Formation Of Porous Alumina Research Articles

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.

Structural Characterization of Anodic Porous Alumina Formed By Galvanostatic Anodizing in Etidronic Acid

Anodizing aluminum in etidronic acid (1-hydroxyethane-1,1-diphosphonic acid) causes the formation of porous alumina at higher voltage more than 200 V. Highly ordered porous alumina with a large-scale cell can be easily fabricated via two-step constant voltage anodizing in etidronic acid. However, extremely little has been reported on the growth behavior during galvanostatic anodizing in etidronic acid and its structural characterization. In the present investigation, we demonstrate the details of the anodizing behaviors and nanostructures of the porous alumina formed via galvanostatic anodizing in etidronic acid under various operating conditions. High-purity aluminum plates were electrochemically polished, and then were anodized at a constant current density of 0.005-500 Am-2 in 0.03-3 M etidronic acid solutions (273-333 K). After anodizing, the specimens were immersed in a 0.20 M CrO3/0.51 M H3PO4 solution (353 K) to dissolve the porous alumina, and the aluminum dimple array was exposed to the surface. The surface and cross-section of the anodized specimens were examined by field-emission scanning electron microscopy (FE-SEM). The cell size (interpore distance) of the porous alumina was calculated using image analysis software. As the electropolished aluminum specimens were anodized galvanostatically under various conditions, the voltage linearly increased, gradually decreased, and then reached plateau value (U p). However, excess current density led to a burning phenomenon and the formation of non-uniform porous alumina. Figure 1 shows the changes in the plateau voltage, U p, and the current density, i a, during anodizing in a 0.3 M etidronic acid solution. The plateau voltage greatly increased with the current density at each temperature, and various porous alumina films were successfully fabricated by anodizing at a wide voltage range measuring 7-218 V. Figure 2 shows the relationship between the cell size, D c and the anodizing voltage, U a. The cell size linearly increased with the anodizing voltage at each solution temperature and current density. A linear proportional constant of 2.5 nmV-1 was obtained, which is similar to the self-ordering condition via constant voltage anodizing. Figure 1

Investigation of the Ordering of Porous Anodic Alumina Formed by Anodization of Aluminum in Selenic Acid

The formation of porous alumina by galvanostatic anodization in selenic acid has been investigated. The influence of the electrolyte concentration on the morphological features and the self-ordering behavior of the porous oxide film has been studied. The best pore arrangement is obtained upon anodization in 0.5 M H2SeO4. Using chronopotentiometry, gravimetric analysis and FESEM it is shown that the highest degree of pore ordering is caused by an optimal combination of the two mechanisms that lead to pore initiation: mechanical instability and field-assisted dissolution. In addition, by means of Raman spectroscopy it is found that the optical properties of porous alumina anodized in selenic acid do not exhibit parasite effects due to electrolyte incorporation during anodization. Hence, these templates are of great interest to fabricate and investigate functional nanostructures for optoelectronics.

Features of the formation of porous alumina mask for local plasma etching of semiconductors

The features of formation of porous alumina masks for silicon nanoprofiling were investigated. The influence of micromorphology on the property of the local etching of silicon on the interface of an anodized double-layer structure of Al-Ti was shown. The correlation between the aspect ratio of alumina pores and the geometrical parameters of the porous silicon was investigated.

Synthesis of porous oxide ceramics using a soft responsive scaffold

Ceramic materials have significant utility. Developing synthetic protocols that are facile and provide low energy alternatives to traditional methods remains a major driver in materials synthesis. We present here the adaptation of a method recently developed in our group for the synthesis of porous silica using a non-ionic emulsion template. The silicate materials are porous on both the nanometre and micrometre length scales and surface-to-volume ratios may be readily modified by altering the volume fraction of the emulsion template. Switching the silica precursor for an alumina or titania precursor resulted in the formation of porous alumina and titania materials which were prepared as thin films or monoliths. The pores formed in the amorphous alumina materials were ~0.8 μm and ~50 nm, with a primary particle size of 50–100 nm. The titania materials had pores on one length scale only: ~0.8 μm, with a smaller primary particle size of 20–60 nm. As-synthesized materials were investigated using scanning electron microscopy and X-ray diffraction.

Process Parameter Control for the Formation of Porous Silicon through Nanoporous Alumina Using Pulsed Anodization

This paper discusses the formation of porous silicon through porous alumina using pulsed anodization for various parameters of anodization, type of acid for the formation of porous alumina, and their characterization using photoluminescence. The porous alumina has been synthesized using sulfuric, oxalic and phosphoric acids and luminescent porous silicon through the porous alumina has been obtained in each case. The porous silicon through porous alumina formation has also been investigated using pulsed anodization. The duty cycle and frequency of the square wave pulse have been varied and the optical properties of the porous silicon, so formed, through the porous alumina, have been investigated.

An Effect of Transfer Velocity on Formation of Porous Alumina during Unidirectional Solidification

An Effect of Transfer Velocity on Formation of Porous Alumina during Unidirectional Solidification

Effect of Silica on Formation of Porous Alumina during Unidirectional Solidification in Hydrogen Atmosphere

A porous alumina with cylindrical pores was fabricated by unidirectional solidification in pressurized H2 or H2-Ar mixed gases using alumina feed rod with 15%SiO2-Al2O3 composition. The effect of silica additive on the formation of pores during the solidification was investigated. The porosity of the samples increases with increasing hydrogen partial pressure under a fixed total pressure and decreases with increasing total pressure under a fixed hydrogen partial pressure. No cylindrical pores are formed under 10%H2-90%Ar atmosphere. On the other hand, when the solidification is performed under 50%H2-50%Ar or 100%H2 atmosphere, the porosity and pore size for porous alumina fabricated using 15%SiO2-Al2O3 feed rod are larger than those for the samples fabricated using 99.99%Al2O3 feed rod. Many small facet shape pores are formed due to vaporization of silica component on the cooling step during the solidification. When a solidification is performed under 10%H2-90%Ar atmosphere using 15%SiO2-Al2O3 feed rod, an excess hydrogen atom in the solid phase is lack to make the cylindrical pores due to decreasing the hydrogen solubility in solid phases by silica addition then, non-porous alumina is formed. As a result, the porosity for porous alumina fabricated using 15%SiO2-Al2O3 feed rod is not proportional to square root of hydrogen partial pressure under a fixed total pressure.

Preparation and Characterization of Ca-Containing Anodic Alumina

Ca-containing anodic alumina (CAA) has been successfully prepared by anodizing Al film in an alkali solution at a constant voltage and subsequently electro-depositing calcium salts on and into anodic alumina. This paper investigated the anodic behavior of Al, deposition behavior of calcium salt, and microstructure of CAA. The results show that the anodic behavior of Al in Na3PO4 electrolyte can be described as three continuous stages as of initial growth of the compact barrier layer, formation of porous alumina and further development of its pores and columnar holes. It is also found that cell voltage of electro-deposition process plays an important role at the deposition behavior of calcium slat and the microstructure of Ca-containing anodic alumina (CAA). The higher the cell voltage is, the faster the deposition rate, and the more calcium being deposited at the surface of anodic alumina and into the columnar holes or at the walls of the holes of anodic alumina. It is expected that Ca-containing anodic alumina films are promising substrates for fabricating functional bio-coatings for prosthetic applications.

In Situ Real-Time Infrared Spectroscopy Study of Formation of Porous Anodic Alumina on Si

We have investigated in situ and in real-time the formation process of anodic porous alumina on silicon using infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). We have collected IR absorption spectra of the interface between a porous alumina film and a Si substrate to elucidate the chemical processes involved in the porous-alumina formation. On the basis of the detailed analysis of IRAS spectra together with the variation of the anodic current density, we demonstrate that the chemical process at the alumina–Si interface proceeds as follows: As the pore reaches the alumina–Si interfacial region, inhomogeneous etching takes place. It produces small holes or cracks in the “barrier layer” at the bottom of the alumina pore, and through those holes or cracks the electrolyte pours into the vicinity of the Si substrate surface to form nanodots. With further anodization, oxidation of the Si substrate surface occurs and finally peels the porous alumina film off the Si substrate. IRAS data are consistent with the results of observation by a field emission scanning electron microscope. The present results show that MIR-IRAS is a powerful tool for in situ monitoring chemical processes at semiconductor–solution interfaces.

Anodization of nanoimprinted titanium: a comparison with formation of porous alumina

Electrochemically prepared ordered porous alumina has become one of the most important nanotemplate materials to date. Naturally the questions arises whether other valve metals such as Ti, Ta, Nb, V, Hf or W can also be used for fabricating ordered pore arrays. We investigate in detail the electrochemical anodization of titanium in different electrolytes and its potential and temperature dependence. It turns out that due to the semiconducting properties of titania, a mirror image of the behavior of the electrically insulating porous alumina seems to be impossible. So-called porous titania in literature corresponds to the pitting regime of aluminum where pores are created due to dielectric breakdown of titania or alumina, respectively. Below the breakdown potential of titania, only thick barrier layers can be obtained. However, by nanoimprint of titanium and successive anodization below the breakdown potential, monodomain porous titanium oxide with a pore depth of 60nm on a cm2-scale can be prepared. We discuss in detail the growth mechanism of porous structures of titanium and compare it with that of porous alumina.

Formation of porous alumina with oriented pores

Adopting the theories describing the bubble forming in the metal–hydrogen solidification process, porous alumina with oriented pores was prepared by combining a foaming method with sol-gel technology. The bubble forming process in the sol-gel is different from that in the metal–hydrogen system. Samples were calcined at temperature from 800 to 1200C. The volume-shrinkage and compressive-strength increased with increasing calcination temperature. The porous alumina exhibited a bimodal pore structure when prepared below 1200°C.