Enhanced thermoelectric performance of heavy-metals (M: Ba, Pb) doped misfit-layered ceramics: (Ca2−xMxCoO3)0.62 (CoO2)

Abstract

We present the temperature dependent thermoelectric properties of polycrystalline misfit-layered ceramics; (Ca2−xMxCoO3)0.62 (CoO2), (M: Ba and Pb, x=0, 0.1, 0.2), fabricated by sol–gel method followed by spark plasma sintering technique. The X-ray diffraction results confirmed the substitution of Ba and Pb in the lattice of (Ca2CoO3)0.62 (CoO2) at Ca-site. An improved grain-alignment was observed at small scale with Ba-doping, which helped increasing the electrical conductivity but for the Pb-doped specimens, the electrical conductivity was suppressed by catastrophic grain-alignment. The bivalent metallic-doping induced a spin-entropy enhancement which resulted in an enhanced thermopower. For Ba-doped specimens, the simultaneous increase in the electrical conductivity and the thermopower resulted in an increased power factor exhibiting the highest value of 527μW/mK2. On the other hand, Pb-doping increased the thermopower but on the expense of electrical conductivity. Although, Pb-doping decreased the electrical conductivity but on the other hand, the enhanced thermopower and the suppressed thermal conductivity were sufficient for achieving ZT value higher than that of pure (Ca2CoO3)0.62 (CoO2). Among all the pure and doped samples, the highest ZT value of ∼0.33 at 1000K was achieved by Ba-doping which is about 100% higher than that of the pure (Ca2CoO3)0.62 (CoO2).