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

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 degrees 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 Si3N4 reaction yield were optimised in the study of a set of mixed gels that were dispersed using water and ethanol as dispersing liquids. The Si3N4 reaction yield at 1450 degrees C was further increased to a value of about 96% of the conversion ratio of silica into Si3N4 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 Si3N4 nuclei density number is increased, yielding nearly spherical particles of Si3N4 powders with sub-micron sizes. (C) 1999 Elsevier Science S.A. All rights reserved.