Oxygen Diffusion Model Research Articles

Correlation between diffraction patterns and surface morphology to the model of oxygen diffusion into ITO films

Indium tin oxide (ITO) films post-annealed in air usually have higher resistivity compared to the ITO films post-annealed in vacuum. This is likely due to the incorporation of oxygen atoms into the ITO films during post-annealing treatment in air. In this paper, we studied mainly the electrical properties of ITO films as a function of post-annealing temperature and post-annealing ambient. Our results show that the ITO films post-annealed in vacuum have lower resistivity compared to the ITO films post-annealed in air. From the results, we relate the model of oxygen diffusion with the AFM images and the (4 0 0)/(2 2 2) XRD peak ratio. We observe that the ITO films post-annealed in vacuum have larger grain size through the AFM images. The reduction of grain boundary scattering leads to higher conductivity. Furthermore, the ITO films post-annealed in vacuum have rather constant (4 0 0)/(2 2 2) XRD peak ratio. This result indicates that there is less oxygen atoms diffuse into the ITO films in vacuum. The ITO film post-annealed at 500 °C in vacuum has a grain size of ∼125 nm and resistivity of ∼3.49 × 10 −4 Ω cm.

Electrical Characteristics of Metal/High-k MOS Devices Using Effective Oxygen Control for Sub-1nm EOT

In this study, we have studied the mechanism of equivalent oxide thickness (EOT) increase after annealing using oxygen diffusion model from W gate electrode. It has been revealed that proper thickness W insertion into metal/high-k interface could depress EOT increase after post metallization annealing (PMA) and achieve small CV hysteresis.

A Transient Diffusion Model of the Cornea for the Assessment of Oxygen Diffusivity and Consumption

Oxygen diffusivity and consumption in the human cornea have not been directly measured yet; current models rely on properties measured in vitro in rabbit corneas. The aim of this study was to present a mathematical model of time-dependent oxygen diffusion that permits the estimation of corneal consumption and diffusivity. The current oxygen diffusion model was extended to include the temporal domain and was used to simulate in vivo noninvasive measurements of tear oxygen tension in human corneas. The new model reproduced experimental data successfully, provided values for corneal diffusivity and consumption, and described the relationship between oxygen consumption and oxygen tension in the cornea. Estimated values were three times higher than those reported previously in in vitro rabbit experiments. This model allowed for the further investigation of oxygen transport in the cornea, including a better mathematical description and a determination of the transport properties of the cornea and the specific oxygen uptake rate of the tissue. The combination of this model and tear oxygen tension measurements can be useful in determining the individual oxygen uptake rate and exploring the relationship between oxygen transport and corneal abnormalities.

PEM Fuel Cells Modeling and Analysis Through Current and Voltage Transient Behaviors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In the last 15 years, polymer electrolyte membrane (PEM) fuel cells have received much attention, mostly through experimental and empirical studies in scientific and industrial research. In most of the works, attention has been given to the steady state analysis of the PEM fuel cells. However, considerable efforts still need to be done to explain different transient behaviors of PEM fuel cells. This paper presents an analysis of the double layer charging effect and reactant diffusion through the cathode gas diffusion layer on voltage transients after sudden current variations. These transient phenomena have typical time durations of less than 5 s. The double layer charging dynamic explains the main voltage transient behaviors when the cathode inlet pressure is constant. In this case, a bicriteria optimization procedure is proposed for numerical characterization of the double-layer charging capacitance. When the air pressure is variable, a pseudo 2-D modeling of oxygen diffusion through the cathode gas diffusion layers, based on the Stephan–Maxwell multicomponent diffusion equations, is used to explain its contribution to the voltage transient overshoots/undershoots. </para>

2D Model for Diffusion of Oxygen with Biochemical Reaction During Biofilm Formation Process in Static Aqueous Medium

2D Model for Diffusion of Oxygen with Biochemical Reaction During Biofilm Formation Process in Static Aqueous Medium

Oxygen Diffusion through Natural Extracellular Matrices: Implications for Estimating “Critical Thickness” Values in Tendon Tissue Engineering

Oxygen is necessary for maintaining cell proliferation and viability and extracellular matrix (ECM) production in 3-dimensional tissue engineering. Typically, diffusion is the primary mode for oxygen transport in vitro; thus, ensuring an adequate oxygen supply is essential. In this study, we determined the oxygen diffusion coefficients of 3 natural ECMs that are being investigated as construct scaffolds for tendon tissue engineering: small-intestine submucosa (SIS), human dermis (Alloderm), and canine fascia lata. Diffusion coefficients were determined using a standard diffusion cell system. The ranges for each matrix type were: SIS: 7 x 10(-6) - 2 x 10(-5) cm2/s, Alloderm: 1.9 - 3.1 x 10(-5) cm2/s, and canine fascia lata: 1.6 - 4 x 10(-5) cm2/s. We used the experimental oxygen diffusivity data for these natural ECMs in a mathematical model of oxygen diffusion through a cell-seeded scaffold to estimate the critical size of cell-seeded scaffold that can be cultured in vitro.

An in silico bioreactor for simulating laboratory experiments in tissue engineering

This paper presents a software framework for the computational modeling of tissue engineering experiments, aimed to supplement and extend the empirical techniques currently employed in tissue engineering. The code included a model of cell population dynamics coupled to a finite element model of oxygen diffusion and consumption at the macroscale level, including the scaffold and the culture medium, and at the level of the scaffold microarchitecture. Cells were modeled as discrete entities moving in a continuum space, under the action of adhesion and repulsion forces. Oxygen distribution was calculated with the transient diffusion equation; oxygen consumption by cells was modeled by using the Michaelis-Menten equation. Other phenomena that can be formulated as a differential problem could be added in a straightforward manner to the code, due to the use of a general purpose finite element library. Two scaffold geometries were considered: a fiber scaffold and a scaffold with interconnected spherical pores. Cells were predicted to form clusters and adhere to the scaffold walls. Although the code demonstrated the ability to provide a robust performance, a calibration of the parameters employed in the model, based on specific laboratory experiments, is now required to verify the reliability of the results.

Measurements of the Effective Diffusion Coefficient of Oxygen in Pancreatic Islets

The effective diffusion coefficient of oxygen in pancreatic islets is an important parameter that is needed for the development of models of viability and function of pancreatic islets that are transplanted to cure diabetes, especially when encapsulated in devices. We have developed a method for estimating the maximum oxygen consumption rate (Vmax) and the oxygen permeability in tissue (αtDt) in a suspension of spherical aggregates of different sizes from measurements of the bulk oxygen partial pressure in batch oxygen consumption experiments. The method made use of a model of oxygen consumption and diffusion in the spheres and a parameter estimation scheme. Measurements made with three different batches of rat islets yielded batch averages of Vmax = 3.40 ± 0.77 × 10-8 mol/(cm3 s) and αtDt = 1.34 ± 0.36 × 10-14 mol/(cm mm Hg s). Using a literature estimate for the solubility (αt) yields Dt = 1.31 ± 0.36 × 10-5 cm2/s at 37 °C. These oxygen permeability and diffusion coefficient values are ∼38% and ∼47% of their values in water, respectively.

Density of capillaries and the oxygen diffusion model in the porous silk fibroin film

In order to obtain porous silk fibroin films (PSFFs) fit for the repair of different tissues and organs and design the configuration of the PSFFs more rationally, a model of the oxygen diffusing system of the capillary was built, and also the equations of the model were solved. Moreover, the relationships between the distribution of the oxygen concentration and each affecting factors were discussed, a method was developed to estimate the density of the capillaries in the tissue, and hereby discussed the characteristics of the oxygen diffusion in the tissues around the open capillaries.

Evidence for an Oxygen Diffusion Model for the Electric Pulse Induced Resistance Change Effect in Transition-Metal Oxides

Electric-pulse induced resistance hysteresis switching loops for Pr0.7Ca0.3MnO3 perovskite oxide films were found to exhibit an additional sharp "shuttle tail" peak around the negative pulse maximum for films deposited in an oxygen-deficient ambient. The resistance relaxation in time of this "shuttle tail" peak as well as resistance relaxation in the transition regions of the resistance hysteresis loop show evidence of oxygen diffusion under electric pulsing, and support a proposed oxygen diffusion model with oxygen vacancy pileup at the metal electrode interface region as the active process for the nonvolatile resistance switching effect in transition-metal oxides.

Electrospun dye-doped polymer nanofibers emitting in the near infrared

The authors report on the fabrication and characterization of near infrared fluorescent nanofibers. The nanofibers are composed by an organic dye dispersed in a poly(methylmethacrylate) inert matrix and realized by electrospinning. They exhibit diameters down to 70nm, with average values in the range of 170–480nm, depending on the process parameters, and photoluminescence emission peaked at 865nm. The temporal behavior of the emission under ultraviolet excitation in air can be described by an oxygen diffusion model with a characteristic time τ in the range of 400–1200s, depending on the fiber size, which correspond to a photostability longer than (0.4–1.2)×105 excitation laser pulses. These results open the way for large volume and cost-effective realization of infrared-emitting nanofibers, which are promising candidates as nanoscale infrared light sources.

Retinal Oxygenation and Oxygen Metabolism in Abyssinian Cats with a Hereditary Retinal Degeneration

To investigate the effects of a hereditary retinal degeneration on retinal oxygenation and determine whether it is responsible for the severe attenuation of retinal circulation in hereditary photoreceptor degenerations. Seven adult Abyssinian cats affected by hereditary retinal degeneration were studied. Oxygen microelectrodes were used to collect spatial profiles of retinal oxygenation in anesthetized animals. A one-dimensional model of oxygen diffusion was fitted to the data to quantify photoreceptor oxygen utilization (Qo(2)). Photoreceptor Qo(2) progressively decreased until it reached zero in the end stage of the disease. Average inner retinal oxygen tension remained within normal limits at all disease stages, despite the observed progressive retinal vessel attenuation. Light affected photoreceptors normally, decreasing Qo(2) by approximately 50% at all stages of the disease. Loss of photoreceptor metabolism allows choroidal oxygen to reach the inner retina, attenuating the retinal circulation in this animal model of retinitis pigmentosa (RP) and probably also in human RP. As the degeneration progresses, there is a strong relationship between changes in the a-wave of the ERG and changes in rod oxidative metabolism, indicating that these two functional measures change together.

Improved oxygen diffusion model to explain the effect of low-temperature baking on high field losses in niobium superconducting cavities

Radio-frequency (rf) superconducting cavities made of high purity niobium are widely used to accelerate charged particle beams in particle accelerators. The major limitation to achieve rf field values approaching the theoretical limit for niobium is represented by “anomalous” losses which degrade the quality factor of the cavities starting at peak surface magnetic fields of about 100mT, in the absence of field emission. These high field losses are often referred to as Q drop. It has been observed that the Q drop is drastically reduced by baking the cavities at 120°C for about 48h under ultrahigh vacuum. An improved oxygen diffusion model for the niobium-oxide system is proposed to explain the benefit of the low-temperature baking on the Q drop in niobium superconducting rf cavities. The model shows that baking at 120°C for 48h allows oxygen to diffuse away from the surface, and therefore increasing the lower critical field towards the value for pure niobium.

Review of the frontier workshop and Q-slope results

Over the last few years, significant progress has been made to produce field emission free niobium surfaces. Nowadays, the major limitation towards achieving the critical field in radio-frequency (rf) superconducting cavities made of bulk niobium of high purity is represented by the so-called “high field Q-slope” or “ Q-drop”. This phenomenon is characterized by a sharp decrease of the cavity quality factor, in absence of field emission, starting at a peak surface magnetic field of the order of 100 mT. It has been observed that these losses are usually reduced by a low-temperature “in situ” baking, typically at 100–120 °C for 24–48 h. Several models have been proposed to explain the high field Q-slope and many experiments have been conducted in different laboratories to validate such models. A three-day workshop was held in Argonne in September 2004 to present and discuss experimental and theoretical results on the present limitations of superconducting rf cavities. In this paper, we will focus on the high field Q-slope by reviewing the results presented at the workshop along with other experimental data. In order to explain the Q-drop and the baking effect we will discuss an improved version of the oxygen diffusion model.

Analysis of aerotactic band formation by Desulfovibrio desulfuricans in a stopped-flow diffusion chamber

Aerotactic band formation by Desulfovibrio desulfuricans (DSM 9104) was studied in a stopped-flow diffusion chamber. This chamber allowed us to create reproducible, steep oxygen gradients in a flat capillary, time-lapse video recordings and spatio-temporal analysis of band formation. The cells formed two types of bands. Bands of the first type evolved quickly after starting the experiment and were located near the oxic-anoxic interface. Bands of the second type typically appeared several minutes later and a few millimeters inside the initially anoxic volume of the capillary. Band formation depended on metabolism and could be stimulated by lactate addition, and thus appears to be energy taxis. Mathematical modeling of oxygen diffusion and respiration within the chamber revealed that bands formed preferentially at oxygen concentrations close to 4% air saturation. The swimming speed of the cells was determined by digital single-cell tracking and found to be highest (up to 58 mum s(-1)) close to the oxic-anoxic interfaces. Motility patterns were influenced by surfaces, at which cells accumulated. Bioconvection sometimes occurred if very dense bands had formed. The ecological implications of these two phenomena are unknown.

Defects and oxygen diffusion in PuO 2− x

A thermochemical model of defects formation is proposed. Using this model, defect species concentrations are determined. The model allows for the calculation of the non-stoichiometry of PuO 2− x , as functions of temperature and oxygen pressure. A model of oxygen diffusion is proposed and the oxygen self and chemical diffusivities are calculated for temperatures in the (900–1400 °C) range and partial pressure of oxygen P O 2 ∈ (1–10 −25 atm). This work shows that, given appropriate parameters, the model is able to describe the non-stoichiometry and oxygen diffusivity in a reasonable way.

Corneal oxygenation during contact lens wear: comparison of diffusion and EOP‐based flux models

Purpose: The aim of this study was to compare corneal oxygen flux values derived from an oxygen diffusion model, with estimates from a model in which equivalent oxygen percentage (EOP) values were substituted for the post‐lens tear film oxygen tension in Fick's law.Methods: A previously described five‐layer corneal oxygen diffusion model was found to artefactually allow theoretical oxygen consumption, when the predicted oxygen tension fell to zero. Consequently, an eight‐layer diffusion model was constructed, with consumption set to zero at points within the cornea, where predicted oxygen tension falls to zero. Post‐lens tear layer thickness was corrected to more contemporary estimates. The eight‐layer and EOP‐based anterior corneal oxygen flux estimates were compared across the range of commonly encountered contact lens Dk/t values.Results: The eight‐layer model overcomes deficiencies in the five‐layer model and provides predicted values that are remarkably similar to the EOP‐based model. Open and closed eye anterior corneal oxygen flux in the absence of contact lens wear was estimated at 7.8 and 7.6 μL/cm2/hr for the open eye and 6.0 and 6.1 μL/cm2/hr for closed eye for the diffusion and EOP‐based models, respectively.Conclusions: The diffusion model supports the EOP model in that there is minimal oxygen benefit to be gained by increasing Dk/t above the Holden‐Mertz criteria of 24 and 87 times 10−9 (cm/sec) (ml02/ml.mmHg) during open and closed eye wear, respectively. The eight‐layer model is suitable for further definition of corneal oxygenation during contact lens wear.

Multiscale modeling of oxygen diffusion through the oxide during silicon oxidation

We investigate at the atomic scale the oxygen diffusion process occurring during silicon oxidation. First, we address the energetics of several oxygen species in the oxide using density-functional calculations. Our results support the interstitial ${\mathrm{O}}_{2}$ molecule as the most stable oxygen species. We then adopt a classical scheme for describing the energetical and topological properties of the percolative diffusion of the ${\mathrm{O}}_{2}$ molecule through the interstitial network of the oxide. By studying a large set of disordered oxide structures, we derive distributions of energy minima, transition barriers, and the number of connections between nearest-neighbor minima. These distributions are then mapped onto a lattice model to study the long-range ${\mathrm{O}}_{2}$ diffusion process by Monte-Carlo simulations. The resulting activation energy for diffusion is found to be in agreement with experimental values. We also extend our atomic-scale approach to an oxide of higher density, finding a significant decrease of the diffusivity. To address the ${\mathrm{O}}_{2}$ diffusion directly at the $\mathrm{Si}\ensuremath{-}\mathrm{Si}{\mathrm{O}}_{2}$ interface, we construct a lattice model of the interface which incorporates the appropriate energetic and connectivity properties in a statistical way. In particular, this lattice model shows a thin oxide layer of higher density at the interface, in accord with x-ray reflectivity data. We carry out Monte-Carlo simulations of the ${\mathrm{O}}_{2}$ diffusion for this model and obtain the dependence of the diffusion rate on oxide thickness. For oxide thicknesses down to about $2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, we find that the presence of an oxide layer of higher density at the $\mathrm{Si}\ensuremath{-}\mathrm{Si}{\mathrm{O}}_{2}$ interface causes a drop of the ${\mathrm{O}}_{2}$ diffusion rate with respect to its value in bulk $\mathrm{Si}{\mathrm{O}}_{2}$, in qualitative agreement with the observed oxidation kinetics.

A finite difference model of O2 transport in aortic valve cusps: importance of intrinsic microcirculation.

Recent studies have reported the presence of a microcirculation within the tissue of aortic valves. To test the hypothesis that this vascular bed is needed to satisfy the oxygen demands of the cusp tissue, a two-dimensional (2D) finite difference model of oxygen diffusion was developed. The in vivo environment was modeled for vascular and avascular cusps using thickness data from precise radiographic measurements of fresh porcine valves, and O2 diffusivity (DO2) and O2 consumption (VO2) values from experimental data. The location and density of the cusp vasculature were determined by the model to prevent oxygen levels from falling to zero. Validation of the model was performed by simulation of the experimental measurements of cusp DO2 and VO2. For a test cusp with uniform thickness, the model returned simulated DO2 and VO2 measurements within 1.43% and 0.18% difference of the true parameter values, respectively. For native cusps, the simulated DO2 measurements were sensitive to thickness variations (-38 to +21% difference), whereas the VO2 measurements were minimally affected (8% difference). An improved DO2 measurement technique was found to reduce these errors to <5% and is recommended for analysis of experimental data. In the avascular case, the model predicted large regions of hypoxic tissue, whereas in the vascular case, the model predicted vessel locations and densities similar to what was experimentally observed in porcine cusps. Overall, the in vivo model developed in this study confirmed the need for an intrinsic microcirculation in the thicker basal regions of aortic cusps.

Effects of the platelet structures on the melt textured growth YBCO superconductors

Melt textured growth YBCO superconductors were fabricated by the top seeding method using Sm1.8 (Sm/sub 1.8/Ba/sub 2.4/Cu/sub 3.4/O/sub 7-x/) seed. The relationship between the Y211 particles and the platelet structures was investigated by micro-structural analysis using SEM. The microstructures of melt textured YBCO superconductors have been examined by TEM and HRTEM. The results of TEM studies clarified the direction of crystal growth and a variety of micro-defects such as twin structures and stacking faults that might behave as pinning centers. Our studies were focused on the stacking faults among those micro-defects by HRTEM and formation mechanism of the stacking faults was studied. The stacking faults formed during the tetragonal to orthorhombic transition that occurred at 450/spl deg/C in oxygen annealing. The platelet structures were clearly observed by SEM due to the chemical etching effects. The lengths of stacking faults were increased as the oxygen annealing time increased from 1 hr to 50 hr. The stacking faults were considered to relate to the oxygen contents, as the platelet structures were. The results suggested an oxygen diffusion model for the formation of the stacking faults.