Preparation of hexagonal micro-sized α-Al2O3 platelets from a milled Al(OH)3 precursor with NH4F and NH4Cl additives

Hexagonal α-Al2O3 platelets with about 1-μm diameter and 0.2-μm thickness are prepared by the calcination of a milled Al(OH)3 precursor with NH4F and NH4Cl additives. The influences of the ball milling treatment and the composite additive on the microstructure of α-Al2O3 micro-powders are studied. The results indicate that the particle size distribution of α-Al2O3 is highly dependent on the aggregate size of the Al(OH)3 precursor and that the morphology of α-Al2O3 can be significantly favored by the addition of 5 wt.% NH4F and 5 wt.% NH4Cl as additives. The growth velocity of the (0001) plane is significantly reduced because of the accelerated growth rate of other crystal faces caused by the increase of gas phase mass transfer. The combined effect of composite additives and precursor ball milling treatment leads to the generation of well-developed α-Al2O3 platelets with uniform size distribution.