[Research Paper] 대한금속 ·재료학회지 (Korean J. Met. Mater.), Vol. 56, No. 1 (2018), pp.66-71 66

DOI: 10.3365/KJMM.2018.56.1.66

Charge Transport and Thermoelectric Properties of P-type Bi2-xSbxTe3 Preparedby Mechanical Alloying and Hot Pressing

Kyung-Wook Jang1, Hyeok-Jin Kim2, Woo-Jin Jung2, and Il-Ho Kim2,*

1Department of Materials Science and Engineering, Hanseo University, Seosan 31962, Republic of Korea2Department of Materials Science and Engineering, Korea National University of Transportation, Chungju 27469, Republic of Korea

Abstract: Bi2-xSbxTe3 (x = 1.4–1.7) 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. Bi0.3Sb1.7Te3 hot-pressed at 698 K exhibited a maximum power

factor of 3.4 mWm-1K-2 at 323 K and a low thermal conductivity of 0.8 Wm-1K-1. The maximum dimensionless

figure of merit (ZTmax = 1.4) and the average performance (ZTave = 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 = α2σTκ-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 = α2σ) and a low thermal

conductivity.

Bi2Te3 and Sb2Te3 have layered structures with the order

-Te1-Bi(or Sb)-Te2-Bi(or Sb)-Te1-, and cleavage planes are

easily formed along the basal plane perpendicular to the c-

axis due to weak van der Waals bonding between Te1-Te1

[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 Bi0.5Sb1.5Te3 prepared by ball milling and hot

pressing (HP), and Xiao et al. [12] reported a ZT = 0.8 for

p-type Bi0.5Sb1.5Te3 prepared by MA and HP. In the present

study, Bi2-xSbxTe3 (x = 1.4–1.7) solid solutions were

synthesized by MA and sintered by HP. The microstructure,

charge transport characteristics, and thermoelectric properties

were analyzed.*Corresponding Author: Il-Ho Kim

[Tel: +82-43-841-5387, E-mail: ihkim@ut.ac.kr]

Copyright ⓒ The Korean Institute of Metals and Materials

67 대한금속 ·재료학회지 제56권 제1호 (2018년 1월)

2. EXPERIMENTAL PROCEDURE

Bi2-xSbxTe3 (x=1.4–1.7) solid solutions were synthesized

using the MA method. Bi (purity 99.999%, 5N Plus), Sb

(purity 99.999%, LTS) and Te (purity 99.999%, 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-Kα Radiation

(λ = 0.15405 nm). The diffraction pattern was measured in

the θ-2θ mode (2θ of 10–90°) 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 mechanically-

alloyed powders of Bi2-xSbxTe3 (x=1.4–1.7) All the

diffraction peaks corresponded to the ICDD standard

diffraction data for Bi2Te3 (PDF# 15-0863) or Sb2Te3

(PDF# 15-0874), indicating that mechanically-alloyed

powders of Bi2-xSbxTe3 were successfully synthesized

without any residual elements or secondary phases. In

addition, diffraction peaks were broadened by MA,

Fig. 2. (a) XRD patterns of Bi2-xSbxTe3 solid solutions hot-pressedat 698K, and (b) enlarged diffraction peaks of the (015) planes.

Fig. 1. XRD patterns for mechanically-alloyed powders of Bi2-xSbxTe3 (x = 1.4–1.7).

Kyung-Wook Jang, Hyeok-Jin Kim, Woo-Jin Jung, and Il-Ho Kim 68

possibly due to grain refinement and residual stress.

Figure 2 shows the XRD patterns of Bi2-xSbxTe3 (x=1.4–

1.7) hot-pressed at 698 K. Unreacted elements and secondary

phases were not identified after HP. Diffraction peaks were

sharpened because the residual stress caused by MA was

reduced and the crystallinity of the particles was improved.

Figure 2(b) shows the enlarged diffraction peaks of the (015)

plane for each specimen. Because the ionic radius of Sb (138

pm) is smaller than that of Bi (146 pm) [13], an increase in the

Sb content shifted the diffraction peaks to higher angles; thus,

the successful substitution of Sb for Bi was confirmed, and the

lattice constant was expected to decrease.

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

and every element was homogeneously distributed without

secondary phases.

Table 1 shows the chemical compositions, lattice

constants, and relative densities of Bi2-xSbxTe3. The

specimen hot-pressed at xxx K is referred to as “HPxxxK”.

The actual compositions were similar to the nominal

compositions. The lattice constants a and c decreased as

the Sb content increased. The decrease in the c-axis was

larger than that in the a-axis. All specimens had average

Fig. 3. SEM images and EDS line scans of the fractured surfaces ofBi2-xSbxTe3 hot-pressed at 698 K.

Table 1. Chemical compositions, lattice constants, and relative densities of Bi2-xSbxTe3.

Specimen Actual CompositionLattice Constant

Relative Density [%]a [nm] c [nm]

Bi0.6Sb1.4Te3:HP648K - 0.4300 3.4554 97.9

Bi0.5Sb1.5Te3:HP648K - 0.4295 3.4489 97.0

Bi0.4Sb1.6Te3:HP648K - 0.4294 3.4409 96.3

Bi0.3Sb1.7Te3:HP648K - 0.4280 3.4382 97.0

Bi0.6Sb1.4Te3:HP673K - 0.4295 3.4483 95.0

Bi0.5Sb1.5Te3:HP673K - 0.4291 3.4257 94.9

Bi0.4Sb1.6Te3:HP673K - 0.4284 3.4162 97.4

Bi0.3Sb1.7Te3:HP673K - 0.4281 3.4035 97.0

Bi0.6Sb1.4Te3:HP698K Bi0.53Sb1.58Te2.89 0.4297 3.4981 95.6

Bi0.5Sb1.5Te3:HP698K Bi0.48Sb1.65Te2.87 0.4293 3.4893 95.0

Bi0.4Sb1.6Te3:HP698K Bi0.38Sb1.76Te2.86 0.4281 3.4778 96.0

Bi0.3Sb1.7Te3:HP698K Bi0.26Sb1.92Te2.82 0.4270 3.4684 94.5

Fig. 4. Variation of the carrier concentration and the mobility of Bi2-xSbxTe3 at room temperature with the Sb content.

69 대한금속 ·재료학회지 제56권 제1호 (2018년 1월)

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 Bi2-xSbxTe3. In this study, both carrier

concentration and mobility increased with increasing Sb

content. In the p-type (Bi,Sb)2Te3, BiTe and SbTe antisite

defects are dominant defects and act as acceptors via

[14]. As

the Sb content increases, the number of SbTe increases and

thereby the carrier concentration increases.

Figure 5 shows the electric conductivity of Bi2-xSbxTe3.

The electrical conductivity of all specimens except the one

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.

Figure 6 presents the Seebeck coefficients of Bi2-xSbxTe3.

The positive Seebeck coefficient confirms p-type

conduction, like the positive Hall coefficient. Except for

Bi0.6Sb1.4Te3, 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)π2kB2m*Te-1h-2(π/

3n)2/3, where kB: 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 Bi2Te3 at room temperature is 0.14–0.16 eV [16]

BiBi SbSb( ) VTe 2e′ VBi VSb( ) Bi′Te Sb′Te( ) 4h •

+ +→+ +

Fig. 5. Temperature dependence of the electrical conductivity ofBi2-xSbxTe3.

Fig. 6. Temperature dependence of the Seebeck coefficient of Bi2-xSbxTe3.

Fig. 7. Temperature dependence of the power factor of Bi2-xSbxTe3.

Kyung-Wook Jang, Hyeok-Jin Kim, Woo-Jin Jung, and Il-Ho Kim 70

and the bandgap energy of Sb2Te3 is 0.25–0.30 eV [17], the

bandgap energy increases as the Sb substitution increases.

Thus, the temperature of the intrinsic transition shifts to

higher temperatures.

Figure 7 shows the power factor (PF) of Bi2-xSbxTe3.

According to the relation PF = α2σ [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, Bi0.3Sb1.7Te3 showed the highest PF = 3.4

mW·m-1·K-2 at 323 K.

Figure 8 presents the thermal conductivities of Bi2-

xSbxTe3. The thermal conductivity is composed of the

lattice thermal conductivity (κL) and the electronic thermal

conductivity (κE), which can be calculated using the

Wiedemann-Franz law (κE = LσT) [19]. In this study, the

Lorenz number was assumed to be L = 2.0 × 10-8 V2·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 Bi2-xSbxTe3. The ZT values increased with increasing

Sb content. The highest ZT value was obtained for

Bi0.3Sb1.7Te3 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 BixSb2-xTe3 prepared by

encapsulated melting (EM) and HP, and their data are

compared in Fig. 9; ZT = 1.1 was obtained for Bi0.4Sb1.6Te3

prepared by EM-HP. In the present study, the maximum ZT

(ZTmax) = 1.4 and the average ZT (ZTave) = 1.2 were

achieved for Bi0.3Sb1.7Te3 prepared by MA-HP.

Consequently, the MA-HP process is suitable for realizing

superior thermoelectric performance.

4. CONCLUSIONS

Bi2-xSbxTe3 (x = 1.4–1.7) solid solutions were prepared

Fig. 8. Temperature dependence of (a) the thermal conductivity and(b) the lattice and electronic thermal conductivities of Bi2-xSbxTe3.

Fig. 9. Dimensionless figure of merit of Bi2-xSbxTe3.

71 대한금속 ·재료학회지 제56권 제1호 (2018년 1월)

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. Bi0.3Sb1.7Te3

hot-pressed at 698 K showed a ZTmax = 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 2015.

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, 16484 (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).