Volume expansion during reaction sintering of gamma-Bi12SiO20

Preparation of gamma-Bi12SiO20 from a compacted mixture of alpha-Bi2O3 and SiO2 powders is described. A very large volume expansion, related to the ph ase formation, is observed during heat treatment at 600 degrees C. It is sh own that the dedensification results from a preferential diffusion of bismu th and oxygen ions towards SiO2, through the layer of gamma-Bi12SiO20 which forms around a silica grain. The expansion begins when gamma-Bi12SiO20 gra ins form a continuous skeleton. When bismuth oxide grains are isolated in t he skeleton, expansion and reaction rates are proportional. A quantitative model is proposed to describe this situation assuming an isotropic matter t ransfer and no coalescence between the gamma-Bi12SiO20 grains. (C) 1999 Els evier Science Ltd. All rights reserved.