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.
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