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

Abstract This work presents the impact of different methodologies in introducing the transition metal dopants into the thermoelectric Ca 3 Co 4 O 9 ceramics and their resultant nanostructure and electrical transport properties. The polycrystalline ceramics samples are with designed nominal composition of cation substitution of Ca 3 Co 4-x Cu x O 9 (x = 0, 0.01, 0.05, 0.1) and cation non-stoichiometric addition of Ca 3 Co 4 Cu y O 9 (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 Ca 3 Co 4-x Cu x O 9 samples. By contrast, the Seebeck coefficient was significantly increased especially in the low-temperature regime in samples of Ca 3 Cu y Co 4 O 9 , when the Cu addition level is over y = 0.05. While the grain size and crystal texture status are almost unchanged among Ca 3 Co 4-x Cu x O 9 samples, non-stoichiometric Cu addition substantially triggered the grain growth, and essentially change the misfit relationship between the Ca 2 CoO 3 and CoO 2 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.