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Impacts of both high pressure and Te–Se double-substituted skutterudite on the thermoelectric properties prepared by HTHP
Journal of Alloys and Compounds 615 (2014) 1056–1059
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Impacts of both high pressure and Te–Se double-substituted skutterudite on the thermoelectric properties prepared by HTHP Hairui Sun a, Xiaopeng Jia a, Le Deng b, Pin Lv a, Xin Guo a, Bing Sun a, Yuewen Zhang a, Binwu Liu a, Hongan Ma a,⇑ a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 June 2014 Received in revised form 22 July 2014 Accepted 22 July 2014 Available online 1 August 2014 Keywords: Thermoelectric properties Dislocation Co4Sb11.3Te0.7 xSex HTHP
a b s t r a c t A series of double-substituting with Te and Se on skutterudite compounds Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) has been prepared successfully by high-temperature and high pressure (HTHP) method. Comparing with other methods, the synthesis time was sharply reduced to half an hour. Microstructure and thermoelectric properties of these series samples were seriously investigated. Co4Sb11.3Te0.7 xSex with abundant grain boundaries, disorder in lattice orientation and dislocations were obtained after HTHP. The experimental results suggest that with the synthetic pressure increasing, relative larger absolute values of Seebeck coefficient were maintained. More importantly, the thermal conductivity of Co4Sb11.3Te0.7 xSex was observed to decrease monotonously with the pressure increasing. The minimum value is 1.8 W m 1 K 1 at room temperature for Co4Sb11.3Te0.6Se0.1 synthesized at 3.5 GPa. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric technology has received more and more interests because of their promising applications in directly converting both the existing heat in nature and waste heat generated in industry or automobile. Thermoelectric devices have numerous advantages, such as being silent, high reliability, and without any moving parts and long period of operation [1,2]. The maximum efficiency of material for thermoelectric application can be quantified by the dimensionless figure of merit, ZT = a2rT/j, where a, r, j, T are the Seebeck coefficient, electrical conductivity, thermal conductivity and the temperature in Kelvin, respectively [3,4]. However, the interdependence of these physical parameters makes it difficult to enhancing ZT value. An increase in r usually leads to an increase in j. An introduction of phonon scattering mechanism to reduce j often simultaneously produces scattering on the electrons and hence decreases r. Therefore, the challenge for the enhancement of ZT value is to reduce j significantly while protecting against obvious deterioration of r [5–8]. Among many thermoelectric materials, skutterudite-based materials have attracted great attention for their potential thermoelectric application in power generation at intermediate temperatures. Co4Sb12-based skutterudites have been noticed because of ⇑ Corresponding author. Fax: +86 431 85168858. E-mail addresses: [email protected] (H. Sun), [email protected] (H. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.07.173 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
their unique crystal structure and good thermoelectric properties, and believed to have strong potential for thermoelectric application. But their thermal conductivities are relatively high compared with other thermoelectric materials, rendering less appealing thermoelectric material performance. Nevertheless, there are two chemical approaches to enhance the thermoelectric performance of Co4Sb12 compounds: one is filling the cage of skutterudite structure [9,10], and another is atom substitution [11,12]. Research findings indicate that Te substitution is effective both in increasing the power factor by adjusting the carrier concentration and in decreasing the thermal conductivity by enhancing the point–defect scattering and the electron–phonon scattering [13,14]. Many approaches under high pressure have been conducted to synthesize Co4Sb12 compounds by substituting, but, regrettably, the perfect performance under high pressure returns back to the initial state when the pressure is unloaded. HTHP method has been proved to be an effective and potential processing approach to prepare thermoelectric materials [15–17], which can improve thermoelectric properties obviously and maintain the excellent properties to ordinary pressure. This method could tune thermoelectric properties splendidly, and the synthesis time was sharply reduced to half an hour. In our experiment, the microstructure and thermoelectric properties of Co4Sb11.3Te0.7 xSex has been studied carefully. The experimental data exhibited high Seebeck coefficient and low thermal conductivity, which indicated that processing materials by HTHP
H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
is an effective way to enhance their thermoelectric performance. The effects of synthetic method on the microstructure and thermoelectric properties are the focuses in this paper. The thermoelectric properties at room temperature were seriously studied such as Seebeck coefficient, electrical resistivity, power factor, and thermal conductivity. 2. Experimental procedure The Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) samples were prepared with Co, Sb, Te and Se (99.99% in purity) powders as raw materials. These powders were weighed according to the stoichiometry, and then mixed in an agate mortar in a glove box under the nitrogen gas atmosphere. The mixtures were shaped to a cylinder with about 3–4 mm thick and 10.5 mm in diameter by press, and then the pole shaped samples were assembled for HTHP synthesis. The samples were prepared in a China-type large volume cubic high-pressure apparatus (CHPA) (SPD6 1200) with a sample chamber of 23 mm on an edge at 1000 K and 1–3.5 GPa, and the synthesis time was selected as half an hour. X-ray diffraction (XRD) measurements with Cu Ka (k = 1.5418 Å) radiation were performed on an X-ray diffractometer (D/MAX-RA) in the range 2h from 20° to 80°. The Seebeck coefficient was determined from thermoelectromotive force given by the temperature difference
Fig. 1. XRD patterns of Co4Sb11.3Te0.7 xSex synthesized by HTHP.
1057
within 3–5 K between the two ends of the sample with a home-made instrument. The error in a measurement did not exceed 5% at room temperature. The electrical resistivity was measured by typical DC four-probe configuration. The thermal diffusivity (k) was measured based on the laser flash technique (Netzsch LFA-427) at room temperature. The morphology and microstructure of samples were performed on field emission scanning electron microscope (SEM, JEOL JSM-6700F) and a highresolution transmission electron microscopy (HRTEM JEOL JEM-2100F). Scanning TEM and energy-dispersive X-ray spectroscopy analyses (STEM-EDX) were taken by a FEI TECNAI F20 transmission electron microscope with an accelerating voltage of 200 kV. All the measurements were carried out at normal temperature and pressure.
3. Results and discussion Fig. 1 shows the XRD patterns of Co4Sb11.3Te0.7 xSex, which demonstrates that the major phase is Co4Sb12 phase. All of the diffraction peak positions and (h k l) values match very well with the standard diffraction data of pure Co4Sb12 (PDF NO. 78-976), and no significant difference can be observed. Obviously, the different stoichiometric ratios of Co4Sb11.3Te0.7 xSex have been successfully synthesized by HTHP. In addition, compared with the traditional method, the processing time of HTHP method reduced from several days to half an hour. We performed Scanning TEM and energy-dispersive X-ray spectroscopy analyses (STEM-EDX) analysis to determine the atom existence and ratio in Co4Sb11.3Te0.6Se0.1. Co, Sb, Te and Se elements are found to be present, and the atomic ratio of Co, Sb, Te and Se is 25.23, 69.97, 4.08 and 0.72, respectively, as shown in Fig. 2(A). Through these mappings pictured at Fig. 2(B), the elements of the sample are evenly distributed. All of these data provide powerful evidence for that Te–Se double-substituted skutterudite was successfully synthesized via HTHP. Fig. 3 shows the SEM micrograph of the fractured surface for Co4Sb11.3Te0.7 xSex sample, which was synthesized at 1 GPa by HTHP. The micrograph revealed that the resulting material was crack-free and had a uniform microstructure. The sample possesses homogeneous grain size, compact structure and having a mean diameter about 4 lm, which indicates that both the double-substituted and the method of HTHP may restrain the growth of particles and homogenize the particle size. The effect of the tiny particle size
Fig. 2. (A) EDX spectrum of Co4Sb11.3Te0.6Se0.1. (B) STEM images and the corresponding STEM-EDX elemental mappings of Co4Sb11.3Te0.6Se0.1.
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H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
Fig. 3. SEM micrograph of Co4Sb11.3Te0.7 xSex synthesized by HTHP.
on the thermal conductivity is striking [18,19]. There are abundant grain boundaries in this sample in the SEM micrograph, which agrees with the advantage of HTHP synthesis of introducing abundant grain boundary. Fig. 4(A) gives the HRTEM micrograph of Co4Sb11.3Te0.7 xSex synthesized by HTHP. We can observe that the lattices orientation is in a disorderly growth pattern, and we can see many dislocations appeared in lattices as shown in Fig. 4(B), which are commonly
existed inside our samples. Maybe the high synthetic pressure can caused the more formation of the disorder and the dislocations, which could enhance the phonon scattering, and then reduce lattice thermal conductivity to some extent [20]. Fig. 5(A) shows synthesized pressure dependence of absolute Seebeck coefficient for Co4Sb11.3Te0.7 xSex. As expected, the absolute values of Seebeck coefficient increase almost linearly with the synthesis pressure rise. The enhancement of Seebeck may be due to the compositional difference and high pressure. Coincidentally, the result agrees with the advantages of high-pressure synthesis that the excellent properties gained at high pressure can be maintained to ordinary pressure. The maximum absolute value of Seebeck coefficient is 220.8 lV K 1 for Co4Sb11.3Te0.6Se0.1 occurring at 3.5 GPa. Fig. 5(B) represents synthesized pressure dependence of electrical resistivities for Co4Sb11.3Te0.7 xSex measured at room temperature. As we see, the value of the electrical resistivity increased when the synthetic pressure rise. It is easy to notice that all the samples possess low electrical resistivity, and the minimum value is 0.7 mX cm. The result was attributed to the high pressure, which decreases the carrier concentration and enhance the mobility of carriers. In addition, the abundant grain boundaries might be a negative factor for the reduction of electrical conductivity. Fig. 6 exhibits the power factors (PF), which were calculated by absolute Seebeck coefficient (a) and electrical resistivity (q): PF = a2/q. Co4Sb11.3Te0.7 xSex tend to relatively large PF values at room temperature mainly because of the low electrical resistivity. The maximum value of PF reaches 14.6 lW cm 1 K 2. The relationship between thermal conductivity and synthesis pressure is shown in Fig. 7. Both the synthesis pressure and
Fig. 4. (A) HR-TEM image of Co4Sb11.3Te0.7 xSex prepared by HTHP. (B) HR-TEM image of Co4Sb11.3Te0.7 xSex prepared by HTHP including several clear dislocations.
Fig. 5. Synthesized pressure dependence of (A) absolute Seebeck coefficient and (B) electrical resistivity for Co4Sb11.3Te0.7 xSex prepared by HTHP.
H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
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conductivity is 1.8 W m 1 K 1 at room temperature for Co4Sb11.3 Te0.6Se0.1 synthesized at 3.5 GPa. 4. Conclusions
Fig. 6. Synthesized pressure dependence of power factor for Co4Sb11.3Te0.7 xSex prepared by HTHP.
Above all, a series of Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) skutterudite alloys were successfully prepared by HTHP. We have in-depth study in microstructure and thermoelectric properties of the samples. As expected, with increasing pressure, the Seebeck coefficients and electrical resistivity increased, but the thermal conductivity decreased greatly. The decreased thermal conductivity is mainly ascribe to the lattices orientation disorder and dislocations, the enhanced point-defect scattering caused by the larger mass and radius fluctuations between Sb, Te and Se atoms and tiny particle size. What is more, the method of HTHP could save the synthesis time distinctly. Consequently, combining the method of HTHP with double-substituting could exhibit a higher thermoelectric performance, and optimize the thermoelectric properties significantly. The minimum value of thermal conductivity for Co4Sb11.3Te0.7 xSex is 1.8 W m 1 K 1 at room temperature. Acknowledgement This work was financially supported by National Natural Science Foundation of China (51171070, 51071074 and 51301024). References
Fig. 7. Synthesized pressure dependence of thermal conductivity for Co4Sb11.3Te0.7 xSex measured at room temperature.
Te–Se double-substituted in Co4Sb11.3Te0.7 xSex skutterudites play a large role, and the thermal conductivities exhibit a remarkable decrease with the synthesis pressure rise. Theoretically, the higher pressure could enforce the lattice defect scattering more strongly, and the thermal conductivity will lower. The disorder in lattices and dislocations (see Fig. 3) are probably the main reason [20]. As for the enhanced point-defect scattering, which caused by the larger mass (121.76, 127.60 and 78.96 for Sb, Te and Se, respectively) and ionic radius (76 pm, 97 pm, 50 pm for Sb, Te and Se, respectively) fluctuations between atoms in Co4Sb11.3Te0.7 xSex may be having an effect, too [21,22]. The point-defect can enhance the phonon scattering, so as to reduce thermal conductivity. Besides, the effect of the tiny particle size as shown in Fig. 3 on lattice thermal conductivity should not be neglected. The results indicated that high pressure and double-substituted could reduce the thermal conductivity effectively. The minimum value of thermal
[1] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’quinn, Nature 413 (6856) (2001) 597–602. [2] J. Yang, T. Caillat, MRS Bull. 31 (2006) 224. [3] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, Z.F. Ren, J.P. Fleurial, P. Gogna, Adv. Mater. 19 (2007) 1043–1053. [4] Y.L. Zhang, C.L. Hapenciuc, E.E. Castillo, T. Borca-Tasciuc, R.J. Mehta, C. Karthik, G. Ramanath, Appl. Phys. Lett. 96 (2010) 062107. [5] G.J. Snyder, E.S. Toberer, Nat. Mater. 7 (2008) 105–114. [6] J.R. Szczech, J.M. Higgins, S. Jin, J. Mater. Chem. 21 (2011) 4037–4055. [7] M.S. Toprak, C. Stiewe, D. Platzek, S. Williams, L. Bertini, E.C. Muller, C. Gatti, Y. Zhang, M. Rowe, M. Muhammed, Adv. Funct. Mater. 14 (2004) 1189–1196. [8] W.Y. Zhao, P. Wei, Q.J. Zhang, C.L. Dong, L.S. Liu, X.F. Tang, J. Am. Chem. Soc. 131 (2009) 3713–3720. [9] J.L. Mi, X.B. Zhao, T.J. Zhu, J.P. Tu, Mater. Lett. 62 (2008) 2363–2365. [10] H. Li, X.F. Tang, Q.J. Zhang, Appl. Phys. Lett. 94 (2009) 102114. [11] W.S. Liu, B.P. Zhang, J.F. Li, H.L. Zhang, L.D. Zhao, J. Appl. Phys. 102 (2007) 103717. [12] L. Deng, X.P. Jia, T.C. Su, Y.P. Jiang, S.Z. Zheng, X. Guo, Mater. Lett. 65 (2011) 1057–1059. [13] X.Y. Li, L.D. Chen, J.F. Fan, W.B. Zhang, T. Kawahara, T. Hirai, J. Appl. Phys. 98 (2005) 083702. [14] W.S. Liu, B.P. Zhang, J.F. Li, H.L. Zhang, L.D. Zhao, J. Appl. Phys. 102 (2007) 103717. [15] D.A. Polvani, J.F. Meng, S.N.V. Chandra, J. Sharp, J.V. Badding, Chem. Mater. 13 (2001) 2068–2071. [16] S.V. Ovsyannikov, V.V. Shchennikov, G.V. Vorontsov, A.Y. Manakov, A.Y. Likhacheva, V.A. Kulbachinskii, J. Appl. Phys. 104 (2008) 053713. [17] T.C. Su, X.P. Jia, H.A. Ma, J.G. Guo, Y.P. Jiang, N. Dong, L. Deng, X.B. Zhao, T.J. Zhu, C. Wei, J. Alloys Comp. 468 (2009) 410–413. [18] S. Bhattacharya, T.M. Tritt, Y. Xia, V. Ponnambalam, S.J. Poon, N. Thadhani, Appl. Phys. Lett. 81 (2002) 43–45. [19] D.M. Rowe, V.S. Shukla, N. Savvides, Nature 290 (1981) 765–766. [20] X. Yan, W.S. Liu, H. Wang, S. Chen, J. Shiomi, K. Esfarjani, H.Z. Wang, D.Z. Wang, G. Chen, Z.F. Ren, Energy Environ. Sci. 5 (2012) 7543–7548. [21] W.S. Liu, B.P. Zhang, L.D. Zhao, J.F. Li, Chem. Mater. 20 (2008) 7526–7531. [22] J. Yang, G.P. Meisner, L. Chen, Appl. Phys. Lett. 85 (2004) 1140–1142.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Impacts of both high pressure and Te–Se double-substituted skutterudite on the thermoelectric properties prepared by HTHP Hairui Sun a, Xiaopeng Jia a, Le Deng b, Pin Lv a, Xin Guo a, Bing Sun a, Yuewen Zhang a, Binwu Liu a, Hongan Ma a,⇑ a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 June 2014 Received in revised form 22 July 2014 Accepted 22 July 2014 Available online 1 August 2014 Keywords: Thermoelectric properties Dislocation Co4Sb11.3Te0.7 xSex HTHP
a b s t r a c t A series of double-substituting with Te and Se on skutterudite compounds Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) has been prepared successfully by high-temperature and high pressure (HTHP) method. Comparing with other methods, the synthesis time was sharply reduced to half an hour. Microstructure and thermoelectric properties of these series samples were seriously investigated. Co4Sb11.3Te0.7 xSex with abundant grain boundaries, disorder in lattice orientation and dislocations were obtained after HTHP. The experimental results suggest that with the synthetic pressure increasing, relative larger absolute values of Seebeck coefficient were maintained. More importantly, the thermal conductivity of Co4Sb11.3Te0.7 xSex was observed to decrease monotonously with the pressure increasing. The minimum value is 1.8 W m 1 K 1 at room temperature for Co4Sb11.3Te0.6Se0.1 synthesized at 3.5 GPa. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric technology has received more and more interests because of their promising applications in directly converting both the existing heat in nature and waste heat generated in industry or automobile. Thermoelectric devices have numerous advantages, such as being silent, high reliability, and without any moving parts and long period of operation [1,2]. The maximum efficiency of material for thermoelectric application can be quantified by the dimensionless figure of merit, ZT = a2rT/j, where a, r, j, T are the Seebeck coefficient, electrical conductivity, thermal conductivity and the temperature in Kelvin, respectively [3,4]. However, the interdependence of these physical parameters makes it difficult to enhancing ZT value. An increase in r usually leads to an increase in j. An introduction of phonon scattering mechanism to reduce j often simultaneously produces scattering on the electrons and hence decreases r. Therefore, the challenge for the enhancement of ZT value is to reduce j significantly while protecting against obvious deterioration of r [5–8]. Among many thermoelectric materials, skutterudite-based materials have attracted great attention for their potential thermoelectric application in power generation at intermediate temperatures. Co4Sb12-based skutterudites have been noticed because of ⇑ Corresponding author. Fax: +86 431 85168858. E-mail addresses: [email protected] (H. Sun), [email protected] (H. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.07.173 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
their unique crystal structure and good thermoelectric properties, and believed to have strong potential for thermoelectric application. But their thermal conductivities are relatively high compared with other thermoelectric materials, rendering less appealing thermoelectric material performance. Nevertheless, there are two chemical approaches to enhance the thermoelectric performance of Co4Sb12 compounds: one is filling the cage of skutterudite structure [9,10], and another is atom substitution [11,12]. Research findings indicate that Te substitution is effective both in increasing the power factor by adjusting the carrier concentration and in decreasing the thermal conductivity by enhancing the point–defect scattering and the electron–phonon scattering [13,14]. Many approaches under high pressure have been conducted to synthesize Co4Sb12 compounds by substituting, but, regrettably, the perfect performance under high pressure returns back to the initial state when the pressure is unloaded. HTHP method has been proved to be an effective and potential processing approach to prepare thermoelectric materials [15–17], which can improve thermoelectric properties obviously and maintain the excellent properties to ordinary pressure. This method could tune thermoelectric properties splendidly, and the synthesis time was sharply reduced to half an hour. In our experiment, the microstructure and thermoelectric properties of Co4Sb11.3Te0.7 xSex has been studied carefully. The experimental data exhibited high Seebeck coefficient and low thermal conductivity, which indicated that processing materials by HTHP
H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
is an effective way to enhance their thermoelectric performance. The effects of synthetic method on the microstructure and thermoelectric properties are the focuses in this paper. The thermoelectric properties at room temperature were seriously studied such as Seebeck coefficient, electrical resistivity, power factor, and thermal conductivity. 2. Experimental procedure The Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) samples were prepared with Co, Sb, Te and Se (99.99% in purity) powders as raw materials. These powders were weighed according to the stoichiometry, and then mixed in an agate mortar in a glove box under the nitrogen gas atmosphere. The mixtures were shaped to a cylinder with about 3–4 mm thick and 10.5 mm in diameter by press, and then the pole shaped samples were assembled for HTHP synthesis. The samples were prepared in a China-type large volume cubic high-pressure apparatus (CHPA) (SPD6 1200) with a sample chamber of 23 mm on an edge at 1000 K and 1–3.5 GPa, and the synthesis time was selected as half an hour. X-ray diffraction (XRD) measurements with Cu Ka (k = 1.5418 Å) radiation were performed on an X-ray diffractometer (D/MAX-RA) in the range 2h from 20° to 80°. The Seebeck coefficient was determined from thermoelectromotive force given by the temperature difference
Fig. 1. XRD patterns of Co4Sb11.3Te0.7 xSex synthesized by HTHP.
1057
within 3–5 K between the two ends of the sample with a home-made instrument. The error in a measurement did not exceed 5% at room temperature. The electrical resistivity was measured by typical DC four-probe configuration. The thermal diffusivity (k) was measured based on the laser flash technique (Netzsch LFA-427) at room temperature. The morphology and microstructure of samples were performed on field emission scanning electron microscope (SEM, JEOL JSM-6700F) and a highresolution transmission electron microscopy (HRTEM JEOL JEM-2100F). Scanning TEM and energy-dispersive X-ray spectroscopy analyses (STEM-EDX) were taken by a FEI TECNAI F20 transmission electron microscope with an accelerating voltage of 200 kV. All the measurements were carried out at normal temperature and pressure.
3. Results and discussion Fig. 1 shows the XRD patterns of Co4Sb11.3Te0.7 xSex, which demonstrates that the major phase is Co4Sb12 phase. All of the diffraction peak positions and (h k l) values match very well with the standard diffraction data of pure Co4Sb12 (PDF NO. 78-976), and no significant difference can be observed. Obviously, the different stoichiometric ratios of Co4Sb11.3Te0.7 xSex have been successfully synthesized by HTHP. In addition, compared with the traditional method, the processing time of HTHP method reduced from several days to half an hour. We performed Scanning TEM and energy-dispersive X-ray spectroscopy analyses (STEM-EDX) analysis to determine the atom existence and ratio in Co4Sb11.3Te0.6Se0.1. Co, Sb, Te and Se elements are found to be present, and the atomic ratio of Co, Sb, Te and Se is 25.23, 69.97, 4.08 and 0.72, respectively, as shown in Fig. 2(A). Through these mappings pictured at Fig. 2(B), the elements of the sample are evenly distributed. All of these data provide powerful evidence for that Te–Se double-substituted skutterudite was successfully synthesized via HTHP. Fig. 3 shows the SEM micrograph of the fractured surface for Co4Sb11.3Te0.7 xSex sample, which was synthesized at 1 GPa by HTHP. The micrograph revealed that the resulting material was crack-free and had a uniform microstructure. The sample possesses homogeneous grain size, compact structure and having a mean diameter about 4 lm, which indicates that both the double-substituted and the method of HTHP may restrain the growth of particles and homogenize the particle size. The effect of the tiny particle size
Fig. 2. (A) EDX spectrum of Co4Sb11.3Te0.6Se0.1. (B) STEM images and the corresponding STEM-EDX elemental mappings of Co4Sb11.3Te0.6Se0.1.
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H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
Fig. 3. SEM micrograph of Co4Sb11.3Te0.7 xSex synthesized by HTHP.
on the thermal conductivity is striking [18,19]. There are abundant grain boundaries in this sample in the SEM micrograph, which agrees with the advantage of HTHP synthesis of introducing abundant grain boundary. Fig. 4(A) gives the HRTEM micrograph of Co4Sb11.3Te0.7 xSex synthesized by HTHP. We can observe that the lattices orientation is in a disorderly growth pattern, and we can see many dislocations appeared in lattices as shown in Fig. 4(B), which are commonly
existed inside our samples. Maybe the high synthetic pressure can caused the more formation of the disorder and the dislocations, which could enhance the phonon scattering, and then reduce lattice thermal conductivity to some extent [20]. Fig. 5(A) shows synthesized pressure dependence of absolute Seebeck coefficient for Co4Sb11.3Te0.7 xSex. As expected, the absolute values of Seebeck coefficient increase almost linearly with the synthesis pressure rise. The enhancement of Seebeck may be due to the compositional difference and high pressure. Coincidentally, the result agrees with the advantages of high-pressure synthesis that the excellent properties gained at high pressure can be maintained to ordinary pressure. The maximum absolute value of Seebeck coefficient is 220.8 lV K 1 for Co4Sb11.3Te0.6Se0.1 occurring at 3.5 GPa. Fig. 5(B) represents synthesized pressure dependence of electrical resistivities for Co4Sb11.3Te0.7 xSex measured at room temperature. As we see, the value of the electrical resistivity increased when the synthetic pressure rise. It is easy to notice that all the samples possess low electrical resistivity, and the minimum value is 0.7 mX cm. The result was attributed to the high pressure, which decreases the carrier concentration and enhance the mobility of carriers. In addition, the abundant grain boundaries might be a negative factor for the reduction of electrical conductivity. Fig. 6 exhibits the power factors (PF), which were calculated by absolute Seebeck coefficient (a) and electrical resistivity (q): PF = a2/q. Co4Sb11.3Te0.7 xSex tend to relatively large PF values at room temperature mainly because of the low electrical resistivity. The maximum value of PF reaches 14.6 lW cm 1 K 2. The relationship between thermal conductivity and synthesis pressure is shown in Fig. 7. Both the synthesis pressure and
Fig. 4. (A) HR-TEM image of Co4Sb11.3Te0.7 xSex prepared by HTHP. (B) HR-TEM image of Co4Sb11.3Te0.7 xSex prepared by HTHP including several clear dislocations.
Fig. 5. Synthesized pressure dependence of (A) absolute Seebeck coefficient and (B) electrical resistivity for Co4Sb11.3Te0.7 xSex prepared by HTHP.
H. Sun et al. / Journal of Alloys and Compounds 615 (2014) 1056–1059
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conductivity is 1.8 W m 1 K 1 at room temperature for Co4Sb11.3 Te0.6Se0.1 synthesized at 3.5 GPa. 4. Conclusions
Fig. 6. Synthesized pressure dependence of power factor for Co4Sb11.3Te0.7 xSex prepared by HTHP.
Above all, a series of Co4Sb11.3Te0.7 xSex (x = 0.05, 0.075 and 0.1) skutterudite alloys were successfully prepared by HTHP. We have in-depth study in microstructure and thermoelectric properties of the samples. As expected, with increasing pressure, the Seebeck coefficients and electrical resistivity increased, but the thermal conductivity decreased greatly. The decreased thermal conductivity is mainly ascribe to the lattices orientation disorder and dislocations, the enhanced point-defect scattering caused by the larger mass and radius fluctuations between Sb, Te and Se atoms and tiny particle size. What is more, the method of HTHP could save the synthesis time distinctly. Consequently, combining the method of HTHP with double-substituting could exhibit a higher thermoelectric performance, and optimize the thermoelectric properties significantly. The minimum value of thermal conductivity for Co4Sb11.3Te0.7 xSex is 1.8 W m 1 K 1 at room temperature. Acknowledgement This work was financially supported by National Natural Science Foundation of China (51171070, 51071074 and 51301024). References
Fig. 7. Synthesized pressure dependence of thermal conductivity for Co4Sb11.3Te0.7 xSex measured at room temperature.
Te–Se double-substituted in Co4Sb11.3Te0.7 xSex skutterudites play a large role, and the thermal conductivities exhibit a remarkable decrease with the synthesis pressure rise. Theoretically, the higher pressure could enforce the lattice defect scattering more strongly, and the thermal conductivity will lower. The disorder in lattices and dislocations (see Fig. 3) are probably the main reason [20]. As for the enhanced point-defect scattering, which caused by the larger mass (121.76, 127.60 and 78.96 for Sb, Te and Se, respectively) and ionic radius (76 pm, 97 pm, 50 pm for Sb, Te and Se, respectively) fluctuations between atoms in Co4Sb11.3Te0.7 xSex may be having an effect, too [21,22]. The point-defect can enhance the phonon scattering, so as to reduce thermal conductivity. Besides, the effect of the tiny particle size as shown in Fig. 3 on lattice thermal conductivity should not be neglected. The results indicated that high pressure and double-substituted could reduce the thermal conductivity effectively. The minimum value of thermal
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