Porous Anodic Alumina Films Research Articles

Structure and Optical Properties of thin Porous Anodic Alumina Films Synthesized on a Glass Surface

The structure and luminescent properties of thin nanoporous aluminum oxide films obtained by anodization of aluminum films thermally deposited on glass have been investigated. The pore size and the interpore distance depend on the anodization voltage. For all studied samples the highest emission intensity obtained at the excitation wavelength equal to 330 nm. This behavior of luminescence curves caused by defect F+ luminescent centers (O− oxygen vacancies). The presence of porous alumina films on the glass surface increases the optical absorption in the visible light region. The oscillations on the spectra are caused by Fabry-Perot interference on the anodic alumina oxide film/glass interface. The suggested technique can be used for obtaining porous aluminum oxide films on other substrates, including Indium-Tin-Oxide, and can be applied in the technology of light-emitting devices and infrared-visible-ultraviolet detectors.

Morphology of anodic alumina films obtained by hard anodization: Influence of the rate of anodization voltage increase

The influence of the anodization voltage ramp on the morphology and thickness homogeneity of porous anodic alumina films was studied. The samples were prepared in the oxalic acid at 120 V during the hard anodization process. As a nondestructive characterization method, the smallangle Xray scattering technique was used. The analysis of diffraction patterns allows determining the mean value and dispersion of interpore distance and the channel tortuosity with high accuracy. The increase of voltage ramp at the initial stage of hard anodization process was shown to lead to reduction of mechanical deformation (tortuosity) of anodic alumina film during crystallization.

Nanomechanical characterization of porous anodic aluminum oxide films by nanoindentation

Nanostructured thin films have gained great interests in recent years due to their significantly enhanced properties and great potential for various applications. Nanoindentation techniques are commonly used to measure nanomechanical properties of thin films and the surface layers of bulk materials. In this article, nanoindentation tests coupled with computer modeling were proposed to characterize nanostructured porous anodic aluminum oxide (AAO) films grown on glass. A three-dimensional nanoindentation model was developed. The Young's modulus and hardness were measured by nanoindentation using a Berkowich pyramidal sharp tip indenter. The effects of the substrate and porous structure of the film on the coating measurements were investigated. The dilemma for extracting the mechanical properties of a porous structural film in nanoindentation has been pointed out, and an alternative approach of combined modeling/experimental provided a justification by considering the porous structure of the film and minimizing the influence of the substrate. Computer modeling results were validated by experimental data.

Fabrication of self-ordered nanoporous alumina with 500–750 nm interpore distances using hard anodization in phosphoric/oxalic acid mixtures

A hard anodization (HA) technique is employed using different mixtures of phosphoric/oxalic acid for fast fabrication of alumina nanopore arrays in voltages higher than 200 V. The mixtures enable to avoid the breakdown of porous anodic alumina (PAA) in the high voltages. It is revealed for the first time that continuously tunable pore intervals (Dint) from 500 to 750 nm can be controlled by varying the concentrations of oxalic acid at anodization voltages (Uanod) from 230 to 360 V, far beyond the Uanod in the single electrolyte of phosphoric acid or oxalic acid. The ratios of interpore distance, pore diameter and barrier layer thickness to anodization voltage are in the range of conventional HA process for each acid mixture. In this approach, the PAA film growth rate is 26 µm/h, being 7 times larger than that in typical mild anodization.

Effects of the voltage and time of anodization on modulation of the pore dimensions of AAO films for nanomaterials synthesis

Highly-ordered and hexagonal-shaped nanoporous anodic aluminum oxide (AAO) of 1 μm thickness of Al pre-deposited onto Si substrate using two-step anodization was successfully fabricated. The growth mechanism of the porous AAO film was investigated by anodization current-time behavior for different anodizing voltages and by visualizing the microstructural procedure of the fabrication of AAO film by two-step anodization using cross-sectional and top view of FESEM imaging. Optimum conditions of the process variables such as annealing time of the as-deposited Al thin film and pore widening time of porous AAO film were experimentally determined to obtain AAO films with uniformly distributed and vertically aligned porous microstructure. Pores with diameter ranging from 50 nm to 110 nm and thicknesses between 250 nm and 1400 nm, were obtained by controlling two main influential anodization parameters: the anodizing voltage and time of the second-step anodization. X-ray diffraction analysis reveals amorphous-to-crystalline phase transformation after annealing at temperatures above 800 °C. AFM images show optimum ordering of the porous AAO film anodized under low voltage condition. AAO films may be exploited as templates with desired size distribution for the fabrication of CuO nanorod arrays. Such nanostructured materials exhibit unique properties and hold high potential for nanotechnology devices.

Optical Fiber Fabry–Perot Humidity Sensor Based on Porous Al2O3 Film

An optical fiber humidity sensor with porous anodic alumina (PAA) film on single-mode fiber tip is proposed and experimentally demonstrated. Relative-humidity (RH) sensing is correlated with the shift of interference fringe due to capillary condensation, which results in the change of effective refractive index of PAA film when exposed to different RH environments. Experimental result shows that sensitivity of 0.31 nm/%RH can be realized with very good repeatability in range of 20%RH–90%RH. It can be concluded that fiber-optic RH sensors with PAA film of thinner thickness, smaller aperture size, and aspect ratio would present promising sensing performance.

Modelling the growth process of porous aluminum oxide film during anodization

Currently it has become important for the development of metamaterials and nanotechnology to obtain regular self-assembled structures. One such structure is porous anodic alumina film that consists of hexagonally packed cylindrical pores. In this work we consider the anodization process, our model takes into account the influence of layers of aluminum and electrolyte on the rate of growth of aluminum oxide, as well as the effect of surface diffusion. In present work we consider those effects. And as a result of our model we obtain the minimum distance between centers of alumina pores in the beginning of anodizing process.

Two-dimensional porous anodic alumina for optoelectronics and photocatalytic application

Fabrication of porous anodic alumina film structures using anodizing, sol-gel synthesis and photolithography is reported. The structures receive interest as planar waveguides due to strong photoluminescence of the embedded trivalent lanthanides. Mesoporous structures comprising sol-gel derived titania in porous anodic alumina play a role of effective catalyst for water purification.

A new self-ordering regime for fast production of long-range ordered porous anodic aluminum oxide films

Porous anodic aluminum oxide (AAO) films with large interpore distance (Dint, 307–415nm) have been fabricated by using a new self-ordering regime in oxalic acid based electrolyte systems. The new regime corresponds to the stage with a stable voltage during constant-current anodization process, accompanied by high anodization voltage (Ua, 135–183V) and current density (Ja, 300–500 A m–2) which facilitate the high speed growth of long-range ordered porous AAO films. In addition, the formation mechanism of the self-ordering regime, together with the influence of anodization conditions on the microstructural parameters of AAO films have been investigated in detail. Experimental results showed that the value of the stable Ua during constant-current anodization can be adjusted by varying the anodization conditions. Accordingly, the self-ordering window of the as-prepared porous AAO films can be modulated, widening the regulating range of Dint to a large extent.

Multidimensional Helical Nanostructures in Multiscale Nanochannels.

We have investigated the various morphological changes of helical nanofilament (HNF; B4) phases in multiscale nanochannels made of porous anodic aluminum oxide (AAO) film. Single or multihelical structures could be manipulated depending on the AAO pore size and the higher-temperature phase of each molecule. Furthermore, the nanostructures of HNFs affected by the chemical affinity between the molecule and surface were drastically controlled in surface-modified nanochannels. These well-controlled hierarchical helical structures that have multidimensions can be a promising tool for the manipulation of chiral pores or the nonlinear optical applications.

Opaque White Anodic Oxide Film Formed on Aluminum

Anodic porous alumina film, which is formed by anodizing of aluminum in an appropriate acidic solution, has been studied on the protection or design of aluminum surfaces owing to commercial applications. The appearance and surface quality of aluminum products (e.g., architectural materials, electronic devices) are key points as with surface properties such as a corrosion resistance, hardness and wear resistance. Although dyeing, painting and electrolytic coloring are well known as industrial methods to provide color variation for aluminum surface, integral coloring, which produces color by anodizing step itself without use any pigment or metallic salt, is also applied in terms of the simplicity and production cost. In our previous work, it was reported that alumina films with irregular porous structure formed in Ematal bath or chromic acid showed opaque white appearance [1, 2]. Irregular porous structure in the direction of pore depth, which contained branching, curving and distortion of pores in anodic alumina, produced the roughening of the metal-oxide interface and made the appearance of the film more opaque [1, 2]. However, Ematal bath containing expensive potassium titanium oxalate deteriorates easily. In terms of reduction of environmental burdens, it is required to avoid using harmful chemical compound such as chromic acid. Under such background, we studied a low-cost alternative anodizing method to form opaque white oxide film. In addition, we also investigated in detail the relation between the appearance and porous structure of oxide film. Anodizing was conducted using high purity (99.99 %) aluminum sheets in various electrolytes (phosphoric acid, sulfuric acid, critic acid, and Ematal bath). Whiteness of the obtained samples was evaluated by spectrophotometry (KONICA MINOLTA, CM-5) based on standard of American Society for Testing Materials (ASTM E313-73). When anodizing was carried out in phosphoric acid at 60 °C at a constant current density of 200 Am−2, anodized specimen indicated whiteness value of 39. Surface appearance was gray in color. The aluminum specimen anodized in Ematal bath showed whitish appearance compared with that treated in phosphoric acid. Whiteness value of aluminum specimens anodized in Ematal bath increased from 35 to 47 with increasing final arrival voltage from 150 V to 350 V during constant current anodizing. On the other hand, whiteness value increased effectively using two-step anodizing; long first anodizing and short second anodizing were conducted in phosphoric acid and subsequently Ematal bath. Under optimized conditions, whiteness value increased to 58. The more increase of the thickness of total oxide film and barrier layer and the degree of branching, the more white appearance indicated. Thus, it is confirmed that the appearance of anodized specimen was strongly affected by these factors. [1] S. Ono, S. Chiaki, T. Sato; J. Met. Finish. Soc. Jpn., 26, (10), 456-460 (1975). [2] S. Ono, T. Sato; J. Met. Finish. Soc. Jpn., 31, (3), 134-139 (1980).

Characterization of Porous Anodic Aluminum Oxide Films by Luminescence Methods - A Review

This review is a summary of applications of photoluminescence and galvanoluminescence techniques used to determine important characteristics of the porous anodic aluminum oxide films. Both photoluminescence and galvanoluminescence of porous anodic oxide films formed on highly reflective aluminum surfaces in either organic or inorganic electrolytes feature clearly pronounced interference maxima that can be used as a tool for determining oxide film thickness and inherent optical parameters. Following this finding, two methods (based on a particular observation angle) for determining such properties are developed and presented. Developed methods can be used for either galvanoluminescence or photoluminescence measurements, however only galvanoluminescence technique can be used to estimate the thickness of porous oxide films on aluminum during the anodization. Keywords: Aluminum, alumina, anodization, galvanoluminescence, photoluminescence, porous anodic aluminum oxide film.

Comparative study of porous anodic alumina: effects of aluminium deposition methods

This article reports on the comparative study of the fabrication of porous anodic alumina films by anodisation of the aluminium films on glass substrates which were deposited by direct current sputtering and electron beam evaporation methods. The relationship between surface morphology of the deposited aluminum films and porous anodic alumina films was investigated. A more uniform and ordered porous anodic alumina was obtained by fabricating from electron beam evaporation deposited aluminium film with smaller and compact grains. Two-step anodisation was used to further improve the quality of porous anodic alumina compared to one-step anodisation. The optical transmittance spectra within wavelength of 370–800 nm were obtained and the optical properties were studied.

A novel method for fabricating self-ordered porous anodic alumina with wide interpore distance using phosphoric/oxalic acid mixed electrolyte

A novel method for fabricating highly ordered porous anodic alumina (PAA) with continuously tunable interpore distance using phosphoric/oxalic acid mixed electrolyte was presented. PAA films with different pore diameters and interpore distances were fabricated by changing anodizing potential and concentration of oxalic acid, characterized by field emission scanning electron microscope (FESEM). Experimental results showed oxalic acid in mixed electrolyte could not only suppress the breakdown of PAA at high potential but also enlarge the continuously tunable interpore distance from 300 to 500nm. The proportionality constant between interpore distance and anodizing potential is 2.48 at the potential from 180 to 200V, 2.31 at the potential from 130 to 170V. The optimal content of oxalic acid and potential has a second power function relationship. Our results could extend the applications of PAA templates in ordered nano-material fabrication field.

Highly Ordered Porous Anodic Alumina with Large Diameter Pores Fabricated by an Improved Two-Step Anodization Approach.

The aim of this study is to prepare highly ordered porous anodic alumina (PAA) with large pore sizes (> 200 nm) by an improved two-step anodization approach which combines the first hard anodization in oxalic acid-water-ethanol system and second mild anodization in phosphoric acid-water-ethanol system. The surface morphology and elemental composition of PAA are characterized by field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectrometer (EDS). The effects of matching of two-step anodizing voltages on the regularity of pore arrangement is evaluated and discussed. Moreover, the pore formation mechanism is also discussed. The results show that the nanopore arrays on all the PAA samples are in a highly regular arrangement and the pore size is adjustable in the range of 200-300 nm. EDS analysis suggests that the main elements of the as-prepared PAA are oxygen, aluminum and a small amount of phosphorus. Furthermore, the voltage in the first anodization must match well with that in the second anodization, which has significant influence on the PAA regularity. The addition of ethanol to the electrolytes effectively accelerates the diffusion of the heat that evolves from the sample, and decreases the steady current to keep the steady growth of PAA film. The improved two-step anodization approach in this study breaks through the restriction of small pore size in oxalic acid and overcomes the drawbacks of irregular pore morphology in phosphoric acid, and is an efficient way to fabricate large diameter ordered PAA.

Effect of pore depth on tribological behavior of anodic alumina films under nano-thin film lubrication

Abstract A number of parameters such as the dimple depth, dimple area density, and dimple shape, affect the tribological performance of dimpled surfaces, and a comprehensive tribological characterization of the depth-dependent nature of their material properties would be useful. However, only surfaces with dimples with depth-to-diameter ratios lower than 1 have been investigated. In this study, porous anodic alumina (PAA) films with high pore depth (PDEP)-to-pore diameter (PDIA) ratios (approximately 2.8–30) were prepared. The as-prepared films had the same PDIA value (50 nm) but different PDEP values (140 nm, 350 nm, 700 nm, 1100 nm, and 1500 nm). Perfluoropolyether was used as the lubricant to improve the tribological properties of the as-prepared films. The effects of the PDEP value on the tribological properties of the as-prepared films were investigated systematically. It was found that the antiwear performance of the PAA films was closely related to the combined effects of flexibility, mobility, and inherent lubricity of PFPE molecules and PDEP values in nanoporous structures as a reservoir for lubricants and wear particles. For PDEP-to-PDIA ratios greater than 1, under the conditions studied, the friction performance of the PAA films improved with an increase in the PDEP value. Further, after a critical PDEP value, the applied load had little effect on the friction coefficient and antiwear life of the films. The results of this study should aid the fabrication of stable films for industrial applications and act as a bridge between science and engineering on the micro/nanoscale.

COMPARISON AND CONSTRUCTION OF "MILD" AND "HARD" ANODISATION REACTORS FOR THE SYNTHESIS OF POROUS ALUMINA

The mild anodization (MA) reactor is exemplified for its operational simplicity and its excellent control over the experimental parameters that are involved in the anodization process. This method provides porous anodic alumina films with a regular cell-arrangement structure. This offers a better cost-benefit ratio than the other equipment configurations that are used to fabricate nanoporous structures (i.e., ion beam lithography). Conversely, the hard anodization (HA) reactor produces oxides at a rate that is 25 to 35 times faster than the MA reactor. The produced oxides also have greater layer thicknesses and interpore distance, and with a uniform nanopore spatial order (> 1000). In contrast to MA reactors, the construction of an HA reactor requires special components to maintain anodisation at a high potential regime. Herein, we describe and compare both reactors from a technical viewpoint.

A Facile Method for Controllable Fabrication of Porous Anodic Aluminum Oxide Templates with Multiple-Scale Structures

Porous anodic aluminum oxide (AAO) templates with diverse nano/submicron/micron multiple-scale structures have been fabricated by a facile and controllable two-step anodization process. The first step anodization has been achieved by using a simple competitive growth process with high anodization voltage (Ua) in ethanol-oxalic acid mixture electrolytes. The effects of Ua (300–600 V), anodization current density (Ja, 100–200 A m−2), and the proportion of ethanol adding (0.3 M oxalic acid : ethanol = 1 : 1, 3 : 2, V/V) on the submicron/micron structures of AAO templates have been investigated in detail. The second step anodization has been conducted under relatively lower Ua of 30 V and 40 V, respectively. The effects of Ua, etching process, and anodization time on the nanostructures of as-fabricated porous AAO templates have been studied carefully. In addition, related intrinsic formation mechanisms have also been investigated. Based on these quantitative experimental results, the fabrication of multiple-scale porous AAO films with diverse nano/submicron/micron structures can be more controllable and designable, facilitating their practical applications in simple template synthesis of complex multiple-scale functional materials.

A Facile Method for Improving the Process of Porous Anodic Aluminum Oxide Film Preparation

A new and facile method for improving the procedure of anodic aluminum oxide (AAO) film preparation was presented, which was based on an electrochemical detachment procedure. The detachment and pore-opening were performed just through one step in a solution of HClO4and C2H5OH at a voltage of 40V. The as-obtained AAO film was characterized by scanning electron microscopy (SEM) and scanning probing microscopy (SPM). The process of detachment and pore-opening was also discussed.

Controllable Fabrication of Networked Three-Dimensional Nanoporous Anodic Alumina Films on Low-Purity Al Materials

Networked three-dimensional (3D) nanoporous alumina nanostructures—consisting of vertical cylindrical stem pore arrays (170–310 nm in diameter) and periodical transverse branched pores (20–80 nm in diameter) interspaced regularly by 190–220 nm across the vertical pore walls—were controllably fabricated from low purity Al materials (99.0%, 99.3%, and 99.56%) by anodization in a phosphoric acidic solution at 110–190 V. The formation of transverse pores depended predominantly on the purity of the Al base materials and the corrosion resistance of the anodic porous alumina films, which can be mainly attributed to anodic and/or chemical dissolution of impurities such as Fe, Cu, Zn, Mg, and Mn incorporated in the Al base materials. Moreover, the effects of the purity of Al materials on the growth and morphology of porous alumina films in oxalic and sulfuric acid solutions were also investigated by a two-step anodization of Al sheets with different purities, namely 99.0%, 99.3%, 99.56%, and 99.999% Al. The alumina films grew more slowly in all electrolytes with decreasing Al purity, which can be attributed to the lower corrosion resistance of the PAA films containing small quantities of Fe oxides and/or hydroxides produced during anodization.