Development of Transport Properties Characterization Capabilities for Thermoelectric Materials and Modules
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Development of Transport Properties Characterization Capabilities for Thermoelectric Materials and Modules Karla R. Reyes-Gil, Josh Whaley, Ryan Nishimoto, and Nancy Yang Sandia National Laboratories, 7011 East Ave, Livermore, CA 94551
Figure of Merit (ZT)
ABSTRACT Thermoelectric (TE) generators have very important applications, such as emerging automotive waste heat recovery and cooling applications. However, reliable transport properties characterization techniques are needed in order to scale-up module production and thermoelectric generator design. DOE round-robin testing found that literature values for figure of merit (ZT) are sometimes not reproducible in part for the lack of standardization of transport properties measurements. In Sandia National Laboratories (SNL), we have been optimizing transport properties measurements techniques of TE materials and modules. We have been using commercial and custom-built instruments to analyze the performance of TE materials and modules. We developed a reliable procedure to measure thermal conductivity, seebeck coefficient and resistivity of TE materials to calculate the ZT as function of temperature. We use NIST standards to validate our procedures and measure multiple samples of each specific material to establish consistency. Using these developed thermoelectric capabilities, we studied transport properties of Bi2Te3 based alloys thermal aged up to 2 years. Parallel with analytical and microscopy studies, we correlated transport properties changes with chemical changes. Also, we have developed a resistance mapping setup to measure the contact resistance of Au contacts on TE materials and TE modules as a whole in a non-destructive way. The development of novel but reliable characterization techniques has been fundamental to better understand TE materials as function of aging time, temperature and environmental conditions. INTRODUCTION Within the last century, solid-state thermoelectric materials have been developed that are capable of converting heat directly into electrical energy. One such material, Bi2Te3, is a doped semiconductor originally developed in the 1950’s to replace the compressor based refrigeration system [1]. Bismuth Telluride (Bi2Te3) has the highest ZT for T between 25 °C and 150 °C when compared to other materials (Figure 1) [2]. Thermoelectric materials, such as Bi2Te3 alloys, generate useful electrical energy when a temperature gradient is applied inducing net migration of charge across the gradient. Directly converting thermal energy into electricity can come with an array of benefits in comparison to more commonly used energy generation methods, such as the Rankine cycle or Co-generation cycle. These benefits can include decreased environmental impact through the reduction of carbon emissions, greater reliability due to minimal moving parts, and the ability to generate electricity from heat that may have otherwise been wasted (such as automotive waste heat recovery). However, standardization of measurements of thermoelectric materials propertie
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