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.     

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.     

Optimized observation of tungsten tracers for investigation of formation of porous anodic alumina

Improved methods are presented for investigating the flow of anodic alumina during the formation of porous anodic films on aluminium in phosphoric acid. In particular, the use of tungsten nanolayer tracers with increased flatness is shown to result in enhanced definition of the influences of flow on the tracer distribution, as observed by electron microscopy. Additionally, taper sectioning of films, by ultramicrotomy, and parallel sectioning of films, by sputtering using an Ar⁺ ion plasma, enable tungsten distributions across cells to be determined. In the case of an Al–3.5 at.%W alloy, the flow results in an inner alumina layer lining the pore walls that is relatively free of tungsten species and an outer cell layer of comparatively high tungsten content. Both sectioning procedures also show the presence of fine cell boundary bands of relatively high tungsten content, which are suggested to result from the transport of tungsten, enriched in the alloy immediately beneath the film, toward the cell boundary by the alloy/film interface.     

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.     

Study of initiation and development of local burning phenomena during anodizing of aluminium under controlled convection

A study of burning during galvanostatic anodizing of aluminium in sulphuric acid is presented. Experiments are performed under controlled convection in wall-jet configuration, while in situ measuring local anode temperatures. Burning is a breakdown phenomenon manifesting itself in the local temperature evolutions and in the electrochemical behaviour of the electrode. Undulating oxide surfaces with protruding oxide hillocks, displaying a twofold microstructure, are formed. Focusing on short anodizing times the initiation of burning is determined in the period of declining potential following the maximum potential, linking burning to the development of the final porous structure. The influence of enhanced Joule heat generation during the onset of anodizing is investigated by anodizing electrodes covered by barrier-type oxide films, formed by an a priori performed barrier anodisation. Increased initial oxide thicknesses intensify burning, in the extreme case resulting in non-quenched phenomena involving quasi complete concentration of current in the burning area. However, the occurrence of burning during experiments on a priori barrier anodized electrodes at reduced current density exclude the conditions of heat transfer as key parameter for triggering burning. Results indicate the existence of a critical barrier layer thickness above which burning is encountered during the period of development of the regular porous structure.     

Growth of multioxide planar film with the nanoscale inner structure via anodizing Al/Ta layers on Si

An Al/Ta bilayer specimen prepared by a successive sputter-deposition of a 150-nm tantalum layer and a 180-nm aluminium layer onto a silicon wafer is anodically processed in a sequence of steps in oxalic acid electrolytes, at voltages of up to 53 V, which generates a 260-nm alumina film with well-ordered nanoporous structure. Further potentiodynamic reanodizing the specimen to 220 V causes the simultaneous growth of a 65-nm tantalum oxide layer beneath the alumina film and an array of oxide ‘nanocolumns’ (∼50 mn wide, ∼80 nm apart, ∼7 × 10⁹ cm⁻² population density) penetrating the alumina pores and reaching precisely to the top of the alumina film. The complete filling of the alumina pores is assisted by the high Pilling-Bedworth ratio for Ta/Ta₂O₅ and a substantially increased transport number for tantalum species (0.4), which is an average value of all migrating tantalum ions with different oxidation states. The nanocolumns are shown to be composed of a unique, regular mixture of Ta₂O₅ (dominating amount), suboxides TaO₂ and TaO_x (0.5 < x < 1), Al₂O₃, metallic Ta and Al aggregates, tantalum diboride (TaB₂) and oxidized boron from the electrolyte. The ionic transport processes determining the self-organized growth of these planar oxide nanostructures are considered and described conceptually.     

Formation of porous silicon at elevated temperatures

The features of electrochemical formation process of porous silicon (PS) at the temperatures above the room temperature have been studied. It was found that besides electrochemical dissolution, chemical etching takes part in the formation process of PS even for concentrated HF electrolyte. The role of chemical etching increases with temperature causing an increase of the porosity and the crater depth. The temperature dependence of chemical etching rate has been established. Obtained results enable to conclude that OH⁻ ions play a major role in the chemical etching. Electrochemical etching allows to fabricate PS with good surface quality at the temperatures at least below 65 °C provided that HF electrolyte is concentrated.     

Adsorption of organic compounds at the aluminium oxide/aqueous solution interface during the aluminium anodizing process

The paper reports a systematic study concerning the role played by different organic compounds (glycolic acid, oxalic acid, glycerol) on the electrochemical oxidation process of aluminium in sulphuric acid solution. The result of the experiment as a whole provides a clear indication of the deep influence exerted by the three selected organic compounds on the aluminium oxide structure, leading to the formation of a more compact oxide layer. Electrochemical impedance spectroscopy results appear of particular interest; they are discussed in the light of a model proposed by Bojinov. The relevant parameters, half-jump distance a and cross-capture section S, are found consistently related to variations in the oxide structure (porosity). This gives a sounder basis to the physically correct nature of the assumptions underlying the Bojinov model, also suggesting that organic compounds are adsorbed at the aluminium oxide/aqueous solution interface.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Electrochemical impedance spectroscopic study of barrier layer thinning in nanostructured aluminium

The electrochemical behaviour of electropolished and anodised aluminium was studied by electrochemical impedance spectroscopy (EIS). Freshly electropolished aluminium behaves as a pure capacitor exhibiting Warburg impedance at low frequencies. Storage of the electropolished aluminium, even in an air-tight bottle, results in the reconstruction of a uniform compact barrier layer. The impedance response of a stored electropolished aluminium as well as anodised aluminium after oxide removal, done by chemical etching, exhibits only a capacitive loop in the complex plane. The effect of the oxide layer thickness on the impedance data was investigated for layers formed during anodising at a cell potential of 15 or 23 V. Impedance measurements carried out over a wide range of frequencies gave useful information on the efficiency of the thinning of the barrier layer at the bottom of porous aluminium oxide layers. The rate of thinning of the barrier layer was estimated for samples anodised at different voltage.     

Characterisation of porous and barrier layers of anodic oxides on different aluminium alloys

Electrochemical impedance spectroscopy (EIS) provides a powerful tool for obtaining detailed information on the electrochemical properties of both porous and barrier layers on different aluminium alloys. The impedance value at a given frequency can serve to calculate the electrochemical parameters of the oxide layers represented by each component of the equivalent circuit (EC) which reproduces the behaviour of the studied systems. It is thus possible, with these parameters, to analyse the effects of any factor on the sealing and ageing processes of anodic aluminium oxide layers. Electrochemical results are completed with a detailed analytical study of the oxide layers by XPS, with gravimetric determinations of the changes experienced in the anodising and sealing processes, and with microstructural characterisation of the anodic films by scanning electron microscopy (SEM).     

The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon

The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced.     

Surface and interface analysis of nanostructured porous silicon layers fabricated at low temperatures from highly doped silicon substrate: application in optical filters

The influence of low temperature of the electrolyte on the surface and interface of porous silicon and its application in optical devices has been studied. Porous Si–bulk Si interface profile was observed by means of the high resolution SEM images. At low temperatures, a decrease in the interface roughness and an increase in the pore size has been observed. These results were used to fabricate optical filters for UV and visible regions at low temperatures. As compared to the filters fabricated at room temperature an increased optical response is reported for the filters formed at low temperature.     

Multi-layer configuration for the cathode electrode of polymer electrolyte fuel cell

This paper conducts a one-dimensional theoretical study on the electrochemical phenomenon in the dual-layer cathode electrode of polymer electrolyte fuel cells (PEFCs) with varying sub-layer thicknesses, and further extends the analysis to a triple-layer configuration. We obtain the explicit solution for a general dual-layer configuration with different layer thicknesses. Distributions of the key quantities such as the local reaction current and electrolyte overpotential are exhibited at different ratios of the ionic conductivities, electrochemical kinetics, and layer thicknesses. Based on the dual-layer approach, we further derive the explicit solutions for a triple-layer electrode. Sub-layer performances are plotted and compared. The results indicate that the layer adjacent to the electrolyte membrane may contribute a major part of the electrode faradic current production. The theoretical analysis presented in this paper can be applied to assist electrode development through complicated multi-layer configuration for cost-effective high performance electrodes.     

Electrochemical microstructuring of ultra-thin passive films on aluminium

Special optical sensors require thin film devices with a layer structure of the type glass/Al/Al₂O₃ with thicknesses of oxide and metal in the range of some nm. Structures comparable with a printed circuit board were written by electrochemical potential control with a scanning droplet device. Dots, lines, or areas of different thickness ratios Al/Al₂O₃ or even with gradients can be written. Insulating lines become possible by complete oxidation of the Al metal.     

Improving the adhesion between epoxy coatings and aluminium substrates

Two different methods were followed to improve the adhesion and durability of the adhesion of a commonly used epoxy coating on an aluminium substrate. The first method was by application of a thin polymeric layer, having a thickness of around 10 nm, on the aluminium substrate prior to application of the epoxy coating. The functional groups in the polymers were chosen so as to be capable of chemisorption to the oxide surface and should also to be capable of being involved in the curing reaction of the epoxy resin. These polymers were poly(acrylic acid) (PAA), poly(ethylene-alt-maleic anhydride) (PEMah) and poly(vinyl phosphonic acid) (PvPA). An investigation of the interphasial region between the epoxy coating and the aluminium substrate in the final cured system showed that the polymeric layers were indeed involved in the curing reaction with the epoxy.

For the poly(ethylene-alt-maleic anhydride)-based system, this resulted in the formation of a cured, mixed poly(ethylene-alt-maleic anhydride)/epoxy interphasial region between coating and substrate while for the two other polymers, a weakly cured interphasial region was formed. The second method of adhesion and durability improvement was by hydration of the aluminium substrates, performed by immersion in boiling water. This procedure results in the formation of a porous pseudoboehmite oxyhydroxide layer. The epoxy coating was found to be capable of fully penetrating into the layer. The adhesion of the epoxy coatings was tested initially and after exposure to 40 °C water and 40 °C 5% acetic acid. The poly(ethylene-alt-maleic anhydride)-based system resulted in a very good initial adhesion and durability in presence of water for the epoxy coating, while the systems based on the other two polymers did not. The pseudoboehmite-based system also resulted in very good initial adhesion and durability in the presence of water. None of the improved systems were, however, found to be able to withstand 40 °C 5% acetic acid and showed severe corrosion underneath the epoxy coating.     

In situ ATR-FTIR studies of the aluminium/polymer interface upon exposure to water and electrolyte

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with the Kretschmann configuration was applied for in situ studies of the transport of water and ionic species through a polymer film to an aluminium/polymer interface. The time dependent intensity changes of the infrared bands of water were used to follow the transport of water to the aluminium/polymer interfacial region and a NaSCN solution was employed as model electrolyte to follow the transport and accumulation of thiocyanate ions. Apart from water sorption and ion transport, the main processes identified were corrosion/oxidation of the aluminium surface and swelling of the polymer film. The method proved to be useful for detailed in situ studies of changes at a polymer coated metal surface, such as oxidation and surface film formation on the metal. It should also be possible to study the effects of defects and pores in the polymer film on the transport properties of water and ions to the metal/polymer interface, as well as adsorption and other chemical reactions and physical interactions in the metal/polymer interfacial region.     

Fabrication and structural control of anodic alumina films with inverted cone porous structure using multi-step anodizing

Porous anodic alumina (PAA) film has recently attracted much attention as a key material for the fabrication of various nanostructures. In this study, a multi-step anodization and leaching process was employed to produce three-dimensional nanometer scale structured film. During the leaching process, the porous alumina film was dipped in phosphoric acid solution for pore widening. Each anodization process was followed by this leaching process. This method produced alumina film with multi-step structure. Meanwhile, with five-step film production, the structure showed inverted cone structure. We produced the low aspect ratio pores of this structure, which would be applicable for fabrications of nanomaterials. In addition, the aspect ratio was controlled by changing the anodization duration.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

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.