Abstract
Abstract In this paper, spherical, smooth and unagglomerated ultrafine amorphous powder particles were prepared by ultrasonic spray pyrolysis (USP) of easy-handling aqueous aluminum nitrate salts increasing the precursor solute concentration to 0.5 mol L −1 and reducing the pyrolysis temperature to 700 °C. The transformation of the USP alumina powders into α-Al 2 O 3 was studied using combination of X-ray diffraction, electron microscopy, infrared spectroscopy, BET surface area, thermogravimetry and differential thermal analysis. A downward shift of the onset temperature of α-phase transformation to 900 °C has been detected using a larger precursor solution concentration and performing a milling before calcination due to an increase in the surface density of defects, in surface area and in anisotropic particle shape. Additional post-milling of the low calcined powders allowed the preparation of agglomerate-free pure ultrafine α-Al 2 O 3 powder particles (∼100 nm, 28 m 2 g −1 ), free of vermicular microstructures.
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