Layered cobalt oxide epitaxial films exhibiting thermoelectric ZT = 0.11 at room temperature
Among many thermoelectric materials, oxide-based materials draw significant interest due to their environmental compatibility. In particular, layered cobaltite, Na0.75CoO2, shows a large thermoelectric power factor parallel to the layers. However, the thermal conductivity (κ) is rather high (5−7 W m−1 K−1), and therefore, its thermoelectric figure of merit ZT is small (~0.03) at room temperature. Here we show that substituting Na+ ion with Ba2+ ion in NaxCoO2 drastically reduces the κ while keeping the large power factor, resulting in a large enhancement in ZT. We fabricated epitaxial films of Na0.75CoO2 by reactive solid-phase epitaxy method and performed ion-exchange treatment from Na+ to Ba2+ to synthesize Ba0.27CoO2 films. The room temperature electrical conductivity (σ), thermopower (S), and power factor of the c-axis oriented Ba0.27CoO2 films along the in-plane direction were 2310 S cm−1, +72 μV K−1, and 1.2 mW m−1 K−2, respectively, while the κ along the in-plane, which was clarified by measuring the κ of the c-axis inclined (55°) film, was 3.3 W m−1 K−1. This yields a ZT (= S2·σ·T·κ−1) value along the in-plane as high as 0.11 at room temperature, which is the highest among the oxide thermoelectric materials except oxychalcogenide, (BiO)(CuSe).