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Producing high-proportion α-Si3N4 powder in a fluidized bed is an advanced method that is low cost and high in production. Traditional investigations on silicon powder nitridation isolated the phase transformation from the nitridation process, which brings difficulties in predicting and controlling the produced α/β phase ratio. In this paper, the silicon nitridation process was related to the final α/β phase ratio, and multiple related parameters were discussed. A thermal gravimetric analysis instrument was used to detect the detailed nitridation process. Silicon powders with different specific surface areas (i.e., diameters) were used at temperatures just below their melting point (1410 °C). The nonsharp interface model was implemented to simulate the nitridation and predict the ratio of α/β-Si3N4. Some key factors on diffusion and reaction were analyzed. The α/β ratios from simulations were highly consistent with those of X-ray diffraction tests. Mechanisms of powder diameter and thermal diffusion on the production of α/β-Si3N4 were summarized.

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The formation of α/β-Si3N4 relates to the reaction process in the direct nitridation method, which can produce cheap ultrafine Si3N4 powder on a large scale in a fluidized bed. Complete nitridation of silicon powder was conducted in a 1 atm nitrogen atmosphere at 1410 °C, which is slightly lower than the silicon melting point of 1414 °C. Comprehensive analyses with thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy show several-stage nitridation kinetics of silicon powder. Compared with previous research projects, more reaction details can be observed. The reaction undergoes evaporation and multiple melting–freezing processes, causing the deviation of the actual reaction curve from the theoretical logarithm curve in a solid–gas reaction. According to heat flow curves and their derivative to time curves, the reaction can be divided into 4 main stages. The repeated melting–freezing phenomenon and the produced α/β-Si3N4 phase were investigated. The increase of particle size increases the liquid–gas and liquid–solid–gas reaction time which causes β-Si3N4 to increase rapidly.