Vapor Phase Growth of Si₃N₄ Whisker by Nitridation of the SiO₂-C-Na₃A1F₆ System

Abstract

The vapor phase growth of Si₃N₄ whisker was studied by nitridation of the SiO₂-C-Na₃A1F₆system in flowing N₂ gas at temperatures between 1350 and 1450°C. Si₃N₄ whiskers were grown both at the inside of the graphite sample cylinder with caps and at the outside of it. At the former place, only α-Si₃N₄ whisker grew in the length of average 5 mm with diameters of 0.02∼0.2 μm, while at the latter place, β-Si₃N₄ whisker grew mainly in the length of severa l mm with diameters up to 1.0 μm. The α-type whisker was obtained in a fairly good yield at 0.02∼0.04 molar ratio of NaaAlF, to SiO₂, wherears the O-type whisker was obtained mainly in a high yield at 0.5∼1.0 molar ratio. It was suggested from the thermo dynamical calculation that Si₃N₄ whiskers were produced by the following reaction mechanisms.
It is considered that crylolite (Na₃A1F₆) plays the following roles; 1) to act as a flux and accelerate the reaction of SiO₂ with C, 2) to evolute SiF₄ gas from the inside of fluorosilicate melt and theoreby to promote evolution of SiO gas, 3) to promote the growth of β-type and β'-sialon whiskers.
The α-ty pe whisker obtained in the investigation was identified to have preferentially grown along the directions of [1010] and [1120].