Coated Conductor Samples Research Articles

Quench Protection Study of a Large Scale REBCO Magnet With Additional Copper Tapes

REBCO coated conductors (CCs) are expected to be applied to high field magnets due to their high longitudinal tensile strength and excellent critical current characteristics in high fields. For large scale magnets with large stored energy, quench protection is a key issue. Although various quench protection methods have been proposed for REBCO magnets, the most traditional and straight forward approach is to increase the copper stabilizer and reduce the current density. This approach leads to a significant increase in conductor length to generate a required magnetic field in case a small bore diameter magnet, while the conductor increase would not be serious in case a large bore diameter one. Based on this approach, we have successfully fabricated a practical cryogen-free superconducting magnet (CSM) with a large bore of 200 mm-diameter and a central field of 5 T using REBCO CCs laminated with 0.3 mm-thick copper tapes. For further study, we designed a MJ-class 10 T-CSM with the same inner-diameter as the 5 T-CSM and investigated the feasibility of the magnet through quench tests in this study. We conducted quench tests on short samples of copper-plated REBCO CCs with copper tape soldered on different surfaces, and quench tests on a small coil assuming an energy recovery method with external resistance. In the short sample tests, it was found that the temperature rise was suppressed regardless of the position of the copper tape lamination, and the normal zone propagation velocity was improved when the copper tape was laminated to the substrate side. In the small coil tests, the decay time constant after shutdown was varied to find the conditions that can protect the magnet with high stored energy. We confirmed that quench protection of 10 T-CSM was feasible under practical conditions.

Rapid assessment of REBCO CC angular critical current density J c(B, T = 4.2 K, θ) using torque magnetometry up to at least 30 tesla

Detailed design of REBa2Cu3O (REBCO)-based magnets ideally relies on knowledge of the full angular and wide temperature range characterization of the critical current of the REBCO coated conductors (CC) at high magnetic fields. In practice, however, obtaining data by the commonly used electrical transport technique is expensive, tedious, and difficult, due to high critical current values that exceed 2000 A for -plane (θ = 90 deg). The conductors are often damaged during angular transport measurements at angles approaching the ab-plane. Therefore, so far, REBCO magnets have been designed without full data sets. Here, we present results for more than twenty samples of CC all made to the same advanced pinning specification produced by SuperPower Inc. For this, we employed torque magnetometry, benchmarking the results to transport measurements mostly made away from ab-plane, finding good agreement with scaling factors ≈1–1.3. What is striking is the huge variety of properties exhibited by the samples, even for CC made recently. Given the huge attention now being paid to the effects of screening currents and to the torques generated by offsets between the tape plane and the local magnetic field vector, our data set suggests some caution in detailed design of such magnets without having data of the type presented here.

Concepts of static vs. dynamic current transfer length in 2G HTS coated conductors with a current flow diverter architecture

This paper uses both experimental and numerical approaches to revisit the concept of current transfer length (CTL) in second-generation high-temperature superconductor coated conductors with a current flow diverter (CFD) architecture. The CFD architecture has been implemented on eight commercial coated conductor samples from THEVA. In order to measure the 2D current distribution in the silver stabilizer layer of the samples, we first used a custom-made array of 120 voltage taps to measure the surface potential distribution. Then, the so-called ‘static’ CTL () was extracted using a semi-analytical model that fitted well the experimental data. As defined in this paper, the static CTL on a 2D domain is a generalization of the definition commonly used in literature. In addition, we used a 3D finite element model to simulate the normal zone (NZ) propagation in our CFD samples, in order to quantify their ‘dynamic’ CTL (), a new concept introduced in this paper and defined as the CTL observed during the propagation of a quenched region. The results show that, for a CFD architecture, is always larger than , whereas when the interfacial resistance between the stabilizer and the superconductor layers is the same everywhere. We proved that the cause of these different behaviors is related to the shape of the NZ, which is curved for the CFD architecture, and rectangular otherwise. Finally, we showed that the normal zone propagation velocity (NZPV) is proportional to , not with , which suggests that the dynamic CTL is the most general definition of the CTL and should always be used when current crowding and non-uniform heat generation occurs around a NZ.

Effect of annealing on HTS tapes with a cerium oxide layer inserted between the REBaCuO and silver layers

This paper presents extensive electrical, microstructural and chemical characterizations of HTS coated conductor samples in which an oxide layer (CeOx) has been added between the superconductor (GdBaCuO) and the silver (Ag) layers, in an attempt to increase the interfacial resistance between these two conductive layers. This increase of interfacial resistance is required to realize the current flow diverter (CFD) architecture in HTS tapes. All samples in this paper have been characterized before and after performing an annealing in oxygen atmosphere. The purpose of the annealing was to reduce the interfacial resistance generated by the CeOx layer, in order to achieve a proper value for the CFD architecture. Samples with different thicknesses of CeOx, namely 0, 10, 35 and 100 nm, have been produced and characterized. The critical current and the critical temperature have been measured to determine the quality of the superconducting layers, while cross-section transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) have been used to monitor the impact of the annealing. The results show a clear degradation of the superconducting layer for samples with a thick layer (≥35 nm) of CeOx after annealing at 450 °C. According to the EDS results, a reduction of the amount of barium is observed in the superconducting layer, which could explain the observed reduction of the critical current.

Effect of Annealing on HTS Tapes with a Cerium Oxide Layer Inserted between the REBaCuO and Silver Layers

This paper presents extensive electrical, microstructural and chemical characterizations of HTS coated conductor samples in which an oxide layer (CeOx) has been added between the superconductor (GdBaCuO) and the silver (Ag) layers, in an attempt to increase the interfacial resistance between these two conductive layers. This increase of interfacial resistance is required to realize the current flow diverter (CFD) architecture in HTS tapes. All samples in this paper have been characterized before and after performing an annealing in oxygen atmosphere. The purpose of the annealing was to reduce the interfacial resistance generated by the CeOx layer, in order to achieve a proper value for the CFD architecture. Samples with different thicknesses of CeOx, namely 0, 10, 35 and 100 nm, have been produced and characterized. The critical current and the critical temperature have been measured to determine the quality of the superconducting layers, while cross-section transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) have been used to monitor the impact of the annealing. The results show a clear degradation of the superconducting layer for samples with a thick layer (≥ 35 nm) of CeOx after annealing at 450 oC. According to the EDS results, a reduction of the amount of barium is observed in the superconducting layer, which could explain the observed reduction of the critical current.

Effective Measuring Position of Hall Probe and <inline-formula> <tex-math notation="LaTeX">$j_\mathrm{c}$</tex-math> </inline-formula> Characterization of Rectangular HTS Thin Films

Scanning Hall probe (SHP) system is a powerful instrument to investigate the planar distribution of areal critical current density (j c ) and defects of high temperature superconducting (HTS) thin film samples. The distance between the effective measuring position and probe bottom (h 1 ) is usually unknown for the commercialized Hall probes. However, h 1 value has a significant influence on the measurement results, according to the calculation of the magnetic flux density (B 0 ) above the planar center of a rectangular HTS thin film sample. In this paper, a method was proposed to obtain h 1 by using a series of copper strips and analyzing the relationship of B 0 against the parameters, such as sample width and current. Then, SHP system was employed to characterize the j c distribution of an HTS thin film fabricated via chemical solution deposition. The relationship of B 0 and h 1 could also be utilized to obtain the average j c value (j c B0 ). A series of commercialized coated conductor samples were measured with both SHP and the traditional four-probe method. The feasibility of preliminary evaluation for rectangular HTS thin film samples by using j c B0 was discussed based on its correlation with the results of four-probe method.

Critical current survival in the YBCO superconducting layer of a delaminated coated conductor

A high-temperature superconducting coated conductor can be practically applied in electric equipment due to its favorable mechanical properties and critical current (Ic) performance. However, the coated conductor can easily delaminate because of its poor stress tolerance along the thickness direction. It would be interesting to investigate whether the Ic of the delaminated YBa2Cu3O7−δ (YBCO) layer can be preserved. In this study, coated conductor samples manufactured through the metal organic deposition route were delaminated by liquid nitrogen immersion. Delaminated samples, including the YBCO layer and silver stabilizer, were obtained. Delamination occurred inside the YBCO layer and near the YBCO–CeO2 interface, as suggested by the results of scanning electron microscopy (SEM) and x-ray diffraction. A scanning Hall probe system was employed to measure the Ic distribution of the original sample and the delaminated sample. It was found that approximately 50% of the Ic can be preserved after delamination, which was verified by Ic measurements using the four-probe method. Dense and crack-free morphologies of the delaminated surfaces were observed by SEM, which accounts for the Ic survival of the delaminated YBCO layer. The potential application of the delaminated sample in superconducting joints was discussed based on the oxygen diffusion estimation.

Interfilament Resistance at 77 K in Striated HTS Coated Conductors

Striating HTS coated conductor (CC) tapes into narrow filaments offers the possibility of reducing the tapes' magnetization losses without unreasonably decreasing their current-carrying capability. However, realizing well-separated striations presents technological challenges, especially if the number of filaments is large and/or if a thick layer of metallic stabilizer is present. In these situations, the filaments can be easily coupled and their effectiveness to reduce magnetization losses is strongly diminished or eliminated. While the onset of coupling is well visible from magnetization loss measurements, the actual path of the coupling current is unknown. In this contribution we present a systematic study of the transverse resistance in HTS CC samples in order to get a deeper understanding of those paths. The measured samples differ in terms of manufacturer (SuperPower and SuperOx), and presence and thickness of stabilizer material. In addition, oxidation is used as a means to increase the resistance between the filaments in non-stabilized samples. The results are interpreted with a chain network model. This work provides useful insights on the factors determining the transverse resistance in striated HTS CCs, thus indicating ways to improve the effectiveness of the striation process for AC loss reduction.

Modeling and Comparison of In-Field Critical Current Density Anisotropy in High-Temperature Superconducting (HTS) Coated Conductors

The development of high-temperature superconducting (HTS) wires is now at a stage where long lengths of high quality are commercially available, and of these, (Re)BCO coated conductors show the most promise for practical applications. One of the most crucial aspects of coil and device modeling is providing accurate data for the anisotropy of the critical current density J c (B, θ) of the superconductor. In this paper, the in-field critical current density characteristics J c (B, θ) of two commercial HTS coated conductor samples are experimentally measured, and based on these data, an engineering formula is introduced to represent this electromagnetic behavior as the input data for numerical modeling. However, due to the complex nature of this behavior and the large number of variables involved, the computational speed of the model can be extremely slow. Therefore, a two-variable direct interpolation method is introduced, which completely avoids any complex data fitting for J c (B, θ) and expresses the anisotropic behavior in the model directly and accurately with a significant improvement in computational speed. The two techniques are validated and compared using numerical models based on the H-formulation by calculating the self-field and in-field dc critical currents and the ac loss for a single coated conductor.

Fabrication of 93.7 m long PLD-EuBCO + BaHfO3 coated conductors with 103 A/cm W at 77 K under 3 T

Introduction of artificial pinning centers such as BaHfO3 (BHO), BaZrO3 (BZO) and BaSnO3 (BSO) into REBa2Cu3O7−δ (REBCO) coated conductor (CC) layers could improve the in-field critical currents (Ic) in wide ranges of temperatures and magnetic fields. In particular, a combination of EuBCO+BHO has been found to be effective for attaining high in-field Ic performance by means of IBAD/PLD process in short length samples. In this work, we have successfully fabricated a 93.7m long EuBCO+BHO CC with 103A/cmW at 77K under a magnet field (B) of 3T applied perpendicular to the CC (B//c). The 93.7m long EuBCO+BHO CC had high uniformity of Ic values and n-values without any trend of fluctuations, independent of the external field up to 0.3T. Ic–B–applied angle (θ) profiles of the 93.7m long EuBCO+BHO CC sample showed the high in-field Ic values in all directions of applied magnetic fields especially B//c (at θ∼180°, Ic=157A/cmW) at 77K under 3T. The profiles were about the same as those in a short length sample.

Relation of <i>n</i>-Value to Critical Current for Local Sections and Overall Sample in a SmBCO Coated Conductor Pulled in Tension

Under application of tensile stress to a SmBCO (SmBa2Cu3O7¹¤) coated conductor sample consisting of series electric circuit of local sections, the relation of voltage­current curve, critical current and n-value of the sections to those of overall sample was studied. The change in critical current and n-value with increasing applied stress was different from section to section due to the difference in damage behavior of the SmBCO layer among the sections. When the difference in extent of damage among the sections was small, the voltages developed in all sections contributed to the voltage of overall sample. In this case, the critical current and n-value of overall sample were within the range of the highest and lowest values among the sections. On the other hand, when the damage in one section was far severer than that of other sections, the voltage developed in the most severely damaged section largely contributed to the overall voltage, and hence the voltage­current curves of the most severely damaged section were almost the same as those of overall sample. In this case, critical current of the overall sample was slightly higher and n-value of the overall sample was lower than the critical current and n-value of the most severely damaged section. Accordingly, the decrease in n-value with decreasing critical current in overall sample was sharper than that in sections. This phenomenon was accounted for by the increase in shunting current at cracked part at higher voltage in the most severely damaged section. [doi:10.2320/matertrans.MBW201301]

Minimum Quench Energy Characteristic of YBCO Coated Conductor With Different Stabilizer Thickness

The study on the minimum quench energy of YBCO coated conductor (CC) samples with different copper stabilizer thicknesses at various operating temperatures is performed. In this paper, we analyzed the thermal properties of YBCO CC in the adiabatic condition. Also, we measured the minimum quench energy of the three kinds of YBCO CC samples with different copper stabilizer thicknesses fabricated by Superpower Incorporated at various temperatures and currents. The results through this research will be utilized to determine the operating current of the conduction-cooled HTS magnet and to design the external protection circuit to keep the HTS magnet from being damage by a thermal quench.

Hydrostatic Pressure Effects on the Critical Current of YBCO Coated Conductor Wire

The critical current density of optimally doped bulk YBCO superconductor has been found to increase with the application of external hydrostatic pressure. This paper reports the transport measurements of the critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of YBCO in the form of a coated conductor (CC) wire under hydrostatic pressure up to 0.24 GPa at 77 K. No noticeable change was observed in both the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and n-value of the CC sample in the studied pressure range. Two possible scenarios are discussed to explain the observed effect.

Inkjet Printing, Pyrolysis and Crystallisation of YBa2Cu3O7-δ Precursor Layers for Fully Chemical Solution Deposited Coated Conductors

Abstract A complete reaction path for the pyrolysis of trifluoroacetate-derived YBa2Cu3O7-! precursor solution is reported based on the authors’ investigations of the separate trifluoroacetate salt decompositions using thermal analysis and infrared spectroscopy of evolved gases. Optimal conditions for the pyrolysis of inkjet printed films are proposed, and the influence of pyrolysis on the YBa2Cu3O7-! crystal growth on a fully buffered Ni-5%W substrate is revealed. Optimisation of the annealing step revealed a strong dependence of the superconducting properties on the heat treatment temperature and duration. The critical temperature ( Tc ) and critical current density ( Jc ) of fully chemical solution deposited coated conductor samples were examined by means of inductive and Hall probe measurements.

Comparative study on the critical current performance of Bi-2223/Ag and YBCO wires in low magnetic fields at liquid nitrogen temperature

A comparative study on the critical current performance of Bi-2223/Ag and YBCO coated conductor wires in low magnetic fields at liquid nitrogen temperature was carried out in this work. Five commercial high temperature superconductor wires from different manufacturers were collected. Their critical currents were measured in magnetic fields, ranging from 0 to 0.4 T. On contrary to the common conception, the Bi-2223/Ag samples had better performance than YBCO coated conductor samples in the magnetic fields parallel to the wide surface of superconducting wires within the experimental scope. We also found similar results by collecting the concerned datum from the published literatures to confirm our measurement results. At the present stage, this fact made that the Bi-2223/Ag wires might be the preferred choice for the applications with mainly low parallel fields involved, unless other considerations were prioritized.

Critical current degradation of shortYBa2Cu3O7 − δ coated conductor due to an unprotected quench

The critical current of a short YBa2Cu3O7 − δ (YBCO) coated conductor sample degrades in an unprotected quench performed in anearly adiabatic environment at 30 K. The conductor has Cu stabilizers on both surfaces.The quench is initiated by a heater attached to the sample surface. The amplitude ofthe transport current is fixed as 91% of the sample’s initial critical current. Theduration of the current is increased to simulate an unprotected quench and to reachincreasing and controlled voltage and temperature levels. A peak temperature of490 ± 50 K and a heatingrate of 1800 K s − 1 are measured when the critical current degrades by ∼ 5%. The applied thermal strain on the YBCO layer from 30 to490 K is estimated to be 0.31% and is applied at a strain rate of ∼ 1% s − 1.The rate of temperature change and the time to reach a certain peak temperature, determinedby the current density in the Cu stabilizer, are estimated assuming adiabatic conditions basedon the short sample case. For a Cu stabilizer current density ranging from 1000 to 2000 A mm − 2, achievedin commercial conductors currently available, the quench detection and protection requires a responsetime < 200 ms to limit the peak temperature below 200 K. A Cu stabilizer current density higher than 3000 A mm − 2 may challenge the existing detection and protection techniques for the same 200 K limit.Integrating the substrate as part of the stabilizer may help reduce the stabilizer currentdensity to gain more time for quench detection and protection while maintaining theengineering current density.

Analysis of Temperature Dependent Quench Characteristics of the YBCO Coated Conductor

Studies on (YBCO) coated conductor (CC) are actively underway for the practical application of high temperature superconducting (HTS) devices such as magnets, cables and fault current limiting devices because the CC has higher critical current characteristics under high temperature and magnetic field. However, it is hard to make a long-length of YBCO CC tape with uniform critical current density using the present processing techniques. The hot spot will emerge in a local region if the critical current is non-uniform along the HTS tape, and eventually, the hot spots may cause serious damage to the HTS tape. Therefore, it is important to examine the thermal stability characteristic of the YBCO CC tape from the quench protection point of view. In this paper, the segmental critical current of a YBCO CC sample tape is measured in liquid nitrogen, and a heater is installed at the center of the sample tape in order to examine the normal zone propagation characteristics in the longitudinal direction under the adiabatic condition with a Gifford-McMahon cryocooler. Measurements are performed as a function of heat input and transport current for the operating temperatures of 77 K, 65 K and 40 K, respectively, and the results are discussed in detail.

Thermal stability of YBCO coated conductor with different Cu stabilizer thickness

The studies on the conduction-cooled superconducting magnets are actively underway with rapid advancement in refrigeration technology recently. YBCO coated conductor (CC) is one of the promising conductors for a conduction-cooled superconducting magnet because of increasing the operation temperature of magnets and being able to have cost collectiveness with conventional copper conductor in the future. However, it is known that quench propagation velocity in high-temperature superconductor (HTS) is two or three orders of magnitude slower than that in low-temperature superconductor because of its large heat capacity and the high operating temperature. The hot spot will emerge in local region if a critical current is non-uniform along the length of HTS tape and eventually, it causes permanent destroy for the whole HTS tape. Based on the protection of YBCO CC, it is necessary to determine a suitable stabilizer thickness for YBCO CC so that the temperature of hot spot in local area does not exceed the permissible temperature.In this study, we have established a suitable thermal analysis model, and analyzed minimum quench energy and thermal properties for three kinds of YBCO CC samples with different stabilizer thickness, which are fabricated by Superpower Incorporated, using finite element method.

Characteristics of Simultaneous Quenches in Series-Connected YBaCuO Coated Conductors

This paper deals with characteristics of quenches in series-connected YBaCuO coated conductor (CC) samples to gain design parameters of non-inductive superconducting fault current limiter (SFCL). The extremely rapid superconductor-normal transition and inhomogeneous superconducting material cause the uneven quenches in series-connected CC samples. For this experiment, four stabilizer-free CC samples with 200 mm length had been connected in series to investigate the characteristics of quenches and voltage distribution in series-connected CC samples, and the short-circuit test was performed in the case of having shunt resistors of 363 mOmega in parallel with each CC sample and not, respectively. As the result of the experiment, the use of shunt resistors in parallel with CC wires enables to increase in the uniformity of quenching characteristics and parity of voltage distribution. The test results had been compared with the theoretical predictions by using finite element method (FEM).

Bending strain effects on the critical current in Cu-stabilized IBAD/EDDC processed SmBCO coated conductor tape

Influences of bending strain on the critical current, I c, in Cu-stabilized IBAD/EDDC processed SmBCO coated conductor (CC) tapes were investigated at 77 K and self-field using a small-scale Goldacker type bending test rig. Considering the location of the superconducting layer against the neutral axis, tensile and compressive bending strains were applied to the SmBCO coating layer and the strain effect on I c degradation was discussed. Under tensile bending strain, the I c decreased monotonically but recovered reversibly up to the irreversible strain limit. Under compressive bending strain, however, the I c increased but in some cases it suddenly dropped after straightening due to the delamination of the copper stabilizer. Also, repeated bending strains were applied to Cu-stabilized SmBCO CC samples in order to observe the behavior of I c under cyclic loading. I c Initially decreased with the application of 0.7% bending strain, but when the tape was subjected to repeated bending up to 100 cycles, no more degradation occurred. And finally, the I c almost recovered to I c0 when the tape was straightened.