Porous Anodic Alumina Films Research Articles

Stress Induced by Electrolyte Anion Incorporation in Porous Anodic Aluminum Oxide

The geometry of porous anodic aluminum oxide films depends sensitively on the type of electrolyte anion in the anodizing bath. It has been proposed that near-surface compressive stress due to anion incorporation induces an oxide flow instability that results in the self-ordered patterns of pores. Experimental evidence for anion incorporation-induced stress in Al oxide films, and accumulation of such stress during anodic oxidation, was sought through in situ stress measurements supplemented by X-ray photoelectron spectroscopy (XPS). Compressive stress generated during open-circuit dissolution in phosphoric acid was found to agree quantitatively with the elastic stress, calculated assuming that phosphate ions detected by XPS are incorporated into the oxide. Incorporation of phosphate ions may be driven by the high electric field in the film. Aluminum samples with varying levels of anion incorporation-induced stress were anodized in phosphoric acid, and the near-surface residual stress in the anodic oxide was determined by stress monitoring as the film dissolved at open circuit. The surface stress tracked closely with the stress introduced at open circuit, thereby demonstrating that surface stress induced by anion incorporation accumulates during barrier oxide growth. Finally, it is shown that anion incorporation prior to anodizing can induce large tensile stress relaxations during the barrier to porous oxide transition, which are ascribed to tensile stress generation at the metal-oxide interface.

Effect of Anodizing Medium on The Morphology and Photoluminescent Property of Porous Alumina Film

Anodic alumina films with uniformly distributed self assembled pores of diameters in the nanometric length scale were prepared by dual step anodization technique in oxalic, phosphoric and sulphuric acid medium. The variations of pore morphology and photoluminescent properties were studied with respect to the change in electrolytic medium. The photoluminescence (PL) spectra of the films prepared in different electrolytic medium have been recorded and compared. The PL spectra of the porous anodic alumina (PAA) film prepared in oxalic acid was found to be much stronger than the PAA prepared in phosphoric and sulphuric acid. However, pore size of the PAA films prepared in oxalic and sulphuric acid were significantly smaller than the PAA films prepared in phosphoric acid medium. Presence of oxygen deficient defect centre exerts a strong influence on the PL properties of the films.

Switchable Photonic Crystals Using One-Dimensional Confined Liquid Crystals for Photonic Device Application

Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photoisomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

Effect of Electric Field Strength on Cell Morphology and Anion Incorporation of Anodic Porous Alumina

The effect of electric field strength (E) on cell morphology and anion incorporation of porous anodic alumina films was studied by using TEM. Cell diameter decreased with decreasing voltage until 2V while pore diameter decreased to a minimum at 5V, followed by an increase at 2V. The ratio of cell diameter to pore diameter (dcell / d pore) of the anodic films formed in sulfuric acid and oxalic acid solutions at different constant voltage was in a linear relationship with logarithm of current density (log i). Since E across the barrier layer of porous films varied linearly with log i as in the case of barrier type film, the dcell / d pore ratio was regarded to be proportional to E. From the obtained results, it is deduced that pore diameter is proportional to voltage and inversely proportional to the square of E, whereas barrier layer thickness and cell diameter are proportional to voltage and inversely proportional to E. Accordingly, pore diameter is more dependent on current density than the other cell parameters are. The content of incorporated anion in the films increased linearly with increasing log i, hence E.

Transient Relaxations of Ionic Conductance during Growth of Porous Anodic Alumina Films: Electrochemical Impedance Spectroscopy and Current Step Experiments

Transient electrochemical relaxations during growth of anodic oxide films can provide insight into mechanisms of transport processes. Few such studies have focused on porous anodic oxides, which are of significant interest as functional materials. Conductance relaxations during anodizing of aluminum in acidic porous anodic oxide-forming solutions are investigated, using high voltage impedance spectroscopy and constant current reanodizing experiments. Both experiments follow phenomenology demonstrated in prior studies of anodic barrier films. Low-frequency inductive loops in impedance spectra exhibit time constants with inverse proportionality to current density and characteristic resistances nearly equal to those of high-frequency capacitive features. Potential decays during reanodizing depend on charge passed instead of time. Both sets of experiments are analyzed with a model including conductance relaxations due to electric field-dependent generation and removal of bulk charge-carrying defects. The model is found to be quantitatively consistent with nearly all aspects of experimental behavior; the inverse current density dependence of the inductive time constant and the charge parameterization of potential decays both arise because of exponential electric field dependence of ionic conduction and defect generation/removal rates. Oxide volume changes accompanying defect relaxations may help explain mechanical stress generation during anodizing, which is thought to drive pore formation.

Computational Simulation of Localised Corrosion in Anodised AA 2099-T8 Aluminium-Lithium Alloy

The initiation of localised corrosion in anodised AA2099-T8 alloy is simulated by using Cellular Automata (CA) method. For CA simulation, the system consisting of corrosive electrolyte, porous anodic alumina film and alloy matrix is described as a 500×300 two-dimensional square lattice. Eight types of cells are defined to construct the cellular space, with the Von Neumann neighbourhood (four cells neighbourhood) being selected. The cellular space is constructed such that a void generated by oxidation of the high-copper-containing Al-Fe-Mn-Cu-Li particle exists in the porous anodic alumina film consisting of a porous layer and a barrier layer. It is suggested that localised corrosion is preferentially initiated in the void defect region in the porous anodic film, which is related to increased corrosion product diffusion rate in this region compared with diffusion in other regions of the porous layer.

Analytical 3D migration model of steady-state metal anodizing: the velocity fields and trajectories of inert tracers, metal and oxygen ions

A mathematical model is proposed that describes the velocity fields of metal and oxygen ions and inert tracers in the steady-state porous metal oxidation. The 3D migration model is based on the theory of field-assisted dissolution of oxide at the pore bottom and migration of ions in strong spherical electric fields. The ion motion at a constant velocity in the direction of anodic film growth and their migration in the radial direction (along the electric field lines) are taken into consideration. The effect of mechanical stresses on the motion of ions and inert tracers is ignored. The oxide flows both from the oxide/electrolyte interface and from the oxide/metal interface to the boundary between the barrier and porous layers. These two flows form the porous oxide layers adjacent to the pores and afar from them. A ratio between the volumes of these layers is determined by the transport numbers and oxide porosity. The calculated tracer trajectories agree with the experimental results of Garcia-Vergara et al. on the oxide flow in the barrier layer of porous anodic alumina film.

Direct observation of liquid crystal phases under nanoconfinement: A grazing incidence X-ray diffraction study

ABSTRACTWe have controlled the molecular orientation of nematic and smectic A liquid crystal (LC) phases in a porous anodic aluminium oxide (AAO) film, in which the pore diameter was varied from 20 nm to 100 nm. Surface anchoring to induce planar and homeotropic molecular arrangement was controlled by chemical modification of the AAO inner surface. Direct observation of the molecular orientation of LC phases was performed using grazing incidence X-ray diffraction technique, showing in-plane and out-of-plane molecular orientation, and the corresponding layer orientation. The systematic investigation of LC phases under nanoconfinement will be useful to design various kinds of physicochemical environments to control the orientation of other soft matters.

Effect of Electric Field Strength on Cell Morphology and Anion Incorporation of Anodic Porous Alumina

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is a promising candidate for starting materials of nanofabrication of various devices. Investigations concerning the cell dimensions of porous anodic alumina films have been extensively performed after the confirmation of the classical cell model proposed by Keller et al (1). It was suggested in these studies that cell dimensions, such as pore and cell diameters and barrier layer thickness, primarily depended on the formation voltage, and that they increased linearly with increasing voltage (2). Nevertheless, the importance of current density as a controlling factor of the cell geometry was sometime claimed (3, 4). According to the classical theory of ionic conduction at the high field strength for the anodic barrier film grown on various metals (5, 6), the film thickness of each metal is inversely proportional to the logarithm of ionic current when the film is formed up to the same voltage. Thus, it is indicated that the log of current density I is proportional to the electric field strength E, i.e., the formation voltage / film thickness ratio at the barrier layer, as we reported previously (7). Concerning the anion incorporation into the film, we found that anion content was also proportional to the electric field strength E, i.e., the log of current density i (7). This principal is suggested to be applicable to porous anodic films. The purpose of the present study is the confirmation of the effect of electric field strength as a controlling factor of the cell morphology of porous anodic alumina such as pore diameter / cell diameter ratio that affects to the radius of curvature of cell base pattern and porosity. In addition, effect of electric field strength on incorporation behavior of electrolyte anions into cell wall has been studied. Obtained knowledge could help us to get further insight into self-ordering mechanism of anodic porous alumina. Although pore and cell sizes of the films formed in oxalic acid were larger than those formed in sulfuric acid even at the same voltages, the ratio of cell diameter to pore diameter (dcell / d pore) of both films is in a linear relation with the logarithm of current density (log i). The electric field strength across the barrier layer of porous films varies linearly with the logarithm of current density as in the case of barrier type films. Therefore, dcell / d pore ratio is regarded to be proportional to the electric field strength. From these results, it is deduced that pore diameter is proportional to voltage and inversely propor­tional to the square of the electric field strength, whereas barrier layer thickness and cell diameter are proportional to voltage and inversely proportional to the electric field strength. Accordingly, pore diameter is more strongly affected by current density than other cell parameters are. The content of incorporated anion in the films increases linearly with increasing log i, hence electric field strength. Therefore, it is evident that the anion incorporation is exactly governed by the electric field strength. [1] F.Keller, M.S.Hunter and D.L.Robinson, This Journal, 100, 411 (1953). [2] J.P.O'Sullivan and G.C.Wood, Proc. Roy. Soc. Lond. A.317, 511 (1970). [3] K.V.Heber, Electrochim. Acta, 23, 127 (1978). [4] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J. Electrochem. Soc ., 151, B473 (2004). [5] N. Cabrera and N. F. Mott, Ret. Pror. Phys., 12, 163 (1948). [6] C. J. Dell’Oca and L. Young, J. Electrochem. Soc., 117, 1548 (1970). [7] S. Ono, F. Mizutani, M. Ue, and N. Masuko, PV 2001-22, p.1129, The Electrochemical Society Proceedings Series, Pennington, NJ (2001).

(Invited) Effect of Heat Treatment Conditions on Crystallization of Anodic Alumina Membrane Formed in Phosphoric Acid

Porous-type anodic aluminum oxide films have attracted considerable attention as a key material in the fabrication of several types of devices owing to their potential technological applications. However, conventional anodic alumina films have low chemical resistance because the anodic oxide films obtained by typical anodizing processes are amorphous. Expanding the application of alumina films necessitates enhancement of the chemical resistance of alumina.Previously, we identified the optimized anodizing conditions and the detachment method for suppressing thermal deformation, such as curving and cracking, during heat treatment of the alumina membrane (1-4). In our previous study, anodizing was conducted under an applied voltage of 25 V–185 V, which resulted in the formation of an anodic porous alumina film with pore diameters tunable over a wide range of approximately 30-350 nm. When oxalic acid was used as an electrolyte, the fabrication of a crack-free and flat α-alumina membrane with a diameter of 25 mm, a thickness of 50 μm and a pore diameter of approximately 60 nm was fabricated using the optimized anodizing conditions in oxalic acid at 40 V, followed by subsequent detachment from the substrate and heat treatment (2). In the case of phosphoric acid, the fabrication of α-alumina membrane with a pore diameter of 350 nm was achieved through optimization of various conditions (3).In the present study, the chemical composition and morphological changes of the alumina membrane formed in phosphoric acid during heat treatment were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). The results of analyses clearly showed that the alumina membrane formed in phosphoric acid has unique characteristics such as coexistence of crystalline alumina and aluminum phosphate at high temperature region above around 1100 oC. Therefore, we focused on the effect of heat treatment conditions (e.g., temperature, heating rate and holding time) on the morphological changes of alumina cell wall in comparison with alumina films formed in sulfuric acid and oxalic acid. By controlling of heating conditions, alumina membrane with hierarchically porous structures consisted of main macropores and smaller textural mesopores in the cell walls could be fabricated. Through-hole structure of heated alumina membrane was maintained even at high temperature, and additional mesopores were formed by the selective removal of aluminum phosphate from alumina cell wall.[1] F. Rashidi, T. Masuda, H. Asoh, S. Ono, Surface and Interface Analysis, 45, 1490-1496 (2013)[2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)[3] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)[4] H. Asoh, T. Masuda, S. Ono, ECS Transactions, 69, 225-233 (2015)

Enhanced gas separation factors of microporous polymer constrained in the channels of anodic alumina membranes.

New composite membranes based on porous anodic alumina films and polymer of intrinsic microporosity (PIM-1) have been prepared using a spin-coating technique. According to scanning electron microscopy, partial penetration of polymer into the pores of alumina supports takes place giving rise to selective polymeric layers with fiber-like microstructure. Geometric confinement of rigid PIM-1 in the channels of anodic alumina causes reduction of small-scale mobility in polymeric chains. As a result, transport of permanent gases, such as CH4, becomes significantly hindered across composite membranes. Contrary, the transport of condensable gases (CO2, С4H10), did not significantly suffer from the confinement due to high solubility in the polymer matrix. This strategy enables enhancement of selectivity towards CO2 and C4H10 without significant loss of the membrane performance and seems to be prospective for drain and sweetening of natural gas.

Study the corrosion behavior of both first and second anodized aluminum surfaces electrolytically colored by nickel and tin

Nano porous anodic aluminum (AAO) characteristics are qualified to have several applications in vital fields such as Medicine and agriculture. Characterization of nano porous anodic aluminum oxide (AAO) film over aluminum substrate was studied. Two steps anodization method was employed for building up of ordered nano porous film. Nano wires of metal was prepared by electrodeposition of two different types of nano metals e.g. nano tin and nano nickel. It was observed the deposition of higher metal content after second anodization more than first anodization. Corrosion rate of second anodized aluminum samples that filled by tin after electrolytic coloring step (0.162mm/y) is lower than that of aluminum substrate (0.736mm/y). The effect of nickel and tin was investigated by Energy dispersive X-ray spectroscopy (EDX) and Scanning electron microscopy (SEM).

X-Ray-, Cathodo-, and Photoluminescence of Yttrium–Aluminum Composites on Porous Anodic Alumina Films

Yttrium–aluminum composites doped with terbium were synthesized by precipitation on porous anodic alumina fi lms grown on silicon substrates. The fabricated structures demonstrated x-ray-, cathodo-, and photoluminescence with characteristic bands of trivalent terbium upon excitation by Cu Kα x-rays of energy 8.86 keV, a 180-keV electron beam, and optical UV radiation, respectively. The terbium luminescence bands increased in intensity as the terbium concentration increased from 0.01 to 0.25 mol%. The intensity of a broad band in the blue spectral region with a maximum at 410 nm that was due to photoluminescence of the porous anodic alumina fi lm increased as the excitation wavelength increased from 260 to 340 nm. Simultaneously, the intensities of luminescence bands in the range 480–650 nm associated with Tb 3 + 5 D 4 – 7 F j (j = 3, 4, 5, 6) transitions decreased. The possibility of practical application of the synthesized luminescent structures was discussed.

Molecular Orientation of Liquid Crystals on Topographic Nanopatterns.

Controlling the orientation of building blocks in soft matter on the substrate has been a big challenge in material sciences. We have controlled the molecular orientation of liquid crystal (LC) materials on the porous anodic aluminum oxide (AAO) film having hexagonal pore arrays on the top surface. In our method, anchoring conditions can be varied by changing the pore size (Dp) and the porosity (P). As a proof-of-concept, the orientation of smectic A (SmA) structure at different anchoring conditions was successfully controlled in a sandwich cell consisting of AAO and a glass substrate, which has not been successfully controlled by conventional methods.

Self-Organization Process of Aluminum Oxide during Hard Anodization

The self-organization process of the porous anodic alumina (PAA) during hard anodization (HA) was proposed by synthetically investigating the nanopore morphology, the current density and barrier layer evolution of the PAA films, which were anodized in 0.3M oxalic acid at 150V (the voltage rising from 40V to 150V at the beginning). It was found that the high enough current density in the voltage rising stage can induce the fast film growth, which caused the pores to rapidly enlarge themselves and to reach a relatively ordered rearrangement. The barrier layer thickness showed linearly increase in voltage rising stage and then increased with a decelerated speed in the followed constant anodization stage, where the pores with growth advantage moved down straightly and gradually expanded their cell size to replace the inferior pores around. Accordingly, HA is a unequilibrium process and can be divided into two stages: I, the rapid pore enlargement and rearrangement during voltage rising stage, where the oxidation rate is larger than that of dissolution; II, competing growth of the nanopores during the followed constant voltage anodization, where the oxidation and dissolution rates approach to each other due to the thickened barrier layer. These findings are very helpful for more efficiently controlling the hard anodization process and developing new electrolyte systems to further broaden the interpore distance range.

Formation of Nanoporous Anodic Alumina by Anodization of Aluminum Films on Glass Substrates.

Our research was aimed at the study of aluminum films and porous anodic alumina (PAA) films in thin-film РАА/Al structures for optical sensors, based on metal-clad waveguides (MCWG). The results of the scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies of the structure of Al films, deposited by DC magnetron sputtering, and of PAA films, formed on them, are presented in this work.The study showed that the structure of the Al films is defined by the deposition rate of aluminum and the thickness of the film. We saw that under anodization in 0.3 M aqueous oxalic acid solution at a voltage of 40 V, the PAA film with a disordered array of pores was formed on aluminum films 200–600 nm thick, which were deposited on glass substrates with an ultra-thin adhesive Nb layer. The research revealed the formation of two differently sized types of pores. The first type of pores is formed on the grain boundaries of aluminum film, and the pores are directed perpendicularly to the surface of aluminum. The second type of pores is formed directly on the grains of aluminum. They are directed perpendicularly to the grain plains. There is a clear tendency to self-ordering in this type of pores.

Optical properties of one-dimensional photonic crystals based on porous films of anodic aluminum oxide

The optical properties of one-dimensional photonic crystals based on porous anodic aluminum oxide films have been studied by measuring transmittance and specular reflectance spectra in the visible and UV spectral regions. Angular dependences of the spectral positions of optical stop bands are obtained. It is shown that the reflectance within the first stop band varies from point to point on the sample surface, reaching a level of 98–99% at some points. The dispersion relation for electromagnetic waves in the model of infinite periodic structure is calculated for the samples under study. The possibility of using models with an infinite or finite number of layers to calculate reflectance spectra near the first optical stop band is discussed.

A novel theory interpreting the extremes of current during potentiostatic anodising of Al and the mechanisms of normal and abnormal growth of porous anodic alumina films

The kinetics and mechanism of growth of porous anodic alumina films were studied during potentiostatic Al anodising at suitable ranges of H2SO4 concentrations, temperatures, anodic potentials, stirring rates, distances of Al from stirrer and times. Chronoamperometry, determination of various physical and structural features and kinetic parameters and proper novel models interrelating anions transport numbers with field strength in barrier layer enabled the form1ulation of a novel theory interpreting the observed for first time extraordinary kinetic behaviour and the mechanisms of normal growth and abnormal growth where burning appears. It turned out that key parameter is mean barrier layer thickness defined by inherent thinning in the second transient stage and by thinning and thickening coming from continuous initiation and termination of some pores in this and quasi-steady state stage. In normal growth wholly and locally in Al surface chronoamperometric plots sequentially show low maximum at the boundary of two stages and then shallow minimum and high maximum in the last stage as the rates of barrier layer thinning and thickening are balanced at the extremes while the first prevails in transient stage and among the two last extremes. In abnormal growth the first two extremes merge in transient stage as thinning rate prevails endlessly and burning appears. It first occurs after certain conditions thresholds at position with highest local current density, as a rule around the lower Al side where stirring efficiency is also highest. New conditions and methods for high rates normal growth and desired properties of films are reliably predicted for first time, vital for Al anodising electrochemistry.

Evidence for electrohydrodynamic convection as a source of spontaneous self-ordering in porous anodic alumina films.

A comparative study of self-ordering behaviour of anodic alumina films fabricated in a series of diluted (down to 0.05 M) oxalic acid electrolytes allowed developing a relationship between the supporting electrolyte concentration and self-ordering voltages for the formation of porous oxide materials. Besides its practical importance, this work elucidates some fundamental principles of porous alumina formation, e.g. it suggests that the cell patterning arises from the electrohydrodynamic (EHD) convection process rather than the interfacial tension gradients near the anode surface (Marangoni-type instability).

Indirect oxidation of aluminum under an AC electric field

Anodic porous alumina films can be formed by indirect oxidation under an alternating-current electric field without a direct electrical connection.