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.

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.