High-temperature thermoelectric properties of Na/sub x/CoO/sub 2-δ/ single crystals

Abstract

The thermoelectric properties of Na/sub x/CoO/sub 2-/spl delta// single crystal have been evaluated at temperatures (300<T< 800 K) for the first time. The Na/sub x/CoO/sub 2-/spl delta// 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 (/spl rho/), the thermoelectric power (S) and the in-plane thermal conductivity (/spl kappa/) are measured in the range of 300 K to 800 K. The estimated power factor (PF=S/sup 2//spl middot//spl rho//sup -1/) and the figure-of-merit (Z=S/sup 2//spl middot//spl rho//sup -1//spl kappa//sup -1/) are about 2.4 mWm/sup -1/ K/sup -2/ and 0.12 mK/sup -1/ at 300 K, respectively. These factors increase with temperature, and reach the value of PF=7.7 mWm/sup -1/ K/sup -2/ and Z=1.5 mK/sup -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/sub x/CoO/sub 2-/spl delta// could be a very promising material for use in the high-temperature thermoelectric power generation devices.