Nanometric ammonium aluminum carbonate hydroxide (AACH) with a particle size less than 5 nm was produced from ammonium aluminum sulfate and ammonium hydrocarbonate under an optimized adding condition, and phase transformations of the AACH on heating and the effect of ␣-Al 2 O 3 seeding on the transformation kinetics were investigated. The phase transformation sequence of AACH on heating was found to be AACH 3 amorphous , where the formation temperatures of -Al 2 O 3 and ␣-Al 2 O 3 are 850°and 1050°C, respectively. The phase transformation sequence on heating of the AACH seeded with 5 wt% ␣-Al 2 O 3 seed crystals 100 nm in diameter was found to be AACH 3 amorphous was not observed, and the -Al 2 O 3 3 ␣-Al 2 O 3 transformation temperature was reduced to 900°C by the seeding. By calcining the ␥-AlOOH, the AACH, and the seeded AACH at their lowest temperatures to transform completely to ␣ -Al 2 O 3 (i.e., about 1200°, 1100°, and 900°C, respectively), ␣-Al 2 O 3 powders have been obtained with mean particle sizes of about 150, 70, and 30 nm, respectively. The ␣-Al 2 O 3 powder produced from the seeded AACH has the highest sintering reactivity. After sintering at 1400°C for 2 h, relative densities of the powder compacts of ␣-Al 2 O 3 obtained by calcining ␥-AlOOH, AACH, and the seeded AACH are 74.3%, 95.0%, and 98.5%, respectively.
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