Fabrication of Grain-Aligned Bulks and Thick Films of Misfit Layered Cobalt Oxides by a Magneto-Scientific Process
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Fabrication of Grain-Aligned Bulks and Thick Films of Misfit Layered Cobalt Oxides by a Magneto-Scientific Process S. Horii1, T. Okamoto1, T. Kumagai1, T. Uchikoshi2, T. S. Suzuki2, Y. Sakka2, J. Shimoyama1, and K. Kishio1 1 Department of Applied Chemistry, University of Tokyo, Tokyo 113-8656, Japan 2 National Institute of Materials Science, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT We report the preparation of grain-aligned [Ca2CoO3-δ]0.62CoO2 (Ca349) thick films and the conversion of the easy axis of magnetization from the a-axis to the c-axis. The thick films were fabricated by a simultaneous usage of electrophoretic deposition and magnetic alignment methods (MEPD) at high deposition rate with the order of 10 µm/min. Moreover, a multi-layered thick film of Al2O3/Ca349/ Al2O3/Ca0.9La0.1MnO3/ Al2O3 was also fabricated by the MEPD method by the optimization of condition of each suspension. The conversion of the easy axis was performed crystallochemically for a [Bi2Sr2O4]0.55CoO2 (BiSr222) compound with the easy axis parallel to the a-axis in order to fabricate c-axis grain-oriented bulks by the magnetic alignment method. The substitution of Ca for Sr in the [(Bi0.5Pb0.5)2Sr2O4] block layer induced the change of the easy axis into the c-axis direction, and the magnetic anisotropy was increased by the partial substitution of rare earth elements of Pr, Nd, Tb and Dy for Ca. Using a compound of [(Bi0.5Pb0.5)2(Ca0.8Pr0.2)2O4]0.55CoO2, we have successfully prepared the c-axis grain-aligned bulk by the magneto-scientific method. Our present results indicate that the magneto-scientific method is one of the practical processes for production of thermoelectric modules. INTRODUCTION Recent development of evaluation of thermoelectric (TE) properties and crystal structures in cobalt oxides has been encouraged by the discovery of high TE properties in NaxCoO2 [1] which showed both a highly conductive behavior and a large TE power. This surprising behavior in strongly electron-correlated systems has brought us high expectation of application of cobalt oxide systems for TE materials especially in high temperatures. In addition to NaxCoO2, misfit layered cobaltites, such as [Ca2CoO3-δ]0.62CoO2 (Ca349) [2, 3] and [BiSrO2]1.10CoO2 (BiSr222)[4], were reported as p-type TE compounds, while CaMnO3 [5], LaNiO3 [6] and (ZnO)mIn2O3 [7] were candidates as n-type TE oxides. Especially, it has been reported in single crystals that layered cobaltites such as NaxCoO2, Ca349 and BiSr222 showed practical figures of merit (ZT) above 800 K[8], all of which possess CdI2-type of CoO2 layers parallel to the ab plane and showing high electrical conductivity and Seebeck coefficient. From a viewpoint of practical use of thermoelectric materials, polycrystalline bulks are preferred rather than single crystals. This is because growth of crystals with large size is difficult in the case of metal oxides and the production cost of the growth is extremely high. Therefore, control of orientation and density in polycrystals is very important in ord
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