Diffusive Transport Processes Research Articles

Electron emission from silicon tip arrays controlled by np junction minority carrier injection

The authors demonstrate for the first time the injection of electrons across an n-type to p-type silicon junction and their subsequent tunneling from approximately 1 μm tall p-type silicon points into a vacuum gap. The diffusive flow of these minority carriers in the p-type material is controlled by the application of a bias voltage in the form of a base contact metallization contact on the p-type silicon, in analogy with a bipolar junction transistor. Using an array density of 4×106 tips/cm2, the authors measured a maximum average current of 1 nA per tip. Increasing the base contact bias voltage from 0 to ∼1 V changes the emission from a supply limited regime typically observed with p-type silicon emitters, bringing the emitted current back to a linear Fowler–Nordheim characteristic similar to that observed previously by photon generation of carriers in p-type silicon tips. The authors finally note that in our short tips, minority carrier flow should be a nondissipative largely adiabatic diffusive transport process which is followed by extraction into vacuum. A novel heat extraction mechanism for future cooling applications is thus anticipated.

Adsorption of polycyclic aromatic hydrocarbons from water using petroleum coke-derived porous carbon

Porous carbons were prepared from petroleum coke by KOH chemical activation, characterized and used as adsorbents for uptaking a mixture of polycyclic aromatic hydrocarbons (PAHs): naphthalene, fluorene, phenanthrene, pyrene and fluoranthene from aqueous solutions. The specific surface area (SSA) of these carbons ranges from 562 to 1904 m 2/g, while their point of zero charge (pH PZC) varies from 2.6 to 8.8. The equilibrium adsorption of PAHs on all four carbons follows the non-linear Freundlich equation well. For any given PAH in the group, the adsorption capacity parameter K f , increases with the SSA and pH PZC of the carbons, confirming the roles of dispersive interactions. For any given carbon, the value of K f follows the order of naphthalene > fluorene > phenanthrene > pyrene. This dependence of K f on molecular size suggests a certain degree of molecular sieving behavior of these carbons toward large PAHs. Under the condition studied, the uptake process is likely controlled by diffusive transport processes. And, it is unlikely that the competitive adsorption played any important roles in determining equilibrium adsorption of the mixed PAHs. Overall, the petroleum coke-derived porous carbon is very effective in adsorbing these PAHs.

Morphologic dating of fault scarps using airborne laser swath mapping (ALSM) data

Models of fault scarp morphology have been previously used to infer the relative age of different fault scarps in a fault zone using labor‐intensive ground surveying. We present a method for automatically extracting scarp morphologic ages within high‐resolution digital topography. Scarp degradation is modeled as a diffusive mass transport process in the across‐scarp direction. The second derivative of the modeled degraded fault scarp was normalized to yield the best‐fitting (in a least‐squared sense) scarp height at each point, and the signal‐to‐noise ratio identified those areas containing scarp‐like topography. We applied this method to three areas along the San Andreas Fault and found correspondence between the mapped geometry of the fault and that extracted by our analysis. This suggests that the spatial distribution of scarp ages may be revealed by such an analysis, allowing the recent temporal development of a fault zone to be imaged along its length.

Diffusive boundary layers do not limit the photosynthesis of the aquatic macrophyte, Vallisneria americana, at moderate flows and saturating light levels

Hydrodynamic models of mass transport assume that diffusive processes next to the surface limit transport and that there are no biological and chemical processes that control the supply and demand of the scalar. The validity of these assumptions was examined by measuring the momentum boundary layer (via particle image velocimetry) and the concentration boundary layer (via O2 microsensors) over the leaves of Vallisneria americana. The O2 flux (Jobs) was highest at x = 2 cm downstream from the leading edge of the leaf and was 1.8 to 1.4 times higher than Jobs measured at the trailing edge of the leaf at 0.5 cm s−1 and 6.6 cm s−1 mean velocity (U), respectively. The maximum Jobs was 0.44 ± 0.07 (mean ± SE) vs. 0.50 ± 0.09 µmol m−2 s−1 at 0.5 vs. 6.6 cm s−1. Interestingly, the surface O2 potential (D[O2] = [O2]surface ‐ [O2]bulk) was also unimodal at the low velocity (D[O2]max = 36 ± 5 mmol m−3 at x = 3 cm) but was uniform at the higher velocity (D[O2] = 9 ± 0.7 mmol m‐3). An analysis of the time scale of nutrient diffusion (τD) vs. nutrient uptake (τup) through the measured diffusion boundary layer revealed that uptake was always the slower process (i.e., τD < τup; τD and τup increased with x and decreased with U). Under moderate water velocities and saturating irradiance, uptake rates rather than diffusive transport processes appear to control mass transfer rates regardless of the location on the leaf and the water velocity.

A stereological perspective on placental morphology in normal and complicated pregnancies

Stereology applied to randomly-generated thin sections allows minimally-biased and economical quantitation of the 3D structure of the placenta from molecular to whole-organ levels. With these sampling and estimation tools, it is possible to derive global quantities (tissue volumes, interface surface areas, tubule lengths and particle numbers), average values (e.g. mean cell size or membrane thickness), spatial relationships (e.g. between compartments and immunoprobes) and functional potential (e.g. diffusive conductance). This review indicates ways in which stereology has been used to interpret the morphology of human and murine placentas including the processes of villous growth, trophoblast differentiation, vascular morphogenesis and diffusive transport. In human placenta, global quantities have shown that villous maturation involves differential growth of fetal capillaries and increases in endothelial cell number. Villous trophoblast is a continuously renewing epithelium and, through much of gestation, exhibits a steady state between increasing numbers of nuclei in cytotrophoblast (CT) and syncytiotrophoblast (ST). The epithelium gradually becomes thinner because its surface expands at a faster rate than its volume. These changes help to ensure that placental diffusing capacity matches the growth in fetal mass. Comparable events occur in the murine placenta. Some of these processes are perturbed in complicated pregnancies: 1) fetoplacental vascular growth is compromised in pregnancies accompanied by maternal asthma, 2) changes in trophoblast turnover occur in pre-eclampsia and intrauterine growth restriction, and 3) uteroplacental vascular development is impoverished, but diffusive transport increases, in pregnant mice exposed to particulate urban air pollution. Finally, quantitative immunoelectron microscopy now permits more rigorous analysis of the spatial distributions of interesting molecules between subcellular compartments or shifts in distributions following experimental manipulation.

Model of impurity distribution in the atmosphere for a source above the Earth’s surface

Taking into account diffusion and wind transport processes, a model of the distribution of an impurity polluting the Earth’s atmosphere was constructed. On its basis, simple formulas were derived, which, using gas-correlation IR radiometry, allow the estimation of the power and position of impurity sources above the Earth’s surface.

Dynamics and Control of a Tubular Solid-Oxide Fuel Cell

This paper presents a study of the dynamic behavior and control of a tubular solid oxide fuel cell system. A dynamic compartmental model that is based on first principles is developed. The model accounts for diffusion processes, inherent impedance, transport (heat and mass transfer) processes, electrochemical processes, anode and cathode activation polarizations, and internal reforming/shifting reactions, among others. Dynamic outlet voltage, current, and fuel-cell−tube temperature responses of the cell to step changes in external load resistance and conditions of the feed streams are presented. Simulation results show that the fuel cell is a multitime-scale system; some of the cell output responses exhibit consecutive apparent dominant time constants, ranging from ∼0.2 ms to ∼40 s. They also reveal that the temperature and pressure of the inlet air stream and the temperature of the inlet fuel stream strongly affect the dynamics of the fuel cell system. The temperature of the inlet air stream has the stro...

A-horizon dynamics in a historical hedged landscape

In hedged landscapes, the hedge structure induces a modification of soil organization at the landscape scale and another modification of soil geometry and properties at the slope scale. In these environments, the effects of landscape structures on soil pedogenesis and properties are identified, yet the time scale on which the target processes are occurring remains unknown. The aim of this paper is to constrain the space and time changes in hedge structure, within a historical hedged landscape, in order to better quantify the dynamics of A-horizons geometry. To achieve this goal, we implemented various relative methods (soil geometry analysis, historical document analysis) and absolute methods ( 14C, 137Cs). Results highlight the complexity of soil spatial organization within the landscape: A-horizon geometry is clearly influenced by landscape structures, whereas deeper horizons appear not to be influenced by these structures. The explanatory factors of this current organization are not only the present topography and hedgerow network pattern, but also the position of former hedgerows. Organo-mineral horizon changes clearly post-date the establishment of the hedgerow network landscape, which has controlled topsoil horizon geometry. Since 1833, the decrease in hedgerow network density has changed the spatial distribution of soil redistributions through diffusive transport (including tillage translocation) and water erosion processes; locally, it has increased soil erosion processes with respect to the increase in uphill drainage surface areas without changing general soil organization at the landscape scale acquired over the longer term.

Evidence for Instantaneous Oxygen‐Limited Biodegradation of Petroleum Hydrocarbon Vapors in the Subsurface

AbstractPetroleum hydrocarbon vapors biodegrade aerobically in the subsurface. Depth profiles of petroleum hydrocarbon vapor and oxygen concentrations from seven locations in sandy and clay soils across four states of Australia are summarized. The data are evaluated to support a simple model of biodegradation that can be used to assess hydrocarbon vapors migrating toward built environments. Multilevel samplers and probes that allow near‐continuous monitoring of oxygen and total volatile organic compounds (VOCs) were used to determine concentration depth profiles and changes over time. Collation of all data across all sites showed distinct separation of oxygen from hydrocarbon vapors, and that most oxygen and hydrocarbon concentration profiles were linear or near linear with depth. The low detection limit on the oxygen probe data and because it is an in situ measurement strengthened the case that little or no overlapping of oxygen and hydrocarbon vapor concentration profiles occurred, and that indeed oxygen and hydrocarbon vapors were largely only coincident near the location where they both decreased to zero. First‐order biodegradation rates determined from all depth profiles were generally lower than other published rates. With lower biodegradation rates, the overlapping of depth profiles might be expected, and yet such overlapping was not observed. A model of rapid (instantaneous) reaction of oxygen and hydrocarbon vapors compared to diffusive transport processes is shown to explain the important aspects of the 13 depth profiles. The model is simply based on the ratio of diffusion coefficients of oxygen and hydrocarbon vapors, the ratio of the maximum concentrations of oxygen and hydrocarbon vapors, the depth to the maximum hydrocarbon source concentration, and the stoichiometry coefficient. Whilst simple, the model offers the potential to incorporate aerobic biodegradation into an oxygen‐limited flux‐reduction approach for vapor intrusion assessments of petroleum hydrocarbon compounds.

Numerical simulation of single layer polymer light-emitting diodes

A numerical model for the electrical properties for polymeric light-emitting diodes (PLEDs) is presented which accounts for drift- and diffusion transport, recombination and re-emission processes. The current–voltage characteristics of single layer polymer light-emitting diodes are found to be dominated by the space-charge limited bulk hole-conductivity. The device efficiency is found to increase with bias as electron current is strongly reduced by injection barrier. The operating voltage increases with increased thickness of polymer layer. The understanding of these characteristics will facilitate the further optimization of the performance of polymer LEDs.

Gas Transport in Bidisperse Coal Particles: Investigation for an Effective Diffusion Coefficient in Coalbeds

Abstract Much work has been carried out on adsorption capacity of coals. Diffusive transport processes within the coal matrix blocks could be the rate-limiting step for adsorption during gas injection and production operations. Identifying these processes and determining their contributions to overall mass transport is a complex and time consuming procedure. The paper presents numerical diffusion models in varying coal particles and investigates transport mechanisms. For this purpose, the coal particle is represented as a microporous solid penetrated by a network of larger interconnected macropores. The solid adsorbs the bulk of the gas. A simple relationship between the apparent and intrinsic Fickian diffusion coefficients is derived in the case of single-component (methane) transport and Langmuir-type adsorption. Mass transport in the bidisperse coal particle is significantly influenced by the adsorption in the microporous solid. The investigation is then extended to study concentration dependence of the microporous solid diffusion for binary (methane-CO2) mixtures using the Maxwell-Stefan formulation. It is found that co-diffusion of the gas molecules enhances the gas mass transport in the solid in the presence of competitive sorption dynamics, while counter-diffusion may diminish the gas mass transport. The influence of lateral interactions among the adsorbed molecules in the solid phase is discussed. The work finds application in modelling CBM and CO2-ECBM processes. Introduction As an unconventional natural gas resource, coalbed methane receives worldwide attention. Deep coal seams that are not accessible for mining are suitable for in situ gas production using conventional drilling, well completion and gas recovery technologies. Hence, a vast amount of natural gas is globally available. Unlike conventional gas resources, however, the gas storage, flow and transport processes in coalbeds are quite complex mainly due to the intricate nature of coal(1–6). Coalbeds are porous media often characterized by a bimodal pore structure: a primary structure consisting of micro- and meso-scale pores, and a secondary structure with macropores and interconnected natural fractures. The microporous coal has an extremely large internal surface area and a strong affinity for certain naturally occurring chemical species such as methane, carbon dioxide, nitrogen and water. At high coalbed pressures, therefore the majority of the natural gas-in-place, in particular, methane, exists abundantly at an adsorbed liquid-like state in the microporous solid(1). Depressurizing the coalbed may yield a significant volume of natural gas. However, the initial stage of recovery is often dominated by water production, whereas, the latter stage is under the influence of diffusional resistances of the primary pores of the coal matrix. Treatment and disposal of the produced water is expensive and its long-term environmental impact has not been clearly understood yet. Injection of a second gas with much higher adsorption capacity, on the other hand, could possibly enhance recovery by maintaining the overall reservoir pressure, thus, keeping the water production at a minimum level. Additionally, injecting a second gas would promote methane desorption by lowering the partial pressure of the coalbed methane in the migrating gas phase.

Transport of Energetic Particles by Microturbulence in Magnetized Plasmas

Transport of energetic particles by the microturbulence in magnetized plasmas is studied in gyrokinetic simulations of the ion temperature gradient turbulence. The probability density function of the ion radial excursion is found to be very close to a Gaussian, indicating a diffusive transport process. The particle diffusivity can thus be calculated from a random walk model. The diffusivity is found to decrease drastically for high energy particles due to the averaging effects of the large gyroradius and orbit width, and the fast decorrelation of the energetic particles with the waves.

Measurement of Diffusion in Ternary Liquid Mixtures by Light Scattering Technique and Comparison with Taylor Dispersion Data

Results of diffusion coefficient measurements by dynamic light scattering (DLS) in the ternary liquid systems, glycerol–acetone–water (GAW), and cyclohexane–methanol–toluene (CMT), are reported. Data for the GAW system are compared with Taylor dispersion (TD) measurements in overlapping concentration regions close to the critical solution point at 298.15 K. A fit of the intensity autocorrelation function (ACF) could be used to predict values and explain the physical character of the corresponding diffusion coefficients. In the vicinity of the critical solution point, the DLS measurements reveal two and more hydrodynamic relaxation modes with well separated characteristic relaxation times. From the ACF, at least two effective diffusivities, D1 and D2, can be experimentally determined. Theoretical predictions reveal that they may result from pure mass diffusion and pure thermal diffusion transport processes. A possible physical meaning of the modes D1 and D2 in the ternary liquid mixtures is discussed. When we compare the transport modes from DLS with the Taylor dispersion results, only the slowest mode represents mass diffusion, and this mode agrees very well with one of the eigenvalues of Fick’s diffusion matrix. There is no relation between the mass diffusion mode from DLS and any one of the four diffusion coefficients obtained by TD.

On the Role of Diffusion in the Oxidation of Fe, Ni, Co and the Fe-Sn Alloy by Calcium and Sodium Sulfates

Diffusion processes play a dominant part in the macro kinetics of Fe, Ni and Co oxidation by calcium and sodium sulfates. Here, the reaction product forms a compact covering which spatially divides the reagents on the surface in the same way as in the oxidation and sulfidization of metals by oxygen and sulfur. Therefore, it is possible to assume in advance that interaction of metals with calcium and sodium sulfates will be determined not by the actual chemical reaction properly but by the diffusion transport processes.

06/01900 Experimental studies of a direct methanol fuel cell: Ge, J. and Liu, H. Journal of Power Sources, 2005, 142, (1–2), 56–69

In direct methanol fuel cells (DMFCs), the optimum characteristics of ion-exchange membranes are investigated at high concentrations of methanol feed up to 7 M by modifying the diffusion coefficient and the ionic conductivity of the polyelectrolyte material. A Nafion membrane is modified by the incorporation of layered double hydroxide (LDH) nanoplatelets with different Mg2+:Al3+ ratios. When the feed concentration of methanol is lower than 3 M, the DMFC is controlled by the ionic conductivity of the polyelectrolyte membrane because methanol cross-over is not relatively significant. When the feed concentration is high, however, the diffusion coefficient of methanol is the key factor that determines the performance of the fuel cell. This is due to a high concentration gradient of methanol across the polyelectrolyte membrane. The open-circuit voltage is increased by the decreased diffusion coefficient in LDH/Nafion nanocomposite membranes at methanol feed concentrations up to 7 M; apparently because methanol cross-over is suppressed by the incorporation of LDH. The maximum power density of the DMFC is determined by the two competing transport processes of ion conduction and methanol diffusion, especially at a relatively high methanol concentration, that can provide optimum operating conditions in the membrane.

06/01906 Preparation of carbon-supported PtRu nanoparticles for direct methanol fuel cell applications — a comparative study: Deivaraj, T. C. and Lee, J. Y. Journal of Power Sources, 2005, 142, (1–2), 43–49

In direct methanol fuel cells (DMFCs), the optimum characteristics of ion-exchange membranes are investigated at high concentrations of methanol feed up to 7 M by modifying the diffusion coefficient and the ionic conductivity of the polyelectrolyte material. A Nafion membrane is modified by the incorporation of layered double hydroxide (LDH) nanoplatelets with different Mg2+:Al3+ ratios. When the feed concentration of methanol is lower than 3 M, the DMFC is controlled by the ionic conductivity of the polyelectrolyte membrane because methanol cross-over is not relatively significant. When the feed concentration is high, however, the diffusion coefficient of methanol is the key factor that determines the performance of the fuel cell. This is due to a high concentration gradient of methanol across the polyelectrolyte membrane. The open-circuit voltage is increased by the decreased diffusion coefficient in LDH/Nafion nanocomposite membranes at methanol feed concentrations up to 7 M; apparently because methanol cross-over is suppressed by the incorporation of LDH. The maximum power density of the DMFC is determined by the two competing transport processes of ion conduction and methanol diffusion, especially at a relatively high methanol concentration, that can provide optimum operating conditions in the membrane.

A hybrid lattice Boltzmann model for solid–liquid phase transition in presence of fluid flow

In the present Letter, a hybrid lattice Boltzmann methodology is developed for simulating convection–diffusion transport processes pertinent to melting/solidification problems. The model is derived by coupling a modified thermal lattice Boltzmann model with an adapted enthalpy-porosity technique. The hydrodynamic variables are simulated using a single particle density distribution function, whereas the thermodynamic variables are obtained from an enthalpy density distribution function. An exhaustive comparison with other standard methods reveals that the proposed technique is computationally more convenient to be implemented for solving topologically-complicated phase boundary evolution problems during solidification of pure materials, such as the growth of an equiaxed dendrite in an undercooled melt.

Role of canopy‐scale photochemistry in modifying biogenic‐atmosphere exchange of reactive terpene species: Results from the CELTIC field study

A one‐dimensional canopy model was used to quantify the impact of photochemistry in modifying biosphere‐atmosphere exchange of trace gases. Canopy escape efficiencies, defined as the fraction of emission that escapes into the well‐mixed boundary layer, were calculated for reactive terpene species. The modeled processes of emission, photochemistry, diffusive transport, and deposition were highly constrained based on intensive observations collected in a Loblolly Pine plantation at Duke Forest, North Carolina, during the CELTIC field study. Canopy top fluxes for isoprene and α,β‐pinene were not significantly altered by photochemistry as calculated escape efficiencies were greater than 0.90 for both species. β‐caryophyllene emission and photochemistry were added to the canopy model as a surrogate for the reactive sesquiterpene class of species. β‐caryopyllene escape efficiencies of 0.30 were calculated for midday summertime conditions. Urbanization scenarios were also performed to assess the impact of pollution on modifying biosphere‐atmosphere exchange. Modest changes in escape efficiencies were calculated for a wide range of anthropogenic hydrocarbon and NOx mixing ratios suggesting a simple parameterization of escape efficiency in terms of grid cell NOx may be possible for incorporating the impact of canopy scale photochemistry within biogenic emission processing systems for regional air quality and climate models. The inferred magnitude of sesquiterpene ozonolysis reactions has important implications on both daytime and nighttime radical formation in the canopy.

Optimum ionic conductivity and diffusion coefficient of ion-exchange membranes at high methanol feed concentrations in a direct methanol fuel cell

In direct methanol fuel cells (DMFCs), the optimum characteristics of ion-exchange membranes are investigated at high concentrations of methanol feed up to 7 M by modifying the diffusion coefficient and the ionic conductivity of the polyelectrolyte material. A Nafion membrane is modified by the incorporation of layered double hydroxide (LDH) nanoplatelets with different Mg 2+:Al 3+ ratios. When the feed concentration of methanol is lower than 3 M, the DMFC is controlled by the ionic conductivity of the polyelectrolyte membrane because methanol cross-over is not relatively significant. When the feed concentration is high, however, the diffusion coefficient of methanol is the key factor that determines the performance of the fuel cell. This is due to a high concentration gradient of methanol across the polyelectrolyte membrane. The open-circuit voltage is increased by the decreased diffusion coefficient in LDH/Nafion nanocomposite membranes at methanol feed concentrations up to 7 M; apparently because methanol cross-over is suppressed by the incorporation of LDH. The maximum power density of the DMFC is determined by the two competing transport processes of ion conduction and methanol diffusion, especially at a relatively high methanol concentration, that can provide optimum operating conditions in the membrane.

The Effect of Grain-Boundary Diffusion on the Oxidation of Low-Chromium Steels

Even though the oxidation behavior of steels is generally considered as to be widely understood, a closer look reveals some open questions, e.g. regarding the influence of the substrate grain size on the overall oxidation kinetics. At temperatures below 570°C the main constituent of the oxide scale formed on top of low alloy steels is magnetite. As shown by gold marker experiments it grows outward and inward at the same time, the latter exhibiting a gradual transition to the more stable spinel compound FeCr2O4. As indicated by intergranular-oxidation attack below the superficial scale, inward oxide growth seems to be driven by oxygen transport along the grain boundaries serving as fast diffusion paths. This is supported by thermogravimetric oxidation tests in air on low-Cr steels with varying grain size: The smaller the grains the higher the oxidation rate. Recently, a numerical model for the diffusive transport processes based on the finite-difference approach has been developed, which distinguishes between fast grain-boundary diffusion and bulk diffusion. Qualitatively, it is capable to predict the relationship between substrate grain size and inward oxide growth kinetics. Together with the thermodynamic tool ChemApp and in combination with a data set for the Fe-Cr-O system the mechanism-based simulation of the overall oxidation process of low-Cr steels is possible.