Degradation of REBCO coated conductors due to a combination of epoxy impregnation, thermal cycles, and quench: Characteristics and a method of alleviation

Abstract

REBa2Cu3Ox (REBCO) coated conductors are an enabling technology for >1 GHz nuclear magnetic resonance spectrometers that need >23.5 T solenoids, compact high-field fusion reactors beyond the International Thermonuclear Experimental Reactor, and 20 T class accelerator magnets. An in-depth understanding of stress and strain management is critically needed for these applications. Here, we report experimental data and an analysis of damages of REBCO-coated conductors in several scenarios relevant for their applications in high-field magnets and unique to their multi-layer thin film structure. The examined scenarios include failure induced by quench, failure induced by epoxy impregnation, and failure induced by a combination of epoxy impregnation and quench. The root cause is film blistering and peeling induced by thermally interfacial stresses characteristic of multilayered heteroepitaxial thin film structures on thick substrates utilized. The quantitative stress analysis is introduced using simple and easy to use formulas and clearly indicates how materials and structural characteristics affect the magnitude and the distribution of stresses and deflections. Finally, a simple method is introduced for removing degradation of REBCO coated conductors due to epoxy impregnation. Our data and analysis help our understanding of the practical limit of REBCO-coated conductors and can be utilized as a guidance for the physical design of REBCO coated conductors and high-current cables assembled from them and for minimizing damages.

An initial magnet experiment using high-temperature superconducting STAR® wires

“…Although the star® wires can be resilient to the mechanical deformation and the resulting strain during the test (figures 7 and 17), we should avoid conductor deformation during magnet operation. Built on the significant experience that the community has gained [10,[64][65][66][67][68][69][70][71][72], a vacuum-pressure impregnation technique should be developed to support individual rebco tapes in a star® wire.…”

Structural Finite Element Analysis of REBCO Tape Delamination with Solid-Shell Element under Various Loads

“…Critical current degradation tends to occur before mechanical delamination, and does so either abruptly or gradually depending on where the delamination occurred [3,4]. The danger of delamination caused by transverse tensile stresses during thermal cooldown (particularly in epoxy-impregnated coils), thermal quenches, and Lorentz forces is well documented in literatures [5,6]. Experiments to measure the transverse tensile strength of REBCO 1241 (2022) 012032 IOP Publishing doi:10.1088/1757-899X/1241/1/012032 2 tapes have shown that delamination can occur at stresses within the range of 10 to 100 MPa [7][8][9].…”

National round robin test (RRT) for delamination strength testing YBCO HTS wires at room temperature (RT) and 77K

“…Besides, their values are a little lower than the minimums of the measured ones. The reason for reduced MDS value at 77 K is supposed to be caused by delamination stresses induced by thermal mismatch of constituent layers of CCs [28,29]. From the point of view of safety of CC applications, the statistical MDS values analyzed by three-parameter Weibull distribution are more meaningful than the averages by traditional statistical method on engineering CC manufacturing and magnet application design.…”