Doping and Alloying Trends in New Thermoelectric Materials
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DOPING AND ALLOYING TRENDS IN NEW THERMOELECTRIC MATERIALS Sim Loo†, Sangeeta Lal†, Theodora Kyratsi‡, Duck-Young Chung‡, Kuei-Fang Hsu‡, Mercouri G. Kanatzidis‡, Timothy P. Hogan† †
‡
Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI Chemistry Department, Michigan State University, East Lansing, MI
ABSTRACT New thermoelectric bulk materials such as CsBi4Te6 have shown superior properties to traditional materials, however, optimal performance requires continuing investigations of doping and alloying trends. A recently modified high throughput measurement system is presented for doping and alloying investigations in several new thermoelectric materials. The modification includes a four-probe configuration for more accurate measurements while maintaining a relatively short sample preparation time. The system is fully computer controlled and provides flexible contacts to accommodate various sample dimensions. Optimal compositions are then identified for further investigations in thermoelectric prototype modules. The most promising materials will be further characterized for electrical conductivity, thermoelectric power, thermal conductivity, and Hall effect measurements as a function of temperature.
INTRODUCTION The renewed interest in the area of thermoelectrics has resulted in a number of new material systems. The most promising of these require additional characterization for optimization of the figure of merit through doping and alloying trends. This is a multi parameter problem that requires the production of a large number of samples to understand such trends. Investigations of repeatability in fabrication further extend the number of measurements required. Several recently developed fabrication techniques [1, 2] have successfully produced large number of interesting TE materials for investigation. A corresponding increase in the rate of sample characterization is required to fully benefit from this increase in production, and to avoid a backlog of unmeasured samples. The thermoelectric figure of merit, ZT, is most influenced by the thermoelectric power through the S2 term in ZT = S2σT/κ. This work was, therefore, initiated by designing and assembling a computer controlled system for measuring temperature dependent thermoelectric power data on 36 samples in a single temperature run (typically 80K to 350K) [3]. A sketch of the sample stage and a picture showing a series of mounted samples are shown in Figures 1, and 2, respectively. Thermoelectric power measurements on standard Bi2Te3 samples cut from the same ingot show agreement within ~20% of one of the samples measured in an independent higher accuracy system [3] as shown in Figures 3 and 4. The Bi2Te3 ingot is made by Tellurex Corporation and the sample number indicated the sample location on the sample stage. This system also incorporated a two-probe electrical conductivity function, however, it was desirable to maintain a large enough geometry of the more promising samples for subsequent device fabrication. G
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