2-1: Concepts and methods for improved thermoelectric materials by nano-scaled interactions and hybrids

TUM: Thomas Fässler, Peter Müller-Buschbaum, Bernhard Rieger
UofA: Frank Hegmann, Arthur Mar, Eric Rivard
Doctoral Candidates: Ebru Alime Üzer, Kerstin Mayer

This project focuses on the synthesis of next generation TMs via a hybrid approach by combining inorganic and organic components into a single device. The Nilges, Mar and Fässler Teams will examine novel classes of substances, encompassing chalcogenides, pnictides, and tetrelides that take advantage of new concepts to improve TM performance. An enticing idea is thermopower (the voltage induced by a temperature difference) which can be enhanced in materials that exhibit charge density waves (CDW) or Peierls distortions (PD) in their substructures or materials following the Phonon Glas-Electron Crystal (PGEC) concept. To date, promising chalcogenides like Ag10Te4Br3 or AgBiSe2 have been identified in which CDW or PD introduces a strong modulation of the density of states at the Fermi level. Large subunits are rearranged, thereby directly affecting TM performance. Pnictidesbased on the CaAl2Si2-type structures (e.g. CaxYb1–xZn2Sb2) or ternary pnictides AM4Pn2 (A = Sr, Eu; M = Cu, Ag; Pn = As, Sb), which can be regarded as “stuffed” versions of the CaAl2Si2-type, offer greater control over the transport properties (e.g., p- or n-type doping) by controlled atomic substitution. Zintl phases can satisfy the conflicting requirements of good electrical conductivity (through a regularly structured covalently bonded anionic network of metalloids) and poor thermal conductivity (through doping or introduction of disorder in intervening cations), while Clathrates based on Ge- or Sn-containing clusters have structures containing large cages within which electropositive alkali- or alkaline-earth guest cations can “rattle”, again reducing thermal conductivity. It may turn out that the best TM candidatesexhibit several of these structural features (CDW, defects, disorder, large cages), and our combined efforts will enhance our ability to design/develop ideal systems. A novel approach to the field of thermoelectrics is to combine layered transition metal oxides that show large Seebeck coefficients such as NaCo2O4 with intrinsic semiconductors such as silicon and germanium. The prototype of such a hybrid layered material exists in form of the crystal inter growth of the Zintl Phase Cs4Ge9 and Cs2WO4 or Cs3VO4 which combines on an atomic level both types of materials.

In addition, hybrid TMs will be studied by the Müller-Buschbaum group by combining newly synthesized inorganic nanoparticles with conductive polymers. In this hybrid approach an additional possibility to induce disorder to the systems arises from the limited order in the polymer phase as well as from the morphology of the inorganic component with respect to the polymer.