Search

Thermal desorption of deuterium in tungsten pre-irradiated with the inert gas (helium, neon, argon) is modelled by TMAP (Tritium Migration & Analysis Program). Each TMAP model incorporates three activation energies linked with three types of traps respectively. First-principles method is performed to calculate the activation energies of deuterium detrapping from different kinds of defects. By comparing the activation energies modelled in TMAP with that obtained from first-principles method, the types of traps are determined. The pre-irradiation of helium can suppress the deuterium retention in vacancy clusters. Neon and argon pre-irradiation suppresses deuterium retention in mono-vacancies due to decreasing the number of mono-vacancies caused by the occupation of the inert gas atoms into mono-vacancies. The pre-irradiation of neon can facilitate deuterium retention in vacancy clusters through generating vacancy clusters during the injection process. Argon pre-irradiation facilitates deuterium retention mainly through generating voids, which are of a larger scale than vacancy clusters and can serve as effective traps. [ABSTRACT FROM AUTHOR]

The recombination of one Frenkel pair in tungsten has been investigated through first-principles simulation. Two different recombination types have been identified: instantaneous and thermally activated. The small recombination barriers for thermally activated recombination cases indicate that recombination can occur easily with a slightly increased temperature. For both of the two recombination types, recombination occurs through the self-interstitial atom moving towards the vacancy. The recombination process can be direct or through replacement sequences, depending on the vertical distance between the vacancy and the 〈1 1 1〉 line of self-interstitial atom pair. [ABSTRACT FROM AUTHOR]

The effects of hydrogen induced vacancy formation in tungsten have been investigated with a first-principles method based on the density functional theory. Hydrogen is demonstrated to assist the formation of vacancy and such effect is enhanced with increasing of hydrogen concentration. Using the energy of a single tungsten atom for the bulk as the reference energy, a spontaneous vacancy formation in tungsten is observed when more than two hydrogen atoms are introduced. These results provide a good explanation for creation of super-abundant vacancies in tungsten and can contribute to understand the mechanism of hydrogen bubble formation. [ABSTRACT FROM AUTHOR]

We have performed a first-principles investigation with a zero point energy correction on dissolution and diffusion of hydrogen (H) in a Σ5 (3 1 0)/[0 0 1] molybdenum (Mo) symmetrical tilt grain boundary (STGB). H prefers to stay at the interstitial site in the vacant space of the Mo GB with a negative solution energy (−0.42 eV) and segregation energy (−1.16 eV), which are decreased in the presence of vacancy in the GB. Furthermore, the H solution energy at the vacancy in the Mo GB is lower than that in the Mo bulk, suggesting a larger trapping capability for H of the vacancy in the Mo GB. The dissolving stability of H in the Mo GB can be explained by the low charge density that the GB provides for H. Kinetically, H prefers to easily diffuse within the vacant space along the Mo GB with a small diffusion barrier (0.04 eV), and to migrate to the GB from the bulk. These results provide a systematically exploration and analysis of H behaviors in the Mo GB at an atomic scale, which suggest that the Mo GB can serve as a trapping center for H, similar to the vacancy. In particular, a detailed comparison of H dissolution and diffusion behaviors in a Mo GB with those in a tungsten GB is performed. This can be helpful in understanding the experimentally observed H bubble formation in the Mo GB from the microscopic view. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated diffusion and permeation behaviors of hydrogen (H), deuterium (D), and tritium (T) in bulk molybdenum (Mo) by a first-principles calculations combined with simplified models. The H diffusion energy barrier with quantum correction is shown to be 0.12eV, in good agreement with the value of 0.11eV obtained from experiment. According to the diffusion theory presented by Wert and Zener, the diffusion coefficients of H, D and T are estimated, respectively. The H diffusion coefficient is found to be , which is also basically consistent with the experiment. Permeability Φ can be approximately evaluated in terms of Φ = SD, where S and D are the solubility and the H diffusion coefficient, respectively. We found that H has the low solubility in bulk Mo, consistent with the results at the temperature from 900K to 1500K in the earlier experiment. The permeability of H isotopes are calculated to be one order of magnitude larger than those of the experimental values. Such discrepancy should stem from that the “defect-free” Mo in the present calculations is considered. The permeation flux of H isotopes are finally examined. At the temperature of 1200K, the fluxes of H, D, and T are calculated to be 1.27×10−6 molm−2 s−1, 8.95×10−7 molm−2 s−1, and 7.30×10−7 molm−2 s−1, respectively. [Copyright &y& Elsevier]

We investigate the stability, diffusion, and impurity concentration of nitrogen in intrinsic tungsten single crystal employing a first-principles method, and find that a single nitrogen atom is energetically favourable for sitting at the octahedral interstitial site. A nitrogen atom prefers to diffuse between the two nearest neighboring octahedral interstitial sites with a diffusion barrier of 0.72 eV. The diffusion coefficient is determined as a function of temperature and expressed as D(N) = 1.66 × 10-7 exp(-0.72/kT). The solubility of nitrogen is estimated in intrinsic tungsten in terms of Sieverts' law. The concentration of the nitrogen impurity is found to be 4.82 × 10-16 A-3 at a temperature of 600 K and a pressure of 1 Pa. A single nitrogen atom can easily sit in an off-vacancy-centre position close to the octahedral interstitial site. There exists a strong attraction between nitrogen and a vacancy with a large binding energy of 1.40 eV. We believe that these results can provide a good reference for the understanding of the behaviour of nitrogen in intrinsic tungsten. [ABSTRACT FROM AUTHOR]

Abstract: A first-principles computational tensile test has been preformed to investigate the effects of hydrogen on a tungsten grain boundary. It has been found that the maximum ideal tensile strength of the tungsten grain boundary with hydrogen atom segregation was 32.85 GPa, which was about 9% lower than that of the clean tungsten grain boundary (36.23 GPa). This indicated that the theoretical strength of the tungsten grain boundary became weaker in the presence of the hydrogen atom. Atomic configuration analysis showed that the grain boundary fracture was caused by the interfacial bond breaking. The Griffith fracture energy was calculated to be 161 meV/Å2 (2.58 J/m2) and 155 meV/Å2 (2.48 J/m2) for the tungsten grain boundary without and with the hydrogen atom segregation, respectively. The solution energy of the hydrogen atom in a fracture free surface was −0.31 eV, which was 0.08 eV lower than that of the hydrogen atom in a tungsten grain boundary. This indicated that hydrogen was a grain boundary embrittler according to the Rice-Wang thermodynamic theory. The Bader charge analysis suggested that the physical origin for hydrogen-induced embrittlement was the charge transfer induced by hydrogen in the tungsten grain boundary. [Copyright &y& Elsevier]

Helium (He) has a synergistic effect on hydrogen (H) retention in metallic plasma-facing materials for nuclear fusion devices, which further makes a potential impact on hydrogen bubble formation in the materials. Using first-principles calculations, we investigated the interactions between H and He in molybdenum. The calculated results show that the binding energies of H are decreased at the pre-formed He-vacancy complex compared to those at the monovacancy without He. This indicates H trapping at the He-vacancy complex is energetically less favorable and the plasma-facing material will have less H retention. Most importantly, we revealed the critical characteristics of hydrogen concentration with the presence of He based on a thermodynamic model and the calculated energies of H m HeV complexes. The existence of He could apparently enhance the critical hydrogen concentration beyond which an initial stage of hydrogen bubble formation will occur. Our findings provide explicit evidences for the suppressing effect of helium on hydrogen bubble formation in molybdenum. [ABSTRACT FROM AUTHOR]

Using a first-principles method, we have investigated site preference and diffusion properties of carbon (C) in copper (Cu). A single C atom energetically prefers to occupy the octahedral interstitial site (OIS) instead of the tetrahedral interstitial site. Double C atoms tend to be paired up at the nearest neighboring OIS’s along the 〈1 0 0〉 direction mediated by a bridging Cu atom with a distance of 3.85 Å and a binding energy of 0.15 eV. This suggests that an indirectly attractive interaction between C atoms exists, which might lead to a local higher concentration of C in Cu. We demonstrate that the OIS → OIS path is the optimal diffusion path of C in Cu with diffusion barrier of 0.93 eV. By the estimation of pre-exponential factor according to a classical thermodynamics model, the diffusion coefficient as a function of temperature has been determined, which is 3.68 × 10 −15 m 2 /s at a typical temperature of 600 K. The results provide a good reference to understand the behavior of C in Cu. [ABSTRACT FROM AUTHOR]

We have investigated the effects of strain on the dissolution and diffusion properties of helium (He) in tungsten (W) using a first-principles method. We show that the solubility and diffusion of He can be enhanced (suppressed) with increasing tensile (compressive) strain. This can be attributed to the variation of the effective volume and the p -projected DOS of He induced by the strain. We demonstrate that the strong electronic interaction between He and W atoms plays a key role in the poor solubility of He in W. The tetrahedral interstitial site is always the preferred site of He under all strains. The same rule is believed to hold for other inert gas elements in strained W. [ABSTRACT FROM AUTHOR]

Tungsten (W) and W alloys are considered as the most promising candidates for plasma facing materials in future fusion reactor. Grain boundaries (GBs) play an important role in the self-healing of irradiation defects in W. Here, we investigate the stability of point defects [vacancy and self-interstitial atoms (SIA’s)] in a Σ5(3 1 0) [0 0 1] tilt W GB by calculating the energetics using a first-principles method. It is found that both the vacancy and SIA are energetically favorable to locate at neighboring sites of the GB, suggesting the vacancy and SIA can easily segregate to the GB region with the segregation energy of 1.53 eV and 7.5 eV, respectively. This can be attributed to the special atomic configuration and large available space of the GB. The effective interaction distance between the GB and the SIA is ∼6.19 Å, which is ∼2 Å larger than that of the vacancy-GB, indicating the SIA are more preferable to locate at the GB in comparison with the vacancy. Further, the binding energy of di-vacancies in the W GB are much larger than that in bulk W, suggesting that the vacancy energetically prefers to congregate in the GB. [ABSTRACT FROM AUTHOR]

Abstract: We employ first-principles total energy method based on the density functional theory with the generalized gradient approximation to investigate the ideal tensile strengths of a bcc tungsten (W) single crystal systemically. The ideal tensile strengths are shown to be 29.1, 49.2 and 37.6GPa for bcc W in the [001], [110] and [111] directions, respectively. The [001] direction is shown to be the weakest direction due to the occurrence of structure transition at the lower strain and the [110] direction is strongest. The results can provide a useful reference for W as a PFM in the nuclear fusion Tokamak. [Copyright &y& Elsevier]

Abstract: We perform first-principles calculations based on density functional theory to investigate energetics and site preference of He in a bcc-W Σ =5 grain boundary (GB). The segregation energy is calculated to be −1.37eV, indicating that He prefers to segregate in the WGB. The formation energy of He in the WGB is positive and thus He is quite hard to dissolve in the WGB, similar to its behavior in the bulk. Because of its closed-shell electronic structure, He is shown to preferably occupy either interstitial or substitutional site with larger space provided by the GB, changing the GB electronic structure. Moreover, segregation of He gives rise to the WGB expansion. These structure variations can have a large effect on the mechanical properties of the W GB. [Copyright &y& Elsevier]

Abstract: Using a first-principles computational tensile test (FPCTT), we have investigated the effect of helium (He) on the structure and bonding properties of tungsten (W), which is a promising plasma-facing material in nuclear fusion Tokamak. Density of states results reveal the underlying reason that the substitutional site for He is the most energetically favorable, while the tetrahedral interstitial site is more favorable than the octahedral interstitial one. The FPCTT shows that the ideal tensile strength is 29.1GPa at the strain of 14% along the [001] direction for intrinsic W, while it decreases to 28.2GPa at the same strain when one impurity He atom is introduced. A local bond-breaking region around He forms in the tensile process due to the presence of He, which suggests He will have a large effect on the bonding properties of W. [Copyright &y& Elsevier]