Browsing by Author "Vlad, Alina-Gabriela"

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Open Access
In situ oxidation study of metallic alloys from UHV to atmospheric pressures
(2009) Vlad, Alina-Gabriela; Strunk, Horst (Prof. Dr.)
Oxidation of metals and metallic alloys is of crucial importance for different technological applications such as heterogeneous catalysis, microelectronic devices, gas sensors and high-temperature resistant coatings. In general, in situ studies of the oxide stability at industrially-relevant environmental conditions, together with precise structural and chemical investigations are desired, in order to achieve a fundamental understanding of the processes governing the aforementioned applications. The present thesis deals with the oxide formation on CoGa(100) and NiAl(110) surfaces. In situ surface-sensitive x-ray diffraction measurements were performed using synchrotron radiation at ANKA (Karlsruhe) and ESRF (Grenoble) and high resolution core level spectroscopy measurements were performed at MAX-Lab (Lund). The structure and formation of surface and bulk gallium oxide on CoGa(100) was investigated from UHV to 1 bar oxygen pressure at different temperatures. Using a multi-method approach, we have achieved an atomic-level understanding of the ultra-thin gallium oxide structure. The formation of bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K. The stability of the ultra-thin aluminium oxide layer on NiAl(110) was studied at different conditions of oxygen pressures and temperatures. The thickness of the ultrathin aluminum oxide can not be further increased by multiple oxidation cycles, as previously suggested, and it only exists as a 5 Angstrom thick layer. Epitaxial gamma-alumina was found to grow and its orientation relationship is strongly dependent on the oxidation conditions. Further oxidation leads to a coexistence of epitaxial gamma- and polycrystalline delta-alumina. The transformation of metastable phases to alpha-alumina was complete at 1370 K. The fcc-hcp martensitic-like transformation of the initial gamma-Al2O3 to epitaxial alpha-Al2O3 was observed. A continuous epitaxial alpha-alumina layer formed between the substrate and the polycrystalline oxide scale, having a thickness of about 150 nm.