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A7: Developments in measurement techniques and preparation methods
The most reliable evaluation of the thermoelectric figure of merit (ZT) is based on the Harman method because the physical properties are measured on the same sample and in the same direction. However, any practical realization of this procedure at elevated temperatures becomes challenging due to unavoidable heat exchange between the sample and its environment.
We report the first successful implementation of this method for the simultaneous measurement of the figure of merit, the Seebeck coefficient, and the electrical and thermal conductivities on the same sample in a temperature range from 240 K to 720 K, which covers most practical thermoelectric applications ranging from cooling to energy harvesting in both low and intermediate temperatures. The system we have developed (ZT-Scanner) automatically measures, under vacuum conditions, the properties of specimens with cylindrical or rectangular (~25 mm2) cross sections from 1 to 6 mm long. The reproducibility of any measurement was found at the level of 1-2 % of the absolute values of the four properties, in an arbitrary temperature range, targeted by the experiment. A novel two-sample iterative calibration (2SIC) has been developed which precisely evaluates, regardless of the sample size, geometry and thermal conductivity, the unavoidable heat exchange between the specimen and the environment during the measurements. We report detailed results of the successful implementation of 2SIC procedure on thermoelectric characterization of bismuth telluride based materials from 240 K to 450 K as well as of a large group of materials including lead telluride, skutterudites, MgSi2, and SiGe from 300 K to 720 K.
The developed ZT-Scanner in combination with the novel 2SIC procedure solve the persisting problem of accurate (< 3 %) determination of ZT and its components and could serve as a key instrument in the evaluation of well-established and new thermoelectric materials.