水素吸蔵合金 Mg/Cu 超積層体における初期活性化に伴う微細組織と水素吸蔵特性の変化

Abstract

Mg/Cu super-laminates prepared by repetitive fold and roll method using conventional two-high roll mill was investigated on changes of micro/nano-structures and hydrogen absorption properties during initial activation. The results of in-situ XRD analyses suggested that the hydrogen absorption/desorption mechanism of Mg/Cu super-laminates was as follows. In the hydrogenation process at 573 K, intermetallic phase Mg2Cu is formed through interdiffusion between Mg and Cu layers, and then the Mg2Cu decomposes into MgH2 and MgCu2 through disproportionation. In the subsequent dehydrogenation process at 573 K, the MgH2 dessociates H2 and metallic Mg. The Mg reacts with MgCu2, and then Mg2Cu is formed again. Therefore, Mg2Cu phase absorbs and desorbs hydrogen reversibly through this sequence. The results of TEM observations revealed structure changes of Mg/Cu super-laminates during initial activation. Interesting points of micro/nano-structures are summarized as follows: In as-rolled Mg/Cu super-laminates, (1) laminated structures in size of sub-micrometer thickness are constructed of Mg and Cu layers, (2) lattice defects are dense. In initial activated super-laminates, (1) laminated structures are preserved, (2) pores with a diameter of sub-micrometer distribute uniformly between Mg2Cu layers, which leads to large surface area. In Mg/Cu super-laminates during initial activation, (1) laminated structures are preserved, (2) spongiform structures composed of MgH2 and MgCu2 are formed. The spongiform structures at the first hydrogenation are coarse and become fine at the second hydrogenation, i.e. the diffusion distance for hydrogen absorption reaction becomes shorter after initial activation. Hydrogen absorption test reveled that Mg/Cu super-laminates absorb hydrogen much faster than Mg2Cu powder and initial activation improve kinetics of both super-laminates and powder, although super-laminates absorb hydrogen much faster than powder, again. Micro/nano-structures of Mg/Cu super-laminates such as large surface area, dense defects and short diffusion distance for the reaction enhances kinetics and enable Mg/Cu super-laminates to absorb hydrogen very fast, which would contribute to achieve high performance hydrogen storage materials.