Effects of Zr and V additions on the stability and migration of He in bcc W: A first-principles study

The effects of zirconium (Zr) and vanadium (V) on the dissolution, diffusion and trapping behaviors of helium (He) in body-centered cubic (bcc) tungsten (W) have been investigated using first-principles simulations. We found that Zr and V atoms usually prefer to disperse into W-rich region rather than aggregate in W host lattice. Energetically, the relatively stable position for an interstitial He remains the tetrahedral interstitial site in W with the presence of Zr/V atom. The solution energy of the He atom is reduced because of the attractive interaction between He and Zr/V atom. Compared to W with Zr, the stronger attractive interaction between V and He results in the lower He solution energy in W with V. Kinetically, the He atom diffusion to Zr/V has a lower energy barrier than that of He diffusion to W along the optimal diffusion path TIS–TIS. In addition, the existing Zr/V atom improves the ability of Zr/V-Vacancy complex to capture He atom. These results provide incontrovertible evidence that the existing Zr/V atom has great influence on the behavior of He atom in bulk W.     

Vacancy trapping mechanism for multiple hydrogen and helium in beryllium: a first-principles study

The microscopic mechanism for H and He trapping by vacancy defects and bubble formation in a Be host lattice is investigated using first-principles calculations. A single He atom prefers to occupy a vacancy centre while H does not. He can segregate towards the vacancy from the interstitial site much more easily than H. Both H and He exhibit lower diffusion barriers from a remote interstitial to a vacancy with regard to their diffusion barriers inside a perfect Be solid. Up to five H or 12 He atoms can be accommodated into the monovacancy space, and the Be-He interaction is much weaker than Be-H. The physical origin for aggregation of multiple H or He atoms in a vacancy is further discussed. The strong tendency of H and He trapping at vacancies provides an explanation for why H and He bubbles were experimentally observed at vacancy defects in materials. We therefore argue that vacancies provide a primary nucleation site for bubbles of H and He gases inside Be materials.     

Effects of Cr on H and He trapping and vacancy complexes in V in a fusion environment: a first-principles study

First-principles density functional theory was used to investigate the interaction between hydrogen (H) and helium (He) in V–Cr alloy, which is a potential structural material for use in fusion reactors. When vacancies are present in the V–Cr alloy, a single He atom prefers to occupy the octahedral site near the Cr atom rather than vacancy centre, which differs from the cases of iron and tungsten. Because of the decrease of the electron density around the He atom, there was a strong interaction between He and H. In the vicinity of He-vac complexes, H atoms tend to stay in the tetrahedral site rather than occupy the octahedral-interstitial site. A single He-vac complex can trap as many as six H atoms, which is more than can be trapped by an empty vacancy in the V–Cr alloy because of the electronic density redistribution of vacancy vicinity. This strong attraction explains the enhanced retention of H and He observed near the surface of V and V-based alloys under both sequential and simultaneous bombardments. The results provide useful insight into the application of the V-based alloys as candidate structural materials in fusion Tokamaks.     

铌对钨中氦行为影响的第一性原理研究

采用第一性原理计算方法系统研究了合金化元素铌(Nb)对钨(W)中氦(He)溶解和扩散行为的影响.研究发现,Nb的存在显著降低了He在W中的溶解能,Nb可以作为W中He捕陷中心,在最稳定位置处Nb对He的捕陷能达到0.37 eV.通过电荷分析发现,这主要是因为Nb的存在引起了W中电荷密度的重新分布.而He在W中的扩散能垒将随着He—Nb间距离的缩小逐渐降低,这从动力学上表明He被Nb捕陷是可行的.因此,Nb的存在将有利于W中He的聚集成泡.     

Atomistics of helium diffusion in copper and tungsten

Using previously determined interatomic potentials, the activation energy for migration of a single substitutional helium atom in copper and tungsten has been determined. The mechanism of migration involves the jump of the helium atom out of the vacancy concurrent with the jump of a host atom into that vacancy. The helium then occupies the vacant site created by the jumping host atom, resulting in a substitutional helium at a distance of √2 r ₀ (√3r ₀) from its original site in copper (tungsten). The rate-limiting step in the process is found to be the jump of the helium out of the vacancy, the activation energy for which is 2.15 eV in copper and 4.69 eV in tungsten.     

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mössbauer effect measurements have been performed using sources of¹¹⁹Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·10¹⁶ He/cm² leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.     

Energetics of He and H Atoms in W–Ta Alloys: First-Principle Calculations

Properties of various defects of He and H atoms in W-Ta alloys are investigated based on density functional theory. The tetrahedral interstitial site is the most configured site for self-interstitial He and H in W and W-Ta alloys. Only a single He atom favors a substitutional site in the presence of a nearby vacancy. However, in the coexistence of He and H atoms in the presence of the vacancy, the single H atom favors the tetrahedral interstitial site(TIS) closest to the vacancy, and the He atom takes the vacancy center. The addition of Ta can reduce the formation energy of TIS He or H defects. The substituted Ta affects the charge density distribution in the vicinity of the He atom and decreases the valence electron density of the H atoms. A strong hybridization of the H s states and the nearest W d state s exists in W_(53)He_1 H_1 structure. The sequence of the He p projected DOS at the Fermi energy level is in agreement with the order of the formation energy of the He-H pair in the systems.     

Nucleation and growth of helium bubble at(110) twist grain boundaries in tungsten studied by molecular dynamics

Migration of He atoms and growth of He bubbles in high angle twist grain boundaries(HAGBs) in tungsten(W) are investigated by atomic simulation method. The energy and free volume(FV) of grain boundary(GB) are affected by the density and structure of dislocation patterns in GB. The migration energy of the He atom between the neighboring trapping sites depends on free volume along the migration path at grain boundary. The region of grain boundary around the He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries. The He atoms aggregate on the grain boundary plane to form a plate-shape configuration. Furthermore, high grain boundary energy(GBE) results in a large volume of He bubble. Thus, the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary, the dislocation patterns and the free volume related migration path on the grain boundary plane.     

Molecular dynamics simulations of point defects in plutonium grain boundaries

A modified analytic embedded atom method (MAEAM) potential is constructed for fcc updelta-Pu. Molecular dynamics (MD) simulations with the potential are performed to investigate the interactions between two symmetrical tilt grain boundaries (GBs) and point defects such as He atom, vacancy and self-interstitial atom (SIA) in Pu. The calculated results show that point defect formation energies are on average lower than those in the lattice but variations from site to site along the GBs are very remarkable. Both substitutional and interstitial He atoms are trapped at GBs. Interstitial He atom is more strongly bound at the GB core than the substitutional He atom. The binding energy of SIA at GB core is higher than those of He atom and vacancy. GB core can bind many He atoms and SIAs due mainly to the fact that it contains many vacancies. Compared with He atom and SIA, the vacancy far from GB core is difficult to diffuse into the core. The GBs can act as sinks and sources of He atoms and SIAs, which may be a reason for the swelling of Pu after a period of self-irradiation because of the higher concentration of vacancy in the bulk.     

改善Si3N4俘获层过擦现象的第一性原理研究

采用第一性原理方法对如何改善电荷俘获存储器的过擦现象进行了研究. 过擦是由于氮空位中Si原子对电荷的局域能力弱导致, 因此, 在Si₃N₄超胞中分别建立了以C, N, O替换氮空位中的Si原子的缺陷结构作为本文的研究模型. 分别计算了擦写之后体系的巴德电荷分布、相互作用能、态密度, 借以分析替位原子对过擦的影响. 巴德电荷分布的计算结果表明, Si₃N₄在O替位128号Si后的过擦现象被明显改善; C替位128号Si也可以改善过擦, 但由于C替位对电荷的局域作用变弱, 不利于电荷的存储实现; N替位128号Si则不能改善过擦; 而在162和196号Si位置, 三种原子的替换均无法改善过擦现象. 相互作用能的研究表明, 在128号Si位置, 三种原子都能够和氮空位形成团簇, 在体系中稳定存在. 特别地, O替位Si后, 体系中两缺陷的相互吸引作用最弱, 从而写入的电荷能够短暂的打破O团簇的稳定性, 实现电荷重构, 将电荷局域在O团簇周围. 此外, 态密度的分析结果表明O在128号Si位置能够在Si₃N₄禁带中引入深能级缺陷, 深能级局域电荷的能力强. 以上分析证明, O替位可以很好的改善Si₃N₄中的过擦现象. 本文的研究结果为电荷俘获存储器改善过擦提供了一种方法, 对提高器件的电荷保持特性和优化存储窗口具有指导意义.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位；悬挂键的存在极大的增强了Pt在graphene空位处的吸附；B替位掺杂有利于Pt原子在杂质附近的吸附.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位;悬挂键的存在极大的增强了Pt在graphene空位处的吸附;B替位掺杂有利于Pt原子在杂质附近的吸附.     

Direct identification of atomic binding sites on a crystal

The atomic resolution of the field ion microscope, in conjunction with its ability to remove and identify individual atomic layers, allowed us to map unambiguously the unit cell of the (111) plane of tungsten and to determine directly the location of single tungsten atoms adsorbed on this plane. Adatoms have been observed to occupy two binding sites only. The predominant site corresponds to a normal lattice position. The second site is of similar symmetry, in that the adatom sits between three first layer atoms; however, at this position the adatom is located above an atom in the second rather than the third lattice layer. The former site is favored energetically, but only by ≈ 0.5 eV. All observations have been made at high fields, however, it is shown from studies of migration and other effects that the binding sites identified in the field ion microscope are typical of a normal, field free environment.     

钨中氢同位素热脱附实验的速率理论模拟研究

本文采用基于速率理论的模拟方法研究钨材料中氢同位素氘的热脱附谱. 热脱附数据来源于520 K下受等离子体辐照的多晶钨, 入射离子能量为40 eV, 剂量为1× 10²⁶ D/m². 通过调节速率理论中的俘获能、俘获率等参数, 最终获得与实验相符合的热脱附拟合谱. 拟合结果表明, 钨中俘获的氘存在于三种俘获态, 俘获能分别为1.14 eV, 1.40 eV和1.70 eV, 相应脱附温度峰值为500 K, 600 K和730 K. 这三个俘获能分别应对应于第一原理计算得到的空位俘获第3–5个氢原子的俘获能(含零点振动能修正)、空位俘获第1–2个氢原子的俘获能, 空位团簇对氢原子的俘获能. 模拟结果表明, 在本辐照实验条件下, 钨中空位及空位团簇是氘在钨中的主要俘获态.     

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.     

Towards understanding the carbon trapping mechanism in copper by investigating the carbon-vacancy interaction

We propose a vacancy trapping mechanism for carbon-vacancy (C-V) complex formation in copper (Cu) according to the first-principles calculations of the energetics and kinetics of C-V interaction. Vacancy reduces charge density in its vicinity to induce C nucleation. A monovacancy is capable of trapping as many as four C atoms to form CnV (n=1,2,3,4) complexes. A single C atom prefers to interact with neighboring Cu at a vacancy with a trapping energy of 0.21 eV. With multiple C atoms added, they are preferred to bind with each other to form covalent-like bonds despite of the metallic Cu environment. For the CnV complexes, C2V is the major one due to its lowest average trapping energy (1.31 eV). Kinetically, the formation of the CnV complexes can be ascribed to the vacancy mechanism due to the lower activation energy barrier and the larger diffusion coefficient of vacancy than those of the interstitial C.     

Interaction between impurity nitrogen and tungsten: a first-principles investigation

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^-7exp (-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 Å^-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.     

Ab initio study of formation, migration and binding properties of helium-vacancy clusters in aluminum

Ab initio calculations based on density functional theory have been performed to study the dissolution and migration of helium, and the stability of small helium-vacancy clusters He_nV_m (n, m=0-4) in aluminum. The results indicate that the octahedral configuration is more stable than the tetrahedral. Interstitial helium atoms are predicted to have attractive interactions and jump between two octahedral sites via an intermediate tetrahedral site with low migration energy. The binding energies of an interstitial He atom and an isolated vacancy to a He_nV_m cluster are also obtained from the calculated formation energies of the clusters. We find that the di- and tri-vacancy clusters are not stable, but He atoms can increase the stability of vacancy clusters.     

Influence of helium on the evolution of irradiation-induced defects in tungsten: An object kinetic Monte Carlo simulation

Understanding the evolution of irradiation-induced defects is of critical importance for the performance estimation of nuclear materials under irradiation. Hereby, we systematically investigate the influence of He on the evolution of Frenkel pairs and collision cascades in tungsten (W) via using the object kinetic Monte Carlo (OKMC) method. Our findings suggest that the presence of He has significant effect on the evolution of irradiation-induced defects. On the one hand, the presence of He can facilitate the recombination of vacancies and self-interstitial atoms (SIAs) in W. This can be attributed to the formation of immobile He-SIA complexes, which increases the annihilation probability of vacancies and SIAs. On the other hand, due to the high stability and low mobility of He-vacancy complexes, the growth of large vacancy clusters in W is kinetically suppressed by He addition. Specially, in comparison with the injection of collision cascades and He in sequential way at 1223 K, the average sizes of surviving vacancy clusters in W via simultaneous way are smaller, which is in good agreement with previous experimental observations. These results advocate that the impurity with low concentration has significant effect on the evolution of irradiation-induced defects in materials, and contributes to our understanding of W performance under irradiation.     

Nonlinear conductivity in CaRuO3 thin films measured by short current pulses

We investigate the effects of carbon (C) on hydrogen (H) solubility in copper (Cu) using a first-principles method. We show C can increase the solution energy of H in the bulk Cu originated from the charge density redistribution, which leads to a weak repulsion between H and C in Cu. On the contrary, we demonstrate the C-vacancy (C-V) complex can serve as a trapping centre of H, and one C-V complex can hold up to six H atoms. Moreover, it is found that C can effectively decrease the solution energy of a single H in the vacancy, 0.68 eV lower than that of H in the C-free vacancy, changing the solution process of H in the vacancy from endothermic to exothermic. This can be attributed to the strong bonding interaction between H and C in the vacancy. Based on analyzing the role of C in different metals, we propose that the effects of C on the H solubility in the vacancy mainly depend on the difference between the H-C interaction and the C-metal atom interaction. These indicate that C plays a key role in H trapping behavior in Cu.