Development of multilayer textured Ca3Co4O9 materials for thermoelectric generators: Influence of the anisotropy on the transport properties

Multilayer Ca3Co4O9 (349) thick thermoelectric (TE) materials were fabricated by hot-pressing stacked dense and strongly textured single-layer samples. Microstructure and volume quantitative texture investigations were undertaken by using scanning electron microscopy and neutron diffraction techniques, respectively. The results show a bulk density similar to single-layer samples, but remarkable texture strength reinforcement. The electrical resistivity, ρ, and Seebeck coefficient, S, were reproducibly measured in directions parallel (ρc and Sc) and perpendicular (ρab and Sab) to the mean c-axis. ρ   showed a high anisotropy ratio ρc/ρabρc/ρab of 13.5 and 8.8 at 300 and 900 K, respectively, and ρab kept the same values whereas ρc decreased in the multilayer samples. Sab and Sc unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio κab/κc=2.7κab/κc=2.7 at 900 K. The resulting figure-of-merit ZT   is then noticeably anisotropic, with ratio ZTab/ZTc=4.6ZTab/ZTc=4.6. ZTab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication.