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Nucleation with Simultaneous Chemical Reaction in the Vapor-Phase Synthesis of AlN Ultrafine Powders

Using the classical (one-component) homogeneous nucleation theory approach of a constrained equilibrium distribution of sub- critical particles, a nucleation equation that considers the effect of heterogeneous reaction is derived for the AlN system. In the absence of more detailed information about the aluminum nitride (AlN) reaction kinetics, a simplified surface reaction model is formulated. Clusters, which can be formed by AlN and pure Al, are assumed to grow from the condensation of Al vapor followed by surface reaction between condensed Al and absorbed NH3. The composition of any subcritical particle is assumed to be determined by the rates at which Al and AlN species are added to their surface and thus made entirely dependent upon local gas concentrations. This assumption resulted in important simplifications that allowed this development to be cast in a form resembling the one used by the classical theory for one-species nucleation. An analysis intended to evaluate the dependence of this nucleation equation on temperature, concentration of reacting species, and degree of aluminum supersaturation is presented. Considerable changes of the nucleation rate were observed between low and high concentration ratio of reacting species. These changes are directly related to the changes of the surface energy and bulk free energy terms determined as a function of the free-Al content in a nucleus.