Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdH_x films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure-optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal-hydrogen systems.

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MgHx thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.@en

Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50 nm thick Mg₇₀Ti₃₀ film capped with a 30 nm Pd catalytic layer we demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amount of hydrogen in the vicinity of the detector. We report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. We further show that the sensing behavior in oxygen rich gas mixtures and in normal air can be improved by adding silver to the catalytic Pd cap layer.

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We report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic hydrogen detectors. Due to the change in optical properties of Mg-based alloys on hydrogen absorption, a drop in reflectance by a factor of 10 is demonstrated at hydrogen levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg-Ni and Mg-Ti based alloys, we find that the latter has superior optical and switching properties. Using a 50 nm thick Mg₇₀Ti₃₀ film capped with a 30 nm Pd catalytic layer as the sensing layer, we report on the sensitivity of the fiber optic detector for hydrogen both in argon and oxygen, the temperature behavior and the reversibility.

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The optical switching from shiny metallic to "black" absorbing states of 30 nm Mg2Ni/50 nm TM/10 nm Pd (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Pd) trilayer thin films upon hydrogenation is used to determine the hydrogen absorption/desorption kinetics at moderate H2 pressure and room temperature. A strong correlation is observed between these kinetics and the enthalpies of solution of H in TM. A diffusion model based on chemical potentials of hydrogen in metals has been developed that agrees qualitatively with the experimental data, both from kinetics and hydrogen concentration profile points of view. The influences of the solution enthalpy and the diffusion coefficient of H in TM and that of the TM layer thickness are discussed with respect to this model. Finally, we conclude with potential applications of these trilayers, from optical hydrogen detection to reinvestigation of diffusion coefficients of H in TM.@en

Hydrogenography, an optical high-throughput combinatorial technique to find hydrogen storage materials, has so far been applied only to materials undergoing a metal-to-semiconductor transition during hydrogenation. We show here that this technique works equally well for metallic hydrides. Additionally, we find that the thermodynamic data obtained optically on thin Pd-H films agree very well with Pd-H bulk data. This confirms that hydrogenography is a valuable general method to determine the relevant parameters for hydrogen storage in metal hydrides.@en

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.

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In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPd_y with 0.023 ≤ y ≤ 0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5 bar H₂ within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5 eV ≤ℏω≤6.0 eV corresponding to 2500 nm≥λ ≥ 210 nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH₂.

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