Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing

Abstract

Plate-like particles of Ca₃Co₄O₉ have been prepared in which Bi and Na were partially substituted for Ca to improve the thermoelectric performance. Successfully fabricated dense and highly textured ceramics have been obtained by combining the templated grain growth technique (TGG) with hot-pressing (HP). Ca₃Co₄O₉ single crystals, which have alternating layers of Co–O and Ca–Co–O in the direction of the c-axis, show high electrical conductivity along the layer compared with that across the layer. Hence, a highly oriented TGG + HP specimen showed high electrical conductivity compared with the specimen sintered under uniaxial pressure (UP + PLS). The improved electrical conductivity with high Seebeck coefficient of the highly oriented specimen (TGG + HP) gave a high thermoelectric power factor of 5.9 × 10⁻⁴ W m⁻¹ K⁻² at 1073 K. The figures-of-merit at 773 K and 1073 K were calculated to be 8.54 × 10⁻⁵ K⁻¹ (ZT = 0.066 at 773 K) and 1.69 × 10⁻⁴ K⁻¹ (ZT = 0.18 at 1073 K), respectively. These values are quite high among Ca–Co–O polycrystalline systems reported so far.