Carbothermal Reduction of Titania in Different Gas Atmospheres
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TITANIUM carbide has high-temperature strength retention, excellent oxidation resistance, a low thermal expansion coefficient, high wear resistance, a high melting point, and light weight.[1] This advanced engineering material is used in the metalworking, electrical, electronic, automotive, and refractory industries. Titanium carbide is similar to titanium metal in appearance and behavior and has higher corrosion resistance.[2] Titanium carbide is commercially produced primarily by the reduction of titania by carbon in a temperature range between 1700 °C and 2100 °C. Other methods include synthesis of TiC from metallic titanium and carbon; chemical reaction between TiCl4, H2, and C; self-propagating high-temperature synthesis; reaction of titanium sulfide with carbon; reaction of titanium hydride with carbon; chemical vapor deposition; and the sol-gel method.[3] These methods are currently under development or have limited applications. Production of titanium carbide is expensive. The cost of high-quality TiC powder is a major factor hindering wide application of titanium carbide. Therefore, developing a more efficient and less expensive process for titanium carbide is an attractive way to expand the application of this advanced material. Titanium carbide and oxycarbide can be chlorinated at temperatures of 200 °C to 500 °C,[4] which are much MOHAMMAD A.R. DEWAN, PhD Student, GUANGQING ZHANG, Lecturer, and OLEG OSTROVSKI, Professor, are with the School of Materials Science and Engineering, the University of New South Wales, UNSW Sydney, NSW 2052, Australia. Contact e-mail: [email protected] Manuscript submitted May 21, 2008. Article published online December 2, 2008. 62—VOLUME 40B, FEBRUARY 2009
lower than temperatures of conventional chlorination of rutile, which are in the range of 800 °C to 1100 °C. Energy-efficient conversion of titania to titanium oxycarbide will stimulate development of low-temperature chlorination processes. It is established that the kinetics of carbothermal TiO2 reduction is affected by the reaction temperature, molar TiO2/C ratio, TiO2 grain size, and initial bulk density.[1,5,6] Gas atmosphere also has a strong effect on the TiO2/C reactivity[6,7] and the rate of TiO2 reduction by methane containing gas.[8,9] However, published data on the effect of gas compositions on the TiO2 carbothermal reduction and their interpretation are to some extent conflicting. Vodop’yanov et al.[7] came to the conclusion that carbothermal reduction of titania to TiC is the fastest in the CO gas atmosphere. This contradicts experimental results and calculations conducted by Zhang and Ostrovski,[8] which showed that Ti2O3 formed in the course of TiO2 reduction, rather than being reduced further to TiC or titanium oxycarbide, was oxidized by CO to Ti3O5, when CO concentration in an Ar-CO gas atmosphere was more than 10 vol pct. Chou and Lin[6] studied carbothermal reduction of TiO2 in a helium atmosphere and observed the decreasing reduction rate and declining reaction yield (measured as TiC/TiO2 ratio) wit
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