Practical Superconductors Research Articles

Modelling current voltage characteristics of practical superconductors

Based on recent experimental results, and in the light of fundamental physical properties of the magnetic flux in type-II superconductors, we introduce a practical expression for the material law to be applied in numerical modelling of superconducting applications. Focusing on the computational side, in this paper, previous theory is worked out, so as to take the celebrated form of a power-law-like dependence for the current voltage characteristic. However, contrary to the common approach in numerical studies, this proposal suits the general situation of current density flow with components either parallel or perpendicular to the local magnetic field, and different constraints applying on each component. Mathematically, the theory is generated from an elliptic locus defined in terms of the current density vector components. From the physical side, this contour establishes the boundary for the onset of entropy production related to overcritical current flow in different conditions. The electric field is obtained by partial differentiation and points perpendicular to the ellipse. Some numerical examples, inspired by the geometry of a two-layer helical counter-wound cable are provided. Corrections to the widespread use of the implicit isotropic assumption (physical properties only depend on the modulus of the current density vector) are discussed, and essentially indicate that the current carrying capacity of practical systems may be underestimated by using such simplification.

Effects of fluctuating magnetic fields on a superconducting bulk rotor shielded with superconducting rings

We study the effect of a fluctuating magnetic field, which is one of the technical problems for trapped magnetic fields in a bulk superconductor, to realize a practical bulk superconductor rotating machine. Previous research and other's research has shown that fluctuating magnetic fields reduce the strength of trapped magnetic fields in superconducting bulk modules [1, 2]. This deters development of applications of AC rotating machines because superconducting bulk modules are always exposed to a fluctuating magnetic field. Therefore, it is necessary to develop a method to control decrease of the trapped magnetic field. We propose a method to use the shielding ring of a superconducting wire to achieve this goal and the effects are confirmed experimentally [3]. We are now building test equipment for examining the performance of a shielding ring in a bulk rotating machine. This paper reports the test result for the shielding ring applied to the bulk superconducting rotor that is a part of the test equipment.

Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications.

High-performance MgB2 superconducting wires for use under liquid-helium-free conditions fabricated using an internal Mg diffusion process

MgB2 has a superconducting transition temperature (Tc) of 39 K, which is much higher than that for practical metallic superconductors. Thus, it is hoped that MgB2 can not only replace metallic superconductors, but can be used under liquid-helium-free conditions, for example, at temperatures of 10–20 K that can easily be achieved using cryocooling systems. However, to date, the reported critical current density (Jc) for MgB2 wires is not high enough for large-scale applications in liquid-helium-free conditions. In the present study, successful fabrication of high-performance MgB2 superconducting wires was carried out using an internal Mg diffusion (IMD) process, involving a p-dimethylbenzene (C8H10) pre-treatment of carbon-coated B powder with nanometer-sized particles. The resulting wires exhibited the highest ever Jc of 1.2 × 105 A cm−2 at 4.2 K and 10 T, and an engineering critical current density (Je) of about 1 × 104 A cm−2. Not only in 4.2 K, but also in 10 K, the Jc values for the wires fabricated in the present study are in fact higher than that for Nb–Ti wires at 4.2 K for the magnetic fields at which the measurements were carried out. At 20 K and 5 T, the Jc and Je were about 7.6 × 105 A cm−2 and 5.3 × 103 A cm−2, respectively, which are the highest values reported for MgB2 wires to date. The results of a detailed microstructural analysis suggested that the main reason for the superior electrical performance was the high density of the MgB2 layer rather than just the small grain size, and that the critical current could be further increased by suitable control of the microstructure. These high-performance IMD-processed MgB2 wires are thus promising superconductors for applications such as magnetic resonance imaging and maglev trains that can operate under liquid-helium-free conditions.

Excellent Jc in the low-temperature sintered MgB2 superconductors consisted of uncompleted MgB2 phase and residual Mg

Though low-temperature sintering of MgB2 superconductors below Mg melting point can effectively depress volatility of Mg and increase sintering density, its development was limited in recent years for the reason that it usually took very long time to form complete MgB2 phase with excellent Jc at low temperature. In present work, significantly improved Jc was surprisingly obtained in the MgB2 samples sintered at 575 °C for only 5 h after short-time ball milling, even though formation of MgB2 phase is not completed and lots of residual Mg is still present in these samples. The grain connectivity in prepared samples is obviously improved compared to referred MgB2 sample sintered at high temperature, which is responsible for the improvement of Jc. The method developed in present work seems bring a new opportunity for the development of low-cost practical MgB2 superconductors with improved Jc without using expensive nanometer-size dopants or high-temperature sintering.

Enhanced upper critical field, critical current density, and thermal activation energy in new ytterbium doped CeFeAsO0.9F0.1 superconductor

In this report, we have investigated the essential physical properties of Ce0.7Yb0.3FeAsO0.9F0.1 superconductor such as field dependent critical current density (Jc), thermal activation energy (U0), and upper critical field (Hc2). From the isothermal magnetization curves and size of the superconducting grains, the critical current density Jc of 2.3 × 106 A/cm2 at 2 K, 0.5 T was estimated using the Bean's model for this Yb doped superconductor. A gradual decrease of Jc and absence of peak effect were found on increasing magnetic field up to 5 T. Thermal activation energy (U0/kB = ∼2500 K) calculated from Arrhenius plots at low magnetic field (0.5 T) indicates a strong flux pinning potential might be co-existing with applied magnetic field. Our results suggest that this new Yb doped superconductor is a possible practical high temperature superconductor under certain magnetic field and temperature.

Materials Chemistry and the Development of Practical High Temperature Superconductor Wires and Electric Power Applications

not Available.

MgB2, a two-gap superconductor for practical applications

Abstract

Multi-Pole Components of Magnetic Field in Dipole Magnets Wound With High $T_{\rm c}$ Superconductor Tape and Feasibility of Their Accelerator Applications

Applications of high superconductors to accelerator magnets are attractive from the viewpoint of cooling cost reduction and that of generating very high magnetic field at low temperature. However, in magnets using cosine-theta like coils, three-dimensional shape of coil ends and magnetic field quality might be concerns, because most of practical high superconductors are with flat tape shape. We studied the field quality of dipole magnets wound with high superconductor tapes. Case studies on magnetic field design were made for dipole magnets wound with coated conductors: we designed two magnets and calculated multi-pole components of the magnetic field to evaluate their field qualities. The torsion/bending tolerance of coated conductors was studied experimentally to clarify the feasibility of three-dimensional winding. The influence of the shielding current in high superconductor tapes on field quality was discussed based on the numerical electromagnetic field analysis of superconductors.

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity inoxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers(1111, 122, 11, 111) are available. They share several characteristics with cupratesuperconductors that compromise easy applications, such as the layered structure,the small coherence length and unconventional pairing. On the other hand, theFe-based superconductors have metallic parent compounds and their electronicanisotropy is generally smaller and does not strongly depend on the level of doping,and the supposed order parameter symmetry is s-wave, thus in principle not sodetrimental to current transmission across grain boundaries. From the applicationpoint of view, the main efforts are still devoted to investigate the superconductingproperties, to distinguish intrinsic from extrinsic behaviors and to compare thedifferent families in order to identify which one is the fittest for the quest forbetter and more practical superconductors. The 1111 family shows the highestTc,huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitionsreminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropicwith sharper resistive transitions as in low temperature superconductors, but with about half theTc of the 1111 compounds. An overview of the main superconducting propertiesrelevant to applications will be presented. Upper critical field, electronic anisotropyparameter, and intragranular and intergranular critical current density will bediscussed and compared, where possible, across the Fe-based superconductor families.

AC Losses in High Tc Superconductors

The AC loss characteristics of high Tc superconductors are reviewed. AC losses are organized according to their generating mechanism and the magnetic field components generating them. Useful analytical formulae of hysteresis losses are presented for the superconductor slab exposed to a parallel magnetic field, for the thin strip exposed to a transverse magnetic field, and for the strip and ellipse carrying transport currents. Experimental and numerical methods for evaluating AC loss are reviewed, and these methods are applied to quantify AC losses in some practical superconductors. The AC loss characteristics of single-coated conductors are shown together with the reduction of their AC losses through striation. The AC loss characteristics of mono-layer as well as multi-layer superconducting power transmission cables are explained, and reduction of their AC losses by means of using narrower coated conductors is discussed.

MgB 2 superconductors with abnormally improved Jc sintered after autoxidation of milled original powders

An autoxidation treatment of short-time milled original powders was introduced for the synthesis of MgB2 superconductors and the critical current density Jc of the sintered MgB2 bulks was measured. It is unusually found that the undoped MgB2 bulks sintered with those autoxided milled original powder exhibit abnormally excellent Jc (above 1×104 A cm−2 even at 3.5 T, 20 K). Combined with the investigation of sintering process, it was found that the autoxidation treatment of the milled powders affects the subsequent sintering process dramatically and finally leads to the formation of MgB2 nanocrystalline with lots of dislocation and self-generated MgO nanoinclusions embedded in them. This unique microstructure brought up a significant improvement of Jc at high fields. Besides, the formation mechanism of this unique microstructure during the sintering process was also discussed in detail. It suggested that the MgO preformed by the reaction between Mg and B2O3 in the interface between Mg particles and B particles might serve as nuclei for the heterogeneous nucleation of the MgB2 phase and finally be included in the MgB2 grains as they grew up. The present method provides possible windows for the development of practical MgB2 superconductors without adopting expensive nanodopants.

Superconducting–magnetic heterostructures: a method of decreasing AC losses andimproving critical current density in multifilamentary conductors

Magnetic materials can help to improve the performance of practical superconductors onthe macroscale/microscale as magnetic diverters and also on the nanoscale as effectivepinning centres. It has been established by numerical modelling that magneticshielding of the filaments reduces AC losses in self-field conditions due to decouplingof the filaments and, at the same time, it increases the critical current of thecomposite. This effect is especially beneficial for coated conductors, in whichthe anisotropic properties of the superconductor are amplified by the conductorarchitecture. However, ferromagnetic coatings are often chemically incompatible withYBa2Cu3O7 and(Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, inMgB2 conductors an iron matrix may remain in direct contact with the superconducting core.The application of superconducting–magnetic heterostructures requires consideration of thethermal and electromagnetic stability of the superconducting materials used. On one hand,magnetic materials reduce the critical current gradient across the individual filamentsbut, on the other hand, they often reduce the thermal conductivity between thesuperconducting core and the cryogen, which may cause destruction of the conductor in theevent of thermal instability. A possible nanoscale method of improving the critical currentdensity of superconducting conductors is the introduction of sub-micron magnetic pinningcentres. However, the volumetric density and chemical compatibility of magneticinclusions has to be controlled to avoid suppression of the superconducting properties.

Influence of magnetic materials on the transport properties of superconducting compositeconductors

Magnetic materials can help to improve the performance of practical superconductors onthe macro/microscale as magnetic diverters and also on the nanoscale as effectivepinning centres. It has been established by numerical modelling that magneticshielding of the filaments reduces ac losses in self-field conditions due to decouplingof the filaments and, at the same time, it increases the critical current of thecomposite. This effect is especially beneficial for coated conductors, in whichthe anisotropic properties of the superconductor are amplified by the conductorarchitecture. However, ferromagnetic coatings are often chemically incompatible withYBa2Cu3O7 and (Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, inMgB2 conductors an iron matrix may remain in direct contact with the superconducting core.The application of superconducting–magnetic heterostructures requires consideration of thethermal and electromagnetic stability of the superconducting materials used. On the onehand, magnetic components reduce the critical current gradient across the individualfilaments but, on the other hand, they often reduce the thermal conductivity between thesuperconducting core and the cryogen, which may cause the destruction of the conductor inthe event of thermal instability. A possible nanoscale method of improving the criticalcurrent density of superconducting conductors is the introduction of sub-micron magneticpinning centres. However, the volumetric density and chemical compatibility of magneticinclusions has to be controlled to avoid suppression of the superconducting properties.

X-Ray Micro-Tomography as a Tool for Quantitative Characterization of Advanced Materials Manufacturing Processes

The potential of X-ray micro-tomography in characterizing the relevant material issues as identified during manufacturing processes was investigated. For this purpose, the 3D X-ray micro-tomography (XµRT) analysis was applied to the characterization of structural integrity of Nb3Sn superconducting wires of differing topology and the evolution of the density distribution of ceramic samples manufactured by Field Assisted Sintering Techniques (FAST). The latter technique was used to consolidate the ceramic, metal (Ni) and composite powders (MgB2). The usual scanning electron microscope (SEM) technique was enhanced by the high resolution XµRT in order to describe the volumetric density distribution before sintering and at different moments of the thermal cycle. Two types of samples sintered Ni and high density MgB2 superconductors with typical volume of 0.8÷1.5⋅10-9 m3 were sampled at space resolution around 5µm. For the sintered Ni, the 3D reconstructed volumes revealed the grain connectivity, necks formation and particle rearrangement in the densification process. The XµRT analysis was essential in explaining the differences in superconducting properties of MgB2 samples in terms of different volumetric structures. For the Nb3Sn multifilamentary wire which is at basis of many practical superconductors, the 3D tomography enabled the determination of the number of inter-filament contacts as well as the twist-pitch parameter. Advantages of our method versus the invasive etching techniques for the determination of the twist-pitch parameter were outlined.

Predicting AC loss in practical superconductors

Recent progress in the development of methods used to predict AC loss in superconductingconductors is summarized. It is underlined that the loss is just one of the electromagneticcharacteristics controlled by the time evolution of magnetic field and current distributioninside the conductor. Powerful methods for the simulation of magnetic flux penetration,like Brandt’s method and the method of minimal magnetic energy variation, allow usto model the interaction of the conductor with an external magnetic field or atransport current, or with both of them. The case of a coincident action of ACfield and AC transport current is of prime importance for practical applications.Numerical simulation methods allow us to expand the prediction range fromsimplified shapes like a (infinitely high) slab or (infinitely thin) strip to morerealistic forms like strips with finite rectangular or elliptic cross-section. Anothersubstantial feature of these methods is that the real composite structure containing anarray of superconducting filaments can be taken into account. Also, the case of aferromagnetic matrix can be considered, with the simulations showing a dramaticimpact on the local field. In all these circumstances, it is possible to indicate howthe AC loss can be reduced by a proper architecture of the composite. On theother hand, the multifilamentary arrangement brings about a presence of couplingcurrents and coupling loss. Simulation of this phenomenon requires 3D formulationwith corresponding growth of the problem complexity and computation time.

Critical current of an MgB2 coil with a ferromagnetic matrix

It is of great interest for magnet designers to know the maximum current that a coil can beoperated with, i.e. the critical current of the coil. Magnesium diboride is a newcomer inthe field of practical superconductors. Iron is a possible matrix metal for use inMgB2 conductors. Its ferromagnetic nature brings several new problems into the computation ofthe critical current of the coil. In this paper we show, by means of finite element methodcomputations, that the measurement situation for the short-sample critical current in the case of anMgB2/Fe wire differs considerably from that for a conventional superconductor.Also, we study a computation of the critical current of anMgB2/Fe coilbased on the engineering permeability. The computational model presented is applied to a small solenoidalMgB2/Fe coil. On the basis of the results, we propose a non-linear, conductor geometry dependentshift for the measured short-sample critical current curve to give values for the criticalcurrent of the coil.

Reversible axial-strain effect in Y–Ba–Cu–O coated conductors**Contribution of NIST, an agency of the US Government, not subject to copyright.

The recently discovered reversible strain effect in Y–Ba–Cu–O (YBCO) coated conductorscontrasts with the general understanding that the effect of strain on the critical-current densityJc in practical high-temperature superconductors is determined only bycrack formation in the ceramic component. Instead of having a constantJc as a function of strain before an irreversible drop when cracks form in the superconductor,Jc in YBCO coated conductors can decrease or increase reversibly with strain over asignificant strain range up to an irreversible strain limit. This reversible effect is present insamples fabricated either with rolling-assisted biaxially textured Ni–W substrates orwith ion-beam-assisted deposition on Hastalloy substrates. The reversibility ofJc with strain is observed for thin as well as thick YBCO films, and at two very differenttemperatures (76 and 4 K). The reversible effect is dependent on temperature and magnetic field,thus indicating its intrinsic nature. We also report an enhancement of the irreversible strain limitεirr where the reversible strain effect ends and YBCO cracking starts. The value ofεirr increases from about 0.4% to more than 0.5% when YBCO coated conductors arefabricated with an additional Cu protection layer.

Microstructure and Properties of Groove-Shaped Artificial Pinning Centers Introduced by Microfabrication

Effective flux pinning centers are required for the development of practical superconductors in order to achieve high critical current density. In the case of metallic superconductors, a drawing process introduces artificial pinning centers in metallic superconductors. However, artificial pinning centers cannot be introduced into high-Tc superconductors through the drawing process because these superconductors are brittle. We consider the microfabrication technique, instead of the drawing process, in order to introduce artificial pinning centers into the superconductors. Groove-shaped artificial pinning centers are introduced into a 0.5 /spl mu/m thick evaporated Nb film on a sapphire substrate by photolithography. The groove-shaped pattern with a spacing of 4 /spl mu/m was introduced by etching on a 0.9 /spl times/ 0.9 mm region of the Nb film. Nb films with varying groove depths were prepared, and the groove depth dependence on critical current density was investigated for the width of the hysteresis curve of the magnetic moment. The hysteresis curves of the specimens with grooves were measured with a SQUID magnetometer. The width of the hysteresis curve decreases with an increasing groove depth for over 70% of the film thickness.

Electromagnetic, atomic structure and chemistry changes induced by Ca-doping of low-angle YBa2Cu3O7–δ grain boundaries

Practical high temperature superconductors must be textured to minimize the reduction of the critical current density $J_{gb}$ at misoriented grain boundaries. Partial substitution of Ca for Y in $YBa_2Cu_3O_{7-\delta}$ has shown significant improvement in $J_{gb}$ but the mechanisms are still not well understood. Here we report atomic-scale, structural and analytical electron microscopy combined with transport measurements on $7^{\circ}$ $[001]$-tilt $Y_{0.7}Ca_{0.3}Ba_2Cu_3O_{7-\delta}$ and $YBa_2Cu_3O_{7-\delta}$ grain boundaries, where the dislocation cores are well separated. We show that the enhanced carrier density, higher $J_{gb}$ and weaker superconductivity depression at the Ca-doped boundary result from a strong, non-monotonic Ca segregation and structural rearrangements on a scale of ~1 nm near the dislocation cores. We propose a model of the formation of $Ca^{2+}$ solute atmospheres in the strain and electric fields of the grain boundary and show that Ca doping expands the dislocation cores yet enhances $J_{gb}$ by improving the screening and local hole concentration.