| TiFe based hydrogen storage alloy is a typical representative of AB type hydrogen storage alloy,which has practical reversible hydrogen storage characteristics,low cost,stable hydride formation,and is not easy to spontaneous combustion.However,the hydrogen absorption activation of TiFe based hydrogen storage alloys is relatively difficult,especially the first activation at low temperature,and it is sensitive to gas impurities,which limit its practical application.It is found that element substitution,mechanical alloying,and catalyst addition can significantly improve the activation performance of TiFe alloy.On the basis of previous studies,this paper intends to explore the effects of different Y element addition on the microstructure,phase structure,activation properties,kinetics,thermodynamics and other hydrogen storage properties of Ti22-xYxFe16Mn3Cr(x=0?0.4)alloy by adding rare earth elements and transition group elements.The as-cast Ti22-xYxFe16Mn3Cr(x=0?0.4)alloy was prepared by vacuum induction melting.Then the as-cast alloy was milled in argon atmosphere for 0 h,0.5 h,0.75 h,1 h,3 h.The effects of Y addition and ball milling on the phase structure and hydrogen storage properties of the alloy were studied.The activation of as-cast and milled alloys at different temperature was studied,including 483 K,443 K and 403 K.The thermodynamics and kinetics of hydiding/dehydiding at 383 K,363 K,343 K and 323 K were studied for all as cast and ball milled alloy samples.By using XRD,SEM,TEM and other material characterization methods,it is found that the as-cast Ti22-xYxFe16Mn3Cr(x=0?0.4)alloy is composed of TiFe main phase and the second phase Mn2Ti phase,Ti(Fe,Mn,Cr)solid solution,Ti phase and Y phase;the ball milled alloy is composed of TiFe phase,Ti(Fe,Mn,Cr)solid solution and so on.It is found the width of XRD diffraction peak of the as-milled alloy increases with the increase of milling time suggesting the grain size of the alloy powder decreases.The morphology of the alloy was studied by SEM,and obvious cracks were observed on the surface of the alloy,which are beneficial to the increase of the reaction rate.While,the particle size of alloy powder decreases and the agglomeration phenomenon becomes obvious gradually.HRTEM observation shows that ball milling leads to nanocrystallization and amorphization of the alloy.The nanocrystalline boundary and the phase boundary introduced by the second phase provide a large number of channels for hydrogen diffusion,which promotes the improvement of hydrogen storage performance of the alloy.The results show that the higher the temperature is,the less the first hydrogen absorption and the shorter the activation incubation period of the alloy.The activation of the alloy is more difficult at 403 K and the activation incubation time of the as-cast Ti21.7Y0.3Fe16Mn3Cr alloy was 707 min.However,the alloys at 443 K and 483 K are fully activated after being activated twice and the activation incubation time was 130 min and 43 min,respectively.In addition,at the same temperature,when Y<0.3,the maximum hydrogen absorption of the alloy increases and the maximum hydrogen absorption of 1.4 wt%was achieved at 323 K.The kinetic model of hydrogen absorption and desorption is threedimensional diffusion model of hydrogen atom,and the kinetic equation is 1-[(1-?)1/3]2=kt.At the same time,with the same Y content,the higher the temperature is,the higher the platform pressure is,and the hysteresis factor will first decreases and then increases,and the maximum hysteresis factor is 0.537.The Y3 alloy has the best comprehensive hydrogen storage performance.Compared with the as cast alloy,the time required for hydrogen absorption and desorption activation of the ball milled Ti21.7Y0.3Fe16Mn3Cr alloy is significantly reduced,and the activation incubation time of the ball milled Ti21.7Y0.3Fe16Mn3Cr alloy is only 4 min.However,when the ball milling time reaches 3 h,the agglomeration phenomenon is obvious,the activation incubation period of the alloy becomes longer.The control mechanism of hydrogen absorption and desorption kinetics after ball milling for 3 h is kinetic model of surface nucleation,and the kinetic equation isln(1-?)=kt.However,it is a three-dimensional diffusion model of hydrogen atom when the milling time is less than 3 h.In addition,the enthalpy change and entropy change of ball milled alloy are slightly larger than those of as cast alloy,and the hysteresis of PCT platform is smaller.The optimum milling time is 0.75 h and the enthalpies of hydrogen absorption and desorption are 22.943 kJ·mol-1H2 and 26.215 kJ·mol-1H2,respectively. |
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