Transport Critical Current Research Articles

High-field Josephson effect caused by the inside-grain vortices

We present a theoretical prediction of the new effects for critical currents across single grain boundaries in polycrystalline superconductors being under an applied magnetic field. It was shown that Josephson oscillation should be observed not only in the case of the increasing of the magnetic flux through the junction but also when vortex penetrates inside grain. In the intergrain Josephson junction the critical transport current is strongly dependent on the normalized grain size ratio, grain anisotropy ratio and grain coupling strength. These results imply that achieving a high degree of texture along these parameters is important for the obtaining of very high critical currents in pure polycrystalline samples. Also, it is shown that in contrast to the well-known Fraunhofer dependence, the period of oscillations corresponds to adding flux quantums per two grains.

Supercurrents in magnesium diboride/metal composite wire

We have fabricated a series of ex situ copper sheathed powder-in-tube MgB2 wires with 20% by volume Ag, Pb, In, and Ga metal added to the MgB2 powder. We find the transport critical current of these wires increases significantly with the addition of specific metals to the core filament. In particular, the critical current density (JC) of the MgB2/Ga(20%) wire is in excess of 5×104A/cm2 at 10K in self-field, nearly 50 times that of the MgB2/Ag(20%) wire. The temperature dependent JC of all wires is well described as an ensemble of clean S/N/S junctions in which the relevant parameters are the average thickness of the N layer, the critical temperature of the S layer, and a scaling term related to JC at zero temperature. Eliminating the differences in the filament microstructure as the primary cause of the enhanced JC, we suggest that JC is determined by the magnitude of the proximity effect induced superconductivity in the normal metal layer, which is known to be proportional to the electron–electron interaction in N. We present one-dimensional material specific calculations that support this, and zero-field cooled DC magnetic susceptibility data that confirm an increased number of well-connected superconducting grains exist in the composite wires that contain metal additions with large electron–electron interactions and long electron mean free paths.

Flux pinning effects of Y 2O 3 nanoparticulate dispersions in multilayered YBCO thin films

The flux pinning effects of Y 2O 3 nanoparticulate inclusions in YBa 2Cu 3O 7− δ (Y123 or YBCO) thin films using (Y 2O 3/Y123) × N multilayer structures were studied. The multilayer films were made with pulsed laser deposition (PLD) on SrTiO 3 and LaAlO 3 substrates with a Y 2O 3 nanoparticulate ‘pseudo-layer’ thickness ranging from 0.2 to 1.4 nm, and YBCO layer thickness varying from 7 to 50 nm. Scanning electron microscopy images showed well-defined nanoparticle formation on film surfaces, with an approximate number density of (0.8–1.6) × 10 11 particles/cm 2 depending on Y 2O 3 thickness. Minor reductions in the critical temperature ( T c) were measured for each increase in Y 2O 3 pseudo-layer thickness. Transport critical currents (77 K, self-field) of 3–5 MA/cm 2 were consistently achieved for composite films with ⩽0.6 nm Y 2O 3 pseudo-layer thicknesses. Magnetic J c measurements using vibrating sample magnetometry ( H ⩽ 9 T, @70 and 77 K) showed a degradation of film properties for Y 2O 3 pseudo-layer thickness greater than 0.6 nm. A comparison to Y211/Y123 multilayer films showed the thinner Y 2O 3 pseudo-layer films exhibited similar properties.

Dependence of transport critical current of magnetic field processedBi2Sr2CaCu2O8/AgMg tapes on the background magnetic field and magnetic field direction

The background magnetic field dependence of the critical current(Ic) is studied on magnetic field processed (MFP)Bi2Sr2CaCu2O8+δ (Bi-2212) multifilamentary tapes in increasing and decreasing magnetic fields. The criticalcurrents are measured in magnetic fields up to 5 T at 4.2 K, with several orientations ofthe applied magnetic field relative to the tape plane. Two field dependences ofIc are found: a stronger one when the magnetic field is applied perpendicular to the tapeplane () and a weaker one when it is applied parallel to the tape plane . The angular dependence of Ic as a function of Bsinθ, which is the component of the magnetic field perpendicular to the tape plane, exhibits thatIc(Bsinθ) coincides with in the high-field region, indicating that the tape behaves as a two-dimensional (2D) superconductor.Because of the existence of misalignment in Bi-2212 grains, an average misalignment angleφavg is introduced and is found within the range of4.5°–6° for the MFPBi-2212 tape. φavg is independent of the magnetic field, indicating that it is an intrinsic property of the tape. Todetermine the critical current behaviour for all orientations of the magnetic field, a scaling functionf(θ) is introduced. The field-angle dependence of the critical current can be scaled to one curvewhere the field applied perpendicular to the tape plane at 4.2 K within the field rangebetween 0 and 5 T, both in increasing and decreasing fields, which is of primary importance.

The perpendicular low-frequency susceptibility of Bi-2223/Ag tapes

The low-frequency susceptibility of six Bi-2223/Ag superconducting tapes with the same 61 filaments but different cross-sectionalaspect ratios is measured at 77 K as a function of the amplitude of the perpendicularly applied field,Hm, using a carefully designed and calibrated high-field ac susceptometer with necessarysample-length corrections. The measured results are compared with two-dimensional calculationsperformed based on the critical state model with constant critical current density or the−1 internal susceptibility at low fields. It is shown that although the fill fraction is about 0.55for the filament-containing core, the measured low-field agrees well with the calculated critical state when the core is assumed to be completely superconducting, when the aspect ratio of thecore ranges from 13 to 32. This indicates a strong magnetostatic coupling among thefilaments. The same coupling is theoretically shown to reduce the field where maximum occurs, . However, the experimental is much higher than that even without coupling when the transport critical currentdensity is used for the model calculation. This suggests that ac magnetization arises fromall superconducting grains and the links between them, whereas the transport current islimited by the filament cross-section where grains are the worst linked, and thegreat discrepancy between magnetic and transport results is a consequence of thenon-uniformity of the superconducting structure of the tape. Therefore, transport criticalcurrent can be significantly improved by ensuring the uniformity of the grain links.

Effects of high-intensity ultrasound on Bi2Sr2CaCu2O8+x superconductor

Slurries of powdered Bi2Sr2CaCu2O8+x (BSCCO-2212) superconductor in high-boiling alkanes were irradiated with intense ultrasound. Significant enhancements of magnetic irreversibility as well as transport critical current are reported. The effects are dependent on the concentration of the slurry and are optimal for 1.5wt% slurry loading. Electron microscopy shows that ultrasonic treatment leads to a change in grain morphology and intergrain welding. The observed enhancement of superconducting properties is consistent with the limitations in critical currents in BSCCO superconductor being due to intergrain coupling rather than intragrain pinning strength.

Quantitative analysis of the critical current due to vortex pinning by surface corrugation

The transport critical current of a Niobium (Nb) thick film has been measured for a large range of magnetic field. Its value and variation are quantitatively described in the framework of the pinning of vortices due to boundary conditions at the rough surface, with a contact angle well explained by the spectral analysis of the surface roughness. Increasing the surface roughness using a Focused Ion Beam results also in an increase of the superficial critical current.

Transport critical current on Fe-sheathedMgB2 coils

Helical windings of Fe-sheathedMgB2 wireswith different dimensions were fabricated by a wind-and-react process. Wires with a cross-sectional areaof 1 mm2 and 20% superconductor core were made using the powder-in-tube (PIT) method andultra-fine starting precursors. Very good grain connectivity as well as high core densitywas obtained by using this method. At 20 K and self-field, the critical currentof a coil with an ID of 12 mm and 12 turns was 360 A, which is equivalent to aJc of1.8 × 105 A cm−2. At a 6 T external magnetic field and 4.2 K, a coil with an ID of 140 mm and 10 turns possessed aJc of1 × 104 A cm−2. These results are a step towards the application ofMgB2 wires in the temperature range of 20–30 K and high magnetic fields.

Role of excess Mg and heat treatments on microstructure and critical current of MgB2 wires

We present a detailed analysis of the effect of heat treatments on the microstructure, magnetization, and transport properties of MgB2 wires produced by the powder-in-tube method. We have used commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as sheath material. We measure the dc transport critical current that can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire, depending on the annealing parameters. We correlate the changes in the critical current with changes in the microstructure, as determined from scanning and transmission electron microscopy analysis. We show through magnetization measurements of short annealed wires that inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg and in inhomogeneous weak-link limited current flow, rendering the critical state model inapplicable. We discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the unannealed wires, where excess Mg promotes the recrystallization. The loss of Mg during the heat treatment may be precluded by annealing long wire lengths with a high heating rate.

Hysteresis and relaxation in TlBa2Ca2Cu3Oy superconducting polycrystals

We study the hysteresis and relaxation of both intragranular and intergranular properties of TlBa2Ca2Cu3Oy (Tl-1223) superconducting polycrystals between 80 and 120 K. The samples have been prepared using a technique involving the mixing of grains of different sizes before the final sintering. The grains show a sizeable reversible magnetization, while vortices inside the grain behave as three-dimensional objects. The transport critical current is strongly hysteretic, with features that distinguish our Tl-1223 samples from ‘standard’ YBa2Cu3O7−δ (YBCO), (Hg, Re)Ba2Ca2Cu3O8+δ (HBCCO) and (Bi, Pb)2Sr2Ca2Cu3O10−δ (BSCCO) polycrystals. The preparation method improves the transport properties of the samples. The relaxation of the transport critical current density, in the presence of trapped fields, is reported here for the first time in this system, as far as we know. A phenomenological model can qualitatively describe the transport properties, where the intragrain magnetization affects the intergranular junctions, but a precise quantitative description is not achieved. The differences in the shape of the transport measurements, for different polycrystalline systems, are also well described by the model.

Fabrications and properties of MgB/sub 2//stainless-steel tapes by PIT process

We have successfully fabricated single-filament composite MgB/sub 2//Stainless-Steel (SS) tapes using powder-in-tube processes such as swaging and cold rolling. The transport critical currents, that were measured by a standard dc four-probe method, I/sub c//spl sim/316 A and /spl sim/105 A were observed at T=4.2 K with H=0 G for as-rolled and sintered MgB/sub 2//SS tapes, respectively. In addition, the isothermal magnetizations M(H) of sintered MgB/sub 2//SS tapes and commercial MgB/sub 2/ powder were measured at temperatures T between 5 and 50 K in fields up to 6 T, employing a PPMS. The critical current density (J/sub c/) values obtained from the M(H) data, using Bean model, were >5 /spl times/ 10/sup 5/ A/cm/sup 2/ and >5 /spl times/ 10/sup 7/ A/cm/sup 2/ at T=10 K with H=0 G for the sintered MgB/sub 2//SS tapes and the commercial MgB/sub 2/ powder, respectively. On the other hand, we investigated the cross sections of both the sintered and the as-rolled tapes, using SEM and EPMA. No evidence of significant diffusion between MgB/sub 2/ and SS tube was found in the SEM/EPMA. However, traces of reaction can be seen in the interface region between MgB/sub 2/ core and SS tube for the sintered MgB/sub 2//SS tapes, from the EPMA line profiles. In addition, impurity phases such as MgO or MgB/sub 4/, which can be generated inside the superconductor MgB/sub 2/ core during fabrication, were observed by XRD analyses.

Comparing powder magnetization and transport critical current of Bi,Pb(2223) tapes

The magnetic field dependence of the critical current in (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x/ tapes is compared with the magnetization response of isolated grains extracted from the tapes. Special attention is paid to the low-field behavior. The goal of the experiment is to test the widely-used hypothesis that current paths in these tapes contain both weak- and strong- linked branches, which in low field act in parallel. The data agree with this hypothesis; at temperatures above 50 K the powder magnetization drops off exponentially from the self-field to the irreversibility field, while the transport and magnetization currents in the intact tapes show an extra low-field component. Below 50 K the powder behavior becomes less straightforward, but the parallel-path picture in the tapes still holds.

High critical currents in powder in tube MgB/sub 2/wires: influence of microstructure and heat treatments

MgB/sub 2/ wires were produced by the powder-in-tube method, using commercial MgB/sub 2/ powder with 5%at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. Depending on the parameters of the heat treatments, the dc transport critical current can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire. Analysis of the zero-field-cooling dc magnetization demonstrates that these large variations are due to changes in the connectivity of the wires. We discuss the optimization of the annealing conditions in terms of the changes in the microstructure, as determined from TEM and SEM analysis. We show that annealing can strongly improve the connectivity by eliminating most of the micro-cracks present in the un-annealed wires. In contrast, inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg.

Reduction in secondary phases and increased transport i c of (bi,pb)2223/ag tapes sintered with rapid heating rates

In a two-step heat treatment with one intermediate rolling the final transport critical current, (I/sub c/ at 77 K, self field, 1/spl inodot//spl grave/V/cm), of 37 filament (Bi,Pb)2223/Ag tapes increases linearly with the heating ramp rate: 20 A to 40 A, (/spl sim/20,000 Acm/sup -2/), for 20/spl deg/C/hour to 400/spl deg/C/hour. The principle benefit of the rapid ramp rate is likely to come from ramping fast though 800/spl deg/C to 832/spl deg/C in the 1st heat treatment. SEM and XRD analysis on the superconductor-Ag interface show a decrease in the amount of secondary phases, Cu/sub 2/O, Bi/sub 2/(Sr,Ca)/sub 2/Cu/sub 1/O/sub x/, Bi(Sr,Ca)O/sub x/, with increasing ramp rate and critical current.

Search for a silver-sheathed 1212-type superconductor with a grain colony microstructure: M/sub 1/Sr/sub 2/Y/sub 1-x/Ca/sub x/Cu/sub 2/O/sub z/ with M = (Bi,Pb), (Cd,Pb), and (Fe,Pb)

We have investigated the possibility of forming a silver-sheathed 1212-type high-critical-temperature superconductor (HTS) with the same type of grain colony microstructure (GCM) that develops in the silver-sheathed Bi-2223 (Ag/Bi-2223) composite conductor. The basis for this study is that a silver-sheathed 1212-type superconductor having a GCM should exhibit 1) good long-length transport critical current properties, 2) better in-field performance than Ag/Bi-2223 due to the significantly shorter blocking layer distance in 1212-type as opposed to 2223-type HTS phases, and 3) long-length transport current capacity equaling or exceeding that of Ag/Bi-2223 in present-day applications. Three series of M/sub 1/Sr/sub 2/Y/sub 1-x/Ca/sub X/Cu/sub 2/O/sub z/ (M-1212) compositions with M = (Bi,Pb), (Cd,Pb), and (Fe,Pb) were prepared in silver-sheathed/flat wire form by the powder-in-tube method. For M = (Bi,Pb), we were able to obtain over 80% conversion to a 1212-phase in a relatively short time (6 h) for oxygen partial pressures in the range from 0.08 to 0.21 atm and temperatures from 900/spl deg/C to 940/spl deg/C. The (Bi,Pb)-1212 phase grew with a GCM that had the same type of fiber texture found in Ag/Bi-2223 composites. However, within the range of processing conditions tolerated by a silver sheath, no transport critical current was achieved in any of the Ag/(Bi,Pb)-1212 specimens, including ones that showed the presence of an HTS phase. Various oxygen doping treatments performed at very high (ca. 88 atm O/sub 2/), intermediate (1 atm O/sub 2/), and low oxygen partial pressures (0.01 atm O/sub 2/) failed to induce a global HTS phase that carried a transport current. The preponderance of evidence from these studies indicated that any superconductivity detected in Ag/(Bi,Pb)-1212 specimens was most likely due to trace amounts of either Bi-2201 or Bi-2212. For M = (Cd,Pb) and (Fe,Pb), M-1212 phases in appreciable quantity did form in the silver-sheathed composites after heat treatment, but those M-1212 phases had granular microstructures. In the case of Ag/(Cd,Pb)-1212, the superconductivity that was detected in several specimens appeared to originate from a 2212-like phase. In the case of Ag/(Fe,Pb)-1212, there was clear evidence for the formation of a superconducting 1212 phase, but neither the Ag/(Cd,Pb)-1212 nor the Ag/(Fe,Pb)-1212 specimens exhibited a measurable transport critical current, presumably (in the latter case at least) because of the absence of a GCM.

Transport properties of hot-forming textured Bi:2223 and single domain YBa 2Cu 3O y fabric materials

The electrical performance of a hot-forming Bi:2223 compounds prepared by modifying the characteristics of the starting precursor powders and of YBa 2Cu 3O y superconducting fabrics are investigated. The single domain Y123 fabrics are processed by an infiltration process from precursor Y 2O 3 cloths which having dimensions of thick films (∼100 μm) and posses the structure replica of precursor Y 2O 3 fabrics. Transport critical currents exceeding 2.1×10 4 A/cm 2 at 77 K, 0 T and 3.9×10 4 A/cm 2 at 65 K in self field are measured for Bi-2223 samples. For fabric samples, transport J c measurements in self field showed the transition between the superconducting state and the flux-flow regime to occur at a critical current density ( J c) of 5.5×10 4 A/cm 2 at 77 K. Quench tests at 77 K on the fabric samples using pulsed currents up to 5 J c with a duration of 25 ms are discussed.

Destructive and non-destructive determination of the transport current density radial distribution: Application to Bi-2212 textured rods

Destructive and non-destructive methods to estimate the radial distribution of the transport critical current, J c( r), of long cylindrical superconductors are presented. The non-destructive method is based on the measurement of self-field AC losses as a function of the current amplitude, Q( I 0) and takes into account the E– J characteristics of the material. Both methods have been used to derive the J c( r) profiles of long and thin Bi-2212 rods textured by laser-induced zone melting techniques. The obtained results have been correlated with the microstructure of the samples and their critical temperature.

Influence of mechanical reinforcement of MgB 2 wires on the superconducting properties

Monofilamentary MgB 2 wires with a mechanical tough Nb/Cu/steel or Fe/steel sheath were prepared and characterized. The steel content was varied to investigate the reinforcement effect and the consequences for the superconducting properties of the wires, which were heat treated to achieve dense and homogeneous filaments. The use of Nb as first sheath layer, having a smaller thermal expansion coefficient than MgB 2, requires the application of higher amounts of steel to achieve compressive pre-stress on the filament in comparison to Fe as first wall material. With raised steel content in the sheath the critical transport currents show field dependent significant critical current and irreversibility field degradations. First I c vs. axial stress and strain experiments confirmed this observation of pre-stress induced degradations. Consequences for improved wires and for future applications will be discussed.

Pulsed transport critical currents of Bi2212 tapes in pulsed magnetic fields

If high-TC superconductors are ever to be used in high-field applications, it is vital that the critical surfaces can be mapped under high-field conditions. However, the latest superconductors have high currents even at fields over 20 T, making accurate measurements very difficult due to the thermal and mechanical problems. In this paper, we compare measurements on BSCCO-2212 tape using a number of different methods, particularly an innovative pulsed transport current and pulsed field mode. We show how the analysis of the voltage signal from BSCCO-2212 tape in pulsed conditions may be used to extract the critical current in quasi-stationary conditions. The effect of a metallic substrate on the results is also briefly discussed.

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube process. After annealing, we measured transport jc values up to 1.1 × 105 A cm−2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with seven filaments fabricated by two-axial rolling, and up to 5 × 104 A cm−2 at 4.2 K in 1 T in a MgB2/Fe tape with seven filaments. For higher currents these multifilamentary wires and tapes quenched due to the insufficient thermal stability of the filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.