Onset coarsening-coalescence kinetics of γ-type related Al2O3 nanoparticles: Implications to their assembly in a laser ablation process

An onset coarsening-coalescence event based on the incubation time of cylindrical mesopore formation and a significant decrease of specific surface area by 50% and 70% relative to the dry pressed samples was determined by N2 adsorption-desorption hysteresis isotherm for two Al2O3 powders having 50 and 10 nm in diameter respectively on an average and with γ-type related structures, i.e. γ- and its distortion derivatives δ- and/or θ-types with {1 0 0}/{1 1 1} facets and twinning according to transmission electron microscopy. In the temperature range of 1100-1400 °C, both powders underwent onset coarsening-coalescence before reconstructive transformation to form the stable α-type. The apparent activation energy for such a rapid coarsening-coalescence event was estimated as 241 ± 18 and 119 ± 19 kJ/mol, for 50 and 10 nm-sized particles, respectively indicating easier surface diffusion and particle movement for the latter. The size dependence of surface relaxation and onset coarsening-coalescence of the γ-type related Al2O3 nanoparticles agrees with their recrystallization-repacking upon electron irradiation and accounts for their assembly into nano chain aggregates or a close packed manner under the radiant heating effect in a dynamic laser ablation process.