Sol-gel synthesis, microstructure, and thermoelectric properties of Ca2.8-xBixDy0.2Co4O9+δ

Small-sized Ca2.8-xBixDy0.2Co4O9+δ (0 ≤ x ≤ 0.1) powders with a plate-like morphology were synthesized via the citric acid-assisted sol-gel method. The structural and thermoelectric properties of Ca2.8-xBixDy0.2Co4O9 samples were studied with an emphasis placed on the Bi content and the fabrication process. The as-sintered Ca2.8-xBixDy0.2Co4O9 samples exhibited a single Ca3Co4O9+δ phase and a plate-like morphology. With increased Bi content, the grain size of the sintered Ca2.8-xBixDy0.2Co4O9 samples decreased, whereas the density of the sintered Ca2.8-xBixDy0.2Co4O9 samples increased. The incorporation of Bi up to x = 0.075 yielded high electrical conductivity. Meanwhile, the Seebeck coefficient decreased with increases in Bi content. The largest power factor (2.18 × 10−4 W m−1 K−2 at 800 °C) was obtained for the twice-sintered Ca2.725Bi0.075Dy0.2Co4O9. The partial substitution of Bi for Ca and the twice sintering were a highly effective route for improving the thermoelectric properties of Ca2.8Dy0.2Co4O9.