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.

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 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/K2m, obtained in 0.10 K-doped annealed fibers at 650 oC.

This paper reports the microstructure, magneto-resistivity, electrical and superconducting properties of Bi-2212 fibers with Na + ions incorporated into a superconducting matrix prepared by a polymer solution method and additionally textured through the laser floating zone process. XRD patterns showed that Bi-2212 phase is the major one with mostly (00ℓ) diffractions due to the grain alignment, independently of Na content. SEM micrographs showed that samples are composed of well-stacked and oriented grains. The irreversibility field (Hirr), upper critical magnetic field (H c2 ), coherence length (ξ), and activation energies (U) have been calculated using magneto-resistivity measurements and explained based on the thermally activated flux flow (TAFF) model. Considering the resistivity-temperature graph for zero field, T c values tend to increase from 84.8 K (for the pure sample) to 93.2 K (for 0.075Na sample), slightly decreasing for higher content. Besides, transition temperature width (ΔT c = T c onset -T c offset ) decreases with the increment in the Na content and reaches its minimum value (ΔT c =3.7 K) in 0.075Na sample. However, broadening of superconducting transition has been observed with applied field and T c values decreased to 76.1 K for the pure sample and 86.8 K for 0.075Na sample. Likewise, the activation energies of the samples also decreases significantly with the increase of the magnetic field and the activation energies of the Na-containing samples are found to be higher than the pure sample at each magnetic field value. H c2 (0) values are calculated as 33.8, 43.8, 50.1, 33.1, and 21.4 T for 0.0, 0.075, 0.10, 0.20 T Na samples, respectively. As a consequence, referring to all experimental results and theoretical findings, the superconducting characteristics improve regularly with Na-doping until x=0.075 due to increment in the interaction of superconducting clusters, decrement in weak-links and stabilization of charge-carriers in CuO 2 conducting planes.

Bi2Sr2−xCsxCo2Oy materials with 0 ≤ x ≤ 0.15, have been fabricated via the classical ceramic technique. XRD results have indicated that undoped and Cs-substituted samples are composed of Bi2Sr2Co2Oy phase as the major one. Microstructural studies have demonstrated the formation of a liquid phase, which allows a drastic grain growth. This factor is responsible for a drastic improvement of relative density, reaching about 95% of the theoretical one for 0.125 Cs content. On the other hand, electrical resistivity has been reduced up to 14 mΩ cm at 650 °C for 0.125 Cs content, around 40% lower than the obtained in undoped samples. As a consequence, Seebeck coefficient has been decreased due to the raise in charge carrier concentration. The highest power factor at 650 °C (0.21 mW/K2 m) has been found for 0.125 Cs substituted sample, about 40% larger than the obtained in undoped samples, and very similar to the notified in single crystals (0.26 mW/K2 m). Magnetisation with respect to temperature results have demonstrated that measured samples have a paramagnetic property above 50 K, except 0.10 Cs. Magnetic hysteresis curves have shown that the slopes and the magnitudes have increased with decreasing temperature.

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