Earticle

This study explores the enhancement of magnetic field homogeneity in Nuclear Magnetic Resonance (NMR) magnets through optimized ferromagnetic shimming design. Traditional shimming techniques, which involve attaching ferromagnetic materials to the magnet bore, often lack manufacturability considerations, limiting their effectiveness in real-world applications. To address this, we employ a topology optimization (TO) approach using the Solid Isotropic Material with Penalization (SIMP) scheme for design parametrization. The proposed optimization framework includes volume and perimeter constraints to improve the practical manufacturability of the shim. Numerical analysis demonstrates that the TO-based design method achieves superior magnetic field homogeneity, achieving 0.45 ppm with integer thickness shims, compared to conventional designs. This approach also effectively reduces manufacturing complexity by minimizing design sensitivity to the thickness and placement of individual shimming elements. The proposed design framework is broadly applicable to superconducting magnets in NMR and MRI systems, where high magnetic field homogeneity is essential. This study presents a significant advancement in ferromagnetic shimming technology, offering a viable solution for enhancing the performance and manufacturability of high-precision magnetic field devices.

High-temperature superconductors (HTS), owing to their high critical current and critical magnetic field, are utilized in various superconducting applications such as superconducting power cables and high-field magnets. However, REBCO (Rare Earth Barium Copper Oxide) tapes face significant challenges in long-length fabrication compared to low-temperature superconducting wires. Consequently, joining REBCO tapes is essential for their application in superconducting devices. The most widely used method for joining REBCO tapes is mechanical pressure soldering, which inevitably introduces contact resistance at the joint interface. Since superconducting devices operate in cryogenic environments where maintaining minimal losses is critical, minimizing joint resistance is essential. In this study, we evaluated the effect of heating temperature on joint resistance in the mechanical pressure soldering process by selecting four types of solders (Pb37Sn63, In52Sn48, Sn42Bi57Ag1, and In66.3Bi33.7). Lap joint samples were prepared by increasing the heating temperature in 10°C increments above the melting points of each solder, and the surface resistance of each sample was measured. The results were analyzed to identify the appropriate heating temperature.

As global environmental regulations intensify, the electrification of the mobility industry is accelerating, thereby increasing the demand for high-power electric propulsion systems. Among the various candidates for application, superconducting motors have attracted considerable attention owing to their potential for delivering exceptionally high-power density. In this study, we fabricated two types of GdBa2Cu3O7-δ (GdBCO) superconducting magnets and evaluated their trapped field performance using a 14T superconducting magnetization system. A single grain bulk magnet, grown by the top seeded melt growth (TSMG) method, exhibited a trapped field of 1.93T at 20K. The stacked tapes, fabricated by commercial GdBCO tapes, showed a maximum trapped field of 1.25T at 20K. While the single grain bulk magnet has an advantage in trapped field performance, the stacked tapes offer benefits in terms of simple manufacturing processes and shape flexibility. These results suggest that simultaneous improvement of trapped field performance and motor design optimization could establish superconducting motors as a core technology for nextgeneration electric propulsion systems.

The Small Punch Test (SPT) was developed to evaluate the softening and embrittlement of materials such as power plants and nuclear fusion reactors by taking samples in the field. Specimens used in the SPT are very thin and small disk-shaped compared to specimens for general tensile test, and thus have economic advantages in terms of miniaturization and repeatability of the test. The cryogenic SPT can also be miniaturized and has a significantly lower heat capacity than conventional universal test machines. This leads to reduced cooling and warm-up times. In this study, the cryogenic SPT was developed by modifying the existing room temperature SPT to be cooled by liquid nitrogen using a super bellows and a thermal insulation structure. Since the cryogenic SPT was first developed, basic experiments were conducted to verify the effectiveness of it. For the validation, aluminum alloy 6061- T6 specimens were tested for mechanical properties at room and cryogenic temperature. The results of the corrected tensile properties from the SPT experiment results were compared with known room temperature and cryogenic properties. Based on the correction results, the effectiveness of the cryogenic SPT test was confirmed, and the surface fracture characteristics of the material were analyzed using a 3d image scanner. In the future, we plan to conduct property evaluation according to the development of various alloy materials.

Accurate measurement of contact resistance in no-insulation (NI) high-temperature superconducting (HTS) coils is crucial for their performance evaluation. The sudden discharge test is a widely used method for measuring the contact resistance of NI HTS coils. However, there are several issues regarding the data acquisition system (DAQ) during the test. In particular, the DAQ must operate at a sampling rate exceeding 500 S/s. An infinite impulse response (IIR) filter can introduce a phase delay into the filtered signal compared to the original data. For instance, applying a 4-Hz low-pass Butterworth filter to a high-frequency signal may lead to measurement errors due to this delay. Moreover, remnant field induced by the screening current after the current transportation introduces uncertainty of conducting sudden discharge tests. This paper proposes the qualified settings of DAQ system configuration for sudden discharge test and suggests an alternative method for measuring the contact resistance of NI HTS coils.

In recent years, the interaction force between a permanent magnet and a closed superconductor coil has been gradually investigated in depth. The principle and application potential of an energy storage/convertor composed of a magnet and a closed superconducting coil have been proved. However, the study on the force between a magnet and a non-closed superconducting coil (superconducting roll stack) has hardly been reported in previous literature. The behavior of this kind of interaction and its influence to the interaction force between a permanent and a closed superconducting coil are also still unclear. In this paper, first we investigated the interaction force between a magnet and a superconducting roll stack. Then, a series of experiments were designed and conducted to clarify the factors affected the interaction force, including the geometrical parameters of the superconducting roll stack and the magnetic field density at the roll stack. Moreover, the comparison of the interaction forces between the magnet and roll stack or a closed coil was also introduced.

High-entropy alloys (HEAs) consist of five or more principal elements with atomic fraction between 5 % and 35 %, randomly distributed throughout the lattice points. Recently, a simple body-centered cubic structure HEA superconductor, Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6, was discovered and classified as a type-II superconductor with a critical temperature of approximately 8 K. Ta-Nb-Hf-Zr-Ti HEA superconductors exhibit a large critical current density of 1 MA cm-2 and demonstrate robust superconductivity against irradiation-induced disorder. We have synthesized HEA superconductors based on the composition Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 with various Ce doping concentrations (0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 4 wt%, 8 wt%), investigating the effects of Ce addition on the crystal structure and superconductivity. X-ray diffraction and energy dispersive x-ray spectroscopy results indicate the complex Ce addition effects on the crystals. Notably, while the superconducting transition temperature remains largely unaffected by the presence of magnetic impurities, it is found to be closely related to the lattice constant, which changes with varying Ce doping concentrations.

Recently, research on applying composite materials to various industrial fields is being actively conducted. In particular, composite materials fabricated by Fused Deposition Modeling 3D printers have more advantages than existing materials as they have fewer restrictions on manufacturing shape, reduce the time required, weight. With these advantages, it is possible to consider utilizing composite materials in cryogenic environments such as the application of liquid oxygen and liquid hydrogen, which are mainly used in an aerospace and mobility. However, FDM composite materials are not verified in cryogenic environments less than 150K. This study evaluates the characteristics of composite materials such as tensile strength and strain using a UTM (Universal Testing Machine). The specimen is immersed in liquid nitrogen (77 K) to cool down during the test. The specimen is fabricated using 3D print, and can be manufactured by stacking reinforced fibers such as carbon fiber, fiber glass, and aramid fiber (Kevlar) with base material (Onyx). For the experimental method and specimen shape, international standards ASTM D638 and ASTM D3039 for tensile testing of composite materials were referenced.

In this work, the effects of GdFeO3(GFO) addition on the superconducting properties of GdBa2Cu3O7-x (GdBCO) bulk samples were investigated. A series of GdBCO + GFO composites (0  1.0 wt.%) were synthesized using the solid-state reaction (SSR) method. XRD analyses revealed peak shifts to low angles and increases in the c-axis lattice constant with increasing GFO content. The superconducting transition temperature (Tc), determined from temperature-resistivity measurements, showed a gradual decrease as GFO was added. At low concentrations (≤ 0.2 wt.%) Tc and superconducting transition temperature width (ΔTc) remained close to those of the pure sample, whereas higher concentrations (≥ 0.5 wt.%) led to a pronounced Tc suppression and broadening of ΔTc. Excess conductivity analysis further indicated that the 3D fluctuation region diminished with increasing GFO concentration, suggesting that GFO incorporation weakens interlayer coupling in the GdBCO system.

High-TC superconductivity (HTSC) has been the central issue in the field of condensed matter physics for decades. An essential part of the research on superconductivity is finding new exotic superconductors. It was recently suggested that Ir-substituted La0.7Sr0.3MnO3 (LSMIO) is a new high-TC superconductor. However, systematic studies to experimentally verify the superconductivity have not been done. Here, we report the growth processes of LSMIO thin films and their electrical transport properties. We observed a clear negative correlation between the intensity of the laser utilized for film deposition and the Curie temperature of the deposited film. We attributed this effect to the suppression of Sr concentration in the LSMIO films as the laser intensity increased. However, our LSMIO films show conventional ferromagnetism instead of HTSC. To realize the HTSC in LSMIO systems, further exploration of diverse compositions of LSMIO compounds is essential.