Diffusion Of Hydrogen Atoms Research Articles

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica**Project supported by the Science Challenge Project, China (Grant No. TZ2016003-1-105) and the CAEP Microsystem and THz Science and Technology Foundation, China (Grant No. CAEPMT201501).

Understanding hydrogen diffusion in amorphous SiO2 (a-SiO2), especially under strain, is of prominent importance for improving the reliability of semiconducting devices, such as metal–oxide–semiconductor field effect transistors. In this work, the diffusion of hydrogen atom in a-SiO2 under strain is simulated by using molecular dynamics (MD) with the ReaxFF force field. A defect-free a-SiO2 atomic model, of which the local structure parameters accord well with the experimental results, is established. Strain is applied by using the uniaxial tensile method, and the values of maximum strain, ultimate strength, and Young’s modulus of the a-SiO2 model under different tensile rates are calculated. The diffusion of hydrogen atom is simulated by MD with the ReaxFF, and its pathway is identified to be a series of hops among local energy minima. Moreover, the calculated diffusivity and activation energy show their dependence on strain. The diffusivity is substantially enhanced by the tensile strain at a low temperature (below 500 K), but reduced at a high temperature (above 500 K). The activation energy decreases as strain increases. Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO2, which may be utilized to improve the reliability of semiconducting devices.

Effect of Mg shell on MgH2 dehydrogenation by morphological and mathematical analysis

Magnesium (Mg) formation during dehydrogenation of nanocrystalline magnesium hydride (MgH2), exhibiting a bimodal particle size distribution (4 ± 1 and 16 ± 8 μm) at 320 and 350°C, was investigated using X-ray diffraction patterns, cross-sectional scanning electron microscopy (SEM) and transmission electron microscopy. Magnesium hydride dehydrogenation consists of an initial period of slow hydrogen gas (H2) release (incubation period). Following incubation, hydrogen release increases significantly. SEM images of the dehydrogenating magnesium hydride particles are graphically divided into several concentric annuli. The radially outward flux of hydrogen released within the particles was estimated by developing a quantitative approach. During incubation, the absence of a magnesium shell necessitates slower hydrogen atom diffusion through magnesium hydride as depicted by the flux estimation. Beyond incubation, the enhanced hydrogen release is due to the presence of a magnesium shell. Thus, the effect of the absence/presence of a magnesium shell on magnesium hydride dehydrogenation was studied using the estimated fluxes.

Development and implementation of algorithms with diffusion tensor images to evaluate brain connectivity

Diffusion-weighted magnetic resonance imaging (DWI) is the use of specific MRI sequences, which uses the diffusion of Hydrogen atoms to generate contrast and it allows the mapping of the diffusion process of molecules in vivo and reflects interactions with macromolecules, fibers, and membranes among other. Hydrogen atom diffusion patterns (quantification of anisotropy) can reveal microscopic details about tissue architecture, either normal or in a diseased state. A special kind of DWI, diffusion tensor imaging (DTI), has been used extensively to map white matter tractography in the brain. Tractography is a procedure that is used to highlight neural tracts (axon), its fibers position estimation in brain areas has broad potential implications in cognitive neuroscience fields. An algorithm based on diffusion tensor Image is developed and implemented in order to evaluate brain connectivity in different regions of interest. The major objective of this work is represent two-dimensional and three-dimensional connectivity between areas thereby show the potential of the DTI. Results shows how Connectivity Matrix provides statistical data on the pattern of anatomical relationships, this connectivity pattern is formed by synapses that represent the cross correlations and the flow of information.

Identification of stable adsorption sites and diffusion paths on nanocluster surfaces: an automated scanning algorithm

The diverse coordination environments on the surfaces of discrete, three-dimensional (3D) nanoclusters contribute significantly to their unique catalytic properties. Identifying the numerous adsorption sites and diffusion paths on these clusters is however tedious and time-consuming, especially for large, asymmetric nanoclusters. Here, we present a simple, automated method for constructing approximate 2D potential energy surfaces for the adsorption of atomic species on the surfaces of 3D nanoclusters with minimal human intervention. These potential energy surfaces fully characterize the important adsorption sites and diffusion paths on the nanocluster surfaces with accuracies similar to current approaches and at comparable computational cost. Our method can treat complex nanoclusters, such as alloy nanoclusters, and accounts for cluster relaxation and adsorbate-induced reconstruction, important for obtaining accurate energetics. Moreover, its highly parallelizable nature is ideal for modern supercomputer architectures. We showcase our method using two clusters: Au18 and Pt55. For Au18, diffusion of atomic hydrogen between the most stable sites occurs via non-intuitive paths, underlining the necessity of exploring the complete potential energy surface. By enabling the rapid and unbiased assessment of adsorption and diffusion on large, complex nanoclusters, which are particularly difficult to handle manually, our method will help advance materials discovery and the rational design of catalysts.

Investigating possible kinetic limitations to MgB2 hydrogenation

An investigation is reported of possible kinetic limitations to MgB2 hydrogenation. The role of H–H bond breaking, a necessary first step in the hydrogenation process, is assessed for bulk MgB2, ball-milled MgB2, as well as MgB2 mixed with Pd, Fe and TiF3 additives. The Pd and Fe additives in the MgB2 material exist as dispersed metallic particles in the size range ~5–40 nm diameter. In contrast, TiF3 reacts with MgB2 to form Ti metal, elemental B and MgF2, with the Ti and the MgF2 phases proximate to each other and coating the MgB2 particulates with a film of thickness ~3 nm. Sieverts studies of hydrogenation kinetics are reported and compared to the rate of H–H bond breaking as measured by H-D exchange studies. The results show that H–H bond dissociation does not limit the rate of hydrogenation of MgB2 because H–H bond cleavage occurs rapidly compared to the initial MgB2 hydrogenation. The results also show that surface diffusion of hydrogen atoms cannot be a limiting factor for MgB2 hydrogenation. Instead, it is speculated that it is the intrinsic stability of the B–B extended hexagonal ring structure in MgB2 that hinders the hydrogenation of this material. This supposition is supported by B K-edge x-ray absorption measurements of the materials, which showed spectroscopically that the B–B ring was intact in the material systems studied. The TiF3/MgB2 system was examined further theoretically with reaction thermodynamics and phase nucleation kinetic calculations to better understand the production of Ti metal when TiB2 is thermodynamically favored. The results show that there exist physically reasonable ranges for which nucleation kinetics supersede thermodynamics in determining the reactive pathway for the TiF3/MgB2 system and perhaps for other additive systems as well.

Unconventional hydrogen permeation behavior of Pd/BCC composite membranes and significance of surface reaction kinetics

As one of hydrogen purification techniques, membrane separation has a significant potential to directly obtain high-purity hydrogen from the mixed gas produced from various catalytic reactions. Specifically, dense metallic membranes exhibit several advantages including good mechanical strength under pressurized conditions, thermal stability, and high hydrogen selectivity. Conventionally, their permeation behavior is predicted from a numerical model known as the Sievert's law, which describes diffusion of hydrogen atoms through a metal layer. This study questions the validity of previous permeability trends of the Pd/BCC composite membranes and reveals the importance of surface reactions that significantly affect the permeation behavior of such membranes. A new permeation model developed, considering both the surface reactions at the catalytic layers and bulk diffusion through the metal layers, exhibits good correlation with the experimental permeation characteristics of the Pd/BCC composite membranes. Moreover, the diffusivity coefficients of BCC metals as a function of temperature is determined with higher accuracy than those reported in previous studies having temperature range inconsistency between hydrogen solubility and diffusivity. The experimental data along with the proposed model successfully accounts for the unique permeation characteristics of BCC metal membranes coated with catalytic layers and advances fundamental understanding of the permeation characteristics of the composite membrane, thereby accelerating the adoption and application of the composite membrane permeation model.

Surface coverage dependent mechanisms for the absorption and desorption of hydrogen from the W(1 1 0) and W(1 0 0) surfaces: a density functional theory investigation

Herein we investigate absorption and desorption of hydrogen in the sub-surface of tungsten via density functional theory. Both the near-surface diffusion and recombination of a bulk hydrogen atom with a hydrogen atom adsorbed upon the W(1 1 0) and W(1 0 0) surfaces are investigated at various surface adsorption coverage ratios. This study intends to model the desorption processes occurring during thermal-desorption spectroscopy experiments and the absorption of hydrogen during gaseous or low energy atomic exposure. Since the diffusion and recombination processes are expected to change as the hydrogen coverage of the surface varies, different coverage ratios were investigated on both surfaces. We found that at a saturation coverage of hydrogen on both surfaces, the activation barriers for the recombination of molecular hydrogen are below 0.8 eV. On the contrary, below saturation, the activation barriers for recombination rise to 1.35 eV and 1.51 eV depending on the coverage and on the orientation of the surface. Regarding the absorption of atomic hydrogen from the surface into the bulk, the activation barrier raises from less than 1.0 eV at saturation to around 1.7 eV below saturation on both surfaces. These results indicate that surface mechanisms certainly play a significant role in the kinetics of desorption of hydrogen from tungsten; it is also expected that surface mechanisms affect the total amount on hydrogen absorbed in tungsten during implantation.

Fracture toughness properties of HIC susceptible carbon steels in sour service conditions

Hydrogen Induced Cracking (HIC) in carbon steels is a well-studied mechanism, where diffusing hydrogen atoms accumulates at the steel imperfections/laminations to create gaseous hydrogen with very high pressure, leading to initiation and growth of internal cavities, so-called HIC. Measurements of relevant fracture toughness properties of non-HIC resistant steels in hydrogen environment is critical to predict and assess the initiation and growth of HIC. The present work attempts to quantify the effect of hydrogen on the fracture toughness properties (KQ and CTOD) of an API X42 pipeline steel under simulated H2S in-service conditions. The fracture toughness properties are measured in TL and SL directions: perpendicular and parallel to the pipeline wall thickness, respectively, following ASTM E1820, standard. Since the X42 is a non-HIC resistant steel, the measurement of the fracture toughness properties in the SL direction is more relevant in terms of HIC initiation and growth than fracture toughness properties in the TL direction. Indeed, parallel to the thickness of the pipeline wall, X42 steel shows microstructural features prone to HIC formation and growth. Steady state H2S in-service conditions were simulated by charging the specimen for 48 h using a special electrolytic solution and then tested (ex-situ) to evaluate the fracture toughness properties. The steady state H2S environment was obtained by measuring the Hydrogen Concentration (CH) in the bulk of the specimen, using Thermal desorption Spectroscopy at three levels of CH. It was observed that the KQ was not affected in the SL direction, while it was reduced in the TL direction for 1.5 ppmw of CH. The CTOD showed mixed results in the TL direction while it was significantly reduced in the SL direction reaching a saturation at 1 ppmw of CH. Besides, microstructural analyses showed that the presence of inclusions coalescence in form of dimples promote the early failure, which is more pronounced in the hydrogen environment especially at higher levels of CH.

Nonadiabatic Dynamics of Hydrogen Diffusion on Cu(001): Classical Mapping Model with Multistate Projection Window in Real Space.

Diffusion of atomic hydrogen on metallic surfaces is a longstanding research topic of both fundamental and practical interests. However, full understanding of the microscopic mechanisms and development of effective strategy for surface dynamics control at the molecular level remain elusive. In this paper, we propose a new nonadiabatic multistate model for surface diffusion based on a real space decomposition scheme by generalizing the classical mapping theory of Meyer and Miller. The model suggests a general multistate perspective on real-time surface dynamics by mapping it into spatially disjointed windowing functions, which feature the explicit nonadiabatic controllability. Within this framework, the first nonadiabatic molecular dynamics simulation is performed for atomic hydrogen diffusion on the Cu(001) surface, and the nonequilibrium effect of lattice distortion is studied.

Modulating hydrogen diffusion on metal surfaces by nonadiabatic transitions

Nonadiabatic transitions may be used as a promising tool for dynamical control. However how it could be applied to and affect surface diffusion remains largely unexplored. Here a nonadiabatic model was proposed based on the classical mapping theory to introduce multistate couplings in addition to the bare surface diffusion. By performing nonadiabatic molecular dynamics simulation on a benchmark system of atomic hydrogen diffusion on the Cu (001) surface, it is demonstrated that nonadiabatic transitions could modulate diffusion dynamics in a robust way, i.e. either suppressing or promoting it. Depending on the design for the coupling regime in the nonadiabatic model, simulation results show that aside for the nonadiabatic damping effect, the diffusion constant of H atom could be enhanced by a factor of 2-6 in the temperature range of T = 500-600 K. The effect of nonadiabatic transitions may provide an explanation to the significant discrepancy between experimental measured diffusion constant and previous theoretical predictions. By highlighting the role of nonadiabatic effects, in particular under nonequilibrium conditions, this work sheds light on the development of new molecular control schemes for practical applications.

Molecular and Atomic Hydrogen Diffusion Behavior by Reaction Kinetic Analysis in Projection Range of Hydrocarbon Molecular Ion for CMOS Image Sensors

In this study, two types of hydrogen diffusion behavior in the projection range of a hydrocarbon molecular ion after high‐temperature heat treatment for the passivation of interface state defects at SiO2/Si interface of the CMOS image sensor is presented. The hydrogen peak concentration in the hydrocarbon ion projection range is observed by secondary ion mass spectrometry analysis after silicon epitaxial growth and the subsequent high‐temperature heat treatment. Moreover, the hydrogen peak concentration strongly depends on heat treatment time after epitaxial growth and the subsequent heat treatment. Two dissociation activation energies by reaction kinetic analysis using the results of the time dependence of hydrogen diffusion behavior are also determined. Assuming two dissociation reactions, the activation energy from the projection range of a hydrocarbon molecular ion is derived. The results of reaction kinetic analysis show that the dissociation activation energies are 0.79 eV for molecular hydrogen and 0.42 eV for atomic hydrogen. The dissociation activation energy of 0.79 eV indicates molecular hydrogen diffusion activation energy, whereas that of 0.42 eV indicates atomic hydrogen diffusion from the projection range of a hydrocarbon molecular ion. Therefore, it is believed that the molecular and atomic hydrogen diffusion behavior of the hydrocarbon molecular ion implanted silicon epitaxial wafers can contribute to the effective reduction in the density of interface state defects at the SiO2/Si interface.

Hydrogen Distribution in Partly Desorbed MgH<sub>2</sub> Samples

The structure of MgH2 samples has been investigated by the neutron diffraction method at room temperature and 5 K. Samples of MgH2 have been prepared with vacuum extraction technique at high temperature. Obtained neutron data demonstrated that samples contain coexisting Mg and MgH2 phases in different rate. The distribution of hydrogen atoms in the structure of the samples is comparable at both temperatures. Collected neutron data and results of X-ray analysis show that microstructure of the samples is different at room and at low temperature. Non-stability of partly desorbed MgH2 samples after low temperature treatment has been discussed on basis of different diffusion of hydrogen atoms in varied microstructure.

A 3-D TCAD Framework for NBTI—Part I: Implementation Details and FinFET Channel Material Impact

The time kinetics of interface trap generation and passivation ( $\Delta {N} _{\textsf {IT}}$ ) and its contribution ( $\Delta {V} _{\textsf {IT}}$ ) during and after negative bias temperature instability (NBTI) stress is calculated by using Sentaurus TCAD. The framework consists of Sentaurus process for the formation of realistic device structures and to obtain material and strain information, while Sentaurus device is used to implement the double-interface reaction–diffusion (RD) model for trap kinetics. Capture–emission depassivation (CED) model is used for the inversion layer hole and oxide electric field ( ${E} _{\textsf {OX}}$ )-assisted breaking of hydrogen (H) passivated defects at channel/gate insulator interface. Multistate configuration (MSC) model is used for diffusion of atomic and molecular hydrogen (H and H2) and further reactions with other H passivated defects in gate insulator bulk. Quantum effects are considered for hole density, and band structure calculation is used including mechanical strain for CED model parameters. Electron capture in the generated traps during recovery is calculated using transient trap occupancy model (TTOM) and implemented as a postprocessor. Measured data from silicon (Si) and silicon germanium (SiGe) p-channel FinFETs are used for validating the TCAD framework.

Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors

Gasochromic based optical hydrogen sensors have attracted much attention as normal temperature sensors. The behaviors of the hydrogen diffusion largely affect the reaction process and the sensitivity. However, few researches focused on the influence of hydrogen diffusion in gasochromic films. Here, we report a method to pre-split the H2 molecule and separate the hydrogen molecule and hydrogen atom diffusion process by deposition of porous PdCl2/SiO2 catalyst films on WO3. The response time of WO3 at 844 Pa H2 partial pressure reduced from 800 s to 200 s. The stability of WO3 film was largely improved to be more than 160 cycling lifetime. A validation experiment was designed and proved the diffusion of the hydrogen atom and the two-step model of H2 reaction on WO3 films, on the basis of which the diffusion coefficient of the hydrogen atom has been estimated. Moreover, the PdCl2 doping SiO2 catalyst exhibits substrate versatility, which can also coat on the nano-structured WO3.

Diffusion of hydrogen atoms in silicon layers deposited from molecular beams on dielectric substrates

In the paper, the processes occurring during low-temperature growth of non-hydrogenated amorphous Si and polycrystalline Si films on multilayer Si3N4/SiO2/c-Si substrates from molecular beams under conditions of ultrahigh vacuum are studied in detail. Diffusion of hydrogen atoms from a dielectric layer into the growing film is shown to accompany the growth of a silicon film on a Si3N4 layer deposited by CVD or on a SiO2 layer obtained by thermal oxidation of a silicon wafer. The process of hydrogen migration from the dielectric substrates into the silicon film is studied using FTIR spectroscopy. The reduction of IR absorption at the bands related to the NH bonds vibrations and the increase of IR absorption at the bands relating to the SiN bonds vibrations in IR spectra demonstrate that hydrogen atoms leave Si3N4 layer during Si deposition from a molecular beam. The absorption band assigned to the valence vibrations of the SiH bond at ∼2100 cm–1 emerging in IR spectra obtained at samples deposited both on Si3N4 and SiO2 layers indicates the accumulation of hydrogen atoms in silicon films. The difference in chemical potentials of hydrogen atoms in the dielectric layer and the silicon film explains the transfer of hydrogen atoms from the Si3N4 or SiO2 layer into the growing silicon film.

Analysis of Unusual and Instantaneous Overshoot of Response Transients in Gas Sensors

Hydrogen is known to diffuse through particular planes of ZnO crystals. However, in spite of being an established gas sensor material, such diffusion related effects have not been reported in gas sensor properties of ZnO in the literature to the best of our knowledge. Here, we report a prominent overshoot observed in the response transients of single crystalline large ZnO microrods sensor upon exposure to hydrogen gas. This overshoot intensity and time has been found to vary systematically with the temperature as well as gas concentration. In spite of larger size of rods (diameter ≈1–3 µm), it shows a very high response of nearly 950% for 1000 ppm hydrogen at 300 °C. The observed overshoot is very prominent at temperatures below 300 °C. This unusual behavior has been attributed to the bulk diffusion of atomic hydrogen into the subsurface of ZnO microrods. The coefficient of diffusivity, estimated from the time of overshoot, is found to be typically 10−15 m2 s−1, at 250 °C, and its thermal activation energy has been found to be 0.22 eV. The lack of grain boundaries within the microrods and surface gradient driven diffused hydrogen species have been proposed as the origin of unusual overshoot profiles.

Progressive modelling and experimentation of hydrogen diffusion and precipitation in anisotropic polycrystals

Hydrogen embrittlement of zirconium alloys is a major concern in nuclear industry. It is known that diffusion of hydrogen atoms in zirconium lattice depends on the localized state of stresses, the patterning of which is significantly affected by the elastic and plastic anisotropy of zirconium crystals. This study focuses on developing a model to understand the effects of such localized stress fields on hydrogen diffusion and precipitation. For this purpose, a crystal plasticity finite element code is updated and coupled with a series of newly developed subroutines for simulating stress-assisted-diffusion of hydrogen atoms in zirconium polycrystals. An electron backscatter diffraction experiment is also conducted on a hydrided Zircaloy-2 sample to compare the measured distribution of the hydride phase with the calculated one. It is shown that even in the absence of macroscopic loads, thermal residual stresses within zirconium grains can disturb the uniform distribution of hydrogen atoms by inducing large hydrostatic stresses at the vicinity of grain boundaries. Such stress concentrations can further affect the sequence of hydrides nucleation. Also, it is shown that by alternating local stress fields, hydride induced transformation strain can change the distribution of the hydrogen atoms in zirconium matrix.

First-Principles Investigation of Atomic Hydrogen Adsorption and Diffusion on/into Mo-doped Nb (100) Surface

To investigate Mo doping effects on the hydrogen permeation performance of Nb membranes, we study the most likely process of atomic hydrogen adsorption and diffusion on/into Mo-doped Nb (100) surface/subsurface (in the Nb12Mo4 case) via first-principles calculations. Our results reveal that the (100) surface is the most stable Mo-doped Nb surface with the smallest surface energy (2.75 J/m2). Hollow sites (HSs) in the Mo-doped Nb (100) surface are H-adsorption-favorable mainly due to their large adsorption energy (−4.27 eV), and the H-diffusion path should preferentially be HS→TIS (tetrahedral interstitial site) over HS→OIS (octahedral interstitial site) because of the correspondingly lower H-diffusion energy barrier. With respect to a pure Nb (100) surface, the Mo-doped Nb (100) surface has a smaller energy barrier along the HS→TIS pathway (0.31 eV).

Nucleation and growth of carbon nanoforms on the surface of metallic plate-substrates and the mechanism of their doping with the clusters of ferromagnetic atoms

The present work deals with the special experiments on SEM-EDX study of morphology, chemical composition and topological transformations of the initial ground surface of the bulk iron plate-substrate after its interaction with the ethanol vapor pyrolysis products at high temperatures in the closed-loop and open cycle reactors. Our experiments have shown that the mechanism of formation of Fe clusters-doped CNFs on plate-substrate surfaces may be represented as a process, the first stage of which is a protonation of the substrate subsurface layers caused by diffusion of hydrogen atoms facilitating the formation of 3D nano-groups of Fe atoms assembled in the characteristic clusters with magic numbers of atoms, depending on the thermodynamics of the metal. The spontaneous coalescence of these clusters into giant Fe-clusters at comparatively low temperatures and formation of iron nano-droplets at comparatively high temperatures results in the formation of a nanopatterned surface with the uniformly distributed catalytic centers of CNFs nucleation. The second stage of the process is a nanoparticle-guided growth through the VLS or VS (at low temperatures) growth mechanisms in which the one cluster provides nucleation of only one CNF particle so that the sizes of the nucleation centers determine the basic size of the CNF nanoparticles.

Atomistic study of hydrogen behavior around dislocations in α iron

Atomic-level studies have been performed to investigate the hydrogen behavior near a 1/2<111>{110} edge dislocation and a 1/2<111> screw dislocation in α iron. Molecular statics analysis has been carried out to determine the stress distributions and hydrogen binding energy around the dislocation cores. The results reveal that the hydrogen binding is more sensitive to the hydrostatics stress than the shear stress around the edge dislocation, while the shear stress plays a leading role on the hydrogen binding around the screw dislocation. In addition, nudged elastic band (NEB) calculations have been applied to explore different migration paths and the corresponding migration energy barriers of a single hydrogen atom at the dislocation cores. It is of interest to note that both edge and screw dislocations are not able to offer fast pipe diffusion of hydrogen atoms along the dislocation line, as normally considered. Instead, hydrogen atoms prefer to diffuse slowly along oblique paths crossing the edge dislocation line on the slip plane and spiral paths surrounding the screw dislocation line. Furthermore, molecular dynamics simulations have been performed to study the diffusion behavior of hydrogen at the dislocation cores, which verifies the results of NEB calculations.