Fabrication and thermoelectric properties of Ca3−xDyxCo4O9+δ system

Journal of Alloys and Compounds 376 (2004) 58–61

Fabrication and thermoelectric properties of Ca3−x Dyx Co4 O9+δ system Dongli Wang a,b , Lidong Chen b,∗ , Qun Wang b , Jianguo Li a b

a School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, PR China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 125 Dingxi Road, Shanghai 200050, PR China

Received 22 September 2003; received in revised form 19 December 2003; accepted 22 December 2003

Abstract Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45) polycrystalline samples have been prepared using a sol–gel process followed by SPS sintering. Their thermoelectric properties have been carefully studied from room temperature to 1000 K. The substitution of Dy for Ca results in increasing of both thermopower and electrical resistivity, which could be attributed to the decreasing of carrier concentrations. The Dy substituted samples (x = 0, 0.3) have lower thermal conductivity than Ca3 Co4 O9+δ due to their lower electronic and lattice thermal conductivities. The dimensionless figure of merit ZT reaches 0.27 at 1000 K for the sample of Ca2.7 Dy0.3 Co4 O9+δ . © 2004 Elsevier B.V. All rights reserved. Keywords: Oxides; Chemical synthesis; Electric properties; Thermal conductivity

1. Introduction Thermoelectric energy conversion is widely recognized as a promising technology for both electric power generation in terms of waste heat recovery and cooling of various electronic devices. The performance of a thermoelectric material is usually characterized by the figure of merit Z = S 2 /ρκ, where S, ρ and κ are thermopower, electrical resistivity and thermal conductivity, respectively. At present, the materials with the highest thermoelectric performance are intermetallic compounds, such as (Bi, Sb)2 (Te, Se)3 [1,2], filled skutterudites [3–5] and Si–Ge alloy [6,7], which have the values of ZT about 1. But their applications to power generation systems have been limited because they are easily decomposed or oxidized at high temperature in air. With respect to high-temperature operation in air, the advantages of metal oxides are very obvious for their excellent thermal and chemical stability. However, the previous guiding principles predicted that the metal oxides are not suitable for TE applications because of their low mobility [8]. In 1997, Terasaki and others reported a new TE oxide material, which didn’t follow the previous guiding princi∗

Corresponding author. Tel.: +86-21-52412522. E-mail address: [email protected] (L. Chen).

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ples [9]. In their work, a layered cobalt oxide, NaCo2 O4 , is suggested as a potential p-type TE material with a large thermopower of about 100 ␮V K−1 and a low resisitivity of about 0.2 m cm at 300 K. Since then, extensive studies have been carried out on these Co-based oxides with a layered structure by focusing on their thermoelectric transport [10–14]. Ca3 Co4 O9+δ is another Co-based oxide which shows good thermoelectric performance [15]. Very recently, Shikano and Funahashi reported that the single crystals of (Ca2 CoO3 )0.7 CoO2 with a Ca3 Co4 O9+δ structure have a dimensionless figure of merit ZT nearly 0.83 at 973 K [16]. Thus Ca3 Co4 O9+δ has a high potential for practical application in thermoelectric power generation. However, the ZT values of the polycrystalline bulk materials are relatively low [15,17]. For practical use their thermoelectric performance must be further improved. One approach is partial substitution for Ca site by metals such as alkali metals, alkaline earth metals or rare earth metals. Previous experiments showed that the substitution of Bi for Ca is effective in improving thermoelectric properties [18]. In the present study, the substitution of Dy for Ca was performed and the effects of Dy substitution on the thermoelectric properties were systematically investigated.

D. Wang et al. / Journal of Alloys and Compounds 376 (2004) 58–61

59

2. Experimental x=0.45

Intensity(arbitrary unit)

Ca3−x Dyx Co4 O9+δ (x = 0.0–0.45) powder was prepared by using wet chemical method. As the materials, reagent grade CaCoO3 , Dy2 O3 , Co and citric acid monohydrate were weighed in specific proportion to obtain the composition of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45). They were dissolved in concentrated HNO3 and deionized water. The citrate solution was dehydrated and nitrates were removed as nitrous gases, and the resulted carbonaceous xerogel was crushed and calcined at 823 K for 2 h in air. After cooling, samples were ground and calcined at 1173 K in the flow of oxygen gas for 12 h. For Spark Plasma Sintering (SPS), the powders were placed in a graphite die, and heated up to 973 K at a rate of 100 K/min and then kept at this temperature for 5 min under a pressure of 50 MPa in vacuum. The obtained samples were annealed at 1173 K for 36 h under an O2 flow to eliminate the carbon on the surface. Powder X-ray diffraction data at room temperature were collected on a diffractometer (Rigaku RINT2000) with Cu K␣ (λ = 0.15406 nm) radiation in the range of 5–80◦ . The relative densities of all the samples were measured using Archimedes’ method. Phase composition analysis was made with EPMA (SHIMADZU 8705QH2 ). The electrical resistivity (ρ) and thermopower (S) were measured along the pressed plane from room temperature to 1000 K. The temperature dependence of ρ was measured using a standard four-probe method. For the thermopower measurement, two Pt–Pt/Rh thermocouples were attached to both end surfaces of the bar-shaped sample, and the temperature gradient in the sample was generated by passing cooling air in a tube mechanically attached with one end of the sample. Thermoelectromotive forces measured as a function of temperature difference give a straight line and its slope is the thermopower. The thermal conductivity was determined from the specific heat capacity and the thermal diffusivity measured by laser flash technique (NETZSCH LFA427).

x=0.3

x=0.15

x=0

10

30

40

50

60

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2 Fig. 1. XRD patterns of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45). Impurity phase is pointed out by the asterisk.

Dy is partially substituted for Ca and the solid solution range of Dy in the Ca3−x Dyx Co4 O9+δ is as much as x = 0.3. Bulk densities of the annealed SPS samples are plotted as a function of Dy doping content (denoted as x) in Fig. 2. The relative densities of all the samples are in the range of 95–97% of the theoretical X-ray density. Thus the SPS technique we employed is quite effective for fabricating densely sintered Ca3 Co4 O9+δ -based compounds. The temperature dependence of thermopower (S) of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45) is shown in Fig. 3. The thermopower values are all positive, indicating hole conduction. For all the samples, the S values increase with increasing temperature over the measured temperature range. Meanwhile, the S values increase with the increase of Dy content. At 1000 K, the thermopower value of Ca3 Co4 O9+δ is 170 ␮V K−1 , while for the samples with x = 0.15, 0.3, 0.45 Dy substitution, they reach 180, 190, 195 ␮V K−1 , respectively. The electrical resistivity versus temperature of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45) in the range of

3. Results and discussions

100

Relative density (%)

The X-ray powder diffraction spectra of oxides Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45) are presented in Fig. 1. Except for the sample of x = 0.45 with a small amount of impurity marked by asterisks, all peaks can be indexed as the Ca3 Co4 O9+δ phase with monoclinic symmetry [14]. As-prepared and O2 annealed SPS samples show XRD patterns almost identical with those of the starting powders. This indicates that the decomposition or the reduction of the Ca3 Co4 O9+δ phase during SPS process is not serious. EPMA analyses performed on Ca2.7 Dy0.3 Co4 O9+δ show that it has a homogeneous structure and no impurity phases such as Dy or Co oxides are detected. The composition of the matrix is Ca:Dy:Co = 2.70:0.33:3.92, which is very close to the nominal composition. This indicates that

20

95

90

85

80

0

5

10

15

Dy doping content x ( mol %) Fig. 2. Relative densities of the samples as a function of Dy doping content x.

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D. Wang et al. / Journal of Alloys and Compounds 376 (2004) 58–61

Ca3-xDyxCo4O9+δ x=0 x=0.15 x=0.3 x=0.45

190 180

-1

x=0 x=0.15 x=0.3 x=0.45

170 160

11

10

150 9

140

300 400 500 600 700 800 900 1000 1100

x=0 x=0.15 x=0.3 x=0.45

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15

10

300

400

500

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900 1000

Temperature (K) Fig. 4. Temperature dependence of electrical resistivity of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45).

2.5

3.0

3.5

ogy with Nax Co2 O4 compound, the edge shared CoO2 layer was considered to be responsible for the electric conduction, whereas the rock-salt type Ca2 CoO3 layer was regarded as a charge reservoir to supply charge carriers into the CoO2 layer [20]. For the Ca3 Co4 O9+δ compound, the effective carrier is hole. The substitution of trivalent Dy3+ for divalent Ca2+ would decrease the hole concentrations. And because the substitution is in the Ca2 CoO3 layer, the conduction path would not be disturbed. Thus with the increasing of Dy content, the carrier concentrations are progressively decreased. Consequently, both the thermopower and the electrical resistivity systematically increase while the activation energy is kept constant. Temperature dependence of thermal conductivity κ of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3) is presented in Fig. 6. For the Dy-free sample, the thermal conductivity decreases with increasing temperature. However, for the samples with Dy substitution the thermal conductivities show weak temperature dependence. The substitution of Dy for Ca causes the decrease of thermal conductivity and with increasing Dy content the value of the thermal conductivity decreases. At room temperature, the value of thermal conductivity is 2.6 W m−1 K−1 for x = 0, 1.9 W m−1 K−1 for x = 0.15

-1

3.0

Ca3-xDyxCo4O9+δ x=0 x=0.15 x=0.3

-1

Ca3-xDyxCo4O9+δ

2.0

Fig. 5. The relation between ln σT and 1000/T.

Thermal Conductivity (Wm K )

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1.5

1000/T (K )

Fig. 3. Temperature dependence of thermoelectric power of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3 and 0.45).

300–1000 K is presented in Fig. 4. For x = 0 sample, the ρ–T curve shows metallic-like behavior (i.e., dρ/dT > 0) below 450 K while semiconducting-like behavior (i.e., dρ/dT ≤ 0) above 450 K. For the samples with Dy substitution, however, they all show semiconducting-like behavior in the measured temperature range. The Dy substitution for Ca results in the increase of electrical resistivity, and with the increase of Dy content the electrical resistivity increases. For all samples, the relationship between ln ρ and 1/T is not linear in the measured temperature range. But the plots of ln(σT) versus 1/T lie on straight lines above about 600 K, which is shown in Fig. 5. This indicates that the oxides are in hopping conduction at high temperature. The slopes of the plots for all the oxides are almost the same, with an E0 of 0.114 eV above 600 K. This indicates that the Dy substitution for Ca does not change the activation energy of the hopping process and also not change the transport mechanism. Ca3 Co4 O9+δ is a layered oxide, which consists of alternating stacks of triple rock salt-type Ca2 CoO3 layers and single CdI2 -type CoO2 layers along the c axis [19]. By anal-

1.0

-1

Temperature (K)

Resistivity (mΩ cm)

Ca3-xDyxCo4O9+δ

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-1

200

ln σT (S K cm )

-1

Thermoelectric power (µVK )

210

2.5 2.0 1.5 1.0 0.5 0.0

300 400 500 600 700 800 900 1000

Temperature (K) Fig. 6. Temperature dependence of thermal conductivity κ of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3).

D. Wang et al. / Journal of Alloys and Compounds 376 (2004) 58–61

Ca3 Co4 O9+δ -based compounds, which results in increase of both thermopower and electrical resistivity. The Dy substituted samples (x = 0, 0.3) have lower thermal conductivities than Ca3 Co4 O9+δ due to their lower electronic and lattice thermal conductivities. The dimensionless figure of merit ZT reaches 0.27 at 1000 K for the sample of Ca3−x Dyx Co4 O9+δ (x = 0.3).

0.30 Ca3-xDyxCo4O9+δ

0.25

x=0 x=0.15 x=0.3

ZT

0.20

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0.15 0.10 0.05 0.00

Acknowledgements 400

500

600

700

800

900

1000

Temperature (K) Fig. 7. Temperature dependence of ZT of Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3).

and 1.69 W m−1 K−1 for x = 0.3. Thermal conductivity can be expressed by the sum of lattice component (κl ) and electronic component (κe ) as κ = κl + κe . The κe values can be estimated from Wiedemann–Franz’s law as κe = LT/ρ, where L is the Lorentz number (2.45 × 10−8 V2 K−2 for free electrons). Hence, κl can be obtained from κ to κe . At room temperature, the electronic component (κe ) for Ca3−x Dyx Co4 O9+δ (x = 0, 0.15, 0.3) is 0.068, 0.059 and 0.044 W m−1 K−1 , respectively; while the lattice component (κl ) is 2.57, 1.85 and 1.64 W m−1 K−1 , respectively. Thus the observed decrease in thermal conductivity with increasing Dy content could mainly be attributed to the decrease of lattice thermal conductivity (κl = κ −κe ) due to the increase of phonon scattering by Dy substitution. Thermoelectric performance is generally evaluated by the dimensionless figure of merit ZT (S 2 T/ρκ). Fig. 7 shows the temperature-dependent ZT for the Dy substituted Ca3 Co4 O9+δ samples. The ZT values for all oxides tend to increase with increasing temperature and the Dy substitution results in the increase of ZT value. The sample Ca3−x Dyx Co4 O9+δ (x = 0.3) has the largest figure of merit, and the value is 0.27 at 1000 K.

4. Conclusion Dy-substituted Ca3 C4 O9+δ oxides have been prepared using sol–gel process followed by SPS sintering and their thermoelectric properties have been studied. The substitution of Dy for Ca decreases the hole concentrations in

This work was financially supported by The National High Technology Research and Development Program of China (863 Program) under Grant No. 2001AA323070.

References [1] D. Perrin, M. Chitroub, S. Scherrer, H. Scherrer, J. Phys. Chem. Solids 61 (2000) 1687. [2] O. Yamashita, S. Tomiyoshi, K. Makita, J. Appl. Phys. 93 (2003) 368. [3] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325. [4] G.S. Nolas, M. Kaeser, R.T. Littleton, T.M. Tritt, Appl. Phys. Lett. 77 (2000) 1855. [5] L.D. Chen, T. Kawahara, X.F. Tang, T. Goto, T. Hirai, J. Appl. Phys. 90 (2001) 1864. [6] M.W. Heller, R.D. Nasby, R.T. Johnson Jr., J. Appl. Phys. 47 (1976) 4113. [7] G.D. Mahan, Solid State Phys. 51 (1998) 81. [8] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [9] H. Yakabe, K. Kikuchi, I. Terasaki, et al., Proceedings of 16th International Conference on Thermoelectrics, IEEE, Piscataway, NJ, 1997, p. 523. [10] T. Terasaki, Physics B 328 (2003) 63. [11] R. Funahashi, I. Matsubara, H. Ikuta, et al., Jpn. J. Appl. Phys. 39 (2000) L1127. [12] R. Asahi, J. Sugiyama, T. Tani, Phys. Rev. B 66 (2002) 155103. [13] R. Runahashi, M. Shikano, Appl. Phys. Lett. 81 (2002) 1459. [14] W.S.N. Murayama, J. Mater. Res. 15 (2000) 382. [15] S. Li, R. Funahashi, I. Matsubara, K. Ueno, H. Yamada, J. Mater. Chem. 9 (1999) 1659. [16] M. Shikano, R. Funahashi, Appl. Phys. Lett. 82 (2003) 1851. [17] I. Matsubara, R. Funahashi, T. Takeuchi, S. Sodeoka, J. Appl. Phys. 90 (2001) 462. [18] S. Li, R. Funahashi, I. Matsubara, K. Ueno, H. Yamada, Chem. Mater. 12 (2000) 2424. [19] A. Masset, C. Michel, A. Maignan, M. Hervieul, et al., Phys. Rev. B 62 (2000) 166. [20] Y. Miyazaki, M. Onoda, T. Oku, et al., J. Phys. Soc. Jpn. 71 (2002) 491.