For Pd thin films, microstructural changes involved during hydrogen cycling provide the information needed to predict and optimize the film's mechanical strength. In this paper, a systematic study of the morphology, microstructure, texture, and stress has been performed on Pd thin films during hydrogen loading and deloading cycles at room temperature. Pd thin films of similar morphology were prepared by magnetron sputtering on substrates of different compliances, i.e., Si-oxide, Titanium (Ti) and Polyimide (PI). The evolution of the morphology, grain-orientation distribution (texture), state of stress, and dislocation densities are analyzed for each of the film substrate types for 20 hydrogen loading/deloading cycles. The lattice expansion and contraction caused by the transition from Pd to Pd-hydride and back result in a strong stress increase. This stress increase stabilizes after a few cycles by grain boundary motion that leads to a gradual enhancement of the (111) texture and changes in the dislocation density for Pd films that are strongly clamped on to an oxidized Si(100) wafer substrate with an intermediate layer (Ti or PI). For Pd on PI, the stress is also partly released by a crack-based (crack widening/growth/propagation) pathway. Pd films on Ti and PI do not buckle or blister after 20 hydrogen cycles. By providing a sufficiently compliant substrate the traditional problems of buckle-delamination of a film on a stiff substrate are mitigated.

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Hydrogen has been contemplated as a desirable energy source of the future with enormous possibilities to create a carbon-neutral society. Since palladium (Pd) readily absorbs hydrogen even at low pressure and room temperature, Pd and its alloys are suitable for hydrogen production, purification, storage, gas sensors, and fuel cell catalyst. However, primary requirements for industrial applications are not always satisfied, such as usability under operation conditions, minimum capital cost, and sustained hydrogen embrittlement. Therefore, developing stable Pd-based thin films to investigate correlations between microstructural features and mechanical properties of a material is of great importance for many hydrogen-related technologies. In this thesis particular interest has been focused on the stability of a series of magnetron sputtered Pd thin films of different nanostructures i.e., non-voided compact and nano-voided open columnar morphology. The X-ray diffraction (XRD) analysis methods are advanced, utilizing the tailored microstructures of the Pd films suitable to investigate the interplay between microstructure and hydrogenation properties of Pd-based thin films. Interpretation of the stress state and microstructural changes during hydrogen cycling are studied utilizing XRD line-profile analysis and the deformation mechanisms are systematically discussed. The change in dislocation density by the generation and annihilation of dislocations at interfaces reflects the difference in film-substrate interaction. The insertion of an intermediate layer between the Pd film and a rigid substrate can prevent buckle-delamination that is caused by the large volume expansion due to hydrogen absorption but it also changes the hydrogen absorption performance. The different effects on the absorption properties in the case of compliant (polyimide) and rigid (titanium) intermediate layers are illustrated. The results of this work showed that the strong clamping usually suppresses or reduces hydrogen absorption, whereas, the flexible layer enhances the lifetime of Pd thin films when exposed to prolonged hydrogen during cycling. The research in this thesis deepens the understanding about an appropriate combination of film microstructure and choice of the intermediate layer to strengthen Pd-based thin films. @en

In this work, we studied the mechanical and thermal stability of ~100 nm Pd thin films magnetron sputter deposited on a bare oxidized Si(100) wafer, a sputtered Titanium (Ti) intermediate layer, and a spin-coated Polyimide (PI) intermediate layer. The dependence of the film stability on the film morphology and the film-substrate interaction was investigated. It was shown that a columnar morphology with elongated voids at part of the grain boundaries is resistant to embrittlement induced by the hydride formation (α↔β phase transitions). For compact film morphology, depending on the rigidity of the intermediate layer and the adherence to the substrate, complete transformation (Pd-PI-SiO₂/Si) or partly suppression (Pd-Ti-SiO₂/Si) of the α to β-phase was observed. In the case of Pd without intermediate layer (Pd-SiO₂/Si), buckling delamination occurred. The damage and deformation mechanisms could be understood by the analysis of the stresses and dislocation (defects) behavior near grain boundaries and the film-substrate interface. From diffraction line-broadening combined with microscopy analysis, we showed that in Pd thin films, stresses relax at critical stress values via different relaxation pathways depending on film-microstructure and film-substrate interaction. On the basis of the in-situ hydriding experiments, it was concluded that a Pd film on a flexible PI intermediate layer exhibits free-standing film-like behavior besides being strongly clamped on a stiff SiO₂/Si substrate.

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In this work, the effect of the deposition parameters of a magnetron sputtered Ti adhesive intermediate layer on the morphology and hydrogen absorption properties of thin Pd films of about 100 nm was investigated. The insertion of an adhesive layer between a Pd film and a rigid substrate usually suppresses or reduces hydrogen absorption. In this study, it is shown that by tuning the surface topography of the intermediate layer the morphology, crystallographic texture and hydrogen absorption properties of the Pd film can be controlled. The surface topography of the Ti layer was characterized using atomic force microscopy. The surface topography strongly depends on the Ti deposition conditions and can vary from widely spread large islands to densely packed small-grained islands depending on thickness (between 1 and 6 nm) and sputter pressure (0.4 and 3 Pa). TEM and XRD analysis led to the conclusion that rough Ti intermediate layers result in Pd films with an open columnar structure with small voids, and a weak and broad (111) texture. Smooth Ti intermediate layers promote the formation of Pd films with a dense columnar structure with fewer voids, and a strong and sharp (111) texture. Changes in the Pd adatom surface diffusion and shadowing effects are the main cause of the observed differences. Pd films with an open columnar morphology and weaker texture show better hydrogen absorption properties with respect to absorption capacity and kinetics of the films with dense columnar morphology. By tuning the surface topography of the Ti adhesive layer, Pd films with controlled morphology and texture can be prepared such that no delamination from the substrate occurs without compromising on absorption properties.

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