Thermoelectric Properties of Composite PbTeThermoelectric Properties of Composite PbTeThermoelectric Properties of Compo
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Thermoelectric Properties of Composite PbTe – PbSnS2 Materials Steven N. Girard,1 Jiaqing He,2 Vinayak P. Dravid,2 and Mercouri G. Kanatzidis1 1 Department of Chemistry, Northwestern University, Evanston IL 60208 2 Department of Materials Science and Engineering, Northwestern University, Evanston IL 60208 ABSTRACT The thermoelectric (Pb1-mSnmTe)1-x(PbS)x where m = 0.05 and x = 0.08 has been shown to produce PbS nanostructures that effectively scatter phonons, enhancing ZT. As Sn substitution is increased, a new phase of PbSnS2 precipitates. We find that incorporation of PbSnS2 in PbTe results in a significant reduction in lattice thermal conductivity around 0.6 W/mK at room temperature. We present preliminary characterization and thermoelectric properties. INTRODUCTION Historically, the utilization of thermoelectric materials and devices has been limited by low efficiencies. The thermoelectric efficiency is related to the figure of merit ZT, defined as ZT= S2σT/κ, where S is the thermopower or Seebeck coefficient, σ is the electrical conductivity, T is the operating temperature, and κ is the total thermal conductivity (a sum of the electronic κelec and lattice κlat vibrations). The conventional thermoelectric PbTe has an optimized ZT around 650 K of ~0.8, corresponding to an efficiency of around 10%.1 The incorporation of nanostructures can increase phonon scattering, reducing κlat and increasing ZT.2,3 Nanostructured layered materials, superlattices, and multilayers can exhibit extremely low values of lattice thermal conductivity, which could be utilized in thermoelectric composites to enhance figure of merit.4-6 Through the high-temperature synthesis of an appropriate combination of PbTe, SnTe, and PbS, we present here a promising thermoelectric composite of PbTe – PbSnS2. PbSnS2 (mineralogical name teallite) is a layered material (space group Pnma) comprised of Sn-Pb bilayers approximately 0.6 nm in thickness which form a superstructure along the a axis. High resolution transmission electron microscopy reveals the PbSnS2 segregates into coherent lamellar structures 50 – 100 nm in thickness that extend 100 nm – 15 µm in length. EXPERIMENT Samples of PbTe – PbSnS2 were synthesized initially using PbTe, SnTe, and PbS starting materials prepared using high-purity starting elements by conventional high temperature solidstate method. Starting materials were flame sealed in fused silica ampoules at a residual vacuum of ~10-4 Torr and heated to 1100º C in a box furnace. Samples were inverted several times at the melt and allowed to cool with the furnace door open to allow rapid solidification. The resulting ingots were free of bubbles and cracks and could be easily cut and polished for properties measurements. Purity of phases and homogeneity along the ingot was confirmed by powder Xray diffraction (PXRD).
DISCUSSION Initially, (Pb1-mSnmTe)1-x(PbS)x where 0.04 > x > 0.30 and m > 0.05 was synthesized to better understand the size effect and thermoelectric properties of Sn substitution on the established hi
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