Constant-Phase-Element Behavior Caused by Resistivity Distributions in Films

Abstract

A normal power-law distribution of local resistivity with a uniform dielectric constant was found to be consistent with the constant-phase element ͑CPE͒. An analytic expression, based on the power-law resistivity distribution, was found that relates CPE parameters to the physical properties of a film. This expression worked well for such diverse systems as aluminum oxides, oxides on stainless steel, and human skin. Good values for film thickness were obtained, even when previously proposed expressions could not be used or yielded incorrect results. The power-law model yields a CPE impedance behavior in an appropriate frequency range, defined by two characteristic frequencies. Ideal capacitive behavior is seen above the upper characteristic frequency and below the lower characteristic frequency. A symmetric CPE response between the characteristic frequencies can be obtained by adding a parallel resistive pathway.Deterministic mathematical models for impedance spectroscopy are intended to provide information concerning physical processes such as kinetics, mass transfer, and reaction mechanisms, as well as material properties, such as permittivity and conductivity. The constant-phase element ͑CPE͒ model, which is purely a mathematical description, may adequately represent impedance data, but it gives no insight into the physical processes that yield such a response. However, capacitance values are often extracted from CPE data using such expressions asdeveloped by Brug et al. 1 or C HM = Q 1/␣ R f ͑1−␣͒/␣ ͓2͔ presented by Hsu and Mansfeld. 2 The parameters R e and R t in Eq. 1 represent the Ohmic and charge-transfer resistances, respectively, and the parameter R f in Eq. 2 represents the film resistance. The subscripts B and HM refer to the authors of the respective papers. Often, the capacitance values obtained are used to estimate the thickness of dielectric layers. Recently, Hirschorn et al. 3 associated these expressions unambiguously with either surface or normal time-constant distributions and demonstrated the importance of using the correct formula that corresponds to a given type of distribution. Limitations of Eq. 2 for predicting layer thickness were discussed. For a blocking system, where the film resistance is infinite, Eq. 2 cannot be used as it would yield an infinite capacitance. When the distribution of physical properties is broad, Eq. 2 may yield an inaccurate estimate of capacitance. Their work demonstrated the importance of developing physically reasonable models that can result in CPE behavior. Normal distributions of time-constants can be expected in systems such as oxide films, organic coatings, and human skin. Such normal time-constant distributions may be caused by distributions of resistivity and/or dielectric constant. Hirschorn et al. 4 used a powerlaw distribution of resistivity ␦where the parameters 0 and ␦ are the boundary values of resistivity at the interfaces, to show that, for Ͻ ͑ ␦ ⑀⑀ 0 ͒ −1 , an analytic expression for the impedance could be obtained asThe function g was evaluate...

Electrochemical characterisation of a martensitic stainless steel in a neutral chloride solution

“…However, the CPE parameter, Q, cannot be attributed to a capacity. Similar values of Q ox were already reported for a Fe17Cr stainless steel [39]. To explain the CPE behaviour, the authors proposed a normal distribution of local resistivity through the passive film with strong differences in resistivity between the metal/film interface and the film/electrolyte interface, due to the semiconductor character of the passive layer.…”

Electrochemical synthesis of oxide nanotubes on Ti6Al7Nb alloy and their interaction with the simulated body fluid

“…For α = 1, Z CPE corresponds to the capacitor, for α = 0, it takes part of the resistor, and for α = −1, it corresponds to the inductor [36,38,39]. In all the cases, the simplest equivalent circuits were chosen in order to achieve lower confidence intervals for the estimated parameters and to capture all qualitative features of the impedance spectra [35].…”

Constant-phase-element behavior caused by inhomogeneous water uptake in anti-corrosion coatings

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