Ba_1/3CoO₂: A Thermoelectric Oxide Showing a Reliable ZT of ∼0.55 at 600 °C in Air

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba_1/3CoO₂. We fabricated Ba_1/3CoO₂ epitaxial films by the reactive solid-phase epitaxy method (Na_3/4CoO₂) followed by ion exchange (Na⁺ → Ba²⁺) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba_1/3CoO₂ epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m^-1 K^-2 and the thermal conductivity gradually decreased to ∼1.9 W m^-1 K^-1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.