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ABSTRACT: Single crystals of ( Ca <sub>2</sub> CoO <sub>3</sub>)<sub>0.7</sub> CoO <sub>2</sub> were grown using a modified strontium chloride flux technique, and their electrical and thermal properties were determined. Growth conditions were established for obtaining plate-like single crystals of a relatively large size (5×10×0.05 mm <sup>3</sup>). At 973 K, thermoelectric power S and electrical resistivity ρ of the specimens are ≈240 μV K<sup>-1</sup> and ≈2.3×10<sup>-5</sup> Ω m , respectively, and their thermal conductivity κ is nearly 3 W m<sup>1</sup> K<sup>-1</sup>, as determined by mathematical extrapolation. The figure of merit ZT=S<sup>2</sup>T/ρκ derived therefrom is ≈0.87 at 973 K. The relatively low κ is likely the result of a misfit structure between the CoO <sub>2</sub> layer and the Ca <sub>2</sub> CoO <sub>3</sub> slab. © 2003 American Institute of Physics.
Applied Physics Letters 04/2003; · 3.84 Impact Factor
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Yuqin Zhou,
Ichiro Matsubara,
Shigeru Horii,
Tomonari Takeuchi,
Ryoji Funahashi, Masahiro Shikano,
Jun-ichi Shimoyama,
Kohji Kishio,
Woosuck Shin,
Noriya Izu,
Norimitsu Murayama
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ABSTRACT: Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity. © 2003 American Institute of Physics.
Journal of Applied Physics 02/2003; 93(5):2653-2658. · 2.17 Impact Factor
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ABSTRACT: Bi2Sr2Co2Oy (BC-222) whiskers were grown with an excellent Seebeck coefficient (S) and electrical resistivity (ρ) for power generation applications at high temperatures in air. The S value of these long BC-222 whiskers reached almost 300 μV K−1 at 973 K. Thermal conductivity (κ) was measured and found to be suppressed to a low value of about 2.0 W m−1 K−1. This is considered due not only to a phonon–phonon interaction but also other scattering processes. The resulting dimensionless thermoelectric figure of merit ZT( = S2T/ρκ) was more than 1.1, which corresponds to a conversion efficiency of almost 10% at 973 K in air. © 2002 American Institute of Physics.
Applied Physics Letters 08/2002; 81(8):1459-1461. · 3.84 Impact Factor
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ABSTRACT: We have controlled in-plane stress in the CoO2 sublattice of (Ca2CoO3)xCoO2 by cationic substitution of alkaline earth metals at the Ca site. The effects of the magnitude of the stress on thermoelectric properties have been investigated by using highly grain-oriented polycrystalline thin films. The in-plane compressive stress is released by the partial substitution of Sr for Ca. With increasing in-plane stress, the magnitude of thermoelectric power and resistivity increased. © 2002 American Institute of Physics.
Applied Physics Letters 06/2002; 80(25):4729-4731. · 3.84 Impact Factor
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ABSTRACT: Bi- and Na-substituted Ca3Co4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of Bi3+ and Na+ for Ca2+, as well as Bi3+ and Na+ double substitution, cause both the electrical conductivity (σ) and thermoelectric power (S) to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit ZT ( = S2σT/κ) reaches 0.32 at 1000 K in the double-substituted sample. © 2002 American Institute of Physics.
Applied Physics Letters 05/2002; 80(20):3760-3762. · 3.84 Impact Factor
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ABSTRACT: Polycrystalline samples of Bi <sub>2.2-x</sub> Pb <sub>x</sub> Sr <sub>2</sub> Co <sub>2</sub> O <sub>y</sub> (x=0, 0.4, and 0.8) with a layered structure have been prepared using a hot pressing technique and their thermoelectric properties methodically investigated. It is found that the substitution of Pb for Bi is a fairly effective means of improving the thermoelectric properties of the Bi <sub>2.2-x</sub> Pb <sub>x</sub> Sr <sub>2</sub> Co <sub>2</sub> O <sub>y</sub> system owing to resistivity (ρ) decreasing and thermoelectric power (S) increasing simultaneously. The thermal conductivity (κ) at high temperature also decreases markedly with Pb substitution. As a result, for the x=0.8 sample the dimensionless figure of merit, ZT(=S<sup>2</sup>T/ρκ), increases with temperature and reaches 0.26 at 1000 K, which indicates that this material is a strong candidate as a high-temperature thermoelectric material. © 2002 American Institute of Physics.
Journal of Applied Physics 05/2002; · 2.17 Impact Factor
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Journal of Materials Research. 04/2002; 17(05):1092 - 1095.
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ABSTRACT: Three kinds of single-crystalline whiskers of Bi2Sr2CoO6 (BC-221), (Bi2Sr2O4)x(CoO2)2 (BC-222), and (Ca2CoO3)xCoO2 (Co-225) phases were grown from the surfaces of precursor plates by annealing in an O2 gas flow. The phases of the whiskers can be controlled by the Co content of the precursor plates. The average compositions of the whiskers are Bi2.5(Sr, Ca)3.3CoO6+δ, Bi3.0(Sr, Ca)3.1Co2.0O9+δ, and (Ca, Sr, Bi)1.9Co2.0O5+δ. Although the Co-225 whiskers are 1.2 mm at the longest, the BC-221 and the BC-222 whiskers reach lengths of as much as 8.0 mm. Microstructural observations indicate that the growth points of the whiskers are their bases. The length of the whiskers depends on the crystallization status of the precursor plates.
12/2001;
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Tsunehiro Takeuchi,
Takeshi Kondo,
Tsuyoshi Takami,
Hirofumi Takahashi,
Hiroshi Ikuta,
Uichiro Mizutani,
Kazuo Soda,
Ryoji Funahashi, Masahiro Shikano,
Masashi Mikami,
Syunsuke Tsuda,
Takayoshi Yokoya,
Shik Shin,
Takayuki Muro
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ABSTRACT: With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca_3Co_4O_9, Na_0.6CoO_2, and Bi_2Sr_2Co_2O_9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5–2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E_F in these compounds. The density of states at E_F is finite but negligibly small at room temperature, because E_F is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near E_F determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence. Physical Review B 69(12), 2004, 125410
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ABSTRACT: Ca3−xNaxCo4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. With the substitution of Na+ for Ca2+, the resistivity (ρ) monotonically decreased, while the thermoelectric power (S) slightly increased simultaneously. Element substitution was demonstrated to be an effective means to adjust the thermal conductivity (κ). These results suggest that there exist a potential for improving the thermoelectric performance of Ca3Co4O9 system.
Solid State Communications.
