Processing effects on the thermoelectric properties of Bi<sub>2</sub>Ca<sub>2</sub>Co<sub>1.7</sub>O<sub>x</sub> ceramics

Abstract

Bi2 Ca2 Co1.7Ox bulk polycrystalline ceramics were prepared by the solid state method and by directional growth. Moreover, the effect of annealing on the textured materials has been studied. Microstructure has shown randomly oriented grains in the classical sintered materials while in the textured samples those were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the textured-annealed samples showed much lower amount of secondary phases than the as-grown ones. These microstructural changes are reflected on the thermoelectric properties which increase with the grain orientation and with the decrease on the secondary phase content mainly due to the electrical resistivity reduction. As a consequence, a raise on the power factor of about 6 and 9 times, compared with the classically sintered samples, was obtained for the as-grown and textured-annealed ones, respectively. The maximum power factor obtained at 650 oC in the textured-annealed samples (~ 0.31 mW/K2 m) is about 50 % higher, at the same temperature, than the obtained in one of the most used methods for texturing this kind of materials, the sinter-forging process.