Alloying Substitution On Structure And Properties Of Y Based AB₂ Hydrogen Storage Alloys

Because of relatively high hydrogen storage capacity and excellent hydrogen absorption properties at room temperature,the AB₂-type rare earth hydrogen storage alloys are widely used in energy conversion and storage,chemical,electronic,aerospace and other military and civil fields.However,for the rare-earth based AB₂-type hydrogen storage alloys,such as rare earths-iron system,rare earth-cobalt and rare earth-nickel system,etc.,the hydrogen-induced amorphization and disproportionation usually occurred in the hydrogen absorption process..As a result,the poor reversible hydrogen storage performance hinders its further commercial application.In this thesis,the rare earth Y based AB₂-type hydrogen storage alloys were investigated by using A or B side elemental substitution,the changes of phase structure in the process of hydrogen absorption and desorption process as well as the effect on the hydrogen storage properties were studied to obtain high-performance reversible rare earth hydrogen storage material.In the materials prepration,the alloy was prepared by arc-melting,and then treated by high temperature annealing.Phase structure and composition of the alloys were characterized by XRD,SEM and EDS methods.The hydrogen storage properties of the alloys were measured by the volume method.In addition,the first principle calculation was conducted to further study the role of alloying elements Main conclusions have been drawn as following:The YFe₂ alloy was firstly modified by A-side Zr substitution.All Y-Zr-Fe alloys remained single C15 Laves phase structure at as-annealed,hydrogenated and dehydrogenated states,indicating that the hydrogen-induced amorphization and disproportionation have been eliminated.With the increasing of Zr content,the Y-Zr-Fe alloys showed the decrease in the lattice constants and hydrogenation capacity,but the increase in the dehydriding equilibrium pressure.The Y-Zr-Fe alloys presented good cyclic hydrogen adsorption properties,although the hydrogen storage capacity showed a small reduction after the cyclic hydrogenation,it almost remained constant at last.The alloy Y_0.9Zr_0.1Fe₂ showed maximum initial hydrogenation capacity of 1.87 wt.%H,while the alloy Y_0.5Zr_0.5Fe₂ showed highest desorption capacity of1.26 wt.%H with obvious dehydriding plateau.Based on experiment analysis and first principle calculation of binding energy,the reasons for improving the dehydrogenation of Y-Zr-Fe alloys was related to the decrease volume of the tetrahedral interstices in the unit cell.On the basis of Y-Zr-Fe system,the effect of A side?Ti?and B side?Co?substitution on the phase structure and hydrogen storage performance of the alloys were investigated.It is found that the C15 single phase structure can be maintained by Co substitutuin in the Y-Zr-Fe-Co system,and no structure changes can be observed in the de-/hydrogenation.With the increase of Co content,the lattice constants and the hydrogen absorption capacity were both decreased,and the hydrogen desorption capacity of the alloys shown decreased compare to those of the Y-Zr-Fe system at the same temperature.In the Y-Zr-Ti-Fe system,Ti element did not effectively replace the A side element Zr,but instead second phase that can not absorb hydrogen.As a result,the hydrogen storage performance of Y-Zr-Ti-Fe system was worse than those of the Y-Zr-Fe system.In addition,to the B-side substituion by Mo element and the A-side substitution by Zr element conducted on the YFe₂ alloy.It is found that the Mo substitution can effectively inhibit the generation of the amorphization and disproportionation.Athough high hydrogen absorption capacity was found in the Y-Fe-Mo system,the desorption pressure and the desorption capacity were very low.In addition,the effect of Zr element on the YNi₂ system was also studied.When the Zr substitution reached 30%,the amorphization still present in the hydrogen absorption process,and the reason for irreversible in Y-Zr-Ni system was related to the small tetrahedral interstices.