Nanostructured Thermoelectrics

Finding alternative energy sources is one approach to satisfying the ever growing demand for energy; another is reclaiming energy that is often wasted in everyday life. For example, most cars convert only about 25% of the chemical potential energy from gasoline into kinetic energy to move the car. The other 75% is converted to ‘waste’ heat in the form of mechanical friction and exhaust gases. Thermoelectrics hold significant potential as a means for increasing energy efficiency and improving our use of various energy sources because they operate on the principal of converting ‘waste’ heat directly into electricity. In this project we are developing the tools and processes needed to accurately characterize the ability of thermoelectric materials to efficiently convert waste energy into usable form.

Recent advances in nanostructured thermoelectric materials have increased the conversion efficiency of the thermoelectric devices to the point that they may soon become cost-effective for widespread use. Several scientific developments have resulted in significant improvements in the thermoelectric figure of merit, ZT, which directly affects conversion efficiency. These developments, each with a distinct mechanism for enhancement, involve fabrication of reduced dimensional structures, structures containing nanoscale inclusions, and lamellar structures.  

Characterizing a thermoelectric material requires precise measurements of electrical conductivity, thermal conductivity, and thermopower (the voltage produced per degree temperature difference across the material). To address this need, we are developing new measurement tools and techniques that can be applied to both conventional and unconventional thermoelectric materials, allowing for a more accurate determination of the key properties, including ZT.

We have designed an innovative heat pipe, incorporating high-precision thermometers that can locally probe temperatures ranging from room temperature to 800 K with an accuracy of a few milliKelvin. The design is versatile, allowing for measurements on samples of different structure and composition, ranging from nanowires grown in templates with a length of a few tens of nanometers, to macroscopic materials containing nanostructures. This instrument can also be used in conjunction with a superconducting magnetic for magnetotransport measurements, and thus allow for determination of charge carrier density. By providing a complete set of measurements, our approach will enable the comprehensive characterization of nanostructured materials needed to advance commercially-promising thermoelectric technologies.