Anisotropic Surface Free Energy Research Articles

Faceting of copper surfaces at 1000°C

Polished copper surfaces were observed to facet on annealing at 1000°C in wet hydrogen atmospheres. It was found that the quantity hi p H 2O p H 2 must be greater than zero for extensive faceting to occur, though some faceting, particularly at the sample edges, can occur for In p H 2O p H 2 as small as −3.5. The criteria for faceting to occur by the reduction of surface free energy are discussed and it is shown that facets near the (111) orientation are thermodynamically stable. The effect of anisotropy of surface free energy on facet nucleation is discussed. The outlines of lens-shaped facets were measured. The shapes and sizes of the lens-shaped facets are consistent with a volume diffusion mechanism of facet formation. The observed profiles of the curved surfaces near separated linear facets are compared to theoretically calculated profiles. The calculations by Mullins of facet profiles are extended slightly to facilitate the comparison. The observed profiles are in reasonable agreement with the calculated profiles for facet formation by surface diffusion with the surface diffusion coefficient the same on both the low index and the complex surfaces.

The thermal rearrangement of field-evaporated iridium surfaces

The field-ion microscope was used to study the formation of facets on field-evaporated iridium surfaces both in ultra-high vacuum and in the presence of impurities. From the size of the (111) facets formed at 990° C, it is estimated that the surface-free energy anisotropy is greater than 4 percent. The activation energy for (111) facet growth was measured to be 45 kcal/mol or approximately 0.3 ΔH v (heat of vaporization). Gaseous impurities can either lower or raise the rate of faceting, which may explain the variations in the activation energy for surface mass transport reported by others in the literature.

Decanted interfaces and growth forms

Experiments have been performed on eutectic alloys in which the liquid metal is rapidly separated from the solid during solidification. They prove that a film of liquid remains on a decanted surface even under rapid decanting conditions. It is shown that the {111} and {100} faceted structures which are sometimes observed on decanted surfaces of pure metals can originate from the subsequent solidification of the remanent liquid film. The previous assumption that the facets represent the actual form of the interface during growth is repudiated. It is suggested that an anisotropic surface free energy favors the production of stepped, low-indexed crystallographic planes over an irrational surface on the free surface of metal crystals.