Influence of silicon on the growth of barrier-type anodic films on titanium

Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti-Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ∼60 V on the Ti-6 at.% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at.% silicon. The crystalline oxide, nucleated at a depth of ∼40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.     

The anodic oxidation of superimposed niobium and tantalum layers: theory

Perriere, Rigo and Siejka have investigated the anodization of superimposed layers of niobium and tantalum, and their results are interpreted here in terms of known processes for the individual metals. Niobium oxide is less resistive to the passage of anodizing current than tantalum oxide, and it can then be shown that, when the niobium layer is superimposed, the metal order will be conserved during anodization. When the tantalum layer is superimposed, however, a partial inversion occurs, with fingers of the substrate niobium oxide pushing their way through the tantalum oxide. The latter is thus converted to a mesh, whose holes are filled with niobium oxide; because the current flows preferentially through this niobium oxide, the tantalum oxide in the mesh is almost completely divorced from the anodizing process. It then acts as a good inert, immobile marker for the purpose of measuring the transport numbers of metal and oxygen, and thus confirms the immobility of the noble gas markers used previously. The tantalum oxide originally within the holes mixes with the niobium atoms pushing their way through; these tantalum atoms then migrate outwards with the niobium atoms, but at approximately two-thirds the rate. The observed movement of the tantalum atoms thus depends on their state of aggregation within the anodic film. The experiments of Perriere et al. with oxygen isotopes can be interpreted on the basis that the oxygen order is conserved throughout the migration process     

Formation of porous anodic films on Ti-Si alloys in hot phosphate-glycerol electrolyte

Porous anodic films, with pore size of ∼10 nm, have been developed by anodizing of magnetron sputtered Ti-25 at.% Si alloy at constant formation voltages in glycerol electrolyte containing dibasic potassium phosphate at 433 K. The films, of amorphous structure, contain titanium and silicon species, as units of TiO₂ and SiO₂, throughout the film thicknesses, with negligible amounts of phosphorus species. The silicon is enriched in the film relative to the composition of the alloy, the level of enrichment suggesting that anion migration is increased in comparison with amorphous film growth at ambient temperature. In contrast to the behaviour of the alloy, essentially barrier films were formed on commercially pure titanium in the glycerol electrolyte, when a main anodic reaction was generation of oxygen, which was probably promoted by the development of anatase.     

Growth of anodic oxide films on oxygen-containing niobium

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

Control of morphology and surface wettability of anodic niobium oxide microcones formed in hot phosphate–glycerol electrolytes

We report the fabrication of superhydrophobic surfaces with a hierarchical morphology by self-organized anodizing process. Simply by anodizing of niobium metal in hot phosphate–glycerol electrolyte, niobium oxide microcones, consisting of highly branched oxide nanofibers, develop on the surface. The size of the microcones and their tip angles are controlled by changing the applied potential difference in anodizing and the water content in the electrolyte. Reduction of the water content increases the size of the microcones, with the nanofibers changing to nanoparticles. The size of microcones is also reduced by increasing the applied potential difference, without influencing the tip angle. The hierarchical oxide surfaces are superhydrophilic, with static contact angles close to 0°. Coating of the anodic oxide films with a monolayer of fluoroalkyl phosphate makes the surfaces superhydrophobic with a contact angle for water as high as 175° and a very small contact angle hysteresis of only 2°. The present results indicate that the larger microcones with smaller tip angles show the higher contact angle for water.     

Light induced electropolymerization of poly(3,4-ethylenedioxythiophene) on niobium oxide

The photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered niobium. Photocurrent Spectroscopy was employed to study the optical properties of Nb/Nb₂O₅/PEDOT/electrolyte interface in a large range of potential, and to get an estimate of the band gap and flat band potential of both the oxide and the polymer. Scanning Electron Microscopy was used to study the morphology of PEDOT. Both the optical and morphological features of the photoelectrochemically grown polymer were compared with those showed by PEDOT electropolymerized on gold conducting substrate.     

Influence of silicon species on the electric properties of anodic niobia

The influence of incorporation of silicon species on the electric properties of anodic niobia, formed in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, has been examined using sputter-deposited Nb-Si alloys containing 5 and 17 at.% silicon. The potential dependence of the capacitance of anodic niobia, originating from its n-type semiconducting properties, becomes less significant by incorporation of silicon species. In addition, the leakage current decreases with increasing silicon content in the alloy. The thermal stability of the anodic niobia is also enhanced by silicon species; the capacitance and leakage current, which increase significantly for niobium, are little influenced by annealing up to 523 K. The silicon species are incorporated in the inner 72% of the film thickness, as a consequence of immobility of the species in growing anodic niobia. The immobility of silicon species is associated with a strong Si⁴⁺O bond, which may also contribute to the reduction of leakage current.     

Amorphous-to-crystalline transition of silicon-incorporated anodic ZrO2 and improved dielectric properties

Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm⁻³ ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr-16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO₂, are developed even at low formation voltages. The outer crystalline layer on the Zr-16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO₂. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr⁴⁺ ions outwards and O²⁻ ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.     

Photocurrent spectroscopy study of passive films on hafnium and hafnium-tungsten sputtered alloys

Anodic and air-formed films on sputtered Hf and W-Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH)₂. This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials (V_f>10 V). In the case of W-Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising in different electrolytes. A PCS analysis of films grown on sputtered alloys is performed on the basis of previously proposed correlation between the band gap of anodic films and difference of electronegativity of their constituents.     

Complex anticorrosion coating for ZK30 magnesium alloy

This work aims at developing a new complex anticorrosion protection system for ZK30 magnesium alloy. This protective coating is based on an anodic oxide layer loaded with corrosion inhibitors in its pores, which is then sealed with a sol–gel hybrid polymer. The porous oxide layer is produced by spark anodizing. The sol–gel film shows good adhesion to the oxide layer as it penetrates through the pores of the anodized layer forming an additional transient oxide–sol–gel interlayer.The thickness of this complex protective coating is about 3.7–7.0 μm. A blank oxide–sol–gel coating system or one doped with Ce³⁺ ions proved to be effective corrosion protection for the magnesium alloy preventing corrosion attack after exposure for a relatively long duration in an aggressive NaCl solution.The structure and the thickness of the anodized layer and the sol–gel film were characterized by scanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with the complex coating was tested by electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET), and scanning ion-selective electrode techniques (SIET).     

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

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

Galvanostatic anodization of pure Al in some aqueous acid solutions Part I: Growth kinetics, composition and morphological structure of porous and barrier-type anodic alumina films

The growth kinetics of anodic films formed on the surface of high purity Al by anodization under galvanostatic conditions at current densities in the range 5–75 mA cm^–2 in thermostatically controlled and vigorously stirred solutions of chromic, sulfuric, phosphoric, citric, tartaric and oxalic acids at different temperatures, were studied. It has been shown that chromic acid solution produces a typical barrier type oxide growth at any given temperature, while the specific kinetic curve representing the combined barrier/porous type film growth is observed when the anodization process is carried out in a nonstirred chromic acid solution. The oxide growth in the rest of the anodizing solutions occurs in different ways depending on the bath temperature. Barrier oxide growth is observed at temperatures lower than 30 °C. Above this temperature, combined barrier/porous oxide growth is observed. In all cases, the slope of the linear part of the potential against time curves, and therefore the rate of barrier oxide growth, increases with increasing anodizing current density and acid concentration, while it decreases with increase in temperature. The composition and surface morphology of the anodic films have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM).     

Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium

Anodizing of niobium has been investigated to develop niobium solid electrolytic capacitors. Chemically polished niobium specimens were anodized in a diluted phosphoric acid solution, initially galvanostatically at i_a = 4 A m⁻² up to E_a = 100 V, and then potentiostatically at E_a = 100 V for t_pa = 43.2 ks. During the galvanostatic anodizing, the anode potential increased almost linearly with time, while, during potentiostatic anodizing, the anodic current decreased up to t_pa = 3.6 ks, and then increased slowly before decreasing again after t_pa = 30.0 ks. Images of FE-SEM and in situ AFM showed that nuclei of imperfections were formed at the ridge of cell structures before t_pa = 3.6 ks. After formation, the imperfection nuclei grew, showing cracking and rolling-up of the anodic oxide film, and crystalline oxide was formed at the center of imperfections after t_pa = 3.6 ks. The growth of imperfections caused increases in the anodic current between t_pa = 3.6 and 30.0 ks. Long-term anodizing caused a coalescence of the imperfections, leading to decreases in the anodic current after t_pa = 30.0 ks. As the imperfections grew, the dielectric dispersion of the anodic oxide films became serious, showing a bias voltage dependence of the parallel equivalent capacitance, C_p, and a dielectric dissipation factor, tan δ. The mechanism of formation and growth of the imperfections, and the correlation between the structure and dielectric properties of anodic oxide films is discussed.     

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.     

多孔氧化铝模板的制备及应用

以磷酸溶液为电解液、以高纯铝为阳极,采用两步阳极氧化法制备氧化铝模板。扫描电子显微镜(SEM)对其表面形貌分析表明,氧化铝膜为多孔结构,膜孔径随着阳极氧化电压的增大而不断增大。对阳极氧化电流密度变化分析证实,铝的阳极氧化经历了三个阶段:阻挡层的生成、多孔层的形成和多孔层的稳定生长。以制备的氧化铝膜为阴极、锌片为阳极,以硝酸锌和硼酸的混合液为电解液,采用交流电沉积方法制备了针状氧化锌纳米线。     

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.     

A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al-10 wt.%Si and Al-10 wt.%Si-3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al-10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl₂ particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions.     

Surfactant-assisted growth of anodic nanoporous niobium oxide with a grained surface

Nanoporous niobium oxide film with a maximum thickness of 520 nm was prepared by anodizing niobium in a mixture of 1 wt% HF, 1 M H₃PO₄, and a small amount of Sodium Dodecyl Sulfate (SDS) surfactant. The porosity of the anodic niobium oxide prepared without SDS is irregular with the surface of the oxide suggesting a grained surface pattern rather than an ordered porous structure. A proper amount of SDS addition can prepare a pore arrangement with stripe patterns. The pore depth and surface pattern were strongly affected by the concentration of SDS and bath temperature. We found that the addition of SDS surfactant facilitated improvement in the chemical resistance of niobium oxide, leading to the formation of pores with a longer length compared to those prepared without a SDS surfactant. This can be in part ascribed to the protection of the surface by the physical adsorption of SDS on the surface due to a charge-charge interaction and be in part attributed to the formation of NbO bonding on the outermost oxide layer by SDS. When anodization was carried out for 4 h, the surface dissolution of niobium oxide was observed, which means that the maximum tolerance time against chemical dissolution was less than 4 h.     

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.     

Anodic Oxidation of Nitric Oxide on Au/Nafion®: Kinetics and Mass Transfer

The rate determining step for the anodic oxidation of nitric oxide on Au/Nafion^® was experimentally and theoretically found to beNO + Au Au–NO_ads The anodic oxidation of nitric oxide was first order with respect to nitric oxide. The reaction rate constant increased from 3.3×10^–5 to 9.6×10^–5cm s^–1 as the applied potential increased from 0.74 to 0.77V. The anodic oxidation of nitric oxide was controlled by the electrochemical kinetics when the anodic potential was less than 0.8 V. When the potential was greater than 1.0 V, it was located in the mass transfer region. The limiting current increased from 1184 to 1589A with increase in gas flow rate from 250 to 750ml min^–1 when the potential was set at 1.05 V and the concentration of nitric oxide was 100 ppm. The diffusion resistance in the gas diffusion layer can be neglected for gas flow rates greater than 750 ml min^–1. The diffusivity of nitric oxide and the equivalent diffusion layer thickness within the porous electrode were evaluated to be 3.43×10^–4cm²s^–1 and 0.051 cm, respectively.