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

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 approximately 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.