EFFECT OF GAS-PHASE ON SIC AND SI3N4 FORMATIONS FROM SIO2

During the synthesis of SiC, Si3N4 and sialon whiskers by carbothermal reduction of SiO2, a localized formation of amorphous phases or Si2N2O powders was observed beneath these whiskers. Because these whiskers were formed by the vapour/solid mechanism, the controlling gas phase was of primary importance to obtain whiskers of tailored morphology and chemistry. To elucidate the effect of the gas phase composition on the reaction mechanisms of SiC and Si3N4, the oxygen partial pressure was measured during the synthesis with a ZrO2 solid electrolyte. The carbothermal reduction of SiO2, as well as evolution of gases, were accelerated by a formation of a molten fluorosilicate with an auxiliary halide bath. The oxygen partial pressure steadily increased with increasing temperature and reached a maximum level of 10(-11)-10(-12) atm in the early stage of reaction at 1623 K, then decreased again towards the end of reaction in both cases. Effects of the gas phase on the SiC and Si3N4 formations were not the same: p(CO) and p(CO2) and their ratio were important factors in the SiC formation, while the higher p(OO2) formed an oxynitride phase in the Si3N4 formation.