Influence of Addition of AlF₃ on Thermal Decomposition of Gibbsite and Phase Transition of the Intermediate Alumina to α-Al₂O₃

Abstract

The addition of aluminum fluoride to gibbsite retarded the phase change to boehmite by chemisorption of hydrogen fluoride on gibbsite particles. In the specimen with addition of aluminum fluoride, the spinel type phase transformed directly to α-Al₂O₃, without passing through κ- and θ-Al₂O₃ phases. The formed α-Al₂O₃ consisted of clear, hexagonal plate-like particles about 1 μm, Without taking over a relic of the gibbsite particle. The formation of automorphic crystals suggests that a surface diffusion also contributes to the mechanism of this material transport in addition to the vapor phase transport.