Influences of NH4F additive and calcination time on the morphological evolution of α-Al2O3 from a milled γ-Al2O3 precursor

The influences of an NH₄F additive content and calcination time on the morphological evolution of α-Al₂O₃ were investigated by using commercial γ-Al₂O₃ precursor as raw material. The results showed that the morphological evolution of α-Al₂O₃ was highly related to the addition of NH₄F, the additive content, and the calcination time. Square-like α-Al₂O₃ powders with a primary crystal size of ~200 nm were formed from the milled γ-Al₂O₃ precursor with 1 wt.% NH₄F at 1300°C for 2 h, whereas round cake-like, hexagonal platelets, slender-like, and other irregularly shaped α-Al₂O₃ pieces in the size range of 0.2–2 μm were synthesized from the milled γ-Al₂O₃ precursor with 20 wt.% NH₄F addition under the same calcination. Both morphological regularity and particle size distribution of α-Al₂O₃ were greatly facilitated by increasing the calcination time from 2 to 5 h. The addition of NH₄F may lead to an alteration of the α-Al₂O₃ growth process by a change in the scale of the developing microstructure.