Investigation On The Phase Structure And The Hydrogen Absorption Properties Of Mg₂Ni Hydrogen Storage Alloys

In order to improve its hydrogen storage properties, the Mg_2.4Ni hydrogen storage alloys were selected as the object of this work based on the review of the research and development of the Mg₂Ni system alloys both from home and abroad. The multi-component Mg₂Ni type hydrogen storage alloys were prepared by medium induction furnace under vacuum. The (Mg_2.4Ni)_100-xRE_x(RE=La,Ce,Nd,Sm; x=0,5,10,15,20) alloys were designed. The high purity helium was used as protecting atmosphere to prevent the volatilization of magnesium during the melting. The amorphous ribbon with the length of consecutive, thickness 25³5um, and the width of 3¹0mm was obtained by single-roll rapid quenching. The structure and morphology were observed and analyzed by XRD and SEM. The electrochemical properties of alloys were measured by electrochemical tester. The crystallization temperature of the amorphous phase was measured by DSC. The influence of the alloy composition and quenching techniques on the amorphous phase thermal stability was analyzed. Hydrogen absorption capacity and kinetics curve of the as-cast and quenched alloys were measured by P-C-T curve tester. The obtained results are as follows:The addition of rear earth element La into the alloys can greatly increase the formation capability of the amorphous phase in the (Mg_2.4Ni)_100-xLa_x(x=0,5,10,15,20) alloys. The phase structure changes obviously with the addition of La, and the main Mg₂Ni phase changes to LaNi₃ phase and LaMg₃ phase with the increasing La content. The influnce of La content on the heat stability of the amorphous is smaller. The activation performance of the as-cast and quenched alloys is favorable, but the maximum discharge capacity changes complicatedly. The kinetics of hydriding becomes worse and the amount of hydrogen absorption becomes less in the as-quenched alloys, the main reason is the hydrogen absorption property of the LaNi₃ phase becomes worse after quenching.For the (Mg_2.4Ni)_100-xCe_x(x=0,5,10,15,20) alloys, the phase structure changes obviously with the addition of Ce, the influence of quenching techniques and Ce content on the heat stability of the amorphous is smaller, and the thermal stability of the amorphous is better, the crystallization temperature is about 580℃. The additon of Ce leads the maximum discharge capacity to decreasing in the as-cast alloys. The as-cast alloy of (Mg_2.4Ni)₉₀Ce₁₀ has the bigest hydrogen absorption amount of 3.6wt% in 150min, the obtained results show that the Ce7Ni₃ phase and CeMg₃ phase are the key hydrogen absorption phases.For the (Mg_2.4Ni)_100-xNd_x(x=0,5,10,15,20) alloys, the structure and the kinetics of hydriding are obtained. It is seen that the as-quenched (Mg_2.4Ni)_100-xNd_x(x=5,10) alloys reveale a nanocrystalline microstructure embedded in the amorphous matrix, the as-quenched (Mg_2.4Ni)_100-xNd_x(x=15,20) alloys are typical of amorphous structure. In the as-cast alloys, the amount of hydrogen absorption first increases and then decreases with rising Nd content, the (Mg_2.4Ni)₉₀Nd₁₀ alloy has the biggest hydrogen absorption amount which is 3.4wt% in 150min. The obtained results show that the NdNi₇ phase and NdNi₁₇ phase are the key hydrogen absorption phases. The kinetics of hydriding is worse and the amount of hydrogen absorption is less after quenching. The as-quenched alloy (Mg_2.4Ni)₈₅Nd₁₅ has the biggest hydrogen absorption amount of 1.9wt% in 150min.The (Mg_2.4Ni)_100-xSm_x(x=0,5,10,15,20) alloys are researched systemically. The addition of rear earth element Sm into the alloys can optimize the hydrogen kinetics properties and increase the amount of hydrogen absorption. Elevating temperature also can improve the kinetics of hydriding in the same pressure, because the higher the temperature is, the bigger the hydrogen atom diffusion coefficient will be;It is favourable for the hydrogen atoms spreading and dissolving. The as-quenched alloy (Mg_2.4Ni)₉₀Sm₁₀ appears Mg₂NiH_0.3 phase,SmH₂ phase and some Mg₂NiH₄ phase after hydrogen absorption for the first time and the tenth. After hydrogen disabsorbs completely, phase abundance and phase intensity greatly increase for the alloys of repeatedly absorb-disabsorb hydrogen, it illustrates that more amorphous crystallize, which is the mechanism of the hydrogen absorption amount falls incessantly the kinetics of hydriding becomes sostenuto worse. Some of the amorphous (Mg_2.4Ni)₉₀Sm₁₀ crystallize after ball milling, discharge capacity declines sharply and hydrogen absorption amount declines greatly.