Charge Transport and Thermoelectric Properties of P-type Bi 2-x Sb x Te 3 Prepared by Mechanical Alloying and Hot Pressing

Transcription

1 [Research Paper] 대한금속 재료학회지 (Korean J. Met. Mater.), Vol. 56, No. 1 (2018), pp DOI: /KJMM Charge Transport and Thermoelectric Properties of P-type Bi 2-x Sb x Te 3 Prepared by Mechanical Alloying and Hot Pressing Kyung-Wook Jang 1, Hyeok-Jin Kim 2, Woo-Jin Jung 2, and Il-Ho Kim 2, * 1 Department of Materials Science and Engineering, Hanseo University, Seosan 31962, Republic of Korea 2 Department of Materials Science and Engineering, Korea National University of Transportation, Chungju 27469, Republic of Korea Abstract: Bi 2-x Sb x Te 3 (x = ) solid solutions were synthesized by mechanical alloying (MA) and consolidated by hot pressing (HP), and their charge transport and thermoelectric properties were examined. The relative densities of the hot-pressed specimens were higher than 96% on average. As the Sb content was increased, the lattice constants decreased, which confirmed that mechanical alloying using a planetary mill was successful in synthesizing solid solutions. The carrier concentration increased with increasing Sb content, and the specimens with x 1.5 behaved as degenerate semiconductors. All specimens showed p-type conduction, which was confirmed from the positive values of the Seebeck coefficient and the Hall coefficient. The increased Sb content caused a shift in the peak values of the Seebeck coefficient to higher temperatures and enhanced the power factor. As the Sb content increased, the electronic thermal conductivity increased, and the lattice thermal conductivity decreased. Bi 0.3 Sb 1.7 Te 3 hot-pressed at 698 K exhibited a maximum power factor of 3.4 mwm -1 K -2 at 323 K and a low thermal conductivity of 0.8 Wm -1 K -1. The maximum dimensionless figure of merit (ZT max = 1.4) and the average performance (ZT ave = 1.2) were obtained at 323 K. (Received July 5, 2017; Accepted October 30, 2017) Keywords: thermoelectric, bismuth telluride, solid solution, mechanical alloying, hot pressing 1. INTRODUCTION Thermoelectric materials have attracted attention because of environmental concerns, diminishing energy resources and growing energy demands. Their application for power generation and electronic cooling have been studied for several decades [1,2]. The efficiency of a thermoelectric material is described by the dimensionless figure of merit, ZT = α 2 σtκ -1, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity [3]. Therefore, superior thermoelectric materials should have a high power factor (PF = α 2 σ) and a low thermal conductivity. Bi 2 Te 3 and Sb 2 Te 3 have layered structures with the order -Te 1 -Bi(or Sb)-Te 2 -Bi(or Sb)-Te 1 -, and cleavage planes are easily formed along the basal plane perpendicular to the c- axis due to weak van der Waals bonding between Te 1 -Te 1 *Corresponding Author: Il-Ho Kim [Tel: , ihkim@ut.ac.kr] Copyright c The Korean Institute of Metals and Materials [4]. Because the thermoelectric properties in the direction parallel to the basal plane are superior to those along the c- axis, the single crystal has anisotropic thermoelectric properties [5]. Mechanical alloying (MA) has several advantages over conventional melting and grinding techniques, such as avoiding the phase separation that can occur during melting, and has been applied to synthesize nanosized powders [6]. Several methods based on MA such as MA- HP [7,8], MA-hot extrusion [9], MA-spark plasma sintering [10], and MA-mechanical deformation [11] have also been employed to optimize thermoelectric and mechanical properties. Poudel et al. [3] obtained a ZT = 1.4 for p-type Bi 0.5 Sb 1.5 Te 3 prepared by ball milling and hot pressing (HP), and Xiao et al. [12] reported a ZT = 0.8 for p-type Bi 0.5 Sb 1.5 Te 3 prepared by MA and HP. In the present study, Bi 2-x Sb x Te 3 (x = ) solid solutions were synthesized by MA and sintered by HP. The microstructure, charge transport characteristics, and thermoelectric properties were analyzed.

2 67 대한금속 재료학회지제 56 권제 1 호 (2018 년 1 월 ) 2. EXPERIMENTAL PROCEDURE Bi 2-x Sb x Te 3 (x= ) solid solutions were synthesized using the MA method. Bi (purity %, 5N Plus), Sb (purity %, LTS) and Te (purity %, 5N Plus) powders were weighed to the stoichiometric ratio and mechanically alloyed at 300 rpm using a planetary mill. The synthesized powders were sintered using HP in a graphite die with an internal diameter of 10 mm at temperatures ranging from 648 K to 698 K under a pressure of 70 MPa for 1 h in a vacuum. An X-ray diffractometer (XRD; Bruker D8-Advance) was used to analyze the phases of the mechanically-alloyed powders and hot-pressed specimens using Cu-K α Radiation (λ = nm). The diffraction pattern was measured in the θ-2θ mode (2θ of ) with a step size of 0.02 and a scan speed of 0.4 s/step. Lattice constants were evaluated from the XRD data for a rhombohedric hexagonal crystal structure. A scanning electron microscope (SEM; FEI Quanta400) and an energy dispersive spectrometer (EDS; JSM-7000F) were used to analyze the fractured surfaces and the compositions of the specimens. The Hall coefficients, carrier concentrations, and mobilities of the specimens were measured using the van der Pauw method (Keithley 7065) at room temperature in a 1 T magnetic field at a 50 ma electric current. The Seebeck coefficient and the electrical conductivity were measured using the temperature differential and the 4-probe method (Ulvac- Riko, ZEM-3) in a He atmosphere. The thermal conductivity was obtained from the density, heat capacity, and thermal diffusivity measured using the laser flash method (Ulvac-Riko, TC-9000H). PF and ZT were evaluated at temperatures ranging from 323 K to 523 K. 3. RESULTS AND DISCUSSION Figure 1 presents the XRD patterns of the mechanicallyalloyed powders of Bi 2-x Sb x Te 3 (x= ) All the diffraction peaks corresponded to the ICDD standard diffraction data for Bi 2 Te 3 (PDF# ) or Sb 2 Te 3 (PDF# ), indicating that mechanically-alloyed powders of Bi 2-x Sb x Te 3 were successfully synthesized without any residual elements or secondary phases. In addition, diffraction peaks were broadened by MA, Fig. 1. XRD patterns for mechanically-alloyed powders of Bi 2- xsb x Te 3 (x = ). Fig. 2. (a) XRD patterns of Bi 2-x Sb x Te 3 solid solutions hot-pressed at 698K, and (b) enlarged diffraction peaks of the (015) planes.

3 68 Kyung-Wook Jang, Hyeok-Jin Kim, Woo-Jin Jung, and Il-Ho Kim Fig. 4. Variation of the carrier concentration and the mobility of Bi2xSbxTe3 at room temperature with the Sb content. the successful substitution of Sb for Bi was confirmed, and the lattice constant was expected to decrease. Fig. 3. SEM images and EDS line scans of the fractured surfaces of Bi2-xSbxTe3 hot-pressed at 698 K. Figure 3 presents the SEM images and EDS line scans of the fractured surfaces of the Bi2-xSbxTe3 solid solutions. All specimens contained randomly-oriented plate-like grains possibly due to grain refinement and residual stress. Figure 2 shows the XRD patterns of Bi2-xSbxTe3 (x= ) hot-pressed at 698 K. Unreacted elements and secondary and every element was homogeneously distributed without secondary phases. phases were not identified after HP. Diffraction peaks were Table 1 shows the chemical compositions, lattice sharpened because the residual stress caused by MA was constants, and relative densities of Bi2-xSbxTe3. The reduced and the crystallinity of the particles was improved. specimen hot-pressed at xxx K is referred to as HPxxxK. Figure 2(b) shows the enlarged diffraction peaks of the (015) The actual compositions were similar to the nominal plane for each specimen. Because the ionic radius of Sb (138 compositions. The lattice constants a and c decreased as pm) is smaller than that of Bi (146 pm) [13], an increase in the the Sb content increased. The decrease in the c-axis was Sb content shifted the diffraction peaks to higher angles; thus, larger than that in the a-axis. All specimens had average Table 1. Chemical compositions, lattice constants, and relative densities of Bi2-xSbxTe3. Specimen Actual Composition Bi0.6Sb1.4Te3:HP648K Bi0.5Sb1.5Te3:HP648K Bi0.4Sb1.6Te3:HP648K Bi0.3Sb1.7Te3:HP648K Bi0.6Sb1.4Te3:HP673K Bi0.5Sb1.5Te3:HP673K Bi0.4Sb1.6Te3:HP673K Bi0.3Sb1.7Te3:HP673K Bi0.6Sb1.4Te3:HP698K Bi0.5Sb1.5Te3:HP698K Bi0.4Sb1.6Te3:HP698K Bi0.3Sb1.7Te3:HP698K Bi0.53Sb1.58Te2.89 Bi0.48Sb1.65Te2.87 Bi0.38Sb1.76Te2.86 Bi0.26Sb1.92Te2.82 Lattice Constant a [nm] c [nm] Relative Density [%]

4 69 대한금속 재료학회지제 56 권제 1 호 (2018 년 1 월 ) Fig. 5. Temperature dependence of the electrical conductivity of Bi 2-x Sb x Te 3. Fig. 7. Temperature dependence of the power factor of Bi 2-x Sb x Te 3. with x = 1.4 showed degenerate semiconductor behavior, which decreased with increasing temperature and increased with increasing Sb substitution. This was due to the increase in the carrier concentration caused by Sb substitution, as shown in Table 1. Fig. 6. Temperature dependence of the Seebeck coefficient of Bi 2- xsb x Te 3. densities higher than 96% of the theoretical density. Figure 4 presents the variations in carrier concentration and mobility at room temperature based on the amount of Sb substitution in Bi 2-x Sb x Te 3. In this study, both carrier concentration and mobility increased with increasing Sb content. In the p-type (Bi,Sb) 2 Te 3, Bi Te and Sb Te antisite defects are dominant defects and act as acceptors via Bi Bi ( Sb Sb ) + V Te + 2e V Bi ( V Sb ) + Bi Te ( Sb Te ) + 4h [14]. As the Sb content increases, the number of Sb Te increases and thereby the carrier concentration increases. Figure 5 shows the electric conductivity of Bi 2-x Sb x Te 3. The electrical conductivity of all specimens except the one Figure 6 presents the Seebeck coefficients of Bi 2-x Sb x Te 3. The positive Seebeck coefficient confirms p-type conduction, like the positive Hall coefficient. Except for Bi 0.6 Sb 1.4 Te 3, the Seebeck coefficient decreased with increasing Sb content because the carrier concentration increased at low temperatures. The temperature where the maximum value of the Seebeck coefficient was observed shifted higher with increasing Sb content; the peak values were obtained at temperatures from 323 K to 423 K. For a p-type degenerate semiconductor, the Seebeck coefficient can be expressed as α = (8/3)π 2 k 2 B m * Te -1 h -2 (π/ 3n) 2/3, where k B : Boltzmann constant, h: Planck constant, m * : effective carrier mass, e: electronic charge, n: carrier concentration, and T: absolute temperature [15]. Therefore, as the temperature increases, the value of the Seebeck coefficient increases and the carrier concentration increases rapidly due to the intrinsic transition at a certain temperature. The reduction in Seebeck coefficient due to the increase in carrier concentration is larger than the increase of the Seebeck coefficient due to rising temperature. Therefore, the Seebeck coefficient shows a peak value at a certain temperature. Because the bandgap energy of Bi 2 Te 3 at room temperature is ev [16]

5 Kyung-Wook Jang, Hyeok-Jin Kim, Woo-Jin Jung, and Il-Ho Kim 70 Thus, the temperature of the intrinsic transition shifts to higher temperatures. Figure 7 shows the power factor (PF) of Bi 2-x Sb x Te 3. According to the relation PF = α 2 σ [18], as the Seebeck coefficient and the electrical conductivity increase, PF increases. In this study, the PF values decreased with increasing temperature and increased with increasing Sb content, which caused an increase in the electrical conductivity, and thereby an increase in the PF. Accordingly, Bi 0.3 Sb 1.7 Te 3 showed the highest PF = 3.4 mw m -1 K-2 at 323 K. Fig. 8. Temperature dependence of (a) the thermal conductivity and (b) the lattice and electronic thermal conductivities of Bi 2-x Sb x Te 3. Fig. 9. Dimensionless figure of merit of Bi 2-x Sb x Te 3. and the bandgap energy of Sb 2 Te 3 is ev [17], the bandgap energy increases as the Sb substitution increases. Figure 8 presents the thermal conductivities of Bi 2- xsb x Te 3. The thermal conductivity is composed of the lattice thermal conductivity (κ L ) and the electronic thermal conductivity (κ E ), which can be calculated using the Wiedemann-Franz law (κ E = LσT) [19]. In this study, the Lorenz number was assumed to be L = V 2 K-2. As the temperature increased, the thermal conductivity increased due to bipolar conduction. As the Sb substitution increased, the temperature at which bipolar conduction occurred shifted to high temperatures. As shown in Fig. 8(b), the electronic thermal conductivity increased with increasing Sb content owing to the increased carrier concentration. The substitution of Sb for Bi caused a reduction in the lattice thermal conductivity owing to alloy scattering of electrons and phonons [20]. Figure 9 presents the dimensionless figures of merit (ZT) for Bi 2-x Sb x Te 3. The ZT values increased with increasing Sb content. The highest ZT value was obtained for Bi 0.3 Sb 1.7 Te 3 despite its high thermal conductivity at 323 K, due to its having the highest PF. Jung and Kim [21] examined the ZT values of p-type Bi x Sb 2-x Te 3 prepared by encapsulated melting (EM) and HP, and their data are compared in Fig. 9; ZT = 1.1 was obtained for Bi 0.4 Sb 1.6 Te 3 prepared by EM-HP. In the present study, the maximum ZT (ZT max ) = 1.4 and the average ZT (ZT ave ) = 1.2 were achieved for Bi 0.3 Sb 1.7 Te 3 prepared by MA-HP. Consequently, the MA-HP process is suitable for realizing superior thermoelectric performance. 4. CONCLUSIONS Bi 2-x Sb x Te 3 (x = ) solid solutions were prepared

6 71 대한금속 재료학회지제 56 권제 1 호 (2018 년 1 월 ) by MA and HP. The solid solutions were synthesized using a planetary mill, and were consolidated by HP without cracks or secondary phases. The positive Hall and Seebeck coefficients indicated p-type characteristics. As the Sb content increased, the temperature at which the intrinsic transition and bipolar conduction occurred shifted to higher temperatures. In the cases of x 1.5, the temperature dependence of the observed electrical conductivity was similar to that of degenerate semiconductors. Bi 0.3 Sb 1.7 Te 3 hot-pressed at 698 K showed a ZT max = 1.4, and excellent thermoelectric properties could be achieved via the MA- HP process. ACKNOWLEDGMENT This work was supported by a grant from Hanseo University in REFERENCES 1. J. F. Li, W. S. Liu, L. D. Zhao, and M. Zhou, NPG Asia Mater. 2, 152 (2010). 2. S. Bae, S. Lee, H. S. Sohn, and H. S. Lee, Met. Mater. Int. 23, 1056 (2017). 3. Y. S. Lim and S. Lee, Korean J. Met. Mater. 55, 651 (2017). 4. J. R. Drabble and C. H. L. Goodman, J. Phys. Chem. Sol. 5, 142 (1958). 5. L. G. Schulz, J. Appl. Phys. 20, 1030 (1949). 6. Y. Q. Yu, B. P. Zhang, Z. H. Ge, P. P. Shang, and Y. X. Chen, Mater. Chem. Phys. 131, 216 (2011). 7. J. Y. Yang, X. A. Fan, R. G. Chen, and W. Zhu, J. Alloy. Compd. 416, 270 (2006). 8. Y. Ma, Q. Hao, B. Poudel, Y. C. Lan, B. Yu, D. Z. Wang G. Chen, and Z. F. Ren, Nano Lett. 8, 2580 (2008). 9. H. S. Kim and S. J. Hong, Curr. Nanosci. 10, 118 (2014). 10. D. Li, R. R. Sun, and X. Y. Qin, Internet. 19, 2002 (2011). 11. D. H. Lee, J. U. Lee, S. J. Jung, S. H. Baek, J. H. Kim, D. I. Kim, D. B. Hyun, and J. S. Kim, J. Electron. Mater. 43, 2255 (2014). 12. Y. Xiao, J. Yang, G. Li, M. Liu, L. Fu, Y. Luo, W. Li, and J. Peng, Intermet. 50, 20 (2014). 13. E. Clementi, D. L. Raimondi, and W. P. Reinhardt, J. Chem. Phys. 47, 1300 (1967). 14. J. H. Son, M. W. Oh, B. S. Kim, S. D. Park, B. K. Min, M. H. Kim, and H. W. Lee, J. Alloy. Compd. 566, 168 (2013). 15. G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008). 16. H. Kohler, Phys. Stat. Sol. B 74, 591 (1976). 17. H. T. Langhammer, M. Stordeur, H. Sobotta, and V. Riede, Phys. Stat. Sol. B 123, K47 (1984). 18. L. Hu, H. Gao, X. Liu, H. Xie, J. Shen, T. Zhu, and X. Zhao, J. Mater. Chem. 22, (2012). 19. H. Cailat, A. Borshchevsky, and J. P. Fleurial, J. Appl. Phys. 80, 4442 (1996). 20. S. K. Bux, J. P. Fleurial, and R. B. Kaner, Chem. Commun. 46, 8311 (2010). 21. W. J. Jung and I. H Kim, J. Korean Phys. Soc. 69, 1328 (2016).