Studies On Migration And Diffusion Of Two-Dimensional Clusters And Defect Clusters

In the present work, the appropriate empirical interatomic potentials are employed to describe the interactions between atoms in typical bcc metals, alloy and UO2. And thus the migration and diffusion behaviors of defect clusters in bulk and atomic clusters on metal surfaces have been studied systematically, which are consisted with the experimental results.In Fe-Cr alloy systems, it is found for the first time that the Cr migration in Fe is controlled by a vacancy-assistant mechanism, and the corresponding minimum energy paths of Cr-vacancy (Cr-V) clusters and Cr interstitials inα-Fe have been determined. A substitutional Cr atom can migrate to a nearest-neighbor vacancy through an energy barrier of 0.56 eV but this simple mechanism alone is unlikely to lead to the long-distance migration of Cr unless there is a supersaturated concentration of vacancies in the system. The Cr-vacancy clusters can lead to long-distance migration of a Cr atom that is accomplished by Fe and Cr atoms successively jumping to nearest-neighbor vacancy positions, defined as a self-vacancy-assisted migration mechanism, with the migration energies ranging from 0.64 to 0.89 eV. In addition, using the NEB method, a mixed Cr-Fe dumbbell interstitial can easily migrate through Fe lattices, with the migration energy barrier of 0.17, which is lower than that of the Fe-Fe interstitial. The on-site rotation of the Cr-Fe interstitial and Cr atom hopping from one site to another are believed to comprise the dominant migration mechanism. The calculated binding energies of Cr-V clusters are strongly depend on the size of clusters and the concentration of Cr atoms in clusters.The Xe-Xe, Xe-O and Xe-U interatomic potential were described using a potential model by Yakub. The calculated energy barriers in Xe-U-O systems show that the potential of Yakub gives a better description of the migration properties. A single vacancy can migrate through the vacancy mechanism. Moreover, the energy barrier of a U-vacancy is higher than that for an O-vacancy. The net migration of an O-divacancy needs to overcome an energy barrier of 0.85eV, while the dissociation energy of the divacancy is only 0.05eV, which suggests that the O-divacancy tends to dissociate. The migration behaviors of defect clusters become more complex and various by introducing fission gases, xenon. In the case of the interstitial Xe atom migration without vacancies at nearest neighboring sites, the energy barrier is very high after introducing an O-vacancy at the nearest neighboring site, the migration barrier of interstitial Xe atom is lowered by 1.50eV via vacancy-assisted migration mechanism. The Xe atom is easy to be trapped at the U-vacancy, becoming a substitutional atom. The relatively large energy implies that, if a Xe is trapped at a U-vacancy, it can hardly lead to a long distance migration. On the other hand, the energy of a Xe trapped at the O-vacancy is lower. As a result, the substitutional Xe atom at the O-vacancy is expected to move to the octahedral interstitial site via the direct migration mechanism. The octahedral interstitial site was found to be the most stable configuration for Xe. For a defect cluster including two or more vacancies, vacancy aggregation did not happen and divacancy is more likely to dissociate. Particularly, for a defect cluster including an interstitial Xe atom and the O-divacancy, the octahedral interstitial site is unstable for Xe due to the existence of the O-divacancy. The energy barrier for the interstitial Xe to form a substitutional Xe atom and an interstitutional U atom was calculated to be 0.16eV.The dimer method presented here for effective finding the minimum energy path has been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. One atom of the trimer jumps to the next nearest neighbor position, leading to a non-compact triangular trimer with a higher energy barrier. In addition, it is of interest to note that the dimer shearing is a dominate migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters.The dynamic self-diffusion behaviors of clusters on a bcc (110) surface have been investigated using molecular dynamics simulations based on a modified analytic embedded-atom method. The stable configurations of clusters are predicted to be close-packed islands configurations for Fe and W cluster size up to nine atoms or even larger by the quenched MD simulation. The characterizations of diffusion behavior, such as the migration energy, the diffusion coefficient and the diffusion prefactor, can be obtained by the MD simulation for a long enough time. The migration energies show an interesting and oscillating behavior with increasing cluster size. As compared to the structures with extra atoms at the periphery, the compact geometric configurations of clusters (four- and seven-atom clusters) have an obviously higher migration energy. Moreover, the NEB method is employed to obtain the energies of the rate-limiting steps in the migration processes of W4, W5, W6 and W7, and also determine the minimum energy paths of the net migration process for tetramer and pentamer, respectively. The diffusion mechanism of 2D small clusters containing more than two atoms is achieved by the migration of extra atoms at the periphery, the dimer-shearing mechanism and the changes of the cluster shape.Surface diffusion of atomic clusters and lattice diffusion of defect clusters could be achieved with the dimer method and NEB method for finding a minimum energy path. In principle the methods could be used to establish the understanding of migration and diffusion behaviors in the atomic scale effectively.