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.     

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.     

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.     

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.     

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.     

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.     

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.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

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.     

Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.     

Formation mechanism and packing options in tubular anodic titania films

Titania nanotube arrays were synthesized via anodic oxidation of titanium foils in glycerol electrolyte containing NH₄F at anodization voltage ranging from 10 V to 30 V. The structural parameters of self-organized periodic arrays of titania nanotubes were determined by small-angle neutron scattering and scanning electron microscopy techniques. Transmission electron microscopy and electron diffraction studies of single-standing nanotubes revealed the presence of nanocrystalline titanium oxide phases with oxidation states lower than +4 (TiO, Ti₂O₃). Several assumptions on growth and self-organization mechanism of nanotube arrays have been made.     

An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium

In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

Stress-affected and stress-affecting instabilities during the growth of anodic oxide films

The objective of the current paper is to (re-)address the question whether internal stress is a fundamental parameter driving some generic cases of growth instabilities commonly encountered during the growth of anodic oxide films, namely breakdown and pore initiation. This has been done by unraveling possible correlations between a key electrochemical characteristic of the instability event and the internal stress evolution, the latter being measured in situ during the very same anodising experiment. As such, we have been able to make more conclusive statements as compared to the merely speculative arguments in the literature whether these instabilities have a mechanical origin or not. In the case of breakdown, the two well-documented types of breakdown events encountered during galvanostatic Zr anodising were both found to be stress-affected: instantaneous compressive internal stresses were identified as the driving force for both the densifying phase transformation responsible for type-I breakdown, as well as for the buckling-induced delamination events observed during type-II breakdown. Pore initiation in anodic Al₂O₃ on the other hand was found not be stress-affected. Instead, pore formation is rather believed to induce itself a modification in the mechanical behaviour, and was therefore classified as stress-affecting.     

A study on the formation of titania nanopillars during porous anodic alumina through-mask anodization of Ti substrates

The formation of anodic titania during porous anodic alumina (PAA) through-mask anodization has been analysed for varying anodization conditions on mechanically polished bulk Ti surfaces. Titania nanopillars were formed through the porous masks in both oxalic and phosphoric acid electrolytes. For applied potentials above 40 V the titania formed along narrow channels through the alumina pore bottoms resulting in root-like attachments of the titania pillars to the Ti substrate. We further demonstrated that high-field anodization can be used for PAA through-mask anodization. The formation of titania changed with increased current density which resulted in more efficient oxide growth through the alumina pores. When the Al/Ti samples were immersed in the electrolyte without exclusively exposing the Al surface to the electrolyte the titania formed solely on top of the alumina pore bottoms which resulted in that the titania structures were detached from the Ti substrates during selective removal of the PAA templates.     

Selective formation of porous layer on n-type InP by anodic etching combined with scratching

The selective formation of porous layer on n-type InP (0 0 1) surface was investigated by using scratching with a diamond scriber followed by anodic etching in deaerated 0.5 M HCl. Since the InP specimen was highly doped, the anodic etching proceeded in the dark. The potentiodynamic polarization showed the anodic current shoulder in the potential region between 0.8 and 1.3 V (SHE) for the scratched area in addition to the anodic current peak at 1.7 V (SHE) for the intact area. The selective formation of porous layer on the scratched are was brought by the anodic etching at a constant potential between 1.0 and 1.2 V (SHE) for a certain time. The nucleation and growth of etch pits on intact area, however, took place when the time passed the critical value.The cross section of porous layer on the scratched area perpendicular to the or [1 1 0] scratching direction had a V-shape, while the cross section of porous layer on the scratched area parallel to the or [1 1 0] scratching direction had a band structure with stripes oriented to the or direction. Moreover, nano-scratching at a constant normal force in the micro-Newton range followed by anodic etching showed the possibility for selective formation of porous wire with a nano-meter width.     

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.     

Decay of capacitance on ceasing formation of anodic oxide films on tantalum

The decrease with time in the capacitance of anodic oxide films on tantalum after ceasing film growth by reducing the field in the oxide was investigated. No dependence was observed of the amount and rate of change in relative dielectric permittivity on the initial current density. The kinetics of this effect are therefore different from those for the decay of the latent ionic conductivity.     

Effect of aluminum annealing on the galvanoluminescence properties of anodic oxide films formed in organic electrolytes

The galvanoluminescence (GL) properties of anodic oxide films formed in organic electrolytes were investigated at different aluminum annealing temperatures. The results of the spectral measurements showed two different types of GL sources: carboxylate ions incorporated in oxide films during the anodization and the molecules AlH, AlO, Al₂, AlH₂, also formed during anodization process and already recognized in the case of inorganic electrolytes. The latter was related to gamma alumina crystalline regions formed by annealing of the aluminum samples at temperatures above 500 °C.