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Signature Redacted Department of Metallurgy Gau, R E Da Signature of Professor in Charge of Research THERMODYNAMIC PROPERTIES OF THE GROUP VIa SULFIDES: CrS, Mo2 S3 AND WS2 by JOHN PATRICK HAGER B. S., Montana School of Mines (1958) M. S. , School of Mines and Metallurgy University of Missouri (1960) Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF SCIENCE at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY August, 1969 Signature of Author Signature redacted Department of Metallurgy gau, r e da Signature of Professor in Charge of Research Signature of Chairman of Departmental Committee Graduate Research nchives Signature redacted INST. 7 DEC 2 1969 4IBRARIES -~ U ii THERMODYNAMIC PROPERTIES OF THE GROUP VIa SULFIDES: CrS, Mo 2S 3 and WS 2 by John Patrick Hager Submitted to the Department of Metallurgy on August 18, 1969, in partial fulfillment of the requirements for the degree of Doctor of Science. ABSTRACT The thermodynamic properties of the metal-saturated phase of the Group VIa sulfides have been determined through a study of the effect of gas composition on the sulfidizing action of (H2S g) + H 2 (g) ) mixtures passed over heated metal samples. The following equations for the standard free energy of formation of CrS (c) ' Mo2S3(c) and WS2(c) were obtained: AFCc) ( 250) = -48,190 ( 760) + 13.27( 0.52)T; cal/l/2 g-mole S2(g) (1375-1570 0 K) AF 0 ( 220) = -41,730( 890) + 17.39( 0.59)1R; cal/l/2 g-mole S2(g) 102S3 (c)2(g (1365-1610 0 K) AF ( 220) = -40,110( 920) + 18.64( 0.63)T; cal/l/2 g-mole S WS 2 (c) 2(g) (1370-15650K) An additional study of the thermodynamic properties of Cu2S ) pro- vided a means of evaluating the experimental technique. The composition of the sulfide phase, in equilibrium with iii chromium at 1420 0 K, was determined to be stoichiometric chromous sul- fide. By correlating the entropies of formation of the Group VIa sulfides with the entropies of formation observed for the refractory metal carbides, nitrides and oxides, estimated values of the entropy of formation at 298 0K for the mono-, sesqui- and di-sulfide phases of the refractory metals were obtained. Thesis Supervisor: Professor John F. Elliott Title: Professor of Metallurgy iv. TABLE OF CONTENTS Chapter Page Number Number ABSTRACT .......................... ........... ii LIST OF FIGURES ........ ........... vi LIST OF TABLES ...................... vii ACKNOWLEDGEMENTS ....... ..... .... ix I INTRODUCTION ............ ... .......... 1 A. Equilibrium Methods ................. 1 B..Outline of This Study ................................ 4 II LITERATURE SURVEY ... ............................ 6 A. Hydrogen-Sulfur System ......... ........ 6 B. Chromium-Sulfur System .......................... 6 C. Molybdenum-Sulfur System ........... ............ 9 D. Tungsten-Sulfur System ............................ 13 E. Copper-Sulfur System............... 15 III APPARATUS AND EXPERIMENTAL PROCEDURE .............. 19 A. Description of Apparatus ....................... 19 1. Furnace and sample loading mechanism ........ 19 2. Temperature control and measurement ......... 21 3. Gas mixing system ....................... 22 4. Gas analysis apparatus ......................... 25 B. Experimental Procedure ............... 0.......... 25 1. Materials and material preparation .......... 25 2. Flowmeter calibration .................. 27 3. Gas analysis procedure ........................ 30 4. Operating procedure .................... 37 5. Analysis of sulfide phase composition ....... 42 IV REDUCTION OF EXPERIMENTAL DATA ...................... 44 A. Data Analysis - General Considerations ............ 44 B. Corrections for Dissociation of Hydrogen Sulfide. 44 C. Thermodynamic Computations ..................... 53 D. Estimation of Uncertainty ......................... 56 1. Uncertainty in experimental measurements .... 56 2. Uncertainty in AF* ......................... 57 R ... ... ... .. 3. Uncertainty in AS* and AH 0.. ... 59 R R 4. Uncertainty in AF*, AS* and AH* of forma- tion of MeXSY ***................ ........ 59 V RESULTS ..... .... ..... ........................ 60 A. Chromium Sulfide ............................... 60 v. Chapter Page Number Number B. Molybdenum Sulfide ....................... 62 C. Tungsten Sulfide ,..,................. 64 D. Copper Sulfide ........ .......... 64 VI DISCUSSION ................ .......... 67 A. Evaluation of the Experimental Technique..... 67 B. Comparison of the Results for CrS,(c) Mo2S3(c) and WS2(c) with those of Previous Studies ....................................... 70 C. Estimation of the Entropies of Formation of the Refractory Metal Sulfides ................ 75 VII SUGGESTIONS FOR FURTHER WORK ............. 79 A. Measurement of the Activity of Chromium in Cr-Mo and Cr-W Solid Solutions ............ ,.. 79 B. Investigation of the Sublimation of Tungsten Disulfide .................................. 80 C. Study of the Higher Sulfides of Chromium through (H2 So + H 2 ) Equilibration ....... 81 REFERENCES ......................... 83 APPENDIX A - TABULATION OF EXPERIMENTAL DATA .... 87 APPENDIX B - CORRECTION OF PH2 H2(g) RATIOS FOR DISSOCIATION' OF H2S . .. 89 APPENDIX C - ANALYSIS OF CHROMIUM SULFIDE DATA .. 93 APPENDIX D - ANALYSIS OF MOLYBDENUM SULFIDE DATA. 101 APPENDIX E - ANALYSIS OF TUNGSTEN SULFIDE DATA .. 108 APPENDIX F - ANALYSIS OF COPPER SULFIDE DATA .... 115 APPENDIX G - GRAVIMETRIC ANALYSIS OF CHROMIUM SULFIDE LAYERS ..................... 121 BIOGRAPHICAL NOTE ................................ 123 vi. LIST OF FIGURES Figure Page Number Number 1 Schematic View of Furnace and Sample Loading Mechanism............................................. 20 2 Correlation between Thermocouple and Sample Temperature .......................................... 23 3 Schematic View of Gas Train ................... ,.......... 24 4 Schematic View of Gas Analysis Apparatus ............. 26 5 Free Energy Change for Reactions (V-1), (V-5) and (V-9): CrS(c) + H2(g) = Cr(c) + H2S(g) 1/3Mo 2 S3(c) + H (g) = 2 3 2 / Mo (c) + H 2 S(g) 1/2WS2(c) + H2(g) = 1/2W (c) + H2S(g)............ 61 6 Free Energy Change for Reaction (V-13): 66 Cu2 S(y) + H2(g) = 2Cu(c) + H2S(g) ............... vii. LIST OF TABLES Table Page Number Number I Thermodynamic Properties of Sulfur and Sulfur- Hydrogen Gases . ......................... ........ 7 5 II Phases in the Cr-S System According to Jellinek .. 8 III Results from Previous Investigators for the Equilibrium: CuS + H =2Cu + HS ..... 18 2 (y) 2 (g) (c) 2 (g) IV Stoichiometric Relations and Partial Pressure Expressions for the Dissociation of H2S(g) into H 2 (g) plus S2(g) ' ... ............ 48 V Stoichiometric Relations and Partial Pressure Expressions for the Dissociation of H2S(g) into H 2 (g) plus S2(g) plus HS(g) ... 2(g) . ... 51 VI The Standard Free Energies, Enthalpies and Entropies of Formation of the Sulfides CrS (c), Mo2S3(c)' WS2(c)' Cu2 S per 1/2 Gram-Mole S2(gas) .................... 63 VII Thermodynamic Properties for the Reaction 1/3Mo23(c) + H2(g) 2/3Mo(c) + H2S(g) ''" " 71 VIII Standard Free Energy Change for the Reaction . .74 (c) 2(g) 2(c)......... 's.. IX Heat of Formation of WS2(c) at 298 0 K .................. 75 X Standard Entropies of the Elements at 298*K According to Stull and Sinke 4 6 ... ...... ............ .. 77 XI Comparison of Experimental and Estimated Values for the Entropy of Formation of the Group VIa Sulfides per 1/2 g-mole S2(g) 77 XII Estimated Standard Entropies of Formation at 298*K for the Mono-, Sesqui- and Di-sulfidephases of the Refractory Metals .................................... 78 A-I Tabulation of Experimental Data ............ 87 B-I Corrections for Dissociation of H2S(g) * .. 911' -~ I viii. Table Page Number Number Experimental Data and Calculated values of AF* C-I R for 93 CrS(c) + H2(g) = Cr(c) + H (2)S).. .... ... and Calculated Values of AF* D-I Experimental Data R for 101 1/3Mo2 S3(c) + H2(g) = 2/3Mo (c) + H(2gS (g) Data and Calculated Values of AF* E-I Experimental R for 1/2WS2(c) + H 2(g) = 1/2W + H (c)g2S) ' , 108 Values of AF* F-I Experimental Data and Calculated R for Cu2S( 115 + H = 2Cu c) + H2S (g) G-I Data for Determination of Molar Cr/S Ratio for CrX S Phase in Equilibrium with Chromium at 1420 0K ................. 122 -~ U ix. ACKNOWLEDGMENTS The author wishes to express his sincere appreciation to Professor John F. Elliott for his counsel and assistance throughout the course of this study. Special thanks is also expressed to Professor John Chipman and Dr. T. R. Meadowcroft, formerly Assistant Professor of Metallurgy, now Research Associate, The Steel Company of Canada, Hamilton, Ontario, for their interest and many helpful suggestions. The following persons who have contributed to the successful completion of this project are also gratefully acknowledged: Mr. Frank Woolley for many helpful discussions; Mr. Theodore Strow for his assistance in chemical analyses; Mr. George Prince and Mr. Walter Correia for their glassblowing services; Miss Barbara Westdyke for her assistance in proofreading the manuscript; Miss Catherine A. Walsh for her excellent preparation of the final manuscript; The National Science Foundation for their financial support of this project. I, INTRODUCTION The Group VIa metals, Cr, Mo and W, are characterized by high melting points and, accordingly, are utilized as high-temperature materials. The compatibility of these metals, at high temperature, with sulfur-con- taining gases, liquids or solids is of practical interest. In addition, several compounds formed between sulfur and the VIa metals have found application as high-temperature lubricants and catalysts. Thermodynamic measurements
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