Bi-based high Tc superconductors and enhancement of critical current density

Sammanfattning: Basic bulk high temperature superconductors in the Bi-Sr-Ca-Cu-O system and the BiPb-Sr-Ca-Cu-O system were prepared through the solid state reaction route and the coprecipitation method. The formation of 110 K phase promoted by partial Pb substitution for Bi was studied. Effects of heat treatment on 110K phase formation were studied. Bulk materials with the zero resistance temperature of 103K was achieved. Glass phase within grain boundary and dislocations were observed by TEM. Single crystal of Bi2.2Sr1.8Ca0.75Cu1.8Ox was grown with step slow cooling and using a sealed cavity technique. The feature of single crystal was studied. The reactivity of the Bi-Pb-Sr-Ca-Cu-O system with oxides such as Al2O3, SiO2, ZrO2 and MgO was investigated. The sequence of the reactivity of these oxides with BPS CCO superconductors has been found to be SiO2 > ZrO2 > Al2O3 > MgO. For enhancement of critical current density Jc a chemical doping of Ca2CuO3 was developed. HIPing technique was attempted to densify Bi-based bulk materials. The grain alignment for increasing Jc for bulk materials was achieved through the "powder in tube" method and "dipping-rolling method". The Jc was significantly enhanced up to 6500 A/cm2. The "dipping-rolling method" is promising for making long tapes. The magnetic field dependence of Jc for Bi-based bulk materials was characterized in terms of three regimes: Josephson weak-link regime, remnant percolation-path regime and flux-flow regime. Flux line shear mechanism was demonstrated to be a main pinning mechanism for the bulk Bi-based material.

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