Molecular Dynamics Simulation Of Energetic Hydrogen And Isotopes Bombarding The Crystalline Graphite

Due to their low bremsstrahlung and compound radiation, and their excellent thermal conductivity, carbon-based materials have been extensively used in most of the current major tokamak fusion reactors. While carbon-based materials have high erosion yield and high capacity to trap hydrogen/tritium, which disadvantages block their further applications. There are a large amount of experimental and simulation studies on carbon-based materials, but the atomic-scale mechanism remains obscure and poorly being understood. In the thesis, the works summarized below have been done:In the chapter3, molecular dynamics simulation is applied to investigation of energy exchanges between single hydrogen and graphite(001). Both Brenner’s reactive empirical bond order potential and Ito’s interlayer intermolecular potential are adopted to represent "ABAB" stacking of graphite. In addition to energy transfer processes, this work analyses various kinds of possible processes, which are the absorption on the upside graphite surface, reflection, absorption on the downside graphite surface and penetration. The simulation results show that the interlayer interaction has a big influence on the reflection, especially when the incident energy is larger than20.0eV. The reflection coefficient increases evidently, compared with that in single graphene sheet case. The interlayer interaction has little influence on the absorption on the upside graphite surface, compared with the process of reflection. If the incident hydrogen has a kinetic energy more than25.0eV, it can penetrate the four-sheet graphite at some striking locations. When the incident energy is larger than28.0eV, the energy transferring to the first graphene sheet is more than to the second graphene sheet. These results would be helpful for understanding the chemical erosion of carbon-based materials.In the chapter4, molecular dynamics simulation is applied to the investigation of the isotopic effects during the course of a hydrogen isotope atom bombarding the crystalline graphite containing four graphene sheets. The simulation results reveal that the mass of the incident species has a big influence on the absorption on and the reflection from the upside graphite surface, the peaks of which move to higher end side of incident energy as the mass increases. The absorption coefficient of the incident tritium is larger, compared with that of the incident either hydrogen or deuterium. To penetrate the four-sheet graphite at some striking locations, deuterium and tritium need more kinetic energy. It is found that both the mass and the incident energy of the incident species affect the energy transfer to background substrate. These results would be important for understanding the tritium retention occurring in fusion devices.