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 Ca3Co4O9 ceramics and their resultant nanostructure and electrical transport properties. The polycrystalline ceramics samples are with designed nominal composition of cation substitution of Ca3Co4-xCuxO9 (x = 0, 0.01, 0.05, 0.1) and cation non-stoichiometric addition of Ca3Co4CuyO9 (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 Ca3Co4-xCuxO9 samples. By contrast, the Seebeck coefficient was significantly increased especially in the low-temperature regime in samples of Ca3CuyCo4O9, when the Cu addition level is over y = 0.05. While the grain size and crystal texture status are almost unchanged among Ca3Co4-xCuxO9 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.