AgTlTe : High ZT Materials With Extremely Low Thermal Conductivity

We have studied the thermoelectric properties Ag9TlTe5 and AgTlTe. The thallium compounds have extremely low thermal conductivities around 0.25 WmK from room temperature to about 700 K. The thallium compounds indicate very high ZT values; especially the highest ZT value of Ag9TlTe5 is 1.23 obtained at 700 K. Ag9TlTe5 is a unique material combining extremely low thermal conductivity and relatively low electrical resistivity. Introduction Thallium compounds have attracted attention as new thermoelectric materials because they have very low thermal conductivities. For instance, the thermoelectric properties of some thallium compounds, such as Tl2SnTe5, Tl9BiTe6, and thallium-filled skutterudites (Tl0.22Co4Sb12) have been reported. The maximum ZT values of these thallium compounds are ZT = 0.6 at 300 K for Tl2SnTe5, [1] ZT = 1.2 at around 500 K for Tl9BiTe6, [2] and ZT = 0.8 at 800 K for Tl0.22Co4Sb12. [3] On the other hand, some groups have reported the phase equilibria for the Tl2Te-Ag2Te pseudo-binary system. [4-6] A comparison of the data reported in the literature confirms that the Tl2Te-Ag2Te pseudo-binary system contains four ternary compounds: Ag3TlTe2, AgTlTe, Ag8Tl2Te5, and Ag9TlTe5. In our previous study, we measured the thermal conductivity of polycrystalline AgTlTe and confirmed the extremely low values. [7] In this paper, we report on the thermoelectric properties of AgTlTe and Ag9TlTe5, which we selected for study as novel thermoelectric materials. Especially, Ag9TlTe5 seems to have a thermoelectric figure of merit comparable to those of state-of-the-art materials. Experimental For polycrystalline sample synthesis, the appropriate amounts of starting materials were sealed in silica tubes, melted at 1173 K, and annealed at 673 K for a few days. The starting materials were Ag2Te and Tl2Te (supplied by Furuuchi Chemical Co. Ltd.). The obtained samples were crushed and pressed into pellets, followed by sintering slightly below their melting temperatures. This preparation method is simple and easy enough that any scientist in the field of solid-state physics or chemistry could prepare the compounds. A photograph of a polycrystalline sample of Ag9TlTe5 is shown in Fig. 1. The polycrystalline samples were taken from the sintered specimens for the measurements of their physical properties. The density of the samples was calculated from the measured weight and dimension. The crystal structure was investigated by the powder X-ray diffraction method at room temperature using Cu Kα radiation. The chemical composition of the samples was determined by the EDX analysis. The melting temperature of the samples was determined by the TG-DTA measurement. The electrical resistivity (ρ) and Seebeck coefficient (S) were measured using a standard four-probe method with a temperature gradient of about 5 K in a helium atmosphere. The thermal conductivity (κ) was evaluated from the thermal diffusivity, heat capacity, and sample density. The thermal diffusivity was measured using a laser flash method in vacuum. The heat capacity data were obtained from the database. Figure 1: A photograph of a polycrystalline sample of Ag9TlTe5, together with a 100-yen coin. 20 30 40 50 60 70 Ag9TlTe5 [8] In te ns ity (a rb . u ni t) Diffraction angle, 2θ (degree) Ag9TlTe5 In te ns ity (a rb . u ni t) 20 30 40 50 60 70 AgTlTe [9] Diffraction angle, 2θ (degree) AgTlTe Figure 2: X-ray diffraction patterns of Ag9TlTe5 and AgTlTe, together with the literature data. [8,9] Results and discussion The X-ray diffraction pattern of our polycrystalline sample agrees well with the powder pattern calculated from the reported crystal structure. The X-ray diffraction patterns of Ag9TlTe5 and AgTlTe are shown in Fig. 2. Ag9TlTe5 exhibits a hexagonal crystal system with the space group R-3c. On the other hand, AgTlTe exhibits an orthorhombic crystal system with the space group Pnma. The hexagonal lattice parameters of Ag9TlTe5 are a= 1.1431 nm and c= 4.1945 nm. The unit cell contains 12 molecules. Ag9TlTe5 has a large crystal cell with a complex structure. The orthorhombic lattice parameters of AgTlTe are a= 0.8800 nm, b= 0.4880 nm, and c= 0.7770 nm. The unit cell contains 4 molecules. The lattice parameters of the compounds are almost the same as those reported in the literature. [8,9] The chemical compositions determined by the EDX analysis do not deviate from the stoichiometric compositions. The structures of Ag9TlTe5 and AgTlTe are shown in Fig. 3. Figure 3: The structure of (a) Ag9TlTe5 and (b) AgTlTe. The green, yellow, and blue balls represent tellurium, silver, and thallium, respectively. In Ag9TlTe5, silver atoms occupy randomly 70 % of the sites. The electrical properties of Ag9TlTe5 and AgTlTe are shown in Fig. 4. The electrical resistivities (ρ) of Ag9TlTe5 and AgTlTe decrease with temperature across the whole temperature range, indicating semiconducting characteristics. The values of the Seebeck coefficient (S) of Ag9TlTe5 and AgTlTe are positive, indicating that the majority of the charge carriers are holes. The Seebeck coefficient decreases with temperature, for Ag9TlTe5 it reaches a minimum at around 650 K, and for AgTlTe it reaches a minimum at around 550 K. Both the electrical resistivity and Seebeck coefficient of AgTlTe are higher than those of Ag9TlTe5, which is because that the carrier concentration of AgTlTe is lower than that of Ag9TlTe5. The electrical resistivity value of Ag9TlTe5 at 700 K is 2.63x10 Ωm, which is higher by about an order of magnitude or more than those of state-of-the-art thermoelectric materials. This high electrical resistivity indicates a contribution from both the low carrier concentration and polycrystalline sample’s microstructure such as cracking and/or grain boundary resistance. The Seebeck coefficient value of Ag9TlTe5 at 700 K is 319 μVK. A maximum power factor (S/ρ) of 3.87x10 WmK was calculated at 700 K. The power factor of Ag9TlTe5 is higher than that of AgTlTe in the whole temperature range. The values of the power factor of Ag9TlTe5 are lower than those of state-of-the-art thermoelectric materials. Although its electrical performance is not very high, Ag9TlTe5 exhibits a very high value of the dimensionless figure of merit because of its extremely low thermal conductivity.