High-Temperature Thermoelectric Properties of NaxCoO2-δ Single Crystals

Abstract

We measure the high-temperature thermoelectric properties of the NaxCoO2-δ single crystal for the first time. The NaxCoO2-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=S2·ρ-1) and the figure-of-merit (Z=S2·ρ-1κ-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 ZT exceeds the criterion of 1. The PF and Z exceed the values of typical “high-temperature thermoelectric materials”, such as p-type FeSi2 and Si0.7Ge0.3. These results suggest that NaxCoO2-δ could be a very promising material for use in high-temperature thermoelectric power generation devices.