The Critical Current Density in High Critical Temperature Superconductors
Abstract
Critical current density, j_{ rm c}, is an important parameter for determining the usefulness. This work focuses on the understanding of various phenomena related to the j_{ rm c} in type II superconductor. Various methods to enhance j_{rm c} by introducing pinning are also considered. In particular, the effect of secondary phase addition and mechanical treatment on the microstructure and j _{rm c} of various high T _{rm c} superconducting system is investigated. Fine inclusions (<0.1 μm) can be introduced by secondary phase additions. An enhancement in j_{ rm c} is always observed associated with the presence of fine inclusions. These cavities are found to interact strongly with flux lines in a high T _{rm c} superconductor. However, the cavities are found ineffective to pin a large number of flux lines. Dislocations and other structural defects are introduced by consolidating Bi_2Sr _2CaCu_2O _{rm x} by hot isotatically pressing (HIP). Samples HIPed for 15 min. contained a high density of dislocations and showed a substantial higher j_{rm c} than the samples HIPed for 45 min. and 120 min., where most of the dislocations were annihilated during the recovery process. Various methods of determining the irreversibility line are also considered. Using the criterion of a constant j_{rm c}, the irreversibility line obtained from magnetic hysteresis measurements was found to improve with the enhancement of flux pinning and reduction of interlayer spacing. The results can be best explained by the model proposed by Kim et al (1) and Clem (2). Magnetic relaxation of various type II superconductors is also reported. The non-logarithmic of decay of magnetization can be understood by assuming a non-linear U-j relationship. A method to extract U-j relationship from magnetic relaxation experiments is also developed. The effect of flux pinning on the U-j relationship is also investigated. Melt-processed YBa_2Cu _3O_{rm x} samples with strong levitation force are also fabricated. The levitation force was found to improve with the use of a seed crystal, an increase in domain size, and with enhanced flux pinning.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......259S
- Keywords:
-
- CURRENT DENSITY;
- Engineering: Materials Science; Physics: Condensed Matter