Thermoelectric properties in single crystals of Ca-Co-O system

Single crystals of Co-based layered oxide, Ca₃Co₄O₉ (Co349), were grown using a flux method and their electrical and thermal properties were investigated. Values of thermoelectric power S are quite large above 773 K and reach almost 250 μV·K^-1 at 973 K, and their thermal conductivity κ is nearly 3 W·m^-1·K^-1, as determined by mathematical extrapolation. The relatively low κ is likely the result of a misfit structure between the CoO₂ layers and the Ca₂CoO₃ slabs. The figure of merit for (Ca,Sr,Bi)₂Co₂O₅₊₀ with Co349 structure can be estimated as greater than 1.1. Co349 should be a promising material for thermoelectric power generation.