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

Thermoelectric properties of gamma-Sr0.32Na0.21CoO2 epitaxial films, which were fabricated on the (0001)-face of alpha-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with gamma-Na0.8CoO2 epitaxial films, were measured to clarify the effect of Sr2+-ion exchange of gamma-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (sigma), Seebeck coefficient (S), and thermoelectric power factor (PF=S-2 sigma) of the gamma-Sr0.32Na0.21CoO2 film (300 K) were 8.9x10(2) S cm(-1), +120 mu V K-1, and 1.2x10(-3) W m(-1) K-2, respectively. These values were relatively large compared to those of gamma-Na0.8CoO2 epitaxial films (sigma=1.2x10(3) S cm(-1), S=+95 mu V K-1, and PF=1.1x10(-3) W m(-1) K-2). The gamma-Sr0.32Na0.21CoO2 epitaxial film exhibited much improved chemical stability against moisture compared to the gamma-Na0.8CoO2 epitaxial film. The Sr2+-ion exchange of gamma-Na0.8CoO2 is vital for practical thermoelectric applications. (c) 2006 American Institute of Physics.