Conduction In Anodic Oxide Films Research Articles

Dielectric Properties and Ionic Conductivity of Anodic Oxide Films on Tantalum

We have investigated certain aspects of the dielectric properties of anodic oxide films on tantalum in connection with Dignam’s theory that a special current-driven polarization process is responsible for the transients shown by the ionic current on changing the electric field in the oxide. In his experiments, which were claimed to prove this theory, films were first held at potentials giving fields too low to produce significant ionic current. When voltage pulses were applied large enough to produce fields giving ionic current, extra current was observed to flow for a short time. This was taken as due to the decay of the dielectric polarization built up due to the ionic current, and partial numerical agreement with the model was found. In the present experiments the discharge current flowing after longer periods of voltage application or of ionic current flow was investigated with the aim of finding if more polarization was indeed produced by ionic current flow. The current was not found to be larger after discharge from fields giving ionic current, as required by Dignam’s model. It is argued that structural changes in the glassy oxide are responsible for the transients.

Electroconduction in the Zr|ZrO2| electrolyte system

Electroconduction of anodic oxide films on zirconium in 0.1 M Na2SO4 is studied. The films are formed in a galvanostatic regime on single-crystal and polycrystalline iodide-refined zirconium, as well as on electrodes manufactured from a rod obtained by hot extrusion of zirconium iodide melted in an arc furnace and from a plate of iodide-refined zirconium of the I-100 brand. Electrophysical properties of the films are compared on the basis of a model of Frenkel defects, which constitute a system of noninteracting donor centers in the oxide, and a model that describes the formation of a space charge from the donor centers in the oxide (exponential distribution of traps over the films’ bulk). It is shown that experimental current-voltage curves cannot be described by a single model throughout the entire voltage range. At low voltages (<12 V), the Frenkel model is more preferable. In terms of this model, the experimental results can be linearized in the lnI vs. U 1/2 coordinates. At higher voltages, it is more convenient to describe experimental data within the space-charge model in the I vs. U2 or ln(I/U) vs. U coordinates. It is discovered that the technique, which is used for preliminary metallurgical treatment, and the structure of the substrate metal affect parameters of electron conduction in anodic oxide films.

Ionic conduction in anodic oxide films on titanium in mixed solutions of aqueous electrolytes and diethylene glycol

Steady state kinetic data have been obtained for anodic growth of films on titanium in mixed solution (1:1 by volume) of 100 mol m −3 aqueous orthophosphoric acid with diethylene glycol (DEG), 100 mol m −3 aqueous sodium dihydrogen orthophosphate with DEG and 100 mol m −3 aqueous disodium hydrogen orthophosphate with DEG at different temperatures and current densities. The constant A in the Guntherschulze and Betz equation is temperature dependent while B is temperature independent. The constant A also depends on the nature of the electrolyte. The data indicate a temperature-independent Tafel slope which rules out the applicability of the single-barrier theory of Cabera and Mott. The data have been analysed in terms of Dignam's equation. The effects of current density, temperature and composition of electrolyte on various parameters of Dignam's equation have been examined.

Modeling of High Field Ionic Conduction in Anodic Oxide Films

Experimental data for the stepped current and open‐circuit transient methods were fitted to one form of the modeling equations given by Dignam. A good representation of the data was obtained except that extreme linearity was found for vs. data for the peak field in stepped current experiments where some degree of nonlinearity was expected. Another question involves the role of electrolyte incorporation in causing the observed curvature of steady‐state vs. plots. It is shown that if a model is preferred in which the history effects are due to structural changes in the glassy oxide rather than to the special current‐driven polarization model of Dignam, then one can obtain equations equivalent to Dignam's by writing the activation energy for ionic transport as a function of a parameter describing the structural condition of the oxide, except that some relations between parameters which follow naturally from Dignam's model do not then appear.

The Open‐Circuit Transient Method of Investigating Ionic Conduction in Anodic Oxide Films: Its Relation to Other Techniques

A refinement of a set of equations which we proposed recently to describe high field ionic conduction in anodic films under general conditions is used to predict data during the self‐discharge on open circuit by ionic conduction of the capacitor metal/oxide/solution. The ionic conduction process is history dependent with limiting cases of steady‐state conditions and conditions in which the field is stepped as rapidly as possible to new values starting from steady‐state conditions. The computed results indicate an appreciable degree of relaxation toward steady‐state conditions during the self‐discharge under some circumstances.

Models for Ionic Conduction in Anodic Oxide Films

It is argued that the history effects, which are an important feature of the behavior of the high field ionic conductivity of anodic oxide films on tantalum and several other metals, are more probably due to structural effects than to a current‐driven component of dielectric polarization. The argument is developed, in particular, for films, since a knowledge of the structure of is available from recent work by Roth et al. and others. It is shown that equations can be formulated, on a phenomenological basis, which describe the experimental results for ionic conduction.

Ionic conduction in anodic oxide films on aluminium in molten LiNO 3 + KNO 3 + NaNO 3 electrolyte

Steady-state ionic conduction studies in the anodic growth of films on aluminium in molten LiNO 3 + NaNO 3 + KNO 3 have been made. The breakdown voltage of the film is 73 V, unaffected by change of cd or temperature. The film-forming characteristics of anodic oxide films on aluminium in molten salt electrolytes are different from that of anodic films formed in aqueous electrolytes. The Tafel slope decreases with increase of temperature. Dignam's equation for ionic conduction has been examined critically. His parameter μ * depends on temperature, whereas i 0 and φ are temperature-independent, which explains the variation of Tafel slope with temperature.

Ionic conduction in anodic oxide films on tantalum in molten LiNO 3 + KNO 3 + NaNO 3 electrolyte

The kinetics of anodic growth of film on tantalum in molten LiNO 3 + KNO 3 + NaNO 3 have been studied by employing various cd ratios. Dewald's double-barrier theory of oxide growth accounts satisfactorily for our data. The results are in disagreement with the single-barrier control theory in which space charge is neglected. The kinetics is independent of the electrolyte used.

The effect of neon bombardment on the corrosion of Zr

Bombardment with 65 keV neon ions has been used to simulate the effects of high neutron doses on oxidized Zr. The electrical conductivity of anodic oxide films was enhanced for doses above 10 μA-min on a few square millimetres, but their ultraviolet absorption was not detectably altered for doses up to 100 μA-min. Doses above 10 μA-min sputtered away the surface of the oxide film with an anisotropic sputtering ratio of roughly 0·1. Thinly oxidized specimens that had suffered doses of 1 μA-min and above, when exposed to steam at 250 °C at 1 atm, corroded at a considerably enhanced rate. This effect appeared to be independent of dose rate and to be an increasing function of dose. A thick anodic film showed no effect after 20 μA-min, suggesting that damage to the metal was important. Bare zirconium and Zircaloy 2 specimens were irradiated to doses high enough to remove the air-formed film by sputtering. These also showed enhanced subsequent corrosion.

The Electronic Conductivity of Defective Anodic Tantalum Oxide Films

The conditions under which anodic oxide films on tantalum showed electronic conductivity were studied using an electrochemical printing‐out technique with starch‐iodide as the redox indicator. The sensitivity for the detection of electronic charge was approximately 1 millicoulomb/cm2. The effect of alloying (contamination), thermal oxidation, plastic deformation, and surface topography were investigated with regard to their contribution to the electronic conductivity of anodic oxide films on mechanically treated (“rough”) tantalum surfaces. It was concluded that impurities alone were responsible for the electronic conductivity even in cases where supposedly clean tantalum had been abraded with tantalum. Roughness by itself was not a sufficient condition for electronic conductivity.

A Redox Printing Technique for the Study of the Electronic Conductivity of Anodic Oxide Films on Valve Metals

A printing method using iodide‐starch as the redox indicator in a solidified electrolyte was developed for applications to various valve metals and their anodic oxide films. It was used to study the electronic conductivity of anodic oxide films on tantalum and niobium and to detect defective sites in oxide films on aluminum. Applied to titanium, zirconium, and silicon, as well as the other valve metals, the technique was used for an evaluation of the surface condition of the metal substrates. The technique also was useful in cases where the oxide films corroded under the influence of the printing electrolyte.

An Ellipsometric Study of Steady‐State High Field Ionic Conduction in Anodic Oxide Films on Tantalum, Niobium, and Silicon

The classical theory of ionic conduction in solids at high field strengths (106 to 107 v cm−1) predicts that the relation between the ionic current density I and the field strength should be where the activation energy , is the magnitude of the charge on the ions, a is half the distance between successive sites occupied by the ions, and is a constant. Deviations from this supposed law have been reported in various forms. New experimental results for steady ‐state conditions are reported which were obtained by in situ ellipsometry. These confirm for tantalum and establish for the first time for niobium that the deviations may be simply and accurately described by taking the activation energy to be nonlinear in E in a way which may be represented over the experimental range of by . Data were also obtained for silicon, but were not sufficiently accurate to detect nonlinearity. Models are discussed which might give this effect. A model in which ions move fairly freely in channels with infrequent trapping by a coulombic potential leads to a law of the form , analogous to the Schottky and Poole‐Frenkel laws for electronic currents. Such a law fits the data well enough for the model to be considered as realistic.

Conduction and Rectification in Anodic Oxide Films

Electronic conduction in anodic oxide films of all thicknesses on all metals studied (Ta, Nb, W, Zr, Al) can occur at certain flaws in the films. Flaw conduction usually dominates the behavior of thick films; it is characterized by marked hysteresis and unusual memory and temperature effects. Flaw conduction is shown to occur by at least two mechanisms: one the opening of physical holes and the other involving some kind of electronic junction. Conduction in the bulk of the film dominates on thin films of Ta2O5, Nb2O5, and WO3. Such conduction is not entirely uniform, and depends strongly on formation conditions, film thickness, temperature, and film history. The mechanism of this bulk conduction appears to involve primarily electron injection into an insulator containing traps; rectification occurs when the contacts produce an asymmetric barrier.

Heterogeneities in Anodic Oxide Films on Tantalum - Their Detection and Electrical Characteristics

A print-out technique was used to study the electronic conductivity of anodic oxide films and, in particular, the electronic conductivity associated with heterogeneities in the oxide film. Tantalum foil was used in the received state, after chemical polishing, abraded, heated in air, and carburized to show the electronic current of defects as a function of formation voltage (film thickness), field strength, and pretreatment.

Kinetics of Recombination of Frenkel Defects in Anodic Oxide Films on Tantalum

The changes with time in the ionic conductivity of anodic oxide films on tantalum have been investigated. The results are discussed in relation to the theory that Frenkel defects are present in the films and that they recombine due to the electrostatic attraction between the vacant cation sites and the interstitial ions.