Increase of electric performances in Bi2Sr2-xRbxCo2O8+δ laser grown ceramics induced by annealing

Abstract

Bi2Sr2-xRbxCo2O8+δ materials with 0 ≤ x ≤ 0.125 were fabricated via the classical solid-state route, and textured using the laser floating zone method. As-grown materials were composed of different secondary phases due to the incongruent melting of this compound. These secondary phases were drastically reduced through an annealing procedure, consisting in heating at 800 °C for 12 h. XRD has not detected any secondary phase on the annealed samples, due to the overlapping of thermoelectric and main secondary phase diffraction peaks. SEM observations have confirmed this microstructural evolution, and shown that Rb-substitution further decreases the secondary phases content and enhances grain orientation. These microstructural features, together with the possible increment of charge carrier concentration due to Rb-doping, decreased electrical resistivity, while Seebeck coefficient has only been slightly reduced. Consequently, the Rb doped samples reached higher power factor (PF) values than the pure ones. The biggest PF values at room temperature (0.11 mW/K2m) are higher than the reported for single crystals (0.07 mW/K2m), while at 650 °C (0.20 mW/K2m), they are also higher than the obtained in textured samples and measured along the conducting plane (0.14 mW/K2m).