In-field Critical Current Density Research Articles

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.

Enhancement of Critical Current Density by Establishing a YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x Quasi-Trilayer Architecture Using the Sol-Gel Method

We developed a solution-derived method to establish a YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x quasi-trilayer architecture. Using the method, nano-scale pinning sites were induced into the quasi-trilayer architecture and yielded an apparent improvement in the in-field critical current density (Jc) of high-quality YBa2Cu3O7−x (YBCO). The improvement in the in-field Jc of the films was closely related to the thickness of the LaAlO3 (LAO) interlayer. In this paper it is demonstrated that when the nominal interlayer thickness approximates 20 nm, which is slightly higher than the roughness of the YBa2Cu3O7−x surface, the LaAlO3 interlayer is discontinuous due to synchromesh-like growth of the LaAlO3 layer on relatively rough YBa2Cu3O7−x surface resulting from the mobility of the solution. Nanoscale defects, such as particles, some amorphous phases, and especially their concomitant lattice defects (such as stacking faults and plane buckling) arise in YBa2Cu3O7−x layers. These nanoscale defects could play a role in flux pinning and thus enhancing Jc. The effective non-vacuum solution to induce vortex pinning into YBa2Cu3O7−x films could be a reference for the further design of an optimal pinning landscape for higher Jc.

Artificial 2D Flux Pinning Centers in MgB2 Induced by Graphitic-Carbon Nitride Coated on Boron for Superconductor Applications

Systemic investigation was carried out on the microstructure, superconducting properties, and flux pinning mechanism of MgB2 in situ fabricated with magnesium and g-C3N4 coated boron as precursors. The encapsulation of the boron powders with g-C3N4 was achieved by polycondensation of urea on boron powders. The g-C3N4 decomposes during the MgB2 fabrication to induce two-dimensional few-carbon layer, dispersed nanoparticles, and carbon-rich phases in the matrix to enhance the flux pinning force and Hirr of MgB2, which accounts for the in-field critical current density (Jc(H)) increase compared to the pure MgB2. The carbon layers acting as artificial two-dimensional flux pinning centers, have demonstrated high flux pinning efficiency to increase the Jc(H) of MgB2 superconductors.

Enhancement of the in-field critical current density of trifluoroacetate metal organic deposition derived (Y0.77Gd0.23)Ba2Cu3Oy films by annealing of CeO2 buffered R-Al2O3 substrates

We investigate the superconducting properties of (Y0.77Gd0.23)Ba2Cu3Oy [(Y,Gd)BCO] films on as-grown and annealed (600 °C–1000 °C) CeO2 buffered R-plane sapphire (R-Al2O3) substrates using trifluoroacetate metal organic deposition (TFA-MOD). The annealing was found to improve the crystallinities of not only the CeO2 buffer layers but also the (Y,Gd)BCO films. Although the anneal had no apparent effect on the pinning centers of the (Y,Gd)BCO films, the in-field Jc of the (Y,Gd)BCO films grown on the CeO2 buffered substrates improved with increasing annealing temperature. The present results are discussed in terms of the crystallinities and Jc values of the (Y,Gd)BCO films.

Giant proximity effect in single-crystalline MgB2 bilayers

Although giant proximity effect (GPE) can shed important information on understanding superconducting pairing mechanisms and superconducting electronics, reports on the GPE are few because the fabrication of the junctions with GPE is technologically difficult. Here, we report a GPE in the single-crystalline MgB2 bilayers (S′/S), where the S′ is the damaged MgB2 layer by cobalt (Co)-ion irradiation and the S is the undamaged MgB2 layer. Superconducting properties of the S′ is remarkably degraded by the irradiation, whereas those of the S is uninfluenced by the irradiation. The degraded superconductivity in the S′ is fully recovered by increasing the thickness of undamaged MgB2 layer S despite almost ten times larger thickness ~ 95 nm of S′ than the superconducting coherence length ξab(0) ~ 8.5 nm of the S, indicating a presence of GPE in the S′/S MgB2 bilayers. A diffusion of electrons in the S′ into the S can reduce a pair breaking scattering in the S′, and the similar electronic structures of S′ and S layers and a finite attractive electron-electron interaction in the S′ are thought to be origins of unpredicted GPE between the same superconducting materials. Both upper critical field (μ0Hc2) and in-field critical current density (Jc) of S′/S bilayers show a significant enhancement, representing a strong correlation between S′ and S. These discoveries provide the blue print to the design of the superconducting multilayers for fundamental researches on the mechanism of the GPE as well as their technological applications.

Effects of boron powder by hydrocarbon gas treatment on the structural and superconducting properties of in situ-processed MgB2 polycrystalline materials

Carbon-based organic molecular gas is a well-known chemical agent for controlling inorganic structures of MgB2 single-crystal material. The utilization of such a gas can also be applied to polycrystalline materials. So far, however, few studies have dealt with the methodology and the effects, especially the structural mechanism. Herein, we have focused on hydrocarbon gas as an active carbon source for bulk and wire materials consisting of MgB2 polycrystals. A starting material, amorphous or crystalline boron powder, was heated and exposed to the molecular gas in argon atmosphere. While the treated powder showed no major change in its structure, as characterized by X-ray diffraction profiles, its usage as a precursor powder for MgB2 bulks resulted in a marked increase in the in-field critical current density. In order to examine and help identify the underlying causes, we have carried out further structural analysis with the Halder Wagner method, considering the integral breadths of peaks in the X-ray diffraction profiles. Finally, the gas treatment was found to improve the in-field transport properties, even for MgB2 wires and densified conductors, which are made from crystalline boron powder.

Investigation of the longitudinal magnetic field effect in SmBa2Cu3Oy films with various shaped artificial pinning centers

The longitudinal magnetic field (LMF) state, in which the magnetic field is applied parallel to the applied current direction and ab-plane of SmBa2Cu3Oy (Sm123) films with shaped artificial pinning centers (APCs) of various shapes is investigated. The in-field critical current density (Jc) of pure Sm123 films in the LMF state and films with Y2O3, Sm2BaCuO5, BaHfO3, and a multilayered structure with short BaHfO3 nanorods were measured in the configuration. The measurement revealed that there was significant Jc enhancement only in the multilayered film. From the viewpoint of flux pinning, 3D flux pinning is key to improving in-field Jc in the LMF state. However, the Sm123 films with APCs, such as Y2O3 and Sm2BaCuO5, did not have a larger Jc than the pure Sm123 films in the LMF state. This indicates that the APCs prevent the current flowing along the external magnetic field. Both of the 3D flux pinning and the pure layer in which current can flow without disturbance are important for the Jc enhancement in the LMF state.

Broad temperature study of RE-substitution effects on the in-field critical current behavior of REBCO superconducting tapes

The critical current behavior of REBa2Cu3O7−δ (REBCO, with RE = Yb, Y, Ho, Dy, Gd, Eu, GdYb, DyY, GdY, EuY, SmY, GdEu, GdSm) superconducting tapes has been systematically studied for the temperature range of 10–77 K, magnetic field range of 0–5 T and field orientation (angle between the field and tape) of 0°–180°. We report that the RE size dependencies of in-field critical current density (Jc) at low temperatures (below 40 K) are significantly different from those at 77 K. In a magnetic field of 1–3 T applied at 77 K, the Jc of REBCO films increases with the RE ionic radius (r). EuBCO film, with the largest r in this study, shows the highest Jc at 77 K in a magnetic field above 1 T, while YbBCO film, with the smallest r, has the lowest Jc at 77 K. At temperatures below 40 K, however, the trend of in-field Jc as a function of r reverses. There are crossovers among the functions of critical current density versus temperature for various RE ions and mixes. RE mixed films such as GdYBCO, EuYBCO, SmYBCO and GdEuBCO exhibit enhanced self-field critical current densities correlating with effective low field random pinning due to their suitable ion size variance. Strongly enhanced pinning by both random and correlated defects has been obtained in GdYbBCO films. Those pinning centers are especially effective at low temperatures. In a magnetic field of 5 T applied at 10 K, GdYbBCO gives the highest critical current density compared with other REBCO films, being 1.9 times greater than GdEuBCO film. This work provides a guideline regarding how to optimize RE ions in REBCO films for given application temperatures and magnetic fields.

Influence of chemical doping on flux pinning of GdBa2Cu3O7−z films

ABSTRACTEpitaxial GdBCO films with Cobalt (Co) and BaZrO3 (BZO) doping were prepared on (00l) LaAlO3 substrate by a self-developed fluorine-free polymer-assisted metal-organic deposition (FF-PA-MOD) approach. The effect of different doping ways on structure and properties of GdBCO films was investigated. Our results show that in-field critical current density (Jc) values of doped films have been distinctly improved compared to the pure film. It is noted that Co-doped film has much larger in-field Jc values than BZO-doped film, especially at higher temperatures. Furthermore, Co-doped GdBCO film shows significantly enhanced surface smoothness and grain connectivity. It is suggested that obviously enhanced Jc–B properties of the film may be attributed to improved surface morphologies and effective flux pinning centres by dilute Co doping. This study may offer another simple and efficient way to obtain the GdBCO-based coated conductors with high current densities.

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.

Flux Pinning Properties of Bi,Pb2223 Films With Columnar Defects by Xe Irradiation

High-quality Hf- and Zr-doped Bi,Pb2223 films on SrTiO3 substrate were fabricated by RF magnetron sputtering and two-step annealing. In order to investigate flux pinning properties of columnar defects, Xe-ions were irradiated onto the Hf- and Zr-doped Bi,Pb2223 films along the c -axis. Although the critical temperature Tc was decreased by heavy ions irradiation, the in-field critical current density Jc in the magnetic field parallel to the c -axis was increased drastically. The enhancement of Jc was achieved in a wide range of magnetic fields over the matching magnetic field Bφ . In the angular dependence of Jc , the improvement of Jc was observed in almost all angle region of magnetic field orientation except for the vicinity of the ab -plane. However, the peak centered at the c -axis did not appear in the angular dependence of Jc . This behavior was quite different from REBa2Cu3O y films installed columnar defects. The angular dependences of Jc in the Hf- and Zr-doped Bi,Pb2223 films were determined by the magnetic field component along the c -axis. These results are originated from the strong two-dimensional nature of the Bi,Pb2223 film.

Enhancement of In-Field Critical Current Density of BaZrO3-Added (Y, Gd) BCO-Coated Conductors by Using a Multi-Coating TFA-MOD Method

We have investigated the in-field critical current properties in BaZrO3-added (Y, Gd) Ba2Cu 3O7-δ (YGdBCO)-coated conductor fabricated by using a multi-coating trifluoroacetates metal-organic deposition (MOD) method. We compared these properties with EuBCO-coated conductor fabricated by the pulsed laser deposition (PLD) method. The sample using the 30-nm once-coat-layer-thickness d once shows superior in-field J c down to 4.2 K than that of the previous standard coating using 170–nm-thick layer for each coating. From the analysis of E – J characteristics, an analytical expression for J c as a function T and B has been derived. It was confirmed that the high J c region in sample with d once = 30 nm is widely spread in high-temperature and high-magnetic-field region. By comparison with the J c properties of the BaHfO3 -added PLD CCs, the minimum J c, which is estimated from magnetic field angle dependence, shows even higher value up to 5 T of magnetic field at 65 K and up to 3 T at 77 K. From these results, the new MOD-YGdBCO process using the thin once-coat-layer-thickness is very promising for the practically important mid field region such as 3 to 5 T.

Trifluoroacetate metal organic deposition derived (Y0.77Gd0.23)Ba2Cu3Oy films on CeO2 buffered R-plane Al2O3 substrates

(Y0.77Gd0.23)Ba2Cu3Oy [(Y,Gd)BCO] films were grown on CeO2 buffered R-plane sapphire (R-Al2O3) substrates using trifluoroacetate metal organic deposition (TFA-MOD). Annealing of the CeO2 buffered R-Al2O3 substrates was performed to control the crystallinity and surface morphology of the CeO2 buffer layer. The annealing treatment led to a significant improvement in the crystallinity and surface morphology of the CeO2 buffer layer. A (Y,Gd)BCO film grown on the CeO2 buffer layer with high crystallinity and an atomically flat surface exhibited high self-field (at 77 K) and in-field (at 20 K, 9 T, μ0H∥c) critical current densities (Jc). Annealing of the CeO2 buffer layer thus enabled enhancement of not only the self-field Jc for the (Y,Gd)BCO film but also the in-field Jc.

Strain characteristics of BMO doped REBCO coated conductors fabricated by hot-wall PLD

REBCO coated conductors (CCs) are expected to show high performance for superconducting applications due to their high tensile strength and high critical current density under magnetic fields. Fujikura Ltd. (Fujikura) has developed REBCO CCs with high performance by ion beam assisted deposition (IBAD) and pulsed laser deposition (PLD) method. It is generally known that the critical current (Ic) of a common superconducting wire varies with the applied longitudinal tensile strain, but that of Fujikura’s commercial CC hardly varies due to the peculiarities of the REBCO crystal orientation. Recently, we have developed BaMO3 (BMO, M : Zr or Hf) doped REBCO CCs in order to enhance their in-field critical current density. In this study, we investigated the strain dependence of the BMO doped REBCO CCs. As a result, it was confirmed that the Ic change with strain increased by BMO doping, even though the crystal orientation did not change.

Tackling the agglomeration of Mg2Si dopant in MgB2 superconductor using cast Mg–Si alloy

The work demonstrates a method to tackle the agglomeration of Mg2Si dopant in chemical doping in MgB2 superconductor to enhance its in-field critical current density. Mg-Si alloy which has uniformly distributed Mg2Si particles in it is prepared by casting technique. The alloy is used as the Mg source for the preparation of MgB2 samples doped with various weight percentage of nano carbon. XRD patterns show peaks of Mg2Si in all Si containing samples. SEM images show a microstructure with distinctly smaller grains and reduced crystallinity in doped samples. HRTEM analysis confirms that the method led to better distribution of Mg2Si particles within the MgB2 grains as compared to the normally prepared sample. It is found that doping has not severely affected the transition temperature of the samples and at the same time enhanced the in-field critical current density.

Superconducting transition width (ΔTc) characteristics of 25 mol% Zr-added (Gd, Y)Ba2Cu3O7−δ superconductor tapes with high in-field critical current density at 30 K

The superconducting transition width (∆Tc) characteristics of REBa2Cu3O7−δ (REBCO and RE = Gd, Y) superconductor tapes with Zr content of 25 mol% with high lift factor (ratio of critical current density (Jc) at 30 K, 3 T (B||c) to the Jc at 77 K, 0 T) has been determined. In this work, heavily doped (Gd, Y)Ba2Cu3O7−δ superconductor tapes with 25 mol% Zr addition were fabricated by metal organic chemical vapor deposition using a reel-to reel process. The optimal chemical composition range of (Gd, Y)Ba2Cu3O7−δ superconductor tapes with Zr content of 25 mol% to achieve critical current densities above 3.5 MA cm−2 at 77 K in zero applied magnetic field has been determined. A superconducting transition width (∆Tc) as narrow as 0.4 K and an onset critical transition temperature (Tc-onset) as high as 92 K were obtained in the 25 mol% Zr-added (Gd, Y)BaCuO superconductor tapes. Based on the mapped compositional phase diagram of the ∆Tc and lift factor, ∆Tc in the range of 0.7–0.9 K is observed in 25 mol% Zr-added (Gd, Y)BaCuO superconductor tapes with a high lift factor.

Enhanced superconducting properties of YBa2Cu3O7−δ thin film with magnetic nanolayer additions

Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)0.5(CeO2)0.5 and pure La0.7Sr0.3MnO3 layers were incorporated into YBa2Cu3O7−δ (YBCO) thin films as bilayer stacks for magnetic flux pinning enhancement. The films show high epitaxial quality, suggested by XRD and TEM study. The critical temperature Tc of the bilayers is about 90K, which is close to that of pure YBCO films, while both the self-field Jcsf and in-field critical current density Jcin-field are largely enhanced. Among all samples, the film with VAN cap layer shows the highest Jc values in all field ranges. This study demonstrates an effective way towards the tunable pinning effect for YBCO coated conductors by both defect and magnetic pinning.

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.

Influence of random point defects introduced by proton irradiation on the flux creep rates and magnetic field dependence of the critical current density Jc of co-evaporated GdBa2Cu3O7−δ coated conductors

We report the influence of random point defects introduced by 3 MeV proton irradiation (doses of 0.5 × 1016, 1 × 1016, 2 × 1016 and 6 × 1016 cm−2) on the vortex dynamics of co-evaporated 1.3 μm thick, GdBa2Cu3O7−δ coated conductors. Our results indicate that the inclusion of additional random point defects reduces the low field and enhances the in-field critical current densities Jc. The main in-field Jc enhancement takes place below 40 K, which is in agreement with the expectations for pinning by random point defects. In addition, our data show a slight though clear increase in flux creep rates as a function of irradiation fluence. Maley analysis indicates that this increment can be associated with a reduction in the exponent μ characterizing the glassy behavior.

Requirements to achieve high in-field critical current density at 30 K in heavily-doped (Gd,Y)Ba2Cu3Ox superconductor tapes

Critical current densities (Jc) above 15 MA cm−2 have been achieved at 30 K, 2.5 T (B||c) in (Gd,Y)–Ba–Cu–O tapes with 20–25 mol.% Zr addition. The optimal compositions of Gd + Y, Ba + Zr and Cu to achieve lift factor in critical current above 6 at 30 K, 2.5 T (B||c) have been determined by development of a composition map of the performance of heavily-doped (Gd,Y)–Ba–Cu–O tapes. An inverse relationship is found between the critical current density at 77 K, 0 T and the lift factor in critical current at 30 K, 2.5 T (B||c). It is discovered that a Jc of 15 MA cm−2 at 30 K, 2.5 T (B||c) is achievable only in tapes with a Jc between 2.5 and 3.8 MA cm−2 at 77 K, 0 T.