Evaluation of a thermoelectric material using duo-frequency impedance spectroscopy method

Abstract

The impedance spectroscopy (IS) method is an efficient technique to evaluate the dimensionless figure of merit (zT) of thermoelectric materials by measuring some impedance values at a wide frequency range. The conventional IS method uses the technique of fitting measured data to estimate zT; however, in our previous work, we showed that zT can also be directly calculated using the measured thermoelectric resistance and ohmic resistance of a sample. Although we successfully estimated zT from two points of impedances whose imaginary components are 0 mΩ, we needed to measure many points of impedances to determine the trend of frequency dependence of impedances and determine specific frequencies. Moreover, discussions on the error of the measured zT caused by the setting values of current frequencies were insufficient. In this paper, we propose the duo-frequency IS method to evaluate the error of the measured zT by relating the phase angle of the measured impedance to the error of zT. Furthermore, appropriate current frequencies are predicted using the proposed method to reduce the measurement time by reducing the data points for calculating zT. The proposed duo-frequency IS method was used to measure the temperature dependence of zT, resistivity, thermal conductivity, and specific heat of a Bi2Te3 bulk material from 125 K to 300 K; the method yielded an error of less than 1% for the measured zT. The results of this study were in good agreement with those obtained in previous studies by using the steady-state method. The proposed duo-frequency IS method simplifies the evaluation of the thermoelectric properties of thermoelectric materials and elucidates the measurement errors caused by the setting frequencies.