Microstructures And Hydrogen Storage Properties Of Non-stoichiometric Zr-based Laves Phase Alloys

ZrV₂ alloy is one of the typical AB₂ Zr-based Laves phase hydrogen storage alloys which preserve high storage capacity,easy activation,fast absorption rate and low equilibrium pressure,possessing potential applications in fields of getter materials and storage and separation of hydrogen and its isotopes.However,the hydrogen absorption kinetics and capacity need to be improved to meet the requirements of practical applications in the nuclear industry.Researches on the improvement of hydrogen storage properties of Zr-based alloys mainly focus on adjusting the phase constituent and microstructures via methods including alloying,melt-spinning and surface modification,which usually cause a decrease of hydrogen storage capacity.The hydrogen storage capacity and absorption kinetics can hardly be improved together.Defects induced by non-stoichiometry and the element content variation in alloys can probably improve the hydrogen absorption capacity and kinetics.However,there is a lack of researches about the effect and mechanism of non-stoichiometry on Zr-based hydrogen storage alloys.In this work,the non-stoichiometric Zr_0.9Ti_xV₂?x=0,0.2,0.3,0.4?Laves phase alloys are designed bases on the AB₂-type Zr_0.9Ti_0.1V₂ alloy.The role of annealing and melt-spinning on microstructure,phase structure and hydrogen storage properties of Zr_0.9Ti_x V₂ have been comparatively studied.The effect of non-stoichiometry on hydrogen absortion capacity,kinetics,PCT characteristics,equilibrium pressure and stability of hydrides have been discussed combined with the microstructure and phase evolution.The non-stoichiometric Zr_0.9Ti_0.1V_x?x=1.7,1.9,2.1,2.3?Laves phase alloys are designed and the effect of V element content in B site on microstructure,hydrogen absortion kinetics and thermodynamics has been discussed.Based on the above,the Zr_0.9-xTi_0.4+x.4+x V_1.7?x=0,0.1?alloys with modified compositions?high Ti content and low V content?are designed.The influence of phase constituent adjustment on hydrogen storage properties of alloys has been investigated.It is attempted to further improve the hydrogen absorption capacity on the premise of fast absorption kinetics.The influencing mechanism of non-stoichiometry on microstructures and hydrogen storage properties of Zr-based Laves phase alloys have been proposed from the above investigations.XRD?SEM?EDS and TEM techniques have been applied to analyze the phase consitituents,microstructures,element distribution and defects in alloys.Ti content variation,annealing treatment and melt-spinning affact the phase constituents and microstructures of Zr_0.9Ti_xV₂ alloys evidently.The phase constituent of as-cast alloys includes C15 Laves phase ZrV₂,V-BCC and?-Zr/?-Zr,the microstructure consists of thick V-rich dendrites and eutectic structure.Annealing treatment can increase the content of Laves phase and reduce dendrites.Melt-spinning can refine the grains and increase the boundries evidently due to the rapid quenching.The content of ZrV₂ in alloys:annealed>melt-spun>as-cast.The lattice parameters of major phases in annealed alloy are higher than that in corresponding melt-spun ribbons.With the increase of Ti content,the content of ZrV₂ decreases,the content of V-BCC and Zr increases,and the lattice parameters of major phases increase.High density of dislocations,twins and stacking faults are observed in annealed Zr_0.9Ti_xV₂ alloys.Hydrogen storage characteristics including the P-t and PCT curves for each sample are obtained from a Sievert type apparatus.The hydrogenation thermodynamics parameters are calculated by using Van't Hoff equation based on the PCT curves test.Ti content variation,annealing treatment and melt-spinning affact the activation behaviors,hydrogenation properties of Zr_0.9Ti_xV₂ alloys evidently.After the degassing at high temperature,the first hydrogen absorption rate decreases with increasing Ti content,both for annealed alloys and melt-spun ribbons.The melt-spun ribbons show lower activation temperture,shorter incubation period and faster rate than annealed alloys due to its higher specfic surface area and grain refinement.Zr_0.9Ti_x V₂ alloys attain 90%of the saturated hydrogen content within 10 seconds at500 ^oC.The saturated hydrogen contents of melt-spun ribbons are 80⁹0%compared to annealed alloys due to the lower ZrV₂ content and smaller lattice parameters.With the increase of Ti content,the slope of PCT curves become lower and the width of platform increases,the equilibrium pressure decreases,hydrogen absorption capacity and the stability of hydrides increase.The absotption kinetics of Zr_0.9Ti_0.1V_x alloys have been fitted and the reaction rate constants have been calculated by using Hirooka equation.The phase constituents of Zr_0.9Ti_0.1V_x alloys consist of C15 Laves phase ZrV₂,V-BCC and a little Zr₃V₃O.With the decrease of V in B site,the content of ZrV₂ and V-BCC increases and decreases,respectively,which leads to the improvement of hydrogen absorption capacity and kinetics,and the reduction of enthalpy change during hydrogen absorption.High density of dislocations,twins and stacking faults observed in Zr_0.9Ti_0.1V_1.7 are located in C15-ZrV₂ phase which makes it the best hydrogen storage properties.The phase abundance adjustment for Zr_0.9-xTi_0.4+xV_1.7?x=0,0.1?with high Ti content gives rise to the improvement on hydrogen storage capacity.The phase constituents consist of C15 Laves phase ZrV₂,V-and Zr-solid solutions.The content of ZrV₂ phase decreases but its unit cell volume increases with increasing Ti content.Zr_0.8Ti_0.5V_1.7 exhibit hydrogen storage capacity up to 2.83 wt%.XPS and Ar-ion etching have been adopted to investigate the elements and state in surface and subsurface of unactivated alloys,the main chemical compositions in alloy surface are oxides which slow down the initial hydrogen absorption,the thickness of oxides layer is no more than 16.8 nm.The apparent activation energy is obtained by using Arrhenius equation,the apparent activation energies of the fast hydrogen absorption stage are calculated to be 3⁵ kJ/mol.The three phases form their hydrides,the hydrogen release stage with temperature raising in DSC curve corresponds to the decomposition of VH_0.81,ZrV₂H_x and ZrH₂,respectively.