Phase Formation Mechanism And Hydrogen Absorption/Desorption Properties Of Mg-Ni-Y Alloys

Magnesium-based alloys have attracted much attention because of their high storage capacity,abundant resources,low cost and so on.They have been considered as one of the most hopeful hydrogen storage materials for practical applications.However,the high dehydrogenation temperature,slow hydrogen absorption and desorption kinetics of Mg-based alloys are major obstacles in application.The researches have focused on improving the hydrogen absorption and desorption kinetics by alloying,adding catalyst components,mechanical alloying,surface modification and melt-spinning for Mg-based hydrogen storage materials.The effects of grain refinement on hydrogen absorption and desorption kinetics are needed to be further made clear.The correlation of hydrogenation/dehydrogenation kinetics and microstructure is still not explicit for the Mg-based melt-spun ribbons.In this dissertation,Mg₂Ni-based alloys were prepared by Y substitution followed by rapid solidification.The phase compositions,microstructures,surface states,hydrogen absorption/desorption kinetics and thermodynamics of Mg₆₇Ni_33-xY_x?x = 0,1,3,6,10?melt-spun ribbons have been investigated.Meanwhile,phase transformations and microstructure evolutions in hydrogenation/dehydrogenation processes have been also paid attention.It has been discussed the hydrogenation/dehydrogenation mechanism based on the microstructure evolution of Mg-Ni-Y melt-spun ribbons.The work of this dissertation aims to provide reference value and certain theoretical guidance for the development of Mg-based hydrogen storage materials.The main research contents and conclusions are as follows:Phase composition,microstructure and phase constitutes of Mg₆₇Ni_33-xY_x?x = 0,1,3,6,10?ingots have been investigated in this work.The results show that the percent of Mg₂Ni,Mg and MgNi₂ in the as-cast Mg₂Ni alloy are 90.1,3.6 and 6.3 wt%,respectively.With the addition of Y element,MgYNi₄ and e phase(Mg₂₄Y₅)are observed in the x = 3,6,10 alloys.The addition of Y element reduces the secondary dendrite arm spacing of Mg₂Ni and the lamellar spacing of the eutectic structure.The three-dimensional?3D?morphology characteristics of MgYNi₄ in solidification state have been explicated.The formation mechanism of tetrahedral MgYNi₄ is discussed based on solidification and crystallographic theories.Based on the step-scanned high resolution powder XRD pattern,the phase structures of melt-spun Mg-Ni-Y ribbons have been determined with the help of Rietveld refinement.The variation of lattice parameter and contents of various phases with composition are analyzed.With the addition of Y content,the percent of Mg₂Ni decreases from 97.4 wt% in x = 0 to 88.5 wt% in x = 6,the percent of Mg and MgYNi₄ are increased.The lattice distortion of Mg₆₇Ni_33-xY_x?x = 1,3,6?alloys is 0.14,0.26 and 1.93%,respectively.The reduced grains are observed in the microstructures of melt-spun ribbons by SEM and transmission electron microscopy?TEM?.Connected shrinkage porosities are observed in the melt-spun Mg-Ni-Y ribbons.The phase composition and elements chemical valence states of melt-spun Mg-Ni-Y ribbons on surface and subsurface are analyzed by X-ray photoelectron spectroscopy?XPS?.Oxides including MgO,NiO and Y₂O₃ are detected on the surface of melt-spun ribbons,the content of the dominant MgO decreases on the subsurface of melt-spun ribbons.The major oxide removed by activation is MgO.The activation properties and hydrogenation/dehydrogenation kinetics of melt-spun Mg₆₇Ni_33-xY_x?x = 0,1,3,6,10?ribbons are analyzed based on the hydrogen absorption and desorption kinetics curves of Mg-Ni-Y ribbons at different temperatures.The initial hydrogen absorption capacities of melt-spun Mg₆₇Ni_33-xY_x?x = 0,1,3,6?ribbons are 1.00,0.78,1.22 and 1.81 wt%.The hydrogen absorption and desorption rates and capacities of the alloys increase in the hydrogenation/dehydrogenation cycles,the capacities are 2.75,3.75,3.3 and 3.25 wt%,the ribbons are fully activated after three hydrogenation/dehydrogenation cycles.The hydrogen absorption capacities of activated Mg₆₇Ni_33-xY_x?x = 0,1,3,6?ribbons are 3.15,3.79,3.78 and 3.45 wt%,the capacities reach the 60% of maximum hydrogen absorption capacities.The refined grain improves the hydrogen absorption/desorption kinetics signally,and the shrinkage also has positive influence on the hydrogen absorption/desorption kinetics.The plateaus are obvious in the hydrogenation/dehydrogenation pressure-composition-temperature?PCT?curves of Mg-Ni-Y alloys,and they show obvious hysteresis effect.The hydrides formation enthalpies of melt-spun Mg₆₇Ni_33-xY_x?x = 0,1,3,6?ribbons are-55.5,-50.5,-46.9 and-48.6 kJ/mol,the decomposition enthalpies are 62.3,59.4,58.6 and 58.8 kJ/mol.Comparing with the as-cast Mg₂Ni alloy,the formation and decomposition enthalpies are decreased.The thermal decomposition and phase transformation of the hydrogenated Mg-Ni-Y ribbons in dehydrogenation process are analyzed by DSC and XRD.The dehydrogenation phases are Mg₂NiH₄,Mg₂NiH₀.3,MgH₂ and YH₃.The dehydrogenation activation energy of melt-spun ribbons are calculated by Kissinger equation based on the DSC data.The decrease of dehydrogenation temperature and improvement of hydrogenation/dehydrogenation kinetics are related with the low dehydrogenation activation energy.The microstructures of the hydrogenated and dehydrogenated ribbons are observed by transmission electron microscopy?TEM?and SEM.The hydrogenation/dehydrogenation mechanism of melt-spun ribbons is discussed based on the microstructure evolution in hydrogenation/dehydrogenation processes.The amorphous and nanocrystalline are also observed in the melt-spun Mg₆₇Ni_33-xY_x?x = 0,1,3,6?ribbons,the contents of amorphous are increased in hydrogenated ribbons for the “hydrogen induced amorphization?HIA?”.The ribbons of x = 6 with much amorphous structure are crystallized and many stacking faults are observed.Hydrides can nucleate and hydrogenate near the shrinkage pores inside the melt-spun ribbons.The initial hydrogenation of Mg-Ni-Y melt-spun ribbons are 3-dimensional diffusion controlled nucleation and growth process before activation.It is 1-dimensional for the hydrogenation process and 3-dimensional for dehydrogenation process after activation.