Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH)₂ platelets as reactive templates

Abstract

Textured bulk ceramics have been prepared of various cobaltites including the common CdI₂-type CoO₂ layer (i.e., [Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624[CoO₂]: CCO-4-layer, [Bi₂M_2−xO₄]_p[CoO₂] (M₂ = Sr₂: BSCO, M₂ = Ca₁Sr₁: B(SC)CO, M₂ = Ca₂: BCCO)) by using β-Co(OH)₂ (CdI₂-type) platelets as reactive templates, and their thermoelectric (TE) properties have been examined. All the specimens were found to be highly c-axis aligned ceramics. Especially for CCO-4-layer, B(SC)CO and BCCO, the present study is the first report on the fabrication of textured ceramics. Our textured ceramic specimens exhibited larger values of electrical conductivity (σ) than the ceramics prepared by a conventional solid-state reaction (SSR specimens): e.g., σ_ab = 0.46 × 10⁴ S m⁻¹ for the textured specimen and σ^SSR = ∼0.26 × 10⁴ S m⁻¹ at 1060 K for the SSR specimen for BCCO. This study indicated that our preparation method using β-Co(OH)₂ templates is a versatile and effective technique to fabricate cobaltites, including CoO₂ layers, with a high degree of orientation and improved TE properties.

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Introduction

Thermoelectric (TE) power generation for energy recovery from waste heat is expected to be an environmentally benign technology in terms of saving resources and reducing CO₂ emission in to the atmosphere. The TE conversion efficiency is characterized by the figure of merit, Z ≡ σS²/κ, where σ, S and κ are electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. Thus, it is essential to develop TE materials exhibiting high σ, high S, and low κ simultaneously in order to realize highly efficient TE devices.

Recently, layered cobaltites with a CoO₂ layer and a block layer stacked alternately were found to show high p-type TE performance: e.g., Na_x[CoO₂]¹ (denoted as NCO in this paper), [Ca₂CoO₃]_0.62[CoO₂]^2–4 (denoted as CCO-3-layer [Ca₂CoO₃] is a triplicate rock salt-type (RS) layer), [Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624[CoO₂]⁵ (denoted as CCO-4-layer, [Ca₂(Co_0.65Cu_0.35)₂O₄] is a quadruplicate RS layer), and [Bi₂M_2−xO₄]_p[CoO₂] (M = Sr,^6,7 Ca,⁸ or Ba;⁹ denoted as BSCO and BCCO for M = Sr and Ca, respectively; [B₂M_2−xO₄] is a quadruplicate RS layer). Since these cobaltites are chemically stable and do not include hazardous elements such as Te, Se and Pb, the cobaltites are potential materials as TE devices to be used in air atmosphere at high temperatures.

TE properties of single crystalline particles of NCO¹ and CCO-3-layer³ were reported to be highly anisotropic; i.e., the value of σ along the CoO₂ layer is higher than that of the direction perpendicular to the CoO₂ layer. Thus, the fabrication of textured bulk ceramics with a large value of σ would be the key technology for the application of these layered cobaltites for actual TE devices. Koumoto's group^10,11 used Bi-doped CCO-3-layer single crystals synthesized by a polymerized complex method, and fabricated the textured CCO-3-layer ceramics with improved TE properties compared to a randomly oriented CCO-3-layer ceramic. Shikano and Funahashi¹² prepared a textured CCO-3-layer ceramic by using flux-grown large CCO-3-layer single crystals as ingredients. However, a polymerized complex method and a flux method would be difficult to apply to industrial scale processes, because of the compositional restrictions and high production costs.

Alternatively, we have proposed the reactive templated grain growth (RTGG) method^13–15 as a more convenient and effective fabrication method. Our previous studies used precipitation-prepared Co₃O₄ or Co(OH)₂ platelets^16,17 as reactive templates and indicated that the proposed method gave highly textured NCO¹⁸ and CCO-3-layer^19,20 ceramics with high TE properties. We deduced that the formation mechanism of a textured CCO-3-layer ceramic would be based on the topotactic conversion: {001}Co(OH)₂ → {111}Co₃O₄ → {001}CCO-3-layer. This is considered to be the appropriate mechanism in terms of their crystal structures; i.e., Co(OH)₂, Co₃O₄ and CCO-3-layer have CoO₂ layers composed of edge-sharing CoO₆ octahedra along the {001}, {111} and {001} planes, respectively.

In the case of the misfit-layered cobaltite with a quadruplicate RS layer, the fabrication of textured ceramics is not currently well established. However, cobaltites with a quadruplicate RS layer would also exhibit a strong anisotropy in TE properties, because of their similarity to NCO and CCO-3-layer in the crystal structure. Thus, we expect that our proposed method would be a more versatile process than the others for textured TE ceramics with various compositions. In this study, we extended the RTGG method using Co(OH)₂ platelets as reactive templates in the preparation of the textured ceramics of CCO-4-layer, BSCO and BCCO. In addition, we have fabricated a textured BSCO ceramic in which half of the Sr sites in the RS layer are substituted by Ca (i.e., [Bi₂(Sr₁Ca₁)_2−xO₄]_p[CoO₂]: denoted as B(SC)CO). This paper examines the microstructures and TE properties of the RTGG-prepared ceramics, and discusses the applicability of the RTGG method for the preparation of the textured cobaltite ceramics.

Experimental

Preparation and characterization of textured ceramics

The reactive templates used in this study were β-Co(OH)₂ platelets with developed {00l} planes (0.5 µm in diameter and an aspect ratio of 5),²⁰ for which the synthesis method was reported in detail elsewhere.¹⁶

Textured ceramics were prepared by the RTGG method by a similar procedure to that described in ref. 20. The high-purity powders of CuO (99.9%, Koujundo Chemical Lab. Co., Ltd.), SrCO₃ (99.9%, Koujundo Chemical Lab. Co., Ltd.), Bi₂O₃ (99.99%, Koujundo Chemical Lab. Co., Ltd.) and CaCO₃ (99.99%, Ube Material Industries, Ltd.) were used without further purification as complementary reactants. Reactive templates (β-Co(OH)₂) and these reactant particles were mixed in a given ratio (see Table 1) in a ball mill for 72 ks in an ethanol–toluene solution. Then, polyvinyl butyral (binder) and di-n-butyl phthalate (plasticizer) were added to the mixture and mixed in a ball mill for another 3.6 ks. The mixed slurry was tape-cast by a doctor-blade technique, and the obtained sheets were dried in air at room temperature. The sheets (50 mm width, 80 mm length and ∼100 µm thickness) were stacked and pressed at 10 MPa at 353 K into a monolithic plate (∼3 mm thickness). Then, the organic substances in the plate were burnt out at 873 K in air for 7.2 ks. Finally, the compact was sintered at 1123–1193 K for 7.2 or 72 ks with uniaxial pressing (UPS, 1.96–19.6 MPa) in O₂ atmosphere (see Table 1). In this study, the sintered specimens are denoted as RTGG-UPS specimens.

Table 1 Preparation conditions of textured ceramics

RTGG-UPS specimen^a	Molar mixing ratio of starting materials	Temperature^b /K	Pressure^b /MPa	Time^b /ks
a RTGG: reactive templated grain growth method, UPS: sinterd with uniaxial pressing.b Sintered in O2 atmosphere.
CCO-4-layer	Co(OH)2∶CaCO3∶CuO = 4.35∶3.00∶1.05	1193	19.6	72
BSCO	Co(OH)2∶Bi2O3∶SrCO3 = 2.00∶1.00∶2.00	1123	9.8	7.2
B(SC)CO	Co(OH)2∶Bi2O3∶SrCO3∶CaCO3 = 2.00∶1.00∶1.00∶1.00	1153	9.8	7.2
BCCO	Co(OH)2∶Bi2O3∶CaCO3 = 2.00∶1.00∶2.00	1173	1.96	7.2

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X-Ray diffraction (XRD, Model RINT2200, Rigaku Co., Cu-Kα radiation) was conducted to determine the crystalline phases of prepared RTGG-UPS specimens. In order to determine the degree of orientation of the specimens, Lotgering's factor (f)²¹ was calculated using the XRD patterns, where f = (P − P₀)/(1 − P₀), where P = ΣI(001)/ΣI(hkl) and P₀ is P for a randomly oriented powder specimen. The value of P₀ for CCO-4-layer was calculated by using the reported XRD data,⁵ and those for BSCO, B(SC)CO and BCCO specimens were derived by the diffraction peak intensities measured for the crushed powder of RTGG-UPS specimens. The microstructures of the specimens were observed by scanning electron microscopy (SEM, Model Sigma-V, Akashi Ltd.).

Measurements of transport properties

A rectangular shape specimen, of which the longitudinal direction was parallel to the ab plane, was machined out from a textured ceramic. Electrical conductivity (σ_ab, ab: in-plane) was measured using a four-probe method with a dc current of 2 mA. The Seebeck coefficient (S_ab) was determined as the slope of the relationship between ΔV and ΔT, where ΔV is the thermoelectric voltage generated between both ends of a specimen with the temperature difference ΔT of 1–5 K. These measurements were carried out in the temperature range between 573 and 1073 K. Thermal conductivity (κ_ab) was calculated from the relationship κ_ab = DαC_p, where D, α and C_p are the density, thermal diffusivity and the heat capacity, respectively. Here, the thermal diffusivity was estimated by the ac calorimetric method associated with the optical pulse heating.²² The figure of merit (Z_ab) was calculated from Z_ab ≡ σ_abS_ab²/κ_ab.

Results

Microstructures

Table 2 summarizes the value of f, density and open porosity for the RTGG-UPS specimens. The specimens had an open porosity of 2.3–6.9% and f of 0.673–0.954. For BSCO, B(SC)CO and BCCO (denoted as Bi-layered-cobaltites) specimens, the values of f is underestimated because the crushed powders have platelike shape so that their XRD patterns are not those of randomly oriented samples but rather slightly oriented in the {00l} plane.

Table 2 Characteristics of the textured ceramic specimens

RTGG-UPS specimen^a	Orientation degree, f	Density/kg m^−3	Open porosity (%)
a RTGG: reactive templated grain growth method, UPS: sintered with uniaxial pressing.
CCO-4-layer	0.954	4720	3.8
BSCO	0.673	6190	4.1
B(SC)CO	0.929	6480	2.3
BCCO	0.831	5690	6.9

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Fig. 1 shows the XRD pattern measured on the surface parallel to the casting plane of the CCO-4-layer (RTGG-UPS) specimen. The specimen showed strong diffraction peaks from the {00l} planes (f = 0.954). The powder XRD measurement using the crushed powder of the ceramic revealed that the specimen was nearly a single phase of CCO-4-layer with traces of Co₃O₄ and CCO-3-layer (data not shown here). Fig. 2 shows an SEM photograph of the fracture surface perpendicular to the casting plane of the CCO-4-layer (RTGG-UPS) specimen. The ceramic was composed of platelike grains of ∼6 µm in diameter and an aspect ratio of ∼8, aligned along the casting plane.

Fig. 1 XRD pattern measured on a surface parallel to the casting plane of a CCO-4-layer (RTGG-UPS) specimen: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 2 SEM photograph of a fracture surface perpendicular to the casting plane of a CCO-4-layer (RTGG-UPS) specimen: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 3(a), (b) and (c) show the XRD patterns measured on the surfaces parallel to the casting plane of the BSCO, B(SC)CO and BCCO (RTGG-UPS) specimens, respectively. All the specimens showed preferred {00l} orientation. The values of f were 0.673, 0.929 and 0.831 for BSCO, B(SC)CO and BCCO (RTGG-UPS) specimens, respectively (see Fig. 3 and Table 2). Fig. 4(a), (b) and (c) show the XRD patterns measured for the crushed powers of the BSCO, B(SC)CO and BCCO (RTGG-UPS) specimens, respectively. No impurity phases were observed in all the three specimens, which could account for the formation of polycrystals with ordered compositions. In addition, the results that the d spacing of {00l} plane and the volume of RS layer shrank continuously with increasing amount of Ca would be consistent with the smaller ionic radius of Ca than that of Sr.

Fig. 3 XRD patterns measured on a surface parallel to the casting plane of RTGG-UPS specimens of (a) BSCO, (b) B(SC)CO and (c) BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 4 XRD patterns measured for the crushed powders of RTGG-UPS ceramic specimens of (a) BSCO, (b) B(SC)CO and (c) BCCO; RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 5(a), (b) and (c) show SEM photographs of the fracture surfaces perpendicular to the casting plane of the BSCO, B(SC)CO and BCCO (RTGG-UPS) specimens, respectively. It was found that the grains of BSCO (RTGG-UPS) specimen were not well aligned along the casting plane (see Fig. 5(a)), while the grains of B(SC)CO and BCCO (RTGG-UPS) specimens were almost perfectly aligned (see Fig. 5(b) and (c)). In addition, the BSCO (RTGG-UPS) specimen was composed of small grains of ∼3 µm diameter and an aspect ratio of ∼3. On the other hand, B(SC)CO and BCCO (RTGG-UPS) specimens were composed of grains with larger size and stronger anisotropy: i.e., the grains were ∼6 µm in diameter and an aspect ratio of ∼8 for B(SC)CO and ∼4 µm in diameter and an aspect ratio of ∼5 for BCCO.

Fig. 5 SEM photographs of the fracture surfaces perpendicular to the casting plane of RTGG-UPS specimens of (a) BSCO, (b) B(SC)CO and (c) BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Transport properties

Fig. 6 shows the temperature dependences of σ_ab and S_ab measured for the CCO-4-layer (RTGG-UPS) specimen. The value of σ for the RTGG-UPS specimen is higher than that of the ceramic prepared by the conventional solid-state reaction (SSR specimen); σ_ab = 1.48 × 10⁴ S m⁻¹ at 550 K for the RTGG-UPS specimen, while σ^SSR = ∼0.62 × 10⁴ S m⁻¹ at 300 K for the SSR specimen.⁵ On the contrary, the value of S_ab for the RTGG-UPS specimen is comparable to that of the SSR specimen; S_ab = 136 µV K⁻¹ at 550 K for the RTGG-UPS specimen, while S^SSR = ∼150 µV K⁻¹ at 300 K for the SSR specimen.⁵

Fig. 6 Temperature dependences of in-plane electrical conductivity (σ_ab) and Seebeck coefficient (S_ab) of CCO-4-layer (RTGG-UPS) specimen: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Figs. 7 and 8 show the temperature dependences of σ_ab and S_ab for the BSCO, B(SC)CO and BCCO (RTGG-UPS) specimens. The experimental error for both measurements were less than 10%. The slope of σ_ab for the three specimens seems to be nealy constant at temperatures between 550 and 1060 K. On the other hand, the magnitude of S_ab increases with increasing Ca content in RS layers (i.e., S_ab, _BCCO > S_ab, _B(SC)CO > S_ab, _BSCO). This indicates that the carrier concentration in the CoO₂ plane decreases with increasing Ca content.

Fig. 7 Temperature dependence of in-plane electrical conductivity (σ_ab) of RTGG-UPS specimens of (a) BSCO, (b) B(SC)CO and (c) BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 8 Temperature dependence of in-plane Seebeck coefficient (S_ab) of RTGG-UPS specimens of (a) BSCO, (b) B(SC)CO, and (c) BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

In order to elucidate the effect of the alignment of the specimens, Fig. 9 shows a comparison of σ(T) and S(T) curves between RTGG-UPS and SSR specimens of BCCO. Here, SSR specimen were prepared by a conventional SSR using reagent grade Co₃O₄, Bi₂O₃ and CaCO₃ as starting mterials. The values of f, density and open porosity for the SSR specimen were 0.604, 6180 kg m⁻³ and 5.1%, respectively. The value of σ_ab for the RTGG-UPS specimen (0.46 × 10⁴ S m⁻¹ at 1060 K) is about 1.8 times that of SSR specimen (0.26 × 10⁴ S m⁻¹ at 1060 K). However, the values of S_ab was found to be almost independent of the synthesis techniques. On the other hand, both σ_ab and S_ab for the present BSCO specimen, which was not well aligned, even using the RTGG method, are comparable with those of the polycrystalline specimen prepared by a conventional SSR.²³

Fig. 9 Comparison of temperature dependences of electrical conductivity (σ) and Seebeck coefficient (S) of RTGG-UPS and SSR specimens of BCCO: RTGG, UPS and SSR represent ‘the reactive templated grain growth method’, ‘sintered with uniaxial pressure’ and ‘conventional solid state reaction’, respectively.

Fig. 10 shows the temperature dependences of κ_ab for CCO-3-layer,²⁰ CCO-4-layer, and BCCO (RTGG-UPS) specimens. The value of κ_ab of the CCO-4-layer specimen was close to that of the CCO-3-layer specimen, while that of BCCO was about half of those of these two specimens.

Fig. 10 Temperature dependence of in-plane thermal conductivity (κ_ab) of RTGG-UPS specimens of CCO-3-layer, CCO-4-layer, and BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Fig. 11 shows the temperature dependence of Z_ab for CCO-3-layer,²⁰ CCO-4-layer, and BCCO (RTGG-UPS) specimens. All the specimens exhibited similar value of Z_ab, and the values of Z_ab reached 1.14 × 10⁻⁴ and 1.22 × 10⁻⁴ K⁻¹ at 1060 K for the CCO-4-layer and BCCO specimens, respectively. Here, for the calculation of Z_ab, the values of κ_ab was estimated by linear extrapolation of the measured values as shown in Fig. 10.

Fig. 11 Temperature dependence of in-plane figure of merit (Z_ab) of RTGG-UPS specimens of CCO-3-layer, CCO-4-layer, and BCCO: RTGG and UPS represent ‘the reactive templated grain growth method’ and ‘sintered with uniaxial pressure’, respectively.

Discussion

Formation of textured cobaltite ceramics

The cobaltites with quadruplicate RS layers were prepared with the maintained orientation of β-Co(OH)₂ platelets as shown in Figs. 1, 2, 3 and 5. In the previous studies,^19,20 we deduced the formation mechanism of a textured CCO-3-layer ceramic as follows. First, β-Co(OH)₂ decomposes into Co₃O₄, which has a layer composed of edge-sharing CoO₆ octahedra (denoted as layer A) and a layer composed of CoO₆ octahedra and CoO₄ tetrahedra (denoted as layer B). In the next transformation, the layer A is maintained to form the CoO₂ layer of CCO-3-layer, while the layer B reacts with complementary elements, Ca and O, to form the RS layer of CCO-3-layer.

In the case of CCO-4-layer, its formation mechanism might be similar to that of CCO-3-layer because of the similarity in their crystal structures: i.e., the layer A maintains to form the CoO₂ layer and the layer B reacts with Ca, Cu and O to form the RS layer. The lower value of f for CCO-4-layer (0.954) than that of CCO-3-layer (>0.99²⁰) may be attributed to the larger distance between CoO₂ layers due to the larger size of RS layers of CCO-4-layer (1.267 nm) than that of CCO-3-layer (1.084 nm) and the distance between the A layers of Co₃O₄ (0.8084 nm).

In contrast, the formation mechanisms of Bi-layered-cobaltites could be different from those of CCO-3-layer and CCO-4-layer since there would be practically no Co in the RS layer in their system. A significant atomic reconstruction must accompany the formation reaction of Bi-layered-cobaltites even if the layer A and/or B of Co₃O₄ serve to form a CoO₂ layer with the maintained orientation. In addition, the ingredient Bi₂O₃ (mp ∼1123 K), provides a liquid-phase which might cause rearrangement of grains and disturb the texture.²⁴ The assumptions mentioned above could account for the lower values of f (0.673–0.929: see Fig. 3) of Bi-layered-cobaltites than those of CCO-3-layer (>0.99²⁰) and CCO-4-layer (0.954). On the other hand, the reasons for the large differences in f among Bi-layered-cobaltites (i.e., B(SC)CO (0.929) > BCCO (0.831) > BSCO (0.673) as shown in Fig. 3) are currently unkown.

The RTGG method using β-Co(OH)₂ was found to give textured ceramics of various layered cobaltites with CoO₂ layers. In future studies, it is necessary to clarify the conversion mechanisms for preserved orientation.

Transport properties

The values of σ of CCO-4-layer (RTGG-UPS) and BCCO (RTGG-UPS) were higher than that of SSR specimens (see refs. 5 and 8 and Fig. 9). These results are consistent with the previous study for CCO-3-layer, and probably due to the reduction in the electron scattering at grain boundaries. On the other hand, the RTGG-UPS specimens of CCO-4-layer, BSCO and BCCO exhibited similar S for SSR specimens (see refs. 5, 8 and 23 and Fig. 9) These results are consistent with the report that the dependence of S on the degree of orientation is small.^10,19,20

The values of S for the Bi-layered-cobaltites were S_ab, _BCCO > S_ab, _B(SC)CO > S_ab, _BSCO (Fig. 8). The possible interpretation is that the volume shrinkage in the RS layers causes the emission of excess oxygen and a decrease in valence of Co ions in the CoO₂ layer thereby resulting in a decrease of the carrier density. The other possibility is that Co would enter in the RS layers in the case of B(SC)CO and BCCO because the ionic radius of Co ion (Co³⁺: 0.065 nm²⁵) is closer to Ca²⁺ (0.0100 nm²⁵) and/or Bi³⁺ (0.0103 nm²⁵) than Sr²⁺ (0.0118 nm²⁵). Such replacement in the position of Co ions would induce the change in carrier deisity of the CoO₂ plane. In order to further understand the change in TE properties, it is necessary to examine the valence of Co ions by Hall mobility measurements.

Finally, BCCO has a lower value of κ_ab than CCO-4-layer and CCO-3-layer (Fig. 11). It is expected that the heavy Bi element and weak Bi–O bonds would suppress the lattice vibration.

The present study indicates that the RTGG method using β-Co(OH)₂ platelets is a versatile and effective method to prepare textured ceramics of various cobaltites with improved TE properties. It is important to clarify the conversion mechanisms through in-situ crystallographic analyses and optimal fabrication conditions found for the fabrication of ceramics with improved texture.

Conclusion

We have fabricated the textured cobaltites composed of a CoO₂ layer and quadruplicate rock salt-type layer (i.e., [Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624[CoO₂]: CCO-4-layer, [Bi₂M_2−xO₄]_p[CoO₂] (M₂ = Sr₂: BSCO, M₂ = Ca₁Sr₁: B(SC)CO, M₂ = Ca₂: BCCO) by using β-Co(OH)₂ platelets as templates. All the ceramic specimens (RTGG-UPS specimens) were c-axis aligned ceramics of nearly single-phase and the values of f were 0.954, 0.673, 0.929 and 0.831 for CCO-4-layer, BSCO, B(SC)CO and BCCO, respectively. It was shown that textured ceramics with high value of f would result in higher values of σ than the ceramics prepared by the conventional solid-state reaction (SSR specimens). On the other hand, the values of S of RTGG-UPS specimens were comparable to those of SSR specimens, which reflects the small anisotropy in S. The values of Z_ab reached 1.14 × 10⁻⁴ and 1.22 × 10⁻⁴ K⁻¹ at 1060 K for the CCO-4-layer and BCCO specimens, respectively. In conclusion, our preparation method using β-Co(OH)₂ platelets is a versatile and effective method to prepare textured ceramics of various cobaltites with improved TE properties.

Acknowledgements

We would like to acknowledge Drs R. Asahi and Y. Seno of Toyota CRDL for their helpful discussion. H. Itahara would like to acknowledge Prof. K. Koumoto of Nagoya Univ. and Dr A. Maignan of Caen Univ. for discussions on the layered cobaltites. This work was partly carried out by joint research and development with the International Center for Environmental Technology Transfer in 2002–2003, commissioned by the Ministry of Economy, Trade and Industry.

References

1. I. Terasaki, Y. Sasago and K. Uchinokura, Phys. Rev. B., 1997, 56, R12685.
2. S. Li, R. Funahashi, I. Matsubara, K. Ueno and H. Yamada, J. Mater. Chem., 1999, 9, 1659.
3. A. C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer and B. Raveau, Phys. Rev. B, 2000, 62, 166.
4. Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii and T. Kajitani, J. Phys. Soc. Jpn., 2002, 71, 491.
5. Y. Miyazaki, T. Miura, Y. Ono and T. Kajitani, Jpn. J. Appl. Phys., 2002, 41, L849.
6. R. Funahashi, I. Matsubara and S. Sodeoka, Appl. Phys. Lett., 2000, 76, 2385.
7. H. Leligny, D. Grebille, O. Pérez, A. C. Masset, M. Hervieu and B. Raveau, Acta Crystallogr., Sect. B, 2000, 56, 173.
8. A. Maignan, S. Hébert, M. Hervieu, C. Michel, D. Pelloquin and D. Khomskii, J. Phys.: Condens. Matter, 2003, 15, 2711.
9. M. Hervieu, A. Maignan, C. Michel, V. Hardy, N. Créon and B. Raveau, Phys. Rev. B, 2003, 67, 45112.
10. J-W. Moon, D. Nagahama, Y. Masuda, W-S. Seo and K. Koumoto, J. Ceram. Soc. Jpn., 2001, 109, 647.
11. Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W-S. Seo and K. Koumoto, J. Mater. Chem., 2003, 13, 1094.
12. M. Shikano and R. Funahashi, Appl. Phys. Lett., 2003, 82, 1851.
13. T. Tani, J. Korean Phys. Soc., 1998, 32, S1217.
14. T. Takeuchi, T. Tani and Y. Saito, Jpn. J. Appl. Phys., 1999, 38, 5553.
15. T. Tani, S. Isobe, W.-S. Seo and K. Koumoto, J. Mater. Chem., 2001, 11, 1.
16. H. Itahara, S. Tajima and T. Tani, J. Ceram. Soc. Jpn., 2002, 110, 1048.
17. C. Xia, H. Itahara, J. Sugiyama and T. Tani, J. Ceram. Soc. Jpn. , submitted.
18. S. Tajima, T. Tani, S. Isobe and K. Koumoto, Mater. Sci. Eng. B, 2001, 86, 20.
19. T. Tani, H. Itahara, C. Xia and J. Sugiyama, J. Mater. Chem., 2003, 13, 1865.
20. H. Itahara, C. Xia, Y. Seno, J. Sugiyama, T. Tani and K. Koumoto, Proceedings of 22nd International Conference on Thermoelectrics, La Grande Motte, 2003, in press.
21. F. K. Lotgering, J. Inorg. Nucl. Chem., 1959, 9, 113.
22. T. Yamane, S. Katayama and M. Todoki, Rev. Sci. Instrum., 1995, 66, 5305.
23. G. Xu, R. Funahashi, M. Shikano, I. Matsubara and Y. Zhou, J. Appl. Phys., 2002, 91, 4344.
24. H. Itahara, K. Fujita, J. Sugiyama, T. Tsukada and T. Tani, Proceedings of 22nd International Conference on Thermoelectrics, La Grande Motte, 2003, in press.
25. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.

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