Optimisation of Adhesion at Transition Metal-Oxide Interfaces by Processing at Well-Chosen Oxygen Activity
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ABSTRACT Transition metal-oxide interfaces suffer within their thermodynamic stability range Gibbs' adsorption and show important changes in chemical composition with oxygen activity. As a consequence, specific free interfacial energy and adhesion energy also vary with oxygen activity. Adhesion at a given non-reactive transition metal-oxide interface can then be optimised by establishing the proper oxygen activity during processing or by a post-treatment at the interface. In the present work, the approach of Gibbs' adsorption is extended to crystalline, anisotropic (special) transition metal-oxide interfaces. It is demonstrated that interfacial energy varies with oxygen activity. The variation in energy is studied for different adsorption energies, temperatures and interfacial planes. INTRODUCTION Precise control of structure and chemistry upon processing is crucial for many technological applications of metal-oxide interfaces [1,2]. Interface properties have often been experimentally optimised, and it remains uncertain, if further improvement is possible. In the present work, we demonstrate the influence of the thermodynamic equilibrium parameters: temperature and oxygen activity on the interfacial energy and, based on these results, propose temperature-oxygen activity domains for processing in which best adhesion at the interface is expected. Even though most technical interfaces are obtained in non-equilibrium, equilibrium considerations can also be extended to those processes, by selection of most favourable conditions during fabrication or upon post-treatments. Non-reactive metal/oxide interfaces are thermodynamically stable over a wide range in temperature and oxygen activity ranging from the redox-equilibrium of the less noble metal of the oxide to that of the more noble metal. Within this stability range, the interface always separates the same two bulk phases, and no new (bulk) reaction products form. Changes in the chemical bulk composition are given by the corresponding phase diagram and in general remain small. At the interface, however, changes in the chemical composition can be much more important because of interaction with the second phase across the boundary, which, through electron transfer or hybridisation, may locally at the interface stabilise compositions, which are different from the bulk composition and which may be even out of the stability range of bulk compositions. Defect concentrations can also be much larger than in the bulk. Reasons for this are both, stabilisation effects by the other phase and different structural/charge constraints at the interface. In the following, a model is presented, which relates variations in free interfacial energy through Gibbs' adsorption isotherms to changes in oxygen chemical potential. In this approach, flat, special (low index plane) interfaces with simple geometry are considered. Adsorption to the interface is most important for the first atomic layers on both sides of the interface and concerns all species. Non-stoichiometry in the oxide at the inte
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