Degradation free epoxy impregnation of REBCO coils and cables

Abstract

In applications utilizing high-temperature superconductors (HTS) under high mechanical loads as high-field magnets or rotors of generators and motors, the rare-earth-barium-copper-oxide (REBCO) tapes have to be stabilized mechanically. This is achieved using support structures of structural materials and filling the voids in the support through the impregnation of the tapes. The impregnation prevents movement of the tapes and distributes mechanical loads evenly. With high mechanical strengths and low sensitivities to rapid temperature changes, epoxy resins are desired materials for the impregnation of superconductor tapes. However, a strong decrease of the current-carrying capabilities was observed in previous epoxy-impregnated REBCO coils. In this work the thermal expansion mismatches between epoxy resins and REBCO tapes are identified as the cause of these degradations. Fillers are used to reduce the thermal expansions of glues and resins. Mixtures with varying filler contents are analyzed systematically. Their thermal expansions and the corresponding degradations of short REBCO tape samples are measured. A mixture of epoxy resin and filler is found which allows degradation-free impregnation of REBCO tapes. This mixture is validated on a 1.2 m long 15 × 5 Roebel-assembled-coated-conductor (RACC) cable from Industrial Research Limited (IRL).

Electro-mechanical properties of REBCO coated conductors from various industrial manufacturers at 77 K, self-field and 4.2 K, 19 T

“…The readouts diverge as the strain gauge onto the free piece of REBCO tape follows the coated conductor's thermal expansion while the strain gauge on the sample is mainly influenced by the thermal expansion of the spring material. With −0.17 % at 4.2 K and −0.16 % at 77 K, the thermal expansion of the spring (Ti-6Al-4V) is very low, much lower than the coated conductors' thermal expansion [13]. The REBCO tapes samples are therefore strained in tensile direction.…”

Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

“…KIT proposed [94] a blend of araldite epoxy with quartz powder in a 1:1 ratio for degradation-free impregnated RE123 tapes/coils, targeted to Roebel cables. The blend is not the best match with RE123 conductors in terms of thermal contraction from RT to 77 K nor has its effectiveness against delamination been demonstrated with full-scale coils.…”

Delamination Characteristics in Wide REBCO Coated Conductor Tapes Under Transverse Loading

“…However, transverse/radial tensile stress due to cool down and centrifugal forces during operation are of greater concern. With regard to coil applications, critical current, I c , degradation of epoxy impregnated SC coils made of CC tapes adapting different deposition processes and different width has been reported elsewhere [5]- [7]. The I c degradation was associated with the characteristic delamination of the CC tape having a multilayer structure.…”

“…It may occur mainly due to the coefficient of thermal expansion (CTE) mismatch among its constituent layers during cool down to its operating temperature [6]. On the other hand, other research group demonstrated that the I c degradation in SC coils cannot be solely attributed to the CTE difference; incompatibility between the impregnating material such as pure epoxy-resin and the SC film can also induce negative effect when both in contact [7]. Delamination of CC tapes can be further mitigated by thermal cycling [6], [8].…”