The influences of fluorides on the transformation of α-alumina crystals

Abstract

Influence of addition of aluminum fluoride (AlF3) and ammonium fluoride (NH4F) on transformation of α-Al2O3 from boehmite was investigated. Transformation of α-Al2O3 occurred at about 1050°C and 950°C, when boehmite contained AlF3 and NH4F additives respectively, which was 150°C and 300°C lower than that without any additives. The formed α-Al2O3 consisted of clear, hexagonal plate-like particles about 2-3 µm in diameter and 200 nm in thickness. However, the formed hexagonal platelet particles gradually became blunt at edges and decreased in sizes at the higher calcinating temperatures. The transfer of α-Al2O3 morphology was due to the diminution and disappearance of fluorine. The fluoride coated on the surfaces were firstly decomposed and then fluorine entered the lattices of crystals was volatilized. The fluoride with tiny particles attached on the surface had a same chemical bonding of F–Al–O with that in the lattices of crystals.