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The control of the γ−α alumina transformation temperature using ionic additives has been the subject of several studies in the past decades. In this sense, the influence of these additives is usually explained considering only kinetic and diffusional parameters. Effects of these impurities on energetics of polymorphism and surface energies are usually disregarded. In this work, the thermodynamic effect of Zr, Mg and Mn on the γ−α transformation is reported on Al2O3 powders prepared by the polymeric precursor method. Adsorption microcalorimetry coupled with high temperature oxide melt solution calorimetry showed that Zr ions dramatically decrease the surface energy of γ-Al2O3, increasing the stability of the transition phase as which has been described to have its stability considerably related to the surface energy. By the same techniques, Mg was shown to increase the thermodynamic stability of γ-alumina, also increasing the transformation temperature. The role of Mg on the control of the transformation temperature, however, was also related to kinetic factors, such as the bulk diffusion decreasing. The effect of Mn on decreasing the transformation temperature, however, was considered to be almost completely related to diffusion parameters, and vacancies generated by the substitution of Al by Mn would be responsible for the increase in bulk diffusion, allowing the system to reach a predetermined critical particle size at lower temperatures. These important conclusions opened new perspectives in the role of additives in the hightemperature processes that should now on be evaluated from both thermodynamic effects and kinetics viewpoints.

Estudo da influência de aditivos na transformação de fase gama-alfa da alumina.