LOW-TEMPERATURE SYNTHESIS OF SINTERABLE SIC POWDERS BY CARBOTHERMIC REDUCTION OF COLLOIDAL SIO2

Beta-SiC powders were synthesised by carbothermic reduction of carbon-black doped silica gel. The morphological characteristics of the resulting powders depend on the morphology of precursor carbon black as well as the reaction conditions. A model for particle coarsening during carbothermic reduction is presented, suggesting that surface diffusion may be responsible for the formation of polycrystalline aggregates, while at higher reaction temperatures volume diffusion is operative. Processing consisting of decarburization, HF treatment and sedimentation but no milling is required to convert crude reaction products into sinterable powders. After doping with boron and carbon they can be sintered to 98-99% of theoretical density which, in general, is better than or comparable to results obtained with commercial-beta-SiC powders which were also included in the present study.