High temperature thermoelectric properties of the ... - Springer Link

J Mater Sci: Mater Electron (2013) 24:4406–4410 DOI 10.1007/s10854-013-1417-6

High temperature thermoelectric properties of the Ca32xBxCo4O9 system S. Demirel • S. Altin • M. A. Aksan

Received: 19 June 2013 / Accepted: 23 July 2013 / Published online: 2 August 2013 Ó Springer Science+Business Media New York 2013

Abstract The Ca3-xBxCo4O9 system (x = 0.0, 0.05, 0.75 and 1.0) has been prepared using the solid-state reaction technique. Structural, microstructural, transport properties (temperature dependence of resistivity, thermoelectric power) of the samples fabricated were investigated in details. The structural/microstructural investigations showed that complex and multiphase crystallographic structure was formed with the B-substitution. A decrease in the grain size was observed in the B-substituted samples. The resistivity increased with the substitution due to decrease in the grain size and weak-link among the grains. It is found that that the thermoelectric power is insensitive to the B-substitution. The highest power factor value was obtained in the x = 0.0 sample, however, it decreased in the B-substituted samples.

1 Introduction Thermoelectric materials are efficiently, economical and environment friendly, promising materials used to convert waste-heat energy directly to electrical energy. Thermoelectric performance of a material is determined by dimensionless figure of merit ZT = S2/qj, where S is the Seebeck coefficient, q the electrical resistivity and j the thermal conductivity. The criteria for technological applications should be ZT C 1. Oxide-based materials with high thermoelectric performance have been attracted great attention due to its chemical stability, high resistivity against oxidization, low S. Demirel S. Altin (&) M. A. Aksan Fen Edebiyat Fakultesi, Inonu Universitesi, Fizik Bolumu, 44280 Malatya, Turkey e-mail: [email protected]

123

toxicity and low cost. Cobaltites, NaxCo2O4, Bi2Sr2Co2Ox, Ca3Co4O9, display large thermoelectric power performance and low resistivity with high ZT coefficient [1]. But, evaporation of Bi and Na with increasing temperature ([300 K) restricts the technological applications of the NaxCo2O4, Bi2Sr2Co2Ox systems. It was found that Ca3Co4O9 has high chemical and thermal stability even at high temperatures (*1000 K), which make a suitable material for the thermoelectric applications [2]. Crystal structure of Ca3Co4O9 consists of misfit layered structure stacking along the c-axis. Ca3Co4O9 has two monoclinic subsystems; one is CdI2-type (CoO2) layer and the other is rocksalt-type (Ca2CoO3) layer. These two ˚ subsystem have similar unit cell parameters (a = 4.83 A ˚ ˚ and c = 10.83 A) but b1 = 4.55 A for (Ca2CoO3) and ˚ for (CoO2) [3]. The reason of good transport b2 = 2.81 A properties of Ca3Co4O9 at high temperatures is still not understood exactly but it was suggested that strong electron correlation and spin-entropy relation may be responsible for the transport properties. The (CoO2) layers dominate the electronic and magnetic behavior of the system, whereas (Ca2CoO3) layers act as a charge reservoir which supply charge carriers into the (CoO2) layers [4]. Therefore, substitutions/dopings to the (CoO2) layers has strong effect on the transport and magnetic behavior of the system, whereas substitutions/dopings to the (Ca2CoO3) layers may also influence significantly the transport and magnetic behavior. Ca3Co4O9 in different forms such as single-crystal or thin film should be fabricated for the thermoelectric applications. Fabrication of the single-crystalline material is very difficult, high-cost and its size can be small so that it cannot be used for the applications. Polycrystalline Ca3Co4O9 material shows low ZT value since it has randomly oriented grains and so high q. In order to improve

J Mater Sci: Mater Electron (2013) 24:4406–4410

the ZT value, the polycrystalline materials have been substituted/doped by some elements such as Bi, Y, Sr, Ag, Ln, Dy, Lu, Gd etc. for Ca and Ga, Mn, Fe, Cu, Cr, Ni, Zn etc. for Co [5–12]. Particularly, the Ag-substitution has positive effect on the transport properties both at high temperatures and low temperatures [13]. In this study, the Ca3-xBxCo4O9 compound, where x = 0.0, 0.05, 0.75 and 1.0, has been fabricated using conventional solid-state reaction technique. Structural, microstructural, transport properties, including temperature dependence of resistivity, thermoelectric power and power factor, of the samples fabricated were investigated.

2 Experimental section Polycrystalline Ca3-xBxCo4O9 (where x = 0.0, 0.5, 0.75 and 1.0) samples were synthesized using conventional solid-state reaction technique. CaCo3, Co2O3, and B2O3 powders were first mixed in an agate mortar for 2 h to give the nominal composition. The mixture was calcined in alumina crucibles at 900 °C for 24 h under air. Then, the mixture was grinded and calcined in air at 900 °C for 36 h under oxygen atmosphere. The calcination was repeated twice with an intermediate grinding for phase purity. The powders were pressed into pellets and finally the pellets sintered at 900 °C for 36 h and cooled to room temperature. Phase identification was performed using X-ray powder diffraction (XRD). XRD patterns were recorded with a ˚) Rigaku RadB Dmax diffractometer, using CuKa (1.5406 A radiation. The surface morphology and compositional characterization of the samples was examined using Leo EVO-40 VPX scanning electron microscope (SEM) and BRUKER X-flash detector 4010 energy dispersive X-ray spectroscope (EDX). Electrical resistivity and thermoelectric power measurements were simultaneously carried out by the standard dc four-probe technique in a LSR-3 measurement system (Linseis GmbH) in the steady state mode between 300 and 1000 K. The power factor, S2/q, was calculated using the electrical resistivity and thermoelectric power results in order to determine the thermoelectric performance of the samples. It should be noted that any resistivity and thermoelectric power data for x = 1.0 could not be obtained due to its high resistivity value out of the measurement range of the instrument.

3 Results and discussion Figure 1 shows XRD patterns of Ca3-xBxCo4O9 (x = 0.0, 0.5, 0.75 and 1.0). For x = 0.0, all the diffraction peaks

4407

were assigned to pure Ca3Co4O9 phase, indicating a good crystallization. No impurity phases were observed in the sample. In the case of x = 0.5, although the main phase was found as Ca3Co4O9, some impurity phases, such as Co3O4, Co3(BO3)2, were also detected. In addition, the peak intensity of the main phase decreased compared to the unsubstituted sample. When the substitution level was further increased to x = 0.75 and x = 1.0, the main phase, Ca3Co4O9, suppressed and many impurity phases such as Co3O4, Co3(BO3)2, CaB2O4, were observed in both the samples. The crystal symmetry of the x = 0.0 sample was determined as monoclinic and unit cell parameters was ˚ , b = 4.299 A ˚ and calculated to be a = 4.761 A ˚ c = 10.757 A. For the x = 0.5 - B substituted sample, the crystal symmetry was also found as monoclinic and the ˚ , b = 4.455 A ˚ and unit cell parameters, a = 4.722 A ?2 ˚ c = 10.635 A. When the ionic radii of Ca and B3? is ˚ and rB3? = 0.27 A ˚ ), both considered (rca2? = 1.00 A occupation of the Ca-sites by the B3? ions and incorporation of the B3? ions into the interstitial sites in the unit cell results naturally in a significant distortion of the crystal structure of Ca3Co4O9. Therefore, we were not able to calculate the unit cell parameters due to this distortion, so it is difficult to make a discussion on the lattice parameters of highly substituted samples. The SEM photographs of the Ca3-xBxCo4O9 samples are shown in Fig. 2a–d. For unsubstituted sample (x = 0.0), randomly oriented plate-like grains in size of around 4–5 lm were obtained, Fig. 2a. The atomic composition was found to be Ca3.12Co3.87Ox from EDX analysis which is very close Ca3Co4O9. It is observed that the grain size reduced to *0.5 lm and amount of porosity increased with increasing the B-concentration in the system. The EDX analysis revealed that the atomic composition of the B-substituted samples are Ca2.33B1.17Co3.45Ox for x = 0.5, Ca2.13B1.37Co3.5Ox for x = 0.75, Ca1.81B1.54 Co3.64Ox for x = 1.0, respectively. SEM results suggested that the B-substitution influenced the microstructural evaluation of the Ca3Co4O9 system. The grain size was reduced by the substitution of B, which leads to the reduced resistivity of the materials fabricated. Temperature dependence of the resistivity (q–T) of Ca3-xBxCo4O9 between 300 and 1000 K is shown in Fig. 3a. Any resistivity data could not be recorded for x = 1.0 since the x = 1.0 sample has the high resistivity value out of the measurement range of the resistivity system. The resistivity increased with increasing the B-concentration in the system. It should be noted that as the temperature increased, the resistivity decreased monotonically in the B-substituted samples. We believe that structural defects, grain boundaries act as scattering centers for the charge carriers. The increase in amount of the porosity

123

4408


Fig. 1 XRD pattern of samples

Fig. 2 SEM photographs of the samples Ca3-xBxCo4O9. a x = 0, b x = 0.5, c 0.75 and d x = 1

and so weak-link among the grains in the B-substituted samples causes the increase in the resistivity and impaired the electrical transport properties of the samples. In addition, the substitution of trivalent B3? for divalent Ca2?

123

decreases the carrier (hole) concentration in the system, which leads to an increase in the resistivity. Figure 3b shows temperature dependence of thermoelectric power (S–T) for the Ca3-xBxCo4O9 system.


Positive S in the samples was obtained over the measured temperature range, indicating a p-type conductivity. The thermoelectric power for all the samples showed similar trend: S gradually increased with temperature. The samples exhibited almost the same S value with temperature, suggesting that the thermoelectric power is seem not so sensitive to the B-substitution. But, it should be noted that the thermoelectric power of the x = 0.5 and x = 0.75 - B substituted samples increased above 850 K, compared to the x = 0.0 B-substituted sample. Similar result was reported by other researchers [14]. It is well known that decrease in the carrier concentration corresponds to an increase in S. However, in the present study, although the carrier concentration decreased by the B-substitution, S remained almost unchanged. Thus, thermoelectric power cannot be interpreted by the variation of the carrier concentration. High temperature thermoelectric power can be interpreted by using the modified Heikes equation; k B g3 x S¼ ð1Þ e g4 1 x

4409

where g3 and g4 are the number of configuration of the Co3? and Co4? ions, respectively, x is the fraction of the the Co3? and Co4? sites [11, 15]. When Co3? and Co4? are considered in low spin state, g3 = 1 and g4 = 6. From Eq. (1), it would conclude that S increases with decreasing x. It is expected that the lower concentration of Co4?, higher S value. The thermoelectric power results indicated that the B-substitution for Ca in the Ca3Co4O9 system did not change the concentration of the Co4? ions. Temperature dependence of the power factor, S2/q, is presented in Fig. 3c. S2/q for all the samples increased monotonically with temperature. It is obvious that the B-substitution significantly influenced S2/q. The highest S2/q was obtained for x = 0.0 but it decreased in the B-substituted samples. When the thermoelectric power is considered as almost same for the samples, it can be concluded that the decrease in S2/q with the B-substitution is related to directly to deterioration in the microstructures of the B-substituted samples and so increase in q.

Fig. 3 Temperature dependence of a resistivity, b thermoelectric power and c power factor of samples fabricated

123

4410


4 Conclusion The B-substituted Ca3Co4O9 samples have been fabricated by using the conventional solid-state reaction technique. The B-substitution lead to a deterioration in the structure and/or microstructure of the Ca3Co4O9 system. Complex and multiphase crystallographic structure was obtained with the B-substitution. Decrease of the grain size and thus weak-link among the grains caused the increase in the resistivity for the B-substituted samples. Thermoelectric power was obtained to be positive for all the samples, suggesting that major carriers are dominated by holes. The results showed that the thermoelectric power is insensitive to the B-substitution. An increase in S is expected with decreasing Co4?, according to the Modified Heikes formula. But, it was found that the B-substitution for Ca did not change the concentration of the Co4? ions. The highest power factor value was obtained for the unsubstituted sample, while it decreased in the B-substituted samples.

4.

5.

6.

7.

8.

9.

10. Acknowledgments The authors would like to thanks Prof. A. Sotello and his Group for high temperature measurements. This study has been supported by Inonu University under Grant No. IUBAP2011-116.

References 1. Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W.S. Seo, K. Koumoto, Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing. J. Mater. Chem. 13, 1094–1099 (2003) 2. E.S. Reddy, J.G. Noudem, S. Hebert, C. Goupil, Fabrication and properties of four-leg oxide thermoelectric modules. J. Phys. D Appl. Phys. 38, 3751–3755 (2005) 3. R. Moubah, S. Colis, C. Leuvrey, G. Schmerber, M. Drillon, A. Dinia, Synthesis and characterization of Ca3Co4O9 thin films

123

11.

12.

13.

14.

15.

prepared by sol–gel spin-coating technique on Al2O3 (001). Thin Solid Films 518, 4546–4548 (2010) Y. Wang, Y. Sui, P. Ren, L. Wang, X. Wang, W. Su, H. Fan, Strongly correlated properties and enhanced thermoelectric response in Ca3Co4–xMxO9 (M = Fe, Mn and Cu). Chem. Mater. 22, 1155–1163 (2012) G.D. Tang, Z.H. Wang, X.N. Xu, L. Qiu, L. Xing, Y.W. Du, Thermoelectric properties of Ca3Co4O9 with Lu substitution. J. Mater. Sci. 45, 3969–3973 (2010) H.Q. Liu, X.B. Zhao, F. Liu, Y. Song, Q. Sun, T.J. Zhu, F.P. Wang, Effect of Gd-doping on thermoelectric properties of Ca3Co4O9?d ceramics. J. Mater. Sci. 43, 6933–6937 (2008) T. Sun, H.H. Hang, Q.Y. Yan, J. Ma, Effect of Ag-doping on the crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition. J. Alloys Compd. 511, 133–138 (2012) Sh. Rasekh, M.A. Torres, G. Constantinescu, M.A. Madre, J.C. Diez, A. Sotelo, Effect of Cu by Co substitution on Ca3Co4O9 thermoelectric ceramics. J. Mater. Sci. Mater. Electron. 24, 2309–2314 (2013) J.C. Diez, M.A. Torres, Sh. Rasekh, G. Constantinescu, M.A. Madre, A. Sotelo, Enhancement of Ca3Co4O9 thermoelectric properties by Cr for Co substitution. Ceram. Int. 39, 6051–6056 (2013) Y.-H. Lin, C.-W. Nan, Y. Liu, J. Li, High-temperature electrical transport and thermoelectric power of partially substituted Ca3Co4O9–based ceramics. J. Am. Ceram. Soc. 90, 132–136 (2007) N.V. Nong, C.-J. Liu, M. Ohtaki, High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9?d. J. Alloys Compd. 509, 977–981 (2011) L. Xu, F. Li, Y. Wang, High-temperature transport and thermoelectric properties of Ca3Co4-xTixO9. J. Alloys Compd. 501, 115–119 (2010) Y. Wang, Y. Sui, J. Cheng, X. Wang, W. Su, Comparison of the high temperature thermoelectric properties for Ag-doped and Agadded Ca3Co4O9. J. Alloys Compd. 477, 817–821 (2009) S. Katsuyama, Y. Takiguchi, M. Ito, Synthesis of Ca3Co4O9 ceramics by citric acid complex and hydrothermal hot-pressing processes and investigation of its thermoelectric properties. Mater. Trans. 48, 2073–2078 (2007) W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides. Phys. Rev. B 62, 6869–6872 (2000)