Pinning Properties of Cold-Processed MgB2 Composite Superconducting Tapes

Superconducting tapes from MgB₂ and powders of magnetic Ni, Co, and non-magnetic Ti were fabricated by cold-processed powder-in-tube (PIT) method. The cupronickel was used as sheath material and no heat treatment was applied. The increasing of critical current was observed in the samples with additions of nickel and titanium. The microstructure and phase analysis of tape core was performed by X-ray diffraction spectroscopy. A direct correlation between type and amount of additives, and the critical current of the tapes has been observed.     

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 (J_c) 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 J_c properties. The improved J_c values at 4.2 K and 10 T 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 J_c properties are easily deteriorated and scattered unless the samples are handled with care. The optimized heat treatment temperature (T_opt) of those tape samples at which best performance in the J_c property is obtained decreases by more than 100 °C, compared with that of tapes using the as-received MgB₂ powder.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB<Subscript>2</Subscript> Wires

This study has shown the effect of excessive mechanical deformation by pressing and consecutive heat treatment, respectively, on the microstructure and transport properties of mono-core sintered in situ Fe/MgB₂ wires. These wires were pressed into short tapes under 1 GPa uniaxial pressure. Transport properties like critical temperature, transition width and engineering critical current density of the same samples were determined before and after pressing and after annealing. Phase formation and grain size of the samples were studied with XRD. The SEM analysis indicated that the mechanical deformation reduced initial voids to a large extent but caused cracks, increased the number of grain boundaries and introduced weak links. The subsequent heat treatment on the pressed samples improved the microstructure and the grain connectivity. Possibility of a final densification of the reacted in situ wires was discussed in terms of transport properties.     

The Effect of Cu Addition on the Phase Formation and Critical Current Density in the Sugar Doped MgB2 Superconductor

With the aim of improving the critical current density (J _c ) in the MgB₂ superconductor, minor Cu (3?at%) was doped to the MgB₂ samples in-situ sintered with Mg powder and sugar-coated amorphous B powder. Combined with thermal analysis, phase identification, microstructure observation and J _c measurement, the effect of minor Cu addition on the sintering mechanism, microstructure and critical current density of sugar-doped MgB₂ superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB₂ phase formation and improve the growth of MgB₂ grains during the sintering process of sugar-doped MgB₂ due to the appearance of Mg?CCu liquid at low sintering temperature. On the other hand, the Mg?CCu liquid hindered the reactive C released from sugar entering in the MgB₂ crystal lattice. Hence, the connectivity between MgB₂ grains was improved accompanying with the C substitution for B is decreased. At 20?K, the J _c of co-doped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugar-doped samples.     

High Energy Milled Ex Situ MgB2 as Precursor for Superconducting Tapes Without Critical Current Anisotropy

One lane of the present MgB₂ research is focused on scale-up of the conductor preparation for magnet applications. One limitation is the deformation of long length powder in tube conductors since the composite structure leads to a couple of complications. Therefore, a combination of various methods is commonly used: swaging, drawing, and flat rolling. In dense tapes deformed via distinct routes, a critical current anisotropy with respect to an external magnetic field is observed. The in situ method (unreacted Mg+B) is preferably used for conductor preparation, with the advantage offering more doping possibilities for the precursors to create flux pinning centers and to enhance the upper critical field and supporting a dense filament. In this work, we show in concurrence to the commonly preferred route, the possibility and potential of ex situ conductor preparation schemes, with the option of carbon doping, using high energy milling. Long multifilament tapes with 20 hours milled powder without carbon and with 5?wt% C were successfully deformed to wires and tapes. Tapes with 21 cores show critical current densities with J _c=10⁴?A/cm² at B=8.8?T without any current anisotropy in different field-direction different to the case of the in situ conductors.     

Fabrication of 6-filament MgB2 wires enhanced by high strength 91-filament Cu–Nb composite

We chose high strength and high conductive Cu–Nb composite as strengthening core to improve the mechanical properties of 6-filament MgB₂ wires. The Cu–Nb core become partially dispersion strengthened during the fabrications of the MgB₂ wires. It has been found that this Cu–Nb composite offers good promise of increased strength while maintaining the superconducting properties of the MgB₂ wire. The Young’s modulus of the best wire samples increased significantly to about 130 GPa, which is comparable to those of high strength ferromagnetic materials sheathed wires but without negative ferromagnetic effects. Those mechanical properties were enough to satisfy the low field application needs. The critical current I _c also achieves 200 A (engineering critical current density, J _ce above 1.30 × 10⁴ A/cm²) at 20 K 1 T field. The 91-filament Cu–Nb composite core reinforced wires were fabricated by in situ Powder In Tube method.     

Hysteresis loops of MgB2 + Co composite tapes

The (MgB₂)_1?x Co _x composite tapes fabricated by the ex situ PIT method were investigated. The structural and magnetic properties were characterized. Only MgB₂ and Co phases were observed in the composites while x ≤ 0.2. A superconducting transition temperature was stable for these samples. Enhancement of the critical current density is found for x = 0.1.     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.     

Investigation of Nb–B Diffusion and the Superconducting Properties of MgB2/Nb/Cu Tapes

The Nb?CB diffusion behaviors and their effects on the superconducting properties of MgB₂/Nb/Cu tapes were investigated. Two relevant samples of the Nb?CB diffusion couples and monocore MgB₂/Nb/Cu tapes were prepared with the same standard in situ PIT method, respectively. And both the samples were sintered at 650, 750, 850, and 950°C for 2 hours, respectively. It has been found that Nb?CB interface is invisible in the diffusion couples sintered at low temperatures as 650°C, correspondingly the superconducting properties of MgB₂ tapes are superior to those of the ones sintered at the other three temperatures. With the heating temperature increasing, Nb?CB interface, mainly NbB₂ with a little NbB, appears and becomes thicker gradually in the Nb?CB diffusion couples. The corresponding superconducting properties of MgB₂ tapes follow the similar patterns: within the range from 650°C to 950°C, the higher the sintering temperature, the poorer the superconducting performances, because the Nb?CB diffusion in MgB₂ tapes could cause the property degradations of MgB₂/Nb tape. As extended to the actual fabrications of Nb sheathed MgB₂ tapes or wires, suitable sintering temperature range should be from 650°C to 750°C for the desired performances.     

Properties of MgB2 superconductors with regard to space applications

Thin MgB₂ conductors with diameters down to 50 μm have been developed for application as current leads on a satellite and as LH₂ level sensor. The high transition temperature of 39 K makes applications of MgB₂ at temperatures around 20 K possible, significantly above the temperature margin of low temperature superconductors like NbTi or Nb₃Sn. The absence of weak link behaviour and the fact that there is no need to texture MgB₂ to obtain high critical current densities is a crucial advantage compared to high temperature superconductors. We give an overview of the current status of MgB₂ conductor development and show the potential of these wires for space applications.     

Magnetic Characterization of SiC Doped Bulk MgB2 Superconductor

Flux pinning is a very significant mechanism to improve critical current density in MgB₂ for many applications, such as developing the performance of wires and tapes. In this study, we have done a systematic study of SiC inclusions in the main matrix of MgB₂. Sample production in the form of MgB_2?x (SiC) _x was carried out by using solid state reaction method (x=0,0.01,0.02,0.04,0.06,0.10) in the argon atmosphere. The structural and magnetic properties of the prepared samples were investigated. Structural characterization of samples was performed by the X-ray powder diffraction (XRD) technique and the Scanning Electron Microscopy (SEM) measurements. Magnetic properties were determined by M?CH loops, magnetization-temperature (M?CT), and AC susceptibility measurements. The T _c of the samples was determined by the M?CT measurements while the J _c is determined through the M?CH measurements. SiC inclusions up to some ratios (x=0.06) increase the critical current density by up to about 50%, while the critical temperature T _c remains unchanged in compatible with the literature.     

Filamentary MgB2 Superconductors with Titanium Barriers

Ti diffusion barrier has been applied for several Cu stabilized MgB₂ wires. Pure Ti is well formable metal allowing formation of thin barrier layers which are not reacting with MgB₂. Instead of, Ti is able to purify MgB₂ filaments by absorbing some impurities during the final heat treatment. Ti has comparable coefficient of thermal expansion with MgB₂, which allows heat treatment at higher temperatures than for Nb barrier wires. Consequently, higher critical current densities can be obtained with Ti. Higher Ti resistivity offers a depressing of coupling currents in AC regime. One disadvantage of Ti is the inter-diffusion with copper during annealing and partial contamination of Cu stabilization. Benefits of Ti diffusion barrier have been utilized for the manufacture of fine-filamentary wires with minimal filament diameter of 10 μm. High critical current densities and high resistances to tensile stress and torsion stress at twisting have been demonstrated for these wires. AC loss measurements have shown reduced losses with decreased filament size and with shortened twist pitch.     

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm² at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm² at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm² at 77 K.     

High Critical Current Densities in MgB2 Films Grown on Hastelloy Tape by Hybrid Physical-Chemical Vapor Deposition

We report on the high critical current densities in MgB₂ films directly grown on Hastelloy tapes without any buffer layer by using the hybrid physical-chemical vapor deposition method. MgB₂ films were formed by reaction of Mg metal vapor with the incoming B₂H₆ gas on the heated substrates. In MgB₂ films grown for 10 min at 500 °C in total working pressure 100 Torr with gas mixing ratio H₂:B₂H₆=70:30, we observed the transport critical current density (J _c) was approximately 10⁶ A/cm² at 4 T and 20 K in magnetic fields applied parallel to the substrate plane. This value is higher than those observed in epitaxial MgB₂ films on sapphire substrates grown by using the same method. Magnetic field dependence of J _c of this sample was well explained by the grain-boundary pinning model. Our result opens up a possibility that the coated conductors made of MgB₂ films have a strong potential for high current applications.     

Development of design for large scale conductors and coils using MgB2 for superconducting magnetic energy storage device

MgB₂ wires are commercially available, and their superconducting characteristics have been continuously developed in the last decade. The relatively high critical temperature of these wires has attracted the attention of researchers, especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, as it enables the use of liquid hydrogen for cooling the coils. The sensitivity of multi-filament MgB₂ wires to bending strain makes the design of large-scale conductors and coils for an SMES system technologically difficult, and the careful investigation of the applied strains during manufacturing is required. Two-conventional methods have been introduced for the fabrication of the coils: wind-and-react (W&R) and react-and-wind (R&W). These methods have been demonstrated to be suitable for the production of large-scale MgB₂ magnets to maximize the coil performance. The W&R and R&W methods have been successfully applied to the designs of conductors and coils, and small W&R test coil fabrication, as well as stability demonstrations are performed in this study. Our study is the first to demonstrate the feeding of hundreds of amperes of transport current using multifilamentary MgB₂ wires at around liquid hydrogen temperature in the practical background magnetic field of 2 T. The minimum quench energy and normal zone propagation velocity are also experimentally investigated for the protection of the actual coils for SMES application.     

Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB<Subscript>2</Subscript> Tapes with Open and Closed Ends

Short powder-in-tube tapes of MgB₂ in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB₂ core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB₂ crystallite size, a weak contact between Fe and MgB₂ core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB₂ cores, the detected impurity phase is MgO, while at the interface with Fe, MgB₄ also occurs. Impurity phases found at interface, MgO and MgB₄, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB₂ powder has an important contribution to observed differences, and its removal or control is recommended.     

Fabrication of Superconducting MgB2 Thin Films by Magnetron co-Sputtering on (001) MgO Substrates

We fabricated superconducting MgB₂ thin films on (001) MgO substrates by magnetron rf and dc co-sputtering on heated substrates. We annealed the samples ex-situ and in-situ at temperatures between 450 ^°C and 750 ^°C. The substrate temperature during the sputtering process and the post annealing temperatures play a crucial role in forming MgB₂ superconducting thin films. We achieved a critical onset temperature of up to 27.1 K for the ex-situ and 25.6 K for the in-situ annealed samples at a film thickness of 30 nm. The samples shows an out of plane (0002)-Peak which was determined by x-ray diffraction.     

Fabrication and Properties of Powder-in-Tube Processed MgB2 Tapes and Wires

MgB₂ tapes were fabricated with MgB₂ powder and several sheath materials such as Cu, Cu-Ni, Fe, carbon steel (Fe-C) and stainless steel. High-density MgB₂ cores were obtained for these sheath materials. J _c of the as-cold rolled (non heat treated) tape significantly increased with increasing the cross sectional area reduction by the cold working. Hard sheath materials (Fe-C and stainless steel) are effective to enhance J _c values. These results can be explained by the densification of MgB₂ core. Non heat treated MgB₂/(stainless steel) and MgB₂/(Fe-C) tapes showed extrapolated J _c values of 300–450 kA/cm² at 4.2 K and zero field. MgB₂ tapes show anisotropy in J _c with respect to field orientation. This anisotropy can be explained by the MgB₂ grain orientation. Heat treatment after the cold rolling is effective to enhance J _c values. An order of magnitude higher J _c values were obtained for Fe-C and stainless steel sheathed tapes by the heat treatment. J _c values extrapolated to zero field of MgB₂/(SUS 316) and MgB₂/(Fe-C) tapes reached 1,000 kA/cm² at 4.2 K.     

Enhancement of Critical Current Density in MgB2 Bulk with CNT-coated Al Addition

Bulk MgB₂ sample with carbon nanotube (CNT)-coated Al addition was prepared by conventional solid-state reaction at 900 °C for 30 min. We investigated the effects of Al and C co-doping on the lattice parameter, the microstructure, and the critical current density of MgB₂. The substitution of Al and C atoms for the sites of Mg and B in the MgB₂ lattice resulted in dislocations in the MgB₂ grains, which makes great contributions, along with the nanoscale oxide particles, to the enhancement of critical current density at high field (10³ A cm^?2, 7 T, 5 K) in the co-doped sample. These results contrasted significantly with the measured values of the pure MgB₂ and Al- and C-doped samples. Co-doping introduced more electrons into MgB₂ and decreased both the parameters c and a of MgB₂ lattice, and the used coating technique delayed and shortened the oxidation process of Mg and Al, leading to the decrease in the size and the content of the oxide. These advantages should be responsible for the enhancement of the critical current density as well.