Anodic Aluminum Oxide Layer Research Articles

Self-organization of anodic aluminum oxide layers by a flow mechanism

Anodic oxidation of aluminum in solutions that dissolve the amorphous oxide produces films with hexagonally-ordered patterns of pores. We show using linear stability analysis that an anodization model including viscous oxide flow accurately predicts the scaling relation governing pore spacing. Flow is driven by nonuniform compressive stress near the oxide-solution interface, resulting from anion adsorption on surface growth sites. The interfacial stress layer is approximated as surface stress, leading to analogous instability mechanisms as in the surface-tension driven Marangoni instability of thin liquid films.

Fabrication of a plasma electrolytic oxidation/anodic aluminum oxide multi-layer film via one-step anodizing aluminum in ammonium carbonate

A plasma electrolytic oxidation (PEO)/anodic aluminum oxide (AAO) multi-layer film was fabricated via one-step galvanostatic anodizing of high-purity aluminum in 0.3–2.0 M ammonium carbonate ((NH4)2CO3) solutions at 283–333 K and 25–400 Am−2. Anodizing at higher concentrations and higher temperatures caused the formation of relatively uniform anodic oxide film on the aluminum substrate. Characteristic voltage-time curves with two high-plateau voltages at approximately 250 V and 375 V were obtained during galvanostatic anodizing. A multi-layer structure of an outer PEO layer with a crystalline γ-Al2O3 structure and an inner amorphous AAO layer with nano-cylinerical pores was formed by continuous visible sparking occurring after passing the first voltage plateau region. The whole aluminum surface was covered with the multi-layer structure after reaching the second voltage region. The thickness of the multi-layer increased with time via the further anodizing process at the second plateau voltage. Pore sealing of the inner nanoporous film was achieved by immersion post-treatment in boiling water.

The effects of pore widening and calcination on anodized aluminum oxide prepared from Al6082

In this study, semi-organized anodized aluminum oxide (AAO) is prepared by two-step anodization in oxalic acid using a low-purity aluminum (Al) alloy (Al6082) as the source material. Obtaining ideally ordered AAO layers from Al alloys is confounded by the presence of impurities and requires specialized post-anodization treatment. Here, a process is proposed for obtaining an AAO layer with a semi-ordered pore arrangement by means of calcination and pore-widening procedures. Calcination temperatures of 400–600 °C and pore–widening times of 15–120 min were explored. It was found that calcination improved the pore order of AAO layers from disordered to semi-ordered (defined as being near ideally ordered—one pore surrounded by six neighboring pores). In addition, it was found that calcination at 600 °C resulted in suitable pore widening of the AAO layer. The morphology of the AAO layers was characterized using scanning electron microscopy, which was applied to determine pore diameter, inter-pore distance, pore density, and AAO layer thickness. We found that calcinated and pore-widened AAO layers with pore diameters of 48.7 nm, inter-pore distances of 80.6 nm, and pore densities of 178 pores/μm2 could be prepared. The AAO preparation method proposed here is an economically attractive process suitable for the preparation of AAO layers due to the utilization of an inexpensive Al source.

Fabrication of piezoelectric nanogenerator using 3D-ZnO nanosheets and optimization of charge storage system

In this research work, an easy and novel approach is adopted for fabrication of piezoelectric nanogenerator (PENG) based on 3D-ZnO nanosheets (NSs). The 3D-ZnO NSs were fabricated on anodic aluminium oxide (AAO) membrane. The NSs were separated from Al bottom electrode by AAO layer and Cu thin sheet was used as top electrode. The 3D-ZnO NSs were investigated by FE-SEM, XRD, and Raman spectroscopy. A mechanical force of 3 kgf with 6 Hz was applied on PENG that generated an output voltage of 895 mV, and current of 446 nA with maximum power density 50 nW/cm2. The PENG was then utilized to charge load capacitors and found that the maximum energy could be stored at an optimum load capacitance (CL) of 10 nF in 600 s. The 3D ZnO NSs enhanced stability, and robustness of PENG. The fabricated PENG was continuously operated for 3 h without any degradation in output performance.

Polytechs focuses on polyolefin masterbatches

Robust organic thin-film transistors (OTFTs) with high temperature stability allow device integration with mass production methods like thermoforming and injection molding, and enable operation in extreme environment applications. Herein we elaborate a series of materials to make suitable gate dielectric and active semiconductor layers for high temperature stable OTFTs. We employ an anodized aluminum oxide layer passivated with cross-linked low-density polyethylene (LD-PE) to form a temperature-stable gate capacitor. As the semiconductor, we use quinacridone, an industrial organic colorant pigment produced on a mass scale. Evaporated MoOx/Ag source and drain electrodes complete the devices. Here we evaluate the performance of the OTFTs heating them in air from 100 °C in 25 °C increments up to 225 °C, holding each temperature for a period of 30 minutes. We find large differences in stability between quinacridone and its dimethylated derivative, with the former showing the best performance with only a factor of 2 decline in mobility after heating at 225 °C, and unaffected on/off ratio and threshold voltage. The approach presented here shows how industriallys calable fabrication of thermally robust OTFTs can be rationalized.

Preparation of anodized aluminium oxide at high temperatures using low purity aluminium (Al6082)

A rapid two-step anodization process was developed to prepare semi-ordered nanoporous anodized aluminium oxide (AAO) layers using an inexpensive aluminium (Al) alloy, Al6082 (97.53% Al) as the Al source material. Al anodizing was performed at various anodization voltages (30, 45, 60 V) and temperatures (20, 30, 40 °C) using 0.4 M oxalic acid as the electrolyte. The effects of temperature and voltage on the morphological characteristics of the obtained AAO formed by one- and two-step anodization were investigated. The obtained AAO surfaces were characterized by scanning electron microscopy. Morphological characteristics of importance here were the pore diameter, inter-pore distance, porosity and pore density. An AAO layer with a semi-ordered pore arrangement was prepared using a two-step anodization process, which included an AAO etching step before the second anodization. The obtained AAO had a pore diameter of 43.8 ± 6.0 nm, inter-pore distance of 82.6 ± 19 nm, 25% porosity, pore density of 169 pores/μm2 and layer thickness of 53 μm. Energy-dispersive X-ray spectroscopy results suggested that non-Al elements present in Al6082 were present on the surface of AAO. From an economic perspective, the AAO preparation process proposed in this study makes the fabrication of AAO more attractive.

Effect of Formation Voltage on the Pore Size of Porous Anodic Aluminum Oxide

This work is aimed to clarify the effect of formation voltage on the pore size of porous anodic aluminum oxide (PAAO) layer formed on commercially pure aluminum. The PAAO layers were obtained by anodization process in 0.3 M sulfuric acid solution at constant voltages of 10, 15, 20, and 25 V at 10°C. The structure and morphology of PAAO layers were characterized by using FE-SEM. The pore diameters which were estimated by using ImageJ software were 19.61 + 15.35 nm, 20.03 +13.59 nm, 20.31 + 12.36 nm, 25.06 +12.10 nm, and the wall thicknesses were 33 nm, 49.99 nm, 74.97 nm, 83.33 nm for the PAAO formed at 10, 15, 20 and 25 V, respectively. The structure of the porous oxide layer became more uniform and organized, and the diameter of the pores increased linearly with applied voltage. High anodization voltage is known to cause Joule heating because of the fast movement of electrons and ions. It is believed that the Joule heat was transferred to the bulk electrolyte which results in larger pore diameter and interpore distance. The optimum condition to obtain high order PAAO is at 25 V.

Durable omniphobicity of oil-impregnated anodic aluminum oxide nanostructured surfaces

Recently, various types of porous surfaces have been demonstrated for lubricant (e.g., oil) impregnated omniphobic surfaces. However, the retention of the lubricating liquid within the porous layer and the omniphobic durability still remain challenges. Here, the omniphobic durability of the oil-impregnated surfaces of various types of anodic aluminum oxide (AAO) nanostructures is investigated. The oil impregnation into nanoporous AAO with high porosity enhances droplet mobility by eliminating the pinning site of a contact line on the solid surface, whereas that with low porosity allows the pinning site to result in less mobility. In the cases of nanopillared AAO layers, although the oil-impregnation enhances the repellency to liquids, oil is prone to be depleted by external force such as fluid flow due to the nature of the interconnected oil through the passages between pillars, which limits the omniphobic durability. Among the various types of nanostructured AAO surfaces, the AAO with isolated pore geometry with high porosity exhibits the most durable omniphobicity for a wide range of liquids including organic liquids with low surface tensions. Moreover, the nanoporous AAO surface shows great anti-bacterial adhesion property, reducing the adhesion of bacteria (Escherichia coli K-12) up to 99.2% compared to a bare aluminum surface.

Morphological evolution and burning behavior of oxide coating fabricated on aluminum immersed in etidronic acid at high current density

Micro arcs on metal surface immersed in alkaline electrolyte have been studied by many works, but that in acidic electrolyte are rarely reported. In this work, anodizing of aluminum was carried out in etidronic acid (HEDP) at high current density to investigate the formation behavior of coating. SEM images indicate that the burning of initial film is actually caused by local oxide cracking due to the increase of compressive stress. The soft spark would appear when voltage reached 300 V, then change to the micro arcs at 400 V, and finally manufacture the outer micro-arc oxide (MAO) layer. Hereafter, the automatic extinguishing of micro arcs gives rise to the formation of inner anodic aluminum oxide (AAO) layer. Moreover, it is evidenced that plasma discharge has few contributions to coating thickening. The barrier layer provides the raw materials for the growth of outer MAO layer. Based on these results, the growth behavior of coating processed in HEDP at high current density is proposed and discussed.

Evaluation of surface pretreatment stages of Al 1050 to obtain nanostructured anodic films

Abstract The growth and ordering of the anodic aluminum oxide layer depend strongly on the initial roughness of the substrate and consequently on the different pretreatment stages. The objective of this study was to select the best combination of surface pretreatment stages of Al 1050 to synthesize self-ordered anodic films. The average roughness of the substrate after the pretreatment was used as a selection parameter. Also, the effect of the selected pretreatments on the porous structure obtained by anodic oxidation was evaluated by means of scanning electron microscopy images and Fast Fourier Transforms analysis. The results showed that the consecutive pretreatments Grinding + Polishing + Electropolishing + Acid etching at appropriate conditions presented a degree of synergism that significantly reduced the surface roughness of the Al 1050 substrate to 0.062 μm, allowing the development of a nanoporous film of self-ordered aluminum oxide.

A hard coating with MAO/AAO double layers prepared on aluminum in etidronic acid by DC oxidation

Anodic aluminum oxide (AAO) films and micro-arc oxide (MAO) coatings are generally prepared from different electrochemical regimes. Herein, we prepared a new hard coating with a MAO/AAO double layer structure in etidronic acid (HEDP) by one-step way for the first time. The influence of current densities on the coating structures, elements distribution, phases composition and hardness was systematically investigated by SEM, EDS, XRD and nanoindentation tests. Results showed that the AAO film would not be burnt down until the current density increased up to 8 A/dm2. A steady sparking was observed when the current density was higher than 4 A/dm2, and the visible sparks would automatically quench after a while. The obtained oxide coatings had both the outer volcanic resolidified pool structure and inner honeycomb structure, i.e., a MAO/AAO composite coating. EDS results suggested that the concentration of anion contaminants of MAO layer was lower than that of AAO layer. The estimated growth rate of oxide coating increased from 1 to 36 μm/h with the increase of current densities from 1 to 7 A/dm2. The hardness of inner AAO layer was about 6–8 GPa, which was in contrast with the highest hardness (26.57 GPa) of the outer MAO layer.

Intermediate-temperature sensors based on La0.5Ba0.5MnO3/nanoporous anodic aluminum oxide multilayered film thermistors

Perovskite La0.5Ba0.5MnO3 (LBMO) film thermistors for temperature monitoring over a wide range of intermediate temperatures (200–450 °C) were fabricated on thin Al-foil substrates with a nanoporous anodic aluminum oxide layer at the Al surface (AAO-Al).

Elucidation of the Anodization and Silver Incorporation Impact on the Surface Properties of AA1050 Aluminum Alloy

The recently increased interest toward the modification of anodized aluminum oxide (AAO) layers has motivated the present systematical research. The experiments were comprised of three groups of AA1050 alloy samples: preliminary treated, anodized and further AAO layer modification, using AC-assisted silver incorporation. The respective impact of the surface treatment was assessed using different analytical methods, in order to obtain systematical information on the variation of the surface properties, morphology and performance of the obtained Al-O-Ag films. The analyses included determination of the sample color, hydrophobicity and mechanical strength. These measurements were followed by characterization of their morphology by scanning electron microscopy and composition by energy dispersion X-ray analysis and X-ray photoelectron spectroscopy. These characterizations were completed by the assessment of the samples’ performance, after 168 hours of exposure to a 3.5% NaCl model corrosive medium. The corrosion evaluation tests included electrochemical impedance spectroscopy and potentiodynamic scanning. The comparative analysis of the acquired results has enabled us to do a comprehensive description of the mechanism of the AC-assisted Ag-incorporation and its impact on the modified AAO films. The obtained Al-O-Ag functional layers have shown remarkable reliability, being appropriate for various industrial applications.

High temperature-stability of organic thin-film transistors based on quinacridone pigments

Abstract Robust organic thin-film transistors (OTFTs) with high temperature stability allow device integration with mass production methods like thermoforming and injection molding, and enable operation in extreme environment applications. Herein we elaborate a series of materials to make suitable gate dielectric and active semiconductor layers for high temperature stable OTFTs. We employ an anodized aluminum oxide layer passivated with cross-linked low-density polyethylene (LD-PE) to form a temperature-stable gate capacitor. As the semiconductor, we use quinacridone, an industrial organic colorant pigment produced on a mass scale. Evaporated MoOx/Ag source and drain electrodes complete the devices. Here we evaluate the performance of the OTFTs heating them in air from 100 °C in 25 °C increments up to 225 °C, holding each temperature for a period of 30 minutes. We find large differences in stability between quinacridone and its dimethylated derivative, with the former showing the best performance with only a factor of 2 decline in mobility after heating at 225 °C, and unaffected on/off ratio and threshold voltage. The approach presented here shows how industriallys calable fabrication of thermally robust OTFTs can be rationalized.

Integrity of Alumina Catalytic Support Prepared by Anodization in a High Temperature Steam Environment

As a catalytic support for passive autocatalytic recombiner in nuclear power plants, anodic alumina layer was formed on stainless steel 304 by applying either Al or NiAl diffusion coatings on 304SS, followed by anodization and calcination. The anodic alumina had porous and nano-fibrous morphology and showed comparable hydrogen conversion ratio as the conventional wash-coated catalytic support. Integrity of catalytic supports was evaluated by steam oxidation test at 900 °C for 100 h. The wash-coated catalytic support exhibited oxide breakaway and Fe-oxide formation. On the other hand, α-alumina was formed on anodized catalytic support with Al or NiAl diffusion layers, which showed good adherence to the underlying metal substrate.

Characterization of porous anodic alumina with AC-incorporated silver

The present research is dedicated to the description of the effects of AC electrochemical incorporation of silver on the compositional, morphological and structural characteristics of the obtained modified anodic aluminum oxide (AAO) layers. The AC electrochemical Ag-incorporation was performed on preliminary galvanostatically anodized technically pure (AA1050) aluminum alloy for different durations (up to 900 s). The obtained Al-O-Ag combined layers were submitted to systematic characterizations in order to determine the incorporation efficiency and the respective process mechanism. The performed analyses comprised subsequent determination of: (i) chemical composition, (ii) surface topology and roughness, (iii) layer thickness and uniformity, and (iv) electrochemical behavior in 3.5% NaCl model corrosive medium. The Ag content and the respective incorporation efficiency were estimated by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The topological and morphological modifications of the films were determined by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), respectively. The morphological characterizations were completed by surface and cross-sectional Energy Dispersion X-ray (EDX) analyses thus determining the film thicknesses and elements distribution. All characterizations using these analytical methods were finalized by studying the performance of these films in a model corrosive medium by Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV).The further comprehensive data analysis of the obtained results has enabled to create a conceptual model for the description of the correlations between the Ag-incorporation duration and the resulting compositional, topological and structural features, and the resulting film properties and performance.The AC electrochemical Ag-incorporation has a relatively low efficiency and proceeds with gradual silver deposition in the bottoms of the pores and its further accumulation in an upward direction. The cause for the observed low efficiency is related to the simultaneous occurrence of AAO layer growth and dissolution during the AC polarization, which result in topological and roughness variations, as well.

Preparation of Anodic Aluminum Oxide Masks with Size-Controlled Pores for 2D Plasmonic Nanodot Arrays

Anodic aluminum oxide (AAO) layers with nano-sized pores have been used as shadow masks for the fabrication of two-dimensional (2D) metal nanodot arrays (NDAs). However, the localized surface plasmon resonance (LSPR) of size-controlled NDAs fabricated using AAO masks has not been much studied. In this study, we report on the improved preparation method and utilization of an ultrathin AAO mask for the fabrication of 2D plasmonic metal NDAs. The greatest challenge in preparing an AAO mask is to control the pore diameter and to make it reproducible. AAO masks with size-controlled pores were reproducibly prepared using a two-step chemical wet etching method. Ag NDAs with different dot diameters (42, 60, and 80 nm) and Ag, Cu, and Au NDAs with dot a diameter of 80 ± 5 nm were fabricated on indium tin oxide glass substrates using AAO masks. The wavelengths corresponding to LSPR of 2D metal NDAs were investigated using ultraviolet–visible spectroscopy. Our results show that AAO masks with tunable pores can be used as shadow masks for the fabrication of 2D plasmonic NDAs.

High-resolution, flexible, and transparent nanopore thin film sensor enabled by cascaded Fabry–Perot effect

This Letter reports a method to significantly improve the optical resolution of the anodic aluminum oxide (AAO) nanopore thin film sensor based on multi-cavity Fabry-Perot interference. The newly designed sensor is fabricated by bonding a layer of transparent polymer thin film (pTF), which is polydimethylsiloxane (PDMS), to a transparent AAO thin film to form a flexible pTF-nanopore sensor. In comparison with the AAO nanopore thin film sensor, the pTF-nanopore sensor shows a much-improved quality (Q) factor and optical resolution. Typical thicknesses of a PDMS layer and an AAO layer of the pTF-nanopore sensor are 80μm and 2μm, respectively. The pTF-nanopore sensor used for angle detection shows a sensitivity of 0.4nm/deg with a resolution of 0.2deg. The pTF-nanopore sensor can also be used for temperature monitoring with a sensitivity of 0.2nm/°C and a resolution of 1°C.

3D-Nanorod Fenton-like CuO/TiO2 Photocatalyst By Electrodeposition in AAO Template for Dye Degradation

Recently, nanostructures such as nanorods and nanowires have drawn much interest because of their unique electronic and optical properties. Anodic aluminum oxide (AAO) method is a promising template-assisted technique which ensures high quality and repeatability for fabrication. Morphology of periodical nanoscale pores in AAO templates has been studied in relation with anodizing process for a few decades. The AAO templates are well prepared by a two-step anodization process method in 0.3 M oxalic acid solution at 10 °C and anodizing voltage 40 V. After first-step anodization, AAO layer was etched out in 5 wt.% phosphoric acid and 2.24 wt.% chromic acid at 60 °C for 1 HR. After barrier thinning, the AAO template was immersed into a 5 wt.% phosphoric acid solution at room temperature for 45 minutes. The pore length increases with electrochemical etching time, while the pore size and interpore distance depend on the applied potential. In our work, the AAO template is 45 nm in diameter and the interpore distance and pore length are 100 nm and 1 μm, respectively. In case that the Al2O3 barrier layer is too thick for the electrodeposition, barrier thinning proceeds by limited current is a necessary process to lower the working voltage and to create the nucleation site for the growth of Cu. The Cu/Cu2O composite nanorods are synthesized from aqueous solution prepared from 0.7 M of CuSO4 dissolved in DI water, with applied pulse potential -1.5 V at room temperature. The nanorods were annealed at 400 °C and resulted in the formation of CuO. After that, TiO2 thin film was deposited on the top followed by annealing at 550 °C to form anatase. Etching out the AAO template, we can obtain CuO nanorod arrays standing on TiO2 thin film nanostructure for application in water treatment. Besides, it can also be transferred on other flexible substrate or on the surface of the item promoting self-cleaning property. CuO is a great Fenton-like photocatalyst which is active with H2O2 and produces free radicals reacting with the dye or pollutants. TiO2 is stable in water and possesses good photocatalytic property in anatase phase. CuO is a narrow bandgap p-type metal oxide semiconductor which provides electrons and holes and absorbs infrared light, while TiO2 is a n-type metal oxide semiconductor which absorbs UV light and has high transparency in sunlight. We combine two metal oxide to form heterojunction with AAO and electrodeposition method. When the infrared light is absorbed by CuO nanorods exciting electron and holes, the electrons shift to TiO2 and the holes stay in CuO. These electron-hole pairs can react with H2O2 producing free radicals and start dye degradation. Furthermore, CuO nanorods immobilized on TiO2 film nanostructure can also be applied as gas sensors or photodetectors and for water splitting.

Development and characterization of hydrophobic anodized aluminum layer to act as a long‐lasting protective film in corrosion

This work focuses on the development of a novel hydrophobic process for anodized aluminum AA2024‐T3 which consists of a combination of corrosion inhibitors: sodium metavanadate with a γ‐irradiated fluoropolymer. The anodizing films, formed in sulfuric acid media, were sealed by using different aqueous solutions such as boiling water, potassium dichromate, and cerium nitrate. The corrosion resistance results obtained by electrochemical impedance spectroscopy (EIS) showed that samples sealed with Ce(NO3)3 had a higher average value of total impedance of 1011 Ω cm2 obtained at low frequencies in 0.1 M NaCl solution. Water contact angle measurements revealed best results at 128.8°. X‐ray photoelectron microscopy in conjunction with scanning electron microscopy indicated the formation of a layer of fluoro compound above the anodized aluminum. X‐ray diffraction, infrared spectroscopy, and differential scanning calorimetry demonstrated thermal modifications of the γ‐irradiated fluoropolymer.