Synthesis of Mg(AlH4)2 in bilayer Mg/Al thin films under plasma immersion hydrogen ion implantation and thermal desorption processes

The behavior of hydrogen in Mg/Al bilayer films introduced by 1 kV pulsed hydrogen plasma immersion ion implantation has been studied. The changes in structure and composition of hydrogenated films were analyzed by glow discharge optical emission spectroscopy, X-ray diffraction (XRD) technique and scanning electron microscope (SEM). The XRD studies show that samples implanted to saturation at temperature below 370 K exhibit nanocrystalline magnesium alanate, Mg(AlH4)2, which is potential hydrogen storage material. The hydrogenated films demonstrate homogeneous distribution of matrix elements and hydrogen across their thickness. The thin 3-5 nm thick layer of the natural Al2O3 oxide protects magnesium alanate from the direct contact with air. The correlation between the effusion of implanted hydrogen and the evolution of surface morphology was studied by the thermal desorption plasma spectroscopy and SEM. In 400-600 K temperature region there is only a weak effusion of hydrogen because the surface oxide layer is not permeable. SEM analysis reveals bubbles at a temperature of 500 K and the increase of their number density and size up to a temperature of ∼575 K. The major part of hydrogen effuses at ∼640 K. The effusion process has an activation energy of 1.85 ± 0.05 eV which is very close to the activation energy for hydrogen diffusion in γ-Al2O3.