Effect of divalent cation additives on the γ-Al2O3-to-α-Al2O3 phase transition

The effect on the γ-Al2O3-to-α-Al2O3 phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu2+, Mn2+, Co2+, Ni2+, Mg2+, Ca2+, Sr2+, and Ba2+, 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 (Cu2+ and Mn2+), (2) those with little or no effect (Co2+, Ni2+, and Mg2+), and (3) those with a retarding effect (Ca2+, Sr2+, and Ba2+). The crystalline phase formed by reaction of the additive with γ-Al2O3 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 Al3+ and the additive (Δr): The transition temperature increased as Δr increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al3+ and O2- in γ-Al2O3, suppressing the grain growth of γ-Al2O3, and retarding the transformation into α-Al2O3.