Extremely Low Thermal Conductivity Substances as Novel Thermoelectric Materials
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Extremely Low Thermal Conductivity Substances as Novel Thermoelectric Materials Shinsuke Yamanaka, Ken Kurosaki, Atsuko Kosuga, Keita Goto, and Hiroaki Muta Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan ABSTRACT We have prepared polycrystalline bulk samples of various thallium compounds and measured their thermoelectric properties. The most remarkable point of the thermoelectric properties of the thallium compounds is the extremely low thermal conductivity. The state-of-the-art thermoelectric materials such as Bi2Te3 and TAGS materials indicate relatively low the thermal conductivity, around 1.5 W/m/K. However, the thermal conductivity of the thallium compounds is below 0.5 W/m/K; especially that of silver thallium tellurides is around 0.25 W/m/K at room temperature. This extremely low thermal conductivity leads a great advantage for an enhancement of the thermoelectric performance. In this paper, we report on the properties of some thallium compounds selected for study as novel thermoelectric materials. One of these compounds seems to have a thermoelectric figure of merit comparable to those of state-of-the-art materials.
INTRODUCTION Thallium compounds have been a focus of attention as new thermoelectric materials because they indicate very low thermal conductivity [1-3]. In order to develop high-performance polycrystalline bulk thermoelectric materials, we are currently exploring ternary thallium tellurides in the hope of discovering a material that can provide advanced thermoelectric performance. In this paper, we review the thermoelectric properties of various thallium compounds studied in our group [4-11].
EXPERIMENT We studied two kinds of thallium compounds. The first group is thallium telluride and metal telluride (Tl2Te-MTex) pseudo-binary system. We studied the following six pseudo-binary systems; Tl2Te-Ag2Te [4,5], Tl2Te-Cu2Te [6], Tl2Te-Sb2Te3 [7,8], Tl2Te-Bi2Te3 [9], Tl2Te-GeTe
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[10]. Especially, the Tl2Te-Ag2Te pseudo-binary system indicates a possibility to show an excellent thermoelectric performance. The next group is Tl5Te3 related materials. Figure 1 shows the structure of Tl5Te3. In the case of Tl9BiTe6 or Tl9SbTe6, blue balls indicate thallium, green balls indicate tellurium, and red balls indicate bismuth or antimony. We studied the thermoelectric properties of Tl-Bi-Te, Tl-Sb-Te, Tl-Sn-Te and Tl-Pb-Te systems; that is Tl9BiTe6 [11], Tl9SbTe6 [8], Tl4SnTe3 [10] and Tl4PbTe3 [10]. We prepared polycrystalline high-density bulk samples of the thallium compounds through a typical solid-state reaction. The preparation method is quite easy, simple and reproducible without any special equipments and techniques. To examine the sample characteristics, the X-ray diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) analyses were performed. Electrical resistivity (ρ) and Seebeck coefficient (S) were measured simultaneously by a st
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