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2013 Jpn. J. Appl. Phys. 52 031102(http://iopscience.iop.org/1347-4065/52/3R/031102)
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REGULAR PAPER

Japanese Journal of Applied Physics 52 (2013) 031102
http://dx.doi.org/10.7567/JJAP.52.031102

Thermoelectric Properties of Li-Doped CuONaoki Yoshida, Tomoyuki Naito, and Hiroyuki Fujishiro
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
E-mail: fujishiro@iwate-u.ac.jp
Received October 17, 2012; accepted January 8, 2013; published online February 27, 2013
Thermoelectric properties of CuO doped with alkali metal ions (Liþ , Naþ , Kþ ) were investigated at temperatures up to 1273 K. Among the doped
alkali metal ions, Liþ ion was the most effective to enhance the thermoelectric properties. The maximum dimensionless thermoelectric figure of
merit Z T and power factor P were, respectively, 3:2  102 and 1:6  104 W K2 m1 at 1246 K for the 3.0%-Li-doped CuO. The enhancementof the thermoelectric properties of the CuO system was discussed. # 2013 The Japan Society of Applied Physics

1. Introduction

The thermoelectric technology of using waste heat for power
generation devices has been revived because of the recent
energy crisis along with other renewable energy sources.
Since the discovery of NaCo2 O4 in 1997,1) oxide thermoelectric materials have been intensively studied using
Ca–Co–O and NaCo2 O4 systems2–5) with layered Co–O
octahedra, as a substitute for conventional semiconductors
such as Bi2 Te3 . However, the thermoelectric performanceis low. In material research for thermoelectricity, we focus
on the possibility of a conventional CuO, because it isinexpensive, plentiful and nontoxic. CuO is known to be ametal-deﬁcient p-type semiconductor with a band gap of
1.2 eV and a monoclinic crystal structure, and many studies
have been reported on its physical properties.6–11) Highpurity CuO has a large positive Seebeck coeﬃcient S, butshows both a high electrical resistivity  and a high thermal
conductivity . If the doping or substitution of univalentor trivalent ions is possible for the Cu site, the electricalresistivity and the thermal conductivity might be decreased.
In this case, the thermoelectric eﬃciency may be enhanced if
the Seebeck coeﬃcient S is not greatly deteriorated. Similar
to CuO, ZnO is plentiful and has a simple wurtzite structure,making it a promising material for n-type thermoelectricity.12) The electrical resistivity drastically decreases upon
doping of a small amount of Al3þ and Ga3þ , and the nondimensional thermoelectric ﬁgure of merit ZT (¼ S 2 T =)reaches 0.3 and 0.65 at 1273 K for Zn0:98 Al0:02 O and
Zn0:96 Al0:02 Ga0:02 O, respectively.13,14) For the CuO system,
the electric conductivity  (¼ 1=) and Seebeck coeﬃcient
S have been reported for Li- and Al-doped CuO,15) in which
the Li-doping enhances the p-type electrical conductivity.
However, a systematic investigation has not been performed
as a thermoelectric material with respect to the doping
species and their optimum concentrations.
In this study, we investigated the CuO polycrystals dopedwith alkali metals (Li, Na, K) and measured the electricaland thermal properties. The potential of CuO as a thermoelectric material was discussed.
2. Experimental Procedure

CuO polycrystals doped with an alkali-metal element(Li, Na, K) were prepared by a conventional solid-state
reaction. For Li-doping, CuO (Furuuchi Chemical or DOWA
Mining, 99.9%) and Li2 CO3 (Furuuchi Chemical, 99.9%)
were mixed with a molar ratio of Cu : Li ¼ 1  x : x(0  x  0:07) and were calcined at 760  C for 12 h inair. In this study, we abbreviate the Li-doped CuO as(Cu1x Lix )O. For Na- and K-doping, only x ¼ 0:01 samples
were prepared, using Na2 CO3 (Furuuchi Chemical, 99.9%)
and K2 CO3 (Furuuchi Chemical, 99.9%) as raw materials.
The precursors were re-ground, pressed cold-isostaticallyunder 64 MPa into a bar and then sintered at 1060  C for
10 h in oxygen ﬂow. The measured densities of each sample
were greater than 95% of the ideal density. A rectangularspecimen of about 2  2  7 mm3 was cut from the bar
for the measurements of thermoelectricity. Powder X-ray
diﬀraction measurements were performed at 300 K using
Cu K radiation (Rigaku Multi Flex) in the range of 20 
2  90 with 0.02 steps, and lattice parameters were determined using a Rietveld proﬁle reﬁnement program, RIETAN
2000.16) The electrical resistivity ðT Þ [or electrical conductivity ðT Þ ¼ 1=ðT Þ] and Seebeck coeﬃcient SðT Þ were
simultaneously measured in the temperature range from 300
to 1273 K using an automated measuring system (Ozawa
Science RZ2001i) and the thermoelectric power factor P ¼
S 2 = was calculated. Low-temperature thermal conductivityðT Þ was measured by a steady-state heat ﬂow method from10 to 300 K using a home-made apparatus combined with ahelium refrigerator.17,18) High-temperature thermal conductivity ðT Þ was also measured by a laser ﬂash method(Ulvac-Riko TC-7000) up to 1023 K. ðT Þ was extrapolated
up to 1300 K using a ﬁtting curve for the relationship withT 1 for the experimental ðT Þ results at low and hightemperatures. A dimensionless ﬁgure of merit ZT ¼ S 2 T =
was estimated using these values.
3. Results and Discussion
3.1 Doping effect of alkali metal in CuO on
thermoelectricity

The doping eﬀect of alkali metals in CuO on the electricalconductivity ðT Þ ¼ 1=ðT Þ and Seebeck coeﬃcient SðT Þ
was investigated. Figure 1(a) shows the temperature dependence of ðT Þ of pure (nondoped) CuO and 1.0% alkalimetal (M = Li, Na, K)-doped Cu0:99 M0:01 O. ðT Þ among all
the samples show a semiconducting behavior and increase
with increasing temperature. The plot of the log  vs 1=T
curve is linear with a bending point at T  . The activationenergy at low temperature (T < T  ) is lower than that at
high temperature (T > T  ). T  of the nondoped CuO isabout 720 K. It increases with increasing ðT Þ for the doped
CuO. Li-doped CuO shows the highest ðT Þ, which is aboutfour orders of magnitude larger than that for the pure CuO

031102-1# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 031102
800

(a)

CuO 1.0%-doped

101

Seebeck Coefficient ( µV/K)

102

N. Yoshida et al.

(S/cm)

100
10-1
10-2
-3

10

T*

10-4

nondoped
Li-doped
Na-doped
K-doped

10-5
0.5

1

1.5

2

2.5

(b)

400
200

1000/T (K-1)

nondoped
Li-doped
Na-doped
K-doped

0
-200
0.5

3

CuO 1.0%-doped

600

1

1.5
2
1000/T (K-1)

2.5

3

Fig. 1. (Color online) Temperature dependence of (a) electrical conductivity ðT Þ and (b) Seebeck coeﬃcient SðT Þ of pure (nondoped) and 1.0% alkalimetal (Li, Na, K)-doped CuO. The arrows show the inﬂection temperature T  (see text).

 Naþ
at 400 K. Although the ionic radii of Liþ (0.76 A),þ and K (1.38 A)
 increase with increasing atomic(1.02 A),
number,19) ðT Þ of Cu0:99 M0:01 O decreases for M = Li, K,
and Na in this order. Liþ is of a comparable size to Cu2þ and may be preferable for occupying the vacancy(0.73 A)site of Cu or to replace the Cu2þ site; then the eﬀective holecarriers will be introduced because of ionic size matching.
Another possible explanation of the order of ðT Þ for
Cu0:99 M0:01 O is the diﬀerence in the solubility of thealkaline atoms in CuO; the solubility of Li may be thehighest and that of Na may be the lowest, although noimpurity phases can be conﬁrmed for the Cu0:99 Na0:01 O
sample from the XRD results.
Figure 1(b) shows the temperature dependence of SðT Þof 1.0%-alkali-metal (M = Li, Na, K)-doped Cu0:99 M0:01 O.
The sign of SðT Þ shows p-type semiconducting properties
throughout the entire temperature range. The absolute valueof ðT Þ reﬂects that of SðT Þ; the lower the ðT Þ value
becomes, the higher SðT Þ reaches, as predicted by theconventional theory.20) The inﬂection temperature of SðT Þ
corresponds to the bending point T  in ðT Þ for the doped
samples. In Fig. 1(b), it should be noted that SðT Þ ofnondoped CuO is apparently quite strange; it is a large
positive value around room temperature and changes its sign
at 600 K, becoming negative large values, and then recovers
to positive values at higher temperatures. The type-change
behavior of SðT Þ was reversibly and reproducibly observed
for the nondoped CuO fabricated under the present conditions. No anomalous SðT Þ behavior can be observed, andSðT Þ is positive over the entire temperature range for the
nondoped CuO, which was sintered at a lower temperaturethan 1050  C using the same raw powder. The re-entrantSðT Þ behavior is closely related to the defect/impurity
chemistry of CuO. A negative conducting channel such as
Cu1þ may exist, and the compensating eﬀect occurs at low
and high temperatures. The mechanism of the type change is
not yet understood and a detailed study using the results ofthe Hall eﬀect is in progress.
Figure 2 shows the temperature dependence of the power
factor P ¼ S 2 . PðT Þ for the doped-samples increases withincreasing T . Li-doped Cu0:99 Li0:01 O shows the highest P
value of 7:45  105 W K2 m1 at 1250 K, which resultsfrom the large  value in spite of the S value being thesmallest. In the following subsection, we focus on theFig. 2. (Color online) Temperature dependence of power factor P ¼ S 2 

of 1.0% alkali-metal (Li, Na, K)-doped CuO.

Li-doped CuO and investigate the relationship between thecontents of doped Li and the thermoelectric properties.
3.2 Thermoelectric properties of Li-doped CuO

X-ray diﬀraction measurement was performed in order to
conﬁrm the eﬀect of Li-doping on the lattice parameters.
Figure 3 presents the lattice parameters as a function of the Li
ion content. The crystal structure of the Li-doped CuO was
conﬁrmed to be monoclinic at 300 K without an impurity
peak until x ¼ 0:04. The lattice parameter a seemed toincrease slightly, the lattice parameters b and c decreased
with increasing Li content. The angle  also increased
monotonically with doping. As a result, the unit-cell volume
Vcell decreased with doping and Vcell of the x ¼ 0:04 sample
was 0.11% smaller than that of the nondoped CuO. These
results suggest that Li substituted into the Cu site or occupied
the vacancy site of Cu up to x ¼ 0:04. The obtained lattice
parameters a and c were scattered slightly for the x ¼ 0:02
sample, which may result from the ﬁnite deviation between
experimental and theoretical XRD patterns.
Figure 4(a) depicts the temperature dependence ofthe electrical resistivity ðT Þ for Li-doped (Cu1x Lix )O(0:0005  x  0:05). (Cu1x Lix )O with x ¼ 0:0005 and0.001 shows semiconducting behavior, which indicates theactivation type. For the samples with 0:005  x  0:05,
ðT Þ drastically decreases with increasing x, is minimum at
x ¼ 0:03, and then increases slightly with an increase in x.
For the Cu-site substitution by a Liþ ion, a hole is produced

031102-2# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 031102

N. Yoshida et al.
5.126

3.42

(a)

99.6

(b)

a

99.58

b
4.683

5.125

4.682

3.418

c

c (Å)

a (Å)

b (Å)

3.419

99.54

5.124

99.52

(Cu1-xLix)O
4.681

0

0.01

0.02
0.03
Li-Content

0.04

99.56

(deg)

4.684

3.417
0.05

(Cu1-xLix)O

5.123

0

0.01

0.02
0.03
Li-Content

0.04

99.5
0.05

Fig. 3. (Color online) Lattice parameters (a, b, c, and ) of monoclinic structure for Li-doped (Cu1x Lix )O, as a function of the content of Li ions, x.

105

(a)

(Cu1-xLix)O(b)(m cm)

104

745K
1145K

103

102

101
10-4

10-3

10-2

10-1

Li-Content
Fig. 4. (Color online) (a) Temperature dependence of electrical resistivity ðT Þ for Li-doped CuO (0:0005  x  0:05). (b) ðT Þ at 745 and 1145 K as afunction of Li-content. Dashed line represents the relation  / 1=x.

as a positive carrier, which is described by the chemical
formula Cu2þ Liþ hþ O. The slight increase in ðT Þ for x 
0:04 may come from the introduction of the lattice disorderor solubility limit of Li in CuO. Figure 4(b) shows the value at 745 and 1145 K, as a function of the contents of Li.
In the ﬁgure, the dashed line shows the linear relation  /1=x. Roughly speaking, the  value is inversely proportional
to the nominal carrier concentration x. In a simple relation, 
can be represented as  / 1=exe p , where e is the charge ofan electron, xe the eﬀective carrier concentration, and p themobility of the hole carrier. In order to clarify the xe and pvalues, the measurement of the temperature dependence ofthe Hall eﬀect must be performed.
Figure 5(a) presents the temperature dependence of theSeebeck coeﬃcient SðT Þ of Li-doped (Cu1x Lix )O with
0:0005  x  0:05. For x ¼ 0:0005 and 0.001, SðT Þ is apositive high value at T  700 K because of large ðT Þ
and then sharply decreases with increasing T . For x  0:005,
the SðT Þ value decreases with decreasing x until x ¼ 0:03
as ðT Þ decreases, but slightly increases with increasing x.
The absolute value and temperature dependence of SðT Þare closely correlated with ðT Þ.20) Figure 5(b) shows
the theoretical estimation of S at T ¼ 773 K using the
Heikes formula,21) S ¼ ðkB =eÞ ln½ð1  xÞ=x, where kB is theBoltzmann constant. The results suggest that the theoretical
S value reproduces the measured one qualitatively, which isalso reproduced at 500–1000 K.

Figure 6(a) shows the temperature dependence of thethermal conductivity ðT Þ of Li-doped CuO at T  300 K.
ðT Þ of all the samples shows a broad peak at 60 K, which
results from phonon-phonon scattering, as well seen in
insulating materials.22) ðT Þ decreases with increasing x up
to x ¼ 0:03 and then slightly increases with an increase in x.
ðT Þ of the x ¼ 0:03 sample is one-third that of the nondoped sample, which is a desirable situation for thermoelectricity. The ðT Þ reduction upon the Li-doping seems tobe the eﬀect of the lattice disorder due to the substitution of
Li into the Cu site. Since the electronic thermal conductivity
e , which was estimated using the Wiedemann–Franz law,
was negligibly small, the heat transport was performed
entirely by phonons. Figure 6(b) shows ðT Þ of the typicalsamples at high temperature, as measured by a laser ﬂash
method. The ﬁtting curves, which are proportional to T 1 ,are also shown, extrapolated up to 1273 K. The  values at
1200 K were estimated to be about 60 mW/cm K, independent of the Li-content, x.
Figure 7(a) depicts the temperature dependence of thepower factor PðT Þ of Li-doped CuO. As would be expectedfrom the results of ðT Þ and SðT Þ values, the sample of
x ¼ 0:03 shows the highest performance, P ¼ 1:6  104
W K2 m1 at 1246 K, which is about one order ofmagnitude higher than that of the sample of x ¼ 0:0005.
Figure 7(b) shows the dimensionless ﬁgure of merit ZT of
Li-doped CuO as a function of T . The ZT value increases

031102-3# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 031102

N. Yoshida et al.
800(b)
S (773K) ( µV/K)(a)

Heikes formula
exp.

600

400

200

(Cu1-xLix)O
0

0

0.02

0.04
Li-Content

0.06

0.08

Fig. 5. (Color online) (a) Temperature dependence of the Seebeck coeﬃcient SðT Þ of Li-doped CuO with 0:0005  x  0:05. (b) Measured and estimatedS values at 773 K as a function of Li-content. The dashed line presents the theoretical estimation of S using Heikes formula.21)

Fig. 6. (Color online) Temperature dependence of thermal conductivity ðT Þ of Li-doped CuO at T  300 K and (b) ðT Þ for x ¼ 0:01 and 0.03 at
T  300 K. The ﬁtting curves are also shown.

10-3

x=0.0005
x=0.001x=0.005
x=0.01x=0.02x=0.03x=0.04x=0.05(b)

2Power factor P (W/K m)

(a)
10-4

10-5

(Cu1-xLix)O
10-6
200

400

600

800
T (K)

1000

1200

1400

Fig. 7. (Color online) Temperature dependence of (a) power factor PðT Þ and (b) dimensionless ﬁgure of merit ZT of Li-doped CuO.

with increasing temperature and Li-content, x, and is the
maximum value of 3:2  102 with x ¼ 0:03 at 1246 K.
Although ðT Þ decreased with increasing x at low temperature, ðT Þ was independent of the Li-content at hightemperature. As a result, the enhancement of the ZT value
at x ¼ 0:03 results mainly from the large decrease in ðT Þ
owing to carrier doping, and overcomes the decrease inSðT Þ. Another mechanism should be introduced for the
reduction of ðT Þ at high temperature to enhance the ZT
value in the CuO system.

4. Summary

We have investigated the electrical and thermal propertiesof CuO polycrystals doped with alkali metals (Li, Na, K)
and studied the potential of using CuO as a thermoelectricmaterial. Important experimental results and conclusions
obtained from this study are summarized as follows.
1) Among the alkali metals substituted into the Cu-site,Liþ is the most promising ion for enhancing thethermoelectric performance. The Li ion replaced the031102-4# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 031102

N. Yoshida et al.

Cu-site or occupied the vacancy site of Cu. As a result,eﬀective hole carriers were created.
2) For Li-doped (Cu1x Lix )O, the electrical resistivityðT Þ and Seebeck coeﬃcient SðT Þ decreased withincreasing Li-content up to x ¼ 0:03. Power factorPðT Þ ¼ S 2 = increases with increasing Li-content,
is the maximum of P ¼ 1:6  104 W K2 m1 at
1246 K with x ¼ 0:03, and decreases in performancewith further increase in x.
3) Thermal conductivity ðT Þ decreased with increasing
Li-content up to x ¼ 0:03 at T < 300 K. The  value at
1200 K was estimated to be about 60 mW/cm K, which
was independent of x. The dimensionless thermoelectric ﬁgure of merit ZT ¼ S 2 T = increases withincreasing Li-content, and is the maximum of ZT ¼
3:2  102 at 1246 K with x ¼ 0:03. To enhance theZT value, another scattering center must be introduced.
4) In nondoped CuO, which was fabricated under thepresent conditions, the sign of SðT Þ shows a reentrant
behavior. The mechanism of sign change may comefrom the defect chemistry. The detailed study is inprogress.

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The authors thank Professors Takashi Goto and Rong Tu
of Tohoku University, Japan, for assistance in thermal
conductivity measurement by the laser ﬂash method andfor valuable discussion. This work was performed underthe Inter-university Cooperative Research Program of theInstitute for Materials Research, Tohoku University.

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# 2013 The Japan Society of Applied Physics