Fabrication of fine α-alumina powders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) Original Research Article

In this paper, the phase transformations that occur during the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) into α-Al2O3 have been studied. Amorphous, γ-, and θ-Al2O3 were identified as intermediate products. 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 is ∼30 nm; the size does not depend on the firing atmosphere. However, the atmosphere affects the grain size distribution of the final α-Al2O3 particles. Powders with an average grain size of 60 nm were obtained by firing in vacuum or hydrogen.