Thermoelectric properties of lower concentration K-doped Ca₃Co₄O₉ ceramics

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials. A series of lower concentration K-doped Ca_3-xK_xCo₄O₉ (x=0, 0.05, 0.10, 0.15) polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method. The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope. The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures (300-1026 K). Low concentration K doping causes electrical conductivity to increase up to 23% with little effect on the Seebeck coefficient. Simultaneously, the thermal conductivity of K-doped sample is lower than that of the undoped sample, and the total thermal conductivity reaches a minimum value of approximately 1.30 W·m^-1·K^-1, which may be suppressed mainly by the phonon thermal conduction confinement. The dimensionless figure-of-merit ZT of Ca_2.95K_0.05Co₄O₉ is close to 0.22 at 1026 K, representing an improvement of about 36% compared with that of Ca₃Co₄O₉, suggesting that lower concentration K-doped Ca₃Co₄O₉ series materials are promising thermoelectric oxides for high-temperature applications.