Improvement of flux pinning by thermo-mechanical treatment of Bi-2223/Ag superconducting tapes

Measurements of the transport critical current in a magnetic field (H), hysteresis loops and a.c. susceptibility were performed on Bi-2223/Ag tapes that were subjected to different degrees of mechanical deformation and subsequently sintered. against H for some of the tapes was also measured at different stages of thermo-mechanical treatment. All the measurements (the field of the peak of the pinning force, the irreversibility field obtained from hysteresis loops and the temperature of the peak of a.c. susceptibility for different d.c. fields) clearly show an increase of pinning with increase of the degree of mechanical deformation. Measurements of samples at different stages of thermo-mechanical treatment indicate that the pinning depends not only on the degree of mechanical deformation, but also on the conditions of the grain growth. Sintering can provide imperfect healing of micro- and nano-cracks, introduced upon mechanical deformation. It is proposed that the imperfections introduced by the healing are responsible for the improvement in flux pinning, and not the dislocations introduced upon mechanical deformation.