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.

The development of superconducting joining technology for reacted magnesium diboride (MgB2) conductors remains a critical challenge for the advancement of cryogen-free MgB2-based magnets for magnetic resonance imaging (MRI). Herein, the fabrication of superconducting joints using reacted carbon-doped multifilament MgB2 wires for MRI magnets is reported. To achieve successful superconducting joints, the powder-in-mold method was employed, which involved tuning the filament protection mechanism, the powder compaction pressure, and the heat treatment condition. The fabricated joints demonstrated clear superconducting-to-normal transitions in self-field, with effective magnetic field screening up to 0.5 T at 20 K. To evaluate the interface between one of the MgB2 filaments and the MgB2 bulk within the joint, serial sectioning was conducted for the first time in this type of superconducting joint. The serial sectioning revealed space formation at the interface, potentially caused by the volume shrinkage associated with the MgB2 formation or the combined effect of the volume shrinkage and the different thermal expansion coefficients of the MgB2 bulk, the filament, the mold, and the sealing material. These findings are expected to be pivotal in developing MgB2 superconducting joining technology for MRI magnet applications through interface engineering.

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.

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.

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.

Low activation isotopic boron (11B) based magnesium diboride (Mg11B2) superconductors doped with biomass-derived activated carbon were synthesized using 11B and magnesium powder via solid-state reaction. The effect of carbon doping on the lattice structure and superconducting properties of Mg11B2 bulks were evaluated using X-ray powder diffraction, high resolution transmission electron microscopy, scanning electron microscopy and magnetization measurements. Precise refinement of structural parameters indicates successful substitution of carbon in Mg11B2 bulks. The critical current density (Jc) of carbon doped Mg11B2 synthesized at 650 °C was enhanced more than two times compared with the pure Mg11B2 bulk. Similar improvement was observed for the Mg11B2 bulks heat-treated at 800 °C. This enhancement is due to successful substitution of biomass-derived carbon with high surface area into Mg11B2 lattice. The flux pinning mechanism of pure and doped Mg11B2 bulks were investigated using the Dew-Hughes model. This study provides information regarding enhancement of the Jc of low activation Mg11B2 superconductors suitable for next-generation fusion magnets.

Magnesium diboride (MgB2) has a high critical temperature (Tc = 39 K) as a metal-based superconductor and is expected to be used as a superconducting material for liquid helium-free applied equipment. The superconducting joint between MgB2 wires is an important technology when considering applications of superconducting magnets. In this commentary, we will review the joint techniques that have been applied to MgB2 wires, and show the results. There are several methods for producing MgB2 wires, and various methods using wires produced by the in-situ method, ex-situ method, and IMD method were obtained from information from patents and treatises. Some typical methods and problems of some typical ones will be explained. The superconducting joint of MgB2 wire is relatively easy to accomplish compared to that of other high-temperature superconducting wires, but Ic has not been obtained easily. This is due to the microstructure of the joint interface, and the improvement of those microstructures is considered to be important for the creation of a reliable superconducting joint for MgB2 in the future.

A superconducting joint architecture to join unreacted carbon-doped multifilament magnesium diboride (MgB2) wires with the functionality to screen external magnetic fields for magnetic resonance imaging (MRI) magnet applications is proposed. The intrinsic diamagnetic property of a superconducting MgB2 bulk was exploited to produce a magnetic field screening effect around the current transfer path within the joint. Unprecedentedly, the joint fabricated using this novel architecture was able to screen magnetic fields up to 1.5 T at 20 K and up to 2 T at 15 K and thereby almost nullified the effect of the applied magnetic field by maintaining a constant critical current (Ic). The joint showed an Ic of 30.8 A in 1.5 T at 20 K and an ultralow resistance of about 3.32 × 10-14 Ω at 20 K in a self-field. The magnetic field screening effect shown by the MgB2 joint is expected to be extremely valuable for MRI magnet applications, where the Ic of the joints is lower than the Ic of the connected MgB2 wires in a given magnetic field and temperature.

The incorporation of nanoarchitectonics into the development of nanozymes to achieve target-specific geometry, dense active sites, and cascade catalysis is highly demanded for developing ultrasensitive bioassays. The improved dispersion and uniform distribution of metal active sites onto a three-dimensional (3D) mesoporous carbon support (MC) with a high surface area can lead to enhanced substrate binding, mobility, and collision probability and therefore, increased peroxidase mimetic activity. Herein, we report the fabrication of welldispersed superparamagnetic iron oxide (IO) nanoparticles (NPs) on mesoporous carbon (IO-MC) support with high Fe3+ active sites, high surface area, and ordered mesoporous pore channels that show promising catalytic activity at room temperature. The as-prepared IO-MC shows good nanozyme activity at room temperature with highly favorable Michaelis-Menten constant, Km (0.242 mM) and fast reaction rate (0.193 x 10- 7 MS-1). Finally, we demonstrate the functionality and pre-eminence of IO-MC nanozyme for bioassay. As a proof-of-concept, we develop a superior glucose assay that provides a LOD (limit of detection) of 2 mu M in the spiked sample. These findings suggest that homogeneously dispersed iron oxide NPs on MC show promising potential as nextgeneration nanozyme for developing sensitive bioassays.

Powder processing by ball milling is an effective approach for materials engineering. Although various methods for material processing are available, only high-energy shaker/vibratory or planetary mills have been intensively utilized to develop mechanical milling or alloying routes for structural control of MgB2 superconducting materials. Herein, we have attempted structural modification by using a low-rotation shaker, which is categorized as a low-energy and economical mill in terms of industrial applications. The operation speed was kept constant at 40 rpm, which is much lower than typical conditions employed for planetary mills. Instead of adjusting the low rotational speed, the other processing parameters were controlled to enhance the energy transfer from the balls to powders. The applied milling conditions were ultimately found to cause severe plastic deformation of the raw powders. The shape and size changed drastically, depending on the processing time. The morphological variation of the processed powders as precursors for the MgB2 materials influenced the void structure and the composition including amorphous phases. By considering these results, we also elucidated the mechanism underlying the structural changes upon ball milling and their effects on the transport critical current performance. The present approach for powder processing offers potential as an effective milling route for structural modification of superconducting materials.

A high field magnet is a key element of cryogenic electron beam ion sources (EBISs), which are known for generating highly charged ions through the magnetic compression of an electron beam. Herein, we report the design, fabrication, and evaluation of a 7 T niobium-titanium superconducting magnet capable of persistent-mode operation. The magnet was designed using finite element analysis by considering its magnetic, thermal, and mechanical properties. The designed magnet was then fabricated, assembled, and evaluated for various design parameters in a recondensing-type liquid helium cryostat. After several quench trainings, the magnet reached a target magnetic field of 7 T with an operating current of 200 A, a magnetic field uniformity of 0.24%, and an electron beam focusing length of 1.3 m inside the bore. The magnet was successfully operated in the persistent-mode for 9.5 days (228 hours) and achieved a field-decay rate of 0.42 ppm $\cdot \text{h}^{-1}$ . The magnet evaluation results confirm that our superconducting magnet system can be applied to an EBIS to carry out stable and effective electron beam compression.

Persistent-mode operation is a key feature of magnetic resonance image systems to improve the required field stability. The superconducting joint is known to be beneficial for reducing all the resistant components in an electrically closed-circuit. The joint technique of magnesium diboride (MgB2) multifilamentary wire, however, is the main obstacle to the use of magnet in practical applications. In response, herein, we designed and developed a unique configuration of superconducting joint to further enhance the interconnection of exposed cores between two 18-multifilamentary wires. It was confirmed that developed joint samples achieved high critical current similar to a non-jointed wire. The proposed joint technique was directly applied to the MgB2 single-turn coil and MgB2 magnet for estimating a joint property through persistent-mode operation. This work provides fundamental insights into the design of persistent-mode MgB2 magnets to boost magnetic resonance image systems.

Evaluation and control of amorphous phases in materials are very important for optimizing their properties. Herein, we focus on polycrystalline MgB2 materials prepared with hydrocarbon doping and study the effects of residual amorphous impurities on the superconducting performance. Carbon is known to be an effective element for enhancing the transport critical current under an external magnetic field. The doped samples were prepared under two different nominal conditions, MgB2(C16H10)(x/1)(6) and MgB2-x(C16H10)(x/1)(6), which respectively correspond to additional and substitutional type doping of the MgB2 composition. Regardless of the doping type, both fabrication methods retarded the formation of the MgB2 phase due to the dopant, leading to an increase in amorphous impurities. However, the apparent phenomena that arise from the additional and substitutional types are still elusive. Ultimately, the structural differences due to the impurity effects caused significant changes in the transport critical current performance. The present quantitative analysis of the amorphous impurities thus paves the way to further optimize the doping methodology for MgB2 superconducting materials. (C) 2021 Elsevier B.V. All rights reserved.

We present superconducting joints using 36 filaments magnesium diboride (MgB2) wires in continuation joint orientation and their characterisation results for the first time. One of the joints demonstrated the critical temperature (offset) of 34.4 K and the critical current (I-c) of 38.5 A at 10 K in self-field. Unconventional I-c behaviour of the joint was observed at 20 K in self-field when the I-c was measured in a high to a low field due to the effect of field trapping in the MgB2 bulk within the joint. Elemental mapping at the interface between the MgB2 bulk and the filament revealed oxidation, which was attributed to being responsible for reducing the I-c of the joint. This work will notably contribute towards MgB2 superconducting joint technology development for magnetic resonance imaging (MRI) application. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A superconducting joint of unreacted monofilament internal magnesium diffusion-processed magnesium diboride (MgB2) wires was fabricated by exploiting the phenomenon of magnesium diffusion into the boron layer inside the superconducting joint. Unprecedentedly, the joint was able to carry an almost identical transport current compared to the bare wire in a 2-7 T magnetic field at 20 K. The joint also exhibited very low joint resistance of 2.01 × 10-13 Ω in self-field at 20 K. Among commercially available superconductors, this work is the first to successfully realize a superconducting joint that is capable of transferring current from one conductor to another without any notable degradation under strong magnetic fields. This work demonstrates great potential to apply MgB2 in a range of practical applications, where superconducting joints are essential.

The anisotropy of the critical current density, the n-factor and the irreversibility field of mono-core in-situ MgB2 tapes have been studied at various magnetic field orientations and temperatures. Undoped as well as silicon carbide (SiC) and malic acid (C4H6O5) added tapes were studied. The anisotropy is noticeably influenced by the additives due to different carbon contents and fabrication processes. The malic acid added tape exhibits almost isotropic behavior compared to SiC doped MgB2 tapes due to the homogeneous carbon substitution through the chemical solution route, which paves the way to design MgB2 tapes to be applied for industrial magnet application.

We report an ultra-low resistance superconducting joint using unreacted multifilament MgB2 wires produced by tailoring the powder compaction pressure within the joint with heat treatment conditions. The joint demonstrated an ultra-low resistance of 5.48 x 10^-15 ohms and critical current (Ic) of 91.3 A at 20 K in self-field. The microstructural and composition studies of the joint revealed cracks and a high amount of MgO, respectively. These two features reduced the Ic of the joint to some extent; nevertheless, the joint resistance was not affected by it. Our tailored joining process will play a pivotal role in superconducting joint development.

Dual sintering induced performance variations in MgB2superconductors are studied in detail in relation to MRI applications.