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The thermal decomposition and phase transformations of ammonium aluminum carbonate hydroxide (AACH) to $alpha$-alumina have been studied. Amorphous-, $gamma$-, and $\theta$-alumina were identified as ...intermediate products. A time-temperature-transformation (TTT) diagram for the processes of thermal decomposition and phase transformations in air conditions was determined. The thermal decomposition or transformation of AACH to $alpha$-alumina invariably passes through $\theta$-alumina under any heating conditions. The critical particle size $\theta$-alumina in phase transformation from $\theta$- to $alpha$-alumina was determined by transmission electron microscopy (TEM). Results indicate that the critical particle size $\theta$-alumina in phase transformation from $\theta$- to $alpha$-alumina is about 30nm. To obtain fine $alpha$-alumina powder with particle size of several tens of nanometers we investigated the effect of AACH powder under various atmosphere conditions (e.g., vacuum, hydrogen, sulfurous acid and ammonia gas) on the critical particle size of $\theta$-alumina, in phase transformation from $\theta$- to $alpha$-alumina. The critical size, 30nm, was independent of the thermal decomposition atmosphere. However, the particle size of $alpha$-alumina was dependent on the thermal decomposition atmosphere (60nm in vacuum and hydrogen atmosphere).続きを見る
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