Synthesis and high temperature thermoelectric properties of BaxAgyCa3−x−yCo4O9 compounds

Journal of Alloys and Compounds 484 (2009) 550–554

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Synthesis and high temperature thermoelectric properties of Bax Agy Ca3−x−y Co4 O9 compounds F.P. Zhang, Q.M. Lu ∗ , J.X. Zhang Key Lab. of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, 100124 Beijing, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 13 January 2009 Received in revised form 28 April 2009 Accepted 29 April 2009 Available online 6 May 2009 Keywords: Ca3 Co4 O9 compound Substitution Grain orientation Thermoelectric properties

a b s t r a c t Polycrystalline Bax Agy Ca3−x−y Co4 O9 bulk samples were prepared by citrate acid sol–gel and spark plasma sintering method. The grain orientation, microstructure and high temperature thermoelectric properties of the bulk samples were systematically studied. Small quantities of the secondary phase Ag precipitated within the x ≤ y compound sample matrices, the grain orientation and microstructure of Ca3 Co4 O9 could be optimized. The electrical resistivity was decreased and the thermal conductivity was increased by increasing y/x ratio. The x = y = 0.1 sample exhibited lowest electrical resistivity (7.2 m cm at 973 K) and thermal conductivity (1.41 W/mK at 973 K) simultaneously with high Seebeck coefficient (172 ␮V/K at 973 K), the dimensionless figure of merit could reach 0.29 at 973 K. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Thermoelectric (TE) device can afford a way converting thermal energy into electrical energy directly, and vice versa. The key to realize an applicable TE device consists in finding materials with high Seebeck coefficient ˛, low resistivity , low thermal conductivity  and high dimensionless figure of merit ZT (ZT = ˛2 T/) [1]. Misfit layered cobalt oxide Ca3 Co4 O9 composed of distorted triple rock salt layer Ca2 CoO3 and hexagonal CdI2 -type layer CoO2 has received renewed attention since the discovery of special TE properties of Nax CoO2−ı and (Ca2 CoO3 )0.7 CoO2 single crystals by Fujita et al. in 2001 and Shikano et al. in 2003, respectively [2–12]. By now many kinds of methods including atom doping [13,14], texture optimizing [15,16] and microstructure adjusting [17] have been used to improve its TE properties. Partial substitution for Ca site was one widely used way due to the advantages of convenience and capability of adjusting carrier [5,8,18] based on its strong electron correlation nature [19,20]. The electrical properties of polycrystalline Ca3 Co4 O9 could be enhanced by substitution of Ag for Ca site, by the addition of Ag2 O to Ca3 Co4 O9 polycrystalline matrix [21,22] or by both substitution for Ca site and addition of Ag phase to the matrix at the same time [23,24]. The thermal conductivity could be restrained by substitution of atoms with high mass such as Ba, Dy, Eu, Sr for Ca site [25–28]. The Ca3 Co4 O9 single crystal is highly anisotropic and the TE performance of highly grain-oriented Ca3 Co4 O9 polycrystal

∗ Corresponding author. Tel.: +86 10 67392661; fax: +86 10 67392840. E-mail address: [email protected] (Q.M. Lu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.144

would be much higher than randomly oriented ones [5,8]. It was reported that textured Ca3 Co4 O9 with different grain orientation degrees could be fabricated through changing sintering pressure and holding time during spark plasma sintering (SPS) procedure [29–31], the grain orientation could also be modified by substitution of Ba for Ca [25]. However, there was no report about effects of double substitution of both Ba and Ag for Ca on TE properties of Ca3 Co4 O9 systems. In this paper, polycrystalline Bax Agy Ca3−x−y Co4 O9 (x = 0, 0.05, 0.1, 0.15, 0.2; y = 0, 0.2, 0.15, 0.1, 0.05, 0) compound bulk samples with different phase compositions were prepared by citrate acid sol–gel and SPS method. Grain orientation, microstructure and high temperature TE properties of the bulk samples up to 1000 K were revealed. 2. Experimental details Polycrystalline Bax Agy Ca3−x−y Co4 O9 (x = 0, 0, 0.05, 0.1, 0.15, 0.2; y = 0, 0.2, 0.15, 0.1, 0.05, 0) bulk samples were fabricated by citrate acid sol–gel and SPS method. Stoichiometric ratios of nitrates of Ba, Ag, Ca and Co were dissolved in aqueous solution of citric acid. The solution was heated under incessant stirring at 353 K in order to form the Bax Agy Ca3−x−y Co4 O9 precursor gel. The gel was dried at 393 K in an oven for 12 h. Then the dry gel was ground and calcined at 1073 K for 8 h to remove excess organic compounds and to obtain Bax Agy Ca3−x−y Co4 O9 powder. The powder was pressed into pellets with a diameter of 20 mm within a graphite die at a uniaxial pressure of 30 MPa. Finally the pellet was sintered by SPS (SPS-3.20MK-V) at sintering temperature 1073 K for 5 min under a continuous uniaxial pressure of 30 MPa with a heating rate of 100–120 K/min from room temperature. The phase composition of bulk samples was analyzed by X-ray diffraction (XRD) at room temperature on a Rigaku diffractometor with Cu K␣ radiation in the 2 range of 5–75◦ with steps of 0.02◦ (2) and a time per step of 1 s. The experimentally absolute density of samples was measured using Archimedes method. The relative

F.P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 550–554

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Fig. 2. Lotgering factors F, relative densities dr for Bax Agy Ca3−x−y Co4 O9 bulk samples as a function of x/y, r. Fig. 1. XRD patterns for Bax Agy Ca3−x−y Co4 O9 bulk samples perpendicular to SPS pressure axis.

density dr was calculated from the ratio of absolute density of substituted samples to that of parallel sample. The microscopic structure was observed with scanning electron microscope (SEM) using secondary electron mode and back scattered electron mode (BSE) by JEOL 6500F operated at 25 kV. The electrical resistivity and Seebeck coefficient were measured in He atmosphere from room temperature up to 1000 K using a conventional dc standard four-probe method on ULVAC ZEM-2 system. The specific heat capacity and thermal diffusivity were measured in b using the laser flash technique on ULVAC-RIKO TC-7000 system. The thermal conductivity was calculated from the experimentally absolute density, specific heat capacity and thermal diffusivity values.

3. Results and discussion 3.1. Sample characterization The room temperature XRD patterns of the bulk samples perpendicular to SPS pressure axis were presented in Fig. 1. The main XRD patterns collected for all samples were compatible with the parameters given for Ca3 Co4 O9 (JCPDS card No. 23-0110) and this indicated that the Ca3 Co4 O9 -type compounds were formed. Secondary phase of Ag was found within x ≤ y samples and no other impurity phases existed. The 2 angles of XRD patterns for x = 0 sample (Ag0.2 Ca2.8 Co4 O9 ) were decreased by 0.04◦ , the 2 angles of XRD patterns for x > 0 samples were decreased by 0.24◦ . Since the average ionic radius of Co and O are invariable and, all samples crystallized in the main form of Ca3 Co4 O9 -type structure, the Ca site was inferred to be substituted and the lattice parameters were changed. The Lotgering factors F [32] based on XRD patterns which was used to evaluate the degrees of sample grain orientations are shown in Fig. 2. Here the Lotgering factors are presented as a function of r (r = x/y). The F can be given by F=

P − P0 1 − P0

favored by optimizing r and the crystallite growth was easier at r = 1. As shown below, the favored crystallite grain growth which lead to density and intergranular contact enhancement would decrease barriers impeding carrier transport. 3.2. Electrical transport properties Fig. 5 shows the temperature dependence of the electrical resistivity  for all samples. The  of all samples was reduced with increasing temperature showing the same semiconductor behaviour over the studied temperature range. The average  in high temperature region was increased by increasing r as a whole. The r = 0 sample (Ag0.2 Ca2.8 Co4 O9 ) showed lowest  and this was estimated to be mainly caused by the secondary phase Ag which acted as shortcut circuit during carrier transport process [22,24]. The r = 1 sample exhibited moderately lower  (7.2 m cm at 973 K) and this was attributed to orientation modifying, density and intergranular contact enhancing discussed above. The  as a function of temperature for Bax Agy Ca3−x−y Co4 O9 compound can be described as (T ) =

T  C

exp

E  a

kT

+ 0

where Ea is the activation energy, k is the Boltzmann constant, C can be regarded as a constant, 0 is the electrical resistivity at 373 K and T is the absolute temperature [33]. It could be seen from Fig. 6 that the relationships between  and T for all samples followed

(1)

The calculation details could be found in Ref. [24]. As shown in Fig. 2, the grain orientation of Ca3 Co4 O9 system was modified by changing r. The F value of r = 0 (Ag0.2 Ca2.8 Co4 O9 ) sample was 0.58 and this phenomenon was estimated to be caused by precipitated secondary phase Ag shown in Fig. 3 within white area. Grain orientation optimized bulk samples could be prepared by altering substituting ratio r in sample preparation process. The optimized grain orientation had led to the density enhancement as shown in Fig. 2 and the dr was increased by increasing r as a whole. Fig. 4 shows the microstructures (SEM) of the doubly substituted Bax Agy Ca2.8 Co4 O9 bulk samples. It can be observed that most of the platelet-like particles were preferentially oriented, the r = 1 and 3 samples (Fig. 4(b and c)) showed higher degree of texture comparing with that of r = 1/3 (Fig. 4(a)) sample. The crystallite growth was

(2)

Fig. 3. BSE image for Ag0.2 Ca2.8 Co4 O9 (r = 0) bulk sample.

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Fig. 5. Temperature dependence of resistivity  for Bax Agy Ca3−x−y Co4 O9 bulk samples.

Fig. 6. ln (/T) vs. 1/T for Bax Agy Ca3−x−y Co4 O9 bulk samples.

The Seebeck coefficient ˛ as a function of temperature for all bulk samples is shown in Fig. 7. The ˛ increased monotonically with increasing temperature and the ˛ was reduced in all substituted sample systems comparing with that of unsubstituted sample. Hole carrier conduction was observed. It was shown that the valence of cobalt plays important role in enhancing the ˛ [19] and the ˛ of the Bax Agy Ca2.8 Co4 O9 compounds could be roughly estimated by using the extended Heikes formula [35] based on Hubbard model [35,36] written as ˛=−

k  B

q

· ln



S3 · f S4 · (1 − f )

 (3)

Fig. 4. Cross-section SEM for Bax Agy Ca2.8 Co4 O9 r = 1/3 (a), r = 1 (b) and r = 3 (c) bulk samples.

the small polaron hopping conduction model [34] in high temperature region. The curves of ln(/T) versus 1/T for all samples showed the same slopes above 650 K. However, the Ea between 500 and 650 K for r = 1 sample was increased to 0.09 eV. The band gap was broadened in this temperature region. The energy needed to move one carrier from the highest occupied band to the conducting band in this temperature region was increased. The thermally activated polaron hopping conduction was observed.

Fig. 7. Temperature dependence of Seebeck coefficient ˛ for Bax Agy Ca3−x−y Co4 O9 bulk samples.

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where kB is Boltzmann constant, q is the value of the elementary charge, Si is the number of the degenerated configuration of the Coi+ state in the CoO2 layers and the f represents the concentration fraction of Co4+ holes on the Co sites in these layers. S3 = 1 and S4 = 6 could be given based on magnetic susceptibility measurement of several cobalt oxides with the same hexagonal CoO2 layers [37–39]. For Bax Agy Ca2.8 Co4 O9 samples, the f was inversely proportional to r. As a whole, the ˛ was increased with increasing r subsequently. The ˛ for r = 3 sample exhibited moderately high value (176 ␮V/K at 973 K) and the ˛ for r = 0 sample exhibited lowest value (139 ␮V/K at 973 K) accordingly. 3.3. Thermal conductivity The relationships between thermal conductivity , lattice thermal conductivity L and temperature for all bulk samples are shown in Fig. 8. The carrier contribution kc to the  was calculated by applying the Wiedemann–Franz equation [40] method kc = LT/ with the Lorenz constant 2.45 × 10−8 V2 K−2 . The lattice thermal conductivity L was obtained by subtracting kc from k. The decrease of  corresponds conventionally to increase of  for Ca3 Co4 O9 semiconductor system and it was found that the  in high temperature region was decreased by increasing r as a whole. This means that changing  had brought the  undesirable for these compounds. Promisingly, the r = 1 sample exhibited lowest . The lowest  and L for r = 1 sample in measuring temperature region reached 1.41 and 1.10 W/mK, respectively. The reduction of  lies mainly in reduction of L for Ca3 Co4 O9 system and the L was observed to be decreased by 37.5%. For Bax Agy Ca2.8 Co4 O9 samples, the L could be approximately expressed as [41]: L =

CV v 3

(4)

where CV , , v are specific heat, spectral mean-free path and phonon transfer speed, respectively. Although further work was needed, it could be inferred that the resonance frequencies of Ba and Ag were different [40] and this intensified range of phonons that went through scattering for Ba0.1 Ag0.1 Ca2.8 Co4 O9 compound. The Ba atom with a high mass (137) acted as phonon scattering center and reduced the atomic vibration frequency. The Ag atom with a high mass (108) acted as phonon scattering center with its own accessional frequency which further lowered the L . Furthermore, the large number of atoms in the unit cell (5) lowered the vibration modes that carried heat efficiently [13]. Therefore, the phonon transport was further confined and thus the L was lowered.

Fig. 9. Temperature dependence of dimensionless figure of merit ZT for several Bax Agy Ca3−x−y Co4 O9 bulk samples.

3.4. Dimensionless figure of merit, ZT Fig. 9 shows the temperature dependence of dimensionless figure of merit ZT for the Ba and Ag substituted Bax Agy Ca3−x−y Co4 O9 (x = 0, 0.05, 0.1, 0.15, y = 0, 0.15, 0.1, 0.05) samples. It could be seen that the ZT values were increased with increasing temperature and the x = y = 0.1 sample exhibited enhancement of ZT reaching 0.29 at 973 K. The ZT value was much higher (65% improved) than that of the Ca3 Co4 O9 sample. 4. Conclusions In summary, polycrystalline Bax Agy Ca3−x−y Co4 O9 bulk samples were prepared via citrate acid sol–gel method followed by spark plasma sintering. The grain orientation and microstructure of Ca3 Co4 O9 were optimized. The electrical resistivity was decreased and the thermal conductivity was increased by increasing y/x ratio, the electrical resistivity and thermal conductivity of the Ca3 Co4 O9 based TE oxide material system Ba0.1 Ag0.1 Ca2.8 Co4 O9 were 7.2 m cm and 1.41 W/mK simultaneously with high Seebeck coefficient 172 ␮V/K at 973 K. Enhanced TE property (ZT = 0.29 at 973 K) was obtained. Furthermore, bulk material of Ba0.1 Ag0.1 Ca2.8 Co4 O9 compound was easy to fabricate by means of sol–gel and SPS. Acknowledgments This work was financially supported by National Natural Science Foundation of China under Grant No. 50702003 and Beijing Municipal Commission of Education Foundation under Grant No. JC009001200803. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Fig. 8. Temperature dependence of thermal conductivity  and lattice thermal conductivity L (inset) for Bax Agy Ca3−x−y Co4 O9 bulk samples.

[13] [14]

T.A. Tyson, Z. Chen, Q. Jie, Q. li, J.J. Tu, Phys. Rev. B 79 (2009) 024109. K. Fujita, T. Mochida, K. Nakamura, Jpn. J. Appl. Phys. 40 (2001) 4644. L. Zhang, G.H. Liu, Y.B. Li, J.R. Xu, X.B. Zhu, Y.P. Sun, Mater. Lett. 62 (2008) 1322. N. Li, Y. Jiang, G.H. Li, C. Wang, J.F. Shi, D.B. Yu, J. Alloys Compd. 467 (2009) 444. Y.H. Lin, J.L. Lan, Z.J. Shen, Y.H. Liu, C.W. Nan, J.F. Li, Appl. Phys. Lett. 94 (2009) 072107. J. Pei, G. Chen, D.Q. lu, P.S. Liu, N. Zhou, Solid State Commun. 146 (2008) 283. C.J. Liu, P.K. Nayak, Z.R. Lin, K.Y. Jeng, Thin Solid Films 516 (2008) 8564. M. Shikano, R. Funahashi, Appl. Phys. Lett. 82 (2003) 1581. G. Yang, Q. Ramasse, R.F. Klie, Phys. Rev. B 78 (2008) 153109. J. Pei, G. Chen, D.Q. Lu, P.S. Liu, N. Zhou, Solid State Commun. 146 (2008) 283. H.Q. Liu, X.B. Zhao, T.J. Zhu, Y. Song, F.P. Wang, Curr. Appl. Phys. 9 (2009) 409. Y. Wang, Y. Sui, J. Cheng, X.J. Wang, J.P. Miao, Z.G. Liu, Z.N. Qian, W.H. Su, J. Alloys Compd. 448 (2008) 1. G. Xu, R. Funahashi, M. Shikano, Q. Pu, B. Liu, Solid State Commun. 124 (2002) 73. J. Nan, J. Wu, Y. Deng, C.W. Nan, J. Eur. Ceram. Soc. 23 (2003) 859.

554

F.P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 550–554

[15] E. Guimeau, R. Funahashi, Appl. Phys. Lett. 85 (2004) 1490. [16] M. Prevel, S. Lemonnier, Y. Klein, S. Hébert, D. Chateigner, J. Appl. Phys. 98 (2005) 093706. [17] E. Guilmeau, R. Funahashi, Appl. Phys. Lett. 85 (2004) 1490. [18] J.D. Francis, Science 285 (1999) 703. [19] W. Koshibae, K. Tsutsui, S. Maekawa, Phys. Rev. B 62 (2000) 6869. [20] J. Merino, R.H. McKenzie, Phys. Rev. B 61 (2000) 7996. [21] P. Xiang, Y. Kinemuchi, H. Kaga, K. Watari, J. Alloys Compd. 454 (2008) 364. [22] M. Mikami, N. Ando, R. Funahashi, J. Solid State Chem. 178 (2005) 2186. [23] Y. Song, Q. Sun, L. Zhao, F. Wang, Z. Jiang, Mater. Chem. Phys. 113 (2009) 645. [24] F.P. Zhang, Q.M. Lu, J.X. Zhang, X. Zhang, J. Alloys Compd. 477 (2009) 543. [25] Q.M. Lu, J.X. Zhang, Q.Y. Zhang, 25th International Conference on Thermoelectrics, IEEE Piscateway, 2006, p. 66. [26] D. Wang, L. Chen, Q. Wang, J. Li, J. Alloys Compd. 376 (2004) 58. [27] D. Wang, L. Chen, Q. Yao, J. Li, Solid State Commun. 129 (2004) 615. [28] S. Li, R. Funahashi, I. Matsubara, H. Yamada, K. Ueno, S. Sodeoka, Ceram. Int. 27 (2001) 321.

[29] Y.F. Zhang, J.X. Zhang, J. Mater. Proc. Technol. 208 (2008) 70. [30] Q.Y. Zhang, Q.M. Lu, J.X. Zhang, Rare Met. Mater. Eng. 36 (2007) 413. [31] J.G. Noudem, M. Prevel, A. Veres, D. Chateigner, J. Galy, J. Electroceram. 22 (2009) 91. [32] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [33] M. Ohtaki, D. Ogura, K. Eguchi, H. Arai, J. Solid State Chem. 120 (1995) 105. [34] N.F. Mott, H. Jones, The Theory of The Properties of Metals and Alloys, Dover, New York, 1958, p. 305. [35] P.M. Chaikin, G. Beni, Phys. Rev. B 13 (1976) 647. [36] J. Hubbard, Proc. R. Soc. A 276 (1963) 238. [37] S. Hébert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev. B 64 (2001) 172101. [38] I. Tsukada, T. Yamamoto, M. Takagi, J. Phys. Soc. Jpn. 70 (2001) 834. [39] T. Mizokawa, L.H. Tjeng, P.G. Steeneken, Phys. Rev. B 64 (2001) 115104. [40] X. Shi, H. Hong, C.P. Li, C. Uher, J. Yang, J.R. Salvador, H. Wang, L. Chen, W. Zhang, Appl. Phys. Lett. 92 (2008) 182101. [41] D.M. Rowe, Thermoelectrics Handbook, CRC Press, Boca Raton, FL, 2006, pp. 14–18.