| With continuously growing environment problems and energy demands,fusion energy generated in the International Thermonuclear Experimental Reactor(ITER)is proposed as a promising and ultimate energy source for the human society.As an important component of Tritium Fuel Cycle System in ITER,hydrogen isotopes Storage and Delivery System(SDS)should develop a appropriate tritium storage material for the radioactivity and rareness of tritium.An intermetallic compound of ZrCo is proposed as a promising alternative of uranium for SDS in fusion energy in ITER,since it has the advantages such as non-radioactivation,low pyrophoricity,low equilibrium absorption pressure,high hydrogen capacity,and the capability to trap 3He.However,operation at high H2 pressure and elevated temperature induces disproportionation reaction,which can be described as 2ZrCo +H2?ZrH2+ZrCo2.The hydrogen-induced disproportionation(HID)reaction results in a decrease in the hydrogen capacity of ZrCo alloy and hinders the adhibition of ZrCo alloy in ITER.Besides disproportionation,ZrCo alloy suffers from poor activation behaviors.Alloying is an effective way to improve the initial activation behaviors and anti-disproportionation ability of ZrCo alloy.Therefore,it is necessary to find appropriate alloying elements to obtain optional comprehensive performances of ZrCo alloy.So far,there is paucity of extant studies have been reported to improve both the initial activation behaviors and the anti-disproportionation ability,and the mechanism has not been fully understood.On the basis of the literature review,a series of ZrCo-based alloys were prepared with Cr and Mo substituting Co and Nb and Ta substituting Zr via arcmelting.The structural properties,initial activation behaviors,hydrogen desorption properties and anti-disproportionation properties were systematically investigated using XRD,SEM/EDS,DSC and Sieverts apparatus.The mechanism on improvement in initial activation behaviors and anti-disproportionation ability were proposed.The results obtained in present research demonstrate the high potential of ZrCo-based alloys utilized in storage and delivery of hydrogen isotopes.(1)Effects of Cr,Mo,Nb and Ta substitution on the structure of ZrCo alloy.Zr1-xCoNbx(x=0-0.2)and Zr1-xCoTax(x=0.05-0.15)alloys exhibit ZrCo main phase with CsCl-type structure and ZrCo2 phase,and their corresponding hydride consist of ZrCoH3 phase;ZrCo1-xCrx(x=0.025-0.1)and ZrCo1-xMox(x=0.05-0.2)alloys consist of ZrCo phase and a fraction of ZrCr2 or ZrMo2 phase,and their hydrides contain ZrCoH3 and ZrH phases.Cr and Mo substituting Co increase the lattice constants and lattice volume of ZrCo,while Nb and Ta substitution decrease lattice constants and lattice volume of ZrCo.Cr,Mo and Nb substitution restrain the formation of ZrCo2 phase,while Ta substituting Zr promotes the formation of ZrCo2 phase.(2)Effects of Cr,Mo,Nb and Ta substitution on the initial activation behaviors of ZrCo alloy.Cr,Mo,Nb and Ta substitution shorten the initial incubation period and activation time greatly,which is beneficial to the initial activation behaviors of ZrCo alloy.Cr and Mo substituting Co form ZrCr2 and ZrMo2 phases,ZrCr2 and ZrMn2 can not only catalyze the H2?2H reaction,but also act as the pathways for hydrogen diffusion.Nb and Ta substituting Zr increase the diffusion coefficient of hydrogen in Nb/Ta-alloyed alloys.The apparent activation energy for hydrogenation of ZrCo,ZrCo0.95Cr0.05,Zr0.8CoNb0.2,ZrCo0.85Mo0.15 and Zr0.9CoTa0.1 alloys are determined to be 44.88,40.34,32.72,32.73 and 42.29 kJ mol-1 H2.Cr,Mo,Nb and Ta substitution reduce the energy barrier for hydrogenation,which is beneficial to the improvement of hydrogen absorption kinetics.(3)Effects of Cr,Mo,Nb and Ta substitution on the hydrogen desorption properties of ZrCo-H systems.Cr,Mo,Nb and Ta substitution improve the hydrogen desorption kinetics,decrease the hydrogen capacities of ZrCo alloys and the desorption temperatures of ZrCo-H systems.The apparent activation energy for hydrogen desorption process of hydrides of ZrCo,ZrCo0.95Cr0.05,Zr0.8CoNb0.2,ZrCo0.85Mo0.15 and Zr0.9CoTa0.1 alloys are determined to be 100.55,94.08,80.05,84.58 and 93.82 kJ mol-1 H2.The decreased value of activation energy for hydrogen desorption process mean that Cr,Mo,Nb and Ta substitution are beneficial to improve the hydrogen desorption kinetics.(4)Effects of Cr,Mo,Nb and Ta substitution on the thermodynamic characteristics of ZrCo-H systems.Cr,Mo,Nb and Ta substitution decrease the hydrogen capacities and plateau width of PCT curves.Cr and Mo substituting Co decrease the plateau pressures,increase the change of enthalpy and entropy of ZrCo-H systems and the residual hydrogen in matrix.Nb and Ta substituting Zr increase the plateau pressures,the plateau slope factor and the change of enthalpy and entropy of ZrCo-H systems.(5)Effects of Cr,Mo,Nb and Ta substitution on the anti-disproportionation properties of ZrCo-H systems.Cr,Mo,Nb and Ta substitution improve the anti-disproportionation performances of ZrCo-H systems.The first-principles calculation results show that Cr substituting Co increase the size of 8e tetrahedron,decrease the bond length of Zr-H(8e),accelerate disproportionation reaction,which is not accordance with the experimental results;Mo,Nb and Ta substitution decrease the size of 8e tetrahedron,increase the bond length of Zr-H(8e),improve the anti-disproportionation performances,which are accordance with the experimental results.The extent of disproportionation decrease from 83.68%for ZrCo-H system to 70.52,26.78,8.71,6.46%for ZrCo0.9Cr0.1-H,ZrCo0.85Mo0.15-H,Zr0.8CoNb0.2-H and Zr0.9CoTa0.1-H systems.DSC measurements show that Cr,Mo,Nb and Ta substitution decrease the low temperature hydrogen desorption peak at 380-540 K and reduce the hydrogen atoms in 8e sites,which decrease the driving force of the disproportionation reaction.The apparent activation energy for disproportionation reaction of hydrides of ZrCo,ZrCo0.95Cr0.05,Zr0.8CoNb0.2,ZrCo0.85Mo0.15 and Zr0.9CoTa0.1 alloys are determined to be 167.46,168.28,175.29,202.32 and 218.89 kJ mol-1 H2.The increased value of activation energy for disproportionation reaction suggesting that Cr,Mo,Nb and Ta substitution increase the energy barrier for disproportionation reaction,which is beneficial to the improvement of anti-disproportionation performance. |
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