Investigation of the phase equilibria and phase transformations associated with the Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub y] superconductor
Compositional measurements of the Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub y] (2212) solid solution region were performed in the TEM to separate intergrowth and solid solubility effects. Most of the solid solubility is accounted for by changes in the Sr and Ca concentrations. Changes in the Bi concentration account for the rest. Two substitution schemes were determined: the interchange between Sr and Ca and the substitution of Bi for Sr or Ca. It appears to be unfavorable for Sr or Ca to substitute for Bi. The Cu content of the 2212 phases was nearly constant. The 2212 phase was found with 12 other phases, most of which were also solid solutions. Variations in the overall cation stoichiometry and oxygen partial pressure are accommodated by changes in (1) the amount, types, and composition of the secondary phases, (2) the number of intergrowths within the 2212 phase, and/or (3) the solid solution composition of the 2212 phase. Crystallization of nominal 2212 glasses proceeded in two steps with the formation of Bi[sub 2]Sr[sub 2[minus]x]Ca[sub x]CuO[sub y] (2201) and Cu[sub 2]O followed by Bi[sub 2]Sr[sub 3[minus]x]Ca[sub x]O[sub y], CaO, and SrO. The 2212 phase converts from the 2201 phase with increasing temperatures and was kinetically limited by diffusion below 800[degrees]C. At 800[degrees]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. An eutectic was determined to separate the 2212/2201 phases that are stable at high oxygen partial pressures from the Bi[sub 2]Sr[sub 3[minus]x]Ca[sub x]O[sub y] (23x) and Bi[sub 2]Sr[sub 2[minus]x]Ca[sub x]O[sub y] (22x) phases present at low oxygen partial pressures. At high oxygen partial pressures, it was found that the separation of CaO in the melt and the initial crystallization of alkaline-earth cuprates resulted in a Bi-rich liquid from which it was impossible to form single-phase 2212.
- Research Organization:
- Iowa State Univ. of Science and Technology, Ames, IA (United States)
- OSTI ID:
- 7166732
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360202* -- Ceramics
Cermets
& Refractories-- Structure & Phase Studies
ALKALINE EARTH METAL COMPOUNDS
ALLOYS
BISMUTH COMPOUNDS
BISMUTH OXIDES
CALCIUM COMPOUNDS
CALCIUM OXIDES
CHALCOGENIDES
CHEMICAL COMPOSITION
COPPER COMPOUNDS
CRYSTALLIZATION
CUPRATES
DISPERSIONS
EQUILIBRIUM
EUTECTICS
HIGH-TC SUPERCONDUCTORS
INTERMETALLIC COMPOUNDS
MIXTURES
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
SOLID SOLUTIONS
SOLUTIONS
STRONTIUM COMPOUNDS
STRONTIUM OXIDES
SUPERCONDUCTORS
TRANSITION ELEMENT COMPOUNDS