Effects of K substitution on thermoelectric and magnetic properties of Bi₂Sr₂Co₂O_y ceramic

In this work, Bi₂Sr_2−xK_xCo₂O_x thermoelectric materials with x = 0.0, 0.025, 0.050, 0.075, 0.10, 0.125, and 0.15 have been produced through the classical solid-state reaction. SEM characterization has shown an improvement of microstructure and a porosity decrease with K substitution, leading to very high densities, confirmed by apparent density measurements. These modifications have been reflected in a drastic decrease of electrical resistivity with slight modification of Seebeck coefficient. As a consequence, power factor has been increased in all the K doped samples, reaching the maximum values (0.192 mW/K² m at 650 °C) in 0.075 K containing samples, which is fairly close to the reported values in single crystals. Magnetic results have shown that all samples are paramagnetic above 50 K and below they undergo to an antiferromagnetic phase transition. From temperature dependence of magnetic hysteresis, it has been observed that the slope of curves and the magnitude of magnetization have been increased when the temperature is decreased.