THECLA

Status

ongoing

Summary

Thermoelectric materials can convert heat gradients into electricity (Seebeck effect) and, in an inverse process, use electrical current for active cooling (Peltier effect). Though both phenomena have long been known it is only recently that thermoelectric energy conversion is considered for an increasing number of applications. Of particular interest for a number of applications is the absence of moving parts and thus the silent, vibration-, and maintainance-free operation of thermoelectric converters.

The thermoelectric figure-of-merit, ZT = S²T/(ρκ), is entirely given by materials properties: the thermopower S, the electrical resistivity ρ and the thermal conductivity κ. T is the absolute temperature. A material is considered a promising candidate for thermoelectric applications if ZT > 1. The energy conversion efficiency η of a device is related to ZT: for ZT = 1 and a temperature difference of 100 K between hot and cold side of the thermocouples η can reach values of about 0.1 (10%).

Well established thermoelectrics such as Bi-Sb or Pd-Te based materials are typically on the border between metallic and insulating behaviour, with charge carrier concentrations of the order of 10²⁰ cm^-3. They have been optimized for room temperature applications where ZT values of 1 have been realized. For higher temperatures, however, there is still a strong quest for efficient materials.

New concepts and new materials classes are much explored in recent years. ZT values well above 1 have been demonstrated with laboratory setups. Thus there is real hope that a new generation of thermoelectric materials can indeed be commercialized.

In the project THECLA the material class of clathrates is at focus. The filled-cage crystal structure of this class mimics in an almost perfect way the much discussed phonon glass, electron crystal (PGEC) concept. A PGEC conducts heat as poorly as a glass but electricity as well as a crystal. The former is believed to be realized in clathrates by the strong scattering of the heat-carrying phonons (lattice vibrations) from strong thermal motions of loosely bound guest atoms residing in oversized cages. The latter is attributed to the charge carriers being confined to the essentially covalently bonded framework.

The aim of THECLA is to optimize the thermoelectric performance of type I clathrates by a dual approach. In a first step, well studied starting materials such as Ba₈Ga₁₆Ge₃₀ shall be modified by substitution/doping with other elements to enhance ZT. In a second step, various methods of actificial micro- and nanostructuring of clathrates shall be investigated to further enhance the performance. Published results on simpler model systems have shown that adequate structuring can, on one hand, strongly suppress the phonon mean free path and thus the thermal conductivity and, on the other hand, enhance the thermopower by quantum confinement effects.

Project or cooperation partner

• E. Bauer - Inst. f. Festkörperphysik
• Uni Wien-Inst. f. Physikalische Chemie

TU Wien - Festkörperphysik
Karlsplatz 13
1040 Wien
RKZ: 90401 EU-Zielgebiet: --
techn. Ansprechpartner: Prof. Silke Bühler-Paschen
Tel.: +43 1 / 58801-13716
kaufm. Ansprechpartner: Prof. Ernst Bauer
Tel.: +43 1 / 58801-13160