Mild Anodization Research Articles

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.

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.

Control of the Nanopore Architecture of Anodic Alumina via Stepwise Anodization with Voltage Modulation and Pore Widening.

Control of the morphology and hierarchy of the nanopore structures of anodic alumina is investigated by employing stepwise anodizing processes, alternating the two different anodizing modes, including mild anodization (MA) and hard anodization (HA), which are further mediated by a pore-widening (PW) step in between. For the experiment, the MA and HA are applied at the anodizing voltages of 40 and 100 V, respectively, in 0.3 M oxalic acid, at 1 °C, for fixed durations (30 min for MA and 0.5 min for HA), while the intermediate PW is applied in 0.1 M phosphoric acid at 30 °C for different durations. In particular, to examine the effects of the anodizing sequence and the PW time on the morphology and hierarchy of the nanopore structures formed, the stepwise anodization is conducted in two different ways: one with no PW step, such as MA→HA and HA→MA, and the other with the timed PW in between, such as MA→PW→MA, MA→PW→HA, HA→PW→HA, and HA→PW→MA. The results show that both the sequence of the voltage-modulated anodizing modes and the application of the intermediate PW step led to unique three-dimensional morphology and hierarchy of the nanopore structures of the anodic alumina beyond the conventional two-dimensional cylindrical pore geometry. It suggests that the stepwise anodizing process regulated by the sequence of the anodizing modes and the intermediate PW step can allow the design and fabrication of various types of nanopore structures, which can broaden the applications of the nanoporous anodic alumina with greater efficacy and versatility.

Preliminary study of the effect of electrolyte dilution on the electrochemical behavior of aluminum in black wattle pyroligneous liquor in MILD ANODIZATION in galvanostatic mode

RESUMO O alumínio é um metal muito utilizado comercialmente e normalmente com tratamento superficial de anodização. A anodização também é um processo consolidado e difundido, principalmente uma técnica potenciostática conhecida como Mild Anodization, que utiliza ácidos orgânicos como eletrólitos com aplicação de baixos potenciais para gerar camadas de óxidos semicondutoras. Por outro lado, o licor pirolenhoso, que é um subproduto da produção de carvão vegetal, oriundo da condensação da fumaça dos fornos de pirólise como processo de controle de poluição atmosférica, é pouco conhecido nos meios tecnológicos sendo utilizado basicamente para fins agrícolas e tem principalmente ácidos orgânicos em sua composição, como o ácido acético, o alcatrão e cetonas. Por outro lado, os processos de anodização são feitos normalmente em eletrólitos que demandam tratamento de efluentes dispendiosos e descarte controlado. Assim, o objetivo deste estudo é avaliar o comportamento do licor pirolenhoso como eletrólito de anodização para alumínio 6061, em diferentes concentrações. Para tanto, foram definidos parâmetros de processo (densidade de corrente, temperatura, tempo) e três níveis de concentração do eletrólito – 10%, 25% e 50% v/v. Comparativamente, os mesmos parâmetros de processo foram utilizados para o eletrólito de ácido acético (em concentrações de 0,4%, 1% e 2% v/v). As superfícies após o tratamento eletroquímico foram avaliadas por sua morfologia, através de microscopia eletrônica de varredura, e quanto à molhabilidade, pelo método da gota séssil. Verificou-se que houve anodização do alumínio em licor pirolenhoso, enquanto que em ácido acético houve eletropolimento da superfície. No caso do licor pirolenhoso, os resultados mostraram que, nas condições de anodização propostas, o desempenho do processo foi melhor na concentração de 10% v/v.

The role of mild and hard anodization regimes of iron oxide nanotubes in the photoelectrochemical performance

• Fe 2 O 3 nanotubes (NTs) prepared by electrochemical anodization for solar water splitting applications. • Two regimes of anodization, Mild and Hard, observed in NTs geometrical features ( D p , D int , W , L ) and anodization curves ( j ( t ) and Q ( t )) • Relation between charge curves Q ( t ), nanotubes length ( L ), and introducing porosity ( P ) as a parametric, established to estimate L by an adapted Classical Faraday’s Law. • Estimated L from adapted Classical Faradaýs Law in excellent agreement with the experimental data from SEM. Self-ordered hematite (α-Fe 2 O 3 ) nanotubes have been intensively studied as the ideal geometrical configuration for photoelectrochemical water splitting. In this work, iron oxide nanotubes were produced by a fast electrochemical anodization at different applied voltages (10 to 100 V) and studied their impact on the photoelectrochemical performance of the photoelectrodes. The anodization voltage highly affects the nanotubes length, pore diameter, interpore distance and porosity that ultimately led to different solar performances. The interplay between oxidation and dissolution rates in the electrochemical process at different voltages led to observe two distinct self-ordering NTs growth regimes: (I) mild-anodization regime from 20-60 V, where both NTs linear growth and geometrical structural ordering are observed; (II) hard-anodization regime from 70-100 V, with non-linear decay of nanotubes thickness combined with a decrease of structural ordering. The highest photocurrent of ∼0.5 mA·cm -2 was obtained for sample anodized at 20 V with a porosity of 30 %. A relation between the anodization charge curves Q ( t ), nanotubes length ( L ) and porosity ( P ) was established to estimate L using Q ( t ) curves. L was found to be in excellent agreement with the experimental data from both scanning electron microscopy (SEM) and anodization curves.

Nanoporous WO3 grown on a 3D tungsten mesh by electrochemical anodization for enhanced photoelectrocatalytic degradation of tetracycline in a continuous flow reactor

• A nanoporous WO 3 /W electrode was obtained via mild anodization of W mesh. • Photo-electrocatalytic (PEC) degradation of tetracycline in continuous flow reactor. • PEC was 3x times more efficient than photocatalysis alone. • Continuous flow reactor design is scalable and highly efficient. The efficient removal of organic micropollutants remains a major challenge for conventional water treatment technology. Photocatalysis, while effective, has not been widely adopted due to cost and implementation challenges. Here, an effective alternative is proposed through the integration of photocatalytic and electrochemical degradation of tetracycline (TC) in a continuous flow reactor using a WO 3 /W mesh as photo-electrode. WO 3 nanoporous structures were grown on a tungsten (W) metal mesh via one step anodization in aqueous oxalic acid electrolyte (0.05 mol/L). The prepared materials were then used as 3D array photoanode in a continuous flow photo-electrocatalytic reactor. The combined influence of anodization time and post-treatment annealing temperature on the photo-electrochemical activity of the WO 3 /W mesh was examined through morphological and structural analysis. W mesh anodized for 4 h, and subsequently annealed at 450 °C have a homogenous nanoporous structure uniformly distributed and well adhering onto the W mesh, with average pore diameter of ∼ 80 nm and pore wall thickness of ∼ 250 nm. These materials are transparent, have high conductivity and WO 3 monoclinic phase. Under optimal conditions of recirculating flow rate of 230 mL min −1 and bias potential of + 1.0 V vs Ag/AgCl/KCl (3 mol/L), the photo-electrocatalytic degradation led to a TC removal of up to 90 % after 60 min of treatment, almost 3 times higher than photocatalysis only. The excellent results by integration of photocatalysis and electrochemical degradation in a flow reactor, combined with a scalable WO 3 /W mesh, offers a useful information for upscaling and designing for practical issues in water treatment (such as costs and efficiency).

Highly ordered nanotubular niobium oxide obtained by self-organizing anodization: A study of capacitive behavior

Transition metal oxide nanotubes such as niobium have attracted wide scientific attention due to their unique properties such as high ionic mobility and large specific surface area. This study investigates the capacitive behavior of nanotubular niobium oxide obtained under extremely mild anodization conditions, aiming its application in supercapacitors. The nanotubular niobium oxide was obtained by potentiostatic anodization at 90 V at room temperature, in a glycerol electrolyte containing 0.4 M NH 4 F and 0.9% H 2 O. Field Emission Gun Scanning Electron Microscopy analysis (FEG-SEM) was employed to perform morphological characterization. The capacitive behavior of the obtained material was investigated by cyclic voltammetry and galvanostatic charge/discharge in a three-electrode system. The neutral aqueous electrolyte used was 1 M Na 2 SO 4 , in a potential window from −0.2 to −1.2 V vs. Ag/AgCl and 1 M H 2 SO 4 as an acid aqueous electrolyte in a potential window from −0.25 to −0.75 V vs Ag/AgCl. The FEG-SEM images revealed the presence of a self-organized highly ordered nanotubular niobium oxide layer on niobium substrate. Appreciable capacitance values of 20 mF cm −2 and 103 mF cm −2 were achieved in Na 2 SO 4 and H 2 SO 4 electrolytes, respectively. This great capacitive response can be associated with a large surface area achieved through the nanostructured oxide layer as well as the morphology of vertically aligned nanotubes that promote a directional ions transport, which, consequently, improves the capacitive performance. Besides, the results showed a superior performance, in terms of capacitance modulus and stability, for nanotubular niobium oxide in H 2 SO 4 electrolytes. The results obtained in this study highlight the great potential of the self-organized highly ordered nanotubular niobium oxide for application as a supercapacitor electrode.

Investigation of the AgCl Formation Mechanism on the Ag Wire Surface for the Fabrication of a Marine Low-Frequency-Electric-Field-Detection Ag/AgCl Sensor Electrode.

One of the most widely used electric field sensors for low-frequency electric field detection (LFEFD) in seawater uses the Ag/AgCl electrode. The surface structure of the electrode including AgCl layers plays a critical role in the electrode’s electrochemical performance required for the sensor. In this study, the sequential AgCl formation process under the constant current was examined on the Ag wire in an electrode size for actual applications, and an optimal electrode surface structure was suggested for the LFEFD Ag/AgCl sensor. Upon mild anodization (0.2 mA/cm2) in 3.3 M KCl solution that permits us to follow the AgCl formation process manageably, Ag dissolution from the wire surface begins leaving cavities on the surface, with the accompanied growth of initial Ag grains. During this period, AgCl deposits in sizes of about several micrometers to 10 μm with crystal planes also form primarily along scratch lines on the wire surface, but in a partial scale. Then, with further anodization, the assumed thin AgCl deposits start to form, covering a large portion of the wire surface. They grow to become deposits in sizes of about several micrometers to 10 μm with no clear facet planes next to one another and are connected to form the network structure, representing the main developing mode of the AgCl deposits. While they cover all the surface, AgCl deposits also form on the surface of the already formed ones, making multiple AgCl layers. All these deposits develop through the nucleation process with a relatively high surface energy barrier, and their formation rate is solely controlled by the release rate of Ag+ from the wire, thus by the applied current magnitude. The Ag/AgCl electrode with a thick AgCl layer and many holes in the AgCl surface structure like microchannels is considered to work effectively for the LFEFD sensor in terms of both detection sensitivity and service lifetime.

Nanosurface modification of Ti64 implant by anodic fluorine-doped alumina/titania for orthopedic application

Given that titanium (Ti)-based alloys have been paid more attention for various biomedical applications, the formation of the nanostructured coatings on Ti-based alloys may be a promising strategy for further preclinical developments. Therefore, the present study reports a new hybrid approach that integrates the engineering and scientific aspects into a single operational framework, in which a highly efficient fluorine-doped alumina/titania coating was developed on the Ti64 implant to improve its performance in long-term operation. Besides, due to the effect of surface tuning on the biomineralization of the system, the in-situ doping mechanism and its impacts on the in-vitro bioactivity of the implant were evaluated. From the microstructural observations, a homogeneous mixed oxide nanoporous structure with a pore size between 18 and 29 nm was developed following one-step mild anodization (2 h) of the magnetron sputtered aluminum (Al) thin film on Ti64 substrate in 0.5 wt% NH4F with 4 ml de-ionized water in ethylene glycol at 20 °C using the applied potential of 60 V. The Rietveld refinement technique not only confirmed the formation of these mixed oxide phases but also provided promising refined data on the structural properties of the samples. In addition, improvements in mechanical performance, wettability and in vitro bioactivity of the system were obtained by the proposed nanosurface modification.

Metalizadora Construída em Laboratório: Deposição de Filme de Alumínio sobre Substrato de Vidro e Posterior Anodização para Obtenção de Óxido de Alumínio

In this work, the assembly of a low-cost metallizer is presented, based on physical vapor deposition, to obtain metallic aluminum film deposited over a glass substrate. Later, the Al film is anodized and converted into a porous nanostructured oxide film. The metallic film thickness was measured by profilometry, and the sample position inside the chamber was evaluated. Samples positioned at the center of the sample holder showed considerable thickness and best homogeneity compared to those samples positioned at the edges of the sample holder. A thick metallic film of Al (6 to 7 µm) was deposited over the glass substrate for subsequent anodizing treatment. Mild anodization allowed the total conversion of metallic Al after 7000 s, producing a transparent anodic aluminum oxide (AAO) film adhered to the glass substrate. The AAO morphology was investigated by SEM. Some factors have been listed to describe the low regularity and homogeneity of nanopores in the outer layer of AAO, such as internal defects, compaction of deposited Al film layers and the glass substrate roughness.

(Invited) Formation Efficiency of Anodic Porous Alumina in Sulfuric Acid Containing Alcohol As Additives

The properties of anodic porous alumina films, which are generally formed by the anodization of aluminum in an acid electrolyte, are well known to be affected by the dimensions of their structure (e.g., pore diameter, pore density, and film thickness). For instance, to increase the hardness of anodic films on aluminum, the anodization is industrially carried out at low-temperature to avoid chemical dissolution of the formed oxide film during anodization. Anodization in an electrolyte containing organic compounds is also one approach for improving the performance of anodic oxide film not only for aluminum but also for other light metals such as titanium and magnesium (1).One such initiative is anodization of aluminum at a relatively high current density or high voltage using a common acidic electrolyte (e.g., sulfuric acid, oxalic acid, or phosphoric acid) with an alcohol added. In such studies, the effects of adding an alcohol are summarized as follows: i) the alcohol functions as an anti-freeze agent in the electrolyte, enabling the anodization temperature to be lowered; ii) the alcohol acts as a cooling agent—specifically, the vaporization of alcohol at the bottom of the pore channels removes the heat generated at the aluminum/oxide interface; and iii) nonuniform growth of the anodic film (i.e., burning) is effectively prevented, even during high-field anodization. However, these effects are observed not only for monohydric alcohols with a boiling point less than 100 °C (e.g., methanol and ethanol) but also for polyhydric alcohols with a boiling point greater than 100 °C (e.g., ethylene glycol and glycerol). Therefore, the effect of alcohol addition cannot be reasonably explained solely on basis of the boiling point of the alcohol used as an additive. Understanding the effects of alcohol addition necessitates a systematic investigation comparing types of alcohols, in addition to further fundamental studies.In this study, monohydric alcohols or polyhydric alcohols were added to the sulfuric-acid-based electrolyte to examine their effects on the anodization behavior of aluminum, the coating ratio, the porosity, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various alcohols with different physical properties under mild anodization conditions with a constant current of 100 A·m−2 at 20 °C (2, 3). The influence of the type and concentration of alcohol on the coulombic efficiency of film formation and the structure of the porous alumina under a constant electric charge was also investigated. In terms of basic research, the potential impact of alcohol addition in electrolytes will be discussed. H. Asoh, K. Asakura, H. Hashimoto, RSC Advances, 10, 9026-9036 (2020).H. Asoh, M. Matsumoto, H. Hashimoto, Surface and Coatings Technology, 378, 124947 (2019).M. Matsumoto, H. Hashimoto, H. Asoh, Journal of The Electrochemical Society, 167, 041504 (2020).

Super-fast fabrication of self-ordered nanoporous anodic alumina membranes by ultra-hard anodization

Although the fast fabrication of long-range ordered porous anodic alumina (PAA) membranes has already been realized by hard anodization, achieving super-fast oxide growth rates has been still very challenging. Herein, we report a new generation of the anodization process, called ultra-hard anodization (UHA), in which the mean growth rate of self-ordered PAA reached 2900 µm/h in the first 80 s of the process, resulting in a membrane thickness of 58 µm. By comparison, the same thickness can be obtained using the conventional hard anodization and mild anodization processes in 2400 s and 10 h, respectively. While the current density (J) reaches a high value of 2400 mA/cm2, burning and dielectric breakdown phenomena are avoided by controlling the barrier layer temperature and the diffusion length. In addition, the mechanism of oxide growth is explained on the basis of the high field theory and the diffusion-limited path of ionic species through the nanopores, taking into account the effect of temperature and sprinkling rate of electrolyte on J. By blending 0.3 M oxalic acid with phosphoric or sulfuric acid separately, the UHA process is also performed at target voltages ranging from 85 to 150 V, thereby allowing for a wide range of interpore distances. From a practical point of view, we synthesize high aspect ratio polymer nanowires (up to 320 µm) in the resulting membranes. Our findings open a new approach for the super-fast fabrication of the self-ordered PAA, facilitating the development of nanodevices and their commercial applications.

Development of Nanoporous AAO Membrane for Nano Filtration Using the Acoustophoresis Method.

A concept of a nanoporous anodic aluminum oxide (AAO) membrane as a vibro-active micro/nano-filter in a micro hydro mechanical system for the filtration, separation, and manipulation of bioparticles is reported in this paper. For the fabrication of a nanoporous AAO, a two-step mild anodization (MA) and hard anodization (HA) technique was used. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the surface morphology of nanoporous AAO. A nanoporous structure with a pore diameter in the range of 50–90 nm, an interpore distance of 110 nm, and an oxide layer thickness of 0.12 mm with 60.72% porosity was obtained. Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS) were employed to evaluate AAO chemical properties. The obtained results showed that the AAO structure is of hexagonal symmetry and showed where Al2O3 is dominant. The hydrophobic properties of the nanoporous surface were characterized by water contact angle measurement. It was observed that the surface of the nanoporous AAO membrane is hydrophilic. Furthermore, to determine whether a nanomembrane could function as a vibro-active nano filter, a numerical simulation was performed using COMSOL Multiphysics 5.4 (COMSOL Inc, Stockholm, Sweden). Here, a membrane was excited at a frequency range of 0–100 kHz for surface acoustics wave (SAW) distribution on the surface of the nanoporous AAO using a PZT 5H cylinder (Piezo Hannas, Wuhan, China). The SAW, standing acoustic waves, and travelling acoustic waves of different wavelengths were excited to the fabricated AAO membrane and the results were compared with experimental ones, obtained from non-destructive testing method 3D scanning vibrometer (PSV-500-3D-HV, Polytec GmbH, Waldbronn, Germany) and holographic interferometry system (PRISM, Hy-Tech Forming Systems (USA), Phoenix, AZ, USA). Finally, a simulation of a single nanotube was performed to analyze the acoustic pressure distribution and time, needed to center nanoparticles in the nanotube.

Synthesis and photoluminescence properties of nanoporous anodic aluminum oxide (NAAO): Sm3+ nanocomposite layer: Effect of anodizing parameters (time and voltage), Sm3+ concentrations, and annealing temperatures

In the present work, nanoporous anodic aluminum oxide (NAAO) layers were synthesized by mild and mild-hard anodization methods in a solution of oxalic acid and photoluminescence properties of these oxide layers were compared. Then, the nanoporous anodic aluminum oxide: Sm3+ (NAAO: Sm3+) nanocomposite layer is produced by boiling the NAAO layers in a solution of samarium nitrate and annealing at different temperatures under argon atmosphere. Moreover, effect of anodizing parameters (time and voltage), Sm3+ concentrations, and annealing temperatures on photoluminescence properties of the NAAO: Sm3+ nanocomposite layers were investigated and results indicate composition of SmAlO3. Finally, morphology, phase structure, energy levels, and emission wavelengths of the oxide layers were investigated by field emission scanning electron microscopy (FE-SEM), X-ray diffraction analysis (XRD), and photoluminescence (PL) spectroscopy. Results indicate that the PL intensity of the NAAO layers produced by mild-hard anodization is more than the mild anodization. Moreover, the layers produced at 120 V for 2 h have the most PL intensity. At 1200 °C temperature, 0.7 M concentration of samarium ions the most PL intensity is created. Moreover, with the excitation of the NAAO: Sm3+ nanocomposite layer created at 400 nm wavelength, three main emission bands from the 4f-4f intra-configuration transitions from Sm3+ ions are clearly observed. These transitions include 4G5/2 → 6H5/2, 4G5 2 → 6H7/2, and 4G5/2 → 6H9/2.

Anodic alumina oxide surfaces prepared by dual hard and mild anodization at subzero temperature: Surface microscopic characterization and influence on wettability

Multistage anodizations allow the preparation of new generations of ordered anodic alumina oxide porous structures with large interpore distances, yet the influence of second anodization and pore widening times on pore dimensions and wettability of the obtained structures has not yet been fully studied. The main objective of this study was to characterize the obtained pores (dimensions and morphology), for different anodizations and pore widening times, and to investigate their respective influence on surface wettability. Anodic alumina oxide ordered porous surfaces were prepared using a dual hard and mild anodization process with oxalic and phosphoric acids at subzero temperature, with or without pore widening. Statistical analyses were realized on pore measurements from samples produced with a large range of anodization durations (7 different tested times). The obtained surfaces possessed regular ordered pores of 99 nm and 307 nm for respectively median pore diameter and inter-pore distance, before pore widening, and did not conform to expected theoretical values for either usual lone mild or hard anodization conditions. The slow growth of pore depth (median of 11 nm/min) observed during second anodization could be explained by the subzero electrolyte temperature used. Pore widening effectively increased pore diameter, but had statistically a bigger impact on diameters for shorter second anodization time than for longer ones. Also, pore widening significantly decreased pore depth as well as the oxide barrier thickness. Additionally, porosity increased from 9.57% to 14.8% after 3 hours of pore widening, but did not significantly affect surface wettability, which was however increased when compared to flat control surfaces. The obtained results showed that a dual anodization with different electrolytes at a subzero temperature had a significant impact on pores dimensions. A discussion on the respective effects of surface chemistry and surface roughness in the wetting evolution was proposed.

Optimized nanoporous alumina coating on AA3003-H14 aluminum alloy with enhanced tribo-corrosion performance in palm oil

This study aimed to prepare nanoporous anodic alumina on AA3003-H14 aluminum alloy using a mild anodization process with minimal working voltage and treatment time. The microstructural features, mechanical properties, and tribocorrosion behavior of the coatings were assessed to determine the optimum conditions for the fabrication of nanoporous anodic alumina on AA3003-H14 alloy. The microstructural analysis showed both uniform and nonuniform pore nucleations during anodization in H2SO4/C2H2O4, H2SO4/H2CrO4, and EG/H2O/NH4F electrolytes, where the optimal nanoporous structure with an average thickness, porosity, pore diameter, and interpore distance of 382 nm, 19%, 16 nm, and 35 nm, respectively, was developed after a short-time mild anodization for 30 min at constant potential of 12 V using 15 wt% H2SO4 electrolyte. The XRD profiles indicated formation of a highly-disordered and amorphous nanoporous anodic alumina. Due to the loss of mechanical properties after heat treatment at 450 °C for 90 min, Vickers microhardness of the samples decreased drastically from 124 to 90.5 HV, respectively. The wear analysis indicated an improved tribological behavior by mild electrochemical anodization owing to the development of nanoporous anodic alumina, which acted as the liquid lubricant reservoir and increased the tribological performance of the workpiece. In addition, the heat treated specimen showed the lowest corrosion rate of 0.079 mm.y-1 and utmost corrosion protection efficiency of 95.24% following exposure in palm oil methyl ester (B100). This novel nanoporous configuration not only endows excellent tribological performance in the biofuel combustion environment, but also particularly decreases consumption of liquid lubricants through a facilitated multiphase lubrication mechanism.

Systematic Control of Anodic Aluminum Oxide Nanostructures for Enhancing the Superhydrophobicity of 5052 Aluminum Alloy.

The recent increased interest in the various applications of superhydrophobic surfaces necessitates investigating ways of how this property can be enhanced further. Thus, this study investigated how superhydrophobic properties can be enhanced through the formation of anodic alumina nanostructures on 5052 aluminum alloy. A multistep anodizing process that alternates two different anodizing modes, mild anodization (MA) and hard anodization (HA), with an intermediate pore-widening (PW) process was employed. Multistep anodization was employed in two different ways: an MA → PW → HA process and an HA → PW → MA process. Both routes were conducted with PW durations of 40, 50, and 60 min. The well-defined nanostructures were coated with a self-assembled monolayer (SAM) of FDTS (1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane). The contact angle values of water droplets were maximized in the pillar-like nanostructures, as they have a less solid fraction than porous nanostructures. With this, the study demonstrated the formation mechanism of both nanoscale pillar and nanoscale hierarchical structures, the wettability of the superhydrophobic surfaces, and the relationship between PW duration time with wettability and the solid fraction of the superhydrophobic surfaces.

Combined mild and hard novel fabrication approach for nanoporous membrane

Anodization of aluminum by a hard anodizing or mild anodizing process is usually done to obtain metal top surface finishing. Recently, this process has been becoming famous for the formation of through-hole porous oxide membranes (TH-POM) which act as a template for fabrication of nanowires/tubes. Membranes with different pore diameters, interpore distance, depth of pore and porosity can be obtained by selecting different parameters such as type and concentration of electrolyte, pH, temperature, time and applied voltage. In this research paper, exclusive experiments were studied and conducted to obtain TH-POM which will be template for generation of ordered nanowires by an electrochemical process. This research is carried out using a combination of mild anodization (MA) and hard anodization (HA) with novel heterogeneous control parameters by using DC power supply, magnetic stirrer and data acquisition system (DAQ) and at high temperature in order to minimize the refrigeration cost required for cooling the electrolyte. Excellent ordered membranes of 24–29 nm and 40–50 nm pore diameter were obtained in short time, characterized by SEM and EDS and analyzed by I–V curves obtained using an Arduino-based current/voltage data logger. A thermal stress test is also performed on the TH-POM formed using MA and HA.

Effects of ethanol on the efficiency of the formation of anodic alumina in sulfuric acid

In the interest of enabling practical applications of anodic oxide films, it is important to develop methods to improve the coulombic efficiency of film formation. In this study, ethanol was added to the sulfuric-acid-based electrolyte to examine its effects on the anodization behavior of aluminum, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various concentrations of ethanol under mild anodization conditions (i.e., 20 °C, as opposed to the lower-temperature hard anodization conditions). When a constant current of 100 A·m−2 was applied, the initial voltage and steady-state voltage were strongly dependent on the ethanol concentration that was added. The growth rate of the alumina film was governed by a field-assisted electrochemical process and, thus, was not affected by the addition of ethanol in the sulfuric-acid-based electrolyte. However, the maximum attainable film thickness increased with increasing ethanol concentration while the charge during anodization was constant. Thus, it was concluded the coulombic efficiency of the film formation could be improved by suppressing the chemical dissolution of the anodic film. Furthermore, the degree of acid dissociation in the water-and-ethanol system is thought to play an important role in controlling the formation and resultant structure of the film.

Fabrication of alumina with ordered tapered-nanopore nested in micro-bowl hierarchical structure by a combined anodization

We report that porous anodic alumina (PAA) with ordered tapered-nanopore nested in micro-bowl hierarchical structure can be facilely and controllably fabricated by a combined anodization process. It includes three main steps: I, stable anodization in citric acid to fabricate micro-bowl; II, mild anodization and chemically dissolving PAA to fabricate nanopit pattern; III, multi-step mild anodization and etching pore-widening to fabricate tapered-nanopore. Although the used anodization technology in each step is well established, the smart combination for realizing hierarchical structure is unreported but very effective. It is found that the original interpore distance of the micro-bowl obtained by stable anodization in citric acid can be enlarged by the followed mild anodization, which can be tuned from 0.88 μm to 1.3 μm. The structural parameters of tapered-nanopore can be tailored by changing the mild anodization conditions. Because of the curvature of the micro-bowl, the average number of the tapered-nanopore can be as high as 1.68 × 1015/cm2, which is about 1.5 times more than that (1.16 × 1015/cm2) on the flat surface. This method is simple and effective, which opens a door for boosting investigations into the physical and chemical properties of micro- and nanostructure made of metal or polymer materials as combining with nanoimprinting or electrochemical deposition.