Resulting Pore Structure Research Articles

Textured macro- and mesoporous alumina samples designed in the presence of different surfactant types

This work presents a method that enables the synthesis of porous alumina in the presence of different types of surfactant. The method is based on a sol–gel transition, using surfactants and droplets of a nonpolar phase as templates. The main purpose was to establish a relationship between the templates and the resulting pore structure. The results show that depending on the type of surfactant used (ionic or nonionic), materials presenting variations in the pore family distribution, X-ray powder diffraction patterns and sample morphology were formed. During the synthesis of these alumina samples, the drying step caused loss of the porous structures, so the shrinkage due to different types of surfactants was evaluated.

Towards Tailored Porous Polymers Using Solvent Effects in Catalytic Degradation

AbstractThe potential for tailored synthesis of porous materials using catalytic degradation of block copolymers has been shown, employing an adaptable one‐pot method. Carefully selected solvent systems help to control the pore size and molecular weight of the product. In the degradation of PS–PB–PS, 2nd Generation Grubbs' catalyst creates scissions in the PB carbon–carbon double bonds resulting in the formation of a porous structure. Critically, methanol is used to maintain the integrity of the polymeric structure during this etching. Catalyst deactivation effected by methanol can be limited through using an excess of 1‐hexene; thereby introducing a competitive metathesis process. Altering the ratio of the co‐solvents employed further controls the degradation of PS–PB–PS and hence the resulting pore structures.

Theoretical study on porphyrin based covalent organic polyhedra as a hydrogen storage

We introduce a new covalent organic polyhedron (COP) containing porphyrinyl groups. The porphyrin based COP (PCOP) is designed by modeling and simulation using 6 tetraaldehyde molecules and 8 triamine molecules combined by 24 imine bonds. Probable molecular crystal structures of the modeled PCOP are suggested by the simulated annealing Monte Carlo simulation method. We found that the predicted crystal structures show large surface areas up to over 6000 m2/g and the surface area depends on how to pack PCOP molecules and the resulting pore structure. Grand canonical Monte Carlo simulations predict the hydrogen uptakes of these polymorphs of PCOP and the values are from 98 to 262 mg/g for gravimetric uptake and from 45 to 50 kg/m3 for volumetric uptake at 77 K. Hydrogen uptakes of PCOP crystals are comparable to the best records of metal-organic frameworks (164.1 mg/g for NU-100 and 176 mg/g for MOF-210 at 77 K). Hence, PCOP is expected to be applicable to hydrogen storage.

Pore size assessment based on wall collision broadening of spectral lines of confined gas: experiments on strongly scattering nanoporous ceramics with fine-tuned pore sizes

Wall collision broadening of absorption lines of gases confined in porous media is a recently opened domain of high-resolution spectroscopy. Here, we present an experimental investigation of its application for pore size assessment. We report on the manufacturing of nanoporous zirconia ceramics with well-defined pore sizes fine-tuned from 50 to 150 nm. The resulting pore structure is characterized using mercury intrusion porosimetry, and the optical properties of these strongly scattering materials are measured using femtosecond photon time-of-flight spectroscopy (transport mean free paths found to be tuned from 2.3 to 1.2 μm as the pore size increase). Wall collision line broadening is studied by performing near-infrared (760 nm) high-resolution diode laser spectroscopy of confined oxygen molecules. A simple method for quantitative estimation of the pore size is outlined and shown to produce results in agreement with mercury intrusion porosimetry. At the same time, the need for improved understanding of wall collision broadening is emphasized.

Aluminum Integral Foams with Near‐Microcellular Structure

AbstractIn the present work the influence of the amount and size distribution of blowing agent particles (magnesium hydride, MgH2) on the resulting pore structure of aluminum foams is studied. A modified die casting process called integral foam moulding (IFM) is used where aluminum melt mixed with blowing agent is injected into a mould cavity. High cooling rates at the wall of the die result in the formation of a dense shell, whereas, the decomposition of the blowing agent in the inner region leads to a cellular core. Different particle size fractions of MgH2 are provided by sieving; the resulting pore structures of the foams are analyzed by microcomputed tomography. The results suggest that the pore size distribution represents a direct image of the size distribution of the used powder as well as of the particle density within the melt. This finding allows the production of homogeneous microcellular foams by using a high number of particles with a narrow size distribution.

ChemInform Abstract: Activation of Novel Copper-Carbon Catalysts Using Temperature- Programmed Mass Spectrometry.

An entirely new method of preparing metal–carbon catalysts is reported in which the metal is present as small crystallites distributed in a three-dimensional array throughout the bulk of the carbon matrix. The metal particles are linked by a network of pores which exposes virtually all of the metal surface to the gas phase. By virtue of their structure, these new materials exhibit a remarkable resistance to sintering, even near the melting point of the metal. While many catalytically important metals can be incorporated into our materials, this paper concentrates on the structure of one of the copper–carbon systems. These are made by the reduction of a copper(II) cellulose precursor to produce a copper(0) cellulose material, followed by thermal degradation of the cellulose and activation of the resulting carbon to produce the pore network. New techniques, using a temperature-programmed gas-flow micro-reactor linked to an on-line mass spectrometer, are used to investigate the nature of the critical activation process. The resulting pore structure is studied and a mechanism is proposed to explain the catalytic effect of the copper particles on the formation of the pore network. Poisoning experiments demonstrate that access to the active sites is determined by the pore size, thus giving a measure of size selectivity. Using temperature-programmed oxidation with on-line MS it was demonstrated that the pores can be widened in a controlled manner, so reducing any diffusion-induced limitation on reaction rates. This offers the possibility of designing a catalyst with a specified balance between the conflicting requirements for size selectivity and high reaction rates.

Changing the adsorption capacity of coal-based honeycomb monoliths for pollutant removal from liquid streams by controlling their porosity

Coal-based honeycomb monoliths extruded using methods developed for ceramic materials have been used to retain methylene blue and p-nitrophenol from aqueous solutions. The influence of the filters’ thermal treatment on their textural properties and performance as adsorbents was examined. Characterization by N 2 physisorption, mercury porosimetry and scanning electron microscopy along with adsorption tests under dynamic conditions suggest that, depending on the pollutant and its initial concentration, it can be more convenient to previously submit the monoliths to a simple carbonization or to an additional activation, with or without preoxidation, as a consequence of their different resulting pore structures. Infrared spectroscopy indicates that their different adsorption behaviour seems not to be related to differences in their surface chemical groups. In addition, axial crushing tests show that the monoliths have an acceptable mechanical resistance for the application investigated.

Processing of carbide-derived carbons with enhanced porosity by activation with carbon dioxide

Carbide-derived carbon (CDC) materials were synthesized by high temperature chlorination of biomorphic TiC and SiC ceramic structures. CDC allows the control over the pore size by choosing an appropriate carbide precursor as well as the reaction temperature. However, the pore volume is limited by the fraction of metal in the carbide structure. Since activation procedures such as treatment in CO 2 significantly increase the porosity of the carbon structure it is applied to pyrolyzed paper preforms and CDC materials. The activation procedure raises the specific surface area (SSA BET) and the pore volume of the carbon-based materials. The effect of variation of activation conditions like temperature, time and flow rate of CO 2 on the resulting pore structure is investigated. SSA BET up to 2300 m 2 g −1 and pore volumina of about 1.3 cm 3 g −1 are obtained. Activation times up to 6 h increase the micropore volume whereas longer activation times result in a decreased micropore volume by simultaneous transformation into mesopores. Activated CDCs from biomorphic ceramics are structured materials compared to powders described mainly in the literature.

Electrophoretically Deposited Porous Ceramics and their Characterisation by X-Ray Computer Tomography

Planar zirconia green bodies with unidirectionally aligned pore channels were prepared by an electrophoretic deposition method using an effect which is usually disadvantageous for the EPD from aqueous suspensions: the formation of gas bubbles by electrolysis. Aqueous ZrO2 suspensions containing acetic acid to enable a sufficient gas generation were used for the EPD experiments. The influence of selected parameters – electrolyte content, applied voltage, and kind of deposition electrode – on the pore structure has been investigated. The green bodies were sintered at 1450 °C in air. Optical microscopy and X-ray computer tomography (CT) were used for characterising the porous structures. CT-investigations have the advantage of a three-dimensional characterisation of the samples by a non-destructive method. The resulting pore structure depended to a high degree on the experimental conditions of the EPD. The kind of the deposition electrode – platinum foil or platinum gauze on foil – had a strong influence on the arrangement of the pores. The EPD on Pt-foil led to randomly arranged pores, whereas very regular pore structures were obtained by the use of Pt-gauze.

Characterization and evaluation of hydrophobically modified chitosan scaffolds: Towards design of enzyme immobilized flow-through electrodes

Chitosan scaffolds were fabricated by application of thermally induced phase separation from aqueous solutions of unmodified chitosan and hydrophobically modified chitosan polymer. The final pore structure, in terms of diameter and geometry, were correlated to freezing temperature and freezing time for both the unmodified and hydrophobically modified chitosan polymer. Results showed that the resulting pore structure is strongly dependent upon the freezing temperature and less dependant upon the freezing time. For scaffolds produced from unmodified chitosan, the pore size decreased as expected with decreasing freezing temperature from −5 °C to −10 °C. However, an inconsistency in this trend was observed as the freezing temperature was decreased to −20 °C. Combined analysis of pore size distribution and average pore diameter suggested that the freezing process was mainly mass transfer dominated at −5 °C and −10 °C, but principally heat transfer dominated at −20 °C. In comparison, the scaffolds produced from hydrophobically modified chitosan (butyl-chitosan) followed the expected trend of decreasing mean pore diameter with decreased freezing temperatures throughout the entire temperature range. The scaffolds produced from the unmodified chitosan were more stable and rigid, and possessed average pore diameters that were generally smaller than those fabricated from the hydrophobically modified chitosan. The generally larger pores in the butyl-modified chitosan scaffolds might be explained by increased phase separation rates due to the introduced hydrophobicity of the chitosan polymer. Among the scaffolds fabricated from the butyl-modified chitosan, those produced at −20 °C yielded the most uniform pore structure, the smallest average pore diameters, and the least temporal broadening of pore size distribution.

The porous structures of activated carbon aerogels and their effects on electrochemical performance

The pore structures of carbon aerogels were designed and controlled by changing conditions for both the microemulsion-templated sol–gel polymerization and the KOH activation processes. The resulting pore structures were characterized by means of N 2 adsorption and SEM. A new pore model is proposed and the relationship between the pore structures of the activated carbon aerogels (ACAs) and their electrochemical performance is discussed. The experimental results show that the ACAs which were prepared contain four types of pores: (A) micropores with diameters below 2 nm; (B) small mesopores with diameters from 2 to 5 nm; (C) large mesopores with diameters from 5 to 40 nm; and (D) large pores and channels with diameters over 40 nm. Appropriate activation with KOH can increase the numbers of micropores and small mesopores as well as the BET areas, resulting in an increase in the specific capacitance ( C s) of the ACA samples. The type D pores and channels play an important role in the enhancement of the electrochemical capacitance under high charge–discharge current conditions. Though ACAs with different porous models show a linear relation between C s and BET area separately, the surface areas of micropores of the ACAs with appropriate type D pores and channels have a higher accessibility and efficiency in the storage of energy than those with closed type C pores.

Changes in commercial wood charcoals by thermal treatments

As a continuation of a recent study of commercial wood charcoals as far as their potential production of carbon adsorbents is concerned, we have studied the influence of the final heating temperature ( T) as carbonization variable in the range 250–1000 °C on the yield and on the characteristics of granular chars prepared from two very different charcoals: a holm-oak charcoal manufactured by partial combustion in a charcoal kiln and an eucalyptus charcoal industrially manufactured in a continuous furnace. Our study also includes the changes produced in both charcoals heated at 250 °C in air for 24 h, and their influences on the adsorption of water vapour at 25 °C. The samples were characterized by thermogravimetry, chemical analyses, Fourier transform infrared spectroscopy, densimetric measurements and mercury porosimetry. T affects the char yield, the chemical composition and the porosity of each char series differently. In particular, the total open pore volumes (to helium) of the starting charcoals, 0.475 and 1.044 cm 3 g −1, increase to 0.872 and 1.293 cm 3 g −1 heating up to 1000 and 500 °C, respectively. The changes by carbonization are mainly due to devolatilization; moreover, a moderate structural shrinkage occurs heating the eucalyptus charcoal at T > 500 °C. Concerning the air treatments, the yields do not present a significant difference; carbonyl groups are formed and the resulting pore structures depend on the starting charcoals. Water adsorption is larger for the eucalyptus carbons (approximately type V isotherms) than for the holm-oak carbons (type II isotherms).

Mesoporous Poly(benzimidazole) Networks via Solvent Mediated Templating of Hard Spheres

Mesoporous Poly(benzimidazole) with well-defined porosity and pore diameter of 11 nm was synthesized via a hard templating approach using silica nanoparticles as template. The effect of the template concentration as well as the influence of the cross-linking density on the resulting pore structure was investigated. The samples were characterized by small-angle X-ray scattering and nitrogen sorption, revealing a linear increase of the porosity and surface area with increasing content of the template up to a critical concentration. Exceeding this concentration leads to a breakdown of some pores due to the emerging high interfacial energy. The effect of the cross-linking density was shown to be nonlinear. Samples with more than 20 mol % cross-linker revealed perfect replication of the template, while a lower cross-linking density leads to a breakdown of some pores.

Bi-stable pore size during electrochemical etching of n-type silicon during a thermal ramp

We have investigated the change in size and density of pores during electrochemical etching of n-type (5 Ωcm) silicon under backside illumination and subjected to a thermal ramp. The pore structure was allowed to self organize, and for the parameters reported here this results in macropores with diameters in the ∼ μm order of magnitude range. As the etching progressed under constant current conditions, the electrolyte and the sample heated slowly up in the temperature range 20–60 °C. The resulting pore structure was obtained by scanning electron microscope examination of cross sections of cleaved samples. The temperature ramp caused the pore diameters and pore densities to change abruptly rather than continuously. The change can thus phenomenologically be described as a transition between two stable pore size configurations; bi-stable switching or phase transition. This phenomenon is observed for a range of parameters yielding macropores: current densities between 2–20 mA/cm 2 and varying light intensity. The transition is between known configurations.

Simulation of cement paste microstructure hydration, pore space characterization and permeability determination

An Integrated Particle Kinetics Model (IPKM) to simulate the evolution of cement microstructure during hydration is presented. The advantages of the model are that the space is not divided into discrete pixels, so there is no lower resolution on the size of feature which can be represented and that the kinetics of hydration are an integral part of the microstructure formation. One method to describe the resulting pore structure is as a network of cylindrical tubes. This transformation is analytical, which allows a wide range of pore sizes to be described without resolution limit. From this representation the water permeability can be determined by applying Darcy and Poiseulle laws. The numerical results obtained are compared with experimental data.

Determining Pore Structure and Growth Mechanisms in Templated Nanoporous Low-k Films

AbstractTemplating is one of the most popular methods for generating nanocomposite and nanoporous films and the resultant pore size and pore interconnection length depend strongly on porogen concentration/porosity among other factors. Positronium Annihilation Lifetime Spectroscopy (PALS) analysis has been performed on a series of films produced using increasing concentrations of a type of cyclodextrin (CD) porogen in a modified silsesquioxane host matrix. PALS reveals the relationship between the resulting pore structure (both size and interconnection length) and porosity, which can be used to deduce pore shape. At low porogen concentration, isolated pores are resolved, but the pore size is consistent with a cluster of two or three CD molecules, rather than an individual one. As the porosity increases, the aggregation of the porogen domains appears to be more 3-dimensional (pseudo-random) with gradual increase in pore size. Computer simulations using a random pore growth model show consistent trends for pore size growth, but the agreement is poor for interconnection length. It is a key demonstration of the usefulness of PALS in untangling the fundamental pore structure and its evolution in porosity. PALS characterization of porosity provides novel feedback in the understanding and design of nanoporous materials.

How do roll compaction/dry granulation affect the tableting behaviour of inorganic materials? Microhardness of ribbons and mercury porosimetry measurements of tablets.

The effect of roll compaction/dry granulation on the ribbon and tablet properties produced using different magnesium carbonates was evaluated. The ribbon microhardness and the pore size distribution of tablets were used as evaluation factors. Increasing the specific compaction force resulted in higher microhardness for ribbons prepared with all four magnesium carbonates accompanied with decreased part of fine. Consequently, the corresponding produced tablets displayed a lower tensile strength. A possible correlation between the particle shape, surface area and the resulting pore structure of tablets produced with the four different types of magnesium carbonate was observed. The tensile strength of tablets prepared using granules was lower than tensile strength of tablets produced using starting materials. The partial loss of compactibility resulted in a demand of low loads during roll compaction. However, the impact of changes in the material properties during the roll compaction depended greatly on the type of magnesium carbonate, the specific compaction force and the tableting pressure applied.

Open-pore morphology of i-PP copolymer crystallized from a gel state in supercritical propane

Open-pore morphology was produced in low-density polypropylene ( i-PP) by first crosslinking the i-PP and then crystallizing it from a highly swollen state in supercritical propane. The extent of crosslinking, expressed by an increasing gel fraction and an increasing compression modulus, was found to have a strong effect on pore size but less so on crystal structure. The resulting pore structure, studied using scanning electron microscopy, showed a decrease in pore size with an increase in gel fraction. Typical pore sizes are between 1 and 10 μm. The polymer morphology in the wall, analyzed using wide-angle X-ray diffraction, indicated the presence of α-PP crystals. The experimental results showed that mostly loose chains (sol fraction) contribute to the crystallinity of the polymer. For our specific i-PP, the crosslinked network devoid of sol fraction (after extraction) collapses during cooling and does not produce any open pore structure.

Effect of Compaction Pressure on the Microstructure Evolution of Anode Substrate for SOFC Application

Ni/YSZ cermet anode is required to satisfy various properties as a key component of anode-supported type SOFC, since the cell performance is dependent upon anode properties. Ni/YSZ anode, with complicated structural features, consists of three different phases such as metallic Ni, YSZ ceramic and pores. The correlation between those structural features makes it difficult to independently optimize individual function of anode such as gas permeability, electrical conductivity and electrochemical activity. Microstructure of anode substrate was evaluated in terms of pore structure and metallic phase development by varying compaction pressure. The resulting pore structure went through a gradual transition, which was correspondingly reflected in gas permeability and electrical conductivity of anode substrate. An effort to establish structure-property-processing correlation has been made in this study to achieve improved anode performance.

Silica Nanocasting of Simple Cellulose Derivatives: Towards Chiral Pore Systems with Long‐Range Order and Chiral Optical Coatings

AbstractAqueous solutions of hydroxypropyl cellulose with cholesteric superstructures were employed as templates in a silica nanocasting process, and the resulting pore structures were characterized by polarization microscopy, circular dichroism, X‐ray scattering, transmission electron microscopy, and sorption measurements. Whereas the local pore architecture corresponds to a cast of single cellulose strands and their statistical aggregates, the averaged structure on larger scales keeps the cholesteric character, with the cholesteric pitch just slightly shortened.