Critical Evaluation and Thermodynamic Optimization of the Ti-C-O System and Its Applications to Carbothermic TiO 2 Reduc
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INTRODUCTION
TITANIUM-BASED materials such as titanium carbide and titanium oxycarbide have been considered as advanced engineering materials for several decades. For example, titanium carbide is the advanced engineering ceramic used in electrical and electronic, automotive, and refractory industries;[1,2] titanium oxycarbide as consumable anode plays an important role in the preparation of high-purity titanium in the USTB process.[3,4] Although there are several synthesis methods of these materials,[5–8] carbothermal reduction of TiO2 still remains as the most used industrial process for the production of titanium carbide and titanium oxycarbide.[9,10] In order to understand the complex reduction sequence from TiO2 to titanium carbide in this process and the thermodynamic behavior of interstitial solute (C and O) in titanium metal, it is essential to know the phase equilibria and thermodynamic properties of the Ti-C-O system. In the thermodynamic ‘‘optimization (assessment or modeling)’’ of a chemical system, all available thermodynamic and phase equilibrium data are evaluated simultaneously in order to obtain a set of model equations for the Gibbs energies of all phases as functions of temperature and composition. From these equations, all the thermodynamic properties and phase diagrams can be back ZHANMIN CAO, Associate Professor, WEI XIE, Master Student, GUANGWEI DU, Ph.D. Student, and ZHIYU QIAO, Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] IN-HO JUNG, Associate Professor, is with the Department of Mining and Materials Engineering, McGill University, Montreal, QC H3A 0C5, Canada. Manuscript submitted December 19, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
calculated. In this way, all the data are rendered selfconsistent and consistent with thermodynamic principles. Thermodynamic property data, such as activity data, can aid in the evaluation of the phase diagram, and phase diagram measurements can be used to deduce thermodynamic properties. Discrepancies in the available data can often be resolved, and interpolations and extrapolations can be made in a thermodynamically correct manner. Thermodynamic assessments of the Ti-C and Ti-O binary systems have been performed by many groups.[11–22] However, it is found that the available assessments for the Ti-C system have one critical problem when they are extended to the Ti-C-O ternary system. In the previous assessments of the Ti-C system,[11–15] the d-TiCx phase was always described only considering carbon vacancy (Va), without considering metal vacancy, i.e., (Ti)1(C, Va)1. However, Puska et al.[23] detected the existence of metal vacancies by positron annihilation method and reported the detectable amount of metal vacancies (up to 0.5 pct). Brauer et al.[24] also verified this result by positron annihilation spectroscopy technique. Furthermore, dTiCx phase forms a complete solid solution with d-TiOy which has a large am
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