Materials Transactions, Vol. 57, No. 6 (2016) pp. 1001 to 1005
©2016 The Japan Institute of Metals and Materials
Constituent Element Addition to n-Type Bi2Te2.67Se0.33 Thermoelectric
Semiconductor without Harmful Dopants by Mechanical Alloying
Kazuhiro Hasezaki1,*1, Sena Wakazuki2,*2, Takuya Fujii2,*2 and Masato Kitamura2,*2
1
Department of Mechanical Science, Graduate School of Technology and Science, Tokushima University, Tokushima 770–8506, Japan
Graduate School of Advanced Technology and Science, Tokushima University, Tokushima 770–8506, Japan
2
N-type Bi2Te2.67Se0.33 thermoelectric materials with added constituent elements were prepared without the addition of harmful dopants
using mechanical alloying (MA) followed by hot pressing (HP). All the prepared samples were identified from the Bi2Te3–Bi2Se3 solid-solution
diffraction peak. They were all single-phase n-type semiconductors. The maximum dimensionless figures of merit, ZT, of Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.03 were 0.93 at 440 K, 0.99 at 441 K, and 0.97 at 442 K, respectively. These results indicate that
the figure of merit of Bi2Te2.67Se0.33 doped with constituent elements, prepared using the MA–HP process, is nearly same as the highest value,
i.e., 1.0, reported in the literature. [doi:10.2320/matertrans.M2016031]
(Received January 26, 2016; Accepted March 23, 2016; Published May 25, 2016)
Keywords: eco-materials, bismuth telluride selenide, n-type, dopant, thermoelectrics
1. Introduction
dopants were prepared using an MA–HP process. The thermoelectric properties of the products were investigated.
Bi2Te3-based p- and n-type materials have been widely
used in thermoelectric cooling devices such as mini refrigerators and central processing unit (CPU) coolers for personal
computers. They are also eco-materials because they can be
used to recover exhaust heat below 500 K by thermoelectric
conversion. These materials have a rhombohedral crystal
structure with the R3m space group and anisotropic physical
and thermoelectric properties1). BiTe–BiSe solid solutions
are n-type materials at room temperature1). The efficiency of
a thermoelectric material is defined by the dimensionless figure of merit ZT:
ZT = α2 σκ−1 T
(1)
−1
where α, σ, κ, and T are the Seebeck coefficient (V K ),
electrical conductivity (S m−1), thermal conductivity
(W m−1 K−1), and absolute temperature (K), respectively. The
highest reported dimensionless figures of merit for halide-doped single crystals of n-type Bi2Te2.88Se0.12 and
Bi2Te3–25Bi2Se3 are 1.02,3). The synthesis of n-type BiTe–
BiSe thermoelectric materials generally involves addition of
harmful halide dopants such as CdBr, CdCl2, HgCl2, HgBr2,
and SbBr31,4–7). The addition of constituent elements (selenium, tellurium, and bismuth) without harmful dopants to control the donor properties of n-type BiTe–BiSe thermoelectric
materials would enable the production of n-type Bi2Te3-based
materials without hazardous substances. This would reduce
the recycling energy needed for the recovery and refinement
of elemental resources from discarded thermoelectric modules. In a previous study, n-type Bi2Te3−xSex (x = 0.15–0.6)
compounds were prepared without harmful dopants by mechanical alloying (MA) followed by hot pressing (HP)8–10).
Recently, first-principles calculations showed that Bi6Te8Se
has a high n-type Seebeck coefficient11).
In the present study, n-type Bi2Te2.67Se0.33 thermoelectric
materials with added constituent elements and no harmful
*1
*2
Corresponding author, E-mail: [email protected]
Graduate Student, Tokushima University
2. Experimental Procedure
The MA powders, which contained no harmful dopants,
Bi2(Te2.67)1+xSe0.33,
and
were
Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67Se0.33)1+x (x = 0.0–0.7). Selenium (1 atom%) was
added to all the powders to compensate for vapor loss, because the vapor pressure of selenium is much higher than
those of tellurium and bismuth; the equilibrium vapor pressures of selenium, tellurium, and bismuth are approximately
200, 0.3, and less than 0.1 Pa, respectively, at 623 K12,13).
The amounts of bismuth (99.999%), tellurium (99.9999%),
and selenium (99.999%) grains needed to give the composiBi2(Te2.67)1+xSe0.33,
and
tions
Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67Se0.33)1+x (x = 0.0–0.7) were placed in stainless-steel
vessels (0.5 L) with Si3N4 ceramic milling balls in an argon-filled glove box. The stainless-steel vessel was sealed to
exclude air. Si3N4 milling balls were selected to avoid contamination from oxidation of the MA powder. The weight
ratio of Si3N4 milling balls to raw materials was 20:1.
MA was performed in a Fritsch P-5 planetary ball-mill at a
maximum speed of 180 rpm for 30 h. The resulting MA powder was passed through a 150 µm diameter sieve and set in an
HP mold in an argon-filled glove box. No grains of the constituent elements or particles of other compositions remained
on the sieve mesh surface. The HP mold filled with MA powder was only exposed to air during transportation between the
argon-filled glove box and the HP chamber. The filled HP
mold was set in the HP chamber, and the chamber was evacuated to less than 2 × 10−3 Pa using a diffusion pump.
The MA powder was sintered at 623 K under an axial compressive pressure of 147 MPa by HP in an argon atmosphere.
The sintered compacts were cylinders of height 9 mm and
diameter 10 mm. All sintered compacts were confirmed dense
materials by the examination of optical microscopy. The sintered compacts were cut into disks 1.0 mm thick and 10 mm
in diameter. Only the central disks were used to determine the
structures and thermoelectric properties. The edges of the
1002
K. Hasezaki, S. Wakazuki, T. Fujii and M. Kitamura
disks were not used because HP affects the plastic deformation and anisotropic thermoelectric properties of these disks.
The disks were examined using X-ray diffraction (XRD)
and the Seebeck coefficients and electrical and thermal conductivities were determined.
XRD measurements were performed with a Rigaku Multiflex diffractometer using Cu Kα radiation at a step size of 0.1
and step speed of 1.0 s/step in the Bragg angle range 2θ = 20–90 .
The Seebeck coefficients α were estimated at room temperature from α = ΔV/ΔT, where ΔV and ΔT are the thermoelectric motive force and temperature differences, respectively. A thermal contact system was used to measure ΔV at a ΔT
value of about 4 K. The thermal contact system was constructed using a copper heating chip, copper base plate, and
constantan block type-T (copper–constantan) thermocouple.
The thermal contact system was calibrated using a standard
Seebeck coefficient material (SRM 3451)10,14,15). The accuracy of Seebeck coefficient with this thermal contact system has
been less than ± 2%.
The electrical conductivity σ was measured at room temperature by the four-point probe method using an electrical
resistance system based on a 2182A/6220 instrument
(Keithley Instruments, Inc.). The probe material and spacing
were WC and 1.00 mm, respectively. The accuracy of electrical conductivity with the four-point probe method has been
less than ± 1%. The electrical conductivity measurement system confirmed ohmic contact for each measurement.
The thermal conductivity was measured at room temperature using a static comparison method. The equipment of a
static comparison method consisted of a film heater, sample
and quartz disks, and water-cooled sink. A film heater of
thickness 0.8 mm was used as a heat source in order to prevent the heat radiation toward the side wall. A quartz disk
1.0 mm thick and 10 mm in diameter was used as the reference material (κ = 1.411 W m−1 K−1). The water-cooled sink
was used for the low temperature side. The chamber of the
equipment was evacuated to less than 0.1 Pa using a vacuum
pump. The thermal conductivity was evaluated from the temperature drops of sample and quartz disks between the heater
and water-cooled sink. The accuracy of thermal conductivity
with this static comparison method has been less than ± 1%.
The room-temperature performance of the thermoelectric
material was estimated using the power factor, P = α2σ.
The temperature dependences of the Seebeck coefficients
and electrical conductivities of the compositions that had
high power factors at room temperature were determined.
Sintered compacts were obtained by HP at 623 K under an
axial compressive pressure of 147 MPa in an argon atmosphere, i.e., the same conditions as for production of the sintered cylinders. The dimensions of the sintered compacts
were 10 mm × 10 mm × 7 mm. The sintered compacts were
cut into samples of dimensions 4 mm × 6 mm × 4 mm. The
centers of the rectangular samples were cut out and used to
investigate the thermoelectric properties, as in the case of the
cylinder central disks. The temperature dependences of the
Seebeck coefficient and electrical conductivity were measured using a ZEM-3 instrument (Advance-Riko) in the temperature range 298–523 K. The accuracy of Seebeck coefficient and electrical conductivity with ZEM-3 has been less
than ± 7%, respectively. The dimensionless figures of merit
(ZT = α2σκ−1T) were measured in the range 298–523 K, with
the thermal conductivity fixed at room temperature.
3. Results and Discussion
All the samples of Bi2Te2.67(Se0.33)1+x, Bi2(Te2.67)1+xSe0.33,
and Bi2(Te2.67Se0.33)1+x (x = 0.0–0.7) prepared without harmful dopants were n-type semiconductors. Figure 1 shows the
XRD patterns of disks of Bi2Te2.67Se0.33, Bi2Te2.67(Se0.33)1+0.07,
Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.07. The compositions of all the disks were identified from the Bi2Te3–Bi2Se3
solid-solution diffraction peaks; all the samples were single
phase. No peaks were observed for elemental bismuth, tellurium, and selenium, or other compositions.
Figure 2 shows the relationships between the Seebeck coefficient and x for Bi2Te2.67(Se0.33)1+x, Bi2(Te2.67)1+xSe0.33, and
Bi2(Te2.67Se0.33)1+x (x = 0–0.07) at room temperature. All the
prepared samples had negative Seebeck coefficients. This
shows that Bi2Te2.67(Se0.33)1+x, Bi2(Te2.67)1+xSe0.33, and
Bi2(Te2.67Se0.33)1+x (x = 0–0.07) show n-type conduction. The
Seebeck coefficient was constant at −300 μV K−1 for selenium addition up to x = 0.03 and increased to −180 μV K−1 at
x = 0.04. In contrast, the Seebeck coefficient increased from
−250 to −180 μV K−1 with increasing tellurium addition. The
added selenium and tellurium act as negative carriers. The
Seebeck coefficient of the materials double-doped with selenium and tellurium remained constant at −180 μV K−1. The
selenium-tellurium combination acts as negative carriers.
The relationships between the electrical conductivity and x
Bi2(Te2.67)1+xSe0.33,
and
for
Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67Se0.33)1+x (x = 0–0.07) at room temperature are
shown in Fig. 3. For selenium addition, the electrical conduc-
Fig. 1 XRD
patterns
of
Bi2Te2.67Se0.33,
Bi2(Te2.67)1+0.07Se0.33, and Bi2(Te2.67Se0.33)1+0.06.
Bi2Te2.67(Se0.33)1+0.07,
Constituent Element Addition to n-Type Bi2Te2.67Se0.33 Thermoelectric Semiconductor without Harmful Dopants by Mechanical Alloying
Fig. 2 Seebeck coefficient α versus x for Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67)1+xSe0.33, and Bi2(Te2.67Se0.33)1+x at room temperature.
Fig. 3 Electrical conductivity σ versus x for Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67)1+xSe0.33, and Bi2(Te2.67Se0.33)1+x at room temperature.
tivity was constant at 2 × 104 S m−1 up to x = 0.03 and rose to
7 × 104 S m−1 at x = 0.04. In contrast, the electrical conductivity increased from 3 × 104 to 8 × 104 S m−1 with increasing
tellurium addition. The electrical conductivity of the material
double doped with selenium and tellurium fluctuated and then
remained constant at 8 × 104 S m−1 with increasing element
addition. If the carrier concentration increases as a result of
element addition, the absolute Seebeck coefficient decreases
and the electrical conductivity increases simultaneously16).
The Seebeck coefficient behaviors in Fig. 2 and electrical
conductivity behaviors in Fig. 3 are consistent with the thermoelectric properties. Doping with selenium, tellurium, and a
combination of these provides donor carriers.
The relationships between thermal conductivity x in
Bi2(Te2.67)1+xSe0.33,
and
Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67Se0.33)1+x (x = 0 to 0.07) at room temperature are
shown in Fig. 4. The thermal conductivities of the doped
disks were stable at 1 W m−1 K−1 and slightly higher than that
of Bi2Te2.67Se0.33, i.e., without dopant addition. The thermal
conductivity of a thermoelectric material is generally the sum
of the phonon and carrier components16). These measured
values are lower than 1.45 W m−1 K−1, which is the minimum
reported value, obtained using the crystal growth method
with a traveling heater17). This is because the conductivity of
the phonon component was reduced as a result of the fine
grain size obtained by MA18). Furthermore, the thermal conductivity of the carrier component is proportional to the elec-
1003
Fig. 4 Thermal conductivity κ versus x for Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67)1+xSe0.33, and Bi2(Te2.67Se0.33)1+x at room temperature.
Fig. 5 Power factor α2σ versus x for Bi2Te2.67(Se0.33)1+x, Bi2(Te2.67)1+xSe0.33,
and Bi2(Te2.67Se0.33)1+x at room temperature.
trical conductivity, and follows the Wiedemann–Franz law, so
the slightly higher thermal conductivities of materials with
additional dopants compared with that of Bi2Te2.67Se0.33 is
consistent with the electrical conductivity results in Fig. 2.
Figure 5 shows the plots of power factor versus x for
Bi2(Te2.67)1+xSe0.33,
and
Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67Se0.33)1+x (x = 0 to 0.07) at room temperature. For
selenium and tellurium additions, the maximum power factors were 2.7 × 10−3 and 2.5 × 10−3 W m−1 K−2, respectively,
at x = 0.06. In contrast, the maximum power factor of the
material double doped with selenium and tellurium was 2.7 × 10−3 W m−1 K−2 at x = 0.03. The maximum of the power factor with added elements for Bi2(Te2.67Se0.33)1+0.03 corresponded to the half of sum with that for Bi2Te2.67(Se0.33)1+0.06 and
Bi2(Te2.67)1+0.06Se0.33. The power factor has the maximum at
a carrier concentrations of around 1025 m−3 3). If added elements are proportional to the carrier concentrations, the behavior of the power factor with added elements is consistent.
The temperature dependences of the Seebeck coefficients
and electrical conductivities of rectangular samples of
Bi2(Te2.67)1+0.06Se0.33,
and
Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67Se0.33)1+0.03, which had higher room-temperature
power factors, were measured.
Figure 6 shows the Seebeck coefficient versus temperature
plots for Bi2Te2.67(Se0.33)1+0.06, Bi2(Te2.67)1+0.06Se0.33, and
Bi2(Te2.67Se0.33)1+0.03. The Seebeck coefficients at 300 K of
all the samples, determined using a thermal contact method,
1004
K. Hasezaki, S. Wakazuki, T. Fujii and M. Kitamura
Fig. 6 Seebeck coefficient α versus temperature T for Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.03.
Fig. 8 Power factor α2σ versus temperature T for Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.03.
Fig. 7 Electrical conductivity σ versus temperature T for
Bi2Te2.67(Se0.33)1+0.06, Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.03.
Fig. 9 Dimensionless figure of merit ZT versus temperature T for
Bi2Te2.67(Se0.33)1+0.06, Bi2(Te2.67)1+0.06Se0.33, and Bi2(Te2.67Se0.33)1+0.03.
were almost the same. The Seebeck coefficients of the all
samples were maximum at around 420 K. All the samples
showed the same Seebeck coefficient behavior.
The plots of electrical conductivity versus temperature for
Bi2(Te2.67)1+0.06Se0.33,
and
Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67Se0.33)1+0.03 are shown in Fig. 7. The electrical conductivities of the all samples at 300 K, measured using a fourpoint probe method, were almost the same. The values for all
the samples decreased with temperature.
Figure 8 shows the power factor versus temperature plots
Bi2(Te2.67)1+0.06Se0.33,
and
for
Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67Se0.33)1+0.03. The maximum power factor for
Bi2Te2.67(Se0.33)1+0.06 was 2.8 × 10−3 W m−1 K−2 at 319 K,
that for Bi2(Te2.67)1+0.06Se0.33 was 2.8 × 10−3 at 367 K, and
that for Bi2(Te2.67Se0.33)1+0.03 was 2.3 × 10−3 W m−1 K−2 at
300 K.
Figure 9 shows plots of the dimensionless figures of merit
(ZT = α2σκ−1T) in the range 298–523 K with the thermal conductivity fixed at room temperature. It was presumed due to
Wiedemann-Franz law and high temperature Umklapp processes that the thermal conductivities of these materials are
approximately constant in these temperature ranges19). The
effects of doping with selenium, tellurium, and a combination
of these were nearly identical. The maximum dimensionless
figures of merit of Bi2Te2.67(Se0.33)1+0.06, Bi2(Te2.67)1+0.06Se0.33,
and Bi2(Te2.67Se0.33)1+0.03 were 0.93 at 440 K, 0.99 at 441 K,
and 0.97 at 442 K, respectively. The maximum dimensionless
figures of merit of Bi2Te2.67(Se0.33)1+0.06, Bi2(Te2.67)1+0.06Se0.33,
and Bi2(Te2.67Se0.33)1+0.03 were nearly the same as the highest
value reported in the literature2,3).
These results show that the maximum figures of merit of
n-type Bi2Te2.67Se0.33 with added constituent elements, produced by MA, were nearly the same. It is not necessary to
dope n-type Bi2Te2.67Se0.33 prepared by MA with harmful elements.
4. Conclusion
In the present study, n-type Bi2Te2.67Se0.33 thermoelectric
materials containing additional constituent elements, without
harmful dopants, were prepared using an MA–HP process.
The thermoelectric properties of the materials were investigated. The results are as follows.
(1) All the prepared samples of Bi2Te2.67(Se0.33)1+x,
Bi2(Te2.67)1+xSe0.33, Bi2(Te2.67Se0.33)1+x (x = 0.0–0.7) were
identified from the Bi2Te3–Bi2Se3 solid-solution diffraction peak. They all consisted of a single phase.
(2) The absolute Seebeck coefficient decreased and the electrical conductivity increased simultaneously on addition
of selenium, tellurium, or a combination of these. The
Constituent Element Addition to n-Type Bi2Te2.67Se0.33 Thermoelectric Semiconductor without Harmful Dopants by Mechanical Alloying
selenium, tellurium, and selenium–tellurium combination acted as donor carriers.
(3) The maximum power factors achieved with selenium and
tellurium additions at room temperature were 2.7 × 10−3
and 2.5 × 10−3 W m−1 K−2 at x = 0.06. The maximum
power factor of the material doped with both selenium
and tellurium was 2.7 × 10−3 W m−1 K−2 at x = 0.03.
(4) The maximum dimensionless figures of merit ZT of
Bi2(Te2.67)1+0.06Se0.33,
and
Bi2Te2.67(Se0.33)1+0.06,
Bi2(Te2.67Se0.33)1+0.03 were 0.93 at 440 K, 0.99 at 441 K,
and 0.97 at 442 K, respectively.
These results indicate that the dimensional figure of merit
of Bi2Te2.67Se0.33 doped with constituent elements, prepared
using MA, is nearly the same as the highest dimensionless
figure of merit, i.e., 1.0, reported in the literature.
REFERENCES
1) H. Scherrer and S. Scherrer: Thermoelectric Handbook: macro to nano,
edit by D. M. Rowe, (CRC Press, Taylor & Francis Group, 2006) ch.27.
2) N.K. Abrikosov, T.E. Svechikova and S.N. Chizhevskaya: Inorg. Mater.
14 (1978) 32–34.
3) D. M. Rowe: Thermoelectric Handbook: macro to nano, edit by D. M.
Rowe, (CRC Press, Taylor & Francis Group, 2006) ch.1
4) H. Kaibe, Y. Tanaka, M. Sakata and I. Nishida: J. Phys. Chem. Solids
50 (1989) 945–950.
5) K. Uemura and I. Nishida: Thermoelectric semiconductor and its applications, (Nikkankogyo Shinbunsha, Tokyo, 1988) pp.179
1005
6) H.T. Kaibe, M. Sakata and I.A. Nishida: J. Phys. Chem. Solids 51
(1990) 1083–1087.
7) D. Perrin, M. Chitrob, S. Scherrer and H. Scherrer: J. Phys. Chem. Solids 61 (2000) 1687–1691.
8) M. Fusa, N. Sumida and K. Hasezaki: Mater. Trans. 53 (2012) 597–
600.
9) M. Fusa, N. Sumida and K. Hasezaki: J. Jpn. Soc. Powder Powder Metallurgy 59 (2012) 121–125.
10) M. Fusa, N. Yamamoto and K. Hasezaki: Mater. Trans. 55 (2014) 942–
946.
11) L. Zhao, P.F. Lu, Z.Y. Yu, T. Gao, C.J. Wu, L. Ding and S.M. Wang:
Solid State Commun. 155 (2013) 34–39.
12) R. Cohen, D. R. Lide, and G. L. Trigg, editors. AlP Physics Desk Reference, 3rd edition. (New York: Springer-Verlag New York, Inc., 2003)
pp.831–833.
13) Y. Suga: Netsudennhanndoutai, (Makisyoten, Tokyo, 1966) pp.317.
14) 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., A Mater. Sci. Process. 96
(2009) 511–514.
15) 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)
1983–1992.
16) K. Uemura and I. Nishida: Thermoelectric semiconductor and its applications, (Nikkankogyo Shinbunsha, Tokyo, 1988), pp.149
17) M. Carle, P. Pierrat, C. Lahalle-Gravier, S. Scherrer and H. Scherrer: J.
Phys. Chem. Solids 56 (1995) 201–209.
18) K. Hasezaki, M. Nishimura, M. Umata, H. Tsukuda and M. Araoka:
Mater. Trans. JIM 35 (1994) 428–432.
19) T. M. Tritt: Thermal Conductivity Theory, Properties, and Applications, Kluwer Academic / Plenum Publishers, New York, (2004), pp.9–
17