[show abstract][hide abstract] ABSTRACT: The electrical resistivity of amorphous Fe(100-x)Zr(x) metal alloy films and multilayers has been investigated in a wide temperature and composition range. The overall behavior of the resistivity is consistent with bulk measurements, exhibiting prominent semiconductor-like changes at low temperatures. The transition from positive (metallic) to negative temperature coefficient of resistivity behavior is accompanied by minute changes in magnetoresistance and we can therefore rule out magnetic phase changes as being the cause for the observed changes in the resistivity. Using x-ray absorption and emission spectroscopies we are able to probe the unoccupied and occupied electronic densities of states. The corresponding spectra are found to significantly overlap, as expected for a metallic-like electronic structure and the absence of a band gap. Besides a broadening of the x-ray emission lines expected from an amorphous material, remarkably small differences are observed in the electronic structures when changing the amount of Zr. The resistivity data were modeled and agreement with the Mott variable range hopping model was found, indicating localized electronic states due the disordered structure of the Fe(100-x)Zr(x) alloys.
[show abstract][hide abstract] ABSTRACT: Optical properties of V thin films deposited on MgO substrates have been obtained from spectrophotometric measurements. The V films were coated with a thin Pd layer to protect them from oxidation and to favor absorption of atomic hydrogen. Electrical resistance was recorded while hydrogen pressure was increased slowly up to 750 mbar keeping the temperature constant. Simultaneously, visible and near-infrared transmittance spectra of this Pd/V/MgO system were measured. The spectra were numerically inverted to obtain the spectral behavior of the Pd and V dielectric functions at 22 and 140 °C. Hydrogen concentrations were first determined from the combined effect of hydrogen content on the electrical resistance and on the optical direct transmission of the system. Then, determination of these concentrations was improved using retrieved values of the absorption coefficients of the hydrides and taking into account the structural change of V and the volumetric expansion of Pd. Good agreement is established when considering qualitative correlations between spectral features of the optimized PdHy and VHx dielectric functions and band structure calculations and densities of states for these two transition metal hydrides.
Physica Scripta 11/2012; 86(6):065702. · 1.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: The distribution of hydrogen in Nb/Ta superlattices has been investigated by combined neutron reflectivity and x-ray scattering. We provide evidence to support that strain modulations determined with x-ray diffraction can be interpreted as modulations in hydrogen content. We show that the hydrogen concentration is modulated and favors Nb, in agreement with previous studies. We measure the concentration directly using neutron reflectivity and demonstrate no detectable change in the distribution of hydrogen with temperature, in stark contrast to previous studies.
[show abstract][hide abstract] ABSTRACT: The dipole force components in nanosized metal hydrides are quantitatively determined with curvature and x-ray diffraction measurements. Ab initio density functional theory is used to calculate the dipole components and the symmetry of the strain field. The hydrogen occupancy in a 100-nm-thick V film is shown to be tetrahedral with a slight asymmetry at low concentration, and a transition to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K. When the thickness of the V layer is reduced to 3 nm and biaxially strained, in a Fe0.5V0.5/V superlattice, the hydrogen unequivocally occupies octahedral z-like sites, even at and below concentrations of 0.02 [H/V].
[show abstract][hide abstract] ABSTRACT: Because of its light weight and small size, hydrogen exhibits one of the fastest diffusion rates in solid materials, comparable to the diffusion rate of liquid water molecules at room temperature. The diffusion rate is determined by an intricate combination of quantum effects and dynamic interplay with the displacement of host atoms that is still only partially understood. Here we present direct observations of the spatial and temporal changes in the diffusion-induced concentration profiles in a vanadium single crystal and we show that the results represent the experimental counterpart of the full time and spatial solution of Fick's diffusion equation. We validate the approach by determining the diffusion rate of hydrogen in a single crystal vanadium (001) film, with net diffusion in the  direction.
[show abstract][hide abstract] ABSTRACT: Pulse intensities greater than 10(17) Watt/cm(2) were reached at the FLASH soft X-ray laser in Hamburg, Germany, using an off-axis parabolic mirror to focus 15 fs pulses of 5-70 mu J energy at 13.5 nm wavelength to a micron-sized spot. We describe the interaction of such pulses with niobium and vanadium targets and their deuterides. The beam produced craters in the solid targets, and we measured the kinetic energy of ions ejected from these craters. Ions with several keV kinetic energy were observed from craters approaching 5 mu m in depth when the sample was at best focus. We also observed the onset of saturation in both ion acceleration and ablation with pulse intensities exceeding 10(16) W/cm(2), when the highest detected ion energies and the crater depths tend to saturate with increasing intensity. A general difficulty in working with micron and sub-micron focusing optics is finding the exact focus of the beam inside a vacuum chamber. Here we propose a direct method to measure the focal position to a resolution better than the Rayleigh length. The method is based on the correlation between the energies of ejected ions and the physical dimensions of the craters. We find that the focus position can be quickly determined from the ion time-of-flight (TOF) data as the target is scanned through the expected focal region. The method does not require external access to the sample or venting the vacuum chamber. Profile fitting employed to analyze the TOF data can extend resolution beyond the actual scanning step size. (C) 2011 Elsevier B.V. All rights reserved.
HIGH ENERGY DENSITY PHYSICS. 01/2011; 7(4):336-342.
[show abstract][hide abstract] ABSTRACT: Studies of materials under extreme conditions have relevance to a broad area of research, including planetary physics, fusion research, materials science, and structural biology with x-ray lasers. We study such extreme conditions and experimentally probe the interaction between ultrashort soft x-ray pulses and solid targets (metals and their deuterides) at the FLASH free-electron laser where power densities exceeding 10(17) W/cm(2) were reached. Time-of-flight ion spectrometry and crater analysis were used to characterize the interaction. The results show the onset of saturation in the ablation process at power densities above 10(16) W/cm(2). This effect can be linked to a transiently induced x-ray transparency in the solid by the femtosecond x-ray pulse at high power densities. The measured kinetic energies of protons and deuterons ejected from the surface reach several keV and concur with predictions from plasma-expansion models. Simulations of the interactions were performed with a nonlocal thermodynamic equilibrium code with radiation transfer. These calculations return critical depths similar to the observed crater depths and capture the transient surface transparency at higher power densities.
[show abstract][hide abstract] ABSTRACT: The diffusion rate of hydrogen in Nb was calculated using ab initio molecular dynamics simulations. At low temperatures the hydrogen is strongly trapped in a local strain field which is caused by the elastic response of the lattice. At elevated temperatures, the residence time (τ) of hydrogen in an interstitial site is not sufficient for fully developing the local strain field. This unbinding of the interstitial hydrogen and the strain field increases the hopping rate (1/τ) at elevated temperatures (>400 K). These results call for a revision of the conceptual framework of diffusion of hydrogen in transition metals at elevated temperatures.
[show abstract][hide abstract] ABSTRACT: The influence of titanium and vanadium on the hydrogen transport rate through thin amorphous alumina films is addressed. Only small changes in the transport rate are observed when the Al 2 O 3 are covered with titanium or vanadium. This is in stark contrast to results with a Pd overlayer, which enhances the transport by an order of magnitude. Similarly, when titanium is embedded into the alumina the transport rate is faster than for the covered case but still slower than the undoped reference. Embedding vanadium in the alumina does not yield an increase in uptake rate compared to the vanadium covered oxide layers. These results add to the understanding of the hydrogen uptake of oxidized metals, especially the alanates, where the addition of titanium has been found to significantly enhance the rate of hydrogen uptake. The current findings eliminate two possible routes for the catalysis of alanates by Ti, namely dissociation and effective diffusion short-cuts formed by Ti. Finally, no photocatalytic enhancement was noticed on the titanium covered samples.
Journal of Alloys and Compounds - J ALLOYS COMPOUNDS. 01/2010; 494(1).
[show abstract][hide abstract] ABSTRACT: We present a method, combining optical transmission with electrical resistance, to study the hydrogen uptake in thin transition metal films. The change in optical transmission is used to determine the hydrogen concentration, while the change in resistance serves as an indicator for the ordering of hydrogen. We identify phase boundaries and regions of high and low orders as well as changes in the ordering temperature. As a demonstration of this approach we compare the hydrogen uptake of 50 and 10 nm single-crystal vanadium films. The ordered phase is found to be extended to higher temperatures in the thinner sample.
[show abstract][hide abstract] ABSTRACT: Neutron reflectometry was used to investigate the deuterium uptake in a thin metal superlattice that displays an anisotropic elastic response [ Fe0.5V0.5/V (6/21 monolayers)]. We see evidence of the presence of a coexistence region which has never before been seen in vanadium-based superlattices. The loading process is completely reversible from which we conclude that the phases must be coherent, in stark contrast to bulk VDx , where the two-phase region displays hysteresis. The deuterium-induced volume expansion exhibits changes in slope that correlate with the plateau region and the expansion is comparable and even larger than in the bulk. The spectacular cooperation between the elastic fields seems to be found only in one-dimensionally confined structures with biaxial compressive strain states.
[show abstract][hide abstract] ABSTRACT: Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atomic distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities. Notwithstanding the large lattice mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. On exposure to H2 gas a two-step hydrogenation process occurs with the Ti layers forming the hydride before Mg. From in situ measurements of the bilayer thickness Λ at different hydrogen pressures, we observe large out-of-plane expansions of Mg and Ti layers on hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. On unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.
[show abstract][hide abstract] ABSTRACT: The Bragg peaks in x-ray diffraction experiments are generally taken to be a reliable measure of the average lattice distance in crystals. We show that this assumption can be misleading when determining hydrogen-induced volume changes. Experiments based on simultaneous determination of the position of the Bragg peak and the sample volume of an Fe/V(001) superlattice while expanding the lattice by in situ hydrogen loading serve as an illustration of this counter-intuitive effect.
[show abstract][hide abstract] ABSTRACT: An experimental exploration of the transport rate of hydrogen through amorphous Al2O3 layers is presented. Significant changes in the transport rate were observed when changing the thickness of the oxide from 1 to 3 nm. By coating the oxide with a catalytically active Pd layer, a fast dissociation of hydrogen is allowed, enabling a separation between the limitation imposed by dissociation and the transport through the oxide.
Journal of Alloys and Compounds 01/2008; 464(1-2):L13-L16. · 2.39 Impact Factor