CC Tapes Research Articles

The Critical Current of REBCO Coated Conductors Subjected to a Mechanical Loading at Varying Angles

High-temperature superconducting REBCO coated conductors and cables made from them are required in the building of fusion reactors, high-field magnets, and other high-current and high magnetic field devices. In the construction phase, the coated conductors undergo mechanical loading throughout the coated conductor production, cable manufacturing and when the resulting device is put in operation. Therefore, a simple method for investigating the loss of critical current after bending at different diameters and angles would be helpful. Here, we present a setup designed with simplicity in mind, allowing easy replication of testing conditions. As an example, the behaviour of two conductors has been characterized. We found differences in critical bending diameter when CC tapes were bent at 30 degrees angle and 45 degrees angle. The measurements were complemented by numerical and analytical calculations. The analytical calculations are an idealized 2D model serving as a first approach method for a fast prediction of loading severity. 3D numerical modelling of the measurement setup was performed in finite element software ANSYS, allowing us in-depth investigation of the strain/stress distribution in the multilayer structure of REBCO coated conductors. Scanning electron microscope study of the surface of the superconducting layer on one of the samples helped explain the measurement results.

Review of the advancements in aluminum and copper ultrasonic welding in electric vehicles and superconductor applications

Ultrasonic welding (UW), a most promising solid-state welding technique, is being developed for the automotive industry, more specifically, for joining components in electric vehicle batteries. It has also been used in rare-earth barium copper oxide (REBCO)-based superconductors, which have different stabilizing materials. At present, challenges remain when using UW to join aluminum (Al) and copper (Cu) in either application, and research in these areas are undeniably important for the future use of fully in-line UW applications. This paper provides a review of the current status of UW for joining Al and Cu, specifically in electric vehicle batteries and REBCO-based superconductors. Important issues about process characteristics and critical concerns about weld parameter optimization, joint structure morphology, temperature measurements, joint mechanical properties, and quality of the joints are addressed. Modeling and simulations using the finite element method in predicting the physical phenomena in the dynamic UW process are discussed. The status of electromechanical behaviors in ultrasonically welded joints in superconductors is detailed. Lastly, guidelines and valuable knowledge for the integration of Al and Cu UW in electric vehicle battery assembly and superconductors are provided as a basis for future research directions in these fields. • Process characteristics, critical issues and findings for UW of Al-Cu are explored. • Discussion on UW future integration in EV battery assembly and CC tapes’ joints. • UW in-line process suitability to join various materials’ combinations are examined. • Several aspects of UW that needed further explorations are investigated. • Joint quality control and reliability methods with uniform standards are evaluated.

Evaluation of the Electromechanical Performance of Practical REBCO CC Tapes Under Low Magnetic Fields Using a Continuous Critical Current Measuring System at 77 K

Evaluation of the Electromechanical Performance of Practical REBCO CC Tapes Under Low Magnetic Fields Using a Continuous Critical Current Measuring System at 77 K

Scalable superconductive magnetic bearing based on non-closed CC tapes windings

A new type of scalable high-temperature superconducting magnetic bearing is proposed. The bearing is based on the multilayer open windings made of coated conductor-tapes (CC-tapes). To demonstrate the possibility of manufacturing such a bearing construction we have built a working prototype and studied its characteristics. The manufactured model of bearing includes a stator, consisting of three multilayer rings (CC-tapes pancake coils) of 12 mm CC-tape and a composite rotor consisting of three rings of permanent magnets, each composed of 16 NdFeB 10 × 10 mm permanent magnets. We showed a contactless levitation of rotor relative to stator, as well as rotation of rotor and we measured load curves, namely the dependence of the rotor displacement on restoring force for axial and radial loads. Numerical simulation of bearing load characteristics has been carried out by the finite element method in the Comsol Multiphysics software. The calculation results are in a good agreement with the experimental data.

Magnetization Loss Estimation of HTS Solenoid Coils Wound With CORC

CORC is one of the candidates of the HTS cable for high field magnets. We have been interested in applying CORC to HTS power transformers that require not only large currents but also very low AC losses. Striation and twisting are the solutions for reducing AC losses. We suggested an estimation method for a rough calculation of magnetization losses in a striated CORC coil. First, we measured and calculated the magnetization losses of short samples of straight CC tapes and CORC. Before considering the striation effect, we analyzed the characteristics of slit HTS tapes. In order to emulate the slitting of an HTS tape, we prepared three kinds of HTS tapes with different widths: 2, 4, and 6 mm. Slitting will certainly create the effect of reducing the magnetization loss, but the effect is not directly dependent on the number of the slits nor the applied field magnitude. We measured and compared the magnetization losses of a wide HTS tape to those of narrow HTS tapes. And the narrow tapes were arranged in parallel and measured, so the net width was same as the width of the wide tape. Finally, we applied the measured data of the magnetization loss of several samples to the calculation of the losses of solenoid coils wound with CORC in order to take into account the loss reduction effect of slit HTS tapes.

Reaction of REBaCuO CC Tapes to Neutron Irradiation

Reaction of REBaCuO CC Tapes to Neutron Irradiation

Experimental investigation of the processes of degradation and transition to the normal state in CC-tapes under the action of current pulses

The study of the behaviour of CC tapes under the action of nonstationary current loads was conducted: a transition to the normal state was observed, as well as processes of CC-tape degradation. The obtained data make it possible to solve the problems associated with the creation of high-speed switching devices based on high-temperature superconductors, intended for functioning in superconducting energy storage devices, energy distribution and transmission systems, current limiters, and new types of transport. In addition to superconducting switches, pulsed current loads can occur in the mentioned above systems due to short-circuit conditions or other factors, and can lead to local heating of HTSC, especially if there is an inhomogenity in the critical current in the CC-tapes. In this work we present the results of a study of non-stationary processes second generation HTS tapes heating under the impulse current loads with amplitudes up to 2.2Ic (77K, self-field). Superconducting tapes with a copper protective layer were used as the samples. The I-V characteristics of the samples were measured under the action of current pulses during cooling in liquid nitrogen. Critical parameters (amplitude, rise time of the transport current) of the current pulses, leading to the degradation of the superconductor were obtained. local current carrying characteristics of CC-tapes investigated using Hall probe magnetometry. The features of the influence of local inhomogeneities in the distribution of the critical current of the tapes on the transition of the superconductor to the normal state are revealed.

Variations of the Strain Effect on Critical Current in REBCO Coated Conductor Tapes Depending on Test Probes

In practical application of the 2G CC tapes, the characteristic I c response with respect to strain and magnetic field is important in designing superconducting devices including magnets and coils. In order to elucidate the strain effect on the critical current, I c, along the longitudinal direction of CC tapes, three different kinds of test rigs or methods were employed: a springboard bending beam holder which can apply continuously both tensile and compressive strains to the CC sample attached on the holder, a Goldacker-type bending test rig without any holder to support the sample during bending, and uniaxial tension test method. The strain effect on the I c varied depending on how the strain was applied, but all the probes verified that excessive straining on the REBCO CC tapes does affect the I c resulting in a significant degradation. The strain effect on I c under low magnetic field in both tensile and compressive strain region was examined using the springboard bending beam and a pair of Nd permanent magnets.

Bending properties of solder joint of YBCO coated conductors by etching copper stabilizer

The YBa2Cu3O7−x coated conductor (YBCO CC) is one of the most promising superconducting materials for magnet applications because of its high critical current (Ic) and low dependency of the Ic on the external magnetic field. For magnet applications, plenty of joints are required to meet the connection of relatively short YBCO CC tapes. The bending properties of the joints inevitably become one of the critical factors that determine the performance of the superconducting magnet. In this paper, we fabricated the joints of YBCO CC tapes by etching Cu stabilizer and studied their bending properties under various bending diameter at 77 K and self-field. Compared with the soldering joint, lower joint resistance (Rj) and preferable electrical properties under applying various bending deformation were achieved and proved by the bending strain tests and finite element simulations. This fabrication technique is a unique practicable solution to reduce the Rj and improve the bending properties for the joints of YBCO CC tapes with thicker metal stabilizing layers in second generation high temperature superconductor (2G HTS) applications.

Strain Characteristics of Ic in Brass-Laminated GdBCO CC Tape Under Tension at Various Low Temperature and Magnetic Field Conditions

Transport properties of coated conductor tapes can be affected by stress and strain induced during fabrication, cool-down, and operation. Therefore, it is important to understand the strain/stress dependence of the critical current of the coated conductor tapes in the reversible region under magnetic field considering practical applications such as magnets and coils. In this paper, the strain characteristics of critical current, I c , in high temperature superconducting (HTS)-coated conductor tapes fabricated using reactive co-evaporation by deposition and reaction reinforced with brass-foil laminate were investigated under external magnetic field at various cryogenic temperatures while applying uniaxial tension. Results show that I c dependences on magnetic field and temperature were similar between brass-laminated coated conductor and Cu-stabilized tape. The irreversible strain limit and the irreversible stress limit of brass-laminated coated conductor tape increased as temperature decreased from 77 to 20 K. This behavior resulted from thermal hardening of the coated conductor tape at low temperature.

Investigation of the hysteresis losses in CC tapes after laser filamentation

Study of AC loss has been conducted on the 4 mm wide CC-tapes manufactured by SuperOx. Filamentation was carried out by laser cutting of the commercially available tapes with a copper coating. Also for comparison, original tapes and commercial multifilament tapes (manufactured by SuperOx by using chemical etching method) were studied. Losses were obtained from the magnetization curves measured on a vibration sample magnetometer in the temperature range from 4 K to 77 K. In addition, the current carrying characteristics of tapes were studied, and the effect of filamentation on the critical current value were examined. We present an analysis of the experimental results, as well as a comparison of data for different types of samples. The possibility of decreasing the energy losses due to filamentation tapes was demonstrated.

Joining of CC Tapes With Lead-Free Solders

Overlap joints of the SuperPower 2G HTS tape SCS12050 with surrounding copper stabilizer were prepared, in controlled conditions, using a lead-free solder and two types of fluxes in air atmosphere. A clamping device was developed, in order to control the evolution of temperature during the process. The reflow time (during which the solder melting is expected) and the pressure exerted on the joint area were utilized as parameters in the optimization of the soldering procedure. The melting point of the used Sn96.5Ag3Cu0.5 alloy was 217 °C; soldering temperatures slightly exceeded the latter temperature. Quality of the joint was evaluated by measurement of the voltage–current curve, from which the joint resistance, critical current, and $n$ -value were determined. Resilience against temperature cycling was tested on one of the samples. Electrical characterization is complemented by structural investigation with scanning electron microscopy, which was focused on the microstructure of the soldered joint layer. Surprisingly, the best joints were obtained at the lowest pressures used in these experiments. With regard to the reflow times and used fluxes, a slight reduction in the joint resistance was observed at longer duration of melting conditions and using rosin flux.

Effect of micro-ceramic fillers in epoxy composites on thermal and electrical stabilities of GdBCO coils

Abstract This paper reports the effects of ceramic fillers for epoxy composites used as impregnation materials on the thermal and electrical stabilities of three GdBCO coils impregnated with epoxy alone, with an epoxy/AlN composite, and with an epoxy/BN composite. During cool-down to 77 K, due to the high thermal conductivity of the filler materials, the coils impregnated with the epoxy composites that included the AlN and BN fillers exhibited faster cooling times than the coil impregnated with epoxy resin alone. Moreover, the addition of the filler could facilitate quench heat dissipation as well as ameliorate the discrepancy of thermal contraction between the GdBCO CC tape and the epoxy. In particular, the coil impregnated with the epoxy/BN composite exhibited superior performance in cooling, over-current, and repetitive-cooling tests. Therefore, the epoxy/BN composite could be the most effective impregnating material for the development of highly-stable superconducting rotating machines, with considerably enhanced reliability.

Pursuing low joint resistivity in Cu-stabilized REBa2Cu3Oδ coated conductor tapes by the ultrasonic weld–solder hybrid method

Development of a coated conductor tape joint with good quality and low joint resistivity, Rsj, in terms of transport and mechanical properties, was attempted by direct bonding at the interface of the Cu–Cu stabilizers in overlapped GdBCO CC tapes. In this study, we attempted to achieve a low Rsj by introducing hybrid joining, soldering and ultrasonic welding (UW), and its mechanism was analyzed theoretically. Coated conductor tapes were experimentally joined using various methods of soldering, UW, and combinations of the two. As a result, a much lower Rsj of about 57 nΩ · cm2 was obtained for RCE-DR-processed GdBCO CC tape joints using the hybrid joining method. The mechanical properties of the jointed CC tapes were also evaluated at room temperature and 77 K under self-field. Load–displacement curves of joined CC tapes followed the curve of the single CC tape. Critical current and joint resistance, Rj, of hybrid-joined CC tape were retained after double bending at room temperature up to 20 mm bending diameter.

Reel-to-Reel Selective Electroplating of Cu Stabilizer for Multifilamentary Coated Conductors

A continuous, reel-to-reel DC electroplating process has been developed to produce high-quality Cu stabilizer for multifilamentary coated conductors. Due to the large resistivity difference between the silver-coated superconducting filaments and the insulating oxidized grooves between the filaments, selective electroplating was achieved on laser-striated multifilamentary tapes. Every superconducting filament in the tape was homogeneously copper-coated without continuous deposition of copper on the insulating grooves. The multifilamentary tapes are then fully stabilized without significant reduction in critical current and the benefit from filamentization on AC loss is maintained. The high flexibility of electroplating enabled us to produce multifilamentary coated conductors with copper stabilizer thickness from 5 to 30 μm. A 10-m tape was fabricated to demonstrate the scalability of fully-stabilized, low-loss multifilamentary coated conductors. The stabilized 12-filamentary CC tape is able to achieve four-fold AC loss reduction with minimal reduction in its critical current.

Fabrication of long REBCO coated conductors by PLD process in China

In China, the First National Key Project on CC Program started in 2009, which was focused on developing hundred meter long class CC tapes based on PLD/RABiTS processes. In this project, SJTU mainly worked on all of functional layer deposition process development. Northwest Institute for Non-ferrous Metal Research worked on RABiTS tape fabrication. At the end of the project in 2011, SJTU successfully fabricated hundred meter long CC tapes with over 300A/cm (at 77K, self field) on RABiTS tapes. To develop high performance CC tapes by PLD/IBAD-MgO processes, a pilot CC fabrication line was set up at Shanghai Superconductor Technology Corporation, Ltd. in 2013. High quality long REBCO coated conductors have been successfully fabricated on flexible polycrystalline metal tapes by PLD plus magnetron sputter and IBAD processes. Under optimized conditions, the IBAD-MgO layers showed pure (001) orientation and excellent in-plane texture. The in-plane phi-scan rocking curve is 4–6 degrees. AFM observation showed MgO layer had very smooth surface. The RMS is less 1nm. On the textured MgO layer, sputter deposited single cerium oxide cap-layer showed pure (001) orientation and excellent in-plane texture of 4–6degree. Reel-to-reel PLD process with high deposition rate was already scaled up to 100m/h tape speed. Hundred meters long coated conductor tapes with over 500A/cm performance have been routinely fabricated. And now, the process optimization for kilometer long coated conductor tapes is underway.

Effects of Melting Diffusion and Annealing in Oxygen on Superconducting Characteristics of GdBCO Coated Conductors: Preliminary Results

This study examined the superconducting properties of GdBCO coated conductor tapes before and after melting diffusion and subsequent annealing in oxygen. Such investigation is necessary in order to determine the optimal parameters of these processes for the future development of a superconducting joint between coated conductors. The melting diffusion process under a reduced oxygen partial pressure (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O2</sub> ) at 850 °C for 60 s enabled the maintenance of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis orientation of Gd123 with few secondary phases such as CuO and BaCuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , resulting from partially melted GdBCO. To restore the oxygen lost during the melting diffusion process, an additional oxygenation annealing process was conducted. The critical current of the GdBCO CC tape oxygenation annealed at 600 °C for 50 h and then at 500 °C for 200 h sequentially was 108 A, indicating that the superconducting properties were considerably recovered. Based on the optimal joining parameters obtained in this study, future studies on the superconducting joint of the GdBCO CC will be carried out.

Determination of winding diameter based on bending strain analysis for REBCO coated conductor tapes

In order to recognize the allowable bending diameter in coils, the strain as function of diameters is evaluated. The irreversible strain limits of <TEX>$I_c$</TEX> in the easy and hard bending modes were measured. Strains were calculated at the coating film in the easy bending and at outer edge or inner edge in the hard bending of the CC tape, respectively. The tape geometry subjected to bending procedures is considered from the current industrial spool winding operation. Through the linear superposition of strain induced in different bending modes regarding the expressions, the appropriate design for critical bending diameter is suggested. Results proved that the existence of buckling resulting from bending in hard direction when applied strain exceeded 0.6% is possible. The depicted results showed that the strain limit as a viable parameter should be considered for future purposes.

Investigation of Superconductor Uniformity in CC Tapes by Magnetic Field Mapping

Uniformity of superconducting layer can play important role in various applications of coated conductor tapes. For example a degradation of critical current density at the tape edges causes significant elevation of transport AC loss compared to the uniform tape with the same critical current. Then the information about tape uniformity is essential when a valid AC loss prediction should be obtained by a numerical simulation. For this purpose we investigated experimentally the distributions of magnetic field above the tape surface obtained by Hall probe mapping. An advanced inversion procedure has been developed to resolve the critical current distribution across the tape width. The results obtained for SuperPower̈ CC tapes with non-magnetic substrate are presented and the implications on the AC loss behaviour are discussed.

Establishment of strain measurement system for evaluation of strain effect in HTS tapes under magnetic field

The evaluation of the electromechanical properties of HTS CC tapes is one of the foremost procedures to be done to ensure the applicability of superconducting wires to electric devices. A precise measurement of the stress and strain is important in deriving the mechanical properties under operating environment. Up to now, there is no standard test method yet for the electromechanical property evaluation of HTS tapes under self field and external magnetic field although there are already reports on the different devices used to evaluate these properties. Strain can be measured by adopting a strain gauge or a high resolution double extensometer. In this study, strain effect on <TEX>$I_c$</TEX> in HTS CC tapes under magnetic fields was evaluated. Comparison of advantages and setback of strain measuring devices were discussed. In addition, a dual strain measurement system using both the SG and extensometer may be practical to lessen the burden in case one of the measuring devices does not work well.