Dejan Cvetinović

Critical Parameters of Si Conversion into Si3N4 Nanophase Powder for Plasma Process

Paper presents the results of plasma synthesis of Si 3 N 4 nanophase powder in a DC nitrogen thermal plasma. Silicon powder (particle diameter range 0.1+60 μm, mean mass particle diameter 13.6 μm, specific surface area 1.8 m 2 /g) as a precursor was injected (mass flow rate 0.06+0.12 g/s) at the exit of DC plasma torch (electric power level 78+85 kW, nitrogen mass flow rate 4.5+6.2 g/s, mean mass exit plasma temperature 5540+5950 K) and evaporated in the reactor. At the exit of evaporation section. Si-N mixture was quenched with ammonia (mass flow rate 1.45+1.87 g/s). Ultrafine ivory gray powder was deposited on cold walls of experimental installation and collected in a bag filter. The powder consists of agglomerated spherical particles of diameter 20+30 nn (TEM analysis) and specific surface area in the range 41.9+66.2 m 2 /g (B.E.T. multipoint method). X-ray diffraction pattern analysis indicates that the powder as synthesized is mostly amorphous (52.6+73.4 mas%) with the following crystal fraction composition: α-Si 3 N 4 48+77 mas%, β-Si 3 N 4 20+39 mas%, and free silicon 2.5+13.9 mas%. To analyse the experimental results, numerical simulation of two phase (nitrogen - Si particles) turbulent flow in plasma reactor and thermodynamic analysis of the system silicon - hydrogen -nitrogen were made. Analyse have shown that the mass fraction of free silicon in the synthesized powder depends on: 1) the concentration of unevaporated Si particles in the Si-N mixture before quenching, and 2) the excess of ammonia for the quenching. i.e. the value of the equilibrium temperature of the system after quenching with ammonia.