Synthesis of thermoelectric Ca3Co4O9 ceramics with high ZT values from a CoIICoIII-Layered Double Hydroxide precursor

Materials Research Bulletin 47 (2012) 3287–3291

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Synthesis of thermoelectric Ca3Co4O9 ceramics with high ZT values from a CoIICoIII-Layered Double Hydroxide precursor F. Delorme *, C. Fernandez Martin, P. Marudhachalam, G. Guzman, D. Ovono Ovono, O. Fraboulet CORNING SAS, CETC, 7 bis Avenue Valvins, 77210 Avon, France

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 April 2012 Received in revised form 8 June 2012 Accepted 28 July 2012 Available online 7 August 2012

In this study, a new synthesis route for the Ca3Co4O9 compound using a CoIICoIII-Layered Double Hydroxide compound as a precursor has been demonstrated. Polycrystalline samples have been prepared by solid state reaction and sintered by spark plasma sintering. XRD study shows that the substitution of Co3O4 as a precursor by the CoIICoIII-Layered Double Hydroxide leads to the formation of a single Ca3Co4O9 phase. The thermoelectric properties of the Ca3Co4O9 samples at high temperature have been studied (550–1100 K). The nature of the precursor does not significantly affect the Seebeck coefficient, the electrical conductivity and the power factor. However, the thermal conductivity is decreased when using the CoIICoIII-Layered Double Hydroxide as precursor rather than Co3O4, 1.6 W m 1 K 1 compared to 2.1 W m 1 K 1 at 1073 K respectively. This leads to a 20% ZT improvement at 1000 K, 0.18 compared to 0.15 respectively. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics A. Layered compounds A. Oxides D. Electrical properties D. Thermal conductivity

1. Introduction Thermoelectric materials can convert exhaust heat energy directly into electrical energy. Nevertheless, thermoelectric generation is not widely used at present due to its low energy conversion efficiency h. The efficiency h is a function of the thermoelectric figure of merit ZT = S2sT/k, where T is the absolute temperature, s is the electrical conductivity, k is the thermal conductivity and S is the Seebeck coefficient. As a result, the optimum material for thermoelectric generation should simultaneously exhibit large S, large s and small k. Classical thermoelectric materials such as tellurides cannot be used for vehicles applications due to their low temperature stability. These applications require high temperature (600–800 8C), oxygen resistance and ZT between 1 and 1.5. The suitable material does not actually exist and is the subject of extensive research. The discovery of a large thermopower in the metallic oxide NaxCoO2 has shown that oxides are potential candidates for thermoelectric applications [1]. Due to their good stability they can be used as thermoelements working in oxidizing conditions up to 800 8C. The structure of this material has been previously described when it was studied as potential electrode material for sodium batteries [2,3]: it consists of CoO2 layers, with the CdI2 structure, i.e. formed by edge-sharing CoO6 octahedra, stacked alternatively with partially deficient Na layers. Different

* Corresponding author. Tel.: +33 1 64 69 70 82; fax: +33 1 64 69 75 55. E-mail address: [email protected] (F. Delorme). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.07.037

thermodynamic stable phase domains have been identified depending on the alkali content, differing in the packing sequence of oxygen layers that leads to octahedral or prismatic oxygen environments for Na+ ions, and to a different number of CoO2 sheets (2 or 3) within the pseudohexagonal unit cell. However, other layered cobalt oxides presenting CdI2-type CoO2 layers have been shown to present excellent thermoelectric properties such as Bi2Sr2Co2Oy [4] or Ca3Co4O9 [5]. Ca3Co4O9 belongs to the family of misfit cobalt oxides [6–9]. Its crystal structure consists of CdI2-type CoO2 layers and triple rocksalt [Ca2CoO3] layers stacking alternately along the c-axis. These two kind of layers have similar a, c, and b lattice parameters but different b parameters. To emphasize the incommensurate nature of the structure, Ca3Co4O9 can be written as [Ca2CoO3](b2/b1)[CoO2], where b1 and b2 are two different lattice parameters for the rocksalt subsystem and the CoO2 subsystem, respectively. The edge sharing CoO2 octahedra layers are considered to be responsible for the electrical conduction, whereas the triple rocksalt layers can be regarded as a charge reservoir to supply charge carriers into the CoO2 layers. Classically, the synthesis of the Ca3Co4O9 compound is performed by standard solid state reaction of a mix of oxides and/or salts of each of the different constituting cations. Most of the authors have used CaCO3 and Co3O4 [4,8,10– 22] but a few have reported the utilization of CaCO3 and Co2O3 [6,23], CaO and Co3O4 [7,9] or CaCO3 and CoO [24]. More recently, a few authors have proposed to use sol–gel method [25–28] or coprecipitation of oxalates [29]. Finally, Tani et al. [30] synthesized the Ca3Co4O9 phase using an in situ topotactic conversion of aligned platelet particles of Co(OH)2.

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Layered Double Hydroxides (LDHs), also known as anionic clays or hydrotalcite-like compounds, are a large family of compounds composed of brucite-like layers (Mg(OH)2 layers of the CdI2-type) positively charged owing to the substitution of a part of the M2+ cations by M3+ cations [31]. In order to neutralize these positive charges, anions, associated with variable amounts of water, are inserted between the layers [32]. Their general formula can be written [M(II)1 xM(III)x(OH)2] [An ]x/n
mH2O where M(II) and M(III) are metal cations and An are anions. M(II) is usually a divalent cation that can be Mg2+, Mn2+, Zn2+, Co2+, Cd2+, Cu2+, Ni2+, etc. and M(III) is usually a trivalent cation that can be Al3+, Fe3+, Cr3+, Ga3+, etc. [31]. The anion An can be as simple as inorganic anions such as Cl , F , Br , OH , SO42 or CO32 , to complex as DNA [33,34]. Due to their layered structure and as the An anions can be exchanged, these compounds have found numerous applications as catalysts or catalyst precursors [35,36], as ion exchangers [33], as part of new heterostructured nanohybrids such as inorganic/ inorganic, organic/inorganic or bio/inorganic materials [34,37,38], or for anionic depollution of water [39–41]. The formation of CoIICoIII-LDHs requires to avoid the formation of the b-Co(OH)2 phase and several synthesis routes have been reported [42–48]. The aim of this paper is to propose a new synthesis route for the Ca3Co4O9 compound using a CoIICoIII-LDH compound as a precursor. 2. Materials and methods All the syntheses have been realized using deionised water. A first aqueous solution, containing 0.01 mole of cobalt nitrate (Co(NO3)2
6H2O, VWR, 99.7% purity) in 50 ml of water has been prepared. A second solution has been made introducing 280 ml of a 3.5 M NaOH solution (NaOH solid from Fischer (98%)) in 1000 ml of a 1 M Na2CO3 solution (Na2CO3, Merck, 99.5% purity). Syntheses of the LDHs have been performed using the varying pH method [49,50], introducing the NaOH/Na2CO3 solution drop by drop at a constant rate through a peristaltic pump. pH was monitored by a pH-meter (Mettler DL67 Titrator) and the experiment was stopped when the pH reached a value of 9. The resulting slurry was aged under vigorous stirring during 24 h at room temperature, and then centrifuged at 4000 rpm during 5 min (Eppendorf Centrifuge 5403). The supernatant was eliminated and the samples were washed three times with deionized water at room temperature. Then samples were dried in a furnace at 60 8C overnight (Binder). Ca3Co4O9 samples were synthesized from CaCO3 (Sigma Aldrich, >99% purity) and the previously synthesized LDH or Co3O4 (Sigma Aldrich) for comparison. Stoichiometric amounts of the precursors were thoroughly mixed 5 min at 400 rpm in an agate ball mill (Retsch PM 100). The resulting powder has been heated at 850 8C for 8 h in an alumina crucible at a rate of 5 8C/min and slowly cooled down. Sintering was performed by Spark Plasma Sintering (SPS, FCT Systeme GmbH HP D 25). The synthesized powders were placed in a 20 mm diameter graphite die. A pressure of 70 MPa was applied whereas the temperature was raised at 100 8C/min up to 850 8C for 5 min. Then the sample was cooled at 100 8C/min to room temperature. The obtained pellets were then polished to remove the graphite foils used during the SPS process and cut as bars for thermoelectric properties measurements or core drilled (12.7 mm diameter, 2 mm in thickness) for thermal conductivity measurements. Thermoelectric properties of the sintered samples were determined from simultaneous measurement of resistivity and Seebeck coefficient in a ZEM III equipment (ULVAC Technologies) and thermal conductivity. The thermal conductivity, k, was determined from thermal diffusivity, a, heat capacity, Cp, and

density r, using the following equation: k = r a Cp. The thermal diffusivity was measured using the laser flash diffusivity technique (Netzsch LFA 427) from room temperature to 800 8C in air atmosphere. The thermal diffusivity measurement of all specimens was carried out three times at each temperature. The heat capacity of the materials was measured from room temperature to 800 8C, with a heating rate of 10 8C min 1 in platinum crucibles and in air atmosphere, using differential scanning calorimeter (Netzsch DSC 404 C pegasus). Powder X-ray diffraction (XRD) patterns have been performed on a Philips X’PERT Pro u/2u diffractometer equipped with an X’CELERATOR real time multiple strip detector, using Cu Ka radiation and operating at 45 kV and 40 mA at room temperature. The scans have been recorded from 58 to 1408 (2u) with a step of 0.001678 and a counting time of 40 s per step. The scanning electron microscopy (SEM) observations have been performed using JEOL 7001F or LEO 1550 Field Emission Gun microscope. LDH samples have been sputter-coated with a thin layer of iridium prior observation. 3. Results and discussion Starting from the red color of the cobalt nitrate solution (divalent cobalt in octahedral environment), the suspension is bluish when pH reaches 9 (corresponding to the metastable aCo(OH)2 phase [42,48]). After the 24 h maturation at room temperature, the suspension is dark greenish, and after centrifugation and drying, the powder is darkish due to the presence of different cobalt valence states [47]. Fig. 1 shows the X-ray diffraction pattern of the darkish synthesized powder compared to a pink b-Co(OH)2 powder (Aldrich, 95% purity). The synthesized powder (Fig. 1a) presents very large peaks characteristic of poorly crystalline compounds around 128, 248 and 368 (2u) that are characteristic of the layered structure of CoIICoIII-LDH containing small anions, such as carbonates, nitrates or chlorides, as described by Liu et al. [45] or Hu et al. [48]. The XRD pattern of a b-Co(OH)2 reference powder (Fig. 1b) has been added to show that the synthesized powder, even if poorly crystallized, presents a very different structure. Moreover, it is well known that for LDH compounds crystallinity can be dramatically improved by aging for long durations at higher temperature than room temperature [31], as well as particles size. However, as the CoIICoIII-LDH is synthesized to become the precursor of a Ca3Co4O9 oxide, the smaller the particles, the higher the reactivity. The SEM observations show that the LDH precursor is composed by aggregated small plate-like particles of less than 100 nm (Fig. 2)

4000

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2 Theta (degrees) Fig. 1. PXRD patterns of (a) the synthesized LDH and (b) a b-Co(OH)2 reference.

F. Delorme et al. / Materials Research Bulletin 47 (2012) 3287–3291

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Conductivity (S/m)

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Temperature (K) Fig. 5. Temperature dependence of the electrical conductivity of Ca3Co4O9 samples synthesized from Co3O4 (^) and CoIICoIII-LDH ( ).

Fig. 2. SEM micrograph (secondary electrons detector) of the CoIICoIII-LDH.

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2 Theta (degrees) Fig. 3. PXRD patterns of Ca3Co4O9 samples synthesized from different cobalt sources (a) the synthesized CoIICoIII-LDH and (b) Co3O4.

The synthesis process of the Ca3Co4O9 oxide from the conventional addition of CaCO3 and Co3O4 precursors leads to the formation of a single Ca3Co4O9 phase. Fig. 3 shows XRD patterns of Ca3Co4O9 oxide obtained from the synthesized CoIICoIIILDH (Fig. 3a) and from the conventional Co3O4 precursor (Fig. 3b) using the same synthesis process. These two patterns are similar, demonstrating that it is possible to synthesize the Ca3Co4O9 compound from a CoIICoIII-LDH precursor.

All the sintered samples present a high bulk density with an apparent density value larger than 95% of the theoretical density [7]. The sintering process used in this study is known to lead to ceramics with randomly oriented Ca3Co4O9 particles [22], even when using nanosized precursors [51]. Fig. 4 exhibits the temperature dependence of the Seebeck coefficient (S) of the samples synthesized from Co3O4 and CoIICoIIILDH precursor. The Seebeck coefficient of both samples shows a positive value over the measured temperature range, indicating a p-type conduction. The behavior of the two samples is similar: the Seebeck coefficient slightly increases when temperature increases. The Seebeck coefficient value of the two samples is in the same range but slightly higher for the sample synthesized from the CoIICoIII-LDH precursor, 170 compared to 166 mV K 1 at 650 K and 188 compared to 184 mV K 1 at 1000 K. The temperature dependence of the electrical conductivity (s) of the samples synthesized from Co3O4 and CoIICoIII-LDH precursor from 650 to 1000 K is shown in Fig. 5. The two samples exhibit the same behavior: the electrical conductivity increases with increasing temperature, which is characteristic of a semiconducting-like behavior (dr/dT  0). The electrical conductivity value of the two samples is in the same range but slightly lower for the sample synthesized from the CoIICoIII-LDH precursor, 6760 compared to 7300 S m 1 at 650 K and 9230 compared to 8680 S m 1 at 1000 K. This is consistent with the higher Seebeck coefficient of the sample synthesized from the CoIICoIII-LDH precursor, and thus the two samples present similar powerfactor (=S2s) as shown in Fig. 6. In both cases, the powerfactor increases when temperature increases, up to 3.1  10 4 W m 1 K 2 at 1000 K. 3.5 3

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Temperature (K) Fig. 4. Temperature dependence of the Seebeck coefficient of Ca3Co4O9 samples synthesized from Co3O4 (^) and CoIICoIII-LDH ( ).

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Temperature (K) Fig. 6. Temperature dependence of the power factor of Ca3Co4O9 samples synthesized from Co3O4 (^) and CoIICoIII-LDH ( ).

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Thermal Conductivity (W/m.K)

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Temperature (K) Fig. 7. Temperature dependence of the thermal conductivity of Ca3Co4O9 samples synthesized from Co3O4 (^) and CoIICoIII-LDH ( ).

Fig. 7 exhibits the temperature dependence of the thermal conductivity (k) of the samples synthesized from Co3O4 and CoIICoIII-LDH precursor. The thermal conductivity of the sample synthesized from Co3O4 is stable (2.1 W m 1 K 1) in the temperature range 500–1100 K, whereas the thermal conductivity of the sample synthesized from the CoIICoIII-LDH precursor is decreasing from to at 1100 K, from 2.1 W m 1 K 1 at 573 K to 1.6 W m 1 K 1 at 1073 K. Finally, Fig. 8 shows the temperature dependence of the dimensionless figure of merit ZT of the samples synthesized from Co3O4 and CoIICoIII-LDH precursor. It shows that the ZT value of the sample synthesized from the CoIICoIII-LDH precursor and the ZT value of the sample synthesized from Co3O4 are similar at 650 K (ZT = 0.064) but that at higher temperatures, the ZT value of the sample synthesized from the CoIICoIII-LDH precursor is higher than the ZT value of the sample synthesized from Co3O4, 0.18 compared to 0.15 at 1000 K respectively. Moreover, when temperature increases, the difference between ZT values also increases. As power factors of both samples are similar, the ZT improvement is only related to the thermal conductivity decrease. The origin of this improvement could be in the Ca3Co4O9 particle size. Indeed, theoretical and applied studies seems to indicate that reducing the particle size of thermoelectric compounds would not affect significantly the power factor but will lead to a decrease thermal conductivity due to increased phonon scattering. In 1993, Hicks and Dresselhaus [52,53] have proposed a model that shows a ZT increase of Bi2Te3 when reducing dimensionality from 3D to 2 or 1D nanostructures. More recently, theoretical work on 0.20

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Fig. 9. SEM micrographs (in lens detector) of the Ca3Co4O9 particles synthesized from (a) the CoIICoIII-LDH precursor and (b) Co3O4.

boundary scattering of phonons in amorphous materials indicates that micron and submicron grains could be very beneficial in order to lower the lattice thermal conductivity and yet not deteriorate the electron mobility [54]. Moreover, Zheng et al. [55] have developed another model showing that the bulk thermal conductivity is significantly reduced by decreasing the grain sizes when the grain diameter is less than 500 nm. Experimentally, Poudel et al. [56] have shown that using nanoparticles allows increasing ZT of a Bi2Te3 compound from 1 to 1.4. SEM results seem to indicate that the Ca3Co4O9 particle size is slightly smaller (<1 mm) for the sample synthesized from the CoIICoIII-LDH precursor (Fig. 9a) than the 1–2 mm measured for to the sample synthesized from Co3O4 (Fig. 9b). This is consistent with the recent results published by Delorme et al. [51] showing that Ca3Co4O9 samples synthesized from nanoprecursors exhibit smaller particle size. Moreover, these samples synthesized from nanoprecursors present ZT values close to 0.2 at 1000 K due to lower thermal conductivities as for the samples synthesized from the CoIICoIIILDH precursor. This is consistent with the nanosize of LDHs [31,57] that can be considered as a cobalt nanoprecursor for the synthesis of the Ca3Co4O9 compound.

Co oxide

4. Conclusion

LDH

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Temperature (K) Fig. 8. Temperature dependence of the dimensionless figure of merit of Ca3Co4O9 samples synthesized from Co3O4 (^) and CoIICoIII-LDH ( ).

In this study, a new synthesis route for the Ca3Co4O9 compound using a CoIICoIII-Layered Double Hydroxide compound as a precursor has been demonstrated. Using the varying pH method, a poorly crystalline darkish CoIICoIII-Layered Double

F. Delorme et al. / Materials Research Bulletin 47 (2012) 3287–3291

Hydroxide has been synthesized by stopping the coprecipitation when pH reaches a value of 9. Polycrystalline Ca3Co4O9 samples have been prepared by solid state reaction from Co3O4 and CoIICoIII-Layered Double Hydroxide precursors, sintered by SPS and their thermoelectric properties at high temperature (550– 1100 K) have been studied. XRD patterns have demonstrated that using the same thermal treatments leads for both precursors in the formation of a single Ca3Co4O9 phase. The thermoelectric properties measurements have shown that the nature of the precursor (Co3O4 or CoIICoIII-LDH) does not significantly affect the Seebeck coefficient, the electrical conductivity and the power factor. However, the thermal conductivity is decreased when using the CoIICoIII-Layered Double Hydroxide as precursor rather than CO3O4, 1.6 W m 1 K 1 compared to 2.1 W m 1 K 1 at 1073 K respectively. This leads to a 20% ZT improvement at 1000 K, 0.18 compared to 0.15 respectively. This has to be related to a slightly smaller particle size for the sample synthesized from the CoIICoIII-LDH precursor compared to the sample synthesized from Co3O4 as recently reported for Ca3Co4O9 samples synthesized from nanoprecursors [51]. Acknowledgments The authors would like to thank P. Pradeau, R. Olivon, T. Montigny, B. Deth and G. Cadot for the preparation of the samples. References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685–R12687. [2] C. Fouassier, G. Matejka, J.-M. Reau, P. Hagenmuller, J. Solid State Chem. 6 (1973) 532–537. [3] C. Fouassier, C. Delmas, P. Hagenmuller, Mater. Res. Bull. 10 (1975) 443–450. [4] R. Funahashi, M. Shikano, Appl. Phys. Lett. 81 (2002) 1459–1461. [5] M. Shikano, Funahashi, Appl. Phys. Lett. 82 (2003) 1851–1853. [6] S. Li, R. Funahashi, I. Matsubara, K. Ueno, H. Yamada, J. Mater. Chem. 9 (1999) 1659–1660. [7] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166–175. [8] A.Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Jpn. J. Appl. Phys. 39 (2000) L531–L533. [9] S. Lambert, H. Leligny, D. Grebille, J. Solid State Chem. 160 (2001) 322–331. [10] G. Xu, R. Funahashi, M. Shikano, I. Matsubara, Y. Zhou, Appl. Phys. Lett. 80 (2002) 3760–3762. [11] S. Horii, I. Matsubara, M. Sano, K. Fujie, M. Suzuki, R. Funahashi, M. Shikano, W. Shin, N. Murayama, J.-I. Shimoyama, K. Kishio, Jpn. J. Appl. Phys. 42 (2003) 7018–7022. [12] Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W.S. Seo, K. Koumoto, J. Mater. Chem. 13 (2003) 1094–1099. [13] M. Mikami, S. Ohtsuka, M. Yoshimura, Y. Mori, T. Sasaki, R. Funahashi, M. Shikano, Jpn. J. Appl. Phys. 42 (2003) 3549–3551. [14] Y. Zhou, I. Matsubara, S. Horii, T. Takeuchi, R. Funahashi, M. Shikano, J.-I. Shimoyama, K. Kishio, W. Shin, N. Izu, N. Muroyama, J. Appl. Phys. 93 (2003) 2653–2658.

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