A flexible route to high strength α-alumina and aluminate spheres

The formation of hollow alumina spheres is accomplished by coating polystyrene beads of 3 mum and 50-80 mum diameter with carboxylic acid functionalized alumina nanoparticles (alumoxanes) from aqueous solution 2-8 wt%. The resulting coated beads were heated to 220 degreesC to calcine the alumoxane to porous amorphous alumina before washing with toluene to remove the polystyrene from inside the ceramic coating. The resulting hollow spheres were sintered at 1000 degreesC to form alpha-alumina. The alpha-alumina spheres have been characterized, by SEM (scanning electron microscopy), BET, and hardness measurements, that show the hardness of the hollow alumina sphere (1900 +/- 100 Kg.mm(-2)) approaches that of corundum (ca. 2000 Kg.mm(-2)). Multilayered bi-phasic spheres may be prepared by subsequent coating the alpha-alumina spheres with a solution of a metal-doped alumoxane. After calcining, the mixed metal oxide phase (CaAl12O19, Er6Al10O24, MgAl2O4, Al2TiO5, and Y3Al5O12) forms outside of the alumina sphere resulting in a composite like ceramic bi-layer sphere. Pre-formed hollow alumina spheres were incorporated into a resin and ceramic thin film formed from a 1 wt% A-alumoxane aqueous solution. The hardness of the composites is compared to the matrix materials themselves. (C) 2003 Kluwer Academic Publishers.