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

Abstract

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 Kdoping. SEM-EDS have indicated the randomly oriented plate like grains of different sizes in bulk sample, evolving to longer and well-oriented 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 K-doping 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/K 2 m, obtained in 0.10 K-doped annealed fibers at 650 o C.

Detailed low temperature studies on thermoelectric performance of K-doped Bi2Ca2Co2Oy ceramics fibers

“…It is already reported that the LFZ system improves the grain orientation in the materials causing an improvement of TE-parameters [16,24]. Lotgering factor (LF) is given with ( ) ( ) = --LF P P P P 0 0 formula, in which P is the sum of the peak intensities of preferred orientation, and P 0 is the sum of the peaks related to the randomly oriented grains [25,26].…”

“…In previous works, Bi 2 Ca 2-x K x Co 2 O y with x=0, 0.05, 0.075, 0.10, and 0.125 thermoelectric materials were fabricated via the classical solid-state route, followed by laser floating zone (LFZ) texturing, then their high temperature thermoelectric properties were, in detail, investigated [16]. In this study, the results related to the microstructure, resistivity, Seebeck coefficient, thermal conductivity, and figure of merit, at low temperatures, will be presented, in order to evaluate the low temperature performances of these thermoelectric ceramics.…”