Defects in Decomposed YBa 2 Cu 4 O x (124) Superconductor after Rapid Annealing
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DEFECTS IN DECOMPOSED YBa 2Cu4Ox (124) SUPERCONDUCTOR AFTER RAPID ANNEALING Y. Li*, Y. Gao*, K. L. Merkle*, H. Shi*, and U. Balachandran** * Materials Science Division and Science and Technology Center for Superconductivity ** Materials and Components Technology Division Argonne National Laboratory, Argonne, IL 60439
ABSTRACT Decomposition of 124 into 123 has been studied after rapid annealing at temperatures from 800 0C to 1000 OC. It was found that the superconducting transition temperature (Tc) depended on the annealing temperature and atmosphere. For decomposed samples, fine-scale defects with strong strain contrast are observed in the 123 matrix. High-resolution electron microscopy studies show that the defects are parallel to the (001) planes of the 123 matrix. The length of the defects varies and ranges from 5 nm to 50 nm in the direction parallel to the (001) planes. The defects have been interpreted to be copper oxides, which could be flux pinning centers in these materials.
INTRODUCTION One of the main limitations for the application of the YBa2Cu3Ox (123) superconductor, especially in the form of bulk materials, is its low critical current density. Two areas need attention regarding improvements in the low critical current density (Jc): weak links and flux pinning. Concerning the first aspect, the intergranular Jc is low because of weak link behavior at grain boundaries, as demonstrated by Dimos et al. [1, 2]. Electron microscopic studies [3] indicate that the cause of the weak link is likely to be non-superconducting material at grain boundary dislocation cores. Regarding the second aspect, the low intragranular Jc is due to insufficient flux pinning within each grain to prevent flux creep. It has been demonstrated that the intergranular Jc can be improved through grain texturing [4-6]. However, the high-field Jc appears at present to be limited to values on the order of 104 /cm 2 at 77 K in the textured bulk materials [4], while transport Jc values of the order of 5x10 6 A/cm 2 have been obtained in c-axis oriented thin films [5, 6]. Electron microscopy studies [7] indicate that a number of structural defects exist in the thin films. Since the pinning centers are regions of inhomogeneities in the lattice, it has been suggested that these structural defects perform as the pinning centers to cause the higher intragranular Jc. These results, therefore, indicate that in order to obtain the higher transport Jc in bulk materials, it is essential to find processing procedures by which one can improve the grain alignment and also create small structural defects throughout the materials. Although such methods are not available yet, several methods for individual improvement have been developed, such as sinter-forging for grain alignment, and neutron irradiation [8] and solid-state phase transformation [9] for flux-pinning enhancement. The solid-state phase transformation method [9] makes use of the decomposition of the YBa 2 Cu4 O 8 phase (124) to the YB a2Cu30x phase (123) during rapid annealing. This method
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