Redox-Promoted Tailoring of the High-Temperature Electrical Performance in Ca 3 Co 4 O 9 Thermoelectric Materials by Metallic Cobalt Addition.

This paper reports a novel composite-based processing route for improving the electrical performance of Ca ₃ Co ₄ O ₉ thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca ₃ Co ₄ O ₉ /xCo composites (x = 0%, 3%, 6% and 9% vol.) 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. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%-61% of ρ _th ) reached maximum PF values of around 210 and 140 μWm ^-1 ·K ^-2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca ₃ Co ₄ O ₉ matrix. Although 2ST sintering resulted in rather dense samples (80% of ρ _th ), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca ₃ Co ₄ O ₉ matrix (224 μWm ^-1 ·K ^-2 at 975K).