Effect of Fluorides on Vapor Phase Growth of Silicon Nitride Whiskers

Abstract

The effect of fluorides on vapor phase growth of Si₃N₄, whiskers was studied on the nitridation of the SiO₂-C-fluorides system in a flow of N₂ gas at 1350 and 1400°C. NaF, AIF8, CaF₂, NH₄F and Na₃AlF₆ were used as fluorides.
The addition of NaF, CaF₂ and Na₃AlF₆ to form fluorosilicate melts under the growth conditions were effective for the growth of SiaN, whiikers, but that of AlF₈, which could not form a melt, were ineffective. The amount of α-Si₃ N₄ whiskers formed at the inside of graphite cylinder tended to decrease, while the amount of β-Si₃N₄ whiskers formed at the outside of it to increase with the increase of the amount of fluorides. The addition of Na.. AlF, or AlF, promoted the growth of β Si₃N₄ and β-Sialon whiskers more preferentially than α-type.
The effects of fluorides on the growth of Si₃N₄ whiskers are estimated as follows: 1) to act as flux and accelerate the reaction of SiO, with C, 2) to evolute Si₃N₄ from fluorosilicate melts and thereby to promote the evolution of SiO gas, 3) to decrease the viscosity of silicate melts and to enhance the evolution of SiO gas.
As previously suggested, SiO gas w as considered to exist as an intermediate species in this vapor phase reaction. It is believed that higher Psio in quasi closed system of graphite cylinder with caps than that in open system without caps is more favourable for nitridation.
It was deduced that Si₃N₄ whiskers grew by direct deposition on the su bstrate through vapor-phase reaction of SiO with N₂ and CO, and/or by the precipitation from the supersaturated solution of SiO, CO and N₂ gas in fluorosilicate melts.