High-Temperature Superconductors from the Grain Boundary Perspective
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High-Temperature Superconductors from the Grain Boundary Perspective Susan E. Babcock The high-transition-temperature superconductors (HTS) present a number of challenging materials science problems whose solutions must precede their use in applications. This article offers a view on our collective progress toward resolution of one central HTS issue, the "weak-link" character of high-angle grain boundaries. Why Are HTS Grain Boundaries of Such Interest? The properties of high-angle grain boundaries control the macroscopic electromagnetic character of all superconducting copper oxides that are produced in polycrystalline form. In particular, grain boundaries limit the transport critical current density (Jrt) and determine its dependence on applied magnetic field (H) and temperature (T). Jd is the maximum macroscopic current density that the superconductor can support in the nondissipative state. H and T are the two most important environmental variables for applications of superconductivity. Thus, the Jct(HT) characteristic largely dictates the types of applications for which a superconducting material can be used successfully. High-angle grain boundaries are the immediate obstacle to further development of materials for applications that require large J^ values in high magnetic fields. The problem arises because most high-angle grain boundaries act like Josephsoncoupled weak links.1 The characteristic properties of such junctions are a reduced zero-field J^ value and, more importantly, a strongly magnetic-field-dependent Jcl that can decrease by more than an order
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Figure 1. Jct(ty characteristics of a 22° [001] Josephson junction-coupled bicrystal. The critical current is normalized to the zero applied field value. Note the rapid decrease in Jcf(H) for weak fields of less than 50 Gauss. Subsidiary peaks are characteristic of Josephson coupling. (These results are described in more detail in References 9 and 11.)
Why is Grain Boundary Research in HTS So Challenging? From both the high-field, flux-pinning viewpoint and the low-field, Josephson junction viewpoint, there is strong motivation to develop detailed models of the grain boundary (micro)structure and its effects on grain boundary electromagnetic properties. (Josephson junction characteristics imply the existence of a thin, wetting layer of insulating, normal-metal, or weakly-superconducting material that hinders the supercurrent.) Why, then, do we not have a more thorough understanding of (micro)structure-property relationships for these important defects? A major reason lies in the spatial resolution with which the (micro)structure must
MRS BULLETIN/AUGUST 1992
High Tc Superconductors from the Grain Boundary Perspective
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t 3 6 2
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Applied Field, Tesla Figure 2. I,,(H) curves for flux-grown YBajCujOj-s bicrystals that show flux-pinning characteristics. Complementary /,,(H) curve from a flux-grown single crystal is shown for comparison. Note the the relativel
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