Temperature dependence of dimensionless figure of merit of a thermoelectric module estimated by impedance spectroscopy

Temperature dependence of dimensionless figure of merit zT of a Π-shaped thermoelectric module using bismuth-telluride materials was estimated on the basis of a theory of impedance spectroscopy, which is able to ascertain zT directly using only electrometric measurements from the frequency dependence of the impedance without any calorific measurement. The dependence of the module was measured from 5 mHz to 10 kHz with precise temperature control. From the analysis, the ohmic resistance, the impedance due to the Peltier effect, and the characteristic heat frequency at 300 K were determined under two different boundary conditions (the suspended and fixed conditions of a heat bath). A comparison between these boundary conditions revealed that their difference led to a slight change in the frequency dependence of the measured impedance due to the variation in the heat capacity of the module, and the estimated dimensionless figure of merit was identical for the two boundary conditions (zT ≈ 0.839 at 300 K). The temperature dependence of the dimensionless figure of merit of the module was successfully measured from 20 K to 300 K. The dependence of the thermal conductivity was also estimated with an assumed Seebeck coefficient from the definition of zT. In addition, a new method using only two impedances with repeatability was proposed and demonstrated to estimate the dimensionless figure of merit precisely (zT ≈ 0.845 at 300 K).