Critical Current Density, Flux Creep, and Microstructure in A Bi-Sr-Ca-Cu-O System
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CRITICAL CURRENT DENSITY, FLUX CREEP, AND MICROSTRUCTURE IN A Bi-Sr-Ca-Cu-O SYSTEM*
Donglu Shi, M. S. Boley, J. G. Chert, Ming Xu, M. M. Fang, U. Weip, K. Vandervoort, and J. M. Hong Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 *Work supported by the U. S. Department of Energy, BES-Materials Science, under Contract No. W-31-109-ENG-38.
Abstrac
We measured magnetization hysteresis and zero-field-cooled magnetic relaxation for liquid-quenched Bi-Sr-Ca-Cu-O samples at various temperatures. We found that there is a close correlation between the magnetic behavior and the microstructures of the samples. The magnetization hysteresis greatly increases in the samples with a large amount of crystal defects such as precipitates and stacking faults in the matrix of the material. We also found that the flux creep is reduced as a result of these microstructural changes.
Intro
~ucto
High-To superconductivity may have great potential in practical applications. Therefore, many research efforts are concentrated in areas that deal with the actual application requirements for these new materials. Critical current density, Jc, is one of the most important requirements in large-scale applications of superconductivity. However, as reported extensively, not all high-Tc superconductors in conventionally sintered polycrystalline form carry high transport current (1,2). Therefore, it becomes essential to study the critical current-related problems such as weak links, flux pinning, and flux creep in high-Tc ceramics. Although transport Jc is quite limited in high-Tc materials, they exhibit high intragrain critical current density, especially in YBa2Cu3 07-x (3). In contrast, relatively low intragrain Jc has been observed in Bi-Sr-Ca-Cu-O system (4). It has been suggested that the low critical current density results from the fact that the system lacks pinning centers. In this report, we show that the intragrain current density is greatly 5 2 increased, from approximately 10 A/cm to 10 A/cm 2 at 5 T and at 10 K, by creating a large quantity of crystal defects in the sample matrix. We also report that the flux creep is reduced by the microstructural changes.
Experimental
Detalil
Starting materials of Bi203, SrCO3, CaCO3, and CuO were weighed in the atomic proportions of 2:2:2:3 and 2:2:4:5. The weighed powders were thoroughly mixed by a ball mill in ani agate container for 2-3 h. After ball
Mat. Res. Soc. Symp. Proc. Vol. 169. @1990Materials Research Society
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milling, the mixed powders were dried and calcined in air at 800'C for 20 h. The calcined powders were melted in a platinum crucible and splat quenched between two copper blocks preheated at 200°C. Extremely dense glass sample plates with dimensions of 50 x 40 x 0.5 mm 3 were formed for subsequent crystallization annealing. In this study, the 2:2:2:3 (2223) and 2:2:4:5 (2245) samples were annealed in air at 870°C for various times (up to one day) and slowly cooled to room temperature. Detailed information about the sample preparation can be fou
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