Characterization of MgH2 formation by low-energy ball-milling of Mg and Mg+C (graphite) mixtures under H2 atmosphere

Abstract

In the present work the formation of nanocrystalline magnesium hydride by reactive mechanical alloying (RMA) of a mixture of magnesium and 10 wt.% graphite flakes is reported. The synthesis was done at room temperature under 5 bar of hydrogen using a low energy milling device. Magnesium without graphite was also milled as a reference material. We determine phase evolution by X-ray diffraction (XRD), thermal dehydriding properties by differential scanning calorimetry (DSC) and morphological and microstructural characteristics by laser granulometry, BET surface analysis, optical microscopy and scanning electron microscopy (SEM), at different stages of the milling process. The formation of MgH2 occurs faster in the graphite-added material, due to the lubricant properties of graphite. It reaches a hydrogen capacity of 6.2 ± 0.1 wt.% H in 50 h, whereas the reference material needs 100 h to load 7.1 ± 0.1 wt.% H. During the synthesis, both materials follow the same sequence of microstructural and morphological changes, though in different time scales. After the synthesis, both materials present similar microstructural and morphological characteristics. As regards dehydriding properties, we found that graphite plays a catalytic role in the decomposition of magnesium hydride. The graphite-added material decomposes always at lower temperatures than the reference material. Additionally, we observe that particle size instead of crystallite size is the relevant property that influences hydrogen desorption kinetics.