Superconducting vortices have a normal core and are pinned at imperfections, facilitating large current flow. Applications such as high-field solenoids or superconducting motors rarely use pure materials, as these are brittle, and instead employ superconductors embedded in ductile matrices (e.g., Cu or Ag). Processing superconductors into grains and then embedding in wires can significantly affect their properties, which remain less explored than in pure materials. In particular, the superconducting gap, relevant for vortex pinning, has been little studied in wires. Here, we determine the gap as a function of temperature and magnetic field in NbTi and MgB _2 wires using scanning tunneling microscopy. We find strong gap inhomogeneity, with Δ _NbTi=0.9± 0.6 mV and Δ _MgB_2=1.8± 0.2 mV. The temperature dependence follows BCS theory. Under magnetic field, the gap decreases approximately linearly, deviating from the usual ( 1-H/H_c2) ^2 behavior. We attribute this deviation to gap inhomogeneity arising possibly due to processing the materials into a wire. Our work shows that gap structure studies could complement efforts to improve superconducting properties of wires.

Abstract This article will present a modified method for measuring critical current for superconducting wires and tapes. In this measurement method, the transport current is kept constant, and the magnetic field is increased by a magnet. This method is called the magnetic field sweep method. The research carried out showed that the magnetic field sweep method allows for a better description of the physical phenomena, which occur in superconducting wires made of several superconducting materials, e.g. Nb diffusion barrier and MgB 2 core. The conducted research indicated that the magnetic field sweep method can detect damage to the Nb diffusion barrier, more precisely determine I c in superconducting wires in which an electric field related to transfer length appears (short wires) and identify the magnetic field ranges in which individual pinning centers effectively trap the vortex lattice. All of these are key factors for enabling high critical current density superconducting wires.

Textured magnetocaloric materials offer a cost-effective alternative to single crystals while retaining magnetic anisotropy required for rotational magnetic refrigeration. In this work, a textured DyNiAl intermetallic compound was prepared by directional solidification during arc melting and systematically investigated for low-temperature magnetic refrigeration. Structural characterization confirmed texturing along the c-axis of the main phase identified as DyNiAl, which crystallizes in a hexagonal structure with space group P-62m. Magnetic and heat-capacity measurements revealed pronounced magnetic anisotropy and magnetic transitions at approximately 14 K and 32 K. The compound exhibited magnetocaloric effects below and above 14 K, respectively, with the maximum response near 32 K. A maximum adiabatic temperature change of 10.6 K was obtained for a field change of 0–14 T, while the maximum magnetic entropy change reached ∼18 J/kgK at 5 T for the magnetic field applied along the texture axis. Direct measurements also revealed a rotating magnetocaloric effect of 0.7 K at 1.8 T. These results demonstrate that textured DyNiAl exhibits magnetocaloric properties comparable to those of single crystals while benefiting from simpler and more cost-effective processing, making it a promising candidate for conventional and rotational magnetic refrigeration in the hydrogen liquefaction temperature range.

In our paper, we showed a new physical phenomenon on the boundary of the superconducting MgB2 material and the FexB material in in situ MgB2 wires in an Fe shield. We called this physical phenomenon "the interfacial topological effect" (ITE). This phenomenon generates resistance, which disappears to zero above the temperature of 7.3 K. This resistance is generated by the current flowing on the penetration depth, which was defective by the intermetallic layer (FexB). Moreover, voltage measurements showed that the interfacial topological effect starts to decrease in the field from 27 mT to 70 mT, and in magnetic fields above 70 mT-350 mT, it disappears. The performed studies show that this effect depends on the annealing temperature and the density of the Mg + 2B mixture in wires. Our studies indicated that in situ MgB2 wires in an Fe shield without a FexB layer do not show an interfacial topological effect. The studies carried out using a scanning electron microscope (SEM) and powder X-ray diffraction (XRD) indicate that the interfacial topological effect is created by the penetration of the FexB phase into the MgB2 superconducting material. The EDS analysis indicates that the disappearance of the interfacial topological effect after annealing at higher temperatures may result from the secondary reaction of Mg with FexB material. The SEM-EDS analyses showed that the density of the untreated Mg + 2B material significantly influences the formation and properties of the interfacial topological effect. Studies suggest that the ITE is strongest in the Meissner state and is reduced in the mixed state. The findings from our study carry significant implications not only for MgB2 wires produced by the powder-in-tube (PIT) method in a Fe diffusion barrier and Fe shield but also for MgB2 wires produced by the internal Mg diffusion (IMD) method.

In this study, a new method called Three-Hole Internal Magnesium Diffusion (TH-IMD) was applied to the fabrication of superconducting monofilament MgB2/Fe wires. In the TH-IMD method, three thin magnesium rods were placed within the boron powder in a triangular arrangement instead of a single magnesium rod in the middle of the wire, as in the standard IMD method. The aim of the TH-IMD method is to ensure the complete diffusion of magnesium into the surrounding boron during the heat treatment of IMD wires and to obtain uniform MgB2 structure throughout the core region. To determine optimal heat treatment conditions and wire diameters, transport measurements (R-T and I-V) were conducted under magnetic fields up to 11 T. Microstructural analysis was performed using optical microscopy, while phase composition was determined via XRD.

The paper presents the results of the influence of SiC dopant, annealing temperature, and annealing time on the morphology of MgB2 material in superconducting wires. The results of measurements of critical temperature (Tc), irreversible magnetic field (Birr), resistance in the normal state (Rn), and transport critical current density (Jct) at the temperature range from 15 K to 30 K are presented. The MgB2 material is characterized by the presence of two specific regions. The first region with high density, excess Mg, and rectangular MgB2 grains is located outside the voids surrounding them. The second region occurs inside the ceramic core, away from voids, and its chemical composition corresponds to a stoichiometric Mg to B ratio (1:2), and it is characterized by the presence of spherical grains and lower material density. A higher amount of SiC admixtures (6 at.%) causes an increase in the first region surface area. This kind of structure observation in MgB2 superconducting wires has never been reported previously. The transport measurements showed that higher SiC dopant leads to lower Jct at higher temperatures and high magnetic fields. The studies showed that the point-dominant mechanism and the first region allow for obtaining high Jct at 30 K.

The results of this work show that annealing under 0.4 GPa isostatic pressure does not influence the critical temperature (Tc) of the NbTi wire, which is about 9 K. Moreover, our studies indicate that annealing under pressure of 0.4 GPa reduces the irreversible magnetic field (Birr), and that annealing under any degree of pressure does not lead to Cu diffusion into the NbTi core. Scanning electron microscope images show that the NbTi core in the superconducting wire has NbTi fibers with a diameter of about 200 nm. Our studies show that isostatic pressure heat treatment significantly accelerated the formation of Ti precipitates on the NbTi grain boundaries. Furthermore, the studies indicate that the higher strain density obtained during cold treatment formed a larger number of α-Ti precipitates.

The findings of this study demonstrate that the morphology of in situ MgB2 wires examined by scanning electron microscopy (SEM) is primarily influenced by the applied polishing method, rather than by intrinsic phenomena associated with the synthesis reaction. This observation is of paramount importance for all superconducting ceramic materials. Main purpose of using fractured surfaces is to significantly better characterize of the Fe2B layer microstructure and composition. Furthermore, our paper presents the influence of the annealing temperature and initial filling densities on the diffusion of Fe from the shield into the core of the MgB2 material. Our results show that two intermetallic layers are formed during the solid-state reaction of Mg with B and long annealing time in MgB2 wires in an Fe shield. The first layer is composed of Mg and Fe, which do not form chemical bonds, and the subsequent layer consists of iron borides. To the best of the present author's knowledge, results of this kind pertaining to in situ MgB2 wires within Fe shields have not hitherto been reported. Moreover, it was demonstrated that thermal treatment in the liquid state of Mg facilitates the formation of an iron boride layer between the MgB2 core and the Fe shields. Additionally, the X-ray powder diffraction showed that the initial filling densities strongly affect the boride phases formed in Fe-shielded MgB2 wires.

In our article, we show the n value determined from measurements using the magnetic field sweep method (curves of E from B/Bc) and the current sweep method (curves of E from I/Ic). The results from these two methods allowed for a scientific n value analysis. Moreover, measurements using these two methods allow for a better understanding of the behavior of superconducting wires after the transition from the superconducting state to the normal (resistive) state. Measurements were made for the NbTi wire at 4.2 K, MgB2 at the temperature range from 4.2 to 30 K, and high-temperature superconducting tape at 77 K. The magnetic field sweep method results show that the n value has a constant value independent of an increase in the magnetic field, number of filaments, type of boron, diameter of the wire, and small amount of the dopant. Moreover, research studies point out that the n value in the magnetic field sweep method depends on the type of superconductor material, a large amount of doping at high temperatures, and the low density of pinning centers. Based on the results obtained, it can be indicated that each superconducting material will have a constant n value. Measurements carried out using the current sweep method showed that the n value decreases significantly with an increase in the magnetic field. Moreover, research studies have shown that the n value depends on the number of filaments, type of boron, dopant, wire diameter, and temperature. On this basis, it can be indicated that the n value for the current sweep method depends on the morphology and structure of the superconducting wires. In addition, based on the samples measured in this research, the current sweep method allows for higher n values than the magnetic field sweep method in low magnetic fields. However, the magnetic field sweep method allows obtaining higher n values than the current sweep method in middle and high magnetic fields.

Our results enable a better understanding and explanation of the physical phenomena associated with strong trapping of vortex lattice in superconducting wires. These measurements showed that dislocations oriented perpendicular to the external magnetic field most strongly trapped the vortex lattice in 8 T and much weaker in the 6 T magnetic field. Further research showed that dislocations situated in the angle of 45° to the external magnetic field most strongly trap the vortexes lattice in 7 T magnetic field. Subsequent results showed that dislocations oriented parallel to the external magnetic field trap the vortex lattice most strongly in the 6 T magnetic field and much weaker in the 8 T magnetic field. Previous studies have not reported these results and pinning analyses. Our results are also important for pinning centers created by irradiation for columnar defects.

We present the results of the irreversibility field (B-irr), critical temperature (T-c), and magnetic critical current density (J(cm)) for cylindrical samples encapsulated in carbon and doped with 1-3 at% dysprosium and terbium oxides. All samples were analyzed microstructurally by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). The influence of annealing temperature and time and high isostatic pressure on Jcm was investigated. Our results show that double-doped (C + Tb2O3 and C + Dy2O3) and low isostatic pressure heat treatment (0.1 MPa) decrease the Tc by about 0.5 K compared to the undoped MgB2 (0.1 MPa). The magnetic measurements show that double-doped and heat treatment under high isostatic pressure (HIP - 1 GPa) leads to a decrease in Tc of about 2.5 K compared to the undoped MgB2 sample after the HIP process. Our results show that the HIP process slightly increases J(cm) at 4.2 K and 20 K, and improves Jcm at 30 K in undoped MgB2 samples. Furthermore, our results show that the HIP process and double-doped significantly increase J(cm) at 4.2K and decrease J(cm) at 30K. Our results indicate that the HIP process can produce high field and low-temperature pinning centers in doubly doped MgB2 materials.

Many exciting effects resulting from the coupling of magnetic sublattice with a magnetic field, may be exposed by changing the field. One such phenomenon is the magnetocaloric effect, which is characterized by the absorption or emission of heat in response to changes in the external magnetic field. Magnetic refrigeration based on the magnetocaloric effect has emerged as an attractive alternative to conventional cooling technology that relies on gas compression and expansion. It is not only more efficient and environmentally friendly, but it can also be implemented across a broad temperature range, from ultra-low to a few hundred Kelvin temperatures. One of the areas where magnetic cooling can have a significant impact is hydrogen liquefaction. Hydrogen is one of the most promising candidates for clean energy sources, but it must be liquefied to facilitate storage and transportation, which requires cooling it down to similar to 20 K. The ideal magnetic refrigerant should exhibit consistent magnetocaloric properties across the entire operating temperature range of a cooler. This paper presents novel three-layer composite magnetic refrigerants that provide a uniform magnetocaloric response over a 30 K temperature range. The selected initial HoNi2, DyNi2, and TbNi2 magnetic intermetallic compounds with a Laves phase structure exhibit large magnetocaloric properties in the temperature range of 13-37 K. The composite composition of 23.56 wt% HoNi2 + 18.21 wt% DyNi2 + 58.23 wt% TbNi2 optimized for 2 T magnetic field change, was determined through numerical approach. The composites are manufactured using spark plasma sintering (SPS) and innovative high-isostatic-pressure (HIP) synthesis. The results of isothermal entropy change derived from magnetization data for a 2 T magnetic field change indicated 4.7 J/kgK (47.2 mJ/cm(3)K) and 4.4 J/kgK (44.2 mJ/cm(3)K) in the temperature range of approx. 13-42 K for composites after SPS and SPS + HIP processes, respectively. Despite the sample subjected to HIP showing slightly lower results, the entropy change is nearly uniform over the 30 K temperature range due to enhanced atomic diffusion between neighboring compounds, as confirmed by microscopic studies. Such a uniform magnetocaloric response in similar to 30-K temperature range has never been observed before in layered refrigerants. By utilizing the innovative high-isostatic-pressure synthesis technique, we have paved the way to high-performing magnetic composite materials that can be used in cryogenic magnetic coolers operating over a broad temperature span, expanding the possibilities of what can be achieved and laying the foundation for cost-effective, clean hydrogen energy.

Currently, MgB2 wires made by the powder-in-tube (PIT) method are most often used in the construction and design of superconducting devices. In this work, we investigated the impact of heat treatment under both low and high isostatic pressures on the formation of a layered structure in PIT MgB2 wires manufactured using the Mg coating method. The microstructure, chemical composition, and density of the obtained superconductive wires were investigated using scanning electron microscopy (SEM) with an energy-dispersive X-ray spectroscopy (EDS) analyzer and optical microscopy with Kameram CMOS software (version 2.11.5.6). Transport measurements of critical parameters were made by using the Physical Property Measurement System (PPMS) for 100 mA and 19 Hz in a perpendicular magnetic field. We observed that the Mg coating method can significantly reduce the reactions of B with the Fe sheath. Moreover, the shape, uniformity, and continuity of the layered structure (cracks, gaps) depend on the homogeneity of the B layer before the synthesis reaction. Additionally, the formation of a layered structure depends on the annealing temperature (for Mg in the liquid or solid-state), isostatic pressure, type of boron, and density of layer B before the synthesis reaction.

This study reports results for the morphology, crystal structure and critical parameters of Sm2O3-doped MgB2 wires with low and high initial filling densities. The transmission electron microscope (TEM) images were done for the longitudinal section of MgB2 wires. The results show that the Sm2O3 admixture significantly changes the morphology of the MgB2 material, accelerates the formation of the MgB2 phase, does not form rectangular MgB2 crystallites, does not leave pure Mg, and forms Sm2O3 areas of 10 nm and 20 nm. The effects of Sm2O3 addition on MgB2 formation in superconducting wires were revealed in detail in this study. Additionally, Sm2O3 causes the formation of point pinning regions that significantly increase the critical transport current density at the temperature range from 15 K to 30 K. The TEM images point out that rectangular MgB2 crystallites are formed in undoped MgB2 wires, which have not been previously reported. XRPD results showed that short-term heating allowed obtaining a larger amount of MgB2 phase for the MgB2 wire with high initial filling density. On the other hand, long heating time and high initial density slow down the creation of MgB2 phase when the Mg is in the solid state.

We present the results of the irreversibility field (Birr), critical temperature (Tc), and magnetic critical current density (Jcm) for cylindrical samples encapsulated in carbon and doped with 1-3 at% dysprosium and terbium oxides. All samples were analyzed microstructurally by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). The influence of annealing temperature and time and high isostatic pressure on Jcm was investigated. Our results show that double-doped (C + Tb2O3 and C + Dy2O3) and low isostatic pressure heat treatment (0.1 MPa) decrease the Tc by about 0.5 K compared to the undoped MgB2 (0.1 MPa). The magnetic measurements show that double-doped and heat treatment under high isostatic pressure (HIP - 1 GPa) leads to a decrease in Tc of about 2.5 K compared to the undoped MgB2 sample after the HIP process. Our results show that the HIP process slightly increases Jcm at 4.2 K and 20 K, and improves Jcm at 30 K in undoped MgB2 samples. Furthermore, our results show that the HIP process and double-doped significantly increase Jcm at 4.2K and decrease Jcm at 30K. Our results indicate that the HIP process can produce high field and low-temperature pinning centers in doubly doped MgB2 materials.

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/).

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.