Influence of the composition and temperature of heat treatment of porous glasses on their structure and light transmission in the visible spectral range

The pore structure and light transmission of high-silica porous glasses in the visible spectral range have been investigated as a function of the heat treatment temperature and the composition of the initial two-phase alkali borosilicate glass. The character of light transmission in porous glasses has been analyzed in the framework of the concepts of structural features of their pore space and the processes occurring in the porous glass during heating. It has been demonstrated that an increase in the temperature of heat treatment of porous glasses with different compositions leads to an increase in the pore size and a decrease in their specific surface area (with a nearly constant total porosity), which is associated with the processes of overcondensation of pores due to the rearrangement and the change in the packing density of secondary silica particles. It has been revealed that the introduction of phosphate and fluoride ions into the initial sodium borosilicate glass results in an increase in the light extinction coefficient of porous glasses due to the increase in the sizes of phase-separated inhomogeneity regions in the initial two-phase glasses, the formation of larger pores, and the presence of nanosized microcrystalline phases in porous glasses.     

Influence of Lead Oxide Introduction into the Composition of Phase-Separated Alkali Borosilicate Glasses on the Electrosurface Characteristics of Porous Products of Leaching

A comparative investigation of the structural, adsorption, and electrokinetic characteristics of porous glasses (products of leaching of alkali borosilicate glasses with and without lead oxide) as functions of the leaching conditions and the composition of the equilibrium electrolyte solution is carried out. The introduction of PbO leads to a decrease in the radius of pores, an increase in their specific surface, and a change in the electrochemical characteristics of porous glass.     

Microporosity of Porous Glasses: New Investigation Techniques

Two new methods are developed for investigating the microporosity of porous materials. The first method, namely, diffusion diagnostics, is based on a computer simulation of the kinetics of gas desorption from a porous material into high vacuum. In this method, the kinetics of gas desorption is measured using mass spectrometry. Another method is based on the analysis of the equilibrium isotherms of gas desorption at low pressures. The microporous and mesoporous substructures of the porous glasses prepared by leaching of two-phase alkali borosilicate glasses in a 3 M HCl solution at 100°C are investigated using both the new methods and the classical adsorption methods. A correlation between the porous structure of glasses and their synthesis and phase separation conditions is revealed. It is demonstrated that porous glasses have a through mesoporous substructure with mean pore diameters ranging from 4 to 15 nm and a polymodal microporous substructure with pore sizes ranging from 0.35 to 2 nm. This microporous substructure is formed by ultramicropores of molecular sizes and also medium- and large-sized micropores. It is found that porous glasses contain bottle-shaped pores with sizes corresponding to the micropore range. An increase in the temperature of heat treatment of glasses from 550 to 700°C and an increase in the boron oxide content in the silica phase lead to an increase in the volume of micropores in porous glasses.     

NMR Study of Adsorbate Self-Diffusion in Porous Glasses

The NMR pulsed field gradient technique was used to study molecular transport in porous glasses. The adsorbent materials were produced by leaching phase-separated sodium-boron-silica glasses of different composition, then heat-treating. The pore diameters of the glass samples produced were within the interval 0.8 to 50 nm. Water, methanol, dodecane, and do-decene were used as adsorbates. The coefficients of adsorbate self-diffusion were found to decrease with decreasing pore diameter. In comparison with the neat liquids, a reduction in the adsorbate mobility (up to two orders of magnitude) was observed. For the larger pores, that decrease may be attributed to the tortuosity of the adsorbent, whereas the low diffusivities in the porous glasses with small pores are a consequence of the stabilizing effect of the rigid adsorbent framework.     

Nonexponential Dynamics in Porous Glasses

The dielectric relaxation properties of porous glasses prepared from sodium borosilicate glasses are studied by dielectric spectroscopy over a wide range of frequencies (20 Hz–1 MHz) and temperatures (–100–300°C). The dielectric behavior reflecting the geometric disorder is analyzed within the models describing the non-Debye slowly damped dynamics. It is found that the dielectric response is very sensitive to microstructural and mesostructural features of the porous matrix and the properties of a material filling pores. The response contains information on the dynamics of water molecules in pores, which accounts for the interaction of these molecules with the pore surface.     

Advancing the fabrication of YSZ-inverse photonic glasses for broadband omnidirectional reflector films

A single-step and all-colloidal deposition method to fabricate yttrium-stabilized zirconia (YSZ)-inverse photonic glasses with 3 μm pores was developed. The process is based on electrostatic attraction and repulsion in suspension, controlled by surface charge of polystyrene (PS) microspheres and YSZ nanoparticles, used as pore templates and matrix material, respectively. The pH was used as a tool to change surface charges and particle-particle interactions. Photonic glass films with 3 μm pores yielded broadband omnidirectional reflection over the wavelengths of 1–5 μm, relevant for thermal radiation at temperatures around 1200 °C. These highly porous materials maintained their structural stability and reflectance after being annealed at 1200 °C for 120 h.     

Microporous Substructure of Porous Glasses: Dynamic and Equilibrium Approaches

Porous glasses produced by the leaching of two-phase alkali borosilicate glasses have been studied by dynamic and equilibrium methods for diagnostics of pore morphology. It has been detected the availability of microporous substructure with some kinds of adsorbing micropores of diameter 0.3–2 nm including ultramicropores of molecular size in porous glasses with transporting pores of mean diameter from 4 to 15 nm. The multimodal nanoporous structure of porous glasses detected by kinetic mass spectral method of gas diffusion diagnostics (DD-method) at low temperatures is consistent with the results obtained from analyzing equilibrium desorption isotherms of nitrogen, oxygen and argon at 77.5 K by different calculation techniques including an equilibrium method of gas desorption at low partial pressures (LPED-method). Micropore volume in porous glasses is equal to 6–18% from total pore volume. The dependence of nanoporous morphology of porous glasses on conditions of their production and composition has been established. The diffusion and equilibrium characteristics of different molecules (nitrogen, oxygen, argon) varying in molecule size and quadrupole moment value have been determined for primary and secondary porous substructures of porous glasses at liquid nitrogen temperature at the first time.     

New adsorption-condensation theory for adsorption of vapors on porous activated carbons

A new analytical model to describe equilibrium adsorption of condensable vapors on porous adsorbents is developed. It accounts for the heterogeneous pore structure of the adsorbent, adsorption in the micropores by a pore filling mechanism and adsorption and condensation in the macropores. A gamma-type pore size distribution function is used. Langmuir-type adsorption equations are used to describe both micropore filling and adsorption on the macropore walls. The vapor condensation in the pores is described by the Kelvin equation. The model is successfully tested using isotherm data for adsorption of various condensable vapors on different porous activated carbons and charcoals. All three types (I, IV and V) of adsorption isotherms by the Brunauer classification which are depicted by the porous adsorbents can be described by the model.     

Increased H2 gravimetric storage capacity in truncated carbon slit pores modeled by Grand Canonical Monte Carlo

Hydrogen adsorption in slit shaped pores built up from truncated graphene fragments has been simulated using Grand Canonical Monte Carlo technique and the influence of pore wall edges on hydrogen storage by physisorption has been analyzed. We show that due to the additional gas adsorption at the pore edges the adsorbed gravimetric amount significantly increases (by a factor of two) with respect to models of pores with infinite graphene walls. The contribution of the edges’ adsorption to the total hydrogen uptake is independent of the pore wall shape but it depends on its surface. We also show that the maximum of the excess adsorption shifts towards higher pressures when the edge contribution increases. This information can be used to characterize experimentally structures of porous adsorbents and complement pore size distribution analysis usually performed with gases others than hydrogen. We suggest that porous carbons built from polycyclic hydrocarbons can achieve storage performances required for practical applications.     

Tailoring of a γ-Alumina Membrane with a Bimodal Pore Size Distribution for Improved Permeability

Porous γ-alumina with a bimodal pore size distribution has been developed by adding nanosized polystyrene beads to boehmite sol as templating units. The primary pore diameter is in the range of 4–6 nm and the secondary pore diameter is ca. 50 nm with minor pore shrinkage. The unsupported γ-alumina with different porous structures are characterized using thermogravimetric analysis, Fourier transform infrared spectra, X-ray diffraction, N₂ adsorption/desorption, and transmission electron microscopy. γ-alumina with a bimodal porous structure shows reduced transport resistance compared with γ-alumina with a unimodal porous structure in the dye adsorption test. Although the thickness of γ-alumina thin layer increases when more secondary pores are generated, a γ-alumina membrane with a bimodal pore size distribution shows diminution of transport resistance in the water permeability study also.     

Computational Simulations for the Assessment of the Mechanical Properties of Glass with Controlled Porosity

Porous glass with closed controlled porosity is used as a model system in order to numerically assess the effect of pores on the macroscopic mechanical and fracture behavior of brittle solids. A computational code called OOF, which converts digitalized two-dimensional (2-D) images of materials microstructures into finite element meshes, is adopted, so that the effect of 2-D microstructural features (e.g. pore size and shape) on the global mechanical response of the material can be determined. Firstly, microstructures of porous glass bodies containing isolated pores were considered. These specimens were numerically investigated in terms of fracture initiation and propagation: the numerical model predicted that larger pores initiate fracture, in agreement with experimental results. Then, the effect of porosity on the elastic and fracture properties was thoroughly investigated by means of model two-dimensional microstructures consisting of selected area fractions of pores (equivalent to pore volume fractions in three dimensions) and with prescribed pore shape, orientation and dimensions. In particular, the effect of pore dimension and shape was studied, finding that the critical stress for crack initiation scales with pore dimension and aspect ratio, i.e. oblate and larger pores oriented perpendicularly to the stress direction cause a higher reduction of strength of the specimen. Finally, several 2-D microstructures characterized by different values of area fraction of pores of the same shape were investigated, in order to determine the variation of elastic properties and the fracture response of porous glasses with pore content. The study confirms the suitability of the 2-D OOF code to investigate the mechanical and fracture behavior of porous materials. Issues regarding the limitation of the model due to its 2-D character are also discussed where appropriate.     

Gel permeation chromatography: Physical characterization and chromatographic properties of corning porous glasses

Five porous glasses, manufactured by Corning, reputed to have very narrow pore size distributions, have been characterized by mercury porosimetry, nitrogen adsorption–desorption isotherms, and electron microscopy. The characteristics of these glasses as packing materials in gel permeation chromatography have been determined. Using polystyrene solutes and toluene solvent at room temperature, the glasses in series combination of columns are capable of separating molecular weights from a few thousand to several million. These glasses are compared with other available materials for column packings in gel permeation chromatography.     

Correlations between pore structure parameters and gas permeability of corundum porous materials

Corundum porous materials with different contents of calcium hexaluminate formed in situ were prepared using pure calcium aluminate cement as the calcium source. The surface fractal dimensions of the porous materials were calculated based on the experimental data of mercury intrusion. Correlations between pore structural parameters and the permeability coefficients k₁ and k₂ of the porous materials were then studied based on the grey system theory. The results showed that pores in the corundum porous materials have great fractal characteristics. The surface fractal dimension was a significant pore structural parameter that reflected the complexity of pore shape, pore surface, and pore-size distribution, which had the maximum correlation coefficient with the permeability of this type of porous materials. The apparent porosity and pore-size distribution had relatively high correlation coefficients to the permeability as well. Increasing the apparent porosity and the volume percentage of larger pores, and decreasing the volume percentage of smaller pores all benefited the permeability of the porous materials. In addition, the mean pore size and median pore size showed lower correlation coefficients to the permeability—especially for porous materials with a wide pore-size distribution.     

2D-NLDFT adsorption models for carbon slit-shaped pores with surface energetical heterogeneity and geometrical corrugation

In this work, we show that the standard slit pore model widely used for the characterization of activated carbons may be improved by introducing structural and/or energetical heterogeneity to the surface of pore walls. The existing one dimensional slit pore model assumes graphite-like energetically uniform pore walls. As a result of this assumption adsorption isotherms calculated by the non local density functional theory (NLDFT) do not fit accurately the experimental N₂ data measured for real activated carbons. Assuming a graphene-based structural framework for activated carbons and using a 2D-NLDFT treatment of the fluid density in the pores we consider two options for model pores: energetically heterogeneous (EH) and geometrically corrugated (GC). For testing, we applied these two models to the pore size analysis of porous carbons that were giving poor results of the analysis with the standard slit model. We found that the typical artifacts of the homogeneous slit pore model were eliminated. Also, the agreement of the new models with experimental data was significantly better than that of the standard slit model.     

The significance of physical factors on the adsorption of polyaromatic compounds by activated carbons

The adsorption of three synthetic organic compounds (SOCs) (i.e., phenanthrene, biphenyl, and 2-chlorobiphenyl), with similar physicochemical properties but different molecular conformations (i.e., planar and nonplanar), by an activated carbon and an activated carbon fiber was investigated. The physical characteristics of the carbons were obtained from low temperature nitrogen adsorption isotherms using BET, DR, and DFT models. Their surface chemistry was characterized by water vapor adsorption, pH of the point of zero charge, acid/base uptakes, and elemental analysis. The results indicated that adsorbent pore structure characteristics and adsorbate molecular conformation are important in the adsorption of SOCs by porous carbonaceous adsorbents. To predict the adsorption of SOCs by activated carbons, accurate characterization of pore shape and size distribution in primary micropores is important. The results indicated that adsorbate molecules can access and fill more efficiently the slit-shape pores than ellipsoidal pores, whereas the ellipsoidal pores create higher adsorption potential than slit-shape pores. Both molecular conformation and dimensions of adsorbate affect the adsorption. Planar molecules appear to access and pack in slit-shape pores more efficiently as compared to nonplanar molecules. Nonplanar molecular conformation weakens the interactions between adsorbate molecules and carbon surfaces.     

Porous polymers prepared via high internal phase emulsion polymerization for reversible CO2 capture

A series of porous polymers with different pore volumes, pore sizes, and crosslinking densities were synthesized by high internal phase emulsion (HIPE) polymerization. The crosslinked polymerized HIPEs (polyHIPEs) were formed by the copolymerization of 4-vinylbenzyl chloride and divinylbenzene using water droplets in conventional or Pickering HIPEs as the templates. These porous materials were further modified by quaternization and ion exchange to introduce quaternary ammonium hydroxide groups. The resulting polyHIPEs were utilized as sorbents for reversible CO₂ capture from air using the humidity swing. The effect of pore structure on the CO₂ adsorption and desorption processes was studied. The polyHIPEs containing large pores and interconnected porous structures showed improved swing sizes and faster adsorption/desorption kinetics of CO₂ compared to a commercial Excellion membrane with similar functional groups.     

Diffusion in porous media of a random structure

The present paper deals with diffusion in a porous solid, which is considered as a discrete random medium composed of random structural elements (pores) chaotically connected with each other. Each structural element is characterized by the passage time distribution function calculated with taking into account the correlations of flows passing through the neighboring pores. The analysis of a nonstationary diffusion process, after transition to the limit of times much exceeding the mean passage time of a single pore, enables to obtain general expressions connecting the effective diffusivity in a porous medium with the statistical parameters of structural elements. At the same time the values are calculated which characterize the deviation of the concentration distribution in a porous medium from the gaussian one obtained as a solution of the quasi-homogeneous diffusion equation. To make the general expressions more concrete it is necessary to introduce a model of structural elements (pores) which determines the passage time distribution. The models of straight and corrugated pores considered yield expressions for the effective diffusion coefficient in the Fick and Knudsen regions. A similar analysis of the nonstationary diffusion accompanied by the first order reaction allows to get the expression for the effective rate constant of the catalytic reaction in a porous medium.     

Stress-induced anisotropy during sintering of hierarchical porosity ceramics

There is significant interest in the design and processing of porous ceramics due to their use in a variety of applications including energy storage, catalysis, adsorption, separation, and life science applications. For many of these applications, it is desirable to have a hierarchical porous structure in which there is a distinct difference between sizes of pores. Our previous study has shown that microstructure and properties of porous materials become anisotropic after sinter-forging. In particular, the small interparticle pores (intrinsic pores) orient parallel to the applied compressive stress, in contrast to large pores from pore formers (extrinsic), which orient perpendicular to the applied stress. However, the pore size, for transition from extrinsic to intrinsic behavior, (transient pore size) has not been quantified. In this study, we report on the effect of applied stresses during sinter-forging on the morphology (shape and size) of pores of different size. Based on these results, we propose a two-step approach to predict transient pore size for hierarchically porous ceramics. We use this approach to quantify the effect of applied stresses on the transient pore size. Finally, we postulate that the stress dependence of the transient pore size may be related to sintering stress—a fundamental quantity in continuum models of sintering. In addition, it can be used to calculate the effective surface energy of complex sintering systems.     

Sorption of poly(vinyl acetate) on cellulose. II. The role of the porous structure

The adsorption of poly(vinyl acetate) from benzene solution onto cellulosic materials having various porous structures was measured in an attempt to investigate the role of the pore size distribution in the sorption process. The variety of cellulose porous structures was obtained by combinations of different swelling agents—water, ethylenediamine, sodium hydroxide solution—with different subsequent drying treatments. The pore structure analysis was based on benzene desorption isotherms. The porosity of cellulose is responsible for selective adsorption of the smaller macromolecules from an unfractionated polymer solution. The amount of sorbed polymer increases when the polymer solution contains a greater fraction of lower molecular weight polymer. Only the pores above a certain size are accessible to the polymer. The amount of polymer sorbed is proportional to the area of such pores but is otherwise independent of the effects produced by swelling pretreatments.     

Micromembrane absorber with deep-permeation nanostructure assembled by flowing synthesis

A micromembrane adsorber with deep-permeation nanostructure (DPNS) has been successfully fabricated by flowing synthesis. The nanoparticles are in-situ assembled in membrane pores and immobilized in each membrane pore along the direction of membrane thickness. The nanoparticles with a lower size and thinner size distribution can be achieved owing to the confined space effect of the membrane pores. As a concept-of-proof, the nano ZIF-8 and ZIF-67 are fabricated in porous membrane pores for methyl orange (MO) and rhodamine B (RhB) adsorption. The adsorption rate is increased significantly owing to the enhanced contact and mass transfer in the confined space. The adsorption capacity for the RhB is also increased, since the size of the nanoparticles assembled in membrane pores is smaller with more active sites exposed. This micromembrane adsorber with DPNS has good reusability and can provide a promising prospect for industrial application.