N₂ガス流通下でのSiO₂-Al₂O₃粉末混合物の熱炭素還元

抄録

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.