| The problem of pollution and energy crisis is worsening today. Tritium is not only an important fuel of the fusion energy but also a pivotal target material in neutron generators. How to obtain high performance hydrogen storage material is an urgent problem. The 3He that comes from the decay of tritium in hydrogen storage material can accumulate in the host metal, which always damages the host metal. In this dissertation, the structure and properties of Sc, Er, Y and the relevant metal-hydrogen and metal-helium systems have been studied. The main contents and conclusions of this dissertation are as follows:1. The elastic properties of hexagonal close-packed structure MAx(M=Sc, Er; A=H, He) have been systematically investigated by the density functional theory, where x=0, 0.0313, 0.125, 0.25. The effects of different concentrations of hydrogen and helium on the elastic properties of MAx(M=Sc, Er; A=H, He) systems have been investigated in detail. The elastic constants and elastic moduli of the a-ScHx system almost increase with increasing of hydrogen concentration, which is in agreement with the experimental observation. The elastic constants and elastic moduli almost decrease with the increase of helium concentration for ScHex system. The results show that the elastic constants, Young’s modulus, bulk modulus and shear modulus of a-ErHx systems mainly increase with increasing hydrogen concentration, whereas, those elastic properties of ErHex systems almost decrease with increasing helium concentration. The changes in the charge densities of M(M=Sc, Er) atoms produced by A(A=H, He) atoms have been investigated. It is found that the change mechanism of the elastic properties of a-ErAx(A=H, He) is similar to that of a-ScAx(A=H, He).2. Ab initio calculations based on density functional theory have been performed to investigate the elastic constants, bulk modulus, shear modulus and Young’s modulus of the β-MH2-xHex(M=Er, Sc) systems, with the variety of the x, the ratio of He to M atoms, ranging from 0 to 0.1875. Results show that the changes in the mechanical properties of β-ErH2-xHex and β-ScH2-xHex exhibit two regimes, when x=0~0.13, the ?rst regime shows that the Young’s modulus slightly decreases, the second regime appears when x about equal 0.13, at which the elasticity exhibits a sharp drop, which is in agreement with the experimental observations. It appears that the different He concentrations bring the different degrees of hybridizations for the β-MH2-xHex systems, a key factor that gives rise to the observed two regimes. When the size of the He cluster is large enough, the mechanical properties of metal hydrides degrade significantly.3. The grain boundary energy of(1121) twin boundary, the formation energies of hydrogen and helium defects in tetrahedral and octahedral interstitial sites at the(1121) twin boundary in hcp-Sc have been investigated by ab initio calculations based on DFT(density functional theory). It is found that the formation energies of the tetrahedral interstice H, octahedral interstice H, and tetrahedral interstice He increase significantly towards the(1121) twin boundary plane, while the formation energy of the octahedral interstice He atom near the(1121) twin boundary plane decreases. To analyze these results, the electronic density of states(DOS) of H, He and their nearest neighbor Sc atoms in several tetrahedral and octahedral configurations have been presented. The results show that the electron inteactions between interstitial atom(H or He) and its nearest-neighbor Sc atoms have been changed at the(1121) twin boundary, which affects the formation energies of H and He defects near the grain boundary plane. The formation energies of He-vacancy clusters in the Sc grain boundary also have been calculated, which indicates that the stabilities of He-vacancy clusters depend on the variation of the relaxed vacancy volume, i.e., the larger absolute values of variation in the relaxation vacancy volume lead to the lower formation energies of He-vacancy clusters near the(1121) twin boundary plane.4. A database for the Sc-Sc and Sc-H systems has been constructed, which includes the data from experiments and our DFT calculations. Then, an analytical bond-order potential(BOP) for Sc-Sc interaction has been developed by fitting to the lattice parameters, elastic constants, cohesive energy, structural energy differences and vacancy formation energy. The present BOP model of Sc-Sc system provides good descriptions for the structural properties and energetics of hcp-Sc, which well reproduces the lattice parameters, elastic constants, cohesive energy, structural energy differences and vacancy formation energy. The present potential give that the divacancy con?gurations are stable. Especially, the present potential can reliably describe the structural stability, as well as the point defect properties of hcp-Sc. Furthermore, the ?tted potential are able to reproduce the melting point of Sc. The BOP potential of Sc-H system also has been constructed and these calculations of properties indicate that the interaction of Sc-H not only can model hcp structure Sc-H system, but also can describe the bulk properties of ScH2.5. Based on the s-band model and repulsive pair potential, the empirical potentials for Er-He and He-H have been developed. The atomic configurations and the formation energies of single He defects and small He-vacancy(HnVm, m=0, 1) clusters obtained by the density functional theory(DFT) calculations are used as the fitting database for Er-He system. The results show that Er-He potential almost reproduces these data of DFT calculations. The binding energies of the additional He to HenVm clusters have been calculated by the Er-He potential and DFT. These calculations show that the binding energy of an additional He atom to an interstitial He cluster(Hen) generally increases with increasing cluster size, and the binding energy of a He to a HenV cluster decreases with increasing the cluster size and then remains almost constant for large cluster size in hcp-Er. In addition, two migration paths of the interstitial He between two most stable interstitial sites(T) have been investigated in the present work. The results from both Er-He potential and DFT indicate that the favorite migration path of a tetrahedral interstitial is along T1®BT®T2 path. Furthermore, the formation of He clusters in hcp-Er at the temperatures of 300 and 600 K is further explored using the molecular dynamics(MD) method. It is found that the distribution of He atoms after annealing 0.5 ns at 300 K is almost the same as the initial configuration, but large clusters have formed after annealing at 600 K. In order to test the effect of vacancies on He clustering, five vacancies, in addition to 22 He atoms, are also added randomly to the Er crystal, and the system is simulated for 0.5 ns at 600 K and 800 K using MD method. The present results show that the vacancies act as sinks to trap He ions, enhancing the formation of He clusters, which strongly support the experimental observations. Finaly, combining with the interactions of Er-Er and H-H from Ref. 117, The the formation energies of point defects have been studied by the present Er-He and H-He potentials, and it is shown that the present potentials can provide a good description of the point defect properties in ErH2.6. First, density functional theory calculations are performed to study the structure, the properties of self-interstitial atoms, the properties of H and He defects, as well as the properties of planar defects in hcp-Y. The changes in DOS for the H or He atoms and their nearest neighbour host metal atoms in hcp-Y also have been investigated. The results show that most of con?gurations, e.g., T, O, C, S, BO, BS and BT, are stable, while the BC site is unstable which transfers to BO site. The calculated data of H defects indicate that H atom prefers to occupy the tetrahedral interstice, followed by the octahedral interstice. While He atom will firstly occupy the vacancy of Y atom, if there is no vacancy, He atom prefers to occupy the tetrahedral site in hcp-Y. Second, based on an analytical bond-order potential scheme, the interaction of Y-Y for hcp-Y has been developed. The potential model is fitted to some properties of Y, e.g., the lattice parameters, elastic constants, bulk modulus, cohesive energy, vacancy formation energy, and the structural energy differences. The present potential has ability to reproduce defect properties including the formation energies of self-interstitial atoms, vacancy formation energy, divacancy binding energy, as well as the bulk properties and the thermal dynamic properties. |
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