Preparation of hollow alumina microspheres by mechanofusion and ultrasonic spray pyrolysis

Hollow alumina microspheres with various diameters were prepared by using mechanofusion and ultrasonic spray pyrolysis followed by calcination in air. Core/shell-type hybrid particles, polyrnethylmethacryrate (PMMA) microspheres (diameter (d): 10 mum) coated with a layer of alpha-Al2O3 powders (d: ca. 0.2 mum) containing 5 Wt% SiO2 fine powders (d: ca. 0.011 mum), were easily prepared by utilizing the mechanoftision system. Hollow alpha-Al2O3 microspheres of 10-60 mum in diameter were obtained by calcination of the core/shell microparticles at 1600degreesC for 3 h. On the other hand, hollow alumina microspheres of 0.1-5.0 mum in diameter could be prepared by ultrasonic spray pyrolysis (frequency: 1.6 MHz) of an aqueous Al(NO3)(3) solution by adjusting the flow rate of air as a carrier gas and the concentration of Al(NO3)(3). The hollow alumina microspheres as-obtained were metastable delta-Al2O3, but the post-calcination at 1300degreesC for 1 h in air transformed the product into alpha-Al2O3. These two methods are promising for preparation of hollow ceramic microspheres.