MgB2 Tapes Research Articles

High temperature and low field regime vortex phase diagram of in-situ prepared stainless steel sheathed MgB2 tapes

A magnetic field-temperature vortex phase diagram has been proposed for MgB2 on the basis of magnetoresistance studies performed on an in-situ prepared stainless steel sheathed MgB2 tapes. Thermally activated flux flow (TAFF) behavior is analyzed using Arrhenius relation. Low dissipative part of magnetoresistance with temperature is well described by vortex glass theory. Above critical region, the resistive behavior is influenced by fluctuation conductivity. In addition, a considerable vortex liquid region below HC2 is also observed but TAFF region is found to be quite narrow.

Improved critical current density in ex situ processed MgB2 tapes by the size reduction of grains and crystallites by high-energy ball milling

We have fabricated Fe-sheathed MgB2 tapes through an ex situ process in a powder-in-tube (PIT) technique using powders ball milled under various conditions. Although the ex situ processed wires and tapes using the high-energy ball milled MgB2 powders have been studied and the decrease of grain and crystallite sizes of MgB2 and the critical current density (Jc) improvement of those conductors were reported so far, the use of filling powders milled at a higher rotation speed than previously reported further decreases the crystallite size and improves the Jc properties. The improved Jc values at 4.2K and 10T were nearly twice as large as those previously reported. Those milled powders and hence as-rolled tapes easily receive contamination in air. Thus, the transport Jc properties are easily deteriorated and scattered unless the samples are handled with care. The optimized heat treatment temperature (Topt) of those tape samples at which best performance in the Jc property is obtained decreases by more than 100°C, compared with that of tapes using the as-received MgB2 powder.

Second-phase segregation and micro strain/lattice parameter dependent transition temperature in polycrystalline MgB2

Un-doped, metal-doped, and carbon-doped MgB2 samples were prepared by furnace cooling and quenching to investigate the second phase behavior and the resultant critical current density Jc performance under different heat treatment processes, which is infrequently mentioned, and to explore the strain/lattice parameter dependence of the superconducting transition temperature. To release the residual stress, quenching induced second-phase segregation in these MgB2 samples shows a negative effect on the Jc. Nevertheless, the dislocations and the lattice distortion assisted the enhancement of the high-field Jc in the un-doped and metal-doped MgB2 samples, which indicated that quenching could be technically applied for the fabrication of metal-sheathed MgB2 wires and tapes to obtain excellent Jc. After evaluating the micro strain and the lattice parameters’ (c and a for hexagonal lattice) variation, a dome was observed in the illustration of the strain/lattice parameter c/a dependence of Tc, which differed from the reported linear relation in previous work. This suggests that the c/a ratio and the strain may be the predominant parameters for scaling of the superconducting dome width in the superconducting phase diagram of MgB2.

Magnetic and transport properties of HTS MgB2 wires

Critical current Ic and magnetization M of industrial MgB2 tape and wire were measured in order to estimate their suitability for production of liquid helium free cryomagnetic system. Samples were subjected to bending with different diameters Db and critical current was measured at temperature T = 4.2 K in magnetic fields up to 10 T. The dependences of the critical current on the diameter of the bend Ic (Db) were found and used for optimization of liquid helium free cryomagnetic system. The magnetization of the wire was measured by a vibrating sample magnetometer at temperatures T = 4.2,10, 15, 20, 25 K in magnetic fields up to 14 T. It was found that the magnetization curve M (H) is influenced by ferromagnetic response of the metal matrix. Ferromagnetic contribution had been taken into account and magnetization loops caused by the diamagnetic contribution of the superconducting phase were extracted. Dependencies Ic(T) and Ic(H) were obtained from the data.

Electromagnetic Study of a Round Coil Superferric Magnet

A novel type of superferric magnets suitable to arbitrary multipole orders was proposed by I. F. Malyshev and later by V. Kashikhin. This new topology, which we refer to as round coil superferric magnets (RCSM), allows a great simplification of the superconducting part, which in the simplest case may be composed by a single round coil, which has intrinsically a rather large bending radius allowing the use of strain-sensitive superconductors. INFN is designing and building a prototype of a multipolar corrector magnet based on this geometry and using MgB2 tapes. In this paper, we investigate a number of issues pertaining to the electromagnetic characteristics of RCSM. The RCSM magnetic has inherently even harmonics, in addition to usual odd ones and a solenoidal component. Either (but not both) disappears when integrated using a one-coil or a two-coil specular design. We investigate the effect of saturation on the multipolar components and on the load line, since in RCSM, saturation plays a role that differs both from conventional superferric magnets and from round yoke design types.

Tellurium addition as a solution to improve compactness of ex-situ processed MgB2-SiC superconducting tapes

Ex-situ spark plasma sintering (SPS) was used to obtain dense MgB2-based tapes in a Fe sheath with the starting composition (MgB2)0.975 + (SiC)0.025 + Te0.01. Prior to the SPS procedure of tape formation, the samples were submitted to a series of cold working processes typical for the powder-in-tube technique. The tapes were compared with optimal doped bulk samples (having the same starting composition) and a pristine MgB2 tape. The morphology of the composite samples, the phase structure of both the core and the inner face of the metallic sheath shows the formation of a plethora of traces as a result of interaction between MgB2, additives, and the Fe sheath. Important critical parameters, like critical current density and the irreversibility field, show that there is a field and temperature range where the SiC and Te-added tapes display better critical parameters comparative to either pristine MgB2 tapes in the Fe sheath or SiC and Te doped MgB2 bulk samples.

Influence of the Internal Architecture of MgB2Conductors in the Load Line of Magnet Coils

MgB2 conductors have a multifilamentary superconducting structure encased in sheaths of different materials. Conductors with ferromagnetic sheaths, such as Ni or Fe, redistribute the magnetic field, which can affect the critical current of the superconducting filaments. The internal magnetic field distribution inside the conductor is not considered in general when modeling magnets, where the winding is considered as a bulk. The previous assumption can lead to discrepancies between the experimental results and numerical $I_{c}$ predictions. Hence, a correct analysis of the effects of the ferromagnetic matrix in conductors will lead to savings in cost and weight of the coil design. In this work, the magnetic field distribution inside tapes with different architectures based on existing MgB2 tapes has been obtained. The magnetic field in the filaments is modeled focusing on the influence of the ferromagnetic matrix in the field distribution. The study is extended to round pancake coils based on the prototype coils aimed for its application in wind turbines in the SUPRAPOWER project.

Estimation of hysteretic losses for MgB2 tapes under the operating conditions of a generator

Hysteretic losses in the MgB2 wound superconducting coils of a 550 kW synchronous hybrid scaled generator were estimated as part of the European project SUPRAPOWER led by the Spanish Fundación Tecnalia Research & Innovation. Particular interest was given to the losses caused by the magnetic flux ripples in the rotor coils originating from the conventional stator during nominal operation. To compute these losses, a 2D finite element analysis was conducted and Maxwell’s equations written in the H-formulation were solved considering the nonlinear material properties of the conductor materials. The modeled tapes are made of multiple MgB2 filaments embedded in a Ni matrix and soldered to a high purity copper strip and insulated with Dacron braid. Three geometrical models of single tape cross sections of decreasing complexity were studied: (1) the first model reproduced closely the actual cross section obtained from tape micrographs. (2) The second model was obtained from the computed elasto-plastic deformation of a round Ni wire. (3) The third model was based on a simplified cross section with the superconducting filaments bundled in a single elliptical bulky structure. The last geometry allowed the validation of the modeling technique by comparing numerical losses with results from well-established analytical expressions. Additionally, the following cases of filament transpositions of the multi-filamentary tape were studied: no transposition, partial and full transposition; thereby improving understanding of the relevance of the tape fabrication process on the magnitude of the determination of ac losses. Finally, choosing the right level of geometrical detail, the following operational regimes of the machine and its impact on individual superconducting tape losses in the rotor were studied: bias-dc current, ramping current under ramping background field and magnetic flux ripples under dc background current and field.

Close relation between Mg–Mg bonds and Tc of SiC buffered-MgB2 tapes

A series of superconducting tapes were prepared by growing MgB2 film on Hastelloy tapes buffered with SiC buffer layers. When the SiC buffer layer increases in thickness, the superconducting transition temperature (Tc) of MgB2 tapes show slight increment with no depression detected. This Tc behavior may have been originated from the changes occurred in the local structure of MgB2. From EXAFS analysis, the ordering of the Mg–Mg bonds was systematically increased when the thicknesses of SiC buffer layers were decreased. We also found that unlike the relatively random bond lengths of Mg–B bond, the Mg–Mg bond lengths are expanding when SiC adds its thickness, inferring the clear effect of SiC to the Mg–Mg bond lengths. On the basis of the results shown in this letter we argue that SiC buffer layers have significant effects on the local structure of MgB2 which contributes to the fundamentals of superconducting properties of MgB2 tapes.

Effect of additions of Ca compounds to the filling powder on the reduction of MgO and the critical current density properties of ex situ processed MgB2 tapes

We have studied the effect of additions of Ca compounds, CaC2 and CaH2, to the filling powder on the transport critical current density (Jc) properties of ex situ processed MgB2 tapes. These additives are expected to reduce MgO present in as-received commercial MgB2 powder and improve the grain coupling of MgB2 in the tapes. Heat treatment of the hand mixed powders, MgB2 and the Ca compounds, brings about both disappearance of MgB4 and formation of CaB6 and Mg, compared with heat treated MgB2 powder. They are observed much more clearly for the mixtures with CaH2 addition. The MgO content decreases by the additions of those compounds. Neither of the a-axis and the c-axis parameters of MgB2 are changed by the additions, suggesting no reactions of substitutions of Ca for Mg and C for B in MgB2. The Jc properties of the tape samples using the hand mixed filling powder with CaC2 addition degrade, whereas those with CaH2 addition are slightly improved. When using the ball milled mixtures, slight contraction of the a-axis parameter occurs only by CaC2 addition. Although the additions cause the reactions similar to those observed for the hand mixed powders, the MgO content reduces more clearly, compared with the MgB2 content. The Jc properties of the tapes are enhanced in the high-field and low-field regions by the additions of CaC2 and CaH2, respectively. These enhancements are attributed to the carbon substitution, the improved grain coupling and the increased content of MgB2.

Conceptual design and thermal analysis of a modular cryostat for one single coil of a 10 MW offshore superconducting wind turbine

Superconducting generators show the potential to reduce the head mass of large offshore wind turbines. A 10 MW offshore superconducting wind turbine has been investigated in the SUPRAPOWER project. The superconducting coils based on MgB2 tapes are supposed to work at cryogenic temperature of 20 K. In this paper, a novel modular rotating cryostat was presented for one single coil of the superconducting wind turbine. The modular concept and cryogen-free cooling method were proposed to fulfil the requirements of handling, maintenance, reliability of long term and offshore operations. Two stage Gifford-McMahon cryocoolers were used to provide cooling source. Supporting rods made of titanium alloy were selected as support structures of the cryostat in aim of reducing the heat load. The thermal performance in the modular cryostat was carefully investigated. The heat load applied to the cryocooler second stage was 2.17 W@20 K per coil. The corresponding temperature difference along the superconducting coil was only around 1 K.

Thermal conductivity and stability of commercial MgB2 conductors

This paper presents a study of the thermal transport properties of MgB2 tapes differing in architecture, stabilization and constituent materials. The temperature and field dependence of thermal conductivity, was investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters to describe the 3D heat diffusion process in a winding. Thermal transport properties—even in field—are typically deduced using semi-empirical formulas based on the residual resistivity ratio of the stabilizer measured in absence of magnetic field. The accuracy of these procedures was evaluated comparing the calculated κ values with the measured ones. Based on the experimental thermal conduction properties and critical current surface we determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout allowed us to provide information on how to optimize the thermal stability of MgB2 conductors.

Reducing delamination in MgB2 films deposited on Hastelloy tapes by applying SiC buffer layers

MgB2 is a promising candidate for superconductivity applications because of its metallic nature and its simple structure compared to other oxide superconductors. We have deposited MgB2 thin films on the SiC buffered-Hastelloy substrates by a hybrid physical–chemical vapor deposition (HPCVD) method. SiC buffer layers of varying thicknesses were grown on the Hastelloy substrates by a pulsed laser deposition (PLD) technique. As the SiC buffer layers thicken, the grain size of MgB2 films is distinctly decreased and the grains become much denser. MgB2 film deposited on bare Hastelloy shows a sign of partial delamination at the interface. Interestingly, the delamination was considerably reduced when the SiC buffer layers with the thickness of 170 and 250nm were added into the MgB2 tape. This suggests that SiC buffer layers acted as an effective sticking agent, providing stronger adhesion to the MgB2–Hastelloy interface, hence the reduced delamination.

Development of a 3 T-250 mm Bore MgB2 Magnet System.

The authors had reported components' development of 3 T-250 mm bore MgB2 magnet system. Pre-reacted MgB2 tape wire with copper lamination had n-value related problem due to raw Boron particle size inequality, but it had been corrected. Long MgB2 wires over 3 km had been supplied. All six component coils were made with a wet winding procedure. They were tested individually with the same cooling scheme of conduction cooling as the actual magnet assembly. Though all coils could be ramped to some extent, some coils showed fairly large remnant voltage. Since the voltage distribution over the coil was not even, the uniformity along the wire length may not be good enough. The stability of the coil was verified by its no training performance even with fast ramping. The magnet assembly and its test with conduction cooling were planned. I c of the superconducting joint with this pre-reacted wire was doubled during past one year's development.

Study of the potential of three different MgB2 tapes for application in cylindrical coils operating at 20 K

The goal of this theoretical study is to illustrate the potential of three different MgB2 tapes, developed by Columbus Superconductors, for application in cylindrical coils. First, the distribution of critical currents and electric fields of individual turns is compared when the winding of the model coil is made with tapes having different Ic(B) and anisotropy values. Second, the influence of the winding geometry on basic parameters of cylindrical coils which consist of a set of pancake coils, such as critical current Icmin, central magnetic field B0 and stored energy E, is analysed. The winding geometry of the coils, i.e. the outer winding radius and the coil length, with the same inner winding radius, was changed from a disc shape to a long thin solenoid in such a way that the overall tape length was held constant, and considered as a parameter. Finally, the winding cross-section of the coil is optimized with respect to the constant tape length in order to reach the maximum central field. The results of calculations show that for a given overall tape length and inner winding radius there exists only one winding geometry which generates the maximum central field. The overall tape length, as a parameter, is changed in a broad range from 500 m to 10 km. All calculations were performed using the experimental data measured at 20 K while the effect of the anisotropy in the Ic(B) characteristic of the short samples is taken into account.

Critical Currents of MgB2 Wire and Tape in Magnetic Field under Bending Deformations

Nowadays MgB2 wires are attractive for designing real devices like motors and magnets. The latest production technologies allow to increase critical current value of wires. However, there is a problem of wire performance degradation under bending strain. Thus, there is a problem of manufacturing solenoids with small diameters, especially from ex situ MgB2 wires produced by powder-in-tube (PIT) technology. In this work, influence of bending on critical current Ic multifilament PIT ex situ MgB2 tape and wire has been studied. Critical current dependencies on external magnetic field Ic(H) were measured at liquid helium temperature in the range of fields from 2.5 T up to 8 T. Measurements were carried out in parallel (H||), and perpendicular (H⊥) orientation of external magnetic field to the sample surface. Voltage current characteristics (V-I) on tape samples were measured with bending on diameters D=30; 40; 60; 70; 80; 90;100mm. It was shown that critical current density (Jc) decreases by 24% and by 28% for tape and wire respectively with decreasing bending diameter from 100mm to 60mm. It was found that the tape still had superconductive properties even with bending diameter D=30mm, but its Jc decreased by 44% in comparison with Jc(D=100mm). One more feature that was found is the appearance of resistive component in the voltage current characteristics for all bend diameters in magnetic fields H>3.5 T.

Effects of three different homemade nanocarbons doping on the superconducting properties of MgB2 tapes

The doping effects of three different homemade nanocarbons on microstructure and superconducting properties of in situ powder-in-tube (PIT) processed MgB2/Fe tapes were studied. Compared with pure tapes, all homemade nano-carbon additions led to an order of magnitude enhancement of the supercurrent at 4.2K, 10T. Among them, hollow carbon nanoboxes doped samples exhibited the highest transport Jc–B properties, about 3.4×104Acm−2 at 4.2K, 10T (Je∼104Acm−2). The large Jc improvements of doped samples are thought to be attributed to the enhancement of flux pinning force. It is also interesting that the change of microstructure and critical field of three nanocarbons doped MgB2 samples are not obvious, which mainly because that the performance parameters of these nanocarbons with different morphologies are almost same.

Improved Transport J c in MgB2 Tapes by Graphene Doping

Improved Transport J c in MgB2 Tapes by Graphene Doping

The effect of refinement of carbon-substituted MgB2 filling powder using various starting powders on the microstructure and critical current density of ex situ processed superconducting tapes

We have studied the effect of refinement of homemade carbon-substituted MgB2 (MBC) filling powders prepared by using various commercially available Mg and B powders on the microstructure and critical current density (Jc) of Fe-sheathed MgB2 tapes fabricated through an ex situ process in a powder-in-tube (PIT) technique. The homemade MBC powders contained a substantial amount of MgO as a main impurity phase when using fine Mg powder, whereas the use of coarse Mg powder significantly reduced the formation of MgO. This was independent of the starting B powder. The MgO was present as isolated particles and platelets inside MgB2 crystallites. Weak acid could dissolve the MgO partially, whereas strong acid could dissolve it almost completely. When using the as-sintered MBC powders, the Jc properties of tapes were very poor. However, they were remarkably enhanced by the refinement of these powders with the acid solutions. The quality of the refined MBC powder prepared with low-cost B powder was comparable to that prepared with high-cost B. The Jc values of tapes using these powders reached about 6 kA cm−2 at 4.2 K and 10 T.

Correlation between the Critical-current Anisotropy and Microstructure of ex-situ Processed MgB2 Tapes Using Carbon-substitution Treatment

Correlation between the critical current anisotropy and the textured microstructure of the ex-situ processed MgB2 tapes with the Fe sheath was studied to obtain basic data related to the commercial application of the ex-situ processed MgB2. Ex situ processed MgB2 tapes were fabricated using newly developed technique to substitute carbon for boron in the MgB2 compound through the ex situ route. These MgB2-xCx tapes showed comparable critical current anisotropy to that of pure MgB2 tapes, but in higher magnetic fields because of their higher irreversible critical field than the pure MgB2 tapes. The x-ray pole figure data show that the tape core texturing of the MgB2-xCx tapes was moderate, but it was a slightly higher degree than that of the pure MgB2 tapes. This result was consistent with results of the critical current anisotropy.