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Thermoelectric properties of NaCo2O4
Journal of Alloys and Compounds 315 (2001) 234–236
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Thermoelectric properties of NaCo 2 O 4 Ken Kurosaki*, Hiroaki Muta, Masayoshi Uno, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2 -1, Suita, Osaka 565 -0871, Japan Received 18 September 2000; accepted 16 October 2000
Abstract The thermoelectric properties such as the thermal conductivity, electrical resistivity and Seebeck coefficient of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated in the temperature range from room temperature to 723 K. Polycrystalline samples were prepared by sintering in air followed by hot pressing. The thermal conductivity was calculated from the heat capacity, the experimental density, and the thermal diffusivity measured by the laser flash method. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in He atmosphere. The dimensionless figure of merit, ZT, of NaCo 1.9 Pd 0.1 O 4 was higher than that of NaCo 2 O 4 over a wide range of temperatures, and reached ZT 5 0.045 at 723 K. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Electrical transport; Heat conduction; Thermoelectric power
1. Introduction Thermoelectric materials can directly convert heat into electric energy through thermoelectric power. Since this process is carried out only in materials, the power generating module of these materials has no movement, and power generation using these materials has high reliability. In addition, the thermoelectric device is environmentally friendly because it essentially produces no waste matter. Therefore, thermoelectric materials are studied worldwide for the reuse of exhaust heat from power plants. The reuse of exhaust heat enables us to enhance the efficiency of power generation systems. The efficiency of thermoelectric materials is indexed by the dimensionless figure of merit, ZT ( 5 TS 2 /rk ), where S is the Seebeck coefficient, r is the electrical resistivity, k is the thermal conductivity and T is the temperature. A large magnitude of ZT indicates high efficiency. In recent years, some new compounds, which have high ZT values comparable to state-of-the-art thermoelectric materials, have been discovered, and NaCo 2 O 4 is one of these materials [1]. In the present study, in order to develop thermoelectric materials with a high ZT value, the thermoelectric properties of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were investigated. The thermal conductivity, electrical *Corresponding author. E-mail address: [email protected] (K. Kurosaki).
resistivity and Seebeck coefficient were measured from room temperature to about 723 K, and ZT was evaluated in the same temperature range.
2. Experimental Polycrystalline samples were prepared by mixing appropriate amounts of Na 2 CO 3 , Co 3 O 4 , TiO 2 , Rh 2 O 3 and Pd followed by calcining first at 973 K for 3 days, and then at 1173 K for 10 h. The crystal structures of the samples were analyzed by powder X-ray diffraction using Cu Ka radiation at room temperature. For thermoelectric property measurements, appropriate shapes of the samples were produced by hot pressing at a pressure of 8 MPa and at a temperature of 973 K for 2 h. The density of the samples was calculated from the measured weight and dimensions. The thermoelectric properties were measured in the temperature range from room temperature to about 723 K. The thermal conductivity was evaluated from the heat capacity, the experimental density and the thermal diffusivity measured by the laser flash method using ULVAC TC-7000 in vacuum. The heat capacities of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated from the Neumann–Kopp law using literature data [2] for NaO 2 , Na 2 O 2 , CoO, TiO, Rh and PdO. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in helium atmosphere. The Seebeck
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01277-9
K. Kurosaki et al. / Journal of Alloys and Compounds 315 (2001) 234 – 236
Fig. 1. X-ray powder diffraction NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
patterns
for
NaCo 2 O 4
and
coefficient was measured in a temperature gradient of about 10 K.
3. Results and discussion The X-ray powder diffraction patterns of the samples are shown in Fig. 1. This figure shows that single phase NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 were obtained in the present study. The lattice parameters and X-ray densities of the samples were obtained from X-ray diffraction analysis. The bulk densities of the samples were about 80% of the theoretical density. The sample characteristics are shown in Table 1. The thermal conductivity k was calculated from the measured thermal diffusivity D, the specific heat capacity Cp and density d using the relationship
235
Fig. 2. Temperature dependence of the thermal conductivity for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
crease with increasing temperature, which showed that the major part of the thermal conductivities is composed of phonon contributions. It was found that the thermal conductivities of NaCo 1.9 M 0.1 O 4 were lower than that of NaCo 2 O 4 over the whole temperature range. This may be caused by the production of phonon scattering due to the substitution of M atoms for cobalt atoms. Among all the samples investigated in the present study, NaCo 1.9 Rh 0.1 O 4 has the lowest thermal conductivity. The temperature dependences of the electrical resistivity r of the samples are shown in Fig. 3. For NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Rh and Pd), the electrical resistivities increase slightly with increasing temperature and are typical of simple metallic behavior, while for NaCo 1.9 Ti 0.1 O 4 the electrical resistivity decreases with increasing temperature and shows semiconducting behavior. An almost 100-fold increase in magnitude of the electrical resistivity was obtained by titanium substitution.
k 5 DCp d The temperature dependences of the thermal conductivity of the samples are shown in Fig. 2. These experimental data were normalized to 100% of the theoretical density using the Maxwell–Eucken relationship [3,4]. The thermal conductivities of all samples indicated a systematic deTable 1 X-ray powder diffraction results and bulk densities for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd)
NaCo 2 O 4 NaCo 1.9 Pd 0.1 O 4 NaCo 1.9 Rh 0.1 O 4 NaCo 1.9 Ti 0.1 O 4
˚ Lattice parameter (A)
Bulk density
a
c
g / cm 3
%T.D.
2.834 2.835 2.832 2.838
10.899 10.937 10.919 10.928
3.88 3.88 3.64 3.57
83.7 82.3 76.9 77.7
Fig. 3. Temperature dependence of the electrical resistivity for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
236
K. Kurosaki et al. / Journal of Alloys and Compounds 315 (2001) 234 – 236
Fig. 5 shows the temperature dependences of the dimensionless figure of merit ZT. The ZT of all samples increases with increasing temperature over the whole temperature range. The ZT of NaCo 1.9 Rh 0.1 O 4 is higher than that of NaCo 2 O 4 over the whole temperature range due to its lower thermal conductivity. For NaCo 1.9 Pd 0.1 O 4 , the ZT is higher than that of NaCo 2 O 4 due to reduction of the electrical resistivity by palladium substitution. The maximum ZT value was obtained for NaCo 1.9 Pd 0.1 O 4 and reached ZT 5 0.045 at 723 K.
4. Conclusion
Fig. 4. Temperature dependence of the Seebeck coefficient for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
Conversely, palladium substitution led to a decrease of the electrical resistivity by about one-fourth. The electrical resistivity of NaCo 2 O 4 was higher than reported values [5]. This is probably caused by the difference in sodium content [6]. The temperature dependences of the Seebeck coefficient S of the samples are shown in Fig. 4. Very large values of the Seebeck coefficients were obtained in spite of the relatively low electrical resistivities. This is considered to be caused by the strong electron–electron correlation [7]. The Seebeck coefficient of all samples increased with increasing temperature and has a positive sign over the whole temperature range. Substitution of palladium for cobalt leads to the lowest value of the Seebeck coefficient as in the case of the electrical resistivity.
The thermoelectric properties of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated in the temperature range from room temperature to 723 K. The thermal conductivity is reduced by substitution of metal M for cobalt in all samples. The thermal conductivity of NaCo 1.9 Rh 0.1 O 4 shows the lowest value, which is about two-thirds that of NaCo 2 O 4 . The electrical resistivity is increased by titanium substitution, but is decreased by palladium substitution. The Seebeck coefficient of all samples shows positive values, and increases with increasing temperature. Substitution of palladium for cobalt leads to a decrease of the Seebeck coefficient. The dimensionless figure of merit ZT is enhanced by rhodium and palladium substitutions, and the ZT of NaCo 1.9 Pd 0.1 O 4 reaches a maximum value of 0.045 at 723 K. The ZT of all samples increases with increasing temperature up to 723 K. These materials have potential for good thermoelectric materials at high temperature.
References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (20) (1997) R12685. [2] Japan Thermal Measurement Society, Thermodynamics Database For Personal Computer MALT2. [3] J.C. Maxwell, 3rd Edition, Treaties on Electricity and Magnetism, Vol. 1, Oxford University Press, 1891, reprinted by Dover, New York, 1954. [4] A.E. Eucken, Forsch. Gebiete Ingenieurw. B3, Forschungsheft, No. 353, 1932, p. 16. [5] H. Yakabe, K. Kikuchi, I. Terasaki, Y. Sasago, K. Uchinokura, in: Proceedings of the 16th International Conference on Thermoelectrics, Dresden, Germany, 1997, p. 523. [6] T. Kawata, Y. Iguchi, T. Itoh, K. Takahata, I. Terasaki, Phys. Rev. B 60 (15) (1999) 10584. [7] Y. Ando, N. Miyamoto, K. Segawa, T. Kawata, I. Terasaki, Phys. Rev. B 60 (15) (1999) 10580. Fig. 5. Temperature dependence of the dimensionless figure of merit for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
L
www.elsevier.com / locate / jallcom
Thermoelectric properties of NaCo 2 O 4 Ken Kurosaki*, Hiroaki Muta, Masayoshi Uno, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2 -1, Suita, Osaka 565 -0871, Japan Received 18 September 2000; accepted 16 October 2000
Abstract The thermoelectric properties such as the thermal conductivity, electrical resistivity and Seebeck coefficient of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated in the temperature range from room temperature to 723 K. Polycrystalline samples were prepared by sintering in air followed by hot pressing. The thermal conductivity was calculated from the heat capacity, the experimental density, and the thermal diffusivity measured by the laser flash method. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in He atmosphere. The dimensionless figure of merit, ZT, of NaCo 1.9 Pd 0.1 O 4 was higher than that of NaCo 2 O 4 over a wide range of temperatures, and reached ZT 5 0.045 at 723 K. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Electrical transport; Heat conduction; Thermoelectric power
1. Introduction Thermoelectric materials can directly convert heat into electric energy through thermoelectric power. Since this process is carried out only in materials, the power generating module of these materials has no movement, and power generation using these materials has high reliability. In addition, the thermoelectric device is environmentally friendly because it essentially produces no waste matter. Therefore, thermoelectric materials are studied worldwide for the reuse of exhaust heat from power plants. The reuse of exhaust heat enables us to enhance the efficiency of power generation systems. The efficiency of thermoelectric materials is indexed by the dimensionless figure of merit, ZT ( 5 TS 2 /rk ), where S is the Seebeck coefficient, r is the electrical resistivity, k is the thermal conductivity and T is the temperature. A large magnitude of ZT indicates high efficiency. In recent years, some new compounds, which have high ZT values comparable to state-of-the-art thermoelectric materials, have been discovered, and NaCo 2 O 4 is one of these materials [1]. In the present study, in order to develop thermoelectric materials with a high ZT value, the thermoelectric properties of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were investigated. The thermal conductivity, electrical *Corresponding author. E-mail address: [email protected] (K. Kurosaki).
resistivity and Seebeck coefficient were measured from room temperature to about 723 K, and ZT was evaluated in the same temperature range.
2. Experimental Polycrystalline samples were prepared by mixing appropriate amounts of Na 2 CO 3 , Co 3 O 4 , TiO 2 , Rh 2 O 3 and Pd followed by calcining first at 973 K for 3 days, and then at 1173 K for 10 h. The crystal structures of the samples were analyzed by powder X-ray diffraction using Cu Ka radiation at room temperature. For thermoelectric property measurements, appropriate shapes of the samples were produced by hot pressing at a pressure of 8 MPa and at a temperature of 973 K for 2 h. The density of the samples was calculated from the measured weight and dimensions. The thermoelectric properties were measured in the temperature range from room temperature to about 723 K. The thermal conductivity was evaluated from the heat capacity, the experimental density and the thermal diffusivity measured by the laser flash method using ULVAC TC-7000 in vacuum. The heat capacities of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated from the Neumann–Kopp law using literature data [2] for NaO 2 , Na 2 O 2 , CoO, TiO, Rh and PdO. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in helium atmosphere. The Seebeck
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01277-9
K. Kurosaki et al. / Journal of Alloys and Compounds 315 (2001) 234 – 236
Fig. 1. X-ray powder diffraction NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
patterns
for
NaCo 2 O 4
and
coefficient was measured in a temperature gradient of about 10 K.
3. Results and discussion The X-ray powder diffraction patterns of the samples are shown in Fig. 1. This figure shows that single phase NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 were obtained in the present study. The lattice parameters and X-ray densities of the samples were obtained from X-ray diffraction analysis. The bulk densities of the samples were about 80% of the theoretical density. The sample characteristics are shown in Table 1. The thermal conductivity k was calculated from the measured thermal diffusivity D, the specific heat capacity Cp and density d using the relationship
235
Fig. 2. Temperature dependence of the thermal conductivity for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
crease with increasing temperature, which showed that the major part of the thermal conductivities is composed of phonon contributions. It was found that the thermal conductivities of NaCo 1.9 M 0.1 O 4 were lower than that of NaCo 2 O 4 over the whole temperature range. This may be caused by the production of phonon scattering due to the substitution of M atoms for cobalt atoms. Among all the samples investigated in the present study, NaCo 1.9 Rh 0.1 O 4 has the lowest thermal conductivity. The temperature dependences of the electrical resistivity r of the samples are shown in Fig. 3. For NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Rh and Pd), the electrical resistivities increase slightly with increasing temperature and are typical of simple metallic behavior, while for NaCo 1.9 Ti 0.1 O 4 the electrical resistivity decreases with increasing temperature and shows semiconducting behavior. An almost 100-fold increase in magnitude of the electrical resistivity was obtained by titanium substitution.
k 5 DCp d The temperature dependences of the thermal conductivity of the samples are shown in Fig. 2. These experimental data were normalized to 100% of the theoretical density using the Maxwell–Eucken relationship [3,4]. The thermal conductivities of all samples indicated a systematic deTable 1 X-ray powder diffraction results and bulk densities for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd)
NaCo 2 O 4 NaCo 1.9 Pd 0.1 O 4 NaCo 1.9 Rh 0.1 O 4 NaCo 1.9 Ti 0.1 O 4
˚ Lattice parameter (A)
Bulk density
a
c
g / cm 3
%T.D.
2.834 2.835 2.832 2.838
10.899 10.937 10.919 10.928
3.88 3.88 3.64 3.57
83.7 82.3 76.9 77.7
Fig. 3. Temperature dependence of the electrical resistivity for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
236
K. Kurosaki et al. / Journal of Alloys and Compounds 315 (2001) 234 – 236
Fig. 5 shows the temperature dependences of the dimensionless figure of merit ZT. The ZT of all samples increases with increasing temperature over the whole temperature range. The ZT of NaCo 1.9 Rh 0.1 O 4 is higher than that of NaCo 2 O 4 over the whole temperature range due to its lower thermal conductivity. For NaCo 1.9 Pd 0.1 O 4 , the ZT is higher than that of NaCo 2 O 4 due to reduction of the electrical resistivity by palladium substitution. The maximum ZT value was obtained for NaCo 1.9 Pd 0.1 O 4 and reached ZT 5 0.045 at 723 K.
4. Conclusion
Fig. 4. Temperature dependence of the Seebeck coefficient for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).
Conversely, palladium substitution led to a decrease of the electrical resistivity by about one-fourth. The electrical resistivity of NaCo 2 O 4 was higher than reported values [5]. This is probably caused by the difference in sodium content [6]. The temperature dependences of the Seebeck coefficient S of the samples are shown in Fig. 4. Very large values of the Seebeck coefficients were obtained in spite of the relatively low electrical resistivities. This is considered to be caused by the strong electron–electron correlation [7]. The Seebeck coefficient of all samples increased with increasing temperature and has a positive sign over the whole temperature range. Substitution of palladium for cobalt leads to the lowest value of the Seebeck coefficient as in the case of the electrical resistivity.
The thermoelectric properties of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd) were evaluated in the temperature range from room temperature to 723 K. The thermal conductivity is reduced by substitution of metal M for cobalt in all samples. The thermal conductivity of NaCo 1.9 Rh 0.1 O 4 shows the lowest value, which is about two-thirds that of NaCo 2 O 4 . The electrical resistivity is increased by titanium substitution, but is decreased by palladium substitution. The Seebeck coefficient of all samples shows positive values, and increases with increasing temperature. Substitution of palladium for cobalt leads to a decrease of the Seebeck coefficient. The dimensionless figure of merit ZT is enhanced by rhodium and palladium substitutions, and the ZT of NaCo 1.9 Pd 0.1 O 4 reaches a maximum value of 0.045 at 723 K. The ZT of all samples increases with increasing temperature up to 723 K. These materials have potential for good thermoelectric materials at high temperature.
References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (20) (1997) R12685. [2] Japan Thermal Measurement Society, Thermodynamics Database For Personal Computer MALT2. [3] J.C. Maxwell, 3rd Edition, Treaties on Electricity and Magnetism, Vol. 1, Oxford University Press, 1891, reprinted by Dover, New York, 1954. [4] A.E. Eucken, Forsch. Gebiete Ingenieurw. B3, Forschungsheft, No. 353, 1932, p. 16. [5] H. Yakabe, K. Kikuchi, I. Terasaki, Y. Sasago, K. Uchinokura, in: Proceedings of the 16th International Conference on Thermoelectrics, Dresden, Germany, 1997, p. 523. [6] T. Kawata, Y. Iguchi, T. Itoh, K. Takahata, I. Terasaki, Phys. Rev. B 60 (15) (1999) 10584. [7] Y. Ando, N. Miyamoto, K. Segawa, T. Kawata, I. Terasaki, Phys. Rev. B 60 (15) (1999) 10580. Fig. 5. Temperature dependence of the dimensionless figure of merit for NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M5Ti, Rh, Pd).