Direct Nitridation Synthesis of Quasi-Spherical β-Si₃N₄ Powders with CaF₂ Additive

In this work, the quasi-spherical β-Si₃N₄ powders were synthesized via an efficient direct nitridation strategy with CaF₂ as the catalytic material under NH₃ atmosphere. The effect of CaF₂ on phase composition and crystalline morphology was studied. CaF₂ additive can accelerate the nitridation of silicon powders, and the particles of nitridation products tend to have an equiaxed structure with the CaF₂ additive increasing. When 4 wt% CaF₂ additive or more was added, submicron β-Si₃N₄ particles with quasi-spherical morphology and eminent crystal integrity were obtained. In contrast, irregular α-Si₃N₄ particles appear as the main phase with less than 4 wt% CaF₂ additive. The growth mechanism of Si₃N₄ particles was also discussed. CaxSiyOz liquid phase is crucial in the nitridation of silicon powders with CaF₂ additive.