Browsing by Author "Ellinger, Claus Steffen"

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    In situ oxidation study of flat and stepped binary alloy surfaces
    (2010) Ellinger, Claus Steffen; Dosch, Helmut (Prof. Dr.)
    Binary intermetallic alloys like Fe3Al and NiAl are the basis for so called super-alloys. Super alloys are technologically used in hot and highly reactive environments, like jet engines. Here, the limit of mechanical stability for normal Fe-based steel is reached. To protect the alloys from further corrosion a thin, homogeneous, pure alumina layer is formed on their surfaces. From a scientific point of view low pressure (1E-6 mbar) oxidation of binary alloys is also interesting. On single crystal surfaces like NiAl (110) or CoGa (100) long-range ordered ultrathin surface oxides are formed, which are used as nanotemplates for catalytic reactions or in future microelectronic devices. Therefore fundamental knowledge of the initial oxidation process of binary alloy surfaces is necessary from a technological as well as a scientific point of view. Regarding this process this thesis answers two main questions: 1) How is the intermetallic ordering, responsible for the high mechanical strength, influenced by the formation of a protecting oxide layer? 2) How do surface steps influence the oxidation process? Is it possible to tailor oxide structures via regularly arranged steps? Four model systems were oxidized to answer these questions: the Fe3Al (110) surface, the vicinal NiAl (671) and (430) surfaces, and the stepped CoGa (100) surface with a 0.8 ° miscut. For a structural analysis of the systems in situ surface x-ray diffraction experiments are carried out at the European synchrotron source ESRF in Grenoble, the Angströmquelle Karlsruhe and the Swiss light source in Villingen. For a chemical analysis of the surfaces high resolution core level spectroscopy (HRCLS) measurements were conducted at MAX-Lab in Lund The x-ray data of the clean Fe3Al (110) surface showed, that, due to segregation, the topmost layer of the system has a different intermetallic ordering than the D03 ordered bulk underneath. Oxidations at 1E-6 mbar O2 and temperatures between 400 K and 720 K lead to a vanishing of the intermetallic order within the interfacial region. The oxide-alloy-interface gets slightly rougher but remains crystalline while an 8.4 Angstroms thick oxide film is formed. The analysis of the HRCLS spectra measured during oxidation shows that the oxide composition strongly depends on the oxidation rate and the Al segregation from the bulk. High oxidation rates at oxygen pressures of 1E-6 mbar lead to the formation of Al- and Fe-oxides. For low oxidation rates (1E-8 mbar) pure alumina can be formed. The experiments on the NiAl surfaces show that both clean, regularly stepped, vicinal surfaces are thermodynamically stable. After oxidation at 550 K and 1E-6 mbar O2 an amorphous, 5.5 Angstrom thick oxide film is formed on both surfaces. Annealing the samples above 1100 K leads to a massive (110) faceting and the formation of a long-range ordered oxide on both surfaces. On the (110) terraces of the (671) surface single domain surface oxide, which is known to grow in twin domains on flat NiAl (110), is observed. On the (110) terraces of the (430) surface theta-Al2O3-like oxide structures are observed. Due to preferential strain release at the oxide alloy interface in the direction of the steps it is therefore possible to trigger the growth of certain oxide structures as well as certain domains. Measurements of the oxide growth on the stepped CoGa (100) surface show the formation of the two oxide domains, also known from the flat surface. Thermally induced disorder on the surface might inhibit a single domain growth. However, in comparison with the flat surface the time-dependence of the oxide growth is different. A two-step exponentially reduced growth mode is observed.