PRODUCTION OF FINE, HIGH-PURITY BETA SILICON-CARBIDE POWDERS

Submicrometer SiC (beta-form) powders were synthesized by reacting silica and carbon black at temperatures between 1450-degrees and 1800-degrees-C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400-degrees-C), the dominant mechanism for silicon carbide formation involves the solid-state reaction of silica and carbon. At higher temperatures (above approximately 1400-degrees-C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400-degrees-C, the rate of SiC formation is controlled by the rate of SiO formation. In as-synthesized form, the SiC powders typically contain < 0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large-scale commercial production of SiC, requiring no postfabrication acid leaching or major milling.