Condensation of zinc aerosols

Abstract

Condensation of zinc aerosols in the presence of argon was studied in the heat-pulse cloud chamber; as with cadmium, two distinct series of particles are produced as the wall temperature is increased, first prisms and then spheres. The prismatic particles are monocrystalline and include complicated dendrites and a species not previously seen, the capped prism. Whiskers grow from the basal surfaces of the simple prisms. The spherical particles include mono-, bi-, and poly-crystals.

The production of simple prisms and dendrites is attributed to condensation of solid in the presence of particularly steep temperature and vapour-pressure gradients. Crystallographic evidence supports the view that the spheres condense as liquid droplets. It is concluded that the droplets solidfy from nuclei that grow rapidly into circular {00.1} rafts on the liquid surface, in the manner observed with large sessile drops by Mutaftschiev and Zell. The formation of hillocks on the basal raft or its conversion into a concave dish is evidence of growth in the vapour.

The numbern of basal rafts on a solidified sphere may be identified with the number of nuclei. When unimpeded by other nuclei, a raft grows to a maximum radiusr∼0.55R, whereR is the droplet radius. The ratior/R is used to deduce the orientation of the pyramidal planes which develop at the edge of the raft as it begins to thicken. At low wall temperatures some of these planes tend to be very coarse, while when the growth front reaches the opposite side of the particle, the solid surface adopts a relatively smooth spherical outline.

As the wall temperature is raised, the highest value observed forn increases from 1 to a maximum of 20 to 30 and then falls abruptly as the melting point of the metal is approached. The temporary increase inn may be an effect of drop size but the final fall is ascribed to the failure of supercooled droplets to nucleate until withdrawn from the apparatus. Spheres withn>1 deform on cooling as the result of thermal expansion anisotropy. Forn>2 cracking and slip are observed.