Anodic Aluminum Oxide Surface 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.

Homeotropic orientation of an ion-channel forming mesophase induced by nanotemplate wetting.

Anodic aluminum oxide (AAO) membranes were used as templates to control orientation of an ion-channel forming columnar mesophase obtained by self assembly of a wedge-shaped sulfonate molecule. Inside the AAO structure, the director vector of the mesophase is oriented parallel to the pore axis due to the confinement effect. The molecular arrangement induced by the spatial confinement within the pores is extended over several microns into the remnant film on the AAO surface. The homeotropic alignment of the channels promotes unidimensional ion conduction through the film plane, which is manifested by a considerable increase in conductivity relative to isotropic samples.

Anodic Aluminum Oxide-Based Chemi-Capacitive Sensor for Ethanol Gas.

Alcohol ingested by humans can be analyzed via breath tests; however, approximately 1% can be excreted via the skin. In this paper, we present a capacitive sensor using hydrophobically treated anodic aluminum oxide (AAO) capable of detecting alcohol excreted through the epidermis. The degree of hydrophobicity based on the duration of exposure to 3-aminopropyltriethoxysilane vapor comprising a small number of Si-NH2 functional groups on the AAO surface was confirmed and the optimal exposure time was confirmed to be 60 min. The hydrophobized AAO showed a 4.8% reduction in sensitivity to moisture. Simultaneously, the sensitivity of the sensor to ethanol decreased by only 12%. Lastly, the fabricated sensor was successfully operated by attaching it to an ankle-type breathalyzer.

Tribological Synergism of Anodic Aluminum Oxide Surface Containing Micro-Holes and Nanopores under Lubricated Reciprocation

Micro-drilled aluminum surfaces containing micro-holes were anodized to produce nanopores over the machined and lapped surfaces. The anodized nanopores had an approximate diameter of 30–40 nm and a depth distribution of 20–30 μm from the surface. The diameter and depth of the machined micro-holes were 125 μm and 300 μm, respectively. Anodization itself did not change the surface roughness because the nanopores were very small. Ball-on-disk reciprocating tests were performed under lubricated conditions for 2 h using a frequency of 2 Hz, a load of 2 N, and a travel distance of 5 mm. The results showed that both the micro-drilled and anodized surfaces greatly reduced the coefficient of friction compared with the lapped bare surface; however, the coefficient of friction of the hole-textured specimen was not maintained till the end. Contrary to expectations, the lubricant retention capability of the textured structure declined because of hole failure that occurred during oscillation. This gradually increased friction until the end of the reciprocating test. When the micro-drilled surface was anodized, the coefficient of friction decreased again, implying that non-anodized micro-holes alone were ineffective for reducing friction. The surface hardness of Al increased owing to anodization, and thus the micro-holes remained intact. Therefore, it is concluded in this study that a prerequisite for friction reduction in Al is to increase the hardness to minimize the failure of micro-holes, which can be achieved by anodization. The synergistic lubricant retention capability can be maintained by the presence of both nanopores and micro-holes.

Lubricant-infused anodic aluminum oxide surface (AAO-LIS) for durable slipperiness under harsh conditions

The wettability of surfaces plays a crucial role in determining their applications, and slippery surfaces have been utilized in various fields. Especially, lubricant-infused surface (LIS) has been undergone extensive research over the past decade. However, LIS tends to lose its infused lubricant and consequently lose its slippery properties when exposed to harsh conditions. Therefore, there is a strong need to enhance the durability of the infused lubricant. Herein, two strategies are adopted to improve the durability of slippery properties against centrifugal forces and high-speed shear flows. One strategy is using re-entrant shaped nanocavities, and the other is grafting PDMS brush onto the surface. Anodic aluminum oxide (AAO) is employed to fabricate the re-entrant shaped nanocavities, and the durability of the infused lubricant is improved when the cavities have a smaller diameter. Furthermore, the re-entrant shape of nanocavity significantly improves the durability compared to nanocavity with a constant diameter. The grafting of PDMS brush is essential for enhancing the durability, and an increase in the length of the PDMS brush also improves the durability. The fabricated AAO-based LIS (AAO-LIS) maintains the slippery properties for 50 min under a high-speed water flow of 12 m/s. In addition, it maintains its superior slipperiness for more than a few days when exposed to actual seawater. Furthermore, the frictional drag reduction performance of the fabricated AAO-LIS is assessed. The proposed AAO-LIS maintains a certain drag reduction performance durably even after losing its superior slipperiness, demonstrating its strong potential for practical applications across various industrial fields.

Comment on Pashchanka, M. Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces. Nanomaterials 2021, 11, 2271.

In the review article "Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces" [...].

Reply to Heinschke, S.; Schneider, J.J. Comment on "Pashchanka, M. Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces. Nanomaterials 2021, 11, 2271".

First of all, I would like to thank Silvio Heinschke and Jörg J. Schneider (hereinafter referred to as "the Readers") for preparing their Comment [...].

Redox Polymer-Regulated Hysteresis in Ion Current Rectification through AAO Membranes and Single Nanopipettes

Nanopore technology has promising potential in fields such as high-density energy conversion, stochastic chemical sensing, logic operation and neuromorphic computing, separation and fundamental nanoelectrochemistry due to its rich ion transport features and easy fabrication. Understanding the ion transport dynamics at the solid-liquid interface is essential for further technological advancements. Ion current rectification (ICR) and its time-dependent hysteresis have been found to arise from the asymmetric nanointerfaces (geometry, together with surface charges in polarity and density). While various nanostructures can be fabricated and surface charges can be modified, active controls in ICR and hysteresis during transport applications are lacking. Here, we report actively controlled transport hysteresis in ion current rectification though PEDOT-modified single nanopipettes and anodic aluminum oxide (AAO) membranes. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) provides high conductivity and chemical stability, low operating voltages, and enhanced performances in hydrophilicity, processability, and flexibility. The PEDOT:PSS aqueous dispersion is crosslinked inside a nanopipette or assembled onto one side of the AAO surface. Oxidation and reduction reversibly switch the PEDOT between the positive and neutral form, and thus modulate the space charge in nanopores or across the AAO membranes, correspondingly, the ion transport behaviors. The amplitude and polarity of ICR and the scale of hysteresis through a nanopore or AAO membrane have been altered by oxidizing/reducing the PEDOT substrate by adjusting the external electrical field. In contrast, the surface charge varying on pH, ionic strength, and electrolyte type is commonly adopted in ion transport studies but cannot be externally regulated in real-time. Interestingly, negative differential resistance (NDR) is also found in the PEDOT-modified nanopores in simple KCl systems without external pressure differential or concentration gradients. The NDR peak position and magnitude can be easily adjusted by controlling the redox states of the PEDOT fillings. Figure 1

Neuron-Inspired Nanofluidic Biosensors for Highly Sensitive and Selective Imidacloprid Detection.

Pesticides have caused concerns about food safety due to their residual effects in vegetables and fruits. Imidacloprid, as the frequently used neonicotinoid pesticide, could harm cardiovascular and respiratory function and cause reproductive toxicity in humans. Therefore, reliable methods for portable, selective, and rapid detection are desirable to develop. Herein, we report a neuron-inspired nanofluidic biosensor based on a tyrosine-modified artificial nanochannel for sensitively detecting imidacloprid. The functional tyrosine is modified on the outer surface of porous anodic aluminum oxide to rapidly capture imidacloprid through π-π interactions and hydrogen bonds. The integrated nanofluidic biosensor has a wide concentration range from 10-8 to 10-4 g/mL with an ultralow detection limit of 6.28 × 10-9 g/mL, which outperforms the state-of-the-art sensors. This work provides a new perspective on detecting imidacloprid residues as well as other hazardous pesticide residues in environmental and food samples.

Effect of modified MgAl-LDH coating on corrosion resistance and friction properties of aluminum alloy

The in-situ growing approach was utilized in this article to construct the magnesium–aluminum layered double hydroxide (MgAl-LDH) film on the surface of a 1060 aluminum anodized film. To improve the corrosion resistance and friction qualities of aluminum alloy, the MgAl-LDH coating was treated using stearic acid (SA) and thiourea (TU). The aluminum substrate and anodized aluminum film layer corroded to varying degrees after 24 h of immersion in 3.5% (mass) NaCl solution, while the modified hydrotalcite film layer continued to exhibit the same microscopic morphology even after being immersed for 7 d. The results show that the synergistic action of thiourea and stearic acid can effectively improve the corrosion resistance of the MgAl-LDH substrate. The tribological testing reveals that the hydrotalcite film layer and the modified film layer lowered the friction coefficient of the anodized aluminum surface substantially. The results of the simulations and experiments demonstrate that SA forms the dense LDH-TU interlayer film layer by exchanging NO3– ions between TU layers on the one hand and the LDH-SA film layer by adsorption on the surface of LDH on the other. Together, these two processes create LDH-TU-SA, which can significantly increase the substrate's corrosion resistance. This synergistically modified superhydrophobic and retardant hydrotalcite film layer offers a novel approach to the investigation of wear reduction and corrosion protection on the surface of aluminum and its alloys.

Interfacial Super-Assembly of Intertwined Nanofibers toward Hybrid Nanochannels for Synergistic Salinity Gradient Power Conversion.

Capturing the abundant salinity gradient power into electric power by nanofluidic systems has attracted increasing attention and has shown huge potential to alleviate the energy crisis and environmental pollution problems. However, not only the imbalance between permeability and selectivity but also the poor stability and high cost of traditional membranes limit their scale-up realistic applications. Here, intertwined "soft-hard" nanofibers/tubes are densely super-assembled on the surface of anodic aluminum oxide (AAO) to construct a heterogeneous nanochannel membrane, which exhibits smart ion transport and improved salinity gradient power conversion. In this process, one-dimensional (1D) "soft" TEMPO-oxidized cellulose nanofibers (CNFs) are wrapped around "hard" carbon nanotubes (CNTs) to form three-dimensional (3D) dense nanochannel networks, subsequently forming a CNF-CNT/AAO hybrid membrane. The 3D nanochannel networks constructed by this intertwined "soft-hard" nanofiber/tube method can significantly enhance the membrane stability while maintaining the ion selectivity and permeability. Furthermore, benefiting from the asymmetric structure and charge polarity, the hybrid nanofluidic membrane displays a low membrane inner resistance, directional ionic rectification characteristics, outstanding cation selectivity, and excellent salinity gradient power conversion performance with an output power density of 3.3 W/m2. Besides, a pH sensitive property of the hybrid membrane is exhibited, and a higher power density of 4.2 W/m2 can be achieved at a pH of 11, which is approximately 2 times more compared to that of pure 1D nanomaterial based homogeneous membranes. These results indicate that this interfacial super-assembly strategy can provide a way for large-scale production of nanofluidic devices for various fields including salinity gradient energy harvesting.

The vertically network modifications on Li/Garnet interface for prolonging Li metal batteries life

The Li dendrites can penetrate the hard garnet ceramics in the solid-state lithium metal batteries (SSLMBs), severely impairing the cycle life of the battery. Herein, a facile Li/Garnets interface modification strategy is reported to mitigate the adverse effects of Li dendrite penetration, which refers to the insertion of anodized aluminum oxide (AAO) membranes with vertically aligned nanoscale channels at the Li/Garnets interface. As a result, the AAO membranes can confine the deposited Li metal in channels during cycling and induce the Li metal across the channels to continue to depositing uniformly on AAO surface. Furthermore, the deposited Li metal tends to react with AAO to form Li-Al-O compounds, which strengthens the confinement effects to Li metal. The results demonstrate that the full cell with Li/Garnets interface modified by 300 nm aperture AAO exhibits excellent cycling performance, which remain discharge specific of 93.5mAh g−1 (initial: 138.1mAh g−1) after stable operating 716 cycles at 0.2 C. The Li/Li symmetric battery can stably cycle above 1000 h at 0.5 mA cm−2 and 100 h under 1 mA cm−2. This facile method of directly inserting nanoscale vertically aligned network into Li/Garnets interface provides a new approach to promote application of garnets ceramics in SSLMBs.

Experimental Investigation of Tube-in-Tube Nanocomposite Coated Heat Exchanger

In this paper, heat transfer performance is investigated for plain and nanoparticle coated tube-in-tube heat exchangers. Four types of tubes, i.e. bare copper tube, bare aluminium tube, Cu-Al2O3 nanoparticle coated tube and anodized aluminium tubes are used for performing the experimental investigation. The coating thickness of Cu-Al2O3 nanocomposite surface and anodized aluminium tube varies from 10, 25 and 30 micrometres to 15, 20 and 30 micrometres respectively. The surface is found to be hydrophilic in nature as the contact angle changes from 79.82deg to 55.47deg. The prepared surfaces are characterized by FESEM, EDS and FTIR. By adjusting the hot and cold fluids relative mass flow rates, one may calculate not only the overall heat transfer coefficient but also the efficiency of the tube-in-tube heat exchanger. The Cu-Al2O3 nanocomposite coated surface has the highest overall heat transfer coefficient and efficiency, followed by an anodized aluminium surface, bare copper surface and bare aluminium surface. The aluminium anodic oxide (AAO) surface also exhibits an increased heat transfer coefficient, but to a lesser extent than the nanocomposite coating.

Surface and tribological characterization of anodic aluminum oxide coating containing diamond-like carbon flakes

Surface and tribological characterization of anodic aluminum oxide coating containing diamond-like carbon flakes

Proper matching of lubricants and modifiers: Another key factor for durable anti-icing performance of lubricated surfaces

Slippery liquid-infused porous surfaces (SLIPSs) with anti-icing properties are challenged by the lubricant loss. Many strategies (such as structure optimization, lubricant improvement) are used to retain the lubricant in the porous structure. However, the matching of modifier and lubricant is rarely studied, which is another key factor for durable anti-icing performance of lubricated surfaces. Herein, proper matching of different lubricants and modifiers were investigated. Perfluoropolyether oil (PFPE) and silicone oil (SO) are chosen as lubricants. Linear alkane (OTS) and fluorinated alkane (FAS) are chosen as modifiers. A series of SLIPSs were prepared by infusing lubricants into unmodified and modified anodized aluminum oxide (AAO) surfaces. The suitable matches (OTS with SO, FAS with PFPE) make the prepared SLIPS have high water mobility and enable high affinity between the lubricant and modified AAO surface. The former contributes to improving the anti-icing performance in the simulated glaze icing test, and the latter is beneficial to improving the durability during the icing/deicing cycles. This work provides useful information to determine the suitable pairs of lubricant-modifier for durable anti-icing lubricated surfaces.

Anodic Aluminum Oxide(AAO)-Based Chemi-Capacitive Sensor Toward Ethanol Gas

The transducer elements for such sensor arrays need to be small, low-power, and compatible with conventional microprocessing technology. The transducers include chemiresistive gas sensor, FET gas sensor, capacitive gas sensor, and inductive gas sensor. Of these transducers, chemicapacitors and chemiresistors are the best suited for low-power sensor arrays. The change of electrical properties such as conductance or capacitance upon exposure to analytes can be used to detect chemical and biological agents, respectively. Chemiresistors are simply implemented in many application fields, but they have disadvantages such as instability and cross sensitivity. Whereas chemicapacitors are known to provide superior sensitivity and recovery time because capacitance can response to very thin layers of chemoselective material down to a single molecular monolayer, resulting in the elimination of the duration time for capacitance change. In this work, the room-temperature capacitive ethanol gas sensing properties of AAO (anodic aluminum oxide) were studied and the underlying mechanisms were analyzed based on its electrode configuration and detection frequency. Figure 1 shows the progress of the experimental progress according to each electrode patterns and thicknesses.The sensor sensitivity of the chemocapacitive sensor toward ethanol was affected by the humidity because it has higher dielectric constant than ethanol. Therefore, the surface treatment of AAO was carried out by silanization using APTES and HMDS. The degree of hydrophobicity of AAO surface was determined by measuring the water contact angle by the Sessile Drop method. After surface treatment was performed in each method, the sensitivity of the sensor to the wet air was also measured.Through silanization, similar sensor sensitivity to ethanol was obtained, and at the same time, sensor sensitivity to wet air was minimized as illustrated in Figure 2. In this study, the sensor characteristics were optimized by the electrode pattern and thickness, and the gas selectivity of the sensor was improved through surface treatment. Figure 1

Chemical Structural Coherence Principle on Polymers for Better Adhesion

Composite materials are the most variative type of materials employed in almost every task imaginable. In the present study, a synthesis of a novel perfluoroalkyltriethoxysilane is reported to be used in creating composites with polyhexafluoropropylene—one of the most indifferent and adhesion-lacking polymers existing. The mechanism of adhesion of hexafluoropropylene is proved to be due to chemical structural coherence of perfluoroalkyltriethoxysilane to a link of polyhexafluoropropylene chain. The ability of perfluoroalkyltriethoxysilane to attach to surfaces was studied by FT-IR spectroscopy of modified glass microspheres. Although the perfluoroalkyltriethoxysilane surface modifier allowed partial adhesion of polyhexafluoropropylene, some detachment took place; therefore, the surface nanostructuring was used to increase its specific area by aluminum foil anodizing. An anodized aluminum surface was studied by scanning electron microscopy. The resulting composite consisting of anodized aluminum, perfluoroalkyl surface modifier, and polyhexafluoropropylene layer was proved to be stable, showed no signs of detachment, and is a promising material for usage in harsh environments.

Anodized Aluminum Surface with Topography-Mediated Antibacterial Properties.

Topography-mediated antibacterial surfaces that inactivate bacteria by physical contact have gained attention in recent years. Contrary to conventional antibacterial coatings, topography-mediated antibacterial surfaces do not suffer from coating instability and possible toxicity problems. In this study, a one-step hard anodization process has been deployed to fabricate a topography-mediated antibacterial aluminum surface. By optimizing anodization parameters, such as the concentration of the electrolyte, current density, and anodization time, desirable features of micronanoscale morphology were achieved. The optimum conditions of anodized aluminum that provided pores of a diameter of 151 ± 37 nm effectively killed 100% of E. coli bacteria.

Fabrication of Ni–Mg–Ag alloy electrodeposited material on the aluminium surface using anodizing technique and their enhanced corrosion resistance for engineering application

In this work, synthesis and fabrication of Ni–Mg–Ag alloy materials have been electrodeposited over the nickel strike anodized aluminium material for metal-finishing engineering applications. The comparative study of Ni–Mg alloy and Ni–Mg–Ag alloy over the anodized as well as without anodized aluminium surface have been carried out. The adherence and quality of the deposits were optimized by different experimental effects. The crystallite sizes of 77 nm and 86 nm have appeared in the Ni–Mg alloy and 72 nm and 119 nm are appeared in the Ni–Mg–Ag alloy, respectively. The Ni–Mg–Ag alloys were obtained at 2 A/dm2 and the alloy phases of MgNi2, Mg2Ni and AgMg3 are identified by ICDD card no.01-087-0720 and ICDD card no.00-039-0991. The Tafel corrosion tests were examined using the three-electrode system in 3.5% NaCl solution and Ni–Mg–Ag alloy exhibits a higher corrosion rate (CR) 1.604 × 10−2 mmpy. Thus, a significant enhancement has been achieved by Ni–Mg–Ag alloy on the Ni strike anodized aluminium surface for engineering and automobiles applications in the metal finishing industry.

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

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