Nanostructured Silicon-based Composites for High Temperature Thermoelectric Applications
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1267-DD01-06
Nanostructured Silicon-Based Composites for High Temperature Thermoelectric Applications Sabah Bux1, Richard B. Kaner1 and Jean-Pierre Fleurial2 1 Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California; 2 Power and Sensor Systems Group, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California; ABSTRACT Recently nanostructured bulk silicon and silicon-germanium have achieved large increases in the thermoelectric figure of merit (ZT). The ZT enhancement is attributed to a significant reduction in the lattice thermal conductivity while maintaining relatively high carrier mobility. Siliconbased thermoelectric devices are attractive due to their low-toxicity, thermal stability, low density, relative abundance and low cost of production. Although significant enhancements in ZT have been achieved using the nanostructuring route, additional decoupling of the thermal and electric transport terms is still necessary in order for silicon-based materials to be viable for thermoelectric applications such as waste heat recovery or radioisotope thermoelectric generators. It is theorized that additional increases in ZT could be achieved by forming composites with nanostructured inert inclusions to further scatter the heat-carrying phonons. Here we present the impact of insulating and conductive nanoparticle composites on ZT. The nanostructured composites are formed via ball milling and high pressure sintering of the nanoparticles. The thermoelectric properties and microstructure of the silicon-based composites are discussed. INTRODUCTION For the past 40 years, the National Aeronautics and Space Administration has relied upon radioisotope thermoelectric generator technology to power their deep space probes and long term planetary missions.1 Although the current thermoelectric generator technologies based on PbTe and SiGe materials have been both reliable and successful, a factor of 2 to 3 improvement in the thermal-to-electrical conversion efficiency is highly desirable to enhance or even enable future NASA science missions. Despite great effort over the past 30 years to improve the thermoelectric figure of merit (ZT), the maximum ZT for bulk materials remains near 1.2 over the entire temperature range from 100 to 1500 K.2 There have been two leading strategies in order to decouple the thermal and electronic transport mechanisms to enhance ZT: either examining complex structures such as Skutterudites or Zintl phases or examining lowdimensional materials such as superlattices, nanowires and quantum dot structures. Preliminary results on low-dimensional structures such as single nanowires of Si3, 4 and Bi2Te3 superlattices5 have shown significant improvements in ZT. Further analyses indicated that the ZT enhancements were mainly due to a substantial reduction in the lattice thermal conductivity. Although the low-dimensional materials have demonstrated promising results, they are ill suited for high temperature
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