Grain Boundary Phase Segregation for Dramatic Improvement of the Thermoelectric Performance of Oxide Ceramics

This work presents a novel approach of dramatically increasing the energy conversion efficiency of thermoelectric CaMnO_3-δ ceramics through the combination of lattice dopants substitution and secondary phase segregation at the grain boundaries. The oxide ceramic samples are with the nominal composition of Ca_{1- x}Bi _xMnCu _yO_3-δ ( x = 0, 0.02, 0.03; y = 0.02, 0.04). When Cu is introduced into the Ca_{1- x}Bi _xMnCu _yO_3-δ samples, the grain growth from Bi-doped CaMnO_3-δ grains is accompanied by the limited solubility of Cu ions in the grain interior, whereas Cu mainly formed a CuO secondary phase at the grain boundaries. Cu nonstoichiometry addition subsequently resulted in the increase of the Seebeck coefficient and decrease of electrical resistivity simultaneously. The sample with designed chemistry of Ca_2.97Bi_0.03MnCu_0.04O_3-δ exhibits the power factor of 2.4 mW m^-1 K^-2 at 337 K and figure of merit ZT of 0.67 at 773 K. This ZT of 0.67 is by far the highest ZT reported for various perovskites oxide ceramics. Such enhancements in electrical power factor and the overall ZT are attributed to the synergistic effect of decreasing the carrier concentration to increase the Seebeck coefficient and simultaneously increasing the carrier mobility through the existence of CuO phase at the grain boundaries.