Particle formation paths in the synthesis of silicon nitride powder in a laser-heated aerosol reactor

The Laser-chemical vapour precipitation of Si3N4 powder by reacting SiH4 and NH3 was studied using a newly developed laminar gas mixing system. Thermophoretic sampling enabled monitoring of the formation process on a milli-second basis and shows the existence of two synthesis paths: the nitridation of Si particles and the sintering of directly formed Si3N4. The latter route leads to almost stoichiometric alpha- and beta-Si3N4, with particle sizes up to 150 nm. The Si3N4 particles do not decompose above the thermodynamic decomposition temperature, probably due to kinetic reasons since the residence time is of the order of milliseconds. (C) 1998 Elsevier Science Limited. All rights reserved.