**1. Introduction**

Magnesium (Mg) can store 7.6 mass% of hydrogen after formation of magnesium hydride (MgH2), which has attractive features for hydrogen storage material such as low cost, abundant resource and light weight [1]. However, dehydrogenation temperature is very high (560 K at 0.1 MPaH2) because MgH2 is thermodynamically stable (Δr*H* = −72.8 ± 4.2 kJ mol<sup>−</sup><sup>1</sup> , Δr*S* = −142 ± 3 J K<sup>−</sup><sup>1</sup> mol<sup>−</sup><sup>1</sup> ) [2]. In addition, their hydrogenation/dehydrogenation kinetics is also lower, then the conditions of hydrogenation and dehydrogenation are severe and core-shell-type hydride is formed. In order to obtain MgH2 completely from Mg and effectiveness of hydrogenation/dehydrogenation process, it is necessary to finely pulverize, severe plastic deformation, heat treatment for a long time, and addition of catalyst [3–10].

Mg is a metal and when it reacts with H2, MgH2 forms an ionic bond and a weak covalent bond between Mg-H and the valence number of the ion is indicated as Mg1.91+ and H0.26<sup>−</sup> [11]. The diffusion coefficient of H in MgH2 is several order of magnitude lower when compared to that in Mg: *D*<sup>H</sup> Mg = 7 × 10−11 m2 s<sup>−</sup><sup>1</sup>

(300 K) in Mg and *D*<sup>H</sup> MgH2 = 1.1 × 10−20 m2 s<sup>−</sup><sup>1</sup> (305 K) in MgH2 [12, 13]. Based on these characteristics, powder Mg forms core-shell-type structure hydride, MgH2 as a shell and unreacted Mg remains in the core [14, 15] making progress of completely hydrogenation difficult. On the other hand, the hydrogen partial pressure has a great influence on the progress of the hydrogenation. When the hydrogen partial pressure is high, since MgH2 quickly covers the Mg powder surface, hydrogenation halts and the amount of hydride concentration decreases markedly, whereas when the hydrogen partial pressure is low, the time until MgH2 covers the Mg surface extends, then the hydride concentration increases [16]. Therefore, to accomplish the efficient hydrogenation, the process of hydrogenation should be revealed.

As a result of investigation aiming at efficient hydrogenation, some curious microstructural characteristics were obtained, that is shell of MgH2 and core Mg in addition to MgH2 in the core Mg. In the following, MgH2 on the surface layer named as MgH2(sur) and that in Mg core named as MgH2(int). MgH2(int) formed in the Mg core is distinguished from MgH2(sur). The particle size of MgH2(int) in Mg<sup>−</sup><sup>6</sup> mass%, Al-1 mass%, Zn alloy was larger than that in pure Mg. This result shows the grain size of MgH2(int) would be in correlation with Al concentration. Therefore, in this study, the influence of Al concentration in Mg on formation of MgH2(int) is clarified. To reveal the mechanism of MgH2(int) formation, coarse-type Mg-based hydrogen storage materials will be developed. Bulky hydrogen storage materials are attractive for handling, safety, and applying large module.
