Investigation of the phase equilibria and phase transformations associated with the Bi2Sr2CaCu2Oy superconductor

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

Radiation tolerance of nanocrystalline ceramics: insights from Yttria Stabilized Zirconia.

Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr(+), 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces

Investigation of the phase equilibria and phase transformations associated with the Bi2Sr2CaCu2Oy superconductor

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.</p

The early stages of microstructural development of the colony structure in Bi-2223 tapes.

The current protocol for processing (Bi,Pb)2Sr2Ca2Cu3010-(B i-2223) multifilamentary tapes involves the in situ formation of the primary phase from a suitable mixture of precursor phases. As such, the developments during the first few minutes of heat treatment determine to a large extent the efficiency of primary phase development, competing secondary phase development, texture evolution, and grain-to-grain connectivity. This work documents the development of the liquid phase, secondary phases, defects which may affect alignment and reaction kinetics, and the precipitation of Bi-2223 from the liquid phase

Phase diagram studies in the SrO-CuO-Ti02 system: applications to YBCO coated conductors.

SrTiO3 (STO) is a potential buffer layer material for use in YBa2Cu3Oy (YBCO) coated conductors based on the IBAD MgO process. However, the interactions with YBCO are not yet fully understood and little information exists in the way of phase diagrams. With this in mind, the tie-line between STO and SrCuO2 in the pseudo-ternary system SrO-CuO-TiO2 was investigated. Phase assemblages and compositions were determined by x-ray diffraction and electron microscopy in the temperature range of 1000oC to 1100oC in oxygen partial pressures of 1%, 10%, and 100%. Preliminary results showed that an appreciable amount of copper substitutes into the STO crystal structure. Conversely, Ti substitution into the SrCuO2 phase was not detected

Engineered Microstructures and Transport Properties in YBCO Coated Conductors

Each process used to deposit or make the bi-axially textured template, buffer layer(s), and the superconductor in a coated conductor creates interfaces along which defects or interfacial reactions may result. These defects can be additive and propagate through the entire film structure to affect the growth and properties of the superconducting film. Defects within the films and their corresponding transport properties have been correlated with the differences in the thickness of the underlying buffer layer material. This knowledge can be used to control and engineer the structure of the coated conductor to maximize critical current densities