Porous Alumina Research Articles

Microstructure control in printable porous polymer composites

The microstructure of a porous polymer matrix composite, specifically the organization of polymer, particles, and pores, plays a defining role in the physical properties of the material. Developing materials and processes that allow us to control the microstructure gives us a powerful tool to create materials with designed performance. Here, a series of porous polyvinylidene fluoride (PVDF) and alumina (Al2O3) composites produced through evaporation induced phase separation (EIPS) are demonstrated. This process utilizes solution processable inks and achieves porosity via a simple drying procedure. This enabled the materials to be printed using direct ink writing (DIW) techniques. The effects of particle size and loading on the mechanical properties and microstructures of these materials were studied. It was found that these materials remain significantly more ductile and extensible than what is expected from particle reinforcement. In the most extreme case, the use of 300 nm Al2O3 particles were found to only minimally reduce strain at failure, even at high loadings. We believe this behavior is enabled by how the EIPS process alters the microstructure of the material and appears to preferentially locate particles at the polymer-pore interface.

Understanding the fast formation mechanism of hard nanoporous alumina films on aluminum in acidic solutions containing nitric acid

Nanoporous alumina films formed on aluminum via anodization are widely used for scientific and technical purposes owing to their various advantageous properties, including excellent corrosion and wear resistance, good thermal and electrical insulation, and facile producibility at low cost. Herein, we report a novel approach to fabricate hard porous anodic alumina (PAA) films by adding an appropriate amount of nitric acid to sulfuric-, oxalic-, and phosphoric-acid solutions at critical high current densities. The effects of the nitric additives on the anodizing behavior, film growth mechanism, microstructure, chemical composition, film thickness, microhardness, and appearance of the PAA films formed from various solutions were systematically investigated. The addition of nitric acid enhanced the formation of alumina and resulted in fast growth rates of 196 μm/h and 166 μm/h for sulfuric- and oxalic-based solutions, respectively, while maintaining high hardness of approximately 380 and 430 HV0.1, respectively. The surface hardness of the films decreased to some extent with the increase in nitric acid concentration and/or film thickness owing to the strong corrosive nature of nitric acid during prolonged anodization, although nitrogen inclusion in anodic alumina films was not detected through X-ray photo-electron spectroscopy, which differs from the other acidic anions. In particular, the barrier layer thickness and pore intervals of PAA films increased with the nitric concentration in the aforementioned solutions, leading to a fast growth rate and high hardness inside the films.

High-efficiency, flexible and large-area red/green/blue all-inorganic metal halide perovskite quantum wires-based light-emitting diodes

Metal halide perovskites have shown great promise as a potential candidate for next-generation solid state lighting and display technologies. However, a generic organic ligand-free and antisolvent-free solution method to fabricate highly efficient full-color perovskite light-emitting diodes has not been realized. Herein, by utilizing porous alumina membranes with ultra-small pore size as templates, we have successfully fabricated crystalline all-inorganic perovskite quantum wire arrays with ultrahigh density and excellent uniformity, using a generic organic ligand-free and anti-solvent-free solution method. The quantum confinement effect, in conjunction with the high light out-coupling efficiency, results in high photoluminescence quantum yield for blue, sky-blue, green and pure-red perovskite quantum wires arrays. Consequently, blue, sky-blue, green and pure-red LED devices with spectrally stable electroluminescence have been successfully fabricated, demonstrating external quantum efficiencies of 12.41%, 16.49%, 26.09% and 9.97%, respectively, after introducing a dual-functional small molecule, which serves as surface passivation and hole transporting layer, and a halide vacancy healing agent.

3D printed hierarchical interpenetrating phase composites with multi-scale mechanical energy absorption mechanisms

Lightweight, strong, and energy-absorbing materials are highly sought after in practical engineering applications. To this end, interpenetrating phase composites (IPC) stand as a novel class of composite materials to better leverage the properties of their constituents. Their structural design, however, has been limited to only specific structural features at a certain length scale in the existing studies. Inspired by the hierarchical composite materials in nature, this work proposes a novel concept of hierarchical interpenetrating phase composites (HIPC) by polymer infiltration into 3D printed hierarchical porous ceramic lattices. The hierarchical porous alumina lattices have up to three levels of porosity, including 3D printed macropores (≈300 μm), emulsion-templated micropores (≈10 μm), and sub-micropores (<1 μm) from partial sintering. Upon epoxy infiltration, three-level HIPC is constructed with hierarchically incorporated epoxy domains and alumina-epoxy interfaces, which leads to its unique multi-scale mechanical energy absorption mechanisms. Featuring a particularly porous and lightweight ceramic lattice, three-level HIPC exhibits an excellent combination of high specific compressive strength (64.6 kN m/kg), specific flexural strength (39.2 kN m/kg), and SEA (18.4 kJ/kg). In addition, even with a low ceramic content, three-level HIPC can effectively enhance the thermal conductivity via its hierarchically continuous ceramic microstructure. Therefore, our three-level HIPC has demonstrated impressive mechanical and thermal enhancement with its exquisite hierarchical architecture. The hierarchical structural engineering approach also represents a paradigm shift from constructing single-scale IPCs to multi-scale HIPCs and can be extended to other material combinations for high-performance structural and functional materials.

Study of black protective-decorative nanocomposite anodic coatings on the surface of AMg5 aluminum alloy

A widely used nanocomposite coating is a porous anodic alumina colored by particles of metals or their compounds deposited into the pores. The insertion of light-scattering nanosized particles into the pores changes the optical properties of the anodic oxide, whereas the immobilization of metal nanoparticles in pores ensures their corrosion resistance. We present the results of studying black protective and decorative coatings on the surface of AMg5 aluminum alloy. The surface morphology of the samples was analyzed using atomic force microscopy, and the electrophysical properties were monitored by electrochemical impedance spectroscopy (EIS). The growth kinetics of the anodic coating has been studied, and optimal conditions for the formation of a regularly porous oxide coating 10 – 12 μm thick with a regular pore diameter of 15 ± 5 nm on the alloy surface were determined. It is shown that subsequent electrochemical coloring for 15 min makes it possible to obtain a black color of the coatings due to the deposition of Cu and/or CuO nanoparticles into the pores. Simulation of electrical equivalent circuits makes it possible to separate and calculate the electrical parameters corresponding to different layers and elucidate their regular changes after coloring and hydrothermal treatment. The high corrosion resistance of electrochemically colored anodized alloy samples subjected to hydrothermal treatment has been revealed. The results obtained can be used in the application of protective and decorative anodic coatings for the manufacture, for example, of solar panels due to the high absorption and low reflectivity of black coatings.

Magnetic saturation enhancement of gold-capped nickel nanorods

Nickel nanorods (NRs) capped with gold (Au/Ni) were grown into porous anodic aluminum oxide templates and subsequently transferred onto Au/Si (100) substrates. A high dense 2D array of Ni and Au/Ni nanorods was analyzed by vibrating sample magnetometry; it was found that an increase in 14.8% of the magnetic moment following the deposition of Au caps. In order to further investigate this phenomenon, the magnetic distribution of Au/Ni nanorods was studied by off-axis electron holography. The magnetization and induction strengths were evaluated to be 4.7 × 105 A/m and 0.62 T, respectively, which is equivalent to magnetometry measurements of the Ni NR arrays. Remarkably, a vortex state configuration was imaged in the Au segment by the retrieved magnetic phase of the electron holograms under free lens conditions of the transmission electron microscope column. It was concluded that the magnetic distribution in the Au segment is associated with a ferromagnetic coupling with Ni and correlated with the magnetometry measurements.

A novel slippery liquid infused porous aluminum conductor with excellent corrosion resistance in simulated marine atmosphere

The overhead aluminum conductors are prone to corrosion, especially in areas with marine climates, which interferes with the safe operation of the power system. Herein, slippery liquid infused porous surface (SLIPS) conductors were successfully prepared on pure aluminum (Bare) conductors by anodizing and infusion of silicone oil. The corrosion resistance of the conductors during 30 days of salt spray corrosion and 7 days of energized conditions was studied. The results indicated that the corrosion of the Bare and anodic aluminum oxide (AAO) conductors was more severe under energized (alternating current (AC) and direct current (DC)) conditions. The exacerbation of corrosion by DC energization was remarkably higher than that of AC energization. The AAO conductor showed better corrosion resistance than the Bare conductor, which was attributed to the porous alumina retarding the medium intrusion. Due to the barrier effect provided by the lubricant layer, the SLIPS conductor always presented excellent corrosion resistance than the Bare and AAO conductors. In addition, the mechanical and electrical properties of the SLIPS conductor were comparable to the Bare conductor and could satisfy the basic requirements for practical applications. The prepared SLIPS conductors offer great application prospects for the corrosion protection of aluminum conductors.

From Self-Assembly of Colloidal Crystals toward Ordered Porous Layer Interferometry.

Interferometry-based, reflectometric, label-free biosensors have made significant progress in the analysis of molecular interactions after years of development. The design of interference substrates is a key research topic for these biosensors, and many studies have focused on porous films prepared by top-down methods such as porous silicon and anodic aluminum oxide. Lately, more research has been conducted on ordered porous layer interferometry (OPLI), which uses ordered porous colloidal crystal films as interference substrates. These films are made using self-assembly techniques, which is the bottom-up approach. They also offer several advantages for biosensing applications, such as budget cost, adjustable porosity, and high structural consistency. This review will briefly explain the fundamental components of self-assembled materials and thoroughly discuss various self-assembly techniques in depth. We will also summarize the latest studies that used the OPLI technique for label-free biosensing applications and divide them into several aspects for further discussion. Then, we will comprehensively evaluate the strengths and weaknesses of self-assembly techniques and discuss possible future research directions. Finally, we will outlook the upcoming challenges and opportunities for label-free biosensing using the OPLI technique.

High-performance flexible pressure sensor based on ordered double-level nanopillar array films: Design, development, and modeling

To improve the sensitivity and broaden the range of flexible pressure sensors, we proposed a high-performance flexible pressure sensor based on ordered double-level nanopillar array films. In this work, polypyrrole/multi-walled carbon nanotube/polyurethane (PPy/MWCNT/PU) conductive films with double-level nanopillar array microstructures were prepared by continuously adjusting the oxidation and pore expansion parameters of a porous anodic alumina template (AAO), where the high pillar height was 1243.0 ± 7.0 nm, and the low pillar height was 868.0 ± 4.70 nm. We studied the influence of the structural characteristics of ordered double-level nanopillar array conductive films and a flexible pressure sensor on the sensing performance using finite element simulations, the establishment of sensing mechanism, and sensor performance measurements. The results showed that the pressure was positively correlated with the change rate of strain and the surface microstructure contact area of the ordered double-level nanopillar array films. The sensor sensitivity reached 208.353 kPa−1, and its range was extended by 267%. In addition, the sensor, which could monitor vocalization, drinking pulse and finger movements, could be used in a wide range of applications, including wearable medical monitoring and smart sign language textiles.

Innovative Nanostructured Fillers for Dental Resins: Nanoporous Alumina and Titania Nanotubes.

The possibility of improving dental restorative materials is investigated through the addition of two different types of fillers to a polymeric resin. These fillers, consisting of porous alumina and TiO2 nanotubes, are compared based on their common physicochemical properties on the nanometric scale. The aim was to characterize and compare the surface morphological properties of composite resins with different types of fillers using analytical techniques. Moreover, ways to optimize the mechanical, surface, and aesthetic properties of reinforced polymer composites are discussed for applications in dental treatments. Filler-reinforced polymer composites are the most widely used materials in curing dental pathologies, although it remains necessary to optimize properties such as mechanical resistance, surface characteristics, and biocompatibility. Anodized porous alumina nanoparticles prepared by electrochemical anodization offer a route to improve mechanical properties and biocompatibility as well as to allow for the controlled release of bioactive molecules that can promote tissue integration and regeneration. The inclusion of TiO2 nanotubes prepared by hydrothermal treatment in the resin matrix promotes the improvement of mechanical and physical properties such as strength, stiffness, and hardness, as well as aesthetic properties such as color stability and translucency. The surface morphological properties of composite resins with anodized porous alumina and TiO2 nanotube fillers were characterized by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and X-ray chemical analysis. In addition, the stress-strain behavior of the two composite resins is examined in comparison with enamel and dentin.

Influence of carbon nanotubes on the plasma electrolytic oxidation process of aluminum under “soft” sparking conditions

Plasma electrolytic oxidation of aluminum (PEO) was performed with dispersed multi-walled carbon nanotubes (MWCNTs) into the electrolyte. The effect of the carbon nanotubes concentration, as well as that of the electrical processing conditions, on the PEO process was investigated. From the point of view of the electrical response of the process, results show that the “soft” sparking regime appears earlier in presence of MWCNTs. The process time at which the transition to the “soft” regime occurs decreases linearly with the increase in the MWCNTs concentration. Over a concentration threshold, depending on the applied pulse bipolar current waveform, the PEO process is inhibited. The dispersion of MWCNTs in the electrolyte results in thicker but more porous alumina layers. More specifically, the proportion of the α-alumina phase relative to the γ-alumina phase decreases with the increase in the MWCNTs concentration. Results are explained by considering the increased coating growth kinetics and lower heat accumulation from MWCNTs addition. Finally, the present study gives new insights into the right management of the PEO process under “soft” sparking regime with dispersed carbon-based nanoparticles in the electrolyte.

Structure and Electric Properties of Anodized Aluminum with PEDOT:PSS Conductive Polymer Cathode

Aluminum solid electrolytic capacitors using a conductive polymer as a cathode material has the advantages of low equivalent series resistance (ESR) and high thermal stability compared to aluminum electrolytic capacitors using liquid electrolytes. A disadvantage of aluminum solid electrolytic capacitors with a conductive polymer is the low breakdown voltages, limiting the operating voltage of the capacitors to ∼100 V or less. In this study, for a possible future increment of the withstand voltages of solid electrolytic capacitors, PEDOT:PSS-coated aluminum with an amorphous or crystalline alumina layer formed at 500 V has been characterized using SEM, STEM/EDS, and conductive AFM to understand the distribution of PEDOT:PSS conductive polymer and its influence on the electric properties. The crystalline alumina layer, developed beneath a porous hydrated alumina layer, contains many voids, which increase the leakage current compared to the void-less amorphous alumina layer. However, the crystalline alumina layer with PEDOT:PSS exhibits a higher breakdown voltage than the amorphous one. The limited incorporation of PEDOT:PSS into the inner part of the porous hydrated alumina layer may introduce a resistive surface layer, possibly contributing to the increased dielectric breakdown.

A Novel Polishing Process for Ultra-Smooth Aluminum Surfaces via Anodizing in Sodium Metaborate

A novel ultra-smoothing process for aluminum surfaces was developed using porous alumina formation and subsequent oxide dissolution. A submicron-scale periodic dimpled aluminum surface with an arithmetic mean roughness of 31.5 nm was prepared by anodizing in an etidronic acid solution. This dimpled aluminum specimen was then anodized in a sodium metaborate (NaBO2) solution to form a unique porous alumina film with an extremely flat barrier layer, which differs from the typical hemispherical barrier layer. The outer porous layer became thicker with time during anodizing, whereas the thickness and smoothness of the inner barrier layer were maintained without oxide breakdown. As the porous alumina film was chemically removed in a CrO3/H3PO4 solution, a smooth aluminum surface was exposed. The mean roughness of the aluminum surface drastically decreased to 0.5 nm by short-term anodizing for 15 min and slightly decreased with further anodizing. As a result, an ultra-smooth aluminum surface measuring 0.4 nm in roughness, which is much smaller than that of an electropolished aluminum surface (1.3 nm), was successfully obtained via anodizing in NaBO2 and subsequent oxide dissolution. Our smoothing process was compared with conventional smoothing processes such as electropolishing and barrier oxide formation.

Electrochemical Separation of Porous Anodic Aluminum Oxide with Thick Barrier Layers

We investigated an electrochemical separation method for anodic aluminum oxide (AAO) films possessing thicker barrier layers by electrolysis in sodium chloride (NaCl) and ethylene glycol (EG) solution and subsequent fabrication of through-hole AAO membranes by immersion in phosphoric acid (H3PO4) solution. The AAO films with different barrier layer thicknesses were formed on the aluminum surface by anodizing in sulfuric, oxalic, and H3PO4 solutions at up to 130 V. The AAO-covered aluminum specimens were immersed in NaCl/EG solution, and then constant voltage electrolysis at 10 V higher than the anodizing voltage was performed for the separation of AAO from the aluminum surface. Many nanoscale paths were formed throughout the barrier alumina layer in the initial stage of electrolysis, and then the aluminum substrate electrochemically dissolved through these narrow paths during electrolysis. Although the AAO film formed by anodizing at 130 V was partially fractured by electrolysis, the AAO films formed at up to 80 V were uniformly separated from the aluminum surface. As the separated AAO membrane was immersed in H3PO4 aqueous solution after electrolysis, the barrier layer with narrow paths preferentially dissolved into the solution, and a through-hole AAO membrane could be successfully obtained.

Thermal Transformations of Porous Anodic Aluminum Oxide Formed in Sulfuric Acid/Oxalic Acid Mixed Electrolytes

Thermal Transformations of Porous Anodic Aluminum Oxide Formed in Sulfuric Acid/Oxalic Acid Mixed Electrolytes

Thermal Transformations of Porous Anodic Aluminum Oxide Formed in Sulfuric Acid/Oxalic Acid Mixed Electrolytes

Aluminum anodizing in electrolytes comprising mixtures of several acids opens way to manufacture porous films of anodic aluminum oxide (AAO) with a widely tunable structure period. Study of thermal transformations in AAO films produced in mixed electrolytes is a separate task, as a complex chemical composition of the material can give rise to some specifics in subsequent annealing. Impurity oxalate and sulfate ions were detected in the AAO produced by aluminum anodizing in sulfuric acid/oxalic acid mixed electrolytes. The sulfate weight fraction appears about one order of magnitude higher than the oxalate weight fraction, and it increases as the concentration ratio of sulfuric acid to oxalic acid in the electrolyte increases. In the same way, the crystallization temperature of amorphous AAO to a mixture of low-temperature Al2O3 polymorphs increases in response to increasing concentration ratio of sulfuric acid and oxalic acid. Thus, the component ratio in the mixed electrolyte used influences the composition and thermal transformations of AAO.

Fabrication of porous alumina using a paraffin pore-forming agent for laser pump cavity applications

Fabrication of porous alumina using a paraffin pore-forming agent for laser pump cavity applications

РАЗРАБОТКА ЛИТЕЙНОЙ ТЕХНОЛОГИИ ПОЛУЧЕНИЯ ПОРИСТОГО КОМПОЗИЦИОННОГО МАТЕРИАЛА НА ОСНОВЕ АЛЮМИНИЯ

The work is devoted to the development of technology for obtaining composite material based on aluminium alloy with a given morphology of pores by using hollow metal spheres as pore formers. In the course of work the following researches have been carried out: selection of material-former of hollow metal spheres forming the structure and physical and mechanical properties of porous aluminum; study of influence of binder type on uniformity of conductive coating on the surface of form-former; study of influence of electrolysis modes on continuity of metal coating on the surface of conductive layer of formformer; production of porous aluminum by foundry technology. It is shown that the use of hollow metallized spheres as a pore former allowed to reduce the density of the AK12 alloy prototype by 40%.

Investigation of Thermal Conditions of Porous Aluminum Formation

Investigation of Thermal Conditions of Porous Aluminum Formation

In English

Anodic porous alumina has been studied and used as nanoscale structure, coating, template in different applications. The porous anodic alumina oxide could be described as numerous hexagonal cells and looks like cellular structure. In this work we report about results of study anodizing of aluminum with usage of electrolyte: “oxalic acid electrolyte-matter with carbon nanodots”. It was received anodic aluminum oxide-membrane with aluminum supporting; calcination was used as post treatment. The aluminum substrate allows one to fix the membrane in the cells. Methods: processes of anodizing was provided in 0.3M oxalic acid with addition of colloid system of carbon nanodots, temperature of process was controlled at range of 10 degree Celsius, aluminum foil (anode) and platinum plate (cathode) were used; thickness of aluminum foil was 0.1 µm; morphology and structure of anodic aluminum oxide-membrane were determined with usage of electron scanning microscope; the contact angle between the surface of anodic aluminum oxide-membrane and deionized water was measured with “drop” methodology. Calcium content was monitored with a conductometer. The content of proteins was determined with photometry (micro Lowry’s method). It was found that contact angle of the surface of anodic aluminum oxide-membrane obtained in electrolyte “oxalic acid-matter with carbon nanodots” and deionized water is 38 degrees. Adding colloidal system of carbon nanodots to the acid electrolyte acts as a hydrophilizer, changes the size of the porous surface: as a result, it is possible to control the porosity of the films. Calcination of anodic aluminum oxide-membrane at 500 degree Celsius lead to expansion and thinning of pore walls. Anodic aluminum oxide-membrane was tested for dialysis process for milk whey separation. The membrane obtained in electrolyte: “oxalic acid-matter with carbon nanodots” showed a greater degree of rejection of protein particles in comparison with a similar membrane obtained in electrolyte of oxalic acid. The advantage of using carbon nanodots in acid electrolyte is the simplicity and environmental friendliness of the synthesis. The approach, which involves the addition of a colloidal system with carbon nanomaterial, allows one to avoid using a strongly acidic electrolyte for obtaining membranes with smaller pores. One of the ways for using of anodic oxide aluminum-membrane is the dialysis of biological fluids, for example, milk whey.