Synthesis of near-spherical α-Si3N4 powder via carbon-assisted crystallization of amorphous Si3N4 powder

This study presents a scalable method for fabricating near-spherical silicon nitride (Si₃N₄) particles with a high α phase content. Amorphous Si₃N₄ powder was synthesized using the diimide synthesis method, mixed with graphite powder as a carbon source, and calcined at 1550°C to produce α-Si₃N₄ powder. The effects of varying carbon contents on the phase composition and microstructure of the resulting products were investigated. In the presence of carbon, the Si₃N₄ particles exhibit a near-spherical morphology, with a reduction in the fraction of whisker-like structures. The products show a complex, nonuniform structure with a broad particle size distribution and varying structural characteristics when excessive powder is added. Graphite powder not only provides nucleation sites but also plays a critical role in the phase transition from amorphous Si₃N₄ to the α phase. The nucleation and growth of α-Si₃N₄ are facilitated by the presence of SiO(g), which is reduced through interactions with C(s) or CO(g). It indicates that carbon accelerates the Si₃N₄ phase transition through the vapor-solid mechanism, which is the key to inhibiting the whisker growth and controlling the particle size of α-Si₃N₄ powder.