Keita Goto

Enhancement of thermoelectric figure of merit of AgTlTe by tuning the carrier concentration

The authors tried to enhance the thermoelectric figure of merit of AgTlTe with extremely low thermal conductivity by controlling the carrier concentration. Pd- or Cu-doped AgTlTe was prepared and characterized. Pd did not dissolve in AgTlTe, whereas Cu fully dissolved up to x=0.4 in Ag1−xCuxTlTe. The effect of Cu doping was investigated by measuring the thermoelectric properties of polycrystalline Ag1−xCuxTlTe (x=0−0.4) from room temperature to slightly below the melting temperature. Cu doping resulted in a net increase in the hole concentration of AgTlTe. Cu doping improved the power factor of AgTlTe while keeping the thermal conductivity extremely low, leading to an enhancement of the thermoelectric figure of merit.

Extremely Low Thermal Conductivity Substances as Novel Thermoelectric Materials

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 Bi 2 Te 3 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.

Thermoelectric properties of Ag8Tl2Te5 and AgTl3Te2

Thermoelectric properties of ternary silver thallium tellurides: Ag8T12Te5 and AgT13Te2 were studied above room temperature to slightly below the melting temperature. As for Ag8T12Te5, the effects of Te content on the thermoelectric properties were also studied by characterizing the samples of Ag8T12Tex (x = 5.0, 5.1, 5.2). Both compounds show positive Seebeck coefficient (S). Ag8T12Te5 indicates semiconductor-like behavior, while AgT13Te2 shows metallic one. Both the electrical resistivity ( ) and S of Ag8T12Tex decrease with increasing x, which indicates that the carrier concentration increases with x. Although the amount of Te of Ag8T12Tex strongly influences the electrical resistivity and Seebeck coefficient, the thermal conductivity ( ) values are at a similar level. The of Ag8T12Te5 and AgT13Te2 are very low, especially those of Ag8T12Tex are extremely low, approximately 0.35 Wm-lK-1 around room temperature. The maximum values of the dimensionless figure of merit ZT (=S2T/ ) of Ag8T12Te5.2 and AgT13Te2 were 0.54 at 567 K and 0.06 at 515 K, respectively.

Enhancement of Thermoelectric Figure of Merit in Ternary Silver Thallium Tellurides by Controlling the Carrier Concentration

We tried to improve the thermoelectric figure of merit of a ternary silver thallium telluride, Ag₉TlTe₅, by controlling the carrier concentration. We studied the effect of deviation from the stoichiometric composition by measuring the thermoelectric properties from room temperature to slightly below the melting temperature of polycrystalline Ag₉TlTe₅ with different nominal compositions. A minimal compositional deviation of Ag₉TlTe₅ improved the power factor while the thermal conductivity remained extremely low, leading to an enhancement of the thermoelectric figure of merit

Thermoelectric properties of Pd alloyed AgTlTe

We have tried to enhance the thermoelectric figure of merit of polycrystalline AgTlTe by alloying Pd. In the present study, we prepared polycrystalline samples of Pd-0 at.%, Pd-5 at.%, Pd-10 at.% alloyed AgTlTe and measured the thermoelectric properties. Polycrystalline samples were prepared from appropriate amounts of Ag2Te, Tl 2Te and PdTe1.3. The electrical resistivity (rho), Seebeck coefficient (S), and thermal conductivity (kappa) were evaluated from room temperature to slightly below the melting temperature, and the effect of Pd alloying on the properties of AgTlTe was studied. Pd-5 at.% and Pd-10 at.% alloyed AgTlTe were made of two phases, AgTlTe containing small amount of Pd and PdTe2. Pd alloying improved the power factor but increased the thermal conductivity, leading to a slightly increasing of the thermoelectric figure of merit

Thermoelectric properties of ternary copper thallium telluride: CuTl/sub 9/Te/sub 5/

We prepared high-density polycrystalline samples of Cu/sub 2/Te, Tl/sub 2/Te, and CuTl/sub 9/Te/sub 5/, and measured the thermoelectric properties. The thermal conductivity of CuTl/sub 9/Te/sub 5/ is very low, 0.49 Wm/sup -1/K/sup -1/ at room temperature. CuTl/sub 9/Te/sub 5/ indicates the relatively high figure of merit ZT because of its very low thermal conductivity. The highest ZT value is 0.38 obtained at 590 K for CuTl/sub 9/Te/sub 5/.

Ag/sub 9/TlTe/sub 5/ and AgTlTe: high ZT materials with extremely low thermal conductivity

We have studied the thermoelectric properties Ag/sub 9/TlTe/sub 5/ and AgTlTe. The thallium compounds have extremely low thermal conductivities around 0.25 Wm/sup -1/K/sup -1/ from room temperature to about 700 K. The thallium compounds indicate very high ZT values; especially the highest ZT value of Ag/sub 9/TlTe/sub 5/ is 1.23 obtained at 700 K. Ag/sub 9/TlTe/sub 5/ is a unique material combining extremely low thermal conductivity and relatively low electrical resistivity.

Thermoelectric Properties of Ag-Tl-Te Ternary System

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 Ag 9 TlTe 5 exhibits an excellent thermoelectric figure of merit (ZT= 1.23) because of its extremely low thermal conductivity (around 0.22 Wm −1 K −1 ). In this paper, we studied the thermal conductivity of four kinds of ternary silver thallium tellurides: AgTl 3 Te 2 , AgTlTe, Ag 8 Tl 2 Te 5 and Ag 9 TlTe 5 , for which we found room temperature values of 0.39, 0.26, 0.14 and 0.21 Wm −1 K −1 , respectively. In order to understand the extremely low thermal conductivity, we performed an ultrasonic pulse echo measurement and evaluated some thermophysical properties.

Thermoelectric Properties of Tl 2 Te-Sb 2 Te 3 Pseudo-Binary System

Polycrystalline-sintered samples of thallium based substances, (Tl 2 Te) 100−x (Sb 2 Te 3 ) x (x= 0, 1, 5, 10), were prepared by melting Tl 2 Te and Sb 2 Te 3 ingots followed by annealing in sealed quartz ampoules. The thermoelectric properties were measured from room temperature to around 600 K. The values of the Seebeck coefficient of all samples are positive, indicating a p-type conduction characteristic. The maximum value of the power factor is 6.53×10 −4 Wm −1 K −2 at 591 K obtained for x= 10 (Tl 9 SbTe 6 ), which is about one order lower than those of state-of-the-art thermoelectric materials. All samples indicate an extremely low thermal conductivity, for example that of Tl 2 Te is approximately 0.35 Wm −1 K −1 from room temperature to around 600 K. Although the electrical performance of the samples is not so good, the ZT value is relatively high due to the extremely low thermal conductivity. The maximum ZT value is 0.42 at 591 K obtained for Tl 9 SbTe 6 .