FWF P29279-N27: ThermoWire

Funding institution:
FWF Austrian Science Fund

Project leader:
Silke BÜHLER-PASCHEN         

Project duration:
April 2017 - March 2020

THERMOELECTRIC PROPERTIES OF COMPLEX MESOWIRES (ThermoWire)

Thermoelectric (TE) materials convert electric currents into temperature gradients and vice versa. As such they can be used both for solid state refrigeration and for conversion of waste heat into electricity, processes of great interest for a cleaner environment and a more efficient energy household. However, this prospect has been hampered by the relatively low efficiency of commercially available TE materials, restricting their technical applicability.

The prediction made more than twenty years ago that this can be remedied by nanotechnological means has triggered extensive research efforts worldwide, most focusing on the introduction and stabilization of nanostructures in bulk TE materials. The main problem with bulk nanostructuring, however, is that it usually does not only alter the grain sizes but also the material's density, composition, doping level, amount of foreign phases, and so on. As a consequence our understanding of the effects that nanostructuring has on the charge and heat carriers of the material is very limited. This situation calls for a new approach. ThermoWire shall overcome these limitations by studying TE properties on single, well-shaped mesoscopic (tens of nanometers to micrometers) structures of TE materials of great current interest.

Measurements on single mesoscopic samples hold great promise for the complex TE materials we propose to study here: (1) rattling compounds, (2) materials with strongly correlated electrons, and (3) topological insulators. In these three materials classes mesostructuring is particularly interesting for very different reasons: in (1) it will reveal whether indeed very long-wavelength phonons dominate the heat transport, in (2) it will test whether giant thermoelectric powers are robust against mesostructuring, and in (3)  it will reveal whether high surface conductivities may boost the TE efficiency. 

Unfortunately, the reproducible bottom-up fabrication of mesowires is an unpredictable and tedious undertaking, and for most of the new materials that shall be investigated no routes of production are known. Therefore, the method that ThermoWire shall employ is a top-down approach. Bulk single crystals of selected compounds of all three material classes shall be synthesized, shaped by a focused ion beam technique into mesowires of varying diameters, crystallographic direction, and surface constitution, and measured one-by-one on a dedicated mesowire measurement platform. This will allow us to disentangle the different effects, compare the results with theory, and thus distinctly advance the field. Having recently developed the techniques, we are well-poised to go about these challenging tasks.