Investigation On Preparation And Hydrogen Storage Performance Of Mg-Li Based Alloys

Both of magnesium and Lithium are hydrogen storage materials with high energy density. But the hydrides are relatively stable. In the thesis, the preparation and hydrogen storage performance of the Mg-Li binary alloys were systematically investigated and discussed.(1) The Mg-40at.%Li alloys were prepared by the sintering and subsequently mechanical alloying methods to explore the effects of different heat treatment and ball milling time on the hydrogenation storage properties. The preparation processes were:sintering+annealing+ball milling(1#); sintering+liquid nitrogen quenching+ball milling(2#); only ball milling(3#). The XRD patterns showed that the main phase of all the alloys were Li3Mg7 and the alloying degree improved with increasing ball milling time. But the Li3Mg7 phase disappeared after ab/desorption cycles. The hydrogen storage performance tests show the 3# sample has the largest hydrogen storage capacity and the best properties. It might be resulted from the oxidation of other samples during the heating and cooling procedure.(2) The Mg-Xat.%Li(X=10,25,40)alloys were prepared by ball milling. The XRD patterns showed Li3Mg17, Li3Mg17, Li3Mg7 phrases respectively after 20 h ball milling. The PCI curves indicated that the hydrogen absorption capacity of Mg-Xat.%Li(X=10,25,40)alloys was 6.30 wt.%,5.88 wt.%,5.29 wt.%. The dehydriding activation energy was 162.1 kJ·mol-1,155.4 kJ·mol-1,132.7 kJ·mol-1 calculated by Kissinger equation.(3) The Li3Mg17AlX(X=0,1,5,12) alloys were prepared by vibration ball milling 5 h. The XRD patterns exhibited that the samples form alloys after vibrating ball milling 5 hours and the 4# sample of the biggest aluminum content which contains Li3Mg17, LiAl and Al phases finally turns into Mg3Al12 phrase after several ab/desorption cycles. The SEM images revealed that grain size of the alloys decreased with the increasing aluminum content. The pressure-composition isotherm (PCI) showed that the hydrogen storage capacity reached to 6.82 wt.%,6.31 wt.%,4.54 wt.%,3.12 wt.% and the equilibrium pressure of ab/desorption raised with the increasing of the aluminum content. According to the Van’t Hoff equation, the reaction enthalpy reduced from 97.9 kJ · mol-1 to 87.7 kJ · mol-1 with the rising of the aluminum content. It was suggested that aluminum enhanced the hydrogen desorption kinetics of Mg-Li alloys. The 4# sample performed the best kinetics with a certain loss of hydrogen storage capacity. The dehydriding activation energy is 187.8 kJ·mol-1, 184.1 kJ·mol-1,177.5 kJ·mol-1,176.9 kJ·mol-1 based on the Kissinger equation.