Coated Conductors Research Articles

Uncovering and elimination of the natural separation of interacting molecules of standard REBCO crystals, a panacea for improvement of its delamination strength

The intermolecular interactions characteristics for constituent molecules of standard rare-earth barium copper oxide (REBCO, rare-earth (RE) are gadolinium (Gd), samarium (Sm), and yttrium (Y)) coated conductor (CC) crystals was studied by Hamaker method. On the Hamaker model, we used the deductions for the combining laws of the Hamaker constants for the interacting molecules to uncover and eliminate the intermolecular interactions responsible for introducing specific microstructural defects in the REBCO crystals. The computed results showed that some molecules of the REBCO crystals could separate naturally due to dispersion forces only, thereby creating microstructural defects in the crystals. Under the given parameters, we found out that it is more beneficial to increase the Hamaker constant of the intervening medium,A33 responsible for introducing the microstructural defects than decreasing it to obtain a REBCO crystal that would have robust delamination strength. For instance, by increasing the A33 from 25×10-20 J to 30×10-20 J (i.e., only 0.2%), it corresponds to the improvement of the delamination strength of the bulk REBCO crystals from approximately 95 MPa to 106 MPa (0.12%). The computed results agree reasonably well with the Hamaker constants of similar ceramic oxides and REBCO’s delamination strength found in the literature, which is 9.01-10.20×10-20 J and 10–100 MPa respectively.

Optimization of interim heat-treatment condition on TFA-MOD process for fabrication of Y0.77Gd0.23Ba2Cu3Oy coated conductors with BaHfO3

We have optimized the interim heat-treatment (IHT) time in trifluoroacetate based metal organic deposition process on fabrication of the BaHfO3 (BHO) doped Y0.77Gd0.23Ba2Cu3Oy (YGdBCO) coated conductor (CC) in order to further improve the in-field critical current density (Jc) performance. The in-field Jc values of 0.6 μm-thick BHO doped YGdBCO CCs increased with increasing the IHT time within 90 min. The highest critical current density (Jc) value of 0.86 MA/cm2 at 77 K in 3 T (B//c) was obtained for 0.6 μm-thick BHO doped YGdBCO CC with IHT time of 90 min. CCs with IHT time shorter than 90 min had relative low in-field Jc performance but high self-field Jc performance. It can be caused by the insufficient transformation to BaF2 in IHT which is a reason for the relatively large BHO particles. In the case of applying IHT for longer time than 90 min, coarsening of CuO particles in precursor films was suggested and degradation of self- and in-field Jc performances were observed.

Parallel magnetic field dependence of AC losses in periodically arranged superconductors with ferromagnetic substrates

Unlike superconductors on nonmagnetic substrates (SC/NM tapes) in a parallel magnetic field, the ferromagnetic substrates can cause obvious change of current density and magnetic field distributions in superconductor coated conductor (CC) tapes. In this case AC losses of CC tapes exposed to a parallel magnetic field cannot be neglected. In this paper, we investigate the electromagnetic response and AC losses of CC tapes using H-formulation model. The ferromagnetic loss of the substrate in a CC tape and the effect of magnetic field dependent permeability on AC loss are also considered. It can be found that the AC loss of a CC tape is much larger than that of a SC/NM tape, especially in weak applied magnetic fields. It is an effective way to significantly reduce AC loss of a CC tape by appropriately increasing the width of the ferromagnetic substrate. We also consider the CC tape carrying DC transport current and simultaneously exposed to a parallel AC magnetic field. The influence of the DC transport current on the AC loss is investigated. Moreover, AC losses of CC tape-array systems mainly depend on the arrangement and the vertical interval between CC tapes. We demonstrate that AC loss can be reduced effectively by decreasing the vertical interval between CC tapes.

Adaptive multi-scale electrothermal model of REBCO coated conductors embedded in a commercial power system transient simulator

High temperature superconducting coated conductors (CCs) are wires with ideal nonlinear properties for use as resistive superconducting fault current limiters (rSFCLs) for power systems. However, choosing a conductor architecture that can survive all types of excitations occurring in a specific power system is a big challenge. Firstly, the non-uniformity of the critical current along the length of commercial CCs makes them prone to hot spots. Secondly, the current flowing in a CC-based rSFCL is determined by the specific dynamics of the power system in which it is installed. In order to correctly design the CC architecture, one must be able to compute the electrothermal bevahiour of CCs submitted to realistic power system excitations. This requires coupling a physically-representative CC model with a suitable commercial power system simulator. In this paper, an example of such coupling is presented, with the particularity that the numerical method used to simulate CCs can account for realistic lengths used in rSFCLs (hundreds of meters of CCs), while still being able to track hot spot developments on the micrometer scale, thanks to a dynamic adaptive meshing strategy. The coupling is realized within the EMTP-RV environment through a dynamic link library that contains the physical CC model. A few examples of applications are presented, in which propagation of multiple hot spots triggered by the power system are simulated in very moderate computation times. Besides helping rSFCL manufacturers to better select a proper CC architecture, the proposed model can also be used in regular power system simulations as an accurate rSFCL device model, which is expected to greatly help power system engineers to better plan their integration in the grid.

Transition frequency of transport ac losses in high temperature superconducting coated conductors

Experimental data reveal that the classical description of transport ac losses in high-temperature superconducting (HTS)-coated conductors (CCs), based on investigations at low frequencies, is incomplete in some aspects when transport currents in the kilohertz range are considered. More specifically, above a certain “transition frequency,” the ac losses per cycle no longer increase with the frequency as the theory predicts. Using a finite element model to allow for loss separation, we find that this phenomenon is caused by a combination of several factors that appear only above the transition frequency: the hysteresis and ferromagnetic losses per cycle are no longer independent of the frequency, while the eddy current losses per cycle no longer increase proportionally to the frequency. Based on a circuit model, we propose that the physical reason for this is that when the frequency increases, part of the supercurrent starts migrating into the metallic path. We argue that the current in the metallic path is not an eddy current but a transport current inductively coupled to the superconducting current. Finally, we discuss the relationship between the magnetic material magnetization, the critical current, and the transport current frequency. This study provides explicit insights into the frequency-dependent transport ac losses of HTS CCs in a broad frequency band, which is valuable for the design and optimization of HTS CC-based power devices.

Enhanced Pinning Properties of Sm-Doped GdBCO CCs by the RCE-DR Process

We report the enhanced pinning properties of Sm-doped GdBa 2 Cu 3 O 7-δ (GdBCO) coated conductors (CCs) compared with those of GdBCO CCs by the reactive co-evaporation deposition and reaction process. For this paper, Sm-doped GdBCO CCs could be fabricated successfully using the same processing parameters with GdBCO CCs. While the GdBCO CCs exhibited higher critical temperature (T c ) and larger self-field critical current (I c ) at 77 K, Sm-doped GdBCO CCs possessed higher infield minimum critical current density (J c ) values in the field region of 1-5 T at 77 K most probably due to a refinement of (Gd,Sm) 2 O 3 nanoparticles trapped in the (Gd,Sm)BCO superconducting matrix.

Effect of Cu Edges on Delamination Strength in Cu-Stabilized CC Tapes Under Transverse Tension at 77 K

Delamination frequently occurs as a result of high transverse tensile stresses caused by fabrication and mechanical and thermal cycles during operation. It is generally known that transverse opening and peeling off are the primary delamination failure modes, and delamination strength is typically evaluated using anvil or peel tests. To avoid transverse openings that cause delamination failures in coated conductor (CC) tapes, the tape structure can be changed by developing or adding the fabrication process. Thus, in most studies, delamination strength has been characterized in REBCO CC tapes with Cu-edges and the slit-side one. In this study, the delamination strengths of 12-mm-wide CC tapes without a slit edge and a 4-mm-wide CC tapes without Cu-edges were experimentally measured under transverse tensile loading at 77 K. Results were compared to those obtained during the mechanical delamination of CC tapes with Cu edges. The delamination morphologies were also observed macroscopically.

Superconducting Joint of GdBa2Cu3Oy Coated Conductors by Crystallization of an Additionally Deposited Precursor Layer

A superconducting joint of REBa 2 Cu 3 O y (REBCO) coated conductors (CCs) has been demanded strongly to fabricate long length CCs for high field magnet applications such as nuclear magnetic resonance and magnetic resonance imaging. In the previous reports of superconducting joint, specimens of REBCO CCs were jointed via melting REBCO phases or via solid state diffusion of REBCO phases. In our study, we propose a new method of joint for REBCO CCs. A precursor layer is additionally deposited on GdBCO CC by a metal organic deposition process, and then two pieces of them are stuck together face-to-face and crystallized the precursor to form 123 phase under mechanical pressure in an oxygen atmosphere. The microstructures and temperature dependence of resistance of the jointed sample are characterized by a cross-sectional transmission electron microscopy (TEM) and a four-probe method, respectively. As a result, TEM observation reveals that two CCs are jointed together without formation of secondary phases at the joint interface. Also, temperature dependence of resistance shows Tc onset and Tc zero of 93 K and 82 K, respectively. Consequently, a superconducting joint has been completed successfully. The concept of this method is combining film growth and solid-state diffusion for the additionally deposited precursor layers.

Flux-Pinning Properties of BaHfO3-Doped EuBCO-Coated Conductors Fabricated by Hot-Wall PLD

REBa2Cu3Ox (REBCO, RE = rare earth) coated conductors (CCs) are promising as superconducting wires for high-field magnets because of their high in-field critical current density (Jc) performance and high tensile tolerance. Fujikura, Ltd., has been developing BaHfO3 (BHO)-doped EuBa2Cu3Ox (EuBCO, Eu = Europium) CCs using a hot-wall-type pulsed-laser deposition (PLD) in order to further improve the in-field performance. Although a high deposition rate in the PLD process is necessary for mass production, it has been found that the in-field performance greatly differ depending on the deposition rate. In this study, flux pinning properties of BHO-doped EuBCO CCs fabricated with different deposition rates by the hot-wall PLD on IBAD substrates were investigated in detail. From the scaling characteristics of the flux-pinning force density (Fp) curve, the BHO precipitates in a fast deposition rate REBCO film appeared to behave like random pinning centers, which was almost consistent with a result of transmission electron microscope observation. On the other hand, from a decrease of anisotropy in the magnetic field angle dependence of Jc, it was also confirmed that BHO precipitates were not completely random pinning centers, that is, it has some kind of anisotropy. Considering the critical current value per production time, it was also found that the fast deposition rate is advantageous since the REBCO layer could be thicker.

Voltage–Current Characteristics and Temperature Variation by DC Power Source for the YBCO Coated Conductor Under Spray Cooling Method

This paper proposes the spray cooling method in conducting the extremely low temperature cooling of the superconducting wire, and presents the results of cooling performance tests and characteristics of the spray cooling method. To that end, extremely low-temperature containers were designed and manufactured to enable the testing of immersion, conduction, and spray cooling methods. Then, a cooling performance test and dc transport current test were carried out according to individual cooling methods. The test results are outlined below for each cooling method. Similar trends were shown in the initial cooling performance and cooling maintenance performance of the existing immersion cooling method and the spray cooling method. Similar trends were also shown in voltage-current characteristics according to dc transport current. Thus, in order to observe the cooling characteristics of the spray cooling method further, additional tests were carried out to define temperature characteristics according to spray pressure, and the voltage-current characteristics based on spray duration. The test results confirmed that higher spray pressure resulted in better cooling performance, and longer refrigerant time resulted in more stable conduction of dc. Accordingly, quench voltage was detected as low.

Study on Electromechanical Properties of Solder Jointed YBCO Coated Conductors With Etched Copper Stabilizer Under Axial Tension

The YBa2Cu3O7-x (YBCO) coated conductor (CC) is one of the most promising superconducting materials because of their high critical current (Ic) and low dependency of the Ic on the external magnetic field. For superconducting applications, a large number of joints are required to join relatively short YBCO CC tapes. In this paper, we study the electromechanical behavior of lap-jointed YBCO CC tapes fabricated by etching Cu stabilizer at its service temperatures (77 K) under self-field. The effects of etching Cu stabilizer on Ic and joint resistance (Rj) of YBCO CC tapes were discussed. The Rj was reduced and the Ic values of the joints are consistent with the parent CCs tape by etching the Cu stabilizer. Preferable electrical properties under applying axial tension are achieved and proved by the axial tension tests and finite element simulations. This fabrication technique is a unique practicable solution to reduce joint resistance for YBCO CC tapes with thicker metal stabilizing layers and improving electromechanical performance in second generation high temperature superconductor (2G HTS) devices.

An analytical method for predicting the net (thermal plus intrinsic) residual stresses in multilayered yttrium barium copper oxide coated conductors

In this paper, the net (thermal plus intrinsic) residual stresses in the films of American Superconductor (AMSC 344®) tape was analyzed via a displacement- energy model (DEM) solution approach. The DEM solution approach was developed to predict both the thermal mismatch and intrinsic film stresses in the coated conductor (CC) films thereby avoid the costly experiments usually required to capture the intrinsic film’s stress part in CC films. The analysis was based on the force and moment balances method, classical plate theory, and Hamaker method. On this model, the authors studied the generation of misfit strains in the CC films due to the thermal and free-vibration mismatch between the layers, and further the role of these actions in the residual stresses evolution in the CC films. The results showed that the thermal film stress in the yttrium barium copper oxide (YBCO) films calculated by the DEM solution approach was 460.80 MPa, whereas that calculated by the closed-form solution method was 455.30 MPa. Moreover, for the non-textured yttrium-stabilized zirconium (YSZ) films deposited on the Hastelloy C substrate, the calculated net residual stress by the DEM solution approach was 673 MPa, whereas the measured net residual stress by the x-ray Diffraction ( Method) was 598 ± 75 MPa. Therefore, the DEM solution approach successfully predicted the net residual stresses in the CC films analyzed because the calculated results by the DEM solution approach agree well with those by the closed-form solution and x-ray Diffraction ( methods).

Analysis of thermo-physical properties of materials suitable for thermal stabilization of superconducting tapes for high-voltage superconducting fault current limiters

High-temperature superconducting coated conductors (CC) are attractive materials for high-voltage resistive fault current limiters. Commercially produced CC tapes cannot withstand electric fields over 100 V m−1, and therefore, they have to be modified by addition of thermal stabilization layer. We investigated several composite systems suitable for such thermal stabilization. As a matrix of composites, Stycast resins were used, which contain various contents of powder particles from SiC, SiO2, PbTiO3, ZrW2O8, or synthetic diamond. The thermo-physical properties such as thermal expansion, heat capacity, thermal diffusivity and direct-current electrical conductivity were measured because they are influenced by filler content in the composite. The possible highest amount of the powder can slightly increase the thermal conductivity and decrease the thermal expansion of the system to an acceptable value needed for the coating of CCs. A side effect is simultaneous drop of heat capacity; however, its value stays still high enough for the purpose of heat sinking. Overall, the best results were achieved in system with 20 vol% of SiC filler in Stycast 2850FT + Catalyst 23LV resin matrix. There the contribution of the semiconducting SiC addition to the electrical conductivity of composite was negligible.

Finite-Element Analysis of the Strain Distribution Due to Bending in a REBCO Coated Conductor for Canted Cosine Theta Dipole Magnet Applications

High-current cables using REBCO tapes can be used to develop high-field dipole magnets. However, the strain accumulated during cable fabrication and coil winding may reduce the critical current of the conductor. Therefore, it is important to properly consider the strain when designing high-field magnets. In this paper, we used structural finite-element analysis (FEA) to predict the strain experienced by a REBCO tape during bending in configurations relevant to the fabrication of high-field accelerator magnets, in particular, the mechanical strain generated during cable fabrication and winding in a canted-cos θ dipole configuration. We considered two different cable options: (A) Flat tape that lay in the mandrel channel and (B) a REBCO tape helically wound around a circular copper core, the typical configuration of the conductor in round core cable (CORC). Strain accumulated during tape winding is studied for different core diameters and winding tilt angles. FEA longitudinal strain results were compared with the simulations for configuration A, where higher strain was observed experimentally. Configuration B was verified indirectly by comparing experimentally measured Ic with the one predicted (based on the longitudinal strain) as a function of the bending diameter. Good agreement was found up to a bending diameter of 30 mm. The presented results will help to understand the impact of bending on REBCO tapes and CORC wires to develop high-field magnets.

Establishing a test procedure for evaluating the electromechanical properties of practical REBCO coated conductor tapes by the uniaxial tension test at 77 K

Nowadays, the superior electrical and mechanical properties of second generation (2G) practical coated conductor (CC) tapes make them viable options for various applications such as power cables, magnets and coils. The CC tapes are composite conductors where the constituent layers are directly affected by the mechanical and the electromechanical properties of the superconducting layer under operating conditions. It is therefore important to investigate the strain effect on the critical current (Ic) and to determine the irreversible limits for Ic degradation in CC tapes. However, standards or codes for testing the electromechanical properties of high-temperature superconductors are lacking. Therefore, the establishment of a test method or procedure for determining the electromechanical properties of 2G CC tapes in both engineering and practical aspects is needed. This study is a standardization activity, wherein the strain/stress sensitivity of Ic and the reversible strain/stress limits for Ic degradation of practical CC tapes using uniaxial tension tests were evaluated at 77 K and self-field. The reversible strain/stress limits of CC tapes were defined and the reversible recovery of Ic during loading–unloading was measured when using the 99% Ic (ε) recovery, 99% Ic0 recovery and 95% Ic0 retention criteria. The electromechanically determined irreversible limits were also compared to the mechanically determined stress and strain at the yield point. The obtained reversible stress limits showed a similar value regardless of criteria adopted, excepting Sample 2. However, the irreversible strain limit value obtained using the 99% Ic(ε) recovery criterion showed the largest value as compared to the cases using other criteria. Through repeated tests, the electromechanical test procedure of the CC tapes was evaluated based on statistical estimates. The obtained results can provide a fundamental basis for establishing test procedures comparing the criterion that should be considered in determining irreversible limits, whether normalized to the original state Ic value, Ic0 or determined using the reversible degradation of Ic(ε) at residual strain, when unloaded.

Delamination strength of HTS tape under transverse tensile stress and its enhancement by using different Ag layer depositing temperatures

ABSTRACT For the application of second generation high temperature superconducting coated conductors (CCs) with layered structures, thermal mismatch between different components and electromagnetic force exerted in superconducting layer in a working magnet can cause transverse tensile stress, which would result in delamination behavior. Therefore many research groups have designed experiments to measure the delamination strength and dedicate to improving that. However, the reason of the discrete distribution of measured data has still not get quantitatively studied, besides, there are lack of investigations on the method of changing depositing conditions to improve the delamination strength except by adding an additional metal layer. In this work, we adopt an anvil test device and obtain delamination strengths as 29.6 MPa of YBa2Cu3O7–x (YBCO)/buffer and 114.6 MPa of buffer/substrate by combing energy disperse spectroscopy (EDS) detection. The reason of discretized measurement data on the delamination strength is explained. Moreover, we find that different temperatures during Ag deposition determine the bonding force of Ag and YBCO layer. The Delamination strength between Ag and YBCO layer increases from 4.4 MPa to larger than 114.6 MPa with temperature elevated from 30 °C to 100 °C. Hence we present a novel method for improving the delamination strength of YBCO CCs by setting an optimal temperature of Ag deposition.

Manifestation of granularity in the transport current of coated conductors

The effects of granularity on the angle and field dependence of the transport Jc in coated conductors (CCs) are discussed. The granularity is revealed by scanning Hall probe microscopy of the trapped field profiles in tapes with different architectures. It was found that pulsed laser deposited (PLD) YBCO film on a RABiTS NiW substrate has the most prominent granular morphology in its trapped field profiles. This is complemented by a peculiar behavior of the critical currents when the orientation of the applied field is within a certain angular range around the ab-planes: Jc(θ) becomes independent of the angle and Jc(B) exhibits a peak. These phenomena are explained by the physics of Abrikosov–Josephson vortices at grain boundaries (GB) that leads to a transition from a GB-limited to a grain-limited regime both in Jc(θ) and Jc(B). The effects of granularity are strongly suppressed by the chemical solution deposition route for growing YBCO films on NiW, and are almost absent in PLD-YBCO on ABAD-textured templates. These results identify the collective influence of the GB on the percolative current flow in CCs, which becomes most significant at low temperatures.

A displacement-energy model of studying the delaminating behavior of multi-layer REBCO coated conductor tapes

A displacement-energy model (DEM) based on the classical plate theory and Hamaker method was developed to study the delaminating behavior of multilayer rare-earth barium copper oxide (REBCO) coated conductors (CC) tapes. Hamaker method focused on the extreme manifestation of Casimir effects such as cohesion/adhesion between bodies/surfaces and their associated energies. On this model, we studied the supposed defects (in-plane and interface relative separation displacement) created due to thermal mismatch of layers during deposition processing and free vibration of layers in time, and further the effect of these actions on the damage evolution of the CC tape. In the studied American Superconductors (AMSC 344®), the relatively thin films (silver, YBCO, and buffer layers) were modeled as a membrane, while the substrate (Ni-5W) was modeled as a thin plate (shell). We validated the DEM approach by a full 3-D Finite Element Method (FEM) counterpart model. The calculated results showed that unequal and scattered relatively weaker points located in the delaminating region of the CC are responsible for the delaminating behavior of multi-layer REBCO CC tapes, and the results of the location of weakest points with time indicates that delaminating starts at any of the layers' interface or simultaneously in more than one interface. Furthermore for this kind of REBCO CC tapes (for example AMSC 344®), we found that the most vulnerable interfaces to de-bonding are Ni-5W/Y2O3, CeO2/YBCO, and YBCO/silver stabilizer.

Epitaxial growth of gradient Ce1−xZrxO2 buffer layer for YBa2Cu3O7−δ coated conductors

A Ce1−xZrxO2 (CZO) buffer layer architecture with gradient change of Zr-doped concentration along the thickness direction of CZO films was fabricated on rolling assisted biaxially textured NiW metallic substrate by sol–gel method. With the change of Zr-doped concentration, the lattice parameters of CZO of each layer also show a gradient variation. It can not only effectively reduce interfacial stress between the buffer layer and the metallic substrate, but also improve the surface quality of the CZO film. In this work, an epitaxial growth relationship and dense microstructure are discerned in the gradient CZO film, as determined by the comprehensive analysis of X-ray diffraction and scanning electron microscopy. YBCO superconductor films have been deposited on the gradient CZO buffer layers to evaluate the quality of such a gradient buffer layer. TEM measurements reveal that NiW/CZO/YBCO coated conductor (CC) with excellent epitaxial growth relationship is achieved. The superconducting transition temperature of the sample is 90 K and the critical current density Jc reaches 1.9 MA/cm2.

High current variable temperature electrical characterization system for superconducting wires and tapes with continuous sample rotation in a split coil magnet.

A new state-of-the-art electrical transport measurement system was developed for the characterization of industrially produced coated conductors (CCs). The current leads are rated to a conduct current of up to 1000 A, which opens up the possibility of measuring the critical current Ic of tapes at a wide range of temperatures. The setup operates in a He-gas flow cryostat that provides stable temperatures between 1.8 and 200 K. The setup is equipped with a split-coil magnet that can apply fields of up to 6 T. A continuous rotation of the sample with respect to the magnetic field with an angular resolution of 0.5° enables characterization of anisotropic Ic of different tapes. In the measured voltage-current curves, weak sample heating mostly occurs from the dissipation in the tape during the Ic transition. It is demonstrated that the system can provide reliable data on the properties of CCs at temperatures lower than 77 K for a magnet design and other applications. The results allow the study of vortex pinning for further prospects of engineering the microstructure of the superconducting layer as well as to assess the performance of various tapes with different architectures to achieve optimum performance at different operating temperatures and magnetic fields.