PREPARATION OF ULTRAFINE SILICON-NITRIDE, AND SILICON-NITRIDE AND SILICON-CARBIDE MIXED POWDERS IN A HYBRID PLASMA

Ultrafine Si3N4 and Si3N4+ 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, SiCl4, was injected into an arc jet and completely decomposed in a hybrid plasma, and the second reactant, CH4 and/or NH3, was injected into the tail flame through multistage ring slits. In the case of ultrafine Si3N4 powder synthesis, reaction effieciency increased significantly by multistage injection compared to single‐stage injection. The most striking result is that amorphous Si3N4 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 NH3/SiCl4 molar ratio of ∼ 1.33, although the crystallinity depended on the NH3/SiCl4 molar ratio and the injection method. For the preparation of Si3N4+ SiC mixed powders, the N/C composition ratio and particle size could be controlled not only by regulating the flow rate of the NH3 and CH4 reactant gases and the H2 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. Copyright © 1990, Wiley Blackwell. All rights reserved