High temperature thermoelectric properties of Ca₃Co₄O_9+δ by auto-combustion synthesis and spark plasma sintering

A rapid method for the synthesis of Ca3Co4O9+δpowder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gelmethod where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit theadvantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes comingfrom the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used tofabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||)and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800°C,power factors of 506 μW/m K2(⊥) and 147 μW/m K2(||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.© 2013 Elsevier Ltd. All rights reserved.