Characterization Of Porous Materials Research Articles

Microstructural characterization of porous materials by two-dimensional X-ray refraction topography

X-ray refraction topography determines the internal surfaces and interfaces of non-metallic porous materials within the range of namometer to micrometer dimensions. The method is based on refraction of X-rays caused by interfaces of microstructures in heterogeneous materials. Generally, it permits a non-destructive full volume characterization of the pore topology. The X-ray refraction scanning technique make possible to visualize integral interface properties up to 10 μm spatial resolution by two-dimensional topographic images. It detects the spatially resolved internal surfaces of both, open and closed pores. Comparison to a certified powder reference of uniform grain size and known packing density reveals a quantitative measure for calibration requirements. An investigation on SiC- and Al 2O 3-ceramics illustrate the quantitative characterization of the internal surface density, pore sizes and their spatial distribution as well as local porosity fluctuations.

Characterization of porous materials: past, present and future

Although many different experimental methods are now available for characterizing porous solids, some of the more important techniques have undergone very little change over the past 50 years. Thus, it is still customary to make use of over-simplified theoretical models of gas adsorption and mercury intrusion for the determination of surface area and pore size distribution. Another ongoing problem is the complexity of many porous materials of technological importance. Striking progress has been made in modeling certain types of ordered pore structures, but disordered pore networks present a much greater challenge. To facilitate the computation of pore size distribution, there is still a tendency to focus attention on simple assemblages of cylindrical or slit-shaped pores. Future progress will depend on a more critical appraisal of the significance of derived values of surface area, pore size, porosity, fractal dimension, etc. Other future requirements include the development of a range of new reference materials having uniform pore structures.

Characterization of porous materials: past, present and future

Characterization of porous materials: past, present and future

Characterisation of porous materials by combining mercury porosimetry and scattering techniques

Small-angle neutron scattering (SANS), ultra small-angle neutron scattering (USANS) and mercury porosimetry have been used to fully characterise macroporous chalk as well as Si 3N 4 and BaTiO 3 samples. For the scattering techniques, multiple scattering effects have been studied. In addition, the scattering data have been evaluated by using a generalised form of the indirect Fourier transformation technique suitable for concentrated systems. Pore size distributions are calculated and compared to the mercury porosimetry results.

Ion transmission and characterization of ordered nanoporous alumina

Ordered nanoporous alumina samples with a pore diameter of 70 nm, an array period of 100 nm and several thicknesses were considered as possible masks for pattern transfer by MeV ion lithography. A simple procedure for the sample alignment using a 2 MeV He + beam was utilized. The energy distributions of transmitted ions as well as backscattering spectra were studied in aligned and non-aligned orientations. The best transmission, comparable to the relative surface area covered by pores, was reached for 2 μm thick samples and was independent on ion species. Although the transmission for thicker membranes was generally lower, it significantly depended on the quality of each individual sample. The presented ion beam technique can therefore be used as a tool for the characterization of porous materials. The acceptance angle for transmission through pores and the effective atomic density of samples can be obtained from the experimental data and it is shown that nanoporous alumina can be used as a mask for MeV ion lithography.

Characterization of porous materials with a rigid frame via reflected waves

The inverse problem for waves reflected by porous material is solved at oblique incidence, and an inverse scattering calculation of porosity and tortuosity is given for air-saturated plastic foam samples. The interaction of the sound pulse with the fluid-saturated porous material is described by a time-domain equivalent fluid model. The sensitivity of the porosity and tortuosity is studied and it shows their effect on the reflection coefficient at the first interface. This study shows that porosity is much more sensitive than tortuosity to reflection, especially when the incident angle is less than its critical value, at which the reflection coefficient vanishes. Some advantages and perspectives of this method are discussed.

Characterisation of Porous Materials by NMR

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Experimental determination of porous media properties for predicting absorption of bulk-reacting surfaces

Porous materials are commonly used and located along enclosure boundaries for the purpose of sound attenuation. An accurate representation of the boundary condition is needed for the analytical modeling of these enclosures. The proper characterization of porous materials requires the determination of the effective complex phase speed and effective complex density of the fluid during propagation through the porous material. This work presents a two-step process in which these characteristic properties are determined using a standing wave tube device. The normal incidence impedance of a porous material is obtained for two cases: a zero velocity condition, and a zero pressure condition, behind the material sample. The effective phase speed and density are then obtained from formulas containing the normal incidence impedances. For verification, standing waves are predicted theoretically using the previously obtained parameters as inputs for the case of a two-dimensional slender duct with the porous material lining a side wall. These predictions are compared to experimental measurements of sound pressure in the two-dimensional duct and good agreement is observed for the open-cell foams considered in this work.

Application of the fundamental measure density functional theory to the adsorption in cylindrical pores

In this work we have implemented the fundamental-measure density functional theory due to Kierlik and Rosinberg to describe the adsorption of Lennard-Jones molecules in cylindrical pores. The accuracy of the theory in predicting adsorption isotherms and particle density profiles is checked by comparison with grand canonical Monte Carlo simulations for a wide range of pore sizes, showing very good agreement in all cases. In addition, the theory has been applied to the adsorption in slitlike pores to study the influence of the pore geometry on this property. The results indicate that the confinement of the cylindrical geometry introduces significant differences in the shape of the adsorption isotherms and density profiles. These differences are relevant for the characterization of porous materials.

Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon

Amorphous materials are usually characterized using nitrogen adsorption isotherms at 77 K taken at pressures up to 1 bar to obtain pore size distributions. Activated carbons are amorphous microporous graphitic materials containing pores which can range from nanometers to microns in width and which can, in principle, be tailored to adsorb specific molecules or classes of molecule by changing the method of preparation (the activation process). For the physical chemist, they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. In this paper, we compare pore size distributions of an ultrahigh surface area activated carbon (AX21) determined from nitrogen adsorption measurements up to 0.6 bar at 77 K with those determined from carbon dioxide adsorption measurements up to 20 bar at 298 K. Our analysis employs grand canonical and Gibbs ensemble Monte Carlo simulations together with accurate site−site interaction models of the adsorbates. We find that the calculated pore size distributions for each adsorbate are quite different, and the adsorption of one gas can be estimated from the adsorption of the other gas to within an error of 25% at the highest pressures only. At lower pressures, we speculate that large errors are due to the behavior of nitrogen in carbon micropores in which diffusion is severely limited. To substantiate this speculation, we have calculated the self-diffusion coefficient for nitrogen at 77 K and carbon dioxide at 298 K in carbon slit pores using equilibrium molecular dynamics. The results suggest that nitrogen is diffusionally limited, and possibly frozen, in such pores whereas carbon dioxide remains mobile. We conclude that room-temperature carbon dioxide adsorption isotherms up to the saturation pressure could provide a more accurate characterization of carbon microstructure than nitrogen isotherms at 77 K up to 1 bar.

Characterization of porous materials by finite concentration inverse gas chromatography

Inverse gas chromatography (IGC) is a well established tool for the measurement of physico-chemical properties of solid materials at infinite dilution as well as for the determination of isotherms and surface heterogeneity at higher concentration. A novel combination of IGC and thermal desorption methods allows an advanced characterization of porous materials with organic vapors. Different porous materials are investigated by a combined IGC/flash thermodesorption. The method is applied to separate the micropore contribution to adsorption from the mesopores and the outer surface area using the difference in the adsorption mechanism.

Nitrogen sorption in aerogels

Proper equilibration upon nitrogen sorption at 77.4 K is the key for a reliable characterization of porous materials with respect to their mesopores. In the past, aerogels were often insufficiently equilibrated, thus yielding misleading isotherms and highly erroneous pore size distributions (PSDs). To allow for the extraction of the mean mesopore size with an accuracy better than 10% total run times up to 150 h are required for 2 mm sized samples with porosities above 90%. The total run time needed to measure a well equilibrated isotherm is increasing with mesoporosity, sample size and compliancy of the sample. If highly porous aerogels are well equilibrated and their deformation due to capillary forces upon sorption is taken into account, both, adsorption and desorption branches of the isotherm yield the same mean pore size when calculated via the Kelvin equation for cylindrical pores.

Porous material characterization – ultrasonic method for estimation of tortuosity and characteristic length using a barometric chamber

An ultrasonic method of acoustic parameter evaluation for porous materials saturated by air (or any other gas) is discussed. The method is based on the evolution of speed of sound and the attenuation inside the material when the static pressure of the gas saturating the material is changed. Asymptotic development of the equivalent fluid model of Johnson–Allard is used for analytical description. The method allows an estimation of three essential parameters of the model: the tortuosity, and the viscous and thermal characteristic lengths. Both characteristic lengths are estimated individually by assuming a given ratio between them. Tests are performed with industrial plastic foams and granular substances (glass beads, sea sand) over a gas pressure range from 0.2 to 6 bars at the frequencies 30–600 kHz. The present technique has a number of distinct advantages over the conventional ultrasonic approach: operation at a single frequency, improved signal-to-noise ratio, possibility of saturation the porous media by different gases. In the case when scattering phenomena occur, the present method permits a separate analysis of scattering losses and viscothermal losses. An analytical description of the method is followed by a presentation of the set-up and the measurement procedure. Experimental results and perspectives are discussed.

Characterisation of porous materials by means of nuclear magnetic resonance

Characterisation of porous materials by means of nuclear magnetic resonance

Optical Characterization of Porous Materials

Optical Characterization of Porous Materials

Characterization of Porous Materials by Gas Adsorption at Ambient Temperatures and High Pressure

Activated carbons are amorphous microporous graphitic materials formed (or activated) from a variety of organic precursers using high-temperature steam or acids. The possibility of modifying the activation process to create smaller or larger pores, from nanometers to microns in width, tailored to adsorb specific molecules or classes of molecule make activated carbons important industrial adsorbents. For the physical chemist they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. One aim is to make predictions concerning the adsorption properties for a given material, i.e., a specific microstructure. In this paper we use molecular simulation methods, including Gibbs ensemble simulation, to determine new molecular models for nitrogen, methane, and carbon dioxide and grand canonical ensemble simulation (together with new experimental data for the adsorption of these gases on Vulcan ...

Membrane voltammetry: the metal/electrolyte interface

A straightforward method to generate ensembles of microelectrodes, using a commercial membrane material is reported. Quantitative analysis of the oxidative currents flowing for a solution phase redox probe (in voltammetric and chronoamperometric experiments) shows that a recessed microelectrode model describes transport to the individual elements of the ensemble. Treatment of the recessed electrode geometry in terms of an equivalent inlaid microelectrode geometry is described. This approximation allows the voltage sweep rate dependence of the voltammetric currents and the standard electron transfer rate constant of the redox probe to be obtained using existing theory. Furthermore, we propose that membrane voltammetry may be a useful tool for the characterisation of porous materials, given the inherent sensitivity of electrochemical experiments to mass transport parameters.

Nitrogen adsorption in compliant materials

The adsorption of nitrogen at 77.4 K, a standard tool for the characterization of porous materials, has been carefully reexamined for compliant mesoporous materials. Simultaneous measurements of nitrogen sorption and linear length change of silica aerogels with bulk moduli between 1.8 and 16 MPa reveal a characteristic shape of the isotherm that is due to contraction and reexpansion of the sample in different phases of adsorption and desorption. The capillary pressure upon filling and emptying of mesopores results in a volumetric contraction of up to 50%. Once the mesopores in the bulk of the contracted porous backbone are completely filled, the capillary pressure is released and the material slowly reexpands, accompanied by further adsorption until the sample is fully relaxed. This is the first time that the reexpansion close to saturation has been observed. The lack of this feature in previous experiments is due to insufficient equilibration of the sample upon adsorption. Analysis of the isotherm indicates that, even for compliant samples, a breakthrough radius can be extracted that is in excellent agreement with values derived from other methods. In addition, the isotherm of compliant samples allows for a calculation of bulk modulus.

Evolutionary strategy algorithm for a complete characterization of porous materials using a standing wave tube

A widely used model for describing the attenuation of an acoustical wave propagating in a rigid open-cell porous material is the Johnson–Champoux–Allard (JCA) model. This model is based on five macroscopic parameters describing the porous medium: flow resistivity, porosity, tortuosity, viscous and thermal characteristic lengths. Simultaneously with the development of direct methods for measuring these five parameters, defining and solving inverse problems based on an artificial intelligence approach appears to overcome some of the limitations of the direct methods. In this work, an application of the evolutionary strategy (ES) algorithm for the estimation of the five parameters of a porous material from simple acoustical measurements is presented. First, a number of numerical tests are performed and the results are used like a priori knowledge of how to set up an evolutionary algorithm to solve such a difficult problem in the shortest time. In the second step, the final setup of the evolutionary algorithm is applied for evaluation of the five parameters from experimental measurements. The method seems practical and promising since it is based on simple acoustical measurements and avoids using complicated and unreliable measurement setups.

Characterization of Porous Materials: Experimental Studies of Mercury Intrusion/Extrusion Phenomena

Characterization of Porous Materials: Experimental Studies of Mercury Intrusion/Extrusion Phenomena