A kinetic investigation of MgH₂ formation/decomposition in non-catalysed MgO–Mg thin films

Highlights: • A 100 nm MgO spacing layer inhibits Mg₂Si formation in Mg thin films up to 450 °C. • Formation of Mg(OH)₂ and Mg evaporation during thermal treatments of the layers. • Dendritic formation of MgH₂ in a very low amount. - Abstract: Mg thin films without Pd-cap layer have been grown by e-beam evaporation on fused SiO₂ substrates. A spacing MgO layer was grown between the substrate and the Mg with the purpose of avoiding Mg₂Si formation. Reactivity of the layers has been studied at high temperature in Ar and Ar/H₂ atmospheres in order to investigate their modifications at the conditions used in hydrogenation experiments. Two processes affecting light transmission through the sample have been observed: (1) formation of Mg(OH)₂ or/and MgO nuclei and (2) partial evaporation of Mg crystals from the Mg layer. The MgO spacing layer effectively avoids reaction of the substrates and the Mg layer. Formation of MgH₂ was done at 290 °C and 11 bar H₂ for as long as 1 week. In spite of the long time passed, only small amounts of dendritic MgH₂ nuclei were formed on the films. Decomposition of the MgH₂ was studied by thermal desorption spectroscopy and correlated to the changes in transmitted light intensity during the hydride decomposition. The fraction of sample converted to MgH₂ is only 15 ± 5% of the whole sample. The peak temperature in the H-desorption curve is higher than expected for thin films, closer to that of bulk MgH₂