Carbothermic reduction of SiO2-Al2O3 powder mixture in flowing nitrogen

SiC-AlN powder mixture was prepared by heating a silica-alumina-graphite po wder mixture in flowing nitrogen at temperatures between 1773 and 1973 K, T he reaction mechanism of the carbothermic reduction and nitridation has bee n both thermodynamically and kinetically investigated. The reaction rates w ere determined according to thermol balance and the reaction products were examined by X-ray diffraction. SiO2 was initially reduced into SiC and subs equently Al2O3 was reduced and nitrided into AlN, During reduction of Al2O3 , AlON, Si2N2O, Si3N4 and SiAlON were produced as transitional phases, once graphite is coated by a SiC layer, Al2O3 may also be reduced by SiC, with the consequent formation of the above transitional phases. The reaction rat es of SiC and AlN formation were described by a three dimensional contracti ng-sphere formula for diffusion-control. The activation energies were 429 k J/mol for SiC formation and 611 kJ/mol for AlN formation. The rate-determin ing steps of SiC and AlN formation are probably the diffusion through Sic l ayer formed around graphite particles and AlN layer formed around Al2O3 par ticles, respectively.