Hydrogen Diffusion Mechanism Research Articles

Vapor-deposited iron sulfide films as a novel hydrogen permeation barrier for steel: Deposition condition, defect effect, and hydrogen diffusion mechanism

Hydrogen embrittlement is a serious problem in the oil/gas industry. In this work, various iron sulfide (FeS) films, including iron monosulfide (FeS), pyrite (FeS2), and pyrrhotite (Fe7S8), were synthesized in X80 steel by chemical vapor deposition at 200 °C, 300 °C, 400 °C, and 500 °C. The corrosion resistance and hydrogen permeation properties of the FeS films were investigated through electrochemical methods. Results indicated that FeS films significantly improved the hydrogen barrier properties of the X80 steel, which was closely related to the crystal structure type and defects of FeS films. Defects like microcracks and pinholes during deposition can increase the porosity of the film, resulting in the film properties decreased. Moreover, FeS film (at 300 °C), which had the smallest apparent hydrogen diffusivity (D ˜ 2.64 × 10−7 cm2/s) and apparent subsurface concentration (Capp ˜ 1.12 μmol/cm3), had the best hydrogen barrier properties. The corrosion resistance of FeS film (300 °C) was excellent.

Role of S-S interlayer spacing on the hydrogen storage mechanism of MoS2

Hydrogen storage mechanism and hydrogen storage capacity are the great challenges for the development of hydrogen energy technology. Besides the better catalytic properties, it is crucial to search for suitable material that provides enough space to store H2 molecule. Similar to graphene, MoS2 with S-S layered structure opens up a new way to improve the hydrogen storage capacity. By using the first-principles calculations, in this work, we investigate the hydrogen diffusion mechanism, hydrogenation process and hydrogen storage capacity of MoS2 with S-S interlayer. We find that hydrogen prefers to diffuse into S-S interlayer along the interstitial site (path: IT-IT). H2 molecule is a stable in S-S interlayer because the charge interaction of H-H atoms is stronger than that of H-S atoms. Finally, we predict that MoS2 with S-S layered-by-layered stacking can effectively improve the hydrogen storage capacity.

Theoretical study of tetrahedral site occupation by hydrogen in Pd nanoparticles.

To understand the enhanced effects and new hydrogen absorption properties of metal nanoparticles, we theoretically investigated the hydrogen absorption in Pd nanoparticles, adopting the Pd405 model of ca. 2.5 nm by using density functional theory. Pd405 showed inhomogeneous geometric features, especially near the surface region. The hydrogen absorptions in octahedral (O) and tetrahedral (T) sites near the core region were stable and unstable, respectively, similar to the Pd bulk. We clearly demonstrated the possibility of hydrogen absorption in T sites near the surface of Pd405. The flexible volume change and the difference in hydrogen position relative to the center of mass of the T site that we observed are important factors for stable hydrogen absorption in T sites of Pd nanoparticles. In addition, we discuss the differences in hydrogen diffusion mechanisms in the core and near surface regions, based on the stability of hydrogen absorption in O and T sites.

Volatile loss during homogenization of lunar melt inclusions

Volatile abundances in lunar mantle are critical factors to consider for constraining the model of Moon formation. Recently, the earlier understanding of a “dry” Moon has shifted to a fairly “wet” Moon due to the detection of measurable amount of H2O in lunar volcanic glass beads, mineral grains, and olivine-hosted melt inclusions. The ongoing debate on a “dry” or “wet” Moon requires further studies on lunar melt inclusions to obtain a broader understanding of volatile abundances in the lunar mantle. One important uncertainty for lunar melt inclusion studies, however, is whether the homogenization of melt inclusions would cause volatile loss. In this study, a series of homogenization experiments were conducted on olivine-hosted melt inclusions from the sample 74220 to evaluate the possible loss of volatiles during homogenization of lunar melt inclusions. Our results suggest that significant loss of H2O could occur even during minutes of homogenization, while F, Cl and S in the inclusions remain unaffected.We model the trend of H2O loss in homogenized melt inclusions by a diffusive hydrogen loss model. The model can reconcile the observed experimental data well, with a best-fit H diffusivity in accordance with diffusion data explained by the “slow” mechanism for hydrogen diffusion in olivine. Surprisingly, no significant effect for the low oxygen fugacity on the Moon is observed on the diffusive loss of hydrogen during homogenization of lunar melt inclusions under reducing conditions. Our experimental and modeling results show that diffusive H loss is negligible for melt inclusions of >25 μm radius. As our results mitigate the concern of H2O loss during homogenization for crystalline lunar melt inclusions, we found that H2O/Ce ratios in melt inclusions from different lunar samples vary with degree of crystallization. Such a variation is more likely due to H2O loss on the lunar surface, while heterogeneity in their lunar mantle source is also a possibility. A similar size-dependence trend of H2O concentrations was also observed in natural unheated melt inclusions in 74220. By comparing the trend of diffusive H loss in the natural MIs and in our homogenized MIs, the cooling rate for 74220 was estimated to be ∼1 °C/s or slower.

Hydrogen diffusion in Ti-doped forsterite and the preservation of metastable point defects

The effect of trace concentrations of Ti on the rate and mechanism of hydrogen diffusion in pure forsterite was investigated experimentally. Forsterite doped with 350–400 ppm Ti (predominantly octahedral Ti 3+ , minor tetrahedral Ti 4+ ) was prepared by diffusing Ti into pure synthetic forsterite at high temperature (1500 °C), very low oxygen fugacity (~QFM-5) at atmospheric pressure. The Ti-doped forsterite was then diffusively hydroxylated in a piston-cylinder apparatus at much lower temperatures (650–1000 °C) and higher oxygen fugacities, at 1.5–2.5 GPa, with chemical activities buffered by forsterite-enstatite or forsterite-periclase and partial pressure of H 2 O equal to total pressure. This produced hydrogen concentration-distance profiles of several hundred micrometers in length. Diffusion of hydrogen through the Ti-doped forsterite, even at very high f O 2 , does not lead to redox re-equilibration of the high Ti 3+ /ΣTi ratio set during the synthesis of the starting material at extremely reducing conditions—the metastable point defects are partially preserved. Three main hydroxylated point defects are observed; hydroxyl is associated with Ti 4+ (titano-clinohumite point defects), Ti 3+ (and possibly other trivalent cations), and M-site vacancies. Concentration-distance profiles represent an interplay between diffusion and reaction (i.e., site rearrangement) to form the observed point defects. In all experiments, the concentration-distance profiles of the hydroxylated Ti defects coincide with the concentration-distance profiles of the M-site vacancy substitution, with the same crystallographic anisotropy. This suggests that the macroscopic movement of hydrogen through the crystal is due to one diffusion mechanism (the diffusion of hydroxylated M-site vacancies). The net H diffusion coefficient [log D (ΣH)], between 650–1000 °C, is log D ( Σ H ) = log D 0 ( Σ H ) + ( − 223 ( ± 8 ) kJ / mol 2 . 3 R T ) where the values of log D 0 (ΣH) parallel to [100] and [001] directions are −3.0 ± 0.4 and −2.2 ± 0.4, respectively; diffusion is therefore around one order of magnitude faster along the c axis than along the a axis. The diffusion of hydrogen is slightly faster in Ti-doped forsterite than in pure forsterite. There is no effect of chemical activity or oxygen fugacity on the rate of diffusion. Hydrogen diffusion profiles represent a complex interplay between the movement of H through the crystal lattice and point-defect reactions to maintain charge balance.

Hydrogen diffusion in doped and undoped α-Ti: An ab-initio investigation

The hydrogen diffusion paths in undoped and doped α-Ti were studied by the projector augmented wave method within the density functional theory. Our calculations confirm that in the basal plane, the indirect mechanism of hydrogen diffusion between octahedral sites through a tetrahedral interstitial is more preferable than the direct path. The lowest energy barrier of 0.1 eV was found between the tetrahedral sites along the c axis; however, a hydrogen atom can only oscillate between them. The influence of substitutional impurities (transition- and simple-metal ones) on the main barriers is discussed. It was found that impurities of simple metals hamper the H mobility by increasing the activation energy, whereas most transition-metal solutes (3d-metals of group 5–10) reduce the energy barriers. In general, a deeper understanding of the H diffusion behavior in doped α-Ti is achieved. The temperature-dependent diffusion coefficient and the activation energy for hydrogen atom hopping along the preferential path in pure titanium are found to be in agreement with the experimental data.

The effects of fractality on hydrogen permeability across meso-porous membrane

A fractal theory employing a box-counting method was used to describe hydrogen gas diffusion into membrane pores in the meso-porosity regime. The diffusion of the gas into the membrane pore network confirmed the existence of fractal structure in the system. Two fractal identities to represent irregularity and roughness of pore surface and tortuosity of the membrane were obtained and analyzed. Their influences on hydrogen permeability were also evaluated. The fractal permeability model that reflects different hydrogen diffusion mechanisms was calculated and compared with that of the state of the art Kozeny–Carman equation.

Hydrogen storage in Mg–LiBH4 composites catalyzed by FeF3

Mg–10 mol% LiBH4 composite plus small amounts of FeF3 is investigated in the present work. The presence of LiBH4 during the milling process noticeably modifies the size and morphology of the Mg agglomerates, leading to faster hydrogenation and reaching almost the theoretical hydrogen capacity owing to enhanced hydrogen diffusion mechanism. However, the dehydrogenation of the system at low temperatures (≤300 °C) is still slow. Thus, FeF3 addition is proposed to improve the dehydrogenation kinetic behavior. From experimental results, it is found that the presence of FeF3 results in an additional size reduction of the Mg agglomerates between ∼10 and ∼100 μm and the formation of stable phases such as MgF2, LiF and FeB. The FeB species might have a catalytic effect upon the MgH2 decomposition. As a further result of the FeF3 addition, the Mg–10 mol%LiBH4–5 mol% FeF3 material shows improved dehydrogenation properties: reduced dehydrogenation activation energy, faster hydrogen desorption rate and reversible hydrogen capacities of about 5 wt% at 275 °C.

New Simulation Method to Characterize the Recoverable Component of Dynamic Negative-Bias Temperature Instability in p-Channel Metal–Oxide–Semiconductor Field-Effect Transistors

We introduce a new simulation technique to investigate the recovery characteristics of dynamic negative-bias temperature instability (NBTI) in conventional silicon dioxide (SiO2) dielectric p-channel metal–oxide–semiconductor field-effect transistors (p-MOSFETs) based on the hydrogen diffusion and hole-trapping mechanisms. In this work, a sequence of train pulses on the gate terminal were applied to simulated p-MOSFETs in single and multiple runs, thus emulating repetitive cycles of stress and recovery. The effects of varying the applied stress voltages, temperatures, and durations were then analyzed. The recoverable component, R, of degradation was found to increase when the magnitudes of the applied stress voltage and temperature were increased. Moreover, the R was reduced when the recovery time was increased for a single run. In contrast, the R increased when the recovery time was increased for multiple runs. The normalized R of the simulated device was found to decrease by 0.7% and to increase by 7% with respect to the shortest recovery duration for a single run and for multiple runs, respectively. In addition, we measured the effects of equivalent oxide thickness (EOT) on the R and found that, in our study, the R for a transistor with a smaller EOT exhibited a substantial increase of 84% compared with that for a transistor with a larger EOT. These characteristics of the R of dynamic NBTI could be explained from the perspectives of the reaction–diffusion (R–D) model and hole-trapping mechanism. The results suggested that the underlying connection between these two mechanisms was the interface trap concentration, which represents the permanent components in both mechanisms.

Empirically-specified Environmental Assisted Cracking Model

Based on various mechanisms of hydrogen diffusion in metals, different crack propagation models have been developed. Here, empirically specified model of crack propagation in structure components, which are simultaneously subjected to aggressive hydrogen environment influence and cycling, is presented. The model uses two different approaches to problem solving, i.e. the influence of aggressive hydrogen environment on material and the fatigue. A developed environmental assisted cracking model assumes that either the influence of aggressive hydrogen environment or the fatigue initiates a local fracture of structure component. This model enables estimating a structure component life.

First-principles study of hydrogen storage and diffusion in B2 FeTi alloy

The structural, elastic properties and electronic structure of FeTi and its hydrides, and hydrogen diffusion behavior in FeTi alloy were investigated by the first-principles calculations within the frame work of density functional theory (DFT). The calculated lattice constants for FeTi were in good agreement with the experimental values, and the lattice distortion of hydrides increases with the rise of hydrogen content. The calculation results of formation enthalpy, fusion heat of hydrogen and octahedron interstice size showed that hydrogen atom is more likely to exist in the central site of octahedron with the frame of four Ti atoms and two Fe atoms. The independent elastic constants as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio ν and anisotropy value A) were calculated, and then the ductility and elastic anisotropy of FeTi and its hydrides were discussed in details. In addition, the density of states, electron density difference and population analysis for FeTi and its hydrides were also calculated, which reveals the underlying mechanism of structural stability and chemical bonding. Finally, the activation energy of next nearest neighbor diffusion for H between octahedron interstices in FeTi alloy was calculated to be 2.92eV, clarifying the atomic mechanism of hydrogen diffusion.

Semiempirical quantum model approach for hydrogen adsorption in ZrNi alloys

Abstract Hydrogen storage is an important topic because of its relevance to the future energy economy. Hydrogen diffusivity in materials plays an important role in hydrogen technology both for hydrogen separation and hydrogen storage. To clarify the mechanism of the rate-controlling step, diffusion mechanism of hydrogen in metallic materials is studied by molecular dynamics method. This study performs semi-empirical-quantum molecular dynamic simulations in order to clarify hydrogen atom diffusion behavior in ZrNi alloys materials. We investigate the mechanical properties change associated with temperature variation for ZrNi base alloys and also consider the influence of materials micro-structure change of hydrogen diffusion. Finally, current work presents a theoretically prediction of dynamical diffusion coefficient to compare diffusion kinetics of crystalline and amorphous structure.

Grain size and grain-boundary effects on diffusion and trapping of hydrogen in pure nickel

The impact of grain size on hydrogen diffusion and trapping mechanisms has been investigated for a wide range of grain size of non-textured pure nickel. Both aspects depend mainly on the nature of grain boundaries (GBs). In particular, we illustrate the effects of random and special boundaries on the different defects and trapping sites stored in the GBs, and their consequences on hydrogen transport and segregation. The high-angle random boundaries are considered as disordered phase where the hydrogen diffusion is accelerated, while the special boundaries constitute a potential zone for hydrogen trapping due to the high density of trapping sites as dislocations and vacancies. The predominance of one phenomenon over the other depends on several parameters, such as the grain size, the probability of grain boundary connectivity, the grain boundary energy and the excess of free volume. In addition, our experiments confirm that hydrogen promotes vacancy formation probably in GBs.

Deuterium diffusion in LiOH–water-corroded oxide layer of zirconium alloys

In order to clarify the hydrogen diffusion mechanism in the oxide layer of zirconium alloys, in situ hydrogen isotope diffusion in the oxide layer has been examined. The zirconium alloys used were Zircaloy-2, GNF-Ziron (Zircaloy-2 type alloy with high iron content) and VB (zirconium-based alloy with high iron and chromium contents). They were corroded in 1 or 0.1M LiOH-containing water at 563K, producing oxide layers of 1.1–2.1μm in thickness. The diffusion experiments were carried out in the temperature range from 488 to 633K by using a combined technique of deuterium plasma exposure and nuclear reaction analysis for D (3He,p)4He reaction. From the transient deuterium profiles in the oxide layers, it was concluded the LiOH–water-corroded oxides had a single-layer structure, which was in contrast to the double-layer structure previously observed in steam-corroded oxide layers. The diffusion coefficients in the 1M LiOH–water-corroded oxides evaluated from the deuterium profiles were smaller in the order of Zircaloy-2>GNF-Ziron>VB at 573K. For the 0.1M LiOH–water-corroded oxide of GNF-Ziron, the diffusivity was lower than that of the 1M LiOH–water-corroded oxide by a factor of 1/4. The present diffusion coefficients of the 1M LiOH–water-corroded oxides of GNF-Ziron and VB were approximately 7 times larger than the previous data of the corresponding steam-corroded oxides. The deuterium diffusion properties in the oxides of the three alloys obtained in the in situ experiment were roughly consistent with their hydrogen absorption performances in the LiOH–water-corrosion tests, as well as in the previous steam corrosion tests.

Deuterium diffusion in steam-corroded oxide layer of zirconium alloys

In situ diffusion experiments of the hydrogen isotope deuterium in the oxide layer formed on zirconium alloys were carried out to clarify the hydrogen diffusion mechanism in the layer. The experiments were done in deuterium plasma for the temperature range from 523 to 673 K by using a nuclear reaction analysis for D( 3He,p) 4He. The zirconium alloys used were GNF-Ziron (high iron Zircaloy-2 type alloy) and VB (high iron and chromium alloy), which had been corroded in 673 K H 2O- or D 2O-steam for 10–15 days. The oxide thickness ranged from1.4 to 1.7 μm of pre-transition condition. The results showed that the steam oxides had a double-layer structure composed of the outside non-protective oxide with faster diffusivity and the inside barrier layer with slower diffusivity. The barrier layer thickness was about 0.8–0.9 μm and unchanged for the two alloys. For the in situ deuterium plasma diffusion experiments, the diffusion coefficient of deuterium in the barrier layer of GNF-Ziron was given as, D (cm 2/s) = 4.5 × 10 −13exp (−17,000/RT). The diffusion coefficient in the VB oxide at 573 K was approximately half of that in the GNF-Ziron oxide. This factor for the diffusivity was roughly consistent with their hydrogen absorption performance. For the deuterium release experiments in a vacuum subsequent to the in situ deuterium diffusion experiments, the diffusion in the barrier oxide layer was further retarded, suggesting lower diffusivity than for the case of the in situ deuterium plasma atmosphere.

Thermodynamic and Kinetic Modelling Hydrogenation of Titanium Particles and Sheets

Use of hydrogen as a temporary alloying element in Ti alloys is an attractive approach to improve the mechanical properties of the materials, enhance processability and thereby reduce manufacturing costs. In this paper, the hydrogen diffusion process and the phase transformation both between Ti particles and in Ti sheets were simulated to analyze the mechanism of hydrogen diffusion in different phases (α-Ti, β-Ti and TiHx). With the simulation based on the kinetics and thermodynamics, quantitative behaviors of the hydrogen diffusion and the phase transformation were analyzed. The simulation results provide an insight into the diffusion process and improve the fundamental understanding of the mechanism of diffusion and phase transformation.

The rate of water loss from olivine-hosted melt inclusions

Diffusive water loss from olivine-hosted melt inclusions has been reported previously. This process must be considered when interpreting melt inclusion data. This study measured the rate of water loss from olivine-hosted melt inclusions during heating-stage experiments to test a previous diffusive reequilibration model and the hydrogen diffusion mechanism that controls the rate. Olivine-hosted melt inclusions were heated to a constant temperature in reduced Ar gas in a heating stage for a few hours, and unpolarized Fourier transform infrared spectra were repeatedly measured through the inclusions. Water loss occurred rapidly in the experiments. Within a few hours, the water absorbance at 3,500 cm−1 wavenumber decreased by half. The observed water loss rate can be explained by the diffusive reequilibration model and hydrogen diffusion in olivine coupled with metal vacancy. The beginning of water loss was different in the low- and high-temperature experiments. At low temperatures (1,423 and 1,437 K), water loss did not occur in the initial 1 or 2 h. At high temperatures (1,471–1,561 K), water loss began immediately. The initial time period without water loss at low temperatures may be explained by a hydrogen fugacity barrier in the host olivine. At low temperatures, the internal pressure may be lower than the equilibrium pressure of melt inclusion and olivine, causing lower hydrogen fugacity in the melt inclusion than in the olivine, which will delay the water loss from the melt inclusion. The tested model and diffusivity were used to estimate the rate of water loss during homogenization experiments and magma eruption and cooling. For 1-h homogenization experiment, the model shows that large inclusions (50 μm radius) in large olivines (500 μm radius) are robust against water loss, while large or small inclusions (50–10 μm radius) in small olivines (150 μm radius) may suffer 30–100% water loss. For natural samples, the correlation between water concentration and melt inclusion and olivine sizes may be helpful to infer the initial water concentration, degree of diffusive reequilibration, and magma cooling rate.

The application of the analysis of potential driven particle diffusion to the strength of materials

The problems of diffusion in solid are widely treated as those of numerical analyses such as mechanisms of hydrogen diffusion which causes embrittlement, boundary micro crack initiation and growth under high temperature creep condition and stress and electro-migration due to vacancy diffusion in LSI circuit. In this paper, the significance of α multiplication method which we proposed to analyze the particle diffusion problems under potential gradient and the results obtained from this method were discussed for the problems of hydrogen embrittlement, vacancy diffusion which results in creep crack damage and deformation, and line failure in LSI circuit. Furthermore, temperature characteristic of the sensitivity of particle diffusion and concentration such as hydrogen and vacancy were discussed based on this proposed analysis.

First-principles study of hydrogen diffusion mechanism in Cr2O3

The system energy of H atom occupying different positions in Cr2O3 crystal lattice is calculated by adopting the first-principles calculation method based on density functional theory in this paper. The results indicate that the most stable position of H atom in Cr2O3 crystal lattice locates at the bilateral positions of the center of the unoccupied O octahedral interstice. The reason resulting in this situation is analyzed by comparing the change of Cr2O3 lattice distortion and density of states in Cr2O3_H system when H atom locates at different positions in octahedral interstice. The diffusion activation energy of H atom is 0.73 eV, which is determined by seeking the diffusion path and transition state of H atom in Cr2O3 crystal lattice. The effective attempt frequency of H atom in Cr2O3 crystal lattice is also calculated by using molecular dynamics. Combining with diffusion activation energy data, the diffusion coefficient of H atom in Cr2O3 crystal is determined.

Thermodynamics approach to the hydrogen diffusion and phase transformation in titanium particles

The hydrogen diffusion and phase transformation in a titanium particle were studied based on thermodynamic calculation. The mechanisms of hydrogen diffusion in different phases (α-Ti, β-Ti and TiH x ) were analyzed. A mobility database was developed for titanium–hydrogen system based on the experimental works on hydrogen diffusion coefficient reported in literature and the fundamental of diffusion. To implement the calculation, a commercial software package for the simulation of diffusion-controlled phase transformation was used. The hydrogen diffusion process, hydrogen distribution, phase transformation and phase growth rate during hydrogenization of a titanium particle at temperatures of 560 K, 800 K and 1000 K were discussed. The thermodynamics and kinetics analysis provided quantitative insight into the diffusion process and improved the understanding of diffusion mechanism and phase transformation.