Effect of Na-doping on thermoelectric and magnetic performances of textured Bi2Sr2Co2Oy ceramics

Bi2Sr2-xNaxCo2Ox (x = 0.0, 0.025, 0.050, 0.075, 0.100, and 0.125) samples were prepared through the solid-state route and textured using the laser floating zone technique. Microstructural analysis of as-grown samples showed well oriented grains and a relatively high amount of secondary phases due to their incongruent melting. Annealing procedure has drastically decreased the number and amount of secondary phases. Moreover, Na-doping has further decreased the secondary phases content and improved grain alignment. These modifications have been reflected in a large decrease of electrical resistivity with the annealing procedure. The maximum power factor values have been obtained in 0.075 Na-doped annealed samples, 0.20 mW/K2m, which are much higher than the best values obtained in textured materials through hot uniaxial pressing. Magnetic properties were very similar for all samples, with paramagnetic Curie temperature and effective magnetic moment values of −48.6 K and ≈2 μB, respectively.