Micro-patterning in anodic oxide film on aluminium by laser irradiation

Microstructures such as microchannels and microchambers were fabricated on anodic oxide film of aluminium with laser direct patterning technique. The aluminium substrate covered with porous type anodic oxide film was dyed with organic dyes followed by hydrothermal treatment. The coloured and pore sealed film shows much higher absorption then as-formed porous film, that makes possible to pattern various types of microstructures on the film by removing the upper part of the film surface by laser irradiation. Structure of the crystallized hydroxide layer on the surface of the anodic oxide film, formed during pore sealing, as well as the partial removal of anodic oxide film from the surface is described. Microstructures with depth lower than 1-27 μm were patterned successfully on 16 and 32 μm thick anodic oxide film surfaces, using various combinations of laser power and laser scanning speed. The affect of the laser scanning speed and laser power density on the shape of the microstructures is also explained.     

Development of three-electrode type micro-electrochemical reactor on anodized aluminum with photon rupture and electrochemistry

Photon rupture with a focused single pulse of pulsed YAG-laser irradiation was used to fabricate an aluminum electrochemical micro-reactor. Porous type anodic oxide film formed on aluminum specimens was irradiated in solutions with a pulsed Nd-YAG laser beam through a convex lens to fabricate micro-channels, micro-electrode, and through holes (for reference electrode, solution inlet, and outlet). During irradiation, specimens were moved by a computer controlled XYZ stage. After irradiation, the surface of the micro-channel and through hole were again treated to form anodic oxide film and the surface of the micro-electrode was treated electrochemically to provide an Au layer. The calculated volume of the micro-reactor including micro-channel and through holes is about 1.5 μl. The cyclic voltammogram of the micro-electrochemical cell was measured in K₃Fe(CN)₆/K₄Fe(CN)₆ with both static and flowing solution at different scanning rates. The anodic and cathodic peak currents were measured and the values depended on scanning rate and ion concentration when the solution was static. With the flowing solution, limiting currents were observed and the anodic limiting current was increased with the cubic root of the solution flow rate.     

Growth of porous anodic films on sputtering-deposited aluminium incorporating Al-Hf alloy nanolayers

Formation of porous anodic films on sputtering-deposited aluminium incorporating Al-Hf tracer layers has been examined at constant current in sulphuric and phosphoric acids. Hafnium was selected as the tracer species since the migration rates of Hf⁴⁺ and Al³⁺ ions are similar in barrier-type anodic alumina. The distribution of hafnium in the films was determined using ion beam analysis, scanning electron microscopy and transmission electron microscopy. Increases in the anodizing voltage and barrier layer thickness accompany the oxidation of hafnium and the migration of Hf⁴⁺ ions through the barrier layer region of the porous film. Hf⁴⁺ and Al³⁺ ions that migrate to the pore bases are lost to the electrolyte. Other Hf⁴⁺ ions are incorporated into the cell walls. For films formed in phosphoric acid, with relatively thick barrier layers, channelling of the ion current leads to accelerated outward transport of Hf⁴⁺ ions toward the pore base, while a U-shaped inner edge of the hafnium distribution beneath the pores is associated with more slowly transported hafnium species. The tracer behaviours for films formed in both acids are consistent with the transport of Hf⁴⁺ ions in the barrier layer regions by a combination of flow of film material and ion migration, the flow being a key factor in the development of the pores. The percentage losses of Hf⁴⁺ and Al³⁺ ions from the films to the electrolyte are relatively similar, correlating with their similar migration rates, and contrast with the retention in the film of slow migrating W⁶⁺ ions, found previously, due to a more dominant role of flow.     

Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm⁻³ K₂HPO₄ + 0.2 mol dm⁻³ K₃PO₄/glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m⁻². The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.     

Local deposition of polypyrrole on aluminum by anodizing, laser irradiation, and electrolytic polymerization and its application to the fabrication of micro-actuators

Polypyrrole was deposited at selected areas on aluminum by anodizing, laser irradiation, and electrolytic polymerization, and the application of the technique for fabricating micro-actuators was attempted. Aluminum specimens covered with porous type anodic oxide films were irradiated with a pulsed Nd-YAG laser to remove the oxide films locally, and then thin Ni layers were deposited at areas where film had been removed. Polypyrrole could be successfully deposited only on the Ni layer by anodic polarization of the specimens in pyrrole monomer solution, and a polypyrrole/Ni bilayer structure could be obtained by dissolution of the aluminum substrate and anodic oxide film in NaOH solutions. The bilayer structure was found to be inactive to doping and dedoping of ions during anodic and cathodic polarization. A three-layer structure, nitrocellulose/Ni/polypyrrole, fabricated by electrolytic polymerization after nitrocellulose coating on a Ni layer detached from the aluminum substrate, showed ion-doping and -dedoping activity, suggesting the possibility of fabricating micro-actuators in this manner.     

Fabrication of three-dimensional platinum microstructures with laser irradiation and electrochemical technique

Three-dimensional (3D) platinum microstructures were fabricated by successive procedures: aluminum anodizing, laser irradiation, nickel/platinum electroplating, and removal of the aluminum substrate, the oxide films, and the nickel metal layer. Aluminum plates and rods were anodized in an oxalic acid solution to form porous type oxide films. The anodized specimens were immersed in a nickel electroplating solution, and then irradiated with a pulsed Nd-yttrium aluminum garnet (YAG) laser beam to remove the anodic oxide film with a three-dimensional XYZθ stage. The specimens were cathodically polarized in the nickel and a platinum electroplating solution to form the metal micropattern at the laser-irradiated area. The electroplated specimens were immersed in NaOH solution to dissolve the aluminum substrate and the oxide films, and then immersed in HCl solution to dissolve the nickel deposits. A platinum grid-shaped microstructure, a microspring, and a cylindrical network microstructure with 50-100 μm line width were obtained successfully.     

Sealing effects of anodic oxide films formed on Mg-Al alloys

Mg alloys were anodized in alkaline NaOH solutions with various additives as a non-chromate method. Specimen AZ91 was anodized at a potential that produced a strong surface dissolution reaction and generated a large amount of Mg(OH)₂. The effect of sealing after anodizing was investigated, focusing on the effects of sealing time, temperature and solution conditions. The current density decreased with increasing A1(OH)₃ concentration in 1 M NaOH solution during anodizing; sparking occurred at potentials above 80 V. The best corrosion resistance with anodizing in 1 M NaOH solution occurred at a potential of 4 V, which caused the strongest active dissolution reaction. The sealing effect improved with increasing time and temperature, and corrosion resistance was proportional to the relative ratio of Mg(OH)₂. If the oxygen thickness observed by EDX equaled the film thickness, the film formed at 4 V in 1 M NaOH was 10–15 Μm thickness. The optimum corrosion resistance in sealing at various solutions after anodizing was 1M-NaOH solution.     

Photoluminescence emission of nanoporous anodic aluminum oxide films prepared in phosphoric acid

The photoluminescence emission of nanoporous anodic aluminum oxide films formed in phosphoric acid is studied in order to explore their defect-based subband electronic structure. Different excitation wavelengths are used to identify most of the details of the subband states. The films are produced under different anodizing conditions to optimize their emission in the visible range. Scanning electron microscopy investigations confirm pore formation in the produced layers. Gaussian analysis of the emission data indicates that subband states change with anodizing parameters, and various point defects can be formed both in the bulk and on the surface of these nanoporous layers during anodizing.     

Characterization of aluminum gallium oxide films grown by pulsed laser deposition

Aluminum gallium oxide (AGO) films were prepared on conventional c-plane sapphire by pulsed laser deposition (PLD). In the current PLD-AGO studies, target composition or growth temperature is usually the main deposition variable, and the other growth conditions are fixed. This would make it difficult to fully understand the theory and characterization of AGO films. In this study, several growth parameters such as target composition, gas atmosphere, laser repetition frequency, growth pressure, and substrate temperature (T_s) were all modulated to realize and optimize the AGO growth. When the (Al_xGa_1-x)₂O₃ target with the Al content larger than 20?at% was used, a serious target poisoning phenomenon occurred, leading to the extremely unstable growth rate. In comparison to the AGO film grown with argon atmosphere, the higher transparency was reached in the film prepared with oxygen atmosphere due to the relative abundance of oxygen. Because of the homogeneous oxygen reduction, the AGO film with the higher crystal quality was obtained at a higher laser repetition frequency. With an increment of growth pressure, the Al content of AGO film was increased. The growth of AGO film at the higher T_s would cause the higher bandgap value, smoother surface, and growth rate degradation. Additionally, the crystal quality of AGO film can be also improved both by increasing the growth pressure and T_s. The better characterization can be reached in the AGO film grown using the (Al_0.05Ga_0.95)₂O₃ target with oxygen atmosphere at the working pressure of 2?×?10^?1 Torr, the laser repetition frequency of 10?Hz, and the T_s of 800?°C. When the metal-semiconductor-metal photodetector fabricated with this AGO active layer, the best performance including the photocurrent of 7.56?×?10^?8 A, dark current of 1.59?×?10^–12 A, and photo/dark current ratio of 4.76?×?10⁴ (@5?V and 240?nm) were achieved.     

Effect of aluminum annealing on the galvanoluminescence properties of anodic oxide films formed in organic electrolytes

The galvanoluminescence (GL) properties of anodic oxide films formed in organic electrolytes were investigated at different aluminum annealing temperatures. The results of the spectral measurements showed two different types of GL sources: carboxylate ions incorporated in oxide films during the anodization and the molecules AlH, AlO, Al₂, AlH₂, also formed during anodization process and already recognized in the case of inorganic electrolytes. The latter was related to gamma alumina crystalline regions formed by annealing of the aluminum samples at temperatures above 500 °C.     

Fabrication of highly ordered pore array in anodic aluminum oxide

Highly ordered pore array in anodic aluminum oxide was fabricated by anodizing pure aluminum. The order of a pore array was affected by anodizing voltage, electrolyte temperature, and first anodizing time. A regular pore array with mean diameter of 24 nm and interpore distance of 109 nm could be formed by two-step anodization at 40 V., oxalic acid concentration of 0.3 M and electrolyte temperature of 15 ‡C. The measured interpore distance showed linearity with anodizing voltage. The diameter of pores was adjusted by pore widening treatment in a 5 wt% phosphoric acid solution at 30 ‡C after two step anodization. The mechanism of self-arrangement of pores could be explained by the repulsive interaction between the pore walls.     

Growth of anodic oxide films on oxygen-containing niobium

The present study is directed at understanding of the influence of oxygen in the metal on anodic film growth on niobium, using sputter-deposited niobium containing from about 0-52 at.% oxygen, with anodizing carried out at high efficiency in phosphoric acid electrolyte. The findings reveal amorphous anodic niobia films, with no significant effect of oxygen on the field strength, transport numbers, mobility of impurity species and capacitance. However, since niobium is partially oxidized due to presence of oxygen in the substrate, less charge is required to form the films, hence reducing the time to reach a particular film thickness and anodizing voltage. Further, the relative thickness of film material formed at the metal/film interface is increased by the incorporation of oxygen species into the films from the substrate, with an associated altered depth of incorporation of phosphorus species into the films.     

A flow model of porous anodic film growth on aluminium

The development of pores in a classical porous anodic film formed on aluminium in phosphoric acid solution is investigated. The study employs a tungsten tracer layer that is incorporated into the anodic film from the aluminium substrate, followed by detection of the tracer by transmission electron microscopy and Rutherford backscattering spectroscopy. Distortions of the tungsten layer on entry into the film and retention of tungsten species in the film are compatible with porosity arising mainly from flow of anodic oxide beneath the pore bases towards the cell walls. The behaviour is contrary to expectations of a dissolution model of pore formation.     

Fabrication of micro-dot arrays and micro-walls of acrylic acid/melamine resin on aluminum by AFM probe processing and electrophoretic coating

Micro-dot arrays and micro-walls of acrylic acid/melamine resin were fabricated on aluminum by anodizing, atomic force microscope (AFM) probe processing, and electrophoretic deposition. Barrier type anodic oxide films of 15 nm thickness were formed on aluminum and then the specimen was scratched with an AFM probe in a solution containing acrylic acid/melamine resin nano-particles to remove the anodic oxide film locally. After scratching, the specimen was anodically polarized to deposit acrylic acid/melamine resin electrophoretically at the film-removed area. The resin deposited on the specimen was finally cured by heating.It was found that scratching with the AFM probe on open circuit leads to the contamination of the probe with resin, due to positive shifts in the potential during scratching. Scratching of the specimen under potentiostatic conditions at −1.0 V, however, resulted in successful resin deposition at the film-removed area without probe contamination. The rate of resin deposition increased as the specimen potential becomes more positive during electrophoretic deposition. Arrays of resin dots with a few to several tens μm diameter and 100–1000 nm height, and resin walls with 100–1000 nm height and 1 μm width were obtained on specimens by successive anodizing, probe processing, and electrophoretic deposition.     

Structural changes in soot particles induced by diode laser irradiation

The effects of laser radiation on soot in a vacuum were studied by Raman spectroscopy and transmission electron microscopy. It was found that defective carbon onions could be formed simply by irradiation of focused common diode lasers with intensity much lower (10³-10⁶ W/cm²) than those used in the previous work. A modified Melton model was introduced to analyze the formation of onions. The results show a threshold value of laser intensity (5.49 × 10³ W/cm²), above which sublimation and rearrangement play a dominant role in the formation of onions. Such onions can also be formed with laser intensities below the threshold since neighboring graphene layers may merge and grow by capturing interstitial carbon atoms. The deposited soot is damage resistant to the diode laser radiation in air due to the rapid formation of carbon onions.     

Surface modification of sintered magnesium oxide (MgO) with chromium oxide (Cr2O3) by pulsed laser irradiation in air and liquids

Surface modification of ceramic materials by laser irradiation is widely used as a non-contact, fast and thermally activated process to generate micro and nanostructures. The effects of liquids while surface modification by laser irradiation of ceramic materials under liquid environment are least explored so far. This study reports the effects of pulsed laser irradiation in air and liquids on the microstructure and morphologies of ceramic materials. Chromium oxide (Cr₂O₃) was mixed in different concentrations (3, 5 and 7% in weight) into magnesium oxide (MgO) matrix and was sintered at 1650 °C. The structure and morphology of the sintered ceramic pellets were characterized using X-ray diffraction and scanning electron microscopy. Presence of the spinel magnesium chromium oxide (MgCr₂O₄) was identified in these samples. For surface modification of these samples, laser irradiation is carried out in air and liquids (methanol, isopropyl alcohol and acetone) using 2 ns pulsed lasers (532 nm) of different pulse repetition rates and energies. The microstructure and morphologies of the samples after irradiation was analyzed and their crystalline structure and composition were maintained after laser irradiation. It was observed that the surface morphologies of the ceramic pellets were modified by laser irradiation as a combined effect of the medium (air/liquids), energy fluence and the concentration of the Cr₂O₃ in MgO. Our results show that pulsed laser irradiation especially in liquids is an effective technique for modifying surface morphology of ceramic materials.     

Fabrication of diameter-modulated and ultrathin porous nanowires in anodic aluminum oxide templates

Anodic aluminum oxide (AAO) membranes with modulated pore diameter were synthesized by pulse anodization in 0.3 M sulfuric acid at 1 °C. For AAO growth, a typical combination of alternating mild anodizing (MA) and hard anodizing (HA) pulses with applied potential pulses of 25 V and 35 V was applied. The control of the duration of HA pulses will provide an interesting way to tune the shape of pores and the structure of AAO channels. It was found that a non-uniform length of HA segments in cross section of AAO is usually observed when the HA pulse duration is shorter than 1.2 s. The pulse anodization performed with longer HA pulses leads to the formation of AAO templates with periodically modulated pore diameter and nearly uniform length of segments. Various diameter-modulated metallic nanowires (Au, Ag, Ni and Ag–Au) were fabricated by electrodeposition in the pores of anodic alumina membranes. A typical average nanowire diameter was about 30 nm and 48 nm for MA and HA nanowire segments, respectively. After a successful dealloying silver from Ag–Au nanowires, porous ultrathin Au nanowires were obtained.     

The galvanoluminescence spectra of barrier oxide films on aluminum formed in organic electrolytes

In this paper, for the first time we have presented the results of the galvanoluminescence (GL) spectra measurement obtained from barrier oxide films during aluminum anodization in various barrier film forming organic electrolytes (aqueous solution of citric acid, tartaric acid, malic acid, succinic acid and ammonium tartarate). Galvanoluminescence spectral measurements were performed utilizing spectrograph system based on the Intensified Charge Coupled Device (ICCD) camera, intended for time-resolved detection of GL phenomena dynamics. The spectra were recorded for different values of electrolyte temperature and anodization current density. We have showed that there are strong GL bands in the visible region and the shape of the spectra as well as peak intensity of the GL bands depend on the anodization voltage. The results cleraly indicate the existance of more then one type of GL centra or GL mechanisms in barrier films formed in organic electrolytes.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

Improvement of sensitivity in an interferometry by controlling pore size on the anodic aluminum oxide chip pore-widening technique

The pore size of an anodic aluminum oxide (AAO) chip, as well as uniform pore distribution, is one of the key parameters that should be adjusted, by choosing the appropriate etching conditions, in order to enhance the sensitivity of an interferometer. In this study, the pore size of AAO chips was optimized and characterized in order to lower the detection limit of prostate specific antigen (PSA) in an interferometric immunoassay system. After pore widening for 30–50 min, the AAO pore size was increased approximately 2-fold larger than that before pore widening. A large increase in effective optical density (ΔEOT) was obtained from the AAO chip fabricated by pore-widening technique, which thereby lowered the PSA detection limit. The present study results are not sufficiently validated to enable the immediate application to immunoassay for prostate cancer (PCa) screening, but they do demonstrate that controlling pore size can positively affect the sensitivity and lower the detection limit.