Electron and Phonon Transport in n- and p-type Skutterudites
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Electron and Phonon Transport in n- and p-type Skutterudites Jiong Yang1, S. Wang1, Jihui Yang1, a), W. Zhang2, and L. Chen2 Materials Science and Engineering Department, University of Washington, Seattle, WA 98195-2120, USA 2 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China 1
ABSTRACT Filled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications at intermediate temperatures due to their superior TE and thermomechanical performance as compared to other materials. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K. TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. In this paper we reveal that the underlying reasons for inferior TE properties of the p-type root in their electronic band structures, which result in higher thermal conductivity at elevated temperatures due to bipolar lattice thermal conduction and lower power factor because of heavy valance bands induced strong electron-phonon interactions. We also identify means of improving the power factor and reducing bipolar effect.
a) Author to whom correspondence should be addressed, Electronic mail: [email protected]
INTRODUCTION One of the key challenges on thermoelectric (TE) research is to search for high efficiency TE materials, which should possess high thermopower (α), high electrical conductivity (σ), low thermal conductivity (κ - summation of the electronic κe, lattice κL, and bipolar κb components), and therefore high dimensionless TE figure of merit ZT (=α2σT/κ). Identifying materials with high ZT values has proven to be extremely difficult since these transport properties are correlated with each other: increasing the thermopower usually means lowering the electrical conductivity, and vice versa; the electronic thermal conductivity also relates to the electrical conductivity via the Wiedemann-Franz law. Usually power factor (PF), α2σ, is determined by the electronic band structures (valence band (VB) or conduction band (CB)), and can be optimized as a function of carrier concentration and scattering mechanisms. On the other hand, the lattice thermal conductivity κL is mainly determined by lattice dynamics and phonon scatterings. The ZTs in bulk materials have been improved significantly since 1990s with the majority of maximum values lower than 2.0, still not high enough to meet the need of large-scale industrial applications.[1]
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Skutterudite compounds have been considered as promising candidates for advanced TE applications. Binary skutterudites, in the general formula of MX3 (where M is Co, Rh, or Ir, and X is a pnicogen atom) crystallize in a body-centered-cubic structure with the space group Im3. The structure of MX3 is composed of corner-sharing MX6 octahedra.[2] CoSb3 is the most widely studied binary skutter
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