Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 150 °C. At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection against oxidation

Low-temperature gaseous nitriding and subsequent oxidation of epitaxial Ni/Fe bilayers

Low-temperature gaseous nitriding was applied to epitaxial Ni/Fe bilayers deposited onto a MgO(001) substrate. The pore-free nitride layers produced were subsequently oxidized in oxygen. The samples were analyzed by conversion electron Mossbauer spectroscopy (CEMS), x-ray diffraction (XRD), and Rutherford backscattering spectroscopy in combination with channeling techniques. Nitriding in pure NH3 gas at 300 degrees C led to the formation of a textured E-Fe-nitride layer with a predominant composition of Fe2.07N. The epitaxial relationship of the E-Fe-nitride layer with the MgO substrate was found to be epsilon-Fe2.07N{203}(010)//MgO(001)(110). The nitride layer produced was subsequently oxidized in p(O-2) = 100 mbar at 275 degrees C. While the XRD spectra acquired on the oxidized samples revealed the formation of a Fe-oxide phase with a spinel structure, the GEMS spectral lines could not be interpreted in terms of any Fe-oxide or Fe-hydroxide phase know. It is suggested that the peculiarities in the GEMS data are caused by N atoms incorporated into the oxide lattice

Thermal stability of ultrasoft Fe–Zr–N films

The thermal stability of nanocrystalline ultrasoft magnetic (Fe98Zr2)1−xNx films with x = 0.10–0.25 was studied using thermal desorption spectrometry, positron beam analysis and high resolution transmission electron microscopy. The results demonstrate that grain growth during the heat treatment is accompanied by an increase of the free volume and nitrogen relocation and desorption. All these phenomena can drastically degrade the ultrasoft magnetic properties. The nitrogen desorption has already started at temperatures around 400 K. Nevertheless, most of the nitrogen leaves the sample at a temperature above 800 K. We found that nitrogen out-diffusion is significantly retarded compared with the prediction of the diffusion in bulk α-Fe. A qualitative model is proposed in which the nitrogen out-diffusion in nanocrystalline material is retarded by trapping at immobile defects, namely Zr atoms, and also by voids at grain boundaries. From a certain temperature, nitrogen migrates from the interior of the nanograins to the nanovoids at the grain boundaries and the out-diffusion to the outer surface is controlled by transport between the voids.