Fabrication Of Anodic Aluminum Oxide Research Articles

Influence of Normal-to-High Anodizing Voltage on AAO Surface Hardness from 1050 Aluminum Alloy in Oxalic Acid.

Anodic aluminum oxide (AAO) has been widely applied for the surface protection of electronic component packaging through a pore-sealing process, with the enhanced hardness value reaching around 400 Vickers hardness (HV). However, the traditional AAO fabrication at 0~10 °C for surface protection takes at least 3-6 h for the reaction or other complicated methods used for the pore-sealing process, including boiling-water sealing, oil sealing, or salt-compound sealing. With the increasing development of nanostructured AAO, there is a growing interest in improving hardness without pore sealing, in order to leverage the characteristics of porous AAO and surface protection properties simultaneously. Here, we investigate the effect of voltage on hardness under the same AAO thickness conditions in oxalic acid at room temperature from a normal level of 40 V to a high level of 100 V and found a positive correlation between surface hardness and voltage. The surface hardness values of AAO formed at 100 V reach about 423 HV without pore sealing in 30 min. By employing a hybrid pulse anodization (HPA) method, we are able to prevent the high-voltage burning effect and complete the anodization process at room temperature. The mechanism behind this can be explained by the porosity and photoluminescence (PL) intensity of AAO. For the same thickness of AAO from 40~100 V, increasing the anodizing voltage decreases both the porosity and PL intensity, indicating a reduction in pores, as well as anion and oxygen vacancy defects, due to rapid AAO growth. This reduction in defects in the AAO film leads to an increase in hardness, allowing us to significantly enhance AAO hardness without a pore-sealing process. This offers an effective hardness enhancement in AAO under economically feasible conditions for the application of hard coatings and protective films.

Influence of Anodic Aluminum Oxide Nanostructures on Resistive Humidity Sensing

Humidity nanosensors play a vital role in modern technology industries, including weather forecasts, industrial manufacturing, agriculture, food and chemistry storage. In recent years, research on humidity sensors has focused on different materials such as ceramics, polymers, carbon-based materials, semiconductors, MXenes or triboelectric nanogenerators, each with their own advantages and disadvantages. Among them, anodic aluminum oxide (AAO) is a well-known ceramic humidity sensor material with a long history of research and development. AAO humidity sensors offer advantages such as simple manufacturing processes, controllable nanostructures, high thermal stability and biocompatibility. However, traditional AAO fabrication still has disadvantages like high costs and longer process times. Hence, finding a low-cost and efficient method to fabricate AAO for controlling different nanostructures to meet the requirements is consistently a major research topic. From our previous studies, we have studied the relationship between the AAO capacitive humidity sensor and its nanostructures. In this paper, we explore the effect of an AAO nanoporous structure controlled by an anodization voltage of 20–40 V on the resistive-type humidity sensor performance instead of a capacitive one. We efficiently apply one-step hybrid pulse anodization at 25 °C to significantly reduce the processing time compared to the traditional two-step process under 0–10 °C. The AAO nanostructures and their impact on sensor measurements of humidity at 20–80 RH% will be discussed in detail. An electrical resistive sensing mechanism is established for further performance improvement by controlling anodization voltage.

Advances in the Fabrication of Nanoporous Anodic Aluminum Oxide and Its Applications to Sensors: A Review.

Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement. In this review, we focus on various fabrication methods, pore geometry modification, and recent advances of AAO, as well as sensor applications linked to our environment, daily life, and safety. Pore geometry is concerned with the material composition, applied voltage mold, electrolyte type, temperature, and anodizing time during the fabrication of AAOs and for adjusting their pore size and profile. The applied voltage can be divided into four types: direct current anodization (DCA), reverse pulse anodization, pulse anodization (PA), and hybrid pulse anodization (HPA). Conventional AAOs are fabricated using DCA and mild anodization (MA) at a relatively low temperature (-5~15 °C) to reduce the Joule heating effect. Moreover, the issues of costly high-purity aluminum and a long processing time can be improved using HPA to diminish the Joule heating effect at relatively high temperatures of 20-30 °C with cheap low-purity (≤99%) aluminum. The AAO-based sensors discussed here are primarily divided into electrical sensors and optical sensors; the performance of both sensors is affected by the sensing material and pore geometry. The electrical sensor is usually used for humidity or gas measurement applications and has a thin metal film on the surface as an electrode. On the contrary, the AAO optical sensor is a well-known sensor for detecting various substances with four kinds of mechanisms: interference, photoluminescence, surface plasma resonance, and surface-enhanced Raman scattering (SERS). Especially for SERS mechanisms, AAO can be used either as a solid support for coating metal nanoparticles or a template for depositing the metal content through the nanopores to form the nanodots or nanowires for detecting substances. High-performance sensors will play a crucial role in our living environments and promote our quality of life in the future.

Dual effects of ultrasound on fabrication of anodic aluminum oxide

Ultrasound has been proven to enhance the mass transfer process and impact the fabrication of anodic aluminum oxide (AAO). However, the different effects of ultrasound propagating in different media make the specific target and process of ultrasound in AAO remain unclear, and the effects of ultrasound on AAO reported in previous studies are contradictory. These uncertainties have greatly limited the application of ultrasonic-assisted anodization (UAA) in practice. In this study, the bubble desorption and mass transfer enhancement effects were decoupled based on an anodizing system with focused ultrasound, such that the dual effects of ultrasound on different targets were distinguished. The results showed that ultrasound has the dual effects on AAO fabrication. Specifically, ultrasound focused on the anode has a nanopore-expansion effect on AAO, leading to a 12.24 % improvement in fabrication efficiency. This was attributed to the promotion of interfacial ion migration through ultrasonic-induced high-frequency vibrational bubble desorption. However, AAO nanopores were observed to shrink when ultrasound was focused on the electrolyte, accompanied by a 25.85 % reduction in fabrication efficiency. The effects of ultrasound on mass transfer through jet cavitation appeared to be the reason for this phenomenon. This study resolved the paradoxical phenomena of UAA in previous studies and is expected to guide AAO application in electrochemistry and surface treatments.

Fabrication of anodic aluminum oxide on silica optical fiber with nanopore channels oriented off surface normal

AbstractAluminum coating on silica optical fiber was anodized by applying an electric field off‐axis to surface normal of the fiber coating with simulation of the electric field lines between angularly positioned anode and cathode as a guide. The nanostructure features, including nanopore orientation, of the resultant anodized aluminum oxide (AAO) were characterized using scanning electron microscopy. This unique off‐axis electric field arrangement enabled the formation of AAO with uniformly tilted nanopore channel arrays. Nanopore channels with tilting angles around 10°–20° relative to surface normal of AAO were obtained. These results are in general agreement with the simulated electric field lines between the anode and cathode.

Advanced Nanoporous Anodic Alumina-Based Optical Sensors for Biomedical Applications

Close-packed hexagonal array nanopores are widely used both in research and industry. A self-ordered nanoporous structure makes anodic aluminum oxide (AAO) one of the most popular nanomaterials. This paper describes the main formation mechanisms for AAO, the AAO fabrication process, and optical sensor applications. The paper is focused on four types of AAO-based optical biosensor technology: surface-Enhanced Raman Scattering (SERS), surface Plasmon Resonance (SPR), reflectometric Interference Spectroscopy (RIfS), and photoluminescence Spectroscopy (PL). AAO-based optical biosensors feature very good selectivity, specificity, and reusability.

A Hierarchical Anodic Aluminum Oxide Template.

Anodic aluminum oxide (AAO) templates are widely used for the development of various functional nanomaterials due to their highly ordered and tunable porous structures. Here, we report a new hierarchical AAO (hAAO) template with the hexagonally ordered unit cells and the radially distributed nanochannels. It is formed by integrating the self-assembled polystyrene microsphere template into the AAO fabrication process and rationalized in terms of mechanical stress and electric-field-induced oxide dissolution. The back side of the hAAO template resembles a moth-eye-like nanoarray, which shows good hydrophobicity. A variety of radial nanopillar arrays and moth-eye-like nanoarrays are fabricated by a series of materials and synthesis techniques employing the hAAO template. These unique nanoarrays demonstrate many physicochemical properties that are distinct from those obtained from the conventional AAO template.

Synthesis of Co-Fe Alloy 1D Nanocone Array Electrodes with Aluminum Oxide Template for Water Spliting Reaction.

Nanostructured materials have an enormous application potential because of their novel physical and chemical properties which appear upon reduction of the length scales toward a few nanometer. Among the nano-structured materials, nano-cone arrays have received particular attention in the last years. Due to their specific structure and properties, nano-cones are attractive for very different applications. Producing and designing proper nano-crystalline structures could further improve the catalytic properties and replace expensive precious metals (platinum). Hence, the fabrication of Co alloy nano-cones opens the window to promising applications in the field of electrocatalysis Porous anodic aluminum oxide (AAO) is a popular template for synthesis of ordered arrays of nanostructured materials. By the controlling of anodization conditions or applying an appropriate post-anodization treatment the internal pore structure can be changed. Conical pores can be obtained basing on alternating repetition of the anodization and pore widening process during fabrication of AAO. To obtain free standing metallic nanocones the Al2O3 template was chemically etched in dilute phosphoric acid solution. In this case the process of cobalt electrodeposition was carried out in a two-electrode cell. The working electrode was AAO sample, the anode was a platinum plate. Influence of temperature and H3PO4 and H2SO4 solutions in the first step of anodization was examined. An attempt was made to synthesize Co based alloy nanocones. Acknowledgement This work was financially supported by Polish National Science Center (NCN) under grant UMO-2016/23/G/ST5/04058. The authors would like to acknowledge the COST MP1407 Action for networking.

Fabrication of AAO over aluminum mesh as an effective support for catalytic decomposition of ozone

In this work, anodic aluminum oxide (AAO) over Al mesh was fabricated as a support for monolithic catalyst for ozone decomposition. AAO with the pore sizes of about 9–38 nm could be obtained by increasing the voltage from 20 to 40 V. A mechanism was proposed to address the formation of AAO over Al mesh. Different with the case over Al plate, the key of the formation of porous AAO over Al mesh is to seal the edges of the Al mesh immersed in the electrolyte to keep the conductivity of the interlaced Al wires. Mn–Co nanospheres with the size of about 80 nm were decorated on AAO through a simple dip-coating process. The obtained material then was used as monolithic catalyst for ozone decomposition. It was found out that the developed material showed much better activity than Mn–Co composite decorated on Al mesh only, which is possibly due to the combination of the adsorption ability of AAO and the catalytic property of Mn–Co composite.

Effect of pre-treatment polishing on fabrication of anodic aluminum oxide using commercial aluminum alloy

Aluminum anodizing is a process in which metal is oxidized with an electrochemical method to make a metallic oxide. Because it can be used to manufacture an Anodic aluminum oxide (AAO) with dozens to hundreds of nanoscale pores that are vertically and uniformly arranged, it is widely applied in a variety of fields. In particular, most of the pre-treatment methods in the anodic oxidation processes using a high-purity aluminum adopt surface treatment through electrolyte polishing. In this study, complex polishing was performed using an abrasive film and Magnetorheological (MR) fluid for a commercial aluminum alloy to produce a uniform porous oxidized aluminum. The surface roughness and surface integrity were analyzed after each process to investigate the production behavior of AAO in relation to the pre-treatment of the surface. In addition, a study was conducted on the nano-pore production by the anodizing process in accordance with the pre-treatment polishing, in terms of the work-hardening and residual stress. Thus, in the anodic oxidation process, it was possible to generate uniform nano-pores when surface integrity including surface roughness and residual stress distribution of the aluminum alloys are excellent. Test results showed that the polishing process using the MR fluid did not generate process scratches and pits, but could effectively remove the work-hardening and compressive residual stress, proving that MR fluid polishing method is a suitable pre-treatment process to produce nano-pores in the anodizing process.

Innovative Nanoimprint-Assisted Fabrication of Anodic Aluminum Oxide

Anodic aluminum oxide (AAO) templates possess highly-ordered nanoporous structures, with advantages such as high aspect ratio. However, there are limits to control the growth of the traditional AAO arrays, such as interpore distance, pore sizes and densities. In this study, we introduce an innovative nanoimprint process guided AAO fabrication to overcome these limitations of the traditional AAO arrays.

High Aspect Ratio Single Crystal of Cesium Iodide Column

Cesium iodide (CsI), which is used in the phosphor layer of a scintillator plate, exhibits a relatively high conversion rate of X- or g-rays to visible light. Important requirements for the scintillator materials used in the applications include high light output, transparency to the light it produces, high stopping efficiency, fast response, good proportionality, low cost, and availability in large volume In order to simplify and reduce the cost of the scintillator deposition process, we deposit the scintillator of a CsI into a high aspect ratio anodic aluminum oxide (AAO) template to produce CsI columns of nano and submicron sizes. The scintillator material of CsI columns were formed in the AAO template by a non-vacuum method. When the CsI melt is injected a confined to an AAO channel with a high aspect ratio, it grows as a stable single-crystal column with smooth surfaces, free of dendrites and grain boundaries. In this research, we design a novel cooling-function of electrochemical mold for a high quality AAO fabrication and compare the CsI wire and column formation by non-vacuum injection method in a AAO template.

High Aspect Ratio Single Crystal of Cesium Iodide Column

Cesium iodide (CsI), which is used in the phosphor layer of a scintillator plate, exhibits a relatively high conversion rate of X- or g-rays to visible light. Important requirements for the scintillator materials used in the applications include high light output, transparency to the light it produces, high stopping efficiency, fast response, good proportionality, low cost, and availability in large volume In order to simplify and reduce the cost of the scintillator deposition process, we deposit the scintillator of a CsI into a high aspect ratio anodic aluminum oxide (AAO) template to produce CsI columns of nano and submicron sizes. The scintillator material of CsI columns were formed in the AAO template by a non-vacuum method. When the CsI melt is injected a confined to an AAO channel with a high aspect ratio, it grows as a stable single-crystal column with smooth surfaces, free of dendrites and grain boundaries. In this research, we design a novel cooling-function of electrochemical mold for a high quality AAO fabrication and compare the CsI wire and column formation by non-vacuum injection method in a AAO template.

Porous and mesh alumina formed by anodization of high purity aluminum films at low anodizing voltage

Electrochemical oxidation of high-purity aluminum (Al) films under low anodizing voltages (1–10) V has been conducted to obtain anodic aluminum oxide (AAO) with ultra-small pore size and inter-pore distance. Different structures of AAO have been obtained e.g. nanoporous and mesh structures. Highly regular pore arrays with small pore size and inter-pore distance have been formed in oxalic or sulfuric acids at different temperatures (22–50°C). It is found that the pore diameter, inter-pore distance and the barrier layer thickness are independent of the anodizing parameters, which is very different from the rules of general AAO fabrication. The brand formation mechanism has been revealed by the scanning electron microscope study. Regular nanopores are formed under 10V at the beginning of the anodization and then serve as a template layer dominating the formation of ultra-small nanopores. Anodization that is performed at voltages less than 5V leads to mesh structured alumina. In addition, we have introduced a simple one-pot synthesis method to develop thin walls of oxide containing lithium (Li) ions that could be used for battery application based on anodization of Al films in a supersaturated mixture of lithium phosphate and phosphoric acid as matrix for Li-composite electrolyte.

Innovative Nanoimprint-Assisted Fabrication of Anodic Aluminum Oxide

Anodic aluminum oxide (AAO) templates possess highly-ordered nanoporous structures, with advantages such as high aspect ratio. However, there are limits to control the growth of the traditional AAO arrays, such as interpore distance, pore sizes and densities. In this study, we introduce an innovative nanoimprint process guided AAO fabrication to overcome these limitations of the traditional AAO arrays.

Fast anodization fabrication of AAO and barrier perforation process on ITO glass

Thin films of porous anodic aluminum oxide (AAO) on tin-doped indium oxide (ITO) substrates were fabricated through evaporation of a 1,000- to 2,000-nm-thick Al, followed by anodization with different durations, electrolytes, and pore widening. A faster method to obtain AAO on ITO substrates has been developed, which with 2.5 vol.% phosphoric acid at a voltage of 195 V at 269 K. It was found that the height of AAO films increased initially and then decreased with the increase of the anodizing time. Especially, the barrier layers can be removed by extending the anodizing duration, which is very useful for obtaining perforation AAO and will broaden the application of AAO on ITO substrates.

Ultra-small nanopores obtained by self-organized anodization of aluminum in oxalic acid at low voltages

A simple method of fabrication of anodic aluminum oxide at low voltages (from 5 to 15V) in 0.3M oxalic acid is presented. To overcome the issues concerning the anode's burning, a high velocity stirring was applied. It allowed to achieve nanopores with ultra-small diameters (even 12nm), interpore distance (even 31nm) and tremendously high pore densities (up to 980 pores per 1µm2) with simultaneous high porosity (up to 38%). Moreover, the presented approach (no modifier added to the electrolyte) resulted in the AAO with relatively well arranged ultra-small nanopores.

Evaluation of CO<sub>2</sub> Absorbent/De-Absorbent by AAO/CaO Membrane

This paper discussed the fabrication of anodic aluminum oxide (AAO)/CaO core-shell membrane by anodization and sol-gel processes and evaluated the relationship between the Al2O3/CaO and CO2 by thermodynamic calculation. According to thermodynamic property that CaO can reacts with CO2 forming calcium carbonate (CaCO3) in a lower temperature ranges, CaCO3 can also be decomposed to CaO in a higher temperature ranges. Because the reversible reaction property that CaO can be a CO2 absorbent/ de-absorbent. We address that AAO based on core-shell nanotubes structure to achieve such a demand. The captured CO2 can further convert to fuel material of hydrocarbons.

Fabrication of anodic aluminum oxide with incorporated chromate ions

The anodization of aluminum in 0.3M chromic acid is studied. The influence of operating conditions (like anodizing voltage and electrolyte's temperature) on the nanoporous anodic aluminum oxide geometry (including pore diameter, interpore distance, the oxide layer thickness and pores density) is thoroughly investigated. The results revealed typical correlations of the anodic alumina nanopore geometry with operating conditions, such as linear increase of pore diameter and interpore distance with anodizing voltage. The anodic aluminum oxide is characterized by a low pores arrangement, as determined by Fast Fourier transforms analyses of the FE-SEM images, which translates into a high concentration of oxygen vacancies. Moreover, an optimal experimental condition where chromate ions are being successfully incorporated into the anodic alumina walls, have been determined: the higher oxide growth rate the more chromate ions are being trapped. The trapped chromate ions and a high concentration of oxygen vacancies make the anodic aluminum oxide a promising luminescent material.

Bending and branching of anodic aluminum oxide nanochannels and their applications

During fabrication of anodic aluminum oxide (AAO) membranes, it is observed that sometimes their nanochannels experience bending and branching. In this research the bending and branching behavior of AAO nanochannels was studied. AAO nanochannels were fabricated in 99.56% pure aluminum (Al) in which Si and Fe impurities blocked the propagation of some nanochannels. It was observed that the neighboring channels bend and make branches for anodizing the Al beneath impurities. The authors developed a model that explains the bending and branching of AAO nanochannels. Our experiments show that AAO nanochannel can bend and divide into branches if it experiences nonanodized Al on one side of the nanochannel. Bending and branching effects were also observed when AAO was fabricated using Al with nonplanar surfaces. The authors found that the bending and branching behavior of AAO nanochannels minimizes impurity defects in the AAO membrane. Based on experimental results, a model has been developed for making 3D structure using AAO nanochannels.