Growth Of Anodic Oxide Films Research Articles

Non-Thickness-Limited Growth of Anodic Oxide Films on Tantalum

The non-thickness-limited (NTL) growth phenomenon reported by Melody et al. in which anodic oxide films on tantalum continue to grow in dry glycerol solutions of dibasic potassium phosphate to large thicknesses (tens of micrometers) at low applied voltages (tens of volts) was investigated using mostly constant current growth. The field strengths in the oxide for growth of anodic oxide films at 180°C with constant current density in wet glycerol solutions (1 vol % water) were in the MV/cm range and close to those calculated from the equations fitted to data from growth in aqueous solutions at lower temperatures. At constant current density and 180°C in dry glycerol solutions, the field fell with time and eventually the voltage decreased as the NTL state developed. The current efficiency for the growth of oxide was estimated by comparing thickness increase measured by ellipsometry with that calculated from the charge passed. It was lower in the NTL state. This is consistent with increased electronic conductivity in the oxide. The dielectric losses of the NTL films were much higher than those of films grown in wet glycerol solutions. Those grown in wet glycerol showed the normal, almost frequency independent, tan δ as found for ordinary anodic oxide growth on tantalum. Those grown in dry glycerol solutions showed increasing tan δ as the frequency decreased, consistent with increased electronic conductance through the oxide. It is suggested that the increased electronic and ionic conductivity in the NTL state may be caused by nonstoichiometry induced by the change in hydroxyl ion availability to the oxide suggested by Melody et al. when dry glycerol solutions are used. © 2001 The Electrochemical Society. All rights reserved.

In-situ evaluation of the anodic oxide growth on Hg1−xCdxTe (MCT) using ellipsometry and second harmonic generation

In-situ measurements of ellipsometry and second harmonic generation (SHG) were carried out to monitor the electrochemical growth of native anodic oxide films on Hg1−xCdxTe (MCT). Growth of the anodic oxide was performed using two different methods viz., by linear sweep voltammetry and by applying a constant current density. The influence of scan rate and the magnitude of the applied current density on the properties of the growing films were examined. From the ellipsometry data, we have shown that the measured refractive index value of 2.19 for the oxide film remains unchanged for moderate and high oxide growth rates. Only at very slow growth rates were significant increases in the refractive index observed (n=2.4), indicating an increase in the compactness of the layer. For film thicknesses in excess of ∼1200 A, a non-zero value for the extinction coefficient was found, indicating the incorporation of HgTe particles within the anodic oxide film. SHG rotational anisotropy measurements, performed on the MCT with and without an anodic oxide film showed only the four-fold symmetry associated with the MCT and so confirmed that the oxide was centrosymmetric. However, an increase in the SH intensity was observed in the presence of the oxide and this has been attributed to multiple reflections in the thin oxide film and also to the increase in the χ(2) non-linear susceptibility tensor as a result of charge accumulation at the MCT/anodic oxide interface.

Heterogeneous Growth of Anodic Oxide Film on a Polycrystalline Titanium Electrode Observed with a Scanning Electrochemical Microscope

A scanning electrochemical microscope (SECM) was applied to study anodic oxide film grown on a polycrystalline titanium electrode in deaerated pH 8.4 borate solution. The probe current images of SECM could detect the heterogeneous growth of anodic oxide film, depending on the substrate crystal grains. This heterogeneity increased with increasing the film formation potential at the potential higher than 3 V (SHE). The study of the dependence of film thickness on the substrate grain has been also supported with Raman microprobe spectroscopy. © 2000 The Electrochemical Society. All rights reserved.

アルミニウムアノード酸化皮膜成長のＩｎ‐ｓｉｔｕ ＡＦＭ観察 皮膜表面の平坦化機構

アルミニウムアノード酸化皮膜成長のＩｎ‐ｓｉｔｕ ＡＦＭ観察 皮膜表面の平坦化機構

Formation and accommodation of gold atom clusters and oxygen bubbles during amorphous anodic alumina growth

The growth of anodic oxide films on metals and alloys is of both theoretical value, in particular with relation to the mechanism of ionic transport in amorphous materials, and practical interest, especially in the surface treatment of metals and alloys and the use of anodic films in electronics applications. Most previous work has focused on uniform film growth, with formation of either an essentially single–layered or a duplex oxide. The present work, employing transmission electron microscopy and Rutherford backscattering spectroscopy, addresses the generation of metallic and gaseous phases in association with the usual film growth, and reveals the ability of the labile amorphous film to accommodate these non–oxide phases. Thus, anodizing of a thin Al–0.47 at.% Au alloy, deposited upon an electropolished aluminium substrate, is shown to result in formation of gold atom clusters of ca . 1–5 nm size and fine oxygen bubbles, at high pressures, which are contained by the growing amorphous alumina film. The generation of the phases originates from enrichment of gold in a thin layer of the alloy, probably containing gold–rich precursor clusters, during the oxidation of the alloy layer and the subsequent enforced incorporation of gold atom clusters into the anodic film by the electropolishing film on the aluminium. The final clusters are essentially immobile following incorporation into the growing anodic film. At the time of, and briefly after, the incorporation of clusters into the film, oxygen gas is produced by oxidation of O2- ions from the alumina, which occurs at the cluster–alumina interface prior to electrical isolation of the cluster due to undermining by further film growth. The resultant oxygen bubbles generated within the alumina expand as the film grows, driven by the high gas pressure and assisted by the effective plasticity of the labile amorphous alumina under the electric field.

The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals

The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals

The growth of a porous oxide film of a unique morphology by anodic oxidation of an Al-0.5 wt% Ni alloy

Porous anodic oxide film growth on an Al-0.5 wt% Ni alloy, containing finely dispersed Al 3Ni intermetallic particles, has been investigated by transmission electron microscopy of ultramicrotomed sections. It was found that film growth is influenced greatly by the localised processes proceeding at the intermetallic Al 3Ni particles, with consequent development of a unique film morphology, which has not been observed previously. At the initial stage of anodizing, the majority of the applied current is consumed by dissolution of the Al 3Ni particles at the alloy surface, accompanied by oxygen gas evolution, which leads to the suppression of the forming voltage, at 10 to 15 V, for a sufficiently long period until the particles are removed from the alloy surface. During this period, a thin porous film of fine dimensions, corresponding to the voltage of 10 to 15 V, is formed over the matrix surface generally. After the particles have been removed from the surface, the applied current is used for the growth of the porous oxide over the matrix surface as well as over the walls of cavities formed by dissolution of the particles. As the forming voltage increases, some of the pores grow further, at the expense of other pores in the immediate surrounding regions, with their dimensions controlled precisely by the increasing forming voltage, resulting in the formation of large pores of a ‘‘tear-drop’’ shape. Implications of these findings for forming porous anodic oxide films of controlled morphologies on aluminium and its alloys are discussed.

The mechanism of stress generation during the growth of anodic oxide films on pure aluminium in acidic solutions

The mechanism of stress generation during the anodic oxidation of pure aluminium in aqueous H2SO4 solutions has been explored using a.c. impedance spectroscopy and a beam deflection technique. From the analysis of a.c. impedance data, the resistance of the anodic oxide film was found to decrease as it grew progressively in 0.5 M H2SO4 solution, indicating an increase in the concentration of aluminium vacancies VAl3−(ox) within the film. From measurements of the deflection, compressive stresses were always observed at relatively low current density of 2 mA cm−2 during the growth of the film in aqueous H2SO4 solutions. Based upon the experimental results obtained from this work, it is suggested that stresses developed which determine the sign and magnitude of the deflections, are not distributed entirely over the whole oxide film, but are limited to a narrow region of the aluminium/oxide film interface below 1 nm. The changes in the sign and magnitude of stresses were satisfactorily accounted for in terms of the annihilation of VAl3−(ox) and the generation of oxygen vacancies VO2+(ox) at the aluminium/oxide film interface. From this, a new model for stress generation during the anodic oxidation of valve metals has been developed.

Nucleation and growth of anodic oxide films on bismuth

Abstract Combined voltammetry and potentiostatic transient techniques have been used to study nucleation, formation and growth of thin oxide films on high purity polycrystalline bismuth in a borate buffer solution, pH=9.2. It was shown that the initial step in the formation of the continuous anodic layer of bismuth oxide on bismuth is a nucleation process. The potentiostatic technique was a valuable tool in its study. The oxide film nucleation kinetics were explained reasonably well through a 3D progressive nucleation and growth mechanism under diffusion controlled growth. Nucleation potential ( E AN ), steady-state nucleation rate ( AN 0 ) and the number density of growing centres ( N s ) are determined. A detailed mechanistic interpretation of the nucleation process and thickening of the anodic layer under potentiodynamic conditions was obtained using the criteria of cyclic voltammetry.

Growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions

The present work was conducted to explore the growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions by using a.c. impedance spectroscopy and a beam deflection technique. From the analyses of a.c impedance data, it was found that the reciprocal capacitance of anodic oxide film on pure aluminium increased linearly with increasing film formation potential in both acidic and alkaline solutions, indicating a linear increase in the film thickness with film formation potential. However, as the film formation potential increased, the resistance of anodic oxide film decreased in acidic solution, while it increased in alkaline solution. From the measurements of the deflection, the deflection was observed to move towards only a compressive direction with time in acidic solution, but it showed a transition in the direction of movement from compressive to tensile in alkaline solution. Based upon the above experimental results, it is suggested that the movement of oxygen vacancy through the oxide film contributes to the growth of anodic oxide film on pure aluminium in acidic solution, but the movement of both aluminium vacancy and oxygen vacancy accounts for that oxide film growth in alkaline solution.

On the Kinetics of Growth of Anodic Oxide Films

Two theories, the high field model (HFM), and variants thereof, and the point defect model (PDM), have been advanced to account for the properties of thin anodic passive films (barrier layers). The HFM states that, under potentiostatic conditions, the electric field strength in a passive film decreases as the film thickens, while the PDM proposes that the field strength remains constant but that the potential drop across the metal/film interface decreases as the film grows. These different hypotheses lead to different kinetic laws for film growth. In this work, diagnostic criteria have been derived to determine the electric field‐thickness relationship and hence to identify the appropriate kinetic law. Experiments on the growth of anodic oxide films on zirconium, tungsten, and tantalum under potentiostatic conditions in phosphate buffer solution show that the PDM provides a better account of the experimental data than does the HFM.

Inter–relationships between ionic transport and composition in amorphous anodic oxides

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

Influence of Fe-bearing particles and nature of electrolyte on the hard anodizing behaviour of AA 7075 extrusion products

The hard anodizing behaviour of aluminium alloy 7075 extrusion products has been examined as functions of variation in the impurity (i.e. Si, Mn and Fe) contents in the alloy and the nature of electrolyte used for the anodizing process. A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) revealed that the chemistry, morphology and the mode of distribution of the Fe-rich constituent particles are important in determining the quality and the growth of the hard anodic oxide film. Using the conventional sulphuric acid—oxalic acid electrolyte, it was established that the Al 12(FeMn) 3Si-based particles survive the anodizing process, and this results in a steep rise in bath voltage with time as well as a non-uniform growth of the anodic oxide film. The extent of these effects are, however, greatly reduced when the Fe-bearing particles are aligned in a direction parallel to the growth direction of the anodic film. Studies employing a stronger electrolyte that is, sulphuric acid—oxalic acid—hydrochloric acid, on the other hand, revealed that this electrolyte is capable of counteracting the deleterious effects of Al 12(FeMn) 3Si-based particles by dissolving them during anodizing provided such particles are not too large in size.

Simulation of anodic Pt oxide growth

A microscopic model of the growth of the thin anodic oxide film on Pt is presented. The slow step is migration of a Pt atom from the metal lattice to become a Pt(II) species. Pt(II) species can then rapidly diffuse across the surface. Sites of different types, e.g. terrace atoms or step atoms, have different reactivity, dependent on their number and type of bonds. Monte Carlo simulations are used to predict the kinetics and the surface reconstruction. The treatment of the structures of the oxide and metal is oversimplified by the assumption that they both have simple cubic unit cells with identical lattice parameters. The model predicts correctly the observed direct logarithmic growth law for potential step transients, without any change in behavior at full monolayer coverage. For cyclic voltammetry, it predicts an anodic peak on the first cycle, and plateau behavior for subsequent cycles. The reversible component is found, and interpreted in terms of a reversible reconstruction aided by the rapid surface diffusion. The height distributions predicted by this model agree with recent structural measurements, but the exact topography of the surface after multiple cycles is not reproduced.

A novel marker approach for the determination of transport numbers during the growth of anodic oxide films on tantalum

Abstract From knowledge of the immobility of incorporated silicon species in anodic films on tantalum, developed by high-field ionic conduction, a procedure has been developed for their use as a marker to determine the mobilities of species contributing to film growth on tantalum and other species incorporated from the electrolyte. In the example given, sequential anodizing, firstly in silicate electrolyte and subsequently in sulphuric acid, have been used to determine the transport number of tantalum cations, 0.24, which is an excellent agreement with the value determined by Rutherford back-scattering spectroscopy and chemical sectioning.

Selective formation of β-PbO and α-PbO films on Pb electrodes by potentiostatic anodization in alkaline solutions

Interesting anodic behavior of Pb electrodes at low overpotentials for PbO formation in alkaline solutions is described. It is observed that during the initial growth of anodic oxide film, β-PbO is grown selectively and continued anodization leads to a phase change or the formation of the α-PbO phase. This result is very clear particularly at low overpotentials for PbO formation (ca. between −0.7 V and −0.5 V vs. SCE). An increase in the anodization temperature and/or increase in overpotential for PbO film formation assists the rapid formation of the anodic film with clear observation of phase change. Compositional analysis of anodic oxide films by X-ray diffraction (XRD) and photoelectrochemical techniques supports the above observation since both the PbO modifications have distinct XRD patterns and photoelectrochemical responses. When this observation of anodic PbO film formation is compared with other physical methods of PbO growth, a correlation is found owing to the particular thermodynamic and/or kinetic aspect which controls the selective formation of β-PbO initially, followed by α-PbO. Further, we observe that this phenomenon is not due to acidic or alkaline conditions maintained during film formation by electrochemical methods, rather it is due to the characteristic nature of structural and thermodynamic conditions which influence the growth of one particular phase in preference to the other.

ChemInform Abstract: Effect of UV Radiation on the Growth and Breakdown Voltage of Anodic Oxide Films on Niobium.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: The Effect of Temperature on the Growth of Anodic Oxide Film on Titanium.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Electrochemical kinetic study on the growth of porous anodic oxide films on aluminium

Aluminium specimens were anodized galvanostatically in a thermostated and vigorously stirred bath of H 2SO 4 15% w/v at different bath temperatures and current densities and for long anodization times, sufficient to produce the maximum or near maximum limiting film thickness, and the anodic potential was followed. Results confirmed that the rate controlling step of the steady-state growth of the barrier layer is the charge transport across this layer, which is near solely due to the migration of anions; oxide is produced only in a region adjacent to the Al-Al 2O 3 interface. Taking into consideration the structural features of the barrier layer oxide and by assuming a uniform oxide nature and one kind of migrating anions or cations, it has become possible to formulate simplified high field ion migration electrochemical kinetic equations. They were found to be non-satisfactory when applied at low anodization temperatures, but become applicable at high temperatures or low barrier layer thicknesses. Their application postulated that the charge transfer at the Al-Al 2O 3 interface, Al-3e → Al 3+ , takes place rather through successive “one electron” transfer elementary steps. Also, the charge transport across the barrier layer oxide bulk takes place through a concomitant migration of the ions O 2−, OH − and SO 4 2− . A model for the ions transport was suggested which gives a physical meaning to the field assisted oxide dissolution at pore bases, explains the incorporation of SO 4 2−, SO 3, OH − and H 2O and predicts the change of their concentration, of the oxide nature and of the values of kinetic parameters across the barrier layer; it also predicts that an H + migration may take place during oxide growth.

Kinetics of Growth of Anodic Oxide Films on Titanium in Mixed Solutions of Aqueous Electrolytes and Diethylene Glycol

Kinetics of Growth of Anodic Oxide Films on Titanium in Mixed Solutions of Aqueous Electrolytes and Diethylene Glycol