Porous Anodic Alumina Films Research Articles

Formation of Through-Hole Porous Anodic Aluminum Oxide Layer Locally with 3D Printed Solution Flow Type Microdroplet Cell

In this study, a 3D-prinited solution-flow type microdroplet cell (SF-MDC) is employed as a new technique for the fabrication of porous anodic aluminum oxide (AAO) layer using oxalic acid electrolyte on aluminum. The surface morphology of the porous AAO film was characterized by a scanning electron microscope. The aim of this study was to fabricate a through-hole porous alumina layer in a single step anodizing process and to investigate the influence of anodized voltages and scanning speeds on the thickness and pore structure of alumina layer. The results showed that the pore diameter and interpore distance were directly proportional to the anodizing voltage. The thicknesses of formed AAO films were found to be 35.5, 50.7, and 81.6 μm at scanning speeds of 10, 5, and 2.5 μms−1, respectively. Through-hole porous AAO was successfully fabricated at room temperature without chemical etching. The SF-MDC fabrication technique is proposed as an environmentally attractive and suitable process for the fabrication of porous AAO layers.

Enhanced bonding of polyphenylene sulfide-aluminum alloy composites using combined mild and hard anodizing techniques

High-strength direct joining between polymers and Al has attracted considerable attention, particularly for reducing the weight device. In this study, porous anodic alumina (PAA) films were fabricated on a commercial 6063 Al alloy using with mild and hard anodizing with varying durations, followed by etching. Subsequently, polyphenylene sulfide (PPS) was directly joined with Al through injection molding. The correlations between the interface bond strength of PPS-Al joints and the Al oxidation duration were systematically investigated. The porosity, diameter of nanopore, and wettability of anodic aluminum oxide (AAO) film surface significantly influenced the bonding strength of PPS-Al composite structure. The porosity increased with an etching duration, increasing the bonding strength. However, excessive porosity (>54 %) weaken the films and reduced their bonding strengths. When the durations of mild anodizing (MA), hard anodizing (HA), and etching treatment were set to 30, 30, and 60 min, respectively, the maximum bonding strength reached 25.2 MPa, approximately 77 % of the base-material shear strength. Scanning electron microscopy revealed nanopillars with two different top sizes (approximately 80 and 110–130 nm) on the surface of injection-molded PPS, which were attributed to the different anodizing preparation voltage. These findings are generally significant for the realization for high-strength direct-bonded polymer-Al joints.

A multi-strategy coating for Al alloy based on orientationally arranged nanosheets doped SiO2 coating and corrosion inhibitor loaded porous anodic alumina film

Although aluminum alloys are widely used in various industries, their wear and corrosion resistance are still needed to be enhanced. In this study, a robust anti-corrosion composite coating was prepared on the surface of Al alloy based on multiple reinforcement strategies. The composite coating is consisted of horizontally arranged and functionally modified h-BN nanosheets doped SiO2 coating (SiO2-hBN(NH3+) coating) and corrosion inhibitor MBT loaded porous anodic alumina (PAA-MBT) film. Results show that the composite coating prepared in this work has excellent corrosion resistance due to the effect of the extended penetration path of corrosive medium by h-BN(NH3+) nanosheets, the full-thickness hydrophobicity of the SiO2 coating, the corrosion inhibition of the MBT released from the PAA film and the physical barrier of the entire coating system. In addition, the composite coating also exhibits good mechanical properties including high hardness, high bonding strength and superior wear resistance. This work provides a novel design of protective coating with excellent anti-corrosion resistance and mechanical properties, which has a promising prospect in the development of superior long-term anti-corrosion coatings for aluminum alloy.

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

Wearable electret based metal oxide nanostructure for self-powered respiration monitoring

Exhaled human breath contains information regarding metabolic and physiological data, and its monitoring can play a crucial role in the early clinical diagnosis and treatment of diseases. An energy-autonomous breath monitoring device was designed by anodizing Al sheet (Nose Bridge), present in existing commercial, medical, and industrial masks for monitoring breath. The electrochemically grown porous anodic alumina (PAA) film acts as the electret layer for generating and trapping charge carriers and has the dual function of energy harvesting and sensing. The device is based on the principle of a triboelectric nanogenerator (TENG), where PAA and Polydimethylsiloxane (PDMS) were used as tribo-electrodes, generating a peak-to-peak voltage of 225 V and a short circuit current of 1 μA with a power density of 110.75 mW/m2. PAA with small pore diameter, depth, and long-range ordering exhibit the best performance. The nanoporous structure of PAA provides an exceptionally high surface area of 14.31 m2/g, significantly enhancing the performance. The device exhibits an exceptionally high sensitivity of −15.84/RH% in the 25–90 RH% detection range, with a hysteresis of 2.2% at 65 RH%. This research paves the way for developing personalized healthcare monitoring systems by integrating anodized nose bridges in medical and industrial masks that provide mechanical support and can be used for energy harvesting and breath monitoring.

Effect of Al Polishing Conditions on the Growth and Morphology of Porous Anodic Alumina Films.

The conditions applied during the electrochemical polishing of aluminum were found to be important parameters for the successive formation of nanoporous alumina films. First, a high-purity Al foil was electrochemically polished in an aqueous solution containing C2H5OH and HClO4 at various sets of conditions, such as applied potential (5-35 V), temperature (0-20 °C), and process duration (10-180 s). Extensive studies of the topography of Al after polishing by scanning electron microscopy and atomic force microscopy allow verification of the correlations between conditions applied during the substrate pretreatment and dimensions of the nanopatterns generated on the metal surface. Next, Al polished samples at two different sets of conditions were used as starting materials for anodization. Unpolished Al samples were also anodized for reference. It was confirmed that electropolishing conditions do not significantly affect the oxide growth rate during anodization and the efficiency of anodic film formation. On the contrary, it was proved that the dimensions of the surface texture formed during Al polishing significantly affect the morphology and pore order within the anodic film. Therefore, it can be stated that it is possible to tune to some extent the arrangement of nanochannels within anodic aluminum oxide films by simply changing conditions during the electropolishing procedure..

Optical Fiber Temperature and Humidity Dual Parameter Sensing Based on Fiber Bragg Gratings and Porous Film.

A porous anodic alumina film is proposed to construct an optical fiber temperature and humidity sensor. In the sensor structure, a fiber Bragg grating is used to detect the environment temperature, and the porous film is used to detect the environment humidity. The proposed porous anodic alumina film was fabricated by anodic oxidation reaction, and it is suitable for the use of humidity detection due to its porous structure. Experimental results show the temperature sensitivity of the proposed sensor was 10.4 pm/°C and the humidity sensitivity of the proposed sensor was 185 pm/%RH.

Se-doped Nb2O5–Al2O3 composite-ceramic nanoarrays via the anodizing of Al/Nb bilayer in selenic acid

Novel arrays of Nb2O5-based ceramic nanostructures of various sizes (9–210 nm) and morphologies (dots, goblets, rods) aligned on substrates are fabricated via the anodizing of a thin Nb film through the initially formed porous anodic alumina (PAA) film in 1.5 M selenic acid (H2SeO4) – a new aqueous electrolyte generating extraordinarily thinner PAA pores than any other solutions. Accordingly, the nanostructures formed in the selenic acid are 1.3-fold thinner and better self-ordered than their counterparts formed from the same Al/Nb precursor bilayer in a reference oxalic-acid electrolyte. The nanostructures have a dual (core/shell) composition: the inner material (the core) is stoichiometric Nb2O5, whereas the outer layer (the shell) is a few nm-thick substoichiometric NbOx mixed with Al2O3. The composite-ceramic nanoarrays grow doped with selenium species such as selenate (SeO42−) and selenide (Se2−) anions originating from the electrolyte and migrating inward under the high electric field. The incorporated Se species do not contribute to photoluminescence emission nor hinder the Raman signal from the nanoarrays, which makes them highly promising as Nb2O5-based SERS biosensing substrates. The planar PAA-inbuilt Se-doped Nb2O5–Al2O3 nanostructured ceramic film performs like a high-k low-loss low-leakage-current dielectric promising for on-chip integration. More potential applications of the Se-doped ceramic nanoarrays developed here include biomedical antibacterial coatings, advanced superhydrophobic surfaces, gas-sensing, and catalytic layers.

Understanding the fast formation mechanism of hard nanoporous alumina films on aluminum in acidic solutions containing nitric acid

Nanoporous alumina films formed on aluminum via anodization are widely used for scientific and technical purposes owing to their various advantageous properties, including excellent corrosion and wear resistance, good thermal and electrical insulation, and facile producibility at low cost. Herein, we report a novel approach to fabricate hard porous anodic alumina (PAA) films by adding an appropriate amount of nitric acid to sulfuric-, oxalic-, and phosphoric-acid solutions at critical high current densities. The effects of the nitric additives on the anodizing behavior, film growth mechanism, microstructure, chemical composition, film thickness, microhardness, and appearance of the PAA films formed from various solutions were systematically investigated. The addition of nitric acid enhanced the formation of alumina and resulted in fast growth rates of 196 μm/h and 166 μm/h for sulfuric- and oxalic-based solutions, respectively, while maintaining high hardness of approximately 380 and 430 HV0.1, respectively. The surface hardness of the films decreased to some extent with the increase in nitric acid concentration and/or film thickness owing to the strong corrosive nature of nitric acid during prolonged anodization, although nitrogen inclusion in anodic alumina films was not detected through X-ray photo-electron spectroscopy, which differs from the other acidic anions. In particular, the barrier layer thickness and pore intervals of PAA films increased with the nitric concentration in the aforementioned solutions, leading to a fast growth rate and high hardness inside the films.

Electrochemical Impedance Spectroscopy Study of Porous Anodic Alumina Films Fabricated at Low Temperature

A method of fabrication of AAO film in 0.3 mol/L H2C2O4 by adding 1,2-propanediol(PROH) and at sub-zero temperature has been proposed. The electrochemical characteristics of the barrier and porous layers of AAO films before and after pore-filling are examined using electrochemical impedance spectroscopy (EIS). It is shown that the high and medium frequency range corresponds to the barrier layer properties and the low frequency ranges reflect the sealed porous layer properties. The calculated thickness of the barrier layer is in the range of 1 ~ 18 nm. The resistance (Rb) and the thickness (δb) of the barrier layer increase and the capacitance of the barrier layer (CPEb) decrease with the volume percentage of PROH increasing from in electrolyte from 25 % to 75 %. The surface non-homogeneity of AAO film goes better by pore-filling, leading to a decrease in the capacitance of the porous layer (CPEp) and an increase in the porous layer thickness.

Development of nano structures for solar driven desalination

Now-a-days there is shortage of pure water in several areas. So, the emerging technology solar driven desalination is one of the core practices that employ solar energy for fresh water making in sunny areas with lacking quality of water resources. In this work, the nano structure was developed using two step anodization procedure. Anodization was performed on pure aluminum foil (99.99 % purity) in 0.3 M of oxalic acid at constant voltage and effect of temperature on the morphology of formed porous anodic aluminum oxide (AAO) film was investigated. The temperature is varied between −5 to 25 °C. The characterization of the material is done using Scanning Electron Microscopy (SEM), Energy dispersive X-ray (EDAX), Ultraviolet-Differential reflectance spectroscopy (UVDRS). It is observed that, at −5 °C, micro indentations are formed on surface. At 5 °C, pores are randomly formed on the surface. At 15 °C, nano balls are formed on the surface. Whereas, at 25 °C, complete nano porous structure is formed. Hence, the formed nano structures were applied in solar driven desalination. In comparison to all other temperatures, we have observed that, the nano porous structure developed at 25 °C has 27 % of higher evaporation than the conventional.

Effects of anion incorporation on the local structure of porous-type amorphous alumina prepared by anodization: NMR study of Al coordination numbers

Although anodization is an essential technology that has supported the aluminum industry for nearly a century, the atomistic structure of the formed alumina films has not completely understood. Here we used NMR spectroscopy to clarify the factors affecting local structure of anodic porous alumina films prepared under various electrolytic conditions. For anion-free amorphous alumina films prepared using chromic acid, the local structure does not depend on electrolytic factors. For films prepared using phosphoric acid, the average coordination number was lower than that of anion-free alumina. On the other hand, films prepared using oxalic and sulfuric acids showed the average coordination numbers higher than that of anion-free alumina. We reveal that aluminum coordination number of anodic porous alumina are affected by anion species and anion content. In addition, the depth of anion incorporation into cell walls, or compositional uniformity, is quite important for accurate interpretation of NMR spectra.

Key role of longitudinal disarranged growth in porous anodic alumina: Crystallographic orientation

Porous anodic alumina (PAA) is a typical template for electrochemical deposition. The longitudinal disarranged growth (LDG) in a PAA template directly causes a distortion of deposited nanomaterials. Crystallographic orientation is an important parameter of aluminum (Al) and its influence on PAA longitudinal growth is the key to high-quality PAA templates. In this study, PAA films prepared from polycrystalline pure Al foils were used to investigate the key effects of crystallographic orientation on PAA LDG. A spatial mapping relationship between crystallographic orientation and PAA morphology was built using a scratch marking method with a combination of electron backscatter diffraction and scanning electron microscopy. The results showed that PAA LDG manifested as LDG regions, whose size was positively correlated with grain boundary misorientation. The effect of misorientation on the LDG was explained based on the volume expansion model. This study contributed to a further understanding of the PAA longitudinal growth mechanism.

Ingenious Fabrication of Ag-Filled Porous Anodic Alumina Films as Powerful SERS Substrates for Efficient Detection of Biological and Organic Molecules.

Surface-enhanced Raman scattering (SERS) has been widely used to effectively detect various biological and organic molecules. This detection method needs analytes adsorbed onto a specific metal nanostructure, e.g., Ag-nanoparticles. A substrate containing such a structure (called SERS substrate) is user-friendly for people implementing the adsorption and subsequent SERS detection. Here, we report on powerful SERS substrates based on efficient fabrication of Ag-filled anodic aluminum oxide (AAO) films. The films contain many nanopores with small as-grown inter-pore gap of 15 nm. The substrates are created by electrochemically depositing silver into nanopores without an additional pore widening process, which is usually needed for conventional two-step AAO fabrication. The created substrates contain well-separated Ag-nanoparticles with quite a small inter-particle gap and a high number density (2.5 × 1010 cm−2). We use one-step anodization together with omitting additional pore widening to improve the throughput of substrate fabrication. Such substrates provide a low concentration detection limit of 10−11 M and high SERS enhancement factor of 1 × 106 for rhodamine 6G (R6G). The effective detection of biological and organic molecules by the substrate is demonstrated with analytes of adenine, glucose, R6G, eosin Y, and methylene blue. These results allow us to take one step further toward the successful commercialization of AAO-based SERS substrates.

Photoluminescence properties of anodic aluminium oxide films formed in a mixture of malonic acid and oxalic acid.

In this work, porous anodic aluminium oxide (AAO) films were fabricated by anodization in an electrolyte mixture with various concentration ratios of malonic acid and oxalic acid at room temperature. The photoluminescence (PL) properties of the AAO films before and after annealing from 300°C to 650°C in air or vacuum conditions were investigated, showing a strong PL band in the range 300-550 nm. We observed a weak PL in the AAO film formed in the malonic acid electrolyte, whereas the films fabricated using an electrolyte mixture showed strong PL emissions, exhibiting a maximum. The broad PL band was decomposed into three Gaussian sub-bands, in which the first two sub-bands could be attributed to the luminescence centre oxygen vacancies (F+ and F defect centres), whereas the latter transformed from malonic impurities and oxalic impurities. More interestingly, the redshift of the PL bands occurred with increasing oxalic acid concentration, and the PL wavelength and intensity could be modulated by varying the concentration ratios in the malonic acid and oxalic acid electrolyte mixture.

The effect of dissolution by periodic reverse current on the microstructure of bamboo-shaped porous anodic alumina

• Periodic reverse current with small amplitude can promote the growth of the porous layer. • Periodic reverse current has the effect of pore expansion. • The water reduction speed increases during the reverse voltage stage. • The ions driven by the electric field change with the direction of current. • H+ (H2O) and OH− are the main ions that move in the reverse voltage stage. In this paper, the effects of a cyclic anodizing method with the introduction of periodic reverse current are reported. The effects of the periodic reverse current on the microstructure of bamboo-shaped porous anodic alumina (PAA) film are investigated using positive and negative current triangular waveforms. The results of the experiment showed that when the reverse current in each current cycle was I D ' = -5 mA, the growth velocity was the fastest. The analysis found that the introduction of the periodic reverse current was beneficial to increasing the growth velocity of the porous anodic alumina film and had the additional effect of pore expansion. However, with any further increase of the periodic reverse current, the growth velocity of the porous anodic alumina film gradually decreased, and the period length showed a decreasing trend. The main reason for this phenomenon is that in a larger reverse voltage phase, the ions driven by the electric field are mainly H + (H 2 O) and OH − . Then the reaction that occurs is mainly dissolution. With large pore size and adjustable period length, this porous anodic alumina film can be used as a template for the synthesis of nanomaterials, which have practical significance for the synthesis of nanodevices.

Nanomorphological changes of anodic aluminum oxide fabricated by anodizing in various phosphate solutions over a wide pH range

High-purity aluminum plates were anodized in phosphoric acid and six phosphate solutions over a wide pH range from acidic to alkaline. The current density corresponding to the growth rate of the anodic oxide was minimum when anodizing in near-neutral solutions and increased when the pH value increased or decreased. A typical porous anodic aluminum oxide (PAAO) film consisting of an outer porous layer and inner hemispherical barrier layer, which is based on the Keller–Hunter–Robinson (KHR) model, was formed in acidic solutions. At the pH values closer to the neutral region, a uniform barrier oxide film or a PAAO film with a narrow porous layer was produced owing to the low solubility of aluminum oxide. On the other hand, two types of characteristic PAAO films were formed during anodizing in alkaline phosphate solutions: one was a PAAO film consisting of an outer porous layer with feathered pore walls and inner hemispherical barrier layer, and the second was a PAAO film consisting of an outer porous layer with elongated pores and inner flat barrier layer. We showed that novel PAAO films, which are not based on the KHR model, can be prepared by anodizing aluminum in various alkaline phosphate solutions.

Rapid electrochemical separation of anodic porous alumina films from aluminum surfaces using a highly safe sodium chloride–ethylene glycol solution

A rapid electrochemical separation of an anodic porous alumina (APA) film formed by anodizing an aluminum substrate was achieved using a highly safe sodium chloride (NaCl)/ethylene glycol (EG) mixture. A thick APA film (thickness: 20–80 µm) with an ordered pore structure was formed on the aluminum surface by anodizing the substrate in a 0.3 M sulfuric acid solution at 25 V, and the specimens were subsequently polarized anodically in a 1.0 M NaCl/EG solution. A large current density was measured when a voltage higher than the anodizing voltage of 25 V was applied, and the APA film was completely separated from the aluminum substrate. This may be due to the formation of extremely small through-holes at the bottom barrier oxide layer of the APA film and the subsequent dissolution of aluminum from the substrate. The separation time of the APA film decreased with increasing applied voltage in the NaCl/EG solution, and rapid separation within 0.5 s was successfully achieved via anodic polarization at 36 V. We demonstrate that large-area and thick APA films can also be isolated from the aluminum surface by anodic polarization in the benign NaCl/EG solution. Moreover, an APA through-hole membrane can be successfully fabricated by subsequent immersion in a 0.52 M H3PO4 solution.

Fabrication of nanostructured DLC coatings using anodic alumina films

Diamond-like carbon (DLC) films, deposited by dry processes, such as, physical vapor deposition and chemical vapor deposition, provide high hardness, lubricity, and wear resistance. With improvements in microfabrication technologies, tuning the microscopic surface morphology to control physical properties (such as wettability) of materials has attracted immense attention. However, the strengths of microstructures with uneven surfaces are insufficient for numerous applications. Therefore, nanoscale structures with uneven surfaces and enhanced strength were synthesized using DLC. Anodic porous alumina films with regularly aligned linear nanopores produced by anodic oxidation of Al were used as mask materials to deposit DLC films on Si substrates. Uneven structures (with uniform distributions of DLC nanoscale protrusions) were created on the substrates. Mask materials with large pore diameters and small thicknesses created DLC protrusions with large diameters and heights, respectively. The friction coefficients of nanostructured DLC surfaces with tall protrusions and normal DLC films were similar. Unevenly structured DLC surfaces were slightly more water-repellent than normal DLC films, with greater water repellency for taller protrusions. The Cassie-Baxter model was applied to analyze the surface wettability. This report describes a new, simple, and effective method of fabricating DLC nanoscale structures using anodic alumina films.

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.