Fabrication and thermoelectric properties of layered cobaltite, γ-Sr0.32Na0.21CoO2 epitaxial films

Thermoelectric properties of γ-Sr0.32Na0.21CoO2 epitaxial films, which were fabricated on the (0001)-face of α-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with γ-Na0.8CoO2 epitaxial films, were measured to clarify the effect of Sr2+-ion exchange of γ-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF=S2σ) of the γ-Sr0.32Na0.21CoO2 film (300K) were 8.9×102Scm−1, +120μVK−1, and 1.2×10−3Wm−1K−2, respectively. These values were relatively large compared to those of γ-Na0.8CoO2 epitaxial films (σ=1.2×103Scm−1, S=+95μVK−1, and PF=1.1×10−3Wm−1K−2). The γ-Sr0.32Na0.21CoO2 epitaxial film exhibited much improved chemical stability against moisture compared to the γ-Na0.8CoO2 epitaxial film. The Sr2+-ion exchange of γ-Na0.8CoO2 is vital for practical thermoelectric applications.