Synthesis and thermoelectric properties of Na$_{x}$CoO$_{2}$ single crystal*

Na$_{x}$CoO$_{2}$ has been known to be a potential thermoelectric material because of its large thermoelectric power and low resistivity. Recently, Na$_{x}$CoO$_{2}\cdot $yH$_{2}$O was discovered as a superconductor with a transition temperature T$_{c}$ of about 5K. Further understanding of the electron structure and properties of this material need larger samples of Na$_{x}$CoO$_{2 }$single crystal. In our work, Na$_{x}$CoO$_{2}$ single crystals were grown by flux method. It was found that choosing an appropriate slow cooling temperature region can increase the size of the single crystals grown by this method. The size of the largest single crystal is about 5$\times $4$\times $0.02mm$^{3}$. The structure and thermoelectric properties of these crystals will be discussed.