Preparation and characterization of Bi-based high – Tc superconductors

Abstract

BSCCO ceramic superconductors prepared by powder in tube, solid state reaction and melt quenching methods have been investigated. The aim is to determine the characteristics optimum doping concentration, quenching temperature and quenching medium for BSCCO ceramic superconductors. Studies were carried out on three systems (System I. Bi 1.7 Pb 0.3 Sr 2 Ca 2-x B x Cu 3 O y , System II. Bi 1.6 Pb 0.4 Sr 2 Ca 2 M 1 Cu 3 O y , System III. Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O y . In system I Ag- sheathed BSCCO tapes were prepared by PIT (powder in tube) technique. Samples were prepared by conventional solid state reaction followed by sintering at 850 o C. In system II, Melt quenching method was adopted to prepare BSCCO samples doped with Samarium and Terbium (M=Sm,Tb). In system III, BSCCO samples were prepared by melt quenching method. The synthesized precursors were quenched from 1200 o C, 1250 o C and 1300 o C to room temperature and sandwiched between brass, stainless steel, copper and bronze blocks. Critical temperatures of the samples were determined by resistivity temperature measurements using four probe technique at liquid nitrogen temperatures. Tc values for the samples were in the range (Tc onset ~ 99K – 104K). XRD analyses were done and lattice parameters of the samples were determined by indexing the diffraction peaks. Porosity of the samples was also measured. Surface of the samples was studied by scanning electron microscope (SEM) micrographs at a magnification of X1000. The SEM results indicate that 2223 phase may have been formed and the long grains of this phase might be obtained by quenching at this temperature. The atomic distribution ratio of each component element on the surface of the sample was investigated by energy dispersive spectroscopy (EDS). Bulk density of the samples was calculated by liquid immersion method. Density was found to be > 90% of the theoretical density of BSCCO glasses. The samples were thermo analytically analyzed by differential thermal analyses (DTA) and thermogravimetric Analysis (TGA). Activation energy, reaction rate constant and frequency factor for crystallization were evaluated employing different models. The sample quenched from 1250 o C on brass had the highest value of Tc (onset), highest volume fraction of 2223 phase, highest value of density and narrowest transition width. Multiphase structure of the samples was observed, major high-Tc 2223 phase coexists with small percentage of 2212 phase. Kinetic studies revealed that the thermal stability (ΔT = T x – T g ) increases for the sample quenched from 1250 o C. According to the results obtained it may be said that the structural, superconducting and thermoanalytical properties were optimum for the sample quenched on brass from 1250 o C.