Hydrogen-rich Region Research Articles

A novel method based on composite effects for evaluating the hydrogen embrittlement of steels with low hydrogen diffusion coefficients

The effect of hydrogen on mechanical properties of a twinning-induced plasticity austenitic steel was investigated by slow strain rate tensile tests and scanning electron microscopy after various hydrogen pre-charging conditions. A new method for evaluating hydrogen embrittlement of materials with low hydrogen diffusion coefficient has been proposed. Materials with low hydrogen diffusion coefficients can be regarded as composite materials after pre-charging, i.e., a hydrogen-rich brittle region and a hydrogen-lean ductile region. Based on the rule of mixtures, the influence of the hydrogen on the mechanical properties can be reflected by the contribution of the brittle zone to the overall mechanical properties, which is more realistic and universal compared to the characterization of the mechanical property loss by hydrogen content. This helps to simplify the selection of materials resistant to hydrogen embrittlement by directly comparison linear loss of mechanical properties when the “average” hydrogen content of the brittle region is similar.

Hydrogen inhibited phase transition near crack tip – An atomistic mechanism of hydrogen embrittlement

Hydrogen embrittlement has been extensively studied for decades; however, the atomistic origins of hydrogen embrittlement remain debated. Here, we studied the role of body-centered cubic (bcc) to face-centered cubic (fcc) phase transition at the crack tip on hydrogen embrittlement in alpha iron at room temperature. Molecular dynamics simulations were performed to examine the propagation of [101¯](101) crack. The results indicate that in the absence of hydrogen atoms, the phase transition could release the strain energy and postpone the crack propagation. Hydrogen atoms would form a nanosize hydrogen-rich region ahead of the crack tip and inhibit phase transition, thus enhancing brittle crack growth. A general expression is provided to predict the hydrogen concentration beyond which the phase transition does not occur. In addition, a simplified crack growth model is proposed to predict the risk level during the operation.

Effects of hydrogen direct-injection angle and charge concentration on gasoline-hydrogen blending lean combustion in a Wankel engine

To analyze the effects of hydrogen charge concentration (HCC) and injection angle (IA) on the lean combustion in a gasoline Wankel engine, the present work implemented a numerical simulation model coupling with the kinetic mechanisms and validated with the experimental data. Results found that with the increase in HCC, the penetration of hydrogen injection is enlarged and the area of the high-speed jet flow is expanded. The jet-flow area for IAs of 45° or 135° is larger than that of 90°. Changing IA could obtain the hydrogen distribution at different regions of the rotor chamber and IA of 90° acquires the smallest hydrogen-rich region. Increasing IA brings about the reduced flame speed substantially; the flame area for IAs of 45° and 90° expands with the increment of HCC whereas the contrary pattern is witnessed at the IA of 135°. Smaller IA leads to the major burning occurring untimely, which resulting in less work delivery of the engine. The hydrogen consumption for IAs of 45° and 90° increases as HCC is ascendant while that for IA of 135° is just the reverse. Variations in the mixture distribution and turbulence are the intrinsic mechanism of how the HCC and IA reflects the combustion progress. As hydrogen is injected with larger HCC and smaller IA, a relatively richer mixture and higher turbulent kinetic energy are distributed close in the spark ignition region. The peak combustion pressure reduces and its corresponding crank position delays with the widened IA at any HCC. Considering the fuel combustion and nitric oxide formation, as hydrogen volume fraction is 3% and IA is 45°, the engine could realize the optimized performance under the computational condition. An efficient combustion performance may be performed in engineering application if the hydrogen IA is in accordance with the rotor rotating direction at lower HCC.

Role of hydrogen on the incipient crack tip deformation behavior in α-Fe: An atomistic perspective

A crack tip in α-Fe presents a preferential trap site for hydrogen, and sufficient concentration of hydrogen can change the incipient crack tip deformation response, causing a transition from a ductile to a brittle failure mechanism for inherently ductile alloys. In this work, the effect of hydrogen segregation around the crack tip on deformation in α-Fe was examined using atomistic simulations and the continuum based Rice-Thompson criterion for various modes of fracture (I, II, and III). The presence of a hydrogen rich region ahead of the crack tip was found to cause a decrease in the critical stress intensity factor required for incipient deformation for various crack orientations and modes of fracture examined here. Furthermore, the triaxial stress state ahead of the crack tip was found to play a crucial role in determining the effect of hydrogen on the deformation behavior. Overall, the segregation of hydrogen atoms around the crack tip enhanced both dislocation emission and cleavage behavior suggesting that hydrogen has a dual role during the deformation in α-Fe.

Atomistic study of hydrogen embrittlement during cyclic loading: Quantitative model of hydrogen accumulation effects

Hydrogen embrittlement (HE) is a serious industrial problem that can reduce the ductility, cause catastrophic brittle failures of susceptible materials. Although previous atomistic HE study is able to quantify hydrogen delivery to the crack tip with the stress intensity distribution, no mathematical model has been proposed to quantify the hydrogen effects in the complex loading spectra. In current study, we first employ molecular dynamics (MD) simulation to study propagation behavior of a single crack with pre-charged hydrogen under cyclic loading. The MD simulation demonstrates that hydrogen atoms will diffuse to the crack tip during loading and unloading and form hydrogen rich region. Once hydrogen-rich region forms, cracks can propagate during unloading because of the brittle nature due to hydrogen accumulation. After the crack tip passes through hydrogen-rich region, it becomes blunted, and re-starts to accumulate hydrogen atoms. In a separate simulation, we closely monitor the hydrogen accumulation during ten cyclic loads and found that hydrogen atoms accumulate linearly in a nano-size region ahead of crack tip. Based upon the MD simulation results, we proposed a new model that could be used to quantify hydrogen accumulation effects in cyclic loading, especially minor cycles. Furthermore, the proposed model was used to predict the acceleration factor for crack growth in the presence of hydrogen and it shows excellent agreement with experimental results.

Carbon Dioxide Utilization for an Enhanced Biohydrogen Production of a Biomass Hydrolysate

Bio-hydrogen (H2) production in a thermophilic anaerobic bioreactor with carbon dioxide (CO2) utilization at the headspace was investigated at different ratio of Biomass substrate to Microorganism (B/M). A 3 mL volume of 80% NaOH (m/v) solution was continuously hung at the headspace of the bioreactors to capture carbon dioxide. The utilization of CO2 at its headspace, proved to enhance bio H2 production. The highest Cumulative Biohydrogen Yield (CHY) of 131.81±3.47 mL-H2/gVS was measured at the ratio of 8.10 parts of biomass to 0.90 parts of Acclimatized Seed Sludge (ASS), B/M 9, while 4.17 parts of similar biomass to 0.83 parts of ASS (B/M 5) had a biohydrogen production of 90.70±16.67 mL-H2/gVS. The B/M of 6.12 parts of biomass to 0.88 parts of ASS (B/M 7) produced a CHY of 84.72±18.35 mL-H2/gVS while the control bioreactors without CO2 utilization (without and with biomass substrate) yielded a 0.06±0.035 mL-H2/gVS and a 3.27±0.78 mL-H2/gVS respectively. The mechanism of the biofermentation in this anaerobic reaction produced two possible resulting reaction; the acetogenesis of CO2 with H2 and the hydrogenotrophic methanogenesis. These reactions consumes hydrogen in the process to produce methane or acids. The presence of 80% (m/v) NaOH solution at the headspace inside the bioreactors, utilizes the CO2 producing a hydrogen-rich region in space: The highest average H2 yield of 51.83 mL-H2 after 49.1 h with B/M 9 without methane was due to carbon dioxide utilization in the bioreactors. A univariate ANOVA and Pair-wise Tukey HSD statistical analysis revealed that the CHY of B/M9 was significantly higher than the other B/Ms. The highest yield, 55.85 mL-H2/gVS obtained with the bioreactor of B/M 9 was optimum for H2 production. The results concluded that H2 production is also enhanced by CO2 utilization at the headspace.

Avalanching strain dynamics during the hydriding phase transformation in individual palladium nanoparticles.

Phase transitions in reactive environments are crucially important in energy and information storage, catalysis and sensors. Nanostructuring active particles can yield faster charging/discharging kinetics, increased lifespan and record catalytic activities. However, establishing the causal link between structure and function is challenging for nanoparticles, as ensemble measurements convolve intrinsic single-particle properties with sample diversity. Here we study the hydriding phase transformation in individual palladium nanocubes in situ using coherent X-ray diffractive imaging. The phase transformation dynamics, which involve the nucleation and propagation of a hydrogen-rich region, are dependent on absolute time (aging) and involve intermittent dynamics (avalanching). A hydrogen-rich surface layer dominates the crystal strain in the hydrogen-poor phase, while strain inversion occurs at the cube corners in the hydrogen-rich phase. A three-dimensional phase-field model is used to interpret the experimental results. Our experimental and theoretical approach provides a general framework for designing and optimizing phase transformations for single nanocrystals in reactive environments.

Metallization of magnesium polyhydrides under pressure

Evolutionary structure searches are used to predict stable phases with unique stoichiometries in the hydrogen-rich region of the magnesium/hydrogen phase diagram under pressure. MgH${}_{4}$, MgH${}_{12}$, and MgH${}_{16}$ are found to be thermodynamically stable with respect to decomposition into MgH${}_{2}$ and H${}_{2}$ near 100 GPa, and all lie on the convex hull by 200 GPa. MgH${}_{4}$ contains two H${}^{\ensuremath{-}}$ anions and one H${}_{2}$ molecule per Mg${}^{2+}$ cation, whereas the hydrogenic sublattices of MgH${}_{12}$ and MgH${}_{16}$ are composed solely of H${}_{2}^{\ensuremath{\delta}\ensuremath{-}}$ molecules. The high-hydrogen content stoichiometries have a large density of states at the Fermi level, and the ${T}_{c}$ of MgH${}_{12}$ at 140 GPa is calculated to be nearly three times greater than that of the classic hydride, MgH${}_{2}$, at 180 GPa.

Liquid water oceans in ice giants

Aptly named, ice giants such as Uranus and Neptune contain significant amounts of water. While this water cannot be present near the cloud tops, it must be abundant in the deep interior. We investigate the likelihood of a liquid water ocean existing in the hydrogen-rich region between the cloud tops and deep interior. Starting from an assumed temperature at a given upper tropospheric pressure (the photosphere), we follow a moist adiabat downward. The mixing ratio of water to hydrogen in the gas phase is small in the photosphere and increases with depth. The mixing ratio in the condensed phase is near unity in the photosphere and decreases with depth; this gives two possible outcomes. If at some pressure level the mixing ratio of water in the gas phase is equal to that in the deep interior, then that level is the cloud base. The gas below the cloud base has constant mixing ratio. Alternately, if the mixing ratio of water in the condensed phase reaches that in the deep interior, then the surface of a liquid ocean will occur. Below this ocean surface, the mixing ratio of water will be constant. A cloud base occurs when the photospheric temperature is high. For a family of ice giants with different photospheric temperatures, the cooler ice giants will have warmer cloud bases. For an ice giant with a cool enough photospheric temperature, the cloud base will exist at the critical temperature. For still cooler ice giants, ocean surfaces will result. A high mixing ratio of water in the deep interior favors a liquid ocean. We find that Neptune is both too warm (photospheric temperature too high) and too dry (mixing ratio of water in the deep interior too low) for liquid oceans to exist at present. To have a liquid ocean, Neptune's deep interior water to gas ratio would have to be higher than current models allow, and the density at 19 kbar would have to be ≈0.8 g/cm3. Such a high density is inconsistent with gravitational data obtained during the Voyager flyby. In our model, Neptune's water cloud base occurs around 660 K and 11 kbar, and the density there is consistent with Voyager gravitational data. As Neptune cools, the probability of a liquid ocean increases. Extrasolar “hot Neptunes,” which presumably migrate inward toward their parent stars, cannot harbor liquid water oceans unless they have lost almost all of the hydrogen and helium from their deep interiors.

Interaction of hydrogen with the vacancy-oxygen pair produced in n-type silicon by electron irradiation

Interaction of hydrogen with the vacancy-oxygen pair produced in n-type silicon by 10 MeV electron irradiation was studied by means of depth profile measurements by deep-level transient spectroscopy. Hydrogen was incorporated into electron-irradiated n-type silicon by wet chemical etching. The reaction of the vacancy-oxygen pair with hydrogen leads to the formation of a defect labelled NH1 (Ec-0.31 eV). The depth profiles of the vacancy-oxygen pair and NH1 defect indicate that further reaction of the NH1 defect with hydrogen occurs, especially in the hydrogen-rich region, making the NH1 defect inactive as an electron trap. It was found that the passivation of the vacancy-oxygen pair as an electron trap by hydrogen is caused via the above two steps, transformation into the NH1 defect and association of the NH1 defect with hydrogen.

Hydrogen depth profiling using 1H(15N,.ALPHA..GAMMA.)12C resonant nuclear reaction on water-treated olivine surfaces and characterization of hydrogen species.

Natural olivine crystals (Fo91) were subjected to water treatment under different temperatures covering 25 to 800°C and different pH covering 2 to 12. Hydrogen analysis using 1H(15N, αγ)12C resonant nuclear reaction was carried out and semi-quantitative hydrogen concentration depth profiles were obtained. The hydrogen depth profiles reveal the presence of high hydrogen concentration at the surface region. The depth of the hydrogen-rich region is less than 20 nm under medium to high pH conditions at 25°C. In contrast, hydrogen penetrates into olivine as deep as 200 nm under low pH conditions at 25°C. The penetration depth is independent of temperature up to 800°C under near neutral conditions. Combining this technique and XPS analysis, the speciation of hydrogen at the near surface region has been examined. The thickness of the leached surface layer is less than 20 nm and seems to be independent of the treatment conditions such as temperature and pH, however, the surface species vary with the treatment conditions. Mg and Fe are selectively leached out under low to medium pH conditions at 25°C. H+ or H3O+ occupies the vacant octahedral sites in this case. The high hydrogen concentration within 200 nm depth in the low pH region arises from infiltration of molecular water. Hydrogen is highly localized at etch pits, etch hole, and dislocations in this case. On the contrary, Si is selectively leached out and hydroxyl species are formed under a high pH condition.

A model of hydrogen implantation into graphite including bulk recombination by tunneling

A distinctly new model for bulk hydrogen recombination in hydrogen implanted graphite is presented. The model considers deep H-atom traps (2.8 eV), but the thermal properties of the recombination and release are determined by shallow (few tenths eV) localized vibrational modes within the trapping potential well in combination with tunneling between trap sites. Four sources of hydrogen recombination are considered: i) direct recombination of hydrogen implanted into a hydrogen rich region, ii) recombination of hydrogen displaced from traps by the incident beam, iii) recombination of hydrogen which tunnels through the potential barrier separating adjacent trap sites, and iv) diffusive recombination. A mechanism for thermal relaxation of the localized vibrational modes is included in the model. Interaction of the incident beam with both trapped atomic H and diffusing molecular H 2 is considered in the model. Various implantation parameters and interaction cross sections required by the model are evaluated with TRIMRC calculations. Other parameters are obtained from the literature or from fits to experimental data. The model is found to give good agreement with experimental inventory versus isochronal anneal temperature, saturation inventory versus implantation temperature, and ion-induced release versus ion fluence measurements. Experiments for confirming the model are suggested.

A comparative study of the properties of evaporated a-Si films before and after hydrogenation

The electron spin resonance signal, dark conductivity and photoconductivity of evaporated amorphous silicon before and after hydrogenation have been measured as a function of deposition rates and of annealing temperatures. In order to produce films with good photoconductivity a low deposition rate of about 1 Å s -1 and an annealing temperature above 800 K are recommended. Contamination by residual gases such as oxygen in a high vacuum of 10 -7 Torr during deposition can “poison” the film, diminishing the effect of post-hydrogenation. The poisoning effect can be removed by annealing above 800 K. Hydrogenation reduces the dangling bond density and suppresses the hopping contribution to electron conduction but does not seem to affect the band tail width. Hydrogenation also shifts the Fermi level towards the conduction band. This may be due to the formation of an electron accumulation layer in the hydrogen-rich surface region which affects the position of the Fermi level in the whole film.

A SIMS depth profiling study of the hydration layer formed at polycrystalline tin oxide surfaces by atmospheric exposure

Dynamic SIMS has been used to examine the hydrated surface layer formed on polycrystalline tin oxide films by exposure to atmospheric humidity. A strongly hydrated layer is found to occur within approximately the top 6 Å of the surface with a hydrogen-rich region extending to a depth of about 20 Å into the bulk. The Sn + sputtering yield for the hydrated layer is greater than that of the bulk-oxide material. It is shown that the profile of the positive-ion 121 amu peak, which is attributed primarily to 120SnH +, appears to yield the least ambiguous measure of the extent of hydration. High-temperature annealing of the films is found to affect the bulk-sputtering properties possibly by altering the structure of the film and/or the form of the hydrogen incorporated in the bulk of the film.

Hydrogen effects on the age hardening behavior of 2024 aluminum

It has been found that the fatigue crack growth rate in aluminum alloys increases significantly in the presence of moisture. This phenomenon along with a moisture effect observed in another context has been attributed to 'embrittlement' of the aluminum by absorbed hydrogen generated by the reaction of moisture with freshly exposed aluminum. A description is given of a number of age hardening experiments involving 2024 aluminum. These experiments show that a mechanism related to the segregation of absorbed hydrogen to the coherent theta-double-prime interfaces may account for the observed reduction in fatigue life. It is pointed out that this segregation promotes a loss of coherency in the hydrogen rich region at a fatigue crack tip. Subsequently, the loss of coherency causes local softening and reduces fatigue life.

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E c, the mobility edge at the conduction band.

Synthesis of the Light Elements in Supernovae

view Abstract Citations (27) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Synthesis of the Light Elements in Supernovae Epstein, Richard L. ; Arnett, W. David ; Schramm, David N. Abstract The results of detailed calculations of possible nucleosynthesis in supernova shocks are presented. The shock waves are parametrized in such a way as to cover a wide variety of theoretical possibilities. Only if a strong high ion-temperature precursor develops does any appreciable nucleosynthesis occur. For presupernova stars with a Population I composition in their hydrogen- rich regions, only 7Li and 11B can be produced by the shock in interesting amounts; these isotopes are not efficiently produced by the interaction of galactic cosmic rays with the interstellar medium. If the hydrogen-rich region contains no CNO nuclei (as might be the case in an extreme Population II object), then 7Li and possibly some deuterium could be produced. However, significant deuterium production requires extreme physical and astronomical conditions: (a) the ion precursor would have to develop only when the shock energy exceeds 40 MeV nucleon -1(b) the total energy of the supernova would have to be greater than 2 x 1052 MsN/Mo erg, where MsN is the mass of ejected material. The restrictions on deuterium production would be eased somewhat if the ion precursor were stronger than Colgate found; however, the calculations of Weaver and Chapline may indicate that the precursor is actually less intense. Subject headings: nucleosynthesis - shock waves - stars: supernovae Publication: The Astrophysical Journal Supplement Series Pub Date: May 1976 DOI: 10.1086/190377 Bibcode: 1976ApJS...31..111E full text sources ADS |

Red Giants of Population II. IV

view Abstract Citations (73) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Red Giants of Population II. IV Härm, R. ; Schwarzschild, M. Abstract The evolutionary phases of a Population II star through the helium flash have been recomputed with improved representations for several of the relevant physical processes. The new computations show that the helium flash is less extreme than indicated by the earlier, less accurate computations; the nuclearenergy release reaches only 1011 LQ at the peak, and the rate of evolutionary changes now appears to be too slow for major hydrodynamic effects. On the other hand, the new computations confirm the earlier results indicating that the convective core at its greatest extent (after the flash) misses the hydrogen-rich region by about two density scale heights. Accordingly, in view of the very small overshooting from convective cores found in a recent separate investigation, it now appears highly likely that the helium flash does not lead to mixing in the star. Publication: The Astrophysical Journal Pub Date: August 1966 DOI: 10.1086/148790 Bibcode: 1966ApJ...145..496H full text sources ADS | Related Materials (3) Part 1: 1962ApJ...136..150S Part 2: 1962ApJ...136..158S Part 3: 1964ApJ...139..594H

Crack propagation in the hydrogen-induced brittle fracture of steel

The macroscopic features of hydrogen-induced static fatigue have been identified. The process is one of crack initiation and slow crack growth, followed by cataclysmic crack propagation (fracture). An electrical resistance method was developed to permit analysis of the crack growth kinetics. Specimens containing a high hydrogen concentration in the surface layers were examined. Three distinct stages of crack growth were identified. The first stage was characteristic of crack growth through a pre-existent hydrogen-rich region (surface layer), while the second stage was associated with the gross inward diffusion of hydrogen. The third stage was associated with imminent fracture.