A NOVEL PRECURSOR FOR SYNTHESIS OF ZIRCONIUM TUNGSTATE AND PRELIMINARY STUDIES FOR NANOFIBER PRODUCTION
A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY
BY
BERKER ÖZERCİYES
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN CHEMICAL ENGINEERING
JANUARY 2009
Approval of the thesis:
A NOVEL PRECURSOR FOR SYNTHESIS OF ZIRCONIUM TUNGSTATE AND PRELIMINARY STUDIES FOR NANOFIBER PRODUCTION
submitted by BERKER ÖZERCİYES in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering Department, Middle East Technical University by,
Prof. Dr. Canan Özgen ______Dean, Graduate School of Natural and Applied Sciences
Prof. Dr. Gürkan KARAKAŞ ______Head of Department, Chemical Engineering
Prof. Dr. Güngör GÜNDÜZ ______Supervisor, Chemical Eng. Dept., METU
Assist. Prof. Dr. Bora MAVİŞ ______Co-supervisor, Mechanical Eng. Dept., Hacettepe Univ.
Examining Committee Members:
Prof. Dr. Hayrettin YÜCEL ______Chemical Engineering Department, METU
Prof. Dr. Güngör GÜNDÜZ ______Chemical Engineering Department, METU
Prof. Dr. Işık ÖNAL ______Chemical Engineering Department, METU
Prof. Dr. Macit ÖZENBAŞ ______Metallurgical and Materials Engineering Department, METU
Assist. Prof. Dr. Ayşen YILMAZ ______Chemistry Department, METU
Date ______
SM
I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
Name, Last name : BERKER ÖZERCİYES
Signature : ABSTRACT
A NOVEL PRECURSOR FOR SYNTHESIS OF ZIRCONIUM TUNGSTATE AND PRELIMINARY STUDIES FOR NANOFIBER PRODUCTION
Özerciyes, Berker M.S., Department of Chemical Engineering Supervisor: Güngör Gündüz, Prof. Dr. Co-Supervisor: Bora Maviş, Assist. Prof. Dr.
January 2009, 97 pages
Zirconium tungstate (ZrW2O8) is a ceramic that shows large isotropic negative thermal expansion over a wide range of temperature. This unique property makes it an interesting candidate for applications where thermal expansion mismatch between components constitutes a problem. ZrW2O8 is typically produced by solid-state reaction between zirconium oxide and o tungsten oxide at 1200 C. In some studies, ZrW2O8 precursors have been produced from relatively expensive zirconium and tungsten sources. While the origin of negative thermal expansion has been the main focus in the majority of publications, production of particles with controlled size, distribution and morphology has not been studied extensively.
iv Electrospinning is a simple technique for producing micron/nano sized fibers from polymer solutions. The method can also be used for producing ceramic or polymer/ceramic composite fibers by electrospinning of a mixture of ceramic precursors or ceramic nanoparticles with suitable polymers. Ceramic precursors could be synthesized either by sol-gel or chemical precipitation routes before mixing them with polymer solutions and a final burnout step would be needed, in case the fiber is desired to be composed of the ceramic phase. Electrospinning technique has not been employed to the production of ZrW2O8 ceramic fibers.
In this study a novel precursor for ZrW2O8 from relatively cheaper and abundant starting chemicals, namely zirconium acetate and tungstic acid were used. Experimental details of development of the precursor are presented with a discussion on the effects of solution parameters on the phase purity of the fired product. Besides the solution parameters investigated (i.e. solubility of tungstic acid, adjustment of the stoichiometry, final pH of the solution, ageing time), evolution of the heat treatment protocol was used in the production of phase pure ZrW2O8. Second, the suitability of the developed precursor for producing ZrW2O8 in fiber form was investigated. Preliminary studies involved the adjustment of the viscosity of precursor solution for electrospinning with poly (vinyl alcohol) (PVA). Optimum PVA concentration leading to bead-free nanofiber mats and a method to increase the fiber production rate were reported. The characterization of the products was achieved by SEM and XRD.
Keywords: zirconium tungstate, negative thermal expansion, sol-gel, ceramic fiber, electrospinning.
v ÖZ
YENİ BİR ÖNCÜLLE ZİRKONYUM TUNGSTAT SENTEZİ VE NANOLİF ÜRETİMİ İÇİN ÖN ÇALIŞMALAR
Özerciyes, Berker Yüksek Lisans, Kimya Mühendisliği Bölümü Tez Danışmanı: Güngör Gündüz, Prof. Dr. Yardımcı Danışman: Bora Maviş, Yardımcı. Doç. Dr.
Ocak 2009, 97 sayfa
Zirkonyum tungstat (ZrW2O8), geniş bir sıcaklık aralığında yüksek eş yönlü (izotropik) negatif ısıl genleşme özelliği gösteren bir seramiktir. Sahip olduğu bu üstün özellik, ZrW2O8’i ısıl genleşmenin sorun yarattığı birçok kompozit malzemede kullanabilecek önemli bir aday yapmaktadır. ZrW2O8 genel olarak zirkonyum oksit ve tungsten oksidin 1200°C’de katı hal tepkimeleri sonucu üretilmektedir. Çözelti kimyası yardımıyla, görece pahalı zirkonyum ve tungsten kaynaklarıyla, ZrW2O8’in üretimine yönelik yapılan çalışmalar yapılmış ancak yayınlanan çalışmalarda, çoğunlukla negatif ısıl genleşme özelliğinin nedeni üzerine odaklanılmıştır. Buna karşılık denetlenebilir parçacık boyutu, dağılımı ve morfolojisi üzerine yaygın bir çalışma bulunmamaktadır. Elektro eğirme, polimer çözeltilerinden mikro/nano boyutta lif üretimi için kullanılan görece basit bir yöntemdir. Seramik veya polimer/seramik
vi kompozit lifler, seramik öncüllerden veya uygun polimer eklenmesiyle viskozitesi ayarlanmış seramik nano parçacık içeren çözeltilerden, elektro eğirme işlemi sonucu elde edilebilmektedir. Eğirme işleminde kullanılacak seramik öncüller, çöz-pel veya ortak çökeltme teknikleriyle hazırlanabilir. Eğirme sonrası uygulanacak yakma işlemiyle de istenilen seramik lifler elde edilebilmektedir. ZrW2O8 seramik liflerinin üretimine dair, henüz yapılmış bir çalışma bulunmamaktadır.
Bu çalışmada, ZrW2O8 öncüllerinin görece daha ucuz ve kolay elde edilebilen başlangıç kimyasallarından (zirkonyum asetat ve tungstik asit) üretimi sağlanmıştır. Deneysel gelişim ve yakılan ürünün saflığına etki eden deneysel değişkenler ayrıntılarıyla incelenmiştir. Çözelti değişkenlerinin (tungstik asidin çözünürlüğü, kimyasal oran ayarlaması, pH ve yaşlandırma etkileri gibi) incelemelerin yanı sıra, saf ZrW2O8 üretimine olanak veren önemli ısıl işlem basamakları gerekçelendirilmiştir. Çalışmada ikincil olarak, geliştirilen yeni öncülün elektro eğirme işleminde kullanıma uygun olup olmadığı incelenmiştir. Poli (vinil alkol) (PVA) yardımıyla viskozite ayarı yapılan öncüllerden, elektro eğirme yöntemiyle polimer/seramik nanolif üretimine yönelik ön çalışmalar gerçekleştirilmiştir. Boncuksuz nano lif hasırların oluşumuna olanak veren uygun PVA derişimi belirlenmiş ve lif üretim hızının arttırılmasına yönelik deneysel düzenek iyileştirmeleri ortaya konmuştur. Elde edilen ürünlerin tanımlanmasında SEM ve XRD yöntemleri kullanılmıştır.
Anahtar Sözcükler: zirkonyum tungstat, negatif ısıl genleşme, çöz-pel, çökeltme, seramik lif, elektro eğirme.
vii
To My Exceptional Family
viii ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Prof. Dr. Güngör Gündüz for his expertise, understanding, and patience especially for accepting me as my supervisor and his insightful motivating speech during my thesis research period. I would like to thank my co-supervisor Assist. Prof. Dr. Bora Maviş for his endless helps on many occasions, his great ideas, and numerous helpful discussions. Additionally I would also like to acknowledge Prof. Dr. Üner Çolak for his valuable advice and support. I am very thankful to Korhan Sezgiker and Nasser Khazeni for their great supports in the final experiments and their collaborative manner. I would also like to thank to my laboratory friends Nihan Karakoç, Erkan Biber, Ahmet Göktaş, Burcu Berna Topuz, Gülden Eroğlu, Simge Çınar and Nagihan Keskin for their collaborative, motivating and friendly manner. I would also thank to Seçkin Öztürk and Sedat Canlı from Central Laboratory of METU and Evren Çubukçu and Orkun Ersoy from the Geological Engineering Department of HU, for helping me on SEM analysis and Necmi Avcı from Metallurgical and Materials Engineering Department of METU for helping me on XRD analysis. I also wish thank to Çiğdem Pulatsü for her help supplying the chemicals and equipments on time and Nur Merve Kazaroğlu from Chemical Engineering Department of Hacettepe University for her help in increasing the product efficiency in electrospinning. I would like to thank all of my friends for all their emotional support and motivation during this difficult accomplishment.
ix Last but not least, I express my sincere thanks to my family for the support they provided me through my entire life. This work was supported by TÜBİTAK (The Science and Technical Research Council of Turkey) (Project Number: 107M006) and METU (BAP- 2008-03-04-01 – Scientific Research Project).
x TABLE OF CONTENTS
ABSTRACT……………………………………………………………………………………….iv ÖZ………………………………………………………………………………………………….vi DEDICATION………………………………………………………………………………….viii ACKNOWLEDGEMENTS…………………………………………………………………….ix TABLE OF CONTENTS………………………………………………………………………xi LIST OF TABLES……………………………………………………………………………..xiv LIST OF FIGURES…………………………………………………………………………...xv NOMENCLATURE……………………………………………………………………….….xviii
CHAPTER
1. INTRODUCTION……………………………..……………………………………………1
2. LITERATURE REVIEW…………………………………………………………………..7 2.1 Thermal Expansion…………….……………………………………7
2.2 ZrW2O8 and Origin of Negative Thermal Expansion………………11
2.3 Phase Transitions in ZrW2O8 ...... 16
2.4 Synthesis of ZrW2O8………………………………………………………….17
2.5 Alternative Methods of Synthesis of ZrW2O8…………………………19 2.6 The Electrospinning of Metal Oxide Nanofibers…………………….24
3. EXPERIMENTAL……………………………………………………..……………………27 3.1 Materials……………………………………………………..…………………..27 3.1.1 Zirconium Acetate………………………………………………………..27 3.1.2 Tungstic Acid……………………………………………………………….28 3.1.3 Ammonia Solution……………..…………………………………………28
xi 3.1.4 Polyvinyl Alcohol (PVA)…………………………………………………28 3.2 Precursor Preparation………………………………………………………..29 3.2.1 Preliminary Studies………………………………………………….……29 3.2.2 Sol Preparation…………..………………………………………………..32 3.3 Electrospinning Setup………………………………………………………..36 3.4 Heat Treatment………………………………………………………………..37 3.5 Characterization…………………………………………..……………………38 3.5.1 Particle Size Analysis………………………………….…………………38 3.5.2 Scanning Electron Microscopy (SEM)………………………………38 3.5.3 X-Ray Diffraction (XRD)…………………………………………………39 3.6 Chemical Equilibrium Calculations…………………………………...... 40
4. RESULTS AND DISCUSSIONS…………………………………………………………….41 4.1 Preliminary Studies on the Precursor...... 41 4.2 Sol Preparation………………………………………………………………………46 4.3 Ageing Effect…………………………..…………………………………………….52
5. CONCLUSIONS…………………………………………………………………………………65
RECOMMENDATIONS…….…………………………………………………………………….67
REFERENCES……………………………………………………………………………………….69
APPENDICES
A. PHASE TRANSITIONS IN ZrW2O8………………….……………………………………80
B. SOL-GEL CHEMISTRY – BASIC CONCEPTS………………………….………………84
C. SYNTHETIC XRD PATTERNS…………………………………………….……………….87
xii C-1 Calculated XRD pattern for monoclinic ZrO2…………………………91
C-2 Calculated XRD pattern for monoclinic WO3…………………………91
C-3 Calculated XRD pattern for α-Cubic ZrW2O8…………………………92
D. SAMPLE CALCULATIONS.………………………………………………………………….93
E. CHEMICAL EQUILIBRIUM CALCULATIONS ON THE SOLUBILITY OF TUNGSTIC ACID SOLUTIONS………………………………………………………………..96
xiii LIST OF TABLES
Table 1.1 Linear thermal expansion coefficient of some materials….…………2
Table 2.1 Properties of α-ZrW2O8...... 15 Table 3.1 Properties of the materials used in this work………………………….29 Table 3.2 Electrospinning conditions of preliminary precursor…………………30 Table 3.3 Aging times applied to the ZrAc and ATA solution mixtures. pH is measured after each major mixing step ………………………………………………..32 Table 3.4 Compositions of electrospinning solutions………………………………34
Table 3.5 Improved approaches for the phase pure ZrW2O8 production……36 Table 4.1 Amount of oxide components forming in the ceramic body in the samples produced from precursors aged for different ageing times…………..57
Table C.1 Monoclinic ZrO2 (Space Group: P21/c)……………………………………88
Table C.2 WO3 (Space Group: P21/n)……………………………………………………89
Table C.3 α-ZrW2O8 (Space Group: P213)……………………………………………..90
Table D.1 ZrO2, WO3 and ZrW2O8 the characteristic peaks and their calculated proportional intensities between 21,0°-32,0°…………………………..93 Table D.2 Density and molecular weight values………………………………...... 95
xiv LIST OF FIGURES
Figure 1.1 Phase relation in the system ZrO2-WO3……..………………….……….4 Figure 2.1 The interatomic potential function of a simple harmonic oscillator…………………………………………………………………………………….….…….9 Figure 2.2 The interatomic potential function of anharmonic oscillator...……9 Figure 2.3 Longitudinal (left) and transversal (right) component of the vibration mode……………………………………………………………………………….……10 Figure 2.4 A schematic representation of how transverse thermal vibrations of M-O-M linkages can give rise to NTE in framework oxide structures………12
Figure 2.5 Crystal structure of cubic ZrW2O8 with polyhedral representation.
ZrO6 octahedra are shown in green, and WO4 tetrahedra in red. The oxygen atoms at the vertices of polyhedra are shown as red spheres…………………..14
Figure 2.6 Unit-cell volume of orthorhombic γ -ZrW2O8 and cubic α-ZrW2O8 with respect to temperature over a temperature range of 20–473 K. Data were obtained by heating the metastable orthorhombic phase through the transition and back to the cubic phase…………………………………………………..17 Figure 3.1 Structural formula of zirconium acetate……………………………..…27 Figure 3.2 Structural formula of tungstic acid……………………………………….28 Figure 3.3 Structural formula of PVA………..……………………………………..…..29 Figure 3.4 Temperature profiles that are applied to the electrospun fibers obtained in preliminary precursor studies……………………………………………….31 Figure 3.5 Temperature profiles that are applied to the as-prepared precursors obtained in preliminary studies……………………………………………..31 Figure 3.6 Temperature profiles for the two step heat treatment applied in the production of ZrW2O8 from solutions A through H……………………………..33 Figure 3.7 Temperature profile for the heat treatment applied to the electrospun fibers………………………………………………………………………………..34
xv Figure 3.8 Flow chart of synthesis of ZrW2O8 from solutions A through H………………………………………………………………………………………………………..35 Figure 3.9 The picture of the electrospinning setup……………………..……….37 Figure 4.1 SEM micrographs of electrospun preliminary Precursor/PVA fibers at (a) 800°C, (b) 900°C, (c) 1000°C…………………………………….………………..42 Figure 4.2 XRD pattern of the preliminary precursor/PVA fibers calcined at 800 °C………………………………………………………………………………………………..43 Figure 4.3 SEM micrographs of preliminary precursors heat treated at; (a) 800°C (b) 900°C (c) 1000°C and (d) 1100°C……….…………………………………45 Figure 4.4 XRD patterns of treated preliminary precursor treated at 800, 1000, 1100°C……………………….……………………………………………………………..45 Figure 4.5 The average particle sizes observed in ZrAc-ATA precursor solutions with decreasing pH…………………………………………………………………47 Figure 4.6 The zeta potential of ‘entities’ forming in ZrAc-ATA precursor solutions with respect to decreasing Ph………………………………………………….47 Figure 4.7 The average particle size measurement from ZrAc-ATA solution after ultrasonic mixing………………………………………………………………………….49 Figure 4.8 The zeta potential measurement from ZrAc-ATA solution after ultrasonic mixing………………………………………………………………………………….50 Figure 4.9 The average particle size measurements from ZrAc-ATA solutions during a 3 day period…………………………………………………………………………..52 Figure 4.10 XRD patterns from the heat treated samples that were produced from precursors aged for different time periods (Refer to Table 3.3. for sample labels)………………………………………………………………………………..54
Figure 4.11 Effect of ageing and pH on the amount of ZrW2O8 phase (in volume) produced….…………………………………………………………………………….55
Figure 4.12 Effect of ageing and pH on the amount of ZrO2 and WO3 (in volume) produced………………………………………………………………………………..55
xvi Figure 4.13 Effect of ageing and pH on the amount of ZrW2O8 phase content (in mole) produced…………………………………………………………………..56
Figure 4.14 Effect of ageing and pH on the amount of ZrO2 and WO3 phase content (in mole) produced…………………………………………………………………..56 Figure 4.15 XRD pattern from the sample prepared at a fixed and high pH………………………………………………………………………………………………………59
Figure 4.16 Electrospun nanofibers of ZrW2O8 precursor (7,5 wt% PVA)…………………………………………………………………………………………………..59
Figure 4.17 Heat treated nanorods produced from ZrW2O8 precursor (7,5 wt% PVA)……………………………………………………………………………………………60
Figure 4.18 Heat treated nanorods produced from ZrW2O8 precursor (8,5 wt% PVA)……………………………………………………………………………………………61
Figure 4.19 XRD patterns of phase pure ZrW2O8 produced by modified precursor approaches (refer to Table 3.4. for labels). Pattern of Sample C (a sample with 2 hours of ageing) is given for comparison…………………………..63
Figure A.1 Structure of the γ-orthorhombic phase of ZrW2O8 illustrated with
ZrO6 octahedra and WO4 tetrahedra………………………………………………………82
Figure A.2 The phase transitions of ZrW2O8…………………………..……………..83 Figure B.1 Schematic representation of zeta potential……………..…………….85
Figure C.1 XRD pattern of monoclinic (baddeleyite) ZrO2...... 91
Figure C.2 XRD pattern of monoclinic WO3...... 91
Figure C.3 XRD pattern of α-Cubic ZrW2O8…………………………………..……….92 Figure D.1 Deconvoluted peaks between 21° and 32°……………………..……94 Figure E.1 The distribution of different ionic species in the ATA solutions with respect to pH. pH is increased by ammonia total quantity of which is represented on the right axis………………………………………………………………..97
xvii NOMENCLATURE
Å Angstrom cm Centimeter mm Millimeter µm Micrometer nm Nanometer C Celsius K Kelvin g Gram h Hour kV Kilovolt V Volt MPa Mega Pascal ml Milliliter M Metal PVA Poly Vinyl Alcohol PVAc Poly Vinyl Acetate SEM Scanning Electron Microscope XRD X-Ray Diffractometer / Diffraction
AM2O8 Zirconium tungstate family with negative thermal expansion
xviii
CHAPTER 1
INTRODUCTION
Today’s technological challenges demand the unusual combinations of properties to be met in a single material. In many cases, such functionality can be acquired only by development of new composite materials.
Most materials expand with an increase in temperature (Table 1.1). While a material can meet the certain requirements of a specific application (e.g. dielectric constant, conductivity), the positive thermal expansion coefficient that might accompany its use, can render it inadequate for the application. Therefore to compensate for the undesired positive expansion of some materials that are certainly needed for specific applications, these materials could be used in combination with materials that can contract on heating. Although negative thermal expansion (NTE) is not observed in common engineering materials, limited families of materials have been demonstrated to show this property. One could imagine creating composite materials with negative, zero or positive thermal expansion coefficient with the use of these materials. The adjustable thermal expansion coefficient of the resulting composites would make it possible to match the thermal expansion coefficient of one part to another part in a certain device so that, upon heating or cooling cracks or separation at interfaces could be avoided. Such composites could also be used in applications, where the physical dimension changes limit the useful range of a device (e.g. optical gratings or hard disk read/write heads).
1 Table 1.1 Linear thermal expansion coefficient of some materials 1-3.
-1 -6 MATERIAL αL ((°C) X 10 ) Metals Aluminum 23.6 Copper 17.0 Silver 19.7 Gold 14.2 Tungsten 4.5 Ceramics
Alumina (Al2O3) 7.6
Fused silica (SiO2) 0.5 Boroslicate (Pyrex) glass 3.3 Polymers Polyethylene (high density) 106-198 Polypropylene 145-180 Polystyrene 90-150
Nylon 6,6 144
AM2O8 (Zirconium tungstate family with negative thermal expansion) constitutes a group of compounds that show NTE coefficient. With its large isotropic NTE coefficient zirconium tungstate (ZrW2O8) is the most famous in the family and it shows the property over a wide range of temperature.
Several applications that are based on the use of ZrW2O8 and adjustment of the overall thermal expansion coefficient to desired values have been envisaged or realized. In fields such as electronics (e.g. zero expansion heat sinks, substrates, composite printed circuit boards), dentistry (e.g. dental fillings that would not cause hot/cold sensitivity), optics (e.g. substrates for high-precision optical applications, optical mirrors, fiber optic systems), or
2 house-hold items (e.g. oven-to-table cookware) NTE coefficient materials have been and will be benefited from 4-7.
ZrW2O8 can be synthesized by a variety of methods. While solid state methods have been applied in many of the pioneering work 8-13, the known drawbacks of this approach do not constitute an exception for the ZrW2O8 system. Due to ZrW2O8’s widely accepted potential, alternative synthesis methods such as; sol-gel, non hydrolytic sol-gel 10, 14-21, hydrothermal 10, 22, 23, co-precipitation 24 and combustion methods 10 have been experimented besides the conventional solid state methods. Besides the known disadvantages of methods based on solid state reactions (i.e. the inhomogeneous mixing conditions that can be achieved in micron-scale at best under most common processing conditions and the resulting difficulty in obtaining the right stoichiometry, need for high temperatures, longer processing times, multiple grinding and reheating cycles that are necessary for the pure phase formation) ZrW2O8 production by high temperature solid state reactions presents additional problems. The o fact that ZrW2O8 is thermodynamically stable only from 1105 to 1257 C, limits the lower end of thermal treatments (Figure 1.1). This necessitates that the cooling should rather be made by quenching from high o temperatures. In addition, over 1105 C the increasing volatility of WO3 brings about the problem of attaining the right stoichiometry during heat treatment. Any volatilization shifts the compositions towards the both sides of the
ZrW2O8 line in the phase diagram. This in turn, would decrease the phase purity of ZrW2O8 and cause the formation of oxides of Zr and W independently. Use of excess quantities of W in the initial oxide mixture has been claimed to solve the problem 13. However, since the mixing efficiencies remain low in solid state mixtures, these precautions may still not lead to phase pure materials.
3
25 Figure 1.1 Phase relation in the system ZrO2-WO3 .
Wet chemical methods like sol-gel, deliver some advantages compared to the conventional methods, probably most important being the chemical homogeneity that can be controlled at the atomic scale. Such intimacy quite often leads to lower processing temperatures and times and a better control on the particle size, shape and distribution 26. On the other hand, cost of starting chemicals generally puts a limit on the large scale production based on these methods. In addition, process time can be lengthy due to the necessity of ageing and drying steps that should be carried out with care.
Use of such methods in ZrW2O8 production brought in some processing advantages, like elimination of the multiple grinding and reheating cycles 27. However, starting chemicals used were relatively expensive compared to alternatives like, zirconium acetate (ZrAc) and tungstic acid (TA). In this study a novel precursor based on sol-gel chemistry between ZrAc and TA was developed in order to obtain phase pure ZrW2O8. Further elaboration on
4 this precursor’s chemistry would potentially lead to better control on particle size, distribution and morphology, which is a rather overlooked aspect in the research on ZrW2O8 synthesis. It is clear that, control over morphology, particle size and distribution is the required aspect of the ZrW2O8 research in order to enumerate and expand the fields of use of ZrW2O8 in the future. Fibers with diameters below 500 nm are often called nanofibers. Compared with the regular fibers, due to their smaller diameters nanofibers have higher surface areas, and hence potentially perform better in applications where higher surface area is associated with higher efficiencies or better mechanical properties. In the production of nanofibers, a number of processing techniques have been used. Drawing 28, template synthesis 29, phase separation 30, self-assembly 31 and electrospinning 11, 32-36 are to name some of these techniques. Electrospinning is an attractive method for the preparation of the ultrathin polymer or polymer/ceramic composite fibers. The standard setup for electrospinning process consists of a metal collector, a syringe pump, a metallic needle and a high voltage power supply. If the fiber that would be electrospun is desired to be a polymer/ceramic composite; a polymer solution is mixed with a suitable ceramic precursor or the ceramic nanoparticles before it is loaded into the syringe. Then the viscous solution is driven to the needle tip by a syringe pump. This solution forms a droplet at the tip of the needle. A high electric field is then generated by the high voltage supply between the metal needle and a metallic collector. When the voltage reaches a critical value, the charge overcomes the surface tension of the droplet at the tip of the needle, and the droplet is stretches into a structure which is called Taylor Cone and the electrified jet is produced. During the stretching of the droplet, the fast evaporation of solvent occurs. Finally, the dry fibers with nano scale diameters are collected on the surface of the metallic collector 37. Parameters that affect the electrospinning process can be grouped into two; i) solutions
5 parameters (i.e. molecular weight and distribution of the polymer, molecular architecture of the polymer, concentration, viscosity, conductivity and surface tension of the solution that is going to be electrospun) and ii) process parameters (i.e. electrical potential, flow rate of the solution, distance between needle and metal collector, motion of the collector and ambient parameters like temperature, humidity and air velocity in the chamber) 38-40. A final burnout step could facilitate the transformation of the polymer/ceramic composite fiber to a ceramic fiber with the desired ceramic phase. Electrospinning technique has not been employed to the production of ZrW2O8 ceramic fibers. An investigation on the suitability of the developed precursor for producing ZrW2O8 in fiber form is another objective of this work. Studies involve the adjustment of the viscosity of precursor solution for electrospinning with poly (vinyl alcohol) (PVA).
6
CHAPTER 2
LITERATURE REVIEW
In this chapter, structure of ZrW2O8, origin of negative thermal expansion in ZrW2O8, and production methods of ZrW2O8 are given with a brief review on use of electrospinning in production of metal oxide nano fibers at the end.
2.1 Thermal Expansion In a range of applications, a strict control on the thermal expansion of materials is a necessity. While positive thermal expansion behavior is more common to most engineering materials, negative thermal expansion (NTE) behavior is seen in a limited family of materials. This makes them appealing for composite applications. Linear thermal expansion coefficient can be expressed as;
lf − lo = α l (Tf-To) (1) lo
Δl = α l Δ T (2) lo
where, l0 and lf represent initial and final lengths after an increase in temperature from To to Tf . α l represents the linear coefficient of thermal expansion in one crystallographic axis. The volumetric coefficient of thermal expansion can be defined as;
7 ΔV = α v Δ T (3) Vo
where, ΔV and Vo are the volume change and original volume, respectively, and α v represents the volumetric coefficients of thermal expansion. The magnitude of the coefficient of expansion can vary from low values to high values depending on the class of material (Table 1.1). For metals, the magnitude of linear thermal expansion coefficient ranges between 5x10-6 and 25x10-6 (°C)-1. In ceramics, high interatomic bonding forces result in low coefficients of thermal expansion and values range between 0.5x10-6 and 15x10-6 (°C)-1. Ceramics are generally exposed to the high temperature changes and they are brittle. Therefore, they must have low and isotropic thermal expansion coefficients. Otherwise, they may not resist the non-uniform dimensional change, which is called thermal shock. Most polymers have very high thermal expansion coefficients. They range between about 50x10-6 and 400x10-6 (°C)-1. Highly branched polymers have the highest expansion coefficient values, because their secondary intermolecular bonds are weak and the crosslinking is low. As the crosslinking increases, the value of thermal expansion coefficient reduces. Thermal expansion of materials is a reflection of a change in the average distance between the atoms with increasing temperature and it is closely related to the nature of interatomic potential. When two atoms brought together, attractive forces control the overall potential energy of the system until the distance between the atoms becomes too small. The attractive interactions cause a decrease in the potential energy and bond formation begins. The repulsive interaction between the electrons of atoms that are now very close is reflected on the left side of the minimum on the curve and it rises with a further decrease in the interatomic distance (Figure 2.1).
8
Figure 2.1 The interatomic potential function of a simple harmonic oscillator.
If the interatomic potential function is assumed to be harmonic, when temperature increases from T1 to T2 (T2>T1), no change can be seen between the average distance of two atoms, R. Nevertheless, in reality, the interatomic potential function is anharmonic. When the temperature increase from T1 to T2 (T2>T1), the average distance between two atoms, R1 increases to R2. Therefore, the material expands with an increase in temperature (Figure 2.2).
Figure 2.2 The interatomic potential function of anharmonic oscillator.
9 If the bonds between the atoms are stronger, minimum of the interatomic potential attains lower values and the potential energy curve becomes narrower and more symmetric. As a result; vibration shows more harmonic behavior and so affect the interatomic distance less. Because of this reason, generally, materials that have stronger atomic interactions and bonds in their structure would tend to show low or negative thermal expansion. However there are additional mechanisms that lead to the observation of negative thermal expansion in certain families of materials. Atomic vibrations are the source of anharmonicity of the interatomic potential. Vibration of atoms may occur in two directions. First one is longitudinal and second one is transversal vibrational motion. These two different motions have opposite effects on the interatomic distances observed in materials. While longitudinal motion causes an increase, the transversal motion causes a decrease in interatomic distances (Figure 2.3). These two opposite mechanisms are the reason of positive and negative thermal expansion in materials, respectively.
Temperature Temperature
Figure 2.3 Longitudinal (left) and transversal (right) component of the vibration mode 4.
10 2.2 ZrW2O8 and Origin of Negative Thermal Expansion
Although, small or negative thermal expansion is known to exist over limited temperature ranges also in materials such as elastomers 41 and elemental uranium 42, here the discussion will only be limited to the small or negative thermal expansion in oxides. The four mechanisms causing this phenomena will be discussed in the following paragraphs 4. The first mechanism is exemplified in ferroelectric oxides with the perovskite structure (e.g. BaTiO3, PbTiO3 and other AMO3 oxides). Negative thermal expansion can be seen below their cubic-to-tetragonal phase transition temperatures. At low temperatures, in the tetragonal phase, Ti06 and AO12 polyhedra become more regular and so the average metal-oxygen distance (M-O) decreases 43. Thermal expansion that exists along the a and b axes are accompanied by the thermal contraction that exists along the c axis.
In cordierite (Mg2Al4Si5018), β-eucryptite (LiAlSiO4) and NZP
(NaZr2P3O12), a second mechanism that is similar to the first one can be observed 44. These materials have hexagonal structures and show highly anisotropic thermal expansion. In cordierite and β-eucryptite, thermal expansion occur along a and b axes with thermal contraction along the c axis. In NZP, the opposite behavior occurs, that is thermal expansion along the c axis is accompanied with a thermal contraction along the a and b axes. According to this mechanism negative thermal expansion is only in one or two dimensions. However, in all three structures, the resulting volume expansion is very low and the mechanism offered is not enough to explain the contraction in the unit cell edges alone. The third mechanism for negative thermal expansion is caused by interstitial cations changing sites as a function of temperature within a network. This mechanism is important in further explaining the negative expansion in β-eucryptite and the NZP family, where ionic conductivity is
11 significant. In β-eucryptite, the Li+ cations are mostly on tetrahedral sites. Sleight’s research group showed that the movement of Li+ to octahedral sites could explain the origin of negative volume thermal expansion. The results also indicated that the thermal expansion properties of some members of the NZP family were closely related to the temperature at which the interstitial cations were accommodated. The fourth mechanism for negative thermal expansion is based on the transverse thermal motion of oxygen in M-O-M linkages. If the M-O bonds are sufficiently strong, these bonds demonstrate negligible thermal expansion. The vibration of oxygen, in a strongly bonded M-O-M linkage, is the reason of the decrease in the average M-O-M distance. Because of the vibration of O atom which is perpendicular to the M-O-M linkage, M and M atoms will be pulled towards each other (Figure 2.4). Therefore, when the temperature increases, the vibrations of O atoms will increase and the average displacement of oxygen will increase. The greater the mean square displacement of the O atom, the greater the effective reduction in M-M distance will be.
Figure 2.4 A schematic representation of how transverse thermal vibrations of M-O-M linkages can give rise to NTE in framework oxide structures 45.
12 There are some requirements for the observation of NTE in a material with fourth mechanism: • A high M-O covalency (M = W6+, V5+, Si4+) since strong M-O bonds are needed • Transverse vibrations by coordinate oxygens • Open framework structure with topology to support low-energy transverse vibrational modes • There should be no interstitial framework cations • For lower symmetry and lower volume structures, displacive phase transitions are needed
All these requirements are met in AM2O8 family of compounds and they present large isotropic negative thermal expansion. ZrW2O8 and HfW2O8 are the two members of the family. 8 Cubic ZrW2O8 which was first synthesized by J. Graham et al. in 1959 is important, because it shows a negative thermal expansion over a wide 46-48 temperature range from 4 to 1050 K . Crystal structure of ZrW2O8 consists of corner sharing ZrO6 octahedra and WO4 tetrahedra. ZrO6 octahedral units share all corners with six WO4 tetrahedral units, whereas each WO4 unit shares only three of its four oxygen atoms with the 47 neighboring ZrO6 units (Figure 2.5) . One of four oxygen atoms is the terminal oxygen per WO4 tetrahedron. The arrangement of the WO4 groups is as such that pairs of tetrahedra are positioned along the main three-fold axis of the cubic unit cell with an asymmetric W···O–W bridge. This geometry results in a short W-Oterminal bond (1.7 Å). The distance between this terminal oxygen atom and the W of an adjacent tetrahedron is significantly longer (2.4 Å). The rather rigid polyhedra and the open framework structure permit the vibrational motions of bridging oxygen atoms, which in turn results in the negative thermal expansion observed 49.
In ZrW2O8, the large transversal vibration of the oxygen atom in the middle
13 of the W–O–Zr linkage is another important source of negative thermal expansion. The systematic changes in this angle cause the observed volume changes. The intrinsic flexibility of an ideal ZrW2O8, support a cell reduction from 9.3 Å to 8.8 Å without distortions of the polyhedra 48.
Figure 2.5 Crystal structure of cubic ZrW2O8 with polyhedral representation.
ZrO6 octahedra are shown in green, and WO4 tetrahedra in red. The oxygen atoms at the vertices of polyhedra are shown as red spheres 47.
Raman spectroscopic studies has confirmed the intimate relationship between negative thermal expansion and lattice vibrational modes 50, 51. Moreover, low temperature specific heat measurements 52 and phonon density of states (DOS) studies by neutron scattering 53, 54 have been done for cubic ZrW2O8. The temperature dependence of the lattice constants showed that low-frequency vibrational modes was the reason of NTE 47, 55 behavior for ZrW2O8 . These low-frequency modes are called rigid unit modes (RUMs) and they are the sort of cooperative motion in the open framework structure, where the polyhedra can rotate without distortion 56, 57.
Some properties of ZrW2O8 are listed in Table 2.1. At room temperature, ZrW2O8 can be regarded as an electrical insulator however,
14 ionic conductivity was determined by AC impedance measurements above its phase transition at 430 K 48. This conductivity originates from the disorder of oxygen that occurs at this temperature and is hardly related to Zr4+ or W6+ ions. a 58 Table 2.1 Properties of α-ZrW2O8 . Thermal Expansion -6 -1 α-ZrW2O8 (20-430 K) -8.7 x 10 K -6 -1 β-ZrW2O8 (430-950 K) -4.9 x 10 K
-6 -1 γ-ZrW2O8 (20-300 K) -1.0 x 10 K Bulk Moduli
α-ZrW2O8 69.4 GPa
γ-ZrW2O8 68.0 GPa
Unit Cell Edges
α-ZrW2O8 9.1575 Å
γ-ZrW2O8 9.07 x 27.04 x 8.92 Å
Space Groups
α-ZrW2O8 P213
β-ZrW2O8 Pa3
γ-ZrW2O8 P212121
Density 3 α-ZrW2O8 5.072 g/cm 3 γ-ZrW2O8 5.355 g/cm Refractive index 1.67 Dielectric constant ~10 Dielectric loss ~10-3 Insoluble in water No reaction with 2N HCl (aq) Decomposes in 2N NaOH (aq)
a α-ZrW2O8 at 25°C unless otherwise indicated.
15 2.3. Phase Transitions in ZrW2O8
Zirconium tungstate maintains its negative thermal expansion over two phase transitions; i) while transforming at ambient pressure and 430 K to β-
ZrW2O8 and ii) while transforming at room temperature and pressures above
0.21 GPa to γ-ZrW2O8. The orthorhombic γ phase shows negative volume thermal expansion behavior below room temperature between 20 and 300 K. Nevertheless, the magnitude of the coefficient of thermal expansion decreases slowly and becomes zero around room temperature. Above room temperature, when the transition to the cubic α phase is approached, the cell volume increases. The increase in unit-cell volume at the phase transition is about 5%. This corresponds in magnitude to the volume change that originates from pressure in the cubic-to-orthorhombic phase transition. This transition can also be called as a first-order transition which occurs between 373 and 391 K. Above the phase transition, the value of the coefficient of thermal expansion becomes negative again 47, 59. The relationship between γ- α-β phase transitions can be seen in Figure 2.6 59. The thermal expansion of the γ phase and the low-temperature experimental observations were studied well by Dove et al. 60. More information on phase transitions in
ZrW2O8 system is given in Appendix A.
16
Figure 2.6 Unit-cell volume of orthorhombic γ -ZrW2O8 and cubic α-ZrW2O8 with respect to temperature over a temperature range of 20–473 K. Data were obtained by heating the metastable orthorhombic phase through the transition and back to the cubic phase 59.
2.4 Synthesis of ZrW2O8
Cubic ZrW2O8 can be synthesized by a variety of methods. Solid state 8-13 methods have been traditionally used to produce ZrW2O8 . Other production strategies that have been applied involve the sol-gel, non hydrolytic sol-gel 10, 14-21, hydrothermal 10, 22, 23, co-precipitation 24 and combustion synthesis 10 approaches.
In the solid state synthesis approach, ZrW2O8 is produced from a stoichiometric mixture of ZrO2 and WO3 that are ground mechanically or manually 9-12. During the firing step applied in a furnace, which may take several hours, solid state reactions between the pure oxide components occur gradually 61. Although this approach is widely used, there are some disadvantages, most important of which is the relatively higher temperatures and longer treatment times that are needed for the diffusion of the
17 components that were mixed in solid state. Such a mixing usually does not guarantee the intimacy of the components and increase the diffusion distances. The drawback of an inhomogeneous mixing can only be overcome by increasing the temperature and time of treatment, and with additional intermediate grinding steps, provided that the phase stability region of the target ceramic is wide enough.
ZrW2O8 was first synthesized in 1959 by heating encapsulated mixture 8 of ZrO2 and WO3 at 1200°C followed by a quenching step . In 1967, Chang et al. published a phase diagram of ZrO2 and WO3 binary system. According to this phase diagram, ZrW2O8 is thermodynamically stable between 1380 and 1530 K. The compound melts above 1530 K to ZrO2 and liquid. As it can be seen from the phase diagram (Figure 1.1), at around 1380 K, ZrW2O8 has a lower limit of stability and it has no solubility limit. This necessitates that it should be quenched to the room temperature either by water or liquid nitrogen, otherwise ZrW2O8 decomposes to its native oxides ZrO2-WO3 whenever the temperature is below 1380 K. However once it forms, it can be used at relatively high temperatures, where ZrO2-WO3 are stable, without any decomposition (e.g. around 700oC) 25. Another potential problem is evident for the cases in which, there is a slight shift from stoichiometry. Due to the fact that there is no solubility limit, a slight shift from stoichiometry might give WO3 enough time to volatilize to permanently to destroy the Zr/W ratio before reacting with ZrO2. Considering the mixing inefficiencies and the high temperatures used, this is a very likely problem. To prevent this problem, in the solid state approaches, besides quenching; use of excess
WO3 and repeated grinding and firing procedures have been adapted recently. This renders the technique energy inefficient and cumbersome.
In an effort to synthesize single crystals of ZrW2O8 Kowach utilized a layered self-flux technique 13, in which he elaborated on the problems stated above. In this method, ZrO2 and WO3 were mixed with a 1:2 molar ratio and
18 then this powder mixture was placed in the bottom of a platinum crucible and a layer of excess WO3 covered the mixture. The uncovered crucible was heated in a vertical tube in oxygen atmosphere. A layer of excess WO3 was used to minimize the volatilization of WO3. As the reaction occurred at
1300°C, the interface between the ZrO2 and WO3 mixture and the WO3 layer began to react and form a melt. When the melt flowed towards the bottom of the crucible, the ZrO2 and WO3 mixture reacted in the solid state to form
ZrW2O8.
2.5 Alternative Methods of Synthesis of ZrW2O8
Some of the stated disadvantages of the solid state approaches can be overcome by wet chemical approaches. Possibilities include sol-gel, co- precipitation, hydrothermal and combustion methods. Because chemicals are mixed in a dissolved state, these methods provide an intimate mixture of zirconium and tungsten sources. After the removal of the solvent, or during the firing steps, the desired homogeneity in stoichiometry can be maintained. Sol-gel has been one of the most preferred choices among these wet chemistry based approaches. It can be best described as the chemical synthesis and processing of inorganic materials such as ceramic powders and glasses either from colloidal dispersions or from metal alkoxides. This method has demonstrated the importance of chemical precursors in determining properties of the final products. Reader should refer to Appendix B for further reading on basic sol-gel chemistry concepts that would be useful in interpreting the results that will be presented in this thesis. Sol-gel method is one of the wet chemical routes in the production of
ZrW2O8. Wilkinson et al. have prepared ZrW2O8 at low temperatures by a 21 non-hydrolytic sol-gel method . Tungsten source, WCl6 (tungsten chloride), was dissolved in CHCl3 (chloroform). Zirconium source, Zr(OiPr)4.iPrOH
19 (zirconium isopropoxide) was dissolved in THF (tetrahydrofuran) and iPr2O (diisopropyl ether) solvent mixture. Zirconium solution was slowly added into dark brown tungsten containing suspension and they were stirred for 60 minutes at room temperature. The mixture was cooled in the liquid nitrogen and sealed. Afterwards, it was heated at 110°C for 7 days. Obtained ZrW2O8 raw gel was heated at 500, 600, 740 and 1200°C. It was shown that crystallization starts at around 740°C.
De Buysser et al. synthesized ZrW2O8 by the sol-gel method with the use of EDTA as complexing agent 14. The EDTA method is a two step process. In the first step, the Zr-EDTA complex was synthesized. In the second step, it was dissolved in the W-EDTA solution. The pH of the final Zr– W-EDTA colorless solution was about 4. Then the solution was heated at 60°C for 12 h. Heat treatment was applied at 1180°C for 2 h and quenching was done in liquid nitrogen to avoid the decomposition of ZrW2O8 into ZrO2 and WO3. Results indicated that EDTA sol-gel method was suitable for the synthesis of pure ZrW2O8.
De Buysser et al. synthesized ZrW2O8 by the use of aqueous citrate gel method which can be described as a water based sol-gel method 15.
Zirconium source, ZrOCl2.8H2O (zirconium oxychloride), tungsten source
(NH4)6H2W12O40.xH2O (ammonium metatungstate) and citric acid were used as the reactants. Zirconium and tungsten sources were dissolved in water. Then the citric acid (CA) which is the complexing agent was added to the zirconium solution to avoid precipitation when both solutions were mixed together. It was necessary to determine the ratio of metal ions/complexant agent and pH value. Therefore, experiments were carried out with different metal ions/complexant agent ratios and pH values. The ratio of zirconium ions–citric acid, 1:6 and pH 7, resulted the perfectly clear and colorless solutions. This solution was heated at 60°C for 24 hours. Obtained gel was calcined at 700-800°C and then, the sample was treated at 1180°C for 2 h.
20 It was shown that aqueous citrate-gel method produced a pure and homogenous oxide mixture suitable for the preparation of the negative thermal expansion material, ZrW2O8.
A.W. Sleight et al. synthesized ZrW2O8 by six different routes in a comprehensive work to compare different aspects of the different methods available 10. The first route was combustion method. Appropriate amounts of zirconium and tungsten salts were dissolved in a minimum quantity of 3N nitric acid. The appropriate amount of the urea fuel was added. The mixture was stirred in a Pyrex container, the container was placed in a furnace preheated to 775 K. It was shown that combustion synthesis was rapid but it didn’t produce phase pure product. The second route was co-precipitation method. Zr and W solutions were prepared and mixed with stoichiometric amounts of 1:2. Then, the mixture was stirred at 333 K. The precipitate was filtered, washed and dried in an oven at 353 K. The precipitate was heated at 1073 K for 10 h. X-ray diffraction indicated that the product was amorphous. However, heating the precipitate at 1423 K produced a phase-pure product. The third route was a sol-gel method. Zr and W solutions were prepared and mixed with the stoichiometric amount, 1:2. After that, the stirring was continued for 10 h. Then 125 ml of 6 N HCl was added into the first mixture, and the new mixture was refluxed for 2 days. After the reflux step, the sol was left aside for 3 weeks for gelation step. Finally, the precipitate was filtered, washed and heated in an oven at 353 K. X-ray diffraction indicated that heating the precipitate at 873 K for 10 h resulted the single phase
ZrW2O8 product. Heating to 973 K for 10 h caused the decomposition of
ZrW2O8 into ZrO2 and WO3. The fourth route was hydrothermal synthesis method. Zr and W solutions were prepared and mixed with stoichiometric amounts of 1:2 at 333 K with constant stirring. 250 ml of 6 N HCl was added and stirring was continued for another 3 h. Then the obtained slurry was transferred to a Teflon-lined Parr bomb and heated at 453 K for 15 h. The
21 product was filtered washed and dried at 333 K. After heating the product at
923 K for 2.5 h, single phase cubic ZrW2O8 was produced. The fifth route was direct synthesis from oxides. In this method, ZrO2 and WO3 were ground together in an agate mortar and pestle. Then the powder mixture was heated at 1473 K for 18 h. Results showed that, the product had significant impurities of ZrO2 and WO3. The last route was direct synthesis from salts. In this method zirconium oxynitrate and ammonium metatungstate were ground together in an agate mortar and pestle. The powder mixture was heated at 1473 K for 6 h. Results showed that, the product had significant impurities of ZrO2 and WO3.
Xing et al. synthesized ZrW2O8 nano rods by a rapid low-temperature hydrothermal route 22. Starting materials were zirconium oxychloride
(ZrOCl2.8H2O) and ammonium metatungstate (N5H37W6O24.H2O). After the preparation of the Zr and W solutions, they were mixed slowly and stirred at 60°C for 2 h. Then, to investigate the acidity effect of HCl addition, the different aqueous solutions of HCl (1, 3, 5, 7 and 10 N) were added to the mixed solution, and stirred for another 3 h. Afterwards, the mixture was transferred to a Teflon-lined Parr bomb and heated at 180°C for 6 h. Finally, the product was filtered, washed with deionized water, and dried at 60 °C.
ZrW2O8 nano rods were prepared by heating the precursor
ZrW2O7(OH)2(H2O)2 at 500°C for 6 h. It was seen that the crystallinity of the precursor increased with decreasing acidity. However, reduction of the HCl acidity below a certain limit (<5N) caused the formation of amorphous precursors. Also, results showed that diameter of produced nano rods varied from 40 to 500 nm. In another group of studies, one of the solid oxides was combined with the inorganic salt of the other oxide. Jun-ichi Tani et al. synthesized
ZrW2O8 from different kinds of chemicals such as ZrO2–WO3, 62 ZrO(NO3)2.2H2O–WO3, ZrCl2O.8H2O–WO3, and ZrO2–(NH4)10W12O41.5H2O .
22 It was shown that prepared inorganic precursors from ZrO(NO3)2.2H2O,
ZrCl2O.8H2O and (NH4)10W12O41.5H2O formed ZrO2 and WO3 after firing above 500°C. Also it was found that the content of ZrW2O8 was influenced by the kinds of precursors and mixing methods. Homogeneity and the particle size of starting powders affected the formation rate of ZrW2O8. Heating
ZrCl2O.8H2O–WO3 mixtures at 1200°C for 4 h resulted in phase pure ZrW2O8.
Time spent was much shorter than in conventional solid state (ZrO2-WO3) systems. A systematic work on the cooling effects after heat treatments put forward importance of the heat treatment schedules that were applied. Shin 12 Nishiyama et al. studied cooling rates over the ZrW2O8 . ZrW2O8 was prepared from ZrO2 and WO3 powders by firing at 1200°C in air atmosphere.
Then, synthesized ZrW2O8 was quenched in air, in liquid nitrogen, in water and in a furnace in order to cool it at various rates. Results showed that when cooling rate become slower, a larger proportion of ZrW2O8 decomposed into ZrO2 and WO3. Cooling the sample in a furnace caused positive thermal expansion. However, quenched samples in air, in water and in liquid nitrogen showed negative thermal expansion from room temperature up to 600°C. All this pioneering work indicate the fact that, although it is possible to form ZrW2O8 with solid state reactions, the lower reactions temperatures or shorter heat treatment times at higher temperatures delivered by the wet chemistry based approaches are important assets. However, almost in every case the chemicals were chosen from relatively expensive sources. Both approaches that were experimented had certain limitations in common. While the solid state reactions suffered from lengthy repetitive cooling-grinding- reheating cycles or long durations at high temperatures; the limitation on the wet chemical approaches was the lengthy ageing times that were needed before the firing of the precursors. The toxicity of chemicals used in sol-gel
23 processes is another important limitation on the scale-up of such processes besides the expensive starting chemicals. Although in some cases, solvents used in sol-gel methods were non-aqueous; existence of water-based routes is an encouraging factor. Experimental evidence strongly suggest that phase purity is not only a matter of choosing the right precursor preparation method, but also combining this with the right heat treatment procedures.
2.6 The Electrospinning of Metal Oxide Nanofibers
ZrW2O8 has been proposed as a component in near-zero and adjustable thermal expansion composites that could be obtained by mixing it with another material such as copper, zirconia, polyimide or alumina 18, 63-65. In all these cases, the linear thermal expansion coefficient of the final product depends on the composition of the original mixture of phases. On the contrary to potential advantages in numerous applications, literature on composites based on ZrW2O8 has not been evolved to a wide variety at the moment. The present work that has been reached through open databases only provides some examples to the possibilities. For such examples to emerge in the coming years, not only a better control on phase purity, but also a better control on particle size, distribution and morphology is needed. For example it is still not known, how the preferential growth under hydrothermal conditions would affect the NTE coefficients of the rod-like
ZrW2O8’s. In order to use the precursor prepared in this work in creating
ZrW2O8’s with a different morphology and size, a preliminary work on its electrospinning properties with PVA was performed. For this purpose the literature was surveyed for the possible previous studies on the production of
ZrW2O8, ZrO2 and WO3 nano fibers with the electrospinning method. It was seen that there were a few studies in which the ZrO2 and WO3 nano fibers
24 were produced. Interesting enough, neither electrospinning nor the production in fiber form has been attempted in the ZrW2O8 system yet. F.R. Lamastra et al. produced zirconia/Poly (vinyl alcohol) (PVA) and alumina/ Poly (vinyl alcohol) (PVA) nano hybrids by electrospinning 66. In the experiments, ZrO2 and Al2O3 nano particles were purchased. 5% aqueous dispersion of ZrO2 and 10% aqueous dispersion of Al2O3 were used as starting materials. Then, different percentages of PVA solutions were prepared and they are electrospun with different applied voltage (12, 14 and
16 kV). After electrospinning process, nanohybrid fibrous PVA/ZrO2 and
PVA/Al2O3 mats were obtained. Results indicated that the average fiber size was about 600 nm and the fiber diameter was not significantly affected by applied voltage (12, 14 and 16 kV).
Yanfei Zhang et al. fabricated CeO2-ZrO2 ceramic fibers by combining 67 electrospinning and sol-gel methods . Ce(NO3)3·6H2O (cerium nitrate) and
ZrOCl2·8H2O (zirconium oxychloride) were used as starting materials. Poly (vinyl pyrolidone) (PVP) was used as the fiber forming agent. After electrospinning and calcination at 1000°C for 6 h, the ultra fine Ce0.67Zr0.33O2 fibers with the diameter of 0.4–2 μm were obtained.
Xiaofeng Lu et al. synthesized WO3 nanofibers by using tungstic acid, ethanol and PVP. Electrospinning was applied to the solution. After heat treatment at 500°C for 2 h, orthorhombic WO3 nanofibers with the diameter between 100 and 500 nm were obtained 68. In our group previously, S. Tanriverdi et al. synthesized alumina borosilicate/PVA composite nanofibers by sol-gel and electrospinning methods 69. Sol-gel recipe was improved to obtain homogeneous three component alkoxide solution. Effects of applied voltage and tip to collector distance on the electrospun fiber diameters were studied. Results showed that increasing the applied voltage and tip to collector distance decreased as- spun fiber diameters and alumina borosilicate nano fibers with the diameter
25 down to 300 nm were obtained. Heat treatment was applied at 800, 1000 and 1200°C. The desired crystal structure in fiber morphology was achieved at 800°C and the fiber structure was disappeared at 1000°C. This is a good indication that the precursor chemistry should permit rather low temperature crystallization.
26
CHAPTER 3
EXPERIMENTAL
Here the preparation of novel ZrW2O8 precursors produced in this work is described along with the heat treatment protocols that were developed. The descriptions of the steps taken in characterization of the product are followed by a section on the details of electrospinning setup used in (ZrW2O8 Precursor/PVA) composite nanofiber production.
3.1 Materials
3.1.1 Zirconium Acetate
In the experiments, zirconium acetate is used as the zirconium source. Material is purchased from Sigma-Aldrich. Structural formula and some properties of zirconium acetate are given in Figure 3.1 and Table 3.1, respectively.
Figure 3.1 Structural formula of zirconium acetate.
27 3.1.2 Tungstic Acid
Tungstic acid is used as the tungsten source in the experiments. The chemical is purchased from Sigma-Aldrich. Structural formula and some properties for tungstic acid are given in Figure 3.2 and Table 3.1, respectively.
Figure 3.2 Structural formula of tungstic acid.
3.1.3 Ammonia Solution
In the experiments, ammonia solution is used to dissolve tungstic acid in water. The chemical is purchased from the Merck KGaA. In water, ammonia deprotonates a small fraction of the water to give ammonium and hydroxide ions according to the following equilibrium:
+ − NH3 + H2O NH4 + OH
Some properties of ammonia solution are given in Table 3.1.
3.1.4 Polyvinyl Alcohol (PVA)
PVA is used to adjust the viscosity of the precursor solution as the fiber forming agent in the electrospinning process. The molecular weight of the polyvinyl alcohol was measured at the Central Laboratory of Middle East Technical University (METU) by using Gel Permeation Chromatography (PL-
28 GPC 220), and found to be 23.000 g/mol (Mw). Structural formula of PVA is given in Figure 3.3.
Figure 3.3 Structural formula of PVA.
Table 3.1 Properties of the materials used in this work.
Chemical Molecular Density Composition Appearance Name Weight (g/ml)
Zirconium Acetate Solution – 1.279 at 25°C Zr, 15-16% Liquid
Tungstic Acid 249.5 gr/mol 5.5 at 25°C – Powder
Ammonia Solution – 0.88 at 20°C NH3, 32% Liquid
3.2 Precursor Preparation
3.2.1 Preliminary Studies
To produce ZrW2O8, the preparation of a precursor solution from zirconium acetate (ZrAc) and tungstic acid (TA) is studied. To parallel this, same solution is also combined with PVA for checking its suitability in electrospinning. It was found that Geiculescu and his coworkers reported the preparation of ZrAc gel by a sol-gel method 70. In their study, ZrAc solution was heated at 60°C for 4 days, and a ZrAc gel was obtained. After a heat
29 treatment at 800°C zirconium crystals from the ZrAc gel were obtained. Then, the decomposition of gel and phase transitions in the zirconium crystals were investigated with some characterization techniques. The study of Geiculescu et al. had inspired the preliminary studies on the precursor. A ZrAc solution which contains 1 M Zr4+ was heated at 60°C for 4 days and ZrAc gel was obtained. TA solution containing 2 M W6+ was added to this ZrAc gel. The precursor which contains Zr4+:W6+ in 1:2 molar ratio was prepared as such. PVA solution was prepared by dissolving 10 g
PVA in 100 ml H2O and heating it at 80°C with continuous stirring for 1 h, then cooling it to room temperature and stirring for an additional 24 h.
ZrW2O8 precursor and PVA solution were then mixed and electrospun with the conditions given in Table 3.2.
Table 3.2 Electrospinning conditions of preliminary precursor.
Zr4+:W6+ PVA (Wt. %) Flow Voltage Tip to Collector Rate(ml/h) (kV) Distance (cm)
1:2 5 0.8 15 15
After the electrospinning process, fibers on the aluminum foil were peeled off by the sharp-edged blade and a heat treatment was applied to fiber mat at 800, 900 and 1000°C by using a tubular furnace in air atmosphere. Samples were then cooled within the furnace. The temperature profile applied is given in Figure 3.4.
30 1200
1000 °C 1000 900 °C
800 °C 800
600
400 Tempereture (°C) Tempereture
200
0 0 50 100 150 200 250 300 350 400 450 500 Time (Minutes) Figure 3.4 Temperature profiles that are applied to the electrospun fibers obtained in preliminary precursor studies.
Since after the burnout procedure, the produced quantities were not adequate for quantitative analysis, portions of the preliminary precursor were also fired at 800, 900, 1000 and 1100°C’s without an electrospinning step. Before these high temperature treatments, precursor was dried in an oven at 110°C and the temperature profiles for heat treatments are given in Figure 3.5.
1200 1100 °C 1000 °C 1000 900 °C
800 °C 800
600
Temperature (°C) 400
200
0 0 50 100 150 200 250 300 350 400 450 500 Time (Minutes) Figure 3.5 Temperature profiles that are applied to the as-prepared precursors obtained in preliminary studies.
31 3.2.2 Sol Preparation
After an evaluation of the results from the preliminary studies, the precursor preparation method is modified, reasons of which are discussed in Chapter 4. Briefly; one of the problems is identified as the low solubility levels of the TA in aqueous solutions. With the addition of ammonia, TA can be dissolved in the water to a large extend. This solution is called “Ammonical Tungstic Acid (ATA)” solution 71. ZrAc, which is highly soluble in water and was independently aged in the preliminary studies, is slowly added to ATA solution by a burette without the prior ageing step. Obtained ZrW2O8 precursors are aged for different aging times to investigate the aging effect on the phase purity of ZrW2O8. In each experiment, the ammonia concentration is kept constant and the attained pH is measured after each major solution preparation step (Table 3.3).
Table 3.3 Aging times applied to the ZrAc and ATA solution mixtures. pH is measured after each major mixing step.
SAMPLES AGING TIME pH (ATA) pH (ZrAc+ATA) (HOURS)
Solution A 0.5 9.59 9.04 Solution B 1.0 9.52 8.70 Solution C 2.0 9.86 9.31 Solution D 3.5 9.55 9.05 Solution E 5.0 10.45 9.72 Solution F 7.0 9.44 8.98 Solution G 10.0 10.16 9.39 Solution H 16.0 9.99 9.48
32 After aging the ZrW2O8 precursor for the specified time, it is dried and a two step heat treatment is applied afterwards. The calcined (600oC, 3 hrs), ground, pressed and fired (1200oC, 4 hrs) precursor is then quenched from 1200oC into water. Applied temperature profile for the heat treatment is given in Figure 3.6 and it should be noted that, both in calcination or firing the heating up of the sample is conducted at a rate of 10oC/min.
1400
1200 °C 1200
1000
800
600 °C 600 Temperature (°C) 400
200
0 0 100 200 300 400 500 600 Time (Minutes) Figure 3.6 Temperature profiles for the two step heat treatment applied in the production of ZrW2O8 from solutions A through H.
In another set of experiment, the ageing time is fixed at two hours and instead of use of fixed amount of ammonia in this case, a fixed pH value is targeted regardless of the ammonia used in the solution preparation. The very same solution is also combined with different amounts of PVA to increase its viscosity. Then, the mixture is electrospun with the conditions given in Table 3.4. Fibers collected on the aluminum foil are peeled off by the sharp-edged blade and the heat treatment given in Figure 3.7, is applied to fiber mat and obtained phases are quenched in water. It should be noted that in the fiber heat treatments, the profile is carried out without an interruption between calcination and firing steps. Because the amount of the product was insufficient, XRD studies of electrospun and fired samples could not be performed. In order to produce adequate amount of fiber mat, an
33 auxiliary electrode system is connected to the needle tip during electrospinning 72. Even though a lot more fiber can be obtained in this configuration; during the high temperature burning step the sample adhered on the platinum (Pt) crucible and this rendered it useless. A summary of the experimental procedure applied is presented as a flow chart in Figure 3.8.
Table 3.4 Compositions of electrospinning solutionsa.
Zr4+:W6+ PVA pH pH (Wt %) (ATA) (ZrAc + ATA)
1:2 7.5 10.24 9.88
1:2 8.5 10.24 9.88
a Flow rate: 0,8 ml/h, Electric potential: 15kV, Tip to collector distance: 15 cm
1400
1200
1000
800
600 Temperature (°C) 400
200
0 0 100 200 300 400 500 600 700 Time (Minutes) Figure 3.7 Temperature profile for the heat treatment applied to the electrospun fibers.
34 0.1 M ZrAc 0.2 M H2WO4 Ultrasonic 60 ml H2O 0.4 M NH4OH Mixing 60 ml H2O
0.05 M ZrAc 0.1 M H2WO4 0.2 M NH4OH 120 ml H2O Ultrasonic Mixing Aging
Drying Zirconium Tungstate (110°C, 16 hours) precursor/PVA
Heat Treatment (600°C, 3 hours) Electrospinning
Pressing
Heat Treatment
Heat Treatment (1200°C, 4 hours)
Quenching (In Water)
Figure 3.8 Flow chart of synthesis of ZrW2O8 from solutions A through H.
35 The intermediate results lead to some improvements in the precursor preparation and heat treatment procedure. In order to increase the phase purity of ZrW2O8, four additional approaches are experimented. For these approaches, instead of heating the samples slowly in a tubular furnace, the samples are put immediately in a box furnace in which temperature is already set at 1180 or 1200°C. The time that is spent at the set temperature is decreased to 2 hours. In addition during the preparation of Solution J, the mixing method is also changed such that, ZrAc and ATA solutions are dropped into a separate container simultaneously. Besides; in one of the samples, the pH of the ZrAc-ATA solution is increased as much as 1 unit, and in another the ATA solution was centrifuged until transparency (Solution K). Additionally, solution J, K and L had 2 wt% excess W. The employed conditions are summarized in Table 3.5.
Table 3.5 Improved approaches for the phase pure ZrW2O8 production.
Samples Aging Time pH pH Temperature (hour) (ATA) (ZrAc+ATA) (°C)
Solution I 2 10.10 9.80 1200
Solution J 2 10.10 9.86 1180
Solution K 2 11.44 10.95 1200
Solution L 2 10.30 9.85 1180
3.3 Electrospinning Setup
In electrospinning process, a DC voltage power supply, GAMMA High Voltage Research Inc., USA (Model no: ES30P-20W/DAM) with an electrical potential range from 0 to 30 kV, a multi-syringe pump, New Era Pump
36 Systems Inc. (Model no: NE-1600 Six-Syringe Pump), a syringe needle and a metal collector were used. The metal collector was covered with an aluminum foil. The positive electrode wire was hooked at the metal part of the needle and negative part of the electrode was attached to the metal collector. A plexi glass box was used to cover the electrospinning setup for experimenter’s safety. The experiments were conducted under atmospheric pressure and at room temperature. In the process, 40-45 hours of operation time was needed for enough amounts of fibers deposited on aluminum foil for the characterization. The picture of electrospinning taken during the process is illustrated in Figure 3.9.
Figure 3.9 The picture of the electrospinning setup.
3.4 Heat Treatment
Precursors were heat treated in a high-temperature tubular or box furnace (Protherm), under atmospheric conditions. Since sintered alumina or mullite crucibles can not sustain the thermal shock of quenching and since the possible shifts in stoichiometry in the precursors increases the possibility of formation of the liquid phase, and so the penetration of the samples into the porous ceramic crucible surfaces; a box crucible made of Platinum was preferred.
37 3.5 Characterization
3.5.1 Particle Size Analysis
The particle size and zeta potential values can give detailed insight into the stability, chemical reactivity and dispersion mechanism in a colloidal solution. To understand the interaction between Zr and W ions in the precursor and to investigate the effect of pH on the ZrAc-ATA solution, three sets of zeta potential and particle size measurements were carried out in a wide angle particle size analyzer (Malvern Zetasizer Nano Instrument – Model No: ZEN3500, Malvern Instruments Ltd.). The measurement range is specified to be between 1 nm and 5 µm (5000 nm). In the measurements, two types of cuvettes were used; • Size cell: Only used for the particle size measurements (larger in volume) • Zeta cell: Used for the particle size and zeta potential measurements (smaller in volume) The first experiment is to measure the change of the average particle size and zeta potential in ZrAc-ATA solution with decreasing pH values. In the second experiment change of the average particle size and zeta potential in ZrAc-ATA solution are monitored with respect to time after mixing ZrAc- ATA solution by an ultrasonic mixer. The third experiment is conducted to monitor the change of the particle size in a 3 day period.
3.5.2 Scanning Electron Microscopy (SEM)
Produced ceramic powders and electrospun nano fibers were characterized by scanning electron microscope (Model No: Quanta 400F Field
38 Emission SEM) in Central Laboratory, METU or (Zeiss Evo 50) in Geological Engineering Department, HU.
3.5.3 X-Ray Diffraction (XRD)
Structural characterization products were carried out by X-Ray Diffraction (Model No: RIGAKU – D/Max-2200/PC) by using CuKα (λ = 0.154 nm) radiation in the Department of Metallurgical and Materials Engineering, METU. The power of the XRD was fixed at 40 kV and 200 mA and the patterns were collected from 2θ 10o to 80o. The phases’ distribution within the burned bodies was estimated from the most intense peaks of each of the oxides, just to draw an approximate trend about the quantities of each of the oxides with respect to the investigated parameter. However due to an overlap of the most intense peaks of some oxides with the secondary peaks of others, this rough estimation fails in determining a trend. Therefore the critical regions in XRD patterns were deconvolved using a peak fit software∗. This improved the quality of the estimations, since rather than using point estimations in associating peaks with intensities, the deconvolved peak intensities were used. The crystal lattice parameters and point coordinates of atoms were used to generate the theoretical XRD patterns with the peak positions and relative intensities∗∗. Then these values were used in beginning the deconvolution processes in the overlapped peak regions. The respective data used in these calculations and an example calculation are given in Appendix C and D, respectively.
∗ Peakfit, v. 4.11. ∗∗ Carine Crystallography, v3.1, 1998.
39 3.6 Chemical Equilibrium Calculations
The fact that the ammonia used in improving the solubility of the TA solution was increasing the solution pH more than it should have been was interesting. Therefore, some approximate calculations on the chemical equilibrium were performed using OLI Analysis – Stream Analyzer. The results and conclusions drawn are summarized in Appendix E.
40
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1 Preliminary Studies on the Precursor
During the preliminary studies on the precursor, the produced yellow- colored Precursor/PVA gel was heat treated at 800, 900, 1000°C after electrospinning. SEM micrographs of the calcined gel are presented in Figures 4.1 (a) through (c), respectively.
(a)
41 (b)
(c) Figure 4.1 SEM micrographs of electrospun preliminary Precursor/PVA fibers at (a) 800°C, (b) 900°C, (c) 1000°C.
It can be observed from the SEM micrographs that the crystal growth started when the temperature was increased. In Figures 4.1 (b) and (c), samples, at 900 and 1000°C start to show certain crystal morphologies. On the other hand, in Figure 4.1 (a) at 800°C, samples tend to retain their fiber like morphology with small sintered crystals forming the fiber body. Therefore, when sintering temperature increases, some morphological changes should be expected and fiber morphology would be lost at even higher temperatures. The XRD analysis could only be performed on the sample sintered at 800°C, because there was not enough amount of sample left to carry out the analysis after treating the fiber mat at 900 and 1000°C. The XRD pattern of the sample treated at 800°C is given in Figure 4.2. The results show that ZrW2O8 crystals could not be produced. ZrO2 and WO3
42 were the identified products. This is not unexpected because these two oxides are the thermodynamically stable phases at this temperature.
■ ● ZrO2 ● ■ WO3 ■■ ♦ ZrW2O8
■
■ ● ■ ■ ■
Intensity (a.u.) Intensity ● ■ ●● ● ■ ● ■ ■ ■ ■ ■ ● ■ ● ■ ● ■
10 15 20 25 30 35 40 45 50 55 60 2θ
Figure 4.2 XRD pattern of the preliminary precursor/PVA fibers calcined at 800°C.
In order to be able to see the effect of heat treatment temperature on the crystal structures, the prepared preliminary precursor was heat treated without any prior electrospinning step, directly at 800, 900, 1000, 1100oC. SEM micrographs of the powders obtained are presented in Figure 4.3 (a) through (d), respectively.
43
(a)
(b)
(c)
44
(d) Figure 4.3 SEM micrographs of preliminary precursors heat treated at; (a) 800°C (b) 900°C (c) 1000°C and (d) 1100°C.
It can be seen that similarly, higher temperatures provided more crystal like morphologies. In order to identify the crystals formed, XRD analyses were performed on treated samples at 800, 1000, 1100°C and the results are given in Figure 4.4.
● ■ ● ZrO2 ■■ ■ WO3 ■ ♦ ZrW2O8 ● ● ■ ■ ● ● ● ● ● ■■ ■ ● ■■● 1100° ■ ■■ ● ●■ ■ ■ ■ ■■● ● ■
Intensity (a.u.) Intensity 1000°C
800°C
10 15 20 25 30 35 40 45 50 55 60 2θ Figure 4.4 XRD patterns of treated preliminary precursor treated at 800, 1000, 1100°C.
45 The results indicate that instead of ZrW2O8, again a physical mixture of ZrO2 and WO3 was produced and the reaction to ZrW2O8 did not proceed under the investigated conditions. In the preliminary experiments, it was observed that there was a problem in the preparation of tungstic acid (TA) solution in water (yellowish color). The solubility of the powder in water was very limited. This obviously caused an inhomogeneous mixture for Zr4+ and W6+ species, which in turn means that Zr4+:W6+ ratio which should be 1:2 could not be preserved in the precursor. Due to the improper stoichiometry, the desired product, ZrW2O8, could not be obtained at this stage.
4.2 Sol Preparation
The dissolution of TA in water can be improved by adding ammonia to the solution. Such an addition increases the pH of the solution. After this stage of experiments ammonical tungstic acid (ATA) solutions were used instead of plain TA solutions. ATA solution was then mixed with ZrAc solution before it forms a gel. Before moving forward with the experiments on the effect of ageing, with a series of experiments, the state and behavior of the solution that is forming was investigated. The effect of pH on the colloidal particle sizes∗ observed in the gel was investigated by decreasing the pH of the precursor with acetic acid. The particle size and zeta potential values that were measured are presented in Figures 4.5 and 4.6, respectively.
∗ Here, it should be noted that the term particles does not necessarily mean that these are ‘real’ particles. It would be more appropriate to call them ‘entities’ or ‘clusters’ at this stage.
46 40 5500 Small particles (♦) Large particles (•) 35 5000
4500 30
4000 25 3500 20 3000 15 2500
10
Average Particles Size (nm) Size Particles Average 2000
5 1500
0 1000 11 10 9 8 7 6 5 4 3 2 pH Figure 4.5 The average particle sizes observed in ZrAc-ATA precursor solutions with decreasing pH.
pH 10 9 8 7 6 5 4 3 2 0
-1000
-2000
-3000
-4000
Zeta Potential (mV) -5000
-6000
-7000
Figure 4.6 The zeta potential of ‘entities’ forming in ZrAc-ATA precursor solutions with respect to decreasing pH.
47 It was observed that as the pH is decreased small particles (5 to 25 nm) start to grow after a low pH threshold (i.e. at about 4). Larger particles (4.5 to 5.5 μm) were observed with decreasing pH further below this threshold. This increase can be explained by three possible factors, none having certain dominance over the other. One explanation could be related to the decrease of solubility of TA in the ATA solution. When pH value of TA solution was increased by the addition of ammonia, the solubility of TA in water was increased resulting in a stable ATA solution. However, with the addition of acetic acid, the solubility of TA began to decrease again and particle sizes were increased as the pKa values of TA are reached. Another explanation could be that; by the addition of acetic acid, the interaction between excess ammonia and acetic acid was increased and thus, ammonium acetate formed, which led to an increase of average particle sizes in the solution. The final explanation is related to the species distribution in the solution. As the pH is decreased down to around 4, the surface charge of the entities were observed to change, indicated by the zeta potential values measured. As expected they were becoming more positively charged. Although the values were still extremely negative and so there should be no hindrance against the stability of the sol, it indicates a change in the surface ion concentrations. At high pH values the entities could be envisaged as particles with ∗ 2- Zr(OH)4 cores that are surrounded by the dissociated TA (WO4 ) and various possible ZrAc complexes. Upon decrease of pH one can expect that 0 the high pH dominant surface complexes like Z ([Zr(OH)3Ac]) may shift to - - 2- complexes like Z ([Zr(OH)3Ac2] ), which could move away the WO4 that has
∗ It is proved in a separate group of experiments that in the absence of TA; when a ZrAc solution is mixed with ammonia (keeping the molarity levels same), the system precipitates ‘entities’ that are of similar size compared to the ‘entities’ forming when ZrAc is mixed with ATA solutions.
48 been populating the particles surfaces with high concentrations at high pH. The surfaces would still be highly negatively charged due to Z- complexes, but values would be less negative depending on the pH and so the respective 2- complex type. In addition, the WO4 that were moved away can now meet the protons and associate again causing the loss of solubility (pKa values of acetic acid and TA are close to 4) supporting the first explanation. On point that bears comment is that TA is quite soluble when ammonia is added, but there are still some undissolved or partially dissolved TA remaining in the solution as colloidal particles, which would be neglected at this stage for practical purposes. This should be considered as a stoichiometry homogeneity disturbing factor. To investigate the ultrasonic mixing effect on the entities that may have a tendency to coagulate, or TA that may remain undissolved or partially dissolved in ZrAc-ATA solution, the particle size and zeta potential measurements were carried in ZrAc-ATA solution after a standard ultrasonic mixing protocol, again with respect to changing pH values. The results are presented in Figure 4.7 and 4.8, respectively.
70
60
50
40
30
20 Average particle size (nm) 10
0 10 9 8 7 6 5 4 3 pH Figure 4.7 The average particle size measurement from ZrAc-ATA solution after ultrasonic mixing.
49 pH 10 9 8 7 6 5 4 3 0
-1000
-2000
-3000
-4000
-5000
-6000 Zeta Potential (mV) Potential Zeta
-7000
-8000
-9000
Figure 4.8 The zeta potential measurement from ZrAc-ATA solution after ultrasonic mixing.
The particle sizes observed in the solution were decreased further and the standard deviations got smaller indicating that the mechanical effect of ultrasonic mixing on the particles is positive. Also, a similar increase in particle size at around pH 4 was observed, reasons of which was previously discussed. Nevertheless, the behavior of zeta potential with pH was not systematic indicating that the system that is forming is highly dynamic and its behavior with respect to time can become very important in the final product’s phase purity. Therefore the solution was followed for a longer time period. During a 3 day period particle size measurements were carried out. The results are presented in Figure 4.9. Even if the solution was mixed by ultrasonic mixing after the first day, average particle sizes were found increase from 5±3.8 nm to 26±14 nm in the second day. During the observation period within the instrument, there was a constant decrease in the sizes observed, which was explained by the
50 sticking of the entities on cell walls and loosing their mobility. On the third day, the particles were possibly more coagulated. The larger sized entities could not be measured by light scattering technique used due to the limitations of the principle that the instrument works on. Therefore only small particles, which were suspended in the solution, were visible in the measurement scale after a 60 minute period in the instrument. These results, besides giving us a better understanding of the physical and chemical properties of the solutions, indicate that the solution properties are strongly dependent on the mixing conditions prior to ageing and ageing time. On the other hand the pH will be another controlling parameter, since it is both related to the complete dissolution of the TA and the possible complexation reactions among and on the entities.
40 1. Day 35
30
25
20
15
Average particle size (nm) 10
5
0 0 50 100 150 200 250 300 Time (Minutes)
51 40 2. Day 35
30
25
20
15
10 Average Particle Size (nm)
5
0 0 50 100 150 200 250 Time (Minutes)
5000 3. Day 4500
4000
3500
3000
2500
2000
1500
1000 Average Particle Size (nm)
500
0 0 20406080100120 Time (Minutes) Figure 4.9 The average particle size measurements from ZrAc-ATA solutions during a 3 day period.
4.3 Ageing Effect
To understand the effect of ageing time on the phase purity of
ZrW2O8, prepared ZrAc-ATA solutions were aged for 0.5, 1, 2, 3.5, 5, 7, 10 and 16 hours at room temperature while they were stirred with a magnetic stirrer. Then, the solution was dried at 110°C for 16 hours and the rest of the procedure was followed as it is presented in Figure 3.7. Structural characterization of the produced oxides was performed by XRD and the results are presented in Figure 4.10.
52 In the final product, if the high temperature solid state reaction between ZrO2 and WO3 would not take place according to stoichiometry due to some reason (e.g. mixing inefficiency, WO3 volatilization); native metal oxides (ZrO2 and WO3) can be observed besides and ZrW2O8. A rough estimation of the respective volumetric quantities of the oxides within the body could be made by taking the most intense peaks of each oxide as their representative peak and considering the ratio of each these peak intensities over their summation as the volume percent of the respective oxide. For this procedure, crystal structure parameter of each oxide is taken from the literature and the values were used to synthetically reproduce the theoretical XRD patterns to find the most intense peak of each oxide∗ (Appendix C). To separate the peaks that overlap in the critical regions of the patterns, a “partial” peak fitting procedure was applied∗∗ (Appendix D). The change in oxides’ content (in volume and also in mole) for each sample was plotted with respect to ageing time (Figures 4.11 and 4.12). In addition, on the secondary axis of the ZrW2O8 plot the final pH values of each of the solutions used in ageing were plotted. Same data was also tabulated in Table 4.1.
∗ Carine Crystallography, v3.1, 1998. ∗∗ Peakfit, v. 4.11.
53
8 O
2 2 3 ZrO WO ZrW
♦ ● ■ ♦
● ♦ ♦ s that were produced from precursors ● ♦ ■ ♦ ● ● ■ 3.3. for sample labels). 3.3. for sample ♦ θ ● 2 ● ● ♦ ■ ■ ♦ ● periods (Refer to Table ● ♦♦ ■ ● ♦ ■ ■ XRD patterns from the heat treated sample ■ ♦■ ♦ H G F C B A E D Figure 4.10 aged for different time
10 20 30 40 50 60 70 80 Intensity (a.u.) Intensity
54 100 11 ZrW2O8 90 pH_ZrAc_ATA pH_ATA 10.5 80
70 10
60 9.5 50 pH 9 40
30 8.5
Phase Content (Volume %) %) (Volume Content Phase 20 8 10
0 7.5 0246810121416 Aging Time (hours)
Figure 4.11 Effect of ageing and pH on the amount of ZrW2O8 phase (in volume) produced.
100 ZrO2 90 WO3 80
70
60
50
40
30
Phase Content (Volume %) %) (Volume Content Phase 20
10
0 0 2 4 6 8 1012141618 Aging Time (hours)
Figure 4.12 Effect of ageing and pH on the amount of ZrO2 and WO3 (in volume) produced.
55 100 11 ZrW2O8 90 pH_ZrAc_ATA pH_ATA 10.5 80
70 10
60 9.5 50 pH 9 40
30 8.5 Phase Content (Mole %) %) (Mole Content Phase 20 8 10
0 7.5 0246810121416 Aging Time (hours)
Figure 4.13 Effect of ageing and pH on the amount of ZrW2O8 phase content (in mole) produced.
100 ZrO2 90 WO3 80
70
60
50
40
30 Phase Content (Mole %) (Mole %) Content Phase 20
10
0 0 2 4 6 8 1012141618 Aging Time (hours)
Figure 4.14 Effect of ageing and pH on the amount of ZrO2 and WO3 phase content (in mole) produced.
56 Table 4.1 Amount of oxide components forming in the ceramic body in the samples produced from precursors aged for different ageing times.
pH Time ZrO2 WO3 ZrW2O8 ZrO2 WO3 ZrW2O8 pH (hours) Vol. Vol. Vol. Mole Mole Mole (ATA) (ZrAc (%) (%) (%) (%) (%) (%) + ATA) 0.5 44.12 26.08 29.81 64.64 27.05 8.30 9.59 9.04
1.0 42.71 31.16 26.13 61.24 31.63 7.12 9.52 8.70
2.0 32.50 22.22 45.80 57.18 27.68 15.15 9.86 9.31
3.5 33.48 37.89 28.64 50.92 40.80 8.28 9.55 9.05
5.0 29.60 22.40 48.00 54.22 29.05 16.72 10.45 9.72
7.0 45.50 30.41 24.09 63.54 30.07 6.40 9.44 8.98
10.0 49.64 30.63 19.73 66.12 28.89 5.00 10.16 9.39
16.0 49.76 33.64 16.60 64.85 31.04 4.11 9.99 9.48
It can be seen from Figure 4.11 and 4.13 that the relationship between aging time and the volumetric content of ZrW2O8 phase does not follow a certain trend. Only after aging the ZrAc-ATA solution for 7 hours or more, ZrW2O8 phase content monotonously decreases. On the other hand, the relationship between the solution pH and ZrW2O8 phase content is more systematic and with increasing pH of ATA and ZrAc-ATA solutions, amount of
ZrW2O8 phase in the ceramic body increases. Therefore, it can be concluded that the pH effect is more dominant in the formation of ZrW2O8. The reason could either be related to the solubility of TA particles or the issues related to the heat treatment protocols (e.g. volatility of WO3 etc.). Increasing ammonia amount certainly increases the solubility of TA particles. However our experience showed that a certain portion of the TA may not be dissolving. This is evidenced by the semi-transparent ATA solutions and the irreproducible pH levels that are attained, even though the
57 solutions were prepared by the same routes. We have also demonstrated this by a rough solution equilibrium calculation (Appendix E). Therefore as a first remedy, the ammonia concentration will be increased freely until a certain fixed value of pH is attained in the solution. This would increase chance of preparing the ATA solution with better reproducibility and dissolving as much as TA before ATA is mixed with ZrAc solution. In turn more homogeneous precursor mixtures should be expected.
Figure 4.12 and 4.14 illustrates the quantities of unreacted ZrO2 and
WO3. Under normal conditions the remaining oxides should also bear the original stoichiometry ratio in the precursor solution. However it is seen that the ratio of the remaining WO3:ZrO2 components, which should be 2:1 (in molar quantities) becomes 0.73±0.17 (in volume percentages). After conversion of this volumetric percentages into moles, the molar ratio was found as 0.52±0.12∗. The reason was probably either the early volatilization of WO3 due to inhomogeneous precursor chemistry and/or improper heat treatment schedules, and/or the decomposition of ZrW2O8 crystals due to volatilization of WO3 from the ceramic body again due to the potentially improper heating protocols applied. Therefore any improvement approach should also take care of the problems associated with the heat treatment protocols. In addition, it is also concluded that the aging time should not be longer than 7 hours, since it causes a monotonous decrease in the phase purity; probably due to the advancing hydrolysis of ZrAc and unpredictably changing ionic surroundings of the entities paralleling this. In an initial effort, a ZrAc-ATA solution was prepared by increasing the pH to 9.88 and keeping the ageing time 2 hours. XRD pattern from this sample is given in Figure 4.15.
∗ XRD analysis is based on volumetric quantities. In unit conversions, the used theoretical densities of ZrW2O8, ZrO2 and WO3 are respectively; 5.072, 5.6, 7.46 g/cm3.
58 Resulting ceramic body contains about 64% (in volume) ZrW2O8. Compared to the best results obtained in the previous set (48% in volume) this constitutes a big leap towards the preparation of phase pure product. This solution was mixed with PVA to increase its viscosity to investigate the suitability of the most recent precursor for electrospinning. SEM micrographs of electrospun and heat treated samples are given in Figures 4.16, 4.17 and 4.18.
♦ ♦ ● ZrO2 ■ WO3 ♦ ZrW2O8
♦ ♦
♦ Intensity (a.u.) ■ ● ♦ ♦ ♦ ♦ ♦ ♦ ♦ ● ♦ ♦ ♦ ● ♦ ■■ ♦ ● ♦ ♦ ♦ ■ ■ ♦ ♦ ■ ♦ ● ♦ ● ♦ ♦ ♦ ♦ ♦ ♦ ● ■ ♦ ■ ■■
10 20 30 40 50 60 70 80 2θ Figure 4.15 XRD pattern from the sample prepared at a fixed and high pH.
Figure 4.16 Electrospun nanofibers of ZrW2O8 precursor (7.5 wt% PVA).
59
Figure 4.17 Heat treated nanorods produced from ZrW2O8 precursor (7.5 wt% PVA)
60
Figure 4.18 Heat treated nanorods produced from ZrW2O8 precursor (8.5 wt% PVA)
61 It can be seen from the Figure 4.16 that even though the nanofibers are forming and relatively bead-free, there are occasional areas that have cohered together, probably to coagulated lumps of precursor and unoptimized electrospinning conditions. When burned (Figures 4.17-18), these points where the fibers loose their morphology result in crystals with undefined geometries. On the other hand, the proper regions with defined fiber morphology were found to result in nanorods with very well defined geometry. Unfortunately, since the yield of the precursor drops down significantly after it is mixed with PVA, the amount of samples at the end of the heat treatments were not adequate for XRD analysis. Comparing the obtained morphologies, with the hydrothermally produced nanorods in literature, it can only be said that there is a good chance that the depicted morphologies in micrographs are possibly from ZrW2O8 crystals. Further improvement efforts in obtaining larger quantities of fiber samples were successful. Fiber mats that are relatively thick (i.e. increased flow rates are possible with the use of an auxiliary electrode) and solutions that are better spun (i.e. solutions with poor spinability generally sprays rather than being spun) could be produces. However a sample that was collected for over 40 hours, did stick to the bottom of the Pt crucible due to unforeseen set-up problems. In any case the goal of this work was to synthesize a novel sol-gel precursor for phase pure ZrW2O8. Therefore after finding the positive evidence that the precursor was suitable for electrospinning, the phase purity problem had the priority. Another set of samples were prepared with the modified precursor synthesis approaches given in Chapter 3. A decrease in heat treatment duration from 4 hours to 2 hours in all samples was accompanied with a 20oC decrease in the heat treatment temperature in two of the samples. Except the first sample, the last three samples were prepared with 2 wt% W excess in the precursor level against the problem of WO3 volatilization. In the
62 preparation of Solution I, the only heat treatment procedure was changed. In the preparation of Solution J, the mixing method of ATA and ZrAc solutions was also modified, such that both solutions are mixed in a separate container simultaneously. In the preparation of Solution K, the final pH value of the ATA-ZrAc solutions was increased to an even higher value (about 1 unit higher). Centrifugation of the ATA solution (with 2 wt% excess) prior to mixing to obtain transparent solutions was also essayed in the preparation of Solution L. The XRD patterns of samples prepared with the four modified recipes which had led to 100% ZrW2O8 phase are given in Figure 4.19.
Compared to a sample that yielded 45.8% ZrW2O8 (Sample C) the improvement is visually obvious also.2θ
♦ ZrW2O8 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦♦ ♦♦ ♦ ♦ L ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦♦♦ ♦ ♦ ♦ ♦
K
Intensity (a.u.) J
I
C
10 20 30 40 50 60 70 80 2θ
Figure 4.19 XRD patterns of phase pure ZrW2O8 produced by modified precursor approaches (refer to Table 3.4. for labels). Pattern of Sample C (a sample with 2 hours of ageing) is given for comparison.
63 Obviously the changes that were experimented on the precursor chemistry and heat treatment protocols needs to be systematically elaborated on to sort out the most important parameters that lead to the phase pure product. Nevertheless results prove that precursors and protocols that were developed based on use of ZrAc and TA in this work, can be used in obtaining phase pure ZrW2O8.
64
CHAPTER 5
CONCLUSIONS
Starting from zirconium acetate (ZrAc) and tungstic acid (TA), a novel precursor was synthesized for the production of phase pure ZrW2O8. Solubility of TA constitutes an important problem against its use in such precursors; a problem which was solved by increasing the pH of the TA solutions by ammonia addition. Although ageing of the ZrAc-ATA solutions adversely affected the formation of ZrW2O8 after 7 hours, it did not have a specific significance if the ageing times were kept as short as 5 hours. On the contrary, the pH values attained in the final ATA or ZrAc-ATA solutions followed the trend of final phase purities reached after thermal treatments. It is believed that higher ammonia concentrations guarantee the immediate and long-term (i.e. on-demand) dissolution and dissociation of TA and thus the stoichiometric homogeneity in the precursor.
ZrW2O8 is a ceramic that is thermodynamically stable only at temperatures above 1100oC with no obvious solubility. At these high temperatures WO3 is extremely volatile a factor that jeopardizes the precise control of stoichiometry that is needed for the formation of phase pure
ZrW2O8. Therefore this problem should not only be tackled at precursor level, but also in the heat treatment stage. It is concluded that, instead of cooling the heat treated sample in the furnace, quenching it in water reduces the risk of decomposition of ZrW2O8 into ZrO2 and WO3 and increases ZrW2O8 purity. However even under these improved conditions, the ratio of unreacted WO3 to ZrO2 can be as low as 0.52±0.12, indicating that the
65 prevention of volatilization of WO3 is one of the key factors in phase pure
ZrW2O8 production. Precursors prepared in this study can be used in precursor/polymer composite nanofiber production via electrospinning, upon addition of 7-8 wt% PVA in the solutions. In the most favorable conditions that were experimented, after the final burnout instead of nanofibers nanorods were formed. Use of an auxiliary electrode to increase the electrospinning rates is justified.
66
CHAPTER 6
RECOMMENDATIONS
Since the phase purity problem should be tackled at both precursor level and in heat treatment stages, recommendations for future possibilities should be given under two tittles:
Precursor • There is positive evidence that centrifugation of the ATA solutions prior to its use improves phase purity. The centrifuged ATA solutions do not loose their transparency in time, indicating that they are suitable for stock solution preparations. Provided that the pH of ATA solution is increased sufficiently, the centrifugate molarities do not change a significantly evidenced by ICP and gravimetric methods. The fact that use of excess W source is positively affecting the results, extenuate the potential problems expected with the loss of “W” during centrifugation. • Instead of mixing of the ZrAc solution into the ATA solution, approaches involving simultaneous mixing of the two solutions should be investigated for its effects in the phase purity. • Use of 2 wt% excess W possibly has a positive effect in obtaining phase pure products. This claim should be elaborated on with experimentally structured controlled studies.
67 Heat Treatment • Effects of using a lower heat treatment temperature and shorter reaction times should be checked independently on some precursors
that are known to yield ZrW2O8. • It is also important to check independently the effect of application of a fast heating rate on the phase purity.
• To prevent WO3 volatilization, sacrificial “W” (atmosphere powder) can
be utilized by surrounding the treatment crucible with a WO3 source. Here the treatment crucible would need to be enclosed with a bigger
crucible so that the volatilized WO3 can be enclosed. However, the limitation of oxygen in the firing atmosphere constitutes a potential problem in this preventive method. • Use of two Pt plates instead of a crucible has been proven to yield products with higher purity recently by other groups. Preserving the morphology of the produced nanofibers during heat treatment cycles remains as a challenge in producing oxide nanofibers via electrospinning method. Larger quantities of fiber samples should be produced by electrospinning method and XRD analysis should be carried out to characterize burnt out fibers. On the other hand, production of ZrW2O8 in different forms would enumerate the possible future applications of the material. Co-spinning of the otherwise unspinnable solutions with a shell that is formed of a polymer that is easy to electrospin, constitutes one way of forming coherent structures. When calcined, such structures should be expected to be yield more ceramic that would remain coherent, due to less use of the polymer within the core. The mixing of the polymer with 600oC calcined powders in solvents that can evaporate faster than water, should not only improve the spinability of the mixtures, but also permit a better coherency during the burnout steps.
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79
APPENDIX A
PHASE TRANSITIONS IN ZrW2O8
Open framework structures can show phase transitions as a function of temperature. Below the phase transition temperature, they collapse to a denser structure. During these displacive phase transitions, only M-O-M bond angles change and no bonds are broken or formed 3, 6.
ZrW2O8 has an unusual phase transition at about 430 K from an acentric (α-phase, P213) to a centric structure (β-phase, Pa3). β- ZrW2O8 has a cubic symmetry. The type of phase transition is a reversible order-disorder phase transition and it is induced by the mobilization of the unshared vertex 46-48, 73, 74 of WO4 tetrahedra along the three-fold axis . These WO4 tetrahedra are ordered and all point in an exact direction at room temperature. At the higher temperatures, in β-phase, WO4 tetrahedra become dynamically disordered.
ZrW2O8 undergoes a phase transition above 0.21 GPa at room temperature. This phase transition, which is called the pressure-induced transformation from α-phase to γ-phase (Figure A.1.), is enabled by the migration of oxygen atoms. These O atoms are bonded to only one W atom in the α-phase. The movement of these oxygen atoms allows the structure to relax through coordinated tilting of the WO4 and ZrO6 polyhedra, which is allowed in the γ phase, but it is apparently not possible in the α phase. Thus, the resulting γ phase has no oxygen atoms that are bonded to only one metal atom 75. The α-to-γ phase transition is not a displacive phase transition because during the transition one oxygen atom moves to a new position 58.
During this transition, it was found that the WO4 groups play the dominant
80 role in the system. In the cubic phase, two crystallographically different WO4 groups are aligned along a threefold axis in such a way that their “terminal’’
O-W vectors are collinear. However, in the orthorhombic phase, all WO4 groups are tilted away from the threefold axis, and the threefold symmetry is broken which is accompanied by a 5% reduction in volume. The volume reduction can be explained by the noticeable decrease in two out of three W- W distances. The lengths of these distances in the orthorhombic phase are;
d(W1-W2) = 3.84 Å d(W3-W4) = 3.87 Å d(W5-W6) = 4.10 Å
However, in the cubic phase, this distance is d(W-W) = 4.16 Å. There are two reasons for this decrease in W-W distances. First reason, during the transition, tilting of the WO4 groups destroys all threefold axes and the W···O–W bond angles can deviate from 180°. Therefore, a decrease between the terminal O atom of one pair of WO4 groups and a W atom of an adjacent WO4 pair occurs. Second reason, there is a significant decrease in the non-bonding W···O distance leading to an increase in the bonding nature of this interaction 76.
The γ-polymorph has an orthorhombic structure (P212121) and it has a unit cell nominally three times larger than the cubic phase 59. The cubic form can be obtained back from the orthorhombic phase by heating at 393 K and ambient pressure 77.
81
Figure A.1 Structure of the γ-orthorhombic phase of ZrW2O8 illustrated with 59 ZrO6 octahedra and WO4 tetrahedra .
ZrW2O8 irreversibly amorphizes above 1.5 GPa upon further compression 78-80. According to these studies, the low-energy modes which are in charge for negative thermal expansion behavior in this compound also have an important role. The mechanism of pressure-induced amorphization can be called as a loss of translational or orientational long-range order. In zirconium tungstate compound, the unconcerted rotations of the polyhedra cause the amorphization under pressure 80. The behavior of zirconium tungstate demonstrates the general relationship between negative thermal expansion and pressure induced amorphization in highly flexible framework 81 structures . Amorphous ZrW2O8 shows a rather positive linear thermal expansion coefficient and it can be retained upon pressure release. The amorphous phase recrystallizes to α- ZrW2O8 when heated at ambient pressure and above 873 K 80, 82. It is important to note that the amorphous-
82 to-crystalline phase transition in ZrW2O8 is endothermic and it is accompanied by an increase of entropy 82. A complete breakdown of the 81 framework structure of ZrW2O8 was observed by A. Grzechnik et al. . Just after the completion of amorphization and decomposition to denser components at higher pressures, ZrW2O8 shows the new dense hexagonal
U3O8 type polymorph. In this structure Zr and W atoms are 6-fold coordinated and statistically disordered. This hexagonal phase is found to be stable to at least 1100 K at ambient pressures 81. All the phase transitions that has been observed in ZrW2O8 system are summarized in Figure A.2.
ZrW2O8 β-cubic γ-orthorhombic (ZrO2:2WO3) (P212121) T=1473 K (Pa3) Ambient pressure
α-cubic phase (P213)
Decompose into Amorphous U3O8-Hexagonal ZrO2 and WO3
Figure A.2 The phase transitions of ZrW2O8.
83
APPENDIX B
SOL-GEL CHEMISTRY – BASIC CONCEPTS
Sol is a system which allows chemical species to become stable in the liquid. In the sol, the solid particles whose density is greater than liquid media should be small enough so that the forces responsible for the dispersion of these particles can overcome the gravity. If the conditions which provide the stabilization of the colloidal solid particles are changed slightly, the system may go to destabilization of the sol such as 83; • Precipitation or aggregation of the sol species • Precipitation of unaggregated particles • Formation of a homogeneous gel In the solution, if the repulsive forces are dominant, the colloidal stability can be maintained. Two fundamental mechanisms that provide dispersion stability are 84; • Steric repulsion: A polymer is added to the system which adsorbs onto the particles. As a result, the polymer layer around the particles prevents the contact between particles and keeps them in a homogeneously dispersed state. • Electrostatic or charge stabilization: The surfaces of colloidal particles can be in a charged state, depending on the nature of the surfaces and the solutions conditions (e.g. pH). Depending on their double layer charges species in a system can reach a dispersed state. The formation of colloidal particles is strongly affected by charging around the particles. To estimate the degree of double layer charge, usually a property called zeta potential is measured. The liquid layer surrounding the
84 particle consists of two parts. The first part is an inner region (Stern layer). In this region, the ions are strongly bound. The second part is an outer (diffuse) region. In this region the ions are less firmly bound. There is a notional boundary within the diffuse layer, (i.e. the ions and particles form a stable entity in the diffuse layer). The electric potential at this boundary is described as the zeta potential Figure B.1. 84. The zeta potential can be calculated by the mobilization of the dispersed particles relative to a fluid under the influence of an electric field. The magnitude of the zeta potential is the measure of the particle repulsion. It indicates the degree of repulsion between adjacent and similarly charged particles in dispersion. If all the particles in suspension have a large negative or positive zeta potential values (more positive than +30 mV or more negative than -30 mV) they are normally considered stable in liquid. Therefore particles have a tendency to repel each other and they do not flocculate or coagulate. Conversely, if the particles have low zeta potential values, repulsion force between particles is not strong enough, therefore the particles are not stable in the solution and flocculation or coagulation occurs 84.
Figure B.1 Schematic representation of zeta potential 84.
85 Macromolecular sols are another group of sols that can be used instead of colloidal sols in sol-gel processing. In macromolecular sols, polymerizing or crosslinking between the polymer molecules cause the growth of the molecules. When the molecules continue to grow, the system becomes a semi solid phase which is called the “gel”. These kinds of systems are mostly obtained from organometallics dissolved in alcoholic solvents. The main and basic difference between these systems and polydisperse colloidal sols is the reversibility of the gel. The gel phase derived from macromolecular sols can be redispersed into the solvent, even if the gel is dried 83. The final stage of a sol-gel processing is named “gelation”. It is a state in which colloidal particles or macromolecules form a 3-dimensionally interconnected solid network. Remaining solvent can not be mobilized in the structure. This provides the visco-elastic property to the gelatinous mass. This semi-solid network can be colloidal or polymeric. If this network consists of colloidal particles, the gel is called a “colloidal gel”. If the semi-solid network is made of sub-colloidal macromolecules, the gel is said to be a “polymeric gel”.
86
APPENDIX C
SYNTHETIC XRD PATTERNS
In the calculations, the most intense or characteristic peaks of ZrO2,
WO3 and ZrW2O8 components should be known. Based on structural refinement studies of ZrO2, WO3 and ZrW2O8, theoretical XRD patterns were recalculated∗. Parameters needed are given below; • Atom positions • Space group • Unit cell parameters After the calculations, intensity values and hkl peaks at 2θ angles are obtained. As a theoretical approach, the most intense peaks for ZrO2, WO3 and ZrW2O8 were assumed to be 100%, and then all other intensities were calculated in proportion with the most intense peak.
ZrO2 (monoclinic)
Thermal treatment between 1273 and 1323 K temperatures causes tetragonal zirconia (t-ZrO2) crystalline structure. Raising the temperature at around T = 1373 K causes the formation of ZrO2 crystallites in the monoclinic 85 crystallographic phase (baddeleyite, m-ZrO2) . It is known that at around
1473 K heat treatment was applied to produce ZrW2O8. Therefore, unconverted or decomposed ZrO2 can be in monoclinic (baddeleyite) phase.
The parameters for monoclinic ZrO2 are tabulated in Table C.1.
∗ Carine Crystallography, v3.1, 1998.
87 86 Table C.1 Monoclinic ZrO2 (Space Group: P21/c)
Unit Cell a(Ǻ) 5.1507 b(Ǻ) 5.2028 c(Ǻ) 5.3156 β (°) 99.196 Zr x 0.2742 y 0.0389 z 0.2095 O1 x 0.0630 y 0.3289 z 0.3476 O2 x 0.4491 y 0.7548 z 0.4827
WO3
There are some studies about the crystal structure of WO3 in literature. In the range of temperature for our system, WO3 is in monoclinic phase. The parameters for monoclinic phase of WO3 are tabulated in Table C.2.
88 87 Table C.2 WO3 (Space Group: P21/n)
Unit Cell a(Ǻ) 7.306 b(Ǻ) 7.540 c(Ǻ) 7.692 β (°) 90.881 W(1) x/a 0.2465 y/b 0.0269 z/c 0.2859 W(2) x/a 0.2535 y/b 0.0353 z/c 0.7807
Ox1 x/a 0.0025 y/b 0.0350 z/c 0.2106
Ox2 x/a 0.9974 y/b 0.4636 z/c 0.2161
Oy1 x/a 0.2840 y/b 0.2605 z/c 0.2848
Oy2 x/a 0.2099 y/b 0.2568 z/c 0.7318
Oz1 x/a 0.2827 y/b 0.0383 z/c 0.0046
Oz2 x/a 0.2856 y/b 0.4840 z/c 0.9944
89 ZrW2O8
Refined structural parameters for cubic α-ZrW2O8 at room temperature and ambient pressure are tabulated in Table C.3.
59 Table C.3 α-ZrW2O8 (Space Group: P213)
a = 9,1494 Ǻ x Y Z Zr 0.0011 0.0011 0.0011 W(1) 0.3401 0.3401 0.3401 W(2) 0.6006 0.6006 0.6006 O(1) 0.2055 0.4376 0.4467 O(2) 0.7877 0.5694 0.5549 O(3) 0.4922 0.4922 0.4922 O(4) 0.2323 0.2323 0.2323
90 C-1 Calculated XRD pattern for monoclinic ZrO2
Intensity (%) 1,1,-1 (28.19,100.0) 100
90
80
1,1,1 (31.47,69.8) 70
60
50
40
30
0,0,2 (34.15,20.4) 0,2,2 20 (49.29,19.0) 0,1,1 2,0,0 (24.07,15.1) (35.27,15.1) 0,2,0 2,1,-1 1,1,0 (34.45,12.6) (40.72,12.2) 2,0,-2 (24.46,10.1) 2,1,1 10 1,1,2 0,2,1 (45.49,7.7) 1,0,0 1,0,21,2,-1 (44.81,6.7) 1,0,-2 (38.60,5.6) (41.13,5.0) (17.43,4.6) (41.42,4.4) 2,1,-2 (35.89,3.0) 0,1,-2 2,1,0 0,1,21,2,0 1,1,-2 1,2,1 (48.93,2.2) (39.42,0.9) (38.39,0.0)(38.85,0.2)(39.98,0.4) (43.83,0.0) 2 θ (°) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
Figure C.1 XRD pattern of monoclinic (baddeleyite) ZrO2.
C-2 Calculated XRD pattern for monoclinic WO3
Intensity (%) 2,0,0 (24.35,100.0) 100 0,0,2 (23.11,96.7)0,2,0 (23.58,94.1)
90
80
70
60
50
1,1,0 (16.88,39.7) 40
30
1,0,-2 0,1,2 (25.99,21.5) 20
1,1,-2 10 1,0,2 (28.60,8.4)1,1,2 (26.33,6.5) (28.91,6.2) 1,0,0 (12.11,3.2) 1,2,0 (26.59,0.6) 2 θ (°) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Figure C.2 XRD pattern of monoclinic WO3.
91 C-3 Calculated XRD pattern for α-Cubic ZrW2O8
Intensity (%) 2,0,1 2,1,0 (21.70,100.0) 100
90
80 2,1,1 (23.80,76.8)
70
60
50
40 3,1,2 3,2,1 (36.72,33.3)
3,1,1 30 (32.43,28.2)
3,2,0 2,2,0 3,1,0 3,0,2 4,1,2 20 4,2,2 (27.55,18.3) 3,0,1 (35.34,18.3) 4,2,1 3,2,2 (45.39,16.9) (48.72,17.4) 1,1,1 (30.88,16.5) 4,0,1 (16.77,14.3) 4,1,0 4,-1,0 2,2,1 (40.62,11.3) 2,0,0 4,3,0 10 (29.26,9.1) (19.39,8.3) 4,0,2 4,0,3 4,2,0 3,3,1 (49.79,7.0) (44.23,5.2) 4,0,0 3,3,0(43.06,4.3) 3,3,2 1,1,0 2,2,2 (39.36,2.4) 4,1,1 (46.52,2.4) (13.68,0.2) (33.91,0.6) (41.85,0.5) 2 θ (°) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
Figure C.3 XRD pattern of α-Cubic ZrW2O8.
92
APPENDIX D
SAMPLE CALCULATIONS
The characteristic peaks of ZrO2, WO3 and ZrW2O8 were determined from Appendix C and they were found to be at, respectively 28.19°, 24.35° and 21.70°. However, the most 3 intense peaks of WO3 were between 22.5° and 25.0°. By performing a peak fit between 21.0° and 32.0°, overlapped peaks for 3 possible components in the product were separated. Then, with a rough assumption the quantities for 3 components were calculated.
Table D.1 ZrO2, WO3 and ZrW2O8 the characteristic peaks and their calculated proportional intensities between 21.0°-32.0°.
2θ 2θ 2θ
ZrW2O8 21.70º(100%) WO3 24.35°(100%) ZrO2 28.19°(100%)
23.11°(96,7%) 24.46°(10,1%) 28.60°(1,2%) WO3 ZrO2 WO3 23.58°(94,1%) 28.91°(6,2%)
25.99°(21.5%) 29.26°(9.1%) ZrW O 23.80°(76,8%) 2 8 WO3 26.33°(6.5%) ZrW2O8 30.88°(16.5%) 26.59°(0.6%)
ZrO2 24.07°(15,1%) ZrW2O8 27.55°(18.3%) ZrO2 31.47°(69.8%)
Peak fitting was utilized and the characteristic peaks were deconvoluted. The calculation of sample which was prepared at fixed pH value and with a 2 hour aging time is shown below;
93 Experimental Generated Peak 1 Gauss+Lor Amp Peak 2 Gauss+Lor Amp Peak 3 Gauss+Lor Amp Peak 4 Gauss+Lor Amp Peak 5 Gauss+Lor Amp Peak 6 Gauss+Lor Amp Peak 7 Gauss+Lor Amp Peak 8 Gauss+Lor Amp Peak 9 Gauss+Lor Amp Peak 10 Gauss+Lor Amp Peak 11 Gauss+Lor Amp Peak 12 Gauss+Lor Amp Peak 13 Gauss+Lor Amp Peak 14 Gauss+Lor Amp Intensity (a.u.) Peak 15 Gauss+Lor Amp Peak 16 Gauss+Lor Amp Peak 17 Gauss+Lor Amp Peak 18 Gauss+Lor Amp
21 23 25 27 29 31 2θ
Figure D.1 Deconvoluted peaks between 21° and 32°.
ZrW2O8; 2θ=21.80° 2388.42
WO3; 2θ=23.31° 467.74
ZrO2; 2θ=28.30° 873.20
2388.42 % (In Volume) ZrW2O8 = = 64.04 % 2388.42 + 467.74 + 873.20 467.74 % (In Volume) WO3 = = 12.54 % 2388.42 + 467.74 + 873.20 873.20 % (In Volume) ZrO2 = = 23.42 % 2388.42 + 467.74 + 873.20
94 Table D.2 Density and molecular weight values
Theoretical Molecular Phase Density Weight (g/cm3) (g/mole)
ZrO2 5.6 123.2231
WO3 7.46 231.8393
ZrW2O8 5.072 586.9016
23.42*5.6 ZrO2 = *100 = 1.064 123.2231
12.54*5.6 WO3 = *100 = 0.404 123.2231
64.04*5.6 ZrW2O8 = *100 = 0.553 123.2231
1.064 % (In Mole) ZrO2 = = 52.65 % 1.064 + 0.404 + 0.553 0.404 % (In Mole) WO3 = = 19.99% 1.064 + 0.404 + 0.553 2388.42 % (In Mole) ZrW2O8 = = 27.36 % 2388.42 + 467.74 + 873.20
The calculations were done for all the samples and the results were tabulated at Table 4.1 in Chapter 4.
95
APPENDIX E
CHEMICAL EQUILIBRIUM CALCULATIONS ON THE SOLUBILITY OF TUNGSTIC ACID SOLUTIONS
In the ATA solutions used in this work, 0.4 M ammonia is typically used. The measured pH values then vary between 9 and 10, irreproducibly. This may be due to some TA that may resist dissolution. Because if TA was + 2- to dissociate completely to 2H and WO4 , then the ammonia that would be needed to neutralize all the protons from this dissociation and increase the pH to level like 9 or 10, relatively more ammonia would be needed. This can be demonstrated by a chemical equilibrium calculation. In Figure E.1, the distribution of ions in an aqueous solution that has 0.2 M TA in it is plotted with respect to pH. Ammonia is used as the base to increase the pH. Calculations show that to increase the pH of such a solution to levels between 9.5 and 10, the ammonia concentration that is needed would be over 0.8 M. This is a positive proof that, in fact not all of the TA is dissolving 2- and dissociating to WO4 , and the added ammonia freely increases the pH to the observed levels. Therefore ammonia should be used in excess and regardless of its concentration and a high level of pH should be targeted for increasing the ZrW2O8 ceramic yields. In the calculations it is assumed that in the solution; 0.2 M TA is completely dissolved and this concentration is kept constant regardless of whatever the quantity of ammonia needed to increase the pH. The pKa, and pKb values of TA and ammonia are used as they are entered in the database of OLI Analyzer.
96
Figure E.1 The distribution of different ionic species in the ATA solutions with respect to pH. pH is increased by ammonia total quantity of which is represented on the right axis.
97