Effect of Aluminum Fluoride on Phase Transformation and Morphology Evolution of Alumina

Abstract

In order to verify the formation of gaseous phase fluoride and its influence on the morphological evolution and the growth mechanism of α-Al2O3, the α-phase transformation temperature and morphology of α-Al2O3 powders from the calcination of commercial γ-Al2O3 and Al(OH)3 precursor with the addition of AlF3 additive have been studied through the incorporation of mixed placement experiments, layered placement experiments and separated placement experiments. The formation of gaseous phase fluoride has been experimentally confirmed by means of layered placement experiments and separated placement experiments, especially by the enhancement of the α-phase transformation occurred outside the small crucible in separated placement experiments. AlF3 additive significantly enhances shape anisotropy of α-Al2O3 precursor and lowers the total α-transformation temperature (900°C for Al(OH)3 precursor and 1100°C for γ-Al2O3 precursor ), even though the commercial precursor and AlF3 additive has been completely isolated by small crucible with cover. In addition, different kinds of α-Al2O3 powders are obtained from these various set of experiments, such as hexagonal platelets, irregularly thick flake-like, and spherical-like α-Al2O3 powders with distinct particle size distribution. In particular, the α-Al2O3 grains obtained from Al(OH)3 precursor at the bottom part of the layered placement experiments and the outside part of the separated placement experiment have obvious growth defects, and some tiny holes emerge on the surface of those irregular α-Al2O3 platelets.