Experimental conditions required for accurate measurements of electrical resistivity, thermal conductivity, and dimensionless figure of merit (ZT) using Harman and impedance spectroscopy methods

Abstract

The Harman method is used extensively for the characterization of the dimensionless figure of merit ZT of thermoelectric (TE) materials and devices. However, its accuracy has often been questioned, since in many cases there are relatively high errors associated with the method. The impedance spectroscopy technique, which has recently been shown as a suitable tool to also characterize TE materials and devices, has some similarities with the Harman method and can also directly provide ZT. In order to obtain reliable measurements in both methods, there are some common critical points that must be taken into account, such as the requirement of fully adiabatic conditions and a negligible Joule effect. In this study, we have evaluated the effect of different experimental conditions in the accuracy of both methods using a sample with known TE properties. Our analysis has led to the identification of different sources of errors and other issues that have not been clearly identified to date that can lead to inaccurate results, namely, the need of a homogeneous Peltier effect at the junctions, problems arising from the use of Ag paint, and the selection of the right value for the current perturbation applied to the system. These problems and sources of errors need to be identified and carefully considered if accurate results are to be obtained.