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 Si H4 and NH3 was studied using a newly developed laminar gas mixing syst em. Thermophoretic sampling enabled monitoring of the formation proces s on a milli-second basis and shows the existence of two synthesis pat hs: the nitridation of Si particles and the sintering of directly form ed Si3N4. The latter route leads to almost stoichiometric alpha- and b eta-Si3N4, with particle sizes up to 150 nm. The Si3N4 particles do no t decompose above the thermodynamic decomposition temperature, probabl y due to kinetic reasons since the residence time is of the order of m illiseconds. (C) 1998 Elsevier Science Limited. All rights reserved.