In future electric aircraft applications employing all-superconducting rotating machines, round multifilamentary magnesium diboride (MgB2) wires are a preferrable material for lowering AC loss, due to their small filaments at the macron level and tight twist pitch. Our previous work has investigated AC loss behavior in a 54-filament MgB2 wire with a filament radius of 12.5 mu m, where the filament size was found not ideal for loss reduction. In this work, 3-D AC loss simulations of a twisted, nonmagnetic 114-filament MgB2 wire with a 5 mu m filament radius at 20 K are performed using H-formulation. Three types of AC losses are studied: 1) Transport loss only (Q(t0), with current levels up to 90% of its self-field critical current I-c0), 2) magnetization loss only (Q(m0), with AC field amplitudes and frequencies up to 2 T and 200 Hz, respectively), 3) total AC loss carrying AC current exposed to AC field (Q(total), with AC field also up to 2 T and current levels up to 40% of I-c0). Simulation results show that, for the Q(m0), the simulated hysteresis loss Q(h) of a 5-mm twist pitch, 114-filament wire at 50 Hz, and 200 Hz matches the analytical hysteresis loss equation for a cylindrical superconductor, scaled by 114 (the number of filaments), when B-m <= 0.5 T. Increasing the twist pitch (5 mm versus 10 mm) and filament size (5 mu m in the 114-filament wire versus 12.5 mu m in the 54-filament wire) leads to a higher Q(m0) due to the coupling effect. Moreover, the simulated Q(total) of the 114-filament wire range from 0.22 to 7.48 W/cm3 for i <= 0.4 and B-m <= 0.5 T operated at 200 Hz.

Abstract It is essential to develop lightweight cables with low AC loss in the application of electric aviation. High Purity Aluminum (HPAL), which operates effectively at cryogenic temperature, has been developed to compete with superconductors especially in higher frequencies. HPAL, characterized by 99.999% aluminum purity, achieves a resistivity ratio (RR) up to 1000. It has minimal impurities, dislocations, and defects resulting in remarkably low resistivity at cryogenic temperatures, but at the same time, the mechanical properties of HPAL itself are inadequate for practical application. HyperTech Research developed a multi-stranded HPAL wire with Cu-Ni matrix and Nb barrier to provide sufficient mechanical support. However, we wish to explore the microstructure and performance of HPAL wire under strain due to tensile stress, thermal stress. In this study, we performed Electron Backscatter Diffraction (EBSD) analysis on various HPAL wires to evaluate the impact of tensile stress and annealing on grain size and dislocation density which influence RR. Four samples, including as-drawn, annealed, and tensile-deformed conditions, were prepared and characterized using EBSD and cryogenic resistance measurements. Our chosen recovery anneal was seen to lead to a significant increase in RR (from 391 to 530) for our samples. Conversely, tensile deformation reduced the RR (from 530 to 277). Direct observation of changes in dislocation density analysis was not possible, but this may be possible with better surface preparation, and more aggressive recovery anneals. These results highlight the importance of material heat and mechanical treatment to enhance cryogenic electrical performance, with implications for HPAL use in high-frequency, cryogenically cooled power systems such as electric aircraft propulsion.

In the pursuit of enabling the application of all-superconducting rotating machines in electric aviation, high AC loss in the armature windings where superconductors carry AC currents and exposed to AC/rotating magnetic fields is a critical stumbling block. For lowering AC loss, multifilamentary magnesium diboride (MgB2) wires with fine filaments and tight twist are one promising candidate for aviation applications. In this paper, 3D AC loss simulations of a 54-filament MgB2 wire with a non-magnetic matrix at 20 K are carried out based on the H-formulation. The transport loss carrying AC current without external field, Q(t0), of 12-, 30- and 54-filament wires is firstly obtained, where the current amplitudes range from 20% to 90% of its self-field critical current I-c0. Then the magnetization loss exposed to field amplitudes up to 2 T without current, Q(m0), is presented, where the operational frequency, the twist pitch and resistivity of the matrix are varied to investigate their impacts on Q(m0) and its three loss components (hysteresis loss Q(h), coupling loss Q(c) and eddy current loss Q(e)). Lastly, the total loss, Q(total), of the 54-filament wire with various twist pitches and frequencies is compared, where the current amplitudes vary from 30% to 70% of I-c0 and the field amplitudes are up to 2 T. All simulations use the measured J(c)(B, 20 K) and n(B, 20 K) data of the 54-filament wire. Simulation results show that, the use of the 5 mm twist pitch wire can significantly reduce Q(m0) due to the decoupling of the filaments, where the simulated Q(h) matches well with the analytical hysteresis loss for a cylindrical superconductor multiplied by 54 (the number of filaments). With increasing twist pitch, the filaments become coupled, resulting in a greater increase in both Q(c) and Q(h). Surprisingly, the simulated Q(total) values in the wires with different twist pitches agree well with the sum of Q(m0) and Q(t0) for all different current levels. This implies that Q(total) in an MgB2 wire carrying an AC current exposed to an AC magnetic field can be accurately predicted by knowing Q(m0) and Q(t0) values which are more easily obtained.

High-power electric aircraft motors and generators require high current density conductors to meet the required performance metrics, which are beyond those achievable using ambient temperature metals. MgB2 superconductors and high-purity aluminum (HPAL) hyperconductors are being considered for this work because of their combination of high current density/ampacity in combination with relatively low AC loss and their ability to work at the temperature of liquid hydrogen and above. In this work, we calculated the AC losses of MgB2 and HPAL conductors of various wire architectures in applied magnetic fields of 0.5 T and frequencies, f, up to 1 kHz. In particular, we estimated the contributions of several AC loss components, including eddy current and coupling losses, in AC magnetic fields and frequencies relevant to AC motors and generators in electric aircraft. The losses were then summed up and presented in terms of power loss per length, per unit current [W/(m*A)] in order to fairly compare the AC losses of superconductors to hyperconductors over various frequency regimes. Additionally, we accounted for the anomalous magnetoresistance component of the HPAL conductor, using data from a newly developed composite, measured at 20 K in fields up to 9 T in a PPMS. We conclude that for a 0.5 T, externally applied, time varying field, MgB2 is the lowest loss conductor for f < 180 Hz, while multifilamentary HPAL conductors are better for f > 180 Hz (explored up to 1 kHz), based on our chosen conductor design assumptions. However, the best place to transition from MgB2 to HPAL composites lies somewhere in the 200 Hz to 1 kHz regime, depending on the parameters of the conductors, the value of Bm, and the relative tradeoffs in Je and power loss per unit volume the application demands.

A study on a 4-stage sub-size MgB2 Cable-in-Conduit Conductor (CICC), tested at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP), revealed a 20% degradation in critical current at 4.2 K compared to single-strand data. To address this issue, the mechanical properties of MgB2 wires from Hyper Tech and WST were investigated, and two sub-size CICCs were manufactured using a "close-to-1-ratio"Twente design with smaller diameter wires. These cables demonstrated no significant degradation in critical current after cabling and compaction, nor after electromagnetic load cycling. The results indicate that the close- to-1-ratio cable design is optimal for brittle superconducting materials such as MgB2, Nb3Sn, and BSCCO, as it minimizes mechanical stress and preserves superconducting properties. This design shows significant potential for the application of MgB2 in next-generation fusion reactors, particularly in Poloidal Field (PF) coils, Correction Coils (CC), and feeders.

The high cost of using the niobium (Nb) barrier for manufacturing magnesium diboride (MgB2) mono-and multi-filamentary wires for large-scale applications has become one of the barriers to replacing current commercial niobium-titanium superconductors. The potential of replacing the Nb barrier with a low-cost iron (Fe) barrier for multifilament MgB2 superconducting wires is investigated in this manuscript. Therefore, MgB2 wires with Fe barrier sintered with different temperatures are studied (from 650 °C to 900 °C for 1 h) to investigate the non-superconducting reaction phase of Fe-B. Their superconducting performance including engineering critical current density (Je) and n-value are tested at 4.2 K in various external magnetic fields. The best sample sintered at 650 °C for 1 h has achieved a Je value of 3.64 × 104 A cm−2 and an n-value of 61 in 2 T magnetic field due to the reduced formation of Fe2B, better grain connectivity and homogenous microstructure. For microstructural analysis, the focused ion beam (FIB) is utilised for the first time to acquire three-dimensional microstructures and elemental mappings of the interface between the Fe barrier and MgB2 core of different wires. The results have shown that if the sintering temperature can be controlled properly, the Je and n-value of the wire are still acceptable for magnet applications. The formation of Fe2B is identified along the edge of MgB2, as the temperature increases, the content of Fe2B also increases which causes the degradation in the performance of wires.

Compared to Nb 3 Sn- and NbTi-wound superconducting undulators (SCUs), MgB 2 -wound SCUs are of interest for future electron synchrotron beam light sources owing to their higher temperature operating margin and associated stability. In this study, a three-period undulator consisting of twelve racetrack coils wound with 2nd generation (2 G) multifilamentary advanced-internal-magnesium-infiltration MgB 2 strands were fabricated and tested in liquid and gaseous helium (He) over a temperature range of 4.2 K–20 K. The coil winding cross sections (in each coil) were 5 mm wide and 4.8 mm thick. At 4.2 K, a critical current ( I c ) of 325.7 Amps produced a maximum undulator bore field of 1.19 T. It should be noted that the short, 3-period nature of the coil led to an asymmetry in the field profile (the maximum positive field was 1.19 T, the maximum negative was −0.25 T), suggesting a peak field of 0.72 T in the absence of end effects. Finite element modeling (FEM) results of simulations for a one meter long undulator of otherwise identical design gave 0.85 T (larger because of higher currents enabled by the lower field). But in any case, the I c value coil reached is 94% of that of the short sample (dictated by the 1.19 T positive field for the coil as tested). FEM was performed to study the magnetic field profile, which was validated experimentally. The magnetic field was measured using a Hall probe which was translated along the beam axis during measurement to explore the spatial field variation along the beam travel direction. The spatially alternating field was asymmetric, and the maximum field was more prominent in the positive direction than in the negative direction, the difference being due to broken symmetry, that is, short coil end effects. In this work, we show useful fields are possible for MgB 2 undulators; the use of such conductors can allow a larger thermal margin and enable conduction-cooled operation.

Our research showed that a physical phenomenon appears in MgB2 wires, which has not been reported in previous studies. We have found that the flow of transport current in the current penetration depth of normal metal areas leads to the creation of voltage in magnetic fields from 0 to B-c1. Scanning electron microscope (SEM) images showed that normal areas can be formed at the junction between the superconducting material and the diffusion barrier. SEM photos indicate that this surface is very non-homogeneous and porous. This new physical phenomenon, that is, the voltage created by these normal areas at the current penetration depth is called, in this paper, the interfacial topological effect. Further measurements showed that above B-c1, the voltage disappears and becomes unmeasurable for magnetic fields above 120 mT. This is due to the appearance of a mixed state and current flow at a deeper depth of the superconducting material. From the interfacial topological effect, transport current flows on the outside surface of the superconducting MgB2 wires not only in magnetic fields from 0 to B-c1 but also in the mixed state and disappears in magnetic fields above 120 mT. This indicates that it is related to the superconducting state. The measurements performed by using a physical property measurement system (PPMS) for the low current (100 mA) indicate that the interfacial topological effect is on the boundary of two superconducting materials: the Nb diffusion barrier and the MgB2 material. In addition, the PPMS measurement results point out that the magnetic field eliminates the interfacial topological effect. Further results indicate that the appearance of voltage (the interfacial topological effect) in a MgB2 coil does not mean that the coil is not superconducting and cannot conduct the transport current without loss. Our results indicate that test procedures for MgB2 coils should assume exceeding the magnetic field of 120 mT because in this field, the voltage can disappear to zero and the transport current flows without loss. This is a very important factor for the future production of superconducting coils made of MgB2 wires with the Nb barrier on an industrial scale. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).

An important goal to enable widespread adoption of electric aircraft propulsion is to develop higher power density motors and generators which are at the same time highly efficient. One way to do this is to use conductors that can carry higher currents and/or generate lower losses. One approach to this is the use of superconducting windings. However, here we focus on very low resistance normal state conductors operating at cryogenic temperatures. The resistivity of both aluminum and copper drops quickly with decreasing temperature, such that the resistivity of Cu drops by about a factor of 7, and that of aluminum by 10, by the time we reach 77 K (LN2). OSU and Hyper Tech have teamed to develop a motor with liquid cryogen cooled aluminum windings (LN2 or LNG cooled). It includes a multi-slot stator with direct cryogen cooling. Here we present the results of a simple “single slot” test which explores the temperature rise of a pair of conductors in a slot directly cooled by LN2. These two aluminum bars are made of 1100 commercial purity Al alloy were placed in parallel with a 1.6 mm gap, which behaved as 120 mm long cryogenic flow channel. Current densities up to 75 A/mm2 were explored, with LN2 flow rates ranging from 1.9 g/s to 6.4 g/s. Thermocouples and voltage taps were used to capture temperature and voltage data during the experiment. As a result, we found stable cooling and operation at these flow rates and current densities, and we characterized the temperature gradient which developed along the conductor bars.

Magnetization AC losses of fine-filamentary MgB 2 wires with resistive CuNi sheaths were measured. The effects of varying the number of filaments (114–342, corresponding to effective filament diameters of 14–20 μ m), twist pitch (10–30 mm) and outer sheath material on the total AC loss were studied. For a better understanding of individual loss contributions, the effects of varying applied temperature, magnetic field, and frequencies were examined. It is found that hysteresis loss per volume decreases with the reduced filament size and that coupling current losses play a dominant role. The effect of decoupling by twisting was clearly observed for the smallest twist pitches. Considering the possible degradation of transport currents by twisting, AC losses were also normalized by the critical currents of the same wires. While twisting to short pitch decreases losses significantly, it apparently does not reduce the transport current. Consequently, the fine-filamentary MgB 2 wires with resistive CuNi sheath examined in this paper are excellent candidates for future low loss applications. Unlike ReBCO tapes, round MgB 2 wires enable easy single strand twisting, and the braiding or cabling, of wires into a variety of specific shapes and diameters.

Magnesium diboride (MgB2) magnets have the potential to be the next-generation Liquid-helium-free magnet for magnetic resonance imaging (MRI) application due to their relatively high superconducting transition temperature, high current density and low raw material cost compared with current commercial niobium-titanium (Nb-Ti) magnets. A typical superconducting magnet includes several coils. To produce an ultra-stable magnetic field for imaging in MRI, a superconducting electromagnet operating in a persistent mode is crucial. Superconducting coils of the electromagnet in MRI are short-circuited to operate in the persistent mode by connecting coils with superconducting joints. Persistent joints have been demonstrated for in-situ and ex-situ wires of both mono- and multi-filamentary structures, made predominantly by PIT techniques similar to those used in wire production. To realise further engagement of MgB2 in MRI applications, enhancing the performance of MgB2 superconducting joints is essential. This literature review summarises research and development on MgB2 superconducting joining technology.

We show for the first time the results for small coils, which were wound with unreacted magnesium diboride (MgB2) wires with nano-amorphous isotopic boron (11B) by using combined internal magnesium (Mg) diffusion (IMD) and hot isostatic pressing (HIP) techniques. The small coils with a diameter of 20 mm or 10 mm were annealed under low (0.1 MPa) and high (1.1 GPa) isostatic pressure. The 10 mm coils annealed under high isostatic pressure of 1.1 GPa had significantly increased critical temperature (Tc), irreversible magnetic field (Birr) and transport critical current density (Jtc), with significantly accelerated Mg diffusion and improved homogeneity of the Mg11B2 material. Moreover, our findings show for the first time that the bending of the unreacted IMD Mg11B2 wires significantly affects the synthesis reaction under the low and high isostatic pressure. Our findings also show for the first time that diffusion of Mg during heat treatment under high isostatic pressure is completely different than during heating under low isostatic pressure. Mg diffusion under low isostatic pressure annealing is mainly dependent on the heat treatment temperature, bending diameter and 11B layer density. This Mg diffusion is related to the simultaneous formation of the superconducting phase. However, the diffusion of Mg under high isostatic pressure heat treatment is mainly dependent on the isostatic pressure, 11B layer density and Mg state (liquid or solid). Our results indicate that liquid Mg is first pushed into the 11B layer by high isostatic pressure and then a superconducting phase is formed.

The use of a low annealing temperature during the production of coils made from superconducting materials is very important because it reduces the production costs. In this study, the morphology, transport critical-current density (Jc), irreversible magnetic field (Birr), and critical temperature (Tc) of straight wires and small 2% C-doped MgB2 coils were investigated. The coils were made using the wind-and-react (W&R) method and annealed at various temperatures from 610 °C to 650 °C for 2–12 h. Critical-current measurements were made for both the coils and straight wires at the temperatures of 4.2 K, 20 K, 25 K, and 30 K. During our research study, we determined the process window that provides the best critical parameters of the coils (annealing at a temperature of 650 °C for 6 h). Moreover, we observed that small coils made with unreacted MgB2 wire and then annealed had morphology and critical parameters similar to those of straight 2% C-doped MgB2 wires. Moreover, small-diameter bending of 20 mm and 10 mm did not lead to transverse cracks, which can cause a large reduction in Jc in the coils. This indicates that the processes of optimization of thermal treatment parameters can be carried out on straight MgB2 wires for MgB2 superconducting coils.

Here, we report superconducting Mg11B2 wires made by using the internal Mg diffusion technique with isotopic amorphous nano boron (11B) as the precursor material. We show the influence of annealing temperature and isostatic pressure of 0.1 MPa and 1.1 GPa on Mg diffusion into 11B layer, microstructure of superconducting filament, critical current density (Jc) at 20 K and 25 K, critical temperature (Tc) and irre-versible magnetic induction (Birr) in mono (single-core) -and multi-filament Mg11B2 wires. Our research shows that thermal treatment at 700 degrees C and 0.1 MPa for 60 min yields a superconducting phase with low Tc, Birr and Jc in single-core Mg11B2 wire. A higher annealing temperature (740 degrees C and 0.1 MPa for 60 min) significantly accelerates the diffusion of Mg into the 11B layer and increases the Tc, Birr and Jc. However, the distribution of Mg in 11B layer is very heterogeneous (places with high and low Mg concentration). This leads to heterogeneity in the superconducting material and inhomogeneous Tc, Birr and Jc. Further studies showed that higher annealing temperatures at 770 degrees C and 800 degrees C significantly accelerates the diffusion of Mg into the 11B layer and leads to the increase of Tc, Birr and Jc in single-core Mg11B2 wire. In single-core wire annealing treatment under isostatic pressure of 1.1 GPa for 60 min at temperature of 800 degrees C it causes Mg penetrates into the 11B layer very heterogeneously and large amount of 11B remains unreacted. However, the same heat treatment at 800 degrees C and 1.1 GPa for 60 min in multi-filament wires leads to a complete Mg synthesis reaction with the 11B layer. This leads to increase Birrand Jc. This work shows that the formation and accumulation of pinning centers is essential to further increase Jc in Mg11B2 wires. (c) 2022 Published by Elsevier B.V.

All-superconducting rotating machines have the potential for meeting the high power density and high efficiency required for electrical aircraft applications. However, very high AC loss encountered in superconducting armature windings could hinder their development. Multifilamentary MgB2 wires are one of the promising candidates for the stator windings, due to their potentially low AC loss properties with small filament size and twist pitches. As the first step, the dependence of critical current and n-value on magnetic fields and temperatures Ic(B, T) and n(B,T), which are basic input parameters for AC loss simulation, needs to be measured. In this work, we present transport Ic measurements in three non-magnetic multifilamentary MgB2 wires (MgB2/ Nb/CuNi/CuZn): one large wire with a 0.70 mm diameter and 25 mm twist pitch, and two small wires with a 0.48 mm diameter each anda 10 mm and 30 mm twist pitch respectively. A four-probe direct current method is used to measure Ic of the MgB2 wires with variations in temperature (15 - 35 K) and magnetic field (0 - 5.5 T). Full Ic data for the small wire with 10 mm twist pitch was obtained, and the n-values were mostly less than 20. While the Ic data for the large wire at low fields was more limited due to heating, the n-values were higher and could be up to around 100. The difference is attributed to the different filament sizes. Experiments also found that there is no significant hysteresis in the transport critical current measured by decreasing or increasing magnetic fields due to the non-magnetic sheaths. This non-hysteretic characteristic is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated. From the magnetic-field dependence of critical current density, an empirical expression has been developed that provides suitable extrapolations to lower fields for the large wire.

Successful superconducting joints of reacted magnesium diboride (MgB2) monofilament wires are reported in this paper. The absence of a reliable method to develop superconducting joints between reacted MgB2 wires presents a major obstacle to the wider adoption of MgB2 as a material for magnet winding. A hot uniaxial pressing (HUP) system was exploited for sintering purposes since it can facilitate the formation of condensed in situ bulk on the wire filament. The wires were manufactured with an extra thick barrier material to protect the filament from damage during HUP sintering. The sintering temperature and pressure of the HUP system were varied to comprehend the best-performing joint. The performance of joints could be improved by depreciating the pores within the intermediate bulk of the joint. To prove this point, joints were cut to study their morphology. However, due to sintering in pressurised conditions, the reaction of the in situ intermediate bulk was not completed. The x-ray diffraction result detected a significant unreacted magnesium phase in the intermediate bulk. This work obtained joints of reacted MgB2 wires which can be considered for industrial MgB2 magnetic resonance imaging magnets fabrication.

The oxygen concentration and distribution in the microstructure of MgB 2 - and YBa 2 Cu 3 O 7-δ -based materials affect the formation of nanostructural defects and thus influence the critical current density, upper critical magnetic field and irreversibility field. For MgB 2 oxygen containing additions (Dy-O, Ti-O) in the form of nanograins occurred not to be very effective for an increase of critical current density, j c . Sn-O additions to MgB 2 can provoke even a significant decrease of j c due to a chemical interaction of Sn with MgB 2 . The processing pressure and temperature influence the defect density of the relevant pinning centers and the regularity of their distribution in doped and undoped MgB 2 as well as in MT-YBCO.

MgB 2 wires with 114, 192 and 342 filaments of size 14–19 µ m 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 MgB 2 filaments and short twist pitches.

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 (V 2 O 3 ) to replace the slightly resistive metal matrix typically used for a low AC loss MgB 2 composite wire. The insulators separate the MgB 2 filaments entirely, only allowing transient current sharing to occur with the high purity Nb diffusion barrier or with the metallic state V 2 O 3 . 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.

We show that the structure of multifilament MgB2 wires made by the powder-in-tube (PIT) method can be texturized by annealing the structure under high isostatic pressure. Our results show that we obtained continuous fibers with a uniform diameter of 250 nm in all 36 filaments, a small grain size of approximately 50 nm and a high density of the superconducting material. These results contribute to a significant improvement in the critical current density in high magnetic fields, e.g., 100 A/mm2 at 14 T and 4.2 K.