Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Abstract Carbothermal reduction-nitridation method is an effective means for synthesizing Si3N4 powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si3N4 seeds content on the products were studied. It was found that high-purity α-Si3N4 (>99.0 wt%) was synthesized from C/SiO2 = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO2 and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si3N4 increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si3N4. Furthermore, higher nitriding temperature and Si3N4 seeds content both decreased the grain size and increased β-Si3N4 content. The forming mechanism of non-agglomerated and submicron-sized α-Si3N4 was clarified.