Preparation And Hydrogen Storage Properties Of Mg-Y-based Composites With Nanocrystalline And Amorphous Structure

Mg-based alloys have received tremendous research interest in view of the potential hydrogen storage applications due to their advantages of high gravimetric hydrogen storage density,low cost and abundant resources.In this thesis,the fundamentals of hydrogen storage and improvements in recent years on Mg-base hydrogen storage material were firstly reviewed.Based on the overview of Mg/MgH2 as the hydrogen storage medium,Mg-Y binary alloys were prepared and systematically investigated.Then Mg-Y-M(M=Ni,Co,Mn,Cu,Al)ternary alloys were prepared,and elemental addition on structures and hydrogen storage properties were systematically investigated.In addition,Mg-Y-based hydrogen storage materials were prepared by vacuum rapidly quenching technology,mechanical milling,and adding catalytic Ni and graphite,to seek better and more efficient ways to improve hydrogen absorption/desorption kinetics and reduce H2 desorption temperature.Firstly,Mg24Yx(x=1-5)alloy were prepared by vacuum induction melting,and their microstructures and hydrogen storage properties were studied in detail.Results show that the alloys are composed of Mg24Y5 and Mg phase.First hydrogenation process resulted in an irreversible two-step disproportion reaction:Mg24Y5+ H2 ?Mg + YH2? MgH2 + YH2 + YH3.For further absorption/desorption cycles,MgH2(?)Mg + H2 chemical reaction occurred.Numerous fine dendritic eutectic mixtures of Mg24Y5 and Mg phase were found in Mg24Y3 alloy,which makes the alloy have the best hydrogen absorption rate and lowest desorption temperature.Different with kinetics,Y content has nearly no effect on thermodynamic performance of Mg-Y alloys.Based on Mg24Y3 alloy,Mg24Y3M(M=Ni,Co,Mn,Cu,Al)ternary alloys were also prepared by vacuum.The alloys also have a multiphase structure,and the main phase is Mg24Y5.The addition of Ni significantly enhances the hydrogen absorption and desorption kinetics,the hydrogen desorption peak temperature decreases to 313?,and the desorption activation energy reduces to 64 kJ/mol as well.The addition of Co,Mn,Cu also increases the desorption rate,but Cu adding decreases hydrogen desorption kinetics.Two plateaus are observed in Mg24Y3Ni and Mg24Y3Cu alloys,the lower and wider plateau corresponds to a transformation Mg + HZ(?)MgH2,and the upper and smaller plateau belongs to Mg2Ni + H2(?)Mg2NiH4 and Mg2Cu + H2(?)MgH2 + MgCu2,respectively.Mg24Y3 and Mg24Y3Ni alloy ribbons were prepared by vacuum rapidly quenching technology.The as-spun Mg24Y3 alloys have a nano-crystalline structure,and the structure of the as-spun Mg24Y3Ni is amorphous.Crystallization transformation of the amorphous Mg24Y3Ni alloy was observed during the first hydrogenation.The hydrogen absorption and desorption properties of the as-spun alloys are basically the same as the as-cast alloys.The effect of milling time on structures and hydrogen storage properties of Mg24Y3Ni alloy was systematically studied.Results show that ball-milling process leads to the XRD peaks broaden and shorten,indicating that the alloy transform into disordered structure during ball-milling.The formation of Mg caused by ball-milling reduces the hydrogen storage performance of the alloy.Adding right amount of graphite makes the particle shape of the alloy turn into flakes,and the hydrogen storage properties were also improved accordingly.However,excess graphite reduces the grinding efficiency during ball-milling,thereby decreasing hydrogen absorption and desorption properties.The p-c-T curves of the ball-milled Mg24Y3Ni alloys were also tested by Sievert's-type apparatus,the results show that ball-milling and graphite adding have hardly effect on thermodynamic performance of Mg24Y3Ni alloy.Mg24Y3-Ni-C composites were also prepared by ball-milling,and the effects of graphite and Ni content on the structures and hydrogen storage properties were studied in detail.It is found that the hydrogen sorption properties of the Mg24Y3-5 wt.%Ni composites increased slightly when 3 wt.%graphite was added,the peak temperature for hydrogen desorption decreases to 301?.Excess graphite reduces the grinding efficiency during ball-milling,and significantly decreases the hydrogen absorption and desorption properties.The addition of Ni dramatically promotes overall properties of Mg24Y3-3 wt.%C composite.As Ni content is larger than 3wt.%,the composites have an excellent absorption rate,the absorption saturation can reach above 90%after only 30 s at 100?.Moreover,with the increase in Ni content,the hydrogen desorption activation was significantly decreased,thereby increasing the hydrogen desorption kinetics.The hydrogen desorption peak temperature also down from 375 0C(0 wt.%Ni)to 282 0C(20wt%Ni).Unfortunately,the addition of Ni and graphite reduces the reversible hydrogen storage capacity inevitably.Considering both of kinetics and storage capacity,Mg24Y3-3 wt.%C-5 wt.%Ni has the best overall hydrogen storage performance.