Effect of gas phase on SiC and Si 3 N 4 formations from SiO 2

Abstract

During the synthesis of SiC, Si 3 N 4 and sialon whiskers by carbothermal reduction of SiO 2 , a localized formation of amorphous phases or Si 2 N 2 O 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 Si 3 N 4 , the oxygen partial pressure was measured during the synthesis with a ZrO 2 solid electrolyte. The carbothermal reduction of SiO 2 , 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 Si 3 N 4 formations were not the same: p CO and and their ratio were important factors in the SiC formation, while the higher formed an oxynitride phase in the Si 3 N 4 formation.