Deuterium Distribution in Fe/V Multi-Layered Films.

The recent progress of Atom Probe Tomography (APT) has opened up atomic-scale elemental analysis including hydrogen species. For APT measurements, the use of deuterium is highly recommended, due to its low mobility compared to the fast and quantum mechanically tunneling isotope hydrogen. In addition, deuterium can be distinguished from hydrogen originating from the APT analysis chamber. To date, however, APT studies on materials with high D concentrations are scarce. In this study, the D concentration profile in a Fe/V multi-layered film sample was investigated, and spanned a wide concentration range. The mean hydrogen isotope concentration was alternatively quantified by electromotive force (EMF) measurements on a similar Fe/V film, thus verifying the APT results. The reduction found in the D concentration at the Fe/V interface results from local alloying at the Fe/V interfaces which accompanies a change in the available volume in the V lattice. Even at the same Fe concentration, the shape of the observed D depth profile was asymmetric at high D2 pressures. This indicates a stress impact caused by the deposition sequence.

Transformation Kinetics of LiBH4-MgH2 for Hydrogen Storage.

The reactive hydride composite (RHC) LiBH4-MgH2 is regarded as one of the most promising materials for hydrogen storage. Its extensive application is so far limited by its poor dehydrogenation kinetics, due to the hampered nucleation and growth process of MgB2. Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB2 with varying contents of 3TiCl3·AlCl3 as an additive, and combined kinetic measurements, X-ray diffraction (XRD), and advanced transmission electron microscopy (TEM) to develop a structural understanding. It was found that the formation of MgB2 preferentially occurs on TiB2 nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to ~4.7 × 107 J/m3 by creating an interface between MgB2 and TiB2, as opposed to ~2.9 × 108 J/m3 at the original interface between MgB2 and Mg. The kinetics of the MgB2 growth was modeled by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB2 growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB2. This transition is also reflected in the change of the MgB2 morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl3·AlCl3 results in the best enhancement of the dehydrogenation kinetics.

Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures.

Metal-hydrogen (M-H) systems offer grand opportunities for studies on fundamental aspects of thermodynamics and kinetics. When the system size is reduced to the nanoscale, microstructural defects as well as mechanical stress affect the systems' properties. This is contemplated for the model system of epitaxial niobium-hydrogen (Nb-H) thin films. Hydrogen absorption in metals commonly leads to lattice expansion which is hindered when the metal adheres to a flat rigid substrate. Consequently, high mechanical stress of about -10 GPa for 1 H/Nb are predicted, in theory. However, metals cannot yield such high stresses and respond with plastic deformation, commonly limiting measured stresses to -2 to -3 GPa for 100 nm Nb-H films. It will be shown that the coherency state changes with film thickness reduction, shifting the onset of plastic deformation to larger hydrogen concentrations. Below critical film thicknesses, plastic deformation is fully absent. The system then behaves purely elastic and ultra-high stress of about -10 (±2) GPa can be obtained. Arising stress controls the phase stability of M-H systems, and the coherency state strongly affects the nucleation and growth dynamics of the phase transition. In case of Nb-H thin films of less than 8 nm thickness the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed at 300 K. Related effects can be utilised to optimise metal-hydrides used in applications.

Suppression of Phase Transformation in Nb-H Thin Films below Switchover Thickness.

Hydrogen uptake in metal-hydrogen (M-H) nanosized systems (e.g., thin films, clusters) is both a fundamental and a technologically relevant topic, which is becoming more important due to the recent developments of hydrogen sensors, purification membranes, and hydrogen storage solutions. It was recently shown that hydrogen (H) absorption in nanosized systems adhered to rigid substrates can lead to ultrahigh mechanical stress in the GPa range. About -10 GPa (compressive) stress were reported for hydrogen loaded niobium (Nb) thin films. Such high stresses can be achieved when conventional stress-release channels are closed, e.g., by reducing the system size. In this paper, we demonstrate that the high stress can be used to strongly modify the system's thermodynamics. In particular, a complete suppression of the phase transformation is achieved by reducing the film thickness below a switchover value dso. Combined in situ scanning tunneling microscopy (STM) and in situ X-ray diffraction (XRD) measurements serve to determine the switchover thickness of epitaxial Nb/Al2O3 films in the thickness range from 55 to 5 nm. A switchover thickness dso = 9 ± 1 nm is found at T = 294 K. This result is supported by complementary methods such as electromotive force (EMF), electrical resistance, and mechanical stress measurements in combination with theoretical modeling.

Microstructural Study of MgB2 in the LiBH4-MgH2 Composite by Using TEM.

The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH4-MgH2 composite under the influence of the 3TiCl3·AlCl3 additives. The formation of the MgB2 phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB2 relies on different nucleation centers (Mg or TiB2 and AlB2). The varied nucleation and growth of MgB2 are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB2 and its nucleation centers. This leads to distinct MgB2 morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH4-MgH2. It was found that the formation of numerous MgB2 platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit.

The impact of nanoscale compositional variation on the properties of amorphous alloys.

The atomic distribution in amorphous FeZr alloys is found to be close to random, nevertheless, the composition can not be viewed as being homogenous at the nm-scale. The spatial variation of the local composition is identified as the root of the unusual magnetic properties in amorphous [Formula: see text] alloys. The findings are discussed and generalised with respect to the physical properties of amorphous and crystalline materials.

Insights into Hydrogen Gas Environment-Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable-Energy Positron Annihilation Spectroscopy.

Environmental transmission electron microscopy (ETEM) and variable-energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free palladium (Pd) foils and stressed Pd thin films grown on rutile TiO2 substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures ( pH < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found, and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H2 pressure. The comparison of the two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

Defect generation in Pd layers by 'smart' films with high H-affinity.

In this paper, we demonstrate that the microstructure and the surface of a thin palladium (Pd) film can be intentionally altered by the presence of a subjacent niobium (Nb) film. Depending on the thickness of the Nb film and on the hydrogen gas pressure, defects in the Pd film can be healed or created. To demonstrate this effect, Pd/Nb/sapphire (Al2O3) stacks are studied during hydrogen gas exposure at room temperature by using scanning tunneling microscopy (STM), X-ray diffraction (XRD) and environmental transmission electron microscopy (ETEM). STM shows that hydrogen-induced topography changes in the Nb films depend on the film thickness which affects the height of the Nb surface corrugations, their lateral size and distribution. XRD measurements show that these changes in the Nb hydride film influence the microstructure of the overlaying Pd film. ETEM reveals that the modifications of the Pd film occur due to the precipitation and growth of the Nb hydride phase. The appearance of new defects, interface and surface roughening is observed in the Pd film above locally grown Nb hydride grains. These results can open a new route to design 'smart' catalysts or membranes, which may accommodate their microstructure depending on the gaseous environment.