Lower-Limits of the Nanostructured Approach To a Better Thermoelectric
A team of scientists, headed by Jeffrey Urban of the Molecular Foundry, have achieved ultralow thermal conductivity in polycrystalline thin films of cadmium selenide (CdSe). The results help identify fundamental limits of grain-boundary scattering as a means of improving thermoelectric efficiency.
An efficient thermoelectric material requires low thermal conductivity. Recent studies have proposed that boundaries between crystalline grains in a polycrystalline material can serve to scatter phonons – excitations that transport heat in solid materials – and thus improve thermoelectric performance.
In order to systematically study the lower-limits of this approach to low thermal conductivity, Urban and coworkers fabricated thin films with grain sizes from 3 to 6 nm using mono-disperse CdSe nanocrystals. To their surprise, the team found their nanocrystalline films had lower thermal conductivity even than analogous amorphous solids as described by conventional theory; however these results were understood by applying theories of grain-boundary phonon scattering.
While CdSe is not an intrinsically good thermoelectric material due to low electrical conductivity, the results shed light on the fundamental limits of phonon transport in nanocrystalline materials and will help guide development of nanostructured thermoelectric materials for waste-heat recovery.