Influence of AlF3 and hydrothermal conditions on morphologies of α-Al2O3

Homogeneous α-Al2O3 platelets were synthesized by introducing AlF3 to alumina precursor. The effects of AlF3 additive on the phase transformation and morphology of the prepared α-Al2O3 platelets were investigated. The results show that a single phase of α-Al2O3 with an average particle size of 8 μm can be obtained at 900 °C with 2% AlF3 additive. The transformation temperature decreasing is attributed to introduction of Al3+ vacancy and to the formation of intermediate compound of AlOF, which is considered to accelerate the mass transportation from transitional Al2O3 to α-Al2O3. AlF3 concentration and hydrothermal temperature can also affect the morphology of α-Al2O3. When hydrothermal temperature is 120 °C, the morphology of α-Al2O3 transforms from irregular to flat hexangular platelet with increasing AlF3 concentration. As hydrothermal temperature increases, the morphology of α-Al2O3 with 2% AlF3 additive changes from polyhedron to hexangular platelet and then to vermicular.