Molten salt synthesis of morphology controlled α-alumina platelets

Abstract

Abstract Controlling the morphology of plate-like α-Al 2 O 3 powders is essential since Al 2 O 3 platelets of different sizes and shapes are needed in numerous applications. In this work, non-aggregated α-Al 2 O 3 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 Al 2 O 3 particles were investigated. Al 2 (SO 4 ) 3 precursor reacts with K 2 SO 4 salt to form K 3 Al(SO 4 ) 3 liquid phase at ~625 °C. The α-Al 2 O 3 seeds generated in-situ from the decomposition of K 3 Al(SO 4 ) 3 can serve as low energy nucleation sites, and thus accelerate complete transformation to α-Al 2 O 3 at much lower temperatures, yielding platelets of ≥20 µm diameter. Conversely, γ-Al 2 O 3 precursor is stable until higher temperatures, resulting in the formation of small hexagonal α-Al 2 O 3 platelets (1–2 µm in diameter) in the K 2 SO 4 molten salt. In addition to alumina precursors, the salt species also strongly affect the morphology of Al 2 O 3 particles. Compared with chloride salts (NaCl and KCl), sulfate salts (K 2 SO 4 and Na 2 SO 4 ) create favorable conditions for growth of higher- aspect-ratio α-Al 2 O 3 hexagonal platelets.