Porous Anodic Alumina Films Research Articles

Effect of interfacial nanostructures on shear strength of Al-PPS joints fabricated via injection moulding method combined with anodising

The high-strength direct joining of metal to polymer has attracted increasing attention in the weight reduction of equipment. In this work, porous anodic alumina (PAA) films with different interpore distances and pore diameters were fabricated on the commercial 6063T5 Al alloy via anodising, followed by etching. Then, the glass fibre/mineral filling polyphenylene sulphide (PPS) was directly joined with Al by using injection moulding method. The relationships between the interfacial structure and bonding strength of Al-PPS joints were systematically investigated and discussed. Results showed that the shear strength of the joints was significantly affected by the porosity, wettability, and hardness of the PAA films. With the increase in etching duration, the porosities of the PAA films increased, whilst their contact angles decreased (from 110° to 20°). However, excessive porosity (>45%) could lead to the weakening of PAA films. In general, the shear strength of joints increased first and then decreased with the etching duration. The maximal joint strength of the Al-PPS joints was ~21.2 MPa, which reached ~82% of the theoretical limit (~25.5 MPa). The images observed by scanning electron microscope and atomic force microscope indicated that the depth of PPS injected into the nanopore could reach >500 nm. The fracture surfaces of the Al-PPS joint with the highest strength could be divided into three different zones in accordance with their morphologies and failure mechanism. In general, these results are of great importance to the further design and optimisation of metal-polymer joints fabricated using the presented method.

Fabrication of unique porous alumina films with extremely high porosity and an ultra-flat barrier layer by anodizing aluminum in sodium metaborate

Unique porous anodic aluminum oxide (PAAO) films with a completely flat barrier layer and an extremely porous loofah-like structure were fabricated by anodizing aluminum in an alkaline sodium metaborate solution. High-purity aluminum plates were anodized in a 0.3 M sodium metaborate solution at various applied voltages in the range of 0.1–200 V. A typical PAAO film with a spherical cap barrier layer was formed at voltages lower than 50 V, whereas a PAAO film with a flat barrier layer was formed at voltages higher than 100 V; this film formation was not based on the Keller-Hunter-Robinson model with the spherical barrier layer. The growth interface of the flat barrier layer exhibited an ultra-flat morphology with a minimum roughness value of 0.4 nm, which is much smaller than that of an electropolished aluminum surface. Such barrier layer morphology is expected to be formed by field-assisted dissolution without oxide flow. Although the current density slightly decreased with the applied voltage, a relatively higher current density of 6.8 Am−2 was still measured at the lowest voltage of 0.1 V. The alumina walls and the bottom barrier layer gradually thinned as the applied voltage decreased, and a loofah-like PAAO film with an ultra-high porosity of 0.93 was successfully fabricated at 0.1 V. The anodic oxide consisted of amorphous, anion-free aluminum oxide.

Self-Ordering of Porous Anodic Alumina Fabricated by Anodizing in Chromic Acid at High Temperature

Anodizing Al in chromic acid is a very useful surface finishing process for corrosion protection and nanoscale porous structure fabrication, whereas the self-ordering of porous anodic alumina (PAA) in chromic acid has never been found to date. Herein, we provide a self-ordered PAA film possessing numerous sub100-nm-scale characteristic bumps through anodizing in chromic acid at high temperature. Anodizing of high purity Al plates in a 0.3 M chromic acid solution at conventional low temperatures, such as 293 K, leads to the formation of a disordered PAA film, whereas anodizing at a high temperature of 348 K causes the self-ordering behavior of the pore structure. The PAA film grown in the initial stage possessed the highest regularity, and it decreased with anodizing time due to pore branching during anodizing. A highly ordered PAA film measuring approximately 340 nm in interpore distance can be fabricated by short-term, two-step anodizing in chromic acid at 348 K and 120 V. The ordered PAA film possesses a characteristic nanostructure consisting of hexagonally arranged 100-nm-scale pores and sub100-nm-scale disordered bumps on their pore walls without any electrolyte chromate anion.

Electro-physical properties of porous anodic alumina films for sensitive elements of MEMS

The results of a comprehensive study of porous alumina film electrical properties obtained by anodizing in acid electrolytes have been presented. The relationship was established between the structure of oxide films, the mechanism of their formation, as well as physicochemical characteristics. A targeted effect on the microstructure of the films made it possible to control their properties by varying the technological parameters without a radical change in the method. This made it possible to predict the behavior of nanostructures and properties for sensitive MEMS elements.

Permeability of anodic alumina membranes grown on low-index aluminium surfaces

Porous anodic aluminium oxide (AAO) membranes have various practical applications in separation and purification technologies. Numerous approaches have been utilized to tailor the transport properties of porous AAO films, but all of them assume an isotropic nature of anodized aluminium. Here, the impact of aluminium crystallography on the permeability of AAO membranes is disclosed. A comparative study of AAO membranes formed on low-index aluminium surfaces by anodizing in a sulphuric acid electrolyte is presented. Small-angle x-ray scattering is used to quantify the out-of-plane pore arrangement. AAO grown on an Al(100) substrate possesses a porous structure with minimal point defects and pore tortuosity, providing the highest permeability of individual gases in a series of AAO membranes. These findings can also be applied for the fabrication of highly permeable AAO membranes on polycrystalline Al foils.

Fabrication and characterization of the hierarchical AAO film and AAO-MnO2 composite as the anode foil of aluminum electrolytic capacitor

Anodic aluminum oxide (AAO) film of different regular hierarchical pore structures and AAO-MnO 2 composite of many homogeneously distributed microvoids have been prepared and comparatively characterized, mostly as they were adopted as an anode foil of aluminum electrolytic capacitors. Results showed that the hierarchical pore structure of AAO film is composed of a supernatant layer of larger holes and a rock-bottom layer of smaller holes. The smaller holes' density increases with the reduction of the secondary anodization voltage in the two-step anodization process of the AAO film, which subsequently enlarges the specific capacitance to 7.88 μF/cm 2 and may also increase the leakage current when the AAO film is adopted as the anode foil in an aluminum electrolytic capacitor. The obtained hierarchical AAO-MnO 2 composite, with a multi-layered interweaved structure, has not only significantly brought up the specific capacitance from 7.88 μF/cm 2 to 25.56 μF/cm 2 but also may significantly reduce the leakage current and consequently improve the stability when being used as the anode foil. Compared with merely increasing the surface area of AAO film, the introduction of high dielectric constant compounds into the AAO film without changing the actual surface area can improve the aluminum electrolytic capacitor's capacitance performance more effectively. • The specific capacitance can be 19.15 uF/cm 2 when hierarchical AAO film used in medium aluminum electrolytic capacitor. • An improved method was used to introduce MnO2 into porous AAO film without reducing the real surface area significantly. • The capacitance can be 25.56 uF/cm 2 when AAO-MnO 2 composite used in the medium voltage aluminum electrolytic capacitors.

Bipolar Electrochemical Anodization Route for the Fabrication of Porous Anodic Alumina with Nanopore Gradients.

Porous anodic alumina (PAA) films with homogeneous nanopores are achieved by traditional anodization. Here, we present a unique anodization technique based on bipolar electrochemistry to fabricate PAA films with nanopore gradients. In an oxalic acid solution dissolved in ethylene glycol, a stable bipolar anodization process is realized. The PAA film prepared at 280 V exhibits a continuous change in interpore distance from ∼171 to ∼83 nm over a range of only 5 mm on the aluminum sheet. Higher driving voltages lead to larger interpore distances and steeper nanopore gradients. Further, no direct electrical connection is required for this bipolar anodization.

Current Regulation and Property Study of Porous Anodic Alumina Films with a Periodic Pore Structure.

Porous anodic alumina (PAA) films with periodically modulated pore diameters are prepared by cyclic anodization of Al in a 0.6 M H3PO4 solution at room temperature. Studies have demonstrated that the oscillating current signals have an important effect on the structures of PAA films. Scanning electron microscopy (SEM) images of the PAA film show that when the positive triangle wave current signal is applied, with the increase in the maximum current value, PAA gradually exhibits a symmetrically modulated pore diameter structure, and part of the pores generates slub-like branches. When the maximum current value is 60 mA, the effect of modulation on the pore diameter is the most obvious and the UV reflectance spectrum shows the lowest reflectivity. A sawtooth wave current signal will cause the generation of a V-shaped structure at the junction of adjacent oxide layers. This work provides important guidance for regulating the structure of PAA by changing the current signal.

Controllable wet etching of porous anodic alumina toward highly ordered hierarchical interfaces

Anisotropic etching is a powerful technique for fabrication of micro/nanostructures. However, methods used to perform anisotropic etching, are very limited and costly. Wet chemical etching is a cost-effective and convenient method but it generally forms isotropic features. Here, the anisotropic wet etching of porous anodic alumina (PAA) using phosphoric acid to controllably form various structures is presented. Using photo-patterned SU-8 as protective layers and 5 wt% phosphoric acid as etchants at 60 °C, the exposed areas of PAA are vertically etched along the direction of inherent nanopores , while the areas coated with SU-8 are protected from etching. Arbitrary alumina micropillar arrays are achieved and the length of micropillar is determined by the thickness of the PAA films. In addition to cylindrical pillar arrays, sharp and concentric cone-shaped pillar arrays are obtained upon fine-tuning the etching conditions. The as-prepared super-hydrophilic alumina substrates with considerable surface roughness can be transformed into super-hydrophobic surfaces of 150° water contact angle. Hole arrays have also been fabricated using a reversed pattern protective layer before etching. These substrates are further used as templates to replicate various structures of polymers and thus provide a facile approach to fabricate hierarchical structures for a range of applications. Controllable wet chemical etching, either anisotropic or isotropic, has been performed on porous anodic alumina. A range of hierarchical structures with various interfacial properties have been fabricated and investigated. The reported technique provides a facile and cost-effective approach for the generation of micro/nano-structures for broad applications. • Anisotropic wet etching is demonstrated on porous anodic alumina (PAA). • Various hierarchical structures of PAA are achieved. • The etching behavior depends on the structures of PAA and the concentration of etchants. • Hierarchical polymer structures are fabricated by molding from PAA templates.

Kinetics of the Formation and Dissolution of Anodic Aluminum Oxide in Electrolytes Based on Sulfuric and Selenic Acids

Measurements were made to study the kinetics of the formation and dissolution of porous anodic aluminum oxide in electrolytes based on sulfuric and selenic acids at temperatures from 0 to 30°C. It was shown that there is a significant difference in the kinetics of the dissolution of anodic aluminum oxide at the bottom of the pores during its formation and throughout the walls of the porous structure after the termination of the anodizing. In the anodizing of aluminum in 2.0 M H2SO4 in kinetic mode, the apparent activation energy of the dissolution of anodic aluminum oxide at the bottom of the pores is 53.2 ± 1.8 kJ/mol, whereas in the chemical dissolution of the walls of the porous structure, this quantity is 83.3 ± 2.1 kJ/mol. The observed difference in the dissolution kinetics predicts the increase in maximum accessible thickness of the porous anodic aluminum oxide film with decreasing electrolyte temperature.

(Invited) Formation Efficiency of Anodic Porous Alumina in Sulfuric Acid Containing Alcohol As Additives

The properties of anodic porous alumina films, which are generally formed by the anodization of aluminum in an acid electrolyte, are well known to be affected by the dimensions of their structure (e.g., pore diameter, pore density, and film thickness). For instance, to increase the hardness of anodic films on aluminum, the anodization is industrially carried out at low-temperature to avoid chemical dissolution of the formed oxide film during anodization. Anodization in an electrolyte containing organic compounds is also one approach for improving the performance of anodic oxide film not only for aluminum but also for other light metals such as titanium and magnesium (1).One such initiative is anodization of aluminum at a relatively high current density or high voltage using a common acidic electrolyte (e.g., sulfuric acid, oxalic acid, or phosphoric acid) with an alcohol added. In such studies, the effects of adding an alcohol are summarized as follows: i) the alcohol functions as an anti-freeze agent in the electrolyte, enabling the anodization temperature to be lowered; ii) the alcohol acts as a cooling agent—specifically, the vaporization of alcohol at the bottom of the pore channels removes the heat generated at the aluminum/oxide interface; and iii) nonuniform growth of the anodic film (i.e., burning) is effectively prevented, even during high-field anodization. However, these effects are observed not only for monohydric alcohols with a boiling point less than 100 °C (e.g., methanol and ethanol) but also for polyhydric alcohols with a boiling point greater than 100 °C (e.g., ethylene glycol and glycerol). Therefore, the effect of alcohol addition cannot be reasonably explained solely on basis of the boiling point of the alcohol used as an additive. Understanding the effects of alcohol addition necessitates a systematic investigation comparing types of alcohols, in addition to further fundamental studies.In this study, monohydric alcohols or polyhydric alcohols were added to the sulfuric-acid-based electrolyte to examine their effects on the anodization behavior of aluminum, the coating ratio, the porosity, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various alcohols with different physical properties under mild anodization conditions with a constant current of 100 A·m−2 at 20 °C (2, 3). The influence of the type and concentration of alcohol on the coulombic efficiency of film formation and the structure of the porous alumina under a constant electric charge was also investigated. In terms of basic research, the potential impact of alcohol addition in electrolytes will be discussed. H. Asoh, K. Asakura, H. Hashimoto, RSC Advances, 10, 9026-9036 (2020).H. Asoh, M. Matsumoto, H. Hashimoto, Surface and Coatings Technology, 378, 124947 (2019).M. Matsumoto, H. Hashimoto, H. Asoh, Journal of The Electrochemical Society, 167, 041504 (2020).

Fabrication of Pt-Nanoparticle-Loaded Mesoporous Alumina Coating through Anodizing of an Al-Pt Alloy

Noble metal nanoparticles, usually dispersed in a mesoporous support, have widespread applications. However, uploading nanoparticles into mesoporous materials and preventing the nanoparticles from aggregation during the fabrication and application processes remain major challenges. Herein, we report a novel, simple and low-cost method for fabrication of Pt-nanoparticle-loaded mesoporous alumina coating through anodizing of an Al-Pt alloy (98.5 at% Al and 1.5 at% Pt) prepared by magnetron sputtering. It was found that the Al-Pt alloy could be effectively anodized at voltages between 2.8 and 4.5 V (SCE), in 0.4 M sulfuric acid solution, at room temperature. The anodizing processes resulted in Pt-nanoparticles of 5∼50 nm dimensions uniformly-dispersed in the porous anodic alumina film. The proposed method can be implemented on a large scale and extended to other alloy systems consisting of a valve metal and different alloying elements with Gibbs free energy of oxide formation less negative than that of the valve metal.

Fabrication of an artificial ionic gate inspired by mercury-resistant bacteria for simple and sensitive detection of mercury ion

Bioremediation of mercury pollution by mercury-resistant bacteria requires the key steps of mercury ions (Hg2+) capture and transportation. Inspired by the functions of mercury transporters in mercury-resistant bacteria, we have fabricated an artificial ionic gate, which may further behave as an Hg2+ biosensor. The gate is constructed by using the asymmetrically modified porous anodic alumina (PAA) film capable of forming Hg2+-activated complexes to control ionic current, while the gate works due to the changes between graphene oxide (GO) and different DNA structures. Specifically, single strand poly(T)n DNA on PAA film may transform into duplex structure through T-Hg2+-T coordination chemistry when Hg2+ exists. Compared with the ring by ring delivery via S-Hg intermediates in bacteria, this artificial gate may show better flexibility for Hg2+ transport, because the poly(T)n-Hg2+ complex can be wrapped in a chain structure with controllable DNA length. Furthermore, with the advantages of simple structure, convenient operation and good practicality, the constructed artificial Hg2+-activated ionic gate has been developed as a biosensor for sensitive detection of Hg2+, with the limit of detection (LOD) down to 0.07 fM. The fabricated ionic gate may be used for building an instrument that can absorb and detect Hg2+ from wastewater in the future.

Fast growth of highly ordered porous alumina films based on closed bipolar electrochemistry

Highly ordered porous anodic alumina (PAA) is widely used as a template for preparing one-dimensional nanostructured materials. At present, research efforts to prepare PAA templates with straight channels mainly focus on improving their regularity or increasing the efficiency of the preparation methods. Herein, we introduce a novel anodization method to fabricate highly ordered PAA films based on closed bipolar electrochemistry. To the best of our knowledge, there have been no reports of the use of closed bipolar electrochemistry in PAA film fabrication. Compared to traditional anodization, a faster PAA film growth rate of 1.67 μm min−1 was obtained in 0.75 M oxalic acid when a voltage of 62 V was applied to the driving electrodes. Furthermore, different electrolyte solutions can be employed in the anodic and cathodic cells in the closed bipolar electrochemical system. In addition, no direct electrical connection to the aluminum sheets is required in this approach.

Fabrication of Orderly Porous Anodic Alumina Optical Waveguide Sensor via Interface Hydrophilic Treatment for the Detection of Small and Large Molecules

AbstractIn this paper, an orderly porous anodic alumina (PAA) optical waveguide sensor is presented with a simple and low‐cost fabrication method. Both the gold film and PAA film undergo interface hydrophilic treatment. The refractive index (RI) sensitivity and resolution for the first order of transverse magnetic (TM1) mode reached 64.67 deg RIU−1 and 1.73 × 10−6 RIU, respectively. Small and large molecules are used to verify its sensing feasibility. For the detection of small molecule, the glucose molecule is used and its limit of detection (LOD) of 39.03 mg L−1, that is 0.22 × 10−3 m, is achieved. In addition, bovine serum albumin (BSA) is used as a model of large molecule to adsorb onto the surface of PAA film due to the characteristic of BSA molecule. The results show that the adsorption of BSA onto the PAA surface belongs to 1:1 model of Langmuir adsorption and the LOD reached 2.67 × 10−12 m based on the threefold standard deviation theory and Langmuir fitting. Further, in the experiment of fluorescence enhancement, the LOD as low as 770.72 × 10−15 m for BSA/Cy5 is obtained, owing to the high adsorption capacity of PAA nanostructure and field enhancement of the PAA waveguide.

Aluminum ASA 6061 Anodizing Process by Chromic Acid Using Box–Wilson Central Composite Design: Optimization and Corrosion Tendency

The behavior of anodic porous alumina film formed on an aluminum surface by anodizing in chromic acid solutions was optimized using Box–Wilson experimental design. The film thickness was correlated as a function of temperature, acid concentration, applied voltage, and time. A mathematical model was suggested and optimized. Results of coating thickness indicate that both temperature and time play an important role in the anodizing process. The interaction effect of variables on the film thickness is less pronounced compared with the effect of the main variables. Optimum film thickness obtained during the anodizing process was 10.43 µm at the optimum value of temperature (45 °C) and time (68 min). The polarization measurement of anodized aluminum ASA 6061 in chromic acid shows a high corrosion resistance in the presence of coating. The results confirmed by using surface morphology investigations.

On the Prediction of Well-Ordered Porous Anodic Alumina Films

Porous hexagonal ordered anodic alumina (PAOX) is a long-known porous oxide material. Despite its straightforward experimental approach, a precise understanding of the microscopic processes that le...

(Invited) Highly Durable Platelet-Type Carbon Nanofibers for Oer in Alkaline Electrolyte

Porous anodic alumina films formed by anodizing of aluminum in acid electrolytes have a highly ordered, nanopore channels, which have attracted increased attention as template for various nanomaterials. Utilizing this template, platelet-type carbon nanofibers (pCNFs) have been successfully prepared by simply heating a mixture of the template and polymer powders, such as polyvinyl chloride and polyvinyl alcohol, in argon atmosphere via liquefied intermediate of the polymers (1, 2). The graphitization degree of pCNFs increases with annealing temperature, and interestingly, the neighboring reactive carbon edge sites at the sidewall of pCNFs form loops of several carbon layers at above 2000ºC. Here, we demonstrate that such pCNFs with high graphitization degree and loop structure exhibit extremely high oxidation resistance under oxygen evolution reaction (OER) conditions in highly alkaline electrolyte for a zinc-air secondary battery application.During galvanostatic OER on the state-of-the-art highly active Ca2FeCoO5 electrode with a conventional carbon black conductive agent in 4 mol dm-3 KOH electrolyte consumed the carbon black almost completely within a few days, whereas that with pCNFs annealed at 2400ºC exhibited almost no consumption of the pCNFs even after one month. The carbon nanofibers with platelet structure and high graphitization degree are promising for the air electrode materials in a zinc-air battery. Acknowledgment This work was supported in part by the RISING2 (Research and Development Initiative for Science Innovation of New Generation Batteries 2) of the New Energy and Industrial Technology Development Organization (NEDO), Japan. 1. H. Konno, S. Sato, H. Habazaki and M. Inagaki, Carbon, 42, 2756 (2004).2. H. Habazaki, M. Kiriu, M. Hayashi and H. Konno, Mater. Chem. Phys., 105, 367 (2007).

Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film

Metallic-monolith catalyst support with self-growing porous anodic alumina (PAA) film was prepared by anodizing Al plate. The effect of hydrothermal treatment (HTT) on the crystalline state and textural properties of PAA film was investigated by XRD, BET, SEM and TG. The HTT treatment above 50 °C and the subsequent calcination above 300 °C could convert the amorphous skeleton alumina into γ-alumina and increase the specific surface area ( S BET ). However, SEM images showed the HTT modification was a non-uniform process along the thickness of PAA film. The promotion effect of HTT on S BET was non-linear, and the slope of S BET gradually decreased with the HTT temperature or time increased. The limited HTT effect should be attributed to a changed pore structure caused by an unfavorable pore sealing limitation. Pore widening treatment (PWT) before HTT could break the pore sealing limitation, because of the reduced internal diffusion resistance of hydrothermal reaction. The synergistic combination of PWT and HTT developed a PAA support with a large S BET comparable to commercial γ-alumina. In the catalytic combustion of toluene, the Pt-based catalyst prepared by using the PWT and HTT co-modified PAA support gave higher Pt dispersion and more favorable catalytic activity than that treated by HTT alone. The presence of a bimodal pore structure was suggested to be a decisive reason.

Depth dependent X-ray diffraction of porous anodic alumina films filled with cubic YAlO3:Tb3+ matrix

The presented paper deals with the measurement methodologies of the structural properties of porous anodic alumina (PAA) films filled with YAlO3:Tb3+ composite using X-ray diffraction, atomic force microscopy and scanning electron microscopy. It shows that the deposited material does not uniformly fill the porous volume of the anodic alumina film and the part of it forms a thick layer on the PAA surface. The aim of this work is to show the differences in the XRD response obtained at different angles of incidence of the excitation beam for the PAA/YAlO3:Tb3+ system. Furthermore, this simple approach enables separation of the signal from both regions on the surface and inside the PAA pores, providing more accurate data interpretation. It reveals that the crystallization of the material on the PAA surface and within the pores is different.