Study On The Properties Of La-Mg Based Hydrogen Storage Compounds And The Laves Phase Structures Of Binary Alloys

In this paper, the electronic structures and hydrogen storage properties of La-Mg based alloys, the Laves phase structures of MgZn2-type and MgCu₂-type binary alloys have all been investigated. The specific contents are as the following works. Firstly, the electronic structure of RMg₂Ni₉ ( R=Ca , La ) alloys are calculated by first-principle plane-wave pseudopotential technology based on the density functional theory method. The effects of the electronic structure on the hydrogen storage properties are carried out. Secondly, La_0.7Mg_0.3Ni₂(Co+Mn) alloys are prepared in high frequency induction furnace under Ar atmosphere, the characteristics of alloy phases and the effects of Co and Mn relative contents in the alloys on their electrochemical performance are studied. Thirdly, the partial least square (PLS) method is applied to investigate the effects of chemical bond parameters on Laves phase structures of MgZn₂-type and MgCu₂-type binary alloys.All results are shown in the following:The calculations for CaMg₂Ni₉ and LaMg₂Ni₉ alloy demonstrate that thedifference of density of states between CaMg₂Ni₉ and LaMg₂Ni₉ alloy mainly comes from s and p electrons of Ca and La atom. Near Fermi level, thedensities of states for two alloys are similar which are mainly composed of d electrons of Ni atom, and the density of states at lowest level is made of p electrons of Mg atom. The bonding stability of Mg atom is stronger than that of La atom in LaMg₂Ni₉ alloy, which make the stability of La_xMg_3-xNi₉ (x=1.0-2.0) alloys increase with Mg content increased. CaMg₂Ni₉ and LaMg₂Ni₉ alloy present the characteristic of typical metallicity for their energy band interlace mutually and go through Feimi level. The interactions among Ni atoms in LaMg₂Ni₉ alloy are stronger than those in CaMg₂Ni₉ alloy, more energy will be supplied for bond transferring from Ni-Ni bond to Ni-H bond during hydrogen is absorbed, which is a reason that leads to the increase of plateau pressure for hydrogen absorption in LaMg₂Ni₉ alloy as compared with CaMg₂Ni₉ alloy. The values of electron change of atomic orbita in La (2.51) is greater than that in Ca (1.17), which make the contour maps of the electron density distributions between Ca and Ni2 atom in CaMg₂Ni₉ alloy different from those between La and Ni2 atom in LaMg₂Ni₉ alloy.XRD analysis and electrochemical experiment show that each sample contains at lease two phases which are nearly all hexagonal structure for La_0.7Mg_0.3Ni₂(Co+Mn) alloys. LaNi_3.87Mn_1.13 phase appears and LaNi₅ phase disappears when the ratio of Mn/Co is above 3/2 and 1/1, respectively. The maximum discharge capacities of the electrodes drop approximately linear with the decrease of Co and the increase of Mn. Too much Co or Mn is responsible for the decay of the cycle life and the discharge capacity retention during constant charging/discharging.The diagrams of PLS classification and criterion equation for binary MgCu₂-type and MgZn₂-type Laves phase alloys reveal that Valence electron density affects the phase structures of nontransiton-transition and lanthanide(actinides) alloys and transition-transition alloys mainly. Valence electrons of B atom is the most important factor influencing the phase structures of alloys composed by transition and lanthanide(actinides) element. Besides, the effects of both valence electrons and atomic radius on their Laves phase structures are in opposition to each other.