Single crystals

Abstract

We measure the high-temperature thermoelectric properties of the NaxCoO2-.DELTA. single crystal for the first time. The NaxCoO2-.DELTA. single crystals are prepared by a flux technique, and the resulting flaky single crystals are very thin on the c-axis. The inplane electrical resistivity (.RHO.), the thermoelectric power (S) and the in-plane thermal conductivity (.KAPPA.) are measured in the range of 300K to 800K. The estimated power factor(PF=S2 .RHO.-1) and the figure-of-merit(Z=S2 .RHO.-1.KAPPA.-1) are about 2.4mWm-1K-2 and 0.12mK-1 at 300K, respectively. These factors increase with temperature, and reach the value of PF=7.7mWm-1K-2 and Z=1.5mK-1 at 800K, 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 than NaxCoO2-.DELTA. could be a very promising material for use in high-temperature thermoelectric power generation devices