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, rho, and Seebeck coefficient, S. were reproducibly measured in directions parallel (rho(c) and S-c) and perpendicular (rho(ab) and S-ab) to the mean c-axis. rho showed a high anisotropy ratio rho(c)/rho(ab) of 13.5 and 8.8 at 300 and 900 K, respectively, and rho(ab) kept the same values whereas rho(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 K-ab/K-c = 2.7 at 900K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio ZT(ab)/ZT(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. (C) 2012 Elsevier Ltd. All rights reserved.