Wetting anisotropy and oxygen activity dependency for oxides by liquid transition metals studied through shape changes of liquid Cu inclusions within MgO

The equilibrium shape of liquid metal inclusions in oxides is used to study anisotropy and oxygen partial pressure dependency of the metal-oxide specific interfacial free energy. Liquid copper inclusions are formed within single crystalline magnesia by internal reduction of mixed oxide (Mg,Cu)O. Their equilibrium shape is studied by TEM for various oxygen chemical potential. Precipitates always adopt cube-octahedral morphology, but relative facet size strongly depends on the oxygen chemical potential. The shape evolution with oxygen chemical potential reveals important and anisotropic Gibbs' adsorption of excess oxygen to the interfaces at high oxygen activities. Gibbs' adsorption to crystallographically different MoO-liquid Cu interfaces is discussed in terms of a point defect model recently developed by the author (Backhaus-Ricoult, M., Phil. Mag., 2001, in press), where oxygen activity-dependent formation of interfacial charge transfer clusters between liquid metal and excess oxygen or magnesium is considered, while the relevant site and charge conservation laws in the two-phase system are respected and matter exchange with the surrounding gas atmosphere is allowed. Present experimental results are interpreted in terms of this interfacial defect model and compared with interpretation by other wetting models from the literature. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.