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Normal-incidence irradiation by 100 MeV Ag ions is used to improve flux pinning in previously optimised commercial REBCO tapes from the American Superconductor Corporation. We observe distinct critical-current anisotropy enhancements below and above 40 K. Above 40 K a strong c -axis peak appears in the angular dependence of the critical current, as is usually expected upon the introduction of columnar defects. The critical current is enhanced significantly but only for a limited range of field angles. Close to the parallel-field direction there is no enhancement or even a reduction in critical current. Below 40 K, on the other hand, the enhancement is much broader with respect to field angle, creating an almost isotropic response at 20 K, 3 T. The absence of a prominent c -axis peak does not indicate a lack of pinning, since the absolute value of the critical current still increases by a factor of 2.8 compared to an unirradiated sample. Instead, we postulate that pre-existing point-like pinning centres act to mediate an interaction between the existing planar and newly-introduced columnar pins, broadening both contributions. The point-like pins become less effective with increasing temperature as the coherence length increases, leading to a reduction in this interaction and a separation of the individual peaks relating to planar and columnar pins. At 20 K, we achieve an enhancement in the angular-minimum critical current by a factor of 2.7, in a material that had already been process-optimised for low-temperature pinning. [ABSTRACT FROM AUTHOR]

REBCO coated conductors are exposed to rotating magnetic fields in various high-temperature superconducting (HTS) applications, such as HTS rotating machines and flux pumps. AC loss will be generated in the conductors when they carry AC current under rotating magnetic fields. We define AC loss in the coated conductors with and without current as total AC loss and magnetization loss, respectively. In this work, total AC loss and magnetization loss, in a REBCO tape under rotating magnetic fields and a perpendicular AC standing wave magnetic field are numerically investigated. We employ a 2D finite element method (FEM) based on the ${T} - {A}$ formulation, where ${T}$ and ${A}$ , are the current and magnetic vector potentials, respectively. In the simulations, the external magnetic field amplitude ( $B_{\mathrm {m}}$ ) is up to 500 mT and the reduced AC current ( $i$ = $I_{\mathrm {t}}/I_{\mathrm {c0}}$ ) varies from 0.1 to 0.9, where the $I_{\mathrm {t}}$ and $I_{\mathrm {c0}}$ are the amplitude of the transport current and self-field critical current of the conductor, respectively. Two different types of rotating fields are considered: one is a uniform field with equal amplitudes and phases at each position, and the other being a non-uniform field created by a rotating Halbach array. Different tape widths ranging from 4 mm to 40 mm are considered. Interestingly, the simulation results show substantially higher magnetization loss in the perpendicular standing wave compared to the rotating magnetic fields when $B_{\mathrm {m}}$ is over 100 mT. We attribute the result to the fact that the magnetization loss is propotional to $J_{\mathrm {c}}$ of the conductor at magnetic field amplitudes much greater than the effective penetration magnetic field. Evidently, the instantaneous loss curves of the SW model and RMF-Uniform model at 200 mT exhibited close similarity with evolving $J_{\mathrm {c}}$ values of the two models. Furthermore, when $B_{\mathrm {m}}$ is lower than the effective penetrated field, we observe a pronounced disparity in magnetization loss of the wider tape between uniform and non-uniform rotating fields. This highlights the importance of considering the effects of non-uniform field distribution, particularly at lower magnetic field. We further show that total AC loss under the perpendicular standing wave magnetic field remains greater than that under rotating magnetic fields when $B_{\mathrm {m}} > $ 100 mT and $i < 0.5$ .

The effects of variable cryogenic temperature on the mechanical and electromechanical properties are implemented for REBCO coated conductor (CC) tapes. A versatile facility was used to provide the successive cooling environment from room temperature (about 293 K) to 5 K with a cryogenic-type extensometer for the strain measurement of the tested samples. At different cryogenic temperatures, the mechanical parameters were measured during the REBCO CC tapes being stretched. The cryogenic temperature dependence of mechanical behavior, including elastic limit, offset yield strength and equivalent elastic modulus of the REBCO CC tapes with two different substrates were captured at different cryogenic levels. In addition, the tensile strain dependence of the critical current ( I c ) of REBCO CC tapes with two different substrates at variable cryogenic temperatures was investigated, and the relations between I c degradation and mechanical properties were revealed. The experimental results show that all the investigated mechanical parameters exhibit a temperature dependence. A uniaxial strain dependence of I c is found under all the test temperatures, and the irreversible strains are between elastic limit and offset yield strains regardless of temperature for both two kinds of test specimens. [ABSTRACT FROM AUTHOR]

In this work, the mechanical lap joint of commercial ReBCO tape from SHSC is prepared. The tape is reinforced with stainless steel bar in both side to reduce the effect of bending moment in the joint tensile test. We use Indium and GaInSn as interlayer material, respectively, and different pressures are applied on the joint by jig during testing. The relationship between joint resistance and applied tensile stress was measured at 77 K. The results shows that for all measured joints under pressure, the voltage rise duo to tensile stress is small at first and then sharply grows until a certain value of stress is reached. The maximum tensile stress is higher for the joint which has lower resistance. Comparing two types of mechanical lap joints with Indium and GaInSn, the former joint shows smaller dispersion on joint resistance than the latter under pressure above 40 Mpa. It is also shown that the joint with Indium can tolerate higher tensile stress than the joint with GaInSn by using the Weibull distribution function analysis. [ABSTRACT FROM AUTHOR]

• The electro-mechanical behaviors of coated conductors under different bending modes are numerically investigated through structural modeling. • The obtained strain distributions of the superconducting layer during bending are found to be related to the applied bending force. • The electro-mechanical behaviors of Q-IS strands under bending are analyzed based on the bending strain distributions obtained from structural modeling. The electro-mechanical properties of Rare-Earth Barium Copper Oxide (REBCO) coated conductors (CCs) are crucial for practical structures such as power cables, magnet coils and so on. Many tests including tension and bending tests were conducted to estimate the critical current of REBCO CCs under different loadings. However, standard test method for the electro-mechanical properties of REBCO CCs has not been established, especially for bending tests. Therefore, finite element simulations of bending tests of REBCO CCs are still needed to further analyze the electro-mechanical behavior under loading. Structural responses of REBCO CCs and quasi-isotropic (Q-IS) strands under bending are analyzed using continuum shell element in this study. The numerical results indicate that bending strains obtained from finite element simulations match well with the analytical calculations in pure bending state. Furthermore, the influences of former shapes are also discussed to evaluate the bending tolerance of REBCO CCs in different cases. In addition, the mechanical properties and critical current degradation of Q-IS strands under bending are analyzed. [ABSTRACT FROM AUTHOR]

• A damage analysis method based on CZM and XFEM is presented. • Model well predicts the progressive failure behavior in REBCO coated conductors. • Mutual enhancement exists between interfacial delamination and layer cracking. Rare-earth barium-copper-oxide (REBCO) coated conductors (CCs) are kinds of multiphase, multilayer, and composited superconducting tapes, which are becoming promising candidates for abundant high-energy and high-field applications owing to their outstanding superconducting properties. However, the mechanical failures of REBCO CCs composites, such as cracks in the brittle ceramic layers, and delamination on interfaces among weak multiple layers, have become the essential issues threatening the capability and stable operation of REBCO-based superconducting devices, especially under extreme low temperature and complex electromagnetic fields. In this study, a failure modeling in combination of the interfacial delamination and layer fracture in REBCO CCs composites was established and analyzed numerically by the finite element method. To effectively treat with the two mechanical failure modes, the interfacial delamination response and the fracture behavior were respectively captured using the cohesive zone model and the extended finite element method for the multilayered superconducting CCs. The failures initiation and evolution in REBCO multilayered structures were implemented based on the stress criteria. The characteristics of two failure modes of the multilayered superconducting CCs under uniaxial tension and bending were investigated, and those during cooling process from room temperature to 77 K were then simulated in consideration of residual thermal stresses accumulated. The results illustrated that the progressive mechanical failure behaviors were presented in combination of the interfacial delamination and fracture damages in the REBCO multilayered structure. The crack propagation in the superconducting layer can cause the delamination failure at the corresponding position on the interfaces between the superconducting layer and its adjacent layers, while the delamination damage on the interfaces releases the constraint between the layers further enhances the crack propagation. The residual thermal stress accumulated during cooling was found to significantly affect the mechanical behavior of the REBCO CCs, and the low-temperature makes a failure occurrence earlier compared to that at room temperature. [ABSTRACT FROM AUTHOR]

The uniaxial pressure dependence of the critical temperature causes a reversible effect of strain on the critical current density and the flux pinning strength in many high-temperature superconductors. Recent experiments on patterned coated conductor bridges have shown that the anisotropic nature of the pressure dependence of the critical temperature of rare earth (RE)-\Ba2\Cu3\O7 - \delta (REBCO) has a major impact on the performance of coated conductors under strain. The strain effect on the critical current density is most prominent when the strain is along the [100] and [010] directions of the superconducting film, whereas it almost completely disappears when the strain is along [110]. In this paper, we investigate the correlation between the uniaxial-pressure dependence of the critical temperature and the reversible strain effect on flux pinning in REBCO coated conductors. We show that axial strain has a large effect on the irreversibility field and the pinning force in coated conductors when the [100] and [010] directions of the superconducting film are aligned along the conductor axis. The magnitude of the strain effect in these conductors largely depends on the angle at which the magnetic field is applied. On the other hand, the critical temperature is not expected to change with the axial strain in coated conductors when the [110] direction is aligned along the conductor axis. Indeed, the irreversibility field and the magnetic field dependence of the pinning force of these conductors are almost independent of the axial strain for all angles at which the magnetic field is applied. The minor strain dependence of the critical current measured in these conductors could be caused by the average in-plane grain misalignment of between 6^\circ and 8 ^\circ, which causes a slight variation in the strain alignment with the axes of the superconducting film. The results confirm that the reversible strain effect in REBCO coated conductors is largely determined by the uniaxial pressure dependence of the critical temperature. [ABSTRACT FROM AUTHOR]

Abstract: In the case of 2G coated conductor (CC) tapes, it has been reported that thin–thick CC tapes with IBAD substrate showed a superior electromechanical property even at smaller bending radius compared with the cases of 1G BSCCO tapes. Considering the application of CC tapes it is significant to evaluate the transport property under operating environment, because CC tapes might experience a change in operating pressure that can affect its current carrying capacity due to temperature variation and deformation. This study was focused on the I c degradation behavior in bent CC tapes under pressurized liquid nitrogen. Differently processed YBCO and SmBCO CC tapes with IBAD substrate are used as samples. The bending strain characteristics at elevated pressure levels were evaluated by using the ρ-shaped sample holder which can induce different bending strain values at pressured state. Depressurization and thermal cycling were performed to check the reversibility of I c in CC tapes. Vacuuming tests were also carried out to investigate the characteristics of I c at different LN2 temperature levels. [ABSTRACT FROM AUTHOR]

• Simulates the extent of fabrication defects in coated conductor tapes with different materials and architecture. • The interlayer interactions of layers cause severe lattice mismatch during deposition of films. • The lattice mismatch in the form of losses of bonding strength was computed using modified Hamaker models. • Computed results show that samples with more number of layers suffer comparatively higher losses of bonding strength. In this study, the authors carried out an inspection of the extent of fabrication defects in rare-earth barium copper oxide (REBCO) coated conductors (CC) using the displacement-energy model (DEM) approach. The DEM approach can simulate the creation, propagation, and distribution of interfacial defects during the deposition of films onto a substrate, thereby save researchers the enormous resources needed to carry out similar studies experimentally. Defects in a sample originate from the interlayer interactions such as the thermal mismatch and eigenstates of layers as a function of temperature and time, respectively. The induced strains or gaps (determined by an increase in a lattice constant) between an interface in a sample caused by the interlayer interactions are computed as losses of bonding energy between interacting molecules using modified Hamaker models. The samples used for the simulation are based on the tape architecture of three leading industrial manufacturers of coated conductors (SuperPower Incorporated, Bruker High-Temperature Superconductors, and American superconductors). Also, in the simulation of this study, it assumes the samples from the manufacturers have a similar size (length x width) and fabrication method (chemical deposition method) to determine a tape architecture with comparatively lesser fabrication defects under similar fabrication conditions. Primarily, simulation results show that samples with more number of layers suffer relatively higher losses of bonding energy. According to this study and previous studies, the computed results on the DEM approach are consistent with those on several valuable experiments and models in the literature. [ABSTRACT FROM AUTHOR]

Recent progresses in the second generation REBa 2 Cu 3 O 7 - x (RE123) coated conductor (CC) have paved a way for the development of superconducting solenoids capable of generating fields well above 23.5 T, i.e. the lim it of NbTi-N b 3 Sn-based magnets. However, the RE123 magnet still poses several fundamental and engineering challenges. In this work we review the state-of- the-art of conductor and magnet technologies. The goal is to illustrate a close synergetic relationship between evolution of high-field magnets and advancement in superconductor technology. The paper is organized in three parts: (1) the basics of RE123 CC fabrication technique, including latest developments to improve conductor performance and production throughput; (2) critical issues and innovative design concepts for the RE123-based magnet; and (3) an overview of noteworthy ongoing magnet projects.