Synthesis of Nanocrystalline α‐Al 2 O 3 Powders from Nanometric Ammonium Aluminum Carbonate Hydroxide

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 α‐Al 2 O 3 seeding on the transformation kinetics were investigated. The phase transformation sequence of AACH on heating was found to be AACH → amorphous Al 2 O 3 →γ‐Al 2 O 3 →θ‐Al 2 O 3 →α‐Al 2 O 3 , where the formation temperatures of θ‐Al 2 O 3 and α‐Al 2 O 3 are 850° and 1050°C, respectively. The phase transformation sequence on heating of the AACH seeded with 5 wt%α‐Al 2 O 3 seed crystals 100 nm in diameter was found to be AACH → amorphous Al 2 O 3 →θ‐Al 2 O 3 →α‐Al 2 O 3 . γ‐Al 2 O 3 was not observed, and the θ‐Al 2 O 3 →α‐Al 2 O 3 transformation temperature was reduced to 900°C by the seeding. By calcining the γ‐AlOOH, the AACH, and the seeded AACH at their lowest temperatures to transform completely to α‐Al 2 O 3 (i.e., about 1200°, 1100°, and 900°C, respectively), α‐Al 2 O 3 powders have been obtained with mean particle sizes of about 150, 70, and 30 nm, respectively. The α‐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 α‐Al 2 O 3 obtained by calcining γ‐AlOOH, AACH, and the seeded AACH are 74.3%, 95.0%, and 98.5%, respectively.