First-principles study of the multiple He trapping in defects in vanadium and SiC
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
In fusion environment, large amount of helium (He) atoms are produced by transmutation along with structural damage in the structural materials, causing materials swelling and degrading of physical properties. In this thesis, using first-principles method, I examined the microscopic mechanism of He trapping in vacancies and voids in structural materials (vanadium solid and 6H–SiC composites). In vanadium, a single He atom located in the tetrahedral interstitial site (TIS) turned out to be more stable than that in the octahedral interstitial site (OIS). Helium atoms were placed one by one into the vacancy defects (monovacancy and void) from the remote TISs, and we calculated the trapping energies as a function of the number of He atoms inside the vacancy defects. We found that, the monovacancy and void (about 0.6 mn in diameter) can host up 18 and 66 He atoms, respectively, in vanadium solid. The induced internal pressure by He bubbles in monovacancy and small void increased up to 7.5 GPa and 19.3 GPa, respectively. In vacancy defect, the He–He equilibrium distances decreased with the amount of He atoms incorporated in monovacancy and small void, and the host lattice expanded dramatically. The atomic structures of selected He clusters trapped in vacancies were compared with the gas-phase clusters. In complex 6H–SiC, there are ten kinds of interstitial sites for a single He atom. According to the calculated formation energy, the most stable site is the. R site. , where R site alternates with hexagonal interstitial sites. We explored the interactions between an interstital He atom and HenVam (Va stands for vacancy) clusters (n, m = 1 – 4). We found that the binding energy between He and the HenVam clusters increases with the number of vacancies (e.g., the binding energy is 1.3 eV for He2Va3, and 1.7 eV for He2Va4, respectively). The small void (about 0.55 nm in diameter) in 6H–SiC can accommodate up to 14 He atoms and the corresponding internal pressure is estimated to be 2.5 GPa. The maximum density of He atoms in a small He bubble is about 50 atoms/nm3, which is of the same magnitude as the experimental value 10 atoms/nm 3. Compared to vanadium, a small nanosized void in the 6H–SiC host lattice has a weak tendency for trapping He. When trapped seventy He atoms in small void in vanadium, the nearest vanadium bond expands 22–28 %, and the volume of the void expands by 80%. At the same time, with fourteen atoms encapsulated in a small void in 6H–SiC, the local Si–C bonds explans 1–5%, and the volume of the small void expands about 7%. We suggest that the differences in the cohesive energies in these two systems are responsible for the different He trapping behavior.
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , vi, 35 p.
IdentifiersURN: urn:nbn:se:kth:diva-159153ISBN: 978-91-7595-434-9OAI: oai:DiVA.org:kth-159153DiVA: diva2:782724
2015-02-13, Sal N111, Brinellvägen 23, KTH, Stockholm, 14:00 (English)
Belonoshko, Anatoly, Assoc. Prof.
Vitos, Levente, Professor
QC 201501262015-01-262015-01-222015-01-26Bibliographically approved
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