Migration energy barriers and diffusion anisotropy of point defects on tungsten surfaces

Hao, Jiannan; Jin, Shuo; Lü, Guang-Hong; Xu, Haixuan

doi:10.1016/j.commatsci.2020.109893

Cited by 12 publications

(5 citation statements)

References 71 publications

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“…Exploration by MD of the elementary events involved in the diffusion of nanocavities highlights the role of creating the pair of surface defects, detrapping ad-atom from a nanocavity vertex, and forming a cluster of surface vacancies. The fast surface diffusion of the ad-atom compared to the vacancy agrees with the migration energies calculated by DFT on (110) surface (Hao et al, 2020). It can be explained by the smaller coordination number of the ad-atom compared to the surface vacancy, which has an impact on the strength of the binding of the defect with the surface along its migration path.…”

Section: Discussionsupporting

confidence: 77%

Readdressing nanocavity diffusion in tungsten

De Backer,

Souidi,

Hodille

et al. 2023

Front. Nucl. Eng.

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In nuclear fusion (ITER and the future DEMO), those components that face the plasma are exposed to high temperature and irradiation which, in the long term, modifies their thermal and mechanical properties and tritium retention. Tungsten is a candidate material and is the subject of many studies of microstructure evolution under various irradiation and temperature conditions. One milestone is the characterization of its defect properties. We here readdress the diffusion of nanocavities on broad ranges of size and temperature and compare it with dissociation, a competing process during nanocavity growth. First, at the atomic scale, we used molecular dynamics to explore the variety of elementary events involved in the nanocavity diffusion. Second, an experimental study of ion-irradiated samples, annealed at different temperatures up to 1,773 K, revealed the creation and growth of nanocavities on transmission electron microscopy images. Third, we performed multi-objective optimization of the nanocavity diffusion input of our object kinetic Monte Carlo model to reproduce the experimental results. Finally, we applied a sensitivity analysis of the main inputs of our model developed for these particular conditions—the source term which combines two cascade databases and the impurities whose interaction with the defects is characterised with a supplemented database of density functional theory calculations. Three domains of nanocavity size were observed. The first is the small vacancy clusters, for which atomistic calculations are possible and dissociation is negligible. The second is the small nanocavities, for which we provide new diffusion data and where a competition with the dissociation can take place. The third domain is the large nanocavities, for which, in any case, the dissociation prevents their existence above 1,500 K in the absence of a stabilizing interface.

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Section: Discussionsupporting

confidence: 77%

Readdressing nanocavity diffusion in tungsten

De Backer,

Souidi,

Hodille

et al. 2023

Front. Nucl. Eng.

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“…) 2 ] 1/2 , ∇ s is the surface gradient operator, Ω is the atomic volume of tungsten at temperature T, J s is the surface atomic mass flux, and J I is the SIA flux from the bulk of the nanobubble region toward the He-plasma-exposed surface along the z-direction. For the lateral dimensions of the surface, we have chosen the two Cartesian coordinates, x and y, to be oriented along the [1 10] and [001] crystallographic directions, respectively, because they correspond to the most stable high-symmetry directions for step edges on the W(110) surface [46,47]. In this model, dislocation loop formation and the loop punching process from helium bubbles in the nearsurface nanobubble region are approximated as a flux of SIAs toward the plasma-exposed surface.…”

Section: Surface Morphological Evolution Model For Pfc Tungstenmentioning

confidence: 99%

Effects of surface vacancy-adatom pair formation on PFC tungsten surface morphological response

et al. 2023

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We report a simulation study of the effect of He-irradiation-induced surface vacancy-adatom pair formation on the surface morphological evolution of plasma-facing component (PFC) tungsten and examine a number of factors that impact such evolution. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed, continuum-scale surface evolution model that has been developed following a hierarchical multiscale modeling strategy and can access the spatiotemporal scales of relevance to fuzz formation. The model accounts for the flux of surface adatoms generated as a result of the surface vacancy-adatom pair formation effect upon He implantation, which contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. We have carried out atomic-scale computations of optimal diffusion pathways along and across island step edges on the W(110) surface and calculated Ehrlich-Schwoebel (ES) barriers in adatom diffusion along and across such step edges. This aspect of surface adatom diffusion contributes to anisotropic surface atomic fluxes, terrace and step diffusive currents, and has been incorporated into our PFC surface evolution model, which predicts the formation of preferentially aligned nanoridge stripe patterns on the PFC surface. We establish that these anisotropic diffusive currents accelerate nanotendril growth on the PFC surface and the onset of surface nanostructure pattern formation. We also explore systematically the dependence of the PFC surface morphological response on the surface temperature and He ion incident flux, characterize in detail the resulting surface topographies and growth kinetics, and compare the predicted surface morphologies with experimental observations. Our simulation predictions for the emerging surface nanostructure patterns under certain plasma exposure conditions are consistent with experimental findings in the literature.

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“…This process is used to explain the difference between square and spherical bcc iron nanoclusters depending on the condition (temperature and iron deposition) [41]: at low temperature and high iron deposition, (100) is favoured because of the high migration barrier of adatoms on (100) compared to (110). Similar to iron, the point defects on W surfaces, especially the adatoms, are more mobile on W(110) (≈1.0 eV) than on W(100) and W(111) (≈2.5 eV) [34]. Thus, depending on the temperature of formation of these dust particles, W(111) could be favoured over W(110).…”

Section: Surface Distributionmentioning

confidence: 98%

“…Such a large quantity of tritium, with a low solubility of interstitial hydrogen, requires a high concentration of very deep defects (>1.5 eV of detrapping energy, i.e. vacancy-type defects [33,34]) coming from the dust production. Note that the bulk contribution, in the assumption of a homogeneous distribution of tritium, does not depend on the surface specific area (section 2.1).…”

Section: Impact Of the Bulk Of The Dust Particlesmentioning

confidence: 99%

Modelling tritium adsorption and desorption from tungsten dust particles with a surface kinetic model

et al. 2021

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A kinetic surface model is presented and used to explain the loading and desorption kinetics of tritium retained in micrometre-sized tungsten (W) dust particles. The model describes the sticking of hydrogen isotopes from the gas phase to W surfaces and the desorption from W surfaces. The initial sticking coefficient is set to one and independent of the temperature. The activation energy for desorption depends on the hydrogen coverage of the surface and is parametrised with density functional theory (DFT) calculations for W(100), W(110) and W(111) surfaces. The DFT-parametrised model is successfully compared to experimental results showing that the amount of measured tritium as well as the desorption kinetic can be modelled with only tritium adsorbed on the surface of W dust particles. Then, the model is used to explore possible scenarios to remove the tritium from the W surfaces by exposing the tritiated surfaces to either deuterium and hydrogen. The simulations suggest that it can be possible to remove all the tritium trapped on the W surfaces even at room temperature as soon as the hydrogen or deuterium pressure is higher than the tritium pressure. This gives opportunity to build tritium removal scenarios for ITER.

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Migration energy barriers and diffusion anisotropy of point defects on tungsten surfaces

Cited by 12 publications

References 71 publications

Readdressing nanocavity diffusion in tungsten

Readdressing nanocavity diffusion in tungsten

Effects of surface vacancy-adatom pair formation on PFC tungsten surface morphological response

Modelling tritium adsorption and desorption from tungsten dust particles with a surface kinetic model

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