Effects of AlF₃ and calcination temperature on preparation of hexagonal-shaped α-Al₂O₃

A hexagonal α-Al₂O₃ with good crystallinity and regular morphology was prepared at lower calcination temperature by using boehmite (AlOOH) as precursor and AlF₃ seed as additive.The possible mechanisms were investigated by DSC-TG-MS analysis under the condition of AlF₃ seed coefficient 2%, which indicates that a transition compound (Al-O-F) promotes the nucleation phase transition of α-Al₂O₃ through gas-solid reaction, and releases HF simultaneously. Furthermore, the effects of AlF₃ coefficient and calcination temperature (T) on the phase, morphology, size, and specific surface area were investigated. Results show that light AlF₃ seeds can enhance the crystallinity of α-Al₂O₃. When the AlF₃ coefficient increases from 0.5% to 10%, the hexagonal-shaped structure becomes clear, but its size increases and its specific surface area decreases. Boehmite can completely change into high-crystallinity, hexagonal-shaped α-Al₂O₃ under the best condition of T=850℃, AlF₃ seed coefficient 2%, and constant calcination time of 4 h.