Exploring Complex Chalcogenides for Thermoelectric Applications
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EXPLORING COMPLEX CHALCOGENIDES FOR THERMOELECTRIC APPLICATIONS
Ying C. Wang and Francis J. DiSalvo Department of Chemistry and Chemical Biology, Baker Lab Cornell University, Ithaca, NY 14853, USA
ABSTRACT
Our research on ternary / quaternary chalcogenides for thermoelectric applications has lead to the identification of new interesting compounds and better understanding of the chemistry and physical properties of complex chalcogenides. The chemical, geometric, electronic diversity and flexibility has been well demonstrated in BaBiSe3 and Sr4Bi6Se13 type compounds. This presents both a challenge and more opportunity in controlling and optimizing the thermoelectric properties of these complex chalcogenides, compared with elemental and binary compounds. The importance of multivalley band structure in thermoelectric materials is emphasized. Only compounds with high crystal symmetry have the possibility of having a large number of degenerate valleys in the conduction bands or peaks in the valence bands, respectively. However, most of the complex chalcogenides crystallize in low crystal symmetry. An Edisonian method of exploratory synthesis and characterization may be the working approach to find good thermoelectric materials with ZT higher than 4.
INTRODUCTION
Thermoelectrics (TE) is a classical field discovered and investigated since the early nineteenth century. Thermoelectric phenomena are used in power generation and Peltier Cooling. TE devices have certain advantages over the conventional thermal-mechanical conversion power generator and compressor-based refrigeration devices. TE devices are light, compact, non-emissive, silent, simple, and very reliable. However, present TE devices have low efficiency typically 10% or less of Carnot efficiency [1-4]. Thermoelectric power generators convert heat or a temperature gradient into electricity. TE generators have found applications mainly in which the device reliability is paramount or the environment is very hostile. TE generators are well suited as the electrical power source in deep space missions where the surrounding can be extremely harsh with drastic temperature variation, asteroid debris, and high radiation fields. The Voyager 1 and 2 spacecraft launched in 1977 were each equipped with three multi-hundred Watt radioisotope thermoelectric generators (RTG). The RTGs have been operating without a single failure since [5-6]. TE cooling or Peltier cooling devices function like a heat pump, converting electricity to a temperature gradient. For decades, it has been the holy grail for scientists and engineers to design thermoelectric cooling devices with efficiency comparable to the conventional compressor-based refrigeration devices. While TE devices operate at about 10% of Carnot
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