Journal of ELECTRONIC MATERIALS, Vol. 40, No. 5, 2011
DOI: 10.1007/s11664-011-1527-y Ó 2011 TMS
Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite CHIA-JYI LIU,1,3,4 YU-CHIH HUANG,1 N.V. NONG,1 YEN-LIANG LIU,1 ´ EK2 ´ IC and V. PETR 1.—Department of Physics, National Changhua University of Education, Changhua 500, Taiwan. 2.—Institute of Physics of the Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 2, 182 21 Praha 8, Czech Republic. 3.—e-mail:
[email protected]. 4.—e-mail:
[email protected]
The cobaltite Ca3Co4O9+d has shown large thermopower and is considered to be a good candidate for use as a thermoelectric material. The composition of Ca3Co4O9+d is better expressed as [Ca2CoO3][CoO2]b1/b2 with the misfit-layered structure featuring different periodicities along the b axis, with b1 referring to the b-axis length of the NaCl-type [Ca2CoO3] sublattice and b2 referring to the b-axis length of the [CoO2] sublattice. The crystal structure of Ca3Co4O9+d can be viewed as being of two subsystems, i.e., the distorted NaCltype [Ca2CoO3] sublattice and the CdI2-type [CoO2] sublattice, alternately stacked along the c-axis. In this paper, we report measurements of the electrical resistivity and Seebeck coefficient for a series of misfit-layered oxides Ca3Co4xSixO9+d prepared by solid-state reaction. Structural parameters are refined with the superspace group X2/m(0b0)s0 using powder x-ray diffraction data. With partial substitution of Si4+ for Co3+, the resistivity decreases, while the thermopower increases simultaneously. These results indicate that partial substitution of Si4+ improves the thermoelectric characteristics of Ca3Co4O9+d. Key words: Thermoelectrics, cobaltite, power factor
INTRODUCTION Thermoelectric materials can directly convert thermal energy to electrical energy via the Seebeck effect. Three transport parameters determine the thermoelectric figure of merit Z = rS2/j, where r, S, and j are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. The cobaltites Ca3Co4O9+d exhibit large thermopower, low thermal conductivity, and thermal stability at high temperatures.1–9 These features make the cobaltite Ca3Co4O9+d a potential thermoelectric material. The structure of Ca3Co4O9+d consists of two misfit-layered sublattices, i.e., the distorted NaCl-type [Ca2CoO3] sublattice and the CdI2-type [CoO2] sublattice. These two sublattices have different periodicities in the b direction. Therefore, Ca3Co4O9+d can be expressed as [Ca2CoO3][CoO2]b1/b2, (Received May 6, 2010; accepted January 17, 2011; published online February 12, 2011)
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where b1 refers to the b-axis length of the [Ca2CoO3] sublattice and b2 refers to the b-axis length of the [CoO2] sublattice. In this paper, we report that the thermoelectric power factor of Ca3Co4xSixO9+d is significantly improved upon partial substitution of Si for Co. The power factor is enhanced by 46% at 295 K for Ca3Co3.97Si0.03O9+d as compared with the undoped sample. EXPERIMENTAL PROCEDURES Polycrystalline samples of Ca3Co4xSixO9+d (x = 0, 0.01, and 0.03) were synthesized by the conventional solid-state reaction method. High-purity powders of CaCO3, Co3O4, and SiO2 were quantitatively mixed and ground using a high-energy ball mill for 45 min, followed by calcination at 900°C in air for 24 h twice with intermediate grinding. The phase purity of the resulting powders was examined by a Shimadzu XRD-6000 powder x-ray diffractometer using Fe Ka radiation. No impurities were
Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite
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detected in the x-ray diffraction (XRD) patterns. The XRD data were analyzed using the Jana2006 Rietveld refinement program on the base of fourdimensional (4D) superspace formalism.10 Electrical resistivity measurements were carried out using the standard four-probe technique. Thermopower measurements were performed using steady-state techniques with a temperature difference of 0.5 K to 1 K across the sample. The temperature gradient was measured using a type E differential thermocouple. The thermally generated Seebeck voltage across the sample was measured using a Keithley 2182 nanovoltmeter. The thermopower of the sample was obtained by subtracting the thermopower of the Cu Seebeck probes. The average valence state of cobalt and the oxygen content were determined using iodometric titration. RESULTS AND DISCUSSION Figure 1a shows powder x-ray diffraction patterns of Ca3Co4xSixO9+d (x = 0, 0.01, and 0.03). The lattice constants for the x = 0 sample are a = ˚ , b1 = 4.5532(3) A ˚ , b2 = 2.8192(3) A ˚, c = 4.8242(3) A ˚ , and b = 98.151(5)°. The lattice con10.8236(7) A ˚, stants for the x = 0.03 sample are a = 4.8254(2) A ˚ , b2 = 2.8240(1) A ˚ , c = 10.834(5) A ˚, b1 = 4.5585(2) A and b = 98.082(4)°. Figure 1b shows the Rietveld fits to powder XRD data for Ca3Co3.97Si0.03O9+d using the superspace group X2/m(0b0)s0. Figure 2 shows the temperature dependence of the electrical resistivity for Ca3Co4xSixO9+d (x = 0, 0.01, and 0.03). All the Si-doped samples exhibit smaller electrical resistivities as compared with the undoped sample. The electrical resistivity at 295 K for x = 0, 0.01, and 0.03 was 13.5 mX-cm, 11.6 mXcm, and 10.3 mX-cm, respectively. The Co4+ is supposed to be responsible for the hole carrier transport in the misfit-layered cobaltites with the mixed valence Co3+/Co4+. With Si4+ doping, one should expect a decrease of Co4+ concentration. Iodometric titration results show that the average valence of cobalt is 3.199(4), 3.182(3), and 3.169(3), and the oxygen content is 0.399(8), 0.369(7), and 0.351(7), for x = 0, 0.01, and 0.03, respectively. One can readily see that both the average valence state of Co and the oxygen content decrease slightly with Si4+ doping. This is in agreement with the expectation of a decrease of Co4+ concentration upon partial substitution of Si4+ for Co4+. A decrease of Co4+ concentration is expected to increase the electrical resistivity. However, this does not match the experimental findings, which will be discussed later. Furthermore, the electrical resistivity shows metallike temperature dependence down to ca. 70 K, and a metal–nonmetal transition occurs in the low temperature regime for all the samples. For variable-range hopping transport in a disordered system, the temperature dependence of the resistivity would obey the following relation at low temperatures:11
Fig. 1. (a) Powder x-ray diffraction patterns of Ca3Co4xSixO9+d for x = 0, 0.01, and 0.03. (b) Rietveld fits to powder XRD data for Ca3Co3.97Si0.03O9+d using the superspace group X2/m(0b0)s0.
Fig. 2. Temperature dependence of the electrical resistivity for Ca3Co4xSixO9+d with x = 0, 0.01, and 0.03.
C.-J. Liu, Huang, Nong, Y.-L. Liu, and Petr´ic´ek
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Fig. 3. The logarithm of the electrical conductivity, ln r, varies as T1/4 for Ca3Co3.9Si0.01O9+d. Other samples exhibit similar behavior.
" 1 # T0 dþ1 r ¼ r0 exp ; T
Fig. 4. Temperature dependence of the thermopower for Ca3Co4xSixO9+d with x = 0, 0.01, and 0.03.
(1)
where r0 is weakly temperature dependent, T0 is associated with the localization length, and d is the dimensionality. For a three-dimensional system, the logarithm of the conductivity, ln r, should vary as T1/4. Figure 3 illustrates that the temperature dependence of the electrical resistivity for Ca3Co3.9Si0.01O9+d follows the variable-range hopping process with d = 3 in Eq. 1 in the nonmetallic regime. The other samples also exhibit similar behavior. Even though Ca3Co3.9Si0.01O9+d has a layered structure, it is in the form of polycrystalline bulk and therefore can be considered as a threedimensional system. This electronic disorder behavior might be associated with the positional disorder involved in the CoO layer of the [Ca2CoO3] sublattice.3 Figure 4 shows the temperature dependence of the thermopower for Ca3Co4xSixO9+d (x = 0, 0.01, and 0.03). The sign of the thermopower for all the samples is positive, indicating that the carrier is of hole type. The thermopower at 295 K for x = 0, 0.01, and 0.03 is 132 lV/K, 140 lV/K, and 140 lV/K, respectively. It can be readily seen that the thermopower is enhanced upon Si doping. Figure 5 shows the temperature dependence of the power factor P = rS2 for Ca3Co4xSixO9+d (x = 0, 0.01, and 0.03). The power factor at 295 K for x = 0, 0.01, and 0.03 is 1.3 lW/cmK2, 1.7 lW/cmK2, and 1.9 lW/cmK2, respectively. Compared with the x = 0 sample, the power factor of the x = 0.03 sample is enhanced by 51% and 46% at 156 K and 295 K, respectively. We now turn to discuss the fact that the electrical resistivity decreases with the Si doping, accompanied by a decrease of Co4+ concentration. It is plausible to propose that the Co4+ concentration is not the sole factor to affect the charge transport of Ca3Co4xSixO9+d. The Si doping might occupy a site
Fig. 5. Temperature dependence of the thermoelectric power factor for Ca3Co4xSixO9+d with x = 0, 0.01, and 0.03.
which also contributes to charge transport, however with activated conduction. This would lead to the creation of impurity states within the band gap and enhanced carrier concentration for the activated conduction type. Furthermore, increasing the electrical conductivity of a material would generally decrease the thermopower. A simultaneous increase of the electrical conductivity and the thermopower in Ca3Co4xSixO9+d could possibly be explained by the following scenario, since Ca3Co4O9+d has two sublattices of [Ca2CoO3] and [CoO2], both of which can contribute to the charge transport. Based on first-principle calculations, one of the sublattices is metal-like and the other is of activated type. For materials with more than one type of charge carrier, the diffusion thermopower can be expressed as X ri (2) Si ; S¼ r i where ri and Si are, respectively, the partial electrical conductivity and partial thermopower associ-
Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite
ated with the ith group of carriers. The thermopower of Ca3Co4xSixO9+d can be rewritten as rCa2 CoO3 rCoO2 S¼ SCa2 CoO3 þ SCoO2 : rCa2 CoO3 þ rCoO2 rCa2 CoO3 þ rCoO2 (3) Supposing that the Si doping for Co occurs in the sublattice with activated conduction type and a small band gap, it will result in a decrease of the activation energy. Due to the increase of the Ea
electrical conductivity e kB T faster than the decrease of S kEBaT for activated type conduction. Assuming that the [CoO2] sublattice is responsible for the activated conduction type,12 it is plausible that the second term in Eq. 3 would increase and hence the size of the thermopower for Ca3Co4xFexO9+d would increase, provided that rCa2 CoO3 dominates the electrical conductivity of the first term due to its metallic character. However, the site of Si substitution for Co still needs to be confirmed. According to the Wiedemann–Franz law, the electronic contribution to the thermal conductivity je is given by je ¼ LrT;
(4)
where the proportionality constant L is the Lorenz number and r is the electrical conductivity. An increase of the electrical conductivity by partial substitution of Si for Co would increase je, which might increase the total thermal conductivity of the title materials. Measurements of thermal conductivity of Si-doped cobaltites are in progress. CONCLUSIONS We have measured the electrical resistivity and thermopower of misfit-layered oxides Ca3Co4xSixO9
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(x = 0, 0.01, and 0.03) prepared by conventional solid-state reaction methods. We find that partial substitution of Si for Co leads to improved thermoelectric power factor, enhanced by 46% at 295 K in Ca3Co3.97Si0.03O9+d as compared with the undoped sample. ACKNOWLEDGEMENTS This work is supported by the National Science Council of Taiwan, ROC with Grant No. NSC 98-2112-M-018-005-MY3. V.P. thanks the Praemium Academiae of the Czech Academy of Sciences for support. REFERENCES 1. S. Li, R. Funahashi, I. Matsubara, K. Ueno, and H. Yamada, J. Mater. Chem. 9, 1659 (1999). 2. A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, and J. Hejtmanek, Phys. Rev. B 62, 166 (2000). 3. S. Lambert, H. Leligny, and D. Grebille, J. Solid State Chem. 160, 322 (2001). 4. Q. Yao, D.L. Wang, L.D. Chen, X. Shi, and M. Zhou, J. Appl. Phys. 97, 103905 (2005). 5. C.-J. Liu, L.-C. Huang, and J.-S. Wang, Appl. Phys. Lett. 89, 204102 (2006). 6. C.-J. Liu, J.-L. Chen, L.-C. Huang, Z.-R. Lin, and C.-L. Chang, J. Appl. Phys. 102, 014908 (2007). 7. J.L. Chen, Y.S. Liu, C.-J. Liu, L.C. Huang, C.L. Dong, S.S. Chen, and C.L. Chang, J. Phys. D Appl. Phys. 42, 135418 (2009). 8. N.V. Nong, C.-J. Liu, and M. Ohtaki, J. Alloy. Compd. 491, 53 (2010). 9. C.-J. Liu, P.K. Nayak, Z.-R. Lin, and K.-Y. Jeng, Thin Solid Films 516, 8564 (2008). 10. V. Petr´ic´ek, M. Dus´ek, and L. Palatinus, Jana2006 (Praha, Czech Republic: Inst. of Physics, ASCR, 2006). 11. N.F. Mott and E.A. Davis, Electronic Process in Noncrystalline Materials, 2nd ed. (Oxford: Oxford University Press, 1979). 12. R. Asahi, J. Sugiyama, and T. Tani, Phys. Rev. B 66, 155103 (2002).
