Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography
Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography
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DOI：
10.1103/physrevb.77.024204
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发表时间：
2008-01
影响因子：
3.7
通讯作者：
C. Broedersz;R. Gremaud;B. Dam;R. Griessen;O. Løvvik
中科院分区：
文献类型：
--
作者：
C. Broedersz;R. Gremaud;B. Dam;R. Griessen;O. Løvvik
利用氢谱技术，我们最近绘制了四元系Mg-Ti-Ni-H中大范围组成的热力学性质图。掺钛后，镁镍合金的氢化物生成热发生了显著的变化。对于较小的组成范围，我们得到了氢化热焓H=�40kJmolH2�1，这是在中等温度和压力下储氢所需的热焓。这个热值很令人惊讶，因为它比镁-镍和镁-钛氢化物的H要小得多。Mg-Ti-Ni-H薄膜的纳米结构阻碍了X射线衍射法对氢化物相的直接测定。利用密度泛函理论对不同的氢化反应途径进行了计算，确定了钛掺杂导致的Mg-Ni-H体系的失稳是由于在沉积状态形成了镁镍和钛镍金属间化合物，它们在氢化过程中转变为亚稳态的掺钛镁镍氢化合物相。掺钛的Mg_2NiH_4相可以认为是重掺杂半导体。
Using hydrogenography, we recently mapped the thermodynamic properties of a large range of compositions in the quaternary Mg-Ti-Ni-H system. The enthalpy of hydride formation of Mg-Ni alloys is significantly altered upon Ti doping. For a small range of compositions, we find a hydrogenation enthalpy H = �40 kJ mol H2 �1 , which is the desired enthalpy for hydrogen storage at moderate temperature and pressure. This enthalpy value is surprising since it is significantly less negative than the H of the Mg-Ni and Mg-Ti hydrides. The nanostructure of the Mg-Ti-Ni-H films hinders a direct determination of the hydride phases involved by x-ray diffraction. Using density functional theory calculations for various hydrogenation reaction paths, we establish that the destabilization of the Mg-Ni-H system by Ti doping is due to the formation of Mg2Ni and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg2NiH4 phase upon hydrogenation. The Ti-doped Mg2NiH4 phase can be considered as a heavily doped semiconductor.