Heating-Induced Performance Degradation of REBa₂Cu₃O_7–x Coated Conductors: An Oxygen Out-Diffusion Scenario with Two Activation Energies

Abstract

The degradation of the superconducting properties of REBa 2 Cu 3 O 7−x coated conductors when heated above 150 °C is a topic of concern for any REBa 2 Cu 3 O 7−x -based applications. This study makes it clear the role of thermally activated oxygen out-diffusion processes in the modification of the superconducting properties of commercial coated conductors heat treated in air. We point out that oxygen can diffuse out of the REBa 2 Cu 3 O 7−x layer along two main channels: one associated with the grain boundaries and a second one associated with the superconducting grains. The activation energy is lower for the oxygen outdiffusion events that take place at the grain boundaries, and this is the main cause of the degradation of the critical current at temperatures lower than ∼250 °C. More specifically, through a systematic investigation of the electrical and magnetic properties, we demonstrate that the decrease of the critical current upon heating the coated conductor is initially due to a loss in the transparency of the REBa 2 Cu 3 O 7−x grain boundaries, which determines a reduction of the effective superconductor cross section available for the superconducting current. The degradation of the intragrain properties becomes important only above ∼250 °C and manifests itself through a simultaneous variation of the critical current and of the critical temperature.

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

“…We assumed that a 5-7.5 mm long section in both wires, consistent with the burn-out, absorbed an equal amount of heat under an adiabatic condition. The estimated peak temperature during Ramps 11-19 ranged from 440 to 1040 K, high enough to degrade the critical current of rebco tapes [76,77]. A recent corc® dipole magnet suffered a similar critical-current degradation, possibly also due to the conductor heating [49].…”

Performance degradation due to thermal cycling in soldered Faraday and SuperPower HTS tape stacks

“…As oxygen continues to diffuse out of the REBCO, this ultimately results in a decrease in I c . The work by Bonura [5] suggests that below 250 • C, oxygen diffuses out of REBCO predominantly via grain boundaries, implying our results may also be grain boundary localized effects; in this case, it would be the loss of superconductivity in the grain boundaries rather than the grains that causes the ultimate decrease in I c in the Faraday samples, by reducing grain connectivity and possible current paths. Improved pinning due to oxygen vacancies or the variation in properties of some of these secondary grains which may act as current-blocking defects [17], remains a possibility.…”

“…The initial tape soldering process to manufacture the cable or coil involves significant heat application to the tapes which can result in thermal degradation [3]. Recent works [4,5] have also shown that individual HTS tapes degrade at temperatures even lower than the soldering temperature, via oxygen-out diffusion from the grain boundaries and grains of the REBCO layer. Post manufacturing, another application that involves heat cycling is the operation of a low temperature solder demountable joint to couple superconducting tape stacks, which avoids melting the Pb 37 Sn 63 , but requires elevated temperatures to mount and demount the joint [3].…”

Self-protected high-temperature superconducting demonstrator magnet for particle detectors

“…To solder directly to aluminium, temperatures over 280 • C would be needed to remove the refractory oxide on the surface with reactive chloride fluxes [10]. Exposure to this temperature would permanently degrade the performance of the tapes [13,14]. Next to that, aluminium specific solders flow poorly, and aluminium reacts with Sn-Pb solders causing corrosion at the solder interface [10] leading to bad contact.…”