Effect of Magnetic Field on Critical Current Density in Bulk Superconducting Wires
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EFFECT OF MAGNETIC FIELD ON CRITICAL CURRENT DENSITY IN BULK SUPERCONDUCTING WIRES M. T. Lanagan, U. Balachandran, C. A. Youngdahl, J. T. Dusek, J. J. Picciolo, and R. B. Poeppel, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT Bulk YBa2Cu 3 O7-x (YBCO) wires and tubes were fabricated by an extrusion technique. Critical current density (Jc) was measured as a function of applied magnetic field at 77 K and was found to decrease significantly in fields below 100 G. Jc was dependent on specimen geometry. In addition, when a concentric magnetic field was generated by passing a current though a copper wire, the external field from the wire could interfere constructively or destructively with the magnetic field produced by current in a YBCO tube. The change in electrical properties with magnetic field has been attributed to weak-link behavior at the grain boundaries. Batch-to-batch differences in the field dependence of Jc imply the possibility of reducing the dependence by processing modifications. INTRODUCTION It is generally accepted that transport critical current density is limited by weak links in granular high-Tc superconductors. Magnetic field penetrates the boundaries and thus forms Josephson junctions between the grains. The magnetic field dependence of transport critical current density has been modeled by a network of Josephson junctions at the grain boundaries. 1 The results predict that current is limited by intergranular links and not by intragranular defects such as twins. Broadening of the resistive transition as a function of current density is 2 experimental evidence of weak-link behavior. Chemical inhomogeneity and microcracking at grain boundaries are
potential barriers to current flow. Grain boundary composition has been studied by energy-dispersive x-ray analysis carried out in a scanning transmission
electron microscope. 3 A periodic change in stoichiometry was observed along the grain boundary. On sections of the grain boundary, areas of copper excess and oxygen depletion were observed. The large anisotropy in thermal expansion and the strain associated with the tetragonal to orthorhombic transition result in microcracks between grains. It was reported that microcracking can4 be suppressed if grain size is maintained below 1 gim during sintering. PROCEDURE Wires and tubes of YBCO were formed by plastic extrusion. The YBCO powder was combined with a set of organics and mixed with a sigma-blade blender for homogeneity. The extrusion process consists of placing high pressure (approximately 20 MPa) on the plastic mass and forcing it through a small aperture. The heat treatment schedule consisted of three basic sections. Initially, temperature was ramped slowly to 350'C to remove organics present in the green wire. Sintering was carried out between 900 and 975°C in flowing oxygen, and the specimens were annealed at 4501C for 10 h. Details of the extrusion process may 5 be found elsewhere.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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