부산대학교 도서관

Summary: Crucial to significant applications of large-scale superconducting electric devices for electric power is the development of wires with high critical current densities Jc at temperatures where cryogenic losses are tolerable. To exploit the fundamental current carrying capability of (Bi,Pb)2Sr2Ca2Cu3O x conductor, the most promising present high-T c superconductor, and to understand its current-limiting mechanisms, we determined 2D maps of Jc distributions over polished cross-sections of Bi-Sr-Ca-Cu-O in slab geometry. We employed quantitative magneto-optical imaging of the magnetic flux distributions, supported by determination of the corresponding current distribution based on inversion of the Biot-Savart law, and then correlated the local Jc values to backscattered scanning electron microscopy images and energy dispersive spectrometry maps of the microstructure. The spatial correlation between the images was established by using registration marks cut by a focused ion beam. In a state-of-the-art overpressure-processed Ag-sheathed (Bi,Pb)2Sr2Ca 2Cu3Ox monocore conductor with self-field bulk transport Jc ∼ 41 kA/cm 2 at 77 K, large variations of local Jc were found, the maximum being as high as 5 times the bulk transport Jc. These observations indicate that current percolates through polycrystalline conductors and that the local fraction of current-carrying cross-section is significantly less than unity. A correlated magneto-optical and high-resolution scanning electron microscopy study showed that the basal-plane terminated grain boundaries were responsible for limiting J c. Finally, magneto-optical current reconstruction was employed to investigate the through-process current-limiting mechanisms. It was found that the main controlling factor of Jc during the early stage of heat treatment was poor connectivity, while J c was controlled by flux pinning of the connected fraction in later stages.