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Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te
Journal of Alloys and Compounds 467 (2009) 31–34
Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te M. Chitroub a,∗ , F. Besse b , H. Scherrer b,1 a
Laboratoire des Sciences et G´enie des Mat´eriaux, D´epartement de M´etallurgie, Ecole Nationale Polytechnique 10, Avenue Hassen Badi, BP: 182 El-Harrach 16200 Alger, Algeria b Laboratoire de Physique des Mat´ eriaux, Ecole Nationale Sup´erieure des Mines de Nancy, Parc de Saurupt, 54042 Nancy, France Received 8 November 2007; received in revised form 26 November 2007; accepted 28 November 2007 Available online 8 December 2007
Abstract Hot-pressed samples of the semi-conducting compound CoSb3 -doped Pd and Te were prepared and characterized by X-ray and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 900 K. All samples had n-type conductivity. The dimensionless thermoelectric figure of merit ZT increases with increasing temperature and reaches a maximum value of 1 at 873 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Thermoelectric; Hot pressing; CoSb3 ; Skutterudite
1. Introduction A growth in the commercial applications of thermoelectric devices depends primarily on increasing the figure of merit ZT of the materials used in the devices. The figure of merit is defined as ZT = α2 σT/λ, where α is the Seebeck coefficient, σ the electrical conductivity, and λ the thermal conductivity. Thermoelectric materials used in power generation can be divided into three categories depending on their temperature range of application. Bismuth telluride and its alloys work around room temperature to about 500 K. In the intermediate temperature range (600–900 K), PbTe-based alloys and TAGS (Te–Ag–Ge–Sb) are the most efficient materials. At the highest temperatures (1000–1300 K), Si–Ge alloys are used in power generation devices mainly for space applications. Despite its low efficiency, FeSi2 has also been used in power generation. Thermoelectric devices are reliable, operate unattended in hostile environments
∗
Corresponding author. Tel.: +213 21 97 57 29; fax: +213 21 52 29 73. E-mail addresses: [email protected] (M. Chitroub), [email protected] (H. Scherrer). 1 Tel.: +33 3 83 58 41 61; fax: +33 3 83 58 41 61. 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.144
and are also environmentally friendly; but new more efficient materials are needed to expand their range of applications. Due to strong demand for thermoelectric materials in market, however, alternative materials are urgently required to replace PbTe-based alloys. The binary skutterudite compound cobalt triantimonide CoSb3 is particularly interesting because it displays interesting electrical properties. The overall performance of this material remains however low [1,2] due to an excessive value of its lattice thermal conductivity. By partially introducing guest atoms in the voids or cages of the host CoSb3 structure, the value of the lattice thermal conductivity can be significantly decreased as it has been shown for many different guest species. Many filler elements have been identified such as alkaline earths (Ba, Ca) [3,4], rare-earths (Ce, La, Eu, Yb, Nd) [5–9]. The thermoelectric performance of the partially filled skutterudites can still be improved through doping. For example, the substitution of Co by Ni was found to be particularly efficient [10]. The aim of the present study has therefore been to characterize the semi-conductor compound CoSb3 doped with 1% of Pd and 1% of Te prepared by hot pressing between room temperature and 873 K. The characterization has been done via electrical and thermal measurements of the Seebeck coefficient.
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M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
The results are discussed in terms of the optimal figure of merit of the material. 2. Experimental procedure Sample preparation—nearly single phase, polycrystalline samples of CoSb3 doped with 1 at.% of Pd and 1 at.% of Te were prepared. First, stoichiometric amounts of cobalt (99.998% pure), antimony shots (99.999% pure), palladium powder (99.998%) and tellurium shots (99.999%) were melted in sealed quartz ampoules. The melts were held at 1323 K for about 48 h for homogenization and quenched in water. Resulting ingots were ground in an agate mortar and analyzed by X-ray diffractometry (XRD). The powders were then sieved and only grains with a size of 100 m or less were retained for further processing. The presynthesized powders were then hot-pressed into cylindrical samples. The hot pressing was conducted in graphite dies. The temperature and pressure of hot pressing were 943 K and 100 MPa. The samples (∼12 mm in diameter and ∼2 cm long) were crack-free and of good mechanical strength. XRD analyses were performed on a Siemens D–500 diffractometer using Cu K␣ radiation with silicon as a standard. The chemical composition of the samples was analyzed by electron probe microanalysis (EPMA) that was performed using a CAMECA SX 50 electron superprobe. The standards used for composition analysis at the 1% level wavelength dispersive measurements were Co, Sb, Pd, and Te. Samples about 1 mm thick and 12 mm in diameter were cut from the hot-pressed specimens (perpendicular to the hot pressing direction). They were then mechanically polished (0.25 m) and ultrasonically cleaned in acetone. Samples characterization—this was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 873 K. The electrical resistivity (ρ) was measured using the Van der Pauw technique with a current of 100 mA using a special high temperature apparatus [11]. The Seebeck coefficient (α) of the samples was measured on the same samples as used for resistivity measurements using a high temperature light pulse technique [12]. The error of measurements of the Seebeck coefficient was estimated to be less than ±1%. The thermal conductivity (λ) of the samples was calculated from the measured density, heat capacity and thermal diffusivity values. The thermal diffusivity was measured using a flash diffusivity technique [13]. The heat capacity (C) was measured on several samples using a PerkinElmer differential scanning calorimeter under argon atmosphere and using sapphire as the reference standard. The mass of samples was ∼60 mg and a heating rate of 5 K mn−1 was employed. The overall error in the thermal conductivity value was at about ±10%.
3. Results and discussion We have measured the lattice parameter of Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample by X-ray diffraction and it is ˚ The literature value of CoSb3 is 9.036 A ˚ [14]. equal to 9.038 A. The small increase in the lattice constant may indeed indicate the presence of Pd and Te in the CoSb3 framework. However this could be due to only Te or only Pd substituting for Sb or Co, respectively. It may be due to the uncertainty in the measurements of the lattice constant. XRD profiles of Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample is shown in Fig. 1. The peak positions are being compared to those calculated from the unit cell determined by indexing the powder pattern. This comparison reveals that the skutterudite phases are well crystallized and homogeneous. Back-scattered electron image of the hot-pressed sample surface is shown in Fig. 2. The start composition of this sample is Co23.87 Sb73.88 Pd1.125 Te1.125 . Punctual analysis realized on the surface equal to 25 m2 shows that the actual average composition (at.%) of the matrix is Sb = 73.8625 ± 0.2315, Co = 23.9125 ± 0.1934, Pd = 1.0894 ± 0.0879, Te = 0.9542 ± 0.0965, and of the dark points is Sb = 97.5341 ± 0.2065,
Fig. 1. XRD profiles of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 .
Fig. 2. Back scattered image of the hot-pressed sample surface of Co23.87 Sb73.88 Pd1.125 Te1.125 .
Co = 2.9784 ± 0.1954. As can be seen, the density of our sample was good and the dark points are antimony aggregates. The plot of the electrical resistivity ρ versus the temperature is shown in Fig. 3. The minimum value (10 m) is at 873 K. Seebeck coefficients α of our hot-pressed sample show negative values, representing negative n-type conductivity. The absolute values of Seebeck coefficient |α| as function of the temperature for hot-pressed sample of Co23.87 Sb73.88 Pd1.125 Te1.125 is shown in Fig. 4. Between room temperature and 873 K the absolute value of Seebeck coefficient increases from 114 to 198 V/K.
Fig. 3. Electrical resistivity of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample, the current was perpendicular to the hot pressing direction.
M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
33
From Figs. 3 and 4, the electrical resistivity ρ is seen to have a shape similar to that of α as a function of temperature. This material shows behavior typical of heavily doped (degenerate) semiconductor or poor metal: relatively high resistivity that increases with temperature and a nearly linear increase in the absolute value of the Seebeck coefficient with increasing temperature. In this regime the carrier concentration changes little with temperature since all of the dopants are typically ionized, but excitation of electron hole pairs across the gap is not occurring. The reason for the increase in resistivity with temperature is not a decrease in carrier concentration. It is an increase in the electron scattering rate due in part to increased electron–phonon scattering at high temperature. The results on the thermal conductivity λ are presented in Fig. 5. They vary between 3.70 and 3.47 W/m K with steady minimum. The thermal conductivity is the sum of an electronic contribution (λe ) and a lattice thermal conductivity (λL ). At first, the electronic contribution can be substantial, because this material has relatively high electron densities, and after, as well as the discussion of the resistivity an increase in the electron scattering
rate due in part to increased electron–phonon scattering at high temperature decreases the electronic contribution. λe and ρ are related by the Weidmann–Franz law λe = L0 T/ρ valid for metals, with the lorentz number L0 = 2.44.10−8 W K−2 . If we consider this relation for our hot-pressed samples, λL of Co23.87 Sb73.88 Pd1.125 Te1.125 can be estimated. It is found equal to 2.6 W m−1 K−1 at room temperature. The literature value of λL of CoSb3 at room temperature is in order of 10 W m−1 K−1 [15]. This reduction of the lattice thermal conductivity can be attributed to that Pd and Te would enter the compound substitutionally, replacing Co and Sb, respectively [16,17]. An indication that substitutional doping is occurring is seen in the Seebeck coefficient. The negative values suggest n-type dopants are present in the CoSb3 network, which is intrinsically p-type. Anno et al. [18] and Caillat et al. [19] investigated the effects of doping with Pd, Pt, Ni, and Te on the transport properties of CoSb3 . They have noticed a significant decrease in the lattice thermal conductivity with the increase of carrier concentration. Their analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron–phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3 . The figure of merit Z = α2 /ρλ is plotted as a function of the temperature (Fig. 6). It can be seen that in n-type Co23.87 Sb73.88 Pd1.125 Te1.125 , Z is between 0.5 and 1.09 × 10−3 K−1 . The maximum value is equal 1.09 × 10−3 K−1 at 873 K. This value can be improved by elimination of Sb second phase inclusion if Seebeck coefficient is proportional to the surface of CoSb3 . The dimensionless thermoelectric figure of merit ZT is a function of the electrical resistivity, the Seebeck coefficient and the thermal conductivity (ZT = α2 T/ρλ). The calculated dimensionless figure of merit values for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples are shown in Fig. 7. The curve grows monotonously with temperature. The maximum ZT value of our samples is equal to 0.95 at 873 K. This value approaches 1 and thus the n-type Co23.87 Sb73.88 Pd1.125 Te1.125 is an efficient material. It could be used in a vari-
Fig. 5. Thermal conductivity as a function of the temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples.
Fig. 6. Figure of merit of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples as function of the temperature.
Fig. 4. The absolute values of Seebeck coefficient as function of temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample.
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M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
see a comparison of these materials with CoSb3 prepared under the same conditions. Acknowledgment The authors would like to thank Hamid Medjahed for preparation and thermoelectric property measurements. References
Fig. 7. Variation of the dimensionless thermoelectric figure of merit ZT as a function of temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples.
ety of applications including waste heat recovery from power plants and automobile industry. In the literature, the maximum ZT value is equal to 1.3 at 823 K for n-type In0.2 Co4 Sb12 [20]. Puyet et al. [21] have found ZT = 1 at 1073 K for n-type Ca0.15 Co3.98 Ni0.02 Sb11.80 , and ZT = 0.65 at 543 K for n-type Nd0.1 Co3.92 Ni0.08 Sb12 [15]. For n-type CoSb2.94 Te0.06 ZT is equal to 0.32 at 480 K [16]. 4. Conclusion In this study we performed the synthesis of CoSb3 doped with 1 at.% of (Pd + Te) by using hot-pressed method. It is clarified that we can obtain CoSb3 doped with 1 at.% of (Pd + Te) phase by this method. But there exists an excess of antimony in the hotpressed samples. Thermoelectric properties are comparable to the samples prepared by melt techniques. The discussion of these results leads us to believe that the values of Z can be improved. In fact, the theoretical factor of reduction of lattice part of the thermal conductivity is ten as compared to binary skutterudites [1,22] and the factor of our results is four. In conclusion, we need first, further optimization of the pulverizing and hot pressing conditions for better homogeneity of composition and second, to choose an appropriate additional elements to CoSb3 to strongly reduce the lattice thermal conductivity. A way to keep a high Seebeck coefficient and a low electrical resistivity is to reduce the carrier concentration by substituting. To see the effect of the Pd and Te doping, we need to
[1] C. Uher, Semiconductors and Semimetals, vol. 69, Academic Press, San Diego, 2001, pp. 139–253. [2] G.S. Nolas, D.T. Morelli, T.M. Tritt, Ann. Rev. Mater. Sci. 29 (1999) 89–116. [3] L.D. Chen, T. Kawahara, X.F. Tang, T.C. Goto, T. Hirai, J. Appl. Phys. 90 (2001) 1864–1868. [4] M. Puyet, B. Lenoir, A. Dauscher, M. Dehmas, C. Stiewe, E. M¨uller, J. Appl. Phys. 95 (2004) 4852–4855. [5] D.T. Morelli, G.P. Meiner, B. Chen, S. Hu, C. Uher, Phys. Rev. B 56 (1997) 7376–7383. [6] G.S. Nolas, J.L. Cohn, G.A. Slack, Phys. Rev. B 58 (1998) 164–170. [7] G.A. Lamberton Jr., S. Bhattacharya, R.T. Littleton IV, M.A. Kaeser, R.H. Tedstrom, T.M. Tritt, J. Yang, G.S. Nolas, Appl. Phys. Lett. 80 (2002) 598–600. [8] G.S. Nolas, M. Kaeser, R.T. Littleton IV, T.M. Tritt, Appl. Phys. Lett. 77 (2000) 1855–1857. [9] V.L. Kuznetsov, L.A. Kuznetsova, D.M. Roe, J. Phys. Condens. Matter 15 (2003) 5035–5048. [10] J.S. Dyck, W. Chen, C. Uher, L. Chen, X. Tang, J. Appl. Phys. 91 (2002) 3698–3705. [11] J. Mc Cormack, J.P. Fleurial, Modern Perspectives on Thermoelectric and Related Materials, vol. 234, Materials Research Society, Pittsburgh, 1991, 135-143. [12] C. Wood, D. Zoltan, G. Stapfer, Rev. Sci. Instrum. 56 (1985) 719–722. [13] J.W. Vandresande, C. Wood, A. Zoltan, D. Whittenberger, Thermal Conductivity, vol. 19, Plenium Press, New York, 1988, pp. 445–452. [14] M. Puyet, B. Lenoir, A. Dauscher, M. Dehmas, C.S. Stiewe, E. M¨uller, J. Appl. Phys. 95 (2004) 4852–4855. [15] V. Da Ros, B. Lenoir, A. Dauscher, C. Bellouard, J. Hejmanek, Proceedings of the Third European Conference on Thermoelectrics, Ecole Nationale Sup´erieure des Mines de Nancy, France, September 1–2, 2005, pp. 108–111. [16] K.T. Wojciechowski, Mater. Res. Bull. 37 (2002) 2023–2033. [17] K.T. Wojciechowski, J. Tobola, J. Leszcz´uski, J. Alloy Compd. 361 (2003) 19–27. [18] H. Anno, K. Matsubara, Y. Notohara, T. Sakakibara, H. Tashiro, J. Appl. Phys. 86 (1999) 3780–3786. [19] T. Caillat, A.J. Borshchevsky, P. Fleurial, J. Appl. Phys. 80 (1996) 4442–4449. [20] K. Akai, H. Kurisu, T. Shimura, M. Matsuura, XVI International Conference on Thermoelectric Dresden, Germany, August 26–29, 1997, pp. 334–337. [21] M. Puyet, B. Lenoir, A. Dauscher, H. Scherrer, M. Dehmas, C. Stiewe, E. M¨uller, J. Hejmanek, Proceedings of the Third European Conference on Thermoelectrics, Ecole Nationale Sup´erieure des Mines de Nancy, France, September 1–2, 2005, pp. 150–153. [22] D.T. Morelli, T. Caillat, J.P. Fleurial, A. Borshchevsky, J.W. Vandersande, B. Chen, C. Ulher, Phys. Rev. B 51 (1995) 9622–9628.
Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te M. Chitroub a,∗ , F. Besse b , H. Scherrer b,1 a
Laboratoire des Sciences et G´enie des Mat´eriaux, D´epartement de M´etallurgie, Ecole Nationale Polytechnique 10, Avenue Hassen Badi, BP: 182 El-Harrach 16200 Alger, Algeria b Laboratoire de Physique des Mat´ eriaux, Ecole Nationale Sup´erieure des Mines de Nancy, Parc de Saurupt, 54042 Nancy, France Received 8 November 2007; received in revised form 26 November 2007; accepted 28 November 2007 Available online 8 December 2007
Abstract Hot-pressed samples of the semi-conducting compound CoSb3 -doped Pd and Te were prepared and characterized by X-ray and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 900 K. All samples had n-type conductivity. The dimensionless thermoelectric figure of merit ZT increases with increasing temperature and reaches a maximum value of 1 at 873 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Thermoelectric; Hot pressing; CoSb3 ; Skutterudite
1. Introduction A growth in the commercial applications of thermoelectric devices depends primarily on increasing the figure of merit ZT of the materials used in the devices. The figure of merit is defined as ZT = α2 σT/λ, where α is the Seebeck coefficient, σ the electrical conductivity, and λ the thermal conductivity. Thermoelectric materials used in power generation can be divided into three categories depending on their temperature range of application. Bismuth telluride and its alloys work around room temperature to about 500 K. In the intermediate temperature range (600–900 K), PbTe-based alloys and TAGS (Te–Ag–Ge–Sb) are the most efficient materials. At the highest temperatures (1000–1300 K), Si–Ge alloys are used in power generation devices mainly for space applications. Despite its low efficiency, FeSi2 has also been used in power generation. Thermoelectric devices are reliable, operate unattended in hostile environments
∗
Corresponding author. Tel.: +213 21 97 57 29; fax: +213 21 52 29 73. E-mail addresses: [email protected] (M. Chitroub), [email protected] (H. Scherrer). 1 Tel.: +33 3 83 58 41 61; fax: +33 3 83 58 41 61. 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.144
and are also environmentally friendly; but new more efficient materials are needed to expand their range of applications. Due to strong demand for thermoelectric materials in market, however, alternative materials are urgently required to replace PbTe-based alloys. The binary skutterudite compound cobalt triantimonide CoSb3 is particularly interesting because it displays interesting electrical properties. The overall performance of this material remains however low [1,2] due to an excessive value of its lattice thermal conductivity. By partially introducing guest atoms in the voids or cages of the host CoSb3 structure, the value of the lattice thermal conductivity can be significantly decreased as it has been shown for many different guest species. Many filler elements have been identified such as alkaline earths (Ba, Ca) [3,4], rare-earths (Ce, La, Eu, Yb, Nd) [5–9]. The thermoelectric performance of the partially filled skutterudites can still be improved through doping. For example, the substitution of Co by Ni was found to be particularly efficient [10]. The aim of the present study has therefore been to characterize the semi-conductor compound CoSb3 doped with 1% of Pd and 1% of Te prepared by hot pressing between room temperature and 873 K. The characterization has been done via electrical and thermal measurements of the Seebeck coefficient.
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M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
The results are discussed in terms of the optimal figure of merit of the material. 2. Experimental procedure Sample preparation—nearly single phase, polycrystalline samples of CoSb3 doped with 1 at.% of Pd and 1 at.% of Te were prepared. First, stoichiometric amounts of cobalt (99.998% pure), antimony shots (99.999% pure), palladium powder (99.998%) and tellurium shots (99.999%) were melted in sealed quartz ampoules. The melts were held at 1323 K for about 48 h for homogenization and quenched in water. Resulting ingots were ground in an agate mortar and analyzed by X-ray diffractometry (XRD). The powders were then sieved and only grains with a size of 100 m or less were retained for further processing. The presynthesized powders were then hot-pressed into cylindrical samples. The hot pressing was conducted in graphite dies. The temperature and pressure of hot pressing were 943 K and 100 MPa. The samples (∼12 mm in diameter and ∼2 cm long) were crack-free and of good mechanical strength. XRD analyses were performed on a Siemens D–500 diffractometer using Cu K␣ radiation with silicon as a standard. The chemical composition of the samples was analyzed by electron probe microanalysis (EPMA) that was performed using a CAMECA SX 50 electron superprobe. The standards used for composition analysis at the 1% level wavelength dispersive measurements were Co, Sb, Pd, and Te. Samples about 1 mm thick and 12 mm in diameter were cut from the hot-pressed specimens (perpendicular to the hot pressing direction). They were then mechanically polished (0.25 m) and ultrasonically cleaned in acetone. Samples characterization—this was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 873 K. The electrical resistivity (ρ) was measured using the Van der Pauw technique with a current of 100 mA using a special high temperature apparatus [11]. The Seebeck coefficient (α) of the samples was measured on the same samples as used for resistivity measurements using a high temperature light pulse technique [12]. The error of measurements of the Seebeck coefficient was estimated to be less than ±1%. The thermal conductivity (λ) of the samples was calculated from the measured density, heat capacity and thermal diffusivity values. The thermal diffusivity was measured using a flash diffusivity technique [13]. The heat capacity (C) was measured on several samples using a PerkinElmer differential scanning calorimeter under argon atmosphere and using sapphire as the reference standard. The mass of samples was ∼60 mg and a heating rate of 5 K mn−1 was employed. The overall error in the thermal conductivity value was at about ±10%.
3. Results and discussion We have measured the lattice parameter of Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample by X-ray diffraction and it is ˚ The literature value of CoSb3 is 9.036 A ˚ [14]. equal to 9.038 A. The small increase in the lattice constant may indeed indicate the presence of Pd and Te in the CoSb3 framework. However this could be due to only Te or only Pd substituting for Sb or Co, respectively. It may be due to the uncertainty in the measurements of the lattice constant. XRD profiles of Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample is shown in Fig. 1. The peak positions are being compared to those calculated from the unit cell determined by indexing the powder pattern. This comparison reveals that the skutterudite phases are well crystallized and homogeneous. Back-scattered electron image of the hot-pressed sample surface is shown in Fig. 2. The start composition of this sample is Co23.87 Sb73.88 Pd1.125 Te1.125 . Punctual analysis realized on the surface equal to 25 m2 shows that the actual average composition (at.%) of the matrix is Sb = 73.8625 ± 0.2315, Co = 23.9125 ± 0.1934, Pd = 1.0894 ± 0.0879, Te = 0.9542 ± 0.0965, and of the dark points is Sb = 97.5341 ± 0.2065,
Fig. 1. XRD profiles of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 .
Fig. 2. Back scattered image of the hot-pressed sample surface of Co23.87 Sb73.88 Pd1.125 Te1.125 .
Co = 2.9784 ± 0.1954. As can be seen, the density of our sample was good and the dark points are antimony aggregates. The plot of the electrical resistivity ρ versus the temperature is shown in Fig. 3. The minimum value (10 m) is at 873 K. Seebeck coefficients α of our hot-pressed sample show negative values, representing negative n-type conductivity. The absolute values of Seebeck coefficient |α| as function of the temperature for hot-pressed sample of Co23.87 Sb73.88 Pd1.125 Te1.125 is shown in Fig. 4. Between room temperature and 873 K the absolute value of Seebeck coefficient increases from 114 to 198 V/K.
Fig. 3. Electrical resistivity of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample, the current was perpendicular to the hot pressing direction.
M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
33
From Figs. 3 and 4, the electrical resistivity ρ is seen to have a shape similar to that of α as a function of temperature. This material shows behavior typical of heavily doped (degenerate) semiconductor or poor metal: relatively high resistivity that increases with temperature and a nearly linear increase in the absolute value of the Seebeck coefficient with increasing temperature. In this regime the carrier concentration changes little with temperature since all of the dopants are typically ionized, but excitation of electron hole pairs across the gap is not occurring. The reason for the increase in resistivity with temperature is not a decrease in carrier concentration. It is an increase in the electron scattering rate due in part to increased electron–phonon scattering at high temperature. The results on the thermal conductivity λ are presented in Fig. 5. They vary between 3.70 and 3.47 W/m K with steady minimum. The thermal conductivity is the sum of an electronic contribution (λe ) and a lattice thermal conductivity (λL ). At first, the electronic contribution can be substantial, because this material has relatively high electron densities, and after, as well as the discussion of the resistivity an increase in the electron scattering
rate due in part to increased electron–phonon scattering at high temperature decreases the electronic contribution. λe and ρ are related by the Weidmann–Franz law λe = L0 T/ρ valid for metals, with the lorentz number L0 = 2.44.10−8 W K−2 . If we consider this relation for our hot-pressed samples, λL of Co23.87 Sb73.88 Pd1.125 Te1.125 can be estimated. It is found equal to 2.6 W m−1 K−1 at room temperature. The literature value of λL of CoSb3 at room temperature is in order of 10 W m−1 K−1 [15]. This reduction of the lattice thermal conductivity can be attributed to that Pd and Te would enter the compound substitutionally, replacing Co and Sb, respectively [16,17]. An indication that substitutional doping is occurring is seen in the Seebeck coefficient. The negative values suggest n-type dopants are present in the CoSb3 network, which is intrinsically p-type. Anno et al. [18] and Caillat et al. [19] investigated the effects of doping with Pd, Pt, Ni, and Te on the transport properties of CoSb3 . They have noticed a significant decrease in the lattice thermal conductivity with the increase of carrier concentration. Their analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron–phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3 . The figure of merit Z = α2 /ρλ is plotted as a function of the temperature (Fig. 6). It can be seen that in n-type Co23.87 Sb73.88 Pd1.125 Te1.125 , Z is between 0.5 and 1.09 × 10−3 K−1 . The maximum value is equal 1.09 × 10−3 K−1 at 873 K. This value can be improved by elimination of Sb second phase inclusion if Seebeck coefficient is proportional to the surface of CoSb3 . The dimensionless thermoelectric figure of merit ZT is a function of the electrical resistivity, the Seebeck coefficient and the thermal conductivity (ZT = α2 T/ρλ). The calculated dimensionless figure of merit values for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples are shown in Fig. 7. The curve grows monotonously with temperature. The maximum ZT value of our samples is equal to 0.95 at 873 K. This value approaches 1 and thus the n-type Co23.87 Sb73.88 Pd1.125 Te1.125 is an efficient material. It could be used in a vari-
Fig. 5. Thermal conductivity as a function of the temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples.
Fig. 6. Figure of merit of n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples as function of the temperature.
Fig. 4. The absolute values of Seebeck coefficient as function of temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed sample.
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M. Chitroub et al. / Journal of Alloys and Compounds 467 (2009) 31–34
see a comparison of these materials with CoSb3 prepared under the same conditions. Acknowledgment The authors would like to thank Hamid Medjahed for preparation and thermoelectric property measurements. References
Fig. 7. Variation of the dimensionless thermoelectric figure of merit ZT as a function of temperature for n-type Co23.87 Sb73.88 Pd1.125 Te1.125 hot-pressed samples.
ety of applications including waste heat recovery from power plants and automobile industry. In the literature, the maximum ZT value is equal to 1.3 at 823 K for n-type In0.2 Co4 Sb12 [20]. Puyet et al. [21] have found ZT = 1 at 1073 K for n-type Ca0.15 Co3.98 Ni0.02 Sb11.80 , and ZT = 0.65 at 543 K for n-type Nd0.1 Co3.92 Ni0.08 Sb12 [15]. For n-type CoSb2.94 Te0.06 ZT is equal to 0.32 at 480 K [16]. 4. Conclusion In this study we performed the synthesis of CoSb3 doped with 1 at.% of (Pd + Te) by using hot-pressed method. It is clarified that we can obtain CoSb3 doped with 1 at.% of (Pd + Te) phase by this method. But there exists an excess of antimony in the hotpressed samples. Thermoelectric properties are comparable to the samples prepared by melt techniques. The discussion of these results leads us to believe that the values of Z can be improved. In fact, the theoretical factor of reduction of lattice part of the thermal conductivity is ten as compared to binary skutterudites [1,22] and the factor of our results is four. In conclusion, we need first, further optimization of the pulverizing and hot pressing conditions for better homogeneity of composition and second, to choose an appropriate additional elements to CoSb3 to strongly reduce the lattice thermal conductivity. A way to keep a high Seebeck coefficient and a low electrical resistivity is to reduce the carrier concentration by substituting. To see the effect of the Pd and Te doping, we need to
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