Diffusion Of H2O Research Articles

Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H2O and CO2 transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H2O and CO2 diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO2 hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H+-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO2-induced H+ secretion by individual ionocytes in embryos. H+ secretion by HR cells remarkably increased after a transient loading of CO2 (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H+ secretion of HR cells by about half and limited the CO2 stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO2-induced H+ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO2 transport.

Heat and mass transfer of temperature–vacuum swing desorption for CO2 capture from air

A transient heat and mass transfer model is developed to simulate the desorption step of a temperature–vacuum swing cyclic process for CO2 capture from ambient air. The model incorporates binary CO2 and H2O adsorption equilibrium data, heat of adsorption, heat conduction, gas phase diffusion, and considers CO2 adsorption from both dry and moist air. A set of desorption experimental runs was carried out using a packed bed of amine-functionalized nanofibrillated cellulose sorbent material in the temperature range 20–95°C and at 62mbarabs desorption pressure. Numerically calculated temperature profiles were compared to those obtained experimentally. The simulation results indicate fast gas diffusion and local re-adsorption of CO2 and H2O in colder zones of the reactor. The risk of O2-induced sorbent degradation is substantially reduced under humid air conditions as opposed to dry conditions, since air is purged out of the reactor by co-desorbed H2O before reaching a critical temperature. Further, the CO2 recovery rates at a given purity level are analyzed for the first time for a direct air capture system and it is found that, for typical process conditions, more than 90% of the captured CO2 can be recovered at >99% purity.

Electrochemical Characterization of Ni/Y2O3-ZrO2 Hydrogen Electrode for Water Splitting in Solid Oxide Electrolysis Cells

Ni-Y2O3 stabilized ZrO2 (Ni-YSZ) cermet is a widely used hydrogen electrode for solid oxide fuel cells (SOECs), but detailed studies regarding the water splitting processes are scarce. Here we report the preliminary results on the electrocatalytic activity of Ni-YSZ hydrogen electrode in the SOEC mode as a function of operating temperature and water concentration. The electrode activity for the water splitting reaction is strongly influenced by the water concentration and dc bias. The electrochemical performance for the water splitting reaction increases with the increase of water concentration. On the other hand, the electrode polarization resistance increases gradually with the increase of dc bias regardless of the H2O content, which is particularly pronounced in the low frequency impedance process related to the H2O dissociative adsorption and diffusion processes. The reaction mechanism and kinetics of the H2O dissociation reaction on Ni-YSZ hydrogen electrodes are discussed.

Influence of Ni-YSZ Electrode Thickness on the Behaviour of Commercial Solid Oxide Electrolysis Cells by Mean of Electrochemical Impedance Spectroscopy

Our world is facing energy needs always growing, the mid-term disappearance of important energy resources such as fossil fuels and global warming issues consecutive to massive greenhouse gas production. A possible answer to produce renewable and clean energy consists in combining promising technologies such as solar photovoltaic, wind turbine and fuel cell, thus making an energy mix. Hydrogen is an energy carrier which can be part of this energy mix. High temperature electrolysis (HTE) using a solid oxide electrolyte is a clean way to produce, from water and electricity, hydrogen. However, many studies showed that the electrochemical phenomena related to the Ni-YSZ electrode have a strong influence on cell performances, including the ones associated to gas transport [1-3]. Further investigations on the hydrogen side of the cell are hence needed. In the following work, two commercial cells with different Ni-YSZ thicknesses are considered. A deep analysis is performed in a two-electrode configuration by mean of electrochemical impedance spectroscopy (EIS) with the use of electrical equivalent circuits (EEC), showing that all the main phenomena governing SOEC functioning are related to the Ni-YSZ electrode for both cells, each one characterized by a specific relaxation frequency and capacitance. Indeed, the impedance diagrams measured were deconvoluted, from the highest to the lowest frequencies, into four arcs: a high frequency arc (HF arc), a first middle frequency arc (MF1 arc), a second middle frequency arc (MF2 arc) and a low frequency arc (LF arc). These arcs were respectively associated to charge transfer at the Ni-YSZ electrode, a first H2O diffusion phenomenon, gas conversion at the Ni-YSZ electrode and a second H2O diffusion phenomenon. Thus, changing Ni-YSZ electrode thickness appears to mainly influence the H2O diffusion phenomenon associated to MF1 arc and the H2O gas conversion related to MF2 arc, while phenomena associated to HF and LF arcs are poorly sensitive to this thickness change. This result suggests that the phenomena associated to MF1 and MF2 arcs occur mainly in the volume of Ni-YSZ electrode, contrary to phenomena related to HF and LF arcs. This in the same time allows distinguishing the two H2O diffusion phenomena identified. [1] A. Nechache, M. Cassir, A. Ringuedé, J. Power Sources 258 164 (2014). [2] P. Kim-Lohsoontorn, J. Bae, J. Power Sources, 196, 7161(2011). [3] PK. Patro PK, T. Delahaye T, E. Bouyer E, PK Sinha, Int. J. Hydrogen Energy, 37, 3865 (2012).

Electrochemical Characterization of Ni/Y2O3-ZrO2 Hydrogen Electrode for Water Splitting in Solid Oxide Electrolysis Cells

Ni/Y2O3 stabilized ZrO2 (Ni/YSZ) cermet is a widely used hydrogen electrode for solid oxide fuel cells (SOECs), but detailed studies regarding the water splitting processes are scarce. Here we report the preliminary results on the variation of electrocatalytic activity of Ni/YSZ hydrogen electrodes as a function of operating temperature, water concentration and dc bias. The electrode activity for the water splitting reaction is strongly influenced by varying the water concentration and dc bias. The electrochemical performance for the water splitting reaction decreases with the increase of water concentration. The electrode polarization resistance increases gradually with the increase of dc bias regardless of the H2O content. The decrease of activity is particularly pronounced in the low frequency impedance process related to the H2O dissociative adsorption and diffusion processes. This study clearly demonstrates the reaction mechanism and kinetics of the H2O dissociation reaction.

Thermogravimetric analysis of kinetic characteristics of K2CO3-impregnated mesoporous silicas in low-concentration CO2

The requirement for self-sustained and long-duration human operations in confined spaces including submarines, spacecrafts, or underground citadels has made ambient removal of low-concentration CO2 a critical technology. Mesoporous silica materials have been regarded as promising carriers to support active components for CO2 sorption. The CO2 sorption kinetic of mesoporous silica-supported adsorbent is an important parameter to be assessed. In this paper, K2CO3-impregnated mesoporous silicas were prepared by impregnating K2CO3 on MCM-41, SBA-15, and silica gel (SG) in ethanol solution, respectively. The CO2 sorption experiments were performed in a simulated confined space atmosphere of 1.0 % CO2, 2.0 % H2O, and 293–333 K using thermogravimetric analysis. The kinetic performances of the sorbents were evaluated by fitting the experimental data to the shrinking core model. K2CO3/SG exhibited the optimum carbonation kinetic performance. The apparent activation energies for chemical reaction-controlled region and internal diffusion-controlled region are 3.95 and 64.87 kJ mol−1, respectively. To obtain the specific carbonation kinetic mechanism, a double exponential model was used to simulate the carbonation process of K2CO3/SG. The apparent activation energies for H2O diffusion–hydration and CO2 diffusion–carbonation stages are 8.40 and 4.32 kJ mol−1, respectively. H2O diffusion–hydration is the rate limiting step in the whole carbonation process.

Degassing of H2O in a phonolitic melt: A closer look at decompression experiments

Melt degassing during magma ascent is controlled by the decompression rate and can be simulated in decompression experiments. H2O-bearing phonolitic melts were decompressed at a super-liquidus T of 1323K in an internally heated argon pressure vessel, applying continuous decompression (CD) as well as to date commonly used step-wise decompression (SD) techniques to investigate the effect of decompression method on melt degassing. The hydrous melts were decompressed from 200MPa at nominal decompression rates of 0.0028–1.7MPa·s−1. At final pressure (Pfinal), the samples were quenched rapidly at isobaric conditions with ~150K·s−1.The bubbles in the quenched samples are often deformed and dented. Flow textures in the glass indicate melt transport at high viscosity. We suggest that this observation is due to bubble shrinkage during quench. This general problem was mostly overlooked in the interpretation of experimentally degassed samples to date. Bubble shrinkage due to decreasing molar volume (Vm) of the exsolved H2O in the bubbles occurs during isobaric rapid quench until the melt is too viscous too relax. The decrease of Vm(H2O) during cooling at Pfinal of the experiments results in a decrease of the bubble volume by a shrinking factor Bs:Bs=VmH2OPfinalTrqVmH2OPfinalTfwhere Trq is the run temperature of the sample prior to isobaric rapid quench and Tf is the fictive temperature where the bubbles stop to shrink. In a first approximation, the glass transition temperature (Tg) of the melt can be used for Tf. At a Pfinal of 75MPa and a Tg of ~840K in our experiments, the bubble volume decreases by a Bs of ~2.5. Thus, bubble textures, size distributions and porosities derived from the vitrified samples do not correspond to the conditions prior to isobaric rapid quench. Nevertheless, the volume fraction of bubbles (Xb) in the melt can be calculated by:Xbmelt=Bs⋅Xbglass1+Bs⋅Xbglass−Xbglassusing the measured volume fraction of bubbles in the glass in consideration of Bs. The corrected porosities are within a relative error of <10% equal to the expected porosities at Trq that can be easily calculated from the residual H2O content in the glass.At nominal decompression rates >0.17MPa·s−1 and a Pfinal of 75MPa, the decompression method has only minor influence on melt degassing. SD and CD result in high bubble number densities of 104–105mm−3. Fast P drop leads to immediate supersaturation with H2O in the melt. At such high nominal decompression rates, the diffusional transport of H2O is limited and therefore bubble nucleation is the predominant degassing process. The residual H2O contents in the melts decompressed to 75MPa increase with nominal decompression rate. After homogeneous nucleation is triggered, CD rates ≤0.024MPa·s−1 facilitate continuous reduction of the supersaturation by H2O diffusion into previously nucleated bubbles. Bubble number densities of CD samples with low nominal decompression rates are several orders of magnitude lower than for SD experiments and the bubble diameters are larger. The reproducibility of MSD experiments with low nominal decompression rates is worse than for CD runs. Commonly used SD techniques are therefore not suitable to simulate melt degassing during continuous magma ascent with low ascent rates.

Hydration resistance and mechanism of regenerated MgO&amp;ndash;CaO bricks

Regenerated MgO–CaO refractory brick samples were prepared from spent MgO–CaO bricks and fused magnesia. Hydration resistance was investigated by hot water experiment, and its hydration reaction mechanism and hydration kinetic model were explored. The results showed a superior hydration resistance of regenerated samples. Ca(OH)2 was the only new generated phase after the hot water experiment, and it is indicated that the hydration of free CaO was the key point. Impurity elements Si, Fe, Al in the spent MgO–CaO bricks had enhanced the hydration resistance through decreasing free CaO content and improving the density of regenerated samples. Meanwhile, it is found that hydration reaction mainly occurred on the surface of regenerated samples, which was controlled by diffusion of H2O, and the diffusion rate constant K was 0.70 × 10−6 s−1.

Electron nanoprobe induced oxidation: a simulation of direct-write purification.

Electron beam direct-write has recently taken a large step forward with the advent of methods to purify deposits. This development has opened the door for future direct-write device prototyping and editing. In one such approach, an additional beam scanning procedure removes carbonaceous impurities via oxidation from metal-carbon deposits (e.g., PtC5) in the presence of H2O or O2 after deposition. So far, critical aspects of the oxidation reaction remain unclear; experiments reveal clearly that electron stimulated oxidation drives the process yet it is not understood why H2O purifies by a bottom-up mechanism while O2 purifies from the top-down. The simulation results presented here suggest that the chemisorption of dissolved O2 at buried Pt nanoparticle surfaces controls purification in the top-down case while both the high relative solubility coupled with weak physisorption of H2O explains the bottom-up process. Crucial too is the role that the carbonaceous contaminant itself has on the dissolution and diffusion of O2 and H2O. These results pave the way for simulation driven experiments where (1) the transient densification of the deposit can be accounted for in the initial deposit design stage and (2) the deposition and purification steps can be combined.

Chemical Stability toward O2 and H2O of Si(111) Grafted with —CH3, —CH2CH2CH3, —CHCHCH3, and —CCCH3

The chemical stability of compact monolayers on silicon toward oxidizing agents is a key issue for the use of such monolayers in devices such as solar cells or in the electronics industry. In this work, we investigated the reactivity toward H2O, O2, and OH species of monolayers terminated with a methyl group to unveil the mechanisms that prevent the oxidation of the underlying silicon. Density functional theory calculations were performed to investigate the reaction pathways for the two competing processes involved: diffusion through the monolayer and reaction with the terminal methyl group. Activation energy barriers for the diffusion of H2O and O2 are very sensitive to the monolayer structure, and they increase in the order —CH2—CH2—CH3 < —C≡C—CH3 < —CH═CH—CH3 with energy barriers of 0.0 kcal/mol (0.0 kcal/mol), 35.0 kcal/mol (42.5 kcal/mol), and 57.0 kcal/mol (64.1 kcal/mol), respectively, for H2O (O2). This agrees with ordering of stabilities reported experimentally for these monolayers. The oxidation of the terminal methyl group by O2 is less affected by steric constraints. The formation of the —CH2OOH species has an energy barrier of 56.5 kcal/mol on the rigid —CH3 monolayer, whereas this barrier decreases to 40.7 kcal/mol on the —C≡C—CH3 monolayer. In the case of the methyl monolayer, the abstraction of a H atom of the —CH3 group has smaller energy barriers with singlet O2 and OH reactants, with values of 38.4 and 3.5 kcal/mol, respectively. The high energy barriers of all of the processes investigated indicate that compact monolayers hinder the oxidation of the underlying substrate. The passivating capability of the monolayers correlates with the steric constraints for H2O and O2 diffusion.

Preferential transport effects in premixed bluff-body stabilized CH4/H2 flames

Recent studies of bluff-body stabilized premixed CH4/air flames, using line-imaged Raman/Rayleigh/LIF methods to measure temperature and major species, have revealed significant effects of preferential species transport on scalar structure across the flame and into the recirculation zone. The main observations were that the C/H atom ratio and the equivalence ratio increased going from reactants to products along a radial profile when the reactant flow velocity was sufficiently high to push the flame close to the boundary of the recirculation zone. This phenomenon was attributed to preferential diffusion of H2 and H2O toward the reactants followed by convective transport of the hydrogen-species enriched fluid downstream and beyond the recirculation zone, leaving elevated levels of CO2 and the C/H and C/O atom ratios within the recirculation zone.The present work continues the investigation of this preferential transport phenomenon by considering lean premixed methane flames, with varying levels of H2 addition, stabilized on the same bluff body burner as in the previous study. Results show similar effects of preferential transport as in the methane flames, with the measured equivalence ratio within the recirculation zone higher than in the reactant stream. However, with increasing levels of H2 addition in the fuel, H2 diffuses from reactants toward products rather than the other way around. For the highest level of H2 addition (40% mole fraction) the C/H atom ratio in the products decreases relative to that in the reactants, while the C/O atom ratio still increases. This indicates that there is still preferential transport of H2O into the rapidly flowing reactant stream, causing a net loss of oxygen from products in the recirculation zone. However, differential diffusion of H2 from the reactant stream into the flame more than compensates for this effect on the C/H atom balance.

In-situ Observation of Polymer Electrolyte Fuel Cell Using Deuterium Gas

In order to visualize water distribution in an operating fuel cell, we combined two different methods using neutron as a probe, i.e., a combined method of small-angle &amp; ultra-small-angle scattering (SANS) and radiography imaging. SANS observes water distribution in a membrane electrolyte assembly (MEA), whereas radiography observes bulk water appeared in a gas flow channel (so called "flooding"). The polymer electrolyte fuel cell (PEFC) was specially designed suitable for small-angle neutron scattering by replacing materials with aluminum in order to decrease background scattering. We employed hydrogen gas (H2) and deuterated gas (D2) as a fuel for operation. With exchange of H2 and D2, we aim to perform a contrast variation as for polyelectrolyte film (Nafion). When D gas is used as a fuel, D2O is produced at the cathode and diffuses back to the film. Then the film, originally swollen by H2O, exhibits change of coherent scattering contrast. By changing a fuel gas from H2 to D2, SANS quantitatively detected decrease of scattering intensity at scattering maximum originating from the ion-channel in the electrolyte. After quantitative analyses on scattering intensity, which is related to water ratio (H2O/D2O) in the ion channel, we found that 30 wt% of the total water is replaced by D2O by changing the gas from H2 to D2. In a stationary state of fuel cell operation using D2, the scattering intensity rhythmically oscillates (respiration of fuel cell). The rhythmic oscillation found for the peak intensity is a non-equilibrium and non-linear phenomenon, in which "flooding" in a flow field is a feedback mechanism to slow down chemical reaction or water production by affecting mass transportation of air at the cathode. A valance between two diffusions, (i) back diffusion of D2O from the cathode to the electrolyte and (ii) diffusion of H2O supplied as humidity, determines a time interval of the respiration.

Invited: Confinement Effects on the Structure and Transport of Nafion Ultrathin Films

Polymer electrolyte membrane (PEM)-based fuel cells are being aggressively targeted as an alternative energy source for transportation and stationary power due to their high charge density and low operating temperatures. While the structure, transport and mechanical properties of bulk PEMs have been extensively studied, there is increasing interest on the behavior of these materials at interfaces and under confinement, since there exist many heterogeneous interfaces within an actual fuel cell assembly. One could argue that the performance and ultimate failure of the membrane electrode assembly is greatly impacted by the structure and transport of ionomers confined to extremely small length-scales.Using a combination of neutron and x-ray reflectivity, grazing-incidence small-angle x-ray scattering (GISAXS), quartz crystal microbalance (QCM), polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), as well as measurements capable of measuring thin-film mechanics we have studied the water transport and mechanical behavior as a function of humidity and initial film thickness (< 220 nm). Our previous work has shown that when the polyelectrolyte is confined to thin films there is suppression in both swelling and water diffusivity in addition to considerable changes in the nano-scale structure. More recently, we have developed methods to measure the mechanical properties of these systems, as well as extended our PM-IRRAS method to attempt to answer questions about the roles interfacial mass transport and polymer chain relaxations. The mechanical methods reveal an effect of confinement on the measured stiffness of the material. Specifically, we observe an increase in the modulus with decreasing film thickness and, in the beginning of the hydration process, a visco-plastic response due to swelling. These results lend some explanation to the observed decrease in water uptake with decreasing film thickness. Moreover, we have extended our use of PM-IRRAS to include in-situ vapor switching between H2O and D2O. In this approach, we measure the time-resolved diffusion of H2O into the film (swelling), followed by the diffusive exchange of D2O for H2O (switch), and then then loss of H2O upon drying (deswelling). The experiments enable us to (1) eliminate contributions of swelling and relaxation to the measured transport of water and (2) measure the transport in a pseudo-steady state manner. This method shows promise in answering questions about the dominant mechanism for transport in confined thin films.Collectively, our program aims to provide a comprehensive understanding of the effect of confinement on the structure and transport of ionomer-based materials, and how these effects manifest themselves in the performance and mechanical response of such membranes.

Structural Arrangements of Isomorphic Substitutions in Smectites: Molecular Simulation of the Swelling Properties, Interlayer Structure, and Dynamics of Hydrated Cs–Montmorillonite Revisited with New Clay Models

Three new structural models of montmorillonite with differently distributed Al/Si and Mg/Al substitutions in the tetrahedral and octahedral clay layers are systematically developed and studied by means of MD simulations to quantify the possible effects of such substitutional disorder on the swelling behavior, the interlayer structure, and mobility of aqueous species. A very wide range of water content, from 0 to 700 mgwater/gclay is explored to derive the swelling properties of Cs–montmorillonite. The determined layer spacing does not differ much depending on the clay model. However, at low water contents up to 1-layer hydrate (∼100 mgwater/gclay) the variation of specific locations of the tetrahedral and octahedral substitutions in the two TOT clay layers slightly but noticeably affects the total hydration energy of the system. Using atom–atom radial distribution functions and the respective atomic coordination numbers we have identified for the three clay models not only the previously observed binding sites for Cs+ on the clay surface but also new ones that are correlated with the position of tetrahedral substitution in the structure. The mobility of Cs+ ions and H2O diffusion coefficients, as expected, gradually increase both with increasing water content and with increasing distance from the clay surface, but they still remain 2 to 4 times lower than the corresponding bulk values. Only small differences were observed between the three Cs–montmorillonite models, but these differences are predicted to increase in the case of higher charge density of the clay layers and/or interlayer cations.

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO2 sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO2 nanoparticles (TiO2 NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H2O in the process of peptization both the crystallinity and particle size of TiO2 NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO2 NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO2 NPs/Al showed the short-circuit current (Jsc) of 12.83mA/cm2 and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO2 buffer layer. The stability measurement showed that PSC devices with a TiO2 NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H2O diffusion into the active layers. The observations indicate that TiO2 NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.

Water-enhanced negative bias temperature instability in p-type low temperature polycrystalline silicon thin film transistors

Water-enhanced degradation of p-type low temperature polycrystalline silicon thin film transistors under negative bias temperature (NBT) condition is studied. H2O penetration into gate oxide network and the role of H2O during NBT stress are confirmed and clarified respectively. To prevent H2O diffusion, a combination of a layer of PECVD SiO2 and a layer of PECVD Si3N4 as passivation layers are investigated, revealing that 100nm SiO2 and 300nm Si3N4 can effectively block H2O diffusion and improve device NBT reliability.

Fundamental properties of a-SiNx:H thin films deposited by ICP-PECVD for MEMS applications

In this study, the impact of deposition conditions on the properties of amorphous hydrogenated silicon nitride (a-SiNx:H) films using an inductively coupled plasma enhanced chemical vapor deposition technique (ICP-CVD) is evaluated. Due to the large number of experiments – even when taking only the most important synthesization parameters into account such as the total pressure in the deposition chamber, the substrate temperature, the ICP power and the flow rate ratio of N2/SiH4 – a design of experiments-based approach is chosen. As expected, the deposition rate strongly depends on the ICP power and the N2/SiH4 flow rate ratio, respectively. Films in the field of investigation deposited with a high flow rate of N2 labeled as Type I show relatively low mechanical stress values between −50 and +200MPa, but exhibit a strong drift behavior toward compressive stress. Layers deposited at low nitrogen flow rates (Type II), however, yield large compressive stress and are stable as a function of time. The wet etch rate in hydrofluoric acid shows a gap of over two orders of magnitude when comparing the two a-SiNx:H types, indicating strong differences in the chemical composition. Fourier-transform infrared measurements demonstrate that in Type I films the hydrogen is mainly bonded to nitrogen, in contrast to Type II films, where Si–H bonds dominate. Surface related X-ray photoelectron spectroscopy measurements show that Type II layers have higher relative silicon content, while depth profiles yield that the oxygen content Type I films is above 10at.%. This high oxygen content is proposed to be the result of diffusion of H2O into the layer, causing oxidation, and, as a consequence, the drifting behavior of the intrinsic film stress.

Anisotropy of Water Dynamics in Clays: Insights from Molecular Simulations for Experimental QENS Analysis

We measure H2O dynamics in a well-defined synthetic hectorite clay by the neutron spin echo technique, in the temperature range from 240 to 347 K. The interlayer spaces of this anisotropic material contain two layers of confined water, corresponding to the so-called bihydrated state. We analyze the experimental data in light of parallel molecular dynamics simulations. Simulations demonstrate that H2O diffusion in the direction perpendicular to the clay layers is not negligible and has to be taken into account in the experimental data analysis. A diffusive model with only two fitting parameters D⊥ and D∥ is well adapted for such analysis. A clear physical meaning for the two fitting parameters exists, in view of the geometry of the system. Experimentally, the diffusion coefficients parallel to the clay layers D∥ were estimated to be slowed down by a factor of 5 compared to bulk water. Further, the activation energy of the diffusion process is higher than in bulk water especially toward the lower temperatur...

Two-fold -to-single-fold transition of the conductivity relaxation patterns of proton-conducting oxides upon hydration/dehydration

Hydration/dehydration kinetics of proton conducting oxides has long been believed to be due to the chemical diffusion of H2O (or ambipolar diffusion of H+ and O2 −), causing defect-structure-sensitive properties, e.g., electrical conductivity and mass, of the oxides to relax single-fold. Thus, a mass relaxation with a change of water activity used to be regarded as a measure of the equilibrium water solubility. It has recently been observed, however, that the conductivity and mass relax are not single-fold, but two-fold upon hydration or dehydration in usual oxidizing atmospheres, indicating that the above does not proceed via single-fold chemical diffusion of H2O, but via decoupled two-fold chemical diffusion of H (or ambipolar diffusion of H+ − h+) and O (or ambipolar diffusion of O2 − + 2 h+). In this paper, we will show theoretically and demonstrate experimentally that the two-fold kinetics asymptotically converges to the single-fold as the hole transference number diminishes to zero with decreasing oxygen activity in the ambient. Its implications are discussed particularly concerning the hydration equilibrium constant or equilibrium water solubility as determined from the mass relaxation.

Transport of moisture at epoxy–SiO2 interfaces investigated by molecular modeling

In this work, the transport of water molecules at the interface of an epoxy molding compound to a SiO2 (chip surface) is investigated by molecular modelling. Experi-mental results of bulk diffusion of H2O into a DGGOA/DAIIBA epoxy/hardener system at several temperatures are compared to molecular dynamics results at elevated (artificial) temperatures with respect to activation energies. Molecular modelling allows to trace individual molecules over time. The recorded traces are evaluated in a semi-quantitative way with respect to jump-distances and comparisons are made between bulk diffusion, diffusion at an interface and diffusion at open surfaces of the involved materials. Results show reasonably agreeing activation energies in experiment and simulation and an enhanced transport at open surfaces. A significantly enhanced transport at a perfect interface could not be confirmed in this investigation.