Diffusion Of Gaseous Species Research Articles

Numerical Simulation of Processes in an Electrochemical Cell Using COMSOL Multiphysics

Fuel cells are a promising source of clean energy. To find optimal parameters for their operation, modeling is necessary, which is quite difficult to implement taking into account all the significant effects occurring in them. We aim to develop a previously unrealized model in COMSOL Multiphysics that, on one hand, will consider the influence of electrochemical heating and non-isothermal fluid flow on the temperature field and reaction rates, and on the other hand, will demonstrate the operating mode of the Solid Oxide Fuel Cell (SOFC) on carbonaceous fuel. This model incorporates a range of physical phenomena, including electron and ion transport, gas species diffusion, electrochemical reactions, and heat transfer, to simulate the performance of the SOFC. The findings provide a detailed view of reactant concentration, temperature, and current distribution, enabling the calculation of power output. The developed model was compared with a 1-kW industrial prototype operating on hydrogen and showed good agreement in the volt-ampere characteristic with a deviation not exceeding 5% for the majority of the operating range. The fuel cell exhibits enhanced performance on hydrogen, generating 1340 W/m2 with a current density of 0.25 A/cm2. When fueled by methane, it produces 1200 W/m2 at the same current density. Using synthesis gas, it reaches its peak power of 1340 W/m2 at a current density of 0.3 A/cm2.

Effect of Atmosphere and Sample Thickness on Kinetics, Microstructure, and Compressive Stresses of Chromia Scale Grown on Ni–25Cr

Most high-temperature alloys rely on the formation of a protective chromia scale during service. Generally, the oxidation resistance is mainly influenced or limited by corrosive environments around the alloy. In the present study, the evaluation of kinetic data is connected to microstructural analyses and stress measurements of the Cr2O3-layer formed on the Ni–25Cr model alloy after 1000 °C of isothermal exposure to atmospheres with and without water vapor and a varying (1 and 0.01%) oxygen content. Significant differences in oxide morphology, scale thickness, and oxide grain appearance could be observed. The results, with respect to oxidation kinetics and microstructures, showed a high tendency of variation when water vapor is present in the test gas. It was found that the addition of water vapor to the dry gas leads to decreased oxidation rates and reduced chromia scale thicknesses. The influence of the gas composition on the defect structure, the chromium flux, and diffusion of gas species is discussed on the basis of classical oxidation theory.

The transient start-up response of a universal exhaust gas oxygen sensor to investigate the Nernst equation in platinum/zirconia cells

The universal exhaust gas oxygen sensor (UEGO) is a device used to infer the combustion air-to-fuel ratio of an internal combustion engine by sampling the exhaust gas. The sensor operates using a feedback system to maintain a specified internal condition, and measures the oxygen current required for this. While the steady state operation of the sensor is reasonably well-understood – dominated as it is by the diffusion of gas species – the factors influencing the transient response are not so clear.In this paper a numerical model of a sensor is compared to experimental data. By examining the effect of the inclusion of different aspects into the model, it becomes clear that it is necessary to account for the influence of gaseous species adsorbing onto surfaces, as well as the more traditional approach based on oxygen partial pressure, to correctly capture the transient response of a sensor containing a Pt|YSZ|Pt cell.

Investigating the photo-oxidative and heterogeneous chemical production of HCHO in the snowpack at the South Pole, Antarctica

Environmental context Snowpacks present a surprisingly active environment for photochemistry, leading to sunlight-induced oxidation of deposited organic matter and the subsequent emission of a variety of photochemically active trace gases. We seek to address questions regarding the ultimate fate of organic matter deposited onto snow in the remote regions of the world. The work is relevant to atmospheric composition and climate change. Abstract We investigate snowpack fluxes of formaldehyde (HCHO) into the South Pole boundary layer using steady-state photochemical models. We study two chemical sources of HCHO within the snowpack. First, we study chemical production of HCHO from the processing of methyl hydroperoxide (CH3OOH): photolysis, reaction with the hydroxyl radical (OH•), and by an acid catalysed rearrangement. Assuming surface layer concentration effects for acidic solutes, we show that the acid catalysed production of HCHO within ice could contribute a non-negligible source to the snowpack HCHO budget. This novel source of HCHO complements existing explanations of HCHO fluxes based on physical emission of HCHO from snow. Secondly, we investigate HCHO production from the oxidation of organic matter (OM) by OH• within snow to explain observed fluxes of photochemical origin from the South Pole snowpack. This work shows that laboratory-derived photochemical production rates of HCHO and our standard model are inconsistent with field observations, which has implications for the distribution of OM relative to oxidants within ice particles. We resolve this inconsistency using new laboratory measurements of the molecular dynamics of the OH• photofragment from hydrogen peroxide (H2O2) and nitrate (NO3–) photolysis, which show that only OH produced in the outermost monolayers can contribute to gas phase and surface layer chemistry. Using these new measurements in conjunction with realistic treatments of ice grain size, H2O2 and NO3– distribution within ice grains, diffusion of gas species within solid ice, and observed OM particle size distributions yields snowpack HCHO photochemical production rates more consistent with observations.

Particle–vapor interaction in deposition systems: influence on deposit morphology

We have here presented methods to study interactions of vapors and particles in systems involving simultaneous deposition of vapors and particles. Besides estimating vapor and particle concentration profiles in the boundary layer adjacent to the deposition surface, their deposition rates are also calculated. In particular, we consider formation of porous preforms by deposition of silica particles and germania particles/vapors during the manufacturing of optical fibers. The process conditions not only dictate the relative rates of germania particle and vapor deposition on the deposition surface, but also controls the fraction of germania crystallinity in the resulting deposit. Moreover, the loss and migration behavior of the deposited germania during the sintering of the porous preform is extremely sensitive to the germania crystalline fraction. Our methods predict the germania weight percent deposited during the deposition process as a function of the deposition conditions, along with the fraction of germania that is crystalline/amorphous. The germania loss and migration behavior during the sintering step is also estimated. In predicting the germania loss and migration behavior, we have developed methods to systematically take into account the simultaneous heating of the preform, sintering of the porous preform, diffusion of gas species through the pores and the gas–solid reaction. Based on the methods developed here for deposition and sintering, processes have been developed which have resulted in sintered glasses with very high germania content (50 weight percent), and, without any glass quality issues of glass seeds, blank splitting or glass crizzling (devitrification).

Oxidation mechanisms and kinetics of SiC-matrix composites and their constituents

The oxidation kinetics and mechanisms of SiC-matrix composites fabricated by chemical vapor infiltration, and of their constituents (C or SiC-fibers, C or BN interphases and SiC matrix) are studied on the basis of an experimental approach and modelling. The oxidation of carbon fibers is rate-controlled by a combined diffusion/chemical reaction mechanism at low temperatures and its rate reduced by a 1600°C heat treatment. The oxidation rate of the pyrocarbon is similar to that of the fibers when they have been heat-treated. The oxidation kinetics of both the SiC-based fibers and matrix are parabolic and assumed to be rate-limited by the diffusion of gaseous species in the silica scale. A full kinetics law is given. The occurrence of water in the atmosphere increases the oxidation rate of the fibers and decreases the activation energy, water becoming the main oxidizing agent. The oxidation of the BN-interphase is complex and strongly anisotropic, its kinetics depending on composition, structure and texture. Finally, the oxidation of SiC-matrix composites, depicted for 1D-SiC/C/SiC and 2D-C/C/SiC composites, involves both diffusion of gaseous species in the composite porosity and heterogeneous oxidation reactions. Oxidation occurs through the thickness of the composites at low temperatures which consumes the carbon-based constituents. Conversely, it tends to be limited to near the composite surface at high temperatures, due to the formation of silica-based phases healing the material porosity and preventing the in-depth oxidation of the carbon-based constituents.

Behavior of metallic fission products in uranium–plutonium mixed oxide fuel

Metallic fission products, ruthenium, rhodium, technetium, palladium, and molybdenum, exist in irradiated oxide fuels as metallic inclusions. In this work, the radial distributions of metallic inclusion constituents in the fuel specimen irradiated to a peak burnup of 7–13 at.% were observed with an electron probe microanalysis. Palladium concentration is high at the periphery in all the specimens. Molybdenum shows the same tendency for the 13 at.% burnup specimen. These results showed the significant difference between experimental data and calculations with ORIGEN-2 at such high burnups, which suggested that the migration of palladium and molybdenum was controlled mainly by diffusion of gaseous species containing each metal along the fuel temperature gradient.

The Effect of Porous Composite Electrode Structure on Solid Oxide Fuel Cell Performance: I. Theoretical Analysis

The effect of porous composite electrodes on the overall charge‐transfer process in solid‐state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge‐transfer resistance, electrode thickness, and porosity. A model is presented that accounts for ionic transport within the electrolyte, electronic conduction through electrocatalyst, and charge‐transfer at the electrolyte‐electrocatalyst interface. Diffusion of gaseous species in porous electrodes is assumed to be rapid so as not to be rate limiting. The conduction of electrons in the electrocatalyst is assumed to introduce negligible resistance. The activation overpotential as a function of current density is assumed to be ohmic, and an effective charge‐transfer resistance is defined. The transport equations are solved numerically in two dimensions using a finite difference technique and analytically in one dimension. The analysis predicts that the use of composite electrodes in devices employing solid electrolytes can significantly increase performance under conditions where the intrinsic charge‐transfer resistance is high in comparison to the area‐specific resistance of the electrolyte. The results indicate a low effective charge‐transfer resistance is obtained for relatively thick electrodes with a fine microstructure as long as the porosity is sufficient to ensure negligible concentration polarization.

Effects of reduction of the AlYO containing secondary phases during sintering of AlN with YF 3 additions

The effect of reduction of the AlYO containing secondary phases forming AlN and YN during sintering of AlN(YF 3) embedded in AlN powder has systematically been studied. The sintering was performed in a graphite furnace at 1880 °C. With increased reduction of the secondary phase, the following effects were observed: (a) the chemical composition of the secondary phase changed in the order YAG → YAP → YAM → Y 2O 3 → YN; (b) reduction of the amount or removal of the secondary phase; (c) decreased sinterability of AlN when more than 5 wt% YF 3 was added; (d) reduction in the lattice oxygen content; and (e) increased thermal conductivity. Included in the paper is a discussion of a possible mechanism for the reduction of the secondary phase, which we propose is controlled by the diffusion of gas species (CO/CO 2 and/or Al 2O) through the powder bed and the distance between the AlN samples and a graphite source, as well as a proposed optimal sintering programme for AlN with AlYO containing phases.

Modeling study of effects of temperature profiling on CVI processing of woven graphite preforms with dimethyldichlorosilane

There is currently considerable interest in producing high temperature capability graphitic materials by first weaving graphite fiber tows (yarns) into a “preform” structure, followed by densification via cracking of precursor compounds such as dimethyldichlorosilane within the pores (macro and micro) at high temperature. The model described in this paper addresses this densification process, treating diffusion of gaseous species both within the macropores (spaces between the tows) and the micropores (spaces between individual fibers in the tows) and finite kinetics associated with the cracking of the precursor gas (treated parametrically). The resulting model is used to examine the effects of temperature distributions through cylindrical preforms on ultimate densification distribution. As might be expected (and as observed experimentally), uniform temperature through the preform leads to premature full densification of the pore structure at the periphery of the cylinder (blocking further densification in the interior), leading to severe porosity in the interior regions. Effects of externally imposed nonuniform temperature profiles (possibly via microwave heating) in alleviating this problem are examined, and it is shown that proper profiling can lead to nearly complete uniform densification throughout the preform.

Preparation and reduction kinetics of uniform copper particles from copper(I) oxides with hydrogen

Uniform copper particles were prepared by reduction of copper(I) oxides with hydrogen at 69–111°C. Three types of precursor particles having average modal sizes of O.46, 0.62 and 1.26 μm, respectively were produced by reducing Fehling's solutions of different concentrations with glucose. The reduction of fine copper(I) oxide particles took place through an autocatalytic process, while the reaction rates of rather coarse oxide particles were controlled by the diffusion of gaseous species through the copper layers formed at a later stage of the process. The resulting copper particles retained the original morphology of the oxides. A moderately oxygen-rich layer about 30 Å thick was detected on exposure to air by means of photoelectron surface microanalysis (XPS).

Reduction Kinetics of Cement-bonded Natural Ilmenite Pellets with Hydrogen.

The hydrogen reduction kinetics of ilmenite was studied thermogravimetrically from 1073 to 1273 K with three kinds of cement-bonded natural ilmenite pellets. The kinetic behavior of the reduction of preoxidized ilmenite was researched with the same technique from 973 to 1273 K. The reduction of the pellets proceeded topochemically, but, the behavior of the grains constituting the pellets was more complex. The reduction kinetics of preoxidized pellets was analyzed according to a two-interface kinetic model and the kinetic parameters of the two main reactions Fe3+→Fe2+ and Fe2+→Fe°, which mainly constitute the overall reduction of preoxidized ilmenite pellets, were determined separately. The experimental data of the reduction of unoxidized pellets were interpreted in terms of a modified one-interface kinetic model derived from the aforementioned two-interface kinetic model. Diffusion of gaseous species through product layer and intrinsic chemical reaction were found to be the main rate controlling factors during reduction.

Removal of Carbonaceous Residue with Wet Hydrogen in Greensheet Processing for Multilayer Ceramic Module: II, Reaction in Large Modules Influenced by Pore Diffusion and Mass Transfer

An experimental and mathematical investigation of the removal of carbonaceous residue deposited on the ceramic substrate in the fabrication of multilayer ceramic modules has been conducted. Such carbonaceous residue is formed when the organic binder is pyrolyzed during the firing of greensheets. This paper is the second of a two‐part series that reports on and discusses the principal findings of the investigation. In this paper we describe the removal of carbonaceous residue from large blocks of the multilayer ceramic module in which the diffusion of gaseous species may play a role. A mathematical model is developed using the Law of Additive Reaction Times for fluid‐solid reactions, which incroporates the effects of intrinsic chemical kinetics, pore diffusion, and external mass transfer. This law gives an approximate solution to the governing equations which describe the removal of carbonaceous residue from the MLC module during firing.

Morphology and deposition rates of TiB2 prepared by chemical vapour deposition of TiCl4 + B2H6 system

Titanium diboride (TiB2) deposits were obtained on a graphite substrate by chemical vapour deposition using TiCl4, B2H6 and H2 at deposition temperatures (Tdep) of 1323 to 1773 K and total gas pressures (Ptot) of 4 to 40 kPa. B/Ti molar ratio in the source gases [2B2H6/TiCl4(mB/Ti)] was varied from 0.6 to 4. TiB2 plates were prepared at mB/Ti = 0.6 to 2. The deposition rate increased with increasing Tdep. The largest value of the deposition rate was 1.4 × 10−7 m sec−1 (0.5 mmh−1) at Tdep = 1773 K, Ptot =40 kPa and mB/Ti = 0.6. The activation energies for the formation of CVD−TiB2 plate were 41 to 51 kJ mol−1. These values suggest that the diffusion of gaseous species through the boundary layer is a rate-determining process.

Analysis of the Chemical Vapor Deposition of Titanium Diboride: II . Modeling the Kinetics of Deposition

Deposition rates and efficiencies for the chemical vapor deposition (CVD) of from , , and were experimentally determined as a function of CVD variables. These results, coupled with previously determined thermodynamic equilibrium compositions and deposit morphologies, were used to develop models for the deposition process. At high temperatures the principal rate‐limiting mechanism is the diffusion of gaseous species through a gas boundary layer above the deposition surface. However, low or high fractions in the reactant gas cause a chemical kinetic mechanism to become rate limiting. At lower deposition temperatures equilibrium no longer appears to exist at the substrate surface and a chemical kinetic rate‐limiting mechanism becomes important. A simple linear rate expression was used to calculate an activation energy of for the chemical kinetic mechanism.

Mass transfer model of halogen doped incandescent lamps with application to the W-O-Br system

A mass transfer model of halogen doped incandescent lamps is formulated. In this model, the transport processes occurring in these lamps are represented by diffusion and counter diffusion of gaseous species across the stagnant boundary layer surrounding the filament with simultaneous reactions at the bulb wall and the filament. The model is applied to the 500 watt-120 volt-T3 quartz halogen lamp geometry in which the initial gas phase is argon with small concentrations of oxygen and bromine. The tungsten flux from wall to filament andvice versa and, thus, the conditions of wall blackening are calculated as a function of the initial bromine and oxygen content, filament and wall temperature, total pressure and wall condensate,i.e., W(s), WO2(s), and WO3(s).