Measurement of Seebeck Coefficient and Conductive Behaviors of Bi2Te3−xSex (x = 0 15 0 6) Thermoelectric Semiconductors without Harmful Dopants

Measurement of Seebeck Coef

fi

cient and Conductive Behaviors of Bi

2

Te

3¹x

Se

x

(

x

=

0.15­0.6) Thermoelectric Semiconductors without Harmful Dopants

Mei Fusa

1,+1

, Naoaki Yamamoto

2,+2

and Kazuhiro Hasezaki

3

1_{Interdisciplinary Graduate School of Science and Engineering, Shimane University, Matsue 690-8504, Japan}

2_{Faculty of Materials Science, Shimane University, Matsue 690-8504, Japan}

3_{Department of Energy System, Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan}

A system for measuring Seebeck coefficient was constructed and applied to Bi2Te3¹xSex(x=0.15­0.6) samples without harmful dopants,

prepared by mechanical alloying (MA) followed by hot pressing (HP). The constructed thermal contact method system, using single and multiple ¦Tvalues, gave Seebeck coefficients of a standard reference material (SRM 3451) at room temperature confirmable as¹230«4 and¹232«1 µV/K, respectively. X-ray diffraction patterns and differential scanning calorimetry curves showed that the MA­HP-sintered samples of Bi2Te3¹xSexwere single-phase Bi2(Te,Se)3-related materials. All the Bi2Te3¹xSexsamples weren-type semiconductors. The maximum power

factor was 1.4©10¹3_{W m}¹1_K¹2_{for Bi}

2Te2.8Se0.2sintered at 623 K. These results indicated that doping with harmful materials of Bi2Te3¹xSex

compounds prepared by the MA­HP process is not necessary for carrier control. [doi:10.2320/matertrans.MB201301]

(Received July 1, 2013; Accepted March 12, 2014; Published May 25, 2014)

Keywords: eco-materials, thermoelectrics, Seebeck coefficient, bismuth telluride selenide, n-type

1. Introduction

Bismuth telluride (Bi2Te3)-based alloys have been widely

used as thermoelectric cooling and generating materials. These materials are also eco-materials because they can be used to recover exhaust heat by thermoelectric conversion. The efficiency of a thermoelectric device is expressed by a

dimensionless figure of merit, which is defined as ZT=

¡2_·¬¹1_{T, where ¡, ·, ¬, and Tare the Seebeck coefficient,}

electrical conductivity, thermal conductivity, and absolute

temperature, respectively. ZT is strongly influenced by ¡

because it is proportional to the square of ¡. The

measure-ment of ¡ needs to be particularly accurate. The Seebeck

coefficient of a Bi2Te3 standard reference material (SRM

3451) has been determined by the National Institute of Standards and Technology.1,2)

Solid solutions of Bi2Te3and bismuth selenide (Bi2Se3) are

known to be the best n-type materials for thermoelectric

refrigeration at room temperature.3) Undoped Bi2Te3¹xSex

prepared using a melt-growth process shows p-type

con-duction forx¯0.2 andn-type conduction forx²0.3.4)_The

production of n-type Bi2Te2.85Se0.15 with excellent

thermo-electric properties using a melt-growth process requires the

addition of harmful halide dopants such as CdBr, CdCl2,

HgCl2, HgBr2, and SbBr3.3,5­8)Eco-materials should contain

as few hazardous substances as possible. In a previous study, undoped Bi2Te2.85Se0.15compounds without harmful dopants,

prepared by mechanical alloying (MA) followed by hot pressing (HP), showed n-type conduction.9,10)

In the present study, a system for measuring Seebeck

coefficients was constructed and applied to Bi2Te3¹xSex

(x=0.15­0.6) samples without harmful dopants, prepared

using an MA­HP process. The thermoelectric properties of

the resulting samples were investigated.

2. Experimental Procedure

2.1 Construction of system for Seebeck coefficient measurements at room temperature

The Seebeck coefficients (¡) were measured using a

thermal contact system. Figure 1 shows a schematic diagram of the system constructed for measuring Seebeck coefficients at room temperature. The Seebeck coefficient was estimated by temperature difference and the thermoelectric motive force. The temperature difference was measured between constantan and heat chip. The thermoelectric motive force was measured between sample and heater chip. The system consisted of a cylindrical direct current (DC) heater with a built-in copper heater rod, a copper plate, a constantan block, and the sample or standard Seebeck coefficient material

(SRM 3451).1,2) The boundary of this measurement system

assumed the constant heat flux. The heater materials, size,

shape and output were determined by the preliminarily FEM simulation. The simulated criterion of these parameters was to recover with the original temperature due to high thermal diffusivity of copper if other materials were contacted to heater chip. If the temperature difference between heater and samples containing constantan is assumed less than 10 K, the

Heater

SRM3451 or sample

Cu plate

constantan E(ΔT) ΔV

Cu heating rod

Fig. 1 Schematic diagram of system constructed for measuring Seebeck coefficients at room temperature.

+1_{Corresponding author, E-mail: s119129@mastu.shimane-u.ac.jp.} Gradu-ate Student, Shimane University

[image:1.595.318.532.334.464.2]

materials of low thermal diffusivity less than constantan were confirmed negligible for the difference of temperature due to difference of these materials. The simulated temperature differences between heater and samples and between heater and constantan were less than 1%. The size of heater rod had a length of 100 and 12 mm in diameter. The size and maximum output of the cylindrical direct current heater were 50 mm long and 6 mm in diameter and 50 W. The constantan

dimensions were 35 mm©10 mm©2 mm. The copper plate

was in the square shape of 40 mm and the thickness of

12 mm. The SRM 3451 dimensions were 3.5 mm©2.5

mm©8.0 mm. The chip size of copper heater rod was

columnar 3 mm in diameter. The chip had plane contact to samples. The input power of the copper heater rod was 0.01­ 0.5 W. The BiTe thermoelectric material SRM 3451 was used as the reference sample to determine the accuracy of measurements at around room temperature. The SRM 3451 sample and the constantan block were placed on the copper plate at temperature controlled room in air. It was assumed that these materials have the sameT1, i.e., room temperature.

The copper heater chip was kept at temperatureT2by the DC

heater. It assumed that T2 is not influenced with contact to

materials. Just before the copper heater chip contacts samples or constantan, the copper heater chip contacted every time the copper plate and set the null adjustment of DC voltmeter for the cancellation of copper Seebeck coefficient. The temper-ature difference¦T=T2­T1between the heater chip and the

plate was determined from the contact thermoelectric motive force,E, between the constantan block and the copper heater chip.Ewas used to calculate¦Tbetween the constantan and

copper heater chip using the type T (copper­constantan)

thermocouple voltage relationship. The differential Seebeck coefficient¡ab(V/K) of the sample is given by

¡ab¼¡a¡b¼V_T ð1Þ

where ¡a, ¡b and ¦Vare Seebeck coefficient of sample or

SRM 3451, that of copper and the contact thermoelectric motive force between the sample or SRM 3451 and the heater

chip, respectively. The null adjustment and E measurement

between the constantan block and the copper heater chip

were carried out every time before the¦Vmeasurement. The

deviations in the measurements of the Seebeck coefficients

were estimated for single¦Tvaluefixed at one input power

of the copper heater rod and multiple ¦T value changed

many input powers of that.

2.2 Process and evaluations

The MA samples were Bi2Te3¹xSex; x=0.15­0.6. Se

(1 at%) was added to control the carrier concentration for

n-type conduction without harmful dopants.

The constituent elements Bi (5N), Te (6N), and Se (5N) were put in a stainless-vessel and milled with silicon nitride ceramic balls, using a planetary ball-mill, for 30 h, at a maximum speed of 180 rpm. The resulting powder was passed through a 150 µm diameter sieve, and HP sintered at 623, 673, and 723 K under a mechanical pressure of 147 MPa in an argon atmosphere. All the powder-processing steps were performed in an argon atmosphere, with the exception that the HP mold was exposed to air during transportation

between the argon-filled glove box and the HP chamber. The dimension of sintered compacts were thickness 9 mm and diameter 10 mm. The sintered compacts were cut into disks of thickness 0.8 mm and diameter 10 mm for samples of Seebeck coefficient and electrical conductivity.

The structures of these MA­HP-sintered sample disks were

investigated by X-ray diffraction (XRD) using Cu K¡

radiation in the Bragg angle range 2ª=20­90°. Differential scanning calorimetry (DSC) was conducted by heating to 950 K at a rate of 0.17 K/s in a quartz container under an

argon atmosphere. The thermoelectric motive force¦V was

measured at a temperature difference ¦Tof about 4 K, and

thermal contact method system was constructed so that the time of contact between the copper heater chip and the sample disk was more than 120 s. The selected reason of measurement time is obtained constant "V and "T. The Seebeck coefficients were estimated from eq. (1). The

electrical conductivity · was measured using a Resitest

8340 (Toyo Corporation, Tokyo, Japan) at room temperature, using the van der Pauw method. The performance of the thermoelectric material was estimated using the power factor,

P=¡2·.

3. Results and Discussion

3.1 Evaluation of constructed thermal contact method system

Figure 2 shows the dependence of the thermoelectric

motive force ¦Vof SRM 3451 on the contact time between

the copper heater chip and SRM 3451 at room temperature; the input powers of the copper heater rod were (a) 0.1 W, (b) 0.24 W, and (c) 0.48 W.

The saturated¦Vwas approximately constant after 120 s, andfluctuations were the result of atmospheric changes. The

"V of constantan and other samples also were saturated approximately constant. These results reveals the back side temperature difference between samples and copper plate were not increased. The measurement conditions were selected so that the contact time between the copper heater chip and the sample was more than 120 s.

Plots of the input power of the copper heater rod versus the temperature difference,¦T, between the heater chip and the plate at room temperature are shown in Fig. 3. The temper-ature difference, ¦T=T2­T1, between the heater chip and

(a) p =0.1W

(b) p =0.24W

(c)p =0.48W 2.0

1.5

1.0

0.5

0 200 400 600 800

Time, t /s

Thermoelectric motive force,

⏐

⏐

Δ

V

/ 10

-3

V

Fig. 2 Dependence of thermoelectric motive force (¦V) of SRM 3451 on contact time between copper heater chip and SRM 3451 at room temperature.

[image:2.595.323.530.620.750.2]

the plate is given by the contact thermoelectric motive force,

E, between the constantan block and the heater chip. Ewas

calculated from ¦T between the constantan and the heater

chip using the type T (copper­constantan) thermocouple

voltage relationship. It was confirmed that the constructed

thermal contact system generated ¦Tfrom 1.8 to 8.1 K.

Figure 4 shows the plots of the Seebeck coefficient¡and

¦Tfor SRM 3451 at room temperature. Many temperatures

were measured by thefixed at one input power of the copper

heater rod. The certified reference value of the Seebeck

coefficient and expanded uncertainty for SRM 3451 were

¹230.82«5.38 µV/K at 300.73 K.1) _{The solid line and}

dashed line show the certified reference values of the Seebeck coefficient and the upper and lower expanded uncertainties for SRM 3451. The circles, vertical bars, and horizontal bars are the average of 10 measurements of¡for a single¦T, and the expanded uncertainties of¡and ¦T, respectively.

The constructed thermal contact system for Seebeck

coefficient measurements with a single ¦T gave a value

confirmable as¹230«4 µV/K for SRM 3451.

Figure 5 shows the plots of temperature difference ¦T

against absolute thermoelectric motive force «¦V«for SRM 3451 at room temperature. The measurements of the absolute thermoelectric motive force were carried out 10 times at six different temperatures. The slope of the line in Fig. 5 was

substituted into eq. (1), and the Seebeck coefficient was

calculated. R is the authenticity coefficient, which was

estimated to be 1. The results showed that the constructed thermal contact system for the measurement of Seebeck

coefficients at multiple¦Tvalues gave results confirmable as

¹232«1 µV/K for SRM 3451. The estimations with both

single and multiple ¦T values were within the expanded

uncertainty range of certified reference values for SRM 3451. All the samples of MA­HP-sintered Bi2Te3¹xSex (x=

0.15­0.6) were n-type semiconductors. Figure 6 shows the

XRD patterns of samples of Bi2Te3¹xSex synthesized by

MA­HP, sintered at 623 and 723 K. The circles show the

diffraction peaks from Bi2(Te,Se)3, indicating that all the

samples were identified as single-phase Bi2Te3-related

materials. The samples sintered at 673 K show identical XRD patterns from Bi2(Te,Se)3. All the sample of MA­

HP-sintered Bi2Te3¹xSex were isotropic. These results were

consistent with the observation of uniform finely grain

structure.10)

Figure 7 shows the DSC curves for Bi2Te3¹xSex(x=0.15, 0.4) sintered at 623, 673, and 723 K. The endothermic peak at around 860 K for Bi2Te2.85Se0.15 and 880 K for Bi2Te2.6Se0.4

correspond to the melting points of these samples. The endothermic peak width at around 880 K for the Bi2Te2.6Se0.4

sample corresponds to the difference between the solid and liquid line temperatures of Bi2Te2.6Se0.4.11)

The XRD and DSC results indicate that these MA­

HP-sintered samples were single-phase Bi2(Te,Se)3-related

ma-terials.

Plots of the electrical conductivities at room temperature versus the amount of Se (x) for Bi2Te3¹xSex(x=0.15, 0.4)

sintered at 623, 673, and 723 K are shown in Fig. 8. The electrical conductivity dependences on the amounts of Se were almost constant. However, the electrical conductivity decreases as the sintering temperature increases, because of precipitation of Bi2Te3¹xSexas a result of evaporation of Se and Te.12,13)

Figure 9 shows the Seebeck coefficients at room temper-ature versus the amounts of Se (x) for Bi2Te3¹xSexsintered at 623, 673, and 723 K. The undoped Bi2Te3¹xSex, prepared by

the melt-growth method, shows p-type conduction for

x¯0.2 and n-type conduction for x²0.3.2) MA­

HP-sintered Bi2Te3¹xSexshowedn-type conduction for all values

of x. The maximum Seebeck coefficient was obtained for

Bi2Te2.7Se0.3. This supports the calculations of the Seebeck

coefficients for Bi2Te3¹xSex based on first principles14) and the band structure of the undoped Bi2Te3¹xSexprepared by the melt-growth method.2)

Plots of the power factor (P) at room temperature against the amount of Se (x) for Bi2Te3¹xSexsintered at 623, 673, and

0.1 0.2 0.3 0.4 0.5 0.6 2

4 6 8 10

0

T

emperature dif

ference,

Δ

T

/K

Input power of heater, P / W

Fig. 3 Input power of copper heater rod versus temperature difference (¦T) between heater chip and plate at room temperature.

Seebeck coef

ficient,

α

/ 10

-6VK

-1

Temperature difference, ΔT /K

-200

-210

-220

-230

-240

-250

0 2 4 6 8

Fig. 4 Plots of Seebeck coefficient ¡ and ¦Tfor SRM 3451 at room temperature.

Temperature difference ΔT /K

2 4 6 8 10

0 2.0

1.0

Thermoelectric motive force,

Δ

V

/ 10

-3

V

Y=5.1×10-6_{+(232±1)×10}-6_X R=1

⏐

⏐

[image:3.595.326.529.68.194.2] [image:3.595.77.264.70.192.2] [image:3.595.67.278.249.387.2]

723 K are shown in Fig. 10. The maximum power factor was 1.4©10¹3_{W m}¹1_K¹2 _{for Bi}

2Te2.8Se0.2 sintered at 623 K.

These results indicated that doping with harmful materials of Bi2Te3¹xSexcompounds prepared by the MA­HP process is not necessary for carrier control.

4. Conclusion

In the present study, a system for measuring Seebeck coefficients was constructed and applied to Bi2Te3¹xSex samples without harmful dopants, prepared by MA followed by HP. The results are as follows.

(1) The results obtained with the constructed thermal contact system for measuring Seebeck coefficients from

single and multiple¦Ts, respectively, were confirmable

as ¹230«4 and ¹232«1 µV/K for SRM 3451

(¹230.82«5.38 µV/K) at room temperature.

(2) XRD patterns and DSC curves showed that the MA­

HP-sintered samples of Bi2Te3¹xSex (x=0.15­0.6) were single-phase Bi2(Te,Se)3-related materials.

(3) All the MA­HP-sintered Bi2Te3¹xSex (x=0.15­0.6)

samples weren-type semiconductors.

Intensity / arb.unit

Diffraction angle, 2θ/ degree

20 40 60 80

Bi2Te3-xSex

x=0.6

x=0.5

x=0.4

x=0.3

x=0.2

x=0.15

(015)

(018)

(1010)

(0

111

)

(0015) (205)

(1

10)

(0210)

(1016) (1

1

15)

(21

1)

(0120)

(21

10)

(300)

20 40 60 80

䠖Bi2 (Te,Se)3

x=0.6

x=0.5

x=0.4

x=0.3

x=0.2

x=0.15

Intensity / arb.unit

Diffraction angle, 2θ/ degree

(a) sintered at 623K

(b) sintered at 723K

Fig. 6 XRD patterns of Bi2Te3¹xSexsamples synthesized by MA and HP

sintered at (a) 623 K and (b) 723 K.

0.2 0.4 0.6

1.0 2.0 3.0 4.0

0

Electrical conductivity

,

σ

/

10

4Sm

-1

Bi2Te3-xSex/x

T_s=623K

Ts=673K

Ts=723K

Fig. 8 Electrical conductivity at room temperature versus amount of Se (x) for Bi2Te3¹xSex(x=0.15, 0.4) sintered at 623, 673, and 723 K.

400 500 600 700 800 900

Heat flow

Q

, / arb.unit

Temperature, T /K (a)Bi2Te2.85Se0.15Ts:623K (b)Bi2Te2.85Se0.15Ts:673K (c)Bi2Te2.85Se0.15Ts:723K (d)Bi2Te2.6Se0.4Ts:623K (e)Bi2Te2.6Se0.4Ts:673K (f)Bi2Te2.6Se0.4Ts:723K

Fig. 7 DSC curves of Bi2Te3¹xSex(x=0.15, 0.4) sintered at 623, 673, and

723 K.

0 0.2 0.4 0.6

-400 -300 -200 -100 0

Seebeck coef

ficient,

α

/ 10

-6VK

-1

Bi2Te3-xSex/x

T_s=623K

Ts=673K

Ts=723K

Fig. 9 Seebeck coefficient at room temperature versus amount of Se (x) for Bi2Te3¹xSexsintered at 623, 673, and 723 K.

[image:4.595.318.534.69.279.2] [image:4.595.64.277.70.558.2] [image:4.595.323.531.322.454.2] [image:4.595.325.531.503.641.2]

(4) The maximum power factor was 1.4©10¹3

W m¹1_K¹2_{, for Bi}

2Te2.8Se0.2sintered at 623 K.

These results indicated that doping with harmful materials

of Bi2Te3¹xSex compounds prepared using the MA­HP

process is not necessary for carrier control.

REFERENCES

1) N. D. Lowhorn, W. Wong-Ng, Z. Q. Lu, E. Thomas, M. Otani, M.

Green, N. Dilley, J. Sharp and T. N. Tran:Appl. Phys. A96(2009) 511­514.

2) N. D. Lowhorn, W. Wong-Ng, Z. Q. Lu, J. Martin, M. Green, J. E. Bonevich, E. L. Thomas, N. R. Dilley and J. Sharp:J. Mater. Res.26

(2011) 1983­1992.

3) H. Scherrer and S. Scherrer: Thermoelectric Handbook: Macro to Nano, ed. by D. M. Rowe, (CRC Press, Taylor & Francis Group, 2006) ch.27.

4) D. L. Greenaway and G. Harbeke:J. Phys. Chem. Solids26(1965) 1585­1604.

5) H. Kaibe, Y. Tanaka, M. Sakata and I. Nishida:J. Phys. Chem. Solids

50(1989) 945­950.

6) K. Uemura and I. Nishida: Thermoelectric Semiconductor and Its Applications, (Nikkankogyo Shinbunsha, Tokyo, 1988) p. 179 (in Japanese).

7) H. T. Kaibe, M. Sakata and I. A. Nishida:J. Phys. Chem. Solids51

(1990) 1083­1087.

8) D. Perrin, M. Chitrob, S. Scherrer and H. Scherrer:J. Phys. Chem. Solids61(2000) 1687­1691.

9) M. Fusa, N. Sumida and K. Hasezaki:Mater. Trans.53(2012) 597­ 600.

10) M. Fusa, N. Sumida and K. Hasezaki:J. Jpn. Soc. Powder Powder Metallurgy59(2012) 121­125.

11) L. V. Poretskaya: Inorg. Mater.23(1987) 1606­1608.

12) M. Sakata:Thermoelectric Conversion, (Syokabo Publishing Co., Ltd., Tokyo, 2005) p. 183 (in Japanese).

13) Y. Suga: Thermoelectric Semiconductor, (Makisyoten, Tokyo, 1966) p. 317 (in Japanese).

14) L. Zhao, P. F. Lu, Z. Y. Yu, T. Gao, C. J. Wu, L. Ding and S. M. Wang:

Solid State Commun.155(2013) 34­39.

0.2 0.4 0.6

1.0 2.0 3.0

0

Po

wer f

actor

,

α

2σ

/1

0

-3Wm -1K

-2

Bi2Te3-xSex/x

Ts=623K

Ts=673K

[image:5.595.68.277.72.213.2]

Ts=723K