Interpretation of dielectric properties for materials showing constant-phase-element (CPE) impedance response

Models invoking Constant-Phase Elements (CPE) are often used to fit impedance data arising from a broad range of experimental systems. While the physical origins of the CPE are controversial, a bigger problem remains the interpretation of impedance data in terms of physically meaningful properties such as capacitance or thickness. Four models are used to interpret the CPE parameters associated with the impedance response of human skin and two metal oxides in terms of characteristic frequencies, film thickness, and dielectric constant. These values were compared against independent measurements. The power-law model developed recently by Hirschorn et al. (1, 2) provided the most reliable interpretation. Readers are cautioned that the CPE parameter Q does not provide an accurate value for capacitance, even when the CPE exponent is greater than i0.9.