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.     

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.     

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.     

Self-organized porous titanium oxide prepared in Na2SO4/NaF electrolytes

The anodic formation of nanoporous TiO₂ on titanium was investigated in Na₂SO₄ electrolytes containing low concentrations of NaF (0.1-1 wt.%). It was found that under optimized electrolyte conditions and extended polarization, a self-organized nanostructure consisting of porous TiO₂ is obtained. The porous structure is arranged in sections of arrays with single pore diameters of typically 100 nm and an average spacing of 150 nm. The pores are open at the top and covered by oxide at the bottom. Compared with earlier work, we show that using a neutral NaF electrolyte significantly thicker porous layers can be obtained than in acidic solutions.     

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.     

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.     

Porous niobium oxide films prepared by anodization in HF/H3PO4

Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium foils at 2.5 V in the mixture of 1 wt% HF and 1 M H₃PO₄ for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H₃PO₄.     

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.     

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.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

Electrochromism in spray deposited iridium oxide thin films

Electrochromic iridium oxide thin films were deposited onto fluorine doped tin oxide coated glass substrates from an aqueous iridium chloride solution by pneumatic spray pyrolysis technique. The as-deposited samples were X-ray amorphous. The electrochromic properties of thin films were studied in an aqueous electrolyte (0.5N H₂SO₄) using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. Iridium oxide films show pronounced anodic electrochromism owing to Ir⁺⁴ ↔ Ir⁺³ intervalency charge transition. The reversibility of cyclic process in Ir oxide films is found to be higher, which increases with increasing number of colour-bleach cycles.     

The effect of thickness on the composition of passive films on a Ti-50Zr at% alloy

Anodic films were grown potentiodynamically in different electrolytes (pH = 1-14) on a Ti-50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS).Regardless of the anodizing conditions, passive films on the Ti-50Zr at% alloy consist of a single layer mixed oxide phase containing both TiO₂ and ZrO₂ groups. However, an enrichment of Ti within the passive film, increasing with the film thickness, is detected both by PCS and XPS. This leads to concentration profiles of Ti⁴⁺ and Zr⁴⁺ ions along the thickness, and to different electronic properties of very thin films (more insulating) with respect to thicker films (more semiconducting), as revealed by the photocurrent-potential curves.     

Cyclic oxidation processes during anodizing of Al-Cu alloys

The influence of copper on the morphologies of porous anodic alumina has been investigated under current and voltage control using a sputtering-deposited Al-2.7 at.% Cu alloy and a commercial AA 2024-T3 aluminium alloy anodized in either sulphuric acid electrolyte or the same electrolyte but with addition of tartaric acid. The findings indicate that film development involves repeated formation of embryo cells of anodic alumina at the metal/film interface. During the initial stages of anodizing at constant voltage, cell formation is accompanied by current peaks in the current-time response. The porosity of the resultant films has a lateral aspect due to the layering of embryo cells. The thickness of individual layers is proportional to the formation voltage, with a ratio of the order 1 nm V⁻¹. The cell formation is accompanied by enrichment of copper in the alloy, incorporation of copper species into the anodic film, in low amounts relative to the alloy, and evolution of oxygen. These processes disrupt the formation of the classical pore morphology, characteristic of high purity aluminium, due to continuous formation of fresh embryo cells and re-direction of pores. The main effect of the tartaric acid addition to the sulphuric acid was to reduce the rate of anodizing of the alloys at constant voltage by about 10-20%.     

Oxide film growth on Al-In alloys in a borate buffer solution in conditions of galvanostatic anodising

Anodic oxide films have been formed galvanostatically on Al-In alloys (containing up to 0.074% In) in a borate buffer solution (pH 7.8) at different current densities (20-100 μA cm⁻²). The mechanism, kinetics of growth and properties of formed oxide films have been investigated. The study of charge curves suggests that the growth of oxide films on Al-In alloys occurs by an activation-controlled ionic conduction under the influence of the high electric field through the oxide film according to an exponential law, like on valve metals. The following parameters have been calculated: the constants of the exponential law, ionic conductivity through the film, the effective activation distance for ion movement and the corresponding field strength. The values for the field strength, of the order of magnitude of 10⁶ V cm⁻¹, justify the application of the high field migration mechanism. Properties of anodic oxide films have been determined by means of electrochemical impedance spectroscopy; the resistance and thickness of the oxide film have been found to increase with the increase in the indium content in the alloy and with increased anodic current density. It has been established that the current efficiency in oxide films formation on Al-In alloys is lower than 100%: the increase of the indium content in the alloy, as well as the increase in anodic current density, increases the value of current efficiency.     

Characterisation of titanium oxide film grown in 0.9% NaCl at different sweep rates

The nature of the anodic oxide film that forms on titanium on titanium in 0.9% NaCl has been investigated using a wide range of techniques. A linear relationship was found between the critical current density required for passivation of titanium in 0.9% NaCl and the sweep rate. Anodic oxide films formed on titanium in 0.9% NaCl appear to consist of two layers, an inner compact layer, the growth of which continues to follow a high field growth law, and a porous less protective outer porous layer. XPS and XRD indicated that passive films on titanium consist mainly of TiO₂. However, hydroxides and lower oxides are also present, especially in rapidly grown films. XRD data indicated that in 0.9% NaCl the anodic oxide film is formed through the preferential removal atoms in the plane of (0 0 2) in the course of electrochemical reaction. A model based analysis XPS spectra was proposed to explain the growth rate dependence of the degree of protection offered by anodic oxide films on titanium. XPS clearly demonstrated the present of Ti(III) and Ti(II) cations in the passive film. This is strong evidence that cation migration more likely dominates over anion migration in the growth mechanism of anodic oxide film. XPS data also revealed that the concentrations of Ti(III) and Ti(II) species within the oxide films increased as the oxide/metal interface was approached.     

Embedded space charge in porous alumina films formed in phosphoric acid

The embedded charge in the barrier layers of porous alumina, formed potentiostatically in phosphoric acid was studied as a function of anodizing voltage (30-57 V) and bath temperature (18 and 21 °C). For that, the polarization measurements of as-grown and annealed alumina/Al samples were conducted in the same anodizing bath by anodic potential sweep at a scan rate of 2.6 V/min. The plane capacitor model was used for the assessment of the charge density in the barrier layers of as-grown porous alumina. For the barrier layers of films formed at 18 °C this value equals to 0.747 μC cm⁻² and does not depend on the anodizing voltage. Increase in electrolyte temperature rises the embedded charge density. Polarization measurements carried out in this paper clearly present that the barriers of phosphoric acid films grown at the anodizing voltages lower than 39 V contain a layer of virtual cathode while at higher voltages this layer disappers. The obtained results allow speaking about promising opportunities of potentiodynamic polarization measurements of alumina films in the same anodizing solution before and after annealing for the studies of charges embedded within the alumina barriers and for the regularities of ion transport.     

Inhibition of field crystallization of anodic niobium oxide by incorporation of silicon species

The present work demonstrates effective inhibition of field crystallization of amorphous anodic niobium oxide by incorporation of silicon species from substrate. The field crystallization, detrimental for capacitor application of niobium, occurs during anodizing of magnetron sputtered niobium at 100 V in 0.1 mol dm⁻³ ammonium pentaborate electrolyte at 333 K, while amorphous structure of the anodic oxide is totally retained during anodizing of magnetron sputtered Nb–12 at%Si. Even after prior thermal treatment in air, which accelerates field crystallization of anodic oxide on niobium, no crystallization occurs on the Nb–12 at%Si. Through examination of the crystallization behaviours of anodic films formed on a thin Nb–12 at%Si layer superimposed on a niobium layer as well as on a thin niobium layer superimposed on an Nb–12 at%Si layer, it has been confirmed that air-formed oxide or thermal oxide becomes a nucleation site for crystallization. Modification of the air-formed or thermal oxide by incorporation of silicon species inhibits the nucleation of crystalline oxide. The modification, however, does not influence the growth of crystalline oxide. The growth is suppressed by continuous incorporation of silicon species into anodic film from the substrate during anodizing.     

Structural features of self-organized nanopore arrays formed by anodization of aluminum in oxalic acid at relatively high temperatures

Anodic aluminum oxide (AAO) membranes with a highly ordered nanopore arrangement typically serve as ideal templates for the formation of various nanostructured materials. A typical procedure of the template preparation is based on a two-step self-organized anodization of aluminum carried out at the temperature of about 1-3 °C. In the current study, AAO templates were fabricated in 0.3 M oxalic acid under the anodizing potential range of 30-65 V at a relatively high electrolyte temperature ranging from 20 to 30 °C. Due to a high rate of porous oxide growth, about 5-10-fold higher than in low-temperature anodizing, the process of the template fabrication can be shorten significantly. Similarly to the low-temperature anodization, the best hexagonal pore arrangement is observed for samples anodized at 40 V. With a prolonged duration of the first anodizing step the order degree of triangular nanoporous lattice, observed after the second anodization, improves considerably. The effects of the anodizing potential and the process duration on the structural features of porous anodic alumina such as: pore diameter (D_p), interpore distance (D_c), porosity (P), pore density (n) and anodizing ratio (B_U) were investigated in details at various temperatures. The obtained results were compared with theoretical predictions and data reported in the literature.     

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。     

Effect of electrolyte concentration on morphology,microstructure and electrochemical impedance of anodic oxide film on titanium alloy Ti-10V–2Fe–3Al

Anodic oxide films were fabricated on titanium alloy Ti-10V–2Fe–3Al in ammonium tartrate solutions at the concentrations: 1, 3, 5, 10 and 15 g L⁻¹. The morphological characteristics and microstructures of the films of the alloy were studied by optical microscopy (OM) and Raman spectroscopy (Raman), respectively. The electrochemical impedances of the films in 0.5 mol L⁻¹ H₂SO₄ solution were investigated by electrochemical impedance spectroscopy (EIS). It was showed that different electrolyte concentrations led to different change rates of anodizing forming voltage. The change rate significantly affected the morphology, microstructure and electrochemical impedance of anodic oxide film. When electrolyte concentration was 5 g L⁻¹, anodic oxide film was the most uniform, exhibited by the least and smallest breakpoints on the film. In addition, the amount of crystal phase of the film was the largest at 5 g L⁻¹, showed by the highest intensity of Raman peaks. Furthermore, the electrochemical impedance of the film of the alloy was the greatest at 5 g L⁻¹, demonstrated by the highest values of polarization resistances and lowest values of capacitances. These phenomenon were associated with the minimum value of the change rate of anodizing forming voltage at 5 g L⁻¹.