Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films

Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics. Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime. A σDOS model is proposed, taking the measured substrate-induced stress contribution to the chemical potential into account. This enables the determination of the different sites’ volume fractions and of pure site energy distributions by fitting measured isotherms. Interstitial sites, grain/domain boundary sites and deep traps are distinguished. Dislocations and vacancies are shown to have a minor impact on the films’ trapping of hydrogen atoms, while deep traps are related to the films’ surface. Enhanced binding energies in nano crystalline films can be ascribed to the tensile strain effect of grain boundaries acting on the grains. Measured surface trapping energies fit to the respective bulk values, while the trapping of hydrogen in grain/domain boundaries of the films is significantly increased. This can be interpreted with different grain/domain boundary structures. Different from octahedral interstitial site occupation, tetrahedral site occupation is suggested for grain/domain boundaries of the films

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 D$_2$ pressures. This indicates a stress impact caused by the deposition sequence

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

Hydrogen site occupancy and strength of forces in nano-sized metal hydrides

The dipole force components in nano-sized 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 Fe_0.5V_0.5/V superlattice, the hydrogen unequivocally occupies octahedral z-like sites, even at and below concentrations of 0.02 [H/V]

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 Fe$_{Zahl}$1-xZr$_{x}$ alloys. The findings are discussed and generalised with respect to the physical properties of amorphous and crystalline materials

Flexible Powder Production for Additive Manufacturing of Refractory Metal-Based Alloys

The quality and properties of metal powders are essential for powder metallurgical (PM) processes in general and for additive manufacturing (AM) processing routes in particular. Thus, a variety of atomization technologies were established meeting the multiple needs of the different processing technologies. However, the production of refractory metal alloy powder remains challenging due to their high liquidus temperatures (>2000 °C), the formation of brittle intermetallic phases, as well as the reactivity with and sensitivity to interstitials of the constituting elements. In this contribution, powders made of Mo-20Si-52.8-Ti (at.%) were produced by a novel ultrasonic atomization (UA) process at laboratory-scale using an industrial electrode induction gas atomization (EIGA) process with a modified electrode concept for the first time. UA allows flexibility in alloy composition due to the arc melting-based principle, while the EIGA electrode is PM manufactured from elemental powders to provide similar flexibility on a larger scale. The powders resulting from these two processes were compared with respect to size distribution, sphericity, microstructure and phase constitution, chemical composition, and interstitial impurity content. In addition, several powder batches were produced with the UA process in order to assess the process reliability and stability. The properties, quality, and quantities of UA powders perfectly meet the requests for alloy development for powder bed fusion AM, while the modified EIGA process allows the upscaling of the alloy powder quantities

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

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

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