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Electrical Properties of Cati03

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Electrical Properties of Cati03 The University of New South Wales Faculty of Science and Technology School of Materials Science and Engineering Electrical Properties of CaTi03 A Thesis in Ceramic Engineering by Mei-Fang Zhou Submitted in Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy March 2004 U N b W 2 7 JAN 2005 LIBRARY CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. (Signed) ACKNOWLEDGMENTS The author would like to express her thanks to the following people for their contributions to the completion of this work: Prof. J. Nowotny, my supervisor, for sparking my interest in this thesis project and for providing valuable advice on various aspects of the project. I am grateful for his constant encouragement and great assistance with the research plan, thesis corrections and valuable discussion. In particular, he contributed exceptional expertise in the defect chemistry of amphoteric semiconducting oxides. Prof. C. C. Sorrell, my supervisor, for his quality supervision. The scholarship provided by him has been a key factor in the completion of the project. I am grateful for his valuable advice, helpful discussions and training in scientific research approaches. In particular, he guided me on the crystal chemistry aspects of this work. Dr. T. Bak, my co-supervisor, for his every day guidance in performing this project and many valuable discussions that were so essential for me to understand defect chemistry and the impact of defect disorder on electrical properties. Dr. Lou Vance, ANSTO Materials, for his cooperation in many aspects of this research project, which was performed as part of a collaborative program between UNSW and ANSTO. Mr. J. W. Sharp for contributing his considerable technical and experimental expertise. Ms. Jane Gao for her support with computing and IT-related matters. I also am grateful for her support for my permanent resident application. II Dr. Y. Wang and Ms. I. Bolkovsky for the help with X-ray diffraction and Laser Roman Spectrometric analysis; Dr. C. H. Kong, Ms V. Piegerova and Mr. J. Budden for assistance with application of optical and electron microscopy. Other staff and my friends in the School of Materials Science and Engineering at the University of New South University, for various technical or practical support during the course of my study. Interlibrary Loan Section, Physical Science Library, UNSW for the acquisition of many papers otherwise unavailable. Commonwealth Department of Education, Science and Training, Australia for providing financial support in the form of International Postgraduate Research Scholarship (IPRS). My family members for their constant support and encouragement. Ill For my family ABSTRACT The present thesis studied semiconducting properties of polycrystalline CaTiC>3 at elevated temperatures and in controlled gas phase environment. The research included the determination of both electrical conductivity and thermoelectric power in the gas phase of well defined oxygen activity. The aim of research was the determination of the effect of processing conditions, including temperature and oxygen activity, on semiconducting properties of CaTiC>3, and related charge transport. The present work was performed as a collaborative project between UNSW and ANSTO. The CaTiC>3 specimen used in this study has been used for the fabrication of SYNROC at ANSTO. The measurements of electrical conductivity and thermoelectric power of CaTi03 in the ranges of temperature 973 - 1323 K and under oxygen partial pressure lO'-Kf Pa were applied in this work. These two properties were determined simultaneously in the condition of gas/solid equilibrium. The electrical conductivity data were used for (i) isothermal monitoring of the re­ equilibration kinetics after a new oxygen activity was imposed in the measuring chamber, and (ii) establishment of the equilibrium state. The thermoelectric power (Seebeck coefficient) data were used for the assessment of the conductivity type. The determined experimental data were used for the following assessments/analyses: • Verification of the gas/solid equilibrium • Verification of defect disorder models • Determination of the conductivity components related to different charge carriers (electrons, electron holes and ions) • Verification of the effect of oxygen activity on the n-p transition The obtained experimental data and their analysis result in the following conclusions: V 1. The electrical conductivity within the n-p transition range should be considered in terms of the conductivity components corresponding to electrons, electron holes and ions. The latter may assume a substantial value (up to 50% of the total value of the electrical conductivity). 2. The temperature dependence of the electrical conductivity includes both the mobility and the formation terms. The determined activation energy of the formation of defects and their mobility is 125.3 kJ/mol at 10 Pa and 94.4 kJ/mol at 72 kPa, respectively. 3. It was found that the p(C>2) corresponding to zero value of the thermoelectric power differs substantially from the minimum of electrical conductivity as a function of p(C>2). Therefore, the latter cannot be considered as corresponding to the n-p transition. This phenomenon indicates that the semiconducting properties of CaTiCb differ essentially from other oxide semiconductors which exhibit a good agreement between the minimum of the electrical conductivity and zero value of thermoelectric power. The discrepancy between the two is likely due to different mobility of electrons and electron holes. The possibilities of incorporation of different impurities into lattice structure of CaTiCb to form substitutional solid solution or interstitial solid solution are assessed. Three atom contact models describing the CaTiCb structure are considered and the total effective concentration of the impurity elements affecting electrical properties of CaTiCb was determined. The obtained experimental data were used to derive an equation that can be used for prediction of the electrical conductivity of CaTiCb as a function of temperature and oxygen partial pressure: vpredia = e-'wmll,T(p(O2)x\0rry + 27.81x10''" e~m,MIRT(p(02)x\tfrY + 68.67xl03+r" g-15®10"”' VI NOMENCLATURE A Sum of concentration of foreign ion A' Concentration of acceptors [atomic ratio] An Kinetic constant related to scattering of electrons Ap Kinetic constant related to scattering of electron holes e Elementary charge [ 1.60206x 10'19C] Ea Activation energy [kJ/mol] Ec Energy of the bottom of the conduction band [eV] Ef Fermi energy [eV] Ev Energy of the top of the valence band [eV] F Faraday constant [96500C] AG Change of free energy [J] h Electron holes AF1 Change of enthalpy [J] AHf Formation enthalpy of the predominant defects [J] k Boltzmann constant [8.6167x10'5eV/K, 1.3807x10'23J/K] K Equilibrium constant m Parameter related to defect disorder n Concentration of electrons [atomic ratio] Nn Density of states for electrons [atomic ratio] Np Density of states for holes [atomic ratio] p Concentration of electron holes [atomic ratio] p(C>2) Oxygen partial pressure [Pa] R Electrical resistance [Q] S Thermoelectric power, S [V/K] Sn Seebeck coefficient components related to electrons [V/K] Sp Seebeck coefficient components related to electron holes [V/K] AS Change of entropy [J] T Absolute temperature [K] VII AT Temperature change across a specimen [K] t Transfer number tj Transport number of ions tn Transfer number of electrons tp Transfer number of electron holes AV Change of potential difference across a specimen [V] z Valency of defects [Tixi] Concentration of Ti ions in their lattice site [Vii””] Concentration of fully charged Ti vacancies [atom ratio] [ ] Concentration of defects n Mobility [mVS'1] p Specific resistivity [Q-cm] g Electrical conductivity [cnf'Q'1] Qj Ionic conductivity component [cnf’Q'1] an Electron conductivity component [cnf'Q"1] gp Electron hole conductivity component [cnf’Q'1] VIII TABLE OF CONTENTS SECTION________________________________________________________ PAGE CERTIFICATE OF ORIGINALITY....................................................................................... I ACKNOWLEDGMENTS....................................................................................................... II ABSTRACT...............................................................................................................................V NOMENCLATURE..............................................................................................................VII TABLE OF CONTENTS........................................................................................................IX LIST OF FIGURES..............................................................................................................XIII LIST OF TABLES..............................................................................................................
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