1999 Volume 107 Issue 1242 Pages 153-158
SiC-AlN powder mixture was prepared by heating a silica-alumina-graphite powder mixture in flowing nitrogen at temperatures between 1773 and 1973K. The reaction mechanism of the carbothermic reduction and nitridation has been both thermodynamically and kinetically investigated. The reaction rates were determined according to thermol balance and the reaction products were examined by X-ray diffraction. SiO2 was initially reduced into SiC and subsequently 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 rates of SiC and AlN formation were described by a three dimensional contracting-sphere formula for diffusion-control. The activation energies were 429kJ/mol for SiC formation and 611kJ/mol for AlN formation. The rate-determining steps of SiC and AlN formation are probably the diffusion through SiC layer formed around graphite particles and AlN layer formed around Al2O3 particles, respectively.
