NANOMETRIC Si-BASED POWDERS BY LASER DRIVEN GAS PHASE REACTIONS.

Powders in the nanometric range (10–50 nm) are synthesized by laser driven reactions. Gaseous reactants are irradiated with a high power cw CO2 laser in a cross flow configuration. Size, crystallinity and chemical composition (SiC, Si3N4 and Si/C/N) are controlled through the choice of experimental conditions. At laboratory scale, a production rate up to 100 g/hr (SiC) is achieved, the experiment being stable for several hours. The powders are heat treated for annealing or sintering. The changes in the structure and morphology are investigated by various methods : BET, IR-spectroscopy, X-ray diffraction, TEM, 29Si MAS-NMR, XPS, EXAFS. The as-formed SiC powders appear as a mixture of fine β-SiC crystallites in an amorphous matrix which appears similar to the environment of an α-SiC polytype. The α/β ratio changes with synthesis conditions and with heating treatments. After pressureless sintering at 2050°C, SiC reaches 97% of the theoretical density.