Fabrication and Characterization of Ba1/3CoO2 Epitaxial Films Exhibiting Thermoelectric ZT=0.12 at Room Temperature

Among many thermoelectric materials, oxide-based materials are attractive in terms of environmental compatibility. Layered cobaltite represented Na0.75CoO2 shows a rather large thermoelectric power factor along to the layers. However, the thermal conductivity (kappa) is rather high (similar to 7 W m(-1) K-1), and therefore, the thermoelectric figure of merit ZT is small (similar to 0.03) at room temperature. Hem we show that Ba2+ ion substitution with Na+ ion in Na0.75CoO2 significantly reduces the kappa while keeping the large power factor, resulting in the large enhancement of ZT. We fabricated the epitaxial films of Na0.75CoO2 by the reactive solid-phase epitaxy method and performed the ion-exchange treatment from Na+ to Ba2+. The mom temperature electrical conductivity (sigma), thermopower (S), and power factor of the taxis oriented Ba1/3CoO2 films along the in-plane were 2500 S cm(-1), +72 mu V K-1, and 1.3 mW m(-1) K--2,K- respectively. The kappa along the in-plane, which was clarified by measuring the kappa of the c-axis inclined (55 degrees) film, was 3.3 Wm(-1 )K(-1). The ZT (= S-2-sigma.T.kappa(-1)) along the in-plane was 0.12 at room temperature, which is highest among the oxide thermoelectric materials ever reported.