Enhanced Thermoelectric Properties of Double-Filled CoSb3 via High

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Enhanced Thermoelectric Properties of Double-Filled CoSb3 via HighPressure Regulating Libin Wang,† Le Deng,*,† Jieming Qin,† and Xiaopeng Jia*,‡ †

Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China National Lab of Superhard Materials, Jinlin University, Changchun 130012, China

‡

S Supporting Information *

ABSTRACT: It has been discussed for a long time that synthetic pressure can effectively optimize thermoelectric properties. The beneficial effect of synthesis pressures on thermoelectric properties has been discussed for a long time. In this paper, it is theoretically and experimentally demonstrated that appropriate synthesis pressures can increase the figure of merit (ZT) through optimizing thermal transport and electronic transport properties. Indium and barium atoms double-filled CoSb3 samples were prepared use highpressure and high-temperature technique for half an hour. X-ray diffraction and some structure analysis were used to reveal the relationship between microstructures and thermoelectric properties. In0.15Ba0.35Co4Sb12 samples were synthesized by different pressures; sample synthesized by 3 GPa has the best electrical transport properties, and sample synthesized by 2.5 GPa has the lowest thermal conductivity. The maximum ZT value of sample synthesized by 3.0 GPa reached 1.18.

1. INTRODUCTION

depress lattice thermal conductivities while retaining higher electrical transport properties.16−19 High-pressure high-temperature (HPHT) method, which has been widely used to synthesize thermoelectric materials, has obtained a series of novel results.20,21 HPHT method has the characteristics of short synthesis time and no impurity phase, and some excellent thermoelectric properties under high pressures can be partially retained to atmospheric pressure. The α2/ρ and κ of thermoelectric materials can be mutually controlled. Our previous studies22 found that the α, ρ, and κ can be regulated by synthesis pressures. HPHT method combined with multi atoms filling can coordinate the thermoelectric properties of materials. The combination of HPHT and atoms double filling may optimize electrical and thermal transports simultaneously. In this study, In0.15Ba0.35Co4Sb12 (nominal composition) samples were synthesized with HPHT technology, and satisfactory results were obtained as expected.

With the continuous development of human society, the demand for new energy technologies is becoming more and more urgent. Thermoelectric material is such a material that meets the requirements of new energy technology, which can directly convert heat and electricity.1−4 The formula ZT = α2T/ ρκ is used to determine whether the thermoelectric properties of samples are excellent. In the formula, α represents Seebeck coefficient, ρ represents electrical resistivity, and κ represents thermal conductivity.5−7 This formula also shows that high electrical transport properties (α2/ρ) and low thermal transport property (κ) are necessary conditions for high-performance thermoelectric materials. In a number of thermoelectric materials, binary CoSb3 is one kind of development potential material for power generation applications. Although skutterudite CoSb3 materials have high Seebeck coefficients and electrical conductivities, high thermal conductivity prevents them from becoming enhanced performance thermoelectric materials. The κ of one kind of material contains heat transportation by carrier and heat transportation by phonons transmission. Improving the phonon scattering is the most effective way to reduce κ. In recent years, a number of new methods and techniques have been used in the optimized preparation process, and many materials with enhanced ZT values have been prepared.8−10 For CoSb3 materials with cage structure, single filling11,12 or double filling13−15 can scatter phonons effectively, while with the thermal conductivity reduced, some effects also appeared on the electrical transport performance. So far, nanostructures, point defect engineering, dispersing nanoparticles, and other methods were used to © XXXX American Chemical Society

2. EXPERIMENTAL SECTION The In and Ba double-filled CoSb3 samples were prepared with the elements of indium, barium, cobalt, and antimony (99.9% in purity) powders as sources. These samples were weighed according to the stoichiometry of In0.15Ba0.35Co4Sb12. After they were uniformly ground by an agate mortar in argon protection, the mixed powders were pressed into the 10 mm diameter cylinders. The samples, which prepared on a high-pressure apparatus (SPD 6 × 1200), were heated to 900 K for 25 min. The synthesis pressures were 2.5, 3.0, and 3.5 GPa. The sample was quenched after 25 min of HPHT synthesis Received: April 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b01110 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry process. X-ray diffraction (XRD) measurements with Cu Kα radiation were performed on an X-ray diffractometer (D/MAX-RA). The electrical transport properties including Seebeck coefficient and electrical conductivity were measured simultaneously by a ZEM-3 apparatus in the temperature range of 323−723 K. The thermal transport property was measured on a Netzsch LFA 427 Laser Flash Thermal Constants Measuring Apparatus in the temperature range of 323−723 K. The thermal conductivities were calculated with the formula κ = DCpρ, where D is thermal diffusivity, Cp (Figure S1) is specific heat, and ρ is density.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of In0.15Ba0.35Co4Sb12 samples prepared by different pressures. For In0.15Ba0.35Co4Sb12 sample

Figure 2. SEM images of In0.15Ba0.35Co4Sb12 bulks prepared by (a) 2.5 and (b) 3.0 GPa. EDS results of the selected areas and sample synthesized by 3.0 GPa (c). TEM, SAED (d) and HRTEM images (e) of In0.15Ba0.35Co4Sb12 sample synthesized by 3.0 GPa, and IFFT image (f) corresponding to (e).

which is mainly for further analysis of the structure of these fringes defects. To further reveal these defects, the inverse fast Fourier transform (IFFT) images are also analyzed. Figure 2f shows that a series of lattice structure disorder occurs in this region, which was caused by a slight shift of atoms from normal lattices. These disordered microstructures commonly exist inside the sample. This phenomenon also indicates that a large number of defects can be introduced by HPHT process. These defects can effectively prevent heat transfer in crystals. Although the sample synthesized by 2.5 GPa has more nanoparticles and smaller grain sizes than other samples, which could scatter phonons effectively, it also greatly increases its own resistivity. A large number of nanoparticles seriously hampered the transmission of electrons, and thermally excited minority carriers increased at the temperature of 550 K, which greatly reduced the Seebeck coefficient of the samples. Although the disordered microstructures commonly existed inside the sample synthesized by 3.0 GPa, an obvious enhancement of α2/ρ was obtained; this is because of the remarkable increase in α and almost unchanged ρ simultaneously. The lattice parameter, relative density, carrier concentration, and carrier mobility of In0.15Ba0.35Co4Sb12 were shown in Table 1. The lattice parameter increases with synthesis pressures. There is no regular relationship between relative density and synthetic pressures. The carrier concentration increases with synthesis pressures, while the carrier mobility decreased with the synthesis pressures. Figure 3a shows the electrical resistivity of In0.15Ba0.35Co4Sb12 samples prepared by different pressures between 323 and 723

Figure 1. XRD patterns of In0.15Ba0.35Co4Sb12 bulks.

synthesized by 2.5 GPa, a few tiny miscellaneous peaks were found. Through comparative analysis, these miscellaneous items are determined to be InSb and Ba2Sb3. Figure 1 also shows that all dominant phases are CoSb3 phase, and all peaks are indexed with the pure skutterudite CoSb3 with Im3̅ space group. It also shows that HPHT synthesis method can rapidly prepare skutterudite CoSb3 materials with half an hour. The fracture surface scanning electron microscopy (SEM) images of In0.15Ba0.35Co4Sb12 samples prepared at 2.5 and 3.0 GPa are shown in Figure 2a,b. A large amount of nanoparticle enrichment occurred in Figure 2a, while Figure 2b only shows a small amount of nanoparticles, and the grain sizes of In0.15Ba0.35Co4Sb12 prepared at 2.5 GPa are smaller than In0.15Ba0.35Co4Sb12 synthesized by 3.0 GPa. Fine grains and more abundant nanoscale will increase the resistivity of the sample. The energy-dispersive X-ray spectroscopy (EDS) of the selected regions 1, 2, and 3 in Figure 2a and sample synthesized by 3.0 GPa was shown in Figure 2c. Through the analysis of EDS and XRD, the nanoparticles were identified as InSb and Ba2Sb3. The bright-field transmission electron microscopy (TEM) image was shown in Figure 2d. To further study the crystal structure, selected area electron diffraction (SAED) was used for this sample, and the result was also shown in Figure 2d. Some fringes can be found in Figure 2d (white square marking), and the SAED pattern reveals that this area is polycrystalline. The corresponding microstructure was investigated by means of high-resolution (HR) TEM in Figure 2e, B

DOI: 10.1021/acs.inorgchem.8b01110 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

related to Fermi level, dispersion factor, and state density, etc. Different concentrations and kinds of defects, which were introduced by different synthetic pressures, have direct impact on these physical quantities. The discrepancy of Seebeck coefficient and electrical resistivity is similar to that of pure CoSb3 prepared by high pressures reported by Zhang et al.23 Figure 4a sho ws th e thermal co nduct ivity o f In0.15Ba0.35Co4Sb12 samples synthesized by different pressures between 323 and 723 K. For all samples, the higher the synthetic pressure was used, the higher the thermal conductivity shown. Moreover, the variation trend between thermal conductivity and temperature of all samples remains the same. The thermal conductivity of In0.15Ba0.35Co4Sb12 sample synthesized at 2.5 GPa, which reached 1.72 W m−1 K−1 at 723 K, was far less than other samples. The lattice thermal conductivity of In0.15Ba0.35Co4Sb12 samples was also shown in Figure 4a (hollow symbols). The κ is composed of electron thermal conductivity κe and lattice thermal conductivity κl. Among them, the κl could be effectively reduced by filling or doping. The κe calculations were obtained through Wiedemann−Franz law, κe = LσT,8,9 in which L = 2.0 × 10−8 W Ω K−2.20 The relationship between κl and temperature is the same as κ, and the difference is synthesis pressures have no significant effect on κl of these samples. After combining with the electrical and thermal properties, it is possible to select the appropriate pressure in high-pressure synthesis to optimize the electrical transport performance while retaining the reduced κl. The κl of In0.15Ba0.35Co4Sb12 prepared by 3.0 GPa reached a minimum value of 1.1 W m−1 K−1 at 723 K, which is lower than that of other samples. The main reason for the significant decrease in κl is that heat transfer phonons can be scattered by filled atoms rattling inside the oversized cages24 and the large number of defects introduced by highpressure synthesis. Usually, there are three kinds of phonon wavelengths, namely, long wavelengths, medium wavelengths, and short wavelengths. The three phonons (long wavelengths, medium wavelengths, and short wavelengths) can be scattered by the three structures (point defects and dislocations, nanoparticles, 0.1 to 3 μm particles) separately. Most grain sizes of samples (prepared by 2.5, 3.0, and 3.5 GPa) are all between 0.1 and 3 μm, so they have the same significant effect on long-wavelength phonons scattering. It can be seen from Figure 2a,b that, although sample prepared by 2.5 GPa contains more abundant nanoparticles, there are also many nanoparticles distributed in

Table 1. Lattice Parameter, Relative Density, Carrier Concentration, and Carrier Mobility for In0.15Ba0.35Co4Sb12

lattice parameter (Å) relative density (%) carrier concentration (1 × 1019 cm−3) carrier mobility (cm2V−1 s−1)

sample (2.5 GPa)

sample (3.0 GPa)

sample (3.5 GPa)

9.031 92 93.51 1.57

9.059 637 95.56 4.62

9.074 958 95.51 5.49

78.2

69.3

100.8

K. The electrical resistivity ρ of In0.15Ba0.35Co4Sb12 all rise first and then decrease with the test temperature. The gradual increase in the electrical resistivity of In0.15Ba0.35Co4Sb12 samples at low temperature (∼300−600 K) can be explained, as the carrier will be scattered more strongly by the lattice vibration when the temperature increases. After the electrical resistivity reached maximum value, the ρ decrease with increasing temperature was mainly due to intrinsic excitation. We also found that the ρ of In0.15Ba0.35Co4Sb12 decreases when synthesis pressures increase. Figure 3b shows the Seebeck coefficient of In0.15Ba0.35CoSb3 samples prepared by different pressures between 323 and 723 K. All In0.15Ba0.35CoSb3 samples are n-type semiconductors. Because the carriers occurred thermal excitation at 550 K, the Seebeck coefficient of In0.15Ba0.35Co4Sb12 sample synthesized by 2.5 GPa is lower than others. The absolute α of In0.15Ba0.35Co4Sb12 all increase first then decrease with the increase of temperature. This phenomenon is mainly due to the electrons transferred from valence band to conduction band could leave the same number of holes and which would reduce the absolute α.22 In all samples, the Seebeck coefficient value of In0.15Ba0.35Co4Sb12 sample synthesized at 3.0 GPa attains to the maximum value of −255.6 μV K−1. Although the regulation of synthetic pressure with Seebeck coefficient is not regular, the influence of synthesis pressures on Seebeck coefficient is remarkable. The nano barium compounds caused many barium atoms were not filled into the cage structures of skutterudite and further weakened the optimizing effect of indium barium double filling on the electrical conductivity and Seebeck coefficient of the material. These factors lead to the fact that the Seebeck coefficient of sample pressed 2.5 GPa is not the highest value but between sample pressed 3.0 GPa and sample pressed 3.5 GPa. Usually, thermoelectric materials have the character of high Seebeck coefficient corresponding to high resistivity. However, the Seebeck coefficient of materials is also

Figure 3. Ρ/T curve (a) and α/T curve (b) of In0.15Ba0.35Co4Sb12 samples prepared by different pressures. C

DOI: 10.1021/acs.inorgchem.8b01110 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 4. Temperature dependence of (a) thermal conductivity and lattice thermal conductivity (hollow symbols). (b) ZT values for In0.15Ba0.35Co4Sb12.

■

sample prepared by 3.0 GPa. Therefore, the mediumwavelength phonons of sample prepared by 3.0 GPa and sample prepared by 3.5 GPa will also be strongly scattered. Figure 2d−f showed that sample prepared by 3.0 GPa contains abundant point defects and dislocations, which could scatter short wavelengths phonons more effectively. Therefore, although the sample pressed by 2.5 GPa contains more nanoparticles and fractures than samples pressed by 3.0 GPa, the short-wave phonon scattering is still weaker than that of samples pressed by 3.0 GPa. In conclusion, the lattice thermal conductivity of sample pressed by 2.5 GPa is lower than that of other samples, mainly due to its weakness in the short-wave phonons scattered by defects and dislocations. Figure 4b shows the ZT values of In0.15Ba0.35Co4Sb12 samples synthesized by different pressures between 323 and 723 K. The ZT values of In0.15Ba0.35Co4Sb12 all increased monotonically with the increasing of temperature. Finally, the ZT value of In0.15Ba0.35Co4Sb12 prepared by 3 GPa reached 1.18 at 723 K; the main reason is due to the larger power factor (α2/ρ) of In0.15Ba0.35Co4Sb12 synthesized by 3 GPa.

Corresponding Authors

*Fax: 86-431-85583016. E-mail: [email protected]. (L.D.) *E-mail: [email protected]. (X.-P.J.) ORCID

Le Deng: 0000-0003-1483-0622 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by National Key Research and Development Program of China (No. 2016YFB0303805), National Science Foundation of China (Nos. 51301024, 11464035, and 51171070), and JiLin Provincial Science and Technology Department Project (Nos. 20150520026JH and 20170101045JC).

■

4. CONCLUSION In this study, the skutterudite compounds In0.15Ba0.35Co4Sb12 have been successfully prepared by HPHT technique, and the whole process only took 30 min. Synthesis pressures could optimize the electrical resistivity of samples significantly. All samples had low thermal conductivity, in which the sample synthesized by 3.0 GPa exhibited the best thermoelectric properties due to the better electrical performance it has. Finally, the maximum ZT value of In0.15Ba0.35Co4Sb12 sample synthesized by 3.0 GPa reached 1.18 at 723 K. The above results showed that the proper synthesis pressures can effectively optimize the thermoelectric properties of materials, and this strategy may be applied for improving ZT values of other filled skutterudite compounds or other kinds of thermoelectric materials.

■

AUTHOR INFORMATION

REFERENCES

(1) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457− 1461. (2) Pei, Y. Z.; LaLonde, A. D.; Heinz, N. A.; Shi, X. Y.; Iwanaga, S.; Wang, H.; et al. Stabilizing the Optimal Carrier Concentration for High Thermoelectric Efficiency. Adv. Mater. 2011, 23, 5674−5678. (3) Zhao, L. D.; Tan, G. J.; Hao, S. Q.; He, J. Q.; Pei, Y. L.; Chi, H.; et al. Ultrahigh power factor and thermoelectric performance in holedoped single-crystal SnSe. Science 2016, 351, 141. (4) Li, X. H.; Zhang, Q.; Kang, Y. L.; Chen, C.; Zhang, L.; Yu, D. L.; et al. High pressure synthesized Ca-filled CoSb3 skutterudites with enhanced thermoelectric properties. J. Alloys Compd. 2016, 677, 61− 65. (5) Peng, J.; He, J.; Alboni, P. N.; Tritt, T. M. Synthesis and Thermoelectric Properties of the Double-Filled Skutterudite Yb0.2InyCo4Sb12. J. Electron. Mater. 2009, 38, 981−984. (6) Wang, L.; Cai, K. F.; Wang, Y. Y.; Li, H.; Wang, H. F. Thermoelectric properties of indium-filled skutterudites prepared by combining solvothermal synthesis and melting. Appl. Phys. A: Mater. Sci. Process. 2009, 97, 841−845. (7) Jung, J. Y.; Park, K. H.; Ur, S. C.; Kim, I. H. Thermoelectric and Transport Properties of In-Filled CoSb3 Skutterudites. Mater. Sci. Forum 2010, 658, 17−20. (8) Oudah, M.; Kleinke, K. M.; Kleinke, H. Thermoelectric Properties of the Quaternary Chalcogenides BaCu5.9STe6 and BaCu5.9SeTe6. Inorg. Chem. 2015, 54, 845−849.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01110. Temperature dependence of heat capacity for In and Ba double-filled sample (PDF) D

DOI: 10.1021/acs.inorgchem.8b01110 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (9) Graff, J.; He, J.; Tritt, T. M. Qualifying the Role of Indium in the Multiple-Filled Ce0.1InxYb0.2Co4Sb12Skutterudite. Inorganics 2014, 2, 168. (10) Feng, B.; Xie, J.; Cao, G. S.; Zhu, T. J.; Zhao, X. B. Enhanced thermoelectric properties of p-type CoSb3/graphene nanocomposite. J. Mater. Chem. A 2013, 1, 13111−13119. (11) Zhao, X. Y.; Shi, X.; Chen, L. D.; Zhang, W. Q.; Zhang, W. B.; et al. Synthesis and thermoelectric properties of Sr-filled skutterudite SryCo4Sb12. J. Appl. Phys. 2006, 99, 053711. (12) He, T.; Chen, J.; Rosenfeld, H. D.; Subramanian, M. A. Thermoelectric properties of indium-filled skutterudites. Chem. Mater. 2006, 18, 759−762. (13) Bai, S. Q.; Pei, Y. Z.; Chen, L. D.; Zhang, W. Q.; Zhao, X. Y.; Yang, J. Enhanced thermoelectric performance of dual-element-filled skutterudites BaxCeyCo4Sb12. Acta Mater. 2009, 57, 3135−3139. (14) Li, H.; Tang, X. F.; Zhang, Q. J.; Uher, C. High performance InxCeyCo4Sb12 thermoelectric materials with in situforming nanostructured InSb phase. Appl. Phys. Lett. 2009, 94, 102114. (15) Peng, J. Y.; Alboni, P. N.; He, J.; Zhang, B.; Su, Z.; Holgat, T.; et al. Thermoelectric properties of (In,Yb) double-filled CoSb3 skutterudite. J. Appl. Phys. 2008, 104, 053710. (16) Poudel, B.; Hao, Q.; Ma, Y. C.; Lan, Y.; Minnich, A.; Yu, B.; et al. High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 2008, 320, 634. (17) Zebarjadi, M.; Joshi, G.; Zhu, G.; Yu, B.; Minnich, A.; Lan, Y. C.; et al. Power factor enhancement by modulation doping in bulk nanocomposites. Nano Lett. 2011, 11, 2225. (18) Yu, B.; Zebarjadi, M.; Wang, H.; Lukas, K.; Wang, H.; Wang, D.; et al. Enhancement of thermoelectric properties by modulation-doping in silicon germanium alloy nanocomposites. Nano Lett. 2012, 12, 2077. (19) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; et al. Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 2008, 321, 554. (20) Zhang, J.; Xu, B.; Wang, L. M.; Yu, D.; Liu, Z.; He, J.; et al. Great thermoelectric power factor enhancement of CoSb3 through the lightest metal element filling. Appl. Phys. Lett. 2011, 98, 072109. (21) Rogl, G.; Grytsiv, A.; Rogl, P.; Peranio, N.; Bauer, E.; Zehetbauer, M.; et al. n-Type skutterudites (R,Ba,Yb)yCo4Sb12 (R = Sr, La, Mm, DD, SrMm, SrDD) approaching ZT≈2.0. Acta Mater. 2014, 63, 30−43. (22) Deng, L.; Jia, X. P.; Su, T. C.; Jiang, Y. P.; Zheng, S. Z.; Guo, X.; et al. The thermoelectric properties of Co4Sb12‑xTex synthesized at different pressure. Mater. Lett. 2011, 65, 1057−1059. (23) Zhang, J. J.; Xu, B.; Yu, F. R.; Yu, D.; Liu, Z.; He, J.; Tian, Y. Thermoelectric properties of n-type CoSb3 fabricated with high pressure sintering. J. Alloys Compd. 2010, 503, 490−493. (24) Zhao, W. Y.; Wei, P.; Zhang, Q. J.; Dong, C. L.; Liu, L. S.; Tang, X. F. Enhanced thermoelectric performance in barium and indium double-filled skutterudite bulk materials via orbital hybridization induced by indium filler. J. Am. Chem. Soc. 2009, 131, 3713−3720.

E

DOI: 10.1021/acs.inorgchem.8b01110 Inorg. Chem. XXXX, XXX, XXX−XXX