Structure modification of Mg-Nb films under hydrogen sorption cycles

In the present work we focus our attention on the structural modifications induced by repeated absorption/desorption cycles on Mg-Nb layers. Samples consisting of a 30 microns thick pure Mg or Mg-5 at.% Nb doped films, coated with a 20 nm thick Pd layer were submitted to repeated H(2) sorption cycles in a volumetric apparatus. Isothermal desorption analysis at 350 degrees C was performed to evaluate the amount of absorbed hydrogen. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and electron microscopy techniques (SEM and TEM) were used for the structural characterisation of the samples. Analyses show a deep modification of the material upon cycling. The presence of Nb enhances the structural modifications and induces an initial lattice contraction of the Mg matrix that tends to decrease on cycling via the formation of Nb nanoparticles (with average size of similar to 10 nm). SEM and TEM observations performed in cross section evidenced the formation of a porous structure.