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

• The Fe and Fe/Y doping at the Co- and Ca-sites of Ca3Co4O9+δ were investigated.
• The rising ρ by Y doping can be mitigated by the coupled Fe doping.
• The increased Seebeck coefficient by Y doping can be maintained in co-doped system.
• The co-doped system leads to an improvement of the thermoelectric performance.
• The co-doped system may preserve the merits from each component doping.

A series of Y and Fe co-doped Ca3−xYxCo4−yFeyO9+δ (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 °C) thermoelectric properties were systematically investigated. For the Fe-doped system (x = 0, y ⩽ 0.1), the electrical resistivity (ρ) 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 ρ and S tended to increase with increasing Y dopants, however, the effect is more substantial on ρ than on S, particularly in the low temperature regime. In contrast to ρ and S, the in-plane thermal conductivity (κ) 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 ρ, enhanced power factor over the measured temperature range, and improved ZT at 800 °C as compared to un-doped Ca3Co4O9+δ.