| Magnesium is considered as one of the most ideal onboard hydrogen storage materials because of its mass hydrogen storage density of up to 7.6 wt.%.However,it has not yet been actually used with the limitation of the excessive high decomposition temperature of Mg hydride and the poor kinetics of the hydrogenation and dehydrogenation.In order to improve the above performances and maintain the high mass hydrogen storage density,in this thesis,the as-milled Mg90Al10+x wt.%CeO2(x=0,1,3,5,8)alloys and the as-cast Mg100-y3Aly(y=10,20,30,40)alloys were prepared by alloying Mg with Al,and the as-cast NdmMg90-mAl10(m=2,5,8,10)alloys were prepared by alloying the as-cast Mg90Al10 alloy with Nd.Considering the changes of phase composition,microtopography and crystal structure,the improvement mechanism of hydrogen storage performance of the alloys was clarified.The main results can be concluded:(1)The microstructure and hydrogen storage performance of the as-milled and cast Mg90Al10 alloys were compared.Main phase of the as-milled and cast Mg90Al00 is Mg,and the second phase is A1 and Mg17Al12 respectively.Compared to pure Mg,the hydrides of the as-milled and cast Mg90Al10 alloys have the lower thermodynamic stability and the higher hydrogenation and dehydrogenation rate.The reduction of the thermodynamic stability is due to the fact that the solid solution of A1 in Mg leads to the decrease of Mg cell volume.The improvement of hydrogenation/dehydrogenation reaction kinetics is owing to the decrease of grain size and the generation of multi-phase structure.(2)Adding nano-CeO2 powder in the preparation process of as-milled Mg90Al10 alloy can enhance the milling efficiency.Besides the Mg phase,Al phase and CeO2 phase,there generates a small amount of Mg]7Al12 phase in the as-milled Mg90Al10+x wt.%CeO2(x=1,3,5,8)alloys.The solid solubility of Al in Mg is increased and the grain size is slightly decreased after adding nano-CeO2 powder.Ce02 phase disappears in the process of hydrogenation,which transforms into the Al4 Ce phase,Compared to the as-milled Mg90Al10 alloy,the hydride decomposition conditions of the as-milled Mg90Al10+x wt.%CeO2(x=1,3,5,8)alloys further decrease,the activation performance and hydrogenation and dehydrogenation kinetics are much better.(3)The effect of Al content on the microstructure and hydrogen storage performance of the as-cast Mg-Al alloy was studied.The results show that the abundance of Mg phase gradually decreases and the abundance of Mg17Al12 phase gradually increases in the alloys with increasing A1 content.The hydrogenation and dehydrogenation kinetics is the best for the alloy with the Al content of 10 at.%.However,the addition of an excess of Al leads to the dramatical decrease of hydrogen absorption capacity,and makes the dehydrogenation kinetics get worse.The dehydrogenation enthalpy of the as-cast Mg100-yAly(y=10,20,30)alloys decreases firstly and then increases,which is related to the change of Mg cell volume.In addition,the dehydrogenation enthalpy of hydrides of Mg17Al12 and Mg2Al3 phase is smaller than that of Mg phase,which indicates that the hydrides of these two phases have the lower thermodynamic stability.(4)The effect of Nd content on the microstructure and hydrogen storage performance of the as-cast Mg90Al10 alloys was studied.The as-cast Nd2Mg88Al10 alloy is composed of the main phase of Mg and the second phase of Mg17Al12 and Al2Nd.For the as-cast Nd5Mg85Al10 alloy,Mg17Al12 phase disappears,the content of Al2Nd phase increases and there generates some Mg(Nd)solid solution phase.Besides the Mg phase and Al2Nd phase,the Mgl2 Nd phase and Mg3Nd phase are also generated in the as-cast Nd8Mg82Al10 alloy.The as-cast Nd10Mg80Al10 alloy consists of Mg12Nd,Mg3Nd and Al1.1Mgo.9Nd phases.The result of performance test shows that the activation performance and the hydrogenation and dehydrogenation rate of the alloys can be improved obviously by alloying with Nd,but the dehydrogenation enthalpy is increased. |
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