Investigation of the phase equilibria and phase transformations associated with the Bi2Sr2CaCu2Oy superconductor
- Iowa State Univ., Ames, IA (United States)
The solid solution region and reaction kinetics of the Bi2Sr2CaCu2Oy (2212) superconductor were examined as a function of temperature and oxygen partial pressure. Crystallization studies from the glassy and molten states were undertaken to determine the phase transformation and kinetics associated with the formation of 2212 and other competing phases. Crystallization of nominal 2212 glasses was found to proceed in two steps with the formation of Bi2Sr2-xCaxCuOy (2201) and Cu2O followed by Bi2Sr3-xCaxOy, CaO, and SrO. The 2212 phase converts from the 2201 phase with increasing temperatures. However, its formation below 800 C was kinetically limited. At 800 C and above, a nearly full conversion to the 2212 phase was achieved after only one minute although considerably longer anneal times were necessary for the system to reach equilibrium. In low oxygen partial pressures, the solidus is reduced to approximately 750 C. Solidification studies revealed an eutectic structure separating the incongruently melting 2212/2201 phases at high oxygen partial pressures from the congruently melting Bi2Sr3-xCaxOy (23x) and Bi2Sr2-xCaxOy (22x) phases present at low oxygen partial pressures. During solidification in various oxygen partial pressures, the separation of CaO in the melt and the initial crystallization of alkaline-earth cuprates leaves behind a Bi-rich liquid from which it is impossible to form single-phase 2212. Hence, significant amounts of 2201 were also present in these samples. These problems could be reduced by melt processing in inert atmospheres. Bulk 2212 material produced in this manner was found to possess high transition temperatures, high intergranular critical current densities below 20K, and modest critical current densities at 77K.
- Research Organization:
- Ames Lab., Ames, IA (United States)
- Sponsoring Organization:
- USDOE; National Science Foundation (NSF)
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 10112822
- Report Number(s):
- IS-T--1631; ON: DE94004889
- Country of Publication:
- United States
- Language:
- English
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