Gas Diffusion Process Research Articles

Ionospheric Disturbances Produced by Artificially Chemical Releases

摘要: 考虑中性气体在电离层高度的扩散过程和相应的电离层离子化学过程,研究了利用主动化学物质释放来改变电离层的方法,理论计算了H2O和SF6两种气体释放后电离层随时间的响应过程.结果表明,在电离层高度上气体的扩散过程非常迅速,电离层F区的电子密度有很大程度的减少,而扩散慢且化学反应快的气体对电离层的影响更大,就更加有利于电离层洞的形成. 关键词: 化学释放 / 电离层扰动 / 扩散过程

Electro-responsive Polymer and its Sensor Applications

ABSTRACTEmfit Ltd is manufacturer of electro-responsive ferroelectret sensors. These film-like sensors can be used for various applications, for example non-body-contact vitals signs monitoring, dementia care, actigraphs, visitor count and keypads. The sensor material is based on a polyolefin material manufactured in a continuous biaxial orientation process that stretches the film in two perpendicular directions (machine direction and the transverse direction). The biaxially stretched material is further expanded to over double of its original thickness in high-pressure gas-diffusion process. The structure of Emfit material consists of flat voids separated by thin polyolefin layers. The voids are made by compounding small particles that function as rupture nuclei, and form closed lens like cavities to the film during the biaxial orientation. A permanent electric charge is injected into the material using a corona charging process. The voided internal structure makes the film soft and elastic in the thickness direction. This means that the voids expand and contract in relation to the orientation of the film. The excellent sensor and actuator operations of Emfit film are based on the fact that reduction in the thickness of the film caused by an external force takes place in the gas voids and these are much softer than the polymer layers.

Gas-Diffusion Process in a Tubular Cathode Substrate of an SOFC: I Theoretical Analysis of Gas-Diffusion Process under Cylindrical Coordinate System

In this the first part of a two-part paper, the gas-diffusion process through a thick and porous tubular cathode substrate of a solid oxide fuel cell-(SOFC) was theoretically analyzed using classic Fick’s diffusion equation under the cylindrical coordinate system. The effects of current density, temperature, oxygen diffusivity or porosity, wall thickness, and bulk on the concentration (or pore in this paper) polarization were calculated and are presented graphically. The results clearly show a greater impact on pore polarization by current density, oxygen diffusivity, wall thickness, and bulk but not by temperature. In addition, the limiting current density, which is a characteristic of a material, was also derived based on the solved cylindrical-coordinated diffusion equation. © 2004 The Electrochemical Society. All rights reserved.

Gas-Diffusion Process in a Tubular Cathode Substrate of a SOFC: II: Identification of Gas-Diffusion Process Using AC Impedance Method

The effects of cathodic dc bias, bulk and effective -diffusivity on ac impedance spectra of Siemens Westinghouse Power Corporation’s cathode-supported solid oxide fuel cells were systematically studied over a temperature range of 800 to 1000°C. It was found that the activation process dominated the overall electrode kinetics at 800°C, by which the applied dc bias reduced the electrode resistance considerably. With increasing the temperature to above 900°C, the activation process became effectively activated, leading to a visible arc at the lowest frequency on the impedance spectrum, which is relevant to the pore gas-diffusion process. Under this circumstance, lower bulk and lower effective diffusivity were shown in ac impedance spectra to increase pore gas-diffusion polarization as predicted by the gas-diffusion theory shown in part I of this paper. DC bias was also found to greatly affect the gas-diffusion process as a result of increased dc current. © 2004 The Electrochemical Society. All rights reserved.

Ru-catalyzed anode materials for direct hydrocarbon SOFCs

A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO 2, which avoided interference from steam and CO 2 in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm −2 with dry methane, which was comparable to the peak power density of 769 mW cm −2 with wet (2.9 vol.% H 2O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion process.

Influence of Heat Treatment on Phases, Microstructures and Mechanical Properties of Laminated Fabric (SiC and Aluminosilicate)/Mullite Matrix Composites

Oxidation resistance of SiC satin fabric (25-29vol%) or aluminosilicate plain fabric (25-33vol%)/mullite matrix porous composites was investigated at 1000-1200°C in air for 24h using thermogravimetric analysis. The composites were produced by a polymer impregnation and pyrolysis (PIP) method using a mullite precursor solution of the mixtures of Si(OC2H5)4 and Al(NO3)3. The oxidation rate of the sintered SiC fibers with a stoichiometric SiC composition and well-crystallized microstructure was controlled by the diffusion process of gases through dense SiO2 layer formed on the SiC surface. The activation energy for the oxidation of the SiC fibers was 299kJ/mol. This value was close to the activation energy for the diffusion of oxygen atom in silica glass. The SiC satin fabric/mullite matrix composite heated in an Ar atmosphere showed pseudoductility due to the delamination along the fabric layers. The four point-flexural strength decreased slightly with increasing heat treatment temperature. However, the composite heated in air at 1100-1200°C showed brittle fracture behavior without delamination because of the increased strength of interfacial bond between the oxidized SiC fibers and mullite matrix. On the other hand, the amorphous aluminosilicate fibers/mullite matrix composite showed little weight change during the heat treatment at 1000-1200°C. Crystallization of mullite occurred in the mullite precursor-derived amorphous solid and aluminosilicate fibers at 1100 and 1200°C in air, respectively. However, no microstructural change was observed in the heat-treated composite. The significant pseudoductility and strength of the aluminosilicate plain fabric/mullite matrix composite were maintained after the heat treatment. The fracture energy increased with increasing heat treatment temperature.

Energy Dispersive X-ray Reflectometry of the NO2 Interaction with Ruthenium Phthalocyanine Films

The X-ray reflectrometry technique in its energy dispersive variant (EDXR) is utilized to study the morphological characteristics of dimeric ruthenium phthalocyanine (RuPc)2 thin films, which are being exploited as nitrogen oxide gas sensors. The advantages of the EDXR with respect to its conventional (angular dispersive) counterpart are discussed. The high sensitivity of this technique allowed us to detect minimal variations of the films thickness and roughness as a consequence of the exposure to a NO2 gas flux, providing information on the gas diffusion process through the film and on the mechanisms of interaction between film and gas molecules.

Cluster persistence of two-dimensional soap froth.

We report on the persistence decay of cell clusters in dynamical simulation of two-dimensional soap-froth as a function of the cluster volume fraction. Both slow gas diffusion and fast dynamical topological processes between cells are incorporated in the simulation. While the simulation yields decay rate in good agreement with two-dimensional soap-froth experiment, it shows a faster decay in comparison to measurements and calculations of persistence in other curvature-driven systems. The fraction of individual cells that contain persistent area also decays due to significant cell movements, confirming that the neighbor swapping T1 topological transition is important in froth coarsening.

An Intermediate-Temperature Solid Oxide Fuel Cell Providing Higher Performance with Hydrocarbons than with Hydrogen

The promotion of direct electrochemical oxidation of hydrocarbons in a solid oxide fuel cell was investigated using a ceria-based electrolyte with different noble metals-containing anode at 600°C. The objective was to avoid interference from a large amount of steam and being produced by discharging the cell, because these gases degrade the anode performance, especially at a high fuel utilization. Ru was an effective catalyst for removing these gases from the anode surface due to its high catalytic activity for the steam and reforming of hydrocarbons. The resulting peak power densities reached 750 mW cm−2 with dry methane, which was comparable to the peak power density of 769 mW cm−2 with wet hydrogen. The cell performance was maintained at a high level regardless of the change in methane utilization from 12 to 46% but was significantly reduced by increasing hydrogen utilization from 13 to 42%. The anodic reaction of hydrogen was controlled by the slow surface diffusion of hydrogen, while the anodic reaction of methane was not subject to the onset of such a gas-diffusion process. © 2002 The Electrochemical Society. All rights reserved.

Molecular flow and wall collision age distributions

The use of storage cells has become a standard technique for internal gas targets in conjunction with high energy storage rings. In case of spin-polarized hydrogen and deuterium gas targets the interaction of the injected atoms with the walls of the storage cell can lead to depolarization and recombination. Thus the number of wall collisions of the atoms in the target gas is important for modeling the processes of spin relaxation and recombination. It is shown in this article that the diffusion process of rarefied gases in long tubes or storage cells can be described with the help of the one-dimensional diffusion equation. Mathematical methods are presented that allow one to calculate collision age distributions (CAD) and their moments analytically. These methods provide a better understanding of the different aspects of diffusion than Monte Carlo calculations. Additionally it is shown that measurements of the atomic density or polarization of a gas sample taken from the center of the tube allow one to determine the possible range of the corresponding density weighted average values along the tube. The calculations are applied to the storage cell geometry of the HERMES internal polarized hydrogen and deuterium gas target.

OVERALL SEPARATION FACTORS FOR STABLE ISOTOPES BY GAS CENTRIFUGE

The demand for stable isotopes is stimulating theoretical and experimental research on separation of stable isotopes by gas centrifuge. Most of the stable elements in nature have three or more isotopes. For modern gas centrifuges the separation factors usually are not as close to unity as is the case for the gaseous diffusion process. The overall separation factor for the unit molar weight difference, γ0, is an important parameter for describing the separation performance of a gas centrifuge, and it depends on many variables of the gas centrifuge and physical properties of process gases. In this paper examples of the influence of some parameters on the overall separation factor will be shown.

Development of a Mathematical Model for MAP Systems Applied to Nonrespiring Foods

ABSTRACT A mathematical model for gas diffusion processes in MAP systems was built. The model was applied to MAP systems containing CO2, O2, and N2 with nonrespiring foods. The validation study was done with gelatin, and simulation trials were carried out utilizing Hake (Merluccius australis) fillets. Experimental results confirmed the proposed mathematical model and its numerical solution. The prediction errors obtained in the validation study were under 5%; the same was true with the simulation trials whose errors were always lower than 5%. Both the simulation trials and validation study demonstrated that MAP systems reach equilibrium after a short period of time if film permeability is low.

Semi-volatile organic aerosols - assessment of gaseous and intraparticle diffusion processes

Semi-volatile organic aerosols - assessment of gaseous and intraparticle diffusion processes

Diffusion in Porous Solids: The Parallel Contribution of Gas and Surface Diffusion Processes in Pores Extending from the Mesoporous Region into the Microporous Region

When a species diffusing in a porous solid is adsorbed to some extent, the ratio of adsorbed molecules to gas molecules in the pores may be very high if there is a large surface area associated with the pores. In addition to transport of molecules through the pores by gaseous diffusion, transport by surface diffusion may then become important. Good examples have been reported in the literature. However, there are many cases in which the mode of transport of an adsorbing species is limited to gaseous diffusion. Although the overall process is simpler when gaseous diffusion alone must be considered, the experimental diffusion coefficient determined directly by classical nonequilibrium methods is still not a true diffusion coefficient. Because of the complication of the “adsorption capacity factor”, the experimental value is lower than the true value, and may be as much as orders of magnitude lower. For mesoporous solids this has been recognized by various investigators for some time. For certain microporous...

NIR spectroscopy of porous glass

Near infrared (NIR) spectroscopy has been used for the investigation of porous glass (PG), kinetics of the molecules, adsorption in it, particularly organic substances such as carbon tetrachloride, ethanol, acetone and p-xylene. NIR spectroscopy is quite a sensitive method because it permits to detect small part of monomolecular layer that was adsorbed on the plane surface. PG is a good adsorptive material owing to the existence of quasi-free OH-groups. It was found that adsorption of gas molecules on the surface of PG changes its optical spectrum in two ways: (i) it shifts OH-spectral lines (the value of spectral displacement Δ ω depends upon adsorptive energy); (ii) changes of spectrum are due to mechanical and electrooptical inharmonic features of molecule–OH bond that leads to the appearance of combination frequencies. Calculations were carried out for gas diffusion process in PG taking into account adsorption of molecules: relaxation constant (1/ t) depends upon diffusion coefficient ( D), element thickness ( d) and adsorption coefficient ( k).

液相メタノールにおける一酸化炭素ガスの輸送過程に関する研究

To estimate the effect of transport process of carbon monoxide gas in liquid methanol in a two-step methanol synthesis process which starts with carbonylation of methanol to methyl formate, solution and diffusion process of CO gas and its single bubbles in liquid phase methanol were theoretically and experimentally studied under various pressures and temperatures. Diffusion coefficients of dissolved CO gas in methanol were obtained by comparing the experimental data of CO gas solution in methanol within a small glass tube with theoretical solutions based on one dimensional diffusion model. The diffusion coefficients are between 4.0×10-9 m2/s and 7.6×10-9 m2/s for the temperatures ranged from 30 to 80 °C, and become higher as temperature increases. In the theoretical analysis for the solution of single CO bubbles in methanol accompanying chemical absorption, the single CO bubble was assumed to be stationary and surrounded by infinite liquid methanol. The rate of solution of bubble in liquid is affected strongly by various parameters, such as solubility, diffusion coefficient, chemical reaction constant, bubble radius, especially temperature. The theoretical results show that the rate of solution of CO bubble in liquid methanol was enhanced by chemical reaction, and the total reaction rate can be improved by enhancing the transport process of gas in the liquid. They also show that the reaction constant can be obtained by comparing the experimental data of the variation of bubble radius with time with the theoretical solutions. It was understood that the theoretical nondimensional times for complete solution of the single bubbles in methanol without reaction agree well with the experimental data for the initial diameters around 0.25 mm, and the times become shorter for larger bubble diameters due to the increasing effect of nature convection induced by density difference around the bubble.

Modeling of pyrite oxidation in saturated and unsaturated subsurface flow systems

Pyrite oxidation (FeS2) causes acidification and mobilization of metals. Mathematical modeling of pyrite oxidation in variably saturated reactive flow systems is challenging because (1) it occurs through a complex interplay of multi-phase flow and transport processes, and (2) aqueous concentrations of key species vary over tens orders of magnitude in different redox conditions. Here, we present a general multi-phase reactive transport model for redox processes. Two alternative implementations were made in the TOUGHREACT and TOUGH2-CHEM simulation codes which use sequential iteration and simultaneous solution, respectively. Both codes are used to simulate a fully and a variably saturated pyrite oxidation problem with simple 1-D flow and reaction conditions. Results from both codes indicate that the effects of oxygen partial pressure reduction due to reactions on the fluid flow is not significant under ambient conditions. However, it must be noted that when fluid flow and chemical reactions are strongly coupled, such as when boiling takes place in geothermal reservoirs, this could be essential. The fully simultaneous approach has a complete process description. The sequential iteration approach is found to be more efficient computationally. The oxygen gas diffusion process plays a dominant role in the chemical evolution for pyrite oxidation in unsaturated conditions. An example in 2-D fractured rock is presented to demonstrate pyrite oxidation under complex flow and geochemical conditions. This example shows that pyrite oxidation exerts strong influence on hydrogeochemical evolution in variably saturated flow systems. The alteration of primary rock minerals and the development of secondary mineral assemblages predicted are consistent with field observations. This example serves as a prototype for oxidative weathering processes with broad significance for geoscientific, engineering, and environmental applications.

Sorption and diffusion of low molecular weight gases and vapors in glassy polymers at high concentration of sorbate

A theoretical approach has been developed to describe the sorption and diffusion processes of low weight molecular gases and vapors in polymers at wide ranges of sorbate concentration. The equation of an S-shaped gas sorption isotherm in glassy polymer matrix has been derived. The concentration dependence of the sorbate molecule diffusion coefficient has been established. For an S-shaped sorption isotherm, this dependence is nonmonotonous. The conditions of cluster formation of sorbate molecules have been analyzed within the proposed approach, in which it is possible to determine a correlation between these conditions and parameters of sorption isotherm. The comparison of the experimental and theoretical data provides an assessment of the microscopic characteristics of investigated polymer–vapor systems, such as the distances between vapor molecules in a matrix corresponding to intermolecular repulsion and attraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2314–2323, 1999

Sorption and diffusion of low molecular weight gases and vapors in glassy polymers at high concentration of sorbate

A theoretical approach has been developed to describe the sorption and diffusion processes of low weight molecular gases and vapors in polymers at wide ranges of sorbate concentration. The equation of an S-shaped gas sorption isotherm in glassy polymer matrix has been derived. The concentration dependence of the sorbate molecule diffusion coefficient has been established. For an S-shaped sorption isotherm, this dependence is nonmonotonous. The conditions of cluster formation of sorbate molecules have been analyzed within the proposed approach, in which it is possible to determine a correlation between these conditions and parameters of sorption isotherm. The comparison of the experimental and theoretical data provides an assessment of the microscopic characteristics of investigated polymer–vapor systems, such as the distances between vapor molecules in a matrix corresponding to intermolecular repulsion and attraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2314–2323, 1999

Embrittlement of austempered spheroidal graphite cast iron by contact with water and resulting preventive method

Austempered spheroidal graphite cast iron (ADI) shows an extremely brittle behaviour in wet conditions when the parallel portion of a tensile specimen is covered with wet tissue paper during testing. The ADI specimens treated in the process window have an excellent tensile strength and elongation in the dry condition, however the former decreased to 70–80% and the latter decreased to 10–15% in the wet condition at room temperature. In the fracture surface of a specimen tested in the wet condition, a white portion consisting of brittle fracture, which is the origin of fracture, was observed in the vicinity of the outer surface of the specimen. The tensile properties in the wet condition decreased as the strain rate decreased, and this brittle behaviour was considered to be controlled by the diffusion process of gas. This water embrittlement was concluded to result from the embrittlement effect promoted by plastic deformation. The water embrittlement was decreased by forming a ferrite surface layer which is not embrittled by water or by avoiding direct contact with water by painting.