Effect of Divalent Cation Additives on the γ‐Al ₂ O ₃ ‐to‐α‐Al ₂ O ₃ Phase Transition

Abstract

The effect on the γ‐Al 2 O 3 ‐to‐α‐Al 2 O 3 phase transition of adding divalent cations was investigated by differential thermal analysis, X‐ray diffractometry, and surface‐area measurements. The cations, Cu 2+ , Mn 2+ , Co 2+ , Ni 2+ , Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ , were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu 2+ and Mn 2+ ), (2) those with little or no effect (Co 2+ , Ni 2+ , and Mg 2+ ), and (3) those with a retarding effect (Ca 2+ , Sr 2+ , and Ba 2+ ). The crystalline phase formed by reaction of the additive with γ‐Al 2 O 3 at high temperature was a spinel‐type structure in groups (1) and (2) and a magnetoplumbite‐type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al 3+ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al 3+ and O 2− in γ‐Al 2 O 3 , suppressing the grain growth of γ‐Al 2 O 3 , and retarding the transformation into α‐Al 2 O 3 .