Porous Layer Thickness Research Articles

Heat and mass transfer in the system hydrocarbon gas–porous carbon layer–metal and formation of supersaturated solid carbon solutions

UDC 536.248 It is shown that the carbon layer formed on the catalytic surface of a metal plays an important role in industrial technologies of obtaining carbon nanofibers. The effective heat conduction of porous carbon samples was measured. A mathematical model of the heat and mass transfer inside a carbon layer has been developed and its stationary solutions were investigated. It is shown that a decrease in the thickness of a porous carbon layer on the surface of a metal as a result of its cleaning and a further growth of this layer lead to the formation of a supersaturated solid solution of carbon in the metal.

Self-similarity in coupled Brinkman/Navier–Stokes flows

AbstractIn this paper we derive self-similar solutions of flows through both a porous medium and a pure fluid. Self-similar filtration velocity and hydrodynamic shear profiles are obtained by means of asymptotic analysis in the limit of infinitely small permeability, and for both laminar and turbulent regimes over the porous medium. We show that a spatial length scale, related to the porous layer thickness, naturally emerges from the limiting process and suggests a more formal definition of thick and thin porous media. We finally specialize the analysis to porous media constituted of patterned cylindrical obstacles, which can freely deflect under the aerodynamic shear exerted by the fluid flowing through and over the forest. A self-similar solution for the bending profile of the elastic cylindrical obstacles is obtained as intermediate asymptotics, and applied to carbon nanotube (CNT) forests’ response to aerodynamic stresses. This self-similar solution is successfully used to estimate flexural rigidity of CNTs by linear fit of appropriately rescaled maximum deflection and average velocity measurements.

Control of nanoenergetics through organized microstructures

Heavily doped p-type silicon substrates were etched to form thick porous layers (∼170 µm) and impregnated with magnesium perchlorate to form reactive composites. Characterization of the reactive wave propagation by high speed photography and spectroscopic methods indicated slow propagation rates between 1 and 8 m s−1. Multiscale structures were formed on the same substrates using microfabrication techniques followed by an electrochemical etch based on a random micro-crack pattern observed in lower doped substrates which yielded faster propagating composites. These organized multiscale composites exhibited flame propagation speeds up to 500 m s−1 indicating that reaction propagation can be controlled by structural modifications.

Controlled Ultraviolet (UV) Photoinitiated Fabrication of Monolithic Porous Layer Open Tubular (monoPLOT) Capillary Columns for Chromatographic Applications

An automated column fabrication technique that is based on a ultraviolet (UV) light-emitting diode (LED) array oven, and provides precisely controlled "in-capillary" ultraviolet (UV) initiated polymerization at 365 nm, is presented for the production of open tubular monolithic porous polymer layer capillary (monoPLOT) columns of varying length, inner diameter (ID), and porous layer thickness. The developed approach allows the preparation of columns of varying length, because of an automated capillary delivery approach, with precisely controlled and uniform layer thickness and monolith morphology, from controlled UV power and exposure time. The relationships between direct exposure times, intensity, and layer thickness were determined, as were the effects of capillary delivery rate (indirect exposure rate), and multiple exposures on the layer thickness and axial distribution. Layer thickness measurements were taken by scanning electron microscopy (SEM), with the longitudinal homogeneity of the stationary phase confirmed using scanning capacitively coupled contactless conductivity detection (sC(4)D). The new automated UV polymerization technique presented in this work allows the fabrication of monoPLOT columns with a very high column-to-column production reproducibility, displaying a longitudinal phase thickness variation within ±0.8% RSD (relative standard deviation).

Investigation of a Confined Laminar Impinging Jet on a Plate with a Porous Layer Using the Preconditioned Density-Based Algorithm

The preconditioned density-based algorithm and two-domain approach were used to investigate the fluid flow and heat transfer characteristics of a confined laminar impinging jet on a plate covered with porous layer. In the porous zone, the momentum equations were formulated by the Darcy-Brinkman-Forchheimer model; the thermal nonequilibrium model was adopted for the energy equation. At the porous/fluid interface, the applicability and influence of different hydrodynamic and thermal interfacial conditions were analyzed for the problem. The governing equations were solved by the preconditioned density-based finite-volume method, with preconditioning matrix for equations of porous domain adopted, aiming to eliminate the equation stiffness of porous seepage flows. The effects of Reynolds number, porosity, Darcy number, thermal conductivity ratio, Biot number, and porous layer thickness on the flow pattern and local heat transfer performance were studied. Results indicate that the Reynolds number and porosity don't strongly influence the flow pattern of porous channel, while the Darcy number and porous layer thickness have obvious influence on the flow pattern. The heat transfer performance are greatly influenced by the parameters studied.

Fully Developed Forced Convection in a Parallel Plate Channel with a Centered Porous Layer

In this study, fully developed heat and fluid flow in a parallel plate channel partially filled with porous layer is analyzed both analytically and numerically. The porous layer is located at the center of the channel and uniform heat flux is applied at the walls. The heat and fluid flow equations for clear fluid and porous regions are separately solved. Continues shear stress and heat flux conditions at the interface are used to determine the interface velocity and temperature. The velocity and temperature profiles in the channel for different values of Darcy number, thermal conductivity ratio, and porous layer thickness are plotted and discussed. The values of Nusselt number and friction factor of a fully clear fluid channel (Nu cl = 4.12 and fRe cl = 24) are used to define heat transfer increment ratio $$({\varepsilon _{\rm th} =Nu_{\rm p}/Nu_{\rm cl})}$$ and pressure drop increment ratio $$({\varepsilon_{\rm p} =fRe_{\rm p}/fRe_{\rm cl} )}$$ and observe the effects of an inserted porous layer on the increase of heat transfer and pressure drop. The heat transfer and pressure drop increment ratios are used to define an overall performance $$({\varepsilon = \varepsilon_{\rm th}/\varepsilon_{\rm p})}$$ to evaluate overall benefits of an inserted porous layer in a parallel plate channel. The obtained results showed that for a partially porous filled channel, the value of $${\varepsilon}$$ is highly influenced from Darcy number, but it is not affected from thermal conductivity ratio (k r) when k r > 2. For a fully porous material filled channel, the value of $${\varepsilon}$$ is considerably affected from thermal conductivity ratio as the porous medium is in contact with the channel walls.

Double-layer ITO antireflection electrodes fabricated at low temperature

A double-layer antireflection (AR) coating fabricated using the single Indium tin oxide (ITO) conductive material is proposed. The high refractive index of the bottom ITO layer was achieved by long-throw radio-frequency magnetron sputtering technique at room temperature. On the other hand, the top ITO layer with low refractive index was prepared by applying supercritical CO2 (SCCO2) treatment at 60°C on gel-coated ITO thin films. Porous structures were obtained with the SCCO2 treatment to volatilize or extract organic matters from the films. Their refractive indices were precisely controlled by varying the sputtering power and the operation pressure of the long-throw sputtering and the SCCO2 treatment, respectively. Weighted average reflectance and average transmittance of 4.3% and 86.2% were achieved for the double-layer ITO AR electrode. In addition, the sheet resistance of the AR electrode is 1.14kΩ/□ with 65nm thick porous ITO layer on 75nm thick sputtered ITO layer.

The Influence of Porous Structure on Tribological Properties of Porous UHMWPE

Porous UHMWPE with different character were prepared by hot press molding in Metallographic Sample Mounting. Friction and wear loss of porous UHMWPE samples were studied under different loads and lubricant on an improved tribological tester. The friction coefficient and wear loss of samples with porosity of 5.44% are lower than that of samples with porosity of 10.17%, which attribute to the better porous structure and extrusion effect of lubricant in testing. Among the increasing of loads, friction and wear loss of porous samples are enhance rapidly. It is also found that the thickness of porous layer has little effect on the tribological properties of porous UHMWPE.

Simultaneous overcome of the equilibrium limitations in BSCF oxygen-permeable membrane reactors: Water splitting and methane coupling

The equilibrium limitations of water splitting and the coupling of methane to C2 hydrocarbons (ethane+ethylene) were simultaneously overcome by using a perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) oxygen-permeable membrane reactor. Oxygen produced from thermal water splitting was transported through the BSCF membrane and consumed in the coupling of methane. The BSCF membrane consists of an about 70μm thick dense BSCF layer on an about 0.8mm thick porous BSCF layer as support. By applying the membrane reactor concept instead of a fixed bed reactor without oxygen supply, the methane conversion and C2 yield increased from 3.7% to 26% and 3.1% to 6.5% at 950°C, respectively. In both experiments, the supported 2wt.% Mn–5wt.% Na2WO4 catalyst was used at 950°C. Simultaneously, about 9% of the H2O injected was converted to hydrogen with a production rate of about 3.3cm3min−1cm−2 at 950°C which is higher than 1 m3 (STP) H2 m−2h−1.

Superficially porous particles columns for super fast HPLC separations

Superficially porous silica particles columns (SPSPCs) are manufactured by different companies. The most common have the brand names Halo, Ascentis Express and Kinetex. These columns provide super fast, sharp peaks and moderate sample loading and back pressure. These are available in different chemistries such as C₈, C₁₈, RP Amide and Hilic. Normally, the silica gel particles have 2.7 and 1.7 µm total and inner solid core diameters with 0.5 µm thick outer porous layer, 90 Å pore size and 150 m²/g surface area. They have been used for the separation and identification of low and high molecular weight compounds. The present article describes the state of the art of superficially porous silica particles based columns with special emphasis on their structures, mechanisms of separation, applications and comparison.

Dependence of Dye-Sensitized Solar Cell Impedance on Photoelectrode Thickness

Dye-sensitized solar cells with a wide range of porous TiO2 layer thicknesses (d = 3–18 μm) have been characterized by impedance spectroscopy. Spectra were analyzed using a well-known equivalent circuit model incorporating a transmission line to obtain the distributed resistance and capacitance parameters characterizing the TiO2 layers. No significant dependence on d was found for any of the distributed parameters, reaffirming the validity of this commonly used model. Other elements in the equivalent circuit model which represent the substrate/electrolyte, cathode/electrolyte, and FTO/TiO2 interfaces are also examined and discussed.

TiO2 Nanostructures Obtained in Non-Aqueous Electrolytes at Low Potential

In this paper, the synthesis of TiO2 porous arrays on titanium substrates in non-aqueous solution of ethylene glycol (1 vol.% H2O) + 0.5 wt.% NH4F was investigated. Electrochemical anodization of titanium was carried out at 3V during 39 to 72 hours. The microstructural analysis of samples by scanning electron microscopy and atomic force microscopy suggests that the thickness of porous layers decrease with the anodization time, whereas the polydispersity in the pore size increases by the integration of small pores formed early during the anodization process. At 72 hours the presence of a double wall of the porous is indicative of the incipient formation of nanotubes. By the other hand, the pore depth suggests that the oxide film was formed by the coalescence of 20-50 nm diameter hemispherical titania particles.

Spectroscopic ellipsometry study of porous silicon-tin oxide nanocomposite layers

The layer-by-layer distribution of components in a porous silicon-tin oxide nanocomposite produced by the following three methods is studied by spectroscopic ellipsometry: chemical vapor deposition, atomic layer deposition, and magnetron sputtering. It is shown that, in the nanocomposites fabricated by these methods, SnOx penetrates to a depth more than 400 nm and is nonuniformly distributed over the porous layer thickness. The nanocomposite prepared by magnetron sputtering followed by heat treatment has the maximum penetration depth and the maximum uniformity of layer-by-layer SnOx distribution.

Experimental study on the lift generation inside a random synthetic porous layer under rapid compaction

Lift generation in a soft porous medium under rapid compaction is a new concept for porous media flow. This concept is of extraordinarily broad interest since it applies to such diverse problems as the motion of a red cell in a tightly fitting capillary [8], the lifting forces generated during skiing or snowboarding [22], and the design of a futuristic train track [18,19], in addition to classical lubrication theory applications [13,14]. In this paper, we developed a systematic, experimental approach to examine the pore pressure generation inside a deformable porous medium. To accomplish this task, a novel porous-walled cylinder–piston apparatus was developed. This apparatus was fully instrumented with pressure transducers, an accelerometer and a displacement sensor. Two synthetic fibers with different microstructures and mechanical properties were tested under various, precisely-controlled loading conditions. Enhanced lift was observed for both of them. The results indicated that lift generation inside a compressible porous medium strongly depends on the material properties and loading conditions. Softer material with lower Darcy permeability was able to generate higher pore pressure, although lowering the permeability of a porous medium was usually accompanied by the increase in the solid phase contribution to the total lift and thus led to the decrease in the pore fluid pressure. It was also observed that higher pore pressure was generated if the porous layer thickness was increased. The study presented herein has provided a rigorous approach for experimentally examining the lift generation in a deformable porous medium. It is of significant importance for the designs of rotational squeeze dampers and shock absorbers, as well as for the application of highly compressible porous media for enhanced lubrication.

Kinetic optimisation of open-tubular liquid-chromatography capillaries coated with thick porous layers for increased loadability

The kinetic optimisation of open-tubular liquid chromatography (OTLC) columns has been revisited by taking the thick-film effects for porous coatings on retention, column resistance, band broadening and mass loadability into account. Considering the most advantageous case ( i.e. where the retentive layer allows for the same high internal diffusion coefficient as conventional porous particles), calculations show the need for the development of coating procedures leading to porous films filling up approximately 50–70% of the total column diameter. Furthermore, to achieve optimum kinetic performance for separations of small molecules with total analysis times of less than 8 h ( k′ = 9), total column diameters should be less than 6 μm with lengths typically greater than 0.8 m for N values of 125,000–500,000 at a pressure of 400 bar. The use of elevated temperature LC (90 °C) is also shown to increase the allowable total column diameter to up to 9 μm for a larger range of N values (100,000–880,000).

1.1 μm superficially porous particles for liquid chromatography. Part I: Synthesis and particle structure characterization

Superficially porous particles are characterized by a non-porous particle core surrounded by a thin porous layer. Superficially porous particles have been shown to have chromatographic advantages over traditional totally porous particles by reducing the resistance to mass transfer and the eddy diffusion contributions to the theoretical plate height, particularly for biomolecule separations. Currently, 1.7 μm superficially porous particles are commercially available, but a further decrease in the particle diameter and reduction in the porous layer thickness has the potential to further improve the efficiency of the column packing material. In this study, the synthesis of smaller diameter superficially porous particles was investigated. As the particle diameter was decreased, however, synthesis parameters previously reported were rendered unsuitable due to particle agglomeration, non-uniform coating, and porous layer disintegration. Parameters such as colloidal silica size, drying process, and sintering temperature were investigated to improve the structural characteristics of smaller diameter superficially porous particles. Reported is a synthetic route for production of 1.1 μm superficially porous particles having a 0.1 μm porous layer. Based on the revised method, the particles produced have a surface area, pore diameter, and particle size distribution RSD of 52 m 2/g, 71 Å, and 2.2%, respectively.

Nanomechanical properties of thick porous silicon layers grown on p- and p+-type bulk crystalline Si

Abstract The nanomechanical properties and the nanoscale deformation of thick porous Si (PSi) layers of two different morphologies, grown electrochemically on p-type and p+-type Si wafers were investigated by the depth-sensing nanoindentation technique over a small range of loads using a Berkovich indenter and were compared with those of bulk crystalline Si. The microstructure of the thick PSi layers was characterized by field emission scanning electron microscopy. PSi layers on p+-type Si show an anisotropic mesoporous structure with straight vertical pores of diameter in the range of 30–50 nm, while those on p-type Si show a sponge like mesoporous structure. The effect of the microstructure on the mechanical properties of the layers is discussed. It is shown that the hardness and Young's modulus of the PSi layers exhibit a strong dependence on their microstructure. In particular, PSi layers with the anisotropic straight vertical pores show higher hardness and elastic modulus values than sponge-like layers. However, sponge-like PSi layers reveal less plastic deformation and higher wear resistance compared with layers with straight vertical pores.

Natural Convection in a Cavity Partially Filled with a Vertical Porous Medium

The shear stress jump boundary condition that must be imposed at an interface between a porous medium and a free fluid in an enclosure is investigated. Two-domain approach is founded and finite element method is used to solve the problem. Three stress jump coefficients 0, 1, -1 are analyzed for different Rayleigh number, permeability and thickness of porous layer. Variation of Maximum stream function and Nusselt number show stronger convection and heat transfer when the stress jump coefficient is positive. There is little distinctive in flow and heat transfer when the value of coefficient is equal to 0 and -1.

Fabrication of Gradient Optical Filter Containing Anisotropic Bragg Nanostructure

New gradient optical filters containing asymmetric Bragg structure were prepared from the distributed Bragg reflector (DBR) porous silicon (PSi). Anisotropic DBR PSi displaying a rainbow-colored reflection was generated by using an asymmetric etching configuration. Flexible anisotropic DBR PSi composite films were obtained by casting of polymer solution onto anisotropic DBR PSi thin films. The surface and cross-sectional images images of anisotropic DBR PSi composite films obtained with cold field emission scanning electron microscope indicated that the average pore size and the thickness of porous layer decreased as the lateral distance increased. As lateral distance increased, the reflection resonance shifted to shorter wavelength.

Performance enhancement of polysulfone ultrafiltration membrane by blending with polyurethane hydrophilic polymer

Abstract The aim of this work was to study the effect of two solvents such as N,N′-dimethylformamide (DMF) and dimethylacetamide (DMAc) used in the production of polysulfone (PSf) and polyurethane (PU) blend ultrafiltration membranes, influence permeability and selectivity. PSf/PU membranes were prepared by the phase inversion method using polymer concentrations of 100/0, 20/80 and 0/100 wt%. Permeation performance of the prepared membranes was evaluated in terms of pure water flux, water content and hydraulic resistance. It was found that the membrane composed of 20/80 wt% of PSf/PU in the presence of DMAc showed a water flux of 60.5 l m-2h-1 under transmembrane pressure of 345 kPa, and the water flux of 23.5 l m-2h-1 for 20/80 wt% of PSf/PU in the presence of DMF. With increasing the polarity of DMAc in the casting solution, an increase in porous layer thickness was observed, and then a good water flux of membranes can be obtained. It was found that the polar solvent caused the rapidly demixing of casting solution in coagulation bath and formed porous asymmetric membranes with defective skin layer. The permeation rates of proteins were measured with different molecular weights of the proteins.