Fuel type and fuel/aluminium nitrate molar ratio proved to be of great importance during the prepa_ration of α-Al_2O_3 powders. A stoichiometric amount of urea (U) enabled the formation of α-Al_2O_3 with a surface area of 24 m~2/g directly from the combustion reaction. Monoethanolamine, triethylenetetramine, trishydroxyme_thylaminomethane, and triethanolamine yield amorphous powders. This behaviour was explained by the reaction mechanism, which requires the simultaneous decomposi_tion 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 m~2/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-Al_2O_3 decreases and the amount of у-Al_2O_3 increases.
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