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.