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Thermoelectric properties of P-type Yb-filled skutterudite YbxFeyCo4-ySb12
Intermetallics 19 (2011) 1390e1393
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Thermoelectric properties of P-type Yb-filled skutterudite YbxFeyCo4-ySb12 Chen Zhou a, Donald Morelli a, *, Xiaoyuan Zhou b, Guoyu Wang b, Ctirad Uher b a b
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2011 Accepted 30 April 2011 Available online 12 June 2011
While intensive work has been done on n-type Yb filled skutterudites in the past, very little is known about their p-type counterparts for potential applications as thermoelectric materials. In this paper, we report a systematic study of high temperature thermoelectric transport properties of p-type Yb-filled Fe-compensated skutterudites YbxFeyCo4-ySb12 with the aim to complement the knowledge base for the Yb-filled skutterudite family. The highest ZTmax ¼ 0.6 was found in Yb0.6Fe2Co2Sb12 at 782 K. YbFe4Sb12 exhibits the second highest ZTmax ¼ 0.57 at 780 K, which is much higher than the previous estimate of 0.4 for the same composition. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Thermoelectric properties C. Crystal growth
1. Introduction Thermoelectric (TE) applications call for materials with high TE figure of merit (FOM) Z ¼ S2s/k, where S is the Seebeck coefficient, s the electric conductivity, and k the thermal conductivity. Among many potential TE materials, skutterudite compounds have been identified as good candidates [1]. The unfilled skutterudite has the form MA3, where M stands for the metal element Co, Fe, Ir and A signifies the pnicogen element, P, As, and Sb. The unit cell of a skutterudite is of the space group Im3 and consists of 32 atoms where six out of eight simple cubic metal sublattices are filled with a near square planar ring formed by four pnicogen atoms [2,3]. The large unit cell, heavy atoms, and complex structure imply a possible low thermal conductivity. Experimental measurements also revealed high carrier mobility in IrSb3, RhSb3, and CoSb3 [1,4,5]. Thus the characteristics of a good TE material have been met. The measured thermal conductivities in binary skutterudites, however, are only moderately small [6] and not low enough to yield a high ZT. One remedy for this drawback is to insert foreign atoms (usually rare-earth or alkaline-earth elements) inside the empty voids, thus creating the filled skutterudite RxM4A12 [7e9]. This approach was so successful that it has become the mainstream method in tuning the TE properties of skutterudites since Morelli et al. [10] first demonstrated experimentally an order of magnitude thermal conductivity reduction in CeFe4Sb12 compared to CoSb3. Today, it is not unusual for double-filled (filling using two different atomic species) n-type skutterudites to display ZTs greater than 1.3 [11]
* Corresponding author. Tel.: þ1 517 432 5453. E-mail address: [email protected] (D. Morelli). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.04.015
and single-filled, melt-spun YbxCo4Sb12 attaining similar values [12]. Compared to the well-documented and exciting news of n-type skutterudites, p-type skutterudites have been like a shadow in the corner with most reported ZTs hovering below unity. The filling elements are also limited mostly to cerium, lanthanum, or mischmetal which is a mixture of unseparated cerium, lanthanum and impurities. It was only recently that the didymium-filled p-type skutterudite finally broke the benchmark of ZT ¼ 1 [13]. Yb is one of the most widely used filling elements in n-type skutterudites, but knowledge on p-type Yb filled skutterudite is very limited. In this work, we report high temperature TE transport properties on a series of Yb filled and Fe compensated compounds with the nominal formula YbxFeyCo4-ySb12. With a refined synthesis method, we achieved a maximum ZT of 0.6 in Yb0.6Fe2Co2Sb12 at 782 K, and ZT of 0.57 in YbFe4Sb12 at 780 K. Such values are much higher than the numbers reported by previous workers [14,15]. We hope our work can complement the knowledge of Yb filled skutterudite family, as well as demonstrate the good thermoelectric properties that have previously been overlooked. 2. Experiment Pure element starting materials of Yb (pieces 99.9%), Fe (powder 99%), Co (powder 99.5%), and Sb (shot 99.999%) were weighed according to the stoichiometric ratio and placed inside graphite crucibles covered by graphite lids. The graphite crucibles were placed inside quartz ampoules and sealed under a vacuum of <105 Torr. The graphite crucible serves as a protection layer to deter the spontaneous reaction between Yb and the quartz ampoule, which is otherwise very detrimental to TE properties
C. Zhou et al. / Intermetallics 19 (2011) 1390e1393
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Table 1 Some physical properties of Yb filled Fe compensated p-type skutterudite sample YbxFeyCo4-ySb12 at room temperature. The lower and upper boundaries of estimated number of carrier/formula unit are calculated assuming a variant Yb valence of þ2 and þ3. Minus sign in estimated number of carrier/formula unit indicates the major carrier is electron. Sample ID
x ¼ 0.4
x ¼ 0.6
x ¼ 0.8
x¼1
Nominal composition Estimated No. of carrier per formula unit Observed No. of carrier per formula unit Carrier density at 300 K (1020 cm3) Carrier mobility at 300 K (cm2 V1 s1) Density (g cm3)
Yb0.4FeCo3Sb12 0.2w0.2 0.06 1.479 28.95 7.512
Yb0.6Fe2Co2Sb12 0.2w0.8 0.235 6.03 14 7.635
Yb0.8Fe3CoSb12 0.6w1.4 0.325 8.54 17.3 7.927
YbFe4Sb12 1w2 0.84 21.5 9.53 7.895
especially at high rare earth filling fractions. From our experience, this step is crucial in order to obtain high quality homogeneous skutterudite ingots. The sealed ampoules were heated to 1373 K at a rate slower than 0.5 K/min and held at that temperature for 6 h before rapid quenching in a cold water bath. The quenched samples were annealed at 923 K for 3 days. The annealed ingots were ball milled into fine powders and hot pressed at 873 K for 15 min. The densities of all hot pressed samples are above 95% of the theoretical density. A disk of approximately 1 mm in thickness was cut from the aspressed pellet for thermal diffusivity measurements. The balance of the pellet was sectioned into a rectangular parallelepiped with dimensions of approximately 2.4 mm 2.2 mm 8 mm for Seebeck coefficient and electrical resistivity measurements. Seebeck coefficient and electrical resistivity were measured from room temperature to 800 K under helium protection gas in an ULVAC ZEM-3 system. Thermal conductivity in the same temperature range was obtained from measurements of thermal diffusivity in a Netzsch LF457, specific heat in a Netzsch DSC 200F3, and the density was determined using Archimedes’ law. Another thin slab of dimensions approximately 2.4 mm 1 mm 8 mm was used for Hall measurements. Hall coefficient measurements from 60 K to 400 K were carried out using an AC current in a varying magnetic field from -3T to 3T in a Quantum Design Versalab system. Carrier concentration was calculated based on the Hall coefficient assuming a single carrier model with the scattering parameter of unity. X-ray diffraction patterns for all samples were collected on powders in the range from 20 to 90 of 2q in a Rigaku MiniFlex II using Cu Ka radiation.
3. Results and discussion Four samples of YbxFeyCo4-ySb12 were prepared with x ¼ 0.4, y ¼ 1; x ¼ 0.6, y ¼ 2; x ¼ 0.8, y ¼ 3; and x ¼ 1, y ¼ 4. Table 1 summarizes some basic physical properties of these samples. Here, for the sake of simplicity, we identify our samples by their Yb filling fraction x. Fig. 1 is the x-ray diffractogram for all samples. The characteristic peaks were indexed primarily to skutterudite phases with a trace of Sb impurity phase found in samples x ¼ 0.8 and x ¼ 1. The diffraction pattern of x ¼ 0.4 best matches the reference phase of CoSb3 PDF#03-065-0671 and sample x ¼ 1 best matches that of Yb0.93Fe4Sb12 PDF#00-056-1123, the skutterudite phase with the highest Yb filling fraction found in the database. Diffraction patterns for the remaining samples show a gradual transition from the binary CoSb3 phase to the Yb0.93Fe4Sb12 phase. Fig. 2 shows the lattice constant as a function of Yb filling fraction x. The lattice constant increases nearly linearly with the increasing amount of Yb, which is another good sign of successful rare earth filling. We must point out, however, that this linear expansion may not be the sole result of Yb filling, but a combination of Yb filling and Fe substitution for Co as the ionic radius for Fe2þ is larger than that of Co3þ. Nevertheless, the lattice constant for our x ¼ 1 sample is 9.153 Å which agrees well with 9.156 Å reported by Kuznetsov and Rowe for YbFe4Sb12 [15] and 9.154 Å by Berardan et al. [16]. Fig. 3 depicts the temperature dependence of the Seebeck coefficient. The Seebeck coefficient decreases with increasing Yb filling fraction x as a result of higher hole concentration induced by Fe compensation. The highest Seebeck coefficient was found in sample x ¼ 0.4 with its peak of 158 mV/K at 710 K. The Seebeck
9.16
Lattice constant (Å)
9.14 9.12 9.1 9.08 9.06 9.04 9.02 9 0
0.2
0.4
0.6
0.8
1
x
Fig. 1. X-ray diffraction patterns for all samples. (a) x ¼ 0.4; (b) x ¼ 0.6; (c) x ¼ 0.8; (d) x ¼ 1.
Fig. 2. Lattice constant as a function of filling fraction x. Sample x ¼ 0.4 best matches CoSb3 PDF#03-065-0671 while sample x ¼ 1 best matches Yb0.93Fe4Sb12 PDF#00-0561123. Lattice constants for x ¼ 0 and x ¼ 0.2 are from reference [3,20]. Diffractions for other samples exhibit a gradual transition.
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x=0.4 x=0.6 x=0.8 x=1 Ref
160 140 120
Power Factor (10-6 W cm-1 K-2 )
Seebeck Coefficient (μV K-1)
180
100 80 60 40 20 0 250
350
450
550
650
750
x=0.4 x=0.6 x=0.8 x=1 Ref1 Ref2
35 30 25 20 15 10 5 0
850
250
350
Temperature (K)
coefficient for x ¼ 1 agrees well with that reported by Kuznetsov and Rowe for YbFe4Sb12. Fig. 4 shows the electrical resistivity as a function of temperature. Resistivity also decreases with increasing x as a result of the increasing carrier concentration. Our x ¼ 1 sample shows a resistivity smaller by almost a factor of two compared to that measured by Kuznetsov and Rowe. Therefore, as shown in Fig. 5, the power factor in our x ¼ 1 sample is significantly enhanced compared to YbFe4Sb12 previously reported and is even on par with the power factor of Ce0.9Fe3.5Co0.5Sb12. Power factor decreases with decreasing x value and is mostly influenced by the electrical resistivity as the Seebeck coefficients are comparable in all samples except x ¼ 0.4. Fig. 6 is the temperature dependence of thermal conductivity. The lattice thermal conductivity was obtained by subtracting the electronic part from the total thermal conductivity. The electronic thermal conductivity was estimated by using Wiedemann-Franz law ke ¼ LsT where the Lorenz number L ¼ 2.44 108 WU/K2, a value we believe is reasonable for these degenerate semiconductors. Lattice thermal conductivities follow the 1/T relation and decrease with increasing Yb filling fraction except for sample x ¼ 0.8. The lowest lattice thermal conductivity was found to be 0.48 W/mK in sample x ¼ 1 at 710 K. This value is very close to the
Thermal conductivity (W m-1 K-1)
Electrical resistivity (Ω cm)
0.001
0.0005
0 350
450
550
650
750
850
theoretical limit of 0.2 W/mK that we have calculated assuming a phonon mean-free path equal to one interatomic spacing. Interestingly, the partially filled sample x ¼ 0.6 exhibits a smaller lattice thermal conductivity than the higher Yb filled sample x ¼ 0.8 and is very close to the fully filled sample x ¼ 1. Such a trend was observed previously in Ce-filled skutterudites at low temperatures. It seems to apply also in our case where a combination of different scattering processes (point defect, mass and size defect, and valence difference upon Fe substituting for Co) all contribute to the overall low lattice thermal conductivity [2,17,18]. Fig. 7 is the TE figure of merit ZT as a function of temperature. The maximum ZT was found to be 0.6 in sample x ¼ 0.6 at 782 K as a result of a moderately high power factor and low thermal conductivity. Sample x ¼ 1 exhibits the next highest ZT of 0.57. Such ZT values, although lower than observed in the Ce filled p-type skutterudites [19], are much higher than previously reported ZT of 0.4 in YbFe4Sb12. The room temperature ZTs for our x ¼ 0.6 and x ¼ 0.8 samples also double the values reported by others in materials of similar compositions [14]. We believe that this difference is primarily due to the lower resistivity of our samples, which is a consequence of the minimization of impurities and defects during the synthesis process.
750
850
Temperature (K) Fig. 4. Electrical resistivity as a function of temperature. Ref data are YbFe4Sb12 measured by Kuznetsov et al. [15].
x=0.4 x=0.6 x=0.8 x=1 1/T
3
5.5
0.0015
250
650
Fig. 5. Power factors as a function of temperature. Ref1 are data calculated based on YbFe4Sb12 measured by Kuznetsov et al. [15]. Ref2 is the power factor for Ce0.9Fe3.5Co0.5Sb12 synthesized by us as a control sample.
6
x=0.4 x=0.6 x=0.8 x=1 Ref
0.002
550
Temperature (K)
Fig. 3. Temperature dependence of Seebeck coefficient. Ref data are the Seebeck coefficients of YbFe4Sb12 measured by Kuznetsov et al. [15].
0.0025
450
2
5
1
4.5
0 250
4
450
650
850
3.5 3 2.5 2 1.5 1 250
350
450
550
650
750
850
Temperature (K) Fig. 6. Thermal conductivity as a function of temperature. The inset graph shows the lattice thermal conductivity. 1/T relation is plotted as an aid to understand the graph.
C. Zhou et al. / Intermetallics 19 (2011) 1390e1393
0.7
Acknowledgements
x=0.4 x=0.6 x=0.8 x=1 Ref1 Ref2 Ref3
0.6 0.5
This work is supported by the State of Michigan under a University Research Corridor seed grant. Sample synthesis and characterization are partially 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. The authors thank Dr. Long Zhang for providing data on Ce-filled skutterudite and helpful discussion. We also acknowledge Mr. Brian Wright for assistance in thermal diffusivity measurement.
ZT
0.4
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0.3 0.2 0.1
References
0 250
350
450
550
650
750
850
Temperature (K) Fig. 7. TE dimensionless figure of merit ZT as a function of temperature. Ref1 is the estimated highest ZT of YbFe4Sb12 by Kuznetsov et al. [15]. Ref2 (Yb0.8Fe4Sb12) and Ref3 (Yb0.5Fe2Co2Sb12) are near room temperature ZT reported by Bauer et al. [14].
4. Conclusion We have systematically investigated the high temperature TE transport properties of p-type Yb-filled Fe-compensated skutterudite YbxFeyCo4-ySb12. With the aid of a graphite crucible during synthesis, we are able to obtain high quality ingots with excellent reproducibility. The lattice thermal conductivity is significantly reduced to the level near the theoretical limit upon Yb filling. Fe substitution for Co also depresses the lattice thermal conductivity through additional phonon scattering mechanisms. The highest ZTmax is 0.6 in sample x ¼ 0.6 at 782 K. ZTs for other samples have also been significantly enhanced compared to published results on similar compositions. ZTs in high Yb filling fraction samples (x ¼ 0.8 and x ¼ 1) failed to exceed that of Ce filled skutterudites due to the high electronic thermal conductivity ke, which, in the case of YbFe4Sb12, accounts for 67% of the total thermal conductivity at room temperature and up to 80% at 780 K.
[1] Caillat T, Borshchevsky A, Fleurial JP. Investigations of several new advanced thermoelectric materials at the jet propulsion lab. Atlanta, GA, USA: Publ by SAE; 1993. 245e248. [2] Morelli DT, Meisner GP, Chen BX, Hu SQ, Uher C. Physical Review B 1997;56: 7376e83. [3] Rowe DM. Thermoelectrics Handbook MACRO TO NANO. CRC; 2005-12-09. [4] Caillat T, Fleurial JP, Borshchevsky A. Journal of Crystal Growth 1996;166:722e6. [5] Morelli DT, Caillat T, Fleurial JP, Borshchevsky A, Vandersande J, Chen B, et al. Physical Review B 1995;51:9622e8. [6] Slack GA, Tsoukala VG. Journal of Applied Physics 1994;76:1665e71. [7] Braun DJ, Jeitschko W. Journal of the Less-Common Metals 1980;72:147e56. [8] Braun DJ, Jeitschko W. Journal of Solid State Chemistry 1980;32:357e63. [9] Jeitschko W, Braun D. Acta Crystallographica Section B-Structural Science 1977;33:3401e6. [10] Morelli DT, Meisner GP. Journal of Applied Physics 1995;77:3777e81. [11] Shi X, Kong H, Li CP, Uher C, Yang J, Salvador JR, et al. Applied Physics Letters 2008;92:182101. [12] Li H, Tang X, Zhang Q, Uher C. Applied Physics Letters 2008;93:252109. [13] Rogl G, Grytsiv A, Rogl P, Bauer E, Zehetbauer M. Intermetallics 2011;19:546e55. [14] Bauer E, Galatanu A, Michor H, Hilscher G, Rogl P, Boulet P, et al. European Physical Journal B 2000;14:483e93. [15] Kuznetsov VL, Rowe DM. Journal of Physics-Condensed Matter 2000;12:7915e21. [16] Berardan D, Godart C, Alleno E, Berger S, Bauer E. Journal of Alloys and Compounds 2003;351:18e23. [17] Yang J, Meisner GP, Morelli DT, Uher C. Physical Review B 2001;63:014410. [18] Chen BX, Xu JH, Uher C, Morelli DT, Meisner GP, Fleurial JP, et al. Physical Review B 1997;55:1476e80. [19] Fleurial J-P, Borshchevsky A, Caillat T, Morelli DT, Meisner GP. High figure of merit in Ce-filled skutterudites. In: Proceedings of the 1996 15th International Conference on Thermoelectrics. Pasadena, CA, USA: ICT; March 26, 1996. p. 91e5. 1996-March 29, 1996. [20] Zhao XY, Shi X, Chen LD, Zhang WQ, Bai SQ, Pei YZ, et al. Applied Physics Letters 2006;89:092121.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Thermoelectric properties of P-type Yb-filled skutterudite YbxFeyCo4-ySb12 Chen Zhou a, Donald Morelli a, *, Xiaoyuan Zhou b, Guoyu Wang b, Ctirad Uher b a b
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2011 Accepted 30 April 2011 Available online 12 June 2011
While intensive work has been done on n-type Yb filled skutterudites in the past, very little is known about their p-type counterparts for potential applications as thermoelectric materials. In this paper, we report a systematic study of high temperature thermoelectric transport properties of p-type Yb-filled Fe-compensated skutterudites YbxFeyCo4-ySb12 with the aim to complement the knowledge base for the Yb-filled skutterudite family. The highest ZTmax ¼ 0.6 was found in Yb0.6Fe2Co2Sb12 at 782 K. YbFe4Sb12 exhibits the second highest ZTmax ¼ 0.57 at 780 K, which is much higher than the previous estimate of 0.4 for the same composition. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Thermoelectric properties C. Crystal growth
1. Introduction Thermoelectric (TE) applications call for materials with high TE figure of merit (FOM) Z ¼ S2s/k, where S is the Seebeck coefficient, s the electric conductivity, and k the thermal conductivity. Among many potential TE materials, skutterudite compounds have been identified as good candidates [1]. The unfilled skutterudite has the form MA3, where M stands for the metal element Co, Fe, Ir and A signifies the pnicogen element, P, As, and Sb. The unit cell of a skutterudite is of the space group Im3 and consists of 32 atoms where six out of eight simple cubic metal sublattices are filled with a near square planar ring formed by four pnicogen atoms [2,3]. The large unit cell, heavy atoms, and complex structure imply a possible low thermal conductivity. Experimental measurements also revealed high carrier mobility in IrSb3, RhSb3, and CoSb3 [1,4,5]. Thus the characteristics of a good TE material have been met. The measured thermal conductivities in binary skutterudites, however, are only moderately small [6] and not low enough to yield a high ZT. One remedy for this drawback is to insert foreign atoms (usually rare-earth or alkaline-earth elements) inside the empty voids, thus creating the filled skutterudite RxM4A12 [7e9]. This approach was so successful that it has become the mainstream method in tuning the TE properties of skutterudites since Morelli et al. [10] first demonstrated experimentally an order of magnitude thermal conductivity reduction in CeFe4Sb12 compared to CoSb3. Today, it is not unusual for double-filled (filling using two different atomic species) n-type skutterudites to display ZTs greater than 1.3 [11]
* Corresponding author. Tel.: þ1 517 432 5453. E-mail address: [email protected] (D. Morelli). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.04.015
and single-filled, melt-spun YbxCo4Sb12 attaining similar values [12]. Compared to the well-documented and exciting news of n-type skutterudites, p-type skutterudites have been like a shadow in the corner with most reported ZTs hovering below unity. The filling elements are also limited mostly to cerium, lanthanum, or mischmetal which is a mixture of unseparated cerium, lanthanum and impurities. It was only recently that the didymium-filled p-type skutterudite finally broke the benchmark of ZT ¼ 1 [13]. Yb is one of the most widely used filling elements in n-type skutterudites, but knowledge on p-type Yb filled skutterudite is very limited. In this work, we report high temperature TE transport properties on a series of Yb filled and Fe compensated compounds with the nominal formula YbxFeyCo4-ySb12. With a refined synthesis method, we achieved a maximum ZT of 0.6 in Yb0.6Fe2Co2Sb12 at 782 K, and ZT of 0.57 in YbFe4Sb12 at 780 K. Such values are much higher than the numbers reported by previous workers [14,15]. We hope our work can complement the knowledge of Yb filled skutterudite family, as well as demonstrate the good thermoelectric properties that have previously been overlooked. 2. Experiment Pure element starting materials of Yb (pieces 99.9%), Fe (powder 99%), Co (powder 99.5%), and Sb (shot 99.999%) were weighed according to the stoichiometric ratio and placed inside graphite crucibles covered by graphite lids. The graphite crucibles were placed inside quartz ampoules and sealed under a vacuum of <105 Torr. The graphite crucible serves as a protection layer to deter the spontaneous reaction between Yb and the quartz ampoule, which is otherwise very detrimental to TE properties
C. Zhou et al. / Intermetallics 19 (2011) 1390e1393
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Table 1 Some physical properties of Yb filled Fe compensated p-type skutterudite sample YbxFeyCo4-ySb12 at room temperature. The lower and upper boundaries of estimated number of carrier/formula unit are calculated assuming a variant Yb valence of þ2 and þ3. Minus sign in estimated number of carrier/formula unit indicates the major carrier is electron. Sample ID
x ¼ 0.4
x ¼ 0.6
x ¼ 0.8
x¼1
Nominal composition Estimated No. of carrier per formula unit Observed No. of carrier per formula unit Carrier density at 300 K (1020 cm3) Carrier mobility at 300 K (cm2 V1 s1) Density (g cm3)
Yb0.4FeCo3Sb12 0.2w0.2 0.06 1.479 28.95 7.512
Yb0.6Fe2Co2Sb12 0.2w0.8 0.235 6.03 14 7.635
Yb0.8Fe3CoSb12 0.6w1.4 0.325 8.54 17.3 7.927
YbFe4Sb12 1w2 0.84 21.5 9.53 7.895
especially at high rare earth filling fractions. From our experience, this step is crucial in order to obtain high quality homogeneous skutterudite ingots. The sealed ampoules were heated to 1373 K at a rate slower than 0.5 K/min and held at that temperature for 6 h before rapid quenching in a cold water bath. The quenched samples were annealed at 923 K for 3 days. The annealed ingots were ball milled into fine powders and hot pressed at 873 K for 15 min. The densities of all hot pressed samples are above 95% of the theoretical density. A disk of approximately 1 mm in thickness was cut from the aspressed pellet for thermal diffusivity measurements. The balance of the pellet was sectioned into a rectangular parallelepiped with dimensions of approximately 2.4 mm 2.2 mm 8 mm for Seebeck coefficient and electrical resistivity measurements. Seebeck coefficient and electrical resistivity were measured from room temperature to 800 K under helium protection gas in an ULVAC ZEM-3 system. Thermal conductivity in the same temperature range was obtained from measurements of thermal diffusivity in a Netzsch LF457, specific heat in a Netzsch DSC 200F3, and the density was determined using Archimedes’ law. Another thin slab of dimensions approximately 2.4 mm 1 mm 8 mm was used for Hall measurements. Hall coefficient measurements from 60 K to 400 K were carried out using an AC current in a varying magnetic field from -3T to 3T in a Quantum Design Versalab system. Carrier concentration was calculated based on the Hall coefficient assuming a single carrier model with the scattering parameter of unity. X-ray diffraction patterns for all samples were collected on powders in the range from 20 to 90 of 2q in a Rigaku MiniFlex II using Cu Ka radiation.
3. Results and discussion Four samples of YbxFeyCo4-ySb12 were prepared with x ¼ 0.4, y ¼ 1; x ¼ 0.6, y ¼ 2; x ¼ 0.8, y ¼ 3; and x ¼ 1, y ¼ 4. Table 1 summarizes some basic physical properties of these samples. Here, for the sake of simplicity, we identify our samples by their Yb filling fraction x. Fig. 1 is the x-ray diffractogram for all samples. The characteristic peaks were indexed primarily to skutterudite phases with a trace of Sb impurity phase found in samples x ¼ 0.8 and x ¼ 1. The diffraction pattern of x ¼ 0.4 best matches the reference phase of CoSb3 PDF#03-065-0671 and sample x ¼ 1 best matches that of Yb0.93Fe4Sb12 PDF#00-056-1123, the skutterudite phase with the highest Yb filling fraction found in the database. Diffraction patterns for the remaining samples show a gradual transition from the binary CoSb3 phase to the Yb0.93Fe4Sb12 phase. Fig. 2 shows the lattice constant as a function of Yb filling fraction x. The lattice constant increases nearly linearly with the increasing amount of Yb, which is another good sign of successful rare earth filling. We must point out, however, that this linear expansion may not be the sole result of Yb filling, but a combination of Yb filling and Fe substitution for Co as the ionic radius for Fe2þ is larger than that of Co3þ. Nevertheless, the lattice constant for our x ¼ 1 sample is 9.153 Å which agrees well with 9.156 Å reported by Kuznetsov and Rowe for YbFe4Sb12 [15] and 9.154 Å by Berardan et al. [16]. Fig. 3 depicts the temperature dependence of the Seebeck coefficient. The Seebeck coefficient decreases with increasing Yb filling fraction x as a result of higher hole concentration induced by Fe compensation. The highest Seebeck coefficient was found in sample x ¼ 0.4 with its peak of 158 mV/K at 710 K. The Seebeck
9.16
Lattice constant (Å)
9.14 9.12 9.1 9.08 9.06 9.04 9.02 9 0
0.2
0.4
0.6
0.8
1
x
Fig. 1. X-ray diffraction patterns for all samples. (a) x ¼ 0.4; (b) x ¼ 0.6; (c) x ¼ 0.8; (d) x ¼ 1.
Fig. 2. Lattice constant as a function of filling fraction x. Sample x ¼ 0.4 best matches CoSb3 PDF#03-065-0671 while sample x ¼ 1 best matches Yb0.93Fe4Sb12 PDF#00-0561123. Lattice constants for x ¼ 0 and x ¼ 0.2 are from reference [3,20]. Diffractions for other samples exhibit a gradual transition.
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40
x=0.4 x=0.6 x=0.8 x=1 Ref
160 140 120
Power Factor (10-6 W cm-1 K-2 )
Seebeck Coefficient (μV K-1)
180
100 80 60 40 20 0 250
350
450
550
650
750
x=0.4 x=0.6 x=0.8 x=1 Ref1 Ref2
35 30 25 20 15 10 5 0
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350
Temperature (K)
coefficient for x ¼ 1 agrees well with that reported by Kuznetsov and Rowe for YbFe4Sb12. Fig. 4 shows the electrical resistivity as a function of temperature. Resistivity also decreases with increasing x as a result of the increasing carrier concentration. Our x ¼ 1 sample shows a resistivity smaller by almost a factor of two compared to that measured by Kuznetsov and Rowe. Therefore, as shown in Fig. 5, the power factor in our x ¼ 1 sample is significantly enhanced compared to YbFe4Sb12 previously reported and is even on par with the power factor of Ce0.9Fe3.5Co0.5Sb12. Power factor decreases with decreasing x value and is mostly influenced by the electrical resistivity as the Seebeck coefficients are comparable in all samples except x ¼ 0.4. Fig. 6 is the temperature dependence of thermal conductivity. The lattice thermal conductivity was obtained by subtracting the electronic part from the total thermal conductivity. The electronic thermal conductivity was estimated by using Wiedemann-Franz law ke ¼ LsT where the Lorenz number L ¼ 2.44 108 WU/K2, a value we believe is reasonable for these degenerate semiconductors. Lattice thermal conductivities follow the 1/T relation and decrease with increasing Yb filling fraction except for sample x ¼ 0.8. The lowest lattice thermal conductivity was found to be 0.48 W/mK in sample x ¼ 1 at 710 K. This value is very close to the
Thermal conductivity (W m-1 K-1)
Electrical resistivity (Ω cm)
0.001
0.0005
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theoretical limit of 0.2 W/mK that we have calculated assuming a phonon mean-free path equal to one interatomic spacing. Interestingly, the partially filled sample x ¼ 0.6 exhibits a smaller lattice thermal conductivity than the higher Yb filled sample x ¼ 0.8 and is very close to the fully filled sample x ¼ 1. Such a trend was observed previously in Ce-filled skutterudites at low temperatures. It seems to apply also in our case where a combination of different scattering processes (point defect, mass and size defect, and valence difference upon Fe substituting for Co) all contribute to the overall low lattice thermal conductivity [2,17,18]. Fig. 7 is the TE figure of merit ZT as a function of temperature. The maximum ZT was found to be 0.6 in sample x ¼ 0.6 at 782 K as a result of a moderately high power factor and low thermal conductivity. Sample x ¼ 1 exhibits the next highest ZT of 0.57. Such ZT values, although lower than observed in the Ce filled p-type skutterudites [19], are much higher than previously reported ZT of 0.4 in YbFe4Sb12. The room temperature ZTs for our x ¼ 0.6 and x ¼ 0.8 samples also double the values reported by others in materials of similar compositions [14]. We believe that this difference is primarily due to the lower resistivity of our samples, which is a consequence of the minimization of impurities and defects during the synthesis process.
750
850
Temperature (K) Fig. 4. Electrical resistivity as a function of temperature. Ref data are YbFe4Sb12 measured by Kuznetsov et al. [15].
x=0.4 x=0.6 x=0.8 x=1 1/T
3
5.5
0.0015
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Fig. 5. Power factors as a function of temperature. Ref1 are data calculated based on YbFe4Sb12 measured by Kuznetsov et al. [15]. Ref2 is the power factor for Ce0.9Fe3.5Co0.5Sb12 synthesized by us as a control sample.
6
x=0.4 x=0.6 x=0.8 x=1 Ref
0.002
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Temperature (K)
Fig. 3. Temperature dependence of Seebeck coefficient. Ref data are the Seebeck coefficients of YbFe4Sb12 measured by Kuznetsov et al. [15].
0.0025
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3.5 3 2.5 2 1.5 1 250
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Temperature (K) Fig. 6. Thermal conductivity as a function of temperature. The inset graph shows the lattice thermal conductivity. 1/T relation is plotted as an aid to understand the graph.
C. Zhou et al. / Intermetallics 19 (2011) 1390e1393
0.7
Acknowledgements
x=0.4 x=0.6 x=0.8 x=1 Ref1 Ref2 Ref3
0.6 0.5
This work is supported by the State of Michigan under a University Research Corridor seed grant. Sample synthesis and characterization are partially 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. The authors thank Dr. Long Zhang for providing data on Ce-filled skutterudite and helpful discussion. We also acknowledge Mr. Brian Wright for assistance in thermal diffusivity measurement.
ZT
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1393
0.3 0.2 0.1
References
0 250
350
450
550
650
750
850
Temperature (K) Fig. 7. TE dimensionless figure of merit ZT as a function of temperature. Ref1 is the estimated highest ZT of YbFe4Sb12 by Kuznetsov et al. [15]. Ref2 (Yb0.8Fe4Sb12) and Ref3 (Yb0.5Fe2Co2Sb12) are near room temperature ZT reported by Bauer et al. [14].
4. Conclusion We have systematically investigated the high temperature TE transport properties of p-type Yb-filled Fe-compensated skutterudite YbxFeyCo4-ySb12. With the aid of a graphite crucible during synthesis, we are able to obtain high quality ingots with excellent reproducibility. The lattice thermal conductivity is significantly reduced to the level near the theoretical limit upon Yb filling. Fe substitution for Co also depresses the lattice thermal conductivity through additional phonon scattering mechanisms. The highest ZTmax is 0.6 in sample x ¼ 0.6 at 782 K. ZTs for other samples have also been significantly enhanced compared to published results on similar compositions. ZTs in high Yb filling fraction samples (x ¼ 0.8 and x ¼ 1) failed to exceed that of Ce filled skutterudites due to the high electronic thermal conductivity ke, which, in the case of YbFe4Sb12, accounts for 67% of the total thermal conductivity at room temperature and up to 80% at 780 K.
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