Dielectric Properties Of Anodic Films Research Articles

Effect of Ethylene Glycol Addition to the Electrolytes on Dielectric Properties of Anodic Films Formed on Niobium

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Dielectric properties of anodic films on sputter-deposited Ti–Si porous columnar films

For electrolytic capacitor application of the single-phase Ti alloys containing supersaturated silicon, which form anodic oxide films with superior dielectric properties, porous Ti–7 at% Si columnar films, as well as Ti columnar films, have been prepared by oblique angle magnetron sputtering on to aluminum substrate with a concave cell structure to enhance the surface area and hence capacitance. The deposited films of both Ti and Ti–7 at% Si have isolated columnar morphology with each column revealing nanogranular texture. The distances between columns are ∼500 nm, corresponding to the cell size of the textured substrate and the gaps between columns are 100–200 nm. When the porous Ti–7 at% Si film is anodized at a constant current density in ammonium pentaborate electrolyte, the growth of a uniform amorphous oxide film continues to ∼35 V, while it is limited to less than 6 V on the porous Ti film. The maximum voltage of the growth of uniform amorphous oxide films on the Ti–7 at% Si films is similar for both the flat and porous columnar films, suggesting little influence of surface roughness on the amorphous-to-crystalline transition of growing anodic oxide under the high electric field. Due to the suppression of crystallization to sufficiently high voltages, the anodic oxide films formed on the porous Ti–7 at% Si film shows markedly improved dielectric properties, in comparison with those on the porous Ti film.

Influence of Film Composition on the Structure and Dielectric Properties of Anodic Films on Ti–W Alloys

The influence of tungsten on an amorphous-to-crystalline transition, leading to formation of anatase, and dielectric properties is examined for anodic oxides on Ti–W alloys. The structural change, which occurs at voltages of on high-purity titanium, and provides sites for generation of high-pressure gas oxygen bubbles, is suppressed to progressively increased voltages by increasing tungsten content of the film. Phosphorus species, derived from the electrolyte, also hinder the change. The tungsten species are located in the inner of the film thickness as result of the faster migration of ions relative to that of ions in the growing oxide. Although the reported permittivity of anodic tungsten oxide is higher than that of amorphous anodic titania, the permittivity of the films on alloys decreases gradually with increasing tungsten content.

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi 2S 3 and Bi 2O 3 semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies ( D≈ 10 −16 cm 2 s −1). Solid-state transformation of sulfide to the oxide film is a consequence of OH − ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.

Dielectric properties of anodic films formed on sputtering-deposited tantalum in phosphoric acid solution

The dielectric properties of anodic films formed on tantalum in dilute phosphoric acid solution, at 20 and 85 °C, have been investigated by electrochemical impedance spectroscopy. Dielectric constants of 28.2±0.9 (20 °C) and 30.2±2.1 (85 °C) were determined from the slopes of the variations of reciprocal capacitance of the anodic film and of film thickness, from transmission electron microscopy, with formation voltage. The slightly higher dielectric constant obtained at 85 °C is due to a reduced amount of incorporated phosphorus species in the outer layer of anodic films formed at this temperature, together with a reduced thickness of the outer layer, compared with those of anodic films formed at 20 °C.

Inter–relationships between ionic transport and composition in amorphous anodic oxides

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

Properties of Anodic Films Formed on Reactively Sputtered Tantalum

The technique of reactive sputtering has been used for producing tantalum films with various concentrations of interstitial additives like oxygen, carbon, nitrogen, and hydrogen. A number of compounds between tantalum and these nonmetallic elements also have been deposited. The dielectric properties of anodic films on the tantalum formed in an aqueous solution of citric acid have been related to the composition of the tantalum. It was found that the capacitance density of 130v anodic films remains unaffected by the presence of oxygen and hydrogen, while it decreases as the atomic concentration of nitrogen and carbon in the tantalum increases. The dielectric constant, ε, ofanodic films on is approximately 50% of the value of anodic formed on pure tantalum.