Preparation, Microstructure And Hydrogen Storage Properties Of RE₅Mg₄₁-based Composites ?RE=Pr,Nd,Sm?

As magnesium-based hydrides have a relatively high hydrogen storage capacity,abundant raw material reserves,low cost and many other advantages,they become one of the most promising hydrogen storage materials.Alloying Mg with RE can effectively improve the hydrogen storage behaviors of Mg-based materials.Especially,microstructural optimization of REMg₁₂,RE₂Mg₁₇ and RE₅Mg₄₁ alloys could be used as high performance hydrogen storage materials.To improve the kinetics of Mg-based hydrogen storage materials,the Mg-based binary alloy RE₅Mg₄₁?RE = Pr,Nd,Sm?were fabricated by vacuum induction melting in this work.The microstructures and gaseous storage hydrogen behaviors of the alloys were investigated in detail.Then,the preferred Sm₅Mg₄₁ alloy was milled with different additions characterized by nano-particles to greately improve the hydrogen storage performances.The effects of modification methods on the gaseous hydrogen storage properties and microstructure of RE-Mg-based alloys were studied,including alloying,nanocrystallization and catalysis.The mechanism of hydrogen storage performance improvement is studied in detail in terms of thermodynamics and kinetics.The as-cast RE₅Mg₄₁?RE = Pr,Nd,Sm?alloy is composed of a main phase of 5:41 and a small amount of second phases of 1:3 and 1:12;After absorbing hydrogen,the second phase nano-particles of rare earth hydride with the size less than 100 nm grow uniformly in the MgH₂ nanocrystalline matrix phase,while the second-phase nano-particles of rare earth hydride still exist imbedded in the Mg nanocrystal matrix phase after the desorbing of hydrogen.The reaction pathways were:Pr₅Mg₄₁+PrMg₁₂+ PrMg₃+H₂ ?PrH₂+PrsH18.96+MgH₂???PrH₂+Mg+H₂;NdsMg₄₁+NdMgi2 +NdMg₃+H₂ ?Nd₂H₅+MgH₂???Nd₂H₅+Mg+H₂;Sm₅Mg₄₁+SmMg₃+H₂ ?Sm₃H₇+ MgH₂???Sm₃H₇+Mg+H₂.The rare earth hydride nanoparticles have a hydrogen pumping effect and can exert a catalytic effect on the hydrogen absorption and desorption of RE₅Mg₄₁?RE = Pr,Nd,Sm?alloy,significantly improving the hydrogen absorption and desorption kinetics,and the hydrogen activation energy Ea?de?is 140.595,139.191,135.280 kJ/mol H₂,respectively.The hydrogenation enthalpy ?HHabs?-76,899,-79.375,-76.519 kJ/mol H₂,for RE = Pr,RE = Nd and RE=Sin alloy,respectively.It indicated that alloying Sm and Mg shows the best hydrogen storage performance,and the reversible hydrogen storage capacity reaches 5 wt.%.Prolonging the time of ball milling results in the gradual evolution of microstructure of Sm₅Mg₄₁ alloy from polycrystalline to nanocrystalline and amorphous.With the milling time increasing,the phase content of nanocrystals will increase firstly and then decrease.When milling for 10 hours,The Sm₅Mg₄₁ alloy consists mainly of nanocrystalline phases with a large number of subgrain boundaries and a small amount of amorphous below 2 nm.Further extending the ball milling time will lead to the reducing of nanocrystalline phase,and the amorphous phase content and size will increase.When the ball milling time was 5,10,20 and 30 hours,the activation energies of hydrogenation for the Sm₅Mg₄₁ ball milled alloy were 128.19,112.934,125,902 and 126.928 kJ/mol H₂,respectively.The hydrogenation enthalpy?Hab were-82.3868,77,5161,79.0047 and 80.0761 kJ/mol H₂,respectively.When ball milling for 10 hours,Sm₅Mg₄₁ alloy has the best hydrogen storage property,and the reversible hydrogen storage capacity is 4.9 wt.%.The influences of the catalysts of CoS₂ and MoS₂ nano-particles on microstructure and hydrogen storage behaviors of as-milled Sm₅Mg₄₁ alloy have been compared in this work.The Sm₅Mg₄₁+wt.%M?x= 0,5,10,M = CoS₂,MoS₂?alloys were prepared by milling the mechanical ground as-cast Sm₅Mg₄₁ alloy powders?particle size?75 ?m?with 5 wt.%CoS₂ or MoS₂ nano-particles?particle size ?30 nm?,respectively.The results demonstrate that the CoS₂ and MoS₂ nanoparticles are embedded into the alloy surface,which is nanostructure containing some crystal defects,such as dislocation,grain boundary and twin etc.Those microstructures play a beneficial role in reducing the total potential barrier that the hydrogen absorption or desorption reactions must overcome,hence improving the hydrogen storage kinetics of the alloys.The as-milled alloys are composed of Sm₅Mg₄₁ and SmMg₃ phases,and ball milling refines their crystal grains,The MgH₂ and Sm₃H₇ phases appear after hydrogenation,while Mg and Sm₃H₇ phases exist after dehydrogenation.The dehydriding activation energy of M=CoS₂ and MoS₂ alloys are 101.67 and 68.25 kJ/mol H₂ respectively.The hydrogenation and dehydrogenation enthalpy changes of M=MoS₂ alloy are a little smaller than that of M=CoS₂ alloy,Therefore,the catalyst MoS₂ can improve the as-milled Sm₅Mg₄₁ alloy in hydrogen storage property more effectively than CoS₂.The effects of adding different content of nano-graphite?NG?on the microstructure and hydrogen storage properties of Sm₅Mg₄₁ alloy were studied,Sm₅Mg₄₁+x wt%NG?x = 0,2,4,8,12?composites were prepared by ball milling for 10 hours.The alloy consists of two phases,Sm₅Mg₄₁ and SmMg₃,which are composed of nanocrystalline.Sm₃H₇ and MgH₂ phases are formed after the hydrogen absorbing,and the newly formed Mg phase and the remaining Sm₃H₇ phase appear after the hydrogen desorbing.Adding NG can effectively refine the grains and improve the microstructure of the alloy to form nanocrystals.NG is embedded in the surface and inside of the alloy in the form of nanosheets,which promotes the formation of nanocrystalline structure,precipitated nanocrystalline magnesium and added some crystal defects,including dislocations,grain boundaries and twin crystal.These crystal defects and nanocrystalline boundaries can reduce the total potential barrier needed to overcome during the hydrogen absorbtion and desorption reactions and thus improve the hydrogen storage kinetics of the alloy.For the alloys with x = 2,4,8 and 12,the activation energies of hydrogen desorption were 132.076,128.056,112.882 and 112.348 kJ/mol H₂,respectively.The enthalpy of hydrogen absorption was-79.181,-78.091,-76.296 and-76.089 kJ/mol H₂,indicating that adding NG can increase crystal defects and nano-interface,and reduce the thermodynamic stability of the alloy.