Formation Of Anodic Aluminum Oxide Research Articles

Improvement of corrosion properties of plasma in an aluminum alloy 6061-T6 by phytic acid anodization temperature

We report on the formation of anodic aluminum oxide (AAO) film using phytic acid (C6H18O24P6), a naturally obtainable and non-toxic organic acid electrolyte. When the temperature of the phytic acid electrolytes changes from 0 °C to 20 °C, the pore size increases, and the AAO film becomes less dense because the growth rate of the AAO film increases. In particular, when the temperature of the phytic acid electrolyte is above 15 °C, the AAO film changes dramatically. According to XRD analysis, the alpha-alumina intensity was relatively higher in the AAO film grown at low-temperatures (0, 5, 10 °C) than high-temperatures (15, 20 °C). It was possible to obtain a result that the properties were better. The microstructural change of the AAO film according to the temperature of the phytic acid electrolytes affected the breakdown voltage and the number of contamination particles that are generated, which are important properties for plasma corrosion protection coating materials. The AAO film grown in the low-temperature has the most suitable properties for use as a plasma corrosion protection coating material. Notably, the breakdown voltage of the AAO film is 0.45 kV at 14.9 μm thickness, with no observable crack on its surface after the plasma corrosion test. These findings provide significant evidence to support the application of phytic acid as an electrolyte to grow AAOfilms, and the AAOfilm grown in a phytic acid bath can be applied as a plasma corrosion protection coating material.

Improving structural features of nanoporous alumina using deuterated electrolytes

Nanoporous anodic aluminum oxide (AAO) is a well-known nanoporous material with multiple applications. In the last years new experimental strategies have been developed to produce AAO in order to tune the pore morphology or ordering and discuss its growth mechanism. Here we describe an original approach to synthesize AAOs using deuterated electrolytes (i.e. sulfuric or oxalic acids in deuterated water D 2 O) leading to a pore diameter reduction of around 20 % and an increase in the pore ordering and pore growth. Better pore ordering in D 2 O is correlated to an increase in anion incorporation during the synthesis and the growth rate improvement is related to a decrease in the activation energy of the alumina formation. Moreover, the use of deuterated species allows to discuss the incorporation of hydroxyl groups during the AAO synthesis or the hydration after immersion in H 2 O or D 2 O by Small-Angle Neutron Scattering (SANS), a technique sensitive to deuteration. SANS reveals no changes between AAOs synthesized in H 2 O or D 2 O due to OH incorporation but shows differences after a long immersion time in water (H 2 O or D 2 O), indicating that the hydration is a slow process. This work shows that the use of deuterium is an interesting alternative for the synthesis of AAOs with well-controlled and specific morphologies and, from a fundamental point of view, can bring new general understanding about AAOs formation. • Nanoporous alumina have been synthesized using deuterated electrolytes. • The pore D p reached with deuterated electrolytes is as small as the smallest pore size obtained so far in the literature. • A better hexagonal pore ordering is observed and correlated to an increase in anion incorporation. • The pore growth rate is improved and related to a decrease in the activation energy of the alumina formation. • The use of deuterated electrolytes allows discussing the presence of OH groups within the material using SANS experiments.

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.

Effects of anodization parameters on the corrosion resistance of 6061 Al alloy using the Taguchi method

Anodizing 6061 aluminum alloy (AA6061) in a H2SO4 electrolyte solution is a crucial process in the formation of anodic aluminum oxide (AAO) film for enhancing the material’s corrosion resistance. In this work, the effects of electrolyte concentration, current density, and anodization time parameters on the evolution of corrosion resistance in AA6061 alloy were investigated using the Taguchi method. Each anodization parameter had three levels using an experimental set of L9 orthogonal arrays. Variation in the bias voltage was recorded during the AAO process, and an energy-dispersive spectrometer was used to analyze the variations in the composition of the oxidized films. Variations in surface morphology and cross-sections of the AAO were examined using a scanning electron microscope. Finally, the potentiostatic polarization method was used to judge the relative corrosion resistance of the oxidized films. The oxidized film with the best corrosion resistance (Icorr = 8.516 × 10−11 A/cm2) was obtained by anodizing at a current density of 1 A/dm2 in 1 M sulfuric acid for 20 min.

Formation of Anodic Aluminum Oxide with Branched and Meshed Pores

Anodic aluminum oxide (AAO), with a self-ordered hexagonal array, is important for various applications in nanofabrication including as the fabrication of nanotemplates and other nanostructures. With the consideration, there have been many efforts to control the characteristic parameters of porous anodic alumina by adjustment of the anodizing conditions such as the electrolyte, temperature, applied potential, and Al purity. In particular, impurities in Al are changing the morphology of an alumina film; however, the formation mechanism has not yet been explained. In this work, we anodized a high purity (99.999%, Al(high)) and low purity (99.8%, Al(low)) aluminum foil by a two-step anodization process in an oxalic acid solution or phosphoric acid. It was found that the purity of aluminum foil has influenced the morphology of the alumina film resulting in branched and meshed pores. Also, electrochemical analysis indicated that the branched and meshed pores in the low-purity Al foil formed by the presence of impurities. Impurities act as defects and change the general growth mechanism for pore formation by inducing an electric field imbalance during anodization. This work contributes to the research field of topographical chemistry and applied fields including nanofabrication.

Effect of conductive and non-conductive substrates on the formation of anodic aluminum oxide (AAO) template for mask-less nanofabrication

Anodic aluminum oxide (AAO) templates are generally the most cost-effective mask less nanofabrication processes for synthesis of nanowires, nano-meshes and nanoperiodic structures for several applications. In this present work, the effect of conductive and non-conductive substrates on the nanopores formation by anodic aluminum oxide (AAO) template method was investigated by using the same electrolyte, anodization voltages and time. The experimental results revealed that under same anodization conditions, the pore size, uniformity and aspect-ratio of the nanopores can be controlled better over the non-conductive substrates as compared to the conductive substrates. Furthermore, it was confirmed that under different electrolytic bath conditions (potentiostatic and galvanostatic),the potentiostatic condition causes formation of higher density of nanopores and leads to better uniformity resulting in higher aspect ratio AAO templates in comparison to the galvanostatic condition. Nanopore widening studies were also carried out and results are presented. The results of the present study can open a pathway to develop nanophotonics, optical-MEMS and sensor applications.

Nanoporous anodic aluminum oxide with a long-range order and tunable cell sizes by phosphoric acid anodization on pre-patterned substrates

Nanoporous anodic aluminum oxide (AAO) has been explored for various applications due to its regular cell arrangement and relatively easy fabrication processes. However, conventional two-step anodization based on self-organization only allows the fabrication of a few discrete cell sizes and formation of small domains of hexagonally packed pores. Recent efforts to pre-pattern aluminum followed with anodization significantly improve the regularity and available pore geometries in AAO, while systematic study of the anodization condition, especially the impact of acid composition on pore formation guided by nanoindentation is still lacking. In this work, we pre-patterned aluminium thin films using ordered monolayers of silica beads and formed porous AAO in a single-step anodization in phosphoric acid. Controllable cell sizes ranging from 280nm to 760nm were obtained, matching the diameters of the silica nanobead molds used. This range of cell size is significantly greater than what has been reported for AAO formed in phosphoric acid in the literature. In addition, the relationships between the acid concentration, cell size, pore size, anodization voltage and film growth rate were studied quantitatively. The results are consistent with the theory of oxide formation through an electrochemical reaction. Not only does this study provide useful operational conditions of nanoindentation induced anodization in phosphoric acid, it also generates significant information for fundamental understanding of AAO formation

On characteristics of pore size distribution in hybrid pulse anodized high-aspect-ratio aluminum oxide with Taguchi method

Many anodic aluminum oxide (AAO) templates were traditionally performed by potentiostatic anodization at 0---10 °C to inhibit the Joule's heat enhanced dissolution in aluminum oxide for ordered AAO configuration. In this article, the hybrid pulse anodization has been performed for offering the effective suppression of Joule's heat generation in high-aspect-ratio AAO formation at room temperature. The effect of pulse period, duty ratio, potential and electrolytic concentration on the evolution of pore characteristics was investigated using Taguchi method. The AAO morphology was captured by scanning electron microscope, and analyzed by gray-scale imaging in order to identify the pore size distribution and AAO thickness for Taguchi analysis. Short pulse-off period and low current density improved the uniformity of pore distribution. Moreover, high current density and intermediate pulse period/duty ratio can enhance the reaction rate and resulted in thick AAO. The evolution of AAO configuration and thickness were further correlated and discussed with pulse modulations and electrolytic concentrations.

Formation of Anodic Aluminum Oxide with Serrated Nanochannels

We report a simple and robust method to self-assemble porous anodic aluminum oxide membranes with serrated nanochannels by anodizing in phosphoric acid solution. Due to high field conduction and anionic incorporation, an increase of anodizing voltage leads to an increase of the impurity levels and also the field strength across barrier layer. On the basis of both experiment and simulation results, the initiation and formation of serrated channels are attributed to the evolution of oxygen gas bubbles followed by plastic deformation in the oxide film. Alternating anodization in oxalic and phosphoric acids is applied to construct multilayered membranes with smooth and serrated channels, demonstrating a unique way to design and construct a three-dimensional hierarchical system with controllable morphology and composition.

Effect of oxalic acid concentration on the formation of anodic aluminum oxide using pulse anodization at room temperature

Most anodic aluminum oxide (AAO) or porous anodic aluminum (PAA) films were fabricated using the potentiostatic method from high-purity (99.999%) aluminum films at low temperatures of 0–10°C to avoid dissolution effects at room temperature (RT). In this article, the fabrication of AAO or PAA film from commercial purity (99%) aluminum at RT has been investigated using hybrid pulse anodization under different oxalic acid concentration and process duration. The effect of acid concentration and anodization duration on the formation of AAO size and morphology has been studied. Both pore diameter and oxide layer thickness of AAO films were examined via SEM images and found that both increased with increasing oxalic concentration and anodization duration. In addition, the pore distribution was more concentrated at high electrolytic concentration and anodization duration.

Dissociation of Water During Formation of Anodic Aluminum Oxide

According to model computations at the B3LYP/6-311+G** level, an external electric field can facilitate the heterolytic dissociation of properly oriented water molecules significantly. Depending on the models used, the maximum predicted change of the dissociation energy in the field is ca. -3 to -4 kcal nm mol(-1) V(-1), and decreases with the cosine of the angle between the external field and the breaking OH bond. These microscopic results can be related semiquantitatively to macroscopic observables from mechanistic studies on the pore formation of anodic aluminum oxide, thus lending support to the equifield strength model and field-enhanced water dissociation at the growing oxide surface that has been put forward in these studies.

Fabrication of ordered Ta2O5 nanodots using an anodic aluminum oxide template on Si substrate

Ordered arrays of Ta2O5 nanodots were prepared using anodic aluminum oxide (AAO) as a template to support localized oxidation of TaN. Films of TaN (50 nm) and Al (1.5 μm) were deposited successively on p-type Si wafers and followed by a two-step anodization process at 40 V using oxalic acid as the electrolyte. The first anodization promoted growth of irregular AAO from overlying Al film. After chemical etching, the second anodization was performed to develop well-organized AAO channels and initiate oxidation of underlying TaN film to form tantalum oxide nanodots at the AAO pore bottoms. X-ray photoelectron spectroscopy results confirmed the chemical nature of nanodots as stoichmetric Ta2O5. X-ray diffraction demonstrated the amorphous characteristic of Ta2O5. As shown in field-emission scanning electron microscopy and transmission electron results, the Ta2O5 nanodots exhibited a hillock structure 80 nm in diameter at the bottom and 50 nm in height. We also synthesized 30-nm nanodots by adjusting AAO formation electrochemistry. This demonstrates the general applicability of the AAO template method for nanodot synthesis from nitride to oxide at a desirable size.

Fabrication of Anodic Aluminum Oxide Film on Large-Area Glass Substrate

Fabrication of anodic aluminum oxide (AAO) on 4 in. glass substrate was demonstrated. Using unique structure with quadruple contacts as electrode support during anodization, we reported gradual transformation from opaque aluminum to transparent AAO over the entire substrate. In contrast, partial transformation was observed with typical single contact approach where growth of insulating AAO inhibited subsequent anodization by interrupting current flow from the contact point. Results from optical spectroscope revealed moderate reduction in transmittance after AAO formation. Successful development of large-area AAO on glass substrate opens up opportunities for AAO templated devices that require a large footprint.

Electrical-Bridge Model on the Self-Organized Growth of Nanopores in Anodized Aluminum Oxide

Anodic aluminum oxide (AAO) has attracted a lot of attention for its application on the template-synthesis of nanomaterials. Highly ordered AAO template is essential to its applications. Here, we propose an electrical-bridge model based on the analysis of carrier ion drifts in the oxide layer and the electrical-field distribution. This model can elucidate the AAO growth mechanism, and allow better understanding of the nature of the nanopore self-organization during the anodic formation of AAO. The electrical-field distribution was greatly affected by the thickness of the oxide layer that is determined by the radii of cell and nanopore. The variable bridge resistances are related to anodizing conditions, such as temperature, which influence the ordering. The nanopores acquire uniformity by self-adjustment via the electrical bridge. Based on this model, the formation of hexagonal morphology of AAO, which is more stable than other arrangements, becomes a natural result of self-organization and the effect of anodizing conditions on the ordering is elucidated.

Fabrication of Anodic Aluminum Oxide on Si and Sapphire Substrate

We carried out anodic aluminum oxide (AAO) on a Si and a sapphire substrate. For anodic oxidation of Al two types of specimens prepared were Al(0.5 <TEX>$\mu\textrm{m}$</TEX>)!Si and Al(0.5 <TEX>$\mu\textrm{m}$</TEX>)/Ti(0.1 <TEX>$\mu\textrm{m}$</TEX>)<TEX>$SiO_2$</TEX>(0.1 <TEX>$\mu\textrm{m}$</TEX>)/GaN(2 <TEX>$\mu\textrm{m}$</TEX>)/Sapphire. Surface morphology of Al film was analyzed depending on the deposition methods such as sputtering, thermal evaporation, and electron beam evaporation. Without conventional electron lithography, we obtained ordered nano-pattern of porous alumina by in- situ process. Electropolishing of Al layer was carried out to improve the surface morphology and evaluated. Two step anodizing was adopted for ordered regular array of AAO formation. The applied electric voltage was 40 V and oxalic acid was used as an electrolyte. The reference electrode was graphite. Through the optimization of process parameters such as electrolyte concentration, temperature, and process time, a regular array of AAO was formed on Si and sapphire substrate. In case of Si substrate the diameter of pore and distance between pores was 50 and 100 nm, respectively. In case of sapphire substrate, the diameter of pore and distance between pores was 40 and 80 nm, respectively