Development of novel Mg-based ternary alloys with high hydrogen storage capacity

镁基贮氢合金由于具有高的贮氢容量而具有广阔的应用前景。本研究利用高压实验装置，在1）MgH_2-TM（TM = Mn，Fe，Co，Ni，Cu），2）MgH_2-CaH_2-Ni和3）MgH_2-REH_3（RE = Y，La）体系中，探索了新型镁基复合材料的制备方法。所使用的高压装置能够产生高达6 GPa的伪均衡压力。结果表明，在3 × 5GPa的高压范围内，合成了Mg_2Ni_3H_<3.4>（正交）、Mg_3MnH_<5.6>（单斜）、（Ca_<0.6>Mg<0.4>）_2NiH_x（BCC）、MgCaH_4（BCC）、MgY_2H_<7.8>（FCC）、Mg_3LaH_9（四斜晶系）等六种新的化合物。关于氢含量，发现Mg_3MnH_<5.6>具有4.2质量%的氢。在460 ~ 650 K温度范围内，除MgY_2H_2外，这些新型的高压氢氧化物均沿着氢的脱附而分解<7.8>。MgY_2H_2<7.8>在600 K下部分脱氢后仍能保持其晶体结构。通过第一性原理计算发现，MgY_2H_3<7.8>的放氢来源于O位，而T位上的氢对保持晶体结构起着重要作用。第一性原理计算也支持部分氢脱附后的晶格膨胀。除了探索新的晶粒尺寸外，还研究了氢处理对AZ 31合金晶粒尺寸的影响。通过对加氢和脱氢条件的优化，发现加氢后出现MgH_2、Al、<0.42>Mg_Al_3<0.58>相（分解过程），脱氢后形成固溶体相（复合过程）。发现经受HDDR现象的AZ 31合金具有约100 nm的细晶粒尺寸。对板状AZ 31镁合金进行了氢处理。在这种情况下，对厚度为20 μm的表面积进行HDDR处理。

Mg-based hydrogen storage alloys are promising in view of high storage capacity. In this study, novel Mg-based hydrides were explored in 1) MgH_2-TM (TM = Mn, Fe, Co, Ni, Cu), 2) MgH_2-CaH_2-Ni, and 3) MgH_2-REH_3 (RE = Y, La) systems by using a cubic anvil-type high-pressure apparatus. The high-pressure apparatus used was capable of generating pseudo-isostatic pressure as high as 6 GPa. As a result, novel hydrides of Mg_2Ni_3H_<3.4> (orthorhombic), Mg_3MnH_<5.6> (monoclinic), (Ca_<0.6>Mg<0.4>)_2NiH_x (BCC), MgCaH_4 (BCC), MgY_2H_<7.8> (FCC), Mg_3LaH_9 (tetragonal) were prepared under high pressure range of 3〜5 GPa. With respect to a hydrogen content, Mg_3MnH_<5.6> was found to have 4.2 mass% of hydrogen. These novel high-pressure hydrides decomposed along with hydrogen desorption in the temperature range of 460〜650 K except for MgY_2H_<7.8>. MgY_2H_<7.8> was able to hold its crystal structure even after partial hydrogen desorption at 600 K. From first principle calculations, it was found that, for MgY_2H_<7.8>, emitted hydrogen originated from O-sites, and remaining hydrogen on T-sites played an important role for holding the crystal structure. The first principle calculation also supported the lattice expansion after the partial hydrogen desorption. In addition to exploring novel hydrides, the effects of hydrogen treatments on the grain size of AZ31 alloys were investigated. As a result of optimization of hydrogenation and dehydrogenation conditions, it was found that MgH_2, Al, Mg_<0.42>Al_<0.58> phases appeared after hydrogenation (disproportionation process), and an original solid solution phase was reformed after dehydrogenation (recombination process). The AZ31 alloys subjected to the HDDR phenomena was found to have fine grain size of around 100 nm. The hydrogen treatments were also applied to plate-shape AZ31 samples. In such case, the surface area of 20 μm in thickness was HDDR-treated.