Mater. Res. Soc. Symp. Proc. Vol. 1490 © 2012 Materials Research Society
DOI: 10.1557/opl.2012.1571

Thermoelectric properties of Li-doped Cu0.95-xM0.05LixO (M=Mn, Ni, Zn)
N. Yoshida, T. Naito and H. Fujishiro*(* corresponding author: fujishiro@iwate-u.ac.jp)
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan.
ABSTRACT
Thermoelectric properties of the Li-doped Cu0.95-xM0.05LixO (M=divalent metal ion; Mn,Ni, Zn) were investigated at the temperature up to 1273 K. In the doped divalent metal ions, Zn2+ion was the most effective to reduce the thermal conductivity, and the Ni2+ substitution waspreferable to decrease the electrical resistivity. For the Cu0.95-xNi0.05LixO sample at x=0.03, themaxima of the dimensionless thermoelectric figure of merit ZT and the power factor P at 1246 K
were 4.2×10-2 and 1.6 ×10-4 W/K2m, respectively. The enhancement of the thermoelectric
properties of the Li-doped Cu0.95-xM0.05LixO system was discussed.
INTRODUCTION
The thermoelectric technology using a waste heat for electric power generation devices
has been revised because of recent energy crisis besides other renewable energy sources. Since
the discovery of NaCo2O4 in 1998 [1], oxide thermoelectric materials have been intensively
studied using Ca-Co-O and NaCo2O4 systems with layered Co-O octahedron [2,3], as a substitutefor the conventional semiconductors such as Bi2Te3. However, the thermoelectric performancehas been still low. In material researches for thermoelectricity, we focus on the possibility of aconventional CuO system, because it is inexpensive, plentiful and non-toxic resources. CuO isknown to be a metal deficient p-type semiconductor with a band gap of 1.2 eV with monoclinic
crystal structure, and many studies were reported for the physical properties [4,5]. High-purity
CuO has a large positive Seebeck coefficient S, but shows both a high electrical resistivity ρ anda high thermal conductivity κ. If the doping or substitution of univalent or trivalent ions ispossible for the Cu site, the electrical resistivity and the thermal conductivity might be decreased.
In this case, the thermoelectric efficiency may be enhanced, if the Seebeck coefficient is notdeteriorated so much. Similarly to the CuO system, ZnO is a plentiful and promising material forthe n-type thermoelectricity with a simple wurtzite structure [6]. The electrical resistivity
ρ drastically decreases by the doping of a small amount of Al3+ and/or Ga3+, and thedimensionless thermoelectric figure of merit ZT (=S2T/ρκ) reaches 0.3 and 0.65 at 1273 K for
Zn0.98Al0.02O and Zn0.96Al0.02Ga0.02O, respectively [7,8]. For the CuO system, the electricconductivity σ (=1/ρ) and Seebeck coefficient S were reported for the Li and Al doping, in which
the Li-doping enhances the p-type electrical conductivity [9]. However, a systematicinvestigation has not been performed as a thermoelectric material of CuO as for the doping
species and the optimum doping concentration.
Recently, we have reported the alkali metal substitution for the Cu-site in CuO, in which
Li+ is the most promising ion to enhance the thermoelectric performance. The ZT takes amaximum of 3.2×10-2 at 1246 K for the Cu0.97Li0.03O [10]. To enhance the thermoelectricperformance, the reduction of the thermal conductivity is a possible approach, which is usuallyrealized by the substitution of the element with different ionic radius.

69

In this study, we fabricated the Li-doped Cu0.95-xM0.05LixO (M=Mn, Ni, Zn) andmeasured thermoelectric properties at high temperature. The potential of the CuO system asthermoelectric materials was discussed.
EXPERIMENT
Cu0.95-xM0.05LixO polycrystals doped with a divalent-metal element (M=Mn, Ni, Zn) wereprepared by a conventional solid-state reaction. The raw powders of CuO (Furuuchi Chemical;99.9%), MnO (KOJUNDO; 99.9%), NiO (Furuuchi Chemical; 99.9%), ZnO (KOJUNDO;99.9%) and Li2O (KOJUNDO; 99% up) were mixed with a molar ratio of Cu:M:Li=0.95x:0.05:x (x=0.01, 0.03). The specimen was pressed cold-isostatically under 64 MPa into a bar andthen sintered at 1060ഒ for 10 h in oxygen flow. The measured densities of each sample weregreater than 95% of the ideal density. The rectangular-shaped specimen with about 2 x 2 x 10mm3 was cut from the bar for the measurements of the thermoelectricity. Powder X-ray
diffraction measurements were performed at 300 K using a Cu Kα radiation (Rigaku; Multi Flex)
in the range of 20°ู2θู90° with 0.02° steps. The electrical resistivity ρ(T) and Seebeckcoefficient S(T) 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=S2/ρ was calculated. Low-temperature thermal conductivity κ(T) was measured by asteady-state heat flow method from 10 to 300 K using a home-made apparatus combined withhelium refrigerator [11]. High-temperature thermal conductivity κ(T) was also measured by alaser flash method (Ulvac-Riko; TC-7000) up to 1023 K. κ(T) was extrapolated up to 1300 K
using a fitting curve which was proportional to T-1 for the experimental κ(T) results at low andhigh temperatures. A dimensionless figure of merit ZT=S2T/ρκ was estimated using these values.
DISCUSSION
Thermal conductivity of Cu0.95M0.05O (M=Mn, Ni, Zn)
The doping effect of divalent metal (M) ion in CuO on the thermal conductivity κ(T) wasinvestigated. Figure 1 shows the temperature dependence of κ(T) of pure CuO and Cu0.95M0.05O(M=Mn, Ni, Zn). κ(T) was reduced by the substitution for the Cu-site, in which the Zn
substitution is the most effective. The κ(T) reduction by the divalent metal ion doping seems tobe the introduction of the lattice disorder due to the substitution of the M ion for the Cu site withdifference of the ionic radius. κ(T) of materials can be generally represented by the sum of latticecontribution (κph) and carrier contribution (κe). In the present samples, heat propagates almost
due to phonons (κph) because of the high ρ(T) value. The measured κ(T) shows a broad peak at
low temperature, which results from the phonon-phonon scattering as can be seen in insulating
materials. In the previous paper, we reported that the thermoelectric properties were enhanced by
the Li doping to the CuO matrix [10]. In the later subsection, we report on the Li doping effecton the thermoelectric properties of Cu0.95M0.05O with smaller κ(T).

70

Figure 1. Temperature dependence of the thermal conductivity κ(T) of pure CuO andCu0.95M0.05O (M=Mn, Ni, Zn).
Thermoelectric properties of Li-doped Cu0.95-xM0.05LixO (M=Ni, Zn)
Figures 2(a) and 2(b) show the temperature dependence of the electrical resistivity ρ(T)and Seebeck coefficient S(T) for the Li-doped Cu0.95-xM0.05LixO (M=Ni, Zn). In Fig. 2(a), ρ(T) of
the Cu0.95M0.05O samples decreased with increasing T at T<800 K, compared with pure CuO, butwas comparable with that of the pure CuO at higher temperatures. Similarly to the Li-doped CuO[10], the Li doping can drastically reduce the ρ(T) and the absolute value of ρ(T) decreased withincreasing contents of Li. ρ(T) of the Cu0.95-xNi0.05LixO is lower than that of the Cu0.95-xZn0.05LixO
at identical x, both of which are lower than that of the Li-doped Cu1-xLixO. In Fig. 2(b), S(T)decreases with increasing x. S(T) of the Cu0.95-xNi0.05LixO is lower than that of the Cu0.95xZn0.05LixO at identical x, both of which are lower than that of the Li-doped Cu1-xLixO [10].
Because S(T) is closely correlated with ρ(T) [12].

Figure 2. Temperature dependence of (a) the electrical resistivity ȡ(T) and (b) the Seebeckcoefficient S(T) of pure CuO and Li-doped Cu0.95-xM0.05LixO (M=Ni, Zn).
Figure 3(a) shows the x dependence of the thermal conductivity κ(T) of the Li-doped
Cu0.95-xM0.05LixO (M=Ni, Zn). As shown in Fig. 1, the κ(T) reduction by the Zn substitution waslarger than that by the Ni substitution, both of which κ(T) further decreased with increasing

71content of Li, x. The origin of the additional κ(T) reduction by the Li doping comes from theintroduction of the disorder in the lattice.
Figure 3(b) shows the κ(T) of the Cu0.94Ni0.05Li0.01O sample up to 1000 K. The fitting
curve, which was proportional to T-1, was also shown and was extrapolated up to 1273 K. The
estimated κ(T) value was 52 mW/cmK at 1200 K, which was smaller than that for theCu0.99Li0.01O sample [10] due to the enhanced disorder by the additional Ni substitution.
Figure 4(a) depicts the temperature dependence of the thermoelectric power factor P(T)of the Li-doped Cu0.95-xM0.05LixO (M=Ni, Zn). P(T) of the Cu0.97Li0.03O, which showed thehighest P(T) in the previous paper [10] is also shown. As would be expected from the results of
ρ(T) and S(T) values, Cu0.95-xNi0.05LixO (x=0.03) shows the highest P(T) of 2.0×10-4 W/K2m at1246 K, which is higher than that for Cu0.97Li0.03O.
Figure 4(b) shows the dimensionless figure of merit ZT of the Li-doped Cu0.95-xM0.05LixO(M=Ni, Zn) as a function of T. The ZT value increased with increasing temperature and contentsof Li, and took a maximum value of 4.2×10-2 for Cu0.95-xNi0.05LixO (x=0.03) at 1246 K, which
was about 30% larger than that of Cu0.97Li0.03O. The enhancement of the ZT value was also
confirmed for the Cu0.95-xZn0.05LixO system. The reduction of κ(T) and ρ(T) by the Ni or Zn
substitution besides the Li doping enhances the thermoelectric properties.

Figure 3. Temperature dependence of the thermal conductivity κ(T) of Cu0.95-xM0.05LixO (M=Ni,Zn) at T300 K and (b) κ(T) for Cu0.94Ni0.05Li0.01O up to 1000 K. The extrapolated fitting curve
up to 1300 K is also shown.

Figure 4. Temperature dependence of the (a) power factor P and (b) dimensionless figure of
merit ZT of Cu0.95-xM0.05LixO (M=Ni, Zn).

72

CONCLUSIONS
We have investigated the thermoelectric properties of the Li-doped Cu0.95-xM0.05LixO(M=Mn, Ni, Zn) as thermoelectric materials. Important experimental results and conclusions
obtained from this study are summarized as follows.
1) Among the divalent metal substitution for the Cu-site in Cu0.95M0.05O, Zn2+ is the mostpromising ions to decrease the thermal conductivity. The κ(T) reduction seems to be the effectof the lattice disorder by the substitution of Zn for the Cu site.
2) For the Cu0.95-xM0.05LixO (M=Ni, Zn), the electrical resistivity ρ(T), Seebeck coefficient S(T)and the thermal conductivity κ(T) decreased with increasing contents of Li up to x=0.03.
Power factor P(T)=S2/ρ increased with increasing contents of Li, takes a maximum of
P=2.0×10-4 W/K2m at 1246 K.
3) For the Cu0.92Ni0.05Li0.03O, the dimensionless thermoelectric figure of merit ZT=S2T/ρκ
increased with increasing contents of Li. A maximum of ZT=4.2×10-2 was realized at 1246 K
which was about 30% larger than that of Cu0.97Li0.03O.
ACKNOWLEDGMENTS
The authors thank Prof. Takashi Goto and Prof. Rong Tu of Tohoku University, Japan,
for the assistance of thermal conductivity measurement using Laser flash method and valuable
discussion. This work was performed under the Inter-university Cooperative Research Programof the Institute for Materials Research, Tohoku University. The presentation at the conference
was financially supported by Iwate University.
REFERENCES
1. I. Terasaki, Y, Sasago, and K. Uchinokura: Phys. Rev. B 56, R12685 (1997).
2. R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, and S. Sodeoka: Jpn. J. Appl.
Phys. 39, L1127 (2000).
3. M. Ito, T. Nagira, D. Furumoto, S. Katsuyama, and H. Nagai: Scripta Materialia 48, 403(2003).
4. Y. K. Jeong and G. M. Choi: J. Phys. Chem. Solids 57, 81 (1996).
5. B. Yang, T. Thurston, J. Tranquada, and G. Shirane: Phys. Rev. B 39, 4343 (1989).
6. D. Klimm, S. Ganschow, D. Schulz, and R. Fornari: J. Cryst. Growth 310, 3009 (2008).
7. T. Tsubota, M. Ohtaki, K. Eguchi, and H. Arai: J. Mater. Chem. 7, 85 (1997).
8. M. Ohtaki, K. Araki, and K. Yamamoto: J Electron. Mater. 38, 1234 (2009).
9. S. Suda, S. Fujitsu, K. Koumoto and H. Yanagida: Jpn. J. Appl. Phys. 31, 2488 (1992).
10. N. Yoshida, T. Naito and H. Fujishiro: submitted to Jpn. J. Appl. Phys. (2012).
11. H. Fujishiro, M. Ikebe, T. Naito, K. Noto, S. Kobayashi, and S. Yoshizawa: Jpn. J. Appl.
Phys. 33, 4965 (1994).
12. J. M. Ziman: PRINCIPLES OF THE THEORY OF SOLIDS (CAMBRIDGE UNIVERSITYPRESS, London, 1964) p. 200.

73