We report non-Cu critical current densities of 4.09 * 10^9 A/m^2 at 12 T and 2.27 * 10^9 A/m^2 at 15 T obtained from transport measurements on a Ti-alloyed RRP Nb3Sn wire after irradiation to a fast neutron fluence of 8.9 * 10^21 m^-2. These values are to our knowledge unprecedented in multifilamentary Nb3Sn, and they correspond to a Jc enhancement of approximately 60% relative to the unirradiated state. Our magnetometry data obtained on short wire samples irradiated to fast neutron fluences of up to 2.5 * 10^22 m^-2 indicate the possibility of an even better performance, whereas earlier irradiation studies on bronze-processed Nb3Sn wires with a Sn content further from stoichiometry attested a decline of the critical current density at such high fluences. We show that radiation induced point-pinning centers rather than an increase of the upper critical field are responsible for this Jc enhancement, and argue that these results call for further research on pinning landscape engineering.
Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. After annealing, we measured transport jc values up to 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with 7 filaments fabricated by two-axial rolling, and up to 5 * 104 A/cm2 at 4.2 K in 1 T in a MgB2/Fe tape with 7 filaments. For higher currents these multifilamentary wires and tapes quenched due to insufficient thermal stability of filaments. Both the processing routes and deformation methods were found to…
In DC and AC practical applications of MgB2 superconducting wires an important role is represented by the material sheath which has to provide, among other things, a suitable electrical and thermal stabilization. A way to obtain a large enough amount of low resistivity material in to the conductor architecture is to use it as external sheath. In this paper we study ex-situ multifilamentary MgB2 wires using oxide-dispersion-strengthened copper (GlidCop) as external sheath in order to reach a good compromise between critical current density and thermal properties. We prepared three GlidCop sampl…
Mono- and multifilamentary MgB2/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit high exponential n factors for the resistive transition: n ~ 80 and 40 were found at 5 T and 7 T, respectively. The highest transport jc values o…
We present two magnetometry-based methods suitable for assessing gradients in the critical temperature and hence the composition of multifilamentary superconductors: AC magnetometry and Scanning Hall Probe Microscopy. The novelty of the former technique lies in the iterative evaluation procedure we developed, whereas the strength of the latter is the direct visualization of the temperature dependent penetration of a magnetic field into the superconductor. Using the example of a PIT Nb3Sn wire, we demonstrate the application of these techniques, and compare the respective results to each other …
Seven-core Ag/Fe sheathed Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires were produced by the ex situ powder-in-tube (PIT) method. The relationship between the cold-work deformation process and the superconducting properties of wires were systematically studied. It was found that flat rolling can efficiently increase the density of the superconducting core and largely improve the transport critical current density (Jc) of as-drawn wires. The Jc of the best sample achieved 21.1 kA/cm^2 at 4.2 K in self field, and showed very weak magnetic field dependence in high fields. Our result suggested a …
A comparative study of magnetic behavior in a DC magnetic field up to 19 Tesla has been undertaken on different Nb3Sn samples, i.e. single crystals, polycrystal samples and multifilamentary wires. From the 1st and 3rd harmonics of the AC magnetic susceptibility a peak effect was experimentally observed in the single crystal and in the polycrystal, whereas this phenomenon is absent in the measured wires. The corresponding H vs T phase diagram reveals a different behavior between wires on one hand and polycrystalline and single crystal samples on the other. In particular, from the 3rd harmonics …
Bi2Sr2CaCu2O8+x is the only cuprate superconductor that can be made into a round-wire conductor form with a high enough critical current density Jc for applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed filamentary wires can be doubled to more than 1.4x10^5 A/cm^2 by low temperature oxygenation. Careful analysis shows that the improved performance is associated with a 12 K reduction in transition temperature Tc to 80 K and a significant enhancement in intergranular connectivity. In spite of the macroscopically untextured nature of the wire, overdoping is highly effective in …
We have fabricated prototype Bi,Pb(2223)/Ag superconduting multifilamentary wires with a square cross-section. The AC losses of these wires were measured, compared with those of tapes and also compared with the theory. Wires show largely reduced AC magnetic field losses at 47 Hz, compared to those of tapes in a perpendicular magnetic field. The effects of the twist pitch and of the use of the high resisitive barriers were also investigated. The results show that the use of a twist pitch of about 10 mm is sufficient for decoupling the filaments, and that the use of very short twist pitches does…
The MgB2 superconductor has already demonstrated its applicative potential, in particular for DC applications such as MRI magnets, thanks to the low costs of the raw materials and to its simple production process. However further efforts have still to be made in order to broaden its employment also towards the AC applications such as SFCL, motors, transformers. The main issues are related to the reduction of the AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powders granulometry can play a cruc…
Significant insights for critical current density (Jc) improvement in Bi-2212 super- conductor wires can be obtained by an accurate analysis of the structural and mi- crostructural properties evolving during the so-called partial-melt process, a heat treatment needed to improve grain connectivity and therefore gain high Jc. Here, we report an in-situ analysis by means of synchrotron X-ray and neutron diffraction performed, for the first time, during the heat treatment carried out with the very same temperature profile and reacting oxygen atmosphere in which the Bi-2212 wires are usually treate…
Iron-based superconductors have been considered to be very promising in high-field applications, for which multifilamentary wire and tape conductors with high mechanical strength are essential. In this work, 7-,19- and 114-filament Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires and tapes with silver as matrix and iron as outer reinforcing sheath were produced by the ex situ powder-in-tube method. The mass densities of Sr-122 phase in 7- and 19-filament conductors were investigated by microhardness characterization, which revealed a positive correlation between hardness and transport critical c…
In this letter we demonstrate the development of ternary Nb3Sn multifilamentary conductors with artificial pinning centers (APC) which achieve high critical fields. These recently-developed conductors were tested in a 31 T magnet, and the results showed that their upper critical field (Bc2) values at 4.2 K are 27-28 T, and irreversible field (Birr) values are above 26 T, values similar to or higher than those of best RRP conductors. The non-Cu Jc has been brought to nearly 1200 A/mm2 at 16 T and 4.2 K, comparable to RRP, in spite of the fact that the fine-grain Nb3Sn fractions in filaments are…
We present an extensive irradiation study involving five state-of-the-art Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast neutron fluence of 1.6 * 10^22 m^-2 (E > 0.1 MeV). The volume pinning force of short wire samples was assessed in the temperature range from 4.2 to 15 K in applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated state and after each irradiation step. Pinning force scaling computations revealed that the exponents in the pinning force function differ significantly from those expected for pure grain boundary pinning, and t…
In this letter we demonstrate achievement of record non-Cu critical current density (Jc,non-Cu) in ternary, multifilamentary Nb3Sn conductors by the introduction of artificial pinning centers (APC). In the past two years, we have made great progress in the development of APC Nb3Sn wires. Recent resistivity vs magnetic field measurements confirmed the high upper critical field (Bc2) of ternary APC wires, which at 4.2 K was ~28 T, about 1-2 T higher than present state-of-the-art conductors. In addition to high Bc2, it was found that APC wires have noticeably higher Sn content in the Nb3Sn layers…
The paper presents discussion on up to date results on MgB2 conductors from the point of view of their future dc and ac applications. Basic physical parameters of MgB2 compound relevant to conductors are introduced. Different conductor preparation methods and conductor architectures are presented and attainable critical current densities discussed. Some numerical results on critical currents and ac losses of future multifilamentary MgB2 conductors with magnetic cladding of their filaments are given. Recently observed anomalous decrease of ac susceptibility at 50 K in copper clad Powder-in-tube…
MgB2 tapes and wires were fabricated by the Powder-In-Tube method. Stainless steel and Cu-Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10,000A/cm2 at 4.2K and 5T. A high Jc of about 300,000A/cm2 was obtained by extrapolating the Jc-B curves to zero field. Multifilamentary(7-core) MgB2 wire was successfully fabricated using Cu-Ni tubes. For both tapes and wires the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu-Ni sheath…
Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e holes) arrays, vort…
Recently the interest about Bi-2212 round wire superconductor for high magnetic field use has been enhancing despite the fact that an increase of the critical current is still needed to boost its successful use in such applications. Recent studies have demonstrated that the main obstacle to current flow, especially in long wires, is the residual porosity inside these Powder-In-Tube processed conductors which develops in bubbles-agglomeration when the Bi-2212 melts. Through this work we tried to overcome this issue acting on the wire densification by changing the deformation process. Here we sh…
The critical current density shown by a superconductor at the extreme type-II limit is predicted to follow an inverse square-root power law with external magnetic field if the vortex lattice is weakly pinned by material line defects. It acquires an additional inverse dependence with thickness along the line direction once pinning of the interstitial vortex lines by material point defects is included. Moderate quantitative agreement with the critical current density shown by second-generation wires of high-temperature superconductors in kG magnetic fields is achieved at liquid-nitrogen temperat…
The current distribution and the AC loss in a composite superconducting tape containing a layer from magnetic material is calculated and compared with experiments, showing a very good agreement. The situations of an alternating uniform applied field or a transport current are studied. The newly developed numerical model is an approximation to the critical state model, adapted for the applicability to commercial finite elements codes that solve the vector potential. Substantial feature of this procedure is that it can be carried out in the case when the critical current density in superconducto…
The ability of high-temperature superconductors (HTSs) to carry very large currents with almost no dissipation makes them irreplaceable for high-power applications. The development and further improvement of HTS-based cables requires an in-depth understanding of the superconducting vortex dynamics in presence of complex pinning landscapes. We present a critical current analysis of a real HTS sample in a magnetic field by combining state-of-the-art large-scale Ginzburg-Landau simulations with reconstructive three-dimensional scanning transmission electron microscopy tomography of the pinning la…
The field error due to magnetization of superconductors in a superconducting magnet can significantly compromise the field homogeneity which is extremely important for NMR magnets. This is especially true for high Tc superconducting magnets. In this paper, the error field induced by the magnetization of superconducting wires or tapes is analyzed using a magnetic dipole field model. It uses experimentally measured magnetization of superconducting wires, and eludes the need of assuming screening current and its distribution as in previous methods. The field error of a REBCO coil calculated by th…
High engineering critical current density JE of >500 A/mm2 at 20 T and 4.2 K can be regularly achieved in Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox (Bi-2212) round wire when the sample length is several centimeters. However, JE(20 T) in Bi-2212 wires of several meters length, as well as longer pieces wound in coils, rarely exceeds 200 A/mm2. Moreover, long-length wires often exhibit signs of Bi-2212 leakage after melt processing that are rarely found in short, open-end samples. We studied the length dependence of JE of state-of-the-art powder-in-tube (PIT) Bi-2212 wires and gases released by …
We report herein a high and isotropic critical-current density Jc for BaFe2(As,P)2 epitaxial films. The isotropy of Jc with respect to the magnetic-field direction was improved significantly by decreasing the film growth rate to 2.2 Å/s. The low growth rate served to preferentially align dislocations along the c-axis, which work well as c-axis vortex-pinning centers. Because of the intentional introduction of effective pinning, the absolute Jc at 9 T was larger than that obtained for other iron-based superconductors and conventional alloy superconducting wires.
In this letter we demonstrate that if oxygen can be properly supplied to (Nb-Zr)-Sn wires, ZrO2 precipitates will form during the heat treatment, refining the Nb3Sn grain size markedly. Here, a Nb3Sn subelement was fabricated in which Nb-1Zr alloy was used, and oxygen was supplied via SnO2 powder. The results showed that such a design could supply sufficient oxygen to internally oxidize the Zr in the Nb-1Zr alloy, and that the sample reacted at 650 °C had grain sizes of ~45 nm, less than half the size of the grains in present Nb3Sn conductors. Magnetic measurements showed that the peak of the …
In this letter, we report the first successful fabrication of dense, metal-clad MgB2 superconductor wires as well as the first demonstration of high transport Jc of greater than 36,000 A/cm2 in MgB2. The fabrication technique that we devised uses only ambient pressure, yet produces very dense MgB2 with little loss of stoichiometry. Such a technique can be useful for rapid and reliable compound syntheses and characterizations for future discovery of new or improved superconductors with higher Tc, higher Hc2 or improved flux pinning characteristics. We also show that while inherently weak-link-f…
We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure "in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity. As a result, higher critical current densities J_c were measured over the entire range of applied magnetic fields B_a for all the samp…
The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Dis…
Here we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb$_{3}$Sn, making it suitable for dipole magnets for Future Circular Collider (FCC). A big challenge for the FCC is that a realistic production target for FCC Nb3Sn requires ~30% improvement over current conductor performance. Recent success with internal oxidation(IO) of Nb-Zr precursor has shown significant improvement in the layer J$_{c}$ of Nb$_{3}$Sn wires, albeit the complication of providing an internal O$_{2}$ diffusion path and avoiding degradation of irreversibility field($_{irr}$). We …
Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (un…
The increasing demand for improving the high-field (16-22 T) performance of Nb$_3$Sn conductors requires a better understanding of the properties of modern wires much closer to irreversibility field, H$_{Irr}$. In this study we investigated the impact of Ta, Ti and Hf doping on the high-field pinning properties, the upper critical field, H$_{c2}$, and H$_{Irr}$. We found that the pinning force curves of commercial Ti and Ta doped wires at different temperatures do not scale and that the Kramer extrapolation, typically used by magnet designers to estimate high-field critical current density and…
The nano-SiC doped MgB2/Fe wires were fabricated using a powder-in-tube method and an in-situ reaction process. The depression of Tc with increasing SiC doping level remained rather small due to the counterbalanced effect of Si and C co-doping. The high level SiC co-doping allowed creation of the intra-grain defects and nano-inclusions, which act as effective pinning centers, resulting in a substantial enhancement in the Jc(H) performance. The transport Jc for all the wires is comparable to the magnetic Jc at higher fields despite the low density of the samples and percolative nature of curren…
We have fabricated the Fe(Se, Te) superconducting wire by a special process based on a powder-in-tube method. The pure Fe tube plays the role of not only the sheath but also the raw material for synthesizing the superconducting phases. We succeeded in observing zero resistivity current on the current-voltage measurements for the Fe(Se, Te) wire. Introduction of the pinning centers and fabricating a multi-core wire will enhance the critical current density for the next step.
Domelike magnetic-flux-density distributions previously have been observed experimentally and analyzed theoretically in superconducting films with edges, such as in strips and thin plates. Such flux domes have been explained as arising from a combination of strong geometric barriers and weak bulk pinning. In this paper we predict that, even in films with bulk pinning, flux domes also occur when vortices and antivortices are produced far from the film edges underneath current-carrying wires, coils, or permanent magnets placed above the film. Vortex-antivortex pairs penetrating through the film …
We prepared and characterized monofilamentary MgB2 wires with a mechanically reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments and correspondingly high transport currents up to Jc = 10^5 A/cm^2 at 4.2 K, self field. The reproducibility of the measured transport currents was excellent and not depending on the wire diameter. Using different precursors, commercial reacted powder or an unreacted Mg/B powder mixture, a strong influence on the pinning behaviour and the irreversibility field was observed. The critical transport current density showed a nearly linear tempe…
The powder-in-tube MgB2 wires were prepared by high energy milling of Mg and B powder. The powder was not mechanically alloyed for 2h short milling time. However, the MgB2 grains in wires are very small (20 100nm) and resembled to the dimple after post-heat treatment. The clear evidence for transcrystlline fracture is observed. It indicated that the grain connection was greatly improved and the fluxing pinning was significantly enhanced. Another point to view is no intermediate annealing was adopted during the whole rolling process. The influence of the post-heat treatment on the transport cur…
The effect of fractal clusters on magnetic and transport properties of percolative superconductors is considered. The superconductor contains percolative superconducting cluster, carrying a transport current, as well as clusters of a normal phase, which act as pinning centers. A prototype of such a structure is a high-temperature superconducting wire. It is found that normal-phase clusters can have fractal features that affect the vortex dynamics. The fractal dimension of the normal-phase cluster boundary in YBCO films is estimated. Depinning and transport of vortices in fractal superconductin…
We propose a targeted attack on making the Fe-based superconductor (FBS) AEFe2As2 (122) (AE = alkaline earth, here Ba or Sr) into a much cheaper, higher stability margin, round, twisted and effectively isotropic multifilament conductor suitable for 16-20 T dipole magnet construction. This white paper urges to greatly accelerate progress by providing a fundamental understanding of what controls the transport critical current density, Jc, of 122 polycrystals and then in the wire forms. High grain-to-grain Jc is the one key property not yet fully demonstrated in this new superconductor and this i…
We evaluated the doping effect of two types of multiwall carbon nanotube (CNT) with different aspect ratios on MgB2/Fe monofilament wires. Relationships between microstructure, magnetic critical current density (Jc), critical temperature (Tc), upper critical field (Hc2), and irreversibility field (Hirr) for pure and CNT doped wires were systematically studied for sintering temperature from 650oC to 1000oC. As the sintering temperature increased, Tc for short CNT doped sample slightly decreased, while Tc for long CNT doped sample increased. This indicates better reactivity between MgB2 and shor…
The tilt modulus of a defective Abrikosov vortex lattice pinned by material line defects is computed using the boson analogy. It tends to infinity at long wavelength, which yields a Bose glass state that is robust to the addition of weak point-pinning centers, and which implies a restoring force per vortex line for rigid translations about mechanical equilibrium that is independent of magnetic field. It also indicates that the Bose glass state breaks into pieces along the direction of the correlated pinning centers if the latter have finite length. The critical current is predicted to crossove…
Nb3Sn superconductors have significant applications in constructing high-field (> 10 T) magnets. This article briefly reviews development of Nb3Sn superconductor and proposes prospects for further improvement. It is shown that significant improvement of critical current density (Jc) is needed for future accelerator magnets. After a brief review of the development of Nb3Sn superconductors, the factors controlling Jc are summarized and correlated with their microstructure and chemistry. The non-matrix Jc of Nb3Sn conductors is mainly determined by three factors: the fraction of current-carrying …
We report significant transport critical currents firstly achieved in Sr0.6K0.4Fe2As2 wires and tapes with a Tc = 34 K, which were fabricated through an in-situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. Transport measurements were performed by a standard four-probe resistive method. All the wire and tape samples have shown transport properties. Critical current density Jc was enhanced upon silver addition, and at 4.2 K, a best Jc of ~1200 A/cm^2 (Ic = 9 A) was achieved for 20 % silver added tapes, which is the highest in iron-based wires and t…
A review of current developments in the study of chemical doping effect on the superconducting properties of MgB2 wires and tapes is presented, based on the known literature data and our own results. The critical current density of MgB2 can be improved through various kinds of dopants. Among these dopants, doping with carbon-containing materials seems to be the most effective way to improve the Jc performance. The doping effect of carbon in different forms and carbon-based compounds such as SiC, nano-C, metal carbides, as well as aromatic hydrocarbon and carbohydrate on the Jc-B characteristic…
I introduce a critical-state theory incorporating both flux cutting and flux transport to calculate the magnetic-field and current-density distributions inside a type-II superconducting cylinder at its critical current in a longitudinal applied magnetic field. The theory is an extension of the elliptic critical-state model introduced by Romero-Salazar and Perez-Rodriguez. The vortex dynamics depend in detail upon two nonlinear effective resistivities for flux cutting (ρ_\parallel) and flux flow (ρ_\perp), and their ratio r = ρ_\parallel/ρ_\perp. When r < 1, the low relative efficiency of flux …
If the critical current density Jc is very high for a superconductor, then estimating its value from transport measurements is not very easy. In such cases, the value of Jc, called Jcm for magnetic Jc, is usually obtained from the measured magnetic hysteresis loop measurements by using a proper critical state model such as Bean's model (ref. 1). However, for bulk polycrystalline high temperature superconductors, the values of Jcm are much higher than the values, Jct, obtained from the transport measurements. This is due to the fact that the Jct is interrupted by weakly linked grain boundaries.…
Superconducting transition temperature (Tc) and critical current density (Jc) are two key factors that are not only crucial for probing high-temperature superconducting mechanisms, but also for practical applications. The simple crystal structure of FeSe is very favorable for the fabrication of thin films and wires, but its application is limited by the relatively low Tc and small Jc. A previous study has found that the Tc of FeSe can be significantly enhanced over 40 K by using the protonation method. Here, we present a systematic study of Jc and vortex properties of H+-intercalated FeSe (Hx-…
In modern Nb3Sn wires there is a fundamental compromise to be made between optimizing the intrinsic properties associated with the superfluid density in the A15 phase (e.g. Tc, Hc, Hc2, all of which are composition dependent), maximizing the quantity of A15 that can be formed from a given mixture of Nb, Sn and Cu, minimizing the A15 composition gradients within each sub-element, while at the same time generating a high vortex pinning critical current density, Jc, by maximizing the grain boundary density with the additional constraint of maintaining the RRR of the Cu stabilizer above 100. Here …
The intrinsic properties of MgB$_2$ form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB$_2$ is limited by the upper critical field H$_{c2}$ and its anisotropy. In wires or tapes, the MgB$_2$ grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting c…
Understanding what makes Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_x$ (Bi-2212) the only high critical current density ($J_c$), high temperature superconductor (HTS) capable of being made as a round wire (RW) is important intellectually because high $J_c$ RW Bi-2212 breaks the paradigm that forces biaxially textured REBCO and uniaxially textured (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_x$ (Bi-2223) into tape geometries that reproduce the strong anisotropy of the native crystal structure and force expensive fabrication routes to ensure the best possible texture with minimum density of high angle grain boundaries. The …
122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields Hc2 (> 100 T), low Hc2 anisotropy r (< 2), and high thin-film critical current densities Jc (> 1 MA/cm2 at 4.2 K). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large in-field Jc, which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with transport Jc up to 0.1 MA/cm2 at 10 T and 4.2 K. This value …
Key questions for any superconductor include: what is its maximum dissipation-free electrical current (its `critical current') and can this be used to extract fundamental thermodynamic parameters? Present models focus on depinning of magnetic vortices and implicate materials engineering to maximise pinning performance. But recently we showed that the self-field critical current for thin films is a universal property, independent of microstructure, controlled only by the penetration depth. Here we generalise this observation to include thin films, wires or nanowires of single- or multi-band s-w…
The paper reports the first successful fabrication of MgB2 superconducting tape using a flexible metallic substrate as well as its strong pinning force, which was verified by direct measurement of transport critical current density. The tape was prepared by depositing MgB2 film on a Hastelloy tape buffered with an YSZ layer. The Jc of the tape exceeds 105A/cm2 at 4.2K and 10T, which is considered as a common benchmark for magnet application. The Jc dependence on magnetic field remains surprisingly very small up to 10T, suggesting that the tape has much better magnetic field characteristic than…
We extend the Ginzburg-Landau-Abrikosov-Gor'kov (GLAG) theory to account for the variation of the upper critical field Hc2 with Sn content in A15-type Nb3Sn. Hc2 at the vicinity of the critical temperature Tc is related quantitatively to the electrical resistivity, specific heat capacity coefficient and Tc, with inclusion of the electron-phonon coupling correction, Pauli paramagnetic limiting and martensitic phase transformation of A15 lattices. Hc2 near Tc is then extrapolated to Hc2(0) at temperature 0K, and Hc2(0) versus tin content agrees well with experiments. We find that, as Sn content …
High critical current densities (Jc) > 1 MA/cm2 were realized in cobalt-doped BaFe2As2 (BaFe2As2:Co) films on flexible metal substrates with biaxially-textured MgO base-layers fabricated by an ion-beam assisted deposition technique. The BaFe2As2:Co films showed small in-plane crystalline misorientations (delta fai BaFe2As2:Co) of ~3o regardless of doubly larger misorientaions of the MgO base-layers (delta fai MgO = 7.3o), and exhibited high self-field Jc up to 3.5 MA/cm2 at 2 K. These values are comparable to that on MgO single crystals and the highest Jc among iron pnictide superconducting ta…
High-temperature superconductors (HTS) are finding use as high - current wires for potential application in power devices. The dependence of their basal-plane critical current properties on orientation in a magnetic field can pose important design problems. Here we report material and operating parameter conditions where prototype HTS tape conductors exhibit critical current characteristics that are essentially field-orientation independent. The novel phenomenon is observed for specific magnetic field intensities that depend on the operating temperature, and in materials having strong flux pin…
Seven years after the discovery of superconductivity in magnesium diboride, the fundamental superconducting properties of this compound are well known and the peculiar current transport in polycrystalline materials is essentially understood. Based on this knowledge the ultimate performance of wires or tapes at high magnetic fields will be predicted and compared to state-of-the-art materials and to other superconductors. The key parameter for high field applications is the upper critical field, which can be strongly enhanced by impurity scattering. This fundamental property might be further opt…
The MgB2 coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB2/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (Tc) ranging between 36 and 38 K with superconducting transition width (\DeltaTc) of about 0.3-0.6 K. The highest critical current density (Jc) of 1.34 \times 105 …
