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

Abstract

Abstract Multilayer Ca 3 Co 4 O 9 (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 ( ρ c and S c ) and perpendicular ( ρ ab and S ab ) to the mean c -axis. ρ showed a high anisotropy ratio ρ c / ρ a b of 13.5 and 8.8 at 300 and 900 K, respectively, and ρ ab kept the same values whereas ρ c decreased in the multilayer samples. S ab and S c unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio κ a b / κ c = 2.7 at 900 K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio Z T a b / Z T c = 4.6 . ZT ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication.