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The influences of an NH 4 F additive content and calcination time on the morphological evolution of α-Al 2 O 3  were investigated by using commercial γ-Al 2 O 3  precursor as raw material. The results showed that the morphological evolution of α-Al 2 O 3  was highly related to the addition of NH 4 F, the additive content, and the calcination time. Square-like α-Al 2 O 3  powders with a primary crystal size of ~200 nm were formed from the milled γ-Al 2 O 3  precursor with 1 wt.% NH 4 F at 1300°C for 2 h, whereas round cake-like, hexagonal platelets, slender-like, and other irregularly shaped α-Al 2 O 3  pieces in the size range of 0.2–2 μm were synthesized from the milled γ-Al 2 O 3  precursor with 20 wt.% NH 4 F addition under the same calcination. Both morphological regularity and particle size distribution of α-Al 2 O 3  were greatly facilitated by increasing the calcination time from 2 to 5 h. The addition of NH 4 F may lead to an alteration of the α-Al 2 O 3  growth process by a change in the scale of the developing microstructure.

Influences of NH 4 F additive and calcination time on the morphological evolution of α-Al 2 O 3 from a milled γ-Al 2 O 3 precursor