Precipitation and Pinning in Ca and Sr-Rich High-T_c Superconducting 'Bi_2Sr_2CaCu_2O_8' Ceramics

摘要

From a consideration of the phase equilibrium diagram of the system Bi_2O_3-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_(2+x)Sr_(2-x)CuO_(6+d), Sr_(14)Cu_(24)O_(41-x), and Bi_2Sr_3O_6 in high-temperature superconducting Sr-rich "Bi_2Sr_2CaCu_2O_8" ceramics and Ca_2CuO_3 and a liquid in Ca-rich "Bi_2Sr_2CaCu_2O_8" ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.