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Oxygen Diffusion (oxygen + diffusion)
Selected AbstractsOxygen Diffusion in Yttria-Stabilized Zirconia: A New Simulation ModelJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2004R. Krishnamurthy We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation. [source] Comparative study between two numerical methods for oxygen diffusion problemINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 8 2009Vildan GülkaçArticle first published online: 28 APR 200 Abstract Two approximate numerical solutions of the oxygen diffusion problem are defined using three time-level of Crank,Nicolson equation and Gauss,Seidel iteration for three time-level of implicit method. Oxygen diffusion in a sike cell with simultaneous absorption is an important problem and has a wide range of medical applications. The problem is mathematically formulated through two different stages. At the first stage, the stable case having no oxygen transition in the isolated cell is searched, whereas at the second stage the moving boundary problem of oxygen absorbed by the tissues in the cell is searched. The results obtained by three time-level of implicit method and Gauss,Seidel iteration for three time-level of implicit method and the results gave a good agreement with the previous methods (J. Inst. Appl. Math. 1972; 10:19,33; 1974; 13:385,398; 1978; 22:467,477). Copyright © 2008 John Wiley & Sons, Ltd. [source] Quantifying oxygen diffusion in paraffin oil used in oocyte and embryo cultureMOLECULAR REPRODUCTION & DEVELOPMENT, Issue 12 2009Yvonne M. StokesArticle first published online: 11 AUG 200 Oxygen diffusion through oil is important in the culture of oocytes and embryos. A diffusion coefficient two orders of magnitude smaller than that of oxygen in water has been thought possible, and this has led to concerns of anoxia in cultures. Using an assay for determining the oxygen consumption rate of embryos and oocytes, along with a mathematical model, it is here shown that the oxygen diffusion rate in paraffin oil at 37°C is about two-thirds of that in water at the same temperature. Although not previously recognised for the assay in question, the geometry is such that anoxia does occur for a period of time in excess of 1,hr and, by the completion of the assay, 30,40% of the medium is anoxic. Hence the quantity of oxygen consumed is less than would be consumed in conditions of plentiful oxygen supply. Nevertheless, using a model with a concentration dependent oxygen consumption rate, the oxygen consumption rate can be estimated. Mol. Reprod. Dev. 76: 1178,1187, 2009. © 2009 Wiley-Liss, Inc. [source] Oxygen diffusion in YSZ single crystals at relatively low temperaturesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Ralf Röwer Abstract The 18O tracer experiments were carried out on two cubic YSZ single crystals at temperatures of TD = 423 K and TD = 473 K. The diffusion profiles were determined by the SIMS technique. The diffusion coefficients, the surface exchange coefficients and the activation enthalpy are discussed together with earlier data at higher temperatures. The present oxygen diffusion data demonstrate that the same diffusion process is operating over a wide temperature range. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Gas sensing in microplates with optodes: Influence of oxygen exchange between sample, air, and plate materialBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2005Sarina Arain Abstract Microplates with integrated optical oxygen sensors are a new tool to study metabolic rates and enzyme activities. Precise measurements are possible only if oxygen exchange between the sample and the environment is known. In this study we quantify gas exchange in plastic microplates. Dissolved oxygen was detected using either an oxygen-sensitive film fixed at the bottom of each well or a needle-type sensor. The diffusion of oxygen into wells sealed with different foils, paraffin oil, and paraffin wax, respectively, was quantified. Although foil covers showed the lowest oxygen permeability, they include an inevitable gas phase between sample and sealing and are difficult to manage. The use of oil was found to be critical due to the extensive shaking caused by movement of the plates during measurements in microplate readers. Thus, paraffin wax was the choice material because it avoids convection of the sample and is easy to handle. Furthermore, without shaking, significant gradients in pO2 levels within a single well of a polystyrene microplate covered with paraffin oil were detected with the needle-type sensor. Higher pO2 levels were obtained near the surface of the sample as well as near the wall of the well. A significant diffusion of oxygen through the plastic plate material was found using plates based on polystyrene. Thus, the location of a sensor element within the well has an effect on the measured pO2 level. Using a sensor film fixed on the bottom of a well or using a dissolved pO2 -sensitive indicator results in pO2 offset and in apparently lower respiration rates or enzyme activities. Oxygen diffusion through a polystyrene microplate was simulated for measurements without convection,that is, for samples without oxygen diffusion through the cover and for unshaken measurements using permeable sealings. This mathematical model allows for calculation of the correct kinetic parameters. © 2005 Wiley Periodicals, Inc. [source] Anomalous Oxidation States in Multilayers for Fuel Cell ApplicationsADVANCED FUNCTIONAL MATERIALS, Issue 16 2010James M. Perkins Abstract Significant recent interest has been directed towards the relationship between interfaces and reports of enhanced ionic conductivity. To gain a greater understanding of the effects of hetero-interfaces on ionic conductivity, advanced analytical techniques including electron microscopy (TEM/STEM), electron energy loss spectroscopy (EELS), and secondary ion mass spectrometry (SIMS) are used to characterize CeO2/Ce0.85Sm0.15O2 multilayer thin films grown by pulsed laser deposition. High quality growth is observed, but ionic conductivity measured by impedance spectroscopy and 18O tracer experiments is consistent with bulk materials. EELS analysis reveals the unusual situation of layers containing only Ce(IV) adjacent to layers containing both Ce(III) and Ce(IV). Post oxygen annealing induced oxygen diffusion and mixed oxidation states in both layers, but only in the vicinity of low angle grain boundaries perpendicular to the layers. The implications of the anomalous behavior of the Ce oxidation states on the design of novel electrolytes for solid oxide fuel cells is discussed. [source] Numerical solution of the oxygen diffusion in absorbing tissue with a moving boundaryINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 9 2006Abdellatif BoureghdaArticle first published online: 9 FEB 200 Abstract A problem of oxygen diffusion in absorbing medium is complex. A mathematical model of this problem is presented, which has previously been investigated by Crank and Gupta (J. Inst. Math. Appl. 1972; 10: 19,33) is studied using a different method of solution. Approximate analytical and numerical solutions of its partial differential equations are obtained, which describe the diffusion of oxygen in absorbing tissue. A moving boundary is an essential feature of this problem but the conditions which determine its movements are different. The results are compared with those of Crank and Gupta. In most cases the agreement is fair. Copyright © 2006 John Wiley & Sons, Ltd. [source] Intelligent structure design of membrane cathode assembly for direct methanol fuel cellINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 12 2005K. Furukawa Abstract The performance and the structural model of membrane electrode assembly (MEA) have been developed and experimentally verified with fundamental calculations of the direct methanol fuel cell (DMFC). The model provides information concerning the influence of the operating and structural parameters. The composition and performance optimization of MEA structure in DMFC has been investigated by including both electrochemical reaction and mass transport process. In the experimentation, the effect of Nafion content and loading method in the catalyst layer of cathode for DMFC was investigated. For the spray method electrode (SME), the cell performance and cathode performance using a dynamic hydrogen electrode (DHE) as a reference electrode was improved in comparison with those of the PME electrode by decreasing cathode potential. From ac impedance measurements of the cathode, the adsorption resistance of the SME electrode was decreased compared with that of the PME electrode. The higher cell performance was mostly dependent on the adsorption resistance. In the modelling, the cathode overpotential was decreased with increasing ionomer content, due to increasing ionic conductivity for proton transfer and the larger reaction site. The resistance to oxygen transport was increased at the same time, and became dominant at higher ionomer loadings, leading to an increase in the voltage loss. The ratio of ionomer to void space in the cathode affected the cathode polarization, which had the lowest resistance of oxygen diffusion at the ratio of 0.1,0.2. Copyright © 2005 John Wiley & Sons, Ltd. [source] The potential role of plant oxygen and sulphide dynamics in die-off events of the tropical seagrass, Thalassia testudinumJOURNAL OF ECOLOGY, Issue 1 2005J. BORUM Summary 1Oxygen and sulphide dynamics were examined, using microelectrode techniques, in meristems and rhizomes of the seagrass Thalassia testudinum at three different sites in Florida Bay, and in the laboratory, to evaluate the potential role of internal oxygen variability and sulphide invasion in episodes of sudden die-off. The sites differed with respect to shoot density and sediment composition, with an active die-off occurring at only one of the sites. 2Meristematic oxygen content followed similar diel patterns at all sites with high oxygen content during the day and hyposaturation relative to the water column during the night. Minimum meristematic oxygen content was recorded around sunrise and varied among sites, with values close to zero at the die-off site. 3Gaseous sulphide was detected within the sediment at all sites but at different concentrations among sites and within the die-off site. Spontaneous invasion of sulphide into Thalassia rhizomes was recorded at low internal oxygen partial pressure during darkness at the die-off site. 4A laboratory experiment showed that the internal oxygen dynamics depended on light availability, and hence plant photosynthesis, and on the oxygen content of the water column controlling passive oxygen diffusion from water column to leaves and below-ground tissues in the dark. 5Sulphide invasion only occurred at low internal oxygen content, and the rate of invasion was highly dependent on the oxygen supply to roots and rhizomes. Sulphide was slowly depleted from the tissues when high oxygen partial pressures were re-established through leaf photosynthesis. Coexistence of sulphide and oxygen in the tissues and the slow rate of sulphide depletion suggest that sulphide reoxidation is not biologically mediated within the tissues of Thalassia. 6Our results support the hypothesis that internal oxygen stress, caused by low water column oxygen content or poor plant performance governed by other environmental factors, allows invasion of sulphide and that the internal plant oxygen and sulphide dynamics potentially are key factors in the episodes of sudden die-off in beds of Thalassia testudinum. Root anoxia followed by sulphide invasion may be a more general mechanism determining the growth and survival of other rooted plants in sulphate-rich aquatic environments. [source] How are nitrogen fixation rates regulated in legumes?JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2004Joachim Schulze Abstract Symbiotic nitrogen fixation by the legume- Rhizobium symbiosis is a finely regulated process that involves significant carbon and energy metabolism by the plant. At present, there are three competing theories as to how the regulation of the process is achieved at the whole plant level. Theory one (carbon supply regulation) states that nitrogen fixation rates are regulated by either current nodule assimilate supply or through metabolism of assimilates into compounds that can be used by the bacteroids. A second theory (oxygen supply regulation) assumes that oxygen diffusion into the nodules is tightly regulated and is the principal regulatory factor for nitrogen fixation rates. A third theory (N-feedback regulation) suggests that a product of nitrogen fixation or assimilation exerts a feedback regulatory impact. The paper summarizes experimental data which support or reject the mentioned theories. Moreover, implications of nitrogen fixation under P stress and of phenomena connected with the argon or acetylene induced decline in nitrogenase activity for the understanding of nitrogen fixation regulation are discussed. It is concluded that we currently have no theory that explains all aspects and experimental results concerning the regulation of nitrogen fixation in legumes. With our growing ability to impact the process, e.g., by genetic engineering, this understanding of nitrogen fixation regulation has potential to be translated into agronomically sustainable benefits. Wie werden die Stickstofffixierungsraten bei Leguminosen reguliert? Die symbiontische N2 -Fixierung bei Leguminosen ist ein fein regulierter Prozess, der einen massiven Substanzumsatz in der Pflanze beinhaltet. Gegenwärtig gibt es drei konkurrierende Theorien über die Art, wie die Regulation des Prozesses auf Ganzpflanzenbasis erreicht wird. Eine Theorie (Regulation über die Kohlenstoffversorgung) geht von einer Abhängigkeit bzw. Regulation der N2 -Fixierungsraten entweder durch die Assimilatversorgung der Knöllchen oder von einer Limitierung des nötigen metabolischen Umsatzes der Assimilate in für den Bakteroiden verwertbare Verbindungen aus. In der zweiten Theorie (Regulation über die Sauerstoffversorgung) wird angenommen, dass die Sauerstoffdiffusion in das Knöllchen einer strengen Regulation unterliegt und dies der prinzipielle Regulationsfaktor für die N2 -Fixierungsraten ist. Die dritte Theorie ("N-feedback"-Regulation) besagt, dass ein bisher nicht zweifelsfrei identifiziertes Produkt der Stickstoffixierung oder -assimilation eine "feedback"-Wirkung auf die N2 -Fixierung ausübt. Der vorliegende Artikel fasst die bis heute gesammelten experimentellen Daten zusammen, die für oder gegen die genannten Theorien sprechen. In diesem Zusammenhang wird diskutiert, auf welche Weise die Regulation der N2 -Fixierung unter P-Mangelbedingungen erfolgt. Schließlich wird diskutiert, ob die Klärung der Ursache der Azetylen- bzw. Argon-induzierten Abnahme der Nitrogenaseaktivität zum Verständnis der Regulation der N2 -Fixierung beitragen kann. Es wird festgestellt, dass zur Zeit mit keiner Theorie alle Aspekte der Regulation der N2 -Fixierung erklärt werden können. Mit unseren beispielsweise durch die Gentechnik wachsenden Möglichkeiten, die N2 -Fixierung zu beeinflussen, wird eine solche Theorie nötig sein, um diese Möglichkeiten in landwirtschaftlich nutzbare Anwendungen umzusetzen. [source] Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004Peter Fielitz Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72 wt% Al2O3, 28 wt% SiO2 and 78 wt% Al2O3, 22 wt% SiO2, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope 18O, and the resulting 18O isotope depth distributions were analyzed using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of oxygen diffusion were 363 ± 25 kJ/mol for the low-alumina and 548 ± 46 kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials. [source] Oxygen Diffusion in Yttria-Stabilized Zirconia: A New Simulation ModelJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2004R. Krishnamurthy We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation. [source] Oxidation Protection of MgO,C Refractories by Means of Al8B4C7JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2001Tianming Wang The effect of Al8B4C7 used as an antioxidant in MgO,C refractories and the behavior of Al8B4C7 in CO gas were investigated in the present study. Al8B4C7 was found to react with CO gas, to form Al2O3(s), B2O3(l), and C(s), at temperatures >1100°C. The Al2O3 reacts with MgO to form MgAl2O4 near the surface of the material. At the same time, B2O3(l) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B2O3 and MgO. The coexistence of the liquid and MgAl2O4 makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C. [source] Modeling the Effect of Oxygen on Photopolymerization KineticsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 2 2006Allison K. O'Brien Abstract Summary: A comprehensive one-dimensional photopolymerization model was utilized to investigate the effect of oxygen on the free-radical photopolymerization kinetics. The spatial profiling aspect of the model provided insight into the heterogeneity of the cure kinetics due to oxygen inhibition, specifically the variance in the concentration profile of monomer and oxygen. Double bond conversion was negligible for the top ten microns of the film due to continuous oxygen diffusion, and increased with increasing depth. Similarly, the oxygen concentration decreased with increasing depth due to the competition between oxygen diffusion time and the polymerization rate. The effect of initiation rate on the extent of oxygen inhibition was investigated for various oxygen concentrations. As the initiation rate increased, the polymerization rate increased, and eventually approached that of a sample in an inert environment. Similarly, as the oxygen concentration was decreased, the polymerization rate increased. The effect of varying the initiation rate on the cure profile in the oxygen-exposed film was also studied. It was found that the unpolymerized tacky layer decreased from 50 µm to 5 µm with a 3 order of magnitude increase in initiation rate. Using the pseudo steady state approximation, the relationship between polymerization rate and initiation rate was derived for films in an oxygen environment. A direct relationship between the polymerization and initiation rate was found for films in air. The polymerization model supported this derivation and found that as the oxygen concentration was decreased, the dependence on initiation rate, or alpha, decreased, reaching the accepted value of 0.5 for alpha in inert environments. Double bond conversion versus film depth and cure time. [source] Quantifying oxygen diffusion in paraffin oil used in oocyte and embryo cultureMOLECULAR REPRODUCTION & DEVELOPMENT, Issue 12 2009Yvonne M. StokesArticle first published online: 11 AUG 200 Oxygen diffusion through oil is important in the culture of oocytes and embryos. A diffusion coefficient two orders of magnitude smaller than that of oxygen in water has been thought possible, and this has led to concerns of anoxia in cultures. Using an assay for determining the oxygen consumption rate of embryos and oocytes, along with a mathematical model, it is here shown that the oxygen diffusion rate in paraffin oil at 37°C is about two-thirds of that in water at the same temperature. Although not previously recognised for the assay in question, the geometry is such that anoxia does occur for a period of time in excess of 1,hr and, by the completion of the assay, 30,40% of the medium is anoxic. Hence the quantity of oxygen consumed is less than would be consumed in conditions of plentiful oxygen supply. Nevertheless, using a model with a concentration dependent oxygen consumption rate, the oxygen consumption rate can be estimated. Mol. Reprod. Dev. 76: 1178,1187, 2009. © 2009 Wiley-Liss, Inc. [source] Enhanced oxygen diffusion in Czochralski silicon at 450,650 °CPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2008Can Cui Abstract New 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 & Co. KGaA, Weinheim) [source] Gas sensing in microplates with optodes: Influence of oxygen exchange between sample, air, and plate materialBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2005Sarina Arain Abstract Microplates with integrated optical oxygen sensors are a new tool to study metabolic rates and enzyme activities. Precise measurements are possible only if oxygen exchange between the sample and the environment is known. In this study we quantify gas exchange in plastic microplates. Dissolved oxygen was detected using either an oxygen-sensitive film fixed at the bottom of each well or a needle-type sensor. The diffusion of oxygen into wells sealed with different foils, paraffin oil, and paraffin wax, respectively, was quantified. Although foil covers showed the lowest oxygen permeability, they include an inevitable gas phase between sample and sealing and are difficult to manage. The use of oil was found to be critical due to the extensive shaking caused by movement of the plates during measurements in microplate readers. Thus, paraffin wax was the choice material because it avoids convection of the sample and is easy to handle. Furthermore, without shaking, significant gradients in pO2 levels within a single well of a polystyrene microplate covered with paraffin oil were detected with the needle-type sensor. Higher pO2 levels were obtained near the surface of the sample as well as near the wall of the well. A significant diffusion of oxygen through the plastic plate material was found using plates based on polystyrene. Thus, the location of a sensor element within the well has an effect on the measured pO2 level. Using a sensor film fixed on the bottom of a well or using a dissolved pO2 -sensitive indicator results in pO2 offset and in apparently lower respiration rates or enzyme activities. Oxygen diffusion through a polystyrene microplate was simulated for measurements without convection,that is, for samples without oxygen diffusion through the cover and for unshaken measurements using permeable sealings. This mathematical model allows for calculation of the correct kinetic parameters. © 2005 Wiley Periodicals, Inc. [source] In vitro antimicrobial effect of vitreous endotamponading substancesACTA OPHTHALMOLOGICA, Issue 2009JA MONTERO MORENO Purpose To demonstrate the in vitro bactericidal, bacteriostatic or inert role of endotamponading substances. Methods Clinical isolates of Staphylococcus epidermidis, Staphylococcus aureus and Clostridium sp were cultured on blood agar plates in sulphur hexafluoride (SF6)and perfluoropropane (C3F8)atmospheres and under silicone oil (SO) and liquid perfluorocarbon (LPFC). The same germs were cultured under aerobic and anaerobic atmospheres as controls. Results SO, SF6 and C3F8 did not significantly inhibit the growth of anaerobic bacteria but markedly reduced the growth of aerobic germs, as compared with aerobic conditions. The growth of the bacteria under SO and SF6 and C3F8 was very similar to that observed under anaerobic conditions. LPFC did not affect the growth of aerobic or anaerobic bacteria. Conclusion Endotamponading gases and liquids seem to improve the visual and anatomical outcome of infected eyes and may limit the proliferation of aerobic germs in vitro. It is not known to what extent the antimicrobial effect of these substances would appear in vivo since oxygen diffusion from the blood could allow the growth of aerobic germs. [source] | ||||||||||||||||||||||