Electrical performance tuning in thermoelectric Ca3Co4O9 materials by transition metals additions

This work reports on the effects on high-temperature thermoelectric (TE) properties in bulk, polycrystalline p-type Ca3Co4O9 ceramics, after employing a composite approach consisting of metallic particles additions and two simple sintering schemes. The added Fe, Co and Ni particles are expected to act as porosity fillers upon oxidation in air and provide improved grain connectivity, changing the microstructural features and electrical properties of the resulted materials. The composites have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. The electrical conductivity (σ), Seebeck coeffcient (α) and power factor (PF) values have been investigated between 475 and 975 K, in air flow, and related to the sample’s respective phase compositions, morphologies and microstructures. For the Co additions in the 1ST sintering case, the porous samples reached maximum PF values of around 210 μWm-1K2 , being around two times higher than those of the pure Ca3Co4O9 matrix. For the 1STsintered Fe and Ni added samples, the highest PF values of 80 and 90 μWm-1K-2 have been measured for the 3% vol. Ni and 3 and 6% vol. Fe additions, respectively, very close to some of the best reported values from literature. In contrast, 2ST sintering resulted in much denser samples and more complex phase compositions and microstructures, leading to lower electrical performance. The improvements of electrical properties achieved in the present work are promoted by a simultaneous increase in electrical conductivity and Seebeck coefficient values, stemming from pore filling effects and subsequent microstructural modifications.publishe