Synthesis of γ-Al2O3 nanopowders by freeze-drying

This work studies the synthesis of γ-Al2O3 nanopowders by a freeze-drying method. Aqueous solutions of Al2(SO4)3·18H2O were used as precursors to Al concentrations of 0.76, 1.00 and 1.40 M. Homogeneous spherical granules with diameters ranging from 1 to 100 μm have been obtained. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm. The microstructure of the agglomerates largely depends on the freezing kinetics. After drying amorphous aluminium sulphate powder is obtained that decomposes at 825 °C leading to the formation of γ-Al2O3. Physicochemical study of the freeze-dried powders is performed through particle size distribution and zeta potential measurements. The characterisation of the powders is evaluated considering the influence of processing parameters such as the salt concentration, the freezing rate and the thermal treatment for the synthesis and the dispersing conditions of the obtained powders. By adjusting the dispersing conditions a minimum particle size <30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles.