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.

It was recently shown that phases forming in thin films undergo a coherency state change depending on the film thickness. For Nb-H thin films, the coherency state was reported to change at about 38 nm. In this study the impact of the coherency state on the phase transformation kinetics is investigated for Nb films of two different film thicknesses (25 nm and 80 nm), below and above the state change thickness. The phase transformation in thin metal-hydrogen films can be studied by surface topography analyses via scanning tunneling microscopy (STM) because of the strong local lattice expansion of the hydride precipitates. STM on Nb-H reveals fast phase transformation kinetics for the 25 nm Nb-film, and much slower kinetics for the 80 nm film. This is suggested to be related to the change in the coherency between the Nb-matrix and the hydride precipitates.

We analysed the hydrogen generation during the smouldering of polymeric materials, which are typically used in the household, in the Smoke Density Chamber coupled to a new developed hydrogen sensor to detect early stages of fires. The results of hydrogen generation were compared with the emission of carbon monoxide and smoke during the fire scenarios. Additionally, the results were compared with parameters used in traditional commercial detection systems. In this scenario, the hydrogen sensor showed encouraging results for the detection of fires in earlier phase compared to traditional detectors. Furthermore, we tested the new developed hydrogen sensor in a real room with different fire scenarios. We have also investigated interferences, e.g. steam and cigarette smoke. The hydrogen sensor could detect hydrogen generation in the earliest stage of fire, even before CO and smoke were developed in detectable amounts. Therefore, the hydrogen sensor can be applied for early fire detection in case of pyrolysis. The sensors are quite good for detecting pyrolysis gases. But when it comes to a fast ignition other techniques are more suitable for it. The sensors are best for combination with other techniques, such as smoke detectors.

Most huge forest fires start in partial combustion. In the beginning of a smouldering fire, emission of hydrogen in low concentration occurs. Therefore, hydrogen can be used to detect forest fires before open flames are visible and high temperatures are generated. We have developed a hydrogen sensor comprising of a metal/solid electrolyte/insulator/semiconductor (MEIS) structure which allows an economical production. Due to the low energy consumption, an autarkic working unit in the forest was established. In this contribution, first experiments are shown demonstrating the possibility to detect forest fires at a very early stage using the hydrogen sensor.

A hydrogen sensor based on a silicon Metal/Solid Electrolyte/Insulator/Semiconductor (MEIS) structure with thin layers of the super-ionic conductor LaF3 and Pd gate metal was investigated in a hybrid structure on a resistance heater mounted on a ceramic substrate. The sensor was operated at room temperature, however, a short heating impulse once a day ensures a fast and reproducible dynamic behavior. The sensor was characterized in the concentration range typical for alarm levels up to 40% of the lower flammability limit and shows a logarithmic dependency of the sensor signal from the hydrogen concentration and a mean sensitivity of about 140 mV/decade. The response time (t90) is about 8 s. In a long term test for a period of three month, the sensor was exposed to hydrogen every two weeks. The sensor signal was shown to be stable in dynamic behavior, sensitivity and signal difference.

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.

Nucleation and growth in thin films are studied by using niobium-hydrogen (Nb-H) 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.

Nano-wire arrays of Niobium were produced by small angle sputtering on facetted sapphire, using the self shadowing effect of the facets. A wire width of about 80 nm was adjusted, the mean (maximum) wire height was about 20 nm (30 nm), the length can be in the cm range. Meander-film morphologies of 20 nm mean (26 run maximum) thickness were produced by conventional sputtering onto smooth sapphire substrates at elevated temperatures. The morphology of the wires was investigated with atomic force microscopy (AFM), using contact mode. Meander-films were studied by scanning tunnelling microscopy (STM).Hydrogen loading was performed by instantaneously increasing the hydrogen gas pressure above the solubility limit. Thus, an elongated hydride could be monitored in an about 30 nm thick wire. STM studies on meander-films show the presence of cylindrical hydrides. Local out-of-plane and in-plane expansion can be explained by the formation of hydrides, being coherent with the matrix. This was verified by finite-element calculations. The surface morphology modification associated with these coherent hydrides disappears reversibly, as soon as the hydrogen gas pressure is reduced. This indicates that plastic deformation does not occur in thin wires and meander films. (C) 2007 Elsevier B.V. All rights reserved.

In this paper we present STM images of thin epitaxial Nb-films taken in situ during H, gas-exposure. We find that the in-plane stresses relax by forming misfit dislocations near the substrate-film interface visible as glide steps at the film surface. The orientation of different glide steps at the surface shows that (011) and (121) glide planes are involved. Also, we show that the arrangement of the hydride is irregular in lateral direction and grows vertically through the complete film. (C) 2002 Elsevier B.V. All rights reserved.