Difference between transition metal cation substitution and Nonstoichiometric addition on nanostructure and thermoelectric performance of complex oxide ceramics

Highlights: • Cation addition in Ca₃Co₄Cu_yO₉ hierarchically modified the crystal structure. • Cation addition modified the incommensurability of two subsystems in unit cell. • Ca₃Co₄Cu_yO₉ ceramics are with significantly enhanced Seebeck coefficient. -- Abstract: This work presents the impact of different methodologies in introducing the transition metal dopants into the thermoelectric Ca₃Co₄O₉ ceramics and their resultant nanostructure and electrical transport properties. The polycrystalline ceramics samples are with designed nominal composition of cation substitution of Ca₃Co_4-xCu_xO₉ (x = 0, 0.01, 0.05, 0.1) and cation non-stoichiometric addition of Ca₃Co₄Cu_yO₉ (y = 0, 0.01, 0.05, 0.1), respectively. At low-temperature regime, the electrical resistivity decreased to the lowest values for each set of samples, upon the minute Cu doping with x = 0.01, and the electrical resistivity increase with Cu doping level. The Seebeck coefficient presents little change among Ca₃Co_4-xCu_xO₉ samples. By contrast, the Seebeck coefficient was significantly increased especially in the low-temperature regime in samples of Ca₃Cu_yCo₄O₉, when the Cu addition level is over y = 0.05. While the grain size and crystal texture status are almost unchanged among Ca₃Co_4-xCu_xO₉ samples, non-stoichiometric Cu addition substantially triggered the grain growth, and essentially change the misfit relationship between the Ca₂CoO₃ and CoO₂ layers within the unit cell. The present study demonstrates a novel approach for hierarchically modifying the structure of complex oxide ceramics to tune their thermoelectric properties through cation non-stoichiometric addition.