Impedance spectroscopy models for the complete characterization of thermoelectric materials

Abstract

This paper analyses the use of impedance spectroscopy as a characterization tool applied to thermoelectric materials. The impedance function of the thermoelectric system under adiabatic conditions and Peltier mode operation is calculated by solving the heat equation in the frequency domain. The analysis, focused on the complex plane, provides the required equivalent circuit elements to interpret the impedance measurements. Using this approach, all the relevant thermoelectric parameters and thermal properties can be potentially extracted at a given temperature from the impedance spectra, i.e., the Seebeck coefficient, electrical resistivity, thermal conductivity, figure of merit (zT), specific heat, and thermal diffusivity. This can be done without the need of measuring temperature differences. To validate the models described, impedance measurements have been carried out in single thermoelectric elements and modules, showing an excellent agreement with the theory. The simple nature of the measurements in conjunction with the advantage of obtaining all the important thermoelectric parameters opens up the possibility of establishing impedance spectroscopy as a very useful characterization method for the thermoelectric field.