Interstitial Helium Research Articles

Molecular dynamics study of interstitial He clusters in nickel

This study presents a molecular dynamics analysis focusing on the behavior of interstitial helium (He) clusters in nickel (Ni), examining their formation, stability, and migration energetics. We found that the binding energies of interstitial He with a He cluster are positive and increase with the cluster size, indicating a preference for He atoms to cluster together. While the formation energy increases monotonically with cluster size, binding energy shows non-monotonic. trend Importantly, small He clusters were found to be thermally unstable at reactor operational temperatures (approximately 600 K), with the He2 cluster exhibiting instability even at room temperature. With a binding energy of 0.44 eV for a He4 cluster, we hypothesize that for He bubbles to form via homogeneous nucleation (i.e., through trap mutation) at reactor operating temperatures, the He concentration must be high enough to facilitate the formation of He clusters of at least size 4 or larger. At finite temperatures, He clusters of size 7 and larger trap mutate immediately. However, clusters of size 10 and larger will trap mutate even at 0 K. As expected, interstitial He and small He clusters are highly mobile, and found to be mobile even at temperatures as low as 200 K. Furthermore, the mean squared displacement method has been utilized to determine the activation energies and the corresponding prefactors for clusters ranging from He1 to He6

Effects of helium and vacancy in Ni symmetric tilt grain boundaries by first-principles

The vacancy and helium effects on eight low-Σ symmetric tilt grain boundaries (STGBs) of Ni were investigated by first-principles calculations. The simulations demonstrate that vacancies in Ni matrix could diffuse easily to the grain boundary and helium atoms are quite stable at the grain boundary vacancy sites. Vacancy accumulation at high-energy STGBs could enhance the binding strength while He defects are generally detrimental to grain boundary binding and interstitial helium defects can initiate more severe grain boundary cracking comparing to vacancy-bind-helium. Further electron charge analysis suggested the influence of grain boundary binding greatly relies on the grain boundary atomic structure and the interaction between Ni-d and He-p states. The polarized charge transfer induced by He helium occupation was predicted to be detrimental to the binding strength, which explains the experimentally-observed helium bubbles and helium-induced GB embrittlement.

Behavior of Foreign Interstitial Hydrogen and Helium Atoms at Tungsten/Beryllium Interface from First-Principles Calculations

The behavior of foreign interstitial hydrogen and helium atoms and its effect on the physical properties of the tungsten/beryllium interface structure were computationally studied by first-principles calculations. Briefly, as part of our study of helium irradiation damage and hydrogen detention, the following properties were calculated: (1) the electronic properties of the tungsten/beryllium interface structure with a single interstitial hydrogen or helium atom and Hen vacancy or Hn vacancy complexes, and (2) the isotropy (polycrystalline) elastic modulus (bulk, torsion, Young’s modulus), anisotropy factor and minimum thermal conductivity of the previously described tungsten/beryllium interface systems. This study found that defect atoms are more likely to be concentrated in beryllium, but the tungsten layer is more sensitive to changes in mechanical properties caused by interstitial atoms. The ability of the beryllium vacancies to capture interstitial atoms is smaller than that of the tungsten vacancies. Based on the computational results, a preliminary assumption of the judgment of the tungsten/beryllium interface structure on the resistivity for plasma-facing materials is introduced. These computational studies provide a critical evaluation of the radiation resistivity and hydrogen retention of tungsten/beryllium interface materials. The calculated interface properties can be incorporated into radiation damage resistance property evaluation systems to develop and test tungsten-based composite materials.

A molecular dynamics study on the influence of vacancies and interstitial helium on mechanical properties of tungsten

Tungsten is a prime candidate material for use in plasma facing components in nuclear fusion reactors. This would entail weathering extreme conditions, such as high thermal loads and particle bombardment. It is of vital importance to understand how tungsten mechanically responds to these conditions, and how it is impacted by the defects that form. The current communication considers how the mechanical properties of tungsten are affected by the presence of several nanosized lattice defects: interstitial helium (both scattered throughout the sample and centrally clustered), isolated vacancies and vacancy clusters. All defects were found to lower the yield stress of the crystal, with vacancies and vacancy clusters having negligible influence during the elastic phase. Interstitial helium formed clusters, leading to the displacement of tungsten atoms, and a lowered stiffness at high strains. These negative effects of interstitial helium — along with the decrease in yield stress — were found to be partially negated by the presence of vacancies.

Ab initio study on structural, magnetic phase and helium migration behaviour of FCC Fe 6.25 at.% Cr binary alloys

ABSTRACT Ab initio calculations are implemented to investigate the structural and helium diffusion behaviour in Fe 6.25 at.% Cr binary alloys based on FCC–Fe. Our simulation results confirmed that the increase of Cr contents in the AFM phase is beneficial to strengthen the shear deformation resistance and hardness of the alloys, and also leads to greater lattice distortion. The presence of Cr atoms accelerates the migration of mono-helium atoms between tetrahedral and octahedral gaps in varying degrees, but greatly impedes the migration of helium atoms between tetrahedral interstitials or within the main lattice. The tetrahedral model is more stable than the octahedral model by 0.213 and 0.433 eV under the AFM and NM phase, respectively. The stability of the helium gap models depends on the electrostatic interaction between helium and surrounding metal atoms, and impurity atoms and magnetism do not directly affect its stability. There is a strong bonding interaction between Fe and Cr atoms, and the presence of interstitial helium atom results in net charge redistribution. This study provides valuable information for the interpretation of complex experimental phenomena.

Diffusional Isotope Effect Based on the Transition-State Theory of Interstitial Mechanism in Solids

It is important to be able to interpret the rapidly growing isotopic diffusion data in mineral grains that are measured using high spatial resolution. Based on previous works, the classical transition-state theory, and quantum statistical mechanics, an atomic theory to evaluate the diffusion coefficient (D) and the kinetic isotope effect (β factor) for the interstitial diffusion mechanism is developed with the unified equations for theoretical calculations. Taking helium diffusion in forsterite as an example, the D and β (4He/3He) were calculated at certain upper mantle conditions (0–20 GPa, 0–1500 °C) based on the developed equations. All parameters needed in our theory were obtained by first-principles calculations. Our calculated results for D at 0 GPa show reasonable agreement with most experimental results, indicating acceptable validity of the developed theory and methods. Meanwhile, possible causes for slight differences between calculations and experimental results at ambient pressure were also explored. We find that the β increases with the increase of temperature, but its pressure dependence is quite limited at least in the case of interstitial helium diffusion in forsterite. Additionally, we confirmed that current widely used simulation protocols with a fixed lattice for diffusion fail to accurately estimate the D values under high pressures due to the neglection of the activation volume. However, we also find that such protocols have almost negligible effects on the β factors for isotope fractionations during diffusion. Finally, potential effects of various approximations and calculation protocols involved in previous studies were carefully evaluated, and we find that some commonly used approximations and techniques are oversimplified and probably lead to significant loss of accuracy, especially at low temperatures (≤∼600 °C). However, it is still quite safe to continue adopting these protocols when dealing with high-temperature problems (e.g., ≥1000 °C).

The behaviors of dislocation loops punched by helium interstitials accumulation under the temperature gradient field in tungsten

ABSTRACT Enormous amounts of investigations have been made to understand the helium interstitials accumulation processes in tungsten, which is a promising candidate of plasma-facing materials in fusion reactors. However, the temperature gradient, which almost certainly exists in plasma-facing materials, was often neglected in previous studies. In view of this, the molecular dynamics method is employed to simulate the helium interstitials accumulation processes in tungsten at temperature gradient field in this study. The results show that in the temperature gradient field, the helium bubble formed by helium interstitials accumulation processes also grows via punching out 1/2<111> dislocation loops, which is similar to that at constant temperature. However, under the temperature gradient field, 1/2<111> dislocation loops always were emitted to the higher-temperature region. Combined with the first principle calculation, the formation energy of self-interstitial atom at high temperature should be responsible for this phenomenon.

Segregation and diffusion behaviours of helium at grain boundaries in silicon carbide ceramics: first-principles calculations and experimental investigations

Under a severe neutron irradiation environment, helium behaviour has become a significant concern due to helium-induced degradation of mechanical properties in SiC structural materials. Experimental observations showed that helium prefers to aggregate at grain boundaries (GBs) in SiC ceramics. However, the interaction mechanisms between helium and GBs have remained unclear. By performing first-principles calculations, we studied helium segregation at six symmetric tilt GBs and helium diffusion along and across the GBs. The solution energies of helium at the GBs are much lower than that of helium in grains, indicating preferential helium segregation at the GBs. The negative segregation energies of helium decrease linearly with the excess volume of GBs and the relative charge densities of the interstitial sites, which reveals that helium easily segregates at the GBs having open structures and low charge density interstitial sites. The interstitial helium atoms at GBs are difficult to form clusters due to weak interactions. The diffusion energy barriers of helium along the GBs Σ9(221)[11¯0] and Σ27(552)[11¯0] in the direction of tilt axis [11¯0] are much lower than that of helium in the grains. The two GBs can significantly enhance the helium diffusivity, serving as fast diffusion channels for helium. The calculation results are analysed and compared with our experimental data.

Role of Ni on the helium diffusion, stability and energetics of vacancy–type clusters in Fe–6.25Cr–3.13Ni (at.%) ternary alloys: From first-principles

Ab initio methods based on DFT are utilized to study the role of Ni addition on the interstitial helium diffusion, energetics and stability of HenVm (n = 0–6, m = 0–4) clusters in austenitic Fe–6.25Cr–3.13Ni (at.%) ternary alloys. The limit vacancy concentration considered in our simulation will be as close to the real irradiation conditions as possible. The presence of Ni obviously hinders the long-range migration of an individual helium between tetrahedral and octahedral interstitials, or between tetrahedral gaps, but greatly promotes the movement between octahedral gaps, and the travel between octahedral (tetrahedral) gaps cannot cross the tetrahedral (octahedral) interstitials. The addition of Ni enhances the attractions between interstitial helium, drives the short-range migration and tends to form small helium bubbles; on the other hand, it seriously hinders the trapping of helium by multiple-vacancies, and reduces the risk of helium bubble nucleation. In addition, the effects of Ni on the stability and energetics of helium vacancy complexes are evaluated, and it is confirmed that Ni improves the stability of the cluster models and enhances the irradiation expansion resistance of the alloy configurations, although it is not conducive to the formation of HenVm clusters. Our findings provide a theoretical reference for understanding the thermal helium desorption spectra and helium-induced embrittlement or hardening under irradiation conditions.

Helium impurities and interactions in lithium

We investigate helium interactions with lithium using density functional theory. Like other body-centered cubic (bcc) metals, the lowest-energy site for interstitial helium is a tetrahedral site. However, helium in lithium shows a higher substitutional formation energy than either the tetrahedral or octahedral interstitial formation energies, which is unique. The calculated migration energies of helium in lithium are also very low (∼1 meV), an order of magnitude lower than those in other bcc metals. We find an increase in the binding energy to a vacancy as the number of helium atoms bound to it increases. The extremely low migration energies suggest that helium transport in lithium will be very fast.

First-principles study of helium in austenitic Fe 6.3 at% Cr alloys: Structural, stability, energetics, and clustering with vacancies

The structural, stability and energetics of helium-vacancy-type (HenVm) clusters in Fe 6.3 at% Cr alloys are studied by means of DFT calculations, considering n (m) from 0 (0) to 6 (4). The self-trapping and accumulation of interstitial helium atoms in the vacancy-free lattice caused by strong attraction are more likely to form clusters in Fe 6.3 at% Cr alloys. The trapping capacity of vacancies strengthens with the increase of m, but attenuates gradually with the increase of n, and the addition of Cr prevents the trapping of helium atoms by multi-vacancies, effectively reducing the risk of helium atoms forming clusters. The binding energies and electronic properties confirmed from the macroscopic and microscopic levels respectively that the stability of Hen-clusters gradually decreases with the increase of n. Whether the HenVm complexes are easily formed are determined by the number of n and m. The higher density of the helium bubble nucleate precursor in Fe 6.3 at% Cr alloys, i.e. the small HenVm clusters, supply more annihilation sites for recombination of radiation-induced Frenkel pairs, resulting in better radiation swelling resistance than the bulk γ-Fe. We predict spontaneous emission of vacancies or helium atoms at smaller or larger n/m ratios. In addition, the trends of the binding energy of interstitial helium and monovacancy with helium-vacancy-type complexes are discussed in detail, which provide useful information for the explanation of complex experimental phenomena.

First-Principles Calculations of the Diffusivity of Interstitial Helium in Alpha-U Considering Anisotropy, Isotope Effects, and Quantum Effects

The interstitial migration of helium in alpha-U, which is planned to be used as a tritium storage material for nuclear fusion reactors and as a metallic fuel for advanced nuclear reactors, is studied by first-principles calculations. First, all migration paths are identified using the nudged elastic band method, and the migration barrier is determined for each path considering the thermal expansion of the lattice. In addition, the jump attempt frequencies are determined by applying harmonic transition state theory coupled with vibration analysis. Subsequently, the diffusion coefficient is evaluated numerically by kinetic Monte Carlo calculations using the determined migration barriers and jump attempt frequencies for all migration paths. Diffusion in the [010]-direction is found to be the most unlikely until sufficiently high temperature, while [001]-diffusion is the most dominant diffusion direction through the whole temperature range. The isotope effect, which is important because the beta decay of tritium produces helium-3, is not large and approaches the classical limit as the temperature increases. The quantum tunneling crossover temperature is computed to be approximately 100 K for helium-3 and 86 K for helium-4, which ensures the validity of the present calculation results over a wide temperature range. The diffusion coefficients are obtained as D=(9.67×10−4)×exp(−0.202eVkT)cm2/s for helium-3 and D=(8.48×10−4)×exp(−0.201eVkT)cm2/s for helium-4 over a temperature range from 200 to 900 K.

Development of an interatomic potential for Fe-He by neural network

Ferritic steel is a widely used structural material for both nuclear fusion and advanced fission reactors, yet the presence of helium degrades the mechanical properties of materials. Molecular dynamics (MD) simulation is a commonly used approach to study the mechanism of helium effect in iron from the atomic scale. The accuracy and reliability of MD simulations depends critically on the empirical interatomic potentials. In this work, we develop a high-dimensional neural network potential (HDNNP) to the Fe-He system that incorporates an extensive dataset of 5205 atomic configurations from ab-initio electronic structure calculations. The developed potential not only well reproduces the bulk properties of bcc iron, but also predicts the binding properties and relative stabilities of helium clusters accurately in the iron matrix. Overall, the average error of the binding energies predicted by HDNNP is 0.04 eV per atom as compared to 0.18 eV per atom for previously reported empirical potentials (EPs). The present Fe-He HDNNP is used to perform MD simulations of helium migration in the iron matrix and it is found that migration energies of both interstitial and substitutional helium are highly consistent with ab-initio results.

&lt;i&gt;Ab Initio&lt;/i&gt; Study of Helium in Tantalum: Interaction, Migration, and Clustering with Helium and Vacancies

Ab initio calculations based on the Density Function Theory (DFT) have been performed to study the interaction between helium and helium, helium and vacancy, migration of helium, and the stability of small helium-vacancy clusters in tantalum. The following results are found: (I) The tetrahedral interstitial helium atoms have weak interactions in tantalum, suggesting that no stable covalent bond is formed between this two helium atoms; (II) The stability of small helium-vacancy clusters is investigated. The interstitial helium atom and vacancy to the clusters are found to be positive in almost all case, i.e., all interactions are attractive; (III) The activation energies for a substitutional helium atom migration by the dissociation or vacancy mechanisms are estimated under the irradiation condition.

A physics-based machine learning study of the behavior of interstitial helium in single crystal W–Mo binary alloys

In this work, the behavior of dilute interstitial helium in W–Mo binary alloys was explored through the application of a first principles-informed neural network (NN) in order to study the early stages of helium-induced damage and inform the design of next generation materials for fusion reactors. The neural network (NN) was trained using a database of 120 density functional theory (DFT) calculations on the alloy. The DFT database of computed solution energies showed a linear dependence on the composition of the first nearest neighbor metallic shell. This NN was then employed in a kinetic Monte Carlo simulation, which took into account two pathways for helium migration, the T-T pathway (T: Tetreahedral) and the T-O-T pathway (a second order saddle in both W and Mo) (O: Octahedral). It was determined that the diffusivity of interstitial helium in W–Mo alloys can vary by several orders of magnitude depending on the composition. Moreover, T-O-T pathways were found to dominate the T-T pathways for all alloy compositions for temperatures over about 450 K. Heterogeneous structures were also examined to account for radiation-induced segregation. It was observed that diffusion was fast when W segregated to the grain interior region and Mo to the grain outer region and was slow for the reverse situation. This behavior was explained by studying the energy landscape. Finally, thermodynamic simulations indicated that Mo-rich regions of the alloy were most favorable for binding the interstitial helium and may be the sites for the nucleation of helium bubbles.

First-principles study of helium behavior in nickel with noble gas incorporation

The behavior of helium in nickel with noble gas atom (helium, neon, argon, krypton, and xenon) incorporations is systematically studied by using the first-principles method. The formation energies of noble gas atoms in nickel increase with atomic size increase from helium to xenon. All noble gas atoms considered in this work energetically prefer to stay at the substitutional sites when compared to the interstitial ones. The variations in formation energies among noble gas atoms can be mainly attributed to the steric effects caused by their incorporation. The chemical binding between nickel and noble gas atoms are further identified by their projected density of states. The substitutional noble gas shows a trapping effect on interstitial helium, and their binding energies also exhibit an approximately linear relation with their size. In addition, the effect of noble gas incorporation on helium clustering in nickel is studied. It shows that noble gas atoms attract small helium clusters and further repel the relatively larger ones. The results help to understand the influence of noble gas atoms on the fundamental helium behavior such as helium stability, trapping, and clustering in nickel and are also technologically important for further study on helium bubble nucleation under similar irradiation conditions.

Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film

Abstract He-charged FeCrNi films were prepared by radio frequency (RF) magnetron sputtering technique, and helium desorption behavior, growth process and evolution mechanism of helium bubbles in FeCrNi film were studied by thermal desorption spectroscopy (TDS) spectra combined with microstructure analysis under different annealing treatments. Three different He desorption peaks were observed in TDS spectra, which can be related to the release of interstitial helium atoms from the near-surface regions, helium-vacancy clusters with different sizes in FeCrNi film. TEM analysis showed that helium bubbles can be uniformly distributed in He-charged FeCrNi nanocrystalline films. Under annealing treatments at different temperatures, the size of helium bubbles obviously increases with increasing annealing temperature, corresponding to the significantly decrease in the number density of helium bubbles in FeCrNi films, and the growth mechanism of helium bubbles was suggested to originate from the migration and coalescence (MC) mechanism.

Ferromagnetic effects on non-Arrhenius diffusion of single interstitial helium solute in BCC Fe

Helium diffusion in metals is one of the key atomic processes for bubble nucleation. Ferromagnetic effects play important roles on microstructural evolution in BCC iron. The spin-lattice dynamics simulations are performed to study the ferromagnetic effects on the non-Arrhenius diffusion behavior of single interstitial helium solute in BCC iron. It is found that the ferromagnetic effects significantly reduce the helium diffusivity at low temperatures, but give rise to a statistically negligible influence at high-temperatures. In addition, we propose a coarse-grained formula to describe the non-Arrhenius behavior, and three diffusion modes are well-defined depending on the competition between thermal energy of helium gained from the host atoms’ collisions and its migratory barrier, for instant, helium diffusion reveals Arrhenius- and Einstein-diffusion modes in the low- and high-temperature limits, respectively. Further, the effects of quantum statistics of both phonons and magnons in BCC Fe are also discussed, which is found to give rise to the non-linear temperature dependent thermal energy thus the strong non-Arrhenius behavior in the region of Arrhenius-diffusion mode. The above-mentioned non-Arrhenius diffusion feature of helium in BCC Fe is consistent with the cases of helium diffusion in BCC W [Wen et al., J. Nucl. Mater. 493 (2017) 21; Woo et al., Phys. Rev. E 96 (2017) 032133], and the diffusion parameters are in agreement with the experimental measurement and other calculations results. The current work could help to get a complete understanding of ferromagnetic effects on irradiation damage accumulation in structural materials of fusion reactors.

The effects of packing structure on the effective thermal conductivity of granular media: A grain scale investigation

Structural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Enhancing heat management requires an improved fundamental understanding of thermal transport in granular media. In this study, the effects of packing structure on heat transfer in granular media are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid coupling heat transfer model is adapted into a discrete-element-method to simulate this transport phenomenon. The numerical framework is validated by experimental data obtained using a plane source technique, and the Smoluchowski effect of the gas phase is found to be captured by this extension. By considering packings of spherical SiO2 grains with an interstitial helium phase, vibration induced ordering in granular media is studied, using the simulation method developed here, to investigate how disorder-to-order transitions of packing structure enhance effective thermal conductivity. Grain-scale thermal transport is shown to be influenced by the local neighbourhood configuration of individual grains. The formation of an ordered packing structure enhances both global and local thermal transport. This study provides a structure-based approach to explain transport phenomena, which can be applied in properties modification for granular media.

Depth synergistic effect of irradiation damage on tungsten irradiated by He-ions with various energies

To simulate the uniform depth-distribution of damages introduced by neutrons, He-ions irradiations with different energies were respectively (W-1@16 keV, W-2@70 keV, W-3@200 keV) or successively (W-4@200-70-16 keV) conducted on the recrystallized tungsten. The damage defect distribution and helium behavior in tungsten were investigated by Positron Annihilation Spectroscopy (PAS). Results show that a sufficient number of small-size vacancy clusters containing low ratio of He atoms are produced in tungsten during a single-energy He-ions irradiation with a fluence below 1021 ions m−2 at room temperature (RT). But more irradiation dose will promote the saturation of the interstitial helium around the positron annihilation sites and accelerate the formation of helium-vacancy clusters. The fitted S profile of W-4 presents a platform in the depth range of 27–477 nm. Meanwhile, S-W line of it deviates from the line of W-1, 2 to W-3, and the (S, W) data form clusters in the depth range of 57–230 nm. These changes may be explained by a large quantity captures of injected He ions in defect sites and the formation of large-size HenVm clusters with continuously increasing n/m ratio. Fortunately, successive He-ions irradiation with various energies can, to some extent, achieve a quasi-uniform damage distribution in a certain depth range in metals, which meets the expectation in this paper.