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Electrical properties of α- and β-Ag2Te
Journal of Alloys and Compounds 387 (2005) 297–299
Electrical properties of ␣- and -Ag2 Te Masaki Fujikane∗ , Ken Kurosaki, Hiroaki Muta, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received 8 June 2004; accepted 17 June 2004
Abstract The electrical resistivity and Seebeck coefficient of Ag2 Te were measured from room temperature to about 900 K. The electrical resistivity changes significantly at around 420 K, due to the phase transition from ␣-Ag2 Te to -Ag2 Te. The values at room temperature for ␣ phase are 2.75 × 10−5 m. The Seebeck coefficient is negative in the whole temperature range, showing that the majority of charge carriers are electrons. The value at room temperature is −128 V K−1 . © 2004 Elsevier B.V. All rights reserved. Keywords: Ag2 Te; Thermoelectric; Electrical transport
1. Introduction High performance thermoelectric materials must have large Seebeck coefficient, high electrical conductivity, and low thermal conductivity to retain the heat at the junction and to reduce the heat transfer losses. The effectiveness of a material for thermoelectric applications is determined by the dimensionless figure of merit ZT [1], where T is the absolute temperature and Z = (S2 σ)/κ (S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity). The electrical properties are determined by the power factor, defined here as S2 σ or S2 /ρ, where ρ is the electrical resistivity. The power factor can be optimized as a function of the carrier concentration; therefore, the thermal conductivity must be reduced to obtain maximal ZT. The ZT value of the materials used in the current devices is about 1. In the authors’ group, thermoelectric properties of various materials, such as a molybdenum telluride with Chevrel phase [2,3], thallium telluride [4,5] and rare earth copper oxide [6,7] have been studied. Recently, Hsu et al. have reported that the ZT value of AgPb18 SbTe20 is 2.2 at 800 K [8]. Ag2 Te is used as an addition to control the carrier density of PbTe [9–11]. ∗
Corresponding author. E-mail address: [email protected] (M. Fujikane).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.054
The AgBiTe2 -Ag2 Te composite, Ag3 AuTe2 , and the GeTeAgBiTe2 composite have also been studied as thermoelectric materials [12–14]. Ag2 Te has three structural modifications; the monoclinic ␣-phase occurs below 418 K, the cubic phase exists from 418 to 1075 K, and the ␥-phase is stable from 1075 to 1233 K [15]. Fig. 1 shows the crystal structure of ␣- and -Ag2 Te. In the present study, the electrical properties are studied from room temperature to about 900 K.
2. Experimental Powder of Ag2 Te (99.99%, supplied by Furuchi Chemical Co. Ltd.) was pressed at 160 MPa to form to φ 10 mm pellets. The pellets were sealed in a quartz ampoule and gradually heated to 1073 K. After 48 h, the ampoule was cooled slowly to room temperature. The density of the samples was calculated from the measured weight and dimension. The crystal structure was analyzed by a powder Xray diffraction method at room temperature using Cu K␣ radiation. The chemical composition was determined by an EDX analysis. In the temperature range between room temperature and about 900 K, the electrical resistivity and the Seebeck coefficient were measured simultaneously using an ULVAC ZEM-1 apparatus in helium atmosphere. The See-
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M. Fujikane et al. / Journal of Alloys and Compounds 387 (2005) 297–299
Fig. 1. Crystal structure of Ag2 Te (Ag: white ball, Te: black ball); (a) ␣ phase, (b)  phase.
beck coefficient was measured in a temperature gradient of about 5 K.
3. Results and discussion The X-ray diffraction pattern of the sample is shown in Fig. 2, together with literature data [16]. It is confirmed that a single phase of ␣-Ag2 Te with space group P2/n is obtained in the present study. The lattice parameters of ␣-Ag2 Te evaluated from the X-ray diffraction pattern are shown in Table 1, which well agree with literature data [16]. The bulk density of the sample is about 85% of the theoretical density. The temperature dependence of the electrical resistivity ρ of Ag2 Te is shown in Fig. 3, together with the data of other
Fig. 2. X-ray diffraction patterns of ␣-Ag2 Te, together with literature data [16].
Table 1 Sample characteristics of ␣-Ag2 Te Monoclinic lattice parameters at room temperature a (nm) b (nm) c (nm) β (◦ ) Theoretical density (g/cm3 ) Sample bulk density (g/cm3 ) (%T.D.)
0.8165 0.8935 0.8061 112.810 8.21 6.99 85.1
substances [1,17–24]. The electrical resistivity changes significantly at around 420 K, which is caused by the phase transition from ␣-Ag2 Te to -Ag2 Te. The values of the electrical resistivity are of an order of magnitude of 10−4 (m), which is one order higher than those of state-of-theart thermoelectric materials such as (GeTe)1−x (AgSbTe2 )x “TAGS” and sintered Bi2 Te3 [1,18,19]. Comparing the measured values with that in the literature [17] shows some difference. The low sample density, 85% T.D., causes this difference. The temperature dependence of the Seebeck coefficient S of Ag2 Te is shown in Fig. 4. The values of the Seebeck coef-
Fig. 3. Temperature dependence of the electrical resistivity of Ag2 Te and other substances [1,17–24].
M. Fujikane et al. / Journal of Alloys and Compounds 387 (2005) 297–299
299
4. Conclusion
Fig. 4. Temperature dependence of the Seebeck coefficient of Ag2 Te and other substances [1,17–24].
ficient are negative in the whole temperature range, showing that the majority of charge carriers are electrons. The absolute value of the Seebeck coefficient of -Ag2 Te (418–1075 K) decreases with increasing temperature. The value of the Seebeck coefficient of Ag2 Te at room temperature, which is that of the ␣-phase, equals −128 V K−1 . Fig. 5 shows the power factor, given as S2 /ρ, that defines the electrical performance of thermoelectric materials. It is known that the power factor is required to have an order of magnitude (Wm−1 K−2 ) of about 10−3 for materials used in current devices. The values of the power factor of Ag2 Te reaches an order of magnitude of 10−4 . The maximum value of the power factor is 5.99 × 10−4 Wm−1 K−2 at around 340 K (for the ␣-phase) and 2.68 × 10−4 Wm−1 K−2 at around 660 K (for the -phase). These results suggest that it is better to use ␣-Ag2 Te rather than -Ag2 Te as a thermoelectric material. To understand the thermoelectric properties of Ag2 Te, it is necessary to measure the thermal conductivity. The results will be shown in the near future.
Fig. 5. Temperature dependence of the power factor of Ag2 Te and other substances [1,17–24].
A polycrystalline sintered sample of Ag2 Te was prepared and the electrical properties were measured from room temperature to 900 K. A dense sample with 85% of the theoretical density was obtained in the present study. Both the electrical resistivity and the Seebeck coefficient change significantly at around 420 K. The values of the electrical resistivity are of an order of magnitude of 10−4 (m). This value is one order higher than those of state-of-the-art thermoelectric materials. The Seebeck coefficient is negative, showing n-type characteristics. The values of the Seebeck coefficient are relatively high; −128 V K−1 at room temperature for ␣-Ag2 Te. The maximum value of the power factor is obtained as 5.99 × 10−4 Wm−1 K−2 at around 340 K. It is better to use ␣-Ag2 Te rather than -Ag2 Te for the thermoelectric material.
References [1] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, New York, 1995. [2] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317. [3] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2002) 208. [4] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2002) 279. [5] S. Yamanaka, A. Kosuga, K. Kurosaki, J. Alloys Compd. 352 (2002) 275. [6] S. Yamanaka, H. Kobayashi, K. Kurosaki, J. Alloys Compd. 349 (2002) 321. [7] K. Kurosaki, H. Kobayashi, S. Yamanaka, J. Alloys Compd. 350 (2002) 340. [8] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818. [9] D.M. Rowe, C.M. Bhandari, Modern Thermoelectrics, Holt, Rinehart & Winston Ltd., London, 1983. [10] D.R. Lovett, Semimetals & Narrow-Bandgap Semiconductors, Pion Ltd., London, 1977. [11] I. B. Cadoff, E. Miller, Thermoelectric Materials and Devices, Reinhald Publishing Co., 1960. [12] Y. Takigawa, T. Imoto, T. Sakakibara, K. Kurosawa, Mat. Res. Soc. Symp. Proc. 545 (1999) 105. [13] D.P. Young, C.L. Brown, P. Khalifah, R.J. Cava, J. Appl. Phys. 88 (2000) 5221. [14] I.A. Avramova, S.K. Plachkova, Phys. Stat. Sol. A 179 (2000) 171. [15] R. Paul, Z. Physik. Chem. B31 (1936) 157. [16] National Bureau of Standards Monograph, 19 (1982) 73. [17] N.V. Dong, P.N. Tung, Phys. Stat. Sol. 30 (1968) 557. [18] R.S. Caputo, V.C. Truscello, Proc. IECEC, 1974, p. 637. [19] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325–1328. [20] S. Yamanaka, A. Kosuga, K. Kurosaki, J. Alloys Compd. 352 (2003) 275. [21] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2003) 14. [22] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317. [23] K. Kurosaki, H. Uneda, H. Muta, S. Yamanaka, J. Alloys Compd. 376 (2004) 43. [24] B. Abeles, R.W. Cohen, J. Appl. Phys. 35 (1964) 247.
Electrical properties of ␣- and -Ag2 Te Masaki Fujikane∗ , Ken Kurosaki, Hiroaki Muta, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received 8 June 2004; accepted 17 June 2004
Abstract The electrical resistivity and Seebeck coefficient of Ag2 Te were measured from room temperature to about 900 K. The electrical resistivity changes significantly at around 420 K, due to the phase transition from ␣-Ag2 Te to -Ag2 Te. The values at room temperature for ␣ phase are 2.75 × 10−5 m. The Seebeck coefficient is negative in the whole temperature range, showing that the majority of charge carriers are electrons. The value at room temperature is −128 V K−1 . © 2004 Elsevier B.V. All rights reserved. Keywords: Ag2 Te; Thermoelectric; Electrical transport
1. Introduction High performance thermoelectric materials must have large Seebeck coefficient, high electrical conductivity, and low thermal conductivity to retain the heat at the junction and to reduce the heat transfer losses. The effectiveness of a material for thermoelectric applications is determined by the dimensionless figure of merit ZT [1], where T is the absolute temperature and Z = (S2 σ)/κ (S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity). The electrical properties are determined by the power factor, defined here as S2 σ or S2 /ρ, where ρ is the electrical resistivity. The power factor can be optimized as a function of the carrier concentration; therefore, the thermal conductivity must be reduced to obtain maximal ZT. The ZT value of the materials used in the current devices is about 1. In the authors’ group, thermoelectric properties of various materials, such as a molybdenum telluride with Chevrel phase [2,3], thallium telluride [4,5] and rare earth copper oxide [6,7] have been studied. Recently, Hsu et al. have reported that the ZT value of AgPb18 SbTe20 is 2.2 at 800 K [8]. Ag2 Te is used as an addition to control the carrier density of PbTe [9–11]. ∗
Corresponding author. E-mail address: [email protected] (M. Fujikane).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.054
The AgBiTe2 -Ag2 Te composite, Ag3 AuTe2 , and the GeTeAgBiTe2 composite have also been studied as thermoelectric materials [12–14]. Ag2 Te has three structural modifications; the monoclinic ␣-phase occurs below 418 K, the cubic phase exists from 418 to 1075 K, and the ␥-phase is stable from 1075 to 1233 K [15]. Fig. 1 shows the crystal structure of ␣- and -Ag2 Te. In the present study, the electrical properties are studied from room temperature to about 900 K.
2. Experimental Powder of Ag2 Te (99.99%, supplied by Furuchi Chemical Co. Ltd.) was pressed at 160 MPa to form to φ 10 mm pellets. The pellets were sealed in a quartz ampoule and gradually heated to 1073 K. After 48 h, the ampoule was cooled slowly to room temperature. The density of the samples was calculated from the measured weight and dimension. The crystal structure was analyzed by a powder Xray diffraction method at room temperature using Cu K␣ radiation. The chemical composition was determined by an EDX analysis. In the temperature range between room temperature and about 900 K, the electrical resistivity and the Seebeck coefficient were measured simultaneously using an ULVAC ZEM-1 apparatus in helium atmosphere. The See-
298
M. Fujikane et al. / Journal of Alloys and Compounds 387 (2005) 297–299
Fig. 1. Crystal structure of Ag2 Te (Ag: white ball, Te: black ball); (a) ␣ phase, (b)  phase.
beck coefficient was measured in a temperature gradient of about 5 K.
3. Results and discussion The X-ray diffraction pattern of the sample is shown in Fig. 2, together with literature data [16]. It is confirmed that a single phase of ␣-Ag2 Te with space group P2/n is obtained in the present study. The lattice parameters of ␣-Ag2 Te evaluated from the X-ray diffraction pattern are shown in Table 1, which well agree with literature data [16]. The bulk density of the sample is about 85% of the theoretical density. The temperature dependence of the electrical resistivity ρ of Ag2 Te is shown in Fig. 3, together with the data of other
Fig. 2. X-ray diffraction patterns of ␣-Ag2 Te, together with literature data [16].
Table 1 Sample characteristics of ␣-Ag2 Te Monoclinic lattice parameters at room temperature a (nm) b (nm) c (nm) β (◦ ) Theoretical density (g/cm3 ) Sample bulk density (g/cm3 ) (%T.D.)
0.8165 0.8935 0.8061 112.810 8.21 6.99 85.1
substances [1,17–24]. The electrical resistivity changes significantly at around 420 K, which is caused by the phase transition from ␣-Ag2 Te to -Ag2 Te. The values of the electrical resistivity are of an order of magnitude of 10−4 (m), which is one order higher than those of state-of-theart thermoelectric materials such as (GeTe)1−x (AgSbTe2 )x “TAGS” and sintered Bi2 Te3 [1,18,19]. Comparing the measured values with that in the literature [17] shows some difference. The low sample density, 85% T.D., causes this difference. The temperature dependence of the Seebeck coefficient S of Ag2 Te is shown in Fig. 4. The values of the Seebeck coef-
Fig. 3. Temperature dependence of the electrical resistivity of Ag2 Te and other substances [1,17–24].
M. Fujikane et al. / Journal of Alloys and Compounds 387 (2005) 297–299
299
4. Conclusion
Fig. 4. Temperature dependence of the Seebeck coefficient of Ag2 Te and other substances [1,17–24].
ficient are negative in the whole temperature range, showing that the majority of charge carriers are electrons. The absolute value of the Seebeck coefficient of -Ag2 Te (418–1075 K) decreases with increasing temperature. The value of the Seebeck coefficient of Ag2 Te at room temperature, which is that of the ␣-phase, equals −128 V K−1 . Fig. 5 shows the power factor, given as S2 /ρ, that defines the electrical performance of thermoelectric materials. It is known that the power factor is required to have an order of magnitude (Wm−1 K−2 ) of about 10−3 for materials used in current devices. The values of the power factor of Ag2 Te reaches an order of magnitude of 10−4 . The maximum value of the power factor is 5.99 × 10−4 Wm−1 K−2 at around 340 K (for the ␣-phase) and 2.68 × 10−4 Wm−1 K−2 at around 660 K (for the -phase). These results suggest that it is better to use ␣-Ag2 Te rather than -Ag2 Te as a thermoelectric material. To understand the thermoelectric properties of Ag2 Te, it is necessary to measure the thermal conductivity. The results will be shown in the near future.
Fig. 5. Temperature dependence of the power factor of Ag2 Te and other substances [1,17–24].
A polycrystalline sintered sample of Ag2 Te was prepared and the electrical properties were measured from room temperature to 900 K. A dense sample with 85% of the theoretical density was obtained in the present study. Both the electrical resistivity and the Seebeck coefficient change significantly at around 420 K. The values of the electrical resistivity are of an order of magnitude of 10−4 (m). This value is one order higher than those of state-of-the-art thermoelectric materials. The Seebeck coefficient is negative, showing n-type characteristics. The values of the Seebeck coefficient are relatively high; −128 V K−1 at room temperature for ␣-Ag2 Te. The maximum value of the power factor is obtained as 5.99 × 10−4 Wm−1 K−2 at around 340 K. It is better to use ␣-Ag2 Te rather than -Ag2 Te for the thermoelectric material.
References [1] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, New York, 1995. [2] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317. [3] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2002) 208. [4] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2002) 279. [5] S. Yamanaka, A. Kosuga, K. Kurosaki, J. Alloys Compd. 352 (2002) 275. [6] S. Yamanaka, H. Kobayashi, K. Kurosaki, J. Alloys Compd. 349 (2002) 321. [7] K. Kurosaki, H. Kobayashi, S. Yamanaka, J. Alloys Compd. 350 (2002) 340. [8] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818. [9] D.M. Rowe, C.M. Bhandari, Modern Thermoelectrics, Holt, Rinehart & Winston Ltd., London, 1983. [10] D.R. Lovett, Semimetals & Narrow-Bandgap Semiconductors, Pion Ltd., London, 1977. [11] I. B. Cadoff, E. Miller, Thermoelectric Materials and Devices, Reinhald Publishing Co., 1960. [12] Y. Takigawa, T. Imoto, T. Sakakibara, K. Kurosawa, Mat. Res. Soc. Symp. Proc. 545 (1999) 105. [13] D.P. Young, C.L. Brown, P. Khalifah, R.J. Cava, J. Appl. Phys. 88 (2000) 5221. [14] I.A. Avramova, S.K. Plachkova, Phys. Stat. Sol. A 179 (2000) 171. [15] R. Paul, Z. Physik. Chem. B31 (1936) 157. [16] National Bureau of Standards Monograph, 19 (1982) 73. [17] N.V. Dong, P.N. Tung, Phys. Stat. Sol. 30 (1968) 557. [18] R.S. Caputo, V.C. Truscello, Proc. IECEC, 1974, p. 637. [19] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325–1328. [20] S. Yamanaka, A. Kosuga, K. Kurosaki, J. Alloys Compd. 352 (2003) 275. [21] K. Kurosaki, A. Kosuga, S. Yamanaka, J. Alloys Compd. 351 (2003) 14. [22] K. Kurosaki, A. Kosuga, M. Uno, S. Yamanaka, J. Alloys Compd. 334 (2002) 317. [23] K. Kurosaki, H. Uneda, H. Muta, S. Yamanaka, J. Alloys Compd. 376 (2004) 43. [24] B. Abeles, R.W. Cohen, J. Appl. Phys. 35 (1964) 247.