80

nanotimes News in Brief

11-05 :: May/June 2011

Thermoelectric Materials // Neutron Analysis Explains Dynamics Behind Best Materials

N

eutron analysis of the atomic dynamics behind thermal conductivity is helping scientists at the

Department of Energy‘s Oak Ridge National Labo- ratory gain a deeper understanding of how thermo- electric materials work. The analysis could spur the development of a broader range of products with the capability to transform heat to electricity.

Researchers performed experiments at both of ORNL‘s neutron facilities – the Spallation Neutron Source and the High Flux Isotope Reactor – to learn why the material lead telluride, which has a similar molecular structure to common table salt, has very low thermal conductivity, or heat loss – a property that makes lead telluride a compelling thermoelectric material.

“The microscopic origin of the low thermal conducti- vity is not well understood. Once we do understand it better we can design materials that perform better at converting heat to electricity,” said Olivier Delaire, a researcher and Clifford Shull Fellow in ORNL‘s Neutron Sciences Directorate.

Delaire‘s research, reported in Nature Materials, shows that an unusual coupling of microscopic vibra- tional modes, called phonons, is responsible for the disruption of the dynamics that transport the thermal

energy in lead telluride. In typical crystalline mate- rials, which have a lattice-like atomic structure, the conduction of heat is enhanced by the propagation of phonons along the lattice. The atoms conduct heat by vibrating in a chain, similar to vibrations propaga- ting along a spring.

Delaire‘s team determined through analysis at the SNS that lead telluride, although having the same crystal lattice as rock salt, exhibits a strong coupling of phonons, which results in a disruption of the latti- ce effect and cancels the abilißty to conduct heat.

“The resolution of the SNS‘s Cold Neutron Chopper Spectrometer, along with the high flux, have been quite important to making these time of flight mea- surements,” Delaire said.

By controlling thermal conductivity in thermoelec- trics, less energy is dispersed and more heat can be directed to power generation. Today, thermoelectric materials are used to power the deep-space probes that explore the outer planets and solar system. Cru- ising beyond the range of solar collectors, the crafts‘ reactor thermoelectric generators use heat from decaying radioisotopes to generate power. New, advanced thermoelectric materials could be used to develop more earthly applications, such as