Constant phase elements (CPE) have become common m fitting procedures of electrochemical impedance spectra. The impedance of a CPE is given by:Z(CPE) = Y_o~(-1)(j#omega#)~(-n) (1)While the use of CPE usually increases the "goodness of the fit," the significance of the parameters Y_o and n often is not discussed. Obviously almost any spectrum can be fit to an equivalent circuit (EC) containing a sufficiently large number of CPE. However, this type of approach usually does not produce information concerning the physical properties of the system under investigation.In studies of corroding systems, CPE are most often used to describe the frequency dependence of non-ideal capacitive behavior. The dimension of Y_o is s~n/#OMEGA#, while that of a capacitance (C) is s/#OMEGA# or F. Despite this difference, Y_o is often used as if it were the capacitance of the corroding system. Conversion of Y_o data into C is very important when experimental capacitance data are to be used to determine quantitatively system parameters such as the thickness of anodized layers on Al alloys or the dielectric constant k of low-k materials. Calculations using Y_o values instead of C will result in thickness values that are too low and k values that are too high.
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