Oxidation of fine aluminum particles: thermally induced transformations in particle shells and kinetics of oxide nucleation

This paper investigates the process of oxidation of fine aluminum powder, consisting of spherical Al particles of 'metal core/oxide shell' type, when heated in air at temperatures below 550 degrees C. The highly dispersed aluminum powder 'Alex' used in this work (particle size: 0.05-1.5 mu m and number average particle diameter (DN): 0.11 mu m) was produced by electric explosion of a thin Al wire in argon with subsequent passivation in an oxygen-containing atmosphere. For the first time, the influence of the particle size on the oxidation process has been identified. During the reaction, individual gamma-Al2O3 oxide nuclei grow at the surface of Al particles with diameters less than 300 nm without laterally overlapping to form a protective passivating layer, as typically occurs during the oxidation of micron-sized particles or bulk metal. The localization of gamma-Al2O3 nuclei is determined by the regions where peeling of the primary amorphous oxide film from the surface of the metal core of an Al particle occurs due to the thermal decomposition of aluminum hydroxides (T approximate to 350 degrees C) present within the particle shells. A significant increase in the contributions of the following factors leads to a high oxidation rate: the diffusion within the disordered structure of a particle core and the surface diffusion of cations (Ea = 80.6 +/- 5.3 kJ mol-1, 400-450 degrees C), the surface and grain boundary diffusion of oxygen during the growth of gamma-Al2O3 crystallites (Ea = 108.3 +/- 11.2 kJ mol-1, 470-500 degrees C) and the surface and grain boundary transport of anions through an amorphous and underlying nanocrystalline oxide layer with a constant thickness (Ea = 205.1 +/- 8.6 kJ mol-1, 520-550 degrees C). The results obtained make it possible to expand the theoretical understanding of the manifestations of the size effect in solid-phase reactions and take into account its influence in practice.