Application of Electrically-Conductive Epoxy in No-Insulation Coils for Controlling Contact Resistance

Abstract

This article presents a partial-depth impregnation method using electrically conductive epoxy composites in high-temperature superconductor (HTS) coils. In a previous study, we proposed a wet winding technique using electrically-conductive epoxy composites to control the contact resistance of no-insulation (NI) HTS coils. In that method, the epoxy composites were applied across the entire contact surface between winding turns, allowing the contact resistance to be controlled by adjusting the mixing ratio of electrically-conductive powder. However, this approach led to significant current degradation due to delamination caused by thermal contraction mismatch between the epoxy composites and the HTS tape. The partial-depth impregnation method addresses this issue by allowing the epoxy composites to penetrate only partway between winding turns, thereby minimizing critical current degradation. In this study, HTS coils were dry-wound with 4.1 mm-wide HTS tape and insulated with 3 mm-wide polyimide tape on each turn. By applying the electrically-conductive epoxy composites to the edge of the HTS coil, the epoxy composite penetrated to a depth of 1.1 mm. This configuration allows the current to bypass through the edge of the insulated coil, providing self-protection characteristics. Additionally, the contact resistance can be controlled by adjusting the mixing ratio of the electrically-conductive powders. The feasibility of the proposed impregnation technique was demonstrated through over-current and sudden-discharge tests on the partial-depth impregnated coils with different mixing ratios of electrically conductive powders.