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Thermoelectric properties of Tl9BiTe6
Journal of Alloys and Compounds 352 (2003) 275–278
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Thermoelectric properties of Tl 9 BiTe 6 Shinsuke Yamanaka, Atsuko Kosuga, Ken Kurosaki* Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2 -1, Suita, Osaka 565 -0871, Japan Received 9 September 2002; accepted 28 September 2002
Abstract Polycrystalline-sintered samples of Tl 9 BiTe 6 have been prepared by hot pressing, and the thermoelectric properties have been measured in the temperature range from room temperature to about 700 K. The electrical resistivity is about 10 times higher than those of state-of-the-art thermoelectric materials, and shows a positive temperature dependence. The Seebeck coefficient is positive in the whole temperature range, showing p-type semiconductor characteristics. The maximum value of the power factor of the sample is about 6.2310 24 W m 21 K 22 at 590 K. The thermal conductivity is extremely low comparable to those of state-of-the-art thermoelectric materials. The maximum value of the thermoelectric figure of merit ZT is 0.86 at about 590 K. 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconductors; Electrical transport; Heat conduction; Thermoelectric
1. Introduction The effectiveness of a material for thermoelectric applications is determined by the dimensionless figure of merit ZT (see, for example, Ref. [1]), where T is the absolute temperature and Z 5 (S 2 s ) /k (S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity). The large Seebeck coefficient, high electrical conductivity, and low thermal conductivity are required for high performance thermoelectric materials. The electrical properties are determined by the power factor, defined here as S 2 s or S 2 /r, where r is the electrical resistivity. The power factor can be optimized as a function of the carrier concentration; therefore, the thermal conductivity must be minimized to maximize ZT. The ZT value of the materials used in the current devices is about 1. The compounds Tl 9 BiTe 6 and TlBiTe 2 have been the subjects of several studies because of their interesting features, for example TlBiTe 2 is a non-metallic superconductor [2,3] and Tl 9 BiTe 6 presents high thermoelectric figure of merit [4–7]. In our previous study [8,9], the thermophysical and electrical properties of Tl–Bi–Te compounds have been studied, and it was found that the compounds have a *Corresponding author. Tel.: 181-6-6879-7905; fax: 181-6-68797889. E-mail address: [email protected] (K. Kurosaki).
possibility to show excellent thermoelectric features. In the present study, the polycrystalline-sintered samples of Tl 9 BiTe 6 are prepared by hot-pressing, and the thermoelectric properties, viz. the electrical resistivity, Seebeck coefficient, and thermal conductivity are measured. The potential for thermoelectric applications of Tl 9 BiTe 6 is discussed.
2. Experimental Polycrystalline-sintered samples of Tl 9 BiTe 6 were prepared from appropriate amounts of thallium, bismuth, and tellurium powders. The mixed powders were pressed into pellets followed by melting in sealed quartz ampoules at 823 K for a few hours. The intermediates were hot-pressed at a pressure of 80 MPa at a temperature of 723 K for 3 h under nitrogen atmosphere. The crystal structure of the samples was analyzed by a powder X-ray diffraction method at room temperature using Cu Ka radiation. For measurements of the thermoelectric properties, appropriate shapes of the samples were cut from the hot-pressed pellets. The density of the samples was calculated from the measured weight and dimensions. In the temperature range from room temperature to about 700 K, the thermoelectric properties were measured. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in helium atmosphere. The Seebeck
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01114-3
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S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
coefficient was measured in a temperature gradient of about 10 K. The thermal conductivity was evaluated from the following relationship:
k 5 DCP d where k is the thermal conductivity, D is the thermal diffusivity, CP is the heat capacity, and d is the measured density. The thermal diffusivity was measured by a laser flash method using ULVAC TC-7000 in vacuum. The heat capacity of Tl 9 BiTe 6 was evaluated from the Neumann– Kopp law using literature data [10,11] of Bi 2 Te 3 and TeTl 2 .
3. Results and discussions The powder X-ray diffraction pattern at room temperature of the sample is shown in Fig. 1, together with literature data [4]. It is found that the tetragonal single phase Tl 9 BiTe 6 is obtained in the present study. The lattice parameters evaluated from the X-ray diffraction pattern are shown in Table 1. The tetragonal lattice parameters well agree with literature data [4]. The bulk density of the sample is about 98% of the theoretical density. The sample characteristics are summarized in Table 1. We have also evaluated the melting point and thermal expansion coefficient of Tl 9 BiTe 6 [8]. The results are also summarized in Table 1. The temperature dependence of the electrical resistivity
Fig. 1. Powder X-ray diffraction pattern of polycrystalline Tl 9 BiTe 6 .
Table 1 Sample characteristics and thermophysical properties of hot-pressed Tl 9 BiTe 6 Tetragonal lattice parameters at room temperature X-ray density Sample bulk density
a c
(nm) (nm)
d d
Melting point [8] Volumetric thermal expansion coefficient [8]
Tm aV
(g cm 23 ) (g cm 23 ) (%T.D.) (K) (K 21 )
0.886 1.305 9.15 8.93 97.6 813 8.83310 25
r of hot-pressed Tl 9 BiTe 6 is shown in Fig. 2, together with the data of other substances [1,9,12,13]. The electrical resistivity of hot-pressed Tl 9 BiTe 6 is about 10 times higher than those of Bi 2 Te 3 and TAGS-85, (AgSbTe 2 ) 12x (GeTe) x , which are state-of-the-art thermoelectric materials in the low and high temperature region, respectively. The electrical resistivity of Tl 9 BiTe 6 has a positive temperature dependence. The temperature dependence of the Seebeck coefficient S of hot-pressed Tl 9 BiTe 6 is shown in Fig. 3, together with the data of other substances [1,9,12,13]. The Seebeck coefficient of Tl 9 BiTe 6 is positive in the whole temperature range, showing that the majority of charge carriers are holes. The maximum absolute value of the Seebeck coefficient is about 235 mV K 21 at 590 K, which is at the same level as those of state-of-the-art thermoelectric materials [1,13]. Fig. 4 shows the relationship between the electrical conductivity s and Seebeck coefficient S. This figure gives an indication of the power factor S 2 s, that defines the electrical performance of the thermoelectric materials. It is known that the power factor is required to have an order of magnitude (W m 21 K 22 ) of about 10 23 for the materials used in the current devices. The maximum value of the power factor of hot-pressed Tl 9 BiTe 6 is about 6.2310 24 W m 21 K 22 at 590 K.
Fig. 2. Temperature dependence of electrical resistivity of hot-pressed Tl 9 BiTe 6 .
S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
Fig. 3. Temperature dependence of Seebeck coefficient of hot-pressed Tl 9 BiTe 6 .
Fig. 4. Relationship between electrical conductivity s and Seebeck coefficient S.
The temperature dependence of the thermal conductivity k of hot-pressed Tl 9 BiTe 6 is shown in Fig. 5, together with the data of other substances [1,9,13,14]. Until about
277
Fig. 6. Dimensionless figure of merit of hot-pressed Tl 9 BiTe 6 compared to those of state-of-the-art p-type thermoelectric materials [1,7,15].
600 K, the thermal conductivity of Tl 9 BiTe 6 is independent of temperature, while above this temperature it gradually increases with temperature. At room temperature, the thermal conductivity of Tl 9 BiTe 6 is 0.39 W m 21 K 21 . The electronic contribution to the thermal conductivity estimated from the Wiedemann–Franz law is about 0.15 W m 21 K 21 at room temperature, which suggests that the lattice contribution is only around 0.25 W m 21 K 21 . This extremely low thermal conductivity is of great advantage for a high-performance thermoelectric material. The dimensionless figure of merit, ZT of hot-pressed Tl 9 BiTe 6 is evaluated by using the data of the electrical resistivity, Seebeck coefficient, and thermal conductivity, as shown in Fig. 6. The ZT of purified polycrystalline Tl 9 BiTe 6 by a zone refining method is also shown in this figure [7]. The ZT of hot-pressed Tl 9 BiTe 6 is comparable with those of state-of-the-art p-type thermoelectric materials [1,5], while it is lower than that of zone refined Tl 9 BiTe 6 [7]. The maximum value of ZT of hot-pressed Tl 9 BiTe 6 is 0.86 at about 590 K. This high ZT is due to the relatively high Seebeck coefficient and extremely low thermal conductivity. It was confirmed that Tl 9 BiTe 6 has a potential for thermoelectric applications in the temperature range from 400 to 600 K.
4. Conclusion
Fig. 5. Temperature dependence of thermal conductivity of hot-pressed Tl 9 BiTe 6 .
The thermoelectric properties of hot-pressed polycrystalline Tl 9 BiTe 6 were measured in the temperature range from room temperature to about 700 K. The electrical resistivity is about 10 times higher than those of state-ofthe-art thermoelectric materials. The Seebeck coefficient is positive in the whole temperature range, showing that the majority of charge carriers are holes. The maximum absolute value of the Seebeck coefficient is about 235 mV K 21 at 590 K. Tl 9 BiTe 6 has an extremely low thermal
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S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
conductivity (0.39 W m 21 K 21 at room temperature). The ZT is comparable with those of state-of-the-art p-type thermoelectric materials, and the maximum value is 0.86 at about 590 K.
References [1] D.M. Rowe, in: CRC Handbook of Thermoelectrics, CRC Press, New York, 1995. [2] R.A. Hein, E.M. Swiggard, Phys. Rev. Lett. 24 (1970) 53–55. [3] J.D. Jensen, J.R. Burke, D.W. Ernst, R.S. Allgaier, Phys. Rev. B 6 (1972) 319–327. [4] A. Pradel, J.-C. Tedenac, D. Coquillat, G. Brun, Rev. Chim. Miner. 19 (1982) 43–48. [5] B. Wolfing, C. Kloc, A. Ramirez, E. Bucher, in: Proc. 18th International Conference on Thermodynamics, 1999, pp. 546–549. [6] B. Wolfing, C. Kloc, E. Bucher, Mater. Res. Soc. Symp. 626 (2000) Z341–346.
[7] B. Wolfing, C. Kloc, J. Teubner, E. Bucher, Phys. Rev. Lett. 86 (2001) 4350–4353. [8] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Comp. 351 (2003) 14–17. [9] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Comp. 351 (2003) 279–282. [10] Japan Thermal Measurement Society, Thermodynamics database for personal computer MALT2, (1992). [11] The SGTE Pure Substance and Solution Databases, GTT-DATA SERVICES, (1996). [12] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Comp. 334 (2002) 317–323. [13] R.S. Caputo, V.C. Truscello, Proc. IECEC (1974) 637. [14] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Nucl. Mater. 294 (2001) 179–182. [15] B.C. Sales, D. Mandrus, B.C. Chakoumakos, V. Keppens, J.R. Thompson, Phys. Rev. B56 (1997) 15081–15089.
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www.elsevier.com / locate / jallcom
Thermoelectric properties of Tl 9 BiTe 6 Shinsuke Yamanaka, Atsuko Kosuga, Ken Kurosaki* Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2 -1, Suita, Osaka 565 -0871, Japan Received 9 September 2002; accepted 28 September 2002
Abstract Polycrystalline-sintered samples of Tl 9 BiTe 6 have been prepared by hot pressing, and the thermoelectric properties have been measured in the temperature range from room temperature to about 700 K. The electrical resistivity is about 10 times higher than those of state-of-the-art thermoelectric materials, and shows a positive temperature dependence. The Seebeck coefficient is positive in the whole temperature range, showing p-type semiconductor characteristics. The maximum value of the power factor of the sample is about 6.2310 24 W m 21 K 22 at 590 K. The thermal conductivity is extremely low comparable to those of state-of-the-art thermoelectric materials. The maximum value of the thermoelectric figure of merit ZT is 0.86 at about 590 K. 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconductors; Electrical transport; Heat conduction; Thermoelectric
1. Introduction The effectiveness of a material for thermoelectric applications is determined by the dimensionless figure of merit ZT (see, for example, Ref. [1]), where T is the absolute temperature and Z 5 (S 2 s ) /k (S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity). The large Seebeck coefficient, high electrical conductivity, and low thermal conductivity are required for high performance thermoelectric materials. The electrical properties are determined by the power factor, defined here as S 2 s or S 2 /r, where r is the electrical resistivity. The power factor can be optimized as a function of the carrier concentration; therefore, the thermal conductivity must be minimized to maximize ZT. The ZT value of the materials used in the current devices is about 1. The compounds Tl 9 BiTe 6 and TlBiTe 2 have been the subjects of several studies because of their interesting features, for example TlBiTe 2 is a non-metallic superconductor [2,3] and Tl 9 BiTe 6 presents high thermoelectric figure of merit [4–7]. In our previous study [8,9], the thermophysical and electrical properties of Tl–Bi–Te compounds have been studied, and it was found that the compounds have a *Corresponding author. Tel.: 181-6-6879-7905; fax: 181-6-68797889. E-mail address: [email protected] (K. Kurosaki).
possibility to show excellent thermoelectric features. In the present study, the polycrystalline-sintered samples of Tl 9 BiTe 6 are prepared by hot-pressing, and the thermoelectric properties, viz. the electrical resistivity, Seebeck coefficient, and thermal conductivity are measured. The potential for thermoelectric applications of Tl 9 BiTe 6 is discussed.
2. Experimental Polycrystalline-sintered samples of Tl 9 BiTe 6 were prepared from appropriate amounts of thallium, bismuth, and tellurium powders. The mixed powders were pressed into pellets followed by melting in sealed quartz ampoules at 823 K for a few hours. The intermediates were hot-pressed at a pressure of 80 MPa at a temperature of 723 K for 3 h under nitrogen atmosphere. The crystal structure of the samples was analyzed by a powder X-ray diffraction method at room temperature using Cu Ka radiation. For measurements of the thermoelectric properties, appropriate shapes of the samples were cut from the hot-pressed pellets. The density of the samples was calculated from the measured weight and dimensions. In the temperature range from room temperature to about 700 K, the thermoelectric properties were measured. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in helium atmosphere. The Seebeck
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01114-3
276
S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
coefficient was measured in a temperature gradient of about 10 K. The thermal conductivity was evaluated from the following relationship:
k 5 DCP d where k is the thermal conductivity, D is the thermal diffusivity, CP is the heat capacity, and d is the measured density. The thermal diffusivity was measured by a laser flash method using ULVAC TC-7000 in vacuum. The heat capacity of Tl 9 BiTe 6 was evaluated from the Neumann– Kopp law using literature data [10,11] of Bi 2 Te 3 and TeTl 2 .
3. Results and discussions The powder X-ray diffraction pattern at room temperature of the sample is shown in Fig. 1, together with literature data [4]. It is found that the tetragonal single phase Tl 9 BiTe 6 is obtained in the present study. The lattice parameters evaluated from the X-ray diffraction pattern are shown in Table 1. The tetragonal lattice parameters well agree with literature data [4]. The bulk density of the sample is about 98% of the theoretical density. The sample characteristics are summarized in Table 1. We have also evaluated the melting point and thermal expansion coefficient of Tl 9 BiTe 6 [8]. The results are also summarized in Table 1. The temperature dependence of the electrical resistivity
Fig. 1. Powder X-ray diffraction pattern of polycrystalline Tl 9 BiTe 6 .
Table 1 Sample characteristics and thermophysical properties of hot-pressed Tl 9 BiTe 6 Tetragonal lattice parameters at room temperature X-ray density Sample bulk density
a c
(nm) (nm)
d d
Melting point [8] Volumetric thermal expansion coefficient [8]
Tm aV
(g cm 23 ) (g cm 23 ) (%T.D.) (K) (K 21 )
0.886 1.305 9.15 8.93 97.6 813 8.83310 25
r of hot-pressed Tl 9 BiTe 6 is shown in Fig. 2, together with the data of other substances [1,9,12,13]. The electrical resistivity of hot-pressed Tl 9 BiTe 6 is about 10 times higher than those of Bi 2 Te 3 and TAGS-85, (AgSbTe 2 ) 12x (GeTe) x , which are state-of-the-art thermoelectric materials in the low and high temperature region, respectively. The electrical resistivity of Tl 9 BiTe 6 has a positive temperature dependence. The temperature dependence of the Seebeck coefficient S of hot-pressed Tl 9 BiTe 6 is shown in Fig. 3, together with the data of other substances [1,9,12,13]. The Seebeck coefficient of Tl 9 BiTe 6 is positive in the whole temperature range, showing that the majority of charge carriers are holes. The maximum absolute value of the Seebeck coefficient is about 235 mV K 21 at 590 K, which is at the same level as those of state-of-the-art thermoelectric materials [1,13]. Fig. 4 shows the relationship between the electrical conductivity s and Seebeck coefficient S. This figure gives an indication of the power factor S 2 s, that defines the electrical performance of the thermoelectric materials. It is known that the power factor is required to have an order of magnitude (W m 21 K 22 ) of about 10 23 for the materials used in the current devices. The maximum value of the power factor of hot-pressed Tl 9 BiTe 6 is about 6.2310 24 W m 21 K 22 at 590 K.
Fig. 2. Temperature dependence of electrical resistivity of hot-pressed Tl 9 BiTe 6 .
S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
Fig. 3. Temperature dependence of Seebeck coefficient of hot-pressed Tl 9 BiTe 6 .
Fig. 4. Relationship between electrical conductivity s and Seebeck coefficient S.
The temperature dependence of the thermal conductivity k of hot-pressed Tl 9 BiTe 6 is shown in Fig. 5, together with the data of other substances [1,9,13,14]. Until about
277
Fig. 6. Dimensionless figure of merit of hot-pressed Tl 9 BiTe 6 compared to those of state-of-the-art p-type thermoelectric materials [1,7,15].
600 K, the thermal conductivity of Tl 9 BiTe 6 is independent of temperature, while above this temperature it gradually increases with temperature. At room temperature, the thermal conductivity of Tl 9 BiTe 6 is 0.39 W m 21 K 21 . The electronic contribution to the thermal conductivity estimated from the Wiedemann–Franz law is about 0.15 W m 21 K 21 at room temperature, which suggests that the lattice contribution is only around 0.25 W m 21 K 21 . This extremely low thermal conductivity is of great advantage for a high-performance thermoelectric material. The dimensionless figure of merit, ZT of hot-pressed Tl 9 BiTe 6 is evaluated by using the data of the electrical resistivity, Seebeck coefficient, and thermal conductivity, as shown in Fig. 6. The ZT of purified polycrystalline Tl 9 BiTe 6 by a zone refining method is also shown in this figure [7]. The ZT of hot-pressed Tl 9 BiTe 6 is comparable with those of state-of-the-art p-type thermoelectric materials [1,5], while it is lower than that of zone refined Tl 9 BiTe 6 [7]. The maximum value of ZT of hot-pressed Tl 9 BiTe 6 is 0.86 at about 590 K. This high ZT is due to the relatively high Seebeck coefficient and extremely low thermal conductivity. It was confirmed that Tl 9 BiTe 6 has a potential for thermoelectric applications in the temperature range from 400 to 600 K.
4. Conclusion
Fig. 5. Temperature dependence of thermal conductivity of hot-pressed Tl 9 BiTe 6 .
The thermoelectric properties of hot-pressed polycrystalline Tl 9 BiTe 6 were measured in the temperature range from room temperature to about 700 K. The electrical resistivity is about 10 times higher than those of state-ofthe-art thermoelectric materials. The Seebeck coefficient is positive in the whole temperature range, showing that the majority of charge carriers are holes. The maximum absolute value of the Seebeck coefficient is about 235 mV K 21 at 590 K. Tl 9 BiTe 6 has an extremely low thermal
278
S. Yamanaka et al. / Journal of Alloys and Compounds 352 (2003) 275–278
conductivity (0.39 W m 21 K 21 at room temperature). The ZT is comparable with those of state-of-the-art p-type thermoelectric materials, and the maximum value is 0.86 at about 590 K.
References [1] D.M. Rowe, in: CRC Handbook of Thermoelectrics, CRC Press, New York, 1995. [2] R.A. Hein, E.M. Swiggard, Phys. Rev. Lett. 24 (1970) 53–55. [3] J.D. Jensen, J.R. Burke, D.W. Ernst, R.S. Allgaier, Phys. Rev. B 6 (1972) 319–327. [4] A. Pradel, J.-C. Tedenac, D. Coquillat, G. Brun, Rev. Chim. Miner. 19 (1982) 43–48. [5] B. Wolfing, C. Kloc, A. Ramirez, E. Bucher, in: Proc. 18th International Conference on Thermodynamics, 1999, pp. 546–549. [6] B. Wolfing, C. Kloc, E. Bucher, Mater. Res. Soc. Symp. 626 (2000) Z341–346.
[7] B. Wolfing, C. Kloc, J. Teubner, E. Bucher, Phys. Rev. Lett. 86 (2001) 4350–4353. [8] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Comp. 351 (2003) 14–17. [9] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Comp. 351 (2003) 279–282. [10] Japan Thermal Measurement Society, Thermodynamics database for personal computer MALT2, (1992). [11] The SGTE Pure Substance and Solution Databases, GTT-DATA SERVICES, (1996). [12] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Comp. 334 (2002) 317–323. [13] R.S. Caputo, V.C. Truscello, Proc. IECEC (1974) 637. [14] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Nucl. Mater. 294 (2001) 179–182. [15] B.C. Sales, D. Mandrus, B.C. Chakoumakos, V. Keppens, J.R. Thompson, Phys. Rev. B56 (1997) 15081–15089.