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

During the synthesis of SiC, Si3N 4 and sialon whiskers by carbothermal reduction of SiO 2, a localized formation of amorphous phases or Si2N20 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 Si3N 4, the oxygen partial pressure was measured during the synthesis with a Zr02 solid electrolyte. The carbothermal reduction of Si02, 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 t h e end of reaction in both cases. Effects of the gas phase on the SiC and Si3N 4 formations were not the same: Pco and Pc02 and their ratio were important factors in the SiC formation, while the higher Po02 formed an oxynitride phase in the Si3N 4 formation.