The effects of La_0.7Sr_0.3MnO₃ (LSMO) and BaZrO₃ (BZO) additions on the structural, electrical, and magnetic properties of (Bi, Pb)-2223 superconductors were systematically investigated. Samples with varying concentrations of LSMO and BZO were synthesized via the solid-state reaction method. X-ray diffraction (XRD) analysis confirmed that the Bi-2223 phase was retained after the incorporation of both secondary phases, with only slight changes in lattice parameters and crystallite size. Scanning electron microscopy (SEM) revealed that BZO addition enhanced lattice strain and microstructural homogeneity, whereas LSMO addition introduced structural disorder and weak-link behavior. Magnetization measurements at 65 K demonstrated that BZO addition significantly enhanced the critical current density (J) and flux pinning force density (F_p), with the BZ05 sample exhibiting the highest performance. In contrast, LSMO addition degraded superconducting properties due to increased disorder and reduced grain connectivity. Normalized pinning force analysis indicated that surface pinning is the dominant mechanism across all samples, although additional pinning contribution, depending on the additive material, play a crucial role under high magnetic field. Overall, BZO proved more effective than LSMO in enhancing the superconducting performance of (Bi, Pb)-2223, with the BZ05 composition showing the most promising results.

Electron-doped cuprates have mainly been studied in the T' structure because chemical electron doping tends to destabilize the T phase. Here, we realize electron-doped T-phase La2CuO4 (LCO) monolayers using polar discontinuity at oxide interfaces. Charging-free LCO/LAO/LSCO heterostructures were fabricated by pulsed laser deposition and studied by ARPES. The LCO monolayer exhibits a clear Fermi surface, unlike insulating bulk-like LCO, indicating successful electron doping. Tight-binding analysis shows that the electron doping level increases with LAO buffer-layer thickness, consistent with charge redistribution driven by polar discontinuity. However, thicker LAO layers also degrade the ARPES spectral quality, suggesting additional compensation mechanisms such as surface reconstruction. These results demonstrate polar discontinuity as a promising route to electron-doped T-phase cuprates.

Vanadium trioxide (V₂O₃) has emerged as a promising candidate for smart electrical insulation for use in rare-earth barium copper oxide (REBCO) magnets due to its metal-to-insulator transition (MIT) behavior. V₂O₃ maintains high resistivity at the operating temperature of 77 K to ensure an insulated state. However, it automatically drops to low a resistivity value as the temperature approaches approximately 150 K. Such a dramatic transition allows the turn-to-turn contact resistance (R_ct) in REBCO coils to shift from the insulating to the metallic state, facilitating the effective bypass and redistribution of current during quench events. Thus, V₂O₃ can function as a thermally activated switching layer that combines the rapid electromagnetic response of insulated coils with the enhanced self-protection capability of non-insulated coils. This study experimentally investigates the repeatability and reproducibility of the MIT characteristics of a V₂O₃ insulation layer fabricated using a paste-coating process and sandwiched between REBCO tapes. The thermal-cycle repeatability was evaluated using two independently prepared samples, each subjected to ten continuous thermal cycles under the same measurement conditions. For the first sample, the mean transition temperature (T_rt) was137.32 K, with a standard deviation (σ_Trt) of 1.56 K. The second sample also exhibited a consistent MIT behavior, with a mean T_rt of 142.58 K and a σ_Trt of 1.42 K. These results confirm that the MIT behavior of V₂O₃ insulation is repeatable with small standard deviation under repeated thermal cycling tests. In addition, the pressure dependence of R_ct was examined under applied torque values ranging from 0.1 to 6.0 N·m. The results show that R_ct is highly sensitive to contact pressure. In particular, at 77 K, a substantial decrease in R_ct was observed when the applied torque reached 5 N·m or higher. Finally, the reproducibility of the V₂O₃ insulation material was investigated using ten independently fabricated samples. These samples exhibited a mean insulation thickness of 96.6 ㎛ with a standard deviation of 13.26 ㎛, while their MIT behavior remained reproducible within the tested sample set, with a mean T_rt of 147.09 K. These findings provide short-sample interface-level data for future V₂O₃-based smart-insulation design. However, wound-coil experiments are still required to verify coil-level self-protection performance.

The Electrodynamic Suspension (EDS) systems are gaining significant attention as next-generation high-speed transportation technology due to their inherent self-stabilizing characteristics and superior lift-to-drag ratios. Conventional figure-eight null-flux coils used in EDS systems provide the advantage of simultaneously realizing levitation and guidance within a single structure. However, due to their structural mechanism, the levitation and guidance forces are strongly coupled, which limits the independent enhancement of guidance performance. To address the limitations of conventional cross-connected null-flux coils, this study proposes an independent inner-outer double-circuit asymmetric null-flux coil structure. The guidance characteristics of the proposed structure were quantitatively evaluated and compared with those of the conventional symmetric model using three-dimensional Finite Element Method (3D FEM) analysis. The simulation results demonstrate that the proposed asymmetric coil achieves approximately 6.47% improvement in guidance force at a 50 mm lateral displacement compared to the conventional model (20.08 kN), indicating its strong potential to enhance the driving stability of EDS systems.

This paper proposes a field current control method to improve the flux-weakening operation performance of a no-insulation (NI) high-temperature superconducting (HTS) synchronous motor for electric vehicles. An NI field coil can enhance thermal stability by providing turn-to-turn current bypass paths; however, turn-to-turn leakage current caused by contact resistance and the large electrical time constant of the coil can delay the field flux response. In particular, in the high-speed operating region of a synchronous motor, the armature d-axis current rapidly changes in the negative direction for flux-weakening control, which induces voltage and current in the field circuit. The induced current causes the azimuthal current to deviate from its reference value and increases the back-electromotive-force component, leading the armature q-axis voltage to reach the voltage limit prematurely. In this study, an analytical model combining the lumped circuit of the NI field coil and the d-q equivalent circuit of the synchronous motor is developed, and a proportional-integral (PI)-based field current controller is designed to regulate the azimuthal current. The effectiveness of the proposed control method is verified under an acceleration condition from standstill to 4000 rpm. The analysis results show that, in the maximum torque per ampere operation region, the difference in operating characteristics with and without field current control is insignificant because the variation rate of the d-axis current is limited. In contrast, in the flux-weakening region above approximately 2400 rpm, the proposed control method significantly improves the operating performance. With field current control, the maximum azimuthal current decreases from 197 A to 192 A, and the settling time of the azimuthal current to its reference value is reduced from 90 s to 9.5 s. In addition, by suppressing the increase in back electromotive force, the q-axis voltage margin is secured, and the time required to reach 4000 rpm is reduced from approximately 72 s to approximately 5 s. A comparison under different contact resistivity conditions also confirms that the proposed field current control method maintains similar acceleration performance. Furthermore, a higher contact resistivity reduces both the turn-to-turn leakage current and the variation in the field operating current. These results indicate that the proposed field current control method is effective for improving the high-speed operation performance of NI HTS synchronous motors, and that contact resistivity should be selected by considering not only acceleration performance but also leakage current, loss, and thermal stability.

Recently, wound-rotor synchronous motors (WRSMs) employing no-insulation high-temperature superconducting (NI-HTS) field coils have attracted attention as a means of achieving high power density and high efficiency simultaneously in low-carbon propulsion systems for the marine and aviation sectors. In NI-HTS field coils, the turn-to-turn contact resistance distributes the field current into an azimuthal current and a leakage current, so the azimuthal current that actually generates the magnetomotive force cannot be determined from the terminal current alone. This paper proposes an armature-voltage-equation-based observer that estimates the azimuthal current using only stator-side voltage, current, and rotational speed, without a field terminal voltage measurement or a dedicated flux sensor. The observer separates the flux-linkage-rate and back-EMF components induced by the azimuthal current as unknown components, and computes the azimuthal current from the d- and q-axis compensator outputs. The estimation performance is analyzed in terms of observer convergence, armature-field mutual inductance error, and inverter dead-time voltage disturbance. The results show that the d-axis-output-based and q-axis-output-based methods exhibit complementary error characteristics, and that the q-axis-output-based method maintains the accuracy of the mean estimate even under dead-time voltage disturbance.

This parametric sensitivity study examines the impact of key operating variables on the efficiency of a precooled Linde cycle for nitrogen liquefaction, with particular emphasis on refrigerant mass flow rate, condensing temperature, and cooling power. The investigation covers condensing temperatures from 20℃ to 45℃ and refrigerant mass flow rates from 0.1 to 0.9 g/s. Three refrigerants R290, R134a, and R410a are analyzed to evaluate their effects on the figure of merit (FOM) and the liquid yield (proportion of gas liquefied). Nitrogen is used as the permanent gas to benchmark system performance. The results show that lower condensing temperatures improve the liquid yield, especially at higher refrigerant mass flow rates, although they also increase the work required by the Linde compressor. Among the refrigerants tested, R290 consistently delivers the highest liquid yield and FOM (27-37%), despite requiring more auxiliary compressor work, due to its higher latent heat and greater cooling effect. R134a is the most energy-efficient in terms of compressor work but yields the lowest liquid production. The choice of refrigerant in the auxiliary cooling circuit is therefore a pivotal factor influencing the overall performance of the precooled Linde cycle. This research provides practical insights for optimizing operating parameters and contributes to the development of more efficient precooled Linde systems.

To ensure stable operation and high system reliability of high-temperature superconducting (HTS) rotating systems, researches on uniform thermal management under cryogenic conditions are actively progressing. Conventional solid conduction-based cooling structures using Stainless steel blocks transport heat solely by solid-state conduction. However, they are prone to generating localized temperature gradients due to the structural complexity of the coil, long heat transfer paths, and low thermal conductivity. Accordingly, to achieve more uniform cooling of the HTS coils, this paper proposes an STS bobbin-integrated heat pipe cooling channel for the HTS rotating system. The heat pipe utilizes the latent heat of the working fluid through a phase-change-driven evaporation-condensation cycle. Therefore, it has the advantage of much lower effective thermal resistance than the conventional solid conduction method, effectively suppressing temperature non-uniformity. To evaluate the thermal performance of the proposed cooling channel, three-dimensional numerical analysis and cryogenic similarity experiments using liquid nitrogen are conducted with a fabricated heat pipe specimen under various gravity orientations.