Anodic Aluminum Oxide Layer Research Articles

ENHANCING SURFACE CHARACTERISTICS OF AA2024-T3 AIRCRAFT ALLOY THROUGH SYNERGISTIC ANODIZATION AND CERIUM CONVERSION COATING. PART I: PERFORMANCE IN MODEL CORROSIVE MEDIUM

The present study is devoted to the monitoring of the performance of AA2024-T3 specimens, after the formation of Anodic Aluminum Oxide (AAO) layer and/or deposition of a Ceruim Conversion Coating (CeCC), obtained at 20°C and 50°C, respectively. Although both methods are well described in the literature, there is no sufficient information regarding the effect of their combination on their behaviour in corrosive media. The performance of the respective specimens was elucidated after 24 h of exposure to 3.5 % NaCl model corrosive medium (MCM) by means of Electrochemical Impedance Spectroscopy (EIS) and acquisition of Potentiodynamic Scanning (PDS) curves. Since the combined AAO/CeCC coating primers revealed superior barrier properties, the respective specimens weresubjected to long-term corrosion tests of up to 672 h of exposure to the MCM to evaluate their durability. The results revealed the synergistic effect of the combination of the surface treatment procedures on effective corrosion mitigation.

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.

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.

Effect of Anodizing Voltage and Tobacco Extract Addition on the Structure of Porous Anodic Aluminum Oxide (PAAO) Layer

Porous Anodic Aluminum oxide (PAAO) is a porous oxide layer resulting from anodization. The structure of PAAO is influenced by anodization parameters, i.e., voltage and electrolyte composition. Increasing anodization voltage can affect the process of pore formation and oxide growth during anodization. Adding additives such as ethanol, propanol, and polyethylene glycol (PEG) can increase pore regularity and affect the structure of PAAO. In this study, tobacco extract (TE) was added to the oxalic acid-based anodizing solution. TE has many active compounds that may affect pore formation and oxide growth. Morphological analysis shows decreased pore diameter when adding tobacco extracts with concentrations of 0, 0.1, and 0.5 g/L, namely 43.92, 41.42, and 37.8 nm at anodization voltage 40 V. In anodization with a voltage of 60 V, a decrease in pore diameter was obtained with 46.47, 34.24, and 26.8 nm for adding tobacco extract 0, 0.1, and 0.5 g/L. The thickness of PAAO increases from 6.45 µm to 16.87 µm with increasing anodization voltage and tobacco extract concentration. The increase of tobacco extract concentration can lead to the decrease of the XRD peak intensity, where the sequence of the most significant decrease was observed for the peaks of (111), (220), (200), and (311), respectively. A decrease in the intensity ratio of (111) and (220) AAO peaks indicates the influence of tobacco extract on the anodization process. Further thermal analysis by Thermo-gravimetric (TG) shows an increase in mass loss from 1.47 to 5.37% with increasing tobacco extract concentration from 0 g/L to 0.5 g/L. TG results indicate the incorporation of tobacco extract in the inner pore wall.

Influence of Induced Local Stress on The Morphology of Porous Anodic Alumina at The Initial Stage of Oxide Growth

A characteristic feature of the disordered pore growth at the initial stage of aluminum anodizing is the development of three large groups of pores: the major pores of larger diameter and two groups of minor pores of smaller diameter. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 30 V at 5 °C–40 °C. According to SEM studies, the pore distribution by diameter for the films obtained at 20 and 40 °C has three distinct peaks at ca. 13.5, 17.2, and 20.3 nm. The ratio of the diameter of major pores to the diameter of minor pores of group 1 or group 2 is constant and approximately equal to 1.17 and 1.51, respectively. The generation of local compressive stress influences the development of porous morphology. The distribution of zones with high and low compressive stress levels inside hexagonal cells is shown, and their correlation with the porous morphology is confirmed. The generation of local stress and strains in the anodic alumina layer with a porous, cellular structure is associated with local areas with changes in the geometric properties on its surface.

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching.

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.

A STUDY ON THE MICROSTRUCTURE, WEAR, AND INDENTATION TOUGHNESS OF ANODIZED ALUMINUM OXIDE LAYER MODIFIED WITH A NEW ADDITIVE

In this study, lemon juice at different concentrations as a new additive was poured into the bath of the anodizing process to enhance the mechanical properties of the manufactured aluminum oxide layers. X-ray diffraction (XRD) and field-emission scanning electron microscopy were utilized to detect formed phases and microstructure, respectively. To investigate mechanical properties, microhardness, indentation toughness, and wear tests of various aluminum oxide layers were performed. The XRD patterns showed a crystalline phase of [Formula: see text]-Al2O3 for all oxide layers. The microhardness of modified layers increased up to 62.2% compared to the unmodified layer. However, by increasing the additive concentration to 2.5 vol%, the hardness decreased. This was based on increasing the pore size of layers. The lowest friction coefficient with a value of 0.53, the lowest wear rate, and the highest indentation toughness was also related to the modified aluminum oxide layer when the concentration of the additive in the bath was 0.3 vol%. For this modified layer, the value of COF/[Formula: see text] was the lowest, and the pore size of 50 nm was the lowest among the layers.

Reflection Interference Spectroscopy Technology Monitoring the Synthesis of ZnCl2-ZnO Nanosheets on Nanoporous Anodic Alumina Substrate in Real Time

In situ, real-time, and non-destructive monitoring of the synthesis of nanomaterials is essence crucial for the development and prospective applications of nanoscience and nanotechnology. Reflection interference spectroscopy technology was used to systematically monitor the synthesis process of a transparent (ZnCl2-ZnO)/NpAA composite film which consists of ZnCl2-ZnO nanosheets formed by ZnCl2 precursor solution on the top surface of the substrate layer of nanoporous anodic alumina. Some significant results are found, e.g., the curve of effective optical thickness with time can be divided into three stages, corresponding to the synthesis process of ZnCl2-ZnO; and more, these films generated from ZnCl2 precursor solution with different concentrations, such as 0.05 M, 0.07 M, 0.085 M, or 0.1 M, can be directly distinguished according to the characteristics of the three stages.

Preparation of an Anodic Alumina-Supported Ni Catalyst and Development of a Catalytic Plate Reformer for the Steam Reforming of Methane

A plate-type Al/Fe–Cr alloy/Al-clad substrate was used to prepare a sandwich-structured plate-type anodic alumina catalyst by anodization, post-modification treatment, and metal loading. The as-prepared plate-type catalyst was utilized in the design of a catalytic plate reactor for a methane steam reforming reaction, and a 3D model was developed to simulate the performance of tube-type and box-type reformers. The experimental results of the preparation of the clad materials showed that the hydrothermal treatment and subsequent high-temperature calcination transformed the amorphous skeletal alumina in the conventional anodic alumina layer into γ-alumina, and significantly increased its specific surface area. Simulation results showed that the temperature difference between the channel wall and the center of the gas phase of the thin-walled catalyst was only 30% of that of the particulate catalyst, indicating the potential advantages of the catalytic plate reactor in terms of heat transfer and energy saving. When the length-to-diameter ratio (or length-to-width ratio) of the reaction channel is small and the channel height is large, insufficient transverse mass transfer and backmixing are two major factors affecting reformer performance. For the tube-type channels, a length-to-diameter ratio of 10~35, and a diameter of 5~20 represent favorable choices. In contrast, for the box-type channel, the length-to-width ratio and the height should be set to 2~4 and 2~5 mm, respectively. Additionally, for box-type channels, the number of gas inlet ports has a significant effect on the reformer performance, and the distribution state provided by two inlet ports is close to the ideal distribution state.

Designing porous photonic crystals for MIR spectral region—a deeper insight into the anodic alumina layer thickness versus charge density relation

The mid-infrared region (MIR) is crucial for many applications in security and industry, in chemical and biomolecular sensing, since it contains strong characteristic vibrational transitions of many important molecules and gases (e.g. CO2, CH4, CO). Despite its great potential, the optical systems operating in this spectral domain are still under development. The situation is caused mainly by the lack of inexpensive and adequate optical materials which show no absorption in the MIR. In this work, we present an easy and affordable way to develop 1D photonic crystals (PCs) based on porous anodic alumina for MIR region. The porous PCs were produced by the pulse anodization of aluminum using charge-controlled mode. The first order photonic stopbands (λ 1) were located within ca. 3.5–6.5 μm. Annealing of the material at 1100 °C for an hour has allowed to recover the wavelength range from around 5.8 to 7.5 μm owing to the decomposition of the absorption centers (oxalate anions) present in the anodic oxide framework while maintaining the PC structural stability. The spectral position and the shape of the resonances were regulated by the charge passing under high (U H) and low (U L) voltage pulses, porosity of the corresponding d H and d L segments, and dura tion of the process (t tot). The thickness of the d H and d L layers was proportional to the charge passing under respective pulses, with the proportionality coefficient increasing with the applied voltage. Despite the constant charge (2500 mC cm−2) applied during the anodization, the thickness of anodic alumina (d) increased with applied voltage (10–60 V) and anodizing temperature (5 °C–30 °C). This behavior was ascribed to the different kinetics of the anodic alumina formation prompted by the variable electrochemical conditions. The photonic material can be used in portable nondispersive gas sensors as an enhancement layer operating up to around 9 μm.

Recent Progress of Fabrication, Characterization, and Applications of Anodic Aluminum Oxide (AAO) Membrane: A Review

The progress of membrane technology with the development of membranes with controlled parameters led to porous membranes. These membranes can be formed using different methods and have numerous applications in science and technology. Anodization of aluminum in this aspect is an electro-synthetic process that changes the surface of the metal through oxidation to deliver an anodic oxide layer. This process results in a self-coordinated, exceptional cluster of round and hollow formed pores with controllable pore widths, periodicity, and thickness. After the initial introduction, the paper proceeds with a brief overview of anodizing process. That engages anodic aluminum oxide (AAO) layers to be used as formats in various nanotechnology applications without the necessity for expensive lithographical systems. This review article surveys the current status of the investigation on AAO membranes. A comprehensive analysis is performed on AAO membranes in applications; filtration, sensors, drug delivery, template-assisted growth of various nanostructures. Their multiple usages in nanotechnology have also been discussed to gather nanomaterials and devices or unite them into specific applications, such as nano-electronic gadgets, channel layers, and clinical platforms tissue designing. From this review, the fact that the specified enhancement of properties of AAO can be done by varying geometric parameters of AAO has been highlighted. No review paper focused on a detailed discussion of applications of AAO with prospects and challenges. This review paper represents the formation, properties, applications with objective consideration of the prospects and challenges of AAO applications. The prospects may appeal to researchers to promote the development of unique membranes with functionalization and controlled geometric parameters and check the feasibility of the AAO membranes in nano-devices.

Алюмооксидные нанопористые мембраны как основа функциональных нанокомпозитных материалов

In the present study, we report about results of obtaining nanocomposite materials based on various types of nanoporous alumina membranes (NAM). The NAM were fabricated by two ways: (I) — using the technique of thinning the barrier layer of porous anodic alumina; (2) — by two-layer galvanostatic electrochemical anodization of aluminium samples. Using atomic force microscopy, it was found that a method of thermal decomposition of K2MnO4 leads to the formation of nanoparticles γ-MnO2 with size ~ 20 nm on the surface and in the pores of NAM, and the use of the method of photochemical synthesis of Ag nanoparticles with a size of ~ 30 nm.

Continuous and Real-Time Measurement of Plant Water Potential Using an AAO-Based Capacitive Humidity Sensor for Irrigation Control

Water potential measurement is an essential factor in determining water consumption management and recycling in the agricultural field. We report the development of a continuous water potential measurement system using sensors for water stress analysis in tomato plants with better irrigation plan feedback. The water potential sensor uses the capacitive sensing principle which measures humidity inside an anodic aluminum oxide (AAO) layer. An analog to digital converter with a wireless communication module system records the capacitance data of the sensing system. Calibration data of sensors derived from superabsorbent polymer (SP) and deionized water (DIW) mixtures can represent their water potential value. The method showed good matching of capacitance and water potential values above −7 MPa, matching the result obtained in tomato stem. The measurements were conducted for a few days with the sap flow and water potential sensors connected in series on a tomato stem. When sunlight is sufficient, sap flow increases; meanwhile, water potential decreases. The opposite phenomenon could be observed during the nighttime. With irrigation restricting conditions, both sap flow and water potential signal decrease, triggering the emergency watering signals. This continuous water potential sensing system can quantitatively monitor the plant stem’s water stress and set irrigation schedules to achieve high-quality products in the agricultural field.

Separation of C1-C15 Alkanes with a Disk-Shaped Aluminum Column Employing Mesoporous AAO as the Stationary Phase.

A new type of gas chromatographic (GC) column employing a mesoporous anodic aluminum oxide (AAO) layer as the stationary phase was developed. The gas fluidic channels were fabricated on both sides of an aluminum disk via a mechanical stamping process. The tops of the gas fluidic channels were sealed with a thick aluminum foil and a thin glass liner. The cross section of this fluidic channel is triangular in shape and consists of two aluminum surfaces and one glass surface. The diameter of the aluminum disk is 8.7 cm, and the length of the GC column is 6.0 m. The AAO layer was grown on the aluminum surface and had an average pore diameter of 50 nm and a specific surface area of 4.13 m2 g-1. The thickness of the AAO stationary phase ranged from 6-150 μm. Although thin AAO is insufficient for separating light alkanes, methane and ethane can be separated with a resolution of 4.25 using a 150 μm thick AAO stationary phase at room temperature in less than 100 s. C1 to C15 alkanes can be completely separated within 20 min when using a temperature program ramped from room temperature to 350 °C. Some limitations of this preliminary design, such as peak broadening probably arising from the triangular cross section, not yet being suitable for polar compounds, and the lack of a stationary phase on one-third of the column surface are discussed.

A newly designed Ni[sbnd]P duplex coating on friction stir welding joint of 6061-T6 aluminum

Friction stir welding (FSW) joints often require coatings with good adhesion and corrosion resistance to improve durability. In this research, a newly designed duplex NiP coating was fabricated on the FSW joint of 6061-T6 aluminum alloy. An anodic aluminum oxide (AAO) layer was formed on the joint substrate as the intermediate layer to enhance adhesion, and a NiP layer was then electroplated on top to strengthen the corrosion and wear properties. The bonding strength, anti-corrosion property and wear resistance of the duplex NiP coating were investigated systematically and compared with those of the traditional zincated NiP coating. The duplex NiP coating results in superior adhesion and significantly improved anti-corrosion property than traditional NiP coating. The friction property of the FSW joint with duplex NiP coating is 88 % higher than that of the original FSW joint and 75 % higher than that of the FSW joint coated with traditional NiP coating. The related mechanisms were discussed in detail.

Metal–insulator–metal micro-capacitors for integrated energy storage up to 105 Hz

Metal–insulator–metal (MIM) micro-capacitors for use in integrated energy storage applications are presented. A new, simple and batch Si processing compatible method for the creation of high aspect ratio metallic 3D structures on the surface of a Si substrate is described. The method consists of creating an array of Si nanopillars and then depositing Al at a small angle off the vertical while rotating the sample. Using this method, the effective area of the samples is increased by a factor of 3.8. Various capacitors are created using the described 3D structures as the lower electrode, with anodic alumina and atomic layer deposited HfO2 as the dielectric. Al and Cu top electrodes are also investigated. Large values of capacitance densities as high as 3.2 μF cm−2 are achieved. All capacitors are demonstrated to possess small values of series resistances and stable operation up to a frequency of 105 Hz. These results make the presented MIM capacitors exceed the state-of-the-art while maintaining a simple and integrable fabrication scheme which renders them very interesting for energy storage applications where operational frequencies larger than 1 kHz are required, as is the case in several vibrational energy harvesters.

Nanoporous anodic alumina layers obtained from novel deep eutectic solvent formulations

ABSTRACT The possible fabrication of porous anodic oxide films on aluminium in ionic liquids based on choline dihydrogen citrate eutectic mixtures both with oxalic acid and isopropyl alcohol and ethylene glycol, has been investigated. The anodisation has been carried out in either potentiostatic or galvanostatic regime, at temperatures of 45–80°C, for different process durations. Quite compact, uniform anodic alumina layers have been obtained. Based on AFM and SEM investigations, pore diameters between 50 and 80 nm and interpore distances in the range of 160–200 nm have been estimated, with values influenced by the electrolyte type and anodisation conditions. The highest anodisation rate of about 0.4 µm min−1 has been determined by applying operation temperatures of 60°C. The recorded EIS spectra showed a pure capacitive behaviour and high anodic oxide resistances of 106–107 Ω cm2 order.

Innovative Codeposition of a Ag-Al2O3 Layer: An Attractive Combination of High Durability and Lack of Cytotoxicity for Public Space Applications.

Today, the use of silver in surfaces for public environments is very frequent, as it ensures high antimicrobial activities, avoiding the continuous disinfection of the surfaces themselves. Similarly, thanks to its interesting combination of technological properties, anodized aluminum is widely employed in the production of components for applications in public spaces. Therefore, this work describes a simple method of the codeposition of silver and anodized aluminum to combine the remarkable properties of Al2O3 layers with the antibacterial performances of silver. The effect of silver in modifying the durability features of the anodized aluminum layer was evaluated by means of various accelerated degradation techniques, such as the exposure in a climatic chamber to UV-B radiation or an aggressive atmosphere simulated by the Kesternich test. These analyses showed the good compatibility between Ag and the alumina matrix, whose durability performances were not particularly influenced by silver. Furthermore, the composite layers did not express relevant cytotoxicity activity, as evidenced by Trypan blue flow cytometry analysis and microscopy observations, ensuring the possible use of this material in applications in close contact with humans. This same conclusion was reached by observing an almost negligible ionic release of Ag by the composite layers, even following severe degradation of the alumina matrix due to exposure to a particular acid solution. In conclusion, this work presents an innovative material that can be used in public spaces, thanks to its interesting combination of high durability and low cytotoxicity.

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.

Functionalized aluminum oxide by immobilization of totally organic aromatic polymer spherical nanoparticles

A novel surface protection method of anodic aluminum oxide (AAO) was developed by using a one-pot, one-step reaction with two environmentally friendly pure organic materials. Totally organic aromatic polymer spherical nanoparticles (SNPs) were immobilized on an AAO surface with one-step addition condensation and polymerization using monomers of 1,5-dihydroxynaphthalene and hexamethylenetetramine. Immobilizing the nanoparticles—with a maximum size of approximately 50 nm—on the scale-like structure provided high water-repellency with a maximum contact angle of 118.4°. The water-repellency arose from the Lotus effect. In addition, immobilization of SNPs induced anticorrosivity against acid and an increase of the withstanding voltage by 18.1%. This immobilization technique of SNPs on the AAO could be used as an effective chemical protection method for an AAO substrate. It was demonstrated that SNPs were chemically bonded to AAO. A cross-section of the AAO with the immobilized SNPs was also investigated. The cross-section was a hierarchical structure comprising four kinds of layers (nanoparticle layer, scale-like and reticular layer, sealed layer of AAO, and AAO layer). Immobilization of SNPs on the surface of AAO provided an excellent anticorrosion effect and a higher electric strength compared with that of unmodified AAO.