Download Crystallographic Anisotropy Control of n-type Bi-Te

Materials Transactions, Vol. 45, No. 3 (2004) pp. 918 to 924
#2004 The Japan Institute of Metals
Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials
via Bulk Mechanical Alloying and Shear Extrusion
Sang Seok Kim* and Tatsuhiko Aizawa
Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan
The shear extrusion processing combined with bulk mechanical alloying is proposed to yield the n-type Bi-Te-Se material from elemental
granules. It has well-developed texture so as to improve the electric conductivity and thermoelectric properties. The shear extrusion processing
of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 alloy green compact can afford the preferred orientation factor of anisotropic crystallographic structure: F ¼ 0:67.
The electric resistivity of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 is controlled to be 0:491 105 (m), which is 0.2 times lower than that of hot-pressed
specimen. Maximum power factor is achieved to be 3:31 103 (W/mK2 ) even without any dopants. The bending strength of the material
produced in this work is also improved to be 166 MPa, 1.7 times higher than that of conventional hot-extruded specimens.
(Received September 4, 2003; Accepted January 6, 2004)
Keywords: n-type thermoelectric materials, shear extrusion processing, anisotropy, orientation factor, bismuth telluride, thermoelectric
properties
1.
Introduction
The Bismuth telluride of Bi2 Te3 has a rhombohedral
structure (a ¼ 0:438 nm and c ¼ 3:049 nm) and belongs to
the space group R3 m. This crystallographic structure is
composed of a five-layer stacking sequence unit of Teð1Þ -BiTeð2Þ -Bi-Teð1Þ . Its successive units have the weak van der
Waals bonding at the interface of neighboring Teð1Þ planes.1,2)
Since the cleavage fracture easily occurs along this weak
bonding plane, the single crystal is difficult to use as the
thermoelectric devices. In order to overcome this difficulty,
the powder metallurgy method was selected for commercial
application of Bi-Te materials.
The Bi-Te based compounds have the highest figure of
merit at room temperature. Since the thermoelectric properties along the a-axis is superior to those along the c-axis, the
single crystal has a preferred anisotropy to increase the
thermoelectric properties.3,4) The polycrystalline material
fabricated via the conventional sintering method has homogeneous microstructure. Since each grain has random
orientation, the original crystallographic anisotropy disappears in the sintered polycrystalline materials and their
figure-of-merit becomes lower than that of the single crystal.
In order to attain higher strength and crystallographic
anisotropy at the same time, it is important to exploit a new
method to control texture formation during sintering. Highly
oriented polycrystalline materials by texture formation are
thought to have the same crystallographic anisotropy as the
single crystal.
The alloyed green compacts of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 is
prepared from elemental granule mixture by the bulk
mechanical alloying (BMA). As suggested by Ref. 5), submicron compound particles produced via BMA are thought to
be effective in texture formation during intense shear
straining. The shear extrusion (SE) process is employed to
enhance the texture development. This process is directly
observed to understand the phenomena of grain rotation and
laminated grain growth during intense plastic deformation.
*Graduate
Student, The University of Tokyo
The correlation between the texture and the thermoelectric
properties is discussed to evaluate the controllability of
thermoelectric properties by shear extrusion. The favored
texture formation is attained even by a single-step shearstraining. Higher bending strength than 150 MPa can be
obtained by the fine-grained microstructure after shear
extrusion.
2.
Experimental Procedure
Elemental granules of Bi (99.999%, 2–5 mm), Se
(99.999%, 1–5 mm) and Te (99.999%, 1–5 mm) were
weighed to have the initial molar ratio with the formula of
(Bi2 Se3 )0:05 (Bi2 Te3 )0:95 , before blending. The blended granule mixture was subjected to BMA for the specified number
of cycles (N). Details of BMA were reported elsewhere.5)
The final product is a high-dense alloyed compact ( ¼
25 mm, h ¼ 21 mm) with the relative density, 85–90% of the
theoretical density, or 85–90% T.D.
In the present experiment, as-BMA preform was pulverized and sieved to have the average agglomerated particle
size of 125 mm. The compacted preform ( ¼ 10:5 mm, h ¼
15 mm, 85% T.D.) was prepared from the sieved particles for
shear extrusion. In the shear extrusion process, the crosshead
speed was kept constant at 0.5 mm/min. The holding
temperature was varied ranging from 643 to 713 K in argon
atmosphere. Crack-free sample was fabricated with the size
of (a) L ¼ 45 (mm), W ¼ 10 (mm) and T ¼ 2 (mm), and, (b)
L ¼ 70 (mm), W ¼ 10 (mm) and T ¼ 2 (mm), as shown in
Fig. 1.
The mechanical properties were measured at room temperature by three-point bending test under a crosshead speed of
0.5 mm/min. The van der Pauw method was employed to
measure the electric resistivity at room temperature. Temperature difference by 10 K was applied to both ends of the
sample to measure the Seebeck coefficient at room temperature. Differential thermal analysis (DTA) was performed at
the heating speed of 10 K/min with the Shimadzu Differential Thermal Analyzer. X-ray diffraction analysis (XRD)
was used to investigate the rearrangement of crystal
Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion
(a)
919
(b)
Fig. 1 Fabricated samples via BMA and SE: (a) 45 (mm) 10 (mm) 2 (mm), and (b) 70 (mm) 10 (mm) 2 (mm).
orientation. The grain rotations and texture development
during shear extrusion were observed by scanning electron
microscopy (SEM).
3.
Results and Discussions
3.1 Solid-state synthesis
(Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solution was synthesized by
BMA. In usual, refining and alloying process monotonically
advances in BMA with increasing the number of cycles. The
number of cycles is usually determined when the initial
element mixture is fully reacted to solid solution of
compounds without any residual elements. Figure 2 compares three DTA curves of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 : (1) after
BMA (N ¼ 400), (2) after BMA (N ¼ 800) and (3) BMA
(N ¼ 800) followed by shear extrusion. At N ¼ 400, two
endothermic peaks were observed. The endothermic peak
between 473 K and 573 K corresponds to the melting of
residual Bi. The endothermic peak near 873 K corresponds to
the melting of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solution. When
increasing the number of cycles to N ¼ 800, the first
endothermic peak completely disappeared. The intensity of
the second endothermic peak was enhanced since the solid
Fig. 2 Comparison of three DTA curves for (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 :
(1) After BMA (N ¼ 400), (2) After BMA (800) and (3) BMA (N ¼ 800)
followed by shear extrusion.
state synthesis advances and more fraction of solid solution is
yielded by further cyclic loading in BMA. When employing
shear extrusion after BMA (N ¼ 800), no change was
observed in DTA. It is concluded that (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solution is stable as a stoichiometric
compound without any dissolution. Hence, the number of
cycle, N ¼ 800, was fixed to make the (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solution. After BMA, its green compact
was shear extruded.
3.2 Texture formation by shear extrusion
Figure 3(a) illustrates the slip line field produced by shear
extrusion. After V. M. Segal,6) the plastic flow in the shear
extrusion is described in what follows. The area ABC denotes
for a ‘dead’ metal zone where no plastic deformation takes
place. The material plastically deforms at the range from AO
to the outlet OD. The triangular area ODE in this crosssectional view corresponds to a rigid zone accompanied by
maximum friction along DE. The gray-colored unit cell of
area ODE is continuously affected by the deformation on the
same slip line located at the area OACD. The deformation
history of shear-extruded unit cell is similar to the all
elements. At the near of point ‘O’, severe shear strain is
applied uniformly to the sample, which is pushed down
vertically at the constant velocity. The applied true shear
strain is estimated, " ¼ 1:16. This value is sufficiently high
enough to enforce the fine-grained texture.7) Figure 3(b)
depicts the load-displacement curves measured during shear
extrusion. The curves consist of three different steps
irrespective of the holding temperature during extrusion.
Two frictional forces must be taken into account to consider
the application of loading. During the (A)-region, relatively
low load is applied against the frictional force f1 along the
vertical channel. In this stage, any unit cells of green compact
do not reach the bottom BD. The (B)-region in Fig. 3(b)
corresponds to the transient state where the backpressure
from the channel begins to increase. In the (C)-region, the
applied load monotonically increases with displacement
since unit cells are subjected to plastic strain by high
backpressure.
The plastic work (Wp ) was calculated from the on-line
measured load-displacement curve. Table 1 lists the calculated plastic work of materials during shear extrusion for
920
S. S. Kim and T. Aizawa
(a)
(b)
Wp ¼ Cf ;
where C is the proper constant. Hence, the above monotonic
decrease of Wp with increasing the holding temperature
mainly comes from the monotonic decrease of the flow stress
with temperature.
SEM micrographs at the same position, P, above AO line
are compared in Fig. 4 between (B)-region and (C)-region in
the loading history. In the (B) region, the material has nearly
the same microstructure as the green compact. Low densification took place and elongation of grains was hardly
observed in Fig. 4(a). On the other hand, densification
advanced significantly by the effect of high backpressure.
The elongated grains by plastic deformation were observed in
the (C)-region, as shown in Fig. 4(b).
Significant texture formation is driven by the shear
extrusion in the inside of ABDEO area. As shown in Fig. 5,
SEM micrographs taken at the different positions in this area
are used to describe the texture formation during the shear
extrusion at 663 K. In parallel with the macroscopic plastic
flow around the L-shaped corner in the shear extrusion
process, each constituent grain is well aligned in the intense
shear-strained region. The well-aligned grain inevitably leads
to the high orientation factor and further results in the low
electric resistivity.
3.3
Fig. 3 Shear extrusion process: (a) Slip line solution, and (b) Loaddisplacement relation.
Table 1
ature.
The plastic work by shear straining for various holding temperShear extrusion temperature (K)
Plastic work (J)
643
663
683
713
491
408
312
234
various holding temperature. Wp monotonically decreases
with increasing the holding temperature. In general, Wp is
roughly estimated by
Z
Wp ¼
e "_e dV
ð1Þ
V
where e and "_e are the equivalent stress and strain rate,
respectively, and V, the volume of a material. In the same
extrusion process, "_e might be assumed to be constant,
irrespective of the holding temperature. Further assuming
that the whole materials except for the dead zone does deform
plastically and e is given by the flow stress (f ), eq. (1) is
rewritten by
ð2Þ
Correlation between the texture and electric resistivity
Figure 6 shows the SEM fractographs of the specimens,
shear-extruded at various holding temperatures. In all the
specimens, the whole grains were aligned to have the
preferred orientation along the extruded direction. At the
relatively low temperature, T ¼ 643 K, the size of laminated
grain is about 4 mm. At T 663 K, the uniform grain growth
was observed; the average grain size is 8.5 mm. Relatively
large grain as well as homogeneously laminated grain at
T 663 K enhances electric conductivity by the reduction of
grain boundary scattering along shear-extruded direction.
To measure the bending strength of shear-extruded specimens, three-point bending tests were carried out. Table 2
shows the bending strength at the various holding temperature for the n-type (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 compound.
The average bending strength of shear-extruded specimen
at 663 K reaches about 166 MPa. This value is 1.7 times
higher than that of the conventional hot-extruded n-type
(Bi2 Se3 )0:05 (Bi2 Te3 )0:95 compound.8) This high strength leads
to improvement of handling and machinability in fabrication
of thermoelectric modules and increases reliability of
modules under extreme service condition.
Figure 7 compares the XRD profiles of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 between the powder sample and the extruded
specimens. The former sample was made by pulverizing the
shear-extruded specimen. Higher intensities are detected
along the orientation perpendicular to the shear-extruded
direction: e.g., of (0015) or (0018) in R3 m for Bi2 Te3 . This
texture development along (00l) direction proves that
significant grain rotation should be accompanied with severe
shear deformation to make arrangement of crystalline
orientation. From this XRD observation, the orientation
Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion
(a)
Fig. 4
(b)
Comparison of microstructure at the same sampling point: (a) Material in region (b), and (b) Material in the region (c).
Fig. 5
Microscopic plastic flow in the shear extrusion along the L-shaped channel at 663 K.
921
922
S. S. Kim and T. Aizawa
Fig. 6
Microstructure of the shear extruded (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 compounds at various holding temperatures.
Table 2 Bending strength of the (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 compounds
fabricated by shear extrusion for various holding temperature.
Shear extrusion temperature (K)
Bending strength (MPa)
643
663
683
713
93.1
166.0
118.1
99.0
Fig. 7 Variation of XRD profiles for the shear extruded (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 specimens with increasing the holding temperature with the
reference of the powder sample.
factor (F) is calculated as an indicator of crystallographic
anisotropy. After Ref. 9), F is defined by
F ¼ ðp po Þ=ð1 po Þ
ð3Þ
where p is the ratio of the integrated intensity along (00l)
directions to the whole integrated intensity for all (hkl)
directions for the texture-developed bulk specimen, and po is
the ratio of the integrated intensity along (00l) directions to
the whole integrated intensity for all (hkl) directions for the
powder sample which was used as a reference. The calculated
orientation factor for each specimen was shown in Fig. 8.
At the relatively low sintering temperature, T ¼ 663 K, the
maximum orientation factor of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 is
0.67. This high orientation factor leads to minimization of
electric resistivity up to 0:491 105 (m) by the texture
formation along a-axis.
In general, the electric resistivity () in (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solution has high sensitivity to the
crystallographic orientation or F. In the case of the single
crystal of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 doped with 0.06% HgBr2 ,
F is 1.0 and is 0:95 105 m.10) On the contrary, the
hot pressed sample has F ¼ 0:03 and ¼ random ¼ 2:40 105 m.11) Hence the shear-extruded specimens might
further lower the value of electric resistivity by the control
of F.
Figure 9 depicts the correlation between the orientation
factor (F) and the electric resisitivity () in (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 solid solutions. For F > 0:6, the measured
Crystallographic Anisotropy Control of n-type Bi-Te-Se Thermoelectric Materials via Bulk Mechanical Alloying and Shear Extrusion
Fig. 8 Variation of the orientation factor (F) for the shear extruded
(Bi2 Se3 )0:05 (Bi2 Te3 )0:95 compound at various holding temperatures.
Fig. 9 Correlation between the orientation factor (F) and electric resistivity () in (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 specimen.
resistivity becomes nearly equal to or less than that of the
single crystals with F ¼ 1. Hence, the electric resistivity can
be sufficiently lowered to that for single crystals when F is
increased to be higher than 0.6 by the shear extrusion process,
irrespective of the holding temperature.
923
Fig. 10 Variation of the electric resistivity and Seebeck coefficient with
increasing the holding temperature for the shear extruded (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 .
m , the effective mass, n, the carrier concentration, h, the
Plank’s constant, s, the scattering parameter and C, the
constant. The first term s in the right-hand side of eq. (5)
depicts the scattering contribution and the second term, lnðnÞ,
the carrier concentration contribution to Seebeck coefficient
(), respectively. When the material has the similar texture,
scattering mode is nearly the same. Hence, in this work, the
variation of Seebeck coefficient with the extrusion temperature is mainly controlled by the carrier concentration.
According to Ref. 13), the Seebeck coefficient is not
dependent on the carrier mobility (). By the relationship
of ¼ 1=ðneÞ, lnðÞ. The Seebeck coefficient is
directly related to the electric resistivity when the scattering
parameter is almost the same. Thermoelectric relationship by
eq. (4) reflects on the result in Fig. 10.
The measured power factors are shown in Fig. 11 with
increasing of the holding temperature. At T 663 K, the
power factors become nearly constant, P ’ ð3 103 )
W/mK2 . The maximum power factor is 3:31 103 W/
mK2 . Table 3 compares the thermoelectric properties among
3.4 Power factor
Figure 10 depicts the variation of electric resistivity and
Seebeck coefficient with increasing the holding temperature.
Variation of the Seebeck coefficient with increasing the
holding temperature is similar to that of electric resisitivity.
After Ref. 12), when assuming the Boltzmann distribution for
the electron, the relationship between Seebeck coefficient and electric resisitivity at the temperature T is given by
B
5
2ð2m kB TÞ3=2
¼
s þ þ ln
ð4Þ
2
e
nh3
¼ s lnðnÞ þ C
ð5Þ
where kB is the Boltzmann constant, e, the electronic charge,
Fig. 11 Variation of the power factors (2 =) for the shear extruded
(Bi2 Se3 )0:05 (Bi2 Te3 )0:95 specimen with the holding temperature in the
shear extrusion.
924
S. S. Kim and T. Aizawa
Table 3 Power factors of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 prepared by various
fabrication methods.
5
(10
P.F. (2 =)
m) (mV/K) (103 W/mK2 )
10)
Single crystal
(with doping 0.06% HgBr2 )
Hot pressing11)
(No doping)
Present method
(No doping)
0.950
210
4.64
2.400
238
2.36
0.854
168
3.31
Acknowledgments
the single crystal, the hot-pressed sample and the shearextruded sample. Significant reduction of electric resistivity
takes place by shear extrusion, resulting in high power factor
than that of hot-pressed sample. With doping of BN, Mg2 Si,
or Si3 N4 , the Seebeck coefficient can be increased by
decreasing the carrier concentration and by increasing the
electric resistivity to the same level as the doped single
crystal. The power factor of shear-extruded sample is
improved as comparable to that of the single crystal by
optimization of doping.
4.
Shear-extruded (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 also has high bending strength of 166 MPa, which is 1.7 times higher than that
of conventional hot-extruded specimen. The present shear
extrusion process is also effective for intense shear-straining
of the bulk thermoelectric materials. It is adaptive as a new
processing route to control the thermoelectric properties even
when starting from the single-crystal rod.
Conclusion
The crack-free thermoelectric plate with the thickness of
2 mm was successfully produced by the bulk mechanical
alloying and shear extrusion process. It is noticeable that the
preferred anisotropic crystallography and high strength is
attained simultaneously by shear extrusion. The anisotropy of
specimen is related to the shear extrusion temperature: F ¼
0:52 at T ¼ 664 K, or F 0:54, Fmax ¼ 0:67 at T 663 K.
This highest orientation factor (Fmax ¼ 0:67) leads to
the lowest of the electric resistivity, 0:491 105 (m).
Thermoelectric properties of (Bi2 Se3 )0:05 (Bi2 Te3 )0:95 are
enhanced by shear-extrusion: e.g., higher power factor of
3:31 103 W/mK2 than that of hot-pressed specimens.
Authors would like to express their gratitude to Mr. J.
Niekawa (Tohoku Okano Electric Co. Ltd.) and Mr. Y.
Suzuki (Okano Electric Co. Ltd.) for their help in measurement of thermoelectric properties. This study is financially
supported in part by the Grand-in-Aid from MEXT with the
contract number of #13852008.
REFERENCES
1) J. R. Drabble and C. H. L. Goodman: J. Phys. Chem. Solids 5 (1953)
142.
2) J. D. Shim, T. Y. Seong and J. M. Kim: J. KIMM 27 (1989) 366–373.
3) I. J. Ohsugi, T. Kojima, M. Sakata, M. Yamanashi and I. A. Nishida: J.
Appl. Phys. 76 (1994) 2235–2239.
4) J. Nagao, M. Ferhat, E. Hatta and K. Mukasa: Phys. Status Solidi(b)
219 (2000) 347–349.
5) T. Aizawa, J. Kihara and D. Benson: Mater. Trans., JIM 36 (1995) 138–
149.
6) V. M. Segal: Mater. Sci. Eng. A345 (2003) 36–46.
7) Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto and T. G. Langdon: Scr.
Mater. 35 (1996) 143–146.
8) J. Seo, K. Park and C. Lee: Materials Research Bulletin 33 (1998) 553–
559.
9) F. K. Lotgering: J. Inorg. Nucl. Chem. 9 (1959) 113.
10) I. Ohsugi and M. Sakata: J. Mater. Sci. Soc. Jap. 32 (1995) 298–302.
11) J. Yang, T. Aizawa, A. Yamamoto and T. Ohta: J. Alloys Compds. 312
(2000) 326–330.
12) H. J. Goldsmid: Recent Trends in Thermoelectric Materials Research I,
Semiconductors and Semimetals 69 (2001) 9–13.
13) W. M. Yim and F. D. Rosi: J. Solid State Electronics 15 (1972) 1121.