Layered cobalt oxide epitaxial films exhibiting thermoelectric ZT = 0.11 at room temperature

Abstract

Among many thermoelectric materials, oxide-based materials draw significant interest due to their environmental compatibility. In particular, layered cobaltite, Na_0.75CoO₂, 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 the Na⁺ ion with the Ba²⁺ ion in Na_xCoO₂ drastically reduces the κ while keeping the large power factor, resulting in a large enhancement in ZT. We fabricated epitaxial films of Na_0.75CoO₂ by the reactive solid-phase epitaxy method and performed ion-exchange treatment from Na⁺ to Ba²⁺ to synthesize Ba_0.27CoO₂ films. The room temperature electrical conductivity (σ), thermopower (S), and power factor of the c-axis oriented Ba_0.27CoO₂ 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 (= S²σT/κ) value along the in-plane as high as 0.11 at room temperature, which is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).