Thermoelectric Performance Enhancement of Calcium Cobaltite through Barium Grain Boundary Segregation

We report the dramatic increase of the Seebeck coefficient S and thermoelectric performance of calcium cobaltite Ca3Co4O9+delta ceramics through non-stoichiometric addition of minute amount of Ba. The nominal chemistry of polycrystal pellets are Ca3BaxCo4O9+delta (x = 0, 0.01, 0.05, and 0.1). At 323 K, S of Ca3Co4O9+delta is 135 mu V K-1, whereas S of Ba incorporated Ca3Ba0.05Co4O9+delta is 162.5 mu V.K-1, which is the highest S value near room temperature regime reported for calcium cobaltite. The increase of S for Ca3Ba0.05Co4O9+delta sample is accompanied by the decrease of the electrical resistivity rho, resulting in high power factor S-2/rho of 843 mu W.m(-1) K-2 at 1007 K. Moreover, the thermal conductivities kappa of Ca3Ba0.05Co4O9+delta decrease with the increase of the Ba addition. The figure-of-merit ZT for Ca3Ba0.05Co4O9+delta reaches 0.52 at 1073 K and a factor of 2.5 increment in comparison with undoped Ca3Co4O9+delta. Nanostructure examinations show that the added Ba segregated at the Ca3Co4O9+delta grain boundaries, while the Ca3Co4O9+delta grain interior is free of Ba. Performance enhancement is attributed to the carrier filtering effect caused by the Ba segregation. In addition, Ba segregation promotes the better crystal alignment and the development of crystal texture.