Advances in Thermoelectric Materials

Abstract

Dear colleagues, This Topical Section of physica status solidi (a) focuses on advances on thermoelectric materials. Our aim is to give a brief but broad overview of current achievements from material design to measurement techniques in the field of nanostructured thermoelectrics. The Feature Article by Hébert et al. 1 reviews CdI2 type structures including oxides, sulfides, and selenides. For example, the authors show that oxides in the CdI2 structure have interesting physical properties, in particular, the observed transport properties cannot be explained by standard Boltzmann theory. An improvement of the thermoelectric figure of merit ZT can be achieved by reducing the lattice thermal conductivity. Kurosaki and Yamanaka 2 review the group 13 chalcogenides because of their very low thermal conductivity. For example, silver–thallium–tellurium ternary compounds (Ag9TlTe5) have already a relatively large ZT value due to an extremely low lattice thermal conductivity but these compounds have bad electrical properties. Consequently, one can expect in future further improvements by optimizing the electrical properties. For the characterization of thermoelectric materials, in particular, for nanostructured materials local measurements are a need. Thus local measurements of the Seebeck coefficient are very useful but hard to perform. In the Feature Article by Ziolkowski et al. 3 different measurement techniques are reviewed and discussed. The measurement of the thermal conductivity of thin films is discussed in the article of Völklein et al. 4, and in-plane and cross-plane thermal conductivity is discussed. Most of current efficient thermoelectric materials incorporate rare elements, and the use of abundant materials for thermoelectric applications is a long-term goal. One promising material for high temperature applications is ZnO. In the invited article by Homm et al. 5 different interface geometries of laterally microstructured ZnO-based thin films are investigated and a strong influence of the interface structure on the thermoelectric properties is found. For the same type of material Bachmann et al. 6 calculated in their contribution the phonon transport based on density functional theory. In particular, it is shown that sulfur impurities in ZnO lead to substantial decrease of the thermal conductivity. In two contributions another material system, cobalt antimonide skutterudites (CoSb3), is investigated theoretically by Hammerschmidt et al. 7 and experimentally by Daniel et al. 8. In the latter the influence of stress due to the substrate is discussed. In the theoretical calculations different exchange–correlation functionals are compared. The other contributions investigate the thermoelectric properties of different materials and geometries. Sittner et al. 9 discuss the thermoelectric properties of phase change materials. Ge nanoparticles are investigated by Stoib et al. 10. The contribution by Bartsch et al. 11 focuses on geometrical effects. In particular, the thermoelectric properties of GaAs pillars are investigated. We hope that the Feature Articles and the contributions in this Topical Section give not only an overview of the achievements but also of the challenges ahead of us in thermoelectrics. In this spirit we hope also that this compilation can be an inspiration for your future work. Gießen, December 20, 2012 Christian Heiliger Bruno K. Meyer