High temperature thermoelectric properties of the Ca 32x B x Co 4 O 9 system

Transcription

1 J Mater Sci: Mater Electron (2013) 24: DOI /s High temperature thermoelectric properties of the Ca 32x B x Co 4 O 9 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 Ca 3-x B x Co 4 O 9 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 = S 2 /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, Malatya, Turkey serdar.altin@inonu.edu.tr toxicity and low cost. Cobaltites, Na x Co 2 O 4,Bi 2 Sr 2 Co 2 O x, Ca 3 Co 4 O 9, 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 Na x Co 2 O 4, Bi 2 Sr 2 Co 2 O x systems. It was found that Ca 3 Co 4 O 9 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 Ca 3 Co 4 O 9 consists of misfit layered structure stacking along the c-axis. Ca 3 Co 4 O 9 has two monoclinic subsystems; one is CdI 2 -type (CoO 2 ) layer and the other is rocksalt-type (Ca 2 CoO 3 ) layer. These two subsystem have similar unit cell parameters (a = 4.83 Å and c = Å) but b 1 = 4.55 Å for (Ca 2 CoO 3 ) and b 2 = 2.81 Å for (CoO 2 )[3]. The reason of good transport properties of Ca 3 Co 4 O 9 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 (CoO 2 ) layers dominate the electronic and magnetic behavior of the system, whereas (Ca 2 CoO 3 ) layers act as a charge reservoir which supply charge carriers into the (CoO 2 ) layers [4]. Therefore, substitutions/dopings to the (CoO 2 ) layers has strong effect on the transport and magnetic behavior of the system, whereas substitutions/dopings to the (Ca 2 CoO 3 ) layers may also influence significantly the transport and magnetic behavior. Ca 3 Co 4 O 9 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 Ca 3 Co 4 O 9 material shows low ZT value since it has randomly oriented grains and so high q. In order to improve

2 J Mater Sci: Mater Electron (2013) 24: 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 Ca 3-x B x Co 4 O 9 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 Ca 3-x B x Co4O 9 (where x = 0.0, 0.5, 0.75 and 1.0) samples were synthesized using conventional solid-state reaction technique. CaCo 3,Co 2 O 3, and B 2 O 3 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 CuK a ( Å) 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, S 2 /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 Ca 3-x B x Co 4 O 9 (x = 0.0, 0.5, 0.75 and 1.0). For x = 0.0, all the diffraction peaks were assigned to pure Ca 3 Co 4 O 9 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 Ca 3 Co 4 O 9, some impurity phases, such as Co 3 O 4, Co 3 (BO 3 ) 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, Ca 3 Co 4 O 9, suppressed and many impurity phases such as Co 3 O 4, Co 3 (BO 3 ) 2, CaB 2 O 4, were observed in both the samples. The crystal symmetry of the x = 0.0 sample was determined as monoclinic and unit cell parameters was calculated to be a = Å, b = Å and c = Å. For the x = B substituted sample, the crystal symmetry was also found as monoclinic and the unit cell parameters, a = Å, b = Å and c = Å. When the ionic radii of Ca?2 and B 3? is considered (r ca2? = 1.00 Å and r B3? = 0.27 Å), both occupation of the Ca-sites by the B 3? ions and incorporation of the B 3? ions into the interstitial sites in the unit cell results naturally in a significant distortion of the crystal structure of Ca 3 Co 4 O 9. 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 Ca 3-x B x Co 4 O 9 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 Ca 3.12 Co 3.87 O x from EDX analysis which is very close Ca 3 Co 4 O 9. 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 Ca 2.33 B 1.17 Co 3.45 O x for x = 0.5, Ca 2.13 B 1.37 Co 3.5 O x for x = 0.75, Ca 1.81 B 1.54 Co 3.64 O x for x = 1.0, respectively. SEM results suggested that the B-substitution influenced the microstructural evaluation of the Ca 3 Co 4 O 9 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 Ca 3-x B x Co 4 O 9 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

3 4408 J Mater Sci: Mater Electron (2013) 24: Fig. 1 XRD pattern of samples Fig. 2 SEM photographs of the samples Ca 3-x B x Co 4 O 9. 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 B 3? for divalent Ca 2? 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 Ca 3-x B x Co 4 O 9 system.

4 J Mater Sci: Mater Electron (2013) 24: 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 = 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; S ¼ k B e g 3 g 4 x 1 x ð1þ where g 3 and g 4 are the number of configuration of the Co 3? and Co 4? ions, respectively, x is the fraction of the the Co 3? and Co 4? sites [11, 15]. When Co 3? and Co 4? are considered in low spin state, g 3 = 1 and g 4 = 6. From Eq. (1), it would conclude that S increases with decreasing x. It is expected that the lower concentration of Co 4?, higher S value. The thermoelectric power results indicated that the B-substitution for Ca in the Ca 3 Co 4 O 9 system did not change the concentration of the Co 4? ions. Temperature dependence of the power factor, S 2 /q, is presented in Fig. 3c. S 2 /q for all the samples increased monotonically with temperature. It is obvious that the B-substitution significantly influenced S 2 /q. The highest S 2 /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 S 2 /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

5 4410 J Mater Sci: Mater Electron (2013) 24: Conclusion The B-substituted Ca 3 Co 4 O 9 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 Ca 3 Co 4 O 9 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 Co 4?, according to the Modified Heikes formula. But, it was found that the B-substitution for Ca did not change the concentration of the Co 4? ions. The highest power factor value was obtained for the unsubstituted sample, while it decreased in the B-substituted samples. 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. IUBAP References 1. Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W.S. Seo, K. Koumoto, Thermoelectric performance of Bi- and Na-substituted Ca 3 Co 4 O 9 improved through ceramic texturing. J. Mater. Chem. 13, (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, (2005) 3. R. Moubah, S. Colis, C. Leuvrey, G. Schmerber, M. Drillon, A. Dinia, Synthesis and characterization of Ca 3 Co 4 O 9 thin films prepared by sol gel spin-coating technique on Al 2 O 3 (001). Thin Solid Films 518, (2010) 4. Y. Wang, Y. Sui, P. Ren, L. Wang, X. Wang, W. Su, H. Fan, Strongly correlated properties and enhanced thermoelectric response in Ca 3 Co 4 x M x O 9 (M = Fe, Mn and Cu). Chem. Mater. 22, (2012) 5. G.D. Tang, Z.H. Wang, X.N. Xu, L. Qiu, L. Xing, Y.W. Du, Thermoelectric properties of Ca 3 Co 4 O 9 with Lu substitution. J. Mater. Sci. 45, (2010) 6. 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 Ca 3 Co 4 O 9?d ceramics. J. Mater. Sci. 43, (2008) 7. 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 Ca 3 Co 4 O 9 thin films by pulsed laser deposition. J. Alloys Compd. 511, (2012) 8. Sh. Rasekh, M.A. Torres, G. Constantinescu, M.A. Madre, J.C. Diez, A. Sotelo, Effect of Cu by Co substitution on Ca 3 Co 4 O 9 thermoelectric ceramics. J. Mater. Sci. Mater. Electron. 24, (2013) 9. J.C. Diez, M.A. Torres, Sh. Rasekh, G. Constantinescu, M.A. Madre, A. Sotelo, Enhancement of Ca 3 Co 4 O 9 thermoelectric properties by Cr for Co substitution. Ceram. Int. 39, (2013) 10. Y.-H. Lin, C.-W. Nan, Y. Liu, J. Li, High-temperature electrical transport and thermoelectric power of partially substituted Ca 3 Co 4 O 9 based ceramics. J. Am. Ceram. Soc. 90, (2007) 11. N.V. Nong, C.-J. Liu, M. Ohtaki, High-temperature thermoelectric properties of late rare earth-doped Ca 3 Co 4 O 9?d. J. Alloys Compd. 509, (2011) 12. L. Xu, F. Li, Y. Wang, High-temperature transport and thermoelectric properties of Ca 3 Co 4-x Ti x O 9. J. Alloys Compd. 501, (2010) 13. Y. Wang, Y. Sui, J. Cheng, X. Wang, W. Su, Comparison of the high temperature thermoelectric properties for Ag-doped and Agadded Ca 3 Co 4 O 9. J. Alloys Compd. 477, (2009) 14. S. Katsuyama, Y. Takiguchi, M. Ito, Synthesis of Ca 3 Co 4 O 9 ceramics by citric acid complex and hydrothermal hot-pressing processes and investigation of its thermoelectric properties. Mater. Trans. 48, (2007) 15. W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides. Phys. Rev. B 62, (2000)