Crystallography of metallic aerosol precipitates
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In a previous paper it was concluded from microscopical evidence that the radius of a basal raft nucleated on a freezing droplet of Zn or Cd expands to a fixed fraction of the droplet radius before thickening into a grain. Further studies on polycrystalline spheres from condensation aerosols, together with observations by other investigators working with much larger, sessile drops, confirm that the raft remains quite thin while the radius is expanding. Additional surface features developed by epitaxial growth from the vapour on monocrystalline aerosol spheres show that in the following stage, in which the raft thickens but its upper surface no longer expands, the growth front propagates into the melt by the build-up of layers parallel to c. Evidence of a growth helix is found opposite the basal flat. Its formation is attributed to growth on a screw dislocation generated by the stress that accumulates at the perimeter of the expanding raft. The onset of rapid helical thickening coincides with termination of raft expansion. Differences in evaporation behaviour of particles are considered to depend on whether the dislocation remains in the solidified droplet or is expelled by thermal stresses. Glide of the same dislocations may be involved in the slip previously observed in polycrystals.
KeywordsThermal Stress Surface Feature Epitaxial Growth Screw Dislocation Sessile Drop
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