Theoretical Studies On The Effect Of B,Cu Substituted On The Structure And Property Of Mg₂ Ni

The hydrogen is a green ideal secondary energy resources, which combustion products is just water. So 21st century is called the times of hydrogen resource. But the hydrogen is easy to leak out and explode, so it is difficult to store and transport. So, many researchers are focus on the development of the hydrogen storage materials.A first-principle based on density functional theory and plane wave pseudo-potential (PWP) method were used to research the A₂B-type Mg-based hydrogen storage materials Mg₂Ni. The crystal structure, electron density distribution, density of states (DOS) and the enthalpy of hydrides formation of Mg₂Ni and low temperature Mg₂NiH₄ hydrides were studied. We also investigated the Mg₂Ni alloy doped by B and Cu atoms respectively in order to modify the rate of hydrogen evolution reaction of the alloy, and discussed the changes of crystal structure, electron density distribution, density of states (DOS) and the enthalpy of hydrides formation, that was used to assess the results of the modification. The dissertation consists of four parts as follows.Part one, gives the theoretical studies on the properties of Mg₂Ni and LT-Mg₂NiH₄ regarding the crystal construction, the density of states (DOS), the enthalpy changes of hydrogenation process. As a result, Ni-Ni shows strong interaction in the direction of axis a in the crystal cell of Mg₂Ni. Moreover, the strong interaction between H and Ni atoms was discovered in LT-Mg₂NiH₄. And due to the strong interaction mentioned above between H and Ni atoms, which leads us to Jasen's calculation result. The enthalpy changes of hydrogenation process from Mg₂Ni to LT-Mg₂NiH₄ were also calculated, the calculation result is -51.2781kJ/mol(H2) which is the Leading reason for the high hydrogen stability.Part two and part three shows the studies on the modified properties of B-doped Mg₂Ni and Cu-doped Mg₂Ni respectively. B possess relatively high electronegativity and Cu possess relatively low electronegativity. Furthermore, these two chapters compared the properties of both undoped and B/Cu-doped Mg₂Ni alloy and its undoped and B/Cu-doped hydrides LT-Mg₂NiH₄ in respect to the crystal construction, the density of state (DOS) and the enthalpy changes of hydrogenation reaction. We fond that B-Ni showed strong interaction, which leads to the interaction of Ni-H be weakened. Because of the high electronegativity, the outer electrons structure of Ni was changed that weaken the interaction of Ni-H, which led to the decreasing of hydrogen storage capacity. the energy change of Hydrogen absorption and desorption process significantly improved We also fond that the interaction of Cu-H is weakened compared with the interaction of Ni-H, Because the adsorption capacity of Cu is poor than the the Ni,which lead to the System becomes unstable, more favorable for dehydrogenation.Therefore, the feasibility of modifying the properties of Mg₂Ni alloy was proved.In the fouth part, the changes of crystal structures, electron density distributions, the densi ty of state(DOS) and the hydrogenation enthalpy for B substituting Mg and Cu substituting Ni in Mg₂Ni were discussed., bis-substitution increased the hydrogen storage capacity, and the overall stability of the unit cell has been reduced,so bis-substitution is superior to the mono-su bstitution for decreaseing the dehydrogenation temperature of Mg₂Ni.The novel conclusions and ideas of this work are listed as follows:1.The crystal structure, electron density distribution, density of states (DOS) and the enthalpy of hydrides formation of Mg₂Ni and low temperature phase of Mg₂NiH₄ were calculated in theory, and we also found that H-Ni atoms have been a strong interaction,The reason of difficulty of the dehydrogenation was explained in theory.2.It explained that the properties changes of B-doped and Cu-doped Mg₂Ni by the calculating in theory. We also shows the new properties of the new alloy of B-doped and Cu-doped Mg₂Ni in this paper.3.The crystal structure, electron density distribution, density of states (DOS) and the enthalpy change of hydrides formation of B-dope and Cu-dope Mg₂Ni was calculated, and the possibility of B-doping and Cu-doping to modification of Mg₂Ni was proved in theory. 4.In the case of B substituting Mg and Cu substituting Ni, i.e. B₁Mg₁₁Ni₅Cu₁, the volume of unit cell has a little change compared with the mono-substitutuion. This can improve the circling stability of hydrogenation/ dehydrogenation of Mg₂Ni alloy.