Crystal Growth of Thermoelectric Material NaxCoO2-δ by a Flux Method

NaxCoO2-δ (0.5≤x≤1) crystals were grown by a flux method. The largest NaxCoO2-δ crystal size was 10×10×0.5 mm3, and the crystal phase depended on the composition of the flux. Crystals grown from a flux of NaCl mixed with Na2CO3 and B2O3 and crystals grown from a flux of Na2CO3 mixed with B2O3 were identified as Na0.6CoO2-δ (γ phase) and Na0.9CoO2-δ (α phase), respectively. The structures and compositions of these crystal growths were determined with a powder X-ray diffraction device and an energy dispersive X-ray spectrometer, respectively. The in-plane thermoelectric power values of these two types of crystal increased with temperature and reached 96 µV/K and 87 µV/K at 450 K, respectively.