Tuning thermoelectric properties of Bi2Ca2Co2Oy through K doping and laser floating zone processing

In the present study, thermoelectric Bi2Ca2-xKxCo2Oy ceramic materials (x = 0.0, 0.05, 0.075, 0.10, and 0.125) in different forms (called bulk, as-grown and annealed fibers) have been manufactured via a classical solid-state method and textured using the laser floating zone (LFZ) technique. The identification and characteristics of undoped and doped samples were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns of all samples have shown great similarity, and the major peaks can be assigned to the Bi2Sr2Co2Oy thermoelectric phase, independently of the processing technique and K-doping. SEM-EDS have indicated the randomly oriented plate like grains of different sizes in bulk sample, evolving to longer and welloriented grain structure through K-doping and LFZ. Because of the incongruent melting properties of compound, the high number of secondary phases formed in the as-grown samples. In order to reduce it, an annealing and Kdoping process have been applied. The microstructural evolution is reflected on the electrical properties, and the lowest resistivity values are found in the annealed K-doped fibers. Seebeck coefficient is positive in all cases, pointing out to p-type conduction mechanism. These modifications led to PF values up to 0.162 mW/(K2m), obtained in 0.10 K-doped annealed fibers at 650 degrees C.