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.

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.

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.