Molten salt synthesis of morphology controlled α-alumina platelets

Controlling the morphology of plate-like α-Al₂O₃ powders is essential since Al₂O₃ platelets of different sizes and shapes are needed in numerous applications. In this work, non-aggregated α-Al₂O₃ single-crystal platelets with diameters ranging from ~1 µm to more than 20 µm and thicknesses from ~0.1 µm to 1.3 µm were produced by molten salt synthesis. The effects of alumina precursor and molten salt compositions on phase formation and morphology development of Al₂O₃ particles were investigated. Al₂(SO₄)₃ precursor reacts with K₂SO₄ salt to form K₃Al(SO₄)₃ liquid phase at ~625 °C. The α-Al₂O₃ seeds generated in-situ from the decomposition of K₃Al(SO₄)₃ can serve as low energy nucleation sites, and thus accelerate complete transformation to α-Al₂O₃ at much lower temperatures, yielding platelets of ≥20 µm diameter. Conversely, γ-Al₂O₃ precursor is stable until higher temperatures, resulting in the formation of small hexagonal α-Al₂O₃ platelets (1–2 µm in diameter) in the K₂SO₄ molten salt. In addition to alumina precursors, the salt species also strongly affect the morphology of Al₂O₃ particles. Compared with chloride salts (NaCl and KCl), sulfate salts (K₂SO₄ and Na₂SO₄) create favorable conditions for growth of higher- aspect-ratio α-Al₂O₃ hexagonal platelets.