Synthesis of textured thermoelectric layered cobaltites by reactive templated grain growth

In order to apply layered cobaltites to thermoelectric (TE) devices, we need aligned ceramics to exploit their superior in-plane TE properties. Here, we report the preparation and characterization of c-axis aligned [Ca₂CoO₃]_0.62[CoO₂] (CCO) ceramics by the reactive templated grain growth (RTGG) method. As reactive templates, we synthesized platelike β-Co(OH)₂ particles by a precipitation method. The template particles were mixed with a complementary reactant, CaCO₃, and aligned by a tape-casting. Upon heat treatment, they reacted to form a CCO ceramic with almost perfectly c-alignment (Lotgering´s orientation degree, f > 0.99). The highly textured ceramic shows significantly large in-plane electrical conductivity (σ_ab): i.e., the value of σ_ab was 2.61 × 10⁴ S/m at 1060 K, where the value is around 60 % of that of a single crystal. We conclude that the RTGG method must be an effective and versatile technique for the preparation of large-sized TE materials with high TE performances.