Self-Propagating High-Temperature Synthesis of Silicon Carbide and Silicon Nitride Nanopowders Composition using Sodium Azide and Halides

Abstract

Regularities of self-propagating high-temperature synthesis (SHS) or combustion synthesis (CS) by using “silicon – sodium azide – ammonium hexafluorosilicate – carbon – aluminum” powder mixture in the nitrogen atmosphere were investigated. The thermodynamic analysis of the combustion synthesis was performed. Experimental investigation of the combustion process: the measurement of linear rates of the combustion front propagation and the maximum combustion temperatures was conducted in a laboratory reactor with working volume 4.5 liters. The influence of the components ratio in the initial mixture on the combustion temperature, combustion rate and composition of reaction product was studied. The phase composition of the product synthesized was determined with an X-ray diffractometer. It was disclosed that the SHS product consists of the composition (mixture) of silicon carbide nanopowder with silicon nitride whiskers and a final halide. Investigation of surface topography and morphology of the product particles was carried out with a scanning electron microscope. Optimal mixture for the synthesis of nanoscale composition based on silicon carbide was determined: “14Si+6NaN₃+(NH₄)2SiF₆+15C+Al”. In this case, the SHS product consists of four phases: silicon carbide (β-SiC) – 48.57 wt.%, α-silicon nitride (α-Si₃N₄) – 27.04 wt.%, β-silicon nitride (β-Si₃N₄) – 5.83 wt.%, and sodium hexafluoroaluminate (Na₃AlF₆) – 18.56 wt.%. The average particle size of the composition was in the range of 70–130 nm. It was shown that the composition of the silicon carbide with silicon nitride and the final halide Na3AlF6 playing a role a flux can be used as a modifier of castable aluminum alloys and as a reinforcing phase of aluminomatrix composites.