Molten salt synthesis of α-Al 2O 3 platelets using NaAlO 2 as raw material

Abstract

α-Al 2O 3 platelets were prepared by a molten salt synthesis method when NaAlO 2 was used as raw material. The effects of the stirring rate during the gel preparation, heating temperature, type and addition amount of molten salts, addition of plate-like α-Al 2O 3 seeds, additives such as TiOSO 4 and Na 3PO 4·12H 2O on the morphology of α-Al 2O 3 were studied. High stirring rate during the gel preparation and high heating temperature not only help to restrain the overlapping of α-Al 2O 3 platelets, but also improve the size distribution. When the heating temperature increases to 1200 °C, most of α-Al 2O 3 platelets are hexagonal in its morphology, and the size of platelets becomes relatively uniform. When Na 2SO 4–K 2SO 4 flux is used instead of NaCl–KCl flux, it is easy to obtain α-Al 2O 3 platelets with a big size. When the molar ratio of salt to final Al 2O 3 powders increases to 4:1, most of α-Al 2O 3 platelets are hexagonal, and the overlapping of powders is inhibited. The addition of a small amount of plate-like seeds has a significant effect on the size of α-Al 2O 3 platelets. With the increase of seed amount, the diameter of α-Al 2O 3 platelets tends to decrease. The addition of 5.45 wt.% TiOSO 4 results in the formation of hexagonal α-Al 2O 3 platelets with an average diameter of 5.1 μm and an average thickness of 1.4 μm. Thin α-Al 2O 3 platelets with a discal shape are obtained owing to the co-addition of 0.51 wt.% Na 3PO 4·12H 2O and 3 wt.% TiOSO 4.