High Diffusivity Of Hydrogen Research Articles

Overview of safety challenges associated with integration of hydrogen-based propulsion systems for climate neutral aviation

Electrification through hydrogen-based fuel cells as well as hydrogen combustion in gas turbines is a key strategy in aviation for achieving substantial reduction of emissions. However, this transition presents multifaceted challenges. Besides the development and improvement of technologies required for such hydrogen-fuelled aero engines, the safety of hydrogen storage and distribution systems on aircraft is paramount. Challenges associated with hydrogen in terms of its material properties, the design and selection of components for the conditioning and distribution, as well as the system design are being presented and discussed in this work. This includes the consideration of high diffusivity, flammability and reactivity of hydrogen and the consequences of these traits: hydrogen embrittlement, hydrogen-induced cracking and leakage, for instance. The challenges elaborated in this work are pertinent to both hydrogen fuel cell-based propulsion systems and hydrogen combusting gas turbines. Design considerations were derived and are being outlined in this work. These are transferable to applications in other industries such as automotive and stationary power plants. The need for novel rigorous safety protocols to enable a sustainable future in aviation is being highlighted.

Effect of magnetron sputtering power on structure and properties of Ta films on Cu substrate of neutron production target for accelerator-based boron neutron capture therapy (AB-BNCT)

Tantalum, with a high hydrogen diffusion coefficient, can be used as an interlayer for neutron production target in Accelerator-based Boron Neutron Capture Therapy (AB-BNCT) to prevent hydrogen embrittlement of Cu substrate. Magnetron sputtering technique was employed to deposit Ta films on the Cu Substrate, with sputtering power being a key parameter. This study investigated the effect of sputtering power on the properties of Ta films deposited on Cu substrate through several methods and Monte Carlo simulations (PHITS and SRIM). A turning point was observed at 250 W, beyond and below which increasing sputtering power led to the increase of Ta film thickness, potentially due to changes in the crystalline structure during deposition. The Ta film exhibited superior surface performance at sputtering powers of 300 W and 350 W. The chemical state of Ta was not affected by sputtering power. Thermal conductivity was observed to decrease after Ta deposition. The thickness of Ta film in this study had a negligible effect on neutron energy spectra and angular distribution. Moreover, a Ta film thickness of 20 μm prevented proton deposition and hydrogen embrittlement in the Cu substrate. These findings have important implications for designing and fabricating neutron production target in AB-BNCT.

Explainable machine learning to uncover hydrogen diffusion mechanism in clinopyroxene

Estimating the water content of mantle-derived magma using clinopyroxene (cpx) phenocrysts serves as a valuable constraint on the water budget in deep Earth. Intricate magma processes and the high hydrogen diffusion rate necessitate careful evaluations of whether the water content in cpx preserves its original state. Machine learning (ML) has been utilized to develop a classifier for judging hydrogen diffusion in cpx. Nevertheless, the opaqueness and complexity of most ML models hinder a clear understanding of their classification principles. To elucidate the mechanistic basis of the ML model, the Shapley theory is integrated to determine the contributions of major elements of cpx as features in a linear additive manner. This study achieves superior classification performance using an extreme gradient boosting model and innovatively presents a quantitative evaluation of feature importance at the sample level for each observation. The results indicate that Na plays a predominant role in the diffusion process surpassing other major elements and its associated hydrogen can easily diffuse out of cpx. Our model also identifies various hydrogen association modes in different elemental compositions and puts constraints on the properties of incorporated hydrogen with non-lattice forming elements in cpx. The findings demonstrate that the application of explainable ML methods in mineralogy holds significant potential for advancing the comprehension of geological phenomena.

The anionic Tx defects of Nb2CTx MXene as the effective catalytically active center for the Mg-based hydrogen storage materials

While early transition metal-based materials, such as MXene, has emerged as an efficient catalyst for the Mg-based hydrogen storage materials, their strong interaction with hydrogen resulted in the high hydrogen diffusion barrier, hindering further improvement of catalytic activity. A MXene is characterized by rich anionic groups on its surface, significantly affecting electronic and catalytic functionalities. Using Nb2CTx as an example, we herein illustrate the critical role of anionic Tx defects on controlling hydrogen dissociation and diffusion processes in Mg-based hydrogen storage materials. The hydrogen desorption properties of MgH2 can be significantly enhanced by utilizing Tx controllable Nb2CTx, and it can release 3.57 wt.% hydrogen within 10 min under 240 °C with the reduced dehydrogenation activation barrier. It also realized stable de/hydrogenation reactions for at least 50 cycles. DFT studies combined with kinetic analysis revealed that the catalyst‒hydrogen interaction could be systematically controlled by optimizing surface Tx defect density, accelerating the hydrogen dissociation and diffusion processes at the same time. These results demonstrate that the Tx defects serve as the effective catalytically active centers of Nb2CTx, offering a flexible catalyst design guideline.

Towards quantitative analysis of deuterium absorption in ferrite and austenite during electrochemical charging by comparing cyclic voltammetry and cryogenic transfer atom probe tomography

Hydrogen embrittlement mechanisms of steels have been studied for several decades. Understanding hydrogen diffusion behavior in steels is crucial towards both developing predictive models for hydrogen embrittlement and identifying mitigation strategies. However, because hydrogen has a low atomic mass, it is extremely challenging to detect by most analytical methods. In recent years, cryogenic-transfer atom probe tomography (APT) of electrochemically-deuterium-charged steels has provided invaluable qualitative analysis of nanoscale deuterium traps such as carbides, dislocations, grain boundaries and interfaces between ferrite and cementite. Independently, cyclic voltammetry (CV) has provided valuable analysis of bulk hydrogen diffusion in steels. In this work, we use a combination of CV and cryogenic-transfer APT for quantitative analysis of deuterium pickup in electrolytically charged pure Fe (ferrite) and a model austenitic Fe18Cr14Ni alloy without any second phase. The high solubility and low diffusivity of hydrogen in austenite versus ferrite and potential influence of oxide layer are highlighted to result in clear observable signatures in CV and cryogenic-transfer APT results. The remaining challenges and pathway for enabling quantitative analysis of hydrogen pick up in steels is also discussed.

Numerical Simulation of Hydrogen Diffusion in Cement Sheath of Wells Used for Underground Hydrogen Storage

The negative environmental impact of carbon emissions from fossil fuels has promoted hydrogen utilization and storage in underground structures. Hydrogen leakage from storage structures through wells is a major concern due to the small hydrogen molecules that diffuse fast in the porous well cement sheath. The second-order parabolic partial differential equation describing the hydrogen diffusion in well cement was solved numerically using the finite difference method (FDM). The numerical model was verified with an analytical solution for an ideal case where the matrix and fluid have invariant properties. Sensitivity analyses with the model revealed several possibilities. Based on simulation studies and underlying assumptions such as non-dissolvable hydrogen gas in water present in the cement pore spaces, constant hydrogen diffusion coefficient, cement properties such as porosity and saturation, etc., hydrogen should take about 7.5 days to fully penetrate a 35 cm cement sheath under expected well conditions. The relatively short duration for hydrogen breakthrough in the cement sheath is mainly due to the small molecule size and high hydrogen diffusivity. If the hydrogen reaches a vertical channel behind the casing, a hydrogen leak from the well is soon expected. Also, the simulation result reveals that hydrogen migration along the axial direction of the cement column from a storage reservoir to the top of a 50 m caprock is likely to occur in 500 years. Hydrogen diffusion into cement sheaths increases with increased cement porosity and diffusion coefficient and decreases with water saturation (and increases with hydrogen saturation). Hence, cement with a low water-to-cement ratio to reduce water content and low cement porosity is desirable for completing hydrogen storage wells.

Hardening effect of diffusible hydrogen on BCC Fe-based model alloys by in situ backside hydrogen charging

Hydrogen embrittlement is common in metallic materials and a critical issue in industries involving hydrogen-related processes. Here we investigate the mechanical response upon hydrogen loading of ferritic Fe-16Cr, Fe-21Cr and Fe-4Al alloys. We use a novel in situ setup for electrochemical backside hydrogen charging during nanoindentation. Single-phase ferritic Fe-Cr binary alloys with high hydrogen diffusivity and low solubility, are ideal for in situ studies during hydrogen charging, particularly the effect of diffusible and lightly trapped hydrogen is targeted. The hardness increases linearly with increasing hydrogen content until a quasi-equilibrium state between hydrogen absorption and desorption is reached while Young's modulus remains unaffected. Above this transient region, the slope of the absolute hardness experiences a drastic decrease. The hardness variation in Fe-21Cr is anisotropic as determined for (100), (110) and (111) oriented grains. Increasing the Cr content enhances the hardening effect in (100) orientation: a 16.7 % hardness increase is observed in Fe-21Cr, while Fe-16Cr, shows an increment of 10.8 %. A Fe-4Al alloy increases slightly in hardness by only 4.3 % at the applied current density of 3 mA/cm2. The hardening effect is caused by enhancing dislocation density, as revealed by studying the cross-section underneath the nanoindentation imprints.

The influence of gaseous hydrogen charging on the microstructural and mechanical behavior of electron beam melted and wrought Ti–6Al–4V alloys using the small punch test

The influence of gaseous hydrogen charging at 600 °C on the microstructure and mechanical behavior of wrought and Electron Beam Melted (EBM) Ti–6Al–4V alloys was investigated for hydrogen contents between 0.14 and 1.0 wt%. The small punch test (SPT) technique was used to characterize the mechanical behavior of all specimens. Both EBM and wrought alloys containing ∼6 wt% β and similar impurity levels showed similar phase content and mechanical property changes at all hydrogen contents, regardless of their original microstructural differences. This similarity can be explained by the high hydrogen diffusivity at the high temperature at which gaseous charging was carried out, and is in contrast to previous reports where EBM Ti–6Al–4V was found to be more sensitive to hydrogen embrittlement due to low-temperature electrochemical charging. After hydrogenation, αH and βH solid solutions were formed. The quantity of the αH phase reduced gradually with hydrogen content, while forming βH, α2, and hydrides. It was found that βH saturated at 0.27 wt% hydrogen content. Both alloys demonstrated relatively high strength and ductility up to hydrogen content of 0.2 wt%, i.e. below the βH saturation concentration. Above the βH saturation concentration, the mechanical properties of the maximum load (Pmax), deflection at maximum load (δmax), and absorbed energy (E), degraded significantly due to hydride formation.

Mechanism of tungsten strengthening hydrogen transportation in Nb48Ti27Co25 hydrogen permeable alloy membrane

Substitution of Nb by W in Nb48Ti27Co25 dual phase hydrogen permeable alloy was carried out. The elemental W replacing Nb mainly dissolved into the primary body-centered cubic (bcc)-(Nb) phase to modify its composition and crystal lattice. The W alloying effect on hydrogen transportation was systematically discussed based on hydrogen chemical potentials and first principle calculations. The substitution of Nb by W reduced the hydrogen solubility, and led to an improved intrinsic hydrogen diffusion coefficient, especially at temperature below 573 K, which promoted the hydrogen permeation flux of Nb48Ti27Co25 alloy membrane at 523 K. According to the theoretical study based on density function theory of the Nb–W system, the reduced hydrogen solubility can be attributed to the weakened binding strength between hydrogen atoms and the bcc-(Nb) lattice and the reduced charge gained by hydrogen atoms after W substitution for Nb. In addition, the W occupying the Nb sublattice can change the diffusion path of hydrogen atoms in the bcc-(Nb) lattice, thereby reducing the diffusion energy barrier for high hydrogen diffusivity.

Effective thermal conductivity of dimagnesium iron hexahydride (Mg2FeH6) for heat storage applications

The transient plane source method was applied to measure the effective thermal conductivity in dimagnesium iron hexahydride (Mg2FeH6) prepared in a high-pressure synthesis of 50 temperature-driven de-/hydrogenation cycles. Temperature- and pressure-dependent measurements of the effective thermal conductivity of the as-synthesized Mg2FeH6 powder have been performed. Measurements for as synthesized Mg2FeH6 were carried out between 2 and 100 bar in a temperature range from 50 °C to 300 °C and at 70 bar in a temperature range from 480 °C to 520 °C during the cycle test. The effective thermal conductivity of the as-synthesized Mg2FeH6 varied between 0.39 W m−1 K−1, recorded at 50 °C and 2 bar of hydrogen gas pressure, and 0.54 W m−1 K−1, measured at 300 °C and 100 bar hydrogen pressure. The effective thermal conductivity increased with elevated hydrogen gas pressure and temperature. An evidence was found that the presence of iron prevents the sintering of the powder, resulting in a constant effective thermal conductivity during all accomplished cycles. The advantage of a non-sintered material resulting in higher hydrogen diffusion, which leads to a faster reaction time. For 50 measured de-/hydrogenation cycles between 480 °C and 520 °C, the thermal conductivity was found to be constant at around ~ 1.0 W m−1 K−1 in the dehydrogenated state (70 bar/520 °C) and between 0.7 W m−1 K−1 and 0.8 W m−1 K−1 in the hydrogenated state (70 bar/480 °C).

Hydrogen Diffusion and Hydrogen Embrittlement Failure Behavior of AH36 Marine Steel Subjected to High Heat Input Welding

AH36 marine steel processed through high heat‐input welding is subjected to microstructural characterization, hydrogen permeation test, internal friction test, and slow strain rate tensile test to reveal its hydrogen diffusion and hydrogen embrittlement failure behaviors. Compared with the base metal, the coarse‐grained heat‐affected zone (CGHAZ) exhibits a significantly coarsened microstructure and decreased hydrogen trap density, resulting in a high hydrogen diffusion coefficient and short hydrogen permeation time. The presence of hydrogen yields a hydrogen‐induced Snoek internal friction peak and promotes the movement of the Snoek–Kê–Köster internal friction peak to the low‐temperature region. In addition, B and Kê peaks, which are not found in the base metal, appear in the CGHAZ. With increasing hydrogen content, the plastic loss and brittle fracture of the AH36 marine steel become increasingly significant, and the hydrogen embrittlement sensitivity of the heat‐affected zone increases compared with that of the base metal. The discontinuous yield phenomenon gradually disappears under slow strain rate tension owing to the weak pinning effect of the Cottrell atmosphere on mobile dislocations.

Reduction of iron oxide by hydrogen spillover over Pt/TiO2 and Pt/Al2O3 surfaces

A two-dimensional model is developed to predict the reduction of iron oxide via hydrogen spillover in Pt/TiO2 and Pt/Al2O3 catalytic systems. The reduction over TiO2 is predicted to be independent of the distance between Pt and iron oxide particles, and is explained by the high surface diffusivity of hydrogen on TiO2. In contrast, the characteristic time for diffusion of hydrogen on Al2O3 is similar to the time scales of the experiment and is strongly dependent on the inter-particle distance, which explains the lower and distance-dependent reduction over Al2O3. It is inferred that hydrogen reaches the TiO2/iron-oxide interface in a short time period, and the reduction rate is primarily governed by the reduction kinetics. The kinetics-governed reduction explanation is also valid for Al2O3, however at longer times when hydrogen has reached the Al2O3/iron-oxide interface. For early time periods, the surface diffusion of hydrogen governs its availability at the Al2O3/iron-oxide interface.

An Alternative Method for the Invariant Threshold Force Evaluation in Incremental Step Loading Tests

Abstract Hydrogen embrittlement (HE) affects the main groups of metal alloys used in the industry, varying from steels to nickel-based corrosion-resistant alloys. It is estimated that 25 % of failures that occur in the oil and gas industry is caused by fracture associated with hydrogen. The incremental step loading (ISL) technique, put forward by ASTM F1624-18(2018), Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique, has been widely used for HE susceptibility evaluation. However, testing metallic materials, other than steels with hardness lower than 33 HRC, following the ASTM F1624 method proved that the invariant threshold force (Pth) determination is not possible. It was observed that the Inconel 718 under certain experimental conditions did not reach the advised 5 % of the fast fracture strength, PFFS, force drop as this material has a low crack growth rate because of its high ductility and low hydrogen diffusivity. Although another force drop value could be adopted, it would still be an arbitrarily defined value that could not be applied to other materials without further study. To overcome this problem, a robust and efficient method of easy implementation was developed and validated. The new approach considers the relative variation, VΔF¯, associated with the difference between the input force (set point value) and the measured force as an alternative to a notch fracture strength force drop percentage. This method does not contradict the ASTM F1624 determinations as it too is based on the force drop values. The Pth values obtained for the AISI 4140 steel using the ASTM F1624 standard corroborate the values obtained based on the proposed method. This method allows the assessment of the Pth values in Inconel 718, as they were corroborated by crack nucleation observations at the notch root using a high-resolution scanning electron microscopy after the ISL tests. The obtained results show that the proposed method proved to be robust, efficient, and of easy implementation. Thus, it can be used as an alternative to the Pth evaluation of steels with a hardness less than 33 HRC and for metallic materials not covered by the ASTM F1624 standard.

Hydrogen Isotope Dissolution and Release Behavior of Rare Earth Oxides

Hydrogen release behavior from rare earth oxides (REOs) (Y2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Er2O3, and Yb2O3) exposed to 133 Pa of deuterium (D2) gas or 2 kPa of heavy water (D2O) vapor at 873 K for 5 h was examined using thermal desorption spectroscopy. Hydrogen solubility and diffusivity in Y2O3, Gd2O3, Dy2O3, Er2O3, and Yb2O3 exposed to a deuterium-tritium gas mixture (5% to 7% T, 133 Pa) at 873 K and 973 K for 5 h were determined using a tritium imaging plate method. The structural and morphological properties of sintered disk specimens of those REOs were evaluated using an X-ray diffractometer and a scanning electron microscope. From the obtained results, the REO materials were clearly categorized into two kinds in terms of their crystal structure and hydrogen solubility: Monoclinic specimens of Sm2O3, Eu2O3, and Gd2O3 had relatively high hydrogen solubility and diffusivity, while cubic Y2O3, Dy2O3, Er2O3, and Yb2O3 had lower ones. The present study suggests that the cubic REOs could be suitable in a nuclear fusion reactor as the tritium barrier materials.

NIR-Driven Water Splitting H2 Production Nanoplatform for H2-Mediated Cascade-Amplifying Synergetic Cancer Therapy.

As a newly emerging treatment strategy for many diseases, hydrogen therapy has attracted a lot of attention because of its excellent biosafety. However, the high diffusivity and low solubility of hydrogen make it difficult to accumulate in local lesions. Herein, we develop a H2 self-generation nanoplatform by in situ water splitting driven by near-infrared (NIR) laser. In this work, core-shell nanoparticles (CSNPs) of NaGdF4:Yb,Tm/g-C3N4/Cu3P (UCC) nanocomposites as core encapsulated with zeolitic imidazolate framework-8 (ZIF-8) modified with folic acid as shell are designed and synthesized. Due to the acid-responsive ZIF-8 shell, enhanced permeability and retention (EPR) effect, and folate receptor-mediated endocytosis, CSNPs are selectively captured by tumor cells. Upon 980 nm laser irradiation, CSNPs exhibit a high production capacity of H2 and active oxygen species (ROS), as well as an appropriate photothermal conversion temperature. Furthermore, rising temperature increases the Fenton reaction rate of Cu(I) with H2O2 and strengthens the curative effect of chemodynamic therapy (CDT). The excess glutathione (GSH) in tumor microenvironment (TME) can deplete positive holes produced in the valence band of g-C3N4 in the g-C3N4/Cu3P Z-scheme heterojunction. GSH also can reduce Cu(II) to Cu(I), ensuring a continuous Fenton reaction. Thus, a NIR-driven H2 production nanoplatform is constructed for H2-mediated cascade-amplifying multimodal synergetic therapy.

Improved Accuracy of Thermal Desorption Spectroscopy by Specimen Cooling during Measurement of Hydrogen Concentration in a High-Strength Steel.

Thermal desorption spectroscopy (TDS) is a powerful method for the measurement of hydrogen concentration in metallic materials. However, hydrogen loss from metallic samples during the preparation of the measurement poses a challenge to the accuracy of the results, especially in materials with high diffusivity of hydrogen, like ferritic and ferritic-martensitic steels. In the present paper, the effect of specimen cooling during the experimental procedure, as a tentative to reduce the loss of hydrogen during air-lock vacuum pumping for one high-strength steel of 1400 MPa, is evaluated. The results show, at room temperature, the presence of a continuous outward hydrogen flux accompanied with the redistribution of hydrogen within the measured steel during its exposure to the air-lock vacuum chamber under continuous pumping. Cooling of the steel samples to 213 K during pumping in the air-lock vacuum chamber before TDS measurement results in an increase in the measured total hydrogen concentration at about 14%. A significant reduction in hydrogen loss and redistribution within the steel sample improves the accuracy of hydrogen concentration measurement and trapping analysis in ferritic and martensitic steels.

Hydrides of Laves type Ti–Zr alloys with enhanced H storage capacity as advanced metal hydride battery anodes

The present work was focused on the studies of the effect of variation of stoichiometric composition of Ti–Zr based AB2±x Laves phase alloys by changing the ratio between A (Ti + Zr) and B (Mn + V + Fe + Ni) components belonging to both hypo-stoichiometric (AB1.90, AB1.95) and over-stoichiometric (AB2.08) alloys further to the stoichiometric AB2.0 composition to optimize their hydrogen storage behaviours and performances as the alloy anodes of nickel metal hydride batteries.AB2-xLa0.03 Laves type alloys (A = Ti0.15Zr0.85; B = Mn0.64–0.69V0.11–0.119Fe0.11–0.119Ni1.097–1.184; x = 0, 0.05 and 0.1) were arc melted and then homogenized by annealing.The studies involved probing of the phase-structural composition by X-Ray diffraction (XRD), together with studies of the microstructural state, hydrogen absorption–desorption and thermodynamic characteristics of gas–solid reactions and electrochemical charge-discharge performance, further to the impedance spectroscopy characterization. The alloys were probed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and XRD.These studies concluded that the alloys contained the main C15 FCC Laves type AB2 intermetallic co-existing with a secondary C14 hexagonal Laves phase and a small amount of LaNi intermetallic. The gaseous H storage capacity and electrochemical performances were found to be closely dependent on the stoichiometric compositions of the alloys. High discharge capacities approaching 500 mAh/g were achieved for the AB1.95 alloy. At 500 mA/g current density, the discharge capacity was maintained as high as 80%, with very low capacity retention after 500 cycles. The alloy which had the highest capacity retention after 500 cycles was found to be AB2.0 alloy. Furthermore, AB2.0 alloy showed an excellent cyclic stability together with a high hydrogen diffusion coefficient. Studies of hydrogen diffusion coefficients showed that annealing enhanced the H diffusion rates allowing advanced performance at high discharge current densities. This has been related to a higher content of C15 Laves compound which allows to reach a higher H mobility in contrast with higher storage capacity observed for the C14 Laves type polymorph.

The hydrogen embrittlement sensitivity of duplex stainless steel with different phase fractions evaluated by in-situ mechanical testing

The influence of the austenite (gamma) phase fraction on the hydrogen embrittlement of duplex stainless steel is investigated. Heat treatments are performed to create two duplex stainless steel specimens, containing 50% and 44% of austenite, respectively. Mechanical testing with and without hydrogen charging reveals that significant embrittlement occurs regardless of the austenite fraction. A higher austenite fraction results in a reduced ductility loss under the presence of hydrogen. Samples with a higher ferrite fraction are embrittled more due to their higher hydrogen diffusivity. In-situ tensile tests, interrupted at the ultimate tensile strength, show hydrogen-assisted cracks on the specimen surface both in austenite and ferrite and across the alpha/gamma interface.

Temperature and hydrogen diffusion length in hydrogenated amorphous silicon films on glass while scanning with a continuous wave laser at 532 nm wavelength

Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring the silicon melting point and with calculated data using a physical model as well as published work. Since D-H interdiffusion measures hydrogen diffusion length and temperature within the silicon films by a memory effect, the method is capable of determining both quantities precisely also in multilayer structures, as is demonstrated for films underneath metal contacts. Several applications are discussed. Employing literature data of laser-induced temperature rise, laser scanning is used to measure the H diffusion coefficient at T > 500 °C in a-Si:H. The model-based high temperature hydrogen diffusion parameters are confirmed with important implications for the understanding of hydrogen diffusion in the amorphous silicon material.

On the (mis)behavior of water in the mantle: Controls on nominally anhydrous mineral water content in mantle peridotites

In magmatic settings, water behaves as an incompatible species and should be depleted during melting and enriched during metasomatism. Previous studies have identified correlations between nominally anhydrous mineral (NAM) water content ([H2O]) and indices of metasomatism or melt extraction, seemingly confirming this behavior in the mantle. However in detail, these correlations are ambiguous and do not reflect robust controls on NAM [H2O]. We measured orthopyroxene (opx) and clinopyroxene (cpx) [H2O] in variably hydrated and metasomatized peridotite xenoliths from the Navajo volcanic field (NVF) that sample the Colorado Plateau subcontinental lithospheric mantle (SCLM), an endmember of SCLM hydration and metasomatism. These xenoliths span a wide range of pyroxene [H2O] (opx from 50 to 588 ppm wt. H2O; cpx from 38 to 581 ppm wt. H2O), but NAM [H2O] does not correlate with either indices of melt depletion or metasomatism.Growth of hydrous minerals suggests higher water activity than in anhydrous peridotites, and therefore hydrous-mineral-bearing xenoliths and anhydrous xenoliths should have different NAM [H2O] and water activities. However, when the two groups are compared no significant differences can be found in either NAM [H2O] or water activity. We propose that the high diffusivity of hydrogen in the mantle allows for equilibration of water activity in the mantle over sub-kilometer length scales over geologic time. Such diffusive equilibration reduces water activity variability and results in the blurring and destruction of correlations between NAM [H2O] and indices of metasomatism or melt extraction. As a result of diffusive equilibration of water, there is a large difference in the variability of concentration between NAM [H2O] (spanning ∼2 orders of magnitude) and similarly incompatible elements such as Ce in the same peridotites (spanning ∼4 orders of magnitude). This difference in behavior explains why H2O/Ce ratios in mantle peridotites are highly variable relative to those of basalts.