High-temperature thermoelectric properties of NaxCoO2-δ single crystals

The thermoelectric properties of Na_xCoO_2-δ single crystal have been evaluated at high temperatures (300<T< 800 K) for the first time. The Na_xCoO_2-δ single crystals are prepared by a flux technique, and the 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=S² ·ρ^-1) and the figure-of-merit (Z=S² ·ρ^-1κ^-1) are about 2.4 mWm ^-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 mWm^-1 K^-2 and Z=1.5 mK^-1 at 800 K, and the dimensionless value of ZT exceeds the criterion of 1. The PF and Z exceed the values of typical "high temperature thermoelectric materials". These results suggest that Na_xCoO_2-δ could be a very promising material for use in the high-temperature thermoelectric power generation devices