Fabrication of magnetically doped Bİ-2212 fibers by laser floating zone (LFZ) method, investigation of grain alignment and superconducting properties

In this thesis laser floating zone method, where its efficiency has already been proved in fabricating materials with highly aligned grains, has been applied for the fabrication of superconducting fibers (Diameter:~1.8 mm, Length:~12 cm) having Bi2Sr2CaCu2-xFexO8+? and Bi2Sr2CaCu2-xNixO8+? (x=0, 0.01, 0.03, 0.05 and 0.1) compositions at two different growth rates as 15 mm/h and 30 mm/h. The determination of phase formation and distribution in fabricated fibers has been performed through XRD and SEM/EDX investigations. Alignment degree of the structure has been determined via pole figures. Critical values of the superconductivity have been calculated from electrical (MR-T) and magnetic (M-T, M-H) measurements and effects of doping and different growth rates on superconductivity of the materials prepared have been investigated. Magnetic measurements have been done by applying magnetic field from two different directions with respect to the growing direction of the fibers and the anisotropy obtained has been correlated with the alignment degree of the structures. Furthermore, thermal properties of the fibers fabricated have been investigated by means of thermal conductivity (?) and thermopower (Seebeck cf., ?) measurements. It has been found that among two different growth rates applied the fibers processed at 15 mm/h have more homogeneous phase distribution, higher alignment and, in consequence, better superconducting properties. The highest critical current density (Jc.mag) have been obtained on the samples prepared with 15 mm/h rate (Jc.mag=5.57x105 A/cm2). However this value decreased down to 3.02x105 A/cm2 for the samples prepared at 30 mm/h growth rate. It has been observed that doping of Fe and Ni atoms have different distribution and reaction behaviour on the crystal structure. Thus, different effects on texturing level of the materials were obtained. However, it has been seen that the negative effects of doping on the grain size and phase coordination cause a degradation for critical superconductivity values. Finally, change on the thermal conductivity curves, caused by doping, has been discussed and correlated with the variation in the crystal structure. KEYWORDS: Laser floating zone (LFZ) method, BSCCO superconductor, Bi-2212 fiber, doping, effect of growing rate, grain alignment.