The influence of combustion synthesis conditions on the α-Al2O3 powder preparation
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Fuel type and fuel/aluminium nitrate molar ratio proved to be of great importance during the preparation of α-Al2O3 powders. A stoichiometric amount of urea (U) enabled the formation of α-Al2O3 with a surface area of 24 m2/g directly from the combustion reaction. Monoethanolamine, triethylenetetramine, trishydroxymethylaminomethane, and triethanolamine yield amorphous powders. This behaviour was explained by the reaction mechanism, which requires the simultaneous decomposition of metal nitrate and fuel, as shown by thermal analysis. The use of 50% of the stoichiometric amount of U was unable to trigger a combustion reaction. The resulting powder was amorphous and had a surface area of 424 m2/g. A parabolic correlation between the surface area of combustion-synthesized powder and the U/aluminium nitrate molar ratio was found. Due to U consumption during the hydrolysis side-reaction, 50% of U excess above the stoichiometric ratio is required in order to maximize the exothermic effect of the combustion reaction. The use of U excess higher than 150% of the stoichiometric ratio not only increases the surface area of the powder, but also changes the phase composition: as the U excess increases the proportion of α-Al2O3 decreases and the amount of γ-Al2O3 increases.