Effects of Yttrium and Iron co-doping on the high temperature thermoelectric properties of Ca3Co4O9+δ

A series of Y and Fe co-doped Ca3-xYxCo4-yFeyO9+delta (0 <= x <= 0.3, 0 <= y <= 0.1) samples synthesized by auto-combustion reaction and followed by a spark plasma sintering (SPS) processing with the effects of Fe and Y doping on the high temperature (RT to 800 degrees C) thermoelectric properties were systematically investigated. For the Fe-doped system (x = 0, y <= 0.1), the electrical resistivity (rho) decreased over the whole measured temperature range, while the Seebeck coefficient (S) remained almost the same. For the co-doped system, at any fixed Fe doping content, both rho and S tended to increase with increasing Y dopants, however, the effect is more substantial on rho than on S, particularly in the low temperature regime. In contrast to rho and S, the in-plane thermal conductivity (kappa) is only slightly influenced by Y and Fe substitutions. Among all the investigated samples, the co-doped sample with x = 0.1 and y = 0.03 showed a decrease of rho, enhanced power factor over the measured temperature range, and improved ZT at 800 degrees C as compared to un-doped Ca3Co4O9+delta. (C) 2015 Elsevier B.V. All rights reserved.