High-Temperature Thermoelectric Properties of NaxCoO2-δ Single Crystals

Abstract

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^-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 Z T exceeds the criterion of 1. The PF and Z exceed the values of typical “high-temperature thermoelectric materials”, such as p-type FeSi₂ and Si_0.7Ge_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.