Formation mechanism of Bi-based high-Tc superconductors in the melt quenching method

The authors prepared superconducting Bi0.8Pb0.2SrCaCuxOy (x=1.5, 1.8 and 2.0) ceramics with Tc above 100K by the melt quenching method and examined the effects of the materials (alumina and platinum) of crucible, the concentration of Cu element and the annealing condition on the superconducting properties. They also examined the growth mechanism of superconducting crystals in the melt quenching method. It was found that an excess amount of Cu element, which was estimated as a deviation from the nominal composition of the high-Tc phase, caused a degradation of superconducting properties and both the formation of the low-Tc and high-Tc phases were enhanced under the presence of partially melted liquid phases. The annealed (840C, 250h) sample of Bi0.8Pb0.2SrCaCu1.5Oy exhibited superconductivity with a Tc(zero) of 106K and a critical current density (77K, zero magnetic field) of 250A/cm2 (the melt quenched sample was prepared using a platinum crusible). It was also found that the addition of a small amount of antimony was very effective in increasing the volume fraction of high-Tc phase