Oxygen out-diffusion in REBCO coated conductor due to heating

Abstract

Rare earth barium copper oxide (REBCO) coated conductor has emerged as one of the high T c superconductors suitable for future ultrahigh field superconducting magnet applications. In the design and fabrication of such ultrahigh field REBCO magnets, it is essential to understand the behavior of REBCO coated conductor. The effect of heating on the properties of commercial REBCO coated conductors is very important for many practical reasons. Nevertheless, a comprehensive study on this effect have not yet been presented in the published literature. This work studies a commercial REBCO coated conductor heat-treated at temperatures between 175 °C and 300 °C for various durations. Critical current and lap joint resistivity were measured at 77 K and 4.2 K for the heat-treated samples. We found that critical current degrades with heat treatment time and temperature. This degradation can be described by a one-dimensional oxygen out-diffusion model with a diffusion coefficient of D = 2.5 × 10−6 exp (−1.17eV kT−1) m2 s−1. The heat treatment also causes appreciable increase in joint resistivity. Comprehensive structural and chemical analyses were performed on Cu/Ag/RECBO interfaces by transmission electron microscopy. Our electron energy loss spectroscopy study provided direct evidence of oxygen deficiency in the heat treated REBCO samples. In addition, it is found that the oxygen diffused out of the REBCO layer forms mostly Cu2O at both Ag/REBCO and Cu/Ag interfaces. Cu2O is also observed at grain boundaries of the Ag layer. The oxygen out-diffusion model proposed in this work is used to predict REBCO thermal degradation in several engineering scenarios.

Heating-Induced Performance Degradation of REBa2Cu3O7–x Coated Conductors: An Oxygen Out-Diffusion Scenario with Two Activation Energies

“…Before concluding, we would like to mention that the external Cu layer might play a role in the thermally induced degradation of stabilized CCs. In particular, it has been shown that the Cu may act as a sink for oxygen atoms diffusing out of the RE BCO layer . Furthermore, we verified that annealing Cu-stabilized CCs in an O 2 atmosphere does not reduce the degradation of I c since the external Cu layer behaves as a barrier for the in-diffusion of oxygen atoms from the atmosphere.…”

“…The superconducting properties of the RE BCO, as T c and J c , are directly related to the oxygen content. ,, Therefore, the degradation of I c upon heating the CCs has been qualitatively ascribed to the activation of oxygen out-diffusion processes that modify the oxygen content of the RE BCO layer. ,,, If the oxygen diffusion alters the stoichiometry of the superconducting grains, one would expect a simultaneous variation of I c , T c , and of the c -axis parameter. However, a comparison of the results from Figures and points out that while a degradation of I c is already observed at T HT = 170 °C, neither ⟨ T c ⟩ nor T c onset varies before 220 °C.…”

“…In recent years, high-performance RE Ba 2 Cu 3 O 7– x ( RE BCO, where RE = rare earth element) coated conductors (CCs) have become more and more widespread on the market, opening the possibility to employ them routinely in different domains as high-field magnets (in particular for Tokamak fusion energy reactors, nuclear magnetic resonance devices, and high-energy particle colliders), electric-power generation (superconducting motors), and electric-power distribution (superconducting grids). − All these applications typically require to heat the CC above room temperature, for example, when soldering the current leads, when realizing a joint, or when impregnating a coil. − Increasing the temperature of RE BCO CCs can lead to modifications of their superconducting properties. In particular, a reduction of the critical current has been reported when heating the CC at temperatures as low as ∼150 °C. − Short-lasting overheating in the 200 °C range due to uncontrolled quenches at cryogenic temperatures was observed to degrade the critical current, too . The origin of this phenomenon has been qualitatively ascribed to elemental oxygen out-diffusion processes that modify the stoichiometry of the superconducting layer.…”

“…Recently, a simple one-dimensional model for the oxygen out-diffusion has been proposed to explain the reduction of the critical current in commercial CCs after heating. 11 However, many aspects of this topic remain unclear, in particular in light of the results found in laboratory-scale films. 16,17 Here we present a systematic investigation of the superconducting and crystal lattice properties of commercial REBCO CCs heat treated in the low-temperature range 170− 320 °C, which is at the same time the one of interest for practical applications and the least investigated in REBCO films.…”

Critical Current Measurement of REBCO Cables by Using a Superconducting Transformer

“…The soldering procedure for each joint was at about 210 °C, and lasted less than 15 min. Under this heating condition, the REBCO Ic degradation and the associated joint resistance rise should be small [11]. In addition, the joints were in low field region where the critical current is high, small degradation would not likely compromise the experiment.…”