Nucleation and growth in thin films are studied by using niobium-hydrogen ͑Nb-H͒ as model system. Hydride precipitation in thin films results in local surface topography changes that can be monitored by scanning tunneling microscopy. The local film expansion can be used to detect hydride precipitates, to study their growth, to gain information about their shape and their lattice coherency. With the help of theoretical calculations, it will be shown that cylindrical Nb-H precipitates evolve in early stages. These precipitates are coherent to the matrix as long as the film is thin and the hydride size is below a critical volume. Above this critical volume, a coherent-to-semi-coherent transition occurs. The critical size is controlled by the balance between the elastic energy stored in the coherent precipitate and the energy needed for the formation of dislocations. Consequently, films below 26 nm thickness keep coherency for all hydride precipitate volumes and never get semi-coherent.

No abstract

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.

The kinetics of hydride precipitation in epitaxial Nb films are studied by means of scanning tunneling microscopy (STM) using hydrogen gas loading. Due to the clamped state of thin films, hydride formation results in strong unidirectional out-of-plane film expansion that can be easily detected with STM. Hydrides are found to initially form with cylindrical morphology, leading to typical surface topographies. Their localized expansion allows the analysis of the hydride lattice matching, which is coherent (H1) at the initial stages and semicoherent (H2) at later stages. The volume fraction of H1 and H2 precipitates changes with time. At initial stages, the coherent precipitates dominate, while at later stages semicoherent precipitates become the dominant ones. The relative occurrence of H1 and H2 is bimodal. A maximum occurrence of 30-40 nm sized H1 hydrides is found, which is related to coherency stress between the hydride and the Nb matrix hindering a further hydride growth. It is further demonstrated that for Nb-H films adhered to substrates, the system can be locked in the two-phase region of the phase diagram (here at 10 −4 Pa at about 50% of hydride). This is different from bulk Nb-H, where the complete sample transforms into a hydride when the hydride formation equilibrium pressure is exceeded. Impact parameters on the lateral hydride arrangement are studied. The impact of the Pd-island surface coating and the intrinsic dislocation network on the precipitation density and arrangement appear to be negligible. However, the substrate miscut and, thus, the surface roughness exhibit a strong influence on hydride nucleation. The H1 hydride arrangement along (111) and the directed H2 hydride growth along (111) are governed by the elastically soft matrix lattice orientations.

No abstract