Development Of Tight-Binding Potential Model For W/Cu And The Mechanism Of H Bubble Formation In Tungsten

It’s very essential to develop nuclear fusion energy so as to solve the problems of energy shortage and environmental pollution. Thus the International Thermonuclear Experimental Reactor (ITER) Project was proposed internationally to develop nuclear fusion energy. In the project, plasma facing materials (PFMs) play fairly important roles because they directly face the plasma core and thus are under strong electromagnetic, high-energy neutrons and escaping particles irradiation. Tungsten (W) has been selected as one of the most promising candidates for PFMs due to its good properties of high melting point, high sputtering threshold, low activation under neutron irradiation, low hydrogen (H) isotopes retention, low coefficient of thermal expansion and good thermal stability. However, for practical application, W-based PFMs not only suffer the displacement damage induced by strong bombardment from energetic particles, but also suffer from H or He bubbles formed in the materials. Both H and He bubbles seriously degrade the mechanical properties of the W-based PFMs, and thus influence the performance lifetime of W-based PFMs. Moreover, W is a high-Z element, the fragment of broken bubbles can contaminate the plasma. Therefore, it is necessary to study the formation mechanism of H or He blistering in W. In this article, we theoretically investigate the H diffusion and bubble formation mechanism using the first-principles calculations, trying to propose a formation mechanism of H bubble.Additionally, copper and its alloys are ideal heat sink materials due to their high thermal conductivity, thus the fusion energy deposited in PFMs can be transferred quickly and effectively. However, W and Cu differ significantly in young’s modulus and thermal expansion coefficient. The connection between W and Cu cannot be treated simply by welding because the contact surface between W and Cu will have huge local thermal stress at a certain temperature. To avoid this phenomenon, W/Cu functionally graded materials (FGMs) are used to link W-based PFMs and Cu-based heat sink materials in experiments. However, the W/Cu FGMs are far from well understood especially in atomic scale. Herein, we propose a transferable tight-binding (TB) potential model for W-Cu binary system, which provides a new approach for theoretical study of W/Cu FGMs. This thesis is organized by five chapters.The first chapter is aimed at demonstrating the background of W-based PFMs and W/Cu FGMs, and put forward the problems needed to be solved. The second chapter of this thesis is to survey the theoretical model and the latest development of environment-dependent TB potential model.In the third chapter, we carefully present a new TB potential model for Cu, W and W-Cu binary system. This TB model can accurately predict the cohesive energies, the elastic properties and the thermodynamics properties of corresponding systems.In the fourth chapter, we simulate the irradiated events for Cu material using our newly developed TB model. Our simulations reveal the basic correlation between the evolution of structural defects, the thermal and mechanical properties of copper arising from the irradiation. The results show that the high density of defects considerably degrades the heat transmission capacity of Cu. This finding is of value for the development of heat sink materials in fusion reactor. In addition, on the basis of the experimental results, we construct a W-Cu interface which can be characterized as [110] for W grain and [356] for Cu grain. By performing molecular dynamics simulations, we obtain a W-Cu interface in nanometre size. As W/Cu FGMs play an important role in linking W-based PFMs and Cu-based heat sink materials, the thermal property of this W-Cu interface is an important parameter. Our calculations show that the thermal conductivity in direction parallel to the interface is2.35times than that in direction perpendicular to the interface. So W-Cu interface have an important impact on heat transmission.In the fifth chapter, by using the first-principles calculations, we have studied the diffusion behaviors of H in tungsten under either biaxial strain or isotropic strain. The results indicate that the diffusion pathways of H atom in tungsten strongly depend on the shape of loaded strain. Besides, stacking faults are the kinds of common face defects, which is a possible core of H gathering. Our calculations reveal that H atoms near the stacking fault can easily migrate to the stacking fault with energy cost of less than0.14eV. Furthermore, we also study the H atoms aggregation behavior. The results show that the W-W bond lengths increase and the W-W bonds strength decrease near the stacking fault as the number of trapped H atoms increase. Consequently, accumulation of H atoms tears up the local structure of the stacking fault with breaking of W-W bonds. This offers larger space for the accommodation of more H atoms, forming large H bubbles in the W surface region, thus stacking fault play an important role in the process of H bubble formation.