Fabrication of high-quality joints between Gd–Ba–Cu–O bulk superconductors

Abstract

This work reports a technique for fabricating superconducting joints between GdBCO-Ag bulk superconductors, using YBCO-Ag as an intermediate joining material. The ability to provide reliable joints between multiple bulk superconductors overcomes many of the challenges of fabricating large superconductors or machining hard and brittle bulk superconductors into practical shapes. We report on nine single grains of GdBCO-Ag which have been joined with a YBCO-Ag intermediate. Samples were cut and joined in a variety of c-plane orientations to refine and understand the effect this had on the superconducting properties of jointed samples. The trapped field of pre-jointed and jointed bulk superconductors were compared; the maximum trapped field achieved was 59% of the pre-jointed sample. Further analysis showed that the critical temperature and critical current of the samples were degraded by the jointing process. Microstructural and chemical analysis showed that the jointing process facilitated diffusion of silver towards the joint and in some cases large pores were formed at the joint interface. These factors consequently inhibited current flow across the joint and thus reduced the maximum trapped field achievable when compared to the original unjointed sample.

A route to fabricate low resistance joints between Eu–Ba–Cu–O bulk, single grain superconductors

“…One of the advantages, therefore, is that there is a greater difference between the peritectic temperatures of the bulk superconductor (1046 • C) and the joining intermediate material (989 • C) [11,40]. There is a larger temperature gap between the bulk superconductor and joining intermediate than was present when joining GdBCO-Ag with a YBCO-Ag intermediate and, in the GdBCO-Ag/YBCO-Ag joining study, the best relative trapped field achieved was only 59% of that of the original sample prior to cutting [41]. This temperature gap provides a much greater range of temperature over which YBCO-Ag will be above its peritectic temperature, whereas the peritectic temperature of EuBCO-Ag will not be exceeded.…”

“…There is, however, a disadvantage to this in that a higher joining temperature may cause more degradation of the surrounding bulk, single grain material, which is unavoidably heated during the joining process. In the work to join GdBCO-Ag, it has been shown that the additional heating the sample experiences during the joining process contributes directly to the degradation of the critical current [41]. In addition, greater consideration must be given to the migration of silver during the joining process compared to the use of silver-free YBCO.…”

Optimisation of the processing parameters for the fabrication of high-quality joints between Y–Ba–Cu–O single grain, bulk superconductors

“…Another possible, but less likely reason is the diffusion of silver to any gaps at the joint interface to form a non-superconducting intermediate layer. A final possibility is the degradation of the localised J c in the bulk material due to the multiple heating cycles, as discussed in [40].…”