Nanoscale structure and the hydrogenation of Pd-capped magnesium thin films prepared by plasma sputter and pulsed laser deposition

摘要

The structural and hydrogen storage properties were studied of nanostructured Mg thin films prepared by two different methods, namely plasma sputter deposition and pulsed laser deposition (PLD). Cross-sectional transmission electron microscopy (TEM) shows that in both cases the films grow in the shape of closely-stacked columns extending throughout the film thickness, while containing polycrystalline grains and grain boundary defects. Subsequent hydrogenation leads to a clear reduction in the presence of such defects. Selected area electron diffraction (SAED) on the films confirms the hcp-Mg to rutile tetragonal MgH_2 transformation upon hydrogenation, following the martensitic-like orientation relationship with Mg(0002)//MgH_2(1100)//Si(002). The hydrogen sorption temperatures reduce significantly from approx 670 to approx 475 K by capping the Mg films with a thin Pd layer, which plays a key role in enhancing the rate-limiting process of dissociating the hydrogen molecules at the sample surface. A maximum hydrogen uptake of 4-7.5 wt percent is reached under optimum hydrogen loading conditions of hydrogen pressures between 0.25 and 1.0 MPa at a temperature of approx 470 K for both types of films. Nevertheless, cycling experiments showed that a clear reduction in hydrogen content occurs within only a few cycles due to partial delamination of the top Pd layer, which poses a clear limit in practical applications.