High-Temperature Thermoelectric Properties of NaxCoO2-δ Single Crystals

We measure the high-temperature thermoelectric properties of the Na x CoO 2-δ single crystal for the first time. The Na x CoO 2-δ single crystals are prepared by a flux technique, and the resulting flaky single crystals are very thin on the c -axis. The in-plane electrical resistivity (ρ), the thermoelectric power ( S ) and the in-plane thermal conductivity (κ) are measured in the range of 300 K to 800 K. The estimated power factor ( PF = S 2 ·ρ -1 ) and the figure-of-merit ( Z = S 2 ·ρ -1 κ -1 ) are about 2.4 mW m -1 K -2 and 0.12 mK -1 at 300 K, respectively. These factors increase with temperature, and reach the value of PF =7.7 mW m -1 K -2 and Z =1.5 mK -1 at 800 K, and the dimensionless value of Z T exceeds the criterion of 1. The PF and Z exceed the values of typical “high-temperature thermoelectric materials”, such as p-type FeSi 2 and Si 0.7 Ge 0.3 . These results suggest that Na x CoO 2-δ could be a very promising material for use in high-temperature thermoelectric power generation devices.