System for Simultaneous Harman-Based Measurement of All Thermoelectric Properties, from 240 to 720 K, by Use of a Novel Calibration Procedure

Abstract

The most reliable evaluation of the thermoelectric (TE) figure of merit (ZT) is based on the Harman method, because the physical properties are measured on the same specimen and in the same direction. However, practical realization of this procedure at elevated temperatures is challenging, because of unavoidable heat exchange between the sample and its environment. We report the first successful implementation of this method for simultaneous measurement of the figure of merit, the Seebeck coefficient, and electrical and thermal conductivity on the same specimen in the temperature range 240–720 K, a range which covers most practical TE applications ranging from cooling to energy harvesting at both low and intermediate temperatures. The system we have developed (ZT-Scanner) automatically measures, under vacuum conditions, the properties of specimens from 1 to 6 mm long with cylindrical or rectangular (∼25 mm²) cross sections. The reproducibility of consecutive measurements was found to be 1–2% of the absolute values of the four properties, in any arbitrary temperature range. The novel two-sample system calibration (2SSC) developed resolves, irrespective of specimen size, geometry, and thermal conductivity, the unavoidable heat exchange between the specimen and the environment, assisting these measurements. We report detailed results from successful implementation of 2SSC for TE characterization of bismuth telluride-based materials from 240 to 450 K and of a large group of other materials including lead telluride, skutterudites, Mg, Si, Sn, and SiGe from 300 to 720 K. The developed ZT-Scanner in combination with the novel 2SSC procedure solve the persistent problem of accurate determination of ZT and its components and can serve as an important instrument for precise evaluation of well-established and new TE materials.