Controllable Preparation of Si3N4@MgSiN2 Core–Shell Powders via a “Template Growth” Mechanism in NaCl-KCl Mixed Molten Salt

Si3N4@MgSiN2 composite powder with a core–shell structure was successfully synthesized via the in situ reaction between Mg and α-Si3N4 using a NaCl–KCl mixed molten salt in this study. The effects of process parameters, including the molten salt system, reaction temperature, and Mg/Si3N4 mass ratio, on the morphology, phase composition, and microstructure of the coating layer were investigated. The results indicate that the reaction follows a “template growth” mechanism. Mg-containing species dissolve in the molten salt, diffuse to the surface of Si3N4 particles, and react with α-Si3N4, resulting in a relatively uniform MgSiN2 layer at 1300 °C. The yield of MgSiN2 layer exhibits a linear positive correlation with the Mg/Si3N4 mass ratio, enabling controllable microstructural regulation through adjustment of the starting materials composition. The core–shell powder forms a liquid phase at a relatively low temperature (approximately 1350 °C), demonstrating excellent sintering activity. This work provides a new material foundation for the fabrication of silicon nitride ceramics with high thermal conductivity.