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In the push to develop high power electric aircraft, superconducting technology promises to significantly reduce mass and volume of motors and generators. However, challenges related to AC-loss and thermal management are a significant factor in preventing the proliferation of aerospace superconducting technologies. Increasing the resistance of the metal matrix stabilization has only gone so far in reducing coupling currents for higher frequency applications. In this research, Multiphysics simulations of a single composite filament were used to investigate stability decreases when using very high thermal conductivity electrical insulator (CsI) or metal-to-insulator transitioning material (V2O3) to replace the slightly resistive metal matrix typically used for a low AC loss MgB2 composite wire. The insulators separate the MgB2 filaments entirely, only allowing transient current sharing to occur with the high purity Nb diffusion barrier or with the metallic state V2O3. These simulations show that for these very low AC-loss composites at 20 K, instability will become a major issue due to reductions in current sharing. With higher electrical conductivity metal-to-insulator materials, higher thermal conductivity impregnation materials, and thicker metallic diffusion barriers it may be possible to find a reasonable balance between AC-loss and stability.

MgB2 wires with 114, 192 and 342 filaments of size 14–19 µm manufactured by HyperTech Research, Inc. have been subjected to low-temperature DC measurements. R(T), I–V characteristics, critical currents, stress and strain tolerances of these wires differing by filament architecture and filament size sheathed by resistive CuNi alloys were measured and compared with the literature data. It was found that these fine-filamentary wires have high engineering current densities not reduced by twisting up to 10 mm, sufficient strain tolerances and therefore are promising for future applications where minimized AC losses are required due to resistive sheaths, thin MgB2 filaments and short twist pitches.

MgB2 superconducting wires made using a Mg infiltration method have reached a higher performance than either in-situ or ex-situ mixed powder based routes. Indeed, very high layer Jc coupled with whole-strand Je (critical current per total strand cross section) exceeding 104 A cm-2 at 4.2 K, 10 T have been found for monocore MgB2 wires. However, previous multicore infiltration route wires have not reached their potential for Je due to partially reacted and non-uniform MgB2 layers. This study shows that 18-core MgB2 AIMI wires processed using a low temperature route can attain higher and more uniform Je values due to a more uniform MgB2 reaction layer. The formation of fully reacted, uniform MgB2 layers is attributed to the switch from a liquid-solid to a vapor-solid reaction route.

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc2 and unchanged Birr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M–H), transport critical current density (Jc) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density Jc (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc2 and Birr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05’s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc2 or Birr.

A react-and-wind MgB2 coil segment for a conduction-cooled magnetic resonance imaging (MRI) machine has been fabricated and tested. The coil was developed as part of a collaborative effort on a conduction-cooled, MgB2-based, whole-body MRI image guided radiation therapy device. This study focuses on the fabrication, winding, instrumentation, cooling, and initial critical current (I c ) testing of this near-full-size MgB2 segment coil. The coil was 0.9 m in diameter; the winding pack, 44.0 mm wide × 50.6 mm high, used 1.7 km of an 18 filament MRI-style conductor with Nb chemical barriers, Cu interfilamentary matrices, and an outer monel sheath. The conductor was insulated and reacted before winding onto a stainless steel former. The coil was instrumented with Cernox and E-type thermocouple temperature sensors, strain sensors, and voltage taps. The conduction-cooled coil was mounted in a cryostat capable of accepting coils of up to 0.9 m in diameter and 0.5 m in height. Critical current measurements were performed as a function of temperature during a controlled heating of the coil. The operational target was I = 200 A at 13 K. The full magnet was designed to produce 0.75 T in the imaging area (at I = 200 A), with a maximum field of 1.93 T in the winding. The single segment coil measured here exceeded this operation specification, with an I c of 280 A at 15 K and a maximum field 1.93 T in the winding. The coil was modeled using a finite element method, and a load line plot showed that 100% of short sample was reached at 21.5 K and above. These measurements demonstrate the viability of conduction-cooled MgB2 background coils as replacements for liquid helium cooled NbTi background coils in future MRI devices.

The effect of wire diameter reduction on the critical current density of pristine MgB2 wire was studied. Wires were treated by a hot isostatic pressing method at 570 °C and at pressures of up to 1.1 GPa. It was found that the wire diameter reduction induces an increase of up to 70% in the mass density of the superconducting cores. This feature leads to increases in critical current, critical current density, and pinning force density. The magnitude and field dependence of the critical current density are related to both grain connectivity and structural defects, which act as effective pinning centers. High field transport properties were obtained without doping of the MgB2 phase. A critical current density jc of 3500 A mm−2 was reached at 4 K, 6 T for the best sample, which was a five-fold increase compared to MgB2 samples synthesized at ambient pressure.

In this work we studied the influence of SnO2 doping on the critical fields and temperatures of MgB2 bulk samples. Bulk samples were made by mixing ex-situ MgB2 powder with 5 wt% SnO2 powder and then pressing the mixed powders into pellets using a pressure of 2000 psi. The bulk pellets were sintered at 900°C in a furnace under flowing Ar. The samples were quenched to room temperature after dwelling at 900°C for 5h. XRD, SEM, and magnetic measurements were made on doped and control samples. XRD showed a decomposition of the SnO2 and very slight reductions in the a-axis and c-axis lattice parameters of the MgB2 phase. M-T (Magnetization-Temperature) curves showed a decrease in Bc2 of approximately 1 T in the temperature range of 24 K - 39 K with SnO2 additions as compared to the control samples.

We fabricated MgB2/Fe wire by a powder-in-tube (PIT) technique, using an in situ process. All wire samples were sintered for 30 min at different sintering temperatures ranging from 650 to 1000 °C. We found strong correlations among crystallinity, critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and microstructures for all MgB2/Fe wires. We observed that the sample with the lowest sintering temperature, ~ 650 °C, had a larger lattice strain, Jc, change in resistivity Δρ(ρ300 K–ρ40 K), Hirr, and Hc2, but a lower density and residual resistivity ratio (RRR). Based on the relationships between all these superconducting and microstructure parameters, grain boundaries are likely to be acting as the predominant pinning centers for MgB2, so grain growth of MgB2 corresponds to a reduction of effective pinning. It should be noted that changes in the MgO fraction within the MgB2 matrix were almost independent of the sintering temperature. This indicates that most MgO may be coming from the starting material.

We present a detailed analysis of the effects of excess Mg on the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2) of MgB2 bulks and wires produced by an in situ solid-state reaction method and the powder-in-tube method, respectively. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. The Tc variation for the 10% Mg excess sample was 1.6 K (36.3–37.9 K) when the sintering temperature changed from 650 to 850 °C while it only varied by 0.5 K (37.2–37.7 K) for the normal sample. The low-field Jc for the 10% Mg excess samples sintered at 750 °C increased by a factor of 3, compared to that for the normal MgB2 samples, while the Hc2 for the 10% Mg excess samples sintered at 650 °C reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in the 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.

Various MgB2 wires with different sheath materials provided by Hyper Tech Research Inc., have been tested in the superconducting fault current limiter (SFCL) desktop tester at 24–26 K in a self-field. Samples 1 and 2 are similarly fabricated monofilamentary MgB2 wires with a sheath of CuNi, except that sample 2 is doped with SiC and Mg addition. Sample 3 is a CuNi sheathed multifilamentary wire with Cu stabilization and Mg addition. All the samples with Nb barriers have the same diameter of 0.83 mm and superconducting fractions ranging from 15% to 27% of the total cross section. They were heat-treated at temperatures of 700 °C for a hold time of 20–40 min. Current limiting properties of MgB2 wires subjected to pulse overcurrents have been experimentally investigated in an AC environment in the self-field at 50 Hz. The quench currents extracted from the pulse measurements were in a range of 200–328 A for different samples, corresponding to an average engineering critical current density (Je) of around 4.8 × 104 A cm−2 at 25 K in the self-field, based on the 1 µV cm−1 criterion. This work is intended to compare the quench behaviour in the Nb-barrier monofilamentary and multifilamentary MgB2 wires with CuNi and Cu/CuNi sheaths. The experimental results can be applied to the design of fault current limiter applications based on MgB2 wires.

The transport critical current density (Jc) was measured at 4.2 K for MgB2 monofilamentary and 7-, 19-, and 37-stack multifilamentary strands. Simple, one-step heat treatments (HT) were used, with temperatures of 675 and 700 °C, and times from 10–40 min. Most measurements were performed on 1 m segments of strands wound onto barrel holders. Transport properties of monofilament, 7-, 19-, and 37-stack strands were compared, and the influence of CuNi and monel outer sheaths was investigated. HT optimization studies were performed on various strands. Transport Jcs of 0.8 mm OD strands reached 2 × 105 A cm−2 at 4 T and 4.2 K (1 µV cm−1), and 1.15 × 106 A cm−2 at 4.2 K and zero field. Smaller 10-filament wires with ODs as small as 0.25 mm (40 µm filaments) exhibited good performance in some cases. The temperature and field dependences of the transport Jc were also measured; a typical example was 2 × 104 A cm−2 at 4 K, 20 T.

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

The active quench protection of a 1.5 T MgB2 conduction-cooled MRI magnet operating in persistent current mode is considered. An active quench protection system relies on the detection of the resistive voltage developed in the magnet, which is used to trigger the external energizing of quench heaters located on the surfaces of all ten coil bundles. A numerical integration of the heat equation is used to determine the development of the temperature profile and the maximum temperature in the coil at the origin, or 'hot spot', of the quench. Both n-value of the superconductor and magnetoresistance of the wire are included in the simulations. An MgB2 wire manufactured by Hyper Tech Research, Inc. was used as the basis to model the wire for the simulations. With the proposed active quench protection system, the maximum temperature was limited to 200 K or less, which is considered low enough to prevent damage to the magnet. By substituting Glidcop for the Monel in the wire sheath or by increasing the thermal conductivity of the insulation, the margin for safe operation was further increased, the maximum temperature decreasing by more than 40 K. The strain on the MgB2 filaments is calculated using ANSYS, verifying that the stress and strain limits in the MgB2 superconductor and epoxy insulation are not exceeded.

(Bi,Pb)2Sr2Ca2Cu3O10 tapes have been fabricated by a tubular wire process which offers the advantages of near-net shape manufacturing and a lower ratio of silver to superconductor compared with the conventional powder-in-tube technique. Critical currents as high as 15 A have been achieved at 77 K in short sections of tapes fabricated by this process. The development of critical current with heat treatment time and the magnetic-field dependence of critical current of the tapes made by this process are found to be similar to those of tapes fabricated under identical conditions by the powder-in-tube technique.

We report a new approach to a superconducting joining process for unreacted in situ carbon (C)-doped magnesium diboride (MgB2) wires. To operate a magnetic resonance imaging (MRI) magnet in the persistent mode, the superconducting joints between two conductors are as critical as the other key components. In addition, a stable and reliable joining process enables the superconducting magnet to operate without an external power supply. However, joint results using unreacted in situ C-doped MgB2 wires, which are used for high-field operation, have been limited, and only very poor performance has been obtained. By controlling the pressure inside a joint part, in this study, we successfully obtained current carrying retention in the joint of up to 72% compared to wire without a joint. The closed-circuit resistance of our closed-loop coil was less than 1.8 × 10−13 Ω at 16.7 ± 4.7 K, as measured by the field-decay measurement method. These results indicate that MgB2 has a promising future in MRI application.

To reduce the usage of liquid helium in MRI magnets, magnesium diboride (MgB2), a high temperature superconductor, has been considered for use in a design of conduction cooled MRI magnets. Compared to NbTi wires the normal zone propagation velocity (NZPV) in MgB2 is much slower leading to a higher temperature rise and the necessity of active quench protection. The temperature rise, resistive voltage, and NZPV during a quench in a 1.5 T main magnet design with MgB2 superconducting wire was calculated for a variety of wire compositions. The quench development was modeled using the Douglas–Gunn method to solve the 3D heat equation. It was determined that wires with higher bulk thermal conductivity and lower electrical resistivity reduced the hot-spot temperature rise near the beginning of a quench. These improvements can be accomplished by increasing the copper fraction inside the wire, using a sheath material (such as Glidcop) with a higher thermal conductivity and lower electrical resistivity, and by increasing the thermal conductivity of the wire's insulation. The focus of this paper is on the initial stages of quench development, and does not consider the later stages of the quench or magnet protection.

We report the fabrication of a magnesium diboride (MgB2) coil and evaluate its persistent-mode operation in a system cooled by a cryocooler with solid nitrogen (SN2) as a cooling medium. The main purpose of SN2 was to increase enthalpy of the cold mass. For this work, an in situ processed carbon-doped MgB2 wire was used. The coil was wound on a stainless steel former in a single layer (22 turns), with an inner diameter of 109 mm and height of 20 mm without any insulation. The two ends of the coil were then joined to make a persistent-current switch to obtain the persistent-current mode. After a heat treatment, the whole coil was installed in the SN2 chamber. During operation, the resultant total circuit resistance was estimated to be

In this paper we present the results of critical current (I c) measurements of MgB2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (J c) and complicates the study on critical temperature (T c), upper critical field (B c2), irreversible magnetic field (B irr), pinning force (F p), and pinning centers in superconducting MgB2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB2 wires.

High-performance superconducting joints are essential for realizing persistent-mode magnets. Herein, we propose a concept and fabrication of such superconducting joints, which yielded reliable performance in the operating temperature range of 4.2–25 K. MgB2–MgB2 joints in magnets are known to result in deterioration of localized electrical, thermal, and mechanical properties. To overcome these problems, the ends of the two wires are inserted into a pellet press, which is then filled with a mixture of unreacted magnesium and boron powders, followed by heat treatment. The critical current capacity and joint resistance were precisely evaluated by the standard four-probe method in open-circuit and by field-decay measurements in a closed-loop, respectively. These joints demonstrated up to 66% of the current-carrying capacity of unjoined wire at 20 K, 2 T and joint resistance of 1.4 × 10−12 Ω at 4.2 K in self-field.