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The effects of the size and position of an aluminum bipolar electrode (BPE) on the uniformity of formation of anodic porous alumina in an alternating current electric field were investigated. Anodized specimens were dyed, and the resistance was measured after the specimens were anodized again. Phenomena observed during film formation indicated that the BPEs had unique potential distributions that strongly depended on their length and width. The color variations and electrical resistance of the BPEs were symmetrical and varied from the centers of the BPEs to their ends. When multiple BPEs were processed at the same time, their position in the non-uniform electric field was demonstrated to be an important factor for controlling the uniformity of film formation. The best results were obtained when the BPE was placed at the center of the defined space.

The effect of the addition of alcohol to a Na3PO4-based electrolyte on plasma electrolytic oxidation (PEO) of AZ31B magnesium alloy was investigated. Anodization with spark discharge was conducted in Na3PO4-based electrolyte containing various alcohols (e.g., ethanol, ethylene glycol, and glycerol) at a constant current density of 200 A m−2 and a constant temperature of 25 °C. Voltage–time curves during the PEO process, the film structure, surface roughness, crystallographic structure, composition, corrosion resistance, and withstand voltage were investigated using various analytical equipment and electrochemical measurements. When the electrolyte containing alcohol was used, the initial bending voltage was higher than that observed using the basic electrolyte without alcohol addition, as was the oscillation voltage during the PEO process. For a given amount of electricity supplied, the addition of alcohol into the basic electrolyte tended to increase the thickness and corrosion resistance of PEO films formed while effectively reducing surface roughness. In particular, the addition of a polyhydric alcohol (i.e., ethylene glycol and glycerol) could act not only as a leveler for the formation of compact film but also as an enhancer for film qualities, such as corrosion resistance and withstand voltage. The patterns observed for Na3PO4-based electrolyte containing alcohol also hold for Na2SiO3-based electrolyte containing alcohol.

A porous alumina film decorated with platinum nanoparticles was formed on an aluminum substrate using one-pot-based on bipolar electrochemistry under an alternating current (AC) electric field. Most previous approaches of the fabrication of metal/porous alumina composites required at least two steps: the formation of a porous alumina film based on an anodic reaction (i.e., anodization) and the subsequent deposition of metal onto porous alumina based on a cathodic reaction (i.e., electrodeposition), and two types of electrolytes for anodization and electrodeposition, respectively. In this study, a Pt/porous alumina composite film was formed on an unconnected aluminum substrate based on a well-balanced redox reaction in a single electrolyte of phosphoric acid under an AC electric field. When the AC electric field was applied between Pt driving electrodes, dissolution of the Pt electrode and deposition of Pt on both sides of the unconnected aluminum electrode continuously proceeded in parallel with the forma...

In this study, the electrical potential distributions on aluminum bipolar electrodes (BPEs) subjected to bipolar anodization in a direct current (DC) electric field were investigated. Aluminum sheets were used as BPEs, suspended in an electrolyte solution, and positioned either vertically or horizontally at the center of a cell. The results of reflective interference measurements, depth-profiling analysis, and re-anodization clearly revealed that the thicknesses of the porous alumina films and barrier layers near the aluminum substrates can be taken to represent the potential distributions on the horizontally located BPEs; the thickest anodic alumina was formed at the edge of the anodic pole of the BPE, and its thickness continuously decreased in the direction of the cathodic pole. Furthermore, the area of the anodic reaction was much larger than expected, accounting for 90% of the BPE.

To fabricate a freestanding through-hole Au membrane using an anodic porous alumina template, Au was deposited on the outermost surface of an anodic film followed by the removal of the template. Alumina templates with different dimensions (e.g. diameters and number of pores) were prepared by two-step anodization in the range of 40-80 V and pore-widening. The Au thin films were deposited onto alumina templates with well-controlled surface morphologies by sputter deposition using a commercially available ion sputter coater. After the removal of the alumina template, a variety of Au membranes with nanoholes, nanotubes, or branched pores were obtained, which reflect the morphology of the alumina template. When the sputtered Au penetrates the pores of the alumina film, Au nanotube arrays with an aspect ratio of ∼3 can be fabricated. The present method is much simpler than the traditional template process involving multi-step replication because there is no need to separate the alumina template from the aluminum substrate before Au deposition.

Nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated by optimizing the conditions for anodizing subsequent detachment by modified anodic polarization and through-hole methods and programmed heat treatment in addition to ceramic plates used as the restriction load. Even with the 23% volume shrinkage resulting from the change in density associated with the transformation from the amorphous state to α-alumina, flat α-alumina membranes, each with a diameter of 25 mm and a thickness of 50 μm, were obtained without thermal deformation. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure. Furthermore, when used for filtration, the α-alumina membrane exhibited a level of flux higher than that of commercial ceramic membrane.

We prepared a through-hole α-alumina membrane with a hierarchical porous structure from an anodic porous alumina membrane prepared using phosphoric acid electrolyte as the starting material. By heating the anodic porous alumina membrane to 1400 °C, aluminum phosphate nanoparticles were segregated in the α-alumina matrix at the anion-incorporated outer layer and the high-purity alumina layer forming a cell boundary band sintered as a high-density α-alumina layer at the central core of the pore wall. When the heat-treated membrane was immersed in concentrated hydrochloric acid, a unique hierarchical porous α-alumina membrane structure was formed with straight macropores and mesopores inside the pore wall due to the dissolution of aluminum phosphate nanoparticles. The developed α-alumina membrane can be ultimately used as a multifunctional filter because of its unique hierarchical porous structure and extremely high chemical and thermal durability.

The thickness and corrosion resistance of films formed on AZ31 magnesium alloys through plasma electrolytic oxidation for the same amount of electricity supplied increased with decreasing electrolyte concentration irrespective of the current density. For all electrolytes of phosphate, silicate, and aluminate, the initial bending/breakdown voltage increased linearly with the logarithm of the decreasing electrolyte concentration and exhibited no dependence on the current density. The chemical composition of the films was independent of all electrolysis conditions such as electrolyte concentration, current density, and anodization time. This behavior is explained by a mechanism in which anion incorporation is governed by a chemical reaction/precipitation under plasma but not by the electric field, unlike the case for conventional anodizing of valve metals without sparking. The corrosion resistance of the films tended to increase as a function of the logarithm of the film thickness, irrespective of the electrolyte species and electrolysis conditions.

Alumina is classified as an intermediate oxide that cannot form a glass and is industrially used as a multifunctional material. Intensive studies on the atomistic structure of amorphous alumina have been conducted because a fundamental understanding of its structure can be meaningful to the design of new materials and devices. Here we focused on anodic alumina as a model material for clarifying the atomistic structure of amorphous alumina and prepared anion-free and anion-incorporated porous-type amorphous alumina by anodization using chromic acid electrolyte and typical electrolytes (e.g., sulfuric acid, oxalic acid, and phosphoric acid), respectively. The local structure around aluminum atoms in the anodic alumina was investigated by nuclear magnetic resonance spectroscopy. We found that the structure of anodic amorphous alumina comprises AlO4, AlO5, and AlO6 units with predominant fractions of AlO5. We also observed that the fraction of each unit was 37.7, 54.3, and 8.0%, with an average coordination n...

Remarkably thick nanoporous anodic films have been formed through simple DC anodization by adding hydrogen peroxide as an oxidant to a sulfuric acid electrolyte. This result was not possible when using the previously reported pulsed polarization technique. While this anodic film's growth was significantly promoted by DC anodization, having only sulfuric acid resulted in the preferential anodic dissolution of the substrate material. Transmission electron microscopy of the anodic film's cross-section confirmed that the film has numerous fine pores with a size distribution ranging from 5 nm to 20 nm. Energy-dispersive x-ray spectroscopic analysis of the anodized specimens revealed that the films were composed of a complex chromium-rich oxide. In addition, coating stainless steel with thick nanoporous oxide films through anodization in the optimized conditions confirmed that it could act as a highly suitable host for synthetic hydroxyapatite coating, resulting in the enhanced growth of natural apatite in a simulated body environment. The enhancement of the apatite deposition is attributed to the availability of the empty pore volume in the thick anodic porous films, formed on stainless steel surfaces. These findings suggest a novel functionality to stainless-steel surface modification.

In this study, we use advanced analysis techniques to examine the nanostructures of anodic films formed on aluminum in various electrolytes, including oxalic acid and sulfuric acid, in the original state (without being chemically dissolved) and reveal that pores grow with radially branched nanopores. Nanopores are generated in the main pores for the same reason that initial small pores are generated on the film surface: nanopores are created when the electric field is reduced due to the presence of a thick barrier layer. These radially branched nanopores appear in a form characteristic of a film formed in chromic acid, producing a feather-like pore wall. As a representative model, the atypical porous structures created in the film formed in chromic acid were classified into five categories: initial small pores, branched pores, side holes, spherical voids, and radially branched nanopores. We demonstrate that these structures are not unique to chromic acid films but are a universal phenomenon, particularly radial nanobranching, which occurs naturally during the anodizing of most electrolytes.

The effect of alloying element Ca (0, 1, and 2 wt%) on corrosion resistance and bioactivity of the as-received and anodized surface of rolled plate AM60 alloys was investigated. A plasma electrolytic oxidation (PEO) was carried out to form anodic oxide film in 0.5 mol dm-3 Na3PO4 solution. The corrosion behavior was studied by polarization measurements while the in vitro bioactivity was tested by soaking the specimens in Simulated Body Fluid (1.5xSBF). Optical micrograph and elemental analysis of the substrate surfaces indicated that the number of intermetallic particles increased with Ca content in the alloys owing to the formation of a new phase Al2Ca. The corrosion resistance of AM60 specimens improved only slightly by alloying with 2 wt% Ca which was attributed to the reticular distribution of Al2Ca phase existed in the alloy that might became barrier for corrosion propagation across grain boundaries. Corrosion resistance of the three alloys was significantly improved by coating the substrates with anodic oxide film formed by PEO. The film mainly composed of magnesium phosphate with thickness in the range 30 - 40 μm. The heat resistant phase of Al2Ca was believed to retard the plasma discharge during anodization and, hence, decreased the film thickness of Ca-containing alloys. The highest apatite forming ability in 1.5xSBF was observed for AM60-1Ca specimens (both substrate and anodized) that exhibited more degradation than the other two alloys as indicated by surface observation. The increase of surface roughness and the degree of supersaturation of 1.5xSBF due to dissolution of Mg ions from the substrate surface or the release of film compounds from the anodized surface are important factors to enhance deposition of Ca-P compound on the specimen surfaces.

Anodic porous alumina films have attracted substantial attention and have found various applications because of their unique nanometric-scale porous structure. Though direct oxidation to form anodic porous alumina films has been studied for the protection or design of aluminum surfaces, the development of a practical process for the fabrication of porous alumina on a large number of small aluminum products, such as fine particles, has not yet been achieved. In this paper, we report the successful formation of porous alumina films by indirect oxidation under an alternating-current electric field without a direct electrical connection. Importantly, despite the indirect wireless oxidation, porous alumina films grew uniformly even in the case of aluminum particles used as an object. As with conventional direct-current anodization, film thickness increased with increasing delivered electricity. These results open a new route to technological and scientific applications of aluminum-based products.

The electrical potential gradients on aluminum bipolar electrodes (BPEs) subjected to bipolar anodization in an alternating current (AC) electric field were investigated by performing scanning electron microscopy, dyeing the anodized specimens, and measuring the resistance in the alumina films after re-anodizing the samples. The thickness and morphology of each alumina film varied, which reflected the unique potential distributions on the BPEs in the AC electric field. The electrical resistance was symmetrical and increased linearly from the centers of the BPEs to their extremities. This synthetic approach should lead to the development of various functional materials using bipolar electrochemistry.

Anodization by plasma electrolytic oxidation (PEO) and subsequent apatite coating were performed on a biodegradable AZ31 magnesium alloy to enhance its corrosion resistance and bioactivity in physiological solution. The PEO film itself (~ 48 μm in thickness) exhibited low bioactivity, where only aggregated apatite particles were deposited locally on its surface as a result of the alternative immersion method (AIM) in Ca–phosphate solutions. The uniformity of apatite coating on the PEO film was markedly improved by pretreatment of the film in a dilute NaOH solution. The alkali treatment induced the formation of a nano-size platelet Mg(OH)2 layer on the film surfaces that drastically enlarged the effective surface area for the precipitation of apatite. A uniform apatite layer as thick as 1 μm was successfully deposited on the hydroxide layer after AIM treatment. The enhanced uniformity of the apatite coating on an alkali- and AIM-treated surface significantly improved the corrosion resistance in both simulated body fluid (SBF) and NaCl solution, and the bioactivity in SBF.

Self-ordered nanoporous α-alumina membrane having uniformly sized straight pore arrays was fabricated by anodization of aluminum and subsequent through-hole and heat treatment. The detachment of thick anodic oxide film formed in oxalic acid was performed by combining preliminary loaded heating before stepwise voltage drop and cathodic polarization in phosphoric acid to prevent excess dissolution at the through-hole process. Suppression of excess dissolution of the upper layer of anodic alumina membrane acted an important role to provide symmetrical evenness of top and bottom pore density for the prevention of deformation such as warp and cracking during subsequent heating for crystallization. Thus, a thick, robust and chemically resistant nanoporous crystalline α-alumina membrane was successfully obtained by optimizing preparation conditions.

Effects of alloying element Ca on the corrosion behavior and bioactivity of films formed by plasma electrolytic oxidation (PEO) on AM60 alloys were investigated. The corrosion behavior was studied by conducting electrochemical tests in 0.9% NaCl solution while the bioactivity was evaluated by soaking the specimens in simulated body fluid (SBF). Under identical anodization conditions, the PEO film thicknesses increased with increasing Ca content in the alloys, which enhanced the corrosion resistance in NaCl solution. Thicker apatite layers grew on the PEO films of Ca-containing alloys because Ca was incorporated into the PEO film and because Ca was present in the alloys. Improvement of corrosion resistance and bioactivity of the PEO-coated AM60 by alloying with Ca may be beneficial for biodegradable implant applications.

In the interest of enabling practical applications of anodic oxide films, it is important to develop methods to improve the coulombic efficiency of film formation. In this study, ethanol was added to the sulfuric-acid-based electrolyte to examine its effects on the anodization behavior of aluminum, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various concentrations of ethanol under mild anodization conditions (i.e., 20 °C, as opposed to the lower-temperature hard anodization conditions). When a constant current of 100 A·m−2 was applied, the initial voltage and steady-state voltage were strongly dependent on the ethanol concentration that was added. The growth rate of the alumina film was governed by a field-assisted electrochemical process and, thus, was not affected by the addition of ethanol in the sulfuric-acid-based electrolyte. However, the maximum attainable film thickness increased with increasing ethanol concentration while the charge during anodization was constant. Thus, it was concluded the coulombic efficiency of the film formation could be improved by suppressing the chemical dissolution of the anodic film. Furthermore, the degree of acid dissociation in the water-and-ethanol system is thought to play an important role in controlling the formation and resultant structure of the film.

The fabrication and modulation of hexagonally ordered arrays of deep pores in (1 1 1) GaAs were performed by combining colloidal crystal templating, anisotropic chemical etching, localized anodic etching and isotropic anodic oxidation. High-aspect-ratio triangular pore arrays of GaAs along the [1 1 1] crystallographic direction were induced by anodic etching in HCl through the apex of inverted triangular pyramid prepits, where electric field strength increased under anodic bias. The triangular pore arrays in GaAs changed to triangular pillar arrays during the anisotropic chemical etching. By applying anodic oxidation in a weak acid solution, the shape of pores in GaAs could be modulated from triangular to circular owing to the leveling effect, and a macroporous GaAs layer was formed. Four types of GaAs microstructure, that is, triangular pore arrays, prism-like pillar arrays, circular pore arrays and needle-like tip arrays, could be obtained by optimizing the chemical etching and anodization conditions.

Plasma anodizing is a surface treatment used to form a ceramic-type oxide film on Mg alloys by the application of a high anodic voltage to create intense plasma near the metal surface. With proper selection of the process parameters, the technique can produce high quality oxide with superior adhesion, corrosion resistance, micro-hardness, wear resistance and strength. The effect of alloying element Al on plasma anodizing process of Mg alloys was studied by comparing the anodizing curves of pure Mg, AZ31, and AZ61 alloys while the effect of Ca were studied on AZ61 alloys containing 0, 1, and 2 wt% Ca. Anodizing was performed in 0.5 M Na3PO4 solution at a constant current density of 200 Am−2 at 25°C. Anodic oxide films with lava-like structure having mix composition of amorphous and crystal were formed on all of the alloys. The main crystal form of the oxide was Mg3(PO4)2 as analyzed by XRD. Alloying elements Al and Ca played role in modifying the plasma lifetime during anodization. Al tended to extend the ...

The effects of electrolyte species and their combination during the two-step anodization of anodic oxide films formed in various ammonium salts solutions on aluminum in terms of the crystallinity and dielectric properties for use as an electrolytic capacitor were investigated. The aluminum substrates were pretreated by dipping in boiling water to form a hydrated layer. Anodic films were annealed at 500°C after the first anodization. The CV (capacitance C multiplied by withstand voltage V) of the films formed with a second anodization in ammonium borate was higher than that of the films formed using the same electrolyte for the first and second anodizations. The increase in CV due to the improved crystallinity when using borate for the second anodization and the following electrolytes for the first anodization increased in the order salicylate < succinate < tartrate < phosphate < citrate < adipate < borate. When phosphate or organic electrolytes were used in the second anodization, considerable spike noise was observed. However, the spike noise was significantly suppressed when borate was used in the second anodization. [doi:10.2320/matertrans.L-M2013826]

High-aspect-ratio structures of GaAs pore arrays and pillar arrays with a triangular cross section were fabricated by a combination of the electrochemical etching of a pre-etched (111) GaAs substrate and anisotropic chemical etching. The formation of deep pores with an ordered interpore distance (e.g., 200 nm diameter, 70 μm depth, and high aspect ratio of 350) was successfully accomplished. The inverted pyramid-like prepits with a two-dimensional hexagonal array, which were formed by colloidal crystal templating, could act as initiation sites and guide the growth of pores along the [111] crystallographic direction. Moreover, the diameter of the pores of anodized GaAs could be easily controlled by post-chemical etching. The applied technique using the anisotropic chemical etching demonstrated the feasibility of the present method by the fabrication of high-aspect-ratio prismlike pillar arrays in GaAs. Keywords: GaAs, Microstructures, Electrochemical etching, Anisotropic chemical etching

Nano/microstructure control and electrochemical etching of aluminium substrate using a honeycomb alumina mask fabricated by anodisation with self-assembled spheres aligned on the aluminium surface were studied to directly control the initiation sites of pits. The transfer of the hexagonally ordered pattern of self-assembled spheres to the aluminium substrate could be achieved by substantially suppressed anodic oxide growth under the spheres where selective electrochemical etching proceeded. That is, etch pits are generated only in the thinner areas or holes of the honeycomb alumina mask with a one-to-one correspondence. With this process, improvements in pit distribution density and the homogeneity of pit sizes, while avoiding excessive dissolution of the aluminium surface, could be achieved easily in comparison with the conventional method. The density of pits could also be controlled by changing the diameter of spheres used as an indirect mask.

The microstructures and corrosion resistance of the anodic oxide films formed on AZ31B magnesium alloy by anodizing under continuous sparking in alkaline phosphate electrolyte (Na3PO4) were investigated. Basically, voltage-current characteristic of AZ31B in Na3PO4 solution was the same to that obtained in other sodium salt electrolytes such as Na2SiO3 and NaAlO2. However, the anodic films formed in Na3PO4 solution exhibited the higher corrosion resistance than that of the films formed in other electrolytes because of the incorporation of substantial amount of magnesium phosphate. The microstructures (e.g., diameter and number of pores) of the anodic films formed under sparking accompanied by current or voltage oscillation and gas evolution were strongly affected by the degree of sparking such as size, number, and appearance frequency, not by the formation voltage itself. X-Ray diffraction analysis revealed the incorporation of crystalline Mg3(PO4)2 into the anodic film formed under the sparking condition of dielectric breakdown.

The effect of high-frequency switching electrolysis on the film thickness uniformity of an anodic oxide film formed on Al–Si (AC8A) alloy was studied. The film thickness uniformity was successfully improved by applying high-frequency switching electrolysis with positive and negative voltage cycles and not by direct current anodizing. To attain uniform film growth on an Al–Si substrate with substantial dispersion of crystallized Si particles, the effects of the applied time Ta at a positive voltage as well as of the shape of current waves on film morphology and film thickness uniformity was investigated. When Ta was set close to the time at which the current peak, Tp was observed, the most uniform film growth was achieved. This was explained by the capsulation of Si particles by intermittent oxide growth accompanied by switching electrolysis with appropriate applied intervals. The mechanism of uniform film growth was further interpreted using equivalent circuit models.

The structural features of anodic oxide films formed on an aluminum substrate coated with self-assembled microspheres were investigated by scanning electron microscopy and atomic force microscopy. In the first anodization in neutral solution, the growth of a barrier-type film was partially suppressed in the contact area between the spheres and the underlying aluminum substrate, resulting in the formation of ordered dimple arrays in an anodic oxide film. After the subsequent second anodization in acid solution at a voltage lower than that of the first anodization, nanopores were generated only within each dimple. The nanoporous region could be removed selectively by post-chemical etching using the difference in structural dimensions between the porous region and the surrounding barrier region. The mechanism of anodic oxide growth on the aluminum substrate coated with microspheres through multistep anodization is discussed.

Formation of anodic porous zinc oxide (ZnO) films with high specific surface area was investigated in various electrolytes such as oxalic acid, ammonium borate, and sodium hydroxide solutions. Composition of the anodic film formed in oxalic acid, which was composed of an outer rock-like structure and an inner granular structure, was mainly Zn(OH)2 containing a small amount of ZnO. In contract, crystalline porous ZnO film with a straight cellular structure was formed in a sodium hydroxide solution. At low anodizing voltage of 9 V, formation of thicker films of more than 30 μm was achieved through suppression of extensive gas evolution. Photocatalytic activity of the ZnO films assessed using photodegradation of methylene blue under UV irradiation was comparable with that of TiO2 nanoparticle film (P25).

Self-organized nanoporous semiconductors such as ZnO and SnO2 having high aspect ratio was prepared by a simple, high throughput and non-costly wet electrochemical process without use of catalyst. By using anodization of metal plate of zinc in sodium hydroxide solution, tubular oxide with a pore interval of 10-20 nm and the thickness up to several ten microns was obtained. Photocatalytic activity of anodic film on zinc was improved by increasing annealing temperature up to 400 oC associated with increasing crystallinity, although the specific surface area of the oxide film decreased as a result of decreasing grain size caused by sintering. Anodic film formed on tin in sodium hydroxide solution showed cellular structure with the size of 50 nm consisted of microcrystal and the thickness up to 60 μm in only 1 h. Photo catalytic activity of the anodic SnO2 was ascertained by a methylene blue degradation test under UV irradiation. If sulfuric acid was used as an anodizing electrolyte for tin, a peculiar tubular film composed of mixture of crystalline SnO, SnO2 and Sn3O(OH)2SO4 was found.

We have fabricated self-organized nanoporous zinc oxide having an extremely high aspect ratio by a simple, high throughput and non-costly wet electrochemical process without use of catalyst: anodization of zinc plate in aqueous solutions. By optimizing the anodizing condition, we could obtain a film with the thickness more than 90 mm. BET specific surface area of the anodic porous ZnO was approximately 28.5 m2/g. XRD measurement of the anodic ZnO reveals diffraction spectra of polycrystalline ZnO. From UV and visible light optical absorption spectra of thick anodic ZnO films, the absorption edge and the estimated band gap were 370 nm and 3.35 eV respectively. Photo catalytic activity of the anodic ZnO was ascertained by a methylene blue degradation test under UV irradiation at the wavelength of 365 nm. The proposed process with use of anodization of zinc could be expanded to other metals to obtain various oxides semiconductors, and has potential technological applications in the fields of catalysis, supports, and sensors.

Pore nucleation and growth processes of anodic oxide films formed on aluminum substrate were investigated by atomic force microscopy with focusing on the crystal orientation and surface topography of aluminum substrate. Nanotopographies of electropolished aluminum were extremely affected by the crystal orientation of aluminum substrate. For as-received aluminum, regularly aligned striped structure appeared after electropolishing. On the other hand, aluminum substrate annealed at 300°C exhibited isotropic hexagonal cell structure. From X-ray diffraction patterns, it was confirmed that the preferential crystal orientation of aluminum was changed from (110) to (100) when annealing temperature was higher than 300°C. In the initial stage of anodization, specific nanotopography of electropolished aluminum surface was served as the initiation sites for pore generation, and accordingly it influenced cell arrangement. Using planarized aluminum with less than 0.3 nm asperities, however, a large number of fine pores initiated on the oxide film surface. Thus, the growth of anodic oxide films was seriously influenced by the surface topography of aluminum substrate.

Nanopatterning processes based on a localized anodization of Si and the subsequent chemical etching of SiO2 were developed to fabricate a dot array and a hole array on an Si surface using self-organized anodic porous alumina as a mask. Through the porous alumina mask, regularly arranged metal nanopatterns on the Si surface were fabricated by the electroless deposition of Cu nanodots in a CuSO4/hydrofluoric acid (HF) solution. The periodicity of the Cu dot arrays was determined using the pore interval of the upper anodic alumina. Using self-assembled nanospheres as a mask for an electroless plating of metals such as Cu and Ag on the Si substrate, the patterning of an ordered honeycomb structure and a hexagonally arranged convex array of metals on Si was also developed by the combination of colloidal crystal patterning and wet chemical etching. The proposed patterning processes of the Si surface have potential technological applications in fields that need textured surfaces of controlled nanoscale periodicity and morphology owing to their relative simplicity and low cost.

Dielectric properties of crystalline anodic barrier films formed on aluminum by multiple step anodizing were evaluated with focusing on the effects of each processing stage and electrolytes species. An aluminum sheet was first immersed in a boiling water to form a hydrated oxide and subsequently anodized up to 350 V in a mixture of boric acid—sodium borate solution or ammonium adipate solution. After the first anodizing, the leakage current of the film formed in borate was much higher than that of the film formed in adipate. After the subsequent heating in air at 500°C, the leakage current of the film formed in borate showed few change although that of adipate remarkably increased. These results suggest that the degree of crystallization accompanied by defects formation after the first anodizing of the film formed in adipate is lower than that in borate. The leakage current after the second anodizing of both films in the borate was effectively suppressed; however, it is still high in the film formed in borate. TEM observation indicated that the film formed in adipate was thinner than the other; despite their same withstand voltage, suggesting higher permittivity of the film formed in adipate. In accordance to that, the capacitance of the film formed in adipate was higher than that of borate. These differences in dielectric properties between the films would be attributed to the difference in anion incorporation depth into the films, particularly incorporation depth of borate.

Silicon disk arrays and silicon pillar arrays with a close-packed configuration having an ordered periodicity were fabricated by the electrochemical etching of a silicon substrate through colloidal crystals used as a mask. The colloidal crystals were directly prepared by the self-assembly of polystyrene particles on a silicon substrate. The transfer of a two-dimensional hexagonal array of colloidal crystals to the silicon substrate could be achieved by the selective electrochemical etching of the exposed silicon surfaces, which were located in interspaces among adjacent particles. The diameter of the tip of the silicon pillars could be controlled easily by changing the anodization conditions, such as current density and period of electrochemical etching. Keywords: Silicon, Electrochemical etching, Colloidal crystal, Natural lithography

Nanohole arrays with a 60 nm hole periodicity were fabricated on a Si substrate by the anodization of an aluminum film sputtered on a Si substrate in sulfuric acid and subsequent chemical etching. The transfer of the nanoporous pattern of anodic alumina into the Si substrate was achieved by the selective removal of silicon oxide, which was produced by the anodic oxidation of the underlying Si substrate through the anodic porous alumina used as a mask.

New self-ordering porous alumina films were fabricated in organic acid electrolytes. Highly ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively. Self-organization was achieved at the maximum voltage required to induce high-current-density anodization while preventing burning, i.e., an extremely high-current flow concentrated at local points. The cells of the film grown at a high field must be pressed against each other, so that the self-ordering proceeds with the porous layer growth. When the self-ordering of cell arrangement proceeds, the cells became smaller. To improve the regularity of the cell configuration, a low electrolyte temperature and a relatively high electrolyte concentration were effective for maintaining a high-current-density to prevent burning. Surface flatness was an essential factor for self-ordering, however, the surface oxide film produced by electropolishing an aluminum substrate prevented quick pore growth in the organic acids having a low dissociation constant. It is confirmed that electropolishing followed by alkaline treatment was most appropriate as the pre-treatment in preparing flat surfaces.

The structure and properties of anodic films formed in a relatively low voltage range in ammonium adipate solution have been investigated focusing on the effect of formation voltage. When anodization voltage was identical, the linear relationship of film thickness with the log of the final current density during film growth could be ascertained for all the films formed even at low voltages such as 5 V. The anodization ratio determined from the transmission electron microscopy (TEM) cross section of the film formed at 5 mA cm −2 until voltage increased to 5 V was 1.8 nm/V suggesting a lower electric field strength during anodization than that for the film formed at the voltage higher than 20 V. Furthermore, the transport number of Al 3+ ions and amount of incorporated anions decreased with decreasing formation voltage, particularly at the voltage lower than 10 V. Changes in chemical dissolution rate in the depth of the films in an acid solution indicated that the films formed in the voltage range from 20 to 80 V were composed of one layer and the films formed at voltages lower than 10 V were composed of two layers with different dissolution rates. However, when applied voltage was kept constant following current decay, the film formed one layer, implying the inward migration of the incorporated anions to the film/metal interface. It was suggested that the outer layer with the higher dissolution rate contains electrolyte anions and the inner layer with the lower dissolution rate consists of pure alumina. Thus, it is clarified that the structure and properties of anodic film change with increasing anodization voltage because of the change in electric field strength, even if anodization is conducted at a constant current density.

Formation behavior and structure of newly established anodic films grown on pure magnesium and AZ alloys were studied in organic electrolyte containing 1 mol·dm−3 triethylamine/ethylene glycol solution with varying water content. The growth behavior and appearance of anodic films were significantly affected by water content. In the region between 10 and 30% of water content, compact barrier-type films were obtained both 99.6% Mg and AZ91D. The surface appearance of obtained films was transparent, which was different from those formed usually in aqueous solutions. In addition, these films possessed a high order of corrosion resistance. In the case of electrolyte containing water below 10% or 50%, the appearances were yellowish white and whitish gray, respectively. The composition of anodic films determined by FT-IR and GD-OES analysis showed the incorporation of organic substance into the anodic film. The amount of incorporated organic species increased with decreasing water content in the electrolyte.

The effect of electrolysis conditions on the self-ordering of the cell arrangement of anodic porous alumina grown in malonic acid solution was investigated for the clarification of the mechanism. Because the cell arrangement of porous alumina is affected by the roughness of the substrate surface, the aluminum substrates were pre-treated by electropolishing followed by chemical polishing to modify the substrate surface and make it suitable for uniform porous film growth. By lowering the electrolyte temperature, the high voltage anodizing accompanying high current density without appearance of local current concentration, i.e., burning, could be archived to obtain self-organized porous alumina. Further, current density could be increased more by increasing malonic acid concentration. The porosity of anodic alumina formed in malonic acid solution was 0.07, regardless of the anodizing voltage. From the above results, it was confirmed that high current density, i.e., high electric field strength is the key controlling factor in the self-ordering of anodic porous alumina.