Hydrogen-induced Phase Transformation And Hydrogen Storage Property Tuning Of Mg-based Amorphous Alloys

Owing to the high theoretical hydrogen storage capacity of Mg（7.6 wt.%-H）and low cost,Mg-based hydrogen storage alloys are regarded as promise future hydrogen storage material.The challenge in such material roots in the strong Mg-H bond which results in difficultly sluggish low-temperature de/hydrogenation.It has been confirmed that the long-range disorder amorphous structure in amorphous alloy can provide more types of occupation sites for H atoms.Comparing with crystalline counterparts,amorphous alloys may own higher capacity,faster de/hydrogenation kinetics and lower de/hydrogenation temperature.Thus,aiming to solve the problems in Mg-based hydrogen storage alloys,the present work studied Mg-based amorphous alloy in following four aspects.By cryogenic thermal cycling treatment（CTC）,samples with different rejuvenation or relaxation extent of three Mg-based amorphous alloys were successfully prepared.Under the same CTC parameters,the relaxation enthalpy of Mg-based alloys was found to be quite sensitive to composition,and a long-cycle treatment always led to relaxation.Evident rejuvenation was achieved in Mg₆₀La₁₀Ni₂₀Cu₁₀ and Mg₆₀La₂₀Ni₂₀ amorphous alloys,and the relaxation enthalpy of Mg₆₀La₂₀Ni₂₀ even increased by～19%.It was found that samples with higher relaxation enthalpy owned higher hydrogen absorption rate.However,if CTC cause relaxation,the hydrogenation performance would obviously decay.The structure and performance analysis proves that the CTC does not change the long-range disorder structure of amorphous,and the performance enhancement in the rejuvenated samples should be attributed to the change of short-range structure in local region of amorphous.As the structure of amorphous phase greatly affects the hydrogen storage properties of amorphous alloys,the H-induced amorphous-amorphous phase separation（HIAPS）and H-induced crystallization（HIC）in Mg₆₀RE_xNi_30-xCu₁₀ amorphous alloys were further studied.The HIAPS results in formation of Ni-Cu-rich and Ce-rich amorphous region during hydrogenation.As the Ni-Cu-rich region crystallizes readily at the end of hydrogenation,it thus proves that HIAPS is prerequisite of HIC.This work also proves that HIAPS is rather similar to spinodal decomposition,and this explains why the amorphous alloys owning severer HIAPS shows better hydrogenation kinetics.As the HIAPS is sensitive to RE:Ni radio,it is possible to improve the hydrogen storage properties of Mg-based alloys via HIAPS.To obtain Mg-based alloys with excellent de/hydrogenation performance at lower temperature,the present work used ethanol glycol（EG）as ball milling additives and successfully prepared Mg-based alloy particles with amorphous core and crystalline shell.The as-prepared sample absorbed nearly 3.0 wt.%-H at 70℃ in 1 hour while the pure amorphous Mg-based alloy hardly absorbed hydrogen at 100℃.Most of the amorphous structure in the as-prepared sample crystallizes during hydrogenation and the hydrogenated sample is mainly nanocrystalline.Such nanocrystalline Mg-based hydrides startes to release hydrogen even below 100℃ which is much better than most of the traditional Mg-based alloys.The nanocrystalline shell enhances hydrogen dissociation and diffusion.The preparation of Mg-based alloys with structure of amorphous core and crystalline shell may inspire the development of novel Mg-based hydrogen storage material.To maintain the amorphous structure during de/hydrogenation,the present work milled Mg₆₀La_xNi_30-xCu₁₀ amorphous alloys together with small amount of crystalline hydrogen storage alloys,and the very fine crystalline particles were embedded on the surface of amorphous particles.The crystalline YFe₂、YFe_1.7Al_0.3、YFe_1.3Al_0.7、YFe₃ and LaNi₅ could all enhance the hydrogenation of Mg₆₀La_xNi_30-xCu₁₀ amorphous alloys,and YFe_1.7Al_0.3 was best.The YFe_1.7Al_0.3-added sample absorbed nearly 3.0 wt.%-H in 1 hour at 130℃,and was ten times faster than the amorphous alloys without additive.What’s more,the YFe_1.7Al_0.3-added sample released 2.0 wt.%-H at 250℃ in 1 hour keeping partial amorphous structure stable.By structure analysis and performance comparison,it is believed that the de/hydrogenation of YFe_1.7Al_0.3 itself was the key to performance enhancement of multiphase Mg-based alloy.The hydrogenation kinetics showed that the crystalline hydrogen storage alloys play the role of“hydrogen pump”on the surface of the Mg-based amorphous alloys.