SUPERCONDUCTING BI1.5PB0.5SR2CA2CU3OX CERAMICS BY RAPID MELT QUENCHING AND GLASS CRYSTALLIZATION

A glass of nominal Bi1.5Pb0.5Sr2Ca 2Cu3Ox composition, prepared by rapid quenching of the melt, showed a glass transition temperature of 383°C, crystallization temperature of 446°C, melting temperature of 855°C, and bulk density of 5.69 g/cm3 in air. The activation energy for crystallization of the glass has been estimated to be 292 kJ/mol from nonisothermal differential scanning calorimetry. On heating in oxygen, the glass showed a slow and continuous weight gain starting at 530°C which reached a plateau at 820°C. The weight gained during heating was retained on cooling to ambient conditions indicating an irreversible oxidation step. The influence of annealing conditions on the formation of various phases in the glass has been investigated. The Bi2Sr2Ca0Cu 1O6 phase crystallized out first followed by formation of other phases at higher temperatures. The high-Tc phase, isostructural with Bi2Sr2Ca2Cu3O1 0, was not detected below 840°C, but its fraction increased with the annealing time at 840°C. A sample annealed at 840°C for 243 h in air and furnace cooled showed the highest Tc(R=0) of 107.2 K and a narrow transition width, ΔTc (10%-90%), of 2 K. The high-T c phase does not seem to crystallize out directly from the glass but is rather produced at high temperature by reaction between the phases formed at lower temperatures. The kinetics of 110-K phase formation was sluggish. It appears that the presence of lead helps in the formation and/or stabilization of the 110-K phase.