Anodic oxides impedance

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

FIGURE 2.20 Change of anode/electrolyte impedance spectra with respect to change in anodic current. (From van Herle, J. et al., Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells, 97(40) 565-574, 1997. Reproduced by permission of ECS-The Electrochemical Society.)... 

A remark has to be made that also applies to (b) anodic oxidation of a metal is often a very complex process, involving, for example, adsorption of intermediates in a multi-step mechanism, film formation of insoluble products, etc. In such cases, the impedance plots can be much more intricate and the straightforward relation to the corrosion rate can be obscured. 

AC and DC responses are useful sources of information about Si-Me/Me"" nanostructured system during cathodic nucleation of metals on silicon and also their anodic oxidation. The impedance dependence on AC frequency is size-sensitive due to diffusion contribution to the interfacial impedance. This allows using potentiodynamic multi-frequency AC probing for fast qualitative characterization of the nanostructured interface at initial stages of metal deposition, when application of other techniques is hindered by nonstationar effects. 

Figure 4. Frequency dependence of the capacity obtained from impedance spectroscopy for the untreated carbon film (ref) and the same film anodically oxidized for 300 s at 2.4 V vs. Ag/AgCl (ox).

L. Yoimg, "Anodic Oxide Films 4. The Interpretation of Impedance Measurements on Oxide Coated Electrodes on Niobium," Transactions of the Faraday Society, 51 (1955) 1250-1260. 

FIGURE 3.32. Impedance spectrum measured in 1 10 NH4OH at 3.5 (Left) Thermal oxide (top right) anodic oxide formed by passivating for 30min at 75 Vs (bottom right) anodic oxide after annealing for 15 min at 700 °C in N2. After Schmuki et (Reproduced by permission of The Electrochemical Society, Inc.)... 

Figure 11.18 Impedance data measured for polypyrrole as function of the potential. The polymer film was prepared by anodic oxidation in a perchlorate electrolyte. Bode representation (A) impedance versus frequency and (B) phase angle versus frequency.

Fortunately, a great deal of work has been accomplished in a short time, and notably by aircraft manufacturers as well as adhesives suppliers. There are several important contributions in this area. First, in the area of FPL etch, the important consideration is what kind of bonding surface is provided by the preparation method. The chromic acid/sulfuric acid not only removes air oxide and leaves base metal it also has a chemical potential which produces a very thin anodic type oxide layer of the surface. This oxide layer is porous, due to the dissolving action of the strong acid mixture, and thus the surface produced may be characterized as a thin, porous anodic oxide. (A. W. Smith compared it to a 3V chromic acid anodize based on impedance measurements.) The optimum conditions for this etch as to time, temperature, and composition have been studied at Fokker and by Smith and generally a somewhat higher concentration of sodium dichromate or chromic acid was recommended than was commonly used. 

Films of oxides may also be prepared by sputtering. The most intensively studied material is irO because of its possibie application as an electrochromic materiai (81-83). Fiims prepared by this method seem to be more similar to films prepared thermally, than to those prepared by anodic oxidation (see next section). Aurian-Biajeni et al. (81) have interpreted the two time constants in the impedance spectrum as caused by a two-iayer structure in the oxide. 

M. V. ten Kortenaar, C. Tessont, Z. I. Kolar, H. van der Weijde, Anodic oxidation of formaldehyde on gold studied by electrochemical impedance spectroscopy an equivalent circuit approach, J. Electrochem. Soc., 1999,146,6, pp. 2146-2155. 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

In a number of works, a potentiostatic regime has been used for the experimental and theoretical study of the anodization of aluminum and other valve metals.80 Upon the application of a constant potential step, Va, barrier-forming electrolytes are characterized by a sharp increase in the anodic current to a certain maximum. Both the slope and the maximum are determined by the impedance of the cell circuit. Subsequently, there is a continuous decrease in the anodic current, which is due to oxide growth. The decay of the anodic current can be described by the expression81... 

FIGURE 2.22 (a) Impedance spectra for symmetrical cells prepared without (square) and with (circle) 40 vol% corn starch as pore former in 3% H20/H2 at 850°C. (From Primdahl, S. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 99(19) 793-802, 1999. Reproduced by permission of ECS-The Electrochemical Society.) (b) Influence of anode support porosity on the performance of cells at 800°C. (From Zhao, F. and Virkar, A.V., J. Power Sources, 141 79-95, 2005. Copyright by Elsevier, reproduced with permission.)... 

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