Mechanical activation of monoclinic gibbsite (Al(OH) 3 ) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with ␣-alumina (␣-Al 2 O 3 ) nanocrystallites 2-3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to ␥-Al 2 O 3 and then to ␣-Al 2 O 3 when flash-calcined at a heating rate of 10°C/min in air. ␥-Al 2 O 3 formed at 520°C, and flash calcination to 1100°C led to the formation of an ␣-Al 2 O 3 phase, which exhibited a refined particle size in the range of 100 -200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°-330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to ␣-Al 2 O 3 phase, with both ␦-Al 2 O 3 and -Al 2 O 3 as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4 -0.8 m in size, together with an extensive degree of particle agglomeration.
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