] They showed that the addition of these dopants was detrimental to the remnant polarisation, Pr, of NKN as the undoped material had the highest Pr of 18.8
μC/cm2 and a coercive field of Ec = 9.65 kV/cm. Such high properties have not been seen for undoped NKN elsewhere.[155]
From the present work it is clear that when 0.3wt% Fe2O3 is added to NKN, the hysteresis behaviour changes. It exhibits a slightly higher Pr of 22 μC/cm2, which is desired, although a larger coercive field of 16.5 kV/cm, which is undesirable. This can be seen through the effect of a “lossy” hysteresis loop, which is shown in blue in Figure 5.19. The preferred type of hysteresis loop is considerably more saturated with a much lower coercive field and a higher Pr value, like the one shown in red in Figure 5.19; this is for the 2SBNF formulation. This loop shows that the addition of SBN to NKN is highly advantageous for hysteresis behaviour. Both samples broke down in fields above 40 kV/cm, which is in agreement with other NKN publications.[70, 106, 187]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 118 NKN + 0.3 wt% Fe2O3 (Ts = 1090C) 30 2 SBNF (Ts = 1140C) 20
10
/cm2) 0
-40 -30 -20 -10 0 10 20 30 40 μC
P ( P -10
-20
-30 E (kV/cm)
Figure 5.19. Hysteresis behaviour of NKN + 0.3wt% Fe2O3 (Tsint = 1090°C) and 2SBNF
(Tsint = 1140°C)
30
2 SBNF 20 3 SBNF
4 SBNF 10
0
-40 -30 -20 -10 0 10 20 30 40
C/cm2) μ
-10 P ( P
-20
-30 E (kV/cm) Figure 5.20. P-E hysteresis behaviour for (100-x)NKN-xSBNF (2 ≤ x ≤ 4).
When comparing the hysteresis loop of 2SBNF to 3 and 4 SBNF samples (Figure 5.20), the
4SBNF sample is clearly the poorest formulation in terms of domain switching and Pr and
EC values. The 2SBNF sample has the highest Pr with 25 μC/cm2, although 3SBNF has a Pr of 24 μC/cm2 and they both have identical coercive fields of EC = 8.8 kV/cm. In comparison with pure NKN, which has a Pr of 18.8 μC/mm2 and a Ec of 9.65 kV/cm,[95] these are both significantly improved properties.
The 4SBNF sample shows a lower Psat (19 μC/cm2), which could be related to the second phases visible in the microstructure (Section 5.5). These could cause a larger disruption in the microstructure and domain configuration, thus restricting domain wall movement in larger grains.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 119 Interestingly, the 3SBNF sample, although it has very similar properties to 2SBNF, when under a negative field it does not reach full saturation of aligned domains to the direction of the field. This occurs in a positive field, as shown by the sharp end on the hysteresis loop, although in the negative field end of the loop is visibly more “blunt” and rounded, indicating full saturation has not been achieved. When the field was increased, however, the sample broke down and a fully complete saturated loop could not be acquired. The rounding of loops at the extreme edges can be caused by conduction in the sample,[115] which for such samples is undesirable.
Figure 5.21 shows the switching of 4SBNF as the applied electric field increases, to give a fully saturated loop. A fully saturated loop is preferred as it shows that the domains are fully switched and aligned with the direction of the applied field in both directions. The sample exhibits a Pr and EC of 19 μC/cm2 and 10 kV/cm respectively. These results, however, are marginally enhanced from NKN, which exhibits maximal values of Pr = 18.8
μC/cm2 and EC = 9.65 kV/cm.[95]
25 40 kV/cm 35 kV/cm 20 30 kV/cm 25 kV/cm 15 20 kV/cm 10 15 kV/cm 10 kV/cm 5
5 kV/cm
0 -40 -30 -20 -10 0 10 20 30 40 -5
P(μC/cm2) -10
-15
-20
-25 E (kV/cm) Figure 5.21. Hysteresis behaviour of 4SBNF at varying field strengths.
It is difficult to directly compare these results with published data due to the lack of tetragonal tungsten bronze type (TBT) materials doping perovskite NKN. Similar dopants for the purpose of discussion are the alkaline earth doping of NKN (AE = Mg, Ca, Sr and Ba) of which SBN (Sr0.5Ba0.5)Nb2O6 contains two of these(also on the A-site). Chang et al. reported the effect of adding 0.5 mol% AETiO3 to NKN.[95] The resulting P-E hysteresis behaviour showed a lower EC and higher Pr for undoped NKN than for any of the dopants,
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 120 as discussed earlier. The Pr value they reported for NKN is 18.8μC/cm2 whereas the 2SBNF sample exhibits 25 μC/cm2 (Figure 5.19), showing a significant rise in the remnant polarisation; the coercive field values are nearly identical. This “soft” effect of a relatively high Pr and low EC is a good result for SBN doping of NKN. Similar changes in properties are often found near orthorhombic to tetragonal MPB regions due to the increased number of directions in which the spontaneous polarisation can occur, along with an increased number of domains in the microstructure.[188]
In spite of this enhanced result, similar Pr values have been reported, such as Pr =
25 μC/cm2 and EC =18.5 kV/cm in 94.8NKN-5.2LiSbO3.[115] Although the remnant polarisation value is identical to that of the 2SBNF sample investigated here, the reported coercive field value is more than double that of the 8.8 kV/cm for 2SBNF.
The combined iron addition and SBN doping could have a significant effect on this composition. When 0.5mol% Fe2O3 was added to NKN, Zuo et al reported the Pr value increased from 15.1 to 21.5μC/cm2.[70] This is a significant increase, attributed to the addition of Fe3+ ions to the B-site Nb5+, creating oxygen vacancies. However, it is well documented that acceptor doping (such as in this investigation) creates “hard” piezoelectric materials with oxygen vacancies, which should decrease Pr and increase
EC.[18] This is due to the vacancies causing domain wall pinning, and this in turn causes the coercive field to increase. The relatively small addition of iron in this case may counter this “hardening” effect and optimise NKN. This effect may be seen with larger amounts of iron doping.
In 2011, Li et al. reported that Li0.06(Na0.535K0.48)0.94NbO3 (NKLN) doped with 0.7mol% ZnO as a sintering aid exhibited an optimal Pr value reaching 21 μC/cm2 and EC =7.5 kV/cm.[99] This shows that 2SBNF and 3SBNF formulations exhibit properties comparable to leading NKN dopants.
5.6.2.2 Effect of Sintering Temperature and Time In a subsequent investigation of 2SBNF, the sintering temperature was increased from 1140°C to 1160°C. The resulting hysteresis behaviour was practically identical (shown as blue in Figure 5.22), and so is not advantageous for the material, and yet would cost more - in time and money. A sample sintered at 1140°C for 24 hours (as opposed to the standard 4 hours used for all samples discussed in this chapter) was also investigated. As mentioned in Section 5.6.1.2, the dielectric data did not vary greatly for the 4 hour sample, however it exhibits significantly different hysteresis behaviour. As can be seen by the green line in
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 121 Figure 5.22, both Pr and EC are weakened, down to 23 μC/cm2 and 21 kV/cm respectively.
The increased EC means that the material has become “harder” with the longer sintering time. This is due to more oxygen vacancies being generated in the material.[155] These vacancies must form during the sintering process, where there is more time available at the elevated sintering temperature (in this case, 1140°C) to allow oxygen to escape from the material.
Although hardening is not necessarily a desirable property for doped NKN (as “soft” piezoelectrics are used for ultrasound transducers, sensors and actuators[18]) it does offer a beneficial reason for the longer sintering time. The sample can withstand higher electric fields than other NKN materials, either here in this investigation or reported in the literature.[70, 106, 187] This 2SBNF sample sintered at 1140°C for 24 hours sustained a field of 60 kV/cm, which is higher than any other value reported. The result is shown by the orange line in Figure 5.22.
35 1160C, 4h 30 25 1140C, 4h 20 1140C, 24h 15
1140C, 24h 10 (higher E) 5 0 -60 -50 -40 -30 -20 -10 -5 0 10 20 30 40 50 60
-10 P(μC/cm2) -15 -20 -25 -30 -35 E (kV/cm) Figure 5.22. P-E behaviour for 2SBNF sintered at 1140 and 1160°C (4h) (navy and red respectively), and at 1140°C (24h) at 40 and 60 kV/cm (green and orange respectively)
When looking at the effect of fields applied to the 24 hour sintered sample (Figure 5.23), it is clear that the saturation of domains does not occur as quickly as in 4 hour sintered samples (see Figure 5.21). The “S-shaped” saturation curve is only initially visible at a field of 35 kV/cm, which is nearly the breakdown field of the 4 hour samples. As Figure 5.21 shows for 4SBNF, the saturated “S-shaped” curve is already forming at a field of 15 kV/cm. This shows that there is faster switching in the 4 hour samples than in the 24 hour sample.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 122
5 kV/cm 30 10 kV/cm 15 kV/cm 25 20 kV/cm 25 kV/cm 20 30 kV/cm 15 35 kV/cm 40 kV/cm 10 45 kV/cm 50 kV/cm 5
55 kV/cm 0 60 kV/cm -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 -5
-10 P(μC/cm2) -15 -20 -25 -30 E (kV/cm)
Figure 5.23. P-E behaviour for 98NKN-2SBN + 0.45wt% Fe2O3 (Tsint = 1140°C, 24h)
5.6.3 Impedance Spectroscopy Impedance Spectroscopy was used to investigate the resistivity, conduction processes and activation energies of NKN and the NKN-SBN systems. The data is presented as ρ”- ρ’ plots, as opposed to the Z”- Z’ plots that are usually shown in the literature. This is due to the fact of the sample’s dimensions being included in the data, as defined in Equation 5:1, (A is the surface area of the sample disc, t is its thickness and σ is the conductivity) and so is more easily comparable to other data that is published elsewhere.
Z*A 1 * Equation 5 :1 t
5.6.3.1 Varying SBN Content The ρ”- ρ’ plot for NKNF is shown in Figure 5.24, for varying temperatures. It is clear that there is only one major impedance arc present, which is due to the bulk contribution to polarisation and conduction in the material. There is a small second arc at low frequencies (at the right end of the arc) from the grain boundary contribution, however this is not taken into account in this case. The major contribution is from the grain, which is the main arc, and so the data is extrapolated down to find ρ’max.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 123 As the applied temperature increases, ρ’max and ρ”max both decrease (not only for this composition, but for all of those investigated in this section). This loss in resistivity behaviour is due to the presence of space charge polarisation in the ceramic materials.[189] As the temperature of the sample increases, it is clear that resistivity will decrease (as more thermal energy is available), as shown in Figure 5.24. However the conduction process needs to be identified. 0.14 465ºC 0.12 485ºC 505ºC 525ºC 0.10 545ºC 565ºC 0.08 585ºC
605ºC
m) 0.06 625ºC Ω
k 645ºC
0.04 ρ"(
0.02
0.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24
ρ'(kΩm)
Figure 5.24. ρ”-ρ’ plot for NKN + 0.45wt% Fe2O3, with varying temperatures (465-645ºC)
Investigating the (100-x)NKN-xSBN + 0.45wt% Fe2O3 ceramics (all sintered for 4 hours) shows how the resistivity of the system can change with SBN content. This is represented graphically in Figure 5.25, at 545-550ºC (in the cubic region of the system). As NKN is an insulator, it should have as high a resistivity as possible, or conversely the lowest conductivity possible. The iron-doped NKN composition (x = 0) was found to have a very low resistivity of 0.08 kΩm, however with the addition of 2 and 3 mol% SBN, this significantly increased to 4.7 and 5.0 kΩm respectively (at temperatures 545-550ºC). This shows that the addition of TBT Sr0.5Ba0.5Nb2O6 to NKN is advantageous for the electrical insulative properties of NKN. It must be noted, however, that 4 mol% addition was not so beneficial to the system, as it had a lower resistivity of 0.27 kΩm (but still higher than that of iron- doped pure NKN).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 124 3.0
0 SBN
2.5 2 SBN
3 SBN 2.0
4 SBN
m) 1.5
Ω
k ρ"( 1.0
0.5
0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 ρ'(kΩm)
Figure 5.25. Resistivity spectra for (1-x)SBN + 0.45wt% Fe2O3 (4h) at 545-550°C.
An Arrhenius plot (ln σ – 1000/T) of resistivity data (Equation 5:2) is shown in Figure
5.26. The gradient of the linear relationship yields –Ea/kB. These data are included in
Figure 5.26. By multiplying the gradient by Boltzmann’s constant (kB = 1.38065 × 10−23 J/K),[37] the activation energy is obtained (discussed later).
Ea kBT 0e Equation 5 :2
1000/T 0 0.9 1.0 1.1 1.2 1.3 1.4
-2 0 SBN 2 SBN y = -6.92x + 3.99 -4 3 SBN 4 SBN y = -11.45x + 8.36
σ -6 ln ln
-8 y = -13.49x + 8.02
-10 y = -13.46x + 7.88
-12
Figure 5.26. Conductivity-temperature plots for (100-x)NKN-xSBNF + 0.45wt% Fe2O3, with x = 0, 2, 3 and 4 mol% (navy, red, green, and blue respectively).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 125 The relationship between activation energies with respect to SBN addition is depicted graphically in Figure 5.27. It is observed that pure NKN has Ea = 0.60 eV (± 0.03 eV) showing that the main contribution to polarisation is electronic conduction, and comes from the bulk of the material. This is in agreement with Matsudo et al. who reported an activation energy of 0.65 eV for NKN at temperatures above 390°C.[190] The activation energy increased significantly to above 1 eV with addition of SBN (until 4 mol%), showing a change in conduction process within the material. Although this is higher than that for pure NKN, all of these activation energies for this system show that the conduction process stays the same in the material, in that the process is via oxygen vacancy migration. It is well documented that an activation energy value of 1eV shows
ionic conductivity attributed to doubly-ionized oxygen vacancies VO (as shown in Equation 5:6).[64, 191-192] This suggests that there are oxygen vacancies in the system, even though iron doping helps improve the system density, there are still vacancies present which allow conduction.
1.4
1.2
1.0
0.8
0.6 Q (eV) Q 0.4
0.2
0.0 0 1 2 3 4 x% SBN
Figure 5.27. Activation energies for (100-x)NKN-xSBN + 0.45wt% Fe2O3, with 4 hours sintering time.
Zuo et al. studied the effect of Fe2O3 in NKN and reported Fe3+ ions replacing B-site Nb5+.[70] The similar ionic radii of 0.64Å for Nb5+ and 0.65Å for Fe3+ allow relatively easy substitution without distorting the crystal lattice. This replacement causes oxygen vacancies, and further vacancies can result from SBN doping, aiding the sintering process.
Hence there are two sources of oxygen vacancies. As NKN suffers sodium and potassium evaporation during sintering, a non-stoichiometric structure is results. Defect equations showing the changes (written in Kroger-Vink notation) are given as Equations 5:3-5:7.
Equation 5:7 shows the defect equation for the evaporation of sodium in the system, following Equations 5:3-5:6.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 126
Equation 5 :3
Equation 5 :4
Equation 5 :5
Equation 5 :6
Equation 5 :7
As the SBN and iron-doped NKN have higher activation energies, it suggests that there is a decrease in the concentration of oxygen vacancies in the material in comparison with pure NKN. This is in accordance to the density changes, showing that iron doping creates less vacancies in the system, and so is another way of concluding that SBN and iron doping is beneficial to the NKN ceramic system.[47, 65] Perovskites such as NKN are known to contain intrinsic defects, which are primarily vacancies on the A (in this case, Na and K) sites and the O sites. An increase in the number of A-site vacancies increases the resistivity of perovskites, or conversely decreases the conductivity, which is desirable. As the highest resistivity is observed in the iron-doped 2 and 3 SBN samples, this could be a link to the number of vacancies in the material. This means that free electrons in the material are captured by the electron holes produced by the A-site vacancies, and thus less are available to freely conduct, thus giving insulative properties.
Table 5–1 summarises the data for the (100-x)NKN-xSBN + 0.45wt% Fe2O3 system at 545ºC (paraelectric region). It may be concluded that the optimum addition for this system is 2-3 mol% SBN addition, as it has the highest resistivity for the bulk (and thus also the lowest conductivity values) for conduction.
Table 5–1. Summary of electrical data for (100-x)NKN-xSBN + 0.45wt% Fe2O3 system, at 545ºC.
X mol% SBN ρ’max (Ωm) σ’min (S/m) ± error (S/m) Q (eV) 0 80 0.0125 781.25 x 10-6 0.60 2 4700 0.00021 4.47 x 10-6 1.16 3 5000 0.0002 4.00 x 10-6 1.16 4 270 0.0037 137.04 x 10-6 0.99
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 127 5.6.3.2 Effect of Sintering Time
Figure 5.28 shows a ρ”-ρ’ plot for 2SBN + 0.45 wt% Fe2O3 as a function of sintering time for data collected at 645ºC (in the cubic region of the material), with sintering times of 4, 8, 16, 24 and 72 hours (all at 1140ºC). It is clear that as the sintering time rises from 4 hours to 16 hours, the resistivity of the material increases from 0.75 kΩm for 4 hours to 0.95 kΩm for 16 hours (taking the main arc representing the bulk contribution). It then drops dramatically as the sintering time increases to 24 and 72 hours (light blue and purple markers respectively), where the resistivity significantly decreases to 0.12 kΩm (for the 72 hour sample). 0.60 0.55 4h 8h 0.50 16h 0.45 24h 0.40 72h 0.35
0.30
m) 0.25 Ω
k 0.20 ρ"( 0.15 0.10 0.05 0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
ρ'(kΩm)
Figure 5.28. ρ”-ρ’ plot for 98NKN-2SBN + 0.45wt% Fe2O3, with sintering times of 4, 8, 16, 24 and 72 hours at 1120°C (navy, red, green, blue and purple respectively).
The Arrhenius plot of the data (Figure 5.29) shows that activation energies did not vary greatly, around 1.15 eV (±0.15 eV). With consistent values around 1.0 eV for all samples, the conduction process as expected does not change with sintering time. Hence this suggests the same conduction process as found in the previous section, indicating oxygen vacancy migration.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 128 1.4 1.2 1.0
0.8
0.6
Q (eV) Q 0.4 0.2 0.0 0 10 20 30 40 50 60 70 80 Sintering Time (hrs)
Figure 5.29. Activation energies of 98NKN-2SBN + 0.45wt% Fe2O3, as a function of sintering time.
In order to produce an equivalent circuit for this composition, further work will need to be undertaken. However, under closer inspection, the Cole-Cole plots do not pass directly through the origin (typically less than 50 Ω away from it), meaning that an extra impedance element would need to be included in the circuit.[62]
The 4 hour sintered 2SBNF sample has a higher resistivity (both ρ’max and ρ”max) than the 24 hour sample (shown in Figure 5.28), even though the activation energies are similar. From the P-E hysteresis behaviour, it is clear the 4 hour sample has a more desirable hysteresis loop (ie a higher Pr and lower EC) than that of the 24 hour sample (Figure 5.22). This clearly shows that a longer sintering time is detrimental to the electrical properties of such samples.
5.7 Chapter Summary The novel work on the addition of SBN to NKN yielded beneficial results in terms of sample properties.
Fe2O3 is an effective sintering aid for NKN. The addition of 0.3 wt% Fe2O3 increased the density from 90 to 94% theoretical. For the NKN-SBN system, a slightly higher addition of 0.45 wt% was required, resulting in densities over 96% theoretical in formulations containing 2-4 mol% SBN. These formulations were investigated in this study. These samples required a higher sintering temperature of 1140°C, 40°C higher than that needed for NKN and NKNF. Laboratory XRD showed 4SBNF was the solubility limit for single phase orthorhombic NKN.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 129 In terms of grain morphology, 2SBNF showed equiaxed grain structures with 3 μm size, exhibiting lamellar 180° domains. The 3 and 4 SBNF samples showed bimodal grain size distributions, where grains up to 15 μm in size were found within a predominantly 2 μm grain sized matrix. The larger grains exhibited herringbone domain structures.
The electrical properties of samples prepared with 2-4 mol% SBN additions were very encouraging. NKNF exhibited TC = 457°C, TO-T = 234°C, εr = 336, Pr = 22 μC/cm2 , EC = 16.5 kV/cm, Q = 0.60 eV and ρ = 0.08 kΩm. Typical values for NKN are TC = 420°C, TO-T =
200°C, εr = 460, Pr = 18.8 μC/cm2 , EC = 9.65 kV/cm, Q = 0.65 eV.[29, 95, 111, 165, 190]
The addition of SBN caused TC to decrease by 14.7°C per 1 mol% SBN addition; the rate of
TO-T decrease was 9.0°C per 1 mol% SBN addition. As SBN levels increased, the value of permittitivity at the TC also decreased (the value for 4SBNF was approximately half that of NKNF, 3750 and 6470, respectively). In terms of hysteresis behaviour, the optimal formulation was 2SBNF, with fully saturated loop resulting in Pr = 25 μC/cm2 and EC = 8.8 kV/cm (though 3SBNF had very similar properties). For higher level of additions, 4SBNF properties deteriorated exhibiting Pr = 19 μC/cm2 , EC = 10.0 kV/cm – a smaller remnant polarisation than that of NKNF. Samples with 2SBNF and 3BNF additions exhibit the highest resistivity of 4.7 and 5.0 kΩm, respectively. The 4SBNF formulation has significantly lower resistivity of ρ = 0.27 kΩm, although this is still higher than that for NKNF. The resulting activation energies for 2-4 mol% SBN additions are larger than 1 eV which suggests that the conduction process is through an ion transport process, possibly oxygen vacancy migration.
The transition temperatures of 2SBNF did not change significantly with sintering time, although longer sintering time yielded materials that can withstand higher electrical fields (up to 60 kV/cm) producing fully saturated hysteresis loops. Longer sintering times also resulted in a decreased activation energy, showing that conduction is slightly easier.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 130 6 SYNCHROTRON ANALYSIS OF XSBNF FORMULATIONS
6.1 Introduction As reported in Chapter 5, the laboratory XRD spectra (Figure 6.1) reveal a predominantly orthorhombic structure, as expected for NKN compositions, with 4SBNF showing a transition to tetragonal (indicated by peaks at 45-47° 2θ having identical height). The aim of the synchrotron experiment was to investigate the phase content of orthorhombic and possible tetragonal phases present in the NKN matrix using high resolution XRD. By doing so, the changes in microstructure and unit cell lattice parameters as a function of SBN addition could also be studied. Small amounts of second phase were detected in SEM images, however, which were not revealed by laboratory X-ray diffraction. Figure 6.2 shows the visible second phases in 4SBNF, as light grey and white patches. The dark grey matrix phase is NKN, though orthorhombic and tetragonal phases cannot be distinguished in SEM. The Diamond Light Source was therefore used to investigate the NKN matrix structure and second phases in (100-x)NKN – xSBN + 0.45wt% Fe2O3 ceramics. All samples were investigated as crushed powder, to eliminate sample strain and grain orientation.
Figure 6.1. Laboratory XRD profiles of (100-x)NKN – xSBN + 0.45wt% Fe2O3 formulations (0 ≤ x ≤ 4)
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 131
Figure 6.2. SEM micrograph of 4SBNF polished sample, showing light grey and white second phases in a dark grey NKN matrix phase. Yellow arrow indicates light grey second phase, and red arrow indicates white second phase.
6.2 Synchrotron Data Analysis The synchrotron diffraction profiles are shown in Figure 6.3. When compared with laboratory XRD (Figure 6.1) the difference in resolution (including the presence of second phases) is clear. The difference in the 2θ peak positions is due to the use of radiation of different wavelengths (laboratory XRD λ = 1.54060 Å and synchrotron XRD λ = 1.40000 Å).
20000
18000 4 SBN 3 SBN 2 SBN 16000 1 SBN 0.5 SBN 14000 0 SBN
12000
10000
8000
6000
4000
2000
0 10 12 14 16 18 20 22 24 26 28 30 Two Theta (synchrotron) Figure 6.3. Synchrotron XRD profiles of xSBNF formulations (0 ≤ x ≤ 4)
It is also clear in Figure 6.3 that there is a second phase detected in the 4SBNF sample (the major peaks of this second phase are indicated), and is starting to form at lower SBN additions. Though this is relatively low content in comparison to the 4SBNF peak intensities, it is endeavoured to determine this phase (discussed in section 6.2.4).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 132 6.2.1 NKNF Refinement
Data refinement shows that 0.45 wt% Fe2O3 addition to NKN does not significantly affect the structure. The room temperature synchrotron data indicated that NKN was orthorhombic, as expected. In Figure 6.4, the refined data is very close to the experimental data, where an orthorhombic model was applied. The difference between the two is shown by the grey line at the bottom of each diffractogram. This is confirmed by the small Goodness of Fit (GOF) value of 3.61. This is an extremely close fit considering the large number of data points in the refinement and the large range of 2θ. This shows that low level iron doping does not affect the NKN structure. It is also a good starting point for the refinements of SBN-doped formulations.
(a)
(b)
Figure 6.4. The refinement of NKNF as (a) the full refinement (square root y axis) and (b) peak profiles of 10-50 degrees 2θ (logarithmic y axis). The blue line shows the observed data, the red line shows the calculated model, and the grey line below shows the difference between the two.
The unit cell parameters resulting from this refinement are a = 3.94433Å, b = 5.64356Å and c = 5.67611Å. The pseudocubic monoclinic parameter lengths a’ and b’ for NKNF are 3.9443372 and 4.002125 Å. These are very close values reported by Tellier et al for NKN (a’ = 4.0046 and b’ = 3.9446 Å).[163] This is a good indication that the refinement is successful and that the iron addition does not have an effect on the orthorhombic NKN unit cell size. This is in agreement with the findings of Zuo et al. that Fe2O3 does not change the crystal symmetry (≤ 4 mol%).[70]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 133 6.2.2 0.5 and 1SBNF Refinement Data for 0.5SBNF refined well to an orthorhombic structure with a low GOF of 3.86. Introduction of a tetragonal phase into the refinement model reduced the GOF to 3.02. The refinement suggested that the tetragonal component amounted to approximately 14%, which is significant. In light of this finding, a tetragonal phase was included in the NKNF refinement, but the tetragonal phase did not improve the refinement in any way. This shows that SBN doping is the sole cause of tetragonal phase formation in NKN. This, however, is a valid conclusion as SBN has a tetragonal TBT structure.
When 1 mol% SBN was added to NKNF, the tetragonal phase was more prominent in the material, supporting the argument that the tetragonal phase is present as a result of SBN addition. The proportion of tetragonal phase increased to 21%, and the GOF value was 4.36. There are a number of small undefined peaks between 17 and 20° 2θ, and sporadically along the baseline; this is one reason for the higher GOF value in this case.
Figure 6.5. The refinement of 1 SBNF.
6.2.3 2-3 SBNF Refinements Similarly, refinements of 2SBNF and 3SBNF gave orthorhombic to tetragonal ratios of 91:9 and 92:8, respectively. The GOF values were much lower, at 2.66 and 2.99 for 2 and 3 mol% SBN addition, respectively. Figure 6.6 shows the refinement of 3SBNF. In spite of the very good fit between the experimental data and the refined model (the difference is shown by the grey line in Figure 6.6), there were some peaks between 13 and 20º 2θ that were not refined, giving rise to the slightly higher GOF value. Inclusion of SBN as a separate phase did not improve the model or the refinement.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 134
Figure 6.6. The synchrotron XRD spectrum and refined data for 3SBNF (logarithmic y-axis)
6.2.4 4 SBNF Refinement This refinement was significantly different to previous ones as the second phase was considerably more prominent (Figure 6.7a). By refining the whole diffraction spectrum (5-145º 2θ) with the orthorhombic and tetragonal phases, as previously, the ratio of orthorhombic to tetragonal phase is 83:17, giving a GOF value of 3.37. In order to take into account the second phase, SBN was included in the refinement model. Structural data for SBN was taken from the work of Podlozhenov et al.[193] Inclusion of SBN in the refinement model was successful (as is evident in Figure 6.7b) and the GOF value fell to 2.58, which is the lowest value recorded for all the refinements. The resulting model showed 3% SBN phase in the material.
(a)
(b)
Figure 6.7. Synchrotron diffraction profile for 4SBNF (a) in the range 0-46º 2θ, square-root y-axis, and (b) in the range 10-48º 2θ, logarithmic y-axis.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 135 That the SBN phase was detected in the 4SBNF material is a significant finding. Under laboratory XRD there was no evidence of a second phase (Figure 6.1) and yet was visible in SEM. EDAX analysis on the second phase in 4SBN (Chapter 5, Section 5.5), however, found that this structure was potassium and barium rich, which is not the correct ratio of elements for an SBN phase. This proves that the SBN phase found in this case is not necessarily SBN, but a phase comprising of a very simliar tetragonal tungsten bronze type (TBT) structure.
Following this discovery, data for the 2 and 3 SBNF materials were reinvestigated, including the SBN phase. This was not productive even though the unidentified peaks were in similar positions to those for the SBN peaks (Figure 6.3). It is possible that the amount of second phase in the 2SBNF and 3SBNF samples is lower than 1%, which is the limit of resolution in the TOPAS program.[194]
6.3 Summary of Data from Refinements
6.3.1 Phase Content The refinements gave a good indication of the orthorhombic and tetragonal phases coexisting in the NKN-SBN system. The resulting phase proportions in (100-x)NKN-xSBN
+ 0.45wt% Fe2O3 samples are summarised in Table 6.1 and Figure 6.8. Phase content is accurate to 1% in Topas refinements.[194] It is clear that samples with 0.5 and 1 mol% addition have the highest tetragonal to orthorhombic ratio and the highest GOF value, indicating that these samples warrant further investigation.
Table 6–1. The phase content for xSBNF refinements, including GOF values.
x% SBN Orthorhobic (%) Tetragonal (%) TBT (%) GOF
0 100 0 0 3.61
0.5 85 15 0 3.02
1 79 21 0 4.36
2 91 9 0 2.64
3 92 8 0 2.99
4 82 15 3 2.58
The data are illustrated in Figure 6.8. It is clearly seen that as SBN is added to the iron- doped NKN, the orthorhombic nature of the structure decreases as the tetragonal
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 136 component increases; this is clearly influenced by the SBN, which has tetragonal TBT structure. Although the 1SBNF has a 20% tetragonal component, its presence was only revealed by high resolution synchrotron XRD.
As SBN doping increases to 2 and 3 mol% there is a reduction in tetragonal component to ~10% and corresponding increase in the orthorhombic component. As noted in Chapter 5, the samples prepared with 2-4 mol% SBN have the optimal electrical properties. At this doping level the samples exhibit the largest orthorhombic component and indeed the 4SBNF sample also contains a TBT phase to the level of 3%.
100 90 80
70
60 orthorhombic 50 tetragonal 40 TBT phase
% phase contentphase % 30 20 10 0 0 1 2 3 4 x% SBN Figure 6.8. Phase Content in NKNF as a function of x mol% SBN addition.
This is in agreement with the SEM for xSBNF formulations. Figure 6.2 clearly shows second phases (shown as light grey and white patches within a dark grey NKN matrix); although 2 and 3 SBNF showed second phases, these were very small and sparse, so no conclusive EDAX data could be drawn. As these are not detected by sunchrotron XRD, the phases are too small to be detected and thus are not deemed significant enough to warrant further attention.
6.3.2 Lattice Parameters
6.3.2.1 Orthorhombic Phase The lattice parameters for the orthorhombic phase are shown as a function of SBN addition in Table 6–2 and Figure 6.9. The overall effect of the SBN doping gives rise to a decrease in the b’ parameter (at a rate of 0.0006 Å per 1 mol% addition, and an average increase in the a’ (and c’) parameter of 0.0012 Å per 1 mol%. This shows that the pseudocubic cell changes significantly as a result of SBN addition.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 137 Table 6–2. Pseudo-cubic parameters for orthorhombic phase in xSBNF (accurate to 5dp)
x SBN a’ b’ 0 3.944337 4.002125 0.5 3.944066 4.001212 1 3.944789 4.001336 2 3.947180 3.999996 3 3.948753 4.000606 4 3.947859 3.999075
4.01
4.00
3.99
a' = c' 3.98 b'
3.97 parameter length (A) length parameter 3.96
3.95
3.94 0 1 2 3 4 x% SBN Figure 6.9. The pseudocubic lattice parameters for the orthorhombic region of xSBNF. The cuboid represents the shape of the unit cell (not to scale)
6.3.2.2 Tetragonal Phase The tetragonal pseudocubic lattice parameters are shown in Table 6–3 and Figure 6.10. They are clearly different from the orthorhombic lattice parameters. The tetragonal pseudocubic lattice parameters change dramatically with composition. At low levels of SBN additions a’ > b’, but from 2% SBN b’ > a’. These changes are significant as the ferrooelectric properties are enhanced (as discussed in Chapter 5) in the 2-4 mol% SBN region. For example, 2SBNF exhibits a Pr and EC of 25 μC/cm2 and 8.8 kV/cm.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 138 Table 6–3. Pseudo-cubic parameters for tetragonal phase in xSBNF (accurate to 5dp)
x SBN a’ b’ 0 - - 0.5 3.981284 4.005615 1 3.979612 4.007148 2 3.989798 3.975777 3 3.990946 3.974705 4 3.985617 3.972295
4.015
4.010
4.005 a' = c'
b' 4.000
3.995
3.990
3.985 parameter length (A) length parameter 3.980
3.975
3.970 0 1 2 3 4 x% SBN Figure 6.10. Pseudocubic lattice parameters for the tetragonal region of xSBNF as a function of SBN addition. Cuboids represent the shape of the unit cell (not to scale)
It is difficult to directly compare this data with other equivalent studies. Seo et al reported tetragonal parameters for 5 mol% addition SrTiO3 (ST) to NKN (a = b = 3.39 Å and c = 3.75 Å); they are significantly smaller than values found in this study, although Seo et al. do not indicate whether they relate to a pseudocubic unit cell, nor report data for undoped NKN. Ahn et al. reported the coexistence of orthorhombic and tetragonal phases when
BaTiO3 (BT) was added to NKN.[195] For addition levels between 3 and 6 mol%, both phases were detected. For 96NKN-4BT, the tetragonal parameters were a = 3.9663(4) Å and c = 4.0126(6) Å and samples contained 67.4 % tetragonal component.[195] This is significantly higher than the 15% tetragonal component found in this study for 4SBNF, although lattice parameters are not grossly different.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 139 6.3.2.3 Tungsten Bronze Type Phase This phase was only refined in the 4SBNF formulation. The parameters for this are a = 3.948327(11) Å, b = 5.640070(17) Å and c = 5.670146(17) Å. This is based upon the SBN structure applied to the refinement, however cannot be SBN due to the deficiency of strontium in the second phases found by EDAX analysis (see Chapter 5, Section 5.5).
6.4 Chapter Summary By use of synchrotron X-ray diffraction, second phases were detected and identified. In the 4SBNF formulation a tetragonal tungsten bronze phase was identified; comprising 3% of the sample. More generally, a tetragonal NKN phase was detected in all SBN doped samples, coexisting with the orthorhombic NKN matrix. This tetragonal phase was found to be present at levels up to 20% (for samples prepared with 1 mol% SBN). Single phase NKNF exhibited an orthorhombic strcuture, as expected, with pseudo- cubic parameters a’ = c’ = 3.94437 Å and b’ = 4.002125 Å. Lattice parameters for both phases are given in Tables 6-2 and 6-3, as a function of SBN addition.
The most accurate refinements in the system were achieved for samples prepared with 2-4 mol% SBN, which showed an orthorhombic to tetragonal phase ratio of 9:1 for 2 and 3 SBNF; the 4SBNF samples contained significantly less orthorhombic phase, only 82% along with 15% tetragonal (the remaining 3% was a tetragonal TBT phase).
The 2-4 SBNF formulations exhibit enhanced electrical properties (see Chapter 5) and the lattice parameters change significantly with composition. Low (< 2%) SBN doping generates a tetragonal unit cell with a large b’ lattice compared to a’ (and c’). Higher doping (≥ 2%) leads to a smaller b’ lattice parameter relative to a’ (and c’) and it is this structure of unit cell that is associated with enhanced electrical properties.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 140 7 DEVELOPMENT OF A ROUTE TO PRODUCE ORIENTED NKN THICK FILMS
7.1 Introduction The method of tape casting is a relatively new approach to manufacturing textured electroceramics. It is well known that single crystal ceramics have superior electrical properties due to the lack of defects in their microstructure,[128, 134, 196] but they are, however, difficult to grow and thus very expensive to produce. There has been much effort into texturing ceramics in order to produce ceramics that have electrical properties much like those of the single crystal, rather than bulk ceramics that are most commonly produced. This chapter involves the development of an orientation procedure for NKN ceramics to investigate whether thick film tape casting multi-layering (up to 200μm thickness) is a reliable and viable manufacturing procedure for enhanced electrical properties.
The tape casting process is a relatively new process in this field; a few other research groups have investigated the Templated Grain Growth (TGG) approach for NKN. These include the Toyota group in Japan,[15-16] and a group in America spearheaded by Gary Messing and Susan Trolier-McKinstry, the latter published their first findings in 2010.[197] A method of orienting NKN developed by Saito and Takao (Japan) uses precursor particles, most commonly those of plate-like Sodium Niobate (NN, NaNbO3) particles.[198]
As NN is one of the end members of NKN (those of NaNbO3 and KNbO3), the NN can be utilised to produce oriented NKN. The same group also attempted oriented NKN production using KN precursor particles.[144] In order to produce both these template particles, plate-like precursor particles, K4NbO17 [144, 198] or Bi2.5Na3.5Nb5O18 (BNN) were produced to form the NN and KN template particles.[15, 118, 142, 144] Other templates that have been used to orientate NKN, or other lead-free perovskite materials, have been plate-like Bi4Ti3O12, SrTiO3 and BaTiO3; and acicular
(needle-shaped) Sr2Nb2O7, Al2O3-B2O3 and PbNb2O6.[138, 182-183, 199]
In order to produce oriented NN particles, Bismuth Sodium Niobate (BNN,
Bi2.5Na3.5Nb5O18) plate-like particles first need to be produced. These are converted into NN particles using a topochemical process. This procedure was first reported by Saito et al. (from the Japanese Toyota group) in 2004.[15] They reported that NKN can be oriented using a Reactive Templated Grain Growth (RTGG) process using NN particles. The resulting oriented NKN was found to have enhanced electrical properties in terms of
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 141 d33 of 416pC/N. This was a significant achievement in producing NKN-based materials with such a high piezoelectric d33 constant.
It should be noted, that all published accounts of BNN and NN particle production, are very vague in terms of experimental details. The following procedures have been developed and investigated as independent research. The approach has been developed to ensure optimal results with minimal raw material wastage. Therefore all stages were undertaken in small batches and work on a small scale, which should be possible to scale up on a production line.
Full details of the optimal processing route developed are given in Section 3.2.2.
7.2 The Production of BNN Particles
7.2.1 Crucible Selection
The first step of making Bi2.5Na3.5Nb5O18 (BNN) particles was to identify the most suitable crucible. Initially, small pellets of BNN mixture were heat treated at 1100ºC on a variety of crucible substrate materials (magnesia, alumina and platinum foil) to test if the material would react with the oxides. The pellets reated severely with all the substrates, including the platinum foil. For BNN and NN particle production the Toyota research group use Platinum crucibles,[15] whereas the research group of Gao Feng et al. in China use alumina crucibles.[127] On this basis, small crucibles (no larger than 40mm high) of quartz, alumina and platinum containing BNN were heated at 1100ºC. The results are shown in Figure 7.1. The BNN mixture reacted with the alumina crucible, and proved difficult to remove from it. The quartz crucible did not react with the BNN mixture, however the product was visibly less yellow. This is possibly due to loss of Bismuth from the mixture. The platinum crucible showed little signs of reaction with the mixture and the contents retained the bright yellow colour. The BNN mixture was also relatively straightforward to remove from the crucible using sonication techniques. This shows that Platinum crucibles are evidently more suitable for processing BNN. Consequently, Platinum crucibles were used for all stages of the production.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 142 (a) (b) (c)
Figure 7.1. BNN particles heat treated under identical conditions, using (a) quartz, (b) alumina and (c) platinum crucible.
7.2.2 Preparation of BNN particles
Raw powders Bi2O3, Na2CO3 and Nb2O5 were vibratory milled together and sintered to produce plate-like BNN particles. The full experimental procedure is given in Section 3.2.2. The initial procedure involved mixing the raw powders together and mixing with a 1:1 salt to oxide ratio with NaCl. Zhang et al. reported an optimal BNN sintering temperature of 1130ºC to achieve the largest well defined BNN particles.[141] In this investigation, however, the initial starting temperature was 1100ºC produced particles shown in Figure 7.2. The particles (observed before the hot washing stage) are clearly visible, although highly agglomerated. Individual particles are of sizes up to 20μm in size, which is comparable to that reported in the literature.[16, 142] Figure 7.2 shows evidence of particle melting in the large agglomerates, suggesting the sintering temperature should be lowered.
Figure 7.2. BNN particles before hot washing stage, sintered at 1100ºC.
Furthermore, the partial loss of the yellow colour from the pre-sintered mixture suggesting possible loss of bismuth during sintering, led to a reduction of the sintering temperature to 1050ºC. The resulting BNN particles, shown in Figure 7.3, are better defined and slightly larger (up to 25μm in size) than those shown in Figure 7.2. EDAX analysis of the particles (Table 7.1) shows the chemical composition to be equivalent to
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 143 Bi2.30Na2.83Nb5.16O18.29. Although this is not the exact Bi2.5Na3.5Nb5O18 composition expected for BNN, it is very close.
(a) (b)
X
Figure 7.3. BNN particles sintered at 1050ºC, before hot washing stage. X denotes the area under EDAX investigation.
Table 7–1. The EDAX data for a pre-hot-washed BNN particle.
Element Atomic % O (K) 64.01 Na (K) 9.91 Nb (L) 18.05 Bi (M) 8.04
These particles were then hot washed 8 times and examined again by SEM. The resulting micrographs are shown in Figure 7.4. There are clearly square plate-like particles visible, with sizes ranging 5-25 μm. EDAX results are shown in Table 7.2, where the effective composition is Bi2.32Na2.80Nb5.65O17.79. This is still not the exact composition expected for BNN, as there is a deficiency in the amount of sodium. This is difficult to adjust however, due to the presence of excess sodium in the system during the molten salt synthesis (MSS) process.
(a) (b)
X
Figure 7.4. SEM micrographs showing BNN particles sintered at 1050ºC after hot washing. X denotes the area under EDAX investigation.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 144 Table 7–2. The EDAX data for a hot-washed BNN particle.
Element Atomic % O K 62.28 Na K 9.81 Nb L 19.79 Bi M 8.12
7.2.3 The Effect of Salt to Oxide Ratio In the literature, different salt to oxide ratios are reported. Zhang et al report the optimum ratio to be 1:1,[141] however Chang et al prefer a 3:2 ratio.[142] This aspect needed to be investigated to define the optimal route for BNN particle production.
Figure 7.5 shows the BNN particles produced under the same conditions as the previous samples, but with a 3:2 salt to oxide ratio, instead of the 1:1 utilised previously. Both sintering temperatures of 1050 and 1100ºC were used to allow for direct comparison with the earlier results. Figure 7.5 shows images of the material before hot washing. It is clear that the particles are not well defined as plate-like structures and are not completely formed.
(a) (c)
(b) (d)
Figure 7.5. SEM micrographs showing BNN particles using the 3:2 salt:oxide ratio (a,b) sintered at 1050ºC, and sintered at 1100ºC (c, d).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 145 It is clear from Figure 7.6 that a temperature of 1100ºC is too high for BNN particle production, as the yellow colour of the powder is significantly reduced, showing loss of bismuth from the system. Thus BNN production using the initial powders need a lower temperature than that reported by Zhang et al.[141]
(a) (b)
Figure 7.6. BNN powder using 3:2 salt:oxide ratio used in alumina crucibles, sintered at (a) 1050 and (b) 1100ºC.
From these initial investigations (summarised in Table 7.3), it was found that the optimal conditions to produce BNN particles were: a platinum crucible, with a 1:1 salt to oxide ratio for the MSS route and a sintering temperature of 1050ºC. Repeated batches of BNN particles were made using these conditions ready for the next stage of the process.
Table 7–3. Resulting data from BNN particles in this investigation.
Tsint (ºC) Salt:Oxide Particle Size Desired Structure? 1100 1 : 1 < 20μm Melted and agglomerated 1100 3 : 2 < 25 μm Yes, but faceted 1050 1 : 1 < 25 μm Yes 1050 3 : 2 < 20μm Yes, but faceted
7.3 The Production of NN Particles
The hot-washed BNN particles were mixed with Na2CO3 in a stoichometric ratio and added to NaCl using a 1:1 salt to oxide ratio for the molten salt synthesis (MSS) method. They were stirred together and again heat treated in the furnace for 8 hours at 1050ºC. This process was developed from the initial details reported by the Toyota group.[145]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 146 According to Saito et al.[15] a topochemical reaction occurs and the Bismuth in the system is displaced by the Sodium to produce NaNbO3 plate-like particles.
After heating the BNN particles with Na2CO3 and NaCl for 8 hours at 1050ºC, the platinum crucible was filled with deionised water and heated on a hotplate for 30 minutes at 80ºC. Some of the resulting solution was taken from the crucible using a pipette, and put onto a glass slide. After evaporating to dryness, the product was examined by SEM (Figure 7.7).
Figure 7.7. SEM micrographs of initial NN particles from solution at (a) low and (b) high magnification.
Table 7–4. The EDAX data for chemical compositions of Figure 7.7 (Spectrum 3).
Element Atomic %
O (K) 69.18
Na (K) 18.17
Nb (L) 12.64
The largest platelets visible in Figure 7.7 are NaCl salt crystals from the flux used in the MSS process. EDAX analysis of a smaller grain showed the presence of sodium and niobium (Table 7.4). This equates to NaNbO3 which is being produced; a good indicator that that NN particles are forming. It must also be noted, that there was no sign of bismuth oxide. This shows evidence of successful topochemical Bi to Na conversion.
As the MSS method was yielding NN particles, additional batches were produced in the platinum crucible at 1050ºC. The resultant product was placed in a beaker of deionised water and sonicated to agitate and separate the particles with minimal damage. They
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 147 were then hot washed at 80ºC until there was no reaction with dilute silver nitrate (meaning there were no longer Cl¯ ions present, and the salt flux has been successfully removed). A small sample of the resultant particles was distributed on an SEM stub and examined by SEM (Figure 7.8). EDAX analysis confirmed the particles comprised of sodium and niobium close to the desired ratio for NaNbO3.
Figure 7.8. SEM micrographs showing an NN particle sintered at 1050ºC.
In order to assess the effect of sintering temperature on NN particle production, another batch of BNN particles was mixed with NaCO3 in the way previously described, but then sintered at 1100ºC. After following identical procedure, there was little evidence of the particles after eight hot washing and filtering steps. Sections of filter paper were cut and examined by SEM. Although particles were visible in between the fibres of the filter paper, they were significantly smaller than those prepared by sintering at 1050ºC, showing that larger particles form more easily at the lower sintering temperature.
The optimal procedure for successfully converting BNN particles to NN particles is use of a platinum crucible and sintering temperature of 1050ºC, like the BNN procedure. This resulted in particles approximately 15μm wide and 0.5μm thick, directly comparable to the particles produced by the Toyota group in 2004.[15] The technique developed yields products similar to those reported in the literature[16, 124, 141] and so is potentially viable for subsequent tape casting.
7.4 Development of Tape Casting Procedure
7.4.1 Tape Casting Method Development Tape casting can be undertaken in many ways (see Section 2.4.1.1). It was found that the optimal consistency of the desired slurry was close to that of Tippex correction fluid; this slurry was used for the trial casting experiments. As the available tape casting facility
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 148 required a minimum of 200ml of slurry, two related “micro” procedures were developed requiring only 2ml slurry.
The first arrangement is illustrated in Figure 7.9. A block of 100 x 50 x 15mm stainless steel was milled to provide a slot of 20 x 55 x 0.1mm. The slot provides the track into which the slurry is poured and cast using a glass slide. This procedure, however, does not allow for multiple tapes to be cast from the same slurry (before drying out) because there is only one track available. This method has severe limitations in terms of tape thickness and the rate of production of tapes. Here, only one tape may be cast from the slurry as there are no further die to utilise.
Figure 7.9. Schematic illustration of the block casting procedure.
The second procedure involves the use of parallel tracks on a glass slide. Initially, different types of adhesive tape were used to make the tracks: sellotape, parcel tape, masking tape, scotch tape, and duct tape. It was found that masking tape gave the optimal tape for 100μm thick slurry of correction fluid, and was the easiest to utilise. Alternative substrates were tested but glass slides proved the most effective. The optimal procedure is shown in Figure 7.10.
Figure 7.10. Schematic illustration of the glass slide casting procedure.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 149 7.4.2 Slurry Development In order to cast particles within the NKN matrix for templated grain growth (TGG), the particles and NKN matrix needed to be cast from a slurry. The slurry needs three main additives: binder, solvent and plasticiser. The binder, some form of polymer, is utilised to bind the particles and powders together in low concentrations, typically less than 10% by volume. It should also be soluble in an inexpensive solvent, which together must volatilise during heat treatment, leaving no residues in the film after sintering. Binders need to be flexible yet tough, and so polymers with long chain molecules are utilised. Popular binders include poly(vinyl chloride), polystyrene, poly(vinyl acetate) and polymethacrylates. Plasticisers are added to the slurry to enhance flexibility of the film , such that the tape can be readily removed from the substrate.
(a) (b) (c) (d) (e)
Figure 7.11. Results of various methods of producing slurry; how they formed tapes using the glass slide casting process.
Multiple experiments were undertaken to define the optimum balance of the three additives. The quantities recommended by Mistler et al. were used as a basis for initial fractions of each additive,[125] as shown in Table 7.6. In fact this resulted in a highly viscous slurry, which was not easy to cast, shown as Figure 7.11a. Consequently, the amounts of slurry additives were adjusted (shown as Figure 7.11(b-e)), until optimised performance was achieved. The resulting slurry was cast and subsequently removed relatively easily, and was used in the later experiments (Figure 7.11d).
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 150 Table 7–5 – Volume fractions of components in test slurries.
Slurry Suggested initial Amount used in Chemical Additive amount (g) [125] optimised slurry (g) Binder PVB (Butvar 3.0 3.0 Solvent Ethanol-Toluene 35.0 35.0 Plasticiser Dibutyl Phthalate 5.6 5.6 Powder NKN-5LN and 6LN 100 60
7.4.3 Cast Tape Quality Control From the work on bulk materials (see Section 7.5) it was shown that lithium niobate (LN) is a suitable dopant for NKN; significantly enhanced electrical properties were observed at 5-6% doping levels. A sample of 94NKN-6LN slurry was produced and cast. The depth of 6LN tape parallel and perpendicular to the casting direction was measured using a Contracer. By dragging a needle along the surface of the slide and tape, height differences can be recorded. It was not possible to undertake the same test for the metal die casting due to the lack of a reference height. Figure 7.12a shows the thickness of the cast tape in different directions. Although the depth is not even, the height difference measured along the length of the cast section is 16.2 μm, giving an average depth of tape is 53.3 μm (±8.1 μm). This is an acceptable thickness for thick film casting.
(a)
(b)
Figure 7.12. The thickness of 6LN tape cast on a glass slide (a) along the length and (b) across the width of the slide.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 151 The thickness across the width of the tape was also examined; an exemplar of the width is given as Figure 7.12b. It is evident that there is a raised section at the sides; this is expected as it is like the meniscus, where the slurry is attracted to the surface of the masking tape that makes the casting “track”. Along the main width of the tape, there is relatively uniform thickness of average 54.8 μm (±4.1 μm) for this section, although the average for all width measurements was 53.6 μm. This is concurrent with the average thickness along the length of tape, and so is the average tape thickness for this method.
7.5 Doping NKN with LiNbO3 (LN) Before tape casting of the NKN-LN system could commence, a study of LN doping in NKN using the conventional mixed oxide route was undertaken. The starting powders were weighed and mixed according to their stoichiometric ratio to produce 95NKN-5LN. Samples were dried and wet milled for 24 hours then calcined at 850-930°C for 4 hours. The calcined powder was doped with CuO (0-0.8 wt%) and wet milled for 24 hours before drying and pressing at 100MPa and sintered at 850-1100°C (±180°C/hour) for 2-18 hours.
(a) (b)
Figure 7.13. P-E hysteresis behaviour of (a) NKN + 0.2 wt % CuO and (b) 95NKN-5LN 0.8 wt% CuO (from Azough et al).[200]
Calcination at 930°C and additions of 0.8 wt% CuO to 5LN were optimal to achieve a 95% theoretical density with a very low sintering temperature of 890°C. This result was published in 2011.[200] XRD data showed that this structure had orthorhombic and tetragonal phases coexisting in the material, as expected for this MPB formulation.[33, 162] Resulting hysteresis data for 0.2 wt% CuO doped NKN and 0.8 wt% CuO doped 5LN are shown in Figure 7.13. Copper doping in NKN leads to properties of Pr = 18 μC/cm2 and EC = 15 kV/cm (Figure 7.13a). These properties are similar to those reported by Moon et al. for 1 mol% CuO doping in NKN, although in the present study the remnant polarisation is 4 μC/cm2 higher.[201]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 152 The 0.8 wt% CuO doped 5LN sample, however, has a significantly higher Pr of 27
μC/cm2 and EC = 10 kV/cm. These values are higher than previous investigations of
5LN.[111, 121] A higher Pr value of 28 μC/cm2 was found for 0.4 wt% CuO.
7.6 Tape Casting of Copper-Doped 5LN and 6LN Compositions 95NKN-5LN and 94NKN-6LN powder samples doped with 0, 0.4 and 0.8wt% CuO were prepared using the mixed oxide route described in Section 3.2.1. Copper oxide was added as a sintering aid, as it has proven effective.[33, 83, 202] The powders were mixed with the slurry additives shown in Table 7.6. The full experimental procedure is described in Section 3.2.2.5. Tapes of thickness 20-30μm were prepared by the glass casting method (Figure 7.10).
For the 95NKN-5LN + 0.4wt% CuO bulk material, the optimum sintering temperature was found to be 1090ºC.[200] Sintering temperatures in the range 1030-1150ºC were utilised for film production. The resulting tapes were cut into sections and investigated by XRD and SEM.
A micrograph of undoped 95NKN-5LN (Figure 7.13a) show that a 1030ºC sintering temperature results in a grain size of 1 μm but there is evidence of liquid phase present. When the sintering temperature was increased to 1060ºC, the grains size increased to 3 μm. There was no evidence of liquid phase although the grains are faceted and not of the desired microstructure. This is shown in Figure 7.14b, where the grains are still relatively small and have not sintered and densified, as seen with successful thick films. For the highest sintering temperature of 1090ºC (Figure 7.14c) there was a significant change in the microstructure. The grains were fully formed, rectangular in shape with some evidence of abnormal grain growth - some being over 12μm in size. This microstructure is similar to that obtained in the bulk material.[200]
When 0.4wt% CuO was added to the 5LN as a sintering aid, there were significant changes in the microstructure (Figure 7.14d-f); a 1030ºC sintering temperature was sufficient to generate fully formed grains typically 3μm in size. This is much larger than for samples prepared without the CuO doping at the same temperature (Figure 7.14a). When the sintering temperature was decreased to 1000ºC, sintering was not complete and the microstructure was similar to that shown in Figure 7.14a. This shows that the CuO lowers the sintering temperature by approximately 30ºC.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 153 When the 5LN + 0.4wt% formulation was sintered at 1060ºC and 1090ºC, there was evidence of abnormal grain growth (Figure 7.14e) and combinations of abnormally large grains with fully formed smaller grains in between (Figure 7.14f).
(a) (d)
(b) (e)
(c) (f)
Figure 7.14. SEM micrographs of 95NKN-5LN + x wt% CuO single tapes (thickness 20-30μm) sintered at different temperatures.
X-ray diffraction spectra for the Copper-doped 5LN films are shown in Figure 7.15. All the spectra show fully formed NKN with an orthorhombic structure, even for samples sintered at 1000ºC. This is expected as bulk samples of this composition can be sintered at temperature as low as 890ºC.[200] In the 002 / 200 double peaks at 45º 2θ (Figure 7.15) the first peak is higher than the second, confirming that an orthorhombic phase is the dominant structure.[98, 157] This is true for samples sintered at all temperatures, showing that orthorhombic NKN is fully formed.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 154
1090C
101 110
1060C
1030C
1000C
001 001
002 200
112 211
101 201 210
Intensity 111
15 20 25 30 35 40 45 50 55 60 two theta Figure 7.15. Diffraction patterns of 95NKN-5LN + 0.4wt% CuO at various sintering temperatures.
7.7 Film Thickness In order to investigate the effect of film thickness, a constant sintering temperature of 1030ºC was employed. The masking tape film casting method yields tapes of approximately 50 μm thick, which after sintering are 25-35 μm thick. In order to produce thicker films, two different methods were utilised: (i) The first method involved slicing the tape into strips approximately 8-10mm wide and stacking them on top of each other and pressing them before sintering. This method allows any thickness of film to be produced. (ii) The second method involves casting a film on a slide, allowing it to dry, then applying another layer of masking tape at each side to increase the “channel” depth and casting another film on top of the first film. This double-cast method is more time-consuming than the first technique. In principle this method can be repeated many times.
Since the MPB of the NKN-LN binary system lies between the 5 and 6 mol% LN compositions, it is clear from the XRD data that orthorhombic and tetragonal phases coexist in the system. This presents difficulties in accurately defining changes in the system and the unit cell parameters. It was decided to focus on the 94NKN-6LN composition, as there is a higher degree of tetragonality in the system, although both structural phases are still present.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 155 Single and double cast tapes of 6LN + 0.4wt% CuO were prepared. The XRD spectrum of the single tape (Figure 7.16) shows the double peaks at 22º and 45º 2θ are of the same height. This shows that both orthorhombic and tetragonal phases coexisting in the material.[101, 109, 180-181] This type of microstructure is expected in 95NKN-5LN, not 94NKN-6LN, formulations. The XRD spectrum for double-cast tape is significantly different (Figure 7.16). In the double peaks, particularly those between 44 and 58º 2θ, the second peak is higher than that of the first, indicating a high degree of tetragonality. This is expected for the 94NKN-6LN composition. There is also evidence of an orthorhombic phase coexisting in the structure, denoted by the double peak, showing that this composition is close to the MPB (which enables enhanced electrical properties in perovskite ceramics). It is also significant that the 001/ 100 peak at 22º 2θ is much lower for the single tape than for the double tape, with reference to the large 110 / 101 peak at 32º 2θ. This shows clear differences between the structure for the two tapes; the double-cast material develops a structure that is comparable with that for bulk samples of the same composition.
double tape
101
single tape 110
001 100
002 200
112 211
Intensity
102 201 210
111
15 20 25 30 35 40 45 50 55 60 two theta Figure 7.16. Diffraction spectra of 94NKN-6LN + 0.4wt% CuO single and double-cast tapes, both sintered at 1030ºC.
The XRD spectra for double cast tape and a double stacked and pressed sample of 6LN + 0.4 wt% CuO were identical, showing both methods formed fully sintered 6LN structures. SEM analysis of the stacked sample showed a uniform thickness throughout with a flat surface, unlike that of the double cast sample. As the double cast method was more time consuming, the stacking method was employed for the remainder of the study.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 156 7.8 Tape Casting Oriented 94NKN-6LN Tapes using NN particles In order to create oriented samples, the previously made (oriented) NN particles were added to the slurry before the mixing stage, and then cast as before. Saito et al. reported adding 5 at% particles,[16] however Gao et al. reported the addition of 30% particles to be optimum for NBT-BT compositions.[143] In view of the differences in the literature, initial experiments used 10 wt% particles added to the slurry for each composition. The aim was to use the oriented NN plate-like particles as templates to react with the matrix composition and thus encourage orientation of the matrix during sintering. A schematic diagram of the process is shown in Figure 7.17.
Figure 7.17. Schematic diagram of the RTGG process [after Zhao et al.].[183]
7.8.1 Preparation of Tapes with Addition of 10% NN Particles
7.8.1.1 Orienting 6LN + 0.4 wt% CuO Composition The NN particles were added at the level of 10 wt% to the matrix composition 6LN + 0.4 wt% CuO to form the slurry, which was cast and dried; tapes were subsequently cut and sintered. The initial sintering temperature was 1030ºC as this was adequate for the same composition without the presence of particles. Fired samples were examined by SEM, shown in Figure 7.18. The plan view Figure 7.18(a) shows template particles (indicated by the arrows) between the sub-micron sized matrix grains, suggesting they have not reacted at this temperature. The cross-sectional view of the tape (Figure 7.18b) indicates significant agglomeration of the template particles.
(a) (b)
Figure 7.18. SEM micrographs of 94NKN-6LN + 0.4 wt% CuO single cast tape sintered at 1030ºC, (a) plan view, and (b) cross-sectional view of tape. Yellow arrows indicate visible particles.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 157 This type of agglomeration was also reported by Kimura et al. when they added Ba6Ti17O40 particles to Bi0.5Na0.5TiO3-BaTiO3 (BNTBT). They overcame this by using a dispersant in order to ensure homogeneity in the slurry and a good alignment during the tape casting process, which was a successful undertaking (before (a) and after (b) use of dispersant shown in Figure 7.19).
Figure 7.19. SEM micrographs showing microstructures of NBTBT thick films (a) using no dispersant, and (b) using dispersant (after Kimura et al).[133]
To overcome the agglomeration in this study, a dispersant (that is, a surfactant in solution with a solvent) was employed. The NN particles were mixed with BASF Dispex A40 dispersant and further sonicated. After drying, the particles were added to the slurries and processed according to the standard experimental methods. Initially, films were prepared from 94NKN-6LN + xwt% CuO (with 10% NN), where x is 0, 0.4 and 0.8. After casting and sintering at 1030ºC, the product was of the same colour as the original tape, yet very brittle. SEM (Figure 7.20a) and XRD revealed underdeveloped sintering. The yellow arrows indicate the template particle visible in the sintered tapes. Consequently, the sintering temperature was increased to 1090ºC. Whilst there was some improvement in densification, there was still very limited reaction between the template particles and the matrix (Figure 7.20b). When the sintering temperature was increased further to 1120ºC, it was clear that large grains developed at this higher temperature (Figure 7.20c,d), typically 5μm in size (compared to 1μm in Figure 7.20a,b) and there was no evidence of template particles suggesting that they had reacted with the matrix. In the side elevation view of the 1120ºC sample, shown in Figure 7.20d there was evidence of alignment of the grains in the casting direction, with some grains, growing to 15μm in length.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 158 (a) (b)
(c) (d)
Figure 7.20. SEM micrographs of single tapes cast of composition 94NKN-6LN + 0.4wt% CuO + 10%NN particles sintered at temperatures of (a) 1030ºC, (b) 1090ºC and 1120ºC (c) top view and (d) cross-section. Yellow arrows show template particles.
Accepting that the 1120ºC sintering temperature yielded good quality samples, the next stage was to form thicker samples. This was done by cutting identically sized tapes, stacking them upon one another and pressing them before sintering. This method allows tapes of any thickness to be produced. Initially, four tapes were pressed together to give a total thickness of approximately 100-130μm, and then eight tapes to give thickness of 200- 250μm. The results, after sintering such tapes at 1120ºC, are shown in Figure 7.21.
The microstructures contrast with those for single tape (Figure 7.20). In the four layered sample, shown in Figure 7.21(a-c), there are fully grown grains, with some abnormal grain growth (Figure 7.21a); the side view (Figure 7.21b,c) shows cubic grains in a fully densified layer 90μm thick, with no sign of interlayer defects. In the eight layered sample (Figure 7.21d-f), there was no sign of these large grains in the tape; template particles are evident within the micron grain-sized matrix. In side view (Figure 7.21e,f), however, the layers have densified and are successfully bonded together, giving a full thickness of 225μm. At high magnification (Figure 7.21f) there are suggestions that some particles within the matrix are just beginning to react with the matrix grains. This indicates that layering and pressing the tapes to make thicker samples is detrimental to the microstructure, leading to different microstructural behaviour to single tapes of 25-30μm thickness.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 159 (a) (d)
(b) (e)
(c) (f)
Figure 7.21. SEM micrographs showing the top and side views of 94NKN-6LN + 0.4wt% CuO + 10%NN particles sintered at 1120ºC, with four layers (a-c) and eight layers (d-f). Yellow arrows indicate visible template particles.
For the eight layered samples, the highest sintering temperature examined was 1150ºC. This is very high for NKN production as sodium and potassium are volatile at such high temperatures, and readily form liquid phases. Indeed, when eight layered tape was sintered at 1150ºC, the tape reacted with the platinum foil substrate and became attached. Representative micrographs are shown in Figure 7.22. Whilst there is some abnormal grain growth the matrix grains have increased in size to typically 2 μm, which is larger than in samples sintered at lower temperatures, and the film is approaching full densification. In the side view (Figure 7.22c,d), the thickness of the tape is 210μm, but the particles, again, have not fully reacted with the matrix and so full orientation has not been achieved.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 160 (a) (b)
(c) (d)
Figure 7.22. SEM micrographs of the top and side views of an eight layered 94NKN-6LN + 0.4wt% CuO + 10%NN particles, sintered at 1150ºC. Yellow arrows indicate visible template particles.
XRD spectra of these samples sintered at 1090-1150ºC are shown in Figure 7.23. The spectrum for the 1150ºC sample is significantly different to samples sintered at lower temperatures. The (100) peak at 22º 2θ for the 1150ºC sample is lower than the same peak in the 1090ºC sample, and conversely the 45º 2θ peak (1150ºC sample) is significantly higher than for the samples sintered at lower temperatures. As the ratio of the peaks has changed significantly, it suggests the development of orientation in the sample. The SEM images confirm that grains are larger and more fully formed at 1150ºC than the lower temperatures, although the template particles are not fully reacted and integrated with the matrix. However, the disadvantages of such high sintering temperatures are because of sodium and potassium volatility and that the sample reacted with the platinum substrate.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 161
002 200
0.4CuO_1150C
0.4CuO_1120C
0.4CuO_1090C
001 100
Intensity
110 101
102 201 210
112 211
20 25 30 35 40 45 50 55 60 Two Theta Figure 7.23. XRD spectra for 94NKN-6LN + 0.4wt% CuO + 10 wt% NN particles, 4x layer pressed tapes sintered at 1090, 1120 and 8x layer pressed tape sintered at 1150ºC.
The increase of the (200) peak intensity, and also its shape, is a direct result of <100> texturing.[130, 134] The split peak shape shows tetragonal phase, which is expected for this composition. A similar result was obtained by Yilmaz et al. where the diffraction patterns of 94.5% (Na0.5Bi0.5)TiO3 - 5.5% BaTiO3 ceramics are significantly modified (in terms of peak intensity ratios) that show successful texturing in the ceramic (after 12 hours sintering time); a Lotgering factor of 96% was achieved.[139] This is illustrated in Figure 7.24. (The Lotgering factor, f, is explained and defined in Section 2.4.1). This exact effect is also seen in here, and so shows successful orientation has been accomplished.
Figure 7.24. XRD diffraction patterns for NBTKT with random orientation (blue) and with
5% added SrTiO3 template particles added (pink) (from Yilmaz et al).[139]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 162 F was calculated for the 1150°C sintered 6LN + 0.4 wt% CuO + 10 wt% NN particles sample and compared to a sample of the same formulation without NN particles made with the mixed oxide route (sintered at 890°C); a factor of 67% was found. The two spectra are shown in green (6LN + 0.4 wt% CuO bulk sample) and blue (6LN + 0.4 wt% CuO + 10 wt% NN) in Figure 7.25. The f value 67% indicates a high orientation, and shows that the tape casting method developed in this investigation does work. A comparison can be made with the work of Takao et al. on the orientation of Cu- doped NKN.[16] They reported that 1 mol% CuO doping to NKN resulted in an orientation of 93%, which is very high; other properties include a density of 95% and grains of sizes 10-20 μm. Their samples were sintered at 1100°C, which is 50°C lower than that required in the present study, though Takao et al. used a pure oxygen atmosphere (as opposed to an atmosphere of normal air utilised in this case). This, however, is not a direct comparison as the tapes under investigation here are Li-doped, which have not been reported elsewhere.
1150 C - 10% NN particles 1030 C - no particles
890 C - bulk sample Intensity (a.u.) Intensity
15 20 25 30 35 40 45 50 55 60 65 70 75 two theta Figure 7.25. Comparison of XRD diffraction profiles of 94NKN-6LN + 0.4 wt% CuO (a) double cast sintered at 1030°C, and (b) 8x layered tape with 10 wt% NN particles added, sintered at 1150°C.
The electrical properties of the 1150°C sintered sample was tested as it exhibited the desired orientation. As described previously, the sample reacted with the platinum foil substrate, and so the foil was used as one of the electrodes. The other side of the sample was covered in silver paste as normal (see Section 3.3.7) and silver wires were attached using silver paste. Capacitance and impedance data were collected.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 163 The relative permittivity data for this textured sample is shown in Figure 7.26. At 10 kHz the relative permittivity at room temperature is 300. The inset in Figure 7.26 shows that there is a transition at approximately 70°C, which is the TO-T. The Curie temperature is at ~440°C. This is slightly lower than that reported for bulk NKN; Guo et al. reported
94NKN-6LN to have a TC of 468°C.[203]
500 1 kHz 2000 450 10 kHz 100 kHz 400 1 MHz 1500 350
300
r ε 250 1000 200 0 50 100 150 200 250 300 350 400
500
0 0 50 100 150 200 250 300 350 400 450 Temperature ( C) Figure 7.26. Permittivity data for 94NKN-6LN + 0.4 wt% CuO + 10% NN particles, as a function of applied frequency. Inset shows 1-400°C range under closer inspection.
There was a problem with impedance measurements in that the switching between lower and higher frequencies was discontinuous. This is shown in Figure 7.27. There was enough of the main arc to extrapolate a semi-circle to determine the lower frequency data. The data was modelled using the ZView program.[204] An exemplar fitting (for 545ºC) is shown as Figure 7.28.
1.4 485 C 1.2 505 C 1.0 525 C 0.8 545 C
m) 0.6
Ω
k (
" " 0.4 ρ 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 ρ' (kΩm) Figure 7.27. Cole-Cole plot data for 94NKN-6LN + 0.4 wt% CuO + 10% NN particles, as a function of temperature (485 – 545 °C)
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 164 The green line in Figure 7.28 shows the modelled semi-circle of the Cole-Cole plot from the impedance data. By modelling the data, the resistance and capacitance can be calculated. At a temperature of 545ºC the resistance was 25 kΩ and capacitance 21.6 pF. For textured samples, there is no published impedance data given for direct comparison. According to Yang et al. NKN + 0.25 mol% CuO prepared by the conventional mixed oxide route has a resistance of 20Ω.[205] This shows that 6LN doping gives thick film (thickness 210 μm) properties comparable to bulk NKN.
Figure 7.28. Cole-Cole impedance plot fitting for 94NKN-6LN + 0.4 wt% CuO + 10 wt% NN particles, at temperature 545ºC. Blue spots and red line denote data, green line denotes fitted semi-circle model
7.8.1.2 Orienting 6LN + 0.8 wt% CuO Composition In view of the effect of the amount of CuO addition on NKN-LN microstructure development (ref earlier section 7.5), a similar investigation was undertaken with tapes prepared from 6LN + 0.8wt% CuO powder with 10% NN particles added. Using the multi- layering pressing procedure, the single layer casting followed by cutting and stacking allows more tapes to be prepared, and allows for a higher degree of options of multilayering and potentially a higher degree of orientation in one layer.
Four layered samples were made and sintered at 1030, 1090 and 1120ºC. The 1030ºC sintering temperature was again too low, the tape did not change colour after sintering and was very brittle. SEM confirmed that particles were sub-micron in size.
The sample sintered at 1090ºC exhibited superior microstructure. Figure 7.29 shows micrographs of single and 4 layered samples which had been sintered at 1090ºC. As with the 0.4 wt% copper-doped samples, it would appear that the additional tape thickness is detrimental to the microstructure, so a higher temperature is needed to produce high quality thicker samples. A similar conclusion was also reached in Section 7.8.1.1.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 165 (a) (b)
(c) (d)
Figure 7.29. Comparison of single and four-layered 6LN + 0.8 wt% CuO + 10% NN particles samples sintered at 1090ºC.
Figure 7.30 shows micrographs of 94NKN-6LN + 0.8wt% CuO + 10%NN ceramics produced from four layered tapes. It is clear that the sample still exhibits a very small grain under one micron in size. Thus for thick film casting, higher levels of CuO doping is not beneficial as a sintering aid. Subsequent work continued with 0.4wt% copper doping.
(a) (b)
Figure 7.30. SEM image of four-layered 6LN + 0.8 wt% CuO + 10% NN particles sample sintered at 1120ºC showing (a) as-fired surface and (b) side elevation view.
7.8.2 Adding 15% NN Particles As samples prepared with both 0.4 wt% and 0.8 wt% CuO doped 94NKN-6LN showed some degree of orientation after sintering at high temperatures, the amount of NN particle addition was increased from 10 to 15%. This aim was to increase the degree of orientation in the samples. Single tapes were cast, and it should be noted that the template particles were coated in surfactant before processing.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 166 Samples prepared with 0.4 wt% CuO doping were very difficult to remove from the glass slides after casting, and were very brittle, unlike the flexible tapes cast for 10% particles. However the 0.8 wt% doped tapes were easy to recover after casting. For this reason the
latter were employed in the remainder of this part of the investigation.
110 101 1120C
1090C
001 100
Intensity
002 200
112 211
102 201 210
20 25 30 35 40 45 50 55 60 two theta Figure 7.31. XRD Spectra for 94NKN-6LN + 0.8wt% CuO + 15 wt% NN particles, 4 layer pressed tapes sintered at 1090 and 1120ºC.
The XRD data in Figure 7.31 shows there is minimal difference between the samples sintered at 1190 and 1120°C from 0.8 wt% copper-doped tapes. They both exhibit tetragonal structure. They also show more second phase peaks than those found for 10% NN samples (Figure 7.23). Ideally these peaks would not be detected at all, due to the reaction with the NKN matrix composition and the desired homogeneity and a oriented single phase. This shows that the processing technique needed to be altered to promote reactions between the tapes.
Feng et al. reported the variation of properties in relation to NN template particle addition.[82] Although this is for a different composition, that of 93NKN-7LT (LiTaO3) the effect of over 5 mol% addition of particles shows a fully tetragonal structure; the (001), (110), (102) and (112) peaks disappeared (so there are single peaks as opposed to the split double peaks observed here). This is shown in Figure 7.32. It must be noted that their investigation did not involve tape casting but the material was pressed and sintered as in the conventional mixed oxide route.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 167
Figure 7.32. XRD diffractogram for 93NKN-7LT-xNN particles (here NKN is denoted as KNN) [from Feng et al}.[82]
Feng et al. also noted a second phase distinguishable between 25 and 35 degrees 2θ. This is of K2Li3Nb5O15 which are marked in as diamonds in Figure 7.32. Hence some of this phase occurs in the present samples.
This K2Li3Nb5O15 phase (structure shown as Figure 7.33e) occurs due to the strain in the lattice when Na+ ions are sitting in the K+ A-site and causes octahedral tilting of the Oxygen ions, illustrated in Figure 7.33c. Some of the spaces surrounding the A-site ions can open or close slightly (Na+ and K+ have ionic radii 0.97 and 1.33 Å)[84] to form larger pentagonal interstitial sites, thus forming smaller triangular sites (clearly shown in Figure 7.33d) which the smaller Li+ ions can occupy when diffused into the lattice (Figure 7.33e).
Although this is a viable structure in Lithium-doped NKN, it must be noted that it is found in very small quantities in the XRD diffraction data throughout this investigation. Due to the small amounts detected, no further investigation into this phase was undertaken.
Figure 7.33. Schematic showing the evolution of K3Li2Nb5O15 structure from NKN (from Feng et al.)[82]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 168 7.9 Chapter Summary This chapter was devoted to a new and novel processing route for textured NKN. An RTGG approach has been developed to produce NKN-based tapes with orientation along the <100> direction, using small amounts of material, which can later be upscaled once a suitable formulation has been developed. In the first stage, platinum crucibles were found to be effective for BNN and NN particle production.
For BNN precursor production, the optimal processing route for molten salt synthesis (MSS) involved a sintering temperature of 1050ºC for 2 hours in an NaCl medium using a salt to oxide ratio of 1:1. Resulting particles were up to 25 μm in size, although the average was 15 μm. In order to convert these pre-cursor particles to NN template particles, a sintering temperature of 1050ºC for 8 hours was required (salt to oxide ratio 1:1). Resulting particles were approximately 15 μm wide and 0.5 μm thick.
A simple low-cost method for casting thick films was developed. This involved making a “track” from masking tape along the sides of a glass slide, allowing a small amount of slurry to be deposited onto one side and cast across the “track” using the side of another glass slide. The optimal amount of slurry components (binder, solvent and plasticiser) was 40% to 60% NKN powder. 95NKN-5LN formulations showed that 0.4 wt% CuO doping was a successful sintering aid, which allowed a 30°C reduction in the sintering temperature.
For 94NKN-6LN composition, 0.4 wt% CuO was also the optimum addition of a sintering aid. Sintering at 1120ºC yielded oriented single tapes, although the temperature had to be increased to 1150ºC for layered tapes. After sintering, single tapes were approximately 20- 30 μm thick, and layered samples were up to 250 μm thick.
A Lotgering orientation factor of 67% was achieved for the 1150ºC sintered eight layered sample. This tape exhibited dielectric properties of TC = 440ºC and TO-T = 70ºC, and a resistance of 25 kΩ and capacitance 21.6 pF.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 169 8 CONCLUSIONS
8.1 SBN doping of NKN The use of SBN as a dopant for NKN is novel. It was found to be beneficial when used with the addition of 0.45 wt% Fe2O3 as a sintering aid. Initial results indicated that 4 mol% SBN addition define the single phase solubility limit. Laboratory XRD (Chapter 5) and a higher resolution diffraction study (Chapter 6) showed that there was in fact a second phase present in such materials; this was also observed (in smaller amounts) in lower SBN formulations. A second phase was visible in SEM imaging, where the phase was potassium rich; in 4SBNF another phase was found which was barium rich. All compositions showed orthorhombic structures when investigated by laboratory XRD; at 4 mol% SBN addition, the orthorhombic and tetragonal phases coexisted in the system. Synchrotron XRD showed that in reality the two phases were coexisting in all SBN-doped formulations, in 1SBNF there was up to 20%. The structure of the second phase, particularly that observed in 4SBNF, was refined as a tungsten bronze type (TBT) structure, but not that of SBN. This was present in 3% of the 4SBNF sample, with parameters a = 3.948327(11) Å, b = 5.640070(17) Å and c = 5.670146(17) Å.
Additions of 2-4 mol% SBN achieved the highest densification, yielding over 96% theoretical density. The addition of SBN caused the transition temperatures to decrease,
TC by 14.7°C and TO-T by 9.0°C per 1 mol% addition; initial values for NKNF were 234ºC and 457°C, respectively. The value of the relative permittivity at TC decreased as SBN was added.
NKNF exhibits a Pr of 22 μC/cm2 and EC of 16.5 kV/cm, a higher remnant polarisation than that found for undoped NKN (Pr = 18.8 μC/cm2 and EC = 9.65 kV/cm).[95] The optimal formulation is 2SBNF, which exhibits Pr = 25 μC/cm2 and EC = 8.8 kV/cm. 24 hour sintering of 2SBNF enables it to withstand larger electric fields before breaking down (60 kV/cm as opposed to 40 kV/cm).
Samples prepared with 2SBNF and 3BNF addition exhibit the highest resistivities of 4.7 and 5.0 kΩm, respectively. The 4SBNF formulation has significantly lower resistivity of ρ = 0.27 kΩm, although this is still higher than that for NKNF (0.08 kΩm). The 2SBNF and 3SBNF formulations were associated with the highest activation energies of 1.16 eV. NKNF, in comparison, had a much smaller activation energy of 0.60 eV, showing SBN addition causes the resistivity of NKN to nearly double. Matsudo et al. reported an activation energy for NKN as 0.65 eV at temperatures above 390°C.[190]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 170 In conclusion, the 2SBNF formulation exhibits superior properties to NKN and NKNF, with
TO-T = 210°C, TC = 427°C, Pr = 25 μC/cm2 , EC = 8.8 kV/cm and Q = 1.16 eV. It comprises approximately 90% orthorhombic to 10% tetragonal phases coexisting in the structure. In 2SBNF, the orthorhombic phase has pseudo-cubic parameters a’ = c’ = 3.947180 Å, and b’ = 3.999996 Å; the tetragonal has a’ = c’ = 3.989798 Å, and b’ = 3.975777 Å.
8.2 Synchrotron Analysis of NKN-based Formulations The Rietveld refinement computer modelling program Topas was utilised to determine the structures of the phases present in copper-doped and LiTaO3-doped NKN.
8.2.1 Synchrotron Analysis for Cu and Nb doped NKN
The results for the 99.5NKN-0.5CuO + 0.6wt% Nb2O5 solid sample were different to that for the same formulation as a powder. The solid sample underwent two polymorphic phase transitions, at ~300ºC and ~515ºC, up to 100°C higher than expected. The second phase transition at 515°C was between two tetragonal phases; the one above 515°C was associated with a significantly larger unit cell volume as the a’(and thus c’) parameters increased to a size that was near to cubic. Average pseudo-cubic parameters for each phase are: orthorhombic a’ = c’ = 4.06245 Å, and b’ = 3.97616 Å, tetragonal (300-520°C) a’ = c’ = 4.99557 Å, and b’ = 4.0363 Å, and high temperature tetragonal (>500°C) a’ = c’ = 4.9519 Å, and b’ = 4.4941 Å.
The powder of the same formulation exhibited more phase transitions, although smaller ones. This is partly due to the complete transparency of the sample in powder form. The structure started as orthorhombic (<140°C, a’ = c’ = 4.10680 Å, and b’ = 4.02620 Å), and the tetragonal phase introduced at 140°C (a’ = c’ = 5.02694 Å, and b’ = 4.00747 Å), where the orthorhombic phase decreased in amount (a’ = c’ = 4.05845 Å, and b’ = 3.93863 Å). At 360°C the orthorhombic unit cell increased (a’ = c’ = 4.10285 Å, and b’ = 3.97435 Å) whilst the tetragonal phase changed to a’ = c’ = 4.92752 Å, and b’ = 4.06810 Å.
P-E hysteresis loops were recorded for this copper doped formulation, and resulted in Pr =
19.9 μC/cm2 and EC = 13.5 kV/cm. The remnant polarisation is only slightly higher than that for NKN (18.8 μC/cm2).[95]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 171 8.2.2 Synchrotron Analysis of 94NKN-6LT
Addition of 6 mol% LiTaO3 to NKN leads to the development of a tetragonal structure, although it is still on the MPB with orthorhombic and tetragonal phases coexisting in the system.[33, 162] This was confirmed at lower temperatures (20-200°C) where both phases were detected using Rietveld refinement. A tetragonal phase was observed throughout the temperature range (20-500°C) average pseudo-cubic parameters a’ = 4.98349 Å and b’ = 4.01715 Å. The a’ parameter found here is significantly larger than that reported by Saito and Takao.[112]
With increasing temperature above 200°C, the orthorhombic phase disappears and the material is entirely tetragonal. There is a transition to cubic at approximately 390°C , and at slightly lower temperatures orthorhombic phase was again detected (340-390°C) which has not previously been reported. At low temperatures (20 - 200°C) an orthorhombic phase with average parameters of a’ = 4.0916 and b’ = 4.0172 Å was detected and refined. It was again detected just before the tetragonal to cubic transition at 390°C (340 – 390°C), with a slightly smaller unit cell volume. The existence of an orthorhombic phase just before the transition to a cubic structure has not been detected previously in synchrotron studies of NKN.
8.3 Tape Casting of LN-doped NKN
8.3.1 Tape Casting Procedure Development A new process for tape casting was introduced and developed to minimise powder wastage. It allows formulations to be investigated in small batches, which is useful and cost-effective for developmental research. A “track” was formed on a glass substrate, using masking tape to provide a casting area approximately 50 μm thick. Slurry was poured onto one side and cast with the side of a glass slide, which acts as a doctor blade in the tape casting process. For the slurry, it was found that the optimal amount of slurry components (binder, solvent and plasticiser) was 40% to 60% NKN powder.
8.3.2 Orientation using NN Template Particles In order to produce oriented NN particles, BNN precursor particles were produced using molten salt synthesis (MSS). The optimal method involved a sintering temperature of 1050ºC for 2 hours in a weight to weight ratio of NaCl to BNN mixture. Resulting particles were on average 15 μm diameter.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 172 These BNN pre-cursor particles were converted to NN template particles, at a sintering temperature of 1050ºC for 8 hours. Resulting particles had the same morphology as the BNN particles, approximately 15 μm wide and 0.5 μm thick., which are comparable to the literature.[15]
8.3.3 CuO doped NKN-LN Tape Casting Development The optimal CuO addition for single layered 5LN as a sintering aid was 0.4 wt%, reducing the sintering temperature by 30ºC. In order to produce thicker films, individual films were stacked and pressed before sintering, to produce films up to 250 μm thick (for eight layers). When NN particles were introduced, a sintering temperature of 1120ºC was required to achieve a visibly oriented microstructure. An eight layered sample of the same formulation required 1150°C to accomplish orientation, which resulted in 67% texturing, giving dielectric properties of TC = 440ºC and TO-T = 70ºC, and a resistance of 25 kΩ and capacitance 21.6 pF. In comparison to NKN, the Curie temperature is higher but TO-T is significantly lower (NKN has TC and TO-T of 420°C and 200°C respectively).[29] Yang et al. reported NKN + 0.25 mol% CuO prepared by the conventional mixed oxide route has a resistance of 20Ω,[205] showing the present 6LN thick film is at least comparable to bulk NKN. In fact the film exhibits higher resistance than a bulk sample of NKN.
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 173 9 RECOMMENDATIONS FOR FURTHER STUDY
9.1 NKN-SBN System Electron backscattered diffraction (EBSD) could usefully be undertaken on NKN and NKN doped samples in order to investigate texturing and grain orientation of the samples. Although conventionally processed samples are not formally textured directly, some degree of orientation can be observed and therefore EBSD investigations would be a valuable starting point.
Inagaki et al. reported that three types of domain walls are present in NKN: 60°, 90° and 180°.[206] Detailed TEM investigations of SBN-doped NKN could be undertaken to investigate the nature of domain structures and how they vary as a function of SBN addition. This could be extended to investigate domain switching in SBN-doped NKN in in- situ, in the presence of an electric field, and simultaneously by x-ray diffraction. This technique has already been undertaken on undoped NKN by Hall et al. in 2010.[207]
A priority for further work should be the determination of the TBT phase observed in 4SBNF in Chapter 6. Resolving the second phases visible in SEM images for formulations prepared with 2-4 mol% SBN addition would usefully advance understanding of NKN. As higher resolution analysis is required, it is recommended that samples (such as 4SBNF) are examined by neutron diffraction.
In order to determine the exact composition of NKN-doped formulations, atomic absorption methods could be used. As NKN is soluble, the sintered compositions could be compared to the intended stoichiometric calculations.
A relatively new approach to NKN production is the use of spark plasma sintering (SPS). This process eliminates grain growth as complete rapid sintering can be undertaken in the matter of minutes due to the high ramping rate, short soaking time and lower sintering temperatures, which is especially useful in NKN as it suppresses the volatilisation of sodium and potassium in the system. Zhen et al. reported achieving densities of above 99% theoretical in NKN using this SPS technique, which resulted in enhanced dielectric permittivity and piezoelectric constant properties of 700 and 148 pC/N, respectively.[208]
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 174 9.2 NKN-LN System NKN-LN is a highly investigated system. However, the tape casting aspect is relatively new. The most promising 6LN formulations should be upscaled and tested on full-sized tape casting equipment in order to be able to collect full electrical data for the materials. TEM should also be employed to investigate the domain structures in the oriented tapes produced.
Feng et al. reported a maximum relative density of 98% theoretical when 3-5% NN particles were added to NKN samples. However these samples were not tape cast, but pressed and sintered after NN particle addition.[82] The work should be developed to define the optimal amount of NN particles to be added to the tape slurry. Previous studies suggest the optimal amount varies between 5 and 30%.[82, 127]
The addition of NN particles to NKN using conventional mixed oxide processing route could also be explored. Feng et al. reported that addition of 1 mol% NN particles to 93NKN-7LT improved the properties.[82] Such an investigation could be undertaken with 6LN and 6LT samples.
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175. Pan, H., et al., Effect of V2O5 on the Sintering Behavior, Microstructure, and Electrical Properties of (Na0.5K0.5)NbO3 Ceramics. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2008. 55(5): p. 994-8.
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178. Zuo, R., Z. Xu, and L. Li, Dielectric and Piezoelectric Properties of Fe2O3-doped (Na0.5K0.5)0.96Li0.04Nb0.86Ta0.1Sb0.04O3 Lead-Free Ceramics. Journal of Physics and Chemistry of Solids, 2008. 69(7): p. 1728-32.
179. Bomlai, P., et al., Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na0.5K0.5NbO3 Powders. Journal of the American Ceramic Society, 2007. 90(5): p. 1650-5.
180. Liu, Y., et al., Dielectric and Piezoelectric Properties of (1 - x)(K0.498Na0.498Li0.04)NbO3–x(Bi0.5Na0.5)0.9Ba0.1(Zr0.0055Ti0.9945)O3 Lead-Free Ceramics. Journal of Materials Science, 2010. 45(1): p. 188-91.
181. Wang, Y., et al., Piezoelectric Properties of (Li, Ag, Sb) Modified (K0.50Na0.50)NbO3 Lead-Free Ceramics. Journal of Alloys and Compounds, 2008. 462(1-2): p. 310-4.
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187. Seo, I.-T., et al., Effect of CuO on the Sintering and Piezoelectric Properties of 0.95(Na0.5K0.5)NbO3-0.05SrTiO3 Lead-Free Piezoelectric Ceramics. Journal of the American Ceramic Society, 2008. 91(12): p. 3955-60.
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Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 189 201. Moon, S., et al., Crystal Phases and Electric Properties of (Na0.5K0.5)1- xNb1+x/5O3:yCuO, zLiTSbO3 Piezoceramics. Ceramics International, To be published 2012(doi:10.1016/j.ceramint.2011.04.116).
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203. Guo, Y., K.-I. Kakimoto, and H. Ohsato, Phase Transitional Behavior and Piezoelectric Properties of (Na0.5K0.5)NbO3-LiNbO3 Ceramics. Applied Physics Letters, 2004. 85(18): p. 4121-3.
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206. Inagaki, Y., K.-I. Kakimoto, and I. Kagomiya, Ferroelectric Domain Characterization of Orthorhombic Sodium-Potassium Niobate Piezoelectric Crystals. Journal of the American Ceramic Society, 2010. 93(12): p. 4061-5.
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Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 190 APPENDIX 1 - FIGURES
Figure 1.1. Number of publications cited in refereed journals based on lead-free piezoceramics (between 1050 and November 2008) (from Rödel et al).[3] ...... 10
Figure 1.2. Piezoelectric coefficient – Curie Temperature comparison of PZT materials to BaTiO3 (BT), BNT and NKN based materials in the literature (from Shrout and Zhang).[18] ...... 10
Figure 2.1. Schematic diagram showing different polarisation mechanisms in a material (from Moulson and Herbert).[35] ...... 14
Figure 2.2. Schematic diagrams showing how charge can store on capacitor plates in vacuum (a) and how a dielectric material (grey) in between them can play a role (b) (from Moulson and Herbert).[35]...... 15
Figure 2.3. Polarisability of dielectric constant (ε’r) and loss (ε”r) as a function of frequency (after Moulson and Herbert).[35] ...... 16
Figure 2.4. Structure of PZT cubic Perovskite (left) and how it is affected in its tetragonal form to create a dipole under applied electric field (right) (from Callister).[43] ...... 17
Figure 2.5. Schematic of k parameter (after van randeraat & setterington).[41] ...... 19
Figure 2.6. Variation of εr of BaTiO3 as a function of temperature. Corresponding phase structures are also provided (after Richerson).[2] ...... 20
Figure 2.7. The displacements of ions in BaTiO3 that occur in the cubic-tetragonal transition (from Moulson and Herbert).[35] ...... 21
Figure 2.8. The initial surface charges in a material when spontaneously polarised (right) and how energy is preserved through 180° domains (left) (from Moulson and Herbert).[35] ...... 22
Figure 2.9. Schematic diagrams showing (a) how a polycrystalline ferroelectric material splits into 90º and 180º domains (from Moulson and Herbert).[35] and (b) domain walls in tetragonal BT (A-A’ are 90º walls, B-B’ are 180º walls) (from Jona and Shirane).[42] ...... 22
Figure 2.10. The stress energy in grains in terms of grain size, accompanied by the domain structure visible in the corresponding grains (from Arlt).[57] ...... 23
Figure 2.11. Schematic of typical ferroelectric P-E hysteresis loop, including diagrams showing the domain orientation at various points (from Damjanovic).[60] ...... 24
Figure 2.12. Exemplar impedance plot for yttria-stablised zirconia, showing the three possible contributions for polarisation processes (from Moulson and Herbert).[35] ...... 25
Figure 2.13. Illustration showing the electrode-ceramic interface cross-section, with respective equivalent circuits (from Waser).[64] [el is the electrode-ceramic interface, b is the bulk and gb is the grain boundary contribution] ...... 26
Figure 2.14. An example of (a) a series-parallel circuit which could produce the impedance plot in part (b) (after Moulson and Herbert.).[35] ...... 26
Figure 2.15. The cubic unit cell of Perovskite BaTiO3 (after Richerson).[2] ...... 27
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 191 Figure 2.16. Phase diagram for PZT, showing various properties of PZT at different compositions and the PZT MPB (in green) (after Lee and Bell).[67] ...... 28
Figure 2.17. The phase structures, polarisation temperatures and transition temperatures of Barium Titanate (after Moulson and Herbert).[35] a and c are parameter lengths of the unit cell, and P is the polarisation direction ...... 30
Figure 2.18. The phase diagram of the BaO-TiO2 system, with BaTiO3 MPB shown in green (after Jaffe).[39] ...... 31
Figure 2.19. Graph depicting the change in Curie temperature of BT with x mol% of each dopant on the Ba site (from Yanagida).[40] ...... 32
Figure 2.20. The dielectric constant hysteresis behaviour of KN with respect to temperature (after Megaw).[48] ...... 33
Figure 2.21. Typical antiferroelectric hysteresis behaviour (after Uchino).[4] ...... 33
Figure 2.22. Diagram showing the lattice prarmeters of NN (after Jona and Shirane).[42] Volume cube root indicated in green ...... 34
Figure 2.23. Dielectric Constant, ε, Saturation Polarisation, Ps, and Ec values for LN as a function of temperature (from Jona and Shirane).[42] ...... 35
Figure 2.24. The phase diagram of the NN-KN solid solution system (after Jaffe et al).[39] ...... 37
Figure 2.25. Revised phase diagram for NKN including phase structures (after Baker et al).[86] Dotted lines indicate change in tilting...... 38
Figure 2.26. NKN P-E hysteresis behaviour according to (a) Ichiki et al.[92] and (b) Zhang et al.[31] ...... 38
Figure 2.27. P-E Hysteresis for (Na0.5K0.5)(Nb0.995Mn0.005)O3 with different cooling rates (see text) (from Inagaki et al).[89] ...... 40
Figure 2.28. XRD diffractograms of (a) orthorhombic and (b) tetragonal NKN, with peak identification (after Skidmore et al.).[98] ...... 41
Figure 2.29. Diffraction profiles of <100> and <200> peaks for (1-x)NKN-xLi0.5Bi1.5TiO3 ceramics, at areas of interest (a) 20 ≤ 2θ ≤ 25° and (b) 44 ≤ 2θ ≤ 48° (from Jiang et al).[101] ...... 41
Figure 2.30. Density and relative density of (Na0.52+xK0.48)0.942Li0.058NbO3 with 0 ≤ x ≤ 0.035 (from Li et al).[99] ...... 43
Figure 2.31. (a) Piezoelectric d33 and Qm values, and (b) P-E hysteresis behaviour for 94NKN-6BT + 1 mol% CuO (from Lin, Kwok and Chan.[83]...... 44
Figure 2.32. XRD profiles of (a) (Na0.5K0.5)1-xLixNbO3 (after Niu et al)[114] and (b) Li0.058(Na0.521+xK0.48)0.942NbO3 (after Li et al).[99] ...... 46
Figure 2.33. Piezoelectric and dielectric properties for Li0.058(Na0.521+xK0.48)0.942NbO3 (from Li et al).[99] ...... 47
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 192 Figure 2.34. Lattice parameters and β (angle) value for structures of (Na0.535K0.485)1- xLix(Nb0.8Ta0.2)O3 (0.02 ≤ x ≤ 0.07) (from Shen et al).[102] Shaded area indicates MPB region...... 47
Figure 2.35. Schematic of the general doctor blade tape casting process (from Richerson).[2] ...... 49
Figure 2.36. Schematic Diagram for the waterfall casting process (from Richerson).[2] ... 50
Figure 2.37. SEM micrographs of (a) TiO2 acicular particles, and (b) plate-like BiT particles (after Jing et al.).[127] ...... 51
Figure 2.38. SEM micrographs of BiT particles (a) and the resulting BNKT microstructure (after Jing et al,).[127] ...... 52
Figure 2.39. Hysteresis behaviour of Nb-doped BNKT thick films, showing results for randomly oriented and textured (using BiT particles) (after Hong et al.).[129] ...... 53
Figure 2.40. (a) The orientation of NBTBT as a function of sintering time, and (b) The diffraction patterns for randomly oriented (blue) and 5% added SrTiO3 template particles (pink)in NBTBT (after Yilmaz et al).[137] ...... 53
Figure 2.41. TiO2 template particles, and the resulting BNKT microstructure (after Jing et al).[127] ...... 54
Figure 2.42. Schematic diagram of the BNN topochemical conversion reaction to NN (from Yan et al).[118]...... 56
Figure 2.43. XRD diffraction profiles for 1 mol% CuO doped NKN (a) randomly oriented (Tsint = 1050ºC) and (b) with 10% KN particles added (Tsint = 1175ºC) (from Saito and Takao).[144] ...... 58
Figure 3.1. Flow chart illustrating the processing route and characterisation techniques used for this mixed oxide route study ...... 62
Figure 3.2. Flow Chart illustrating the processing and characterisation techniques used for this tape casting study...... 64
Figure 3.3. Schematic showing set-up of Hot Washing Procedure...... 65
Figure 3.4. Flow chart for the production of templated NKN-based oriented tapes ...... 67
Figure 3.5. Schematic illustrating the Mechanism of Metal Die Casting...... 69
Figure 3.6. Schematic Diagram of Glass Slide Casting...... 69
Figure 3.7. Theoretical Illustration of X-Ray Diffraction (from Callister).[43] ...... 72
Figure 3.8. Schematic Diagram of Diamond Synchrotron (Oxford, UK), with its main components highlighted.[12] (1)Electron gun and linear accelerator; (2) Booster Synchrotron; (3) Storage Ring; (4) Beamlines; (5) Front End; (6) Optics Hutch; (7) Experimental Hutch; (8) Control Cabin; (9) Radiofrequency (RF) Cabin; (10) Diamond House ...... 73
Figure 3.9. Diagram of inside MRI TC-Basic furnace utilized for experiment at Darsebury SRS, station 6.2 (from MRI).[147] ...... 74
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 193 Figure 3.10. Schematic Representation of Beamline I11, showing approximate distances of main components from source (from Thompson et al.).[13] ...... 75
Figure 3.11. An original Gaussian peak fitting of observed data (dotted) and calculated profile (linear) by Rietveld’s refinement method (material unknown) (from Rietveld).[151] ...... 77
Figure 3.12. Schematic Illustration of how an SEM works (from University of South Denmark).[14] ...... 79
Figure 3.13. Representation of Sample prepared to be clamped in between two alumina plates, in preparation for dielectric and impedance analysis...... 80
Figure 4.1. Diffraction spectra of 99.5NKN-0.5CuO + 0.6wt% Nb2O5sintered pellet sample over 20 – 60° 2θ range, as a function of temperature (°C)...... 83
Figure 4.2. Diffraction spectra of 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample in the 200 and 002 peak area under investigation with respect to temperature (ºC)...... 84
Figure 4.3. Peak modelling for data collected at 31ºC for 200 and 002 peaks. The blue line shows the observed data, and the red line shows the calculated model...... 84
Figure 4.4. Lattice parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample. Symbols: Green for orthorhombic phase, red for tetragonal phase, and blue for new tetragonal phase; circle for a, square for b and triangle for c parameters...... 85
Figure 4.5. Profile Refinement for data collected at 307°C. Thick blue line denotes the tetragonal phase component of the refinement...... 86
Figure 4.6. Relationship between orthorhombic Bmm2 unit cell (a0 b0 c0), tetragonal unit cell (at bt ct) and pseudocubic unit cell (a1 a2 a3 β) with atomic positions (from Ishizawa et al).[117] ...... 87
Figure 4.7. Pseudo-cubic parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 sintered pellet sample. Symbols: Green for orthorhombic phase, red for tetragonal phase, blue for new tetragonal phase; square for a’ (and c’) and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison...... 88
Figure 4.8. The diffraction profiles for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 (crushed powder) (°C), as a function of temperature, at 20-60° 2θ...... 89
Figure 4.9. Diffraction profiles for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 (as a crushed powder) as a function of temperature (ºC) at 38-43° 2θ...... 89
Figure 4.10. The Rietveld refinement of the double peak at in data collected at 72ºC (using orthorhombic parameters)...... 90
Figure 4.11. Profile refinement for data collected at 152ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase...... 91
Figure 4.12. Profile refinement for data collected at 296ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase...... 92
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 194 Figure 4.13. Profile refinement for data collected at 344ºC. The top thin blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase...... 92
Figure 4.14. Phase parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 crushed powder sample. Markers: Square for tetragonal phase and triangle for orthorhombic phase; red for a parameter, green for b parameter and blue for c parameter...... 94
Figure 4.15. Pseudo-cubic parameters for 99.5NKN-0.5CuO + 0.6wt% Nb2O5 crushed powder sample. Markers: Red for tetragonal phase and blue for orthorhombic phase; square for a’ (and c’) and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison...... 95
Figure 4.16. P-S hysteresis behaviour for 99.5NKN-0.5CuO + 0.6wt% Nb2O5...... 96
Figure 4.17. Diffraction profiles for 94NKN-6LT as a function of temperature (°C)...... 96
Figure 4.18. Diffraction profiles for 200 and 002 peaks for 94NKN-6LT as a function of temperature (ºC)...... 97
Figure 4.19. The refinement of 94NKN-6LT for data collected at 30ºC. The top blue and red lines denote the observed and calculated lines respectively. The thick blue line in (a) shows the tetragonal model, and the black line (b) shows the orthorhombic phase...... 98
Figure 4.20. The diffraction profile for data collected at 270ºC, with peak broadening on first peak...... 99
Figure 4.21. The Rietveld refinement of 94NKN-6LT for data collected at 334ºC, the thick black line showing the orthorhombic phase component...... 99
Figure 4.22. The refinement at 336ºC, the thick blue line in (a) showing the tetragonal phase component, and the thick black line in (b) showing the orthorhombic phase component...... 100
Figure 4.23. Resulting lattice parameters for 94NKN-6LT powder as a function of temperature. Markers: Green for orthorhombic phase, red for tetragonal phase and orange for cubic phase; square for a parameters, triangle for b parameters and circle for c parameters...... 101
Figure 4.24. Pseudocubic parameters for 94NKN-6LT. Symbols: Green for orthorhombic phase, red for tetragonal phase, and orange for cubic phase; square for a’ and triangle for b’ parameters. The cubes show the pseudo-cubic parameters for the phases for direct comparison...... 102
Figure 5.1. (a) SEM micrograph of Na2CO3 particle and (b) X-ray diffraction spectrum for Na2CO3 with fitting to reference data shown by “stick patterns”...... 105
Figure 5.2. SEM micrograph of Potassium Carbonate powder...... 106
Figure 5.3. SEM micrograph of Niobium Oxide powder and (b) X-ray diffraction spectrum for Nb2O5 with fitting to reference data shown by “stick patterns”...... 107
Figure 5.4. (a) SEM micrograph of SrCO3 particles and (b) X-ray diffraction spectrum for SrCO3 with fitting to reference data shown by “stick patterns”...... 107
Figure 5.5. (a) SEM micrograph of BaCO3 particles and (b) X-ray diffraction spectrum for Na2CO3 with fitting to reference data shown by “stick patterns”...... 108
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 195 Figure 5.6. (a) SEM micrograph of Fe2O3 powder and (b) X-ray diffraction spectrum for Na2CO3 with fitting to reference data shown by “stick patterns”...... 108
Figure 5.7. Relative density of NKN as a function of x% Fe2O3 additions (after Wegrzyn).[176]...... 110
Figure 5.8. Densities of 96NKN-4SBN (Tsint = 1140ºC) as a function of x wt% Fe2O3 and ZnO addition...... 111
Figure 5.9. Relative densities of (1-x)NKN – xSBN + 0.45 wt% Fe2O3 (0 ≤ x ≤ 4), all sintered at 1140°C...... 111
Figure 5.10. Diffraction spectra for (1-x)NKN – xSBN (0 ≤ x ≤ 4). Indexing of orthorhombic peaks based on the work of Wu et al.[165] ...... 112
Figure 5.11. Diffraction spectra for (1-x)NKN – xSBN + 0.45 wt% Fe2O3 (0 ≤ x ≤ 4)...... 113
Figure 5.12. SEM micrograph of thermally etched 2SBNF...... 114
Figure 5.13. SEM micrograph of thermally etched 3SBNF. Arrows indicate herringbone domain structures...... 115
Figure 5.14. SEM backscattered imaging micrographs of second phases found in polished (a) 2SBNF and (b) 3SBNF. Yellow arrows indicate light grey second phase...... 115
Figure 5.15. SEM backscattered imaging micrograph of second phases found in polished 4SBNF. Yellow arrow indicates light grey second phase, and red arrow indicates white second phase...... 115
Figure 5.16. Permittivities of (100-x)NKN-xSBN + 0.45wt% Fe2O3 (4h) (at 100 kHz), with lower temperatures magnified, inset...... 117
Figure 5.17. The transition temperatures of (100-x)NKN-xSBN + 0.45wt% Fe2O3 (4h) .. 117
Figure 5.18. The effect of sintering time on the transition temperatures of 2SBNF...... 119
Figure 5.19. Hysteresis behaviour of NKN + 0.3wt% Fe2O3 (Tsint = 1090°C) and 2SBNF (Tsint = 1140°C) ...... 120
Figure 5.20. P-E hysteresis behaviour for (100-x)NKN-xSBNF (2 ≤ x ≤ 4)...... 120
Figure 5.21. Hysteresis behaviour of 4SBNF at varying field strengths...... 121
Figure 5.22. P-E behaviour for 2SBNF sintered at 1140 and 1160°C (4h) (navy and red respectively), and at 1140°C (24h) at 40 and 60 kV/cm (green and orange respectively) ...... 123
Figure 5.23. P-E behaviour for 98NKN-2SBN + 0.45wt% Fe2O3 (Tsint = 1140°C, 24h) ...... 124
Figure 5.24. ρ”-ρ’ plot for NKN + 0.45wt% Fe2O3, with varying temperatures (465-645ºC) ...... 125
Figure 5.25. Resistivity spectra for (1-x)SBN + 0.45wt% Fe2O3 (4h) at 545-550°C...... 126
Figure 5.26. Conductivity-temperature plots for (100-x)NKN-xSBNF + 0.45wt% Fe2O3, with x = 0, 2, 3 and 4 mol% (navy, red, green, and blue respectively)...... 126
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 196 Figure 5.27. Activation energies for (100-x)NKN-xSBN + 0.45wt% Fe2O3, with 4 hours sintering time, 0 ≤ x ≤ 4...... 127
Figure 5.28. ρ”-ρ’ plot for 98NKN-2SBN + 0.45wt% Fe2O3, with sintering times of 4, 8, 16, 24 and 72 hours at 1120°C (navy, red, green, blue and purple respectively)...... 129
Figure 5.29. Activation energies of 98NKN-2SBN + 0.45wt% Fe2O3, as a function of sintering time...... 130
Figure 6.1. Laboratory XRD profiles of (100-x)NKN – xSBN + 0.45wt% Fe2O3 formulations (0 ≤ x ≤ 4) ...... 132
Figure 6.2. SEM micrograph of 4SBNF polished sample, showing light grey and white second phases in a dark grey NKN matrix phase. Yellow arrow indicates light grey second phase, and red arrow indicates white second phase...... 133
Figure 6.3. Synchrtoron XRD profiles of xSBNF formulations (0 ≤ x ≤ 4) ...... 133
Figure 6.4. The refinement of NKNF as (a) the full refinement (square root y axis) and (b) peak profiles of 10-50 degrees 2θ (logarithmic y axis). The blue line shows the observed data, the red line shows the calculated model, and the grey line below shows the difference between the two...... 134
Figure 6.5. The refinement of 1 SBNF...... 135
Figure 6.6. The synchrotron XRD spectrum and refined data for 3SBNF (logarithmic y- axis) ...... 136
Figure 6.7. Synchrotron diffraction profile for 4SBNF (a) in the range 0-46º 2θ, square- root y-axis, and (b) in the range 10-48º 2θ, logarithmic y-axis...... 136
Figure 6.8. Phase Content in NKNF as a function of x mol% SBN addition...... 138
Figure 6.9. The pseudocubic lattice parameters for the orthorhombic region of xSBNF. The cuboid represents the shape of the unit cell (not to scale) ...... 139
Figure 6.10. Pseudocubic lattice parameters for the tetragonal region of NKNF as a function of SBN addition. Cuboids represent the shape of the unit cell (not to scale) ...... 140
Figure 7.1. BNN particles heat treated under identical conditions, using (a) quartz, (b) alumina and (c) platinum crucible...... 144
Figure 7.2. BNN particles before hot washing stage, sintered at 1100ºC...... 144
Figure 7.3. BNN particles sintered at 1050ºC, before hot washing stage. X denotes the area under EDAX investigation...... 145
Figure 7.4. SEM micrographs showing BNN particles sintered at 1050ºC after hot washing. X denotes the area under EDAX investigation...... 145
Figure 7.5. SEM micrographs showing BNN particles using the 3:2 salt:oxide ratio (a,b)sintered at 1050ºC, and sintered at 1100ºC (c, d)...... 146
Figure 7.6. BNN powder using 3:2 salt:oxide ratio used in alumina crucibles, sintered at (a) 1050 and (b) 1100ºC...... 147
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 197 Figure 7.7. SEM micrographs of initial NN particles from solution at (a) low and (b) high magnification...... 148
Figure 7.8. SEM micrographs showing an NN particle sintered at 1050ºC...... 149
Figure 7.9. Schematic illustration of the block casting procedure...... 150
Figure 7.10. Schematic illustration of the glass slide casting procedure...... 150
Figure 7.11. Results of various methods of producing slurry; how they formed tapes using the glass slide casting process...... 151
Figure 7.12. The thickness of 6LN tape cast on a glass slide (a) along the length and (b) across the width of the slide...... 152
Figure 7.13. P-E hysteresis behaviour of (a) NKN + 0.2 wt % CuO and (b) 95NKN-5LN 0.8 wt% CuO (from Azough et al).[200] ...... 153
Figure 7.14. SEM micrographs of 95NKN-5LN + x wt% CuO single tapes (thickness 20- 30μm) sintered at different temperatures...... 155
Figure 7.15. Diffraction patterns of 95NKN-5LN + 0.4wt% CuO at various sintering temperatures...... 156
Figure 7.16. Diffraction spectra of 94NKN-6LN + 0.4wt% CuO single and double-cast tapes, both sintered at 1030ºC...... 157
Figure 7.17. Schematic diagram of the RTGG process [after Zhao et al.].[183]...... 158
Figure 7.18. SEM micrographs of 94NKN-6LN + 0.4 wt% CuO single cast tape sintered at 1030ºC, (a) plan view, and (b) cross-sectional view of tape. Yellow arrows indicate visible particles...... 159
Figure 7.19. SEM micrographs showing microstructures of NBTBT thick films (a) using no dispersant, and (b) using dispersant (after Kimura et al).[131] ...... 159
Figure 7.20. SEM micrographs of single tapes cast of composition 94NKN-6LN + 0.4wt% CuO + 10%NN particles sintered at temperatures of (a) 1030ºC, (b) 1090ºC and 1120ºC (c) top view and (d) cross-section. Yellow arrows show template particles...... 160
Figure 7.21. SEM micrographs showing the top and side views of 94NKN-6LN + 0.4wt% CuO + 10%NN particles sintered at 1120ºC, with four layers (a-c) and eight layers (d-f). Yellow arrows indicate visible template particles...... 161
Figure 7.22. SEM micrographs of the top and side views of an eight layered 94NKN-6LN + 0.4wt% CuO + 10%NN particles, sintered at 1150ºC. Yellow arrows indicate visible template particles...... 162
Figure 7.23. XRD spectra for 94NKN-6LN + 0.4wt% CuO + 10 wt% NN particles, 4x layer pressed tapes sintered at 1090, 1120 and 8x layer pressed tape sintered at 1150ºC...... 163
Figure 7.24. XRD diffraction patterns for NBTKT with random orientation (blue) and with 5% added SrTiO3 template particles added (pink) (from Yilmaz et al).[137] ...... 163
Figure 7.25. Comparison of XRD diffraction profiles of 94NKN-6LN + 0.4 wt% CuO (a) double cast sintered at 1030°C, and (b) 8x layered tape with 10 wt% NN particles added, sintered at 1150°C...... 165
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 198 Figure 7.26. Permittivity data for 94NKN-6LN + 0.4 wt% CuO + 10% NN particles, as a function of applied frequency. Inset shows 1-400°C range under closer inspection...... 166
Figure 7.27. Cole-Cole plot data for 94NKN-6LN + 0.4 wt% CuO + 10% NN particles, as a function of temperature (485 – 545 °C) ...... 166
Figure 7.28. Cole-Cole impedance plot fitting for 94NKN-6LN + 0.4 wt% CuO + 10 wt% NN particles, at temperature 545ºC. Blue spots and red line denote data, green line denotes fitted semi-circle model...... 167
Figure 7.29. Comparison of single and four-layered 6LN + 0.8 wt% CuO + 10% NN particles samples sintered at 1090ºC...... 168
Figure 7.30. SEM image of four-layered 6LN + 0.8 wt% CuO + 10% NN particles sample sintered at 1120ºC showing (a) as-fired surface and (b) side elevation view...... 168
Figure 7.31. XRD Spectra for 94NKN-6LN + 0.8wt% CuO + 15 wt% NN particles, 4 layer pressed tapes sintered at 1090 and 1120ºC...... 169
Figure 7.32. XRD diffractogram for 93NKN-7LT-xNN particles (here NKN is denoted as KNN) [from Feng et al}.[82] ...... 170
Figure 7.33. Schematic showing the evolution of K3Li2Nb5O15 structure from NKN (from Feng et al.)[82] ...... 170
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 199 APPENDIX 2 - TABLES
Table 2–1. Hysteresis Properties of pure NKN (in order of field threshold, E)...... 39
Table 2–2. Summary of properties and processing of BNN particles (optimal results) with references...... 58
Table 3–1. Details of raw starting powders ...... 61
Table 3–2. Samples prepared in this study...... 63
Table 3–3. Manufacturer’s data for slurry additives ...... 68
Table 3–4. Samples under Investigation at Daresbury SRS, Station 6.2...... 74
Table 4–1. Summary of reported NKN pseudo-cubic parameters (at room temperature). 87
Table 4–2. Average lattice parameters of phases present in Cu doped NKN (sintered pellet) ...... 88
Table 4–3. Average lattice parameters for phases present in Cu doped NKN (powder sample)...... 95
Table 4–4. Average parameters of phases present in 94NKN-6LT...... 102
Table 5–1. Summary of electrical data for (100-x)NKN-xSBN + 0.45wt% Fe2O3 system, at 545ºC...... 128
Table 6–1. The phase content for xSBNF refinements, including GOF values...... 137
Table 6–2. Pseudo-cubic parameters for orthorhombic phase in xSBNF (accurate to 5dp) ...... 139
Table 6–3. Pseudo-cubic parameters for tetragonal phase in xSBNF (accurate to 5dp)... 140
Table 7–1. The EDAX data for a pre-hot-washed BNN particle...... 145
Table 7–2. The EDAX data for a hot-washed BNN particle...... 146
Table 7–3. Resulting data from BNN particles in this investigation...... 147
Table 7–4. The EDAX data for chemical compositions of Figure 7.7 (Spectrum 3)...... 148
Table 7–5 – Volume fractions of components in test slurries...... 152
Margaret Węgrzyn Sodium Potassium Niobate Based Piezoelectric Ceramics Page 200