Improvements of thermoelectric transport properties for a partially substituted Ca ₃Co ₄O ₉ system by spark plasma sintering

Highly-dense and textured Ca ₃Co ₄O ₉ ceramic samples were obtained by a spark plasma sintering (SPS) process. The sintering conditions were modified to optimize the degree of orientation and the bulk densities which are important for their thermoelectric performance. Their thermoelectric properties were measured from room temperature to 1,180 K in air. The maximum power factor was achieved at 1,153 K in the SPS-sintered case because of the maximum electrical conductivity owing to both the degree of orientation and bulk density. In order to improve the thermoelectric performance of this SPS-sintered Ca ₃Co ₄O ₉ system a doping effect with various dopants was also investigated. Ca ₃Co ₄O ₉ samples doped with Ag, Bi, and Na were prepared. Three factors including acceptor doping, a grain boundary phase and elevated mobility caused enhanced electrical conductivities. And these increased electrical conductivities resulted in increased power factors.