Fabrication of highly textured Ca₃Co₄O₉ ceramics with controlled density and high thermoelectric power factors

Ca₃Co₄O₉ ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca₃Co₄O₉ ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)₂ as a template and CaCO₃ as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m⁻¹ K⁻².