Thin Porous Layer Research Articles

Stabilization of Mine Tailings Using Fly Ash and Sewage Sludge Planted with Phalaris arundinacea L.

The impact of plants (Phalaris arundinacea L.) on the leakage of ammonium, cadmium, copper, nitrate, phosphate, and zinc from sulfidic mine tailings covered with wood fly ash and sewage sludge was investigated. Either ash or sludge was placed in contact with the tailings, and ash layers of either low or high compactness were used. It was revealed that an ash/sludge cover effectively decreased the metal leaching from the tailings regardless of the order in which the materials were applied. Plants decreased the amount of leachate and the concentrations of ammonium and phosphate. The presence of ash below the sludge decreased the plant uptake of copper and zinc and nitrate leakage. However, when the ash was added as a thin (1.5 cm) porous layer, roots and air reached the tailings and caused high metal leakage. The results support the use of a vegetated ash/sludge cover in the treatment of mine tailings, provided that the sealing layer is firm enough to prevent root penetration.

The measurement of the shear modulus of a porous polymer layer with two microphones

An experimental method is described for measuring the shear modulus of a thin porous layer. An acoustic excitation with a loudspeaker and a simulation performed with the Biot theory allows measurement without any mechanical excitation.

Scale effect and two-phase flow in a thin hydrophobic porous layer. Application to water transport in gas diffusion layers of proton exchange membrane fuel cells

Pore network simulations are performed to study water transport in gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The transport and equilibrium properties are shown to be scale dependent in a thin system like a GDL. A distinguishing feature of such a thin system is the lack of length scale separation between the system size and the size of the representative elementary volume (REV) over which are supposed to be defined the macroscopic properties within the framework of the continuum approach to porous media. Owing to the lack of length scale separation, two-phase flow traditional continuum models are expected to offer poor predictions of water distribution in a GDL. This is illustrated through comparisons with results from the pore network model. The influence of inlet boundary conditions on invasion patterns is studied and shown to affect greatly the saturation profiles. The effects of GDL differential compression and partial coverage of outlet surface are also investigated.

Oxygen transport in La 2NiO 4 + δ: Assessment of surface limitations and multilayer membrane architectures

The steady-state oxygen permeation through dense La 2NiO 4 + δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La 2NiO 4 + δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in the case when one dense layer, ~ 60 μm in thickness, is sandwiched between relatively thin (< 150 μm) porous layers. Whilst the permeability of such membranes is still affected by surface-exchange kinetics, increasing thickness of the porous supporting components leads to gas diffusion limitations.

Structural, optical, and adsorption properties of ZnO 2/poly(acrylic acid) hybrid thin porous films prepared by ionic strength controlled layer-by-layer method

ZnO 2/poly(acrylic acid) sandwich structures were prepared by layer-by-layer (LbL) self-assembly. The structure and optical behavior of the hybrid films were controlled by changing the surface charge and conformation of the poly(acrylic acid). The buildup of the films was followed by UV–vis absorption and reflection spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), and quartz crystal microbalance (QCM) measurements. It was found that the ionic strength of the polymer solution had a great influence on the film thickness which, in turn, affected the optical properties. The water vapor adsorption isotherms of the films determined by QCM showed an adsorption hysteresis characteristic of porous thin layer structures. The adsorption of water molecules inside the films changed the effective refractive index resulting in a change of the reflection properties. This phenomenon is shown to be exploited for the application of the films as optical sensors. The polarizability of water molecules in the adsorption layer was also determined. It was found that polarization of water molecules in the adsorption layer is much lower than in the liquid water when the surface coverage ( Θ) is low.

Unsteady flows in a capillary lined with a thin porous surface layer by method of fundamental solutions

This paper studies unsteady viscous flows in a capillary using the method of fundamental solutions. The effect on fluid velocity of a thin porous surface layer on the wall of the tube is modelled using a slip boundary condition. Laplace transform is used to handle the time variables in the problem. Fundamental solutions of point force in the transformed domain and in the time domain are derived. This study extends our previous work by developing a sophisticated boundary singularity scheme based on the Method of Fundamental Solution (MFS).

Enhancement of Solid Oxide Fuel Cell Performance by La0.6Sr0.4Co0.2Fe0.8O3 − δ Double-Layer Cathode

cells with either a single-layer or a double-layer cathode were fabricated and tested. The single-layer LSCF cathode was made by stencil printing, while the double-layer cathode was prepared by depositing a thin porous layer on the SDC electrolyte using electrostatic spray deposition (ESD) before stencil printing. The cells were tested with moist hydrogen as fuel and air as oxidant. The maximum power density increased from at and at when the ESD-LSCF layer was introduced. The interfacial area specific resistances (ASRs) of electrodes for both cells were also measured under open-circuit condition. In comparison to that in the single-layer cathode cell, the electrode ASRs were reduced to half with the addition of the ESD-LSCF cathode layer.

The Potential of Pulsed Underwater Streamer Discharges as a Disinfection Technique

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we investigate the effectiveness of pulsed underwater streamer discharges for water disinfection and its scalability to large throughputs. For this paper, we have built a coaxial streamer reactor containing a central cylindrical anode covered by a thin porous layer of almandine. The reactor was supplied by electric pulses with amplitudes of up to 100 kV and pulse durations between 200 and 400 ns at a repetition rate of up to 20 Hz. Different chemical probes have been used as the main diagnostic to determine the production rates and the accumulation of oxidants in the bulk water. It has been found that the number and the length of streamers scaled linearly with the applied voltage amplitude, whereas the pulse duration only affected the streamer length. All oxidant production rates scaled linearly with the number-length product of streamers. OH-radical concentrations of up to 100 mM appear at the streamer-water-boundary. However, because of the limited volume of streamers and the limited range of OH radicals, the most important oxidant with respect to decontamination is <formula formulatype="inline"><tex>$\hbox{H}_{2}\hbox{O}_{2}$</tex></formula> which accumulates in the water and, by interacting with the intense UV radiation and the shock waves from the streamers, produces OH radicals in the bulk water. Placing water-filled cuvettes from different materials in the reactor, we found that the contribution of shock waves to the production of oxidants in the bulk water seems more important than the contribution of UV radiation. The production of oxidants can be enhanced by percolating suitable gases through the active zone of the reactor. Most effective is the use of oxygen. About 20 <formula formulatype="inline"><tex>$\hbox{J/cm}^{3}$</tex> </formula> of electrical energy was required to reduce the concentration of <emphasis emphasistype="italic"> Pseudomonas putida</emphasis> bacteria by six orders of magnitude. However, up to 180 <formula formulatype="inline"> <tex>$\hbox{J/cm}^{3}$</tex></formula> of specific energy input was necessary to reduce the bacterial freight in waste water from a municipal sewage plant by just two orders of magnitude. This has been explained by the high conductivity of waste water which drastically reduces the length of streamers for the same electrical parameters. Therefore, it seems unlikely that underwater streamer discharges alone can become competitive with other advanced oxidation processes like ozonation, UV irradiation, or irradiation with megaelectronvolt electron beams. However, it seems conceivable that a combination of underwater streamer discharges with ozonation or other processes can lead to a more effective and economic decontamination technique. </para>

Mechanical properties of Hi-Nicalon fiber-reinforced celsian composites after high-temperature exposures in air

BN/SiC-coated Hi-Nicalon fiber-reinforced celsian matrix composites (CMCs) were annealed for 100 h in air at various temperatures to 1200 °C, followed by flexural strength measurements at room temperature. Values of yield stress and strain, ultimate strength, and composite modulus remain almost unchanged for samples annealed up to 1100 °C. A thin porous layer formed on the surface of the 1100 °C annealed sample and its density decreased from 3.09 to 2.90 g/cm 3. The specimen annealed at 1200 °C gained 0.43% weight, was severely deformed, and was covered with a porous layer of thick shiny glaze which could be easily peeled off. Some gas bubbles were also present on the surface. This surface layer consisted of elongated crystals of monoclinic celsian and some amorphous phase(s). The fibers in this surface ply of the CMC had broken into small pieces. The fiber–matrix interface strength was characterized through fiber push-in technique. Values of debond stress, σ d, and frictional sliding stress, τ f, for the as-fabricated CMC were 0.31 ± 0.14 GPa and 10.4 ± 3.1 MPa, respectively. These values compared with 0.53 ± 0.47 GPa and 8.33 ± 1.72 MPa for the fibers in the interior of the 1200 °C annealed sample, indicating hardly any change in fiber–matrix interface strength. The effects of thermal aging on microstructure were investigated using scanning electron microscopy. Only the surface ply of the 1200 °C annealed specimens had degraded from oxidation whereas the bulk interior part of the CMC was unaffected. A mechanism is proposed explaining the various steps involved during the degradation of the CMC on annealing in air at 1200 °C.

Natural convection in a horizontal binary fluid layer bounded by thin porous layers

This paper reports an analytical and numerical study of natural convection in a horizontal binary fluid layer confined between two horizontal porous walls. The cavity is heated from the bottom by a constant heat flux while the long side walls are impermeable and adiabatic. The Beavers–Joseph slip condition on velocity is applied at the interface between the fluid and porous layers. Both double-diffusive convection and Soret-induced convection are considered. An analytical model, based on the parallel flow approximation, is proposed for the case of a shallow layer. The flow and heat and mass transfer variables are obtained in terms of the governing parameters of the problem. The critical Rayleigh numbers for the onset of supercritical and subcritical convection are predicted for various hydrodynamic boundary conditions. The results for a fluid layer bounded by solid walls and free surfaces emerge from the present analysis as limiting cases. The study is completed by a numerical solution of the full governing equations.

Microstructural Evolution of Anodic Coating on AZ31 Magnesium Alloy in Alkaline Silicate Solution

The microstructural evolution of the plasma electrolytic anodic coating on AZ31 magnesium alloy in an alkaline silicate solution system was studied. Voltage as high as was observed in the constant current density anodization and caused a characteristic coating transformation from a single thin porous layer to a multilayered microstructure during the anodization process. This transformation was greatly contributed to physical breakdown behaviors of the thin dielectric, during which, closed and open types of defects were observed. Such defect distribution and density in the barrier dielectric thin film were highly likely to affect the sparking behaviors as well as the resultant coating formed. In a run of anodization, a near-amorphous, silicate-containing, dense-to-porous overcoat was observed, under transmission electron microscope, attaching to a fluoride-rich barrier layer. In chemical analyses, fluoride contents were concentrated in the barrier layer along the magnesium substrate, while silicate contents were distributed away from the barrier layer in increasing amounts during the growth evolution.

Deduction of porous material properties using a point source

The acoustic field created by a point source above the plane boundary between two semi‐infinite fluid layers has been previously studied by Brekhovskikh. A straightforward generalization can be performed by replacing the fluid under the source by the fluid equivalent to a rigid‐framed porous medium. This leads to a method of evaluating, from pressure measurements, the surface impedance close to grazing incidence and the Brewster angle of total refraction of the porous medium. The model by Brekhovskikh can also be generalized when a thin porous layer is set under the point source. A pole of the reflection coefficient exists for an angle of incidence close to grazing incidence. This pole is related to an acoustic field similar to a surface wave above the layer. The measurement of this angle provides an evaluation of the surface impedance close to grazing incidence. The point source induces frame vibrations that can be predicted with the Biot theory. The rigidity coefficients of the frame at audible frequencies can be evaluated from measurements of the frame velocity with a laser velocimeter.

Acoustical and micro‐structural properties of recycled grains and fibres

This paper presents a systematic study into the production, characterisation and modeling of the acoustic behaviour of highly heterogeneous, low density porous layers having a complex pore size distribution. A new cold extrusion production method was developed at the University of Bradford to process recycled polymeric fibres and grains so that accurate control of the pore size distribution and the porosity of the resultant porous product could be attained. In this way high values of the acoustic absorption coefficient could be obtained in a relatively thin porous layer throughout the design frequency range . Two approaches were used to model the acoustic performance of the manufactured porous media. The first approach requires the direct numerical integration of the Biot viscosity correction function which depends on the probability density function of the pore size. The other approach assumes a low permeability contrast between the two porous scales so that the acoustic properties could be estimated using the semi‐phenomenological models of Johnson and Lafarge for the viscous and thermal dynamic permeabilities. Numerical results predicted by the two models were then compared with impedance tube experimental data showing good accuracy of the selected prediction methods.

Nanoporous alumina enhanced surface plasmon resonance sensors

The signal enhancement of an easy to fabricate, nanoporous alumina assisted surface plasmon resonance (SPR) sensor is investigated. It is theoretically shown that the presence of a thin (under 200nm) porous alumina layer on top of an aluminum film supporting the surface plasmons, may significantly increase (over one order of magnitude) the sensitivity of the SPR method in the case where the adsorption of relatively small molecules is probed. The comparative experimental investigation of self-assembled monolayer formation on planar metal films and porous alumina layers verifies the theoretical predictions. Based on these results, we discuss the extended applicability of this setup in biosensor and other related applications.

T-RANS Simulations of Subcritical Flow with Heat Transfer Past a Circular Cylinder Surrounded by a Thin Porous Layer

We study flow and heat transfer to a cylinder in cross flow at Re = 3,900–80,000 by means of three-dimensional transient RANS (T-RANS) simulations, employing an RNG k???? turbulence model. Both the case of a bare solid cylinder and that of a solid cylinder surrounded at some fixed distance by a thin porous layer have been studied. The latter configuration is a standard test geometry for measuring the insulating and protective performance of garments. In this geometry, the flow in the space between the solid cylinder and the porous layer is laminar but periodic, whereas the outer flow is transitional and characterized by vortex shedding in the wake of the cylinder. The results from the T-RANS simulations are validated against data from Direct Numerical Simulations and experiments. It is found that T-RANS is very well suited for simulating this type of flow. The transient nature of the flow underneath the porous layer is well reproduced, as well as the influence of vortex shedding on the heat transfer in the downstream stagnation zone. T-RANS results are found to be in much better agreement with DNS and experimental data than results from steady-state RANS.

PROPERTIES OF POROUS SILICON LIGHT-EMITTING STRUCTURE

A study has been made of current-voltage and luminescence characteristics light-emitting Al/ n+–Si/porSi/mono-Si structures with thin and thick porous silicon layers obtained by electrochemical etching. It was established that these structures had greatly improve injection carriers with respect to Al/PS/mono-Si structures.

Detailed Experimental and Theoretical Investigation of the Electron Transport in a Dye Solar Cell by Means of a Three-Electrode Configuration

Dye solar cells have been built in a three-electrode configuration where the nanoporous TiO2 layer has been contacted additionally on the rear side by means of a thin porous layer of metal titanium. With this third electrode, it was proved that under short-circuit conditions and an illumination intensity equivalent to “1 sun”, the differences of the electrochemical potential of the electrons between the two ends of the TiO2 layer reached values of more than 600 meV. The results confirm our former work carried out using an indirect method. As the third electrode could be used as a current collector as well, deeper insight in the transient behavior was achieved. A model has been developed to describe the transient electron transport through the TiO2 matrix, taking explicitly into account the numerous trap states of nanoporous anatase. The model turned out to be in good agreement with the experimental results.

On Darcy-Brinkman Equation: Viscous Flow Between Two Parallel Plates Packed with Regular Square Arrays of Cylinders

Effects of the bounding solid walls are examined numerically for slow flow overregular, square arrays of circular cylinders between two parallel plates. A local magnitudeof the rate of entropy generation is used effectively to determine the flow region affected bythe presence of the solid boundary. Computed axial pressure gradients are compared to thecorresponding solution based on the Darcy-Brinkman equation for porous media in whichthe effective viscosity appears as an additional property to be determined from the flowcharacteristics. Results indicate that, between two limits of the Darcian porous medium andthe viscous flow, the magnitude of μ (the ratio of the effective viscosity to the fluid ˆviscosity) needs to be close to unity in order to satisfy the non-slip boundary conditions atthe bounding walls. Although the study deals with a specific geometric pattern of the porousstructure, it suggests a restriction on the validity of the Darcy-Brinkman equation to modelhigh porosity porous media. The non-slip condition at the bounding solid walls may beaccounted for by introducing a thin porous layer with μ = 1 near the solid walls. ˆ

Physical and electrochemical evaluation of commercial carbon black as electrocatalysts supports for DMFC applications

This work presents results with noble metal catalysts, Pt and PtRu supported on Black Pearl with a higher surface area in comparison with carbon black Vulcan XC-72R and Vulcan XC72. The nanoparticles were synthesized following the alcohol reduction method. Brunauer–Emmet–Teller (BET) surface area analysis, X-ray diffraction (XRD), energy dispersive analysis by X-rays (EDAX), and high resolution transmission electronic microscopy (TEM) experiments were carried out to characterize the materials obtained. Cyclic voltammograms (CV) of catalysts using the porous thin layer electrode technique were obtained for the catalysts surface evaluation and for methanol oxidation to check the electrocatalytic behavior of these nanocatalyst systems.

Poroelastic modeling of seismic boundary conditions across a fracture

Permeability of a fracture can affect how the fracture interacts with seismic waves. To examine this effect, a simple mathematical model that describes the poroelastic nature of wave-fracture interaction is useful. In this paper, a set of boundary conditions is presented which relate wave-induced particle velocity (or displacement) and stress including fluid pressure across a compliant, fluid-bearing fracture. These conditions are derived by modeling a fracture as a thin porous layer with increased compliance and finite permeability. Assuming a small layer thickness, the boundary conditions can be derived by integrating the governing equations of poroelastic wave propagation. A finite jump in the stress and velocity across a fracture is expressed as a function of the stress and velocity at the boundaries. Further simplification for a thin fracture yields a set of characteristic parameters that control the seismic response of single fractures with a wide range of mechanical and hydraulic properties. These boundary conditions have potential applications in simplifying numerical models such as finite-difference and finite-element methods to compute seismic wave scattering off nonplanar (e.g., curved and intersecting) fractures.