In situ determination of the activation energy for restructuring of nanometer aerosol agglomerates

A new approach to the kinetics of the rearrangement of aerosol agglomerates is presented. This analysis is based on the change in free energy per primary particle during restructuring. The excess free energy compared with the final state drives the agglomerate to become more compact which results in an increase in the coordination number, as estimated from the changing fractal dimension of the aerosol agglomerates. The analysis was tested on agglomerates of nanometer silver and copper particles produced by an evaporation-condensation technique and by laser ablation. The assumption that the rearrangement is an activated process was supported by the results. It was concluded that the interaction between the primary particles occurs over a short range. The activation energies were one order of magnitude lower than the bond energies expected from the bulk Hamaker constants for silver and copper. Within the measured size range no significant difference in the activation energies of silver and copper agglomerates was found. A maximum in the activation energy was found for particles about 18 nm in diameter. Larger particles showed decreasing activation energy with increasing diameter. The addition of oxygen (50%) before the agglomeration of the primary particles increased the activation energy by a factor of two and shifted the maximum in the activation energy to 15 nm. The prefactor in the rate coefficient for restructuring increased by four orders of magnitude when oxygen was added. Copyright (C) 1997 Published by Elsevier Science Ltd