Fast superconductive joining of Y-Ba-Cu-O bulks with high quality

Y-based Superconductors(Y-Ba-Cu-O) is considered to be one of the promisingsuperconducting materials owing to its good applications in many fields, and theseapplications need Y-Ba-Cu-O bulks with large dimension and complex shape. However,at present, the mean current density of large bulk decreases significantly compared withthat of the small one and the shape of the bulk is very simple, usually in form of cylinderor hexagonal. In order to meet the need of the practical using of Y-Ba-Cu-O, joiningtechnique has been used. Up to now, a lot of researchers had done a great deal ofinvestigation on soldering, however, almost all of them needed a relatively long time torealize superconductive joining. In this paper, fast superconductive joining technology ofY-Ba-Cu-O was investigated. Three kinds of solders, including sintered Yb-Ba-Cu-O,sintered Y-Ba-Cu-O/Ag and melt Y-Ba-Cu-O/Ag solder were synthesized and differentsoldering thermal cycles were designed respectively. The results show that the solderedbulk using sintered Yb-Ba-Cu-O can partially recover the superconductivity butaccumulation of Yb₂BaCuO₅ (Yb211), which results in decrease in superconductivity,was observed in the bonding zone. In comparison, soldering with Y-Ba-Cu-O/Ag soldersrealized much better superconductive bonding owing to its thinner solder thickness andslower cooling rates during soldering. Moreover, soldering with the melt Y-Ba-Cu-O/Agsolder obtained better performance than soldering with the sintered solder. One plausiblereason for the better performance of melt solder is that the dense structure owing to itspretreatment at higher temperature. The microstructure of the bonding zones wasinvestigated by scan electron microscope and the images indicate that neither Ag particlesnor Y₂BaCuO₅ (Y211) accumulated during the soldering process. The trapped field imagealso reveals that the supercurrent is almost not blocked by the bonding zone.