Porous Alumina Research Articles

Research and fabrication of color filters based on porous anodic alumina

Research and fabrication of color filters based on porous anodic alumina

Tuning the geometry of porous alumina layers via anodization in mixtures of different acids

AbstractPorous anodic aluminum oxide (AAO) layers have been obtained by two-step anodization of high-purity Al in two types of acid mixtures, i.e., in H2C2O4–H3PO4 and, for the first time, in H2SO4–H3PO4 systems. The kinetics of oxide formation was examined by monitoring the current vs. time curves while the morphology of the resulting layers was carefully verified by scanning electron microscopy (SEM). A special emphasis was put on establishing correlations between electrolyte composition, the kinetics and effectiveness of oxide growth, and the morphological features of AAO layers (pore and cell diameter, porosity), as well as pore arrangement. It was confirmed that the addition of H3PO4 to both H2C2O4 and H2SO4 electrolytes results in a significant decrease in oxide growth rate, and worsening of pore arrangement, while the values of pore diameter and interpore distance are much less affected. Moreover, the presence of a small amount of phosphoric acid in the reaction mixture allowed for a noticeable increase in pore ordering if anodization was carried out beyond the self-ordering regime, or performing controlled anodization even at voltages at which the burning phenomenon is typically observed. It is strongly believed that manipulating the electrolyte composition by adding another acid may provide another degree of freedom to control the morphology of the resulting nanostructured alumina layers.

Nanoscopically Confined 1,4-Polybutadiene Melts: Exploring Confinement by Alumina Nanorod and Nanopore Systems.

We present molecular dynamics simulations of a chemically realistic model of 1,4-polybutadiene (PBD) in contact with curved alumina surfaces. We contrast the behavior of PBD infiltrated into alumina pores with a curvature radius of about three times the radius of gyration of the chains to its behavior next to a melt dispersed alumina rod of equal absolute curvature. These confinement types represent situations occurring in polymer melts loaded with nanoparticles due to nanoparticle aggregation. While there are observable differences in structure and dynamics due to the different types of geometric confinement, the main effects stem from the strong attraction of PBD to the alumina surfaces. This strong attraction leads to a deformation of the chains in contact to the surfaces. We focus on temperatures well above the bulk glass transition temperature, but even at these high temperatures, the layers next to the alumina surfaces show glass-like relaxation behavior. We analyze the signature of this glassy behavior for neutron scattering or nuclear magnetic resonances experiments.

Unlocking the Chemical and Structural Complexity of Aluminum Hydroxy Acetates: from Commodity Chemicals to Porous Materials.

Aluminum acetates have been in use for more than a century, but despite their widespread commercial applications, essential scientific knowledge of their synthesis-structure-property relationships is lacking. High-throughput screening, followed by fine tuning and extensive optimization of reaction conditions using Al3+, OH- and CH3COO- ions, has unraveled their complex synthetic chemistry, yielding for the first time the four phase pure products Al(OH)(O2CCH3) ⋅ x H2O (x = 0, 2) (1A and CAU-65, 1B), Al3O(HO2CCH3)(O2CCH3)7 (2), and the porous aluminum salt [Al24(OH)56(CH3COO)12](OH)4 (CAU-55-OH, 3). Structure determination by electron and X-ray diffraction was carried out and the data suggested porosity for 1B and 3, which was confirmed by physisorption experiments. Even the scale-up to the 10 L scale was accomplished for 1A, 1B and 3 with yields of up to 1.1 kg (99%). This study of a seemingly simple chemical system provides important information on both fundamental inorganic chemistry and porous materials.

Environmental barrier coatings on alumina-reinforced CMCs: Experimental validation and numerical modeling of thermal stability

Recent advancements in ceramic matrix composites (CMCs), including SiC/SiC, C/SiC, C/C, Al2O3 reinforced Al2O3, face various challenges at high temperature applications, including grain growth, sintering, and creep deformation which are leading to strength loss. The new generation of Environmental Barrier Coatings (EBCs) must allow the protection of these CMCs when operating under harsh conditions, which can reach up to 1100 °C in some applications, such as gas turbines and certain reforming processes. In this work, 8 wt% yttria stabilized zirconia (8YSZ) applied by Atmospheric Plasma Spraying (APS) on top of a porous alumina oxide matrix with reinforced alumina fibers Al2O3/Al2O3-CMC (OCMC), considering different interfaces, has been studied. Surface morphology and pre-treatment of CMCs is the most challenging part for thermal spray applications. Hence, two different surface structure were used in this work. The results show that the roughness of OCMC can be reduced from Rz = 41.8±6.8 μm to 20.3±1.1 μm with the new porous bond coat and plasma sprayed coating, resulting in a homogeneous surface morphology.. In terms of thermal stability, a numerical model that combines Object Oriented FEM (OOF) and Cohesive Zone Model CZM tools, has been developed to evaluate the effect of the porosity of the real microstructure and the characteristics of the interfacial roughness profile, respectively, in CMC/EBC structures. In addition, other materials, such as La2Zr2O7 and Y2O3, have been numerically studied in order to evaluate their suitability as EBC.

Magnetic properties of bamboo-like Ni-Zn-in nanowires using 3D-AAO templates

In this paper, using three-dimensional porous alumina (3D-AAO) as a template, Ni-Zn-In nanowires array with periodically changing diameters was successfully prepared by direct current electrodeposition method. The effect of deposition voltage on the morphology, crystal structure and magnetic properties of Ni-Zn-In ternary alloy nanowires was studied. The experimental results show that the diameters of the nanowires are 212 nm and 151 nm, and the periodic length is 800 nm, which matches the pore size of the phosphoric acid-etched 3D-AAO template. XRD results indicate that the crystal structure of Ni-Zn-In nanowires can be adjusted by the deposition voltage. The VSM results reveal that the Ni-Zn-In nanowires possess soft magnetic properties and significant magnetic anisotropy, with the easy magnetization direction parallel to the film. The nanowires deposited at −1.5 V exhibit the highest coercivity of 161 Oe and squareness of 0.045. Micromagnetic simulations show that the magnetization reversal mechanism of the Ni-Zn-In nanowires is localized nucleation mode.

Foam forming of highly porous alumina ceramic paper and its enhancement of mechanical and thermal insulation properties in aerogel composites

The strength and toughness of aerogel composites can be enhanced by incorporating alumina ceramic paper as a reinforcing material. However, the production processes often suffer from issues related to poor evenness and low porosity. Herein, alumina ceramic paper with high evenness and porosity was prepared using foam forming technology and utilized as a reinforcing material for alumina-SiO2 aerogel composites. Initially, the hydroxyl groups (Al-OH) of the alumina ceramic fibers were adsorbed onto the hydrophilic moieties of bubbles via hydrogen bonding during the foam forming, which hindered the migration of the fibers and facilitated their dispersion. Subsequently, the alumina ceramic paper was fabricated through the optimization of the squeezing dehydration and defoaming processes. Finally, the alumina-SiO2 aerogel composites were formed via the sol-gel method. The alumina ceramic paper exhibited excellent multifunctional properties, including high porosity and evenness, which significantly enhanced the mechanical and thermal insulation properties of the alumina-SiO2 aerogel composites. In conclusion, highly porous alumina ceramic paper presents significant potential for application in various fields, including automotive, construction, and aerospace.

Study of Thermal Processes in Porous Anodic Aluminum Oxide Filled With Perovskite

Study of Thermal Processes in Porous Anodic Aluminum Oxide Filled With Perovskite

Enhancing Capillary Pressure of Porous Aluminum Wicks by Controlling Bi-Porous Structure Using Different-Sized NaCl Space Holders.

Capillary pressure and permeability of porous media are important for heat transfer devices, including loop heat pipes. In general, smaller pore sizes enhance capillary pressure but decrease permeability. Introducing a bi-porous structure is promising for solving this trade-off relation. In this study, the bi-porous aluminum was fabricated by the space holder method using two different-sized NaCl particles (approximately 400 and 40 μm). The capillary pressure and permeability of the bi-porous Al were evaluated and compared with those of mono-porous Al fabricated by the space holder method. Increasing the porosity of the mono-porous Al improved the permeability but reduced the capillary pressure because of better-connected pores and increased effective pore size. The fraction of large and small pores in the bi-porous Al was successfully controlled under a constant porosity of 70%. The capillary pressure of the bi-porous Al with 40% large and 30% small pores was higher than the mono-porous Al with 70% porosity without sacrificing the permeability. However, the bi-porous Al with other fractions of large and small pores did not exhibit properties superior to the mono-porous Al. Thus, accurately controlling the fractions of large and small pores is required to enhance the capillary performance by introducing the bi-porous structure.

Pinhole small-angle neutron scattering based approach for desmearing slit ultra-small-angle neutron scattering data.

Presented here is an effective approach to desmearing slit ultra-small-angle neutron scattering (USANS) data, based on complementary small-angle neutron scattering (SANS) measurements, leading to a seamless merging of these data sets. The study focuses on the methodological aspects of desmearing USANS data, which can then be presented in the conventional manner of SANS, enabling a broader pool of data analysis methods. The key innovation lies in the use of smeared SANS data for extrapolating slit USANS, offering a self-consistent integrand function for desmearing with Lake's iterative method. The proposed approach is validated through experimental data on porous anodized aluminium oxide membranes, showcasing its applicability and benefits. The findings emphasize the importance of accurate desmearing for merging USANS and SANS data in the crossover q region, which is particularly crucial for complex scattering patterns.

Effects of Heat Treatment on Anodic Porous Alumina Membranes for Mass Spectrometry

Abstract Desorption ionization using through-hole alumina membrane (DIUTHAME) method is a mass spectrometry imaging (MSI) method that uses an anodic porous alumina membrane (APAM) with an array of submicron-sized through-holes as a surface-assisted laser desorption/ionization (SALDI) substrate. The DIUTHAME method is particularly promising for MSI because it does not generate interfering peaks in the low-molecular-weight region. However, the SALDI effect cannot be obtained if the nanostructures of the substrate are destroyed owing to laser irradiation before sample vaporization. APAMs that maintain their structures after irradiation with a high-intensity laser must be fabricated to realize highly sensitive measurements. In this study, we investigated the role of heat treatment suppressing the laser irradiation induced fracture of APAMs. We found that heat treatment at higher temperatures more effectively suppressed APAM fracture associated with laser irradiation. This result was attributed to the reduction of anion-derived impurities in APAMs upon heat treatment. The resulting heat-treated APAM is expected to serve as a substrate for highly sensitive and robust MSI.

Liquid-infused surface on optimized anodic porous aluminum for anti-frost and anti-ice application

Here, we present liquid-infused surfaces (LISs) fabricated on an aluminum substrate using the anodizing method to introduce specific nanofeatures infiltrated by silicone oil to reduce the friction between ice and surface. To achieve various nanostructures on the aluminum substrate, the anodizing conditions are varied. The ice adhesion stress could be significantly plummeted to 21.7 kPa on an LIS with optimized dimensions for nanofeatures. Frost formation tests in environments with a relative humidity of 75 % on the fabricated surfaces with a temperature of −17 ℃ reveal that the fabricated samples can considerably extend the frost formation time. The frosting time for the fabricated sample can reach 47 min, whereas the bare aluminum is completely covered by frost only after 1.5 min

Tailoring the properties of carbon molecular sieves membranes for the separation of propionic acid from aqueous solutions

In the fermentative production of propionic acid (PA), the major problem with batch fermentation systems is the strong inhibitory effect of PA on the production yield; one way to increase the yield is the in-situ removal of PA by using pervaporation. Acetic acid (AA) is the most important by-product in the fermentation; therefore, the membrane should be able to remove selectively PA from an aqueous solution containing AA. Considering that PA is more hydrophobic than AA and their kinetic diameter are 0.480 and 0.436 nm respectively, hydrophobic membranes with main pores in the range of around 0.5–0.6 nm with high permeation are required.Supported thin Carbon Molecular Sieve Membranes (CMSM) were prepared by the dip coating a porous alumina support into a solution containing resorcinol phenolic resin as carbon source. The hydrophobicity was obtained by carbonizing the polymer at temperatures higher than 750 °C and adding polyvinyl butyral (PVB) as pore forming agent and carbon contributor. PA with 88 % of purity was obtained by pervaporation of an aqueous solution containing 5 % of PA and 5 % of AA using a CMSM carbonized at 850 °C containing 1 % of PVB in the dipping solution.

An Experimental Study on the Thermal Performance of a Heat Sink Filled with Porous Aluminum Skeleton/Paraffin Composite Phase Change Material.

Organic phase change material is an ideal solution to solve the heat dissipation problem of electronic devices. However, its low thermal conductivity limits its application. To solve this problem, a new porous aluminum skeleton/paraffin composite phase change material (AS-PCM) was prepared. The effects of porosity and porous aluminum skeletons on temperature control performance were explored. The experimental results show that the addition of AS significantly improves the thermal conductivity of organic PCM, and the thermal conductivity of AS-PCM is 32.3-59.6 times higher than that of pure paraffin. In addition, the temperature difference in AS-PCM with 75% porosity is 1-2 °C lower than that of AS-PCM with 85%, and 5-8 °C lower than that of AS-PCM with 95% porosity. The skeleton structure has an impact on the temperature control performance. The Mcc porous aluminum skeleton/paraffin composite phase change material (MAS-PCM) yields the best thermal performance compared with the Fcc porous aluminum skeleton/paraffin composite phase change material (FAS-PCM). The temperature control time of the MAS-PCM heat sink is increased by 5.3-50.8% relative to the FAS-PCM heat sink. The research results provide a novel approach for improving the thermal conductivity of PCMs.

Boosting the H2/CO2 and H2/N2 selectivities of a graphene oxide membrane by shrinking the membrane interlayer spacing via thermal treatment

Graphene oxide (GO) membranes have been widely studied and used for gas separation processes, but some structural changes are necessary to guarantee high membrane selectivities. The thermal reduction process stands out for reducing the interlayer channel size of the GO nanosheets to improve the molecular sieving mechanism. Herein, thermally annealed GO layers were deposited on the outer surface of porous alumina hollow fiber substrates and the produced composite membranes were tested for selective hydrogen (H2) separations. The influence of the thermal annealing process at different mild temperatures (80 and 160 °C) on the characteristics of the GO structure was investigated towards the improvement of the membrane efficiency for H2 separation. ATR-FTIR results confirmed the elimination of oxygenated groups, and XRD analyses showed the GO interlayer space decreased from 8.71 to 7.38 Å after the thermal annealing process at 160 °C. The GO thermally annealed membrane presented an H2 permeance of 2032.47 ± 101.69 GPU and an H2/N2 selectivity of 12.16 ± 0.30, which is 315% greater than the selectivity of the pristine GO membrane. Hence, the application of the thermal reduction method to produce GO membranes with greater selectivities is a viable alternative to deal with gas separation processes.

Tailoring structural and magnetic properties of NiCu nanowires by electrodeposition

In this work, we investigated the tailoring of structural and magnetic properties of NiCu nanowires through electrodeposition. Continuous (S1) and composition-modulated (S2) wires were fabricated by electrodeposition using porous alumina membranes as a template. Morphological characterization revealed that the total length of the wires was 8 ± 3 µm in both S1 and S2. For the composition-modulated wires, the length of the segments with the lowest and highest Cu concentrations was 1.2 ± 0.4 µm and 226 ± 65 nm, respectively. Mapping by energy dispersive spectroscopy (EDS) revealed that the concentration of copper and nickel varied along the length of the composition-modulated nanowires, while the continuous nanowires contained a relatively constant concentration of both metals. It is demonstrated that the change in Cu concentration along the wire modifies the lattice parameter, average crystallite size (D) and lattice strain (ε) of Ni. This result is pivotal for understanding the magnetic properties of the wires, as nickel is primarily responsible for the magnetic behavior of the wires. From the ferromagnetic resonance (FMR) results, the linewidth and resonance field values for samples S1 and S2 were determined. It was demonstrated that the greater deformation in the nickel lattice in NiCu nanowires increases the angular dependence of the resonance field. Furthermore, the smaller nickel crystallite size was shown to increase spin dispersion and magnetic damping, leading to complex behavior in FMR responses. Finally, it was demonstrated how Cu can influence the magnetic properties such as coercivity (HC) and squareness (MR/MS) of the wires. Overall, this work contributes to understanding the tailoring of structural and magnetic properties of NiCu nanowires through electrodeposition.

Effects of Diamond Content on the Morphology and Compressive Properties of Porous Aluminum Composites.

Porous aluminum (Al) is popular due to its lightweight properties and impact energy absorption. However, it often has a lower mechanical strength than solid Al. To improve the performance, diamond reinforcement was introduced into the matrix. Further, addressing the challenge of interfacial bonding between Al and diamond, coated diamond with varying contents of 5, 10, 15, and 20 wt % was added to the porous Al alloy matrix via the powder metallurgy technique. The porosities were formed by using poly(methyl methacrylate) (30 wt %) as a space holder. The densities of the resultant porous composites ranged from 2.20 to 2.37 g/cm3 and porosities ranged from 33 to 38% for 5-20 wt % diamond contents. Furthermore, the yield strength and plateau stress increased from 21.47 to 29.46 MPa and 14 to 20 MPa, respectively, up to 10 wt % diamond content but declined upon further addition. Similarly, the energy absorption capacity increased from 2.15 to 2.95 MJ/m3 up to 10 wt % diamond content and thereafter decreased. Thus, the addition of coated diamond and alloying elements in Al strengthened the porous Al composites, making it suitable for applications requiring good compressive strength and energy absorption capacity.

On the Occurrence of Domain Boundaries and Defects in Self-Organized Ordered Porous Anodic Alumina Systems.

The formation of domains and defects is a part of every self-organized structure without prepatterning which evolves from the electrochemical oxidation of valve metals like Ti, Al or Ta. Defects and domains, especially in highly ordered porous alumina films (PAOX), are one of the most studied phenomena in the current literature. The focus is on understanding their formation in order to find parameters to minimize their amount for applications that require the most highly organized structures. In this letter we give a solution to this problem by showing that the occurrence of defects and domains is an unavoidable consequence of the underlying self-organized process due to the simultaneous impact of the minimum (MINEP) and maximum (MAXEP) entropy production principle. As a consequence, experimental procedures to manufacture defect-free PAOX-films from a self-organization process face a fundamental limitation.

Fabrication of arrayed V-shape micro-nano hierarchical structures for dew collection

Planer 2D nanopore arrays of porous anodic alumina can be unfolded to designed 3D arrayed micro-nano hierarchical structures by a programmed anisotropic electrochemical anodization, which consists of programed polymer masking and stepped anodization. Arrayed symmetrical and asymmetrical V-shape tiered micro grooves with nanotip-capped nanopore structures are respectively custom tailored to obtain high-efficiency dew collection. It is found that symmetrical V-shape tiered micro grooves with nanotip-capped nanopore structures show collection efficiency (36.176 ± 3.832 mg cm−2 h−1) twice as much as that of pure nanostructured surface, where the environment temperature is ∼18 °C, the relative humidity is ∼86 % and the sample temperature is ∼5 °C. Owning to the larger dew nucleation area, low solid-liquid adhesion and faster dew growth rate, the jumping micro-dews quickly gather and growth to big dews at the bottom of the V-shape tiers as the samples horizontally placed, which can easily fall off as turn the samples to vertical. Due to the simple equipment, low cost and large-area fabrication ability, these arrayed micro-nano hierarchical structures not only have potential to become high efficiency dew collector but also could be a platform for the controlled preparation of 3D complicated micro-nano structures of various materials with advanced functions.

Enhanced drying of water-containing porous aluminum hydroxide particles via a cyclone based on microchannel oscillation

Conventional drying methods for water-containing porous particles typically require overcoming the latent heat of water evaporation, rendering them energy intensive. We present a rapid drying process via a cyclone below the boiling point on the basis of high-speed self-rotation and microchannel oscillation of particles. A high-speed camera recorded the particle revolution and self-rotation speeds within the cyclone, which allowed us to calculate the centrifugal acceleration of water in the particle pores, which reached 23,766 m/s² and periodically oscillated. Consequently, water in macropore with diameters larger than 8.1 μm can be separated into microdroplets by cyclones. Various factors influencing the drying efficiency of the cyclone were investigated at the laboratory scale. In experiments conducted with an airflow rate of 75 m³/h, an air temperature of 80°C, and a particle feed rate of 5.4 kg/h, a significant reduction in the water content was observed, with the water content decreasing from 68.2 % to 32.1 % within only 25 s. Compared with traditional evaporative drying, the drying process via a cyclone for aluminum hydroxide with a capacity of 2.4 tons/day saves 2500 tons of steam and 70,000 kW•h of electricity per year, resulting in a cost reduction of 560,000 CNY. These findings suggest that cyclone drying is a highly efficient and cost-effective method for drying water-containing porous particles, with significant implications for energy savings and sustainability in industrial applications.