Search Results (210)

This paper presents the design and implementation of a high-efficiency, full silicon carbide (SiC)-based center-tapped phase-shifted full-bridge (PSFB) converter for NiCd battery charging applications in railway systems. The converter utilizes SiC MOSFET modules on the primary side and SiC diodes on the secondary side, resulting in significant efficiency improvements due to the superior switching characteristics and high-temperature tolerance inherent in SiC devices. A nanocrystalline-cored center-tapped transformer is optimized to minimize voltage stress on the rectifier diodes. Additionally, the use of a nanocrystalline core provides high saturation flux density, low core loss, and excellent permeability, particularly at high frequencies, which significantly enhances system efficiency. The converter also compensates for temperature fluctuations during operation, enabling a wide and adjustable output voltage range according to the temperature differences. A prototype of the 10-kW, 50-kHz PSFB converter, operating with an input voltage range of 700–750 V and output voltage of 77–138 V, was developed and tested both through simulations and experimentally. The converter achieved a maximum efficiency of 97% and demonstrated a high power density of 2.23 kW/L, thereby validating the effectiveness of the proposed design for railway battery charging applications. Full article

Multilayer thin films composed of cobalt (Co), carbon (C), and chromium (Cr) possess promising electromagnetic properties, yet the combined Co–C–Cr system remains underexplored, particularly regarding its performance as a microwave absorber. Existing research has primarily focused on binary Co–C or Co–Cr compositions, leaving a critical knowledge gap in understanding how ternary multilayer architectures influence electromagnetic behavior. This study addresses this gap by investigating the structure, phase composition, and microwave absorption performance of Co–C–Cr multilayer coatings fabricated via magnetron sputtering onto porous silicon substrates. This study compares four-layer and eight-layer configurations to assess how multilayer architecture affects impedance matching, reflection coefficients, and absorption characteristics within the 8.2–12.4 GHz frequency range. Structural analyses using X-ray diffraction and transmission electron microscopy confirm the coexistence of amorphous and nanocrystalline phases, which enhance absorption through dielectric and magnetic loss mechanisms. Both experimental and simulated results show that increasing the number of layers improves impedance gradients and broadens the operational bandwidth. The eight-layer coatings demonstrate a more uniform absorption response, while four-layer structures exhibit sharper resonant minima. These findings advance the understanding of ternary multilayer systems and contribute to the development of frequency-selective surfaces and broadband microwave shielding materials. Full article

In order to break through the difficulties with a very-low-frequency (VLF) miniaturized antenna with small power capacity and low radiation efficiency, this paper proposes a high-radiation-field-strength magnetic loop antenna based on a nanocrystalline alloy magnetic core. A high-permeability nanocrystalline toroidal core (μ_r = 50,000, B_s = 1.2 T) is used to optimize the thickness-to-diameter ratio (t = 0.08) and increase the effective permeability to 11,000. The Leeds wires, characterized by their substantial carrying capacity, are manufactured through a toroidal winding process. This method results in a 68% reduction in leakage compared to traditional radial winding techniques and enhances magnetic induction strength by a factor of 1.5. Additionally, this approach effectively minimizes losses, thereby facilitating support for kilowatt-level power inputs. A cascaded LC resonant network (resonant capacitance 2.3 μF) and ferrite balun transformer (power capacity 3.37 kW) realize a 20-times amplification of the input current. A series connection of a high-voltage isolation capacitor blocks DC bias noise, guaranteeing the stable transmission of 1200 W power, which is 6 times higher than the power capacity of traditional ring antenna. At 7.8 kHz frequency, the magnetic field strength at 120 m reaches 47.32 dBμA/m, and, if 0.16 pT is used as the threshold, the communication distance can reach 1446 m, which is significantly better than the traditional solution. This design marks the first instance of achieving kilowatt-class VLF effective radiation in a compact 51 cm-diameter magnetic loop antenna, offering a highly efficient solution for applications such as mine communication and geological exploration. Full article

This study investigates the effects of molybdenum (Mo) additions on the crystallization behavior and soft magnetic properties and of Fe_82-xSi₄B₁₂Nb₁Mo_xCu₁ (x = 0–2) nanocrystalline alloys. Molybdenum enhances glass-forming ability (GFA) and magnetic properties by increasing negative mixing enthalpy ($∆{\mathrm{H}}_{\mathrm{m}\mathrm{i}\mathrm{x}}$), mixing entropy ($∆{\mathrm{S}}_{\mathrm{m}\mathrm{i}\mathrm{x}}$), and atomic size mismatch ($\delta$), which stabilize the amorphous phase. X-ray diffraction (XRD) analysis shows that Mo addition improves amorphous phase stability, further enhancing GFA. The simultaneous addition of Mo and Nb increases mixing entropy, promotes nucleation rates, and creates favorable conditions for optimizing nanocrystallization. Upon annealing, this optimized microstructure demonstrated low coercivity and high permeability. Notably, the Fe₈₀Si₄B₁₂Nb₁Mo₂Cu₁ ribbon, annealed at 470 °C for 10 min, exhibited exceptional soft magnetic properties, with a coercivity of 4.54 A/m, a maximum relative permeability of 48,410, and a saturation magnetization of 175.24 emu/g. High-resolution transmission electron microscopy (TEM) revealed an average crystal size of 18.16 nm. These findings suggest that Fe_82-xSi₄B₁₂Nb₁Mo_xCu₁ (x = 0–2) nanocrystalline alloys are suitable for advanced electromagnetic applications pursuing miniaturization and high efficiency. Full article

High-Cu-content (Cu-content > 1.3 at.%) nanocrystalline alloys exhibit wide heat-treatment windows and favorable soft magnetic properties due to the presence of pre-existing α-Fe nanocrystals. By fabricating ribbons with varying thicknesses to tailor cooling rates, distinct structural characteristics were achieved in Fe₈₂B_16.5Cu_1.5 alloy ribbons. Notably, the face-centered cubic (fcc) γ-Fe phase was identified in Fe-based nanocrystalline alloys. The precipitation of the fcc γ-Fe phase originates from a phase-selection mechanism under specific cooling conditions, while its retention in the as-quenched ribbon with a thickness of 27 μm is attributed to kinetic suppression during rapid cooling and the nanoscale stabilization effect. The formation of the fcc γ-Fe phase significantly reduced the saturation flux density (B_s) and increased coercivity (H_c), concurrently destabilizing the residual amorphous matrix. By suppressing the precipitation of the γ-Fe and Fe₃B phases through precise control of ribbon thickness and annealing parameters, the alloy ribbon with a thickness of 16 μm achieved an optimal combination of B_s (1.82 T) and H_c (8.3 A/m). These findings on anomalous fcc γ-Fe phase precipitation provide novel insights into metastable phase engineering and offer structural design guidelines for alloys containing pre-existing α-Fe nanocrystals. Full article

We studied the influence of annealing on the magnetic properties and microstructure of ultrathin (metallic nucleus diameter ≈ 5 μm, total diameter ≈ 19 μm) Heusler-type NiMnGa glass-coated microwires prepared using the Taylor–Ulitovsky method. The as-prepared NiMnGa microwires exhibit unexpectedly strong magnetic anisotropy, characterized by a coercivity exceeding 3 kOe at room temperature. Furthermore, their Curie temperature (T_c) lies above room temperature. Additionally, a spontaneous exchange bias of approximately 120 Oe is observed in the as-prepared sample at 100 K. Annealing the microwires leads to a decrease in coercivity, spontaneous exchange bias, and T_c values. Notably, the annealing process shifts the T_c of the samples closer to room temperature, making them more suitable for magnetic solid-state refrigeration applications. Moreover, the hysteresis observed in the temperature dependence of magnetization for the samples annealed for 1 h and 2 h, along with the magnetic softening observed at around 260 K, is attributed to a first-order phase transformation. The observed changes are discussed in the context of internal stress relaxation after annealing, the nanocrystalline structure of both the as-prepared and annealed samples, the recrystallization process, and the magnetic ordering of phases identified in the as-prepared sample and those appearing during recrystallization. The glass coating on microwires offers benefits like better flexibility and resistance to damage and corrosion. However, it is important to recognize that this coating can substantially alter the microwires’ magnetic characteristics. Consequently, precise control over the annealing process is vital to obtain the specific martensitic transformation needed. Full article

This work demonstrates a modeling technique focused on reproducing the behavior of magnetic cores subject to high voltage pulses. The working principle of the model is based on a magnetic circuit with additional elements that influence the model’s behavior. The elements include a function that defines the response of the model depending on the applied pulse voltage and a component that dominates the transient response. These elements are necessary to replicate the experimentally observed behavior of magnetic cores. The model was developed based on the measured behavior of three nanocrystalline magnetic materials subject to a range of pulse voltages. This modeling technique was created to address the limitations of other models in accurately capturing fast pulse responses. The key limitation of traditional modeling techniques that the proposed model addresses is their inability to capture variations in core response under different applied pulse voltages (magnetization rates). The proposed model has been shown to produce accurate results for magnetization rates between 1 T/μs and 8 T/μs, with potential for further expansion. Implemented in LTspice, this model is both fast and accurate, effectively replicating the behavior of the magnetic core while maintaining simplicity. This work outlines the foundation of this modeling technique, the trends in the parameters that influence its behavior, and its application within a simple pulsed power system. The most notable feature of this model is its ability to operate across a wide range of pulse voltages without requiring adjustments to the model parameters. Full article

Manganese oxides (manganites) are among the most studied materials in condensed matter physics due to the famous colossal magnetoresistance and very rich phase diagrams characterized by strong competition between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating phases. One of the key questions that remains open even after more than thirty years of intensive research is the exact conductivity mechanism in insulating as well as in metallic phases and its relation to the corresponding magnetic structure. In order to shed more light on this problem, here, we report magnetotransport measurements on sintered nanocrystalline samples of the very poorly explored manganites ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{MnO}}_3$ with $x=0.05$ and $x=0.10$, in the temperature range 2–300 K, and in magnetic fields up to 16 T. Our results indicate that both compounds at low temperatures exhibit metallic behavior with a peculiar resistivity upturn and a large negative magnetoresistance. We argue that such behavior is consistent with a Kondo-like scattering on Gd impurities coupled with the percolation of FM metallic regions within insulating AFM matrix. Full article

Iron core loss (P_cm) is the main source of energy dissipation in motors, primarily affected by the stator material, which necessitates the optimization of soft-magnetic materials. In this work, the magnetic characteristics of FeSiB amorphous alloys and their influence on motors were systematically investigated via both experiment and finite-element simulation. It was found that the P_cm of the FeSiB core initially decreased significantly during heating but subsequently increased with a further temperature rise. In particular, after annealing at 460 °C for 10 min, the FeSiB core exhibited the lowest P_cm of 0.11 W/kg (50 Hz, 1 T) and 5.45 W/kg (1 kHz, 1 T), which correlated well with the changes in the magnetization. With the help of the finite-element analysis, the low P_cm of the motor using the FeSiB core was further demonstrated, and was closely associated with the dominance of the stator loss. Additionally, the magnetic flux density cloud and the related electromagnetic torque of the motor were comparatively analyzed to unveil the potential advantages of the current FeSiB core. This work provides an important theoretical basis for the design and development of amorphous/nanocrystalline motors. Full article

The physical stability, molecular mobility, and appearance of nanocrystalline resveratrol in a polyvinylpyrrolidone (PVP) matrix were investigated. Two formulations with resveratrol loadings of 30% and 50% were prepared and characterized using powder X-ray diffraction (PXRD) and time-domain nuclear magnetic resonance (TD-NMR). Samples were studied over time (up to 300 days post-preparation), across temperatures (80–300 K), and under varying humidity conditions (0% and 75% relative humidity). The results demonstrate that the 30% resveratrol–PVP sample is a homogeneous amorphous solid dispersion (ASD), while the 50% resveratrol–PVP sample contained resveratrol nanocrystals measuring about 40 nm. NMR measurements and molecular dynamics (MD) simulations revealed that incorporation of resveratrol into the polymer matrix modifies the system’s dynamics and mobility compared to the pure PVP polymer. Additionally, MD simulations analyzed the hydrogen bonding network within the system, providing insights for a better understanding of the physical stability of the ASD under different conditions. Full article

Medium-entropy alloys (MEAs) exhibit properties comparable or even superior to high-entropy alloys (HEAs). Due to their very good resistance in thermomechanical conditions and corrosive environments and unique electrical and magnetic properties, medium-entropy alloys are good candidates for coating applications. One of the most economically effective methods of producing metallic coatings is electrodeposition. In this work, the structure of an electrodeposited CoFeNi medium-entropy alloy coating on a copper substrate from a metal chlorides solution (FeCl₂ ∙ 4H₂O + CoCl₂ ∙ 6H₂O + NiCl₂ ∙ 6H₂O) with the addition of boric acid (H₃BO₃) was investigated. The coating was characterized by a nanocrystalline structure identified by transmission electron microscopy examination and X-ray diffraction methods. Based on XRD and TEM, the face-centered cubic (FCC) phase of the CoFeNi MEA coating was identified. The high corrosion resistance of the MEA coating in a 3.5% NaCl environment at 25 °C was confirmed by electrochemical tests. Full article

Magnetocrystalline anisotropy has many advantages over shape anisotropy regarding coercivity in permanent magnets, making it a promising approach to enhance the coercivity of AlNiCo magnets. In this work, AlNiCo magnets with NdFeB-nanocrystalline phase were prepared by spark plasma sintering (SPS), and the effect of the NdFeB phase on coercivity was uncovered. AlNiCo powder with a spinodal structure and NdFeB powder with a nanocrystalline structure, which exhibit shape anisotropy and magnetocrystalline anisotropy, respectively, were sintered by SPS. With the advantages of low-temperature densification achieved by the SPS process, the spinodal and nanocrystalline structures were mostly retained. The microstructure analysis indicated that the SPS-ed magnet primarily consisted of AlNiCo regions with a spinodal structure, NdFeB regions with a nanocrystalline structure, and a transition region approximately 1~7 µm wide between them. A significant effect of the magnetic anisotropy of the NdFeB phase on magnetization behavior was found. The hysteresis loop of the SPS-ed magnets became single-phase magnetization, in contrast with the double-phase magnetization observed in the simple mixed powder. As the magnetocrystalline anisotropy of the NdFeB phase possesses higher coercivity, the coercivity of the SPS-ed magnet increased from 1250 Oe (of the AlNiCo raw powder) to 2490 Oe. This work provides valuable information for the coercivity enhancement of AlNiCo magnets. Full article

FeSiBCuNb powders, produced via the gas–water atomization method, typically exhibit a broad particle size distribution and high sphericity. Nanocrystalline soft magnetic composites derived from these powders demonstrate exceptional service stability. In this study, a series of FeSiBCuNb/FeNi nanocrystalline magnetic powder cores (NMPCs) were fabricated under varying compaction pressures and heat treatment temperatures. The effects of these parameters on the soft magnetic properties were systematically analyzed. The findings reveal that optimizing compaction pressure and heat treatment temperature significantly enhances the density of the composite powders, leading to improved magnetic permeability and reduced core loss; when compaction pressure is 1800 MPa and heat treatment temperature is 550 °C, the NMPCs display outstanding magnetic properties with a low H_c of 6.32 Oe, high μ_e of 71.9, a low P_cv of 86.3 kW/m³ at 50 mT and 100 kHz, and 351.5 kW/m³ at 20 mT and 1000 kHz. Therefore, tailoring these processing conditions can enhance the soft magnetic performance of FeSiBCuNb nanocrystalline composites. Full article

Nanocrystalline powders, characterized by a biphasic amorphous nanocrystalline structure, demonstrate outstanding soft magnetic characteristics, including reduced coercivity (H_c), enhanced effective permeability (μ_e), and increased resistivity. However, their high hardness, poor formability, and significant core loss (P_cv) restrict their use in high-performance molded inductors. In this study, FeSiBCuNb/FeSiAl nanocrystalline soft magnetic composites (NSMCs) were fabricated, and the influence of varying the FeSiAl concentration on the microstructure, density, and soft magnetic characteristics of NSMCs was investigated. Then, the underlying mechanisms of these effects were explained. The results demonstrate that FeSiAl exhibits apparent deformation following compression, effectively filling the air gap between the FeSiBCuNb powder particles, thereby enhancing coupling among the magnetic particles. Consequently, the density of the NSMCs was enhanced, leading to a significant improvement in their overall soft magnetic properties. When 50 wt.% FeSiAl is added, the NSMCs display outstanding magnetic properties, including a low H_c of 4.36 Oe, a high μ_e of 48.7, a low P_cv of 119.35 kW/m³ at 50 mT and 100 kHz, and a high DC-bias performance of 73.29% at 100 Oe. Compared to NSMCs without FeSiAl, μ_e increased by 59.4% and P_cv decreased by 66.1%. Meanwhile, the incorporation of ultrafine FeSiAl powder was found to significantly improve the material properties, as the deformable FeSiAl particles effectively fill interparticle gaps during compaction, enhancing density and magnetic coupling. The 50 wt.% FeSiAl composition demonstrated exceptional properties. These advances address critical challenges in high-frequency power electronic applications and provide a practical material solution for next-generation power electronics. Full article

Fe-based amorphous and nanocrystalline alloys, such as FINEMET and its improved variants, are highly valued as green energy-saving materials due to their unique magnetic properties, including high permeability, low coercivity, and near-zero saturation magnetostriction. These characteristics have enabled their extensive use in power electronics and information technology. However, the full potential of these alloys remains unfulfilled due to insufficient understanding of their stress sensitivity. This study focuses on the development history, heat treatment, annealing processes, chemical composition, and underlying mechanisms of Fe-based amorphous and nanocrystalline alloys, aiming to provide insights into stress-induced magnetic anisotropy and guide the development of greener and more efficient soft magnetic materials. Full article