Grain connectivity and flux pinning in Bi-2223/Ag PIT tapes

Abstract

A hot-pressing technique was used to prepare Ag-sheathed Bi-2223 multifilamentary tapes in order to study its effect on grain connectivity and flux pinning. The self field critical current density ( J c) after hot-pressing was significantly enhanced, the maximum increase reached more than double than before. Hot pressing raised the J c ( B=0) from 22 000 A/cm 2 ( I c=18 A) to 56 800 A/cm 2 ( I c=36.5 A) for 81-filament tape and from 26 000 A/cm 2 ( I c=36.2 A) to 56 000 A/cm 2 ( I c=56.6 A) for 19-filament tape. But, the J c in applied magnetic field was not improved or degraded by hot-pressing. The J c retention in magnetic field of 1 T decreased from 33% for the original tape to 21% for the hot-pressed 81-filamentary tapes and remained the same (24%) for 19-filamentary tapes before and after hot-pressing. The effect of cooling rate on connectivity and flux pinning in hot-pressed Bi-2223 PIT tape has been investigated. The fraction of strong links ( I C0 s/ I C0) was improved from 64% to 73% for 81-filament tape due to hot pressing and slow cooling. However it was reduced to 42% for hot-pressed and normally cooled tape. Microstructural analysis showed that hot-pressing chiefly improved grain connectivity, increased core density, improved crystal alignment, recovered micro-cracks and reduced secondary phase impurities. Hot-pressing technique reduced some defects, but at the same time, it created different type of defects, so the whole amount of defects was not much changed. Therefore, hot pressing caused different influence on flux pinning for different original tapes. These results suggest that limitation of J c in Bi-2223 tapes is multiple and potential for further improvement of J c is great.