Phase equilibria, crystal structure and hydriding/dehydriding mechanism of Nd 4 Mg 80 Ni 8 compound

Abstract

In order to find out the optimal composition of novel Nd-Mg-Ni alloys for hydrogen storage, the isothermal section of Nd-Mg-Ni system at 400 °C is established by examining the equilibrated alloys. A new ternary compound Nd 4 Mg 80 Ni 8 is discovered in the Mg-rich corner. It has the crystal structure of space group I4 1 /amd with lattice parameters of a = b = 11.2743(1) Å and c = 15.9170(2) Å, characterized by the synchrotron powder X-ray diffraction (SR-PXRD). High-resolution transmission electron microscopy (HR-TEM) is used to investigate the microstructure of Nd 4 Mg 80 Ni 8 and its hydrogen-induced microstructure evolution. The hydrogenation leads to Nd 4 Mg 80 Ni 8 decomposing into NdH 2.61 -MgH 2 -Mg 2 NiH 0.3 nanocomposites, where the high density phase boundaries provide a great deal of hydrogen atoms diffusion channels and nucleation sites of hydrides, which greatly enhances the hydriding/dehydriding (H/D) properties. The Nd 4 Mg 80 Ni 8 exhibits a good cycle ability. The kinetic mechanisms of H/D reactions are studied by Real Physical Picture (RPP) model. The rate controlling steps are diffusion for hydriding reaction in the temperature range of 100 ∼ 350 °C and surface penetration for dehydriding reaction at 291 ∼ 347 °C. In-situ SR-PXRD results reveal the phase transformations of Mg to MgH 2 and Mg 2 Ni to Mg 2 NiH 4 as functions of hydrogen pressure and hydriding time.