Preparation of Ultrafine Silicon Nitride, and Silicon Nitride and Silicon Carbide Mixed Powders in a Hybrid Plasma

Abstract

Ultrafine Si 3 N 4 and Si 3 N 4 + SiC mixed powders were synthesized through thermal plasma chemical vapor deposition (CVD) using a hybrid plasma which was characterized by the superposition of a radio‐frequency plasma and an arc jet. The reactant, SiCl 4 , was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH 4 and/or NH 3 , was injected into the tail flame through multistage ring slits. In the case of ultrafine Si 3 N 4 powder synthesis, reaction effieciency increased significantly by multistage injection compared to single‐stage injection. The most striking result is that amorphous Si 3 N 4 with a nitrogen content of about 37 wt% and a particle size of 10 to 30 nm could be prepared successfully even at the theoretical NH 3 /SiCl 4 molar ratio of ∼ 1.33, although the crystallinity depended on the NH 3 /SiCl 4 molar ratio and the injection method. For the preparation of Si 3 N 4 + SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH 3 and CH 4 reactant gases and the H 2 quenching gas, but also by adjusting the reaction space. The results of this study provide sufficient evidence to suggest that multistage injection is very effective for regulating the condensation process of fine particles in a plasma tail flame.