The Hydrogen Storage Properties Of Mg3Y/Mg₃Ag/LaMg₂ Ni Alloys

Mg₃RE(RE=La,Ce,Nd,Pr)compound, as a new hydrogen storage alloy system, has been studied in recent years. It was found that all the Mg₃RE (RE=La,Ce,Nd,Pr) alloys have disproportionation reaction under relatively low hydrogen pressure. At the same time, the maximum atom radius could be calculated to be 0.27 ? from the biggest interstitial vacancy of Mg₃La phase lattice,which is smaller than hydrogen atom radius (0.37 ?). Based on this result, Mg₃Y and Mg₃Ag alloys were chosen to be studied in this thesis (The maximum atom radius could be calculated from the biggest interstitial vacancy of the phases, are near or bigger than that of hydrogen atom.). The hydrogen storage properties and the phase transformation of Mg₃Y and Mg₃Ag alloys during the hydrogen absorption/desorption process were studied. Additionally, the hydrogen storage properties of LaMg₂Ni compound have also been studied, which is the phase appeared when Ni was introduced into Mg₃La alloy. The microstructure and phases before and after hydrogen absorption/desorption were identified by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX). The cycles for hydrogenation/dehydrogenation process were measured by pressure-composition isotherm (PCI) system, the basic thermodynamic parameters were also calculated in this thesis.For the LaMg₂Ni alloy, the essential transformation was processed between Mg₂Ni and Mg₂NiH4 phase in the hydrogen absorption/desorption cycle. The appearance of LaH2.46 during the hydrogenation process was helpful not only for the improvement of hydriding kinetics, but also for the stabilizing the hydride. After only one hydrogenation/dehydrogenation, the maximum hydrogen storage capacity of LaMg₂Ni compound reaches 1.96 wt.% at 564 K. The standard enthalpy and entropy for hydriding and dehydriding were calculated according to van't Hoff equation.The as-melted Mg₃Y alloy was composed of Mg₂Y and Mg₂4Y₅ phases. The essential transformation is between Mg?MgH2 and YH2?YH3 in the hydrogen absorption/desorption process for both Mg₂Y and Mg₂4Y₅ phases. The hydrogen storage capacity was determined by the amount of Mg?MgH2 transition. The maximum hydrogen capacity and the standard enthalpy and entropy for dehydriding in Mg₃Y alloy system had also been studied in this work.The hydrogen absorption/desorption processes of as-melted Mg₃Ag alloy included two steps: (1), Mg54Ag17 phase reacted with hydrogen to form Mg₃Ag and MgH2 phase; (2), Mg₃Ag phase continued to absorb hydrogen and formed MgH2 and MgAg phases. Vice versa, the dehydrogenation regenerated Mg₃Ag phase and then transformed to Mg54Ag17 phase. The hydrogen storage capacity of Mg₃Ag alloy reaches the maximum value at 660 K. The mean enthalpy and entropy for dehydriding in Mg₃Ag alloy system are also calculated, and the results show that MgH2 transited from Mg₃Ag is more stable than common MgH2.The maximum atom radius could be calculated from the biggest interstitial vacancy of Mg₃La,Mg₂Y,Mg₂4Y₅ and Mg₃Ag phases are calculated as well in this work. Nevertheless, the disproportionation reaction for Mg₂Y,Mg₂4Y₅ and Mg₃Ag phases happened during the hydrogenation process. It's demonstrated that the disproportionation reaction for hydrogen storage alloys was determined by other effects other than by the size of the accommodated atoms.