The Thermodynamics of the Rhenium-Oxygen and Molybdenum-Oxygen Systems and the Defect Structure of Alpha Tantalum Pentoxide
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This dissertation has been 65—5636 microfilmed exactly as received FOSTER, James Sheridan, 1937- THE THERMODYNAMICS OF THE RHENIUM- OXYGEN AND MOLYBDENUM-OXYGEN SYSTEMS AND THE DEFECT STRUCTURE OF ALPHA TANTALUM PENTOXIDE. The Ohio State University, Ph.D., 1964 Engineering, metallurgy University Microfilms, Inc., Ann Arbor, Michigan THE THERMODYNAMICS OF THE RHENIUM-OXYGEN AND MOLYBDENUM-OXYGEN SYSTEMS AND THE DEFECT STRUCTURE OF ALPHA TANTALUM PENTOXIDE DISSERTATION Presented in Partial Fulfillment of the requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By James Sheridan Foster, B.Met.E., M.Sc The Ohio State University 1964- Approved by Adviser Department of Metallurgical Engineering Dedicated to my devoted wife, Jane Ann, for her encouragement and sacrifice in helping to complete this work, and to my son, Chris, for the tears he so often shed when I left and the smiles he always gave when I returned. ACKNOWLEDGMENTS Many people, professors, administrators, and fellow students Lave made contributions to my education, and, thus, either directly or indirectly to this work. A sin cere thanks goes to all of these people. Only a few, those who made major contributions to this work, are acknowledged here. The greatest contribution to this work was made by my adviser Dr. George R. St.Pierre, who has given much help and guidance. Dr. St.Pierre has also given me con siderable freedom, allowing me to use my judgment in doing research on thermodynamics and defect structures of re fractory metal oxides. Drs. Rudolph Speiser, John Hirth, Robert Rapp, and Gordon Powell have made contributions through many helpful discussions and encouraging remarks. Messrs. Paul Lewis, Ross Justus, Neal Farrar, and Jerris Moeller helped in the construction of my equipment and gave sound technical advice. The early stages of this work were financially supported by the United States Air Force, and, later, I iii was awarded an International Nickel Fellowship. Financial aids including a loan from the Ford Foundation and a grant from the duPont Company were of considerable help. My sincere appreciation goes to each of these bene factors and to Dr. Mars G. Fontana who has always helped me obtain the financial support necessary to complete my education. iv VITA October 14, 1937 Born - Huntington, West Virginia 1962 .......... B.Met.E., The Ohio State University, Columbus, Ohio 1962 .......... M.Sc., The Ohio State University, Columbus, Ohio 1961-1963 • • • Research Fellow, Department of Metallurgical Engineering, The Ohio State University, Columbus, Ohio 1963-1964 . International Nickel Fellow, Depart ment of Metallurgical Engineering, The Ohio State University, Columbus, Ohio FIELDS OF STUDY Major Field: Engineering Studies in Chemical Metallurgy. Professor George R. St.Pierre Studies in Mechanical Metallurgy. Professor John P. Hirth. CONTENTS Page INTRODUCTION . ........................ 1 THERMODYNAMICS OP THE RHENIUM-OXYGEN SYSTEM .............................. 5 Introduction ................................... 5 Literature Survey ............ 7 Experimental Procedure ........................ 14 R e s u l t 3 ......................................... 24 Discussion ..... .......................... 31 THERMODYNAMICS OP THE MOLYBDENUM-OXYGEN SYSTEM .......................... 51 Introduction............................ 51 Literature Survey ............................ 52 Experimental Procedure ........................ 63 R e s u l t s ......................................... 67 Discussion....................................... 74 DEFECT STRUCTURE OF ALPHA TANTALUM PENTOXIDE ................................. 83 Introduction ................................... 83 Literature Survey ............................ 85 Experimental Procedure ........................ 91 Results and Discussion ........................ 96 CONCLUSIONS.......................................... 124 APPENDIX ............ 126 BIBLIOGRAPHY 130 TABLES Table Page 1. Properties of Rhen ium...................... 7 2. Crystal Structures of Rhenium Oxides .... 9 3. Chemical Analyses of Starting Materials................................... 19 4. Free Energy of Formation of Re02 Data .... 28 5. Standard Entropy Values for ReOg and WO2 .............................. 37 6. Free Energy Change for the Reaction 4Re0j(c) + 02(g) = 2Re20 y ( c , l , g ) .......... 4-3 7. Free Energy Changes for the Reactions I. ^Re02(c) = *yRe(c) + Re20r7(g), II. 3Re05(c) = Re02(c) + Re207( g ) ........ 44 8. Standard Heats and Free Energies of Formation of Re02, ReO^, and ..... 47 9. Properties of M o l y b d e n u m .............. 52 10. Crystal Structures of Molybdenum O x i d e s ..................................... 54 11. Temperature Range of Formation for Molybdenum Oxides .......................... 57 12. Thermodynamic Functions for M 0 O2 and M o O j ............................... 59 13. Thermodynamic Functions for the Intermediate Molybdenum Oxides Estimated by Brewer ........................ 62 14. Free Energy of Formation of Mo^O^ Data . 70 vii Table Page 15. Thermodynamic Functions for Mo^O^^(c) .... 82 16. Crystal Structures of 8- and cx-TapO^ from Zaslavskii et a l . 9 7 .......... 88 17. Resistance Versus Oxygen Pressure and Temperature for cx-TapO^................ 97 18. Least Squares Analysis of the Data at Low Oxygen Pressures Using logR = B + nlogPn .............................102 2 19. Slope of logR versus logP0 Curves for High Oxygen Pressures 2 . ............... 102 20. Activation Energies for Resistance of a-TagOj. for Various I s o b a r s ...............103 21. Exponents for Oxygen Pressure Dependence of Electron or Electron Hole Concentration........................... Ill 22. Electrical Conductivities of W0P, WOj, and M o O p .............................. 128 viii ILLUSTRATIONS Figure Page 1. Schematic diagram of the galvanic cell apparatus ............................. 16 2. Experimental Af 9 n values versus temperature . 2 ......................... 30 3. Standard free energy changes for thele reactions —~ *^R eOx + 02 2 ■ReO 0 based on data in the z - xy y z l i t e r a t u r e ............................... 33 4. Revised standard free energy changes for0 the reactions —z - xy ReO x^ + 0o2 = —z - -— xy Re y0 z 48 5. Phase diagram for the rhenium- oxygen s y s t e m ............................ 49 6. Experimental AF?M ~ n \ values reJsus temperature^ 2 ^ 1 1 .............. 72 7. Standard free energy changes for the reactions -■--^-■■•■Ho O + 0o = wz - xy w x 2 -■... .M o 0 wz - xy y z ............................... 76 8. Phase diagram for the molybdenum- oxygen system ............................. 78 9. Schematic diagram of the apparatus used in the a-Ta20,- defect structure determination . ? ....................... 93 10. Log - log plot of resistance versus Pq for a-Ta20 ^ .......................... 99 ix Figure Page Zl 11, LogR versus 10 /T for a-Ta20,- at high oxygen pressures . .............. 104 12. LogR versus 10^/T for a-Ta20,- at low oxygen pressures . .......... 106 +1/& 15. Resistance versus Pq for a-TagO,- at low oxygen 2 ^ p r e s s u r e s .............................. 114 x INTRODUCTION The oxidation of metals is an area of considerable activity in metallurgical engineering. The rate of reac tion between a metal and the oxidizing gases in the environment of its application often determines whether or not the metal is a useful engineering material. The refrac tory metals, tungsten, tantalum, niobium, molybdenum, and rhenium, hgve very high melting points and good strength properties at temperatures well above the upper temperature limits for the iron, nickel, and cobalt base alloys. These metals would be useful at very high temperatures except for their uniformly poor resistance to attack by oxidizing gases. Their behavior varies from the formation of volatile oxides to autogeneous combustion. Faced with this problem, the engineer may attempt to determine the oxidation mechan ism and then try to alter the rate-controlling steps in the reaction. This is a fundamental approach to the oxidation problem. To determine the oxidation mechanism for a metal one needs to know, among other things, the stability ranges of the oxides formed and the transport characteristics of each 1 2 oxide. The stability ranges of the oxides are determined by the thermodynamics of the metal-oxygen system. The transport characteristics of an oxide are often dependent on its defect structure. The defect structure is the com posite of point defects present in the oxide. These point defects may be vacant oxygen or metal sites, interstitial oxygen or metal atoms, etc., and any of these defects may be ionized, contributing electrons to the electrical con duction processes of the oxide. The defects directly participate in the diffusion processes and may be considered to be the mobile species. Usually only one type of defect is present in a particular oxide for a given temperature and oxygen potential. The defect structure of an oxide may be determined by studying the electrical resistance or thermoelectric power over a range of oxygen potentials and temperatures. This method works when the defects are par tially or completely ionized and the intrinsic conductivity of the oxide is relatively small. An alternative method that will work in most cases is the gravimetric technique in which the weight of an oxide sample is monitored over a range of oxygen potentials and temperatures. The identity of the defect is determined from the exponential relation