Preparation And Hydrogenation Kinetics Of Mm-Mg-Ni System A₂B₇-type Hydrogen Storage Materials

The La-Mg-Ni system A2B7-type hydrogen storage alloy is a kind of material with high storage capacity and excellent dynamic characteristics in hydrogen absorbing and desorbing, and it has become an important material in the hydrogen storage research for its application. In the present work, five kinds of alloys such as (La, Ce)0.75Mg0.25Ni3.5,(La, Ce)0.7Mg0.3Ni3.5,(La, Ce)0.8Mg0.2Ni3.5,(La, Ce)0.75Mg0.25Ni3.3Co0.2and (La, Ce)0.75Mg0.25Ni3.0Co0.5were adopted as the target material, and the effects of Mg content, Co content, annealing treatment temperature and solidification rate on their structures and hydrogenation kinetics were investigated systematically.For (La, Ce)1-xMgxNi35(x=0.2,0.25,0.3) alloys, with increasing Mg content, Pr5Co19-type, MgCu4Sn-type contents decreased, and (La, Mg)2Ni7phase contents increased, and the CaCu5-type phase content first reduced and then remained unchanged. Compared to Gd2Co7-type phase, the Ce2Ni7-type phase more easily generated with increasing Mg content. The cell parameters of CaCu5-type phase changed little, indicating that Mg entered into the CaCu5-type phase rarely. Both cell parameters of the Ce2Ni7-type phase and Gd2Co7-type phase decreased with increasing Mg content, but the axial ratio tended to increase. Besides, the Mg content had evident effect on the hydrogenation kinetics. For instance, when the Mg content was0.25, the hydrogenation kinetics of the alloy was the worst. As the Mg content was0.3, the maximum hydrogen absorption capacity of the alloy was measured to be1.52wt.%at300℃with an initial hydrogen pressure of4.5MPa.The effects of Co content on the phase structures and hydrogenation kinetics of (La,Ce)0.75Mg0.25Ni3.5-xCox(x=0,0.2,0.5) alloys were examined systematically. With increasing Co content, each phase in the alloy kept unchanged, but their contents have changed, and the expansion of cell volumes took place owing to the atomic radius of Co being somewhat larger than that of Ni. In addition, the increase in temperature had an evident inhibitive effect on the hydrogenation kinetics. While the Co content was as high as0.5, the maximum hydrogen absorption capacity of the alloy was determined to be2.47wt.%at25℃under an initial hydrogen pressure of5MPa. The addition of a small amount of Co (x=0.2) had little effect on the hydrogenation kinetics of the alloy. However, the hydrogenation kinetics of the alloy can be greatly improved as the Co content was0.5. The expansion of the cell volume would promote the nucleation of the hydride phase and increased the hydrogen diffusion rate.Regarding (La, Ce)0.75Mg0.25Ni35type alloys, the effects of annealing temperature on their phase structure, microstructures, hydrogenation kinetics and thermodynamic properties were investigated. After annealing treatment at850℃, the over-burning of Mg during annealing resulted in the increase in CaCu5-type content. In the case that there was little change with the Mg content, with the increase of annealing temperature, Ce2Ni7-type and Gd2Co7-type phase contents increased. As Mg content was poor, the stability of Ce2Ni7-type phase was worse than that of Gd2Co7-type phase. Annealing treatment had an important influence on the hydrogenation kinetics of alloys. For example, the alloy annealed at850℃had the best hydrogenation kinetics, and the maximum hydrogen absorption capacity of the alloy were measured at25℃and80℃to be1.478and1.449wt.%, respectively, through pressure-composition-temperature (PCT) curves. The plateau of hydrogen absorption/desorption in PTC curves for the alloy was composed of two parts, and the high plateau may be originated from the LaNi5phase and the low plateau may be from the La2Ni7phase. With increasing test temperature, the absorption/desorption plateau pressure of the alloy increased and the hydrogen absorption/desorption hysteresis reduced.The effects of melt spun on the phase structure, hydrogenation kinetics and thermodynamic properties of (La, Ce)0.75Mg0.25N13.5alloys were investigated. With the increasing of quenching rate, the content of the CaCu5-type and MgCu4Sn-type phases increased gradually. When the surface velocity of the copper wheel was20m/s, the content of main phase of LaNi5was determined to be64.69wt.%, and the alloy had the best hydrogenation kinetics in this case probably resulting from microstructural refinement, which stimulated the rapid diffusion of hydrogen into the organization. With further increase of solidification rate (40m/s), the content of LaNi5phase were increased too much, degenerating the hydrogenation kinetics. In the case of the surface velocity of the copper wheel of20m/s, the maximum hydrogen absorption capacity of the alloy were measured at25℃and50℃to be1.480and1.539wt.%, respectively, by PCT measurements.