Thermodynamic Study of Titanium Oxycarbide
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INTRODUCTION
MANY studies on the preparation and applications of titanium oxycarbide were published during the preceding decades.[1–3] Titanium oxycarbide is usually obtained through the carbothermic reduction of TiO2.[4–6] This material attracts considerable interest due to its special physical properties such as high melting points, high conductivity, high hardness, etc. Recently, the consuming anode for titanium metal extraction through the electrolysis process was extensively developed.[7,8] It is regarded as a solid solution of titanium carbide (TiC) and titanium monoxide (TiO), generally denoted as TiCxO1–x, where 0 £ x £ 1, with the same face-centered-cubic structure of TiC. However, the formation mechanism of TiCxO1–x is still not clear, and the thermodynamic measurement of titanium oxycarbide is the concern. Ouensanga reported that the CO equilibrium pressure over the condensed phases b-Ti3O5, TiCxO1–x, and C(graphite) was obtained by the reduction of rutile TiO2 with carbon in a graphite furnace.[9,10] The reduction of TiO2 by graphite produces the oxycarbide TiC0.67O0.33 in equilibrium with b-Ti3O5, CO, and C at 1580 K (1307 C), and the standard free energy of the obtained TiC0.67O0.33 was calculated to be DGof (1580 K) = 254 kJ mol–1. Hashimoto also measured the CO pressures in equilibrium with TiCxO1–x specimens, which were prepared from TiO2 and C in the temperature region from 1573 K to 2200 K (1300 C to 1927 C).[11] He summarized an equation for the standard Gibbs energy of formation of TiCxO1–x as the function of temperature and x value. Both of the preceding studies obtained the thermodynamic data directly by establishing the equilibrium of CO and TiCxO1–x, while there are some problems with this method. First, the measurement is usually feasible only at high temperatures, because at low temperatures, it will take a very long time to reach equilibrium; for BO JIANG, Postdoctoral Student, KAI HUANG and ZHANMIN CAO, Associate Professors, and HONGMIN ZHU, Professor, are with the Department of State Key Laboratory of Advanced Metallurgy, University of Science and Technology, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 10, 2011. Article published online February 8, 2012 3510—VOLUME 43A, OCTOBER 2012
example, at 1580 K (1307 C), it takes more than 10 days to reach the equilibrium of CO pressure.[9–11] Second, the measured composition of the oxycarbide is the value in equilibrium at the temperature, so it usually has a high carbon content (x > 0.5) because the equilibrium is under the saturation of carbon or pure TiC. The highest oxygen value Hashimoto reached in TiCxO1–x was x = 0.9. However, for the application of the consuming anode, the oxygen value is required to be equal to the carbon value, which is C/O 1.[7,8] Therefore, it is more worthwhile to measure the thermodynamic data of the titanium oxycarbide with a high oxygen value, especially around TiCxO1–x (x = 0.5). Maitre obtained the combustion enthalpies of TiC (DH695K(422C) = 1180
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