Effect of Pb Filling and Synthesis Pressure Regulation on the

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

Effect of Pb Filling and Synthesis Pressure Regulation on the Thermoelectric Properties of CoSb3 Le Deng,* Dong-Ni Li, Jie-Ming Qin, and Qian Duan*

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Department of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China ABSTRACT: To take the advantage of the synergy of atom filling and pressure regulation, atom Pb was assumed to play the role of fillers to occupy the Sb-icosahedron voids. In this paper, skutterudite PbxCo4Sb11.5Te0.5 materials have been synthesized by high-pressure and high-temperature (HPHT) method with 0.5 h processing time. Although the increase of Pb filling rate increased the electrical resistivity of samples, it reduced the thermal conductivity of samples significantly. When the synthesis pressure increased, the Seebeck coefficients of Pb0.2Co4Sb11.5Te0.5 increased, and the thermal conductivity decreased. The crystal morphology and structure of samples, which can effectively affect the thermoelectric properties of materials, were investigated with XRD, Raman spectra, SEM mapping, and HR-TEM analysis. cannot be filled under normal pressure, were successfully “pressed” into the cage structure by the high-pressure environment, and the thermal conductivity of skutterudite materials were reduced effectively. If the as-synthesized samples do not show severe reversibility on in situ measurements,13,14 then chemical substitution and pressure tuning are both deemed to be an attractive alternative strategy. In order to further improve the thermoelectric properties of original CoSb3 and investigate the effect of Pb atom as source of filling, PbxCo4Sb11.5Te0.5 samples were prepared by HPHT method. We found that the thermal and electrical transport properties of materials can be optimized by obviously higher prepared pressures, and the sample Pb0.2Co4Sb11.5Te0.5 prepared at 3.5 GPa achieved the highest ZT value, which is mainly due to the synergetic regulation of high synthesis pressure.

1. INTRODUCTION Thermoelectric material is a kind of functional material which can convert electrical energy and thermal energy directly through Seebeck effect or Peltier effect.1−4 It has gradually become a research hotspot due to the advantages of thermoelectric converters without mechanical rotating parts, long service life and low maintenance costs. The purpose of the study is to optimize the thermal transport properties of the material as much as possible without affecting the electrical transport properties, and increased the figure of merit (ZT) value of materials eventually, where the figure of merit, ZT = α2T/ρκ (α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity).5−7 CoSb3-based material is one of the most promising thermoelectric materials at present, but there is still a certain gap with the high ZT value of theoretical calculation. CoSb3 materials have good electrical transport properties, but the relatively high lattice thermal conductivity (∼10 W m−1 K−1 around 300 K) limit the enhancement of ZT values. Therefore, how to reduce lattice thermal conductivity (κph) becomes the key research direction of CoSb3 materials. Two methods can effectively reduce the κph of this kind of compounds. First, heavy atoms with larger atomic radius and higher atomic number, such as rare earth elements,8,9 alkaline earth elements,10 and some other elements (Ge and Sn),11,12 are filled into the Sb12 icosahedral voids of the skutterudite structure. The strong resonance generated by the local perturbation of the filled atom can scatter phonons effectively, which can reduce the lattice thermal conductivity greatly and thus improve the thermal transport properties significantly. The second is to introduce a large number of phonon scattering interfaces by nanocomposite technology. In addition, Nalos et al.11,12 studied Sn and Ge filling by highpressure synthesis method. The filled Sn and Ge atoms, which © XXXX American Chemical Society

2. EXPERIMENTAL SECTION The elements of Pb, Te, Co and Sb (99.9% in purity) powders were weighed according to the stoichiometry of PbxCo4Sb11.5Te0.5 (x = 0.1, 0.2, and 0.3) as sources. The powders of these mixtures were ground by an agate mortar for 1 h in the case of argon as protective gas. The mixed powders were oil-pressed into cylinder with diameter of 10 mm and thickness of 3 mm and then were synthesized under high temperature and pressure by instrument (SPD 6 × 1200). The preparation process was carried out at temperature (900 K) and pressure (1.5−3.5 GPa) for 25 min and then quench under high pressure. The phase constitutions were identified by X-ray diffraction instrument (D/MAX-RA). Microstructure and element distribution of materials were analyzed by scanning electron microscopy (SEM) via Magellan 400 FEI microscope. The nanostructures and defects were analyzed by high-resolution TEM (HRTEM JEOL JEMo2200FS). Received: January 28, 2019

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DOI: 10.1021/acs.inorgchem.9b00270 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) XRD patterns of PbxCo4Sb11.5Te0.5 samples and (b) Raman spectra of Pb0.2Co4Sb11.5Te0.5 prepared at different pressures.

Figure 2. SEM images of samples Pb0.2Co4Sb11.5Te0.5 prepared by (a) 1.5 GPa and (b) 3.5 GPa. (c−f) EDS elemental mapping of Co, Pb, Sb, and Te elements for Pb0.2Co4Sb11.5Te0.5 sample prepared by 2 GPa. (g) HRTEM images of Pb0.2Co4Sb11.5Te0.5 sample synthesized at 3.5 GPa and (h) IFFT image corresponding to the white square marking of (g). The atom filling and high-pressure effect were analyzed by Raman microscope (LabRAM HR800). The Seebeck coefficient and electrical resistivity were measured by ZEM-3, and the thermal diffusivity D was

measured by Netzsch LFA 427 in a the temperature range of 373− 773 K. The thermal conductivity was calculated by the formula of κ = DCpd. The specific heat (Cp) and density (d) were measured by a B

DOI: 10.1021/acs.inorgchem.9b00270 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Linseis STA PT-1750 equipment with the sapphire revision and Archimedes method, respectively.

3. RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of PbxCo4Sb11.5Te0.5 samples. All characteristic peaks are indexed according to the skutterudite structure of CoSb3 (Powder Diffraction File No. 78−976, Joint Committee on Powder Diffraction Standards, [1987]). It can be seen that PbxCo4Sb11.5Te0.5 samples all are single-phase compounds with skutterudite structure. With the increase of synthesis pressures, the XRD peaks’ positions shift to low angles; this indicates that higher pressure effects could be helpful for the Pb filling. In Figure 1b, the collected Raman spectra were used to analysis the effect of atoms filling and high-pressure effect on the optical phonon vibrational modes of Pb0.2Co4Sb11.5Te0.5 samples prepared at different pressures. According to the polarization dependence, these Raman peaks have been identified to different symmetry modes. The corresponding results are compared with earlier theoretical calculations15 and experimental studies.16 In the Raman peaks range of 80−200 cm−1, six Raman peaks were observed. Separate Raman peaks due to Pb and Te doping were not observed, the result is similar to the study by Battabyal et al.16 The Ag energy mode of Pb0.2Co4Sb11.5Te0.5 prepared by 1.5 GPa is higher than that of Pb0.2Co4Sb11.5Te0.5 prepared by 3 and 3.5 GPa, which is presumably due to the higher synthesis pressures can increase the filling ratio of Pb and then make Sb−Sb bond longer. From the analysis of SEM images, we have found that the sample synthesized under 3.5 GPa have smaller grain size. As the particle size decreases, the Raman diffraction peaks move to low wavenumber and become wider and weaker, so the Raman diffraction peaks in Figure 1b decrease with increasing synthetic pressures. Figure 2a,b show the SEM micrographs of Pb0.2Co4Sb11.5Te0.5 samples prepared at 1.5 and 3.5 GPa. The grain size of Pb0.2Co4Sb11.5Te0.5 prepared at 1.5 GPa is smaller than that of Pb0.2Co4Sb11.5Te0.5 prepared at 3.5 GPa, which is corresponding to the results of Yao et al.17 This is mainly due to that the nucleation rate of crystals will increase with the increase of synthesis pressures significantly.18,19 Under the same synthesis time and the same element content, the number of grains synthesized under 3.5 GPa is larger than that of Pb0.2Co4Sb11.5Te0.5 synthesized under 1.5 GPa, while the size of grains is smaller than that of Pb0.2Co4Sb11.5Te0.5 prepared under 1.5 GPa. In order to investigate the chemical homogeneity of Pb-filled sample, we mapped the elemental distributions of Pb0.2Co4Sb11.5Te0.5 prepared at 3.5 GPa by EDS in Figure 2c−f. Figure 2c−f show that Co, Pb, Sb, and Te are distributed in sample Pb0.2Co4Sb11.5Te0.5 synthesized at 3.5 GPa uniformly. The HRTEM image of Pb0.2Co4Sb11.5Te0.5 synthesized under 3.5 GPa is shown in Figure 2g. It shows that abundant dislocations, nanocrystalline polymer, and lattice distortion exist in the sample generally, which is mainly due to the pressure-induced strain. Carriers and phonons have different wavelength distribution ranges. The wavelength distribution range of phonons is much larger than that of carriers. Therefore, grain size in the range of 10−100 nm is more conducive to the coordinated regulation of electrical and thermal transport properties. The corresponding inverse fast Fourier transform (IFFT) reconstructed image shows the distribution of dislocations in Figure 2h. Figure 3a shows the relationship between the electrical resistivity (ρ) and temperature of PbxCo4Sb11.5Te0.5 samples.

Figure 3. Temperature as a function of electrical resistivity (a), Seebeck coefficients (b), and power factors (c) for PbxCo4Sb11.5Te0.5 prepared by HPHT.

In Figure 3a, the electrical resistivity of samples Pb0.1Co4Sb11.5Te0.5 is lower than that of other samples, which indicates the high concentration of Pb filling hinders the electric transmission performance of the sample. The variation of synthesis pressures have little effect on the ρ of Pb0.2Co4Sb11.5Te0.5 samples, which is different from our previous studies of that the ρ of CoSb3 could be increased by the increasing synthesis pressures. Figure 3b shows the relationship between the Seebeck coefficients (α) of samples and temperature. The α of PbxCo4Sb11.5Te0.5 samples are negative, indicating that the samples are N-type semiconductors. The absolute Seebeck coefficients (|α|) of samples Pb 0.1 Co 4 Sb 11.5 Te 0.5 and Pb0.2Co4Sb11.5Te0.5 prepared at 1.5 GPa all increase with temperature. The |α| of other samples increase at first and then decrease with increasing temperature; this shows that they have changed from semimetal to semiconductor at 673 K. Among the Pb-filled samples synthesized at 1.5 GPa, the absolute Seebeck coefficient of Pb0.2Co4Sb11.5Te0.5 is higher than that of other two samples. At the same time, the absolute C

DOI: 10.1021/acs.inorgchem.9b00270 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

produce more dislocations and defects. The κ of Pb0.2Co4Sb11.5Te0.5 prepared at 3.5 GPa reached 2.25 W m−1 K−1 at 673 K. The thermal conductivity includes two parts: One is carrier contribution (κe), and the other is phonons contribution (κph). The κe is calculated by Wiedemann− Franz’s law as κe = LσT,20,21 where the Lorenz number L22 is 2.0 × 10−8 W Ω K−2. The relationship between κph and temperature is shown in Figure 4b. For the samples synthesized at 1.5 GPa, although Pb filling can increase the κe, it can more effectively reduce the κph as phonon scattering centers; the κph decreases significantly with the increase of Pb content. Pb filling into the cage structure of CoSb3 can produce strong vibration point defects. With the increase of the filling amount of Pb, the distribution area of point defects will be wider, the distribution density will be larger, and the scattering effect on phonons will be stronger. The κph of Pb0.2Co4Sb11.5Te0.5 samples prepared at 3 and 3.5 GPa is lower than that of Pb0.2Co4Sb11.5Te0.5 sample prepared at 1.5 GPa, which is mainly due to the higher synthesis pressures can introduce more abundant grain boundaries and defects to scatter phonons more effectively. Figure 4c shows the relationship between the ZT values (ZTs) of PbxCo4Sb11.5Te0.5 samples and temperature. The ZT values of samples except Pb0.3Co4Sb11.5Te0.5 synthesized at 1.5 GPa all increase monotonically with increasing temperature. For the samples synthesized at 1.5 GPa, the ZT value of Pb0.2Co4Sb11.5Te0.5 is higher than that of other two samples. For the same chemical compositions of Pb0.2Co4Sb11.5Te0.5, the ZTs increase with increasing synthetic pressures in the temperature range of 673−773 K. This indicates that higher synthesis pressures can effectively optimize the thermoelectric properties of materials.23,24 A maximum ZT value of Pb0.2Co4Sb11.5Te0.5 sample prepared by 3.5 GPa reached 0.93 at 773 K, the main reason is that the relatively high thermoelectric transport performance was optimized by higher synthesis pressures.

Seebeck coefficient of Pb0.2Co4Sb11.5Te0.5 increase with synthetic pressures. Figure 3c shows the relationship between the power factors (PF) of samples and temperature. Except for sample Pb0.3Co4Sb11.5Te0.5 prepared at 1.5 GPa, the power factors of others all increase with increasing temperature. For Pb0.2Co4Sb11.5Te0.5 samples prepared by different pressures, the PF all increase with increasing synthetic pressures, which indicates that higher synthetic pressures will increase the PF of s k u tt e r u d i t e m a t e r i a l s s i g n i fic a n t l y . T h e P F o f Pb0.2Co4Sb11.5Te0.5 sample showed a maximum value of 27.3 μW cm−1 K−2 at 773 K. Thermal conductivity (κ) of PbxCo4Sb11.5Te0.5 samples is shown in Figure 4a. For Pb0.2Co4Sb11.5Te0.5 samples synthesized at different pressures, the κ of sample synthesized under higher pressure is much lower than that of the sample synthesized under 1.5 GPa. The main reason is that higher synthesis pressures can lead to smaller grains and more abundant grain boundaries; high-pressure quenching can also

4. CONCLUSION In this paper, the skutterudite PbxCo4Sb11.5Te0.5 materials were prepared by HPHT technology. Pb-filling could reduce the thermal conductivity effectively. Higher synthesis pressures can optimize the electrical transport and thermal transport properties of materials simultaneously. Pb0.2Co4Sb11.5Te0.5 sample prepared at 3.5 GPa had a high PF of 27.3 μW cm−1 K−2 and low κ of 2.26 W m−1 K−1 at 773 K, and the highest ZT value of 0.93 has been obtained finally. Our findings suggest that this technical route may be equally effective in the further optimization of other kinds of thermoelectric materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.D.). *E-mail: [email protected] (Q.D.) ORCID

Le Deng: 0000-0003-1483-0622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Key Research and Development Program of China (No 2016YFB0303805), National Science Foundation of China (51301024, 11464035

Figure 4. Temperature as a function of thermal conductivity (a), lattice thermal conductivity (b), and ZT (c) for PbxCo4Sb11.5Te0.5 prepared by HPHT. D

DOI: 10.1021/acs.inorgchem.9b00270 Inorg. Chem. XXXX, XXX, XXX−XXX

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electrical and thermal transport properties of nonstoichiometric TiO1.80. Inorg. Chem. 2017, 56, 11275−11281. (20) Gao, N.; Zhu, B.; Wang, X. Y.; Yu, Y.; Zu, F. Q. Simultaneous optimization of Seebeck, electrical and thermal conductivity in freesolidified Bi0.4Sb1.6Te3 alloy via liquid-state manipulation. J. Mater. Sci. 2018, 53, 9107−9116. (21) Li, J.; Li, X.; Chen, C.; Hu, W.; Yu, F.; Zhao, Z.; Zhang, L.; Yu, D.; Tian, Y.; Xu, B. Enhanced thermoelectric performance of bismuthdoped magnesium silicide synthesized under high pressure. J. Mater. Sci. 2018, 53, 9091−9098. (22) Wang, S. Y.; Yang, J.; Wu, L. H.; Wei, P.; Zhang, W. Q.; Yang, J. H. On intensifying carrier impurity scattering to enhance thermoelectric performance in Cr-doped Ce gamma Co4Sb12. Adv. Funct. Mater. 2015, 25, 6660−6670. (23) Yang, X. X.; Dai, Z. H.; Zhao, Y. C.; Niu, W. C.; Liu, J. Y.; Meng, S. Pressure induced excellent thermoelectric behavior in skutterudites CoSb3 and IrSb3. Phys. Chem. Chem. Phys. 2019, 21, 851−858. (24) Rogl, G.; Grytsiv, A.; Anbalagan, R.; Bursik, J.; Kerber, M.; Schafler, E.; Zehetbauer, M.; Bauer, E.; Rogl, P. Direct SPDprocessing to achieve high-ZT skutterudites. Acta Mater. 2018, 159, 352−363.

and 51171070), Jilin Provincial Education Department (JJKH20181123KJ) and JiLin Provincial Science and Technology Department Project (20150520026JH and 20170101045JC).

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DOI: 10.1021/acs.inorgchem.9b00270 Inorg. Chem. XXXX, XXX, XXX−XXX