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A superconducting joint of unreacted monofilament internal magnesium diffusion-processed magnesium diboride (MgB) 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 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 MgB in a range of practical applications, where superconducting joints are essential.

Structural disorder within grains formed by carbon doping is widely known to enhance the in-field transport critical current performance of MgB2 wire. The structural formation usually requires high-temperature sintering above 600 °C due to low reactivity of the dopant. In this study, we selected a carbohydrate (malic acid) that has a low-decomposition temperature as the carbon dopant and performed low-temperature sintering (below 600 °C) to introduce greater disorder within the MgB2 grains. We revealed that the doping and low-temperature sintering process cause severe disorder, especially along the out-of-plane (c-axis) direction. The disordered formation, in addition to the in-plane disorder combined with well-connected grains, eventually raised the in-field transport performance.

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 × 10−15 Ω 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 the superconducting joints development.

A superconducting joint architecture to join unreacted carbon-doped multifilament magnesium diboride (MgB

LiFePO4 (LFP) as a cathode material for lithium-ion batteries (LIBs) requires the application of carbon composite to compensate its poor electrical conductivity. Thus, it is important to develop an efficient and cost-effective carbon composite process. This work demonstrates a new carbon coating (CC) method involving the direct solventless pyrolysis of the polycyclic aromatic hydrocarbon coronene (C24H12) as a means of coating LFP powder. The electrochemical properties of the resulting LIB cathodes were examined to find that the CLFP cathode exhibited a high specific capacity of 154 mAh/g at 0.05 C, which corresponds to 91% of the theoretical capacity of LFP (170 mAh/g), and LIB cells with the CLFP cathode showed rate capabilities and cycling performance comparable to those of commercial LIB cells. Due to the simple CC process and the associated formation mechanism, we anticipate that this method will have practical applications in the large-scale manufacture of LFP for LIB cathodes with low cost.

We elucidate the microstructural evolution in MgB2 wires fabricated at two different temperatures by the internal Mg diffusion (IMD) method via multiple-scale microstructural observations. The critical current (Ic) of MgB2 wire was enhanced by the IMD method; however, the Ic of MgB2 was not sufficiently high to allow its practical applications. To prepare MgB2 by the IMD method, molten Mg is required to infiltrate the gaps between the B grains and penetrate each B grain. Here, we suggest that controlling the infiltration and penetration behaviors of Mg are key to enhancing the Ic of MgB2 fabricated by the IMD method. One reason for the decrease in Ic is the existence of residual B grains in the MgB2 crystalline region, which has an area fraction of 25% even in the MgB2 wire exhibiting the highest Ic. The amount of residual B grains, which formed due to the insufficient penetration distance of Mg atoms in the B grain, decreased after high-temperature treatment. However, the microstructure of the MgB2 wire fabricated at a temperature higher than the melting point of Mg was non-uniform because Mg infiltrated the gap between the B grains rapidly and non-uniformly. Thus, optimization of heat treatment temperature can effectively control the infiltration and penetration of Mg in the B grains, yielding uniform microstructures.

MgB2, which has a critical temperature $T_{{\rm{c}}}$ of 39 K, is a candidate for the practical application of liquid-helium-free magnetic resonance imaging systems. Since the discovery of MgB2, the performance of MgB2 wires and the technology required to manufacture long-length conductors have been significantly improved. Wires processed by internal magnesium diffusion (IMD) show high performance in comparison with powder-in-tube-processed MgB2 wires. Our group has been developing high-performance IMD wires. In this study, a joint connecting two IMD-processed MgB2 wires was fabricated. The maximum critical current in the connected wires was 1/3 lower than that in a normal IMD-processed wire at each considered magnetic field strength. However, the critical current of the joint at 3 T was equal to that of a normal IMD-processed wire at 10 T, suggesting that this joint would be effective at magnetic fields of around 3 T. Furthermore, a joint resistance of 10–13 Ω was obtained by measuring the current decay at the joint.

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 (Ic) of 38.5 A at 10 K in self-field. Unconventional Ic behaviour of the joint was observed at 20 K in self-field when the Ic 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 Ic of the joint. This work will notably contribute towards MgB2 superconducting joint technology development for magnetic resonance imaging (MRI) application.

Superconducting joints are essential for persistent-mode operation in a superconducting magnet system to produce an ultra-stable magnetic field. Herein, we report rationally designed niobium-titanium (Nb-Ti) superconducting joints and their evaluation results in detail. For practical applications, superconducting joints were fabricated by using a solder matrix replacement method with two types of lead-bismuth (Pb-Bi) solder, including Pb42Bi58 as a new composition. All the joints attained a critical current of >200 A below 1.43 T at 4.2 K. Our optimized superconducting joining method was tested in a closed-loop coil, obtaining a total circuit resistance of 3.25 × 10−14 Ω at 4.2 K in self-field. Finally, persistent-mode operation was demonstrated in an Nb-Ti solenoid coil with a persistent-current switch. This work will pave the way to developing high-performance Nb-Ti superconducting joints for practical applications.

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/16 and MgB2−x(C16H10)x/16, 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.

We have developed a method for the nondestructive characterization of critical current distribution in MgB 2 wires based on magnetic microscopy. This method enables us to evaluate critical currents of MgB 2 wires, even in the condition where they become too large to be measured by the conventional four-probe transport method from the viewpoint of heat generation and/or the limitation from the measurement system. Furthermore, the inhomogeneity in local critical currents in the wires can be also estimated, while it is difficult for the conventional magnetization method, using a SQUID magnetometer. In this paper, by overcoming the influence of magnetic sheath materials, which are widely used for recent high-performance MgB 2 wires, we have succeeded in developing a nondestructive method for the characterization of longitudinal distribution of local critical currents in them. This method will become a fundamental technique for a nondestructive product testing at the stage of full-fledged commercial production in the future.

For the further development of MgB2 wires, investigation of 3-D structures for MgB2 filaments and sheath materials is important. However, typical cross-sectional observation such as optical microscopy and scanning electron microscopy are essentially 2-D, therefore, not enough to understand the 3-D filament structures. In this paper, we have adopted X-ray microtomography and carried out a 3-D analysis for MgB2 wire. X-ray microtomography (Skyscan 1172, Bruker) allows us to visualize the internal filament structure of a centimeter-long MgB2 wire with a high spatial resolution of 1.1 $\mu\text{m}$ , with a nondestructive manner. We investigated the boundary between the sheath material and the MgB2 filament of an internal Mg-diffusion-processed MgB2 wire. Furthermore, by improving the signal-to-noise ratio for the X-ray microtomography, we have also identified the boundary between the MgB2 filament and the hole (air) in spite of the low X-ray absorption coefficient for boron and magnesium. From the 3-D analysis, we can reveal the shape and area of MgB2 filament quantitatively and also their statistical distribution.

This paper reports a diagnostic method for clarifying performance bottlenecks of superconducting wires and tapes for their further performance enhancements. In particular, our achievements by scanning Hall-probe microscopy (SHPM), which worked well for selecting positions for microstructure observation by SEM, TEM, and X-ray CT, are introduced. This hybrid microscopy offers the information of direct relationship between the performance of a practical-scale superconducting wire or tape and its origin in micro-scale or nano-scale structure. As such examples, characterization results for an MgB2 multi-filamentary wire, a commercially available long RE-123 coated conductor, and a striated multi-filamentary coated conductor are reported in this paper through reviewing our past studies.

Simple and efficient characterization of local structural states in materials is desirable for evaluating and tailoring the material performance. Herein, we have focused on micro X-ray diffraction and conducted a local structure characterization for superconducting wires having MgB2 polycrystalline cores. The critical current properties of the superconducting materials were controlled and improved by a low-temperature sintering approach with malic acid doping as a carbon source. The transport performance depending on the sintering and doping effects was found to be closely associated with the characterized properties of the local structures. The evaluation method will therefore be useful to develop composite conductors.

Recently, a project was initiated in Japan to transport a large amount of liquid hydrogen (LH2) from Australia to Japan by sea. It is important to understand the sloshing and boil-off that are likely to occur inside an LH2 tank during marine transportation by ship, but such characteristics are yet to be experimentally clarified. To do so, we combined the liquid level detected by five 500 mm long external-heating-type magnesium diboride (MgB2) level sensors with synchronous measurements of temperature, pressure, ship motion, and acceleration during a zigzag maneuver. During this zigzag maneuver, the pressure of gaseous hydrogen (GH2) in the small LH2 tank increased to roughly 0.67 MPaG/h, and the temperature of the GH2 in the small LH2 tank increased at the position of gaseous hydrogen at roughly 1.0 K/min when the maximum rolling angle was 5&deg, the average rolling and liquid-oscillation periods were 114 and 118 s, respectively, as detected by the MgB2 level sensors, which therefore detected a long-period LH2 wave due to the ship&rsquo, s motion.

We fabricated 100-m-long class-MgB 2 superconducting wires using an internal Mg diffusion (IMD) process. Coronene was added as a carbon source to enhance the in-field critical current properties. We prepared a coil with a 4.8-cm diameter with 12 turns from a 2.6-m wire cut from the long wire. We obtained uniform critical currents (the critical current $(I_{c})$ , the critical current density $(J_{c})$ , and engineering $J_{c}(J_{e})$ ) at different magnetic fields and temperatures, wire structures, compositions, and microstructures of the MgB 2 layer in ten short wires cut from the middle ten turns of the coil. We also took seven pieces of a monofilament wire from the IMD long wire to fabricate a seven-filament IMD MgB 2 wire, and we found no obvious decrease in $J_{c}$ and $J_{e}$ when the diameter of the wires decreased from 1.06 to 0.88 mm. These results suggest that the IMD process could be used to fabricate MgB 2 wires with a uniform structure. Coronene addition is useful for obtaining uniform composition and microstructure in the MgB 2 layer because the B powder is uniformly coated with coronene for the whole wire during the annealing steps in the drawing process and the final heat treatment process. Therefore, the IMD process, together with coronene addition, is expected to fabricate MgB 2 superconducting wires with uniform in-field critical current properties for practical large-scale applications.

Hydrogen is an ultimate energy source because only water is produced after the chemical reaction of hydrogen and oxygen. In the near future, a large amount of hydrogen, produced using sustainable/renewable energy, is expected to be consumed. Since liquid hydrogen (LH2) has the advantage of high storage efficiency, it is expected to be the ultimate medium for the worldwide storage and transportation of large amounts of hydrogen. To make a simulation model of the sloshing of LH2 inside a 2000 liter tank, simulation analyses of LH2 surface oscillation, temperature and pressure inside the tank during a truck transportation have been carried out using a multipurpose software ANSYS CFX. Numerical results are discussed in comparison with experimental results.

We are currently developing an external-heating-type superconducting magnesium diboride (MgB2) level sensor for a liquid hydrogen (LH2) tank. The aim of this study is to investigate the measuring current dependence of the level-detecting characteristics of the MgB2 level sensor for LH2 under a static condition which has not yet been clarified. It was found that the linear correlation coefficient was 0.99 or more, indicating high linearity, regardless of the measuring current at heater inputs of 3 W and 6 W. Moreover, there was no effect of self-heating by the measuring current and it was found that a current of up to 100 mA can be used.

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

It is well known that SiC doping in superconducting MgB2 improves the upper critical magnetic field (Bc2) and the critical current density (Jc) under high magnetic field. However, the relationship between SiC doping and the flux pinning mechanism has not been clarified. In this study, several MgB2 samples with and without SiC doping were prepared by the conventional in situ powder-in-tube method. The critical current densities and the force-displacement characteristics of fluxoids in samples were investigated by an ac inductive measurement (Campbell’s method). The Labusch parameter (αL) and the interaction distance (di) were estimated from the obtained force-displacement profile. It was found that SiC doping enhances the values of αL, but does not change the characteristics of the magnetic field dependence of αL apparently. Namely, αL vs. B3/2 characteristics in the pure samples and SiC doped samples are almost the same. Such a result of αL properties implies that the pinning mechanism in the SiC doped samples could be consistent with the conventional pinning theory. On the other hand, di, which is considered to be proportional to the size of pinning potential, decreases rapidly with increasing magnetic field, especially in the pure samples. For high magnetic field region, the variations of di were deduced to be caused by flux creep. The depth of pinning potential, U0, was estimated by using the values of αL and di. The values of U0 give evidence of that SiC doping can prevent the flux bundles moving to another pinning center under high magnetic field.

The internal Mg diffusion (IMD) process produces a MgB 2 layer with higher density than that achieved with the traditional Powder-in-tube (PIT) method; this makes the IMD process an attractive one for the fabrication of superconducting MgB 2 wires with higher critical current densities ( J c ). We have recently shown that co-addition of SiC and some liquid aromatic hydrocarbons (toluene and dimethylbenzene) can enhance the J c of IMD-processed mono-core MgB 2 wires. In the present contribution, we discuss the J c of the IMD-processed mono-core MgB 2 wires fabricated under different conditions, viz. with pure B powder, B powder with additive-like SiC or toluene, and B powder with SiC + toluene co-addition. The results from these samples indicated that the SiC + toluene co-addition gave the best J c value, as in the previous results. The composition and microstructure suggested that carbon substitution for boron and reductions of grain size by SiC and/or toluene additives are responsible for the J c enhancements of IMD-processed MgB 2 wires.

MgB2 is a promising superconductor for future applications to superconduct wires and tapes used at the liquid hydrogen temperature (20 K). Because the maximal superconducting current (critical current) in MgB2 depends on its microstructure, an “in-situ” process to fabricate MgB2 based on a reaction between Mg and B is effective in controlling the microstructure. However, the critical current in the fabricated MgB2 wires and tapes is not sufficiently high for practical use. This may be attributed to the imperfectness of the microstructural control in MgB2 wires and tapes. In this paper, we will discuss the microstructural features of MgB2 tapes fabricated using a typical in-situ powder-in-tube process. By observing the early stage of microstructural evolution in detail, we can gain insights into the next strategy for improving the microstructure of MgB2 tapes with respect to their applications.

Microstructure in a high-density MgB 2 wire fabricated by an internal Mg diffusion (IMD) process has been investigated by electron microscopy imaging and analysis at different scales. In the IMD process, a pure Mg rod was used as Mg source, and nanosized SiC powders were mixed with amorphous B powders. In the case of a heat treatment at 640 °C for 1 h carried out after rolling and drawing processes, the wire has two microstructural features that degrade critical current density: uncrystallized zones composed mainly of unreacted B and SiC powders, and cracks partly filled with course Mg 2 Si crystals. Those cracks were formed in the uncrystallized zones as well as in crystallized MgB 2 zones. It indicate that the cracks formed by the mechanical milling and drawing remain after the heat treatment.

We applied the addition of both SiC and liquid aromatic hydrocarbon to the internal Mg diffusion (IMD) processed MgB2 wires. The liquid aromatic hydrocarbons used in this work are toluene, p-ethyltoluene and p-dimethylbenzene. The engineering critical current density (Je) and the critical current density (Jc) calculated for the cross-sectional areas of the reacted layer are compared to those of the IMD processed wires without aromatic hydrocarbon addition. All wires with co-addition of aromatic hydrocarbon and SiC show much smaller field dependence of Je (Jc)–B curve similarly to the wires with only SiC addition and, hence, larger Je (Jc) in high magnetic fields compared to the wires processed from pure B. Furthermore, the SiC + toluene co-added wires show larger Je and Jc values in entire magnetic fields which is independent of the heat treatment temperature while that of the other co-added wires is depending on the heat treatment temperature. The highest Je and Jc at 4.2 K and 10 T obtained in this work are 3700 A/cm2 and 48,000 A/cm2, which were achieved for the SiC + dimethylbenzene co-added wires heat treated at 670 °C and the SiC + toluene co-added wires heat treated at 640 °C. Those results indicate that the co-addition of SiC and liquid aromatic hydrocarbon is one of the promising methods to improve the Jc as well as Je of the IMD processed MgB2 wires.

We fabricated FeTe0.5Se0.5 superconducting wires using ex-situ Powder-in-tube method with an Fe sheath. It was amazing that superconducting current was observed in the as-drawn wire without any heat treatments. By heat treatment at 200C for 2 hours, Tczero and Jc at 4.2 K were enhanced up to 9.1 K and 64.1 A/cm2 (Ic = 182.6 mA), respectively. Furthermore, the Jc of FeTe0.5Se0.5 wire heat treated at 200C for 2 hours was not much sensitive to the applied magnetic fields. Therefore, FeTe0.5Se0.5 wires have a great potential for applications., Comment: 15 pages, 5 figures

High Temperature Superconductors (HTS) are now widely used in many applications such as power transmission cables, motors, etc. Recent (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) conductors have been much improved in transport and mechanical properties, and their application as insert coils for high field generation is expected. In order to demonstrate the feasibility and the potential of Bi-2223/Ag tapes for magnet applications, we fabricated a layer-wound coil using Bi-2223/Ag/Cu-alloy tapes. We used two tapes, 330 m- and 210 m-pieces, in order to examine the possibility of extension of conductor length through a lap-joint inside the coil. These two tapes were soldered without removing Cu-alloy laminations. The winding has the dimensions: inner dia. 57.1 mm, outer dia. 88.9 mm, coil height: 153.9 mm, 44 layers (2,355 turns). We tested this coil at 4.2 K in 0 ~ 12 T. The test coil carried 193.3 A (0.1 μV/cm criterion) and generated 3.3 T in the axial direction of the coil in external 12 T (15.3 T in total). The critical current is close to the short sample performance in the perpendicular fields. Splice resistance at the joint stayed about 1 μΩ at 4.2 K, 10 T.

We have fabricated ex situ processed MgB2 round wires and tapes sheathed with various metals using chemically treated powders. The critical current density (Jc) properties of the as-fabricated wires and tapes are not so different, that is, independent of the sheath materials. On the other hand, the transport Jc properties of the heat-treated wires and tapes strongly depend on the sheath materials. Heat treatment of those wires and tapes brings about the sintering of MgB2 and slight enhancement in magnetization hysteresis when using Ta sheath, compared with Fe sheath. In contrast, the transport Jc properties of the wires sheathed with Ta as barrier are drastically deteriorated by heat treatment, whereas Fe-sheathed tapes show drastic Jc enhancement. Microstructural observation reveals that many cracks are generated in the MgB2 core by heat treatment only for the heat treated wires sheathed with Ta as barrier. Such cracks act as obstacles for supercurrent flow.

Several reaction-induced diffusion processes to fabricate high-density MgB2 materials are developed, and the critical current density (Jc) has been notably enhanced. In this study, microstructure in high-density MgB2 wires fabricated by an internal Mg diffusion (IMD) process has been investigated. The inner reacted region of the wire heat-treated at 640°C for 1 h shows dense polycrystalline MgB2 of 20-200 nm in grain sizes. Fine MgO and Mg2Si particles of 10-30 nm in sizes are dispersed in this region. On the other hand, the outer region near the Ta sheath is composed of unreacted B and SiC powders, fine MgO particles and small voids. Sizes of voids in the IMD MgB2 wire are small compared with the PIT MgB2 wire. Oxidation of Mg in the IMD process forms fine dispersion of MgO which may be effective for flux pinning.

We have developed a characterization method of local critical current in iron-sheathed MgB 2 wires. Local homogeneity was visualized by the scanning Hall-probe microscopy (SHPM). The value of local critical current was quantified with the aid of the finite element method (FEM) considering the ferromagnetic properties of the iron sheath. The results suggested that MgB 2 wires fabricated by internal Mg diffusion processes still have large longitudinal inhomogeneity and much higher potential than that estimated by the four-probe transport method. This will be very important information for making a correct strategy for further development of MgB 2 wires.

Field and temperature dependences of critical current density ( J c ) and n -value were evaluated from un-doped and carbon doped MgB 2 wires, supplied by the Hyper Tech Research, Inc. In order to explore the possibility of persistent current mode, the variation of n -values for the different MgB 2 conductors was studied in more detail. It was observed that the n -value decreased gradually with an increasing magnetic field and operating temperature. From the power-law relationship between the J c and the n -value, n ∝ J c m , we found that there was a strong relationship, m = 0.371, for both wires at 4.2 and 20 K. On the basis of our experiments, a larger J c led to a higher n -value, whether there was doping or not.

Uniaxial hot-pressing was applied as an alternative way to further improve the grain connectivity of both un-doped and carbon doped MgB2/Nb/Monel wires. In this study, hot-pressing of 100 MPa resulted in the improvement of the mass density, the critical current density, and the grain connectivity of the wires. However, this also caused additional critical current density anisotropy, which is associated with the texture of a- and b-axes oriented grains. In particular, the anisotropy factors of critical current density for the un-doped and the carbon doped conductors were estimated to be 2.6 and 1.7, respectively, under an external magnetic field of 14 T.