Hydrogen uptake kinetics of Pd coated FeTi films

The kinetics of hydrogen uptake of thin films of FeTi deposited on Si substrates and covered with 20 nm Pd were studied. The films serve as a model system for powdered FeTi, with grains that are (partly) covered with Pd, which serves as a protection for severe oxidation. Two FeTi compositions near the 50/50 composition were studied. The hydrogen uptake kinetics as a function of temperature and pressure were measured by probing the differential pressure between a small hydriding reaction chamber and a reference chamber. Both compositions showed first order kinetics for the largest portion of the uptake. Using results of additional measurements in which the thickness of the layers was varied, a model is proposed in which the uptake proceeds via fast channels through the film, followed by slower diffusion into the bulk. Finally, the influence of oxidation was studied. An FeTi-oxide underneath the Pd layer is a barrier for H diffusion. It was found that by annealing the H uptake rate could be increased. This is probably due to the decomposition of the oxide. Samples partly covered with Pd and partly by FeTi-oxides, obtained by decomposing the fully covered structure by air annealing at 250 °C, showed uptake throughout the entire FeTi film with an even faster rate than in a fully covered film. Some explanation with simple models of the observed phenomena is given.     

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 O₂ and H₂O. 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 300°C in vacuum, air and 10⁻⁵ mbar O₂. The layered structure remains intact up to 150°C. At 200°C in air and O₂, 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 150°C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200°C, 20 nm Pd may still provide sufficient protection against oxidation.     

Anisotropic recoilless fraction and temperature dependent quadrupole interaction of substitutional 129I in Si due to Jahn-Teller distortion

A dope concentration dependent quadrupole relaxation and a large anisotropy of the recoilles fraction are observed for substitutional iodine impurities in silicon, using ¹²⁹I Mössbauer spectroscopy. The results are interpreted in terms of a transition from a static to a dynamic Jahn-Teller distortion.     

H uptake kinetics of FeTi films coated with Ni

The H₂ storage material FeTi can be fully charged through a Ni coating in the course of hours. A mild activation, either by long H₂ exposure at RT, or annealing in 35 mbar H₂ at 150 °C, leads to a full uptake in minutes. The enhancement of the uptake rate after activation is caused by a reduction of the Ni-oxide surface. Ni does not limit the hydrogen uptake rate once the surface is clean. However, the Ni surface is sensitive to contamination, as for instance by CO adsorbed from air.     

Tin fluoride and tin oxide molecules in noble metals

The geometrical and chemical bonding structure of isolated molecules consisting of a number of interstitial gas atoms bound to a substitutional 5sp element atom in a noble metal matrix have been studied utilizing Mössbauer spectroscopy of¹¹⁹Sn and¹²¹Sb in conjunction with ion beam analysis. The structure information obtained is discussed in relation to the structure of conventional molecules and to the constraints introduced by the metal matrix.

     

1D lattice distortions as the origin of the (2 x 2)p4gm reconstruction in gamma'-Fe4N(100): a magnetism-induced surface reconstruction

The (100) surface of magnetic gamma'-Fe4N films epitaxially grown on Cu(100) shows a (2 x 2)p4gm reconstruction. The atom positions determined experimentally are properly accounted for by first-principles total energy calculations, which further reveal that the (2 x 2)p4gm reconstruction requires the topmost Fe4N2 plane to be magnetic and effectively decoupled from the rest of the crystal and that it originates from the perpendicular crossing of one-dimensionally dimerized Fe chains.     

Lattice site location and electronic structure of Te in GaAs

High purity <100> wafers of GaAs were implanted with radioactive^129mTe and stable¹²⁸Te at 110 keV to total doses of 2×10¹⁴ and 2×10¹⁵ Te/cm² respectively and studied with RBS/ channeling and Mössbauer spectroscopy on the 27.8 keV level of¹²⁹I. After implantation and/or annealing at temperatures between 200–300°C the Mössbauer spectra are dominated by a single line. Channeling reveals an appreciable residual damage in the host lattice, but also points to a substitutional position of the Te atoms. After annealing above ≌500°C, where nearly complete lattice damage recovery is obtained, the Te atoms become defect-associated. The results clearly point to the formation of Te_As?V_Ga complexes.     

Evaporation and ripple formation during pulsed laser irradiation of GaAs

The evaporation of As and Ga during pulsed laser irradiation of GaAs single crystals was measured under vacuum. The data were reproduced in a thermal-model calculation. The laser power density useful for annealing was found to be limited both by excessive evaporation and surface ripple formation.