Drastic modification of low temperature thermoelectric properties of Na-doped Bi2Sr2Co2Oy ceramics prepared via laser floating zone technique

In this study, Bi2Sr2−xNaxCo2Oy (x = 0.0, 0.05, 0.075, 0.10, and 0.15) ceramic powders have been fabricated via the classical ceramic route, followed by a texturing process through the laser floating zone technique. XRD patterns show the thermoelectric phase as the major one. In addition, Na-substitution reduces the amount of secondary phases, when compared to the pure sample. SEM observations point out that grain orientation is significantly improved when Na-content is increased. Na-substitution reduces electrical resistivity from 35 (in pure samples) to 19.6 mΩ cm (in Na = 0.05 ones) at around room temperature, while Seebeck coefficient is, approximately, twice measured in Na-free. On the other hand, thermal conductivity is slightly lower in undoped samples (0.83 W/K m), when compared to the Na-substituted ones (1.10–1.40 W/K m) at room temperature, due to their lower electrical conductivity. Finally, ZT values are higher when the Na-content is increased, reaching 0.022 at around 400 K.