Fabrication and synthesis of α-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH)

An α-Al2O3 nanopowder was prepared via the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). The AACH precursor, which had an average crystallite size of 5–8 nm, was fabricated at a reaction temperature of 8 °C with the pH of the aqueous ammonium hydrogen carbonate (AHC) precursor solution set to 10, which afforded the highest [NH4+][AlO(OH)n(SO4)−3−n/2]][HCO2−] ionic concentration. The AACH precursor was transformed to NH4Al(SO4)2, rhombohedral (Al2(SO4)3), amorphous-, θ-, and α-Al2O3 according to the heat-treatment temperature. A time-temperature-transformation (TTT) diagram for thermal decomposition in air was determined. The critical particle size of θ-Al2O3 in the θ → α phase transformation was ∼30 nm. Homogeneous, spherical α-Al2O3 nanopowders with an average particle size of 70 nm were prepared by firing the precursor crystallites at 1150 °C for 3 h in air.