Enhancement of Electrical Properties of the Thermoelectric Compound Ca 3 Co 4 O 9 through Use of Large-grained Powder
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Kangji Chong Osaka Electro-Communication University, Neyagawa, Osaka 572-0833, Japan
Damiel Chateigner CRISMAT-ENSICAEN Laboratory, UMR CNRS 6508, 14050 Cean Cedex, France (Received 11 May 2005; accepted 25 May 2005)
Hot-forged Ca3Co4O9 (Co349) ceramics were synthesized using large-grained powders prepared by a flux-growth method, and their thermoelectric properties and degree of grain alignment were evaluated. Neutron-diffraction experiments evidenced the effect of grain size on the development of the c-axis grain alignment. The optimum grain size was around 7 m in our hot-forging method. The electrical resistivity () in the direction parallel to the pressed-plane was more reduced at higher degrees of orientation. Since was reduced without lowering the Seebeck coefficient (S), the power factor (PF ⳱ S2/) of the Co349 sample was improved and reached 0.8 mW/mK2 at 1073 K using Co349 grains with average size of around 7 m. The thermal conductivity () in the direction parallel to the pressed-plane slightly increased with the increase of the grain size, however the improvement of PF owing to use of large-grained powder outweighed this negative impact on the component of the thermoelectric figure of merit (Z ⳱ S2/).
I. INTRODUCTION
The recent discovery of high thermopower coexisting with low electrical resistivity in layered cobaltites has opened the route to the exploration of thermoelectric materials in oxides.1–5 Due to their high thermal stability, high tolerance to oxidation, and low toxicity, these oxides are promising candidates for practical use in thermoelectric power generation in air at high temperatures. The conversion efficiency of a thermoelectric material is evaluated by the dimensionless figure-of-merit ZT ⳱ (S2/ )T, where S is the Seebeck coefficient, is the electrical resistivity, is the thermal conductivity, and T is the absolute temperature. ZT values of single crystals of NaxCoO2 and Ca3Co4O9 (Co349) along the ab-plane exceed 1 at 1000 K,2,4,5 the estimated criterion for practical application. Due to the layered structure of the Co349 phase, consisting of alternate stacking of a rock-salttype (Ca2CoO3) layer and a CdI2-type (CoO2) layer,6–8 along the c axis is considerably higher than that in the ab-plane.6 Given this anisotropy, texturizing the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10/1557/JMR.2005.0298 J. Mater. Res., Vol. 20, No. 9, Sep 2005
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polycrystalline bulk material is indispensable for the achievement of ZT high enough for practical application, as it is difficult to grow single crystals large enough for the construction of a thermoelectric device. Some research groups have already reported the fabrication of textured Co349 ceramics by reactive templated grain growth,9–11 magnetic alignment,12–14 or utilization of the combined effect of large single crystals and powder.15 The resulting textured samples show reduced compared to non-aligned sintered-Co349 materials. To
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