High Voltage Anodization Research Articles

Formation and microstructures of unique nanoporous AAO films fabricated by high voltage anodization

Nanoporous anodic aluminium oxide (AAO) films with a large-range tunable interpore distance (Dint) and unique microstructures have been fabricated by high voltage anodization (140–400 V) in 0.3 M oxalic acid electrolyte. The influences of anodization conditions (e.g., anodization voltage (Ua) and current density (ia)) on the microstructures of AAO films have been investigated in detail. Experimental results show that there is a linear relationship between Ua and Dint under relatively low Ua. With the increase of Ua, the influences of ia and dehydration of aluminium hydroxide on the Dint may not be ignored, thus resulting in a nonlinear relationship between Ua and Dint. When the anodization is performed at an excessively high Ua of 400 V, an interesting competitive growth of nanochannels inside the AAO film tends to occur, thus forming an AAO film with unique nano/micro morphology at the barrier layer side.

Porous Anodic Alumina with Continuously Manipulated Pore/Cell Size

We used polyethyleneglycol (PEG) as a modulator to manipulate pore and cell sizes in the porous anodic alumina (PAA) fabrication. It is shown for the first time that continuous manipulation of the pore size of PAA can be realized. Combined with the coexistent cell-size controlling effect, the morphology and properties of this important nanoscale template and separation membrane can be precisely regulated. The pore size modulation mechanism is proposed on the basis of the morphological and electrochemical results. The presence of PEG in the electrolyte results in a more compacted structure of the barrier layer alumina (BLA), although the barrier layer thickness does not change considerably. In addition, the additive can obviously restrain the chemical dissolution of alumina and shape smaller pores. These two effects combined with the increased viscosity of the electrolyte slow down the ion transportation and diminish the anodization current. Thus, the burning-down phenomena of the aluminum substrates can be avoided at relatively high voltage anodization, and an interpore distance up to 610 nm can be achieved.

Microstructure promoted photosensitization activity of dye-titania/titanium composites

Abstract Well-constructed titania reactive layer/titanium metal–matrix composites were fabricated for photosensitization substrate application by a controlled electrochemical anodization process. A low-voltage anodization at 20 V produced titania nanotube array with 60–70 nm diameter and 540 nm height, while a high-voltage anodization at 180 V resulted in titania multiporous film with 170–260 nm pore size and 4 μm thickness. Two types of dye-titania/titanium composite electrodes were also prepared through a surface adsorption modification by orange G dye. The surface morphologies and interfacial electric properties of heterogeneous materials were examined by microstructure characterization and electrochemical impedance spectroscopy analysis. The significant decrease of interfacial charge-transfer resistance from 12,000 Ω for microporous titania to 5100 Ω for nanotubular titania accordingly contributed a much lower electrode impedance and higher polarization current for dye-titania/titanium electrode. Additionally, 4 and 1.5 times magnification of photocurrent density and photovoltage were achieved under a visible light irradiation for nanotubular dye-titania/titanium electrode in comparison with microporous one. A better photosensitization activity could be well obtained by tailoring microstructure from micro-composite to nano-composite. The corresponding photocurrent response was more significant than photovoltage in the photosensitivity evaluation of these composite electrodes.

High Voltage Pulse Anodization of a NiTi Shape Memory Alloy

In an attempt to improve the corrosion resistance of a NiTi shape memory alloy, thick oxide films were fabricated on the surface by an anodic oxidation process. High-voltage square shape pulse anodization was carried out in various electrolytes over the whole range of pH values from sulfuric acid, acetate buffer, ammonium pentaborate, sodium diphospho heptoxide, sodium aluminate, and potassium hydroxide. Microscopic observation showed that the films are porous or tile structured in nature and the structural analyses by X-ray diffraction and Raman spectroscopy demonstrated their amorphous nature. From the chemical analyses, the oxide films were determined to consist mainly of titanium as the metal component, whereas the nickel content of the oxide films depends on the pH value of the electrolyte as well as the amount of anions incorporated from the electrolyte. The application of an asymmetrical rectangle waveform was effective in increasing the density of the films. An electrochemical technique was used for screening the corrosion resistance of the anodized films and demonstrating the improvement in the resistance against localized corrosion of NiTi.

Combining Atomic Layer Deposition with a Template-Assisted Approach To Fabricate Size-Reduced Nanowire Arrays on Substrates and Their Electrochemical Characterization

We report a combinational method to fabricate well-aligned size-reduced gold nanowire (Au NW) arrays on a Si substrate over large areas (>1.5 cm2). In this method, we employ a SiO2 atomic layer deposition (ALD) technique to reduce the pores of a porous anodic alumina (PAA) template which serves as a template for the electrodeposition of Au NW arrays and is directly integrated on the Si substrate. Unlike conventional method that simultaneously reduces the pore size and the interpore spacing by decreasing the applied potential during PAA anodization, our method allows for independently tuning these parameters. By using the ALD-reduced PAA template, we can fabricate the size-reduced Au NW arrays directly on the Si substrate with the average diameter reduced from ∼79 to ∼33 nm while their nanowire density and interwire spacing remain constant. The ALD technique enables the fine-tuning of the pore size or the nanowire diameter at an angstrom scale. Electrochemical characterization of the size-reduced Au NW arrays as nanoelectrodes is performed. The double-layer charging current (noise of the nanoelectrodes) decreases with the reduction of the nanowire diameter. Our method could be used to fabricate nanowire arrays with large spacing and small diameters via high voltage anodization and subsequent ALD reduction on the PAA template. This type of nanowire arrays might have potential applications in electrochemical and field-emission devices.

High Voltage Anodization of a NiTi Shape Memory Alloy

In order to improve the corrosion resistance of a NiTi shape memory alloy, thick oxide films were fabricated on the surface by an anodic oxidation process. The microscopic observation showed the films had a porous nature and the structural analyses indicated that the films are amorphous. From the chemical analyses, the oxide films mainly contained titanium as metal component, the nickel content of the oxide films depends on pH of the electrolyte and anions could be incorporated from the electrolyte into the films. The asymmetrically rectangle waveform was effective to increase the density of the films. The electrochemical technique showed that the corrosion resistance of the anodised films could improve the resistance against localized corrosion of NiTi.

Preparation and characteristics of well-aligned macroporous films on aluminum by high voltage anodization in mixed acid

Well-aligned macroporous films on aluminum were prepared by high voltage anodization (HVA) in mixed acid electrolyte consisting of oxalic acid, sodium tungstate, phosphoric acid and hypophosphorous acid. The total concentration (about 10 g l − 1 ) of the electrolyte is much lower than that of normal anodization electrolyte. This anodizing process is characterized by a “saddle” shape change of the current density and short anodization period (around 15 min), comparing to conventional anodizing process. The aligned cell arrangement was markedly affected by the peak current density, which was optimized as 6–10 A dm − 2 . The effects of the concentrations of phosphoric acid, oxalic acid and hypophosphorous acid on the morphologies of the films were discussed in detail. The results showed that the pore diameter ranged from 250 to 500 nm, and increased with increasing of the concentration of phosphoric acid, oxalic acid and hypophosphorous acid. The pore geometry could change from round to hexagonal, which was related to the electrolyte components and their concentrations.

Enhancement of the Electrocatalytic Oxidation of Methanol at Pt∕Ru Nanoparticles Immobilized in Different WO[sub 3] Matrices

The importance of morphology and electrochemical characteristics of tungsten oxide as a support for the catalytic platinum/ ruthenium nanoparticles during electro-oxidation of methanol in acid medium is addressed here. Bimetallic Pt/Ru nanoparticles were immobilized at the same loadings in three different WO 3 matrices. Application of high voltages (up to 40 V at 1 V s -1 ) to tungsten foil in H 2 SO 4 solution containing a small amount of NaF (0.15 wt %) resulted in the formation of highly ordered nanoporous WO 3 . Compact tungsten oxide was produced by application of high voltages to tungsten foil in the absence of NaF. Parallel measurements were done with conventional thin films of microporous tungsten oxide electrodeposited on a glassy carbon substrate from the colloidal Na 2 WO 4 solution in H 2 SO 4 . While redox reactions of the structures generated by high voltage anodization were characteristic of poorly hydrated WO 3 (that could be reduced partially to substoichiometric oxides, WO 3-v ), the conventionally electrodeposited microporous structures behaved more like hydrated WO 3 (that could be reduced to nonstoichiometric hydrogen bronzes, H x WO 3 ). Contrary to compact structures (that were blocking Pt/Ru reactivity), both ordered nanoporous and electrodeposited microporous WO 3 matrices, that were more open and characterized by high surface areas, significantly enhanced the electrocatalytic (chronoamperometric, voltammetric) currents for methanol oxidation. Among important advantages of the system produced by assembling Pt/Ru in nanoporous WO 3 are its rigidity, long-term stability, and the ability to oxidize methanol at less positive potentials.

박막 알루미늄을 이용한 규칙적으로 정렬된 나노급 미세기공 어레이 제조기술 개발

An alumina membrane with nano-sized pore array by anodic oxidation using the thin film aluminum deposited on silicon wafer was fabricated. It Is important that the sample prepared by metal deposition method has a flat aluminum surface and a good adhesion between the silicon wafer and the thin film aluminum. The oxidation time was controlled by observation of current variation. While the oxalic acid with 0.2 M was used for low voltage anodization under 100 V, the chromic acid with 0.1 M was used for high voltage anodization over 100 V. The nano-sized pores with diameter of <TEX>$60\~120$</TEX> nm was obtained by low voltage anodization of <TEX>$40\~80$</TEX> V and those of <TEX>$200\~300$</TEX> nm was obtained by high voltage anodization of <TEX>$140\~200$</TEX> V. The pore widening process was employed for obtaining the one-channel with flat surface because the pores of the alumina membrane prepared by the fixed voltage method shows the structure of two-channel with rough surface. Finally, the sample was immersed to the phosphoric acid with 0.1 M concentration to etching the barrier layer.

Osteointegration of titanium and its alloys by anodic spark deposition and other electrochemical techniques: a review.

Direct osteointegration of titanium and titanium alloy implants is one of the main goals of biomaterial research for den- tal and orthopedic applications. Chemical, mechanical or biological treatments have been investigated to obtain a fast and durable implant to bone bonding. In recent years, to improve osteointegration scientists have focussed their attention on the interface intere- actions between the implant surface and the bone. In particular, many efforts have been concentrated on the modification of the thin oxide layer spontaneously formed on the surface of titanium. This stable oxide layer has been considered responsible for the titanium biocompatibility and for the ability of this material to osteointegrate. The stability of the oxide layer in air is well known. However, when titanium is placed in vivo the titanium oxide layer changes its properties because of the electrolytic nature of the body fluids, whose components are able to interact and bind to the metal surface ox- ide. In particular, titanium dioxide exhibits hydroxyl group formation which, in their deprotonated form, can bind calcium thus promoting osteointegration. Therefore, the rate of osteointegration can be modulated tailoring the oxide layer properties before im- plantation. The introduction of new techniques capable of changing the physicochemical and morphological properties of the oxide film can be a step forward towards developing a new generation of highly compatible surfaces suitable to induce a strong and durable bonding to the bone. Recently, an electrochemical technique, discovered in the early 1930s and widely investigated in the 1960s, has been applied to titanium and its alloys to enhance implant osteointegration. This technique, initially employed to produce a corro- sion-resistant thick oxide film on valve metals, is known as Anodic Spark Deposition or Anodic Spark Discharge (ASD). It consists of a high voltage anodization of titanium in electrolyte solutions, whose ions become embedded in the thickened dioxide layer as a consequence of the melting process at the surface induced by the high plasma temperature. The high temperature is generated by ran- domly produced sparks originated during the process by the dielectric breakdown of the thickened oxide layer. By this technique it is possible to obtain relatively thick oxide layers enriched with the electrolytes originally dissolved in the medium and bringing a micro- porous and oxygen-rich surface. These properties can be finely modulated controlling each parameter of the reaction. Several research groups studied ASD and explored its potential, sometimes by coupling this technique with further treatments such as hydrothermal, thermal and chemical treatments, with the goal of obtaining a new generation of biocompatible and osteoinductive or osteoconduc- tive surfaces. The first studies on ASD were performed by Kurze's research group, who paved the way for the first really effective sur- face treatment developed by Ishizawa et al. Ishizawa patented a new method to coat titanium with thydroxyapatite, through the en- richment of titanium oxide layer with calcium and phosphorus ions via ASD process. In this method, after ASD process titanium oxide layer is treated to a hydrothermal process, which introduces on the surface a thin layer of hydroxyapatite crystals. Kokubo et al considered the ASD technique to improve the titanium dioxide bioactivity, and demonstrated the capability of this modified oxide lay- er to enhance calcium-phosphate nucleation after soaking in SBF. Recently, the same authors of this paper functionalized the Ca and P enriched titanium dioxide by a final alkali treatment, achieving a nano-structured surface that proved to exhibit high mineraliz- ing potential and selective adsorption of protein (fibronectin). ASD can now be considered an effective method for the modification of titanium surfaces. By this method a new generation of implantable surfaces can be therefore designed, which may improve the per- formances of orthopedic and dental implants. (Journal of Applied Biomaterials & Biomechanics 2003; 1: 91-107)

Preparation and characteristics of oxide films on AA339.1 cast aluminum

Ceramic films on AA339.1 cast aluminum alloys (ANSI) were prepared by high-voltage anodization (HVA) in a mixed electrolyte of glutaric acid, sodium tungstate and phosphoric acid. The characteristics of the films and the effect of various factors, such as peak current density, composition and concentration of the electrolytes, on the properties of the films were investigated in detail. In addition, the structure, chemical composition and morphology of the films were investigated by electron diffraction spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the elements in the film are O, Al, Si, P, S, C and that the structure of the film is non-crystalline. Compared with conventional anodic film on aluminum, the films on AA339.1 show good ornamental property and high microhardness. SEM results indicate that the film appears to be divided into loose and compact layers. Many nano-gaps or cracks are observed in the compact layer, which may be a result of a different film formation mechanism from that of conventional anodic films.

Microporosity of dense anodic Al2O3 films

Dense Al2O3 films ≃ 2.5 and 5 μm thick produced by high-voltage and two-step anodization were studied by electron microscopy and small-angle x-ray scattering. The volume microporosity of the films and the mean radius of inertia of pores were evaluated. The results indicate that the densification of porous films in the second step of anodization is a result of two concurrent processes: dense oxide growth and dissolution of the primary porous Al2O3 layer.

Preparation and analysis of films on aluminium by high voltage anodization in phosphoric acid and sodium tungstate solution

A high voltage anodic film on aluminium was prepared in a mixed electrolyte of phosphoric acid and sodium tungstate. The properties, structure, morphology and chemical composition of the film were investigated. It was found that the elements in the film were O, Al, P and W and the main chemical compositions of the film were aluminium oxides with some phosphates and tungstates. Compared with conventional anodic films, the high voltage anodic film showed good hardness and excellent corrosion and heat resistance. Scanning electron microscopy indicated that the film could be divided into two layers, a porous surface layer and a compact underlayer. Many cracks were observed in both layers, which are significantly different from those of conventional anodic films.

Anodizing of cast aluminium: Vollmer, H. Foundry Trade J. Mar. 1991 165, (3428), 164–165

Ceramic films on AA339.1 cast aluminum alloys (ANSI) were prepared by high-voltage anodization (HVA) in a mixed electrolyte of glutaric acid, sodium tungstate and phosphoric acid. The characteristics of the films and the effect of various factors, such as peak current density, composition and concentration of the electrolytes, on the properties of the films were investigated in detail. In addition, the structure, chemical composition and morphology of the films were investigated by electron diffraction spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the elements in the film are O, Al, Si, P, S, C and that the structure of the film is non-crystalline. Compared with conventional anodic film on aluminum, the films on AA339.1 show good ornamental property and high microhardness. SEM results indicate that the film appears to be divided into loose and compact layers. Many nano-gaps or cracks are observed in the compact layer, which may be a result of a different film formation mechanism from that of conventional anodic films.

The photoactivity of porous Tio 2 anodized at a high voltage

The photoactivity of TiO 2 produced by high-voltage anodization as described by Marchenoir et al. was compared to single-crystal TiO 2. The anodic oxide film was found to be highly porous. The photocurrent and quantum yield of the anodic sample is markedly larger on the long-wavelength side of the action spectrum as compared to sinle-crystal rutile. We ascribe this behavior either to the formation of anatase “pockets” in the film or to the incomplete oxidation of Ti during the anodization process.