Modelling of defect formation and evolution in metals and silicon

Defects always exists in a crystal lattice at temperatures above absolute zero. Our knowledge of defect concentration and mobility is crucial, due to their profound influence on the material properties. By the means of thermodynamics it is possible to estimate defect concentrations at the equilibrium conditions. Near the melting point of pure metals the vacancy fraction is typically about 10−4. However, it has been shown that the presence of light impurities may enhance vacancy formation in many metals and metal alloys. The main reason for this phenomenon, often referred to as the superabundant vacancy formation, is the lowering of the vacancy formation energy due to the impurity trapping. In this thesis a theoretical thermodynamics model has been developed to study the equilibrium vacancy concentrations as a function of impurity concentration and temperature. Our model takes into account monovacancy and divacancy thermodynamics as well as the binding energy of each trapped impurity and the vibrational entropy of defects. The diffusion of monovacancies and hydrogen in tungsten is studied due to its relevance to fusion research. Extraordinary thermal and mechanical properties, like high melting temperature, high heat conductivity, low sputtering yield and low hydrogen retention makes tungsten a prime candidate for a divertor plate material in the next step fusion device ITER. At this region of the fusion reactor, the material is exposed to the highest heat and particle fluxes producing damage in the plasma facing components. Open volume defects are known to trap hydrogen and, thus, are the main reasons for hydrogen retention in tungsten. The radioactive hydrogen isotope tritium retention in the fusion reactor is especially undesirable due to efficiency and safety reasons. The molecular dynamics method has been used to simulate the diffusion of hydrogen in tungsten. The commonly accepted and so far used H diffusion migration barrier is revised and a new analysis method to determine diffusion coefficients that accounts for the random oscillation of atoms around the equilibrium position is presented. At high hydrogen concentrations, a dramatic reduction in the diffusion coefficient is observed, due to the neighbouring interstitial