High Porosity Regions Research Articles

Unstructured Pore Network Modeling with Heterogeneous PEMFC GDL Porosity Distributions

This is the first investigation of the liquid water saturation profile dependence on empirically determined heterogeneous polymer electrolyte membrane fuel cell (PEMFC) gas diffusion layer (GDL) porosity distributions. An unstructured, two-dimensional pore network model using an invasion percolation algorithm is presented. Random fiber placements are based on the heterogeneous porosity distributions of six commercially available GDL materials recently obtained through X-ray-computed tomography visualizations. The pore space is characterized with a Voronoi diagram, and simulations are performed with a single inlet liquid water cluster. Saturation profiles are also computed for GDLs with uniform, sinusoidal, and square-wave porosity distributions. Liquid water tends to accumulate in regions of high porosity due to the associated lower capillary pressures. The results of this work suggest that GDLs tailored to have smooth porosity distributions have fewer pockets of high saturation levels within the bulk of the material. Finally, a study on theoretical surface modifications demonstrates that low porosity surface treatments at the catalyst interface result in greatly reduced overall saturation levels of the material.

Influence of Citric and Ascorbic Acids on Electrodeposited Au/FeAu Multilayer Nanowires

AuFe electrodeposits onto a rotating disk electrode were examined to assess the composition and current efficiency behavior when citric acid, ascorbic acid, or a combination of both is present in the electrolyte. Nanowires were subsequently fabricated with pulsed current deposition to create compositionally, multilayered Au/FeAu nanowires. The FeAu layer of the nanowires was porous in a citrate electrolyte, but the porosity decreased with the addition of ascorbic acid in the electrolyte. Comparatively higher side reaction partial current densities in the citric acid electrolyte generated local alkaline conditions, inducing the chemical precipitation of Fe ions in the form of iron oxide nanoparticles inside the recess, which manifested as the regions of high porosity. Nonporous layers observed in the ascorbic acid bath, in combination with decreased and increased current efficiency of the deposits, suggested that a similar mechanism of Fe precipitation inside the nanorecess is less likely.

Ｘ線ＣＴ法による多孔質岩石内ＣＯ２残留ガストラップ現象の分析

CCS (Carbon dioxide Capture & Storage) is expected as one of most efficient methods in order to reduce the amount of exhausted CO2 into the air. In the projects, the estimation of storage volume and the assessment of CO2 trapping effect in the reservoir are necessary. The objective of this paper is to analyze the residual CO2 trapping in the porous media after the storage of CO2 in the aquifer by means of X-ray CT. The CO2 injection and successive water injection test were conducted, and the replacement and trapping process of water — CO2 was analyzed by X-ray CT method. Here, the amount of CO2 replaced in the pore during CO2 injection tests was estimated. The amount of residual CO2 trapped in the rock pore during successive water injection tests was also estimated, and relations between the replaced/trapped CO2 and the porosity distribution of rock were discussed. It was found that more CO2 was stored at the relatively high porosity region, and that the residual ratio at the high porosity region easily decreased during the water injection process. However, it was also found that the approximately 10 % of injected CO2 was still remained after the six to ten times pore volume of water was injected into the rock sample.

Melt channelization in ascending mantle

We study a model for channels of magma flow within mantle undergoing decompression melting. Cylindrical conduits in a viscous, porous, compacting matrix are considered, and it is found that the dynamics of the conduit walls are governed by the competition between melting (caused by decompression) and viscous closure (caused by the reduced pressure in the conduit). There are many similarities with the Röthlisberger channels which transport melt water beneath glaciers. Pressure in these mantle conduits is very nearly magmastatic, and ascent velocities on the order of 100 m a−1 are predicted. Flow from the surrounding porous partially molten matrix into the low‐pressure channel is considered and can supply a continual source of melt. The accumulation region is on the order of the compaction length, and the residual matrix is reduced to very low melt fractions, typically <0.5%. Channels form naturally from porous flow in the matrix because of the enhanced melting rate in regions of higher porosity, which have a larger heat flux from below. The vast majority of melt could be expected to flow eventually into one of these channels, which therefore offer a physical grounding for the conceptual near‐fractional melting models used to explain field observations.

Impact of Conduit Geometry and Bed Porosity on Flow and Dispersion in Noncylindrical Sphere Packings

The three-dimensional velocity field and corresponding hydrodynamic dispersion in pressure-driven flow through fixed beds of solid (impermeable), uniformly sized, spherical particles are studied by quantitative numerical analysis for conduits with different cross-sectional geometries. Packings with average interparticle porosities (bed porosities) of 0.40 < or = epsilon < or = 0.50 were generated in conduits with circular, quadratic, rectangular, and semicircular cross sections utilizing a parallel collective-rearrangement algorithm. The lateral dimensions of the generated packings were chosen to represent typical values encountered in miniaturized liquid chromatography (LC) systems. The interparticle velocity field was calculated using the lattice-Boltzmann (LB) method, and a random-walk particle-tracking method was employed to model advective-diffusive transport of an inert tracer in the LB velocity field. We present the morphologies and corresponding flow patterns for these packings and demonstrate that the porosity distribution and velocity profiles of noncylindrical packings deviate significantly from those of conventional cylindrical packings. This deviation becomes more pronounced at higher bed porosities. Extended regions of high local porosity in the corners of noncylindrical conduits give rise to the formation of fluid channels of advanced flow velocity. The differences in the flow velocity distributions of cylindrical and noncylindrical packings are analyzed, and their impact on the axial hydrodynamic dispersion coefficient is shown. The presented data support the conclusion that LC performance depends critically on the conduit geometry and bed porosity. Our results have particular relevance for microchip-LC, where noncylindrical conduit geometries are prevalent.

Multistage Fatigue Modeling of Cast A356-T6 and A380-F Aluminum Alloys

This article presents a microstructure-based multistage fatigue (MSF) model extended from the model developed by McDowell et al.[1,2] to an A380-F aluminum alloy to consider microstructure-property relations of descending order, signifying deleterious effects of defects/discontinuities: (1) pores or oxides greater than 100 μm, (2) pores or oxides greater than 50 μm near the free surface, (3) a high porosity region with an area greater than 200 μm, and (4) oxide film of an area greater than 10,000 μm2. These microconstituents, inclusions, or discontinuities represent different casting features that may dominate fatigue life at stages of fatigue damage evolutions. The incubation life is estimated using a modified Coffin–Mansion law at the microscale based on the microplasticity at the discontinuity. The microstructurally small crack (MSC) and physically small crack (PSC) growth was modeled using the crack tip displacement as the driving force, which is affected by the porosity and dendrite cell size (DCS). When the fatigue damage evolves to several DCSs, cracks behave as long cracks with growth subject to the effective stress intensity factor in linear elastic fracture mechanics. Based on an understanding of the microstructures of A380-F and A356-T6 aluminum alloys, an engineering treatment of the MSF model was introduced for A380-F aluminum alloys by tailoring a few model parameters based on the mechanical properties of the alloy. The MSF model is used to predict the upper and lower bounds of the experimental fatigue strain life and stress life of the two cast aluminum alloys.

The Role of Microstructure on Ductility of Die-Cast AM50 and AM60 Magnesium Alloys

Die-cast AM50 and AM60 magnesium alloys have been examined to determine the fracture processes in bending and tension and to elucidate the influences of microstructure and porosity distribution on mechanical properties. The effect of section thickness has been explored using 2-, 6-, and 10-mm-thick die-cast plates. The processes of damage accumulation in terms of crack initiation, growth, and linkage leading to eventual failure have been studied qualitatively using progressive tensile straining experiments and three-point bend studies. The presence of a heterogeneous distribution of porosity played a critical role in the observed differences between strains to fracture in tension and in bending. More rapid damage accumulation at lower strains was observed in the high porosity regions with the rate of damage accumulation strongly dependent on the loading mode. Fracture processes at the microstructural level were characterized by scanning electron microscopy using an in-situ bending fixture. Crack initiation and growth occurred predominantly in the interdendritic eutectic regions, both in the presence of porosity and, to a lesser extent, in pore-free regions. The role of porosity volume fraction and distribution on ductility in these alloys has been examined using a modified Brown–Embury model. This model allows the prediction of fracture location in either loading mode by predicting the critical strains for the onset of cracking in differently strained regions of the test samples.

Effect of residual surface stress on the fatigue behavior of a low-alloy powder metallurgy steel

In this study we have characterized the fatigue behavior of a low-alloy powder metallurgy (P/M) sintered steel, composed mainly of martensite and bainite with 10.4% porosity. After each processing step, the residual axial surface stress was measured by X-ray diffraction. Significant compressive surface stresses were generated during the machining of the fatigue specimens. A heat-treatment at 175 °C after machining had no effect on these residual stresses, but polishing the surface resulted in a 20% reduction in compressive stresses. The residual surface stresses had no apparent effect on the fatigue behavior of the sintered steel. Rather, the fatigue behavior was controlled by sub-surface defects consisting of weakly bonded particles located in regions of high porosity.

Effect of a periodic action on average flow in an inhomogeneous liquid-saturated porous medium

The problem of the average flow of a viscous incompressible fluid saturating a stationary porous incompressible matrix under a periodic action is considered. It is shown that a spatial inhomogeneity of the medium porosity leads to an average fluid flow, quadratically dependent on the action amplitude, in the direction of increase in porosity. In particular, this means that wave action on an oil reservoir could lead to fluid flow on the interfaces from low-porosity,weakly permeable collector regions into high-porosity regions, for example, to flow from blocks to fractures in fractured porous reservoirs, which makes it possible to enhance oil production.

Adsorption of human serum albumin in porous silicon gradients monitored by spatially‐resolved spectroscopic ellipsometry

Porous silicon layers with a one-dimensional lateral gradient in pore size are prepared by electrochemical etching and characterized by spectroscopic ellipsometry in the visible to near-infrared region. The ellipsometer is equipped with a micro-spot option giving a lateral resolution of approximately 100 µm. By matching multiple-layer-model calculations to the laterally-resolved variable angle-of-incidence spectroscopic ellipsometry data, the thickness variation along the gradient as well as the in-depth porosity profile is mapped. Upon exposure to a protein solution, protein adsorption occurs on top of the porous silicon layer. At the high-porosity region of the gradient also penetration of protein molecules into the porous layer takes place. Ellipsometry data is recorded after protein exposure and variations of protein adsorption along the porous silicon gradient is modeled as well as the in-depth profile of protein penetration. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mixing-induced precipitation and porosity evolution in porous media

The dynamics of porosity evolution are explored during mineral precipitation that is induced by the mixing of two fluids of different compositions. During mineral precipitation in geological formations, the physical parameters that characterize the porous matrix, such as porosity and specific surface area, can change significantly. A series of coupled equations that determine the changes in porosity is outlined and solved for a 2D model domain using a finite element scheme. Using model parameters equivalent to those for calcite precipitation in a saline system, the evolution of porosity is examined for two types of porous media: (1) an initially homogeneous system and (2) a heterogeneous system containing high porosity regions that serve initially as preferential flow paths. In addition, the influence of two different expressions that relate specific surface area to porosity is explored. The simulations in both domains indicated that porosity was reduced primarily in the regions in which significant degrees of mixing occurred. Although an effective barrier was created in these regions, the fluids bypassed the clogged areas allowing precipitation to continue farther “downstream”. Furthermore, mixing-induced precipitation can account for systems in which some high porosity regions are filled while others remain almost unchanged. Thus, mixing-induced precipitation represents a viable mechanism for the infilling of pores in fractured and porous rocks. The simulations also demonstrate that the choice of functional form for specific surface area plays an important role in controlling porosity patterns by influencing both the kinetics of precipitation and the permeability of the porous medium. As specific surface area is currently one of the least constrained parameters in models of porosity evolution, this result highlights the need for future experimental studies in this field of research.

Analysis of the frost growth on a flat plate by simple models of saturation and supersaturation

When humid air comes in contact with a cold surface maintained below the freezing temperature, heat and mass transfer occur and a frost layer begins to grow. Simple models for the growth of the frost layer on a flat plate are presented and they focus on diffusion and phase change inside the porous media. A simple model was built on the assumption that the water-vapor concentration at the frost surface is saturated, and that the gradient of vapor pressure is the same as the value obtained from the Clausius–Clapeyron equation. It was found that initial porosity of the frost layer affected the characteristics of frost growth. In order to solve the inconsistency of the diffusion resistance of water-vapor in the region of high porosity, the model was modified with the assumption that the concentration of water-vapor at the frost surface was in supersaturation. Two parameters controlling the densification and thickness growth of the frost layer were obtained.

Reliability of Os model ages in pervasively metasomatized continental mantle lithosphere: a case study of Sidamo spinel peridotite xenoliths (East African Rift, Ethiopia)

Re–Os results are reported for a suite of spinel peridotite xenoliths from the Sidamo region of Ethiopia, near the southernmost tip of the Main Ethiopian Rift (MER). The sulfide petrography and platinum group element (PGE) characteristics, as well as the major and trace element geochemistry of these samples have been investigated in previous studies [Earth Planet. Sci. Lett. 153 (1997) 67; Chem. Geol. 196 (2003) 57], and shown to reflect the effects of melt percolation, probably related to the impingement of the mantle plume that initiated rifting. The Os isotopic ratios correlate roughly with indices of melt depletion in the peridotites, providing evidence for a long period of radiogenic ingrowth subsequent to an ancient melt extraction event. Nevertheless, this correlation has been perturbed, probably during the melt percolation process documented by the earlier studies. Consideration of the Os data in conjunction with the sulfide petrography, PGE spectra, and geochemical and textural variations of the rocks indicates that the mechanism of Os mobility changed as the melts migrated upwards through the percolation column. In the high porosity region near the bottom of the column, now represented by the peridotites with granular textures, Os from the passing melts was scavenged by monosulfide solid solution phases. The apparent modification of the Os isotopic ratios in these rocks suggests that large quantities of melt, perhaps 2 to 5 g per gram of rock, passed through the granular facies peridotites. In the shallower, low porosity region represented by peridotites with deformed textures, the melt was highly evolved and volatile rich. These volatiles may have served as the vector for Os mobility evident in the high 187Os/ 188Os ratios of the harzburgitic samples. Despite the effects of recent melt percolation, the main feature of the Os systematics is the general correlation of 187Os/ 188Os with indices of melt depletion. This demonstrates that Os isotopes may still provide useful chronological information, even in regions where the lithosphere has been strongly perturbed by the impingement of a mantle plume. Model ages obtained from this correlation, as well as the Re-depletion age of the most depleted sample, indicate that melt extraction occurred between 2.4 and 2.8 Ga. This is much older than the Pan-African age of the overlying crust [J. African Earth Sci. 26 (1998) 207], and could suggest a major decoupling between the crust and the lithospheric mantle in this region, with the mantle representing an extension of the nearby Tanzanian cratonic root. However, the geochemistry of the Sidamo peridotites does not resemble that of typical cratonic mantle [Earth Planet. Sci. Lett. 96 (1989) 15]. Furthermore, several recent studies [Earth Planet. Sci. Lett. 177 (2000) 319; Ninth Annual V.M. Goldschmidt Conference, Abstract #7389, LPI Contribution No. 971, Lunar and Planetary Institute, Houston (CD-ROM), 1999; Geochim. Cosmochim. Acta 55 (1991) 1421; Science 281 (1998) 2011] have shown that large masses of depleted material can survive in the convecting mantle for hundreds of millions of years. Thus it may be unwise to equate the time of melt extraction with the time of lithosphere stabilization in a given region.

Protein adsorption in porous silicon gradients monitored by spatially-resolved spectroscopic ellipsometry

Porous silicon layers with a one-dimensional lateral gradient in pore size are prepared by electrochemical etching and characterized by spectroscopic ellipsometry in the visible to near-infrared region. The ellipsometer is equipped with a micro-spot option giving a lateral resolution of approximately 100 μm. By matching multiple-layer-model calculations to the laterally-resolved variable angle of incidence spectroscopic ellipsometry data, the thickness variation along the gradient as well as the in-depth porosity profile is mapped. Upon exposure to a protein solution, protein adsorption occurs on top of the porous silicon layer. At the high-porosity region of the gradient also penetration of protein molecules into the porous layer takes place. Ellipsometry data are recorded after protein exposure and variations of protein adsorption along the porous silicon gradient is modeled as well as the in-depth profile of protein penetration.

Lagrangian approach for simulating the gas-particle flow structure in a circulating fluidized bed riser

Lagrangian approach is used to simulate the realistic process of cluster formation in circulating fluidized bed riser. The influence of particle properties, porosity function and gas velocity on the global particle flow structure, and the formation and development of local regions of higher particle concentration are investigated. The collision parameters govern the formation of non-homogeneous flow structure because of the difference of kinetic energy and velocity distribution. Moreover, the porosity function has a significant effect on the clusters. The gas flows preferentially into the region of high porosity, which leads to non-uniform drag force on the particles and affects strongly the particle flow structure. Examination of the cluster microstructure shows that the particle velocity in the cluster is smaller than around it. Segregation of particles with different diameter and density is also predicted.

CT derived porosity distribution and flow domains

Computed X-ray tomography images of dry undisturbed soil samples of the coarse sandy loam of Korkusova Hut (KH) and the fine sand of Hupselse Beek (HB) are analysed concerning the image resolution, the direction of scanning and the evaluation of boundaries between the fast and slow flow domains. The image resolution appears to be a less important factor than the direction of scanning. Additional data sets are recommended for partitioning the fast and slow flow domains. Comparison is made with the tracer breakthrough pictures, magnetic resonance (MR) images and outflow volumes obtained from infiltration outflow experiments. The two soils under study appeared to have essentially different flow dynamics. In the KH sandy loam soil, the fast flow occurs in regions of high local porosity. In case of the HB fine sand, the sample appeared to consist of a dense part, where the flow process took place, and a more porous part, which stayed dry. Flow instability was detected on MR images for the KH soil.

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging withT1Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT1enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT1andT*:2relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,1H, were acquired in a phantom study. Sulfur hexafluoride,19F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.

Microstructural features limiting performance of PM steels: H.Danninger et al. (Technical University of Vienna, Austria.) Int. J. Powder Metall., vol 33, no 4, 1997, 43–53

Pressed and sintered structural steels exhibit mechanical properties that are markedly inferior to those of their wrought counterparts. This is due primarily to the unique microstructure of powder metallurgy products which includes porosity as the pivotal feature. It is shown here that porosity results in an out-of-proportion decrease in the effective load-bearing cross-section and this microstructural parameter is related directly to the mechanical properties. Interconnected porosity is more detrimental than isolated porosity, resulting in isolated sintered necks that are mostly small compared to the powder particles size and are thus unable to sustain higher loads. Also the distribution of porosity is rarely homogeneous, and the high porosity regions act as crack initiation sites particularly in cyclic loading. Fatigue fracture is also caused by singular defects such as secondary pores generated via a transient liquid phase or by slag inclusions. The effect of these inhomogeneities is more pronounced, the higher the strength of the material.

Sims image analysis of D2O migration in ZrO2 thin films

The diffusion of D 2 O into and through oxide films on Zr-Nb alloys was investigated using secondary ion mass spectrometry (SIMS) image analysis. In preparation for this, the microscopic structures of the oxide were studied in relation to the grain structures in the underlying alloy. On alloys containing low concentrations of niobium (1-2.5 wt.%), the oxide was found to exhibit more localized growth, particularly above grain boundaries in the alloy. Such oxide regions appeared to be considerably more porous to D 2 O ingress. By contrast, the oxide film on the Zr 20 wt.% Nb alloy was found to be the most resistant to D 2 O ingress; no local regions of higher porosity could be found.

Channelling Flow and Heat Transfer Characteristics of Porous Insulating Layer.

The porous insulation layer may include inhomogeneous high-porosity regions near the bounding walls, which introduce additional thermal resistance and enthalpy transfer, i.e. channelling effects. The present paper deals with the channelling effects by considering effective fluid layers as models for the high-porosity regions. The paper is divided into two parts. First, boundary conditions at the interface between the porous and fluid layer are discussed, and the Beavers-Joseph condition is extended to take account of rapid velocity change in the porous layer near the interface. Second, on the basis of the extended boundary condition, an analysis is made of the effects of both buoyant channelling flow and the near-wall thermal resistance on the heat transfer characteristics. It is shown that the heat transfer is reduced by the additional thermal resistance, while it can be enhanced by the enthalpy transport due to the channelling flow. Discussion is also given of the critical thickness of the effective fluid layer to eliminate the channelling enthalpy flow.