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He-vacancy interaction and multiple He trapping in small void of silicon carbide
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
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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.

Place, publisher, year, edition, pages
2015. Vol. 457, 36-41 p.
Keyword [en]
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: urn:nbn:se:kth:diva-159217DOI: 10.1016/j.jnucmat.2014.10.062ISI: 000349169100005Scopus ID: 2-s2.0-84910628889OAI: oai:DiVA.org:kth-159217DiVA: diva2:783359
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved
In thesis
1. First-principles study of the multiple He trapping in defects in vanadium and SiC
Open this publication in new window or tab >>First-principles study of the multiple He trapping in defects in vanadium and SiC
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:nbn:se:kth:diva-159153 (URN)978-91-7595-434-9 (ISBN)
Presentation
2015-02-13, Sal N111, Brinellvägen 23, KTH, Stockholm, 14:00 (English)
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Supervisors
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-22 Last updated: 2015-01-26Bibliographically approved
2. First-principles study of defects instructural materials
Open this publication in new window or tab >>First-principles study of defects instructural materials
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, first we focus on the Helium (He) and He bubbles behavior in three kinds of the most promising candidate structural materials for future fusion reactor. These materials are vanadium, silicon carbide (SiC) composites, and reduced activation ferritic-martensitic (RAFM) steels. Second we investigate the intrinsic stacking fault of face-centered cubic (fcc) metals and alloys, with special emphasis on the interfacial energy between fcc and hexagonal close packed (hcp) phases. The present research has been carried out using modern ab initio quantum mechanical tools based on Density Functional Theory.

The microscopic mechanism of He trapping in vacancies and voids in structural materials has been examined using first-principles calculations based on pseudopotential method as implemented in the Vienna ab initio Simulation Package (VASP). For body-centered cubic (bcc) vanadium (paper I), the trapping energies for multiple He atoms in monovacancy and 9-atom small void (about 0.6 nm in diameter) have been investigated. It is found that monovacancy and 9-atom void capture at least 18 and 66 He atoms, respectively. The corresponding internal pressure caused by He cluster is as large as 7.5 and 19.3 GPa. The He-He distance constrained in small void is shorter than in gas-phase Hen clusters. This finding is consistent with the results obtained for the radial distribution function. For hexagonal 6H–SiC (paper II), the interactions between a He (in one vacancy, Va) and HenVam clusters (n, m = 1 – 4) have been investigated. For a specified vacancy number (i.e. m fixed) in HenVam, the bind energy decreases with increasing He atoms, meaning that it becomes increasingly difficult for trapping more He atoms due to the He-He repulsion. This phenomenon is further confirmed by the attractive interaction between a vacancy and HenVam that expands the void space to release He-He repulsive interaction. However, bulk 6H–SiC has a weak capacity to capture He atoms (14 He atoms) due to its brittle property. The estimated internal pressure (2.5 GPa) has the same order of magnitude as the experimental value (0.8 GPa). For ferromagnetic bcc iron (Fe) (paper III), we concentrate on the effect of chromium (Cr) and tungsten (W) alloying elements on the He stable interstitial position, migration energy and trapping energy. The formation energies of He in tetrahedral interstitial site (T-site) and octahedral interstitial site (O-site) with different number of Cr and W atoms have been studied. The He formation energy trends with increasing Cr and W content are non-linear, respectively. It is found that the antiferromagnetic Cr-Cr coupling in bcc Fe transforms to ferromagnetic coupling, and the repulsion between He and W is larger than in pure W host lattice. The He migration energy and the number of He atoms trapped by monovacancy become lower compared to pure Fe due to the additional Cr and W. It is found that Cr and W lead to higher trapping energies for multiple He and slightly hamper He trapping in vacancy compared to pure bcc Fe.

In the second part of the thesis (paper IV) the stacking fault energy (SFE) and interfacial energy of six fcc metals and Fe-Cr-Ni alloys have been studied. SFE γ plays an important role in determining the plastic deformation mechanism of fcc metals and thus is a fundamental parameter describing and understanding the mechanical properties of high-technology alloys. Small SFE favors twinning, and high SFE favors dislocation slip. The formation energy of the interface between fcc(111)/hcp(0001) is a key parameter in determining the SFE when using standard thermodynamic approaches. In this thesis, two other models that are commonly used in the ab initio calculation of the SFE are considered. One is based on the supercell technique with one intrinsic stacking fault pure unit cell, and the other on the axial interaction model. Due to the different conditions for hcp structures in entering the thermodynamic model and the above ab initio models, we differentiate between the actual interfacial energy σ for the coherent fcc(111)/hcp(0001) interface and the "pseudo-interfacial energy (σ∗)", the latter appearing in the thermodynamic expression for the SFE. Using the first-principles exact muffin-tin orbitals method (EMTO) in combination with the coherent potential approximation (CPA), we investigated the coherent and pesudo-interfacial energy for six fcc metal (Al, Ni, Cu, Ag, Pt, and Au) and three Fe-Cr-Ni alloys. It is found the two interfacial energies remarkable differ from each other. Our results form the first systematic first-principles data for the interfacial energies of monoatomic fcc metals and austenitic stainless steels and are expected to be used in future thermodynamic predictions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. x, 66 p.
National Category
Other Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-182124 (URN)978-91-7595-856-9 (ISBN)
Public defence
2016-03-10, Sal B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20160218

Available from: 2016-02-18 Created: 2016-02-16 Last updated: 2016-03-09Bibliographically approved

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