The effect of different anions on the crystallization course of α‐Al 2 O 3 powder in hydrothermal method

Abstract α‐Al 2 O 3 powder with excellent sintering properties plays an irreplaceable role in advanced ceramics. However, the preparation of α‐Al 2 O 3 at low temperature still remains a grand challenge. Herein, aluminum sulfate, aluminum nitrate, and aluminum chloride are used as raw materials for α‐Al 2 O 3 synthesis. In general, the Al 3+ will be hydrolyzed to form AlOOH during the hydrothermal process. But SO 4 2− reacted with Al 3+ and OH − to form Al 4 (OH) 10 SO 4 during the hydrothermal process. During the calcining process, the transition temperatures from AlOOH to α‐Al 2 O 3 were 1100°C, which can be summarized as follows: AlOOH → γ‐Al 2 O 3 → θ‐Al 2 O 3 → α‐Al 2 O 3 . Meanwhile, Al 4 (OH) 10 SO 4 converted into pure α‐Al 2 O 3 at only 1050°C, due to the direct conversion from γ‐Al 2 O 3 to α‐Al 2 O 3 . This spherical α‐Al 2 O 3 was made up by completely uniformed small particles of about 10–20 nm. Finally, an Al 2 O 3 ceramic with 98.07% of theoretical density and .01% water absorption rate was obtained at a temperature of 1600°C. This work offers an intriguing solution toward the development of Al 2 O 3 ‐based advanced ceramics.