Thermoelectric Properties and Magnetic Anisotropies of Magnetically Grain-Oriented Sr- or Bi-doped Ca 3 Co 4 O 9 Thick F
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1044-U09-03
Thermoelectric Properties and Magnetic Anisotropies of Magnetically Grain-Oriented Sr- or Bi-doped Ca 3Co 4O9 Thick Films Shigeru Horii1, Yuhya Yamasaki1, Masayuki Sakurai1, Ryoji Funahashi2, Tetsuo Uchikoshi3, Tohru S Suzuki3, Yoshio Sakka3, Hiraku Ogino1, Jun-ichi Shimoyama1, and Kohji Kishio1 1 Department of Applied Chemistry, The University of Tokyo, Tokyo, 113-8656, Japan 2 National Institute of Advanced Industrial Science and Technology, Ikeda, 563-8577, Japan 3 National Institute for Materials Science, Tsukuba, 305-0047, Japan ABSTRACT We report magnetic properties of [Ca2CoO3-δ]0.62CoO2 (Ca349) powders with various average size and the Bi- and Sr-doping effects on thermoelectric properties for the magnetically grain-aligned and densified Ca349 thick films. Magnetic anisotropy at 300 K depended on the initial average size of Ca349 powders and decreased with the decrease in the size. This presumably suggests that distortion of crystal structure was induced by a ball-milling process and led to the change of magnetic anisotropy. On the Bi- and Sr-doping effects, an obvious enhancement of thermoelectric properties did not emerge in the case of the Sr-doping, whereas the enhancement was observed for the Bi-doped Ca349 thick films. However, a drastic decrease of magnetic anisotropy was caused by the Bi-doping. For usage of the p-type layer in multilayered thermoelectric module, tuning of the Bi-doping levels in which both enhancement of thermoelectric properties and a certain level of magnetic anisotropy are achieved is required. INTRODUCTION Discovery of an excellent thermoelectric (TE) property in NaxCoO2 [1] has triggered the research and development of TE oxides for electric power generation at high temperatures in air. Synthesis of not only NaxCoO2 but also other TE oxides, such as [Ca2CoO3-δ]0.62CoO2 [2, 3] and [BiSrO]1.10CoO2 [4], has been reported as p-type TE compounds. For the improvement of TE properties and practical use in polycrystals for these p-type cobaltites with anisotropic crystal structures, introduction of grain-alignment and densification processes is indispensable. In practice, the grain-orientation using magneto-science[5, 6] and mechanical process [7] and the densification using spark-plasma-sintering and hot-pressing [7] and intermediate cold-pressing [8] have been proposed before and it has been found that each method was effective for the improvement of each TE property due to the increase in electrical conductivity. For the practical use of downsized and integrated TE module, a thin-film technology is preferable rather than a bulk process. However, there are disadvantages of slow-deposition rates, difficulty of formation of highly grain-oriented microstructures with single-crystalline substrates and serious contribution of these substrates with high thermal conductivity. Recently, our group [9] has reported that electrophoretic deposition (EPD) was useful for the fabrication of multilayered thick films and enabled the formation of c-axis grain-oriented microstructure by th
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