LOW-COST HIGH-YIELD SYNTHESIS OF NANOSIZE POWDERS OF SI-BASED NITRIDES AND CARBIDES BY CO2-LASER INDUCED GAS-PHASE REACTIONS

Amorphous Si3N4, Si/N/C and crystalline SiC nano-powders, 5-50 nm in mean diameter, with yields of 40-120 g/h were synthesized via laser induced gas phase reactions of low cost hexamethyldisilazane (HMDS) and NH3 (or H-2). Reaction flame temperatures, which were controlled by the total gas flow rate, greatly influenced powder composition and crystallinity. Amorphous Si3N4 powders (96 wt%) were obtained at 1000 degrees C from HMDS+NH3; beta-SiC (93 wt%) was formed at 1600 degrees C with H-2. Amorphous Si/N/C powders with various N/C ratios (0.1-6) were synthesized at various temperatures. The main impurities in the powders were the organo-ligands such as NHx and CHx (x=1-3) whose existence also resulted in a strong surface absorption of oxygen.