Fabrication of grain-aligned bulks and thick films of misfit layered cobalt oxides by a magneto-scientific process

We report the preparation of grain-aligned [Ca₂CoO _3-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 Al ₂O₃/Ca349/Al₂O₃/Ca _0.9La_0.1MnO₃/ 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]_0.55CoO ₂(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 [(Bi_0.5Pb_0.5)Sr ₂O₄] 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 [(Bi_0.5Pb_0.5)₂Ca _0.8Pr_0.2)₂O₄]_0.55CoO ₂, 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.