Influence of Cr and Fe on Formation of α‐Al ₂ O ₃ from γ‐Al ₂ O ₃

Abstract

The effect of Cr and Fe in solid solution in γ‐Al 2 O 3 on its rate of conversion to α‐Al 2 O 3 at 1100°C was studied by X‐ray diffraction. The δ form of Al 2 O 3 was the principal intermediate phase produced from both pure γ‐Al 2 O 3 and that containing Fe 3+ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr 6+ in γ‐Al 2 O 3 and as Cr 3+ in α‐Al 2 O 3 , with θ‐Al 2 O 3 as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α‐Al 2 O 3 were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α‐Al 2 O 3 formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ‐ or θ‐Al 2 O 3 , within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O 2‐ ions by synchro‐shear.