Critical parameters of Si conversion into Si3N4 nanophase powder for plasma process

Paper presents the results of plasma synthesis of Si3N4 nanophase powder in a DC nitrogen thermal plasma. Silicon powder (particle diameter range 0.1 divided by 60 mu m, mean mass particle diameter 13.6 mu m, specific surface area 1.8 m(2)/g) as a precursor was injected (mass flow rate 0.06 divided by 0.12 g/s) at the exit of DC plasma torch (electric power level 78 divided by 85 kW; nitrogen mass flow rate 4.5 divided by 6.2 gis, mean mass exit plasma temperature 5540 divided by 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 divided by 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 divided by 30 nm (TEM analysis) and specific surface area in the range 41.9 divided by 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 divided by 73.4 mas%) with the following crystal fraction composition: alpha-Si3N4 48 divided by 77 mas%, beta-Si3N4 20 divided by 39 mas%, and free silicon 2.5 divided by 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.