Investigation of the phase equilibria and phase transformations associated with the Bi2Sr2CaCu2Oy superconductor
Scott L. Chumbley
Compositional measurements of the Bi[subscript]2Sr[subscript]2CaCu[subscript]2O[subscript]y (2212) solid solution region were performed in the TEM in order 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. Based on these results, two substitution schemes were determined to occur. The first is just the interchange between Sr and Ca. The second involves 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 phase was found to be nearly constant. The 2212 phase was found with 12 other phases in this work, most of which were also solid solutions. Hence 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 (apparent composition), and/or (3) the solid solution composition of the 2212 phase (actual composition);Crystallization of nominal 2212 glasses was found to proceed in two steps with the formation of Bi[subscript]2Sr[subscript]2-xCa[subscript]xCuO[subscript]y (2201) and Cu[subscript]2O followed by Bi[subscript]2Sr[subscript]3-xCa[subscript]xO[subscript]y, CaO, and SrO. The 2212 phase converts from the 2201 phase with increasing temperatures and was kinetically limited by diffusion below 800°C. 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;From the results of the solidification study, an eutectic was determined to separate the 2212/2201 phases that are stable at high oxygen partial pressures from the Bi[subscript]2Sr[subscript]3-xCa[subscript]xO[subscript]y (23x) and Bi[subscript]2Sr[subscript]2-xCa[subscript]xO[subscript]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. These problems were minimized by developing a melt processing technique using a reducing atmosphere.