Synthesis of single-phase Ca 3 Co 4 O 9 ceramics and their processing for a microstructure-enhanced thermoelectric performance

Single-phase Ca3Co4O9 with a high porosity, having a ZT of 0.09 at 627 °C, was successfully prepared by a simple solid-state reaction using fine powders of CaCO3 and Co3O4 after a calcination at 760 °C for 12 hours. It was excellent either for the preparation of the starting powder for the processing of the Ca3Co4O9 ceramics or for direct processing. The influence of already-reported processing methods (classic sintering, hot pressing, free-edge spark-plasma sintering (SPS) of a sintered pellet) and new methods, such as free-edge SPS of a pellet from just-compacted powder, cold pressing of a sintered pellet without post annealing, and free-edge cold pressing of a sintered pellet with post-annealing, on the preparation of Ca3Co4O9 ceramics was studied. The results showed how the density, the grain morphology and the microstructural anisotropy can all influence the thermoelectric characteristics of Ca3Co4O9 ceramics measured in directions parallel and perpendicular to the applied pressure. While the fully dense (99%) and perfectly textured ceramics prepared by the free-edge SPS of a pellet from just-compacted powder had a ZT of 0.17, the highest ZT of 0.31 was obtained for the free-edge cold-pressed and annealed ceramics with modest texturing and low density (65%), having a very low κ of 0.47 W/mK. The results also showed that thin and irregular-shaped plate-like grains with sharp edges are preferred, while their thickening accompanied by rounding of their shape resulted in a reduced thermoelectric performance. The study revealed both the possibilities and the limitations for enhancing the TE characteristics of Ca3Co4O9 ceramics just through microstructure optimisation.