On the Thermoelectric Potential of Inverse Clathrates
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1166-N06-03
On the Thermoelectric Potential of Inverse Clathrates Matthias Falmbigl1, Peter F. Rogl1, Ernst Bauer2, Martin Kriegisch2, Herbert Müller2, Silke Paschen² 1 2
Institute of Physical Chemistry, University of Vienna, A-1090 Wien, Austria Institute of Solid State Physics, Vienna University of Technology, A-1040 Wien, Austria
ABSTRACT In the context of a general survey on the thermoelectric potential of cationic clathrates, formation, crystal chemistry and physical properties were investigated for novel inverse clathrates deriving from Sn19.3Cu4.7P22I8. Substitution of Cu by Zn and Sn by Ni was attempted to bring down electrical resistivity and lower thermal conductivity. Materials were synthesized by mechanical alloying using a ball mill and hot pressing. Structural investigations for all specimens confirm isotypism with the cubic primitive clathrate type I structure (lattice parameters a = ~1.1 nm and space group type Pm-3n). Studies of transport properties evidence holes as the majority charge carriers. Thermal expansion data, measured in a capacitance dilatometer from 4 to 300 K on Sn19.3Cu1.7Zn3P19.9 2.1I8, compare well with literature data available for Sn24P19.6 2.4Br8 and for an anionic type I clathrate Ba8Zn8Ge38. From the rather complex crystal structure including split atom sites and lattice defects thermal conductivity in inverse clathrates is generally low. Following Zintl rules rather closely inverse clathrates tend to be semiconductors with attractive Seebeck coefficients. Thus for thermoelectric applications the main activity will have to focus on achieving low electrical resistivity in a compromise with still sufficiently high Seebeck coefficients. INTRODUCTION Efficient thermoelectric (TE) power generation and thermoelectric cooling technologies request materials of low electrical resistivity, ρ, and low thermal conductivity, λ, but with a high Seebeck coefficient, S [1]. Thermoelectric application orients itself on the so-called (dimensionless) thermoelectric figure of merit, ZT=S2T/(ρλ), which should exceed ZT>1 for efficient conversion of heat into electric power in a thermoelectric generator (TEG). “Classical” thermoelectric materials based on bismuth and lead tellurides have recently achieved ZT~1.2-1.4 by selective doping, superlattice technologies or nanostructuring but suffer from low thermal stability (Tm(Bi2Te3)=586 °C; ZTmax(Bi2Te3) at 100 °C; ZTmax(PbTe) at 350 °C) [2,3]. Design of thermoelectric generators for automotive applications, however, needs thermoelectric materials that can reliably function at elevated temperatures [4]. “Intermetallic clathrates”, especially compounds with clathrate type I structure, proved that they may fulfill these requirements exploiting the Phonon Glass-Electron Crystal (PGEC) concept developed by Slack based on (a) a sufficiently good electrical conductivity in combination with (b) weakly bound atoms or molecules, which via their “rattling modes” efficiently scatter phonons and thus decrease thermal conductivity [5]. In this respect the compli
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