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Ball milling effects for induced carriers and reduced grain size on thermoelectric properties inBi1− x Sr x CuSeO

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Japanese Journal of Applied Physics 55, 115801 (2016)
http://doi.org/10.7567/JJAP.55.115801

Ball milling effects for induced carriers and reduced grain size on thermoelectric
properties in Bi1%xSrxCuSeO ðx ¼ 0; 0:1ÞShu Mizuno1, Mamoru Ishizawa1, Hiroyuki Fujishiro1*, Tomoyuki Naito1, Hirokazu Katsui2, and Takashi Goto2
1

Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan2*E-mail: fujishiro@iwate-u.ac.jp
Received May 24, 2016; revised July 23, 2016; accepted July 30, 2016; published online October 5, 2016
We have investigated the ball milling effects for thermoelectric Bi1%xSrxCuSeO ðx ¼ 0; 0:1Þ materials. The characteristic rotation speed R for theball milling exists, at which the crystallite size of the starting powder suddenly decreased. The grain size in the bulks sintered using ball-milled
powders also decreased and thermoelectric properties were enhanced mainly by the induced carriers, rather than by the reduction in thermal
conductivity. The ball milling effects of carrier doping on thermoelectricity are discussed. © 2016 The Japan Society of Applied Physics

1.

Introduction

The thermoelectric technology using waste heat for power
generation devices has been revived because of the recent
energy crisis together with other renewable energy sources.
The performance of thermoelectric materials can be written
using the dimensionless ﬁgure of merit, ZT (= S 2T=ρκ),and the power factor P (= S 2=ρ), where S is the Seebeck
coeﬃcient, T is the absolute temperature, ρ is the electricalresistivity, and κ is the thermal conductivity. Most conventional thermoelectric bulk materials with high ZT values, suchas Bi2Te3 and PbTe, exhibit thermal and chemical instabilities in air at elevated temperatures, and contain toxic,
scarce or expensive elements.1–3) Nevertheless, oxide materials could be potential candidates in thermoelectric applications on the basis of their advantages over heavy metallic
alloys in terms of chemical and thermal robustness, although
the present potential is relatively low.
Typical n-type thermoelectric oxides are SrTi0.8Nb0.2O3(ZT = 0.37 at 1000 K),4) Zn0.98Al0.02O (ZT = 0.30 at
1273 K),5) In2−xGexO3 (ZT = 0.45 at 1273 K),6) and so forth.
On the other hand, typical p-type thermoelectric oxides aremainly layered cobaltites: Ti-doped Ca3Co4O9 (ZT = 0.3
at 1000 K),7) NaxCoO2 (ZT = 0.8 at 1050 K),8) and so forth,in which relatively higher Seebeck coeﬃcients and lower
thermal conductivities are realized using the layered structure. However, it is necessary to explore high-performance
thermoelectric oxide polycrystals for practical applications.
Recently, oxyselenides, BiCuSeO, have been paid considerable attention for a relatively higher thermoelectric performance from 700 to 1000 K.9–21) This material crystallizes ina layered crystal structure that is composed of conductive[Cu2Se2]2− layers constituting a conduction pathway for
carrier transport alternately stacked with insulating [Bi2O2]2+
layers acting as a charge reservoir along the c-axis of thetetragonal unit cell, which is similar to layered cobaltites andiron-based superconductors.10,11) The main features of the
BiCuSeO system can be summarized as follows: a moderateeﬀective mass (m+ ∼ 0.6m0), a low hole mobility (2–20
cm2 V−1 s−1) and a relatively high carrier concentration (n ∼1018 cm−3).17) The thermal conductivity κ is low (= 4.6
mW cm−1 K−1 at 873 K),16) resulting from a large Gruneisenparameter (γ ∼ 1.5) and a low Young’s modulus (E ∼ 76.5
GPa).21) The presence of Bi, which is a heavy element andis known to exhibit strong anharmonic vibrations, is also

detrimental for heat transfer. To enhance the thermoelectric
properties, BiCuSeO showed an excellent charge transport
behavior owing to the introduction of hole carriers by
replacing Bi3+ with divalent alkaline-earth ions such as Sr 2+,
Ba2+, Ca2+, and Pb2+ in the insulating [Bi2O2]2+ layer.16,17,21)
In the [Cu2Se2]2− layer, the replacement of the Se2− sitewith the isovalent Te2− for the opening of a small gap inthe electronic structure22) and the deﬁciency of Cu+ 23) werealso investigated by the introduction of hole carriers.
Hole doping also induces point defect scattering, which
reduces the thermal conductivity. The highest ZT of 1.4
was achieved at 923 K for the Bi0.875Ba0.125CuSeO sample
to date.24)
A ball milling technique is widely used to pulverize
powder ﬁnely before spark plasma sintering (SPS) to reduce
the thermal conductivity of bulk materials. For the pristine
BiCuSeO compound, Li et al. fabricated bulk samples by
SPS using BiCuSeO powders ball-milled at 250 rpm forvarious milling times from 0 to 1500 min. They investigated
the thermoelectric properties of the sintered bulks,25) in which
ZT was increased mainly by increasing the electrical conductivity. Pele et al. investigated the milling time (80–600min) and rotation speed (400–800 rpm) dependences of
BiCuSeO powders by mechanical alloying (MA) and, subsequently, the bulk samples were fabricated by SPS using
powders under diﬀerent MA conditions,26) in which the
optimum MA condition exists for the starting powders.
The ball milling procedure, at the same time, introduces adeﬁciency of the metal element and oxygen, and also creates
an amorphous phase. These crystal defects introduce electron
or hole carriers to the crystallites and subsequently sinteredbulks, which are supposed to aﬀect the electrical and thermoelectric properties. However, the ball milling eﬀect on the
crystallite size and the relationship between the initial crystallite size and the subsequent grain size of the sintered bulk
fabricated by SPS have not been clariﬁed systematicallyso far. Also, the mechanism of the charge carrier creation
has not been investigated in detail.
In this study, we performed the ball milling procedure
for the nondoped and Sr-doped Bi1−xSrxCuSeO ðx ¼ 0; 0:1Þ
powders under various rotation speeds (R), in which we
selected Sr as a dopant for the Bi site. Because Sr is a popular
dopant in divalent alkaline-earth ions, we investigated theeﬀects of ball milling on the crystallographic and thermoelectric properties of the Bi1−xSrxCuSeO system.

115801-1© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 115801 (2016)

2.

S. Mizuno et al.

Experimental procedure

To fabricate Bi1−xSrxCuSeO ðx ¼ 0; 0:1Þ materials, Bi(99.9%) and Bi2O3 (99.9%), SrO (99.5%), Cu (99.9%), Se(99.9%), and SrO (99.5%) ﬁne powders were used as raw
materials. These were weighed with a stoichiometoric ratio of
Bi : Sr : Cu : Se ¼ 1  x : x : 1 : 1 and mixed in Ar atmosphere. After the mixed raw materials were evacuated andsealed in a quartz tube, they were heated at 973 K for 12 h andthen furnace-cooled. The obtained precursor was pulverized
and mixed in air for 0.5 h using an automatic mortar. The
Bi1−xSrxCuSeO powder obtained at this stage was named the“0 rpm” powder. Using this powder, four types of Bi1−xSrxCuSeO starting powders of about 9 g were ball-milled using aplanetary ball milling apparatus (Retsch PM100) for 6 h with25 stainless steel (SUS304) balls (0.95 cm in diameter) and avial (77 cm3) under the rotation speed R from 100 to 400 rpm.
The volume ratio of the powder to balls in the vial is about
1 : 11. The apparatus was stopped for 1 min every 1 h andcounter-rotated for milling eﬀectively. These ball-milled
powders were named “R = 100 rpm” to “R = 400 rpm”. The
powders were sintered using the SPS apparatus (NJSLABOX-110C) under a uniaxial pressure of 40 MPa at about
973 K for 10 min in vacuum. The mass density of the obtainedpellets (10 mm in diameter and 9 mm in thickness) was higher
than 93% of the ideal density.
Powder X-ray diﬀraction (XRD) measurements were
performed (Rigaku Multi Flex) at room temperature using
Cu Kα radiation to estimate the crystallite size of the starting
powder using the Scherrer equation,27) and to conﬁrm theimpurity phase in the starting powders and sintered bulks.
The microstructure and chemical composition of sinteredbulks were analyzed by electron probe microanalysis using
a ﬁeld emission gun (FE-EPMA) and scanning electron
microscopy (SEM) coupled with electron backscatter diffraction (EBSD; Oxford NordlysNano). The grain size in thesample was estimated using the software equipped in the
EBSD apparatus. The sintered pellet fabricated by SPS wassliced perpendicularly to the uniaxial pressure direction,
rectangular bars were prepared, and the thermoelectric
properties were measured. We conﬁrmed that the thermoelectric properties were independent of the cutting directionof the bars, although the crystal structure of BiCuSeO is
anisotropic. The electrical resistivity ρ(T ) and the Seebeck
coeﬃcient S(T ) were simultaneously measured at the temperature range from 300 to 973 K using an automated measuringsystem (Ozawa Science RZ2001i) and the thermoelectric
power factor P = S 2=ρ was calculated. The thermal conductivity κ(T ) was measured in vacuum by a laser ﬂash
method (Ulvac-Riko TC-7000) from 300 to 873 K. To
examine the accuracy of κ(T ), we also estimated κ (T) using
the relation κ = DαC, where α is the thermal diﬀusivity,
which was measured by a laser ﬂash method, C is the speciﬁc
heat, which was measured by diﬀerential scanning calorimetry (DSC), and D is the mass density, which was measured
by the Archimedes method. The diﬀerence in κ(T ) between
the two methods was estimated to be about 20%. The ρ(T )and κ(T ) below 300 K were also measured by a conventional
four-terminal method and a steady-state heat ﬂow method,respectively, using a home-made measuring system.28) The
dimensionless ﬁgure of merit, ZT = S 2T=ρκ, was estimatedFig. 1. (Color online) (a) Normalized XRD patterns and (b) (102) peakintensity of the BiCuSeO starting powders, which were ball-milled at various
rotation speeds (R). In (b), the FWHM of the (102) peak of the powders is
also shown in the right ordinates.

using the obtained values, in which the involved uncertainty
is about 20%.
3.

Results and discussion

Figures 1(a) and 1(b) show the normalized XRD patterns and(102) peak intensity of the BiCuSeO (x = 0) starting powders,
respectively, which were ball-milled at various rotation
speeds (R). The full width at half maximum (FWHM) of the(102) peak for the powders is also shown in the right ordinatesof Fig. 1(b). The XRD pattern of the R = 100 rpm powder isnearly identical to that of the R = 0 rpm powder, but that of theR = 200 rpm powder became fairly broadened and the XRD
intensity also decreased. The FWHM of the (102) peaksuddenly increased for the R = 200 rpm powder and increased
gradually with increasing R. For the R = 300 and 400 rpm
powders, the width and intensity of the diﬀraction peakchanged moderately. These ball-milling eﬀects can be seenusually, during the decrease in crystallite size and the creationof the amorphous phase. No impurity phase was detected by
the XRD measurements.
Figure 2 shows EBSD images of the BiCuSeO bulks
sintered by SPS using powders of R = 0, 200, and 400 rpm.
The crystal grains were randomly oriented and the grain size
decreased with increasing R. Note that there are manyelongated crystals in the R = 400 rpm sintered bulk as shownin Fig. 2(c), which were also reported in a previous paper.24)
Figures 3(a) and 3(b) present the crystallite size of theBi1−xSrxCuSeO starting powders ðx ¼ 0; 0:1Þ estimated using
the Scherrer equation and the grain size in the sintered pellets

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Jpn. J. Appl. Phys. 55, 115801 (2016)

S. Mizuno et al.

Fig. 2. (Color online) EBSD images of the BiCuSeO sintered bulks fabricated using powders at (a) R = 0 rpm, (b) R = 200 rpm, and (c) R = 400 rpm.

Fig. 3. (Color online) (a) Crystallite size of the Bi1−xSrxCuSeO
ðx ¼ 0; 0:1Þ starting powders estimated using Scherrer equation and (b) grain
size in the sintered pellets estimated by EBSD observation, as a function of
the rotation speed R.

estimated by EBSD observation, respectively, as a functionof the rotation speed R. In Fig. 3(a), the crystallite size of thestarting powders decreases with increasing R and a steplikedecrease can be clearly observed at R = 200 rpm, which isconsistent with the results in Fig. 1(b). The crystallite size ofthe x = 0.1 series is smaller than that of the x = 0 series,
which originates from the suppression of the grain growth
due to the Sr substitution during the precursor synthesis in thesealed tube. After the subsequent SPS process, the grain sizeof the bulk samples becomes about one order of magnitudelarger than the crystallite size of the starting powders dueto the grain growth. However, the sintered bulk samples
took over the trend of the starting powders; the grain size
estimated by EBSD suddenly decreased at R = 200 rpm forboth series. The decrease in grain size induces the increase in
the number of grain boundaries and, as a result, is expected toFig. 4. (Color online) Temperature dependences of the (a) electricalresistivity ρ(T), (b) Seebeck coeﬃcient S(T), and (c) thermoelectric power
factor P(T) of the BiCuSeO bulk samples fabricated using starting powders
at various rotation speeds (R).

decrease the thermal conductivity. The existence of thecharacteristic rotation speed and milling time is also reported
in other papers.25,26)
Figure 4 shows the temperature dependences of theelectrical resistivity ρ(T ), the Seebeck coeﬃcient S(T ), and115801-3© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 115801 (2016)

S. Mizuno et al.

the thermoelectric power factor P(T ) of the BiCuSeO sinteredbulks fabricated using starting powders of various rotation
speeds (R). In Fig. 4(a), low-temperature (T < 300 K) ρ(T )values are also shown, in which a slight discontinuity can beseen around 300 K from the results obtained using diﬀerent
measuring apparatuses above and below 300 K. The hightemperature ρ(T ) (T > 300 K) of all the samples presents a
semiconducting behavior of ρ(T ) decreasing with increasing
T. The ρ of the bulk without ball milling (R = 0 rpm) is thehighest, but ρ is minimum for the R = 200 rpm sample andthen increases with R. On the other hand, at T < 300 K, theρ(T ) values of the R = 0 and 400 rpm samples show a semiconducting behavior, but those of the R = 200 and 300 rpm
bulk samples show a rather metallic behavior. Since the lowtemperature ρ(T ) reﬂects sensitively the number of carriers,
these results suggest that the carriers were eﬀectively createdduring the ball milling process under the rotation speed from100 to 300 rpm. The increased number of acceptor-like crystal
defects was produced via broken layered structures throughmechanical stress during the ball milling process.
In Fig. 4(b), the S(T) value of the R = 200 rpm sample isthe lowest at T > 473 K and, roughly, all the S(T ) values
decrease with increasing T at higher temperatures. Since
the Seebeck coeﬃcient is inversely proportional to log n(n: carrier concentration) in a classical wide-gap semiconductor model, the reduced S(T ) value might be causedby the enhanced carrier concentration, and the ρ(T ) and S(T )values are closely related to each other. The thermoelectric
power factor P(T ) (= S 2=ρ), as shown in Fig. 4(c), increases
with T, and that of the R = 200 rpm sample is the highest,
which results from both the lowest ρ(T) and not so small
S(T ) values.
We consider the reason why the ρ values of the BiCuSeO
samples decrease during the ball milling process. If the
stoichiometry of the BiCuSeO compound is ideally realized,
in which [Cu2Se2]2− and [Bi2O2]2+ layers are alternately
stacked, there are no mobile carriers and then ρ(T ) becomes
fairly large and S(T ) also shows either a large positive or
negative value. However, the present BiCuSeO bulk samples
showed p-type conductivity owing to the introduction of avery small number of hole carriers, which may originate fromthe nonstoichiometry of the constituent elements. If a lower
level of Cu deﬁciency is introduced to the nondoped
BiCuSeO powder during ball milling, hole carriers are
created as shown by the following defect equation:23)
2BiCu1x SeO ¼ ½Bi2 O2 2þ þ ½Cu2ð1xÞ Se2 ð2þxÞþ xVCu  þ 2xhþ :ð1Þ

If Bi deﬁciency is introduced into the BiCuSeO crystal,hole carriers are also created as shown by the following
defect equation:
2Bi1x CuSeO ¼ ½Bi2ð1xÞ O2 ð2xÞþ þ ½Cu2 Se2 2þ xVBi  þ 2xhþ :ð2Þ

On the other hand, in the cases of Se and O deﬁciencies
during ball milling, which are easy to create because of highvapor pressure, electron carriers should be created from theviewpoint of electrical neutrality.
The results shown in Figs. 4(a) and 4(b) simply suggestthat not the electrons but the holes increased in number inthe powders during ball milling. If electron carriers are

Fig. 5. (Color online) Temperature dependences of the thermal
conductivity κ(T ) values of the BiCuSeO bulk samples fabricated using
starting powders at various rotation speeds (R) at (a) T < 230 K and(b) T > 300 K. (c) Temperature dependence of the thermoelectric ﬁgureof merit, ZT(T), of the BiCuSeO bulk samples.

created during ball milling mainly owing to the Se and O
deﬁciencies, intrinsic holes might be compensated by doped
electrons. As a result, ρ(T ) and S(T ) might be increased. The
possibility that the Cu and=or Bi elements are preferentiallydepleted during ball milling is an open question. The
depletion of [Bi2O2]2+ layers during ball milling may also
introduce hole carriers into the conductive [Cu2Se2]2− layers.
We think that hole carriers are created by the crystal defects
or imperfections on the grain surface. Small amounts of otherimpurity phases, which were not detected by XRD measurements, may be introduced to the ball-milled powders and act
as mobile holes. The applied energy during the milling
process increases proportionally to the square of the rotation
speed, R2, and the shock frequency increases with the numberof balls and the rotation speed R.25) Although the saturation
trend for R > 200 rpm can be seen for the crystallite size ofthe starting powders and the grain size in the bulk samples,thermoelectric properties change intricately depending on thenumber and species of mobile carriers through the rotation
speed R.
Figures 5(a) and 5(b) show the temperature dependencesof the thermal conductivity κ (T ) values of the BiCuSeO
bulk samples at T < 230 K and T > 300 K, respectively.
In Fig. 5(a), a large and sharp κ(T ) peak is seen at low
temperatures for the R = 0 and 100 rpm samples; κ (T ) mustbe zero at T = 0 K, although we cannot conﬁrm the κ(T )peak experimentally. However, the κ(T ) of the samples of
R > 200 rpm sharply decreased with a broad peak owing to115801-4© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 115801 (2016)

S. Mizuno et al.

the increase in crystal defect density. Since the electronic
thermal conductivity κe, which was estimated on the basis of
the Wiedemann–Franz law, was negligibly small, the heattransport was performed entirely by phonons. These results
suggest that the R = 0 and 100 rpm samples are relatively
clean for phonon transport, and that phonon–phonon
scattering, which is inversely proportional to temperature,
is dominant. However, the phonon scattering due to the
crystal defects introduced by ball milling increases for the
R > 200 rpm samples. In Fig. 5(b), κ (T ) at T > 300 K hardly
depends on the rotation speed R at higher temperatures. This
result originates from the dominant phonon scattering due tothe intrinsic lattice vibration at high temperatures. Figure 5(c)shows the temperature dependence of the thermoelectric
ﬁgure of merit, ZT(T ). The ZT values of all the samples
increase with increasing temperature. ZT(T ) is maximum for
the R = 200 rpm sample and then decreases with increasing
rotation speed R, which closely reﬂects the thermoelectric
power factor P(T ). Because κ (T) hardly depends on therotation speed R at higher temperatures, the maximum ZT
of 0.20 was achieved at 773 K for the nondoped BiCuSeO
bulk fabricated using the R = 200 rpm powder.
Next, we consider the ball milling eﬀect for the Sr-doped
Bi1−xSrxCuSeO samples (x = 0.1). Figure 6 shows thetemperature dependences of the electrical resistivity ρ(T ),the Seebeck coeﬃcient S(T ), and the thermoelectric power
factor P(T) of the Bi1−xSrxCuSeO samples (x = 0.1) using
starting powders fabricated at various rotation speeds R. The
ρ(T ) values of all the samples present a metallic behavior.
The ρ of the R = 0 rpm bulk is the lowest and increases withR at low temperatures. On the other hand, the ρ(T ) at
T > 300 K seems to be independent of R. These results
suggest that the charge carriers were mainly created by the Sr
substitution and that the crystal defects introduced by ball
milling reduced the electrical conductivity at low temperatures, which is in clear contrast to that observed in the
nondoped BiCuSeO samples shown in Fig. 4. In Fig. 6(b),the S(T ) values of all the samples increase with increasing T
and the S(T) of the R = 200 rpm sample is the largest. The
P(T ) value, as shown in Fig. 6(c), increases with T, and thatof the R = 200 rpm sample is the highest, which results fromlarger S(T ) values in the samples.
Figures 7(a) and 7(b) show the temperature dependencesof the thermal conductivity κ (T ) values of the Bi1−xSrxCuSeO
bulk samples (x = 0.1) at T < 230 K and T > 300 K, respectively. In Fig. 7(a), a small κ(T ) peak can be seen at low
temperatures for the R = 0 and 100 rpm samples. However,the κ(T ) peak of the R > 200 rpm samples disappeared andthe absolute κ(T ) value decreased. As shown in Fig. 7(b),κ (T ) at T > 300 K slightly decreased with increasing rotation
speed R. Figure 7(c) shows the temperature dependenceof the thermoelectric ﬁgure of merit, ZT(T ), of the Bi1−xSrxCuSeO bulk samples (x = 0.1). The ZT values of all thesamples increased with increasing temperature. The ZT(T )value became maximum for the R = 200 rpm bulk and thendecreased with increasing rotation speed R, which reﬂectedclosely the thermoelectric power factor P(T ). Because κ(T )slightly decreased with the increase in R at higher temperatures, the maximum ZT of 0.46 was achieved at 773 K
for the Bi1−xSrxCuSeO bulk (x = 0.1) fabricated using theR = 200 rpm powder.

Fig. 6. (Color online) Temperature dependences of the (a) electricalresistivity ρ(T), (b) Seebeck coeﬃcient S(T), and (c) thermoelectric power
factor P(T) of the Bi1−xSrxCuSeO bulk samples (x = 0.1) fabricated using
starting powders at various rotation speeds (R).

Figures 8(a) and 8(b) show the electrical resistivity ρ andthe Seebeck coeﬃcient S at 773 K for the Bi1−xSrxCuSeO
samples (x = 0.1) fabricated using starting powders at various
rotation speeds (R), compared with those for the x = 0 bulk
samples. The ρ values at 773 K for the x = 0.1 samples areabout one order of magnitude smaller than those for the x = 0
samples owing to the carrier doping, and the S values at
773 K for the x = 0.1 samples are about 50% smaller thanthose for the x = 0 samples. Although the R dependences ofthe ρ and S values of the x = 0.1 series are relatively small,a small local maximum of the S value can be seen for
R = 200 rpm. Figure 8(c) presents the R dependence of thethermoelectric power factor P at 773 K for the x = 0 and 0.1
series. The P value for the x = 0.1 series is three to four times
larger than that for the x = 0 series, and the maximum P value
can be obtained for the R = 200 rpm bulk for both series. The

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Jpn. J. Appl. Phys. 55, 115801 (2016)

S. Mizuno et al.

Fig. 7. (Color online) Temperature dependences of the thermal
conductivity κ(T ) values of the Bi1−xSrxCuSeO bulk samples (x = 0.1)
fabricated using starting powders at various rotation speeds (R) at(a) T < 230 K and (b) T > 300 K. (c) Temperature dependence of thethermoelectric ﬁgure of merit, ZT(T), of the Bi1−xSrxCuSeO bulk samples(x = 0.1).

Fig. 8. (Color online) (a) Electrical resistivity ρ, (b) Seebeck coeﬃcient S,
and (c) power factor P at 773 K for the Bi1−xSrxCuSeO bulk samplesðx ¼ 0; 0:1Þ fabricated using starting powders at various rotation speeds (R).

maximum P value of 5.5 × 10−4 W K−2 m−1 was achieved at
773 K for the Bi1−xSrxCuSeO bulk (x = 0.1) fabricated using
the R = 200 rpm powder.
Figure 9(a) shows the total thermal conductivity κ, which
was measured by a laser ﬂash method, and the electronic
thermal conductivity κe at 773 K, which was estimated on thebasis of the Wiedemann–Franz law, for the x = 0 and 0.1
series, as a function of the rotation speed, R. The κ of thex = 0.1 series, which is the sum of κe and the lattice thermal
conductivity κlattice (κ = κe + κlattice), is larger than that of thex = 0 series, which results from the large contribution ofκe for the x = 0.1 series; about 30% of κ is κe. κ slightly
decreased with increasing R. Figure 9(b) presents the ZT
value at 773 K for the x = 0 and 0.1 series as a function of R.
The ZT for the x = 0.1 series is about two to three times larger
than that for the x = 0 series, which results mainly from thereduction in thermal conductivity κ, and from the increasein P. The maximum ZT can also be obtained for theR = 200 rpm bulks for the x = 0.1 series. The maximum ZTvalue of 0.46 was achieved at 773 K for the Bi1−xSrxCuSeO
bulk (x = 0.1) fabricated using the R = 200 rpm powder.
4.

Conclusions

We have investigated the ball milling eﬀects for the starting
powders of Bi1−xSrxCuSeO ðx ¼ 0; 0:1Þ on the thermoelectric
properties of the subsequent sintered bulks by SPS. The
important results and conclusions obtained from this study
are summarized as follows.

Fig. 9. (Color online) (a) Total thermal conductivity κ and the electronic
thermal conductivity κe at 773 K for the x = 0 and 0.1 series as a function of
the rotation speed R. (b) ZT values at 773 K for the x = 0 and 0.1 series as afunction of R.

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Jpn. J. Appl. Phys. 55, 115801 (2016)

S. Mizuno et al.

1) The crystallite size of ball-milled Bi1−xSrxCuSeO
powders decreased suddenly at the characteristic rotation
speed of R = 200 rpm and then gradually with increasing R.
The grain size of the Bi1−xSrxCuSeO bulks also showed
the same tendency, depending on the crystallite size of thestarting powders.
2) For the Bi1−xSrxCuSeO (x = 0) material, the electricalresistivity ρ(T ) and the thermal conductivity κ(T ) changed
depending on the rotation speed for the starting powders
at T < 300 K; mobile carriers were created during the ball
milling process, and ρ(T ) showed a metallic behavior. The
κ (T ) peak was observed for the R = 0 and 100 rpm bulks, but
was suppressed for the R > 200 rpm bulks owing to theintroduction of crystal defects. At T > 300 K, the ρ(T ) of theR = 200 rpm bulk was the lowest, and the Seebeck coeﬃcientS(T ) did not decrease. As a result, the power factor P(T )of the R = 200 rpm bulk was the largest. The κ(T ) of thebulks hardly depended on the rotation speed of the starting
powders, suggesting that the phonon scattering mainly
originates from the lattice vibration.
3) For the Bi1−xSrxCuSeO (x = 0.1) material, ρ(T) decreased with carrier doping by Sr 2+ substitution for the Bi3+ site
and the P(T ) value was three to four times larger than thatfor the x = 0 series. κ (T ) was slightly larger than that forthe x = 0 series owing to the increase in the electronic
thermal conductivity κe. The additional increases in P and
the ﬁgure of merit, ZT, were conﬁrmed at R = 200 rpm owing
to the ball milling, similarly to that observed for the x = 0
series.
Acknowledgment

This work was performed under the Inter-university Cooperative Research Program of the Institute for Materials
Research, Tohoku University.

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