Frist Principles Studies On The Interactions Of Nb And Its Vacancies With H

With the development of our society and human civilization, the global energy consuming increases rapidly. The traditional fossil fuel energy resources like oil and coal will be used up in the near future. Moreover, the side effects, like green house effect, bring up by using fossil fuel make our living environment becomes worse than the past. Therefore, searching for clean and renewable energy resources becomes a general conception of our human being, and also a very important task to the people world-wide.Hydrogen (H) is a very common element in nature. The earth contains a large amount of hydrogen resources. Oxidation of H releases good heat amount and the product of the oxidation reaction (H2O) do not have any pollution to the environment. Therefore, H is regarded as a very promising energy resource in the future. In recent decades, a lot of countries have paied special attention to H energy. A large number of research activities have been carried out concerning preparation, purification and storage of H resources. Among various hydrogen production processes, separating H2 using hydrogen selective membranes is regarded as an efficient way from industrial gases. In this method of hydrogen production, the properties of the hydrogen separation membrance (i.e. selectivity, diffusivity, permeability et. al.) are very important, and metallic membrane have a lot of advantages among other hydrogen separation membrane. Pd-based membranes, such as Pd-Ag and Pd-Cu, have been widely used in the hydrogen separation industry. Recently, more and more attentions are given to non-Pd based metallic membranes which are much cheaper and non-pollution. Group V metals (V, Nb, and Ta) are good candidates because of their high hydrogen solubility and diffusivity. However, durability and instability issue of the group V membranes due to hydrogen embrittlement has delayed any large scale industrial applications. Despite intensive studies carried out by many research groups, the mechanisms of H embrittlement and related physical problems remain poorly understood.In recent years, a number of first principles studies have investigated H embrittlement. Four general mechanisms have been proposed:(i) formation of a hydride phase; (ii) enhanced local plasticity; (iii) grain boundary weakening and (iv) blister and bubble formation. However, since H solubility is high in group V metals, those proposed mechanisms may not be equally applicable. The underlying atomic processes and relative importance of the four mechanisms remain uncertain, and it is likely that a combination of these processes may contribute to embrittlement simulianeously. It is widely observed that H-enhanced dislocation mobility is a prelude to the embrittlement and that the fracture planes coincide with the slip plane of the material, which is not the typical situation. Vacancies, being ubiquitously present in solids and having the ability to act as impurity traps, could play a central role in the embrittlement process, but detailed arguments about this role or estimates of its relative importance are totally lacking.In this paper, we use the first principles calculations based on density function theory to study some fundamental physical problems in Nb as hydrogen separation membrane.1) We investigated the interactions between Nb metal and interstitial H atoms. In this part, we first compared the structure and the solution energies of M-H system with the H atom in T site and O site. Then we analyzed the electronic structure of these systems, we find that the H-1s state and Nb-4d,4p,5s states forming bonding states upon H intercalation. we also showed that the Al-H interaction is stronger when H is put into T-site, the energy levels of the H-induced states for T-site H atoms are lower than O-sites. We also calculated the diffusion energy barrier of H in Nb lattice with nudged elastic band method, and the H diffusion coefficient in Nb is then evaluated. The results show that the energy barrier is about 0.17 eV and the diffusion coefficient is approximately 7.8×10-9 m2/s at 500?, which is comparable to the experimental observations.2) We studied the interaction of H and vacancies in Nb metal. We first introduced the grand canonical ensemble statistical model. Then, we calculated the interaction energy between H and vacancies. The calculated trapping energies and solution energies shown that H2 molecule can not be formed in Nb vacancies. Finally, we calculated the vacancy concentration under 573K at various H2 external conditions. The vacancy concentration increases with the external H2 concentration increases. When the H/M ratio is close to 0.6, the Vac-nH concentration is close to 10-3.