Influence of particles size on thermal properties of aluminium powder

The microstructure, chemistry and morphology evolution of aluminium particles of different size are followed during specific thermal treatments under inert or oxidative atmospheres by electron microscopy, X-ray diffraction, differential scanning calorimetry and gas adsorption. Under inert conditions the crystallization of the alumina layer occurs before the melting of the aluminium core. Under oxidative conditions, it is proposed that the oxidation rate is limited by the diffusion of oxygen through the amorphous alumina layer and depends on the particle size, leading to either successive or simultaneous melting of the aluminium core and crystallization of the amorphous layer. Despite the higher reactivity of the powder with smaller particle size, the thickness of the alumina layer is rather independent of the particle size at the end of the first oxidation step, probably because the oxygen diffusion is slowed down by crystallized alumina. Under oxidative conditions, the coalescence of particle aluminium cores is reduced in comparison with inert conditions and this is due to a thicker oxide layer. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.