S.M. Cassidy

Pinning mechanisms in Ag-sheathed Bi(Pb)SrCaCuO tapes

Abstract The pinning properties of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+y (2223) wires have been studied in comparison with the normal powder, and the powder scraped from the 2223 tape by the measurements of irreversibility lines (IL). The IL for both the tape and the powder scraped from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder. This indicates that the enhanced IL is attributable to the defect pinning. TEM studies reveal that the grains within Ag-clad 2223 tape contain high concentrations of dislocations (1010/cm2) and stacking faults that may act as effective pinning sites. The results on the transport and magnetisation Jc suggest that the critical current is controlled by flux pinning at high temperatures, while the weak links limit the critical current at low temperatures.

‘Strong’ grain boundaries in Ag-BSCCO(2223) tapes

Abstract The remarkable improvement of the superconducting properties of polycrystalline BSCCO(2223) when processed into Ag-clad tapes has been studied magnetically. Two factors contribute to the excellent tape performance: Over the investigated range of field and temperature, the grain boundaries show no evidence of being ‘weak-links’. Also, intra granular flux pinning is much stronger than might be expected from single crystal data, and appears to determine the tape critical current.

Self-similarity in the I-V characteristics of granular YBa2Cu3O7

The onset of dissipation in the I-V characteristics of granular YBa2Cu3O7 shows clear evidence for a finite critical current. At currents above critical, dissipation in the weak links is only slightly field- and temperature-dependent.

Dissipation in granular high-temperature superconductors

The current-voltage characteristics of bulk-sintered YBa2Cu3O7 have been studied in detail. The onset of dissipation indicates that an intergranular critical current can be defined. The magnitudes of the resistance of a single grain boundary and of its critical current are estimated. These estimates lead to values of the rnic(0) product for the intergranular weak-links that are of the order of 100 μV, well within the wide range of rnic(0) product measured on bi-crystal grain boundaries by Dimos et al., but two to three orders of magnitude less than that expected from “classical” models of superconducting weak-links.

Self-similarity in the I-V characteristics of granular YBa/sub 2/Cu/sub 3/O/sub 7/

The onset of dissipation in the I-V characteristics of granular YBa/sub 2/Cu/sub 3/O/sub 7/ shows clear evidence for a finite critical current. At currents above critical, dissipation in the weak links is only slightly field and temperature dependent. This self-similarity of the characteristics implies strongly that there is a true critical current, despite the random network nature of these materials. Also, it indicates that the dissipative behavior of the grain boundary weak links is almost field and temperature independent.

Weak links and flux pinning in Ag/BiPbSrCaCuO tapes

Abstract Weak links and pinning properties of Ag-sheathed (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+y (2223) wires have been investigated through measurements of the transport and magnetisation J c and of the irreversibility lines (IL). The IL for both the tape and the scraped powder from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder, suggesting that the enchanced IL is attributable to intragrain pinning. Results on the transport and magnetisation J c of these tapes suggest that the critical current is controlled by flux pinning. Grain boundary weak links are largerly reduced for high quality tapes at high temperatures, while the weak link behaviour becomes pronounced at low temperatures.

Transport critical currents, I-V characteristics and magnetisation of YBa2Cu3O7 thick films

YBa2Cu3O7 thick films doped with Ag have improved critical current performance. Magnetisation measurements and the I-V characteristics show that the Ag reduces the normal state resistance of the intergranular weak-links by about 2 orders of magnitude, and also removes their sensitivity to small magnetic fields.

Transport critical currents, I-V characteristics and magnetisation of Ag-sheathed BiSrCaCuO (2223) tapes

The grain-boundary weak-links in pressed Ag-sheathed BiSrCaCuO (2223) tapes have been studied with magnetic and transport techniques. Compared with unsheathed BiSrCaCuO, there is significant grain alignment, and about two orders of magnitude reduction in the weak-link resistance.

Magnetisation measurements on melt-textured YBa2Cu3O7- delta

Quasi-single crystal specimens were grown from rods of bulk YBa2Cu3O7- delta by a melt-textured (MT) growth process. The materials magnetisation was highly anisotropic which supports the microstructural picture. While M-H data show there is a high bulk critical current density, the flux pinning in MT material differs little from that in single crystals or grains of sintered material.

Transport critical currents, I–V characteristics and magnetisation of Ag-sheathed BiSrCaCuO (2223) tapes

Abstract The grain-boundary weak-links in pressed Ag-sheathed BiSrCaCuO (2223) tapes have been studied with magnetic and transport techniques. Compared with unsheathed BiSrCaCuO, there is significant grain alignment, and about two orders of magnitude reduction in the weak-link resistance.