Deuterium storage in nanocrystalline magnesium thin films

Abstract

Nanocrystalline magnesium deuteride thin films with the β-MgD2 structure were prepared by vacuum evaporation of hexagonal magnesium (h-Mg) samples and thermal annealing in 0.15 MPa D2 atmosphere at 373 K. Thermal desorption spectroscopy analysis indicated that the rate-limiting step in the deuterium desorption was given by the thermal decomposition of the deuteride phase. The activation energy Δg of the β-MgD2→h-Mg+D2 reaction scaled from 1.13±0.03 eV in 650-nm-thick films to 1.01±0.02 eV in 75-nm-thick films most likely as consequence of different stress and defect level. Positron annihilation spectroscopy analysis of the thin-film samples submitted to deuterium absorption and desorption cycles reveal the presence of a high concentration of void-like defects in the h-Mg layers after the very first decomposition of the β-MgD2 phase, the presence of these open volume defects reduces the D2 absorption capacity of the h-Mg thin film.

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

“…The trapping defects can be vacancy-type defects at the interface, e.g., related to the prestage of delamination involving breaking of bonds at the interface, to the diffusion of Pd, creating an intermixed interface area containing vacancies, or to the transformation of the top Pd-Mg layer into Mg 2 Pd which may contain itself positron-trapping defects. Delamination itself does not yet occur at these temperatures, consistent with positron studies on Pd-capped Mg films at the still lower hydrogen sorption temperatures of around 373-390 K. 11 For completeness we note that the full VEPFIT analysis 36 of the positron Doppler curves in Figs. 1, 7, and 8 is consistent with this picture.…”

“…XRD showed that this particular film was hydrogenated partially, attaining a MgH 2 phase fraction of ϳ63% ͑ϳ5 wt % hydrogen loading͒. The estimated change in Doppler S-parameter for the complete transformation of Mg to MgH 2 is therefore ⌬S / S ϳ −4.5%, i.e., close to the reduction of ϳ5% reported by Checchetto et al 11 The reduction in the S-parameter stems from a clear broadening in the valence part of the electron momentum density with formation of the metal hydride, as observed in a Compton scattering study on Mg and MgH 2 powders by Garreau and Loupias. 23 The reduction is therefore primarily related to the change in valence electron momentum distributions ͑EMDs͒ of the bulk metal and metal hydride systems, as supported by the calculated momentum distributions of valence electrons in Mg and MgH 2 shown in Fig.…”

“…In this perspective, positron annihilation methods, 9,10 applied widely in the field of thin film microelectronic active layers, 9 are promising owing to their depth-profiling capabilities typically in the submicrometer range, and their sensitivity to the local electronic properties, the material phase, and the presence and type of vacancyrelated defects. Previous thermal desorption and structural studies on Mg thin films 11 presented complementary positron depth-profiling data, which indicate that open-volume defects appear in the Pd cap layer after the first decomposition of the MgD 2 film, without affecting the catalytic activity of the cap layer. Further, the depth profiles presented in Ref.…”

Meeting challenges in nanomaterials for sustainable energy applications using high‐resolution positron methods

“…Recent studies on semiconductor NCs [11][12][13][14] and metal hydrides [15][16][17] showed the promise of positron annihilation to aid in solving bottlenecks in these fields. The positron can be used as a sensitive probe for detecting the surface composition of semiconductor NCs and the confinement of the upper valence electron states (Fig.…”

Mg-Based Thin Films as Model Systems in the Search for Optimal Hydrogen Storage Materials

“…hydriding/dehydriding cycles was also studied by the positron annihilation spectroscopy (PAS) [58]. After the very first decomposition of MgD2 the increase of void-like defects was observed accompanied by the decrease in deuterium storage capacity.…”