Jc Enhancement Research Articles

Impact of deoxygenation/reoxygenation processes on the superconducting properties of commercial coated conductors

We report the evolution of the superconducting properties of a commercial coated conductor during deoxygenation and reoxygenation processes. By analyzing the changes on the critical temperature, Tc, and critical current density, Jc, at 4 and 77 K, we have identified the conditions that cause a complete deoxygenation of the coated conductor and, also, the reoxygenation conditions that allow a recovery of the superconducting properties. A complete suppression of superconductivity happens at ~ 500–550 °C under a pure argon flow. After a complete deoxygenation, we observed that a reoxygenation process at ~ 400–450 °C in pure oxygen flow allows, not only a full recovery, but even an improvement in Jc, both at 4 and 77 K. Such an increase of Jc is kept or even enhanced, especially at 77 K, in the presence of magnetic fields up to ~ 6 T. A microstructural analysis by transmission electron microscopy did not give evidence of major differences in the densities of Y2O3 nanoparticles and stacking faults between the pristine and reoxygenated samples, suggesting that these defects should not be the cause of the observed enhancement of Jc. Therefore, the combined action of other types of defects, which could appear as a consequence of our reoxygenation process, and of a new level of oxygen doping should be responsible of the Jc enhancement. The higher Jc that can be achieved by using our simple reoxygenation process opens new parameter space for CCs optimization, which means choosing a proper pO2-temperature–time trajectory for optimizing Jc.

Effect of metallic particle addition to the filling powders on the superconducting properties of ex situ processed MgB2 tapes using milled powders

Various metallic particles A were added to the filling powder in ex situ processed MgB2 tapes. The superconducting properties such as critical temperature (Tc) and transport critical current density (Jc) of those sintered tapes depend on the additives. Some additives such as Te can enhance the Jc property, whereas the others such as Mo, Nb and W can cause a Jc deterioration. The Jc enhancement is due to the improved grain connectivity and the enhancement in both upper critical field (Hc2) and irreversibility field (Hirr), whereas the origin of the Jc deterioration is the Tc reduction which is probably brought about by the reaction of the additives with MgB2. The detrimental additives possibly substitute the Mg site, Mg1−yAyB2. In contrast, the other additives which do not cause significant Jc deterioration neither form borides, ApBq, nor cause substitution reaction influencing the superconducting properties. Thus, neither reduction of superconducting phase MgB2 in the core layer nor Tc reduction occurs. Te addition reduces the optimal sintering temperature (Tsin) at which best performance in transport Jc at 4.2 K and 10 T is obtained by only 30 °C, from 710 °C to 680 °C. This Tsin reduction is much smaller than that for Sn addition, about 100 °C. Nevertheless, when sintered at temperatures of 530–560 °C, the Jc values of the tapes with Te addition are around 150 A/mm2, which is comparable to those with Sn addition and twice as large as those of the tapes without addition. Thus, Te is effective for improving the Jc property over a wide range of Tsin among various additives.

Novel ultra-sonic boron refinement in distilled water for cost-efficient fabrication of MgB2 bulk ceramic superconductors

Cheap commercial boron was refined ultra-sonically in distilled water medium for various durations. X-ray diffraction pattern (XRD) indicated impurity-free nature of ultra-sonically refined boron. Field emission scanning electron microscope (FE-SEM) showed refinement of coarse boron to nanoscale. Rietveld phase fraction analysis on XRD pattern of sintered bulk prepared from ultra-sonically refined boron and Mg powder revealed a complete MgB2 phase formation, with a few impurities of MgO. FWHM (Full width at half maximum) is used to estimate crystallite size and microstrain. Tc,onset was around 38.5 K, with a sharp transition (ΔTc ∼ 0.5 K). Critical current density (Jc) of bulks prepared with B ultra-sonicated in distilled water for 15 and 30 min reached 295 and 305 kAcm−2 at 20 K/self-field, respectively. This is 35% increase in Jc showing the cost efficiency of this novel technique. Transmission electron microscopy of bulk MgB2 revealed a high density of nano-sized grains in the bulk prepared of B ultra-sonicated for 30 min. Flux pining diagrams with peak position at b = 0.2 points to a strong grain boundary pinning. The results prove that ultra-sonic treatment in distilled water results in boron refinement and Jc increase. The role of ultra-sonication medium viscosity in the process of MgB2 grain size refinement and Jc enhancement is elucidated.

Superconducting properties and pinning mechanism of filamentary (Sm,Gd,Dy)–Ba–Cu–O doped with Co

In this work, the effect of cobalt (Co) doping on the superconducting properties of filamentary (Sm1/3Gd1/3Dy1/3)1.18Ba2.12Cu3.09−xCoxOy (SGD123, x = 0 − 1.20) superconductors was investigated using samples prepared using a solution spinning method. Its pinning mechanism was also examined. The Co-doped x = 5.0 × 10−4 and 1.0 × 10−3 samples obtained a Jc value higher than that of the pure sample, although the Tc value of the SGD123 filaments slightly decreased with increasing Co content. The anisotropic behavior of the field dependence of the transport Jc can hardly be confirmed for the pure sample. The superconductivity of the pure sample disappeared at an applied magnetic field approximately 12 T, at most. In contrast, Co-doped x = 1.0 × 10−3 sample showed the angular dependence of the transport Jc in an applied magnetic field. Furthermore, the Co-doped x = 1.0 × 10−3 sample rotated at 90° maintained a Jc value higher than 103 A cm−2 in a magnetic field up to 17.5 T. Co doping was effective in enhancing Jc at self-field and high magnetic field. In addition, we attempted to elucidate the pinning mechanism of Co on the Jc enhancement of the SGD123 filament using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements.

Improved critical current density property in ex situ processed MgB2 tapes using filling powders with metallic particle addition

MgB2 powders milled with low-melting-point metallic particles, Bi and In, and Fe-sheathed tapes using those milled powders were sintered at various temperatures. With increasing the amount of those additives, the wear of WC jar and balls during milling process is promoted. Compared with In addition, Bi addition clearly causes the formation of some amorphous or poor crystalline phase in the as-milled state. Concerning the sintered tape samples, the optimal sintering temperature (Tsin), at which best performance in transport critical current density (Jc) at 4.2 K in 10 T is obtained, is 680 °C for the tapes using the milled powders with In addition. This temperature is lower than that by 30 °C for the tapes without additives. Furthermore, In addition promotes the reaction at the interface between the MgB2 core and the Fe sheath. In contrast, Bi addition appreciably causes neither reduction of the optimal Tsin nor the promoted reaction at the interface. Although both additives hardly bring about critical temperature (Tc) reduction, only Bi addition can improve the transport Jc property. This Jc enhancement is mainly attributed to improved grain connectivity. In addition itself can be effective for grain connectivity improvement as well. However, reduced upper critical and irreversibility fields (Hc2 and Hirr) brought about by the addition degrade the Jc property.

Strategy study of critical flux/threshold flux on alleviating protein fouling of PVDF-TiO2 modified membrane

Aiming at the fouling problem of PVDF membrane, one novel integrated strategy was used for the first time, including hydrophilic membrane modification and low-pressure operation. Permeability and anti-fouling ability of PVDF were simultaneously improved by nano-TiO2 embedment. From perspective of operating condition optimization, the concepts of critical flux (JC) and threshold flux (JTH) were recommended as fouling-control tools. The JC and JTH of PVDF-TiO2 membrane were tested by two constant pressure-steps methods, i.e. Constant pressure-step method and Alternating constant pressure-step method. Three indicators, consisting of instant FTTT curves, dFlux/dt condition and TMP-Fluxave curve, were employed to evaluate the JC and JTH of PVDF-TiO2 membrane. In brief, the JC and JTH of PVDF-TiO2 were 10.62 and 23.91 L/(m2·h), respectively, which were nearly 3 times of PVDF membrane. Fouling resistance results displayed that protein irreversible fouling of two PVDF membranes were effectively reduced by JC mode and JTH mode. Besides, reversible fouling of PVDF-TiO2 membrane was controlled well by JC mode meanwhile. The pure water filtration line (PWF line) and the normalized resistance were both used to judge the form of JC of two membranes. As compared with PVDF, it was speculated the enhancement of JC and JTH mainly related to the better hydrophilicity and local hydraulic condition of PVDF-TiO2 modified membrane.

Large Jc Enhancement of Bi-2212 Ceramics by Triple Doping of Li, K, and Na in Bi1.8-x(Li,Na,K)xSr2Ca1.1Cu2.1Oy (x = 0.05, 0.1, and 0.25) Composition

The samples with compositions of Bi1.8-x(Li,Na,K)xSr2Ca1.1Cu2.1Oy (x = 0.05, 0.1, 0.25) were prepared by the solid-state reaction method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and DC magnetization measurements. XRD data indicates that all samples have an ideal ratio of Bi-2212 phase; moreover, some impurity phases such as Bi2CaO4 and Bi4Sr4CaCu3O4 are observed to disappear in the maximum alkaline-doped sample (x = 0.25). SEM observations have shown that, with the increase in the amount of alkaline-doped elements, the grain sizes grow significantly. The highest onset superconducting transition temperature obtained from M-T data belongs to the alkaline-doped (Na, K, and Li) samples at high contents (x > 0.05). It has been found that the sample with x = 0.25 (Na, K, Li) has the largest magnetic hysteresis loop, indicating that high level of triple alkaline element doping positively affects the superconductivity properties of the Bi-2212 phase due to the better intergranular coupling. In addition, Bean’s critical model has been used for determining the critical current density. It has been found that maximum Jc of 141.42 × 104 A/cm2 at H = 1 kOe and T = 15 K is achieved in the x = 0.25 alkaline-doped sample, which is higher than the pure samples obtained by the same initial conditions in literature.

Large artificial ferromagnetic dot arrays for the critical current enhancement in superconducting YBa2Cu3O thin films

In order to enhance and/or control critical current density (Jc) in superconducting YBa2Cu3O (YBCO) thin films, different arrays of 3 µm large ferromagnetic (iron) dots with differing configurations and shapes have been deposited on top of high-quality YBCO thin films post-buffered with a layer of CeO2. Some tremendous Jc enhancement of up to nearly 100% have been obtained at high temperatures and low fields. However, the Jc performance is strongly dependent on the array configurations, shape and amount of ferromagnetic iron involved. We show that it is possible to enhance Jc at high or low magnetic field ranges. The results are clearly different to similar non magnetic array structures used to previously manipulate Jc in YBCO films, which proves the magnetic origin of the changes in Jc we observed. The enhancement is likely due to the flux localization and magnetic pinning effects, rather than magnetic shielding alone, which is effective at relatively low fields only. The results also suggest that the observed Jc changes depend on rather minor variations in initial pinning and corresponding Jc levels of the films. At the same time a common trend for all of the various investigated magnetic arrays could not be established due to explained factors.

Y3Fe5O12 Nanoparticles as Efficient Pinning Centers and Temperature Dependence of the Critical Current in YBa2Cu3O7-δ Nanoparticles

Magnetic measurements have been reported in nanosized YBa2Cu3O7-δ (YBCO) cylindrical tablets doped with different proportions of Y3Fe5O12 (YIG) nanoparticles. X-ray diffraction and magnetization measurements have been achieved to characterize samples and study the influence of temperature T, applied field H, and added YIG on the intergranular critical current JC. The field has been applied perpendicular to the tablet. Temperature exponential decay of JC in YBCO nanoparticles has been observed and analyzed in the collective and correlated model framework. A two-current model has been proposed: the low-T region behavior has been attributed to the trapping effectiveness of ferromagnetic and superconducting nanoarea interfaces. In the high-T region, there are many ferromagnetic traps to ensnare a very small number of vortices leading to a flexing of JC decrease vs. T and to very low values of JC. Field dependence of JC of many tablets of YBCO with different YIG quantities has shown an optimum enhancement of JC for 0.5% of YIG in weight.

Enhanced flux pinning properties in Bi1.6Pb0.4Sr2−xKxCa2Cu3O10+δ compounds

The effect of K substitutions on the structural and superconducting properties of Bi–Pb–Sr–Ca–Cu–O (BPSCCO) system has been investigated. Polycrystalline samples with stoichiometry of Bi1.6Pb0.4Sr2−xKxCa2Cu3O10+δ, where x = (0.00, 0.02, 0.04, 0.06, 0.08, and 0.1), were prepared by using the conventional solid state reaction technique. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) data showed that all samples consisted of high Tc (Bi-2223) and low Tc (Bi-2212) phases, and their volume fractions were found to vary with K content (x). The zero critical temperature was found to decrease from ~ (104.5 to 94.6) K, which was attributed to the decreases in hole carrier concentration. At 65 K, the magnetization critical current density (Jc) of the K-substituted samples was enhanced in comparison with that of the pure one. The Jc enhancement might be attributed to the formation of point defects resulting from the partial substitutions of K into Sr site in the BPSCCO system. In addition, the dominant pinning mechanism was found to be point-like pinning, as described using the Dew-Hughes model.

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.

In-field dependences of the critical current density Jc in GdBa2Cu3O7-d coated conductors produced by Zr irradiation and post-annealing at low temperatures

We report the influence of 6 MeV Zr4+irradiation and post-irradiation annealing (200 °C) in the in-field dependences of the critical current densities Jc of 1.3 thick GdBa2Cu3O7-d coated conductors grown by co-evaporation. Samples were irradiated with 6 MeV Zr4+ and fluences between 2.3 × 1011 cm−2 and 3 × 1012 cm−2. The correlation between the superconducting critical temperature Tc and in-field dependences of Jc has been analyzed. In addition, random disorder introduced by irradiation was reduced by thermal annealed at 200 °C. The analysis of our experimental findings indicates that the optimal irradiation (reducing random disorder by annealing) results in the suppression of the self-field Jc of ≈10% and in-field Jc enhancements nearly doubled at about 5 T. A clear correlation between Tc, disorder and self-field Jc is observed. Additional random disorder and nanoclusters suppress systematically Tc and increase the flux creep relaxation at intermediate temperatures (reducing the characteristic glassy μ value).

Pressure-induced property improvement of magnesium diboride wire

The 4.2 K non-barrier transport Jc values of the powder-in-tube (PIT) in-situ MgB2 wires have been enhanced by cold high pressure densification (CHPD). With respect to the control wire, the 1.5 GPa pressure induced the Jc improvement of the wire at 4.2 K and 10 T by 25 %. The Jc enhancement induced by the CHPD may result from two aspects. First, the CHPD resulted in the reduction of the transverse MgB2 core area. Second, the grain connectivity of the PIT in-situ MgB2 wire was improved by the CHPD, which is reflected by the reduced porosity.

Relevant Pinning for ab-Plane Jc Enhancement of MOCVD REBCO Coated Conductors

For REBa 2 Cu 3 O x (REBCO, where RE = rare earth) coated conductor applications, the prominence of the background magnetic field H in the orientation of ab-plane urges an immediate and proper understanding of the ab-plane pinning and in turn, the critical current density J c tuning. In this work, we present our ab-plane pinning study of metal organic chemical vapor deposited (MOCVD) REBCO coated conductors based on our previous observation of a strong linear correlation between J c at 77 K and J c at lower temperatures T and higher magnetic fields H. In this work, we found that, below ~40 K, besides the weak uncorrelated pinning, the intrinsic pinning due to the layered crystal structure of REBCO with charge reservoir CuO planes also plays an essential role for J c (T, H||ab). Such intrinsic pinning is independent of H, and increases with decreasing T. Combined with the microstructure and J c (T, H, θ) analysis, we found that, at T above ~40 K, strong correlated pinning centers, such as stacking faults and RE 2 O 3 precipitates, govern J c (T, H||ab). Our work provides an understanding of ab-plane pinning in Zr-added REBCO coated conductors and guidance for further J c (T, H) enhancement.

Enhanced critical current density of in situ processed MgB2 bulk superconductors with MgB4 additions

The effects of MgB4 addition on the superconducting properties and the microstructure of in situ processed MgB2 bulk superconductors were studied. MgB4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB4) were heat-treated at 650 ℃ for 1 h in flowing argon. The powder X-ray diffraction (XRD) analysis for the heat-treated samples showed that the major formed phase in all samples was MgB2 and the minor phases were MgB4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB2 decreased as the amount of MgB4 addition increased. MgB4 particles included in a MgB2 matrix is considered to suppress the grain growth of MgB2. The onset temperatures (Tc,onset) of MgB2 with MgB4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB4 addition slightly reduced the Tc,onset of MgB2 to 36.5 K. This result indicates that MgB4 addition did not influence the superconducting transition temperature (Tc) of MgB2 significantly. On the other hand, the small additions of 1-5 wt.% MgB4 increased the critical current density (Jc) of MgB2. The Jc enhancement by MgB4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB4 particles dispersed in a MgB2 matrix.

Tilted BaHfO3 nanorod artificial pinning centres in REBCO films on inclined substrate deposited-MgO coated conductor templates

We grow BaHfO3 (BHO) nanorods in REBa2Cu3O7−x (REBCO, RE: Gd or Y) thin films on metal tapes coated with the inclined substrate deposited (ISD)-MgO template by both electron beam physical vapour deposition and pulsed laser deposition. In both cases the nanorods are inclined by an angle of 21°–29° with respect to the sample surface normal as a consequence of the tilted growth of the REBCO film resulting from the ISD-MgO layer. We present angular critical current density (Jc) anisotropy as well as field- and temperature-dependant Jc data of the BHO nanorod-containing GdBCO films demonstrating an increase in Jc over a wide range of temperatures between 30 and 77 K and magnetic fields up to 8 T. In addition, we show that the angle of the peak in the Jc anisotropy curve resulting from the nanorods is dependent both on temperature and magnetic field. The largest Jc enhancement from the addition of the nanorods was found to occur at 30 K, 3 T, resulting in a Jc of 3.0 MA cm−2.

A route for a strong increase of critical current in nanostrained iron-based superconductors

The critical temperature Tc and the critical current density Jc determine the limits to large-scale superconductor applications. Superconductivity emerges at Tc. The practical current-carrying capability, measured by Jc, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce Tc. Simultaneous increase of Tc and Jc in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both Tc and Jc together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe0.5Te0.5 films. Tc is enhanced due to the nanoscale compressive strain and proximity effect, whereas Jc is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of Jc enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.

Effects of 250 MeV Au-ion Irradiation on the Superconducting Properties of Ba1-xKxFe2As2 Single Crystals

ABSTRACTIn this work, we present an approach towards achieving a detailed knowledge on the effects of irradiation in iron-based superconductors with high-energy heavy ions. Ba1−xKxFe2As2 single crystals (x = 0.42) were irradiated with 250 MeV Au-ions. The crystals were shaped to be thin plates with thickness lower than the ion implantation depth and bases perpendicular to the c-axis. By means of the quantitative magneto-optical imaging technique we attained the local current density in pristine and irradiated sample regions without any model assumption. After irradiation a strong critical current density (Jc) enhancement was measured, overcoming a factor 3 at T = 4 K with an irradiation fluence φ = 9.7 × 109 cm-2. Moreover, correlating point by point Jc and induction magnetic field maps, a contribution of the Au-ion induced defects to the anisotropic out of plane pinning (i.e. pinning in the direction parallel to the ion path) turned up. This contribution shows a maximum at applied fields quite lower than the nominal dose equivalent field. Furthermore, the temperature dependence of the penetration depth was evaluated before and after high-fluence irradiations by investigating the high-frequency behavior of these crystals using a microwave coplanar resonator technique. Fitting the experimental curves with a power law, we found a decrease of the power exponent, more pronounced at the higher fluences, that can supports the s± wave model.

Investigation of the effect of Ag addition on the critical current density of the high-temperature superconductor Nd1.85Ce0.15CuO4

We have synthesized a Nd1.85Ce0.15CuO4 + Ag composite system with the aim of studying the effect of Ag addition in the electron-doped system Nd1.85Ce0.15CuO4 on its superconducting and magnetic properties. Measurements of magnetization using a vibration sample magnetometer indicate a systematic increase in diamagnetic shielding upon Ag addition. A subsequent analysis of the critical current density using the Bean model indicates a small but significant increase in intra-grain critical current density upon Ag addition. From the study of the dependence of electrical resistivity on the temperature and electrical current, an increase in inter-granular critical current has also been evidenced upon Ag addition. These results correlate well with the earlier reports of a similar improvement in the properties on the hole-doped ceramic superconductor/Ag composites. Possible reasons for the enhancement of JC have been described and discussed. Magnetic characterization by AC susceptibility using a SQUID magnetometer has been presented for a representative composition, which enabled delineation of the inter- and intra-granular transitions.

Investigation of the Longitudinal Magnetic Field Effect on Multilayered- Films Fabricated on Single-Crystal and Metal Substrates

We discuss the longitudinal magnetic field (LMF) effect in BaHfO 3 (BHO)-doped multilayered (ML) SmBa 2 Cu 3 Oy (SmBCO) films. We observed critical current density (J c ) enhancement in the form of a J c peak with increasing magnetic fields due to the LMF effect at various measurement temperatures on an ML-SmBCO film fabricated on a single-crystalline substrate. These J c peaks were higher than J c s under self-field, and the J c gain was temperature dependent. We concluded that the J c gain was related to the BHO nanorod diameter and the diameter of flux lines along the ab-plane. We also attempted to determine the J c gain of SmBCO films with a similar film structure that were fabricated on IBAD-MgO-buffered metal substrates.