Sol-gel synthesis, microstructure, and thermoelectric properties of Ca2.8-Bi Dy0.2Co4O9+

Abstract Small-sized Ca 2.8- x Bi x Dy 0.2 Co 4 O 9+ 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 Ca 2.8- x Bi x Dy 0.2 Co 4 O 9 samples were studied with an emphasis placed on the Bi content and the fabrication process. The as-sintered Ca 2.8- x Bi x Dy 0.2 Co 4 O 9 samples exhibited a single Ca 3 Co 4 O 9+ δ phase and a plate-like morphology. With increased Bi content, the grain size of the sintered Ca 2.8- x Bi x Dy 0.2 Co 4 O 9 samples decreased, whereas the density of the sintered Ca 2.8- x Bi x Dy 0.2 Co 4 O 9 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 Ca 2.725 Bi 0.075 Dy 0.2 Co 4 O 9 . The partial substitution of Bi for Ca and the twice sintering were a highly effective route for improving the thermoelectric properties of Ca 2.8 Dy 0.2 Co 4 O 9 .