THE KINETICS AND MECHANISM OP THE CONVERSION OF TITANIUM DIOXIDE TO TITANIUM NITRIDE DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By David Leslie Douglass, B.S., M.S. The Ohio State University 1958 Approved by: * V Adviser Department of Metallurgical Engineering DEDICATION This dissertation is devoted to my family— to my wife who spent many silent nights in the same room with her husband who "wasn't there," and to my two young daughters who didn't see too much of "Daddy" and couldn't understand his Inability to devote more time to them. The patience and forebearance of my family have been a driving force and motivation. To them I am deeply Indebted. il ACKNOWLEDGMENTS The writer wishes to express his gratitude and sincere appreciation to Dr. George R. St.Pierre who acted as thesis adviser and whose sincere interest and many discussions were invaluable and provided much of the stimulus for this work. The contributions of Professor Rudolph Speiser proved extremely valuable and are gratefully acknowledged. Appreciation is expressed for the assistance of Gerald Gordon, fellow graduate student, in providing Innumerable experimental facilities and aids. The many interesting and pertinent.discussions with Lyle L. Marsh, another student, also proved very beneficial, and his keen interest is appreciated. Lastly, the understanding and patience of my wife, Rebecca, during my entire graduate program made this work possible. ill CONTENTS Page INTRODUCTION ................. 1 LITERATURE SURVEY ............................. 2 Preparation of Titanium Nitride ........... 2 Practical Significance of the Conversion . 3 Manufacture of Abrasives . ........... 3 Scaling of Titanium in A i r ........... 4 Surface Hardening of Titanium ......... 4 Recovery of Titanium from Ilmenite . 4 Previous Work ............................. 6 Phase Equilibria ............. 8 Related Studies ............. 10 GAS-SOLID REACTIONS ........................... 11 Rates of Reaction ......................... 11 Dependence upon Gas Pressure ............. 15 MECHANISM OP GAS-SOLID REACTIONS ............. 19 Chemlsorption............................ 19 Llnear-Rate Reactions ......... 20 Parabolic-Rate Reactions . ........... 21 Logarithmic-Rate Reactions . ........... 24 POSSIBLE METHODS OP APPROACH ................. 27 Topochemlcal Studies ..................... 2? iv Page Electrical Conductivity ................... 28 Kirkendall Effects ......................... 29 Self-Diffusion................. 31 EXPERIMENTAL PROCEDURES ....................... 33 M a t e r i a l s ................................ 33 Rate-Constant Determination ............... 34 Topochemlcal and X-ray Diffraction Studies . 35 RESULTS .............................. 38 Nitridizatlon of R u t i l e ................... 38 Topochemlcal and X-ray Studies ......... 38 Reaction Kinetics ..................... 46 Effect of temperature ............. 46 Effect of p r e s s u r e ................. 52 Rates of layer G r o w t h ................. 56 Nitridizatlon of T *2^3 and ............. DISCUSSION .................................. 66 Dissociation of Ti02 to TiO^ ........... 66 Complex Intermediate Oxides ............... 68 Formation of T i O ........................... 69 Nitride Formation from TiO . ........... 71 Comparison of Nitride Growth on Various Starting Materials ............... 74 Effect of Temperature ............. 77 Effect of Pressure ....................... 81 v SUMMARY AND CONCLUSIONS REFERENCES . LIST OP TABLES Table Page I. CHEMICAL ANALYSES OP TITANIUM DIOXIDE . 33 II. X-EAY SPECTROMETER RESULTS SHOWING BOTH TIO AND TIN REFLECTIONS FROM SURFACE LAYERS OF T102 NITRIDED 6 MINS. AT 1010°C IN 1 ATM AMMONIA .... 42 III. THE ABSORPTION OF TIO REFLECTIONS BY THE NITRIDE LAYER FOR VARIOUS THICKNESSES OF THE NITRIDE ..............44 vll LIST OP FIGURES Figure Page 1. Schematic Representation of Experimental Equipment ............... 36 2. Multiple Layers In Partially Reacted Sample (5x) ................... 40 3. Same as Figure 2 (30x).................. 40 4. Lattice Parameter Determination: Plot of a0 versus Sin 6 ............... 43 5. Lattice Parameter-Composition Relation of the TiO P h a s e ............. 45 6. Individual Contributions tc Cumulative Weight Change: Rutile at 1010°C and 1 Atm Ammonia ......................... 47 7. Comparative Weight Losses of Rutile in 1 Atm Ammonia at Various Temperatures 48 8. Test for Parabolic Rate Law. Rutile Reacted at 900° and 950°C .... 49 8.(cont'd.) Rutile at 1010°C and 1 Atm NH^ . 50 8.(concluded) Rutile at 1070°C and 1 Atm NH^ . 51 9. Temperature-Dependency of Rate Constant . 53 10. Comparative Weight Losses of Rutile at 1010°C and Various Ammonia Pressures . 54 11. Test for Parabolic Rate Law. Rutile Reacted at Various Ammonia Pressures at 1010°C . ....................... 55 12. Pressure Dependency of Parabolic Rate Constant at 1010°C ............... 57 13 . Micrographs of Fractured Sections Showing the Thickness of Each layer at Two Different T i m e s ............... 39 viii Figure Page 1^. The Effect of Time on Layer Growth on TIOg at 1010°C .............. 60 1 5 . Test for Parabolic Bate Law. TiO Reacted at 1010°C and 1 Atm Ammonia ....................... 62 16. The Effect of Time on Layer Growth on T12°3 63 lx THE KINETICS AND MECHANISM OP THE CONVERSION OP TITANIUM DIOXIDE TO TITANIUM NITRIDE INTRODUCTION The conversion of titanium dioxide to the nitride has important practical implications in several fields of metallurgical technology. Titanium nitride is a very hard, refractory compound which finds widespread use as the "hard metal" phase in cemented compacts. The nitride is the chief constituent of the hard case on nitrided titan­ ium and offers excellent wear resistance. Nitride for­ mation is also of interest in the high temperature scaling behavior of titanium and its alloys in air. Lastly, the selective formation of titanium nitride from ilmenite, an inexpensive, plentiful iron-titanium ore, gives promise of economical recovery of titanium. LITERATURE SURVEY Preparation of Titanium Nitride Several methods have been used to prepare the "hard metal" nitrides. The metals or metal hydride may be reacted with nitrogen or ammonia. Agte and Moers^ prepared nitrides of titanium, zirconium, and tantalum by reacting high purity powders In pure nitrogen. Other groups, IV-a, V-a, and Vl-a (in the periodic chart), metal nitrides of hafnium, vanadium, niobium, chromium, thorium, and uranium have been produced by reaction of the powders with ammonia.2»3»^»5»6,7»8,9 The hydrides have been used to produce the ni- h, Q trides by reaction with ammonia. *7 However, very high temperatures, 2000°C or more, are required. The most economical method of preparation In­ volves the reaction of the oxides with either nitrogen or ammonia in the presence of carbon. Unfortunately, the formation of the carbides occurs simultaneously. In the case of titanium, the carbide and nitride are lso- morphous, and an impure solid solution of nitride-carbide results. Early work by several investigators showed that titanium nitride could be.prepared by reacting Ti02 with ammonia. ^ 2 3 Deposition of the nitride from the vapor phase has been successfully achieved.^3*14,15#16 a mixture of TiCl^, nitrogen, and hydrogen will deposit titanium ni­ tride on an Incandescent tungsten filament at tempera­ ture of about 1100°-1700°C. Several investigators have produced the nitride by reacting titanium compounds such as the tetrachloride with ammonia.12»17»18 However, the present research was not aimed at the preparation of titanium nitride per se; rather, it was designed to convert the oxide to the ni­ tride for reasons which will be listed in the following section. Practical Significance of the Conversion Manufacture of Abrasives Abrasive manufacturers produce materials such as silicon nitride and titanium nitride from the metal as a starting material. Both silicon and titanium are expens­ ive, and if the abrasive nitrides could be made directly from the plentiful, low-cost oxides, a large economic gain would be realized. A knowledge of the mechanism of the conversion would enable the optimum conditions to be determined for the process. Scaling of Titanium In Air A study of the scaling behavior of titanium in air"^ indicated that the nitride formed either simultan­ eously with the oxides or subsequently from the oxides. Once again, a knowledge of the conversion mechanism would enable more oxidation-resistant or scale-resistant alloys to be designed. Surface Hardening of Titanium Another Important aspect of this problem can be found in an analysis of the surface hardening studies of titanium. The depth of hardening was found to increase by the presence of oxygen.2® This behavior suggests the possible formation of an oxynitride, whose mechanism of formation is unknown. Recovery of Titanium from Ilmenlte Perhaps the most important aspect of the oxide-to- nitride conversion lies in the field of extractive met­ allurgy. Titanium is still an expensive metal in spite of much research to develop new processes and to make present production practices more efficient. The cus­ tomary raw material, rutile, is essentially titanium dioxide and costs about $0.15 per pound of contained titanium. This material is subjected to an expensive, batch-type chlorination to form the tetrachloride, which is subsequently reduced by magnesium or some other costly reducing agent. A significantly cheaper raw