Delamination Strengths of Different Types of REBCO-Coated Conductors and Method for Reducing Radial Thermal Stresses of Impregnated REBCO Pancake Coils

An impregnated pancake coil is composed of a REBa₂Cu₃O_7-δ (REBCO)-coated conductor, a polyimide tape, and epoxy resin, so that a radial thermal stress is generated in the winding by their anisotropic thermal contraction when the coil is cooled from room temperature to 77 K or less. If this radial stress locally exceeds the delamination strength of the REBCO-coated conductor under transverse tensile stress, the coil will be degraded. Although it is important to determine the exact delamination strength of the REBCO-coated conductor to fabricate an impregnated coil that does not suffer from degradation, the delamination strengths reported until now have shown considerable variation due to the evaluation methods used. Therefore, we propose a new method for evaluating the delamination strength using small epoxy impregnated test coils, since the radial stress depends on the ratio of inner to outer diameters regardless of the coil size. The delamination strength can be obtained by investigating the ratios of inner to outer diameters that do not result in degradation. In this paper, the delamination strengths of three types of REBCO-coated conductors made with different manufacturing processes and having different configurations were evaluated. This method showed that a conductor laminated with copper plates had higher delamination strength than the other two kinds of conductors. In addition, impregnated single pancake coils whose inner diameter was 50 mm and whose outer diameter was 100 mm were fabricated using the three types of REBCO-coated conductors. Degradation was avoided in all coils by dividing the coils into a number of winding parts in the radial direction according to the delamination strength of the conductors.