NUCLEATION AND GROWTH OF CESIUM IODIDE AEROSOLS

Experiments on aerosol condensation in the vapour over caesium iodide at 973-1273 K provided morphological data and size distributions to support detailed tests of recent theory. Plumes of particles were obtained by evaporating the pure salt into a flow of unreactive carrier gas. Samples of the flow in the aerosol plume were aspirated to Nuclepore filters for examination by SEM. The interior of particles was observed by transmission optical microscopy of samples collected in immersion oil. The global population was studied by connecting the chamber outlet to differential mobility particle sizing (DMPS) and aerodynamic particle sizing (APS) equipment. The evaporation rate was also measured for theoretical purposes. The particles, which were essentially spherical, showed monomodal size distributions with radii ranging between 0.1 and 4-mu-m. The mean particle size increased and the size distribution broadened with evaporation temperature. The majority of particles consisted of a clear matrix containing one or more granular inclusions. Single granules without the surrounding matrix resembled rounded prisms, suggesting that they represent centres of crystallization in the molten salt droplet. The observations support the boundary-layer model of nucleation and growth adopted in the theory. The evaporation rates indicated condensation boundary-layer thicknesses of the order of 0.1 mm, increasing with temperature. The theoretical particle sizes lie within the limits observed experimentally and show a corresponding dependence on evaporation temperature. The total concentration of particles emitted was predicted to within an order of magnitude. Evaporation at 973 K or less should lead to vapour nucleation at temperatures below the thermodynamic melting point, but the particles observed appear to nucleate as liquid. The absence of morphological change and the minor effect of evaporation temperature on layer thickness indicate that the condensation mechanisms were not influenced by the convective pattern outside the boundary layer.