Grain Boundaries and Jc in High-Tc Oxide Superconductors

Abstract

In metallic superconductors, grain boundaries are effective pinners of magnetic flux and, as such, are partially responsible for high critical current densities achievable in those materials. However, in the highcritical-temperature oxide superconductors, grain boundaries have been identified as the source of problems leading to the low critical current densities exhibited by bulksintered material. The very short superconducting coherence lengths of the oxides make their electrical and magnetic properties exceedingly sensitive to microstructural inhomogeneities, thus placing stringent requirements on the quality of grain boundaries. Recent studies of grain boundary chemistry and its effects on critical current density have shed some light on the problem.10 Jlm Figure 1. A scanning electron micrograph of a fracture surface of YBa2Cup7~ fractured in the Auger system. The platelet grain mOrphology is evident. Transgranular and intergranular areas can be identified on the basis of morphology. For example, the analysis areas marked 1 and 2 are grain boundary surfaces.If you want more Information on this subject, please circle reader service card number 52. LL INTRODUCTIONFrom the beginning, the excitement generated by the discovery of superconductivity at temperatures higher than 77K has been tempered by the realization that, for bulk applications, improvement in one important materials parameter has been obtained at the expense of another. The long-studied, low-transitiontemperature metallic superconductors such as NbTi and Nb Sn readily support high current densities (l05-106 Ncm 2 ) in high magnetic fields (lOT or more), while the new oxide superconductors, with transition temperatures in the range of92 to 124K, typically have critical current densities three to five orders of magnitude lower when prepared in bulk polycrystalline forms. Development of high critical current density (high-J c ) material prepared by powder processing routes would be a major step toward utilizing high-critical-temperature (high-Tc) oxide superconductors in energy-related applications such as power generation, motors, energy storage and transmission.Early success! in obtaining high current density in thin films ofYBa2Ct1a07~ grown epitaxially on SrTiO a single crystal substrates indicated that critical currents are not intrinsically low in that material, and focused attention on grain boundaries and the anisotropy of J c (with respect to both the crystal lattice and an applied magnetic field) as probable causes ofthe poor current densities obtained in sintered material. The highly anisotropic crystal structures of all of the presently known high-Tc oxides lead to strongly anisotropic electron transport properties in both the normal and superconducting states. For example, in or-

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

“…The phenomenon of reduced Jc due to poor electrical connectivity across GBs is called weak-link behavior. Possible sources for producing the weak-link effect include intrinsic anisotropy of the superconducting properties, the presence of structural disorder at grain boundaries [13,14], microcracking due to thermal and transformational stresses [19,20], and the presence of nonstoichiometric material at GBs [21][22][23][24].…”

“…Auger spectroscopy [21,23,24] and analytical electron microscopy [19,22,[25][26][27] studies have shown the presence of grain boundary films and variations in copper and oxygen stoichiometry. More recently, TEM and parallel-detection electron energy-loss spectroscopy (EELS) [28] have been applied to characterize the correlation between the crystallographic misorientations of the Y123 grain boundaries and oxygen deficiency at the boundaries.…”