Carbon Removal and Oxidation of SiC Powder Synthesized by Carbothermic Reduction of Silica

Abstract

Three kinds of SiC powders were synthesized by the carbothermic reduction of SiO₂ at 1973K in an Ar atmosphere from reagent graphite, electrode graphite and carbon black. The oxidation rates of free carbon and SiC component in these powders were measured in an O₂ gas stream at temperatures from 923 to 1573K by the thermogravimetric analysis and the determination of CO₂ evolved. As the powders synthesized from the reagent graphite and carbon black were surrounded continuously by SiC layer, the free carbon could not be removed by oxidation. The free carbon in the powder synthesized from the electrode graphite was partly removed. The activation energy for the oxidation of free carbon was 173kJ/mol. This oxidation was controlled by a chemical process. The oxidation of SiC component in the powder initiated at 1073K. The oxidation rate was significantly dependent on surface state of the powder. Jander's rate equation was applicable to the oxidation rate of SiC. The activation energy varied from 217 to 283kJ/mol. The oxidation of SiC is probably controlled by the diffusion of O^2- ion in the SiO₂ film formed around the SiC particles.