Anodic Alumina Films Research Articles

Silver-Doped Anodic Alumina with Antimicrobial Properties—Synthesis and Characterization

The incessant need for the elimination of pathogenic viruses and multi-drug resistant bacteria has been a critical issue during recent decades, and requires the creation of new antimicrobial materials. Our study describes the production of silver-modified anodic alumina substrates by two methods, and estimation of their bactericidal activity. Aluminum oxide coatings were obtained via an anodization process of low-purity aluminum in an acidic bath for different time periods. The realization of silver infiltration into the pores of the alumina layers was carried out employing two different routes—electrochemical deposition, and in situ thermal reduction. The obtained films were characterized using scanning electron microscopy (SEM). Changes in the surface morphology and thickness of the initial oxide structures after hot water sealing procedure were observed. The presence of silver inside the pores of the alumina layers was also assessed. It was found that silver electrodeposition resulted in greater surface saturation. Large silver accumulations were observed on the thinner anodic films which experienced electroplating for longer time periods. Finally, the antibacterial activity of the modified alumina structures against Gram-negative (Escherichia coli) and Gram-positive (Bacillus cereus) bacteria was evaluated. The results demonstrate that silver deposits acquired by the electrochemical technique improve the bactericidal efficiency of the anodic aluminum oxide (AAO) layers. On the contrary, alumina structures with chemically embedded Ag particles did not show significant antibacterial properties. Overall, the present studies demonstrate that biological activity of silver-doped AAO films depends on the techniques used for their modification.

High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.

Self-ordered nano-concave-regulated Al foil anode for high-stability dual-ion batteries

A highly self-ordered arrangement of a nano-concave morphology was prepared on Al foil by an anodization treatment in 0.3 M oxalic acid or 0.3 M tartaric acid followed by the removal of the resulting anodic alumina film. The foil was then used as the anode of dual-ion batteries. The wettability of the Al surface increased with increasing degree of self-assembly, facilitating a uniform distribution of electrochemically active sites during alloying/dealloying process. The highly self-ordered surface of Al foil anodized in oxalic acid can improve the electrochemical performance significantly compared to the only electropolished Al foil anode and the relatively disorderly surface state of the anodized Al foil obtained in tartaric acid. A 93 mA h g−1 capacity and 98% capacity retention were obtained after 500 cycles at 100 mA g−1, as well as a high reversible capacity of 83 mA h g−1 at 500 mA g−1. Although the capacity of the self-ordered regulated Al foil anode was not the highest among the existing anode materials, its synthesis process was more straightforward and cost-effective.

Investigation of the features of the porous morphology of anodic alumina films at the initial stage of anodization

The work is devoted to the study of the porous structure formation of anodic alumina films at the initial stage of aluminium anodizing. SEM images of the surface morphology of the oxalic acid anodic films were analyzed. It was shown that at the initial stage, both major and minor pores are formed, the diameter ratio of which is about 1.16 and does not depend on the anodizing voltage. The results obtained indicate that the minor pores in the anodic films are located inside hexagonal cells composed of the major pores.

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.

Embedding Cs3Cu2I5 Scintillators into Anodic Aluminum Oxide Matrix for High‐Resolution X‐Ray Imaging

AbstractScintillators are widely utilized as X‐ray imaging detectors. Pixelated scintillator films with optical‐waveguide‐effect are preferred to realize high‐resolution X‐ray imaging. However, it remains a great challenge to rapidly fabricate high‐resolution pixelated scintillator film. Herein, a new strategy is developed to solve the problem by embedding metal halide scintillators into an anodic aluminum oxide (AAO) via a facile hot‐pressing method. Cs3Cu2I5 is selected as the prototype to fabricate pixelated scintillator films with targeted thickness, denoted as Cs3Cu2I5−AAO. The light confinement effect provided by AAO is validated by finite‐difference time‐domain simulations. In particular, the as‐prepared Cs3Cu2I5−AAO film demonstrates higher spatial resolution (10.4 lp mm−1 at modulation transfer function = 0.2), higher UV and X‐ray imaging performances compared to the homogeneous counterpart and commercialized terbium‐doped gadolinium oxysulfide. These results can inspire further research on the design of nanostructured metal halide perovskites for high‐resolution X‐ray imaging.

Forming Hard Anodic Films on Aluminum by Anodization in Oxalic Acid and Alcohol

Porous anodic films have attracted wide attention because of their potential applications in various industrial fields. Reducing their porosity is expected to improve the hardness. This study focused on evaluating the effect of the electrolyte composition on the hardness of anodic alumina films formed on aluminum by anodization under mild conditions. In particular, the effect of adding alcohol (i.e., ethanol and ethylene glycol) to an oxalic acid–based electrolyte was considered. The voltage–time curve, coating ratio (i.e., film formation efficiency), morphology, and Martens hardness of the resulting anodic alumina films were clearly affected by the type and amount of added alcohol. Adding alcohol effectively suppressed the chemical dissolution of the anodic film during anodization without needing to lower the electrolyte temperature, which formed high-strength films with low porosity and thick pore walls. The Martens hardness of the formed films increased by 18% with the addition of 50 vol% ethanol and increased by 25% with the addition of 50 vol% ethylene glycol.

Effect of sealing on characteristics of nano-porous aluminum oxide as black selective coatings

This study aimed to investigate the effect of sealing on improvement of optical characteristics of black anodized aluminum that was first anodized and re-anodized. Various types of selective coatings were studied, nano-porous anodized aluminum was black colored using electrolytic coloring technique. The anodization process was performed to build up the first and second nano anodized aluminum oxide film. Copper, tin and nickel ions were deposited within the porous layer using AC current. The optical characteristics of different types of black coatings were recorded. In case of first anodized black colored samples, the optical and potentiodynamic polarization data, showed that the sealing step was highly effective and essential step. As, it enhanced the absorbance value, decreased the rate of corrosion and increase the stability and protection of colored anodized sample. However, in case of tin coating that was re-anodized sealing can be neglected. The surface was characterized by different analysis; SEM, FTIR and solar absorption. The data revealed that the prepared selective coating can be beneficially used for solar panel industries e.g. solar water heaters and consequently solar desalination units.

Effect of Anodization Treatment on the Thickness, Hardness, and Microstructural Characterization of Anodic Aluminum Oxide Film on AA 6061 and Critical Patent Analysis

Effect of Anodization Treatment on the Thickness, Hardness, and Microstructural Characterization of Anodic Aluminum Oxide Film on AA 6061 and Critical Patent Analysis

Synergistic Effect of Rapid Anodization and Hydration on the Distribution of the Etched Tunnels for Aluminum Foil

The concave structures and the hydrated layer prepared by the anodized treatment and hydration process were investigated for improving the uniform distribution of the etched tunnels during the DC electrochemical etching of aluminum foil. It is indicated that when the anodized time is 20 s and the following hydration time is 90 s, the proportion of merged tunnels and the unetched regions to the etched aluminum foil substantially decreased, thus resulting in the significant increase in the specific surface area. The potentiodynamic polarization and EIS results indicated that the prolonged hydration process can gradually reduce the corrosion resistance of aluminum foil, which ascribe to the transformation of the anodic alumina film into the hydrated layer. Based on the synergistic effect of the hydrated layer and the concave structures, the distribution of etched tunnels on the as-received aluminum foil can be improved observably.

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.

Influence of anodization parameters on film thickness and volume expansion of thick- and large-sized anodic aluminum oxide film

Anodic aluminum oxide (AAO) film with a thickness ranging from 20 to 100 μm was prepared using a large-sized Al plate (4 cm × 10 cm) to investigate the anodization parameter effect on the film thickness and volume expansion factor. A corrosion treatment (voltage = 0 V) was performed to investigate the film dissolution caused by acid. The actual anode surface temperature was also measured to confirm the field-assisted nature of AAO dissolution. The film thickness increases exponentially with temperature, and increases approximately linearly with voltage, duration or concentration. The volume expansion factor gives a first rising and then falling trend with temperature or duration, while it has a nearly linear trend with voltage or concentration. The volume expansion factor increases with the intensified electric field, while its decrease is attributed to the Joule heat-enhanced dissolution. In the case of large film thickness (> 20 μm), the pore confinement effect may be one of the reasons for the change of volume expansion factor. In addition to the conventional parameters, the heat transfer-related parameters (for example, sample size) also greatly affect the AAO film growth.

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.

Preparation and growth mechanism of a gold-coloured Ag@AAO composite film with no angular dependence

Ag-coated anodic aluminium oxide film with a golden colour was successfully synthesized by an ultrahigh vacuum magnetron sputtering system. Free-standing Ag films were obtained by a mechanical stripping process. The results indicate that the Ag films have an ordered close-packed hexagonal pattern similar to that of the anodic aluminium oxide template. The structural colour of the sample is not derived from thin-film interference. UV–Vis reflectance spectra measurements show that the blue purple-spectrum area has a relatively strong absorption. The absorption peak indicates that the golden colour comes from localized surface plasmons. We think the reason, for the gold-coloured Ag@AAO composite film, is the periodic arrangement of Ag particles. This is also the reason why the golden colour does not change with the viewing angle. Further research shows that the Ag atoms initially grew preferentially on the hexagonal vertices of the anodic aluminium oxide template, and then Ag islands grew on the top of the anodic aluminium oxide surface, finally forming narrow Ag nanoholes on top of the wider pores of the template below, with increasing Ag sputtering time. These films show the promise of application in many areas, including decoration, display and anti-counterfeiting.

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.

The Flip-Flop Effect on Anodized Aluminum Oxide Films

The Flip-Flop Effect on Anodized Aluminum Oxide Films

Thin nanoporous anodic alumina film on aluminium for passive radiative cooling

We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98 $$\%$$ within the solar spectrum (0.4–2.2 $$\mu $$ m) and simultaneously exhibits a high average emissivity of 88 $$\%$$ within the atmospheric infrared (IR) transmission window of 8–13 $$\mu $$ m with the peak IR emission approaching 99 $$\%$$ at a wavelength of 10 $$\mu $$ m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $$\mu $$ m. Further, we estimate the average cooling power of NAA/Al to be about 136 W $$\hbox {m}^{-2}$$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.