Theoretical Studies On The Structure And Property Of Ti Substituted Hydrogen Storage Alloy Mg₂Ni

Energy is the cornerstone and driving force for the survival and development of human being. People could not live without energy, and the progress and development of human society would greatly constrained by the energy shortage. In recent years, the problem of energy shortage is becoming more and more serious with the rapid development of China's economy. Hydrogen Energy, which has the qualities of high energy density and no pollution, gains great attentions on its produce, storage and usage. As a result, the 21st century is called the time of Hydrogen Energy. So it is the common target of researchers from all over the world to find new energy resource, which is clean and environment-friendly, to substitute for the traditional hydrocarbon energy.In this thesis, the First-Principles calculations based on the Density Functional Theory(DFT) for the Mg-based Hydrogen Storage Materials, Mg₂Ni, were carried out. The aim is to investigate the changes of crystal and elecronic structures by the metal substitutions. The enthalpy changes in the hydrogenation processes were also discussed.The paper mainly consist of three parts as following:In the first part, the theoretical studis on the crystal structures, electron density distributions, the density of state(DOS) and the enthalpy change of hydrogenation reaction for Mg₂Ni alloy, 3d site Ti substitued Mg₂Ni, and their hydrides, were performed. As a result, we found that the interaction of Ni-Ni is stronger than that of Mg-Ni in the crystal of Mg₂Ni (i.e. Mg₁₂Ni₆). In the unit cell of Mg₁₂Ni₆H₂, Ni and H atoms forms a strong covalent bond. It makes dehydrogenation difficultly. When a Ti atom substitute a Ni (3d) atom, the interation of Ni-Ti is weaker than that of Ni-Ni in Mg₂Ni alloy. The interaction of Ti-H in Mg₁₂Ni₆-xTixH(x=1,2,3)is relatively weaker than that of Ni-H in Mg₁₂Ni₆H₂. It indicates that the sustitution could possibly lowering the temperature of dehydrogenation.In the second part, the study focused on the crystal structures, electron density distributions, the density of state(DOS) and the enthalpy change of hydrogenation reaction for 3b site Ti substitued Mg₂Ni. The results show that the ineractions of Ni-Ti, and Ni-Mg in Ti substituted Ni (3b site) alloy are relatively weaker than those of Ni-Ni and Ni-Mg in Mg₁₂Ni₆. The volume of the unit cell increases after the substitution, which is more favorable for the difusion of H atom in the hydride. In the third part, the changes of crystal structures, electron density distributions, the density of state(DOS) and the hydrogenation enthalpy for Al substituting Mg and Ti substituting Ni in Mg₂Ni were discussed. The influences for mono-substitution and bis-substitution were compared. In bis-substitution case, the ineraction between hydrogen absorbing atom and H atom is relatively weaker than that in the mono-substitution one. The bis-substitution is more propitious for the weakening of strong Ni-H bond, so bis-substitution is superior to the mono-substitution for decreaseing the dehydrogenation temperature of Mg₂Ni..The novel conclusions and ideas of this work are listed as follows:1. The first-principles calaulaions on the crystal and electronic structures of Mg₂Ni and its hydrides were performed. The optimized structure of Mg₁₂Ni₆ alloys and its hydride are consistent with the experimental data of neutron power diffraction.2. The analysis of electron density distribution and density of state(DOS) show that there exsit an strong interaction between Ni-4s and H-1s orbitals, forming a covalent Ni-H bond. It is the main reason for the difficulty of dehydrogenation.3. In the case of Ti substituting Ni(3d) atom of Mg₁₂Ni₆, the volume of the unit cell increases about 4% for one atom substiutio.The interation between Ti and H atoms in Mg₁₂Ni_6-xTi_xH₂(x=1,2,3)is weaker than that of Ni and H atoms in Mg₁₂Ni₆H₂. So it is preferable for lowering the dehydrogenation temperature of Mg₂Ni.4. In the case of Al substituting Mg and Ti substituting Ni, i.e. Al₁Mg₁₁Ni₅Ti₁, 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.