Carbothermal reduction and nitridation of silica: nuclei planar growth controlled by silicon monoxide diffusion on the reducer surface

Abstract

The carbothermal reduction and nitridation of silica as a source of low-cost silicon nitride powders prepared from cheap raw materials, lamp black and fumed silica, was investigated in the temperature range of 1200–1450°C. Carbon and silica with a specific surface area of 27 and 176 m 2 g −1, respectively, were wet mixed to form gels. The degree of mixing of carbon and silica, as given by microscopic observation, and the Si 3N 4 reaction yield were optimised in the study of a set of mixed gels that were dispersed using water and ethanol as dispersing liquids. The Si 3N 4 reaction yield at 1450°C was further increased to a value of about 96% of the conversion ratio of silica into Si 3N 4 and silica loss as SiO(g) was controlled to a value of below 1% by using lamp black with a specific surface area of 390 m 2 g −1. Quantitative analysis of the reaction kinetics of the carbothermal reduction and nitridation of silica reveals that it is of exponential type, as given by the Johnson–Mehl–Avrami law for processes controlled by fast nucleation followed by diffusion-limited crystal growth. By selecting carbon sources of high specific surface area, the Si 3N 4 nuclei density number is increased, yielding nearly spherical particles of Si 3N 4 powders with sub-micron sizes.