Condensation of cadmium aerosols

Abstract

Cadmium aerosols were prepared by the heat-pulse method and fall-out collected at temperatures T from ambient to just above the melting point T _f. Deposits were examined by electron microscopy after coating with Au-Pd. Representative particles were removed for electron and X-ray diffraction studies.

The particles are crystalline. The types, and their proportions and size distributions vary with temperature. At low temperatures particles are monocrystals of high specific surface, chiefly prisms with deep fissures and cavities but also rough spheres and stellate dendrites. At higher temperatures the crystal forms are perfected, the proportion of prisms falls, and polycrystalline as well as monocrystalline spheres are found. Spheres can exceed 50Μm in diameter but the largest prisms are 2 to 3 Μm and disappear at 0.8 to 0.9 T _f. Up to this point spheres have one or more circular {0001} depressions, or “dishes”, depending on the number of crystallites they contain. At higher temperatures they are quite smooth.

It is concluded that smooth spheres are droplets which have supercooled and frozen on the collector, and as such are not aerosol particles. Droplets that freeze in the cloud become dished spheres, and their subsequent growth involves condensation on areas between the dishes. All particles are nucleated from the vapour close to the source, the prisms apparently at T<T _f.