Synthesis of Nanocrystalline α‐Al2O3 Powders from Nanometric Ammonium Aluminum Carbonate Hydroxide

Abstract

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‐Al2O3 seeding on the transformation kinetics were investigated. The phase transformation sequence of AACH on heating was found to be AACH → amorphous Al2O3→γ‐Al2O3→θ‐Al2O3→α‐Al2O3, where the formation temperatures of θ‐Al2O3 and α‐Al2O3 are 850° and 1050°C, respectively. The phase transformation sequence on heating of the AACH seeded with 5 wt%α‐Al2O3 seed crystals 100 nm in diameter was found to be AACH → amorphous Al2O3→θ‐Al2O3→α‐Al2O3. γ‐Al2O3 was not observed, and the θ‐Al2O3→α‐Al2O3 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‐Al2O3 (i.e., about 1200°, 1100°, and 900°C, respectively), α‐Al2O3 powders have been obtained with mean particle sizes of about 150, 70, and 30 nm, respectively. The α‐Al2O3 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‐Al2O3 obtained by calcining γ‐AlOOH, AACH, and the seeded AACH are 74.3%, 95.0%, and 98.5%, respectively.