Investigation On The Microstructure And Electrochemical Performances Of Ball-milled Mg_1-xNd_xNi_0.5（x=0-0.2） Alloys

With a gradually serious problem of the energy shortage and the living environmentbeing destroyed, looking for a new sustainable and environment friendly candidate toreplace the fossil fuel has become more and more important. Therefore, the Mg-basedhydrogen storage materials with high specific capacity and rich sources have a promisingprospect in the electric vehicle as the negative electrode of Ni-MH battery. However, thereare some instinct shortcomings of the Mg-based hydrogen storage materials, so the key ofresearching is ameliorating the integrated hydrogen storage performance.In this paper, the Mg_1-xNd_xNi_0.5（x=0,0.05,0.1,0.15,0.2） alloys were selected as theobject after reviewing the latest achievements both domestic and abroad, and the alloyswere prepared by vacuμm melting. The X-Ray Diffraction （XRD） and High ResolutionTransmission Electron Microscopes （HRTEM+ED） were used to analyze the phasecomposition, structures and phase distribution of the as cast alloys. Meanwhile, theelectrochemical and dynamics performance were investigated by electrochemical testequipment and dynamics workstations. The influence of replacement of Mg by Nd on thecomprehensive hydrogen storage performance was explored consequently. Then differentmechanical alloying technology was used to make the as cast alloys turned to ball millalloys, and the influence of ball milling time （T=0,40,60,80h） on the comprehensivehydrogen storage performance of alloys was investigated. It mainly concludes as follows:The analysis of XRD, SEM+EDS and HRTEM+ED reveals that the as cast Mg1-xNdxNi0.5（x=0,0.05,0.1,0.15,0.2） alloys hold a multiphase containing Nd, Mg₂Ni, NdNiwhen the substitution quantity of Nd increases to0.1, and there is a new phase NdMgNi₄with the x>0.1. It is founded that the element Nd in these alloys exhibits obvious grainrefining, the unit cell distortion of Mg₂Ni is clear, and the cell volume is monotonouslyincreasing. The phase composition of ball mill alloys did not take obvious change after0⁸0h ball milling, but the particle size was decreased from20μm to2μm, and the alloysafter ball milling hold a nanocrystalline and amorphous structure.The electrochemical measurements exhibit that Nd plays an evident impact onimproving the discharge capacity of the as cast alloys, gradually from66mAh·g^-1（Nd0-z） to138.4mAh·g^-1（Nd0.2-z）. But the improvement for the cycle stability is limited, too muchNd is to the disadvantage of the cycle stability. And the discharge capacity of Nd0after80hball milling can reach220.4mAh·g^-1, the ball milling technology dose not play optimum role in the discharge capacity, but shows some improvement on the cycle stability whenthe content of Nd increases from x>0.05.It is founded from the dynamics testing that moderate amount of Nd enhances thedynamics performance of the as cast alloys, the high rate discharge ability （HRD）, limitingcurrent density （IL）, hydrogen diffusion coefficient （D） first mount up and then fall with therising of Nd content, and the best content of Nd is x=0.15, the limiting current density （IL）is301.3mA/g, hydrogen diffusion coefficient （D） is1.93×10-10cm²·s^-1. The dynamicsperformance of the as cast alloys is controlled by the electron transfer ability of alloysurface and the hydrogen diffusion ability. The high rate discharge ability （HRD） of theball milling alloys can be improved by appropriate ball miling time, the limiting currentdensity （IL） is226.8mA/g （Nd0.1-60h）, hydrogen diffusion coefficient （D） is1.12×10-10cm²·s^-1（Nd0.2-80h）. The electrochemical kinetics of the ball-milled alloys is controlled bythe electron transfer ability of alloy surface.