The formation of ultrafine Al2O3, ZrO2 and Fe2O3 by the spray-ICP technique

Abstract Ultrafine aluminium oxide, zirconium oxide and iron oxide have been prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultra-high temperature (the spray-ICP technique). Their particles are roundish, 0.01–0.02 μm in diameter, and homogeneous both in size and in shape. Their major phases are γ-Al 2 O 3 , metastable tetragonal-ZrO 2 and γ-Fe 2 O 3 , respectively. These oxides appear to be formed through the following two reaction processes, (1) the complete decomposition of the droplets of the solution to their component atoms in the ICP flame, (2) the homogeneous nucleation and the successive crystal growth occurring outside the ICP flame. In the process (2), quenching is so rapid that the crystals grown after the nucleation do not transform to high temperature phases, which seems to cause the formation of the oxide of low temperature phase.