Coated Conductors Research Articles

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.

Recent status and progress on HTS strand with quasi-isotropic critical current

The coated conductors (CCs), also known as the second-generation high-temperature superconducting (2G HTS) tape, has attractive potential applications in transmissions at low magnetic field/liquid nitrogen temperatures and high-field applications at low temperatures due to their excellent critical current density and mechanical performance. Its critical current density has strong anisotropy, particularly at 77 K. Because the single tape has limited capacity of carrying current, the cable or strand making of multi-tapes, stacked in parallel, is necessary. Within recent years, several cables, such as the Roebel cable, CORC cable, TSTC cable, TS cable, and the HTS-CroCo cable, were proposed. This paper presents a status and progress on a type of HTS strand made from REBCO tapes. Its quasi-isotropic critical current is experimentally confirmed for liquid nitrogen temperature, mechanical characteristics, AC losses, thermal stability, and quench characterizations are also researched. Finally, the feasible process for the production of practical length of such strand is described by processing 10 m long sample, which is promising for applications of high fields or power transmission.

The influence of cyclical lateral displacements on levitation and guidance force for the system of coated conductor stacks and permanent magnets

For the development of levitation bearings and transportation systems based on coated conductor (CC) tapes it is important to know not only information on levitation force at vertical displacement, but also the data on stability of the system during lateral displacements of superconductor relative to permanent magnets. In this paper, we present new experimental results on investigation of both levitation and guidance forces of CC-tapes stacks subjected to cyclical lateral displacements relative to the position of permanent magnet. For the measurements we used 12 mm wide commercially available CC-tape manufactured by SuperOx. The long tape was cut into pieces 12 mm × 12 mm which then put to the stack. Total number of the single tape in the stack N was varied from 5 to 100. Under lateral displacement we measured the dependencies of levitation and guidance forces on value of displacement for various number of N and found that both levitation and guidance forces tend to decrease as a number of periodical cycles of displacements increase. The rate of reducing of forces was observed to depend on the number of tapes in the stack. We observed that dependencies of levitation and lateral forces on lateral displacements have a hysteresis. The area of hysteresis, which corresponds to the magnitude of energy losses, was found to decrease with increasing of number of elements in the stack. Experimental results are in a good agreement with the theoretical calculations of lateral force.

Comparative Characterization of Magnetic Properties of Textured Ni–5 at.%W Alloy Substrates

The hysteresis loss of Ni–5 at.%W (Ni5W) substrate contributes considerably to the alternating current (AC) loss of coated conductors (CCs) working at low transporting current or low-frequency field. In the present work, the evolution of texture and magnetic properties in Ni5W substrates is analyzed and also the relationship between magnetic properties and the extrinsic characteristics (evolved due to the processing techniques) is determined. By making the comparison, we discovered the magnetic domain structures of Ni5W substrates for the first time, achieving a strip/branched domain structures in the recrystallized substrates. The hysteresis loss has been decreased dramatically for the recrystallized substrates. Interestingly, we also observed that this loss is also related to the annealing temperature in the fully recrystallized substrates. The reduced loss can be mainly attributed toward the decrease of grain boundary, which proves the contribution of grain size toward the reduction of hysteresis loss. This result provides a new insight to tailor the hysteresis loss of Ni5W substrates used for coated conductors.

Trapping Large Magnetic Field by Suppression of Thermomagnetic Instability in Coated Conductor Stacks

A new type of bulk magnet can be fabricated by simply stacking short segments of coated conductors (CCs) of 123-type superconductors. To enhance the trapping field of the CC stack, we irradiated 800 MeV Xe ions to CCs. The fabricated CC stack consists of 80 pieces of irradiated CCs, and it was magnetized by field-cooling method in an applied field 9 T down to 5 K. When we tried to trap a large field by reducing the field at ramp rates faster than 0.5 mT/s, flux jumps occurred, which abruptly suppressed the trapped field. By reducing the magnetic field down to zero at a ramp rate slower than 0.5 mT/s, we have succeeded in trapping 9 T at the center of the CC stack. We also confirmed that the CC stack can sustain the high trapped field for more than 20 min, which demonstrates the practical usefulness of such a staked CC magnet.

Development of surface planarization process using MOD-Gd-Zr-O layer

REBa2Cu3Oy (RE: rare-earth element, REBCO) coated conductors (CCs) have a high critical current density (Jc) under a magnetic field at the liquid nitrogen temperature and a potential performance for high-magnetic field applications. In order to prepare a biaxially highly oriented REBCO, a highly-textured buffer layer is required. The ion-beam assisted deposition (IBAD) technique is one of the promising techniques for preparation of the highly-textured layer even on the non-textured metallic substrate such as Hastelloy™. However, excellent surface smoothness and uniformity are indispensable to the non-textured metallic substrate using the IBAD technique. Therefore, an inexpensive surface planarization process has been required for fabrication of REBCO CCs with low-cost. In this study, we have investigated the surface planarization using the metal-organic deposition (MOD) method for the IBAD-MgO texturing. The Gd-Zr-O (GZO) planarization layer was prepared by MOD method on roughly-polished Hastelloy™ C-276 substrate. The surface roughness values (Ra) of MOD-GZO planarization layer became smaller than that of the Hastelloy™ C-276 with increase in coating times. Minimum Ra of the MOD-GZO planarization layer achieved less than 2 nm, which is smooth enough for obtaining the highly-textured IBAD-MgO film. The in-plane grain alignment of CeO2 film deposited on sputter-LaMnO3 / IBAD-MgO / MOD-GZO / Hastelloy™ C-276 substrate by the pulsed laser deposition was 4°, and high Jc value of 2.5 MA/cm2 (1.8 μm-t) was obtained for MOD-YBCO film fabricated on it. The CCs using MODGZO planarization layer provided the delamination strength over 70 MPa, which was compatible with conventional structure of CCs.

Development of Scribing Technique by using Multiple-laser Beams for Multi-filamentary Coated Conductors

The REBa2Cu3O7-d (REBCO) coated conductors (CCs) with the multi-filamentary structure is very useful for controlling the AC-loss and the shielding current. We have developed laser scribing technique to fabricate multi-filamentary structure by irradiating a rectangular beam using excimer laser source (KrF, λ∼248nm). Since the reflectance of stabilized silver and copper at the wavelength of Nd: YAG or fiber laser (∼ 1 μm) is 98% or more, destructive thermal processing must be required for processing in the wavelength range, which was the reason why we chose the excimer laser as light source. Actually, we successfully fabricated scribing structure with high precision. On the other hand, the improvement of the processing speed has been an important issue to be solved. Although the multi-laser beam system is effective for this purpose, the uniform intensity distribution in the width direction has to be realized. There is no strong Gaussian component in the excimer laser compared with a solid laser. The uniformity is useful for a single beam, however, it is not enough for the multi-beam system. In order to solve this issue, we have modified the beam intensity distribution by using homogenizer. Concretely, the beam intensity in the minor axis direction was flattened by a homogenizer using a plurality of horizontally elongated cylindrical lenses. This modification made the uniform range of laser intensity wider and it was confirmed that multi-laser beams can be applicable as a high-speed scribing technique.

Influence of AC Loss on Stress and Strain of Superconducting Coils

The second-generation (2G) high-temperature superconducting (HTS) coated conductors (CC) are increasingly used in power systems recently, especially in large-capacity superconducting magnetic energy storage (SMES). HTSCC in superconducting energy storage coil is subjected to thermal stress which is caused by thermal contraction due to AC loss. The thermal stress will degrade the critical currents and may cause the displacement of the superconducting coil and insulating epoxy resin. In this work, the AC loss of the superconducting coil is firstly obtained by finite element method (FEM) software COMSOL with the method of iterative conductivity. Furthermore, the effect of AC loss on the superconducting coil is thus analyzed by solving the coupling problems which include magnetic field, mechanical behaviors, and heat transfer in solids. The result shows that the AC loss of the superconducting coil is concentrated in the middle of the coil and the maximum AC loss density is 5.96 × 105 W/m3; the maximum stress laid at the inner radius of the coil and the maximum stress is 0.01 MPa.

Quench and recovery characteristics of second-generation high-temperature superconducting GdBCO coated conductor with various patterns of stabilizers

Abstract This paper investigates the quench and recovery characteristics of second-generation high-temperature superconducting GdBCO coated conductors (CC) with variously patterned stabilizers in over-current conditions. A chemical etching method was used to produce various patterns of stabilizer on GdBCO CC without degrading current-carrying capacity. From the results of the over-current testing of GdBCO CC tapes with variously patterned stabilizers, the tape that had stabilizer with longitudinally disconnected patterns exhibited higher voltage increase and slower recovery time than those that had continuity of stabilizers along the longitudinal direction, because the excessive heat and current could not be transferred through the stabilizers in the former. Overall, uniform heat and current distributions along the stabilizer of GdBCO CC are among the critical factors determining thermal and electrical stabilities of a GdBCO magnet in over-current conditions, especially when quench occurs.

Joint Resistance Characteristics in Ultrasonic Weld REBCO CC Tapes at Various Temperatures and Magnetic Fields

In coil and magnet manufacturing that requires long length 2G coated conductor (CC) tapes, the development of joining methods for CC tapes with zero or low resistance is necessary. Nowadays, although it is said that CC tapes with around 1-km length were produced, it is still inadequate to produce cables and coil windings. Therefore, the development of a stable and reliable joint for long CC tapes having a low joint resistance (R j ) and the understanding of the magnetic field and temperature dependence of R j is essential in the design of high field magnets and coils. In this study, the characteristics of joint resistance in REBCO CC tapes jointed using ultrasonic welding (UW) method were experimentally evaluated in various temperatures and magnetic field. The CC joints were also fabricated by hybrid welding, which incorporates soldering to the UW joint. The magnetic field and temperature dependencies of joint resistance of both types CC joints were measured. The acquired results showed that the joint resistance decreased with a decrease of temperature, and increased slightly as magnetic field applied increased.

Experimental and Numerical Investigation of Shielding Performance of Superconducting Magnetic Shields Using Coated Conductor Tapes

This study investigated the magnetic shielding performance of various configurations of superconducting cylinders based on high temperature superconducting Coated Conductor (CC) tapes. We applied ac magnetic fields with various frequencies and amplitudes to superconducting cylinders with CC tapes. The magnetic shielding performances of the cylinders with a variety of layer number and length of the cylinder were compared. We also performed the numerical simulations, and got the same tendency as the experimental results. We clarified that the shielding factor lineally increases with the layer number of the CC tape. More than five layers of the CC tape wound with the same orientation and angle to cover the gaps of an inner layer achieves 98% attenuation of the magnetic field. However, the simulation and experimental results show that shielding factor is not greatly improved by increasing the length of the cylinder due to the magnetic field penetration from the gaps between each winding of the CC tape. This study demonstrates a good possibility and also the limitation to use CC tapes for the magnetic shielding.

Structural Modeling of REBCO Coated Conductor Tapes in TORT Cables

High-temperature superconductor devices often need conductors with transport capacity obtained by assembling more REBCO (RE = rare earth, B = barium, C = copper, O = oxygen) coated conductor (CC) tapes in parallel, thus forming a cable. One method of cabling flat REBCO CC tapes is the tape-on-round-tube (TORT) concept. In TORT cables, multiple CCs are wound in a helical fashion on a copper tube, through which the coolant (liquid nitrogen or cold helium vapor) may flow. A detailed structural finite-element (FE) analysis using the FE code ANSYS was carried out to investigate the stress-strain behavior of CC tapes under tension and bending loading during the winding process. In addition to the finite-element method (FEM) investigations, the measurements of critical current dependence on the axial and bending load of REBCO CC tapes at room temperature were performed. Finally, the possibility of the application of FEM simulation to predict the critical loads of REBCO CC tapes subjected to mechanical loading during the winding process is assessed.

Evaluation of Actual Delamination Strength of REBCO-Coated Conductors Based on the Weibull Analysis Considering Size Effect

Fujikura has developed production techniques of REBa 2 Cu 3 O y (REBCO, RE = rare earth) coated conductors (CCs) and provided high-performance REBCO CCs with long-length and high homogeneity of the critical current. REBCO CCs have a low c-axis strength (delamination strength) which originates from the layered structure. When an epoxy-impregnated REBCO coil is cooled, the radial thermal stress is generated. The stress could cause the delamination of REBCO CCs and lead to degradation of the coil. Therefore, it is important to evaluate the exact delamination strength of the CCs. In this paper, we evaluated the failure probability of the epoxy-impregnated coils at 77 K. We introduced Weibull analysis considering the stress distribution and the sample size effect, and estimated the actual delamination strength of the REBCO CCs. By using the evaluation method, we confirmed the strength enhancement by process improvement.

Progress of REBCO Coated Conductor Program at SJTU and SSTC

At Shanghai Jiao Tong University (SJTU), REBa2Cu3O7‐σ (REBCO) coated conductor (CC) research program started in 2007. With the support of China's First National Key Project on CC Program, SJTU successfully developed hundred meters long class CC tapes with over 300 A/cm (at 77 K, self-field) based on PLD (pulsed laser deposition)/RABiTS processes in 2011. To develop high-performance CC tapes, in 2012, we switched from RABiTS to ion beam-assisted deposition (IBAD)-MgO tape and a pilot PLD/IBAD process CC fabrication line was set up at Shanghai Superconductor Technology Corporation in 2013, and deposition processes were successfully transferred from lab to fabrication line. Under optimized conditions, the IBAD-MgO layers showed pure (001) orientation and excellent in-plane texture. Atomic force microscopy observation showed MgO layer had very smooth surface. The rms is less than 1 nm. On the textured MgO layer, single CeO2 layer showed pure (001) orientation and excellent in-plane texture of ${\text{4}}^{\circ}$ – ${\text{5}}^\circ$ . Reel-to-reel PLD process was already scaled up to >100 m/h tape speed. Kilometer (Km) long CC tapes with over 300 A/cm have been routinely fabricated. Recently, our research work focused on introducing high-density nanoscale artificial flux pinning centers into REBCO superconducting layer to improve in-field superconducting properties and further expand application areas of REBCO tapes.

Two Level Undercut-Profile Substrate-Based Filamentary Coated Conductors Produced Using Metal Organic Chemical Vapor Deposition

The two level undercut-profile substrate (2LUPS) has been introduced as a concept for subdividing rare-earth-Ba 2Cu3O7 coated conductors (CC) into narrow filaments that effectively reduces the ac losses and improves field stability for dc magnets. The 2LUPS consists of two levels of plateaus connected by a wall with an undercut-profile, which enables a physical separation of the superconducting layer between the plateaus without reducing the effective width of the superconducting layer. In this study, we report for the first time the results of fabrication and characterization of a filamentary CC produced in an industrial setup by SuperPower Inc. using ion beam assisted deposition and metal organic chemical vapor deposition (IBAD-MOCVD) on a 2LUPS realized at the Technical University of Denmark, whereas previous studies discussed the fabrication using alternating beam assisted deposition and pulsed laser deposition (ABAD-PLD). We also present Hall probe scanning measurements performed using a standard THEVA TAPESTAR XL machine that is routinely employed for industrial critical current characterization of long length CCs. From these results is it clear that additional analysis of the measured field profiles are required when characterizing filamentary 2LUPS CC using a standard TAPESTAR setting. Using a model representation of the 2LUPS we calculated the expected magnetization response by means of finite element methods simulations and we find a good agreement with the experimentally observed magnetic profiles.

Development of Long-Length BMO-Doped REBCO Coated Conductors by Hot-Wall PLD Process

Fujikura Ltd., Chiba, Japan, has developed stable and high-throughput production techniques for REBCO coated conductors (CC) by using large-area ion-beam-assisted-deposition (IBAD), and hot-wall type pulsed-laser-deposition (PLD). Recently, we have been attempting to dope BaMO 3 (BMO, M: Zr or Hf) to REBCO layer in order to enhance in-field critical current density (J c ). In the course of optimizing deposition conditions of PLD for BaHfO 3 doped EuBCO films on IBAD templates, “high J c condition” and “high growth rate condition” were found. For samples prepared under these conditions, the magnetic field angle dependence of I c and REBCO thickness dependence were investigated. Finally, we succeeded in fabricating a 300-m-long CC with uniform I c distribution at the high-growth-rate condition. The longitudinal I c homogeneity (defined as standard deviation divided by average) at 77 K evaluated by magnetization method was 2.6%, which is comparable to nondoped conductors. Transport in-field I c for the both end points were 1756 and 1786 A/cm at 30 K, 2 T, respectively.

The Post-Annealing Effect on the Pinning Properties of GdBa2Cu3O 7−δ Coated Conductors via RCE-DR

We report the post-annealing effect on the pinning properties of GdBa 2 Cu 3 O 7-δ (GdBCO) coated conductors (CCs) fabricated by a reactive co-evaporation deposition and reaction process. The GdBCO CCs annealed at 750 °C in the PO 2 of 300 mtorr for 15 min show the most enhanced critical current densities (J c ) for B//c due to the stacking faults (SF) piled up along the c-axis. The minimum J c values of post-annealed samples in relatively higher fields such as 5 T at 77 K and 7 T at 65 K were larger than those of pristine sample; although their minimum J c values were lower in relatively lower fields such as 1 T at 77 K and 3 T at 65 K. In-field J c values for B//ab were closely related to the density of SFs within samples. Analysis results by transmission electron microscopy revealed that the SF density was remarkably reduced after the post-annealing process, which is responsible for enhanced minimum J c values and significantly degraded J c values for B//ab in relatively higher fields.

Effects of chemical etching on structure and properties of Y0.5Gd0.5Ba2Cu3O7-z coated conductors

Corrosion resistance is a crucial property to achieve successful superconducting joints of Y0.5Gd0.5Ba2Cu3O7-z (YGdBCO) coated conductors (CCs). Cu and Ag metallic layers need to be fully removed from the area of conductor to be joint to allow for a superconducting path across the joint. Therefore, when using a wet etching process to remove the metallic layers, the joint performance can be significantly influenced by the etching conditions. The effects of chemical etching with ammonia water and hydrogen peroxide mixture on crystal structure, surface microstructure and critical current (Ic) of YGdBCO CCs were systematically investigated. We found the set of etching parameters that does not affect conductor performance, leaving the Ic of the YGdBCO conductor unchanged upon etching. However, when the etching conditions are not optimal, decrease in Ic was found and the underlying reasons driving the degradation were investigated. Raman spectroscopy and XRD analysis indicated that the reduced Ic is mainly due to oxygen deficiency in the YGdBCO crystal lattice.

The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba–Cu–O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.

Current Capacity of Cu-Sheathed Multifilamentary Coated Conductors Under the Influence of Spatial Variation of Local Critical Currents in Each Filament

We investigated the relationship between current capacity and local critical current distribution of a Cu-sheathed multifilamentary RE-123 coated conductor (CC). Patterning multifilamentary structure on CC will be a promising solution for reducing magnetization to assure spatial homogeneity and its temporal stability of magnet applications such as MRI and NMR. On the other hand, it will become more difficult to maintain the current capacity because a smaller defect can block the current flow in a narrower filament. Permitting electrical coupling among the filaments will work for maintaining current capacity because current can avoid such a defect by flowing into the adjacent filament. However, too small interfilamentary resistance will result in long time constant of filament coupling, which will affect spatial homogeneity and its temporal stability of magnet applications. Therefore, to design a multifilamentary CC satisfying the requirement from magnet applications, it is necessary to understand the quantitative impact of interfilamentary resistance of the multifilamentary CC on its current capacity under the influence of spatial variation of local critical currents. In this study, we estimated global critical current of a Cu-sheathed multifilamentary CC as a function of interfilamentary resistance by considering its local critical current distribution in each filament. As a result, it was confirmed that the electrical coupling among the filaments was very effective to improve the current capacity of such a multifilamentary CC especially for a section with spatially inhomogeneous local critical currents. Furthermore, it was also found that local heat generation could be significantly suppressed even for a section with relatively homogeneous local critical currents.