Enhanced Oxygen Diffusion Research Articles

Modeling of oxygen permeation through a LSCF ion transport membrane

The oxyfuel combustion characteristics have been investigated numerically for stagnation flow conditions inside a membrane reactor. The effect of combustion in the permeate side on the oxygen permeation through a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF-6428) ionic ceramic membrane is presented. The membrane reactor has a simple symmetric design allowing the reduction of the number of coordinates to 2D without reducing the accuracy of the calculations. The results of the present model were validated through comparison with the available literature data. The membrane separates oxygen from oxygen containing stream (typically air). The oxygen transports to a downstream permeate side containing methane, with CO2 as an inert carrier gas. The membrane area is 8cm×8cm and both streams are at 800°C. The numerical simulations were performed using CFD software FLUENT 6.3 with the aid of Gambit 2.2 to construct the mesh. A source sink term has been added to the conservation equations through a series of user defined functions compiled and incorporated to fluent in order to account for the transfer of oxygen across the membrane surface. The simulations were carried out over a wide range of mass fluxes of feed side and permeate side considering the n-type flux equation transport mechanisms and oxidation reaction kinetics (real finite rate combustion of hydrocarbon versus no reaction or cold combustion). It was found that the oxygen permeation flux increases as the permeate side flux increases as a result of reduced partial pressure of O2. Also, it was found that the combustion in the permeate side has a great effect on the oxygen permeation flux. This is due to the increase in the temperature in the permeate side due to combustion which enhances oxygen diffusion and the reduction in the oxygen partial pressure in the permeation side due to the consumption of oxygen in the combustion process. In addition, increasing the partial pressure of oxygen in the air side has a great effect on oxygen permeation flux and overall combustion process. The membrane temperature in all simulations was found to be very close to the inlet flow temperature which should be controlled within the operating temperature of the membrane.

3D Differentiation of Neural Stem Cells in Macroporous Photopolymerizable Hydrogel Scaffolds

Neural stem/progenitor cells (NSPCs) are the stem cell of the adult central nervous system (CNS). These cells are able to differentiate into the major cell types found in the CNS (neurons, oligodendrocytes, astrocytes), thus NSPCs are the mechanism by which the adult CNS could potentially regenerate after injury or disorder. Microenviromental factors are critical for guiding NSPC differentiation and are thus important for neural tissue engineering. In this study, D-mannitol crystals were mixed with photocrosslinkable methacrylamide chitosan (MAC) as a porogen to enhance pore size during hydrogel formation. D-mannitol was admixed to MAC at 5, 10 and 20 wt% D-mannitol per total initial hydrogel weight. D-mannitol crystals were observed to dissolve and leave the scaffold within 1 hr. Quantification of resulting average pore sizes showed that D-mannitol addition resulted in larger average pore size (5 wt%, 4060±160 µm2, 10 wt%, 6330±1160 µm2, 20 wt%, 7600±1550 µm2) compared with controls (0 wt%, 3150±220 µm2). Oxygen diffusion studies demonstrated that larger average pore area resulted in enhanced oxygen diffusion through scaffolds. Finally, the differentiation responses of NSPCs to phenotypic differentiation conditions were studied for neurons, astrocytes and oligodendrocytes in hydrogels of varied porosity over 14 d. Quantification of total cell numbers at day 7 and 14, showed that cell numbers decreased with increased porosity and over the length of the culture. At day 14 immunohistochemistry quantification for primary cell types demonstrated significant differentiation to the desired cells types, and that total percentages of each cell type was greatest when scaffolds were more porous. These results suggest that larger pore sizes in MAC hydrogels effectively promote NSPC 3D differentiation.

Performance enhancement of strontium-doped lanthanum manganite cathode by developing a highly porous microstructure

A simple and promising template method was applied for the development of a highly porous microstructure, referred to as a macroporous structure (having pore size >50 nm), for a solid oxide fuel cell strontium-doped lanthanum manganite (LSM) cathode. Using poly(methyl methacrylate) beads of 2.9 μm size as the pore-forming templates, a porosity value of ca. 63 % was achieved. The electrochemical performance of the developed macroporous cathode was evaluated by measuring electrochemical impedance spectra of the symmetrical cells as well as power density of the full cells. The results indicated that due to enhanced oxygen diffusion and reaction kinetics, the macroporous structure was effective in reducing the interfacial polarization resistance of the LSM cathode and hence attaining a higher cell performance.

Enhanced oxygen diffusion in low barium-containing La0.2175Pr0.2175Ba0.145Sr0.4Fe0.8Co0.2O3−δ intermediate temperature solid oxide fuel cell cathodes

Isotopic tracer diffusion studies have been performed on the perovskite composition La0.2175Pr0.2175Ba0.145Sr0.4Fe0.8Co0.2O3−δ to obtain the diffusion and surface exchange coefficients for oxygen. This material has been identified as a highly active electrocatalytic cathode for intermediate temperature solid oxide fuel cells. The oxygen diffusion coefficients obtained in the 450–650°C temperature range are higher than the ones measured for most of the cathode materials reported in the literature and they agree with those calculated from electrochemical impedance spectroscopy measurements performed on symmetrical cells.

Structural Characterization and Catalytic Activity of Ce0.65Zr0.25RE0.1O2−δ Nanocrystalline Powders Synthesized by the Glycine-Nitrate Process

In this study, Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Gd, Eu, Sm, Nd, Pr, and La) solid solutions were successfully prepared by the glycine-nitrate process and tested for CO oxidation activity. The X-ray diffraction results confirmed the formation of complete Ce0.65Zr0.25RE0.1O2−δ solid solutions. The Raman spectroscopy measurements suggested the presence of oxygen vacancies due to defective structure formation and further evidenced the formation of solid solution. The high-resolution transmission electron microscopy observations showed the nanocrystalline nature of the solid solutions. From X-ray photoelectron spectroscopy analysis it was revealed that the cerium, terbium, and praseodymium are present in +3 and +4 oxidation states. The UV–vis diffuse reflectance spectroscopy indicated that the Pr3+ ions in the Ce0.65Zr0.25Pr0.1O2−δ system provoked a significant increase in the Ce3+ fraction on the surface. H2 temperature-programmed reduction measurements showed an enhanced surface reduction at much lower temperatures for Ce0.65Zr0.25Pr0.1O2−δ sample compared to others, indicating increased oxygen mobility in these samples, which enable the enhanced oxygen diffusion at lower temperatures. Significantly high CO oxidation activity is exhibited by Ce0.65Zr0.25Pr0.1O2 solid solution.

Simulations of REBaCo2O5.5 (RE[dbnd]Gd, La, Y) cathode materials through energy minimisation and molecular dynamics

The GdBaCo2O5+x oxide has been presented as a promising cathode material for solid oxide fuel cells. It presents very high oxygen exchange and diffusion coefficients, two characteristics of utmost importance for an efficient cathode material. Yet the understanding at atomic scale of these two properties is rather limited. Here, we performed calculations to understand the influence of rare-earth nature in REBaCo2O5.5 (REGd, La, Y) on material stability and oxygen diffusion properties. Through energy minimisation, we determined the most energetically favourable distribution of A-site cations and oxygen vacancies. We also investigated with Molecular Dynamics simulations the mechanisms of oxygen diffusion in A-site ordered REBaCo2O5.5. The results confirm that oxygen vacancies essentially lie in the RE-plane and that diffusion is mainly two-dimensional with oxygen moving in the (a,b) plane while diffusion along the c axis is strongly hindered. Between 1300 and 1900K, the activation energy for oxygen diffusion lies in the range 0.69–0.83eV depending on the RE cation nature, values in good agreement with the experimental ones. We show that, in the double perovskite structure, the replacement of Gd by a larger rare-earth ion enhances oxygen diffusion properties but also reduces the stability of the double perovskite structure.

Influence of oxygen transfer in Hf-based high-k dielectrics on flatband voltage shift

We investigated flatband voltage (Vfb) behavior for several Hf-based high-k dielectrics, including HfO2, Mg-, and La-incorporated HfO2, HfSiOx, and Mg-, La-, and N-incorporated HfSiOx, during the reduction (forming gas annealing: FGA) and oxidation annealing (ODA) processes. A negative Vfb shift appeared in all high-k dielectrics as the FGA temperature increased. In contrast, a positive Vfb shift was observed after the introduction of additional oxygen into the high-k layer during ODA. The oxygen diffusion coefficient (D) values of all samples were estimated using Fick's law. The results showed that the D value of the HfO2 dielectric was five times as large as that of the HfSiOx dielectric in ODA at 400°C. Furthermore, the Mg-, La-, and N- incorporated high-k dielectrics exhibited a larger D value compared with the pure high-k dielectrics. These results strongly suggest that the ionicity of high-k dielectrics, which we attribute to a large positive Vfb shift, enhances oxygen diffusion in the high-k layer.

Formation of silicon nanoclusters in buried ultra-thin oxide layers

The peculiarities of buried layer formation obtained by co-implantation of O2 ions with the energy of 130 keV and carbon ions within the energy range of 30-50 keV have been investigated. The corresponding ion doses for carbon and oxygen ions were equal to and , respectively. It has been observed that annealing at 1150 °C results in enhanced oxygen diffusion towards the region with a maximum carbon concentration. Analysis of X-ray diffraction patterns with a SIMS depth profiles inherent to annealed samples suggests formation of Si nanoclusters in the region with maximum concentrations of carbon and oxygen vacancies. The intensive luminescence has been observed with the maximum at 600 nm, which could be associated with silicon nano-inclusions in thin stoichiometric SiO 2 16 cm 10 2 − × 2 17 cm 10 8 . 1 − ×

Stimulated Creation of the SOI Structures with Si Nano-Clustersw by Low–Dose SIMOX Technology

The peculiarities of a buried layer formation obtained by a co-implantation of O2 ions with the energy of 130 keV and carbon ions within the energy range of 30-50 keV have been investigated. The corresponding ion doses for carbon and oxygen ions were equal to 2∙1016 cm-2 and 1.8∙1017 cm-2, respectively. It has been observed that annealing at 1150°C results in enhanced oxygen diffusion towards the region with a maximum carbon concentration. Analysis of x-ray diffraction patterns and TEM images confirm formation of Si nanoclusters in the SiO2 buried layer. The intensive luminescence with the maximum at 600 nm has been observed in the synthesized structures.

Mitochondrial biogenesis in cold-bodied fishes

Mitochondrial biogenesis is induced in response to cold temperature in many organisms. The effect is particularly pronounced in ectotherms such as fishes, where acclimation to cold temperature increases mitochondrial density. Some polar fishes also have exceptionally high densities of mitochondria. The net effect of increasing mitochondrial density is threefold. First, it increases the concentration of aerobic metabolic enzymes per gram of tissue, maintaining ATP production. Second, it elevates the density of mitochondrial membrane phospholipids, enhancing rates of intracellular oxygen diffusion. Third, it reduces the diffusion distance for oxygen and metabolites between capillaries and mitochondria. Although cold-induced mitochondrial biogenesis has been well documented in fishes, little is known about the molecular pathway governing it. In mammals, the co-transcriptional activator peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) is thought to coordinate the three components of mitochondrial biogenesis: the synthesis of mitochondrial proteins, the synthesis of phospholipids and the replication of mitochondrial DNA. Some components of the mitochondrial biogenic pathway are conserved between fishes and mammals, yet the pathway appears more versatile in fishes. In some tissues of cold-acclimated fishes, the synthesis of mitochondrial proteins increases in the absence of an increase in phospholipids, whereas in some polar fishes, densities of mitochondrial phospholipids increase in the absence of an increase in proteins. The ability of cold-bodied fishes to fine-tune the mitochondrial biogenic pathway may allow them to modify mitochondrial characteristics to meet the specific needs of the cell, whether it is to increase ATP production or enhance oxygen diffusion.

Oxygen tracer diffusion in magnesium-doped ZnO ceramics

ZnO samples added with MgO of 1.4–11.7 mol % were used for studying oxygen diffusion by the vapor–solid exchange method. The analyses of oxygen diffusion profiles and impurities were performed by secondary-ion mass spectrometry (SIMS). The results indicate that the increase of MgO concentration enhances oxygen diffusion. The difference between the bulk-diffusion coefficients of oxygen for ZnO samples added with MgO of 1.4 and 11.7 mol % was about two orders of magnitude. It was found that the grain boundary diffusion coefficients of oxygen in ZnO with 11.7 mol % of MgO were larger than those in other samples, by a factor of about 10. Above results indicate that MgO addition into ZnO increases the concentration of effective defects for that promote oxygen diffusion. As the effective defects, the possibilities of vacancy and interstitial of oxygen are discussed.

Efficacy and safety profile of the carotenoid trans sodium crocetinate administered to rabbits following multiple infarct ischemic strokes: A combination therapy study with tissue plasminogen activator

Trans sodium crocetinate (TSC) is a synthetic small-molecule antioxidant that has the ability to enhance oxygen diffusion to hypoxic tissue. Because TSC is a promising drug candidate to treat acute ischemic stroke (AIS), we tested the hypothesis that TSC may be neuroprotective following cerebral ischemia using a rabbit small clot embolic stroke model (RSCEM) using clinical rating scores as the endpoint. TSC or saline was administered IV following the injection of small blood clots into the brain vasculature. Behavior was measured 24 h following embolization in order to calculate the effective stroke dose (P 50) that produces neurological deficits in 50% of the rabbits. A treatment is considered beneficial if it significantly increases the P 50 compared to control. TSC (0.25 mg/kg) given 5 or 60 min following embolization significantly ( p < 0.05) increased P 50 values by 104% and 181%; but not when given 3 h post-embolization (48% increase, p > 0.05). tPA (3.3 mg/kg) produced a significant increase in P 50 when given 1, but not 3 h following embolization. In combination studies, when TSC was administered 1 h and tPA was given either 1 or 3 h following embolization, the group P 50 values were increased by 291% and 140%, respectively. In addition, TSC plus tPA administered 3 h following embolization significantly ( p < 0.05) increased the group P 50 value by 90%. There were no significant effects ( p > 0.05) of either TSC alone or TSC administered in combination with tPA on intracerebral hemorrhage incidence. This study suggests that TSC may be used for the treatment of AIS either alone or when administered before or concomitant with tPA to improve clinical rating scores with a therapeutic window for TSC therapy up to 3 h in rabbits. Moreover, it appears that TSC can be administered with tPA, since the combination did not result in any significant change in intracerebral hemorrhage incidence.

Characterization of corrosion of X65 pipeline steel under disbonded coating by scanning Kelvin probe

Corrosion of X65 pipeline steel under a disbonded coating was studied by scanning Kelvin probe measurements. Three types of specimen were designed and prepared to investigate the effects of immersion time, oxygen concentration and wet–dry cycle on Kelvin potential profile and thus corrosion behavior of the steel. Kelvin potential measured on “intact” area is shifted negatively with time, indicating an increasing water uptake under the “intact” coating. With the increase of the amount of solution, it is expected that the electrolyte concentration and electrochemical reaction rate change, resulting in a significant decrease of interfacial potential. Moreover, there is a more negative Kelvin potential on disbonded area than that on “intact” area. The negative shift of Kelvin potential is attributed to corrosion reaction of steel occurring under the disbonded coating. Due to the narrow geometry of coating disbondment, an oxygen concentration difference exists along the depth of the disbondment. The corrosion behavior under disbonded coating strongly depends on the oxygen partial pressure and local geometry. With continuous purging of nitrogen and removing of oxygen, Kelvin potential tends to be identical throughout the disbonded area. During wet–dry cycle, the thickness of solution layer trapped under disbonded coating decreases due to evaporation of water. With the decrease of solution layer thickness, the measured Kelvin potential decreases, indicating that the effect associated with the reduction of oxygen solubility in the concentrated solution during drying of electrolyte is favored over that related to the enhanced oxygen diffusion and reduction. There exists a critical thickness of solution layer, below which the oxygen solubility is sufficiently low to support the electrochemical corrosion reaction of steel.

Electrochemical methodology for determination of imidazolium ionic liquids (solids at room temperature) properties: influence of the temperature

A set of six imidazolium ionic liquids (1a–b, 2a–c, 3), that were solids at room temperature, were characterized by electrical impedance spectroscopy to obtain information about their polarization resistance (Rp), conductivity (σ) and charge transfer activation energy (Ea). These experiments were performed at different temperatures in a glass micro-cell, equipped with three platinum electrodes. The observed conductivities were due to charge transfer processes of molecular oxygen at the electrode surface and mass transfer processes within the IL matrix. Higher temperatures resulted for all ionic liquids in increased conductivities. X-Ray diffraction of the ionic liquids 2a–c suggested that a higher degree of supramolecular two-dimensional organization, higher density, is related to an easier oxygen-electrode approximation, lower Ea. Two distinct temperatures ranges were observed. The larger conductivity increases in the higher temperature range were explained by melting (ILs 1–2) and fluxional behavior/reorientation phenomena of the ionic liquids and are due to enhanced oxygen diffusion (IL 3). In general, the understanding of imidazolium ionic liquid electrochemical properties could facilitate the development of new applications.

The Role of Zr in the High-Temperature Oxidation of Fe&lt;sub&gt;3&lt;/sub&gt;Al

The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Enhanced oxygen diffusion in Czochralski silicon at 450–650 °C

AbstractNew evidence for the enhanced oxygen diffusion in Czochralski silicon at temperatures ranging from 450 °C to 650 °C has been obtained from oxygen precipitation in prolonged annealing of 750 °C following a variety of pre‐treatments involving slow ramping anneal or single‐step anneal. The two previously proposed fast‐diffusing species of oxygen–vacancy and oxygen–silicon‐interstitial to understand the enhanced oxygen diffusion at low temperatures are questioned. Moreover, the reason as why significant oxygen precipitation can occur at such a low temperature of 750 °C has been elucidated based on the enhanced oxygen diffusion at low temperatures. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Role of ethylene in acclimations to promote oxygen transport in roots of plants in waterlogged soils

Ethylene plays a central role in morphological and anatomical acclimations of plants to soil waterlogging. To acclimate to waterlogging, roots of wetland species, and even some crops, form aerenchyma. Aerenchyma is large interconnected gas spaces that connect from shoot to near the tips of roots. Oxygen moves via diffusion in the root aerenchyma. In many wetland species, a barrier to radial oxygen loss (ROL) that greatly reduces oxygen leakage from basal parts, further enhances oxygen diffusion to the apex. Two main types of aerenchyma are recognized: schizogenous and lysigenous aerenchymas. Schizogenous aerenchyma is formed by cell separation without cell death. Lysigenous aerenchyma results from programmed cell death (PCD). Ethylene accumulation in plants during waterlogging induces lysigenous aerenchyma, although many wetland species form some aerenchyma even in drained conditions. The ethylene accumulation may result in the activation of G-proteins and other signal transduction cascades, of which increased cytosolic Ca 2+ is a component. Finally, PCD leading to selective cell degradation occurs in the root cortex, with cell wall degrading enzymes, such as cellulase and pectinase, also being activated. On the other hand, ethylene does not appear to induce the barrier to ROL, at least in roots of rice. Thus, induction of the barrier to ROL and formation of aerenchyma appear to be regulated by different signals. Further elucidation of signal transduction pathways initiated by ethylene sensing, as well as non-ethylene dependant responses, and of the genes regulated to control formation of the barrier to ROL and aerenchyma, should be priority areas for future research on plant tolerance of waterlogging.

First Principles Calculation of the Mechanism of Oxygen Precipitation in Czochralski Silicon Crystals (Effects of Heavy Boron Doping)

The mechanism of enhancement of the nucleation and growth of oxide precipitates (SiO2) in heavily boron (B)-doped Czochralski (CZ) silicon (Si) crystals was analyzed by first principles calculation. At the initial stage of oxygen precipitation including a small number n of interstitial oxygen (O) atoms with and without one substitutional B atom, i.e., B–On complex and On complex, reduction of the total energy of stable B–On complex formation was greater than that of stable On complex formation from isolated B and O atoms. Other calculations showed that the O atom in B-doped Si diffused as O charge state with a diffusion barrier of about 2.0 eV, which was lower than the barrier of about 2.5 eV for the O charge state in intrinsic Si. This reduction of the diffusion barrier should be the mechanism responsible for the enhanced oxygen diffusion and enhanced precipitate growth in heavily B-doped CZ Si crystals.

Molecular H2O as carrier for oxygen diffusion in hydrous silicate melts

Dissolved water is known to dramatically enhance oxygen diffusion in silicate melts, glasses and minerals. A quantitative theory has been developed to explain this phenomenon by transport via molecular H2O diffusion [Y. Zhang, E.M. Stolper, G.J. Wasserburg, Diffusion of a multi-species component and its role in the diffusion of water and oxygen in silicates, Earth Planet. Sci. Lett., 103 (1991) 228–240.]. Here we report experimental confirmation of the theory for rhyolitic melts by measuring both H2O and 18O diffusion profiles in a single experiment. In sorption experiments at 100 MPa and temperatures from 1041 to 1136 K isotopically enriched water diffused into doubly polished rhyolitic glass wafers. H2O profiles were analyzed by infrared spectroscopy and 18O profiles by SIMS. 18O diffusivities were found to be 1–2 orders of magnitude slower than bulk water diffusivities but 3–4 orders of magnitude faster than Eyring diffusivities calculated from viscosity. The data show that oxygen “self” diffusion under hydrothermal conditions is due to molecular H2O diffusion, not due to the self diffusion of oxygen itself. With this confirmation, experimental data on H2O diffusion in silicate melts can be used to infer 18O diffusion under hydrothermal conditions, and hydrothermal oxygen diffusion data in silicate minerals can be used to infer H2O diffusivity, as long as the concentration or solubility of H2O in the given phase is known.

Enhanced oxygen diffusion in highly doped p-type Czochralski silicon

The locking of dislocations by oxygen has been investigated experimentally in Czochralski silicon (Cz-Si) with different concentrations of shallow dopants. Specimens containing well-defined arrays of dislocation half-loops were subjected to isothermal anneals in the 350–550°C temperature range, and the stress required to bring about dislocation motion at 550°C was then measured. This dislocation unlocking stress was found to increase with annealing time due to oxygen diffusion to the dislocation core. The dislocation unlocking stress was measured in n-type Cz-Si with a high antimony doping level (∼3.4×1018cm−3) and p-type Cz-Si with a low boron doping level (∼1.3×1015cm−3). An analysis of the data taking the different oxygen concentrations into account showed that the rate of increase in dislocation unlocking stress was unaffected by the high level of antimony doping. This indicates that a high antimony doping level has no significant effect on oxygen transport for the conditions used in this experiment. However, in p-type Cz-Si with a high boron doping level (∼5.4×1018cm−3), the dislocation unlocking stress was found to rise at a much faster rate than in Cz-Si with a low boron doping level or high antimony doping level. This enhancement in dislocation locking was by a factor of approximately 60 at 400°C. By performing a numerical simulation to solve the diffusion equation for oxygen transport to a dislocation, the effective diffusivity of oxygen was deduced from the dislocation unlocking data to be 2.7×10−6exp(−1.4eV∕kT)cm2s−1 in the highly boron doped Cz-Si. In the temperature range studied, the effective diffusion coefficient in the highly boron doped Cz-Si was found to be approximately 44 times higher than expected in low boron doped Cz-Si with an identical oxygen concentration.