EFFECT OF IMPURITIES ON THE FORMATION OF SILICON-NITRIDE BY CARBOTHERMAL REDUCTION-NITRIDATION OF FINE HYDRATED SILICA POWDERS

Through the simultaneous carbothermal reduction-nitridation(CRN) of fine hydrated silica powders at 1,450 degrees C for 7 hours, the single phase of alpha-silicon nitride could be synthesized from pure silica and pure carbon black which was treated with hydrochloric acid. When as-received carbon black was used, a constant alpha/beta-phase ratio equivalent to approximate to 9.0 was obtained by heat treatment at 1,430 degrees C for 10 hours under N-2/5%H-2 atmosphere, remaining constant, regardless of the prolonged reaction time. To investigate the effects of the impurities on the formation of Si3N4, various oxides were added to the refined carbon black individually. alpha-Si3N4 seed accelerated the alpha-phase formation, and beta-Si3N4 was formed in the presence of V2O5 and CaO, while beta-SiC appeared in the presence of Fe2O3, NiO and Cr2O3 respectively.