純MgおよびMg-Al-Zn合金における金属内部でのMg水素化物の生成

Abstract

Mg is hydrogenated as core-shell type hydride in which the surface is covered with MgH2 (shell) and unreacted Mg remains at the internal side (core). Therefore, increase of absorption capacity to the theoretical hydrogen capacity is still one of the most important issues for the hydrogen storage materials, although several treatments were proposed to enhance their absorption capacity. In this study, the procedure of the core-shell structure as well as effect of Al concentration in Mg on the growth MgH2 in Mg were investigated using pure Mg and Mg-(3-9)mass%Al-1 mass%Zn alloys (AZ). MgH2 was formed on the surface as well as inside of unreacted Mg core which was apart from both gaseous H2 and the surface MgH2 layer. The inside MgH2 was formed in a granular form on Mg grain boundary and its size increased by applying plastic deformation. Thickness of the surface MgH2 and size of the internal MgH2 increased with an increase in hydrogenation time until the hydride surface was completely covered with MgH2. However, the growth of the surface and internal MgH2 came to a halt after the surface was covered with MgH2. The increase in Al concertation in AZ lead to decrease in thickness of the surface MgH2, prolongation of the halt time and increase in the internal MgH2 grain size. From these results, supplying H from metal side was dominantly contributed for growth of the surface and internal MgH2 because diffusion rate of H in Mg was much higher than that in MgH2. In addition, the growth of internal MgH2 as well as control of surface MgH2 can contribute to the promotion of the complete hydrogenation of Mg based hydrogen storage materials.