Surface Roughening and Unstable Neck Formation in Faceted Particles: I, Experimental Results and Mechanisms

Experimental in situ TEM results described by Rankin and Boatner are surprising because they show initial neck formation between two isolated MgO particles, followed by neck rupture. A quantitative assessment of the original images shows that initial neck growth corresponds to material removed from the rounded edges which contact the neck, and that neck rupture transfers material directly from the neck to the adjacent faceted faces. During the initial neck growth process surface diffusion along the rounded edges of the crystals is apparently the dominant mass transport mechanism, and material is not transferred to the faceted faces because of the energy barrier associated with forming a new atomic layer on top of a faceted surface. This energy barrier can apparently be overcome when a curvature reversal at a facet/rough surface boundary creates a step which makes it possible to add one or more new atomic layers to the faceted face(s). This can move material away from the neck, and thus lead to neck rupture.