Crystallography of metallic aerosol precipitates: dendritic origin of hexagonal prisms in Cd and Zn

Abstract

The aspect ratio was measured on simple hexagonal prisms in the sediment from condensation aerosols of Cd and Zn. For Cd the ratio R=p√3/p, where p and q are the respective lengths of the prism edges parallel and perpendicular to the basal pinacoid, varies symmetrically above and below a most frequent value of 1.2 to 1.3. The corresponding R-distribution for Zn was broader, less symmetrical and peaked at 1.5 to 1.6. Similar results had been found with hexagonal bubbles of Ar in annealed foils of these metals and interpreted as denoting the equilibrium shape. In the condensation of aerosols, however, crystals grow from the vapour under highly non-uniform conditions, and the constancy of aspect ratio is attributed to the operation of a dendritic growth mechanism as put forward by Buckle and Pointon.