Synthesis and sintering of silicon nitride nano-powders via sodium reduction in liquid ammonia

Nano-sized silicon nitride powders were synthesized through the reduction of silicon tetrachloride (SiCl4) by sodium in liquid ammonia at around −40 °C.The product was in uniform particles of several nanometers, and in the amorphous form. The amorphous structure was stable up to 1200 °C and turned to the crystal structure of α-Si3N4 at 1300 °C, and then to β-Si3N4 structure when heated at temperatures higher than 1450 °C.Via spark plasma sintering (SPS) dense bulks with 97.9% relative density of theoretic value and the grain size of 100–300 nm was successfully fabricated at 1500 °C without any sintering additives.