A model for two-component aerosol coagulation and phase separation: a method for changing the growth rate of nanoparticles

In previous studies of particle growth we have synthesized binary metal oxide aerosols and have observed the evolution of internal phase segregation during growth of molten nanodroplets. We have also generated NaCl/metal aerosols in which the metal nanoparticle is enveloped within a salt droplet. In both systems the nanoparticles were grown in the molten state. In this paper we propose a model which incorporates phase segregation in a binary aerosol. The model assumes that complete phase segregation is the thermo dynamically favored state, that no thermodynamic activation energy exists, and that the phase segregation process is kinetically controlled. The results indicate that a steady state behavior can be reached in which the characteristic time for aerosol coagulation and the characteristic time for the growth of the minority phase coincide such that the number of distinct segregated entities within each aerosol droplet is constant. The results suggest what we believe to be an important concept that can be utilized in materials synthesis. This is that the major phase can be used to moderate the growth rate of the minor phase by changing the characteristic encounter frequency and therefore the eventual growth rate of the minority phase. In particular, temperature, which does not play an important role in aerosol coagulation, is seen to be a very sensitive parameter for the growth of the minority phase nanoparticles. We discuss the parameter space necessary for this to occur. (C) 2001 Elsevier Science Ltd. All rights reserved.