Synthesis of textured thermoelectric layered cobaltites by reactive templated grain growth

T. Tani1, H. Itahara1, C. Xia1, J. Sugiyama1, K. Koumoto2

1Toyota Central Research and Development Labs. Inc., 41-1, Nagakute, Aichi 480-1192, Japan

2Nagoya University, Graduate School of Engineering, Department of Applied Chemistry, Nagoya 464-8603, Japan.

In order to apply layered cobaltites to thermoelectric devices, we need aligned polycrystals to exploit their thermoelectric properties with large anisotropy. Here, we report the preparation and characterization of the aligned [Ca2CoO3]0.62[CoO2] (CCO) polycrystals by the reactive templated grain growth (RTGG) technique. Plate-like Co(OH)2 particles were aligned by a tape-casting and reacted with CaCO3 at 1193 K to form CCO polycrystals. Almost perfectly c-aligned CCO bulk polycrystals have been synthesized via a topotactic conversion from the Co(OH)2 platelets (as a reactive template) into the CoO2 layers, while the [Ca2CoO3] layers were in-situ formed by a reaction between CoOx and CaCO3. In addition, it was found that the RTGG-prepared CCO polycrystals showed clear anisotropic properties. We conclude that the RTGG technique must be an effective and practical technology for the preparation of large-sized thermoelectric materials with high thermoelectric performance.