Thermoelectric Properties of Ag-Tl-Te Ternary System
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Thermoelectric Properties of Ag-Tl-Te Ternary System Ken Kurosaki, Atsuko Kosuga, Keita Goto, Hiroaki Muta, and Shinsuke Yamanaka Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan ABSTRACT We have studied the thermoelectric properties of thallium compounds as novel thermoelectric materials. Especially, we focus on the Ag-Tl-Te ternary system, in which we found that Ag9TlTe5 exhibits an excellent thermoelectric figure of merit (ZT= 1.23) because of its extremely low thermal conductivity (around 0.22 Wm-1K-1). In this paper, we studied the thermal conductivity of four kinds of ternary silver thallium tellurides: AgTl3Te2, AgTlTe, Ag8Tl2Te5 and Ag9TlTe5, for which we found room temperature values of 0.39, 0.26, 0.14 and 0.21 Wm-1K-1, respectively. In order to understand the extremely low thermal conductivity, we performed an ultrasonic pulse echo measurement and evaluated some thermophysical properties.
INTRODUCTION Thallium compounds have attracted considerable attention as new thermoelectric materials because of their very low thermal conductivity [1-3]. Thallium’s atomic mass is very heavy, that is about 204.4. High atomic masses reduce the atomic vibration frequencies. In addition, a lot of thallium compounds indicate a large crystal cell with a complicated crystal structure, and as a result the number of atoms in the cell (N) becomes large. Since the fraction of the acoustic mode is one per N, the large N lowers the fraction of the acoustic mode that carries heat efficiency. These characteristics of the thallium compounds are considered to be the reasons of the low thermal conductivity. To develop high-performance bulk thermoelectric materials, we are currently exploring ternary silver thallium tellurides in the hope of discovering a material which can provide advanced thermoelectric performance. We recently studied a silver thallium telluride whose chemical formula is Ag9TlTe5, which indicates a very high ZT value (1.23 at 700 K) caused by its extremely low thermal conductivity (0.22 Wm-1K-1 at 700 K) [4]; this low thermal conductivity value is approximately one-fifth of the value for current state-of-the-art thermoelectric materials such as Bi2Te3. Some researchers have reported the phase equilibria for the Tl2Te-Ag2Te pseudo binary
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system [5-7]. A comparison of the data reported in the literature confirms that the Tl2Te-Ag2Te pseudo binary system contains four ternary compounds: AgTl3Te2, AgTlTe, Ag8Tl2Te5 and Ag9TlTe5. The typical Tl2Te-Ag2Te pseudo binary phase diagram is shown in Figure 1 [7]. In this study, we examine the thermal conductivity of AgTl3Te2, AgTlTe, Ag8Tl2Te5 and Ag9TlTe5. In addition, we performed the ultrasonic pulse echo measurement and evaluated some thermophysical properties such as average sound velocity and Debye temperature.
EXPERIMENT The ternary silver thallium tellurides were prepared by a standard solid-state reaction. Firstly, the appropriate
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