Mechanism of Bi2Sr2CaCu2Ox crystallization and superconducting properties for Bi2Sr2CaCu2Ox/Ag tapes prepared using isothermal partial melting method

The mechanism of Bi2Sr2CaCu2Ox (Bi-2212) crystallization from a partially molten state has been studied by means of thermal analysis, X-ray diffraction (XRD) measurement, and microstructural observation in Bi-2212/Ag tapes. Tapes have been prepared using the isothermal partial melting (IPM) method, under which partial melting and solidification are carried out at the same temperature in an nitrogen atmosphere and 20% oxygen partial pressure( p(O-2)), respectively. The Bi-2212 phase crystallizes by peritectic reaction between (Sr, Ca)CuO2 (1:1 phase) and the liquid phase at processing temperatures (T-p) greater than or equal to 855 degrees C. At the beginning of Bi-2212 crystallization, only the 1:1 and the liquid phases are present in tapes. The Bi-2212 nucleation occurs in the vicinity of 1:1 grains (homogeneous nucleation). This peritectic reaction progresses by recovery of oxygen in the liquid phase. In contrast, the nucleation of the Bi-2212 phase occurs heterogeneously at Tp less than or equal to 850 degrees C. The Bi-2212 phase co-exists with four Bi-free phases ((Sr, Ca)(14)Cu24O41 (14:24 phase), (Sr, Ca)(2)CuO3 (2:1 phase), (Sr, Ca)O (1:0 phase), and the 1:1 phase) and the liquid phase in the early stage of the Bi-2212 crystallization step. Since the precipitation temperature of the Bi-2212 phase is higher than 850 degrees C, the Bi-2212 crystallization progresses by direct precipitation. The crystallization paths affect microstructure and transport critical current density (J(c)) of Bi-2212 tapes. (C) 1999 Elsevier Science B.V. All rights reserved.