ABSTRACT
Thermal conductivity reduction has played a central role in improving the thermoelectric figure-of-
merit, ZT, of materials that already have a good power factor. For bulk materials one major method used
to reduce the thermal conductivity is point defect scattering of phonons, by alloying or adding phonon
“rattlers.” Nanostructured materials such as superlattices and nanowires offer two additional tools to
reduce thermal conductivity: altering the phonon dispersion relation to reduce the specific heat and
average group velocity, and increasing phonon scattering at interfaces and boundaries to reduce the
mean-free path. The thermal conductivity reduction in nanostructures has been exploited experimentally
to improve ZT in some materials described in more detail elsewhere in this Handbook. For example, in
very uniform Bi
Te
/Bi
Te
Se
superlattices of ,6 nm period, Venkatasubramanian and coworkers
were able to reduce the lattice thermal conductivity at room temperature in the cross-plane direction to
about 0.58 W/mK
compared to about 1.7 for bulk Bi
Te
.
