Effect Of Mg Content On Phase Structure And Hydrogen Storage Properties Of Re-Mg-Ni-Based Alloys

In the crystal structure of RE-Mg-Ni-based superlattice alloy,the occupancy orientation and content of Mg elements are very important.Meanwhile,there is a serious problem of Mg volatilization in the preparation of RE-Mg-Ni-based alloys,and it is difficult to accurately control the Mg content.Herein,the infiltration sintering method based on the Mg thermal diffusion process was used to realize the controllable adjustment of Mg content in the La-Ni-based precursor alloy,and the A₂B₇ type La-Mg-Ni-based alloy was prepared.The influence of Mg content on the hydrogen storage capacity and cycle stability of the alloy was studied.At the same time,the effect of the preferred occupation orientation of Mg elements and the content of Mg on the phase structure and hydrogen storage performances of Sm-Mg-Ni-based AB₂ type alloy were studied.La-Ni-based precursor was prepared by induction melting and followed annealing treatment,and the Mg content was adjusted by Mg infiltration sintering method.The effect of Mg content on the phase structure and hydrogen storage performance of the alloy was investigated.The results show that the increase of Mg content makes the La Ni₅ phase in the precursor alloy gradually transform into superlattice structure,and significantly inhibit the phase structure decomposition of superlattice structure in the alloy during hydrogen absorption cycles.When the molar fraction of Mg in A-side elements is adjusted to 0.23,the A₂B₇ type alloy with a composition of La_0.57Sm_0.02Y_0.18Mg_0.23Ni_3.38Al_0.03 is successfully synthesized,which has a biphase structure consisting of Gd₂Co₇ phase and Ce₂Ni₇ phase.At 303 K,the hydrogen storage capacity of the alloy is as high as 1.73 wt%,and the alloy also shows a good cycle stability.After 50 cycles of hydrogen absorption,the alloy still maintains a hydrogen storage capacity of 1.43 wt%,and the capacity retention rate can reach84.1%.Sm-Mg-Ni-based AB₂-type alloy with the composition of Sm_0.63Y_0.17Mg_0.19Ni_2.19Co_0.04Al_0.11 was prepared by induction melting combined with annealing treatment.The Mg content of the alloy was further adjusted by powder sintering method.The results of XRD refinement and simulated calculation show that the preferred orientation of Mg element when it enters the unit cell of Sm Ni₂ phase can be reasonably approximated as the para-position substitution of rare earth elements at 4c site of the body center.With the increasing occupancy of Mg element at 4c sites,the volume of the unit cell shrinks linearly,and the alloy undergoes a phase transformation from Sm Ni₂ phase to Mg Cu₄Sn-type phase,meanwhile,there appears an obvious hydrogen/absorption platform in the PCT curves.The unit cell volume shrinkage makes the initial hydrogen absorption capacity of the alloy decrease,but the structural stability of the alloy is significantly enhanced during the hydrogenation cycles.When Mg elements completely occupied 4c sites,the alloy with a composition of Sm_0.38Y_0.10Mg_0.52Ni_1.93Co_0.03Al_0.05 still maintains a Mg Cu₄Sn-type single-phase structure during hydrogenation cycles,showing excellent cycle stability,and the capacity retention rate reaches 94.2%after 20 cycles.