Anisotropy of the thermoelectric figure of merit (ZT) in textured Ca₃Co₄O₉ ceramics prepared by using a spark plasma sintering process

Textured Ca₃Co₄O₉ ceramics were prepared by spark plasma sintering (SPS) from commercial CaCO₃ and Co₃O₄ powders to investigate the thermoelectric anisotropy of Ca₃Co₄O₉. The degree of orientation of the textured specimens was evaluated by using the Lotgering factor method, and the SPS process was confirmed to be an effective technology to prepare textured Ca₃Co₄O₉ ceramics. Both the power factors and the thermal conductivities were measured in two different directions, parallel (out-of-plane) and perpendicular (in-plane) to the SPS press axis. The difference in anisotropy between the electrical conductivity and the thermal conductivity was investigated for the textured Ca₃Co₄O₉ ceramics. The maximum figure of merit (ZT) value along the direction perpendicular to the SPS press axis was more than 20% higher than it was along the direction parallel to the SPS press axis. It is noted that the strong thermoelectric anisotropy can give rise to an overestimates of ZT when preparing samples in different directions for power factor and thermal conductivity measurements.