Anodic Oxide Growth on Tantalum with AC + DC Fields: I . Experimental Procedures and Results

Abstract

The small signal ac response of the ionic conductivity growth process was described using the ionic admittivity defined as the (complex) ratio of the ac components of ionic current density and field since, unlike the ratio of current and voltage (admittance), this gives results independent of film thickness. Ideally constant ac and dc components of field were approximated by applying a ramp plus an ac voltage increasing with the average film thickness. Growth due to the ac ionic current produces an out of phase component of thickness giving a series capacitance effect. The measured current density includes the electric displacement current density. When corrected for these effects, plots of the real vs. the imaginary part of the ionic admittivity at a given dc current density were well represented as arcs of circles with centers above the real axis. The ionic admittivity could thus be described using A + B/{I + (j coTm)1-~ where I/Zm is the angular frequency (~o) corresponding to the maximum imaginary part and 7(i -0) the central angle subtended by the arc. vm(E, T) was not far from inversely proportional to the dc ionic current density independent of temperature. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2015-03-05 to IP ABSTRACTThe small signal ac response of the ionic conductivity process, investigated in Part I, with its inductive phase relation between ionic current and field, is a manifestation of the dependence of the ionic current density on the previous history of the film. This history dependence is probably caused by structural changes in the oxide. Equations previously formulated to predict the ionic current density for all situations were modified to account for the observed behavior by introducing a distribution of relaxation times. The type of distribution found is consistent with structural changes in a glassy oxide. The relaxation times involved in the adjustment of the structure to a new field are expected to be a function of the field, temperature, and the state of the oxide. For the present experiments, since the state of the oxide corresponded to steady state (growth at constant current and field for long enough), Zm could be fitted by a function of field and temperature alone. The implications of the fact that Tm was close to inversely proportional to the ionic current density independent of temperature are discussed.

Anion-Incorporation Model Proposed for Interpreting the Interfacial Physical Origin of the Faradaic Pseudocapacitance Observed on Anodized Valve Metals—with Anodized Titanium in Fluoride-Containing Perchloric Acid as an Example

“…This similar-shaped EIS spectrum was early measured by De Wit et al for anodized Al, Kelly for passivated Ti, and Keddam and co-workers , for passivated Fe. And now, it is so common for (and only limited to) the valve metals that it has been observed on most valve metals, such as Ti, ,− W, − Nb, ,,, Mo, ,, Al, ,− Fe, ,, Ta, , and Bi …”

“…This type of EIS spectra is featured by a high-frequency capacitive loop, a middle-frequency inductive loop, and a low-frequency vertical capacitive line (Figure a), and it can be modeled by the equivalent electrical circuit shown in Figure b. ,,,, A typical method widely used for analyzing and interpreting this type of EIS spectra is the surface-charge approach (SCA), ,,, which is developed and updated by Bojinov and co-workers based on the point-defect model (PDM) of Macdonald and co-workers, − the ideas of De Wit et al and Young and co-workers, , and the equivalent electrical circuit model shown in Figure b. With SCA, the EIS behaviors relating to the film formation−dissolution have been investigated for anodized Ti, W, , Nb, , Mo, , Bi, Ta, Al, and Fe …”

“…There are many controversies at present when dealing with the ascription of the low-frequency capacitance response in EIS. As an example in Table , a most popular, most representative explanation for the origin of the faradaic pseudocapacitance C 0 is that C 0 results from the low frequency modulation of the film thickness or, say, from the modulation of the film thickness with potential. ,,,− ,− ,, However, this interpretation/statement is only an alternative description of eq , which is still unclear, and is not an answer to the questions raised above. Indeed, eq was developed based on this interpretation/statement, resulting in the quantity of λ with λ < 1 (please see next section).…”

Probing Barrier Oxide Layer of Porous Anodic Alumina by In Situ Electrochemical Impedance Spectroscopy

“…It is worth noting that in situ EIS has been widely applied to study the growth of barrier-type oxide films on the surface of valve metals. [28][29][30] Unfortunately, the collected data cannot be directly used for description the processes occurring during porous AAO formation because they are much more complex. Here EIS is applied for in situ study of the thickness of the barrier layer during the porous AAO films grown on high-purity aluminium.…”