Friend, Christopher Michael (1994) Transport critical current density measurements on high and low temperature superconductors in magnetic fields up to 15 tesla. Doctoral thesis, Durham University.
A probe for measuring the transport critical current density (J(_c)(B,T)) of superconductors has been constructed, extending the magnetic field and temperature range of previous probes. It can measure critical currents up to 500A, from 2K up to 150K, inside the bore of a 17T Tesla magnet. The temperature is held constant during measurement to: ±50mK at 4.2K and below; ±70mK at 20K; ±100mK at 80K and ±200mK at 150K. The J(_c)(B,T) of a Nb(46.5wt%)Ti multifilamentary wire has been measured in transverse and longitudinal fields. The longitudinal orientation has both a higher J(_c) and upper critical field. It is found that the orientation, field and temperature dependence of J(_c)(B,T) is described by a single functional form. In particular, the orientation dependence determined only by the upper critical field, the anisotropy of which is due to variations in the microstructure or flux cutting events. A Bi(_2)Sr(_2)Ca(_2)Cu(_3)O(_x) tape has been measured in three different orientations. An anomaly in J(_c) is observed, only below 10K, at a field B(_2)(T) for each orientation. Above IT the data are best described by the function J(_c)(B,T)=a(_j)(T)exp. There is a crossover to a stronger field and temperature dependence of J(_c) at temperatures above 55K. Close to the irreversibility line there is a markedly different field dependence of J(_c) depending on whether the field is perpendicular or parallel to the tape surface. A new description has been put forward to explain the data. It is suggested that the intergrain material of the tape is a low Tc BSCCO phase, possibly die 2201 or 2011 phase, of which B,(T) is die upper critical field. Above 10K, in fields less than IT, J(_c) is determined by coupling across S-N-S Josephson junctions at the grain boundaries. Above IT, current mainly flows between grains by connections which allow it to remain in the crystal ab-planes. J(_c)(B,T) is then determined by intrinsic properties and follows the exponential functional form. This form is determined by die average field component parallel to the c-axis. It is proposed that collective pinning theory offers the best framework for describing J(_c), in which case the crossover could be due to a pinning phase transition. The J(_c)(B,T) of some Bi(_2)Sr(_2)Ca(_1)Cu(_2)O(_x), multifilamentary wires is explained using the same description of S-N-S coupling and ab-plane connections as for the 2223-tape. However, whereas the tape was highly aligned throughout, each wire filament has aligned grains in the region of die BSCCO-Ag interface but an untextured core. It is in the aligned regions of each filament that most of die ab-plane connections exist. Increasing the number of filaments therefore improves the wire performance in high fields, though phase purity and density are still important. The 19 and 37 filament wires are promising for high field applications below 20K. The results on the BiSrCaCuO tape and wires demonstrate that, in this material, J(_c)(B,T) can be improved with better texturing and connectivity between grains. To advance the theory a better understanding of die average pinning potentials in BSCCO is required.
|Item Type:||Thesis (Doctoral)|
|Award:||Doctor of Philosophy|
|Copyright:||Copyright of this thesis is held by the author|
|Deposited On:||16 Nov 2012 11:00|