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First-principles study of the multiple He trapping in defects in vanadium and SiC
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Dalian University of Technology, China.ORCID iD: 0000-0001-5676-418X
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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. [1], 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.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-159153ISBN: 978-91-7595-434-9 (print)OAI: oai:DiVA.org:kth-159153DiVA: diva2:782724
Presentation
2015-02-13, Sal N111, Brinellvägen 23, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-22 Last updated: 2015-01-26Bibliographically approved
List of papers
1. He-vacancy interaction and multiple He trapping in small void of silicon carbide
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2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 457, 36-41 p.Article in journal (Refereed) Published
Abstract [en]

In fusion environment, large amounts of helium (He) atoms are produced by transmutation along with structural damage in the structural materials, causing material swelling and degrading of physical properties. To understand the microscopic mechanism of He trapping in vacancies and voids, we explored He-vacancy interactions in HenVam (Va for vacancy) clusters (n, m = 1-4) and multiple He trapping in a 7-atom void of silicon carbide (SiC) by first-principles calculations. The binding energy between He and the HenVam clusters increases with the number of vacancies, while the vacancy binding energy gradually increases with the number of He atoms. Furthermore, a small cavity of about 0.55 nm in diameter can accommodate up to 14 He atoms energetically and the corresponding internal pressure is estimated to be 2.5 GPa. The tendency of He trapping in small voids provides an explanation for the experimentally observed He bubble formation at vacancy defects in SiC materials.

Keyword
Atoms, Binding energy, Calculations, Silicon, Silicon carbide, Swelling, Vacancies, Crystal atomic structure, First-principles calculation, Large amounts, Microscopic mechanisms, SiC materials, Silicon carbides (SiC), Small cavities, Structural damages, Vacancy Defects
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-159217 (URN)10.1016/j.jnucmat.2014.10.062 (DOI)000349169100005 ()2-s2.0-84910628889 (Scopus ID)
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved
2. Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid
Open this publication in new window or tab >>Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid
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2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 440, no 1-3, 557-561 p.Article in journal (Refereed) Published
Abstract [en]

Using first-principles methods, we have investigated the microscopic mechanism for He trapping in two kinds of vacancy defects (monovacancy and 9-atom void) inside vanadium host lattice. In the monovacancy, single He prefers to occupy the octahedral site near vacancy rather than vacancy center. Inside vacancy defects, the He-He equilibrium distances range in 1.6-2.2 angstrom. After more He atoms are incorporated, the magnitude of trapping energy decreases and the host lattice expand dramatically. A monovacancy and 9-atom void can host up to 18 and 66 He atoms, respectively, with internal pressure up to 7.5 and 19.3 GPa. The atomic structures of selected He clusters trapped in vacancies are compared with the gas-phase clusters. The strong tendency of He trapping at vacancies and 9-atom voids provides an explanation for experimentally observed He bubble formation at vacancy defects in metals.

Keyword
Equilibrium distances, First principles method, Gas-phase clusters, Microscopic mechanisms, Octahedral sites, Trapping energy, Vacancy Defects, Vacancy trapping
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-159219 (URN)10.1016/j.jnucmat.2013.03.068 (DOI)000323396600070 ()2-s2.0-84885295185 (Scopus ID)
Conference
NuMat Conference, OCT 22-25, 2012, Osaka, Japan
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved

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Licentiate Thesis(1892 kB)171 downloads
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Li, Ruihuan

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