Volume expansion during reaction sintering of γ-Bi12SiO20

Preparation of γ-Bi12SiO20 from a compacted mixture of α-Bi2O3 and SiO2 powders is described. A very large volume expansion, related to the phase formation, is observed during heat treatment at 600°C. It is shown that the dedensification results from a preferential diffusion of bismuth and oxygen ions towards SiO2, through the layer of γ-Bi12SiO20 which forms around a silica grain. The expansion begins when γ-Bi12SiO20 grains form a continuous skeleton. When bismuth oxide grains are isolated in the skeleton, expansion and reaction rates are proportional. A quantitative model is proposed to describe this situation assuming an isotropic matter transfer and no coalescence between the γ-Bi12SiO20 grains.