For many hydrogen related applications it is preferred to use optical hydrogen sensors above electrical systems. Optical sensors reduce the risk of ignition by spark formation and are less sensitive to electrical interference. Currently palladium and palladium alloys are used for most hydrogen sensors since they are well known for their hydrogen dissociation and absorption properties at relatively low temperatures. The disadvantages of palladium in sensors are the low optical response upon hydrogen loading, the cross sensitivity for oxygen and carbon, the limited detection range and the formation of micro-cracks after some hydrogen absorption/desorption cycles. In contrast to Pd, we find that the use of magnesium or rear earth bases metal-hydrides in optical hydrogen sensors allow tuning of the detection levels over a broad pressure range, while maintaining a high optical response. We demonstrate a stable detection layer for detecting hydrogen below 10% of the lower explosion limit in an oxygen rich environment. This detection layer is deposited at the bare end of a glass fiber as a micro-mirror and is covered with a thin layer of palladium. The palladium layer promotes the hydrogen uptake at room temperature and acts as a hydrogen selective membrane. To protect the sensor for a long time in air a final layer of a hydrophobic fluorine based coating is applied. Such a sensor can be used for example as safety detector in automotive applications. We find that this type of fiber optic hydrogen sensor is also suitable for hydrogen detection in liquids. As example we demonstrate a sensor for detecting a broad range of concentrations in transformer oil. Such a sensor can signal a warning when sparks inside a high voltage power transformer decompose the transformer oil over a long period.

Mg₂NiH₄ thin films have been prepared by activated reactive evaporation in a molecular beam epitaxy system equipped with an atomic hydrogen source. The optical reflection spectra and the resistivity of the films are measured in situ during deposition. In situ grown Mg₂NiH ₄ appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg₂NiH₄ phase is kinetically blocked. Hydrogen desorption only takes place when a Pd cap layer is added. The optical band gap of the in situ deposited Mg₂NiH₄ hydride, 1.75 eV, is in good agreement with that of Mg₂NiH₄ which has been formed ex situ by hydrogenation of metallic Pd capped Mg₂Ni films. The microstructure of these in situ grown films is characterized by a homogeneous layer with very small grain sizes. This microstructure suppresses the preferred hydride nucleation at the film/substrate interface which was found in as-grown Mg₂Ni thin films that are hydrogenated after deposition.

The engineering of pure and metal alloy catalysts for hydrogen absorption is needed to improve the kinetics of hydrogen-related devices. We introduce a new route to search for alloys that can yield superior catalytic behavior for hydrogen absorption, using an optical technique to measure the catalytic activity for hydrogen sorption of thin films. The catalytic activity of the noble metals Pd, Ni, Cu, Ag, Pt, and Au is studied as a function of hydrogen pressure and temperature. The rate-limiting step is identified by the pressure dependence and the normal isotope effect for hydrogen absorption. The measured rates and activation energies are correlated to such physical properties as activation barrier for dissociation and heat of solution. The observed compensation effect is explained in the framework of interplay between surface and bulk processes. From the experimentally derived model, a guiding principle for the search for catalysts promoting hydrogen absorption is drawn.

Hydrogen absorption by a thin Mg₂Ni film capped with Pd results in the nucleation of the Mg₂NiH₄ phase at the film/substrate interface. On further hydrogenation, a self-organized two-layer system consisting of a Mg₂NiH_0.3/Mg₂NiH₄ bottom-layer and a Mg₂NiH_0.3 top-layer is formed. This leads to an intermediate optical black state in Mg₂Ni thin films, which transforms from metallic/reflective to semiconducting/transparent upon hydrogenation. This hydrogen absorption behavior is completely unexpected, since the hydrogen enters the film through the Pd-capped film surface. To explain the preferential nucleation of Mg₂NiH₄ at the substrate/film interface, we determine the chemical homogeneity of these thin films by RBS and SIMS. Furthermore by STM, TEM and SEM, we analyze the microstructure. We find that up to a film thickness of 50 nm, the film consists of small grains and clusters of small grains. On further growth, a columnar structure develops. We propose that the nucleation barrier for the formation of the Mg₂NiH₄ phase is smaller for the small loosely packed grains at the interface, while the columnar grain boundaries promote the hydrogen diffusion to the substrate.

A novel optical technique to measure catalytic activity for hydrogen sorption is presented. The catalytic activity of the noble metals of Pd, Ni, Cu and Ag is studied as function of pressure and temperature. The uptake rate is limited by chemisorption as deduced by modelling kinetics. An empirical relation between uptake rate and the measured activation energy is given. The catalytic activity of NbO_x is studied as a function of the oxygen concentration determined from electron spectroscopy. The measured activation energy depends strongly on the oxygen concentration and reaches a minimum of 0.6 eV at the highest oxygen content (x=2.5, i.e. Nb₂O₅). Pure Nb does not display a significant catalytic efect.

Thin films of Mg₂NiH_x which exhibit a remarkable black state with low reflection over the entire visible spectrum were investigated. The films show zero transmission and a low electrical resistivity. It was shown that such a black state is not explicable for a homogeneous layer. The results show that a large absorption coefficient yields substantial reflection.