Magnetic Shielding by Superconducting Y-Ba-Cu-O Prepared by the Modified Quench and Melt Growth (QMG) Process

Magnetic shielding capability has been investigated on Y-Ba-Cu-O crystals prepared by the modified quench and melt growth (QMG) process at 77 K and 4.2 K. Overall critical current densities in large single-grained specimens were estimated to be about 104 A/cm2 based on the Bean model in order to predict shield effect. At 77 K, a single grain cylinder shields the inside up to about 0.2 T of the external field, and this value is the highest among all the values reported so far for tubes fabricated of oxide superconductors. A comparison between shielding characteristics of a single grain and those of a polygrain demonstrated that weaklinking high-angle grain boundaries should be eliminated for high field shielding. Lorentz force, flux creep, and flux jump are discussed.