Birgit Pöter

The new diffractometer for surface X-ray diffraction at beamline BL9 of DELTA.

The experimental endstation of the hard X-ray beamline BL9 of the Dortmund Electron Accelerator is equipped with a Huber six-circle diffractometer. It is dedicated to grazing-incidence X-ray diffraction and X-ray reflectivity experiments on solid surfaces and thin films as well as to powder diffraction measurements. A new set-up for grazing-incidence X-ray scattering of liquids has been built up using a silicon mirror to reflect the incident X-ray to the liquid surface at angles of incidence around the critical angle of total reflection of the sample. X-ray reflectivity measurements of a polymer film and grazing-incidence X-ray diffraction measurements of an epitaxically grown Gd40Y60 film, an oxidized surface of Fe-15at.%Al alloy and aqueous salt solutions are presented and discussed.

Early Stages of Protective Oxide Layer Growth on Binary Iron Aluminides

Abstract The aim of the present study was to investigate the early stages of protective oxide layer growth on Fe-15 at.%Al and Fe-40 at.%Al oxidized at 700 to 900 °C for 5 to 300 min in dry, synthetic air. Emphasis was put on the analysis of the oxide layer with respect to chemical and phase composition using X-ray photoelectron spectroscopy (XPS), grazing incidence XRD with synchrotron radiation and TEM investigations. The results indicate that at all conditions an external oxide layer grows and that this oxide layer consists of an outer, approximately 40 nm thin, Fe2O3-containing layer followed by a nearly pure Al2O3 layer which grows continuously with increasing temperature and time. The Fe2O3 content of the outer oxide layer decreases with time, temperature and Al content of the binary iron aluminide. The phase compositions of oxide layers less than 100 nm in thickness are determined, indicating the formation of hematite (α-Fe2O3) and corundum (α-Al2O3) on Fe-15 at.%Al at 700, 800 and 900 °C after 300 min of oxidation. Moreover, TEM analyses indicate that for both alloys at 700 °C a metastable Al2O3 layer, most likely consisting of θ-Al2O3 or γ-Al2O3 was formed epitaxially at the iron aluminide/oxide interface. Exposure times of 300 min at 700 °C lead to the transformation of metastable Al2O3 to stable α-Al2O3. However, a very thin Al2O3 layer of about 5 nm thickness seems to remain at the iron aluminide/oxide interface, most probably a metastable Al2O3 modification.

In-situ scanning electron microscopy and electron backscatter diffraction investigation on the oxidation of pure iron

Abstract α-iron samples, with a 4 to 6 nm oxide layer on top of the surface due to preparation, were oxidised in situ within a scanning electron microscopy equipped with electron back-scatter diffraction. At 773 and 973 K, two different growth mechanisms during the initial stage of iron oxide formation could be observed, layer-by-layer growth at 773K and island growth at 973 K. The growth mechanism at 973K is correlated to the formation of a thin layer of Fe1–XO (wüstite) prior to Fe3O4 (magnetite) crystallisation whereas at 773K only magnetite was found as newly grown oxide. Magnetite showed a strong epitaxial relationship to α-iron with its {111} plane growing preferentially on the {110} plane of iron.