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Synthesis and thermoelectric properties of the (GeTe)1-x(PbTe)x alloys
Solid State Sciences 13 (2011) 399e403
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and thermoelectric properties of the (GeTe)1-x(PbTe)x alloys S.P. Li, J.Q. Li*, Q.B. Wang, L. Wang, F.S. Liu, W.Q. Ao College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2010 Received in revised form 16 November 2010 Accepted 30 November 2010 Available online 13 December 2010
The Ge-rich (GeTe)1-x(PbTe)x alloys with x ¼ 0.10, 0.14, 0.18 and 0.22 were prepared by induction melting, ball milling and spark plasma sintering techniques. The thermoelectric properties of the samples were investigated. The experimental results show that all samples consist of the solid solutions of the two phases GeTe and PbTe. The samples are of p-type semiconductors. The existence of PbTe solution in GeTe increases its resistivity and Seebeck coefficient slightly, but reduces its thermal conductivity significantly. As result, the figures of merit for the materials can be enhanced. The maximum figure of merit ZT value of 0.81 was obtained in the sample (GeTe)0.82(PbTe)0.18 at 673K. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: PbTe-GeTe alloys Preparation Thermoelectric property
1. Introduction Thermoelectric (TE) materials can provide a direct conversion of thermal and electrical energy. This conversion has been receiving much attention for their potential applications in solid state cooling and power generation. The quality of thermoelectric materials is related to a dimensionless figure of merit ZT ¼ S2sT/k, where S is the Seebeck coefficient, s is the electrical conductivity, T is the temperature in Kelvin and k is the thermal conductivity. A high ZT makes high conversion efficiency. However, the ZT values of the commercially used materials are around 0.8, which corresponds to about 10% of Carnot efficiency [1]. Therefore, many researchers around the world make great efforts to improve the ZT. Although significant enhancement in ZT values have been reported in superlattices [2,3], it is generally difficult to use these superlattices in large-scale energy-conversion applications because of limitations in both heat transfer and cost [4]. Controlling the size and phases in bulk TE material is an effective way to enhance its ZT values. The semiconductors based on lead telluride or germanium telluride are known as appropriate candidate thermoelectric materials for the applications in the medium temperature range from 50 C to 500 C. Many researches have studied the PbTe based or GeTe based alloys. The compound AgPbmSbTe2þm bulk material shows a high ZT of 2.2 at 800K due to the distribution of Ag-Sb nanodots in PbTe matrix, which plays a key role in reducing
* Corresponding author. Tel./fax: þ86 755 26538528. E-mail address: [email protected] (J.Q. Li). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.045
thermal conductivity [5]. The compound (AgSbTe2)1-x(GeTe)x with x ¼ 0.85 has the highest ZT of 1.36 at 700K as the result of its very low value of thermal conductivity [6]. The PbeTeeS ternary system diagram consists of a spinodal decomposition. The (PbTe)1x(PbS)x with the nanostructured multiphase formed by the spinodal decomposition have a maximum ZT of 1.5 at 642K for x ¼ 0.08 [7] due to the contribution of reduction on thermal conductivity, especially on the lattice thermal conductivity. The pseudo-binary PbTe-GeTe phase diagram [8,9] shows that the unlimited mutual solubility of its end-members in both liquid and solid states at high temperature but there is a spinodal decomposition (miscibility gap) below 587 C (860K). The miscibility gap range determined by Hohnke et al. [10] is from about 3.0% to 90.0% PbTe in mole fraction at 673 K. The calculated chemical spinodal range is from about 15% to 70% PbTe in mole fraction at 673 K. The Pb0.36Ge0.64Te alloy was reported to be decomposed into different microstructure states depending on the time of aging treatment [11]. Gorsse et al. obtained the low thermal diffusivity samples in the PbTe-GeTe quasi-binary system utilizing its spinodal decomposition to control the microstructure states through solutioning, quenching and aging treatment [12]. In this paper, we prepared the (GeTe)1-x(PbTe)x samples with two phases based on the spinodal miscibility gap of the PbTe-GeTe pseudo-binary system. The compounds PbTe and GeTe were prepared by high frequency induction melting separately. They were then ball milled into powder with particle size of about 100 nm, separately. The bulk samples (GeTe)1-x(PbTe)x and (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2 were obtained by spark plasma sintering (SPS) the appropriates mixtures of PbTe and GeTe powders. The formation of the two phases in the sample should reduce the thermal
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Fig. 1. Powder XRD patterns for the samples (GeTe)1-x(PbTe)x with (a) x ¼ 0.14 before sintering; (b) x ¼ 0.10 after sintering and (c) x ¼ 0.18 after sintering.
conductivity, which benefits to the enhancement of ZT. The formation of interface between two phases during the ball milling and spark plasma sintering of the mixture powder should benefit the electrical conductivities of the samples.
calorimetry. The bulk density of the sample was calculated from the sample’s geometry and mass.
3. Results and discussion 2. Experiment
3.1. Phase analysis
The elementals Pb, Te and Ge with purity of 99.99% were used as starting materials. The compounds PbTe and GeTe were prepared by high-frequency induction melting in evacuated quartz tubes and then ball milled into the powders with particle size of about 100 nm, separately. Subsequently, the mixture powders with the compositions of (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22) were prepared by ball milling using the milled PbTe and GeTe powders. The ball milling was carried out in a planetary ball mill (QM-4F, Nanjing University, China) using a hard stainless steel vial and balls, at 200 rpm for 5h for the pure PbTe and GeTe powders, while at 200 rpm for 14h for the mixture powders. The weight ratio of balls to powders was kept at about 20:1, and the mill vial was evacuated and then filled with a purified H2 atmosphere to prevent the powders from oxidation during the milling process. The sample powders were consolidated by spark plasma sintering (SPS) at 663 K for 5 min under an axial pressure of 32 MPa. The peak value of impulse was 1200 A during SPS process. The sample with the density of around 6.5 g cm3, more than 95% of relative density without any crack, was obtained in this process. The bar specimen with dimensions of 12.0 mm 5.0 mm 5.0 mm was prepared for the electrical properties measurement and the disk specimen with f12.7 mm 2.0 mm for the thermal conductivity measurement. The phases in the samples were analyzed by X-ray diffraction using a Bruker D8 Advance SS/18 kW diffractometer with CuKa radiation and JADE 5.0 software. The Seebeck coefficient (S) and electrical conductivity (s) were evacuated by the apparatus (ZEM-2, Ulvac-Riko, Japan) in a helium atmosphere. The thermal conductivity (k) was calculated using the equation k ¼ lCpd, where l is the thermal diffusivity, Cp is the heat capacity, and d is the bulk density of the sample. The thermal diffusivity was measured by a laser flash technique (NETZSCH LFA457) in Ar atmosphere. The heat capacity was measured by the differential scanning
The X-ray diffraction analysis shows that there is two phases in every sample before or after sintering. The two phases do have the rhombohedral GeTe and the cubic PbTe crystal structure, but the peak are slightly shifted, so that these are not pure GeTe and PbTe but GeTe(Pb) and PbTe(Ge). Fig. 1 show the powder X-ray diffraction patterns for some representative samples before and after sintering: Fig. 1(a) for the sample (GeTe)0.86(PbTe)0.14 before sintering; Fig. 1(b) and (c) for the samples (GeTe)1-x(PbTe)x with x ¼ 0.10 and 0.18 after sintering, respectively. The broad peaks appear in the X-ray diffraction pattern of the sample before sintering due to the small particle size of the powders and the existence of stress or strain after the ball milling process. It is note that the X-ray diffraction pattern of the PbTe solid solution (marked with “o” in Fig. 1(c)) shifts to higher 2q angle as compared with that of the pure compound PbTe due to the substitution of Ge for Pb in compound PbTe. It means that the PbTe solid solution can be formed in the mixture powders during ball milling and SPS process, which modifies the interface between GeTe and PbTe two phases and benefits to the electrical conductivity. The shift of the XRD pattern for GeTe phase is not obvious, but this phase should be a solid solution of small amount of Pb in GeTe. The amount of the PbTe solid solution in the sample, estimated from the Rietveld refinement for its XRD pattern, is closed to but a little more than the x in the prepared (GeTe)1-x(PbTe)x sample since the solubility of Ge in PbTe is more than that of Pb in GeTe. The low agreement factors and low the Goodness-of-fits (GOFs) indicate that the Rietveld refinements are satisfactory. The Rietveld refinement for the sample (GeTe)0.82(PbTe)0.18 as example, shown in Fig. 2, estimates that this sample contains 28.9 wt. % PbTe solution, a little more than 26.9 wt.% of prepared sample with x ¼ 0.18 (in mol.%). The agreement factors for this refinement are Rwp ¼ 15.53 and Rexp ¼ 9.21 and Goodness-of-fits is GOF ¼ 1.69.
S.P. Li et al. / Solid State Sciences 13 (2011) 399e403
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Fig. 2. Rietveld refinement for the XRD pattern of the sintered sample (GeTe)0.90(PbTe)0.10.
3.2. Thermoelectric properties The temperature dependence of electrical resistivity, Seebeck coefficient, thermal conductivity and figure of merit ZT for the alloys (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22) are given in Fig. 3. The accuracies are 10%, 7% and 5% for the electrical resistivity, Seebeck coefficient, thermal conductivity measurement respectively, which
are shown as the error bars in Fig. 3(a)e(c). Those for the pure compound GeTe prepared by SPS reported in ref. [13] are also given in the figure for comparing. The electrical resistivities for all the studied samples, shown in Fig. 3(a), increase with increasing temperature, showing the degenerate semiconductor behavior due to the positive temperature coefficient similar to that of pure GeTe reported in ref. [13]. The electrical resistivity of (GeTe)1-x(PbTe)x increases with
Fig. 3. Temperature dependence of the (a) electrical resistivity r, (b) Seebeck coefficient S, (c) thermal conductivity k and (d) figure of merit ZT for the samples (GeTe)1-x(PbTe)x with x ¼ 0.10, 0.14, 0.18 and 0.22, the (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2, the pure compound GeTe prepared by SPS reported in Ref. [10] for comparing. The total thermal conductivities for the bulk GeTe prepared by melting reported in Ref. [19] and the GeTe crystalline film reported in Ref. [20] at room temperature are shown in Fig. 3(c) for comparing.
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increasing x at the temperature lower than 450 K. This may be ascribed to the reduction of carrier mobility m and carrier concentration. The reduction of carrier mobility is owing to the increasing of the carrier scattering due to the appearance of the GeTe and PbTe two solutions in the samples. The reduction of carrier concentration may be due to the replacement of the higher carrier concentration compound GeTe (p z 1020cm3e1021cm3) [14] by the lower carrier concentration compound PbTe (n z 1017cm3e1018cm3) [15]. The electrical resistivities for the PbTe-doped samples (GeTe)1-x(PbTe)x are about 2e3 times as that of the pure GeTe reported in ref. [13]. The electrical resistivities of (GeTe)1-x(PbTe)x increase with increasing x due to the increases of the PbTe phase. The formation of the PbTe or GeTe solid solutions, by diffusion of Ge or Pb atoms between PbTe and GeTe particles during ball milling and SPS process, can modify the grain boundaries and enhance the electrical resistivity of the material. Introducing PbTe phase into GeTe increases its electrical resistivity, but it also increases its Seebeck coefficient and reduces its thermal conductivity obviously. As a result, the figures of merit may be enhanced. The Seebeck coefficients of the studied samples, shown in Fig. 3 (b), were found to be positive over the entire temperature range, the same behavior as the p-type GeTe, indicating that the hole-type carriers dominate the thermoelectric transport in the samples. The Seebeck coefficient for GeTe crystal grown from gas phase was lower than 32.5 mV K1 at room temperature [16], closed to that of GeTe prepared by SPS at the same temperature (33.6 mV K1) reported in ref. [13] (star line in Fig. 3(b)). Introducing PbTe phase into GeTe increases its Seebeck coefficient in some degrees as compared with that of the pure GeTe prepared by SPS reported in ref. [13]. This increase is the result of the complex effects of the substitutions of Ge in compound PbTe and Pb in compound GeTe, the grain size and the interaction between both GeTe and PbTe solutions. A small amount of GeTe doping in PbTe prepared by melting [17] or by hot pressing [18] increases the absolute value of Seebeck coefficient, lower thermal conductivity and thus enhance the figure- of- merit. A small amount of PbTe doping in GeTe may have similar effects. The interaction between both p-type GeTe and n-type PbTe solutions may also have positive effect on the total Seebeck coefficient. The temperature dependence of thermal conductivity for the samples (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22), shown in Fig. 3(c), indicates their thermal conductivities decrease with increasing temperature, similar to that of the pure bulk GeTe prepared by melting reported in ref. [19] or SPS reported in ref. [13] and the GeTe crystalline film [20]. The total thermal conductivities at room temperature are 8.7 W m1 K1 (4.7 W m1 K1 of lattice thermal conductivity and 4.0 W m1 K1 of electronic thermal conductivity) for the bulk GeTe prepared by melting reported in ref. [19], 5.9 W m1 K1 (4.35 W m1 K1 of lattice thermal conductivity and 1.55 W m1 K1 of electronic thermal conductivity) for the GeTe crystalline film reported in ref. [20], and 7.0 for the bulk GeTe prepared by SPS reported in ref. [13], also shown in Fig. 3(c). Compared with the thermal conductivity for the bulk GeTe prepared by melting, the reduction of thermal conductivity for the GeTe crystalline film can be attributed to the less concentration of hole in the film [20], while the reduction of thermal conductivity for the bulk GeTe prepared by SPS should be originated from increases of phonon scattering from the nanograin and grain boundaries. The thermal conductivities for all samples (GeTe)1-x(PbTe)x with x > 0 are much smaller than that of the pure GeTe, and decrease with increasing the fraction of PbTe solid solution x except for x ¼ 0.22. It indicates that the existence of the second phase PbTe solid solution in GeTe can reduce its thermal conductivity significantly, which benefits its thermoelectric properties. It is reasonable to believe
that the low thermal conductivity of the samples should be originated from the enhanced phonon scattering due to both the fine nanoscale dispersion and the solute atoms Ge or Pb in PbTe or GeTe solution because of the spinodal decomposition in the pseudo-binary PbTe-GeTe system. The more details were described in ref. [21]. The (GeTe)0.82(PbTe)0.18 sample has the thermal conductivity as low as 2.8 W m1 K1, much lower than 7.1 W m1 K1 of pure GeTe at room temperature. The nanostructures may created by both the nucleation and growth and the chemical spinodal decomposition in the sample with x ¼ 0.18, which is more effective in scatting phonons, similar to the sample (Pb0.95Sn0.05Te)0.92(PbS)0.08 reported in ref. [22]. The figures of merit (ZT) for the studied samples can be calculated, shown in Fig. 3(d), by the equation ZT ¼ S2sT/k, from the above data over the entire temperature range. It appears that the values of ZT for all samples increase with increasing temperature. The sample (GeTe)0.82(PbTe)0.18 reaches the maximum ZT value of 0.81 at 673K, due to it lowest thermal conductivity, relative lower electrical resistivity and rather higher Seebeck coefficient. The thermal conductivity and electrical resistivity of the Ge-rich (GeTe)1-x(PbTe)x are comparable to those of Sn-rich Ge1-xSnxTe alloy [23], but higher Seebeck coefficient of the Ge-rich (GeTe)1-x(PbTe)x makes it higher ZT value. The thermoelectric properties of the Ge-rich (GeTe)1-x(PbTe)x prepared in this work are comparable to those of Pb-rich Pb0.8Ge0.2Te prepared by melt quenching [24]. The TE properties can be further optimized. Attempt to further enhance the thermoelectric properties of (GeTe)1-x(PbTe)x by introducing the third phase AgSbTe2, we prepared and studied the sample (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2. The experimental results are also shown in Fig. 3. Unfortunately, the doped sample showed the higher electrical resistivity and higher thermal conductivity and nearly unchanged Seebeck coefficients as compared with those of the sample (GeTe)0.82(PbTe)0.18, leading to the lower the figures of merit (ZT). 4. Conclusion The p-type semiconductors (GeTe)1-x(PbTe)x with two phases were fabricated based on the spinodal miscibility gap of the PbTeGeTe pseudo-binary system. The existence of PbTe solution in GeTe increases its resistivity and Seebeck coefficient slightly, but reduces its thermal conductivity significantly. It leads to enhance the figure of merit of the materials. The maximum figure of merit ZT value of 0.81, was obtained in the sample (GeTe)0.82(PbTe)0.18 at 673K. Acknowledgments The work was supported by the National Natural Science Foundation of China (Nos: 50871070 and 51003060) and Shenzhen Science and Technology Research Grant (Nos.CXB200903090012A and JC200903120109A). The authors would like to thank Mr. Z.L. Lu for his help in experiment. References [1] C.B. Vining, Nat. Mater. 8 (2009) 83. [2] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001) 597. [3] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2002) 2229. [4] B. Poudel, Q. Hao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z. Ren, Science 320 (2008) 634. [5] Y. Gelbstein, O.B. Yehuda, E. Pinhas, J. Electron. Mater. 38 (2009) 1478e1482. [6] J.R. Salvador, J. Yang, X. Shi, H. Wang, A.A. Wereszczak, J. Solid State Chem. 182 (2009) 2088e2095. [7] J. Androulakis, C.H. Lin, H.J. Kong, C. Uher, C.I. Wu, T. Hogan, B.A. Cook, T. Caillat, K.M. Paraskevopoulos, M.G. Kanatzidis, J. Am. Chem. Soc. 129 (2007) 9780e9788.
S.P. Li et al. / Solid State Sciences 13 (2011) 399e403 [8] S.G. Parker, J.E. Pinnell, L.N. Swink, J. Mater. Sci. 9 (1974) 1829e1832. [9] S.P. Ramachandrarao, S. Misra, T.R. Anantharaman, J. Mater. Sci. 10 (1975) 1849e1855. [10] D.K. Hohnke, H. Holloway, S. Kaiser, J. Phys. Chem. Solids 33 (1972) 2053. [11] L.V. Yashina, V. Leute, V.I. Shtanov, H.M. Schmidtke, V.S. Neudachina, J. Alloys Compd. 413 (2006) 133e143. [12] S. Gorsse, P. Bauer Pereira, R. Decourt, E. Sellier, Chem. Mater. 22 (2010) 988e993. [13] Y. Gelbstein, B. Dado, O.B. Yehuda, Y. Sadia, Z. Dashevsky, M.P. Dariel, Chem. Mater. 22 (2010) 1054e1058. [14] L.E. Shelimova, E.S. Avilov, M.A. Kretova, J. Adv. Mater. 1 (1994) 253. [15] V. Fano, in: D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, FL, 1995, pp. 257e266.
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[16] N.N. Koren, V.I. Levchenko, V.V. Dikareva, A.V. Kazushchik, Phys. Stat. Sol. (a) 84 (1984) K173. [17] J.S. Choi, H.J. Kim, H.C. Kim, T.S. Oh, D. B Hyun, H.W. Lee, 16th International Conference on Thermoelectrics (1997) 228e231. [18] I. Kudman, Metall. Trans. 2 (1971) 163e168. [19] A.N. Krestonikov, A.S. Okhotin, A.A. Aivazov, High Temp 6 (1968) 1088. [20] P. Nath, K.L. Chopra, Phys. Rev. B. 10 (1974) 3412e3418. [21] L.D. Hicks, M.S. Dresselhaus, Phys. Rev. B. 47 (1993) 12727. [22] J. Androulakis, C.H. Lin, H.J. Kong, C. Uher, C.I. Wu, T. Hogan, B.A. Cook, T. Caillat, J. Am. Chem. Soc. 129 (2007) 9780e9788. [23] C.C. Wu, N.J. Ferng, H.J. Gau, J. Cryst. Growth. 304 (2007) 127e132. [24] J.R. Sootsman, J.Q. He, V.P. Dravid, C.P. Li, C. Uher, M.G. Kanatzidis, J. Appl. Phys. 105 (2009) 083718.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis and thermoelectric properties of the (GeTe)1-x(PbTe)x alloys S.P. Li, J.Q. Li*, Q.B. Wang, L. Wang, F.S. Liu, W.Q. Ao College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2010 Received in revised form 16 November 2010 Accepted 30 November 2010 Available online 13 December 2010
The Ge-rich (GeTe)1-x(PbTe)x alloys with x ¼ 0.10, 0.14, 0.18 and 0.22 were prepared by induction melting, ball milling and spark plasma sintering techniques. The thermoelectric properties of the samples were investigated. The experimental results show that all samples consist of the solid solutions of the two phases GeTe and PbTe. The samples are of p-type semiconductors. The existence of PbTe solution in GeTe increases its resistivity and Seebeck coefficient slightly, but reduces its thermal conductivity significantly. As result, the figures of merit for the materials can be enhanced. The maximum figure of merit ZT value of 0.81 was obtained in the sample (GeTe)0.82(PbTe)0.18 at 673K. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: PbTe-GeTe alloys Preparation Thermoelectric property
1. Introduction Thermoelectric (TE) materials can provide a direct conversion of thermal and electrical energy. This conversion has been receiving much attention for their potential applications in solid state cooling and power generation. The quality of thermoelectric materials is related to a dimensionless figure of merit ZT ¼ S2sT/k, where S is the Seebeck coefficient, s is the electrical conductivity, T is the temperature in Kelvin and k is the thermal conductivity. A high ZT makes high conversion efficiency. However, the ZT values of the commercially used materials are around 0.8, which corresponds to about 10% of Carnot efficiency [1]. Therefore, many researchers around the world make great efforts to improve the ZT. Although significant enhancement in ZT values have been reported in superlattices [2,3], it is generally difficult to use these superlattices in large-scale energy-conversion applications because of limitations in both heat transfer and cost [4]. Controlling the size and phases in bulk TE material is an effective way to enhance its ZT values. The semiconductors based on lead telluride or germanium telluride are known as appropriate candidate thermoelectric materials for the applications in the medium temperature range from 50 C to 500 C. Many researches have studied the PbTe based or GeTe based alloys. The compound AgPbmSbTe2þm bulk material shows a high ZT of 2.2 at 800K due to the distribution of Ag-Sb nanodots in PbTe matrix, which plays a key role in reducing
* Corresponding author. Tel./fax: þ86 755 26538528. E-mail address: [email protected] (J.Q. Li). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.045
thermal conductivity [5]. The compound (AgSbTe2)1-x(GeTe)x with x ¼ 0.85 has the highest ZT of 1.36 at 700K as the result of its very low value of thermal conductivity [6]. The PbeTeeS ternary system diagram consists of a spinodal decomposition. The (PbTe)1x(PbS)x with the nanostructured multiphase formed by the spinodal decomposition have a maximum ZT of 1.5 at 642K for x ¼ 0.08 [7] due to the contribution of reduction on thermal conductivity, especially on the lattice thermal conductivity. The pseudo-binary PbTe-GeTe phase diagram [8,9] shows that the unlimited mutual solubility of its end-members in both liquid and solid states at high temperature but there is a spinodal decomposition (miscibility gap) below 587 C (860K). The miscibility gap range determined by Hohnke et al. [10] is from about 3.0% to 90.0% PbTe in mole fraction at 673 K. The calculated chemical spinodal range is from about 15% to 70% PbTe in mole fraction at 673 K. The Pb0.36Ge0.64Te alloy was reported to be decomposed into different microstructure states depending on the time of aging treatment [11]. Gorsse et al. obtained the low thermal diffusivity samples in the PbTe-GeTe quasi-binary system utilizing its spinodal decomposition to control the microstructure states through solutioning, quenching and aging treatment [12]. In this paper, we prepared the (GeTe)1-x(PbTe)x samples with two phases based on the spinodal miscibility gap of the PbTe-GeTe pseudo-binary system. The compounds PbTe and GeTe were prepared by high frequency induction melting separately. They were then ball milled into powder with particle size of about 100 nm, separately. The bulk samples (GeTe)1-x(PbTe)x and (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2 were obtained by spark plasma sintering (SPS) the appropriates mixtures of PbTe and GeTe powders. The formation of the two phases in the sample should reduce the thermal
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S.P. Li et al. / Solid State Sciences 13 (2011) 399e403
Fig. 1. Powder XRD patterns for the samples (GeTe)1-x(PbTe)x with (a) x ¼ 0.14 before sintering; (b) x ¼ 0.10 after sintering and (c) x ¼ 0.18 after sintering.
conductivity, which benefits to the enhancement of ZT. The formation of interface between two phases during the ball milling and spark plasma sintering of the mixture powder should benefit the electrical conductivities of the samples.
calorimetry. The bulk density of the sample was calculated from the sample’s geometry and mass.
3. Results and discussion 2. Experiment
3.1. Phase analysis
The elementals Pb, Te and Ge with purity of 99.99% were used as starting materials. The compounds PbTe and GeTe were prepared by high-frequency induction melting in evacuated quartz tubes and then ball milled into the powders with particle size of about 100 nm, separately. Subsequently, the mixture powders with the compositions of (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22) were prepared by ball milling using the milled PbTe and GeTe powders. The ball milling was carried out in a planetary ball mill (QM-4F, Nanjing University, China) using a hard stainless steel vial and balls, at 200 rpm for 5h for the pure PbTe and GeTe powders, while at 200 rpm for 14h for the mixture powders. The weight ratio of balls to powders was kept at about 20:1, and the mill vial was evacuated and then filled with a purified H2 atmosphere to prevent the powders from oxidation during the milling process. The sample powders were consolidated by spark plasma sintering (SPS) at 663 K for 5 min under an axial pressure of 32 MPa. The peak value of impulse was 1200 A during SPS process. The sample with the density of around 6.5 g cm3, more than 95% of relative density without any crack, was obtained in this process. The bar specimen with dimensions of 12.0 mm 5.0 mm 5.0 mm was prepared for the electrical properties measurement and the disk specimen with f12.7 mm 2.0 mm for the thermal conductivity measurement. The phases in the samples were analyzed by X-ray diffraction using a Bruker D8 Advance SS/18 kW diffractometer with CuKa radiation and JADE 5.0 software. The Seebeck coefficient (S) and electrical conductivity (s) were evacuated by the apparatus (ZEM-2, Ulvac-Riko, Japan) in a helium atmosphere. The thermal conductivity (k) was calculated using the equation k ¼ lCpd, where l is the thermal diffusivity, Cp is the heat capacity, and d is the bulk density of the sample. The thermal diffusivity was measured by a laser flash technique (NETZSCH LFA457) in Ar atmosphere. The heat capacity was measured by the differential scanning
The X-ray diffraction analysis shows that there is two phases in every sample before or after sintering. The two phases do have the rhombohedral GeTe and the cubic PbTe crystal structure, but the peak are slightly shifted, so that these are not pure GeTe and PbTe but GeTe(Pb) and PbTe(Ge). Fig. 1 show the powder X-ray diffraction patterns for some representative samples before and after sintering: Fig. 1(a) for the sample (GeTe)0.86(PbTe)0.14 before sintering; Fig. 1(b) and (c) for the samples (GeTe)1-x(PbTe)x with x ¼ 0.10 and 0.18 after sintering, respectively. The broad peaks appear in the X-ray diffraction pattern of the sample before sintering due to the small particle size of the powders and the existence of stress or strain after the ball milling process. It is note that the X-ray diffraction pattern of the PbTe solid solution (marked with “o” in Fig. 1(c)) shifts to higher 2q angle as compared with that of the pure compound PbTe due to the substitution of Ge for Pb in compound PbTe. It means that the PbTe solid solution can be formed in the mixture powders during ball milling and SPS process, which modifies the interface between GeTe and PbTe two phases and benefits to the electrical conductivity. The shift of the XRD pattern for GeTe phase is not obvious, but this phase should be a solid solution of small amount of Pb in GeTe. The amount of the PbTe solid solution in the sample, estimated from the Rietveld refinement for its XRD pattern, is closed to but a little more than the x in the prepared (GeTe)1-x(PbTe)x sample since the solubility of Ge in PbTe is more than that of Pb in GeTe. The low agreement factors and low the Goodness-of-fits (GOFs) indicate that the Rietveld refinements are satisfactory. The Rietveld refinement for the sample (GeTe)0.82(PbTe)0.18 as example, shown in Fig. 2, estimates that this sample contains 28.9 wt. % PbTe solution, a little more than 26.9 wt.% of prepared sample with x ¼ 0.18 (in mol.%). The agreement factors for this refinement are Rwp ¼ 15.53 and Rexp ¼ 9.21 and Goodness-of-fits is GOF ¼ 1.69.
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Fig. 2. Rietveld refinement for the XRD pattern of the sintered sample (GeTe)0.90(PbTe)0.10.
3.2. Thermoelectric properties The temperature dependence of electrical resistivity, Seebeck coefficient, thermal conductivity and figure of merit ZT for the alloys (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22) are given in Fig. 3. The accuracies are 10%, 7% and 5% for the electrical resistivity, Seebeck coefficient, thermal conductivity measurement respectively, which
are shown as the error bars in Fig. 3(a)e(c). Those for the pure compound GeTe prepared by SPS reported in ref. [13] are also given in the figure for comparing. The electrical resistivities for all the studied samples, shown in Fig. 3(a), increase with increasing temperature, showing the degenerate semiconductor behavior due to the positive temperature coefficient similar to that of pure GeTe reported in ref. [13]. The electrical resistivity of (GeTe)1-x(PbTe)x increases with
Fig. 3. Temperature dependence of the (a) electrical resistivity r, (b) Seebeck coefficient S, (c) thermal conductivity k and (d) figure of merit ZT for the samples (GeTe)1-x(PbTe)x with x ¼ 0.10, 0.14, 0.18 and 0.22, the (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2, the pure compound GeTe prepared by SPS reported in Ref. [10] for comparing. The total thermal conductivities for the bulk GeTe prepared by melting reported in Ref. [19] and the GeTe crystalline film reported in Ref. [20] at room temperature are shown in Fig. 3(c) for comparing.
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increasing x at the temperature lower than 450 K. This may be ascribed to the reduction of carrier mobility m and carrier concentration. The reduction of carrier mobility is owing to the increasing of the carrier scattering due to the appearance of the GeTe and PbTe two solutions in the samples. The reduction of carrier concentration may be due to the replacement of the higher carrier concentration compound GeTe (p z 1020cm3e1021cm3) [14] by the lower carrier concentration compound PbTe (n z 1017cm3e1018cm3) [15]. The electrical resistivities for the PbTe-doped samples (GeTe)1-x(PbTe)x are about 2e3 times as that of the pure GeTe reported in ref. [13]. The electrical resistivities of (GeTe)1-x(PbTe)x increase with increasing x due to the increases of the PbTe phase. The formation of the PbTe or GeTe solid solutions, by diffusion of Ge or Pb atoms between PbTe and GeTe particles during ball milling and SPS process, can modify the grain boundaries and enhance the electrical resistivity of the material. Introducing PbTe phase into GeTe increases its electrical resistivity, but it also increases its Seebeck coefficient and reduces its thermal conductivity obviously. As a result, the figures of merit may be enhanced. The Seebeck coefficients of the studied samples, shown in Fig. 3 (b), were found to be positive over the entire temperature range, the same behavior as the p-type GeTe, indicating that the hole-type carriers dominate the thermoelectric transport in the samples. The Seebeck coefficient for GeTe crystal grown from gas phase was lower than 32.5 mV K1 at room temperature [16], closed to that of GeTe prepared by SPS at the same temperature (33.6 mV K1) reported in ref. [13] (star line in Fig. 3(b)). Introducing PbTe phase into GeTe increases its Seebeck coefficient in some degrees as compared with that of the pure GeTe prepared by SPS reported in ref. [13]. This increase is the result of the complex effects of the substitutions of Ge in compound PbTe and Pb in compound GeTe, the grain size and the interaction between both GeTe and PbTe solutions. A small amount of GeTe doping in PbTe prepared by melting [17] or by hot pressing [18] increases the absolute value of Seebeck coefficient, lower thermal conductivity and thus enhance the figure- of- merit. A small amount of PbTe doping in GeTe may have similar effects. The interaction between both p-type GeTe and n-type PbTe solutions may also have positive effect on the total Seebeck coefficient. The temperature dependence of thermal conductivity for the samples (GeTe)1-x(PbTe)x (x ¼ 0.10, 0.14, 0.18 and 0.22), shown in Fig. 3(c), indicates their thermal conductivities decrease with increasing temperature, similar to that of the pure bulk GeTe prepared by melting reported in ref. [19] or SPS reported in ref. [13] and the GeTe crystalline film [20]. The total thermal conductivities at room temperature are 8.7 W m1 K1 (4.7 W m1 K1 of lattice thermal conductivity and 4.0 W m1 K1 of electronic thermal conductivity) for the bulk GeTe prepared by melting reported in ref. [19], 5.9 W m1 K1 (4.35 W m1 K1 of lattice thermal conductivity and 1.55 W m1 K1 of electronic thermal conductivity) for the GeTe crystalline film reported in ref. [20], and 7.0 for the bulk GeTe prepared by SPS reported in ref. [13], also shown in Fig. 3(c). Compared with the thermal conductivity for the bulk GeTe prepared by melting, the reduction of thermal conductivity for the GeTe crystalline film can be attributed to the less concentration of hole in the film [20], while the reduction of thermal conductivity for the bulk GeTe prepared by SPS should be originated from increases of phonon scattering from the nanograin and grain boundaries. The thermal conductivities for all samples (GeTe)1-x(PbTe)x with x > 0 are much smaller than that of the pure GeTe, and decrease with increasing the fraction of PbTe solid solution x except for x ¼ 0.22. It indicates that the existence of the second phase PbTe solid solution in GeTe can reduce its thermal conductivity significantly, which benefits its thermoelectric properties. It is reasonable to believe
that the low thermal conductivity of the samples should be originated from the enhanced phonon scattering due to both the fine nanoscale dispersion and the solute atoms Ge or Pb in PbTe or GeTe solution because of the spinodal decomposition in the pseudo-binary PbTe-GeTe system. The more details were described in ref. [21]. The (GeTe)0.82(PbTe)0.18 sample has the thermal conductivity as low as 2.8 W m1 K1, much lower than 7.1 W m1 K1 of pure GeTe at room temperature. The nanostructures may created by both the nucleation and growth and the chemical spinodal decomposition in the sample with x ¼ 0.18, which is more effective in scatting phonons, similar to the sample (Pb0.95Sn0.05Te)0.92(PbS)0.08 reported in ref. [22]. The figures of merit (ZT) for the studied samples can be calculated, shown in Fig. 3(d), by the equation ZT ¼ S2sT/k, from the above data over the entire temperature range. It appears that the values of ZT for all samples increase with increasing temperature. The sample (GeTe)0.82(PbTe)0.18 reaches the maximum ZT value of 0.81 at 673K, due to it lowest thermal conductivity, relative lower electrical resistivity and rather higher Seebeck coefficient. The thermal conductivity and electrical resistivity of the Ge-rich (GeTe)1-x(PbTe)x are comparable to those of Sn-rich Ge1-xSnxTe alloy [23], but higher Seebeck coefficient of the Ge-rich (GeTe)1-x(PbTe)x makes it higher ZT value. The thermoelectric properties of the Ge-rich (GeTe)1-x(PbTe)x prepared in this work are comparable to those of Pb-rich Pb0.8Ge0.2Te prepared by melt quenching [24]. The TE properties can be further optimized. Attempt to further enhance the thermoelectric properties of (GeTe)1-x(PbTe)x by introducing the third phase AgSbTe2, we prepared and studied the sample (GeTe)0.82(PbTe)0.18 doped with 3 mol% AgSbTe2. The experimental results are also shown in Fig. 3. Unfortunately, the doped sample showed the higher electrical resistivity and higher thermal conductivity and nearly unchanged Seebeck coefficients as compared with those of the sample (GeTe)0.82(PbTe)0.18, leading to the lower the figures of merit (ZT). 4. Conclusion The p-type semiconductors (GeTe)1-x(PbTe)x with two phases were fabricated based on the spinodal miscibility gap of the PbTeGeTe pseudo-binary system. The existence of PbTe solution in GeTe increases its resistivity and Seebeck coefficient slightly, but reduces its thermal conductivity significantly. It leads to enhance the figure of merit of the materials. The maximum figure of merit ZT value of 0.81, was obtained in the sample (GeTe)0.82(PbTe)0.18 at 673K. Acknowledgments The work was supported by the National Natural Science Foundation of China (Nos: 50871070 and 51003060) and Shenzhen Science and Technology Research Grant (Nos.CXB200903090012A and JC200903120109A). The authors would like to thank Mr. Z.L. Lu for his help in experiment. References [1] C.B. Vining, Nat. Mater. 8 (2009) 83. [2] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001) 597. [3] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2002) 2229. [4] B. Poudel, Q. Hao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J.M. Liu, M.S. Dresselhaus, G. Chen, Z. Ren, Science 320 (2008) 634. [5] Y. Gelbstein, O.B. Yehuda, E. Pinhas, J. Electron. Mater. 38 (2009) 1478e1482. [6] J.R. Salvador, J. Yang, X. Shi, H. Wang, A.A. Wereszczak, J. Solid State Chem. 182 (2009) 2088e2095. [7] J. Androulakis, C.H. Lin, H.J. Kong, C. Uher, C.I. Wu, T. Hogan, B.A. Cook, T. Caillat, K.M. Paraskevopoulos, M.G. Kanatzidis, J. Am. Chem. Soc. 129 (2007) 9780e9788.
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