Synthesis of Si3N4 powder with tunable α/β-Si3N4 content from waste silica fume using carbothermal reduction nitridation

Silicon nitride (Si3N4) powder with tunable α/β-Si3N4 relative contents was synthesized from waste silica fume using carbothermal reduction nitridation (CRN) at a relatively low temperature range of 1400–1500 °C. The effects of the carbon source, CRN temperature and raw materials ratio (Carbon-to-Silica fume) on phase assembly and micro-morphology of the products were investigated. Fabrication of α-Si3N4/β-Si3N4 from waste silica fume by CRN was achieved at temperatures as low as 1400 °C. Carbon black was found to be more suitable than coke and graphite for use as the reductant. Elevated temperature was beneficial for the formation of β-Si3N4, whilst excessive carbon black promoted the formation of α-Si3N4. Thus, the α-Si3N4/β-Si3N4 relative contents could be effectively tailored by adjusting the CRN temperature and carbon concentration of the initial mixtures. In addition, the bi-modal crystals of the whiskers and the short columns strongly influenced the morphologies of the product.