Typical Pore Size Research Articles

Pseudo-interface Rayleigh wave on a permeable porous medium/vacuum interface.

It is well known that the Rayleigh wave propagating along a permeable plane boundary of a poroelastic half-space may have a high-frequency cutoff beyond which the corresponding Rayleigh pole is absent. The present study investigates the specific features of the surface wave propagation during transition through this cut-off frequency. Using a set of experimentally determined mechanical parameter values for water-saturated sintered glass beads in the framework of Biot's theory, this theoretical investigation indicates the following. The Rayleigh wave upper cut-off frequency may occur within a physical frequency range over which the characteristic wavelength far exceeds typical pore size. Beyond the cut-off frequency, the Rayleigh pole migrates onto the non-principal, in other words, unphysical, Riemann sheet. As a consequence, during this transition, the Rayleigh wave transforms into a pseudo-interface wave and radiates part of its energy into the interior of the half-space in the form of P2-wave motion.

Monolithic germanium oxide aerogel with the building block of nano-crystals

In the present paper, we have prepared germanium oxide aerogel using the sol–gel method followed by CO2 supercritical drying. Scanning electron microscopy showed a fine microstructure of the as-prepared aerogel. Results from the nitrogen adsorption and desorption data exhibited that the aerogel had attractive specific surface area and typical pore size distribution of aerogel. X-ray diffraction indicated that the aerogel was composed of nano-crystals with hexagonal structure. X-ray photoelectron spectroscopy confirmed the existence of oxygen vacancies. At room temperature, interesting strong blue luminescence was observed at 425nm under excitation wavelength at 280nm. We proposed an explanation for the phenomenon.

Probing hysteresis during sorption of cyclohexane within mesoporous silica using NMR cryoporometry and relaxometry

The conventional data analysis methods for obtaining a pore size distribution (PSD) from gas sorption data make several critical assumptions that impact significantly on the accuracy of the PSD thereby obtained. In particular, assumptions must be made concerning the nature of the pore-filling or emptying process in adsorption, or desorption, respectively. The possibility of pore–pore interactions is also generally neglected. In this work, NMR cryoporometry and relaxometry have been used to study the adsorption and desorption of cyclohexane within a mesoporous, sol–gel silica catalyst support pellet with the aim of assessing the impact of the aforementioned problems for gas sorption PSDs and developing solutions. The advanced melting effect makes cryoporometry a particularly sensitive probe of adsorbate ganglia spatial distribution. It has been demonstrated that utilising gas sorption scanning curves provided insufficient additional information to alleviate the aforementioned problems with interpreting gas sorption data. The NMR data has shown how the nature of the sorption hysteresis changed with amount adsorbed, due to detectable variations in the mechanisms of pore-filling and emptying along the isotherm. Hence, relating a particular condensation or evaporation pressure to a specific characteristic pore size is not as straightforward as assumed in typical pore size analysis software. However, the NMR techniques reveal the additional information required to improve pore size estimates from gas sorption for disordered solids.

Probing pore characteristics of deformed chalk by NMR relaxation

1H Nuclear Magnetic Resonance (NMR) relaxation time was applied to investigate pores characteristic of chalk cores which were subjected to prior mechanical loading. The NMR relaxation responses of the cores loaded in elastic region (before yield) and in plastic region (after yield) were compared. T1,2 relaxation measurements performed on fully water saturated cores resolved a typical pore size corresponding to large pore body, which is affected by stress application particularly beyond the yield point. The 1/T1,2 relaxation rate also appeared to correlate with the volumetric strain. The NMR signal from the partially saturated cores (Sw<13%), however, showed no respectable change despite being loaded beyond the yield point. The T2 signal was dominated by two relaxation rates, which we argue corresponds to the surface water film (∼10nm) and micropores (∼0.1μm). The point that T1,2 NMR relaxation showed no significant change on partially saturated cores loaded beyond the yield point may indicate that the structure of water film remains unchanged despite progressive decay of pores structure during pore collapse failures.

Nonequilibrium molecular dynamics simulation of water transport through carbon nanotube membranes at low pressure

Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.

Controlled synthesis of mesoporous α-Fe2O3 nanorods and visible light photocatalytic property

Porous α-Fe2O3 nanorods with typical pore size of 2–4nm were controlled prepared by a facile hydrothermal process of Fe(NO3)3·9H2O aqueous solution in the presence of NaOH, followed by a calcination treatment. Contrast experiments indicate that the morphology and crystalline structure of the hydrothermal products depend greatly on the quantity of NaOH. Hematite nanoparticles and microplates were respectively obtained under conditions without or with excess NaOH. The porous α-Fe2O3 nanorods exhibit a high BET surface area of 105.1m2g−1 and a pore volume of 0.13m3g−1. UV–vis measurement shows wide absorption to visible light and an obvious blue-shift of the adsorption edge due to the quantum size effect. The visible-light photocatalytic performances of the as-prepared samples were evaluated by photocatalytic decolorization of methylene blue at ambient temperature. The results indicate that the photocatalytic activity of the porous α-Fe2O3 nanorods is superior to hematite nanoparticles and platelets and exhibit good reusable feature. The photocatalytic process of porous structure is determined to be pseudo-first-order reaction with apparent reaction rate constant of 1.04×10−2min−1. And the optimum photocatalyst dosage is 20mg per 100mL of dye solution. The porous α-Fe2O3 nanorods are considered potential photocatalyst for practical application due to the excellent photocatalytic behavior and good reusability.

Fabrication porous Si3N4 ceramics via starch consolidation–freeze drying process

Si3N4 ceramics with porosity of ~72% were prepared via starch consolidation–freeze drying process. Pre-gelled method was used to reduce the granular size of starch which acted as consolidator/binder and pore forming agent. Polyvinyl alcohol (PVA) was added to control ice crystal growth. The results indicated that the porosities of Si3N4 ceramics were mainly controlled by slurry concentrations, starch/PVA contents had slight influence on the porosities. With higher PVA content, large pores around 35μm were decreased, the microstructure turned more uniform. Typical trimodal pore size distributions were obtained with 0 and 2.5% PVA addition, the sample with 5% PVA only showed bimodal pore size distribution without large pores. The flexural strength was increased from 6.7 to 42.3MPa with increasing PVA content.

Optical measurement of pore scale velocity field inside microporous media

An experimental technique to quantify velocity field at pore scale with in microporous media, formed by packing of microglass spheres of size 200 μm inside a glass micro-model, is presented. A microparticle image velocimetry (μ-PIV) system is used to study velocity fields at four different spatial regions in the microporous medium. A combined particle image velocimetry (PIV) and particle tracking velocimetry (PTV) scheme is used to quantify velocity within a typical pore size of 10–50 μm. The experiments are conducted at four different flow rates. Two different measurement planes are selected for obtaining the detailed pore scale velocity field—one close to the glass wall and the other inside the porous medium at a distance 100 μm below the glass wall. The image processing technique for dealing with noisy data and sparse vector field has been discussed in detail. Probability density functions of transverse and axial velocity components are compared with available results in literature. The pore scale velocity field obtained can provide insight to flow properties in microporous media and can be a powerful tool to validate existing numerical results for flow through porous media.

Evaluation of the nanofibrillar structure of Dioscorea opposite extract for cell attachment

Dioscorea opposite has been widely used in traditional herbal medicine in the Far East, ameliorating symptoms ranging from abdominal swelling to pain. Previous studies have focused on understanding the chemical components that lead to the medicinal effects of the extract. In this study, we examined the nanostructures formed by the soluble and insoluble parts of the sticky excretion from the mucilaginous rhizome of Dioscorea opposite and evaluated their cellular response. Using atomic force microscopy, we found that the soluble extract of the excretion had the capacity to form a nanofibrillar scaffold composed of uniform ∼10 nm nanofibers with a typical pore size of ∼40 nm, while the insoluble extract formed some nanofibers without specific structure. Cellular response to the two types of nanostructures was tested by seeding with HeLa and MC3T3 cells. The observations suggested that the nanofibrillar scaffold formed from the soluble extract provided an excellent platform for HeLa cell attachment and growth and to a lesser degree for MC3T3 cells, while nanofibers from the insoluble extract displayed no cell attachment and growth. Further analysis by direct incubation of the soluble extract with growing cells indicated that components from the extract preferentially bound to HeLa cells, but not to MC3T3 cells, which might help explain the observed preference of HeLa cells on the nanofibrillar scaffold. The nanofibrillar scaffold created from the Dioscorea opposite extract and its ability to sustain the attachment of specific cell types demonstrate the potential for this natural nanomaterial in tissue engineering applications

Morphology and Linear‐Elastic Moduli of Random Network Solids

The effective linear-elastic moduli of disordered network solids are analyzed by voxel-based finite element calculations. We analyze network solids given by Poisson-Voronoi processes and by the structure of collagen fiber networks imaged by confocal microscopy. The solid volume fraction ϕ is varied by adjusting the fiber radius, while keeping the structural mesh or pore size of the underlying network fixed. For intermediate ϕ, the bulk and shear modulus are approximated by empirical power-laws K(phi)proptophin and G(phi)proptophim with n≈1.4 and m≈1.7. The exponents for the collagen and the Poisson-Voronoi network solids are similar, and are close to the values n=1.22 and m=2.11 found in a previous voxel-based finite element study of Poisson-Voronoi systems with different boundary conditions. However, the exponents of these empirical power-laws are at odds with the analytic values of n=1 and m=2, valid for low-density cellular structures in the limit of thin beams. We propose a functional form for K(ϕ) that models the cross-over from a power-law at low densities to a porous solid at high densities; a fit of the data to this functional form yields the asymptotic exponent n≈1.00, as expected. Further, both the intensity of the Poisson-Voronoi process and the collagen concentration in the samples, both of which alter the typical pore or mesh size, affect the effective moduli only by the resulting change of the solid volume fraction. These findings suggest that a network solid with the structure of the collagen networks can be modeled in quantitative agreement by a Poisson-Voronoi process.

Fabrication of highly porous scaffolds for tissue engineering based on star‐shaped functional poly(ε‐caprolactone)

The potential of novel functional star-shaped poly(ε-caprolactone)s of controlled molecular weight and low molecular weight distribution bearing acrylate end groups as material for biomedical applications was demonstrated in this study. The polymers were functionalized via Michael-type addition of amino acid esters containing amino or thiol groups showing the potential for immobilization of biomolecules. Furthermore, scaffolds of different geometries were prepared by uniaxial freezing of polymer solutions followed by freeze drying. Different solvents and polymer concentrations were investigated, resulting in scaffolds with porosities between 76 and 96%. Mechanical properties of the scaffolds were investigated and the morphology was determined via scanning electron microscopy. Scaffolds with interconnected channels were prepared using benzene, 1,2-dichloroethane or dioxane as solvent. The tubular longitudinal pores in honeycomb arrangement extend throughout the full extent of the scaffolds (typical pore sizes: 20-100 µm).

Seismic wave attenuation and dispersion resulting from wave-induced flow in porous rocks — A review

One major cause of elastic wave attenuation in heterogeneous porous media is wave-induced flow of the pore fluid between heterogeneities of various scales. It is believed that for frequencies below [Formula: see text], the most important cause is the wave-induced flow between mesoscopic inhomogeneities, which are large compared with the typical individual pore size but small compared to the wavelength. Various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter-scale mesoscopic heterogeneities. Laboratory and field measurements of seismic attenuation in fluid-saturated rocks provide indications of the role of the wave-induced flow. Signatures of wave-induced flow include the frequency and saturation dependence of P-wave attenuation and its associated velocity dispersion, frequency-dependent shear-wave splitting, and attenuation anisotropy. During the last four decades, numerous models for attenuation and velocity dispersion from wave-induced flow have been developed with varying degrees of rigor and complexity. These models can be categorized roughly into three groups ac-cording to their underlying theoretical framework. The first group of models is based on Biot’s theory of poroelasticity. The second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation. Another group of models is derived using the theory of viscoelasticity. Though all models predict attenuation and velocity dispersion typical for a relaxation process, there exist differences that can be related to the type of disorder (periodic, random, space dimension) and to the way the local flow is incorporated. The differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies. In recent years, some theoretical models of wave-induced fluid flow have been validated numerically, using finite-difference, finite-element, and reflectivity algorithms applied to Biot’s equations of poroelasticity. Application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data.

Exact expression for the effective acoustics of patchy-saturated rocks

Seismic effects of a partially gas-saturated subsurface have been known for many years. For example, patches of nonuniform saturation occur at the gas-oil and gas-water contacts in hydrocarbon reservoirs. Open-pore boundary conditions are applied to the quasi-static Biot equations of poroelasticity to derive an exact analytic expression of the effective bulk modulus for partially saturated media with spherical gas patches larger than the typical pore size. The pore fluid and the rock properties can have different values in the central sphere and in the surrounding region. An analytic solution prevents loss of accuracy from ill-conditioned equations as encountered in the numerical solution for certain input. For a sandstone saturated with gas and water, we found that the P-wave velocity and attenuation in conventional models differ as much as 15% from the exact solution at seismic frequencies. This makes the use of present exact theory necessary to describe patchy saturation, although (more realistic) complex patch shapes and distributions were not considered. We found that, despite earlier corrections, the White conventional model does not yield the correct low-frequency asymptote for the attenuation.

Mesoporous carbons derived from citrates for use in electrochemical capacitors

Two mesoporous carbons were prepared by simple pyrolysis of commercial magnesium or barium citrate and tested as electrode materials for electrochemical double-layer capacitors (EDLCs), denoted MgC and BaC, respectively. The as-prepared carbon materials were characterized by N 2 adsorption, scanning electron microscopy and Fourier transform infrared spectrometry. Nitrogen adsorption measurements demonstrated that the porosity of the prepared carbons was related to the type of metal cation. BaC possesses a typical bimodal pore size distribution (PSD) at 3.8 and about 15 nm, while MgC was between small-size mesoporous and microporous. The carbons were tested as electrode materials using different electrochemical means such as cyclic voltammetry and constant current charge-discharge. Very high specific capacitance (180 F·g −1 for MgC and 171 F·g −1 for BaC) was achieved in an ionic liquid electrolyte. BaC proved to be an excellent electrode material with a high rate performance for EDLC application and exhibited an energy density up to 53.3 Wh·kg −1 and a high maximum specific power density of 20 kW·kg −1 in IL electrolyte. The good capacitive performance of BaC is attributed to its bimodal PSD and hydrophilic surface properties.

Capacitive performance of mesoporous carbons derived from the citrates in ionic liquid

Mesoporous carbons with different pore structures are prepared via a simple pyrolysis process using citrate salts as precursors. BaC-T carbons derived from barium citrate possess large pore volume and typical multimodal pore size distribution (PSD), while MgC-T carbons derived from magnesium citrate possess high specific surface areas due to their dominant small mesopores and micropores. The capacitive performance of the prepared carbons is investigated in ionic liquid and high specific capacitances (maximum of 180.3Fg−1 for MgC-700) are achieved. Experimental data demonstrate that porosity and surface chemistry corporately determine the capacitive performance. BaC-700 and BaC-800 present good rate performance, and exhibit energy densities of about 50Whkg−1. The good capacitive performance of these carbons is attributed to their bimodal PSD with large pore size and appropriate surface properties. The MgC-800 and BaC-800 carbons present good durability of capacitance during three thousand of charge/discharge cycles.

Solid-State NMR Studies of the Local Structure of NaAlH4/C Nanocomposites at Different Stages of Hydrogen Desorption and Rehydrogenation

Structural properties of NaAlH4/C nanocomposites were studied using 23Na and 27Al solid-state NMR. The samples were synthesized by melt infiltration of a highly porous carbon support, with typical pore sizes of 2−3 nm. Physical mixtures of high surface carbon with alanates in different stages of hydrogen desorption show somewhat broadened resonances and a small negative chemical shift compared to pure alanates. This is most likely caused by a susceptibility effect of the carbon support material, which shields and distorts the applied magnetic field. After melt infiltration, 23Na and 27Al spectra are broadened with a small downfield average shift, which is mainly caused by a chemical shift distribution and is explained by a larger disorder in the nanoconfined materials and a possible charge transfer to the carbon. Our measurements show that the local structure of the nanoconfined alanate is the similar to bulk alanate because a comparable chemical shift and average quadrupolar coupling constant is found. In contrast to bulk alanates, in partly desorbed nanocomposite samples no Na3AlH6 is detected. Together with a single release peak observed by dehydrogenation experiments, this points toward a desorption in one single step. 23Na spectra of completely desorbed NaAlH4/C and NaH/C nanocomposites confirm the formation of metallic sodium at lower temperatures than those observed for bulk alanates. The structural properties observed with solid-state NMR of the nanoconfined alanate are restored after a rehydrogenation cycle. This demonstrates that the dehydrogenation of the NaAlH4/C nanocomposite is reversible, even without a Ti-based catalyst.

Modeling Effective Interface Laws for Transport Phenomena Between an Unconfined Fluid and a Porous Medium Using Homogenization

In this chapter of the special issue of the journal “Transport in Porous Media,” on the topic “Flow and transport above permeable domains,” we present modeling of flow and transport above permeable domains using the homogenization method. Our goal is to develop a heuristic approach which can be used by the engineering community for treating this type of problems and which has a solid mathematical background. The rigorous mathematical justification of the presented results is given in the corresponding articles of the authors. The plan is as follows: We start with the section “Introduction” where we give an overview and comparison with interface conditions obtained using other approaches. In Sect. 2, we give a very short derivation of the Darcy law by homogenization, using the two-scale expansion in the typical pore size parameter e. It gives us the definition of various auxiliary functions and typical effective properties as permeability. In Sect. 3, we introduce our approach to the effective interface laws on a simple 1D example. The approximation is obtained heuristically using the two steps strategy. For the 1D problem we calculate the approximation and the effective interface law explicitly and show that it is valid at order O(e 2). Next, in Sect. 4 we give a derivation of the Beavers–Joseph–Saffman interface condition and of the pressure jump condition, using homogenization. We construct the corresponding boundary layer and present a heuristic calculation, leading to the interface law and being based on the rigorous mathematical result. In addition, we show the invariance of the law with respect to the small variations in the choice of the interface position. Finally, there is a short concluding section.

New method for growing silicon carbide on silicon by solid-phase epitaxy: Model and experiment

A new method of solid-state epitaxy of silicon carbide (SiC) on silicon (Si) is proposed theoretically and realized experimentally. Films of various polytypes of SiC on Si(111) grow through a chemical reaction (at T = 1100–1400°C) between single-crystal silicon and gaseous carbon oxide CO (at p = 10–300 Pa). Some silicon atoms transform into gaseous silicon oxide SiO and escape from the system, which brings about the formation of vacancies and pores in the silicon near the interface between the silicon and the silicon carbide. These pores provide significant relaxation of the elastic stresses caused by the lattice misfit between Si and SiC. X-ray diffraction, electron diffraction, and electron microscopy studies and luminescence analysis showed that the silicon carbide layers are epitaxial, homogeneous over the thickness, and can contain various polytypes and a mixture of them, depending on the growth conditions. The typical pore size is 1 to 5 μm at film thicknesses of ∼20 to 100 nm. Thermodynamic nucleation theory is generalized to the case where a chemical reaction occurs. Kinetic and thermodynamic theories of this growth mechanism are constructed, and the time dependences of the number of new-phase nuclei, the concentrations of chemical components, and the film thickness are calculated. A model is proposed for relaxation of elastic stresses in a film favored by vacancies and pores in the substrate.

Novel proton-exchange membrane based on single-step preparation of functionalized ceramic powder containing surface-anchored sulfonic acid

A novel approach to the synthesis of a low-cost proton-exchange membrane (PEM) based on the single-step preparation of a functionalized ceramic powder containing surface-anchored sulfonic acid (SASA) and a polymer binder, is presented for the first time. The added value of this technique, compared with earlier work published by our group, is the adoption of a direct, single-step synthesis, as opposed to a multiple-step synthesis. The latter requires an oxidation step, in order to convert the thiol group into a sulfonic group. SASA powders of different compositions have been prepared and characterized by means of Brunaur–Emmet–Teller (BET), thermogravimetric analysis–differential thermal analysis (TGA–DTG), differential scanning calorimeter (DSC), Fourier transformation infrared (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical techniques. The lowest equivalent weight measured for SASA powders is 1281gequiv.−1. The ionic conductivity of a 100-μm-thick membrane is measured ex situ at room temperature (25±3°C) and the highest proton conductivity is 48mScm−1. The typical pore size, for the SASA powders is less than 10nm and ranges from 2 to 50nm for the SASA-based membranes. The membranes are thermally stable up to 250°C.Direct methanol fuel cells (DMFCs) are assembled with some of the membranes. Preliminary tests showed that the cell resistance for a ∼100-μm-thick membrane ranges between 0.29 and 0.19Ωcm2 from 80 to 130°C, respectively, and that the maximum cell power density with a 1M methanol solution is 127, 208 and 290mWcm−2 at 80, 110 and 130°C, respectively, while the corresponding methanol crossover current density is 0.093, 0.238 and 0.281Acm−2.

Electrosynthesis of manganese oxide films

Nanostructured manganese oxide films for electrochemical supercapacitors were obtained by cathodic electrosynthesis from aqueous NaMnO4 solutions. The proposed deposition mechanism is based on cathodic reduction of MnO4− species. The amount of the deposited material has been controlled by the variation of deposition time at a constant current density. Obtained films were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction analysis, thermogravimetric and differential thermal analysis. It was found that the composition and microstructure of the films depend on the concentration of NaMnO4 in the solutions used for deposition. The films prepared from the 0˙02M NaMnO4 solutions were crack free and porous with typical pore size of about 100–200 nm. Cyclic voltammetry and chronopotentiometry data for the films tested in the 0˙1M Na2SO4 solutions showed capacitive behaviour in the voltage window of 0–1˙0 V versus standard calomel reference electrode. The highest specific capacitance of ∼220 F g−1 was obtained for 90 μg cm−2 films at a scan rate of 2 mV s−1 using Ni substrates. The capacitance decreased with increasing scan rate. The charge storage mechanism is discussed.