Effect of Divalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition |
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Authors: | Kiyoshi Okada Akiyoshi Hattori Taketoshi Taniguchi Akihiko Nukui Rathindra Nath Das |
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Affiliation: | Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Meguro, Tokyo 152-8552, Japan;National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305, Japan;Ceramic Technological Institute, Bharat Heavy Electricals, Limited, Bangalore 560012, India |
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Abstract: | 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 (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 Al3+ and O2? in γ-Al2O3, suppressing the grain growth of γ-Al2O3, and retarding the transformation into α-Al2O3. |
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Keywords: | alumina phase transformation additives |
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