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High thermoelectric performance of In, Yb, Ce multiple filled CoSb3 based skutterudite compounds
Journal of Solid State Chemistry 193 (2012) 31–35
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
High thermoelectric performance of In, Yb, Ce multiple filled CoSb3 based skutterudite compounds Sedat Ballikaya a,b, Neslihan Uzar a, Saffettin Yildirim a, James R. Salvador c, Ctirad Uher b,n a
Department of Physics, University of Istanbul, 34134 Vezneciler, Istanbul, Turkey Department of Physics, University of Michigan, Ann Arbor, MI, 48109, USA c Chemical Sci. and Mater. Systems Laboratory, General Motors Global R&D Center, Warren, MI 48090, USA b
a r t i c l e i n f o
abstract
Available online 28 March 2012
Filling voids with rare earth atoms is an effective way to lowering thermal conductivity which necessarily enhances thermoelectric properties of skutterudite compounds. Yb atom is one of the most effective species among the rare earth atoms for filling the voids in the skutterudite structure due to a large atomic mass, radius and it is intermediate valence state. In this work, we aim to find the best filling partners for Yb using different combinations of Ce and In as well as to optimize actual filling fraction in order to achieve high values of ZT. The traditional method of synthesis relying on melting– annealing and followed by spark plasma sintering was used to prepare all samples. The thermoelectric properties of four samples of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4Sb12, and Yb0.3Ce0.15In0.2Co4Sb12 (nominal) were examined based on the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. Hall coefficient and Seebeck coefficient signs confirm that all samples are n-type skutterudite compounds. Carrier density increases with the increasing Yb þCe content. A high power factor value of 57.7 mW/K2/cm for Yb0.2Ce0.15Co4Sb12 and a lower thermal conductivity value of 2.82 W/m/K for Yb0.2Ce0.15In0.2Co4Sb12 indicate that small quantities of Ce with In may be a good partner to Yb to reduce the thermal conductivity further and thus enhance the thermoelectric performance of skutterudites. The highest ZT value of 1.43 was achieved for Yb0.2Ce0.15In0.2Co4Sb12 triple-filled skutterudite at 800 K. & 2012 Elsevier Inc. All rights reserved.
Keywords: Thermoelectric properties CoSb3 based skutterudites Multiple filled Thermal conductivity High ZT
1. Introduction Thermoelectric (TE) materials convert the heat energy directly into electric power or vice versa. Because of this ability, it is believed that they will play a pivotal role in future technologies utilizing waste industrial heat and converting it into useful electrical power [1]. The performance of a thermoelectric material is determined by the dimensionless figure of merit, ZT, defined as ZT ¼S2sT/k where S, s, k are the Seebeck coefficient, electrical conductivity, total thermal conductivity (k ¼ ke þ kL where ke is the electronic contribution and kL the lattice contribution ) and T stands for the absolute temperature. In other words, to maximize ZT values of TE materials, a high power factor (S2s) value and low thermal conductivity (k) are required. Since the transport parameters S, s, and k are mutually interdependent, it is not a simple task to enhance ZT values of bulk TE materials. The usual strategy is to attempt to lower thermal conductivity while maintaining very good electronic properties (high Seebeck coefficient and
n
Corresponding author. Fax: þ1 734 763 9694. E-mail address: [email protected] (C. Uher).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.03.029
large electrical conductivity). In its extreme form, this is the Phonon Glass Electron Crystal paradigm proposed by Slack [2]. Skutterudite compounds are one of the most prospective TE materials where this approach has proved useful [3]. Binary skutterudites crystallize is the body centered cubic structure which belongs to the space group (Im3). Compounds are represented by the chemical formula MX3, where M is a metal atom such as Co, Rh, or Ir and X is a pnicogen atom P, As, or Sb. The unit cell of skutterudites contains 32 atoms and two large voids that can be filled by foreign species, forming the so-called filled skutterudites [4]. Binary skutterudites have remarkably high carrier mobility compared with other TE materials, however they also possess a rather high thermal conductivity likely due to strong covalent bonding between pnicogen atoms. Filling voids with foreign species is an effective way of reducing the thermal conductivity of skutterudites. Filler atoms in voids are weakly bonded to pnicogen atoms and they behave like Einstein oscillators (rattlers) that scatter heat carrying phonons in a resonant fashion. Among skutterudites, CoSb3 has attracted most of the attention due to its high carrier mobility, relatively low thermal conductivity, good mechanical properties, and readily available chemical constituents [5]. During the past decade, theoretical and experimental efforts have focused on optimization of
32
S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
this compound by identifying most effective filler species (rare earths, alkaline earths, alkali metals and the others), exploring and establishing filling fraction limits and substituting on the metal or pnicogen lattice [6–18]. While all these approaches brought down the thermal conductivity to a more manageable level, it nevertheless still remains well above the estimated minimum thermal conductivity of 0.31 W/m/K [19] and represents the major factor limiting the thermoelectric performance. Although one cannot do much about the electronic part of the thermal conductivity which is tied via the Wiedemann–Franz law to the electrical conductivity, one should attack the lattice part of thermal conductivity. In this respect, the last couple of years have witnessed several innovative approaches being used to reduce the lattice part of the thermal conductivity of skutterudites further. Perhaps the most prospective is the nano-structuring approach where one hopes to create thermodynamically stable and finely dispersed nanometer-scale inclusions in the skutterudite matrix [20,21]. The other possibility proposed recently [22] makes use of Fe that is substituted in small quantities on the Co site. Provided the content of Fe is low enough, one relies on lowering the lattice thermal conductivity and enhancing the Seebeck coefficient sufficiently so that the combined effect of the two more than compensates for the anticipated decrease in the electrical conductivity. Another successful approach is based on multi-filling that makes use of more than one kind of a filler and thus broadens the range of phonon frequencies affected by ‘‘rattling’’ provided the mass and vibration frequency of the filler species are widely different [23–25]. This latter approach is also our motivation for preparing and studying triple-filled CoSb3-based skutterudite compounds using a combination of Yb, In and Ce fillers. Several groups have reported TE properties of double filled and triple-filled skutterudites with In, Ce and Yb fillers [20,26,27]. Among them, Graff et al. [26] prepared similar triple-filled skutterudites with a different content of Yb, Ce and In and they specifically commented on the influence of Ce on transport properties. While they reported on an observation of secondary phases like Yb2O3 and InSb and speculated about their effect on the thermal conductivity, they provided no information on the actual content of such impurity phases. Since secondary phases like Yb2O3 and InSb may change the actual content of the filler species which in turn affects transport parameters, it is important to determine the actual content of fillers in the compounds in order to achieve high values of ZT. In this work we carefully analyze the actual content of Yb, In, and Ce fillers and we show that we can achieve ZT values in excess of 1.4 at 800 K. This is one of the highest ZT values obtained on n-type skutterudite compounds prepared by the traditional method of melting–annealing followed by spark plasma sintering.
2. Experimental process
We use the following starting materials: Co (99.998% powder), Sb (99.9999% shot), In (99.9999 bar), Ce (99.98% ingot) and Yb (99.9% ingot). Their stoichiometric quantities were weighed in a glove box under protective atmosphere of argon. The charge was then loaded into graphite-coated quartz tubes and sealed under the vacuum of 10 4 Torr. The quartz tubes were then placed in a furnace and heated to 1100 1C at the rate of one degree per minute and held at that temperature for 10 h to ensure thorough mixing of the constituents. The temperature of the furnace was then decreased to 740 1C at a rate of four degrees per minute and held there for 10 days to complete the reaction. The furnace was finally turned off and cooled to room temperature. The resulting ingots were ground into fine powders in a glove box and cold pressed into pellets. The compacted pellets were then loaded into quartz tubes again, sealed under a vacuum of 10-4 Torr, heated to 740 1C at a rate of four degrees per minute, and annealed for two weeks to form pure skutterudite phase. The furnace was finally turned off and allowed to cool down to room temperature. The ingots were ground once again in a glove box into a fine powder and loaded into a cylindrical graphite die for Spark Plasma Sintering under a dynamic vacuum. The powders were first cold pressed under a pressure of 50 MPa then the pressure was reduced to 10 MPa. The samples were heated under this reduced load at a rate of 50 oC/min to a final temperature of 640 1C at which point the uniaxial pressure was increased to 50 MPa, with a subsequent isothermal/isobaric hold for 10 min. The power was shut off and the pressure removed and the sample was allowed to cool to room temperature over the course of 2 h. Compacted ingots were cut by a diamond blade saw into disks for thermal diffusivity measurements and into prismatic bars for transport characterization. 2.2. Measurement All high temperature transport measurements were carried out under the dynamic flow of argon in the range of 300–800 K. The Seebeck coefficient and electrical conductivity were measured using a home-built apparatus with a standard four probe potentiometric configuration. The total thermal conductivity k was calculated according to k ¼D r Cp, where D is the thermal diffusivity, Cp the specific heat capacity and r the bulk density. Thermal diffusivity was measured using a Flashline 5000 laserbased apparatus from Anter Corporation. Specific heat was measured by a differential scanning calorimeter (Netzsch-DSC 404 Pegasus) and density obtained from the Archimedes’ method is 93–95% of the theoretical value. High temperature AC Hall effect data (300–800 K) were gathered using our air-bore superconducting magnet (Oxford) that accommodates an oven and Hall effect insert. The measurements were carried out in fields of 71 T and extended from 300 K to 800 K.
2.1. Sample preparation 2.3. Structure analysis Double and triple filled YbxInyCezCo4Sb12 (0rx,y,zr0.18 actual) compounds were synthesized using a classical method of melting and long-term annealing followed by spark plasma sintering.
Phase identity and purity were confirmed by powder X-ray diffraction (PXRD) which was collected using a Scintag X1 XRD
Table 1 Nominal and actual compositions (EMPA) and some relevant transport data. Nominal compounds
Actual compounds (EMPA)
Elect. Cond. (S/cm)
Electron concent. (1020 cm 3 )
Electron mobility (cm2/Vs)
Seebeck coeff. (mV/K)
Power factor (mW/K2 m)
Thermal cond. (W/m/K)
Yb0.2Ce0.15Co4Sb12 Yb0.2In0.2Co4Sb12 Yb0.2In0.2Ce0.15Co4Sb12 Yb0.3In0.2Ce0.15Co4Sb12
Yb0.06Ce0.047Co4Sb11.9 Yb0.11In0.09Co4Sb11.87 Yb0.07In0.094Ce0.065Co4Sb11.92 Yb0.18In0.12Ce0.045Co4Sb11.98
2123 1827 1943 2547
3.49 2.88 5.77 6.56
38 40 21 24
124 146 121 106
33 32 29 29
3.66 3.58 3.49 3.46
S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
diffractometer and an Electron Microprobe Analyzer (Cameca SX100-EMPA) was used to obtain the actual chemical composition of all samples.
3. Results and discussion Nominal and actual compositions (EMPA) of samples and some room temperature transport parameters are presented in Table 1. The actual values of filling fractions of Yb, In and Ce are less than their nominal values. Sublimation of these elements during the
Fig. 1. XRD patterns for YbxInyCezCo4Sb12 compounds.
33
melting process and the formation of phases like Yb2O3, InSb, YbSb and CeSb in the skutterudite matrix are possible reasons for deviations in the nominal and actual stoichiometry. The presence of Yb2O3 and InSb is confirmed by powder X-ray diffraction analysis displayed in Fig. 1. The data confirm the main phase as being the skutterudite structure with the space group Im-3. A signature of the presence of InSb is seen in all In-containing compounds. Fig. 2 shows the EMPA (Electron Micro Probe Analysis) analysis pattern of the sample Yb0.07In0.094Ce0.065Co4Sb11.92. The BSE (Back Scattering Electron) image of the sample surface (500 mm size) indicates that there might be some other secondary phases (circled) in the skutterudite structure such as CeSb and YbSb even though they were not detected by XRD. We have observed similar secondary phases in other compounds that included Yb and Ce. The Hall coefficient of the compounds is shown in Fig. 3. The sign of the Hall coefficient indicates that all samples are n-type semiconductors. Calculated carrier densities (see inset in Fig. 2) using a single parabolic band model indicate an increasing electron concentration with the increasing filling fraction of Yb and Ce. This is not suprising since each Yb þ 2 and Ce þ 3 donate electrons to the skutterudite structure. Temperature dependence of the electrical conductivity is shown in Fig. 4. All samples display a behavior typical of heavily degenerate semiconductors. Electrical conductivity data indicate that Yb and Ce are more effective than In in enhancing the electrical conductivity. The temperature dependence of the Seebeck coefficient of YbxInyCezCo4Sb12 compounds is shown in Fig. 5. Negative sign of the Seebeck coefficient is in accord with the Hall data and confirms the electrons as being the major carrier in these compounds. Power factor versus temperature of the samples is displayed in Fig. 6. The highest power factor of 57.7 (mW/(K2/cm) is achieved with Yb0.06Ce0.047Co4Sb11.9 at 750 K due to the combination of high Seebeck coefficient and electrical conductivity. Total thermal
Fig. 2. EMPA (Electron Micro Probe Analysis) analysis for the Yb0.07In0.094Ce0.065Co4Sb11.92 compound.
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S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
Fig. 3. Hall coefficient and carrier density (inset) of YbxInyCezCo4Sb12 compounds.
Fig. 4. Temperature dependent electrical conductivity of samples.
Fig. 5. Seebeck coefficient versus temperature.
Fig. 6. Power factor of YbxInyCezCo4Sb12 compounds versus temperature.
Fig. 7. Total thermal conductivity and the lattice part of thermal conductivity (inset) versus temperature.
conductivity and its lattice contribution (inset in figure) are displayed in Fig. 7. Total thermal conductivity decreases with the increasing filling fraction likely due to enhanced resonant scattering of phonons. This is consistent with the data of the lattice thermal conductivity versus temperature shown in the inset of Fig.7. The lattice thermal conductivity was determined by subtracting the electronic thermal conductivity ke from the total thermal conductivity k assuming the validity of the Wiedemann–Franz law, ke ¼L0sT, where L0 is the Lorenz number, 2.45 10 8 V2 K 2. Increases in the total thermal conductivity of Yb0.18In0.12Ce0.045Co4Sb11.98 above 700 K are likely due to bipolar contribution as the influence of minority holes increases as one approaches the regime of intrinsic conduction. ZT values of YbxInyCezCo4Sb12 compounds with an average error bar of 15% are presented in Fig. 8. All samples exceed the value of unity above about 650 K. The highest value of the figure of merit ZT ¼1.43 at 800 K is observed for the Yb0.07In0.094Ce0.065Co4Sb11.92 triple filled skutterudite compound and the ZT vs. T curve of this compound shows no distinct tendency to peak implying that even higher values are possible at somewhat more elevated temperatures.
S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
35
17515 and 18190. The authors would like to thank Dr. Jihui Yang for making available to us a Spark Plasma Sintering instrument at the GM Research Laboratories for the densification of samples. We also acknowledge the use of an Electron Micro Probe Analysis (EMPA) instrument that was funded by the NSF Grant under the contract No: EAR-99-11352. Transport measurements conducted at the University of Michigan were supported as part of the Revolutionary Materials for Solid State Energy Conversion, an Energy frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Science under Award Number DE-SC001054.
References
Fig. 8. Thermoelectric figure of merit of YbxInyCezCo4Sb12 compounds versus temperature.
4. Conclusion YbxInyCezCo4Sb12 (0r x,y,zr0.18 actual) triple-filled skutterudites were prepared by a traditional melting and long-term annealing method that was followed by spark plasma sintering. Thermoelectric properties of the compounds were investigated in the temperature range of 300–800 K. The in-situ formed InSb, Sb and Yb2O3 phases are observed in In and Yb containing samples. The presence of these phases is the main reason for deviations in the nominal and actual composition of the compounds. Negative signs of Seebeck and Hall coefficients confirm the dominant role of electrons in all compounds. Based on the carrier density and electrical conductivity data, it is seen that Yb and Ce are more effective than In in enhancing electronic properties. The thermal conductivity is strongly suppressed as a result of multiple filling of Yb, Ce and In. Further additional content of Yb enhances both the electrical conductivity and the electronic part of thermal conductivity. Based on EMPA results and transport data, it follows that using small quantities of Ce and In with Yb (0rx,y,zr0.1 actual) is beneficial for the enhancement of TE performance of skutterudite compounds. The highest ZT¼1.43 was achieved with the Yb0.07In0.094Ce0.065Co4Sb11.92 compound at 800 K. This is one of the highest values of the figure of merit among n-type skutterudite compounds that were prepared using the traditional method of synthesis.
Acknowledgment This work was supported by Scientific Research Projects Coordination unit of Istanbul University with project number of
[1] D.M. Rowe (Ed.), CRC Press, Taylor & Francis, 2006. [2] G.A. Slack, in: D.M. Rowe (Ed.), CRC Handbook of Thermoelectricity, CRC Press, Boca Raton, 1995, p. 407. [3] C. Uher, Recent trends in thermoelectric materials research I, in: T.M. Tritt (Ed.), Semiconductors and Semimetals, 69, Academic Press, 2001, pp. 139–253. [4] W. Jeitschko, D.J. Brown, Acta Cryst., B 33 (1977) 3401–3406. [5] T. Caillat, A. Borschevsky, J.P. Fleurial, J. Appl. Phys. 80 (1996) 8. [6] G.S. Nolas, J.L. Cohn, G.A. Slack, Phys. Rev. B 58 (1998) 164. [7] D.T. Morelli, G.P. Meisner, B.X. Chen, S.Q. Hu, C. Uher, Phys. Rev. B 56 (1997) 7376. [8] G.S. Nolas, M. Littleton, T.M. Tritt, Appl. Phys. Lett. 77 (2000) 1855. [9] Y.Z. Pei, S.Q. Bai, X.Y. Zhao, W. Zhang, L.D. Chen Solid, State Sci. 10 (2008) 1422–1428. [10] L.D. Chen, T. Kawahara, X.F. Tang, T. Goto, T. Hirai, J.S. Dyck, W. Chen, C. Uher, J. Appl. Phys. 90 (2001) 1864. [11] X.Y. Zhao, X. Shi, L.D. Chen, W.Q. Zhang, W.B. Zhang, Y.Z. Pei, J. Appl. Phys. 99 (2006) 053711. [12] M. Puyet, B. Lenoir, A. Dauscher, P. Weisbecker, S.J. Clarke, J. Solid, State Chem. 177 (2004) 2138. [13] X. Shi, W. Zhang, L.D. Chen, J. Yang, C. Uher, Phys. Rev. B: 75 (2007) 235208. [14] B.C. Sales, B.C. Chakoumakos, D. Mandrus, Phys. Rev. B 61 (2000) 2475. [15] T. He, J.Z. Chen, H.D. Rosenfeld, M.A. Subramanian, Chem. Mater. 18 (2006) 759. [16] Y.Z. Pei, L.D. Chen, W. Zhang, X. Shi, S.Q. Bai, X.Y. Zhao, Z.G. Mei, X.Y. Li, Appl. Phys. Lett. 89 (2006) 221107. [17] J. Yang, L. Xi, W. Zhang, L.D. Chen, J.H. Yang, J. Electron. Mater. 38 (2009) 1397. [18] A. Zhou, L. Liu, P. Zhai, W. Zhao, Q. Zhang, J. Appl. Phys. 109 (2011) 113723. [19] G.S. Nolas, G.A. Slack, D.T. Morelli, T.M. Tritt, A.C. Ehrlich, J. Appl. Phys. 79 (1996) 4002. [20] H. Li, X. Tang, Q. Zhang, C. Uher, Appl. Phys. Lett. 94 (2009) 102114. [21] S. Ballikaya, G. Wang, K. Sun, C. Uher, J. Electron. Mater. 40 (2011) 570. [22] C. Uher, C.–P. Li, S. Ballikaya, J. Electron. Mater. 39 (2010) 2122. [23] J. Yang, W. Zhang, S.Q. Bai, Z. Mei, L.D. Chen, Appl. Phys. Lett. 90 (2007) 192111. [24] G. Rogl, A. Grytsiv, A. Grytsiv, N.M. Koblyuk, E. Bauer, S. Laumann, P. Rogl, J. Phys. Condens. Matter 23 (2011) 275601. [25] X. Shi, J. Yang, J.R. Salvador, M. Chi, J.Y. Cho, H. Wang, S. Bai, J. Yang, W. Zhang, L. Chen, J. Am. Chem. Soc. (2011) 133. [26] J. Graff, S. Zhu, T. Holgate, J. Peng, J. He, T.M. Tritt, J. Electron. Mater. 40 (2011) 696. [27] J.Y. Peng, J. He, Z. Su, P.N. Alboni, S. Zhu, T.M. Tritt, J. Appl. Phys. 105 (2009) 084907.
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
High thermoelectric performance of In, Yb, Ce multiple filled CoSb3 based skutterudite compounds Sedat Ballikaya a,b, Neslihan Uzar a, Saffettin Yildirim a, James R. Salvador c, Ctirad Uher b,n a
Department of Physics, University of Istanbul, 34134 Vezneciler, Istanbul, Turkey Department of Physics, University of Michigan, Ann Arbor, MI, 48109, USA c Chemical Sci. and Mater. Systems Laboratory, General Motors Global R&D Center, Warren, MI 48090, USA b
a r t i c l e i n f o
abstract
Available online 28 March 2012
Filling voids with rare earth atoms is an effective way to lowering thermal conductivity which necessarily enhances thermoelectric properties of skutterudite compounds. Yb atom is one of the most effective species among the rare earth atoms for filling the voids in the skutterudite structure due to a large atomic mass, radius and it is intermediate valence state. In this work, we aim to find the best filling partners for Yb using different combinations of Ce and In as well as to optimize actual filling fraction in order to achieve high values of ZT. The traditional method of synthesis relying on melting– annealing and followed by spark plasma sintering was used to prepare all samples. The thermoelectric properties of four samples of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4Sb12, and Yb0.3Ce0.15In0.2Co4Sb12 (nominal) were examined based on the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. Hall coefficient and Seebeck coefficient signs confirm that all samples are n-type skutterudite compounds. Carrier density increases with the increasing Yb þCe content. A high power factor value of 57.7 mW/K2/cm for Yb0.2Ce0.15Co4Sb12 and a lower thermal conductivity value of 2.82 W/m/K for Yb0.2Ce0.15In0.2Co4Sb12 indicate that small quantities of Ce with In may be a good partner to Yb to reduce the thermal conductivity further and thus enhance the thermoelectric performance of skutterudites. The highest ZT value of 1.43 was achieved for Yb0.2Ce0.15In0.2Co4Sb12 triple-filled skutterudite at 800 K. & 2012 Elsevier Inc. All rights reserved.
Keywords: Thermoelectric properties CoSb3 based skutterudites Multiple filled Thermal conductivity High ZT
1. Introduction Thermoelectric (TE) materials convert the heat energy directly into electric power or vice versa. Because of this ability, it is believed that they will play a pivotal role in future technologies utilizing waste industrial heat and converting it into useful electrical power [1]. The performance of a thermoelectric material is determined by the dimensionless figure of merit, ZT, defined as ZT ¼S2sT/k where S, s, k are the Seebeck coefficient, electrical conductivity, total thermal conductivity (k ¼ ke þ kL where ke is the electronic contribution and kL the lattice contribution ) and T stands for the absolute temperature. In other words, to maximize ZT values of TE materials, a high power factor (S2s) value and low thermal conductivity (k) are required. Since the transport parameters S, s, and k are mutually interdependent, it is not a simple task to enhance ZT values of bulk TE materials. The usual strategy is to attempt to lower thermal conductivity while maintaining very good electronic properties (high Seebeck coefficient and
n
Corresponding author. Fax: þ1 734 763 9694. E-mail address: [email protected] (C. Uher).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.03.029
large electrical conductivity). In its extreme form, this is the Phonon Glass Electron Crystal paradigm proposed by Slack [2]. Skutterudite compounds are one of the most prospective TE materials where this approach has proved useful [3]. Binary skutterudites crystallize is the body centered cubic structure which belongs to the space group (Im3). Compounds are represented by the chemical formula MX3, where M is a metal atom such as Co, Rh, or Ir and X is a pnicogen atom P, As, or Sb. The unit cell of skutterudites contains 32 atoms and two large voids that can be filled by foreign species, forming the so-called filled skutterudites [4]. Binary skutterudites have remarkably high carrier mobility compared with other TE materials, however they also possess a rather high thermal conductivity likely due to strong covalent bonding between pnicogen atoms. Filling voids with foreign species is an effective way of reducing the thermal conductivity of skutterudites. Filler atoms in voids are weakly bonded to pnicogen atoms and they behave like Einstein oscillators (rattlers) that scatter heat carrying phonons in a resonant fashion. Among skutterudites, CoSb3 has attracted most of the attention due to its high carrier mobility, relatively low thermal conductivity, good mechanical properties, and readily available chemical constituents [5]. During the past decade, theoretical and experimental efforts have focused on optimization of
32
S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
this compound by identifying most effective filler species (rare earths, alkaline earths, alkali metals and the others), exploring and establishing filling fraction limits and substituting on the metal or pnicogen lattice [6–18]. While all these approaches brought down the thermal conductivity to a more manageable level, it nevertheless still remains well above the estimated minimum thermal conductivity of 0.31 W/m/K [19] and represents the major factor limiting the thermoelectric performance. Although one cannot do much about the electronic part of the thermal conductivity which is tied via the Wiedemann–Franz law to the electrical conductivity, one should attack the lattice part of thermal conductivity. In this respect, the last couple of years have witnessed several innovative approaches being used to reduce the lattice part of the thermal conductivity of skutterudites further. Perhaps the most prospective is the nano-structuring approach where one hopes to create thermodynamically stable and finely dispersed nanometer-scale inclusions in the skutterudite matrix [20,21]. The other possibility proposed recently [22] makes use of Fe that is substituted in small quantities on the Co site. Provided the content of Fe is low enough, one relies on lowering the lattice thermal conductivity and enhancing the Seebeck coefficient sufficiently so that the combined effect of the two more than compensates for the anticipated decrease in the electrical conductivity. Another successful approach is based on multi-filling that makes use of more than one kind of a filler and thus broadens the range of phonon frequencies affected by ‘‘rattling’’ provided the mass and vibration frequency of the filler species are widely different [23–25]. This latter approach is also our motivation for preparing and studying triple-filled CoSb3-based skutterudite compounds using a combination of Yb, In and Ce fillers. Several groups have reported TE properties of double filled and triple-filled skutterudites with In, Ce and Yb fillers [20,26,27]. Among them, Graff et al. [26] prepared similar triple-filled skutterudites with a different content of Yb, Ce and In and they specifically commented on the influence of Ce on transport properties. While they reported on an observation of secondary phases like Yb2O3 and InSb and speculated about their effect on the thermal conductivity, they provided no information on the actual content of such impurity phases. Since secondary phases like Yb2O3 and InSb may change the actual content of the filler species which in turn affects transport parameters, it is important to determine the actual content of fillers in the compounds in order to achieve high values of ZT. In this work we carefully analyze the actual content of Yb, In, and Ce fillers and we show that we can achieve ZT values in excess of 1.4 at 800 K. This is one of the highest ZT values obtained on n-type skutterudite compounds prepared by the traditional method of melting–annealing followed by spark plasma sintering.
2. Experimental process
We use the following starting materials: Co (99.998% powder), Sb (99.9999% shot), In (99.9999 bar), Ce (99.98% ingot) and Yb (99.9% ingot). Their stoichiometric quantities were weighed in a glove box under protective atmosphere of argon. The charge was then loaded into graphite-coated quartz tubes and sealed under the vacuum of 10 4 Torr. The quartz tubes were then placed in a furnace and heated to 1100 1C at the rate of one degree per minute and held at that temperature for 10 h to ensure thorough mixing of the constituents. The temperature of the furnace was then decreased to 740 1C at a rate of four degrees per minute and held there for 10 days to complete the reaction. The furnace was finally turned off and cooled to room temperature. The resulting ingots were ground into fine powders in a glove box and cold pressed into pellets. The compacted pellets were then loaded into quartz tubes again, sealed under a vacuum of 10-4 Torr, heated to 740 1C at a rate of four degrees per minute, and annealed for two weeks to form pure skutterudite phase. The furnace was finally turned off and allowed to cool down to room temperature. The ingots were ground once again in a glove box into a fine powder and loaded into a cylindrical graphite die for Spark Plasma Sintering under a dynamic vacuum. The powders were first cold pressed under a pressure of 50 MPa then the pressure was reduced to 10 MPa. The samples were heated under this reduced load at a rate of 50 oC/min to a final temperature of 640 1C at which point the uniaxial pressure was increased to 50 MPa, with a subsequent isothermal/isobaric hold for 10 min. The power was shut off and the pressure removed and the sample was allowed to cool to room temperature over the course of 2 h. Compacted ingots were cut by a diamond blade saw into disks for thermal diffusivity measurements and into prismatic bars for transport characterization. 2.2. Measurement All high temperature transport measurements were carried out under the dynamic flow of argon in the range of 300–800 K. The Seebeck coefficient and electrical conductivity were measured using a home-built apparatus with a standard four probe potentiometric configuration. The total thermal conductivity k was calculated according to k ¼D r Cp, where D is the thermal diffusivity, Cp the specific heat capacity and r the bulk density. Thermal diffusivity was measured using a Flashline 5000 laserbased apparatus from Anter Corporation. Specific heat was measured by a differential scanning calorimeter (Netzsch-DSC 404 Pegasus) and density obtained from the Archimedes’ method is 93–95% of the theoretical value. High temperature AC Hall effect data (300–800 K) were gathered using our air-bore superconducting magnet (Oxford) that accommodates an oven and Hall effect insert. The measurements were carried out in fields of 71 T and extended from 300 K to 800 K.
2.1. Sample preparation 2.3. Structure analysis Double and triple filled YbxInyCezCo4Sb12 (0rx,y,zr0.18 actual) compounds were synthesized using a classical method of melting and long-term annealing followed by spark plasma sintering.
Phase identity and purity were confirmed by powder X-ray diffraction (PXRD) which was collected using a Scintag X1 XRD
Table 1 Nominal and actual compositions (EMPA) and some relevant transport data. Nominal compounds
Actual compounds (EMPA)
Elect. Cond. (S/cm)
Electron concent. (1020 cm 3 )
Electron mobility (cm2/Vs)
Seebeck coeff. (mV/K)
Power factor (mW/K2 m)
Thermal cond. (W/m/K)
Yb0.2Ce0.15Co4Sb12 Yb0.2In0.2Co4Sb12 Yb0.2In0.2Ce0.15Co4Sb12 Yb0.3In0.2Ce0.15Co4Sb12
Yb0.06Ce0.047Co4Sb11.9 Yb0.11In0.09Co4Sb11.87 Yb0.07In0.094Ce0.065Co4Sb11.92 Yb0.18In0.12Ce0.045Co4Sb11.98
2123 1827 1943 2547
3.49 2.88 5.77 6.56
38 40 21 24
124 146 121 106
33 32 29 29
3.66 3.58 3.49 3.46
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diffractometer and an Electron Microprobe Analyzer (Cameca SX100-EMPA) was used to obtain the actual chemical composition of all samples.
3. Results and discussion Nominal and actual compositions (EMPA) of samples and some room temperature transport parameters are presented in Table 1. The actual values of filling fractions of Yb, In and Ce are less than their nominal values. Sublimation of these elements during the
Fig. 1. XRD patterns for YbxInyCezCo4Sb12 compounds.
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melting process and the formation of phases like Yb2O3, InSb, YbSb and CeSb in the skutterudite matrix are possible reasons for deviations in the nominal and actual stoichiometry. The presence of Yb2O3 and InSb is confirmed by powder X-ray diffraction analysis displayed in Fig. 1. The data confirm the main phase as being the skutterudite structure with the space group Im-3. A signature of the presence of InSb is seen in all In-containing compounds. Fig. 2 shows the EMPA (Electron Micro Probe Analysis) analysis pattern of the sample Yb0.07In0.094Ce0.065Co4Sb11.92. The BSE (Back Scattering Electron) image of the sample surface (500 mm size) indicates that there might be some other secondary phases (circled) in the skutterudite structure such as CeSb and YbSb even though they were not detected by XRD. We have observed similar secondary phases in other compounds that included Yb and Ce. The Hall coefficient of the compounds is shown in Fig. 3. The sign of the Hall coefficient indicates that all samples are n-type semiconductors. Calculated carrier densities (see inset in Fig. 2) using a single parabolic band model indicate an increasing electron concentration with the increasing filling fraction of Yb and Ce. This is not suprising since each Yb þ 2 and Ce þ 3 donate electrons to the skutterudite structure. Temperature dependence of the electrical conductivity is shown in Fig. 4. All samples display a behavior typical of heavily degenerate semiconductors. Electrical conductivity data indicate that Yb and Ce are more effective than In in enhancing the electrical conductivity. The temperature dependence of the Seebeck coefficient of YbxInyCezCo4Sb12 compounds is shown in Fig. 5. Negative sign of the Seebeck coefficient is in accord with the Hall data and confirms the electrons as being the major carrier in these compounds. Power factor versus temperature of the samples is displayed in Fig. 6. The highest power factor of 57.7 (mW/(K2/cm) is achieved with Yb0.06Ce0.047Co4Sb11.9 at 750 K due to the combination of high Seebeck coefficient and electrical conductivity. Total thermal
Fig. 2. EMPA (Electron Micro Probe Analysis) analysis for the Yb0.07In0.094Ce0.065Co4Sb11.92 compound.
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S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
Fig. 3. Hall coefficient and carrier density (inset) of YbxInyCezCo4Sb12 compounds.
Fig. 4. Temperature dependent electrical conductivity of samples.
Fig. 5. Seebeck coefficient versus temperature.
Fig. 6. Power factor of YbxInyCezCo4Sb12 compounds versus temperature.
Fig. 7. Total thermal conductivity and the lattice part of thermal conductivity (inset) versus temperature.
conductivity and its lattice contribution (inset in figure) are displayed in Fig. 7. Total thermal conductivity decreases with the increasing filling fraction likely due to enhanced resonant scattering of phonons. This is consistent with the data of the lattice thermal conductivity versus temperature shown in the inset of Fig.7. The lattice thermal conductivity was determined by subtracting the electronic thermal conductivity ke from the total thermal conductivity k assuming the validity of the Wiedemann–Franz law, ke ¼L0sT, where L0 is the Lorenz number, 2.45 10 8 V2 K 2. Increases in the total thermal conductivity of Yb0.18In0.12Ce0.045Co4Sb11.98 above 700 K are likely due to bipolar contribution as the influence of minority holes increases as one approaches the regime of intrinsic conduction. ZT values of YbxInyCezCo4Sb12 compounds with an average error bar of 15% are presented in Fig. 8. All samples exceed the value of unity above about 650 K. The highest value of the figure of merit ZT ¼1.43 at 800 K is observed for the Yb0.07In0.094Ce0.065Co4Sb11.92 triple filled skutterudite compound and the ZT vs. T curve of this compound shows no distinct tendency to peak implying that even higher values are possible at somewhat more elevated temperatures.
S. Ballikaya et al. / Journal of Solid State Chemistry 193 (2012) 31–35
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17515 and 18190. The authors would like to thank Dr. Jihui Yang for making available to us a Spark Plasma Sintering instrument at the GM Research Laboratories for the densification of samples. We also acknowledge the use of an Electron Micro Probe Analysis (EMPA) instrument that was funded by the NSF Grant under the contract No: EAR-99-11352. Transport measurements conducted at the University of Michigan were supported as part of the Revolutionary Materials for Solid State Energy Conversion, an Energy frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Science under Award Number DE-SC001054.
References
Fig. 8. Thermoelectric figure of merit of YbxInyCezCo4Sb12 compounds versus temperature.
4. Conclusion YbxInyCezCo4Sb12 (0r x,y,zr0.18 actual) triple-filled skutterudites were prepared by a traditional melting and long-term annealing method that was followed by spark plasma sintering. Thermoelectric properties of the compounds were investigated in the temperature range of 300–800 K. The in-situ formed InSb, Sb and Yb2O3 phases are observed in In and Yb containing samples. The presence of these phases is the main reason for deviations in the nominal and actual composition of the compounds. Negative signs of Seebeck and Hall coefficients confirm the dominant role of electrons in all compounds. Based on the carrier density and electrical conductivity data, it is seen that Yb and Ce are more effective than In in enhancing electronic properties. The thermal conductivity is strongly suppressed as a result of multiple filling of Yb, Ce and In. Further additional content of Yb enhances both the electrical conductivity and the electronic part of thermal conductivity. Based on EMPA results and transport data, it follows that using small quantities of Ce and In with Yb (0rx,y,zr0.1 actual) is beneficial for the enhancement of TE performance of skutterudite compounds. The highest ZT¼1.43 was achieved with the Yb0.07In0.094Ce0.065Co4Sb11.92 compound at 800 K. This is one of the highest values of the figure of merit among n-type skutterudite compounds that were prepared using the traditional method of synthesis.
Acknowledgment This work was supported by Scientific Research Projects Coordination unit of Istanbul University with project number of
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