Straight Pores Research Articles

Adsorption on Highly Ordered Porous Alumina

Porous anodic aluminum oxide (AAO) is characterized by a regular arrangement of the pores with a narrow pore size distribution over extended areas, uniform pore depth, and solid pore walls without micropores. Thanks to significant improvements in anodization techniques, structural engineering of AAO allows to accurately tailor the pore morphology. These features make porous AAO an excellent substrate to study adsorption phenomena. In this paper, we review recent experiments involving the adsorption in porous AAO. Particular attention will be devoted to adsorption in straight and structured pores with a closed end which shed new light on fundamental issues like the origin of hysteresis in closed end pores and the nature of evaporation from ink-bottle pores. The results will be compared to those obtained in other synthetic materials like porous silicon and silica.

Improving the Direct Electron Transfer in Monolithic Bioelectrodes Prepared by Immobilization of FDH Enzyme on Carbon-Coated Anodic Aluminum Oxide Films

The present work reports the preparation of binderless carbon-coated porous films and the study of their performance as monolithic bioanodes. The films were prepared by coating anodic aluminum oxide (AAO) films with a thin layer of nitrogen-doped carbon by chemical vapor deposition. The films have cylindrical straight pores with controllable diameter and length. These monolithic films were used directly as bioelectrodes by loading the films with D-fructose dehydrogenase (FDH), an oxidoreductase enzyme that catalyzes the oxidation of D-fructose to 5-keto-D-fructose. The immobilization of the enzymes was carried out by physical adsorption in liquid phase and with an electrostatic attraction method. The latter method takes advantage of the fact that FDH is negatively charged during the catalytic oxidation of fructose. Thus the immobilization was performed under the application of a positive voltage to the CAAO film in a FDH-fructose solution in McIlvaine buffer (pH 5) at 25 oC. As a result, the FDH modified electrodes with the latter method show much better electrochemical response than that with the conventional physical adsorption method. Due to the singular porous structure of the monolithic films, which consists of an array of straight and parallel nanochannels, it is possible to rule out the effect of the diffusion of the D-fructose into the pores. Thus the improvement in the performance upon using the electrostatic attraction method can be ascribed not only to a higher uptake, but also to a more appropriate molecule orientation of the enzyme units on the surface of the electrodes.

Freeze Cast Porous Mullite Ceramics and Recycling of Coal Fly Ash

In order to fabricate porous mullite ceramics with controlled pore structure and improved mechanical strength, a freeze casting route has been processed using camphene mixed with tertiary-butyl alcohol (TBA) and coal fly ash/alumina as the solvent and the ceramic material, respectively. After sintering, the solidification characteristics of camphene and TBA solvent were evident in the pore morphology, i.e., dendritic and straight pore channels formed along the solidification directions of camphene and TBA ice, respectively, after sublimation. Also, the presence of microcracks was observed in the bodies sintered at 1500 oC, mainly due to the difference in solidification volume change between camphene and TBA. The compressive strength of the sintered bodies was found generally to be dependent, in an inverse manner, on the porosity, which was mainly determined by the processing conditions. After sintering at 1300~1500 oC with 30~50 wt% solid loading, the resulting mullite ceramics showed porosity and compressive strength values in ranges of 83.8~43.1% and 3.7~206.8 MPa, respectively.

Comparative evaluation for the sorption capacity of four carbonaceous sorbents to phenol

Sorption kinetics and isotherms of phenol by four carbonaceous sorbents (activated carbon (AC), mesoporous carbon (MPC), bamboo biochar (BBC) and oak wood biochar (OBC)) were compared in this study. MPC has the fastest sorption rate and initial sorption potential, which were indicated by sorption rate constants and initial sorption rate “h” in a pseudo-second-order kinetic model. The ordered and straight pore structure of MPC facilitated the accessibility of phenol. The AC showed the greatest sorption capacity towards phenol with maximum sorption of 123 mg/g as calculated by the Langmuir model. High surface area, complexity of pore structure, and the strong binding force of the π–π electron-donor-acceptor interaction between phenol molecules and AC were the main mechanisms. The BBC and OBC had much slower sorption and lower sorption capacity (33.04 and 29.86 mg/g, respectively), compared to MPC (73.00 mg/g) and AC, indicating an ineffective potential for phenol removal from water.

Durable superhydrophilic coatings formed for anti-biofouling and oil–water separation

Building up unique interfaces by means of layer-by-layer (LbL) assembly is of interest for a variety of practical applications, particularly those that require the advantages of special wettability. Herein, a UV-cured superhydrophilic coating with high stability was constructed in the form of a hydrogen-bonding-based LbL multilayer incorporating silica nanoparticles (SNPs). In our present work, hydrophilic branched poly(ethylenimine) (BPEI) and SNPs were assembled into LbL multilayers in the presence of UV-curable poly(urethane acrylate) (PUA), which is described herein as (PUA–SNPs/BPEI)n. The as-prepared architectures showed good acid, thermal, and mechanical stability following crosslinking by UV exposure. Crosslinking is of great importance for LbL film, considering the weak interactions among polyelectrolytes. Owing to the superhydrophilicity of the coating, the anti-biofouling properties were enhanced to some extent for gram-positive Staphylococcus aureus and gram-negative Pseudomonas aeruginosa contamination. Furthermore, the superhydrophilic polycarbonate track-etched (PCTE) membrane coated with (PUA–SNPs/BPEI)n coatings with different numbers of LbL deposition cycles were also investigated for oil–water separation. To the best of our knowledge, this is the first demonstration of rough structures created on a straight pore PCTE membrane. Commercial PCTE membranes with robust coatings, which allow water to penetrate the membrane while oily substance are intercepted, have been developed for regular oil–water separation with more than 99% efficiency.

Mesoscopic crystallographic textures on shells of a hyaline radial foraminifer Ammonia beccarii

Most foraminifera, simple unicellular protists, create calcareous shells with precisely controlled architectures. We investigated the crystal fabric of porous shells of a hyaline radial foraminifer, Ammonia beccarii. The complex structures of the calcitic shell include micrometric single-crystalline domains around a straight pore ∼2 μm in diameter which are divided by strained intermediate regions. The c axes of calcite in the domains were almost perpendicular to the shell surface. The mesoscopic textures, such as periodic lateral lines with a ∼200–450 nm interval and vertical cracks with a ∼150 nm interval, were observed throughout the single-crystalline domains in the shell wall. Specific mechanical properties, such as a lower Young's modulus and a higher hardness of the shells than those of Iceland spar, are ascribed to the presence of the specific mesoscopic textures.

Etching anisotropy mechanisms lead to morphology-controlled silicon nanoporous structures by metal assisted chemical etching.

The etching anisotropy induced by the morphology and rotation of silver particles controls the morphology of silicon nanoporous structures, through various underlying complex etching mechanisms. The level of etching anisotropy can be modulated by controlling the morphology of the silver catalyst to obtain silicon nanoporous structures with straight pores, cone-shaped pores and pyramid-shaped pores. In addition, the structures with helical pores are obtained by taking advantage of the special anisotropic etching, which is induced by the rotation and revolution of silver particles during the etching process. An investigation of the etching anisotropy during metal assisted chemical etching will promote a deep understanding of the chemical etching mechanism of silicon, and provide a feasible approach to fabricate Si nanoporous structures with special morphologies.

The influence of straight pore blockage on the selectivity of methanol to aromatics in nanosized Zn/ZSM-5: an atomic Cs-corrected STEM analysis study

Here we report the direct atomic scale observation of ZSM-5 pores and the influence of extra-framework Al on methanol to aromatic (MTA).

Homoporous Membranes with Tailored Pores by Soaking Block Copolymer/Homopolymer Blends in Selective Solvents: Dissolution versus Swelling

Extraction homopolymers premixed in aligned films of block copolymers by rinsing with selective solvents has long been used for the preparation of membranes with uniform straight pores (homoporous membranes). It is frequently assumed that only the dissolution of homopolymers contributes to the pore formation. However, in this work, we demonstrate that the effect of swelling plays a significant role in determining the pore sizes. We prepare blended films of block copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and P2VP homopolymers with low molecular weight and anneal the films to perpendicularly align the P2VP microdomains. Rinsing the aligned films in ethanol results in homoporous membranes, and the pore sizes can be tuned by the dosages of P2VP homopolymers. Interestingly, the pore sizes can also be effectively tailored by changing the rinsing temperatures and/or durations because of the significant contribution of the selective swelling of P2VP blocks under strong rinsing conditions i...

Numerical simulation of coalescence phenomena of oil-in-water emulsions permeating through straight membrane pore

Abstract We numerically investigated the demulsification phenomena of oil-in-water (O/W) emulsions using a membrane coalescence process. We simulated the permeation of O/W emulsions through a membrane with a single straight pore using our numerical framework based on the coupled level set and volume-of-fluid (CLSVOF) method. With a low droplet volume fraction, small droplets, and a large pore size, the oil droplets frequently passed through the membrane without attaching to its surface. After the steady coalescence of the droplets on the membrane surface at the permeation side, large oil droplets could be obtained. With a higher droplet volume fraction, larger oil droplets, and smaller membrane pores, the oil droplets easily blocked the membrane pores and were thus forced away by the permeating flow, which yielded relatively small oil droplets. Our numerical work revealed the fundamental dynamics of the coalescence of oil droplets in an O/W emulsion using a simple membrane model.

Effect of porous supports on the permeance of thin film composite membranes: Part I. Track-etched polycarbonate supports

Thin film composite (TFC) membranes for industrial gas separation are often comprised of a thin selective layer (100nm or less) on a porous support that provides mechanical strength. As the selective layer becomes thinner to increase gas permeance and thus reduce the capital cost, the support surface morphology restricts the concentration profile of the penetrant in the selective layer. Such geometric restriction has been demonstrated using modeling and computational simulations. However, there are no rigorous experimental studies that verify the modeling results. This study investigates model two-layer TFC membranes, which are comprised of track-etched polycarbonate (PC) nanofiltration membranes with uniform and straight cylindrical pores as porous supports, and AF1600 with excellent film formation and good stability as the selective layer. The presence of a support can decrease the gas permeance by as much as 85% compared to the freestanding thin film, when the PC support has a pore radius of 47nm and porosity of 2.8%. The gas permeance increases with increasing porosity at a constant pore size and decreasing pore size at a constant porosity. The measured membrane permeances are consistent with the simulation values obtained using three-dimensional (3D) computational mass transport models that predict the steady-state penetrant concentration in the selective layer.

Eggshell penetration by Salmonella Typhimurium in table eggs: Examination of underlying eggshell structures by micro-computed tomography and scanning electron microscopy

Horizontal infection of table eggs by food borne, human infection causative agents such as Salmonella is a serious concern for consumers and industry. In this study, we investigated the relationship between eggshell translucency, mammillary layer abnormalities and pore structure using Computed Tomography (CT) and scanning electron microscopy. The effects of eggshell pore structure, size and number on Salmonella Typhimurium penetration was also investigated. The eggs were infected with S. Typhimurium and were incubated at 37°C for 3 or 6days. Micro CT results comparing shell features to shell translucency found that there was a significantly increased incidence of externally branching pores found in the high translucency score eggshell group, and more straight pores found in the low translucency score group. Different pore structures, the total number of pores and the shell thickness do not appear to play a role in the horizontal infection of eggs by the S. Typhimurium strain used in this study. While it is likely that the presence of shell pores is responsible for shell penetration, other unknown shell factors must also play a role, and eggshells with a higher incidence of shell pores are not penetrated at a higher rate.

Monolithic Membranes with Designable Pore Geometries and Sizes via Retarded Evaporation of Block Copolymer Supramolecules

We develop a retarded evaporation approach for the alignment of cylinder-forming block copolymer supramolecular monoliths, 3-n-pentadecylphenol (PDP) hydrogen-bonded polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP). A variety of highly ordered, aligned morphologies are produced by varying the dosages of PDP in the supramolecules. Treatment of the aligned supramolecular monoliths in hot ethanol leads to the dissolution of PDP and the selective swelling of P4VP, yielding enlargeable ordered mesopores along the original P4VP/PDP domains. Particularly, from supramolecular monoliths aligned in the morphology of perpendicular cylinders and gyroids, we obtain highly ordered monolithic membranes containing enlarged straight pores and bicontinuous pores, respectively. The straight and gyroidal pores were filled with phenol–formaldehyde resol and further carbonized to produce well-defined carbon nanostructures including nanofibers and reversed gyroids, demonstrating the pore accessibility and the promising templatin...

Tailoring the porous nanostructure of porous anodic alumina membrane with the impurity control

Porous Anodic Alumina (PAA) membranes have been fabricated from pure and doped aluminum films (Pure Al, Al–0.5wt.%Cu and Al–1wt.%Si) by the one-step anodization method at room temperature. Formation of the PAA has been performed by three different electrolytes, namely: sulfuric (1 M), oxalic (0.3 M) and phosphoric (0.75 M) acids. For this purpose, potentiostatic anodization in a two-electrode cell configuration has been carried out. High quality alumina with straight pores have been obtained starting from pure aluminum metal films. It is found that, different types of branched structures are formed by aluminum films doped with impurities such as Si and Cu. The relationship between the electrochemical current response and the obtained pore architectures are discussed. Oxide formation rates, barrier layer thickness, vertical and horizontal inter-pore distances are investigated. The present synthesized templates have a large surface area make it one of the mostpopular nanomaterial with applications across a gamut of areas including, energy storage, sensors and biosensors.

Characterization and design of sound absorber materials

Sound absorbing materials are widely used to mitigate noise in indoor environments. Foams and fiberglass are commonly used for passive noise control in the automotive, aerospace industries and for architectural design. The physics of sound absorption in porous materials is not typically included in introductory acoustics courses. However, characterization and design of sound absorbing materials and modeling of their properties can be valuable for students interested in applied acoustics. In this laboratory-oriented course at the Catholic University of America, we cover design of multilayered sound absorbers and experimental procedures for testing such materials. These include an introduction to data acquisition using LabVIEW and post-processing (with Matlab) of recorded sound as well as microphone calibration, and measurement of sound pressure level, and the frequency response function of a speaker. The second part of the course focuses on room acoustics, acoustic properties of materials, impedance tube measurements, modeling of sound propagation in porous media, and design of sound absorbers. We validate the Zwikker and Kosten model using a sample with straight cylindrical pores and apply the Delany-Bazley model to predict acoustic behavior of fibrous materials. Finally, multilayered materials are designed using the impedance translation theorem and tested with impedance tube measurements.

Mixed matrix membranes comprising of matrimid and mesoporous COK-12: Preparation and gas separation properties

Novel ordered mesoporous COK-12 type silica particles were used as fillers in polyimide (Matrimid) based mixed-matrix membranes (MMMs). The highly ordered 2D hexagonal structured filler materials with short and straight pores were synthesized at room temperature under quasi-neutral pH. The gas permeation and SEM results of the MMMs confirmed a homogenous filler dispersion and defect free synthesis of membranes. The presence of large mesopores in the fillers leading to faster diffusion of penetrant gas resulted in higher gas permeabilities in comparison to pristine Matrimid membrane. The potential of the membrane under mixed-gas conditions and different operating temperatures was also evaluated. The membranes showed decreasing trend of activation energy of permeation with the addition of filler. The ease of synthesis, highly ordered structure with short and straight pore channels and improved gas permeation properties makes COK-12a promising filler for industrial gas separations.

Optical Property of Porous Silicon Produced by Metal-Assisted Etching

A number of ideas have been applied to improve the conversion efficiency of crystalline silicon (Si) solar cells. Antireflection of Si surface is an important technology to increase the photocurrent density of solar cells. Both random pyramid structures produced by anisotropic etching of crystalline Si wafers and antireflection coatings which have lower refractive index than Si are generally used for commercial solar cells. Common antireflection coatings using Si nitride have such problems as high manufacturing cost and limited antireflection effect only in a specific range of wavelength. Previously, antireflection coatings of porous Si formed by stain etching using a mixture solution of hydrofluoric acid and nitric acid or by anodic etching were reported1, 2. This method can control the refractive index by changing the structure of the porous Si, so it can decrease the reflection of Si surface in a wide wavelength range. We reported that macro porous Si formed by a metal assisted etching, which is immersing metal catalyst modified Si in a hydrofluoric acid (HF) aqueous solution, can be used as a texture layer for antireflection of solar cells3. The macro porous layers consisting of pores in µm-size are expected to damage p-n junction and reduce the conversion efficiency of solar cells. In this study, we investigated the optical properties of porous Si thin films, which were thinner than p-n junction. Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. The Si wafers were immersed in 1 mM (M = mol dm-3) silver nitrate (AgNO3) solution including 0.15 M HF at 278 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 6.6 M HF solution including hydrogen peroxide (H2O2) at 298 K for 1 ~ 60 s. The samples after etching were immersed in a 13 M nitric acid and then 7.3 M HF to remove silver nanoparticles remaining in the bottom of the pores and the Si oxide film, respectively. A multi-photometric system (Otsuka Electronics, MCPD-7000KA) with an integrating sphere was used to measure reflectance of Si surface. A spectroscopic ellipsometer (Otsuka Electronics, FE-5000) was used to analyze the optical constants of the porous Si thin film. Fig. 1 shows that an uniform porous Si thin film consisting of straight pores, few tens nm in diameter and 250 nm in length, was formed by the etching for 15 s. The structure of porous Si was changed with HF and H2O2 concentrations of the etching solution and etching time. The pore size was consistent with the size of Ag nanoparticles as shown in Fig. 1. Thus, the porosity of porous Si can be evaluate as the coverage of Ag nanoparticles on Si before etching. The coverage, that is porosity, can be controlled by deposition time of Ag. The reflection of porous Si surface was much lower than mirror polished Si wafer before metal assisted etching (Fig. 2). As the porosity increased, interference waveform in the reflectance spectra appeared and the peak interval of reflectance spectra decreased. On the other hand, it was possible to control the thickness of the porous Si thin film by changing the etching time. As porous layer thickness increased, the reflectance decreased and the amplitude of the interference waveform decreased. The reflectance reached lower than 5.5%, which is much lower than 10% of a pyramidal structure, in the wavelength range of 400-1000 nm. From these results, we assumed that the porous Si functions as an optical thin film of a smooth single-layer. The refractive index of porous Si thin film at the porosity of 33 ~ 56% was observed as 1.1 ~ 2.2. The attenuation factor (factor that affects light absorption) of porous Si thin film was ~100 times higher than that of crystalline Si. The thin porous Si layers produced by the metal assisted etching are expected to work as suitable antireflection films for crystalline Si solar cells. ACKNOWLEDGEMENT The present work was partly supported by JSPS KAKENHI (26289276). The authers wish to express their thanks to Otsuka Elecronics Co., LTD for ellipsometric analysis. REFERENCES 1. R. R. Bilyalov, R. Lüdemann, W. Wettling, L. Stalmans, J. Poortmans, J. Nijs, L. Schirone, G. Sotgiu, S. Strehlke, C. Lévy-Clément, Sol. Energy Mater. Sol. Cells 60, 391-420 (2000). 2. L. Remachea, E. Fourmonda, A. Mahdjoubb, J. Dupuisa, M. Lemitia, Mater. Sci. Eng. B 176, 45–48 (2011). 3. S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, H. Matsuda, Electrochem.Commun. 5, 632 (2003). Figure 1

Surface Attachment of Gold Nanoparticles Guided by Block Copolymer Micellar Films and Its Application in Silicon Etching.

Patterning metallic nanoparticles on substrate surfaces is important in a number of applications. However, it remains challenging to fabricate such patterned nanoparticles with easily controlled structural parameters, including particle sizes and densities, from simple methods. We report on a new route to directly pattern pre-formed gold nanoparticles with different diameters on block copolymer micellar monolayers coated on silicon substrates. Due to the synergetic effect of complexation and electrostatic interactions between the micellar cores and the gold particles, incubating the copolymer-coated silicon in a gold nanoparticles suspension leads to a monolayer of gold particles attached on the coated silicon. The intermediate micellar film was then removed using oxygen plasma treatment, allowing the direct contact of the gold particles with the Si substrate. We further demonstrate that the gold nanoparticles can serve as catalysts for the localized etching of the silicon substrate, resulting in nanoporous Si with a top layer of straight pores.

Investigation on the Microfluidics in PEM Water Electrolyzers

A sustainable and robust electrochemical energy storage and/or a hybrid system to accommodate daily, even hourly changes of renewable resources becomes more critical. An advanced polymer electrolyte membrane water electrolyzer (PEMWE) has been shown to be effective energy storage medium by producing hydrogen/oxygen from water with electricity from renewable sources. When the renewable resources are available, hydrogen will be produced and stored, such that it can later provide a constant power supply with a PEM fuel cell, which is a reverse device of the electrolyzer. This entire portfolio will make renewable and hybrid energy systems effective to provide reliable and multiscale energy whenever needed. In proton exchange membrane water electrolyzers (PEMWEs), water is electrochemically splitting into oxygen and hydrogen. The oxygen generated at the anode side and the circled water flowing over the liquid/gas diffusion layer yield two-phase transport conditions in micro-channels and pores of LGDL, which significantly impact the performance. Due to limitations of the design of conventional PEMWEs and LGDLs, the in-situ phenomena of two-phase flow has few been explored in operating PEMWEs. In this research, an innovative design of a transparent PEMWE is developed coupled with the thin and well-tuned titanium LGDLs with straight pores. The high-speed and micro-scale visualization system (HMVS), and electrochemical impedance spectroscopy will be used for in-situ characterizations. Visualization results are used for better understanding the behavior of two-phase flow and its effects on the PEMWE performance. Patterns of gas-bubble formation, evolution, departure and transport are identified. The effects of property of materials, including LGDL pore sizes, channel dimensions, are also investigated.

Impedance tube experiments as a tool to teach acoustics of porous media

As a practical matter, sound propagation in many everyday situations includes interactions with porous materials. Such materials include textiles, foams, gravel, grass-covered ground, asphalt, and trees. Modeling sound propagation in complex environments, such as passenger cabins or urban spaces, with porous materials like these is essential to understanding realistic acoustics problems. The theory behind the sound absorption in porous material is not typically included in introductory acoustics courses. This is, at least in part, attributable to the mathematical complexity of the topic. Despite this barrier, acoustic properties of porous material can be easily introduced to typical undergraduate students using simple experiments to demonstrate the acoustic effects of different materials. This presentation describes acoustic property measurements of rigid-frame porous material with straight cylindrical pores as well as foam and soil included in an “Introduction to Acoustics” class at the Catholic University of America. The apparatus is composed of a LabVIEW measurement interface, a pair of microphones, and a simple aluminum impedance tube. The modular design allows students to adapt the software or hardware for other experiments. These hands-on experiences are intended to impart a conceptual understanding of the theory of sound absorption and the acoustic performance of sound absorbing (porous) media.