The Relation Between Structure And Properties In Novel YFe₂-Based Rare Earth Alloys

As the rare earth-transition metal alloys,such as AB₅、AB₃ and AB₂ alloys,could be used to store hydrogen under normal temperature and pressure,they were considered as one kind of ideal materials for hydrogen storage.Among them,AB₂ type Y-based alloys generally show C14 MgZn₂ hexagonal or C15 MgCu₂ structure,and their theoretical hydrogen storage capacity reaches the level of 1.8².4 wt.%.However,lattice distortion,hydrogen induced amorphization and disproportionation are extremely easy to appear during the reaction with hydrogen.This leads to the variation of lattice structure or the formation of yttrium hydrides,which worsens the cycle performances,and makes Y-based AB₂ alloys fail to meet the requirements of hydrogen storage materials.Present research focus on designing new YFe₂ rare earth alloy systems by element substitution,non-stoichiometric design and change the morphology and microstructure with the aim to realize high capacity reversible de-/hydrogenation.Al was used to substitute Fe in YFe₂ alloy and the phenomenon of lattice distortion,hydrogen induced amorphization and disproportionation were successfully restrained and reversible de-/hydrogenation cycle is realized.Although lattice expansion and extraction accompanied by hydrogen atoms enter into/come out the lattice,the structure keep stable.YFe_1.7Al_0.3 alloy shows the highest capacity of 1.38 wt.%,however,it decreases with the increasing content of Al atoms in the alloy.In addition,capacity loss could be observed after the first de-/hydrogenation cycle.The desorption PCT curves appear to a sloping line,which means a decreasing desorption equilibrium pressure during the dehydrogenation process.Therefore,a problem of incomplete desorption under normal condition exists in the YFe_2-x-x Al_x alloys.In order to overcome the weakness of YFe_2-xAl_x alloys,non-stoichiometric Y-Fe-Al alloys were designed and prepared for further research.It shows that the C15 Laves phase could be retained in both hypo and hyper-stoichiometric alloys after several de-/hydrogenation cycles.Hypo-stoichiometric alloys shows good kinetic properties and high capacity of 1.63 wt.%.Hyper-stoichiometric alloys show increased hydrogen desorption equilibrium pressure,especially in YFe_1.9Al_0.3alloy.The excess Fe atoms in the lattice could be found to substitute Y on A side,decreasing the tetrahedral site size and weakening the bonding between alloy and H atoms.Therefore,YFe_1.7+x.7+x Al_0.3 alloy shows higher desorption equilibrium pressure and desorption capacity.Based on the above results,Zr,Ti and V atoms were used to substitute Y on A side,forming Y-M-Fe-Al quaternary alloys.In certain composition range,they maintain C15 Laves structure even after de-/hydrogenation cycles.The substitution on A side reducing the size of tetrahedral sites,meanwhile,weakening the bonding between alloys and H atoms,which enhancing the desorption equilibrium pressure of the hydrides.Therefore,it could be found that the desorption capacity increases drastically under normal conditions.The relation among the decrement of tetrahedral site size,hydride formation enthalpy and the increment of desorption pressure could be fitted with a quadric surface,which reveals that reducing the tetrahedral site size is a more efficient way to raise the pressure during dehydrogenation.The desorption equilibrium pressure of alloys related closely to the local stress and strain of interstitial site that H atom occupied.Therefore,Y-Zr-Fe-Al dual-phase alloy was designed and prepared to introduce extra lattice strain caused by interaction force between the two phases.The composition of the phases is Y_0.803Zr_0.197Fe_1.683Al_0.317（Y-rich phase）and Y_0.196Zr_0.804Fe_1.694Al_0.306（Zr-rich phase）,respectively,and both phases show the C15 Laves phase structure.The desorption equilibrium pressure of Y-rich phase was found to be much higher than that of pure Y_0.8Zr_0.2Fe_1.7Al_0.3 alloy,which demonstrates the positive effect of the introduced local stress of H atom.Based on the finite element analysis result,the relation between the strain and the desorption equilibrium pressure was established.In addition,the kinetic property of the dual-phase alloy was successfully enhanced by mechanical alloying.