Anodic Oxide Formation Research Articles

Combinatorial surface nanostructuring in aluminium-niobium system

Co-deposition at glancing angles is used for deposition of an AlNb thin film library with a compositional resolution of 0.8 at.% mm−1 and a total compositional spread of 55 at.%. Atomic self-shadowing leads to nanostructure formation along the compositional gradient. The morphology of the obtained different nanowires is fundamentally studied together with their crystallinity in the cubic system and is linked to specific AlNb nanostructured alloys. Composition and morphology of the nanowires, naturally emerging as coupled variables due to the deposition geometry, are decoupled using the deposition distance. This allows identifying conditions for reproducing a single wire for further applications. Electrical properties are mapped across the library by scanning Kelvin probe microscopy and four point resistivity measurements. Nanowires with low work function are identified at low Nb concentration. Contact angle measurements are performed along the library before and after anodic oxide formation, and these changes are mapped as a function of composition. The presented compositional mappings are relevant for alloys selection in applications where surface tuning or functionalization is paramount.

ELECTROCHEMICAL SYNTHESIS OF POROUS CRYSTALLINE TANTAL OXIDE

Self-organization of nanoscale structures during electrochemical processing is most pronounced during the formation of porous anodic metal oxides (aluminum, titanium, tungsten, niobium, tantalum). These oxides contain arrays oriented perpendicular to the pore substrate. A distinctive feature of these films is a high degree of orderliness in the arrangement of pores and the possibility of controlled variation of the pore diameter in a wide range (from 10 to 150 nm). In this work, the features of the electrochemical formation of nanoporous oxide coatings on tantalum in acid-fluoride and organic electrolytes are investigated. Tantalum foil with a thickness of 0.1 mm and a purity of 99.99 % was used as a working electrode. For the formation of tantalum oxides, 1 M H2SO4 solutions with the addition of HF (0.1 M; 0.25 M; 0.5 M; 1 M) and an organic electrolyte were used: EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NH4F, EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NaF. Polarization studies were carried out on a P-45X potentiostat. in potentiodynamic mode. The reference electrode is saturated silver chloride. The magnitudes of the potentials are given relative to the normal hydrogen electrode. The morphology of the obtained coatings was studied using scanning electron microscopy using a JSM-7001F microscope. It is shown that the use of fluoride ion activator and electrolytes of different nature allows at the initial stage of anodizing to provide conditions for the formation of a crystalline anodic oxide film with different surface morphology. By varying the anodizing regime on tantalum, it is possible to synthesize porous films of amorphous or crystalline types. To obtain a crystalline porous oxide of tantalum with a developed surface, it is advisable to use an organic aprotic electrolyte composition: EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NH4F.

Morphological evolution and burning behavior of oxide coating fabricated on aluminum immersed in etidronic acid at high current density

Micro arcs on metal surface immersed in alkaline electrolyte have been studied by many works, but that in acidic electrolyte are rarely reported. In this work, anodizing of aluminum was carried out in etidronic acid (HEDP) at high current density to investigate the formation behavior of coating. SEM images indicate that the burning of initial film is actually caused by local oxide cracking due to the increase of compressive stress. The soft spark would appear when voltage reached 300 V, then change to the micro arcs at 400 V, and finally manufacture the outer micro-arc oxide (MAO) layer. Hereafter, the automatic extinguishing of micro arcs gives rise to the formation of inner anodic aluminum oxide (AAO) layer. Moreover, it is evidenced that plasma discharge has few contributions to coating thickening. The barrier layer provides the raw materials for the growth of outer MAO layer. Based on these results, the growth behavior of coating processed in HEDP at high current density is proposed and discussed.

Polarization: the key to anodic oxide formation

In this theoretical treatment, the approach to the anodic aluminum oxide structure is based upon the premise that it is a self-assembled, highly ordered, nano-scale network of individual cells. The individual cells are regarded from the point of origin to understand 1) how the oxide is ordered and 2) how the oxide network continues to grow after the substrate surface is reconstructed by oxide. By considering the conditions that surround the anodizing process, specifically, the effects of polarization and the applied current bias, a mechanistic approach to the formation and growth of the anodic aluminum oxide is presented

Review on the Formation of Anodic Metal Oxides and their Sensing Applications

Review on the Formation of Anodic Metal Oxides and their Sensing Applications

Anodization of Aluminum in Highly Viscous Phosphoric Acid. PART 2: Investigation of Anodic Oxide Formation and Dissolution Rates

Anodization of Aluminum in Highly Viscous Phosphoric Acid. PART 2: Investigation of Anodic Oxide Formation and Dissolution Rates

ВЛИЯНИЕ ЭЛЕКТРОХИМИЧЕСКОЙ ПОЛИРОВКИ ТАНТАЛА НА ДИЭЛЕКТРИЧЕСКИЕ ПАРАМЕТРЫ АНОДНОГО ОКСИДА

Изучено анодное поведение Та-электрода в 0.1 M водном растворе H3PO4 после стадии электрохимической полировки поверхности металла. Установлены оптимальные условия проведения процесса электрохимической полировки в электролите HF:C3H7OH:H2SO4. На основе анализа циклических вольтамперных кривых сделано заключение, что эффективность процесса формирования анодного оксида тантала близка к 100 %. Данные импедансной спектроскопии подтверждают образование оксидной пленки с высокими диэлектрическими характеристиками. Проведен расчет диэлектрической проницаемости сформированного оксида, дана оценка фактора шероховатости поверхности тантала после ее электрохимической полировки.

An efficient and low-cost photoanode for backside illuminated dye-sensitized solar cell using 3D porous alumina

This paper presents a new strategy for developing back-side illuminated dye-sensitized solar cells (DSSCs) using nanoporous alumina substrate containing Cu impurities. The presence of a traces of Cu in the aluminum leads to the formation of 3D nanoporous anodic aluminum oxide (AAO) structure which consisted of an array of cylindrical pores interconnected by horizontal holes. Under different anodizing conditions, four different AAO substrates were fabricated and applied for developing DSSCs. The current-voltage characteristics of the DSSCs were measured and compared. Results obtained indicated that the performance (photocurrent density, open-circuit photovoltage and efficiency) of DSSCs based on 3D AAO was significantly enhanced compared to those of FTO/TiO2. The efficiencies of DSSCs based on alumina P90A, P90B and OX60 were improved by 172%, 51%, and 274%, respectively. The enhancement in photocurrent and efficiency is attributed to the reduction in the carrier recombination, uniform distribution of the dye molecules inside the vertical tubes, horizontal interconnections of pores as well as a reduction in pore size. This indicates that AAO based DSSCs can contribute significantly to energy harvesting from the sun.

Ozone Generator Aluminum Electrode Soldering

The creation of an entirely soldered ozone electrode component is proposed and implemented for the first time by the authors. Electrode construction with a ribbed plate insert within it is the most perfect heat exchange device from the point of view of effective heat exchange and uniform distribution of liquid flow through its section. Stages are considered for electrode preparation for soldering and a standard heating cycle is proposed for soldering and electrode with eutectic silumin. It is shown that as a result of the soldering thermal cycle metal structure becomes coarse-grained and favorable for subsequent electrochemical formation of a nanostructured anodic oxide coating 50–60 μm thick.

Self-Assembly of Metal Oxide Nanoparticles in Liquid Metal toward Nucleation Control for Graphene Single-Crystal Arrays

Summary We have achieved the spontaneous assembly of alumina nanoparticles on a liquid metal surface and further synthesized a very-large-scale graphene single-crystal array. The effective electrical field established as a result of the electrical charge of the alumina nanoparticles in the liquid metal ensures the ordered self-assembly of the alumina islands and lays the foundation for precise spatial control of graphene nucleation, leading to the formation of graphene single-crystal arrays. In addition, controlling the density of the alumina in the liquid metal can precisely tune the periodicity of the arrays. This simple strategy, which is unlike the usual solid/aqueous solution system, opens up new territory for nanoparticle assembly. Focusing on the origin of the nanoparticle interaction that drives the self-assembly of ordered structure in the liquid metal system is a promising avenue for triggering more interesting applications.

New insights into the oxidation rate and formation of porous structures on silicon

The rate of oxide formation on silicon was investigated by in situ I–V measurements, one after the other within 300 s, and as a result the current or I–V gap was used to compare the relative oxidation rate during pore formation at different anodizing currents. The study was based on the electrochemical etching of p-type silicon using HF electrolyte treated with surfactant to obtain pores with straight walls. The study showed that the rate of oxidation during the formation of macropores, which have straight walls and relatively deep structures, decreases as the pores grow down with time, where it shows that the etching rate decreases as the pores grow deep. However, electropolishing resulted in a relatively high and constant rate of oxide formation with time. These observed properties are related to the expected diffusion limitation of oxide-forming (OH−) molecules, which reach the electrolyte-pore tip while etching occurs. Anodic oxide formation rate at the electrolyte-pore tip, which was created under complex kinetic conditions, depends on the pore structure, pore depth, diffusion rate of oxide-forming species, and anodizing current. The fast I–V method is an advanced approach and can be used to study pore formation mechanisms in different semiconductor materials.

Microelectrochemical investigation of anodic oxide formation on the aluminum alloy AA2024

The present paper deals with the influence of the most relevant intermetallic phases (S- and Θ-phases) of the aluminum alloy AA2024 under anodizing conditions in citric acid. The study is a combinatorial approach of localized microelectrochemistry and complementary material diagnostic using SEM/EDX-analysis. The EDX-analysis is carried out quantitatively. It is shown that different individual electrochemical processes, e.g. anodic oxide formation and corrosion effects, superimpose and dominate the anodizing process at different time scales. The complexing effect of citric acid on copper can be excluded at the chosen acidic pH value. Though, the formation of obviously dense barriers such as oxide films leads to a oxidation behavior in citric acid similar to the anodizing in neutral electrolytes.

Integration of electrochemical and synchrotron-based X-ray techniques for in-situ investigation of aluminum anodization

Anodization of aluminum alloys AA 6082 and AA 7075 was investigated in-situ with integrated electrochemical and synchrotron-based X-ray reflectivity (XRR) methods providing complementary information about the anodic processes taking place on the alloys. The stepwise potentiostatic polarization measurements reveal dynamic processes of the anodic oxide formation and dissolution, and the following electrochemical impedance spectroscopy measurements detect the break of the native oxide and the growth of typical two-layer anodic oxide film, while the XRR measurements show the growth of entire anodic oxide film whose thickness increases linearly with the increasing applied potential. The results indicate that while a stable anodic oxide can be formed on the both alloys with a similar growth factor, AA 7075 shows a thinner thickness of the barrier layer and a lower resistance of the oxide film. The electrochemical results suggest both localized and uniform anodic dissolution processes, which are more pronounced on AA 7075, demonstrating the effect of alloying elements on the growth of anodic oxides.

Effects of anodization parameters on the corrosion resistance of 6061 Al alloy using the Taguchi method

Anodizing 6061 aluminum alloy (AA6061) in a H2SO4 electrolyte solution is a crucial process in the formation of anodic aluminum oxide (AAO) film for enhancing the material’s corrosion resistance. In this work, the effects of electrolyte concentration, current density, and anodization time parameters on the evolution of corrosion resistance in AA6061 alloy were investigated using the Taguchi method. Each anodization parameter had three levels using an experimental set of L9 orthogonal arrays. Variation in the bias voltage was recorded during the AAO process, and an energy-dispersive spectrometer was used to analyze the variations in the composition of the oxidized films. Variations in surface morphology and cross-sections of the AAO were examined using a scanning electron microscope. Finally, the potentiostatic polarization method was used to judge the relative corrosion resistance of the oxidized films. The oxidized film with the best corrosion resistance (Icorr = 8.516 × 10−11 A/cm2) was obtained by anodizing at a current density of 1 A/dm2 in 1 M sulfuric acid for 20 min.

Partial currents of anodic oxidation of copper in alkaline media according to RRDE data. II. Experiment

Anodic oxidation of copper in aqueous alkaline media is studied using the method of chronoamperometry on a rotating ring–disk electrode in a wide range of potentials. Partial currents of processes of anodic oxide formation, active copper dissolution, and chemical dissolution of Cu(I) oxide taking into account the possibility of its chemical growth are calculated. The current of chemical oxide dissolution is registered at all the studied potentials. In the cathodic range of potentials, anodic dissolution of copper is thermodynamically impossible, but, after the disk electrode polarization is switched off, oxide is formed via the chemical route. In the range of potentials where anodic copper dissolution is thermodynamically possible, the current of anodic oxide formation prevails over the currents of the other partial processes. The dependence of partial currents on time within the studied time interval (120 s) is not systematic, while their dependence on the disk polarization potential is very significant: the rate of oxide formation repeats the shape of the cyclic voltammogram, the rate of active copper dissolution through the oxide pores increases with potential, and the rate (and rate constant) of chemical dissolution of Cu(I) oxide are maximum at the potentials of its anodic formation. If the thermodynamic activity of the oxide layer does not reach unity within the anodic oxidation period, then the rate constant of chemical copper oxidation exceeds the rate constant of chemical oxide dissolution. The result is chemical oxidation of copper by traces of molecular oxygen in the solution.

Oxide Microstructural Changes Accompanying Pore Formation During Anodic Oxidation of Aluminum

Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two types of experiments: potentiodynamic anodizing and stress measurements during dissolution of the anodic films. Cyclic voltammetry during anodizing in sulfuric acid shows that pore formation coincides with the appearance of localized ohmic- conducting regions close to the oxide-solution interface of individual pores. These defects induce large current increases which are apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base. It is argued that interface defects are generated by surface forces in the oxide that accompany flow contributing to pore initiation. Stress measurements reveal that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. Such defects are explained by increases of local conduction current density due to the concentration of anodizing current in pores. Both sets of experiments reveal that pore formation generates locally defective oxide, which may help explain the phenomenon of burning during high-rate anodizing.

In-Situ Monitoring of Metal Dissolution during Anodization of Tantalum

The dissolution of Ta in acidic media during both potentiodynamic and potentiostatic anodic oxide formation was quantified in-situ by ICP-MS. Potentiodynamic polarization yields a constant dissolution rate of 4 pg s−1 cm−2 (5 fm s−1) while a continuous dissolution rate decay was observed during potentiostatic anodization. The rate of Ta species release increased with applied potential in good agreement to the measured current density transients. A higher percentage of the charge carriers was identified as responsible for generation of soluble Ta species in potentiostatic regime as compared to potentiodynamic oxide growth. The current efficiency of anodization was determined from the combined investigation to exceed 99.9%.

Flow Instability Mechanism for Formation of Self-Ordered Porous Anodic Oxide Films

Experimental studies have suggested that oxide flow plays an important role in the growth of porous anodic oxide films on reactive metals. Here a mathematical model for anodic oxide formation is presented that includes viscous flow of oxide, along with high-field ionic conduction and interfacial oxygen transfer and metal dissolution reactions. Flow is driven by near-surface compressive stress, as has been detected by measurements of residual stress distributions in anodic alumina. Linear stability analysis of the model shows that pattern formation at the solution interface is possible near the critical value of a parameter expressing competition between stabilizing electrochemical oxide formation and destabilizing flow. According to the critical parameter value, pattern formation requires that the Faradaic efficiency for oxygen transfer at the solution interface must be close to the transport number for electrical migration of oxygen ions across the film. This relationship between efficiency and transport number is indeed suggested by experimental data. A scaling ratio of pore separation to voltage, as characteristically found in anodic films, is predicted by the wavelength at the critical point.

Characterization of nanoporous anodic aluminum oxide formed on laser pre-treated aluminum

Technical purity aluminum was subjected to laser pre-treatment. Subsequently, self-organized anodization in sulfuric acid was performed in order to form nanoporous oxide layer. The influence of the laser pre-treatment operating conditions and grain size of the direct solidified aluminum on the formation of the nanoporous anodic aluminum oxide was studied on the base of FE-SEM images quantitative analyses. It was found there is no significant influence of laser pre-treatment on pore diameter, interpore distance and thickness of the grown oxide. Furthermore, fast Fourier transform-based quantitative image analysis showed that arrangement of the nanoporous alumina grown on the laser pre-treated aluminum is comparable to the arrangement of nanopores formed on high-purity aluminum without any pre-treatment. These findings were supplemented by the circularity analysis of the grown nanopores. This means that laser-treated aluminum elements can be anodized without significant decreases in the quality of the anodic oxides.

(Invited) Anodic Oxide Formation and Oxygen Evolution on Metals Such As Al and Ta - Experiment and Simulation

This presentation recalls the peculiarities of anodic polarization valve metals namely aluminium and Tantalum. Both metals are very unnoble and react immediately with oxygen or water. They form an anodic oxide that hinders further transport of anions and cations through this film. Upon higher polarization the electrical field in the oxide exceeds the critical value of oxide formation field strength enabling metal cations and oxygen anions to migrate within the high field gradient in a thermally activated and field supported hopping process. This mechanism that is known as the high field mechanism of oxide formation nicely explains the most important aspects of oxide formation but some of the kinetic observations require a more sophisticated description that is called the extended high field model. As a result of the ions injected into the adjacent phases through the interface, a space charge is formed that drastically changes the local field strength. Since the potential drop over the entire film thickness remains constant, other regions of the film must compensate the reduced interface potential drop. This leads to a temporal imbalance of the driving forces within the oxide. For a limited time the charge carrier concentration can become extremely high, resulting in an experimentally observed maximum in current density named overshoot. This overshoot is not only observed in potentiostatic experiments but also in potentiodynamic scans. Both types of experiments can be satisfyingly simulated on the basis of this model. Under specific conditions however, oxide formation is not the only occurring process, meaning that the current efficiency in terms of oxide formation is below 100%. There are at least two different charge transport mechanism leading to oxygen evolution. The first one is the more obvious one in which the oxide breakdown field strength is exceeded resulting in a, at least temporal and/or local, destruction of the oxide film, that allows charge flow into this side reaction. The second one is less obvious since direct elastic tunnelling of electrons is not possible over a range of a few nm. A large number of defects in the insulating oxide film which can also be described as states in the band gap may allow charge transport in an unexpected way. The importance of these findings, the model behind, and the industrial applications will be discussed.