Abstract
We studied Combustion Synthesis （CS） of Si3N4 from a mixture of Si powder with/without NaCl and Si3N4 as diluent at pressurized nitrogen. The effect of premixing before CS on reaction temperature, α-Si3N4 content, and grain shapes of CSed products was mainly examined. As a result, with prolonging premixing time, α-Si3N4 content of the products increased to 84％ under the conditions of 30 mass％ NaCl additive and 120 min premixing. As the premixing time prolonged, the reaction temperature also increased slightly and the particle size of product decreased obviously. From these results, the effect mechanism of NaCl on the CS reaction was finally derived, in which the longer premixing time caused larger contact area between Si and NaCl as the raw materials. When the surface of Si particle is highly coated by NaCl, α-Si3N4 can be synthesized firstly due to the lower reaction temperature caused by the melting and evaporation of NaCl. Therefore, the increase of premix time improves the dispersion of NaCl on Si Surface, thereby restrains the phase transformation of α-Si3N4 to β-Si3N4 and meanwhile restricts the grain growth occurring at high temperature period.
