Combustion synthesis of Si3N4 powders with high sintering activity under low nitrogen pressures

In this study, silicon nitride (Si₃N₄) powders with high α-phase content and rod-like crystal morphologies were synthesized via self-propagating high-temperature synthesis (SHS) in a low-pressure nitrogen atmosphere (0.2 MPa), using silicon and Si₃N₄ powders as reactants, with magnesium powder serving as an igniter. The influence of different ratios of silicon to silicon nitride on the phase composition, crystal morphology, and sintering activity of the synthesized Si₃N₄ powders was systematically investigated. The results demonstrated that controlling the silicon-to-silicon nitride ratio under low-pressure conditions significantly affects the formation of α-phase Si₃N₄ and the resulting microstructure of the powders. A low-pressure nitrogen atmosphere was adopted based on kinetic and thermodynamic analyses, providing a feasible environment for SHS. Notably, in situ formation of magnesium nitride (MgSiN₂) during the reaction not only acted as an effective sintering aid but also improved the thermal conductivity of the resulting Si₃N₄ ceramic. The powder produced from a reaction using a raw material composition of 65 % Si minimizes both silicon infiltration and melting to the greatest extent, and consequently, the sintered silicon nitride ceramics exhibit the highest thermal conductivity. The low-pressure environment effectively moderated the reaction rate and temperature, promoting the formation of fine α-phase Si₃N₄ particles with rod-like morphologies. This innovative method offers a cost-effective and environmentally friendly approach for producing high-quality Si₃N₄ powders with high sintering activity.