Overview of various strategies and promising new bulk materials for potential thermoelectric applications

Recently, there has been a renewed interest in thermoelectric material research. There are a number of different systems of potential thermoelectric (TE) materials that are under investigation by various research groups. Some of these research efforts focus on minimizing lattice thermal conductivity while other efforts focus on materials that exhibit large power factors. An overview of some of the requirements and strategies for the investigation and optimization of a new system of materials for potential thermoelectric applications will be discussed. Some of the newer concepts such as low-dimensional systems and Slack's phonon-glass, electron-crystal concept will be discussed. Current strategies for minimizing lattice thermal conductivity and also minimum requirements for thermopower will be presented. The emphasis of this paper will be to identify some of the more recent promising hulk materials and discuss the challenges and issues related to each. This paper is targeted more at "newcomers" to the field and does not discuss some of the very interesting results that are being reported in the thin film and superlattice materials. Some of the bulk materials which will be discussed include complex chalcogenides (e.g. CsBi4Te6 and pentatellurides such as the Zr1-xHfxTe5 system), half-Heusler alloys (e.g. TiNiSn1-xSbx), ceramic oxides (NaCo4O2), skutterudites (e.g. YbxCo4-xSb12 or EuxCo4-xSb12) and clathrates (e.g. SrxGa16Ge30). Each of these systems is distinctly different yet each exhibits some prospect as a potential thermoelectric material. Results will be presented and discussed on each system of materials.