Diffusion Of Hydrogen In Tungsten Research Articles

Hydrogen interactions with intrinsic point defects and hydrogen diffusion in tungsten doped Al2O3

In light of the extensive use of tungsten (W) in fusion experimental devices and the importance of hydrogen isotopes permeation, based on the first principle approaches we study the effects of W doping on the formation energies of intrinsic point defects, hydrogen interactions with such defects, and hydrogen diffusion in bulk of α-Al2O3. Our investigation demonstrated that W doping enhances the formation of oxygen and aluminum vacancies (Vo and VAl) in α-Al2O3. In W doping α-Al2O3, hydrogen primarily exists in the form of Ho− (hydrogenation of the bulk VO3−) and Hi− (H interstitial) defects, while it hardly occupies VAl. Hydrogen is firmly trapped by Vo with a formation energy −3.35eV, and Hi− diffusion become extremely challenging because W doping significantly raises the Hi− diffusion barrier to 4.44 eV. Our findings indicate that W doping is beneficial for hydrogen permeation resistance in α-Al2O3.

Hydrogen interaction with vacancy defects in tungsten: Unraveling the influence on diffusion mechanisms and mechanical properties

This study investigates hydrogen diffusion in tungsten at high temperatures using molecular dynamics simulations. It explores the effect of different hydrogen concentrations on diffusion mechanisms, the interaction of hydrogen atoms with vacancy defects, and their impact on the mechanical properties of tungsten. The activation energy for hydrogen diffusion in perfect bulk tungsten with 0.1 % hydrogen concentration is calculated as 0.235 eV, with a diffusion pre-exponential factor of 5.336 × 10-8 m2/s at temperatures ranging from 1000 to 3000 K. The investigation reveals two factors contributing to the increase in activation energy with temperature: migration paths involving TIS-TIS and TIS-OIS-TIS pathways, and an increase in migration energy. Three diffusion regimes are identified, with trapping effects dominating at temperatures below 1500 K, TIS-TIS pathway dominating at intermediate temperatures, and TIS-OIS-TIS pathway becoming more prominent at higher temperatures. Increasing hydrogen concentration up to 2 % (H/W concentrations in fusion environments) has minimal effect on diffusion in perfect tungsten. However, in a W-H-V system with a 1 % vacancy defect, increasing hydrogen concentration increases the effective diffusion coefficient and activation energy. The presence of hydrogen atoms and vacancy defects reduces the tensile strength and elastic modulus of tungsten, with hydrogen atoms having a greater impact on mechanical properties compared to vacancies. The degree of reduction in mechanical properties is proportional to the concentration of hydrogen atoms and vacancy defects.

Atomic layer deposition of tungsten nitride films as protective barriers to hydrogen

Nanoscale films of tungsten nitride (WN) were deposited as environmental barrier coatings (EBCs) by particle atomic layer deposition (ALD) on zirconia nanoparticles and yttria stabilized zirconia micropowders. Hydrogen diffusion in tungsten (W) was investigated computationally using density functional theory and experimentally using differential thermal analysis of the ALD samples in hydrogen at temperatures > 1000 °C. Reaction of hydrogen with the underlying material was delayed but not eliminated by the ALD film, consistent with the low computationally predicted hydrogen diffusion barrier in bulk W of 0.19 eV. This is the first study of WN ALD films as EBCs for hydrogen above 1000 °C.

Molecular dynamics studies of hydrogen diffusion in tungsten at elevated temperature: Concentration dependence and defect effects

Influence of hydrogen concentration and defects introduced by neutron irradiation on hydrogen diffusion in tungsten has been investigated by molecular dynamics simulation at elevated temperatures. Hydrogen diffusion is shown to be significantly restrained at high concentrations due to spontaneous formation of platelet-like hydrogen clusters. For neutron irradiation defects, self-interstitials, mono-vacancies and vacancy clusters are considered. By clustering and acting as dislocation loops, self-interstitials show considerable trapping effects on hydrogen, leading to the suppression of hydrogen effective diffusion and the change of diffusion model in which hydrogen mainly diffuses along dislocation lines instead of hopping between tetrahedral interstitial sites. Moreover, an equation connecting hydrogen diffusion parameters and the total length of dislocation loops is empirically established. Different influences of mono-vacancies and vacancy clusters on hydrogen diffusion have been carefully identified. With the same vacancy concentration, hydrogen diffusivity is lower with mono-vacancies than that with vacancy clusters because more isolated trapping sites are provided by mono-vacancies. This work is not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for larger scale simulations as input data.

Atomistic simulation of mechanical properties of tungsten-hydrogen system and hydrogen diffusion in tungsten

Molecular dynamics simulation is used to systematically investigate mechanical properties of tungsten-hydrogen system and hydrogen diffusion in tungsten. It is found that the tensile strength of tungsten is decreased seriously by the formation of hydrogen bubbles, and the intrinsic mechanism is discussed by deformation dislocations. The present calculation also reveals that the hydrogen clusters in tungsten would be hard to diffuse, and may finally accumulate and form bubbles. In addition, the uniaxial and isotropic tensile strains have different effects on the diffusivity of hydrogen clusters, and the mean square displacements of H clusters diffusing along several directions under uniaxial and isotropic tensile strains are derived and compared with each other. The simulated results agree well with experimental evidence and calculated data available in the literature.

Embedded-atom method potential for modeling hydrogen and hydrogen-defect interaction in tungsten

An embedded-atom method potential has been developed for modeling hydrogen in body-centered-cubic (bcc) tungsten by fitting to an extensive database of density functional theory (DFT) calculations. Comprehensive evaluations of the new potential are conducted by comparing various hydrogen properties with DFT calculations and available experimental data, as well as all the other tungsten–hydrogen potentials. The new potential accurately reproduces the point defect properties of hydrogen, the interaction among hydrogen atoms, the interplay between hydrogen and a monovacancy, and the thermal diffusion of hydrogen in tungsten. The successful validation of the new potential confirms its good reliability and transferability, which enables large-scale atomistic simulations of tungsten–hydrogen system. The new potential is afterward employed to investigate the interplay between hydrogen and other defects, including [1 1 1] self-interstitial atoms (SIAs) and vacancy clusters in tungsten. It is found that both the [1 1 1] SIAs and the vacancy clusters exhibit considerable attraction for hydrogen. Hydrogen solution and diffusion in strained tungsten are also studied using the present potential, which demonstrates that tensile (compressive) stress facilitates (impedes) hydrogen solution, and isotropic tensile (compressive) stress impedes (facilitates) hydrogen diffusion while anisotropic tensile (compressive) stress facilitates (impedes) hydrogen diffusion.

First principles study of inert-gas (helium, neon, and argon) interactions with hydrogen in tungsten

We have systematically evaluated binding energies of hydrogen with inert-gas (helium, neon, and argon) defects, including interstitial clusters and vacancy-inert-gas complexes, and their stable configurations using first-principles calculations. Our calculations show that these inert-gas defects have large positive binding energies with hydrogen, 0.4–1.1 eV, 0.7–1.0 eV, and 0.6–0.8 eV for helium, neon, and argon, respectively. This indicates that these inert-gas defects can act as traps for hydrogen in tungsten, and impede or interrupt the diffusion of hydrogen in tungsten, which supports the discussion on the influence of inert-gas on hydrogen retention in recent experimental literature. The interaction between these inert-gas defects and hydrogen can be understood by the attractive interaction due to the distortion of the lattice structure induced by inert-gas defects, the intrinsic repulsive interaction between inert-gas atoms and hydrogen, and the hydrogen-hydrogen repelling in tungsten lattice.

Concentration dependent hydrogen diffusion in tungsten

The diffusion of hydrogen in tungsten is studied as a function of temperature, hydrogen concentration and pressure using Molecular Dynamics technique. A new analysis method to determine diffusion coefficients that accounts for the random oscillation of atoms around the equilibrium position is presented. The results indicate that the hydrogen migration barrier of 0.25 eV should be used instead of the presently recommended value of 0.39 eV. This conclusion is supported by both experiments and density functional theory calculations. Moreover, the migration volume at the saddle point for H in W is found to be positive: ΔVm ≈ 0.488 Å3, leading to a decrease in the diffusivity at high pressures. At high H concentrations, a dramatic reduction in the diffusion coefficient is observed, due to site blocking and the repulsive H-H interaction. The results of this study indicates that high flux hydrogen irradiation leads to much higher H concentrations in tungsten than expected.

Hydrogen diffusion and vacancies formation in tungsten: Density Functional Theory calculations and statistical models

The interaction of hydrogen with tungsten is investigated by means of the Density Functional Theory (DFT) and statistical methods based on the transition-state theory and thermodynamics. This model yields temperature-dependent data that can help understanding macro-scale experimental results. Within this model, the concentrations of trapped hydrogen atoms at thermodynamic equilibrium are established. Taking into account the configurational entropy, hydrogen is shown to induce vacancy formation below 1000K. Based on this model, TDS spectra are simulated with a basic kinetic model to provide some better insight into the desorption process of hydrogen. Finally, revised mechanisms for hydrogen diffusion in tungsten are proposed; we conclude that the discrepancy existing between the experimental diffusion coefficient measured by Frauenfelder (1969) and the one calculated by DFT would be reconciled provided one uses two different diffusion regimes that would depend on temperature and vacancies concentration.

First-principles calculations of hydrogen solution and diffusion in tungsten: Temperature and defect-trapping effects

The solubility and diffusivity of hydrogen in tungsten are fundamental and essential factors in the application of this tungsten as a plasma-facing material, but data are scarce and largely scattered, indicating some important factors might be missed. We perform a series of first-principles calculations to predict the dissolution and diffusion properties of interstitial hydrogen in tungsten and the influence of temperature and the defect-trapping effect. The interstitial hydrogen always prefers the tetrahedral site over the temperature range 300–2700K, and its migration path mainly advances via the nearest-neighboring tetrahedral sites. Our results reveal that both solution and activation energies are strongly temperature dependent. The predicted solubility and diffusivity show good agreement with the experimental data above 1500K, but present a large difference below 1500K, which can be bridged by the trapping effects of vacancies and natural trap sites. The present study reveals a dramatic effect of temperature and defect trapping on the hydrogen solution and diffusion properties in tungsten, and provides a sound explanation for the large scatter in the reported values of hydrogen diffusivity in tungsten.

First principles investigation of cluster consisting of hydrogen–helium atoms interstitially-trapped in tungsten

We evaluate the binding energies of mixed helium and hydrogen clusters consisted of interstitially trapped atoms in bcc tungsten by first-principles calculations based on density functional theories. It is shown that helium-rich interstitially-trapped clusters have the positive binding energies and the low electron-density region expand as the number of helium in the cluster increase. Thus, the helium-rich interstitially trapped clusters can act as a trapping site for hydrogen, and interstitially trapped helium interrupts or disturbs the hydrogen diffusion in tungsten.

Structure, stability and diffusion of hydrogen in tungsten: A first-principles study

Using a first-principles method, we have investigated structure, stability and diffusion of hydrogen (H) in tungsten (W). We found that single H atom prefers to occupy the tetrahedral interstitial site with the formation energy of ∼−2.45 eV. Two H in the tetrahedral interstitial sites form a pairing cluster along the <1 1 0> directions with the H–H distance of ∼2.22 Å, while the corresponding binding energy is only 0.02 eV, indicating a very weak attractive interaction. This suggests that H itself is not capable of trapping other H atoms to form a H 2 molecule. The kinetics of H in intrinsic W is discussed, and the diffusion barrier of H that jumps between the tetrahedral interstitials is calculated to be 0.20 eV.

Nucleation rate of kink-antikink pairs in a driven and overdamped deformable chain

The equilibrium nucleation rate of thermally activated kink-antikink pairs, in nonlinear deformable substrate potential systems coupled to an external applied field, is determined analytically at low temperature, and in the limit of strong damping. We focus our attention on a class of parametrized one-site potential ${V}_{\mathrm{RP}}(u,r)$ whose shape can be varied as a function of parameter $r$ and which has the sine-Gordon shape as the particular case. We derive the driven kink velocity as well as the average velocity of the displacement of a particle as a function of an applied field. We show that for a given temperature this average velocity not only depends on the external field, but also on the shape parameter $r.$ The model is used to describe the diffusion of atoms on metallic surfaces. Numerical values are estimated for the diffusion of hydrogen in tungsten (W) and ruthenium (Ru) substrates.

An atom-probe field-ion microscope study of 200-eV 1H+2 ions implanted in tungsten at 29 K

The atom-probe field-ion microscope (FIM) technique was used to study the low-temperature diffusive behavior of hydrogen (1H) in tungsten. Tungsten FIM specimens were implanted in situ with 200-eV 1H+2 ions, at a specimen temperature of 29 K. The specimens were analyzed chemically on an atomic scale, via the atom-probe FIM technique, during the controlled pulse field evaporation of the (110) planes. No hydrogen events were detected at depths below the surface which corresponded to the calculated mean projected range of either 1H or 1H2. The mean projected ranges were calculated employing a modified version of Biersack’s and Haggmark’s Monte Carlo program entitled trim (Transport of Ions in Matter). The experimental results set a lower bound of (1–10)×10−18 cm2 s−1 on the diffusivity of hydrogen (1H) in tungsten at 29 K. This extremely high diffusivity of hydrogen in tungsten at 29 K, when compared with the value extrapolated from ∼1100 K on an Arrhenius plot, suggests strongly that the diffusion of hydrogen in tungsten should be described by a nonclassical model. In addition, experiments were performed which demonstrated the adsorption of hydrogen on the surface of tungsten FIM tips (initially cleaned atomically by field evaporation) from the ambient pressure in the FIM.

Solution and Diffusion of Hydrogen in Tungsten

Using mass spectroscopic and ultrahigh vacuum techniques, solution and diffusion of hydrogen in tungsten was investigated for pressures between 600 and 10−8 Torr and temperatures between 1100 and 2400 K. Solubility and diffusion constants are derived from degassing rates of a solid cylinder which was pre-loaded with hydrogen at ≈600 Torr. The solubility constant, S=2.9×10−1×exp (−24000/RT) Torr liter/cm3Torr1/2, and the diffusion constant, D=4.1×10−3×exp (−9000/RT) cm2/sec, are obtained, which in conjunction with literature values for the permeation constant P are consistent with the equation P=SD. Comparison to theory indicates that the solubility and diffusion constants are characteristic of interstitially dissolved hydrogen. Expressions are derived for the concentration of interstitial hydrogen as a function of pressure and temperature. Semiquantitative values for the total hydrogen concentration at low pressures are derived from H2 pressure changes which result when a sample is flashed between selected, high temperatures. Below 10−4 Torr, the total hydrogen concentration appears to be several orders of magnitude higher than the concentration of interstitial hydrogen, indicating that hydrogen is held in, and diffuses via both interstitial sites and a second kind of site of unknown nature. A semiquantitative analysis of diffusion is given for the case of the diffusing species held in, and diffusing via, two different kinds of sites.