Frequency-domain Harman technique for rapid characterization of bulk and thin film thermoelectric materials

Abstract

Nanostructured thermoelectrics, often in the form of thin films, may potentially improve the generally poor efficiency of bulk thermoelectric power generators and coolers. In order to characterize the efficiency of these new materials it is necessary to measure their thermoelectric figure of merit, ZT. The only direct measurement of ZT is based on the Harman technique and relies on measuring the voltage drop across a sample subjected to a passing continuous current. Application of this technique to thin films is currently carried out as a time-domain measurement of the voltage as the thermal component decays after switching off an applied voltage. This work develops a technique for direct simultaneous measurement of figure of merit and Seebeck coefficient from the harmonic response of a thermoelectric material under alternating current excitation. A thermocouple mounted on the top surface measures voltage across the device as the frequency of the applied voltage is varied. A thermal model allows the sample thermal conductivity to also be determined and shows good agreement with measurements. This technique provides improved signal-to-noise ratio and accuracy compared to time-domain ZT measurements for comparable conditions while simultaneously measuring Seebeck coefficient. The technique is applied to both bulk and thin film thermoelectric samples.