Search Results (274)

This paper presents a method for estimating moisture content (%MC) in power transformers. It primarily relies on the analysis of the statistical properties of relaxation times characterizing the dielectric frequency response (DFR), which is fitted as a sum of rational functions using the Vector Fitting (VF) tool. The DFR is modeled as a sum of Debye terms (accounting for materials exhibiting different relaxation times due to multiple polarization processes) characterized by poles and residues provided by VF. These parameters are then used to derive statistical factors that correlate with the shape of the dielectric response curve in the context of the Havriliak–Negami (HN) model, which is known for its effectiveness in characterizing materials with multiple relaxation times. By correlating the statistical factors with the HN model parameters, substantial insights into the insulation condition can be achieved. A moisture index (MI) is proposed from these parameters, which, when combined with conductivity, allows for accurate %MC estimation in the solid insulation system (cellulose). The combined MI and conductivity capture combined effects on moisture behavior, addressing both conductivity and polarization losses at different frequencies. The proposed method provides an efficient and straightforward non-invasive approach to insulation assessment without complex optimization algorithms. Experimental work on transformers at varying moisture levels provides validation of the proposed approach and demonstrates strong correlation with industry standards. The results confirm its reliability for moisture evaluation in transformer monitoring. Full article

To reveal the electromagnetic response characteristics and hydro-thermal evolution mechanism of frozen soil under microwave irradiation, we used remolded frozen soil prepared from undisturbed parent soil collected in Hegang, China, as the research object. We conducted dielectric parameter tests across the 715–1150 MHz and 2250–2650 MHz frequency bands and 1.5 kW microwave heating tests on specimens with three gravimetric water contents (15%, 20%, and 25%) paired with a coupled numerical simulation of electromagnetic field-heat transfer-moisture migration. The results show that water content is the dominant factor controlling the dielectric response of frozen soil. The dielectric loss and water content sensitivity of frozen soil in the low-frequency band (dominated by unfrozen water) are significantly higher than those in the high-frequency band (dominated by ice phase and soil matrix). Microwave-induced temperature rise exhibits a three-stage characteristic, as follows: slow temperature rise, isothermal plateau at the freezing point, and rapid temperature rise. Specimens with a lower initial water content show a higher temperature rise efficiency in the late heating stage, with a maximum rate of 1.112 °C·s⁻¹ for the 15% water content specimen. Mass loss is negatively correlated with initial water content, with a maximum value of 1.8 g after 120 s of irradiation. In addition, the non-uniformity of the electromagnetic field results in a temperature field pattern characterized by a high-temperature core at the specimen center and lower temperatures at the edges. This study provides fundamental theoretical support and technical guidance for the application of microwave thawing technology in geotechnical engineering, particularly for frozen soil foundation treatment in cold regions. Full article

Soluble solids content (SSC) is one of the most important indicators of sweetness, ripeness, and market quality in sweet cherries. However, conventional SSC determination is destructive, labor-intensive, and unsuitable for rapid or large-scale quality assessment. Therefore, there is a need for fast, non-destructive, and data-driven sensing approaches that can estimate internal fruit quality without damaging the sample. This study aimed to develop a non-destructive approach for SSC prediction in sweet cherries by combining open-ended coaxial probe dielectric spectroscopy with deep learning models. An open-ended coaxial probe measurement system was designed and developed to determine the dielectric properties of sweet cherries and was coupled with an Agilent E4991A impedance analyzer operating over a frequency range of 5–3005 MHz. A total of 10,080 dielectric measurements and 2100 reference SSC measurements were collected over 26 experimental days. The dielectric constant (ε′), loss factor (ε″), and loss tangent (tan δ) were extracted and used to construct separate ε′, ε″, tan δ, and integrated combined datasets. Six deep learning architectures, namely convolutional neural network (CNN), long short-term memory (LSTM), bidirectional long short-term memory (BiLSTM), gated recurrent unit (GRU), CNN-LSTM, and convolutional long short-term memory (ConvLSTM), were trained and optimized using Bayesian optimization and early stopping. CNN achieved the best performance on the tan δ dataset (test R² = 0.9099, RMSE = 0.8354 °Brix, MAE = 0.6599 °Brix), whereas GRU yielded the highest accuracy on the integrated combined dataset (test R² = 0.8622, RMSE = 1.0331 °Brix, MAE = 0.7958 °Brix). ConvLSTM provided the most consistent performance across all four datasets (test R² = 0.8081–0.8651), demonstrating strong predictive capability and practical computational efficiency. These findings confirm the potential of reduced-range dielectric spectroscopy combined with deep learning for rapid, non-destructive SSC assessment in sweet cherries. Full article

A lightweight and high-efficiency microwave-absorbing material was developed via an in situ solvothermal pyrolysis strategy by anchoring sphere-like Fe₃O₄ nanostructures onto bamboo-derived porous carbon (BPC). The resulting composites preserve the intrinsic anisotropic honeycomb architecture of bamboo while introducing uniformly distributed magnetic nanoparticles, enabling synergistic dielectric–magnetic loss. Electromagnetic parameters, alongside impedance matching, were successfully modulated through the optimization of precursor concentrations. Of the evaluated materials, BPC-0.9 stood out for its intense attenuation, recording an RL_min of −45.17 dB at a 1.8 mm thickness. Furthermore, a significant effective absorption bandwidth of 6.65 GHz was attained by the BPC-0.6 sample at only 2.2 mm. Several factors contribute to the boosted efficiency, starting with conductive and interfacial polarization losses paired with multiple scattering events. Furthermore, magnetic loss components, encompassing eddy current effects as well as natural and exchange resonances, play a pivotal role in optimizing the material’s response. Furthermore, radar cross-section (RCS) modeling reveals a substantial reduction of 19.9 dB·m², verifying the material’s viability for real-world stealth technologies. Our findings offer a straightforward methodology for fabricating magnetic carbon structures from biomass with adjustable dielectric responses, underscoring their potential in high-performance energy conversion and low-density microwave absorption. Full article

In this study, the sintering behavior and electrical properties of 0.7Pb(Zr_0.46Ti_0.54)O₃ (PZT)–0.1Pb(Zn₁/₃Nb₂/₃)O₃ (PZN)–0.2Pb(Ni₁/₃Nb₂/₃)O₃ (PNN) piezoelectric ceramics with different Pb(Fe₂/₃W₁/₃)O₃ (PFW) doping contents were investigated to obtain a formulation that can be co-fired with silver (Ag) electrodes below 900 °C for multilayer ceramics. PFW was introduced as a sintering aid, which effectively reduced the sintering temperature of the ceramics from 1200 °C to 850 °C. The sample with x = 0.12 exhibited the largest average grain size of 1.72 μm, achieving excellent comprehensive properties with piezoelectric constant (d₃₃) = 477 pC/N, planar electromechanical coupling factor (k_p) = 0.68, dielectric loss tangent (tanδ) = 0.0154, and relative density of 98.2%. Furthermore, the feasibility of fabricating piezoelectric actuators based on this optimized composition was verified. Multilayer piezoelectric devices were prepared via screen printing combined with a carbon-based sacrificial layer method. No obvious interdiffusion was observed at the interface between the Ag internal electrodes and the ceramic matrix. The 9-layer device attained a high d₃₃ = 1470 pC/N and produced a large displacement of 5.5 μm (corresponding to a strain = 1.83%) with a voltage of 500 V. The thickness of the multilayer piezoelectric film was approximately 0.3 mm. Through this, the feasibility of manufacturing a multilayered actuator with an Ag electrode was confirmed through the composition of 0.58PZT–0.1PZN–0.2PNN–0.12PFW. Full article

Detecting changes in the permittivities of materials has important applications in electronic information, materials science, biomedicine, and many other fields. However, existing detection methods are limited by factors such as sample thickness and resonance intensity, making it difficult to achieve sensitive dielectric constant detection at desired frequency bands. This paper proposes a method for detecting the dielectric constant changes in samples based on destructive interference of spoof surface plasmon polaritons (SSPPs) in a dual-path transmission structure, which forms a characteristic absorption peak at the SSPPs’ cutoff frequency. Specifically, by utilizing the dependence of the SSPPs’ phase on the periodic unit, a constant π phase difference is formed at the cutoff frequency through the periodic unit number difference between the two paths, resulting in a cutoff frequency absorption peak. When the sample is coated on the SSPPs’ dual-path structure, the boundary conditions are altered, leading to a cutoff frequency shift, thereby enabling dielectric constant detection at the specified frequency. Simulation results show that, with proper structural design, the normalized characteristic frequency shift reaches 10.8%/${\epsilon }_S$ and further demonstrates dramatic robustness against initial phase difference, sample thickness and sample loss. In summary, this work provides a novel high-precision and high-robustness method for detecting dielectric constant changes in samples at specified frequencies. Full article

With the advancement of computing and transmission technologies, there has been a growing demand for quartz oscillators and resonators, whose performance is evaluated by the quality coefficient (Figure of Merit, FoM). High-frequency, miniaturized fabrication is the design goal, and process optimization and innovative design methods need to be emphasized. Based on the 1978 Institute of Electrical and Electronics Engineers standard definition, the old design parameters of AT-cut quartz crystal sheet are retained to analyze the structural loss factor, dielectric loss factor, frequency, admittance, quality factor, and error value with the fundamental frequency increased from 76.8 to 96 MHz. In this study, COMSOL Multiphysics is used to simulate and analyze the quartz resonator by introducing the femtosecond laser quartz microvia machining technique from the literature, improving the electrode and inverted wet etching process, and incorporating the structural loss factor and dielectric loss factor into the quartz resonator model to observe the changes in the quality factor, the percentage of the quality factor error, and the values of the eigen-frequency and the error of the frequency. We analyze the trend of loss factor, frequency value, and error value and analyze the process advantages and disadvantages of femtosecond laser drilling electrodes, coated electrodes, inverted wet etching, inverted dry etching, and single-side and double-side etching to provide a reference for the design of future process components. Full article

Alumina is a commonly used ceramic material known for high permittivity, low dielectric loss, good thermal conductivity, and low cost. In the development of electronic devices, the size effect of powdery materials is crucial, particularly in applications involving composite materials. This study introduces the field-enhancement method (FEM) to measure the resonant frequency ($f_0$) and the quality factor (Q) of alumina powders packed in a Teflon container and placed on top of the central rod in the proposed cavity. The measured resonant condition ($f_0$ and Q) is mapped to a contour plot and simulated using a high-frequency structure simulator (HFSS). The contour mapping technique allows the researchers to obtain the effective complex permittivity of alumina–air composites. The complex permittivity of the alumina powder is retrieved using a hybrid model and the effective medium theories (EMTs), respectively. The Landau–Lifshitz–Looyenga (LLL) model is compared with the results using the hybrid model for its applicability. The dielectric constant and the loss tangent of the alumina powder are found to increase as the powder size reduces. A power relation is found to fit the obtained permittivity, covering sizes ranging from nanometers to micrometers, and a surface-charge scaling argument is proposed to explain the observed trend. This finding opens a new avenue for manipulation of permittivity in composite materials and has potential applications in stealth/absorber technology and as a self-limiter for grain growth during sintering. Full article

Interlayer bond strength is critical for ensuring the safety and durability of 3D-printed concrete (3DPC) structures. However, there remains a lack of real-time prediction methods addressing interlayer performance under the combined effects of interval time and environmental factors during the in situ printing process. To address this issue, this study conducted experiments considering various printing interval times and environmental conditions, incorporating monitoring of dielectric constant and water evaporation, alongside interlayer splitting tensile tests. By integrating the SHAP interpretability algorithm with nonlinear regression analysis, the results indicate that the printing interval time is the dominant factor inducing interlayer strength decay (with a contribution rate of 68.6%), while relative humidity emerges as the primary environmental variable (with a contribution rate of 21.3%). Mechanism analysis reveals that prolonged printing intervals intensify the hydration of the lower deposited layer, leading to reduced interfacial moisture content and loss of plasticity. Furthermore, environmental evaporation significantly regulates this process, with high-humidity environments notably mitigating the moisture loss and strength reduction caused by time delays. Based on the correlation mechanism between moisture and strength, a dimensionless general prediction model for 3DPC interlayer strength was established, incorporating printing interval time and an evaporation index (goodness of fit, R² = 0.96). Consequently, a digital twin quality inversion scheme based on companion specimen monitoring and printing timestamps was proposed. This study quantifies the intrinsic relationships among printing interval time, environmental conditions, and interlayer strength, offering a novel approach for determining the construction window and achieving non-destructive quality prediction for 3DPC in complex environments. Full article

This study investigates the combined effects of thermal and moisture aging on PVC-insulated low voltage (LV) photovoltaic (PV) cables using an accelerated-aging design to represent realistic PV operating conditions commonly encountered in hot and humid climates. Thermal aging was carried out at 90 °C for five aging cycles, with each thermal cycle followed by controlled moisture injection to simulate moisture stress. The degradation behavior was evaluated using broadband dielectric spectroscopy, FTIR analysis, and Shore D hardness measurements. Changes in dielectric dissipation factor (tanδ) and real permittivity (${\epsilon }^{\prime }$) were analyzed over a wide frequency range, with 100 kHz selected for its high sensitivity to aging-induced oxidation-related dipolar and interfacial polarization mechanisms. Degradation indices (DI) and degradation rates (DR) were derived from tanδ and correlated with mechanical and chemical changes. The results showed a 5% and 7% increase in tanδ at 100 kHz and in hardness, respectively, with decreases of 68% and 75% in the carbonyl and hydroxyl indices, respectively. Three distinct aging stages were identified: early thermo-oxidation with limited functional impact; mid-stage dehydrochlorination and moisture interaction; and late-stage chain scission, plasticizer loss, and insulation stiffening. The findings demonstrate the importance of climate-specific aging assessment and confirm the effectiveness of integrated electrical, mechanical, and chemical diagnostics for PV cable condition monitoring. Full article

This study focuses on the reliability issue of magnetic fluid (MF) in the magnetic fluid sealing technology for the upper guide bearing (UGB) of hydro-generators and proposes selection schemes for MF suitable for different models of hydro-generators. By analyzing the performance indicators of five base fluids and MFs, including the acid value, flash point, oxidation stability, magnetorheological performance, breakdown voltage, dielectric loss factor and volume resistivity, the influencing factors of the insulating performance of MFs and their mechanism in sealing the UGBs of hydro-generators are investigated. The results show that, when the spindle speed is below 27 rpm, the viscosity of the MF is dominated by the magnetic field strength, while, when the speed exceeds 27 rpm, the viscosity of the MF is dominated by the shear rate. In addition, the addition of magnetic nanoparticles (MNPs) causes the breakdown voltage of the base carrier liquid to fluctuate in the range of 31.2–55.9 kV, the dielectric loss factor to fluctuate in the range of 2.5 × 10⁻⁴–6.7 × 10⁻³, and the volume resistivity to fluctuate in the range of 2.8 × 10¹¹–2.6 × 10¹² Ω·m. The research results provide a theoretical basis for the application of high-efficiency and stable magnetic fluid sealing technology. Full article

To support the development and computer simulation of radio frequency (RF) and microwave (MW) thawing processes, this study characterized the dielectric properties and penetration depth of chicken breast across a frequency range of 10–3000 MHz and temperatures from −20 °C to 10 °C. The influence of three RF anode voltages and four MW power levels on heating rates was also evaluated. Results showed that both the dielectric constant and loss factor decreased with increasing frequency, with the most significant reduction occurring between 10 and 60 MHz. In contrast, these properties increased with temperature, exhibiting a sharp rise during the phase transition zone (−5 to 0 °C). Penetration depth decreased with frequency and was consistently higher under RF than MW exposure. High-precision regression models (R² > 0.97) were established to describe these relationships. RF heating achieved more uniform temperature distribution compared to MW, which showed pronounced center-corner temperature differences. By integrating experimental measurements with mathematical modeling, this work provides key insights and reliable data for optimizing RF and MW thawing strategies in industrial applications. Full article

This study presents the design of a compact low-index-contrast structure capable of trapping electromagnetic waves within a highly confined region. The energy storage performance of the geometries was enhanced using a genetic algorithm that employed a binary (present/absent) assignment of insulating cylinders. Time-domain simulations performed with MEEP were experimentally validated using precisely positioned $\theta$-phase alumina rods, resulting in a 13 dB increase in receiver-antenna power within the microwave regime. The measurements showed a correlation exceeding 99% with the computational results over a dielectric constant range of 4.5 to 5.0. A quality factor of Q $=3.6\times {10}^3$ was measured at 5.08 GHz in the experiment. Subsequently, machine learning techniques were applied, further increasing the Q value to approximately ${10}^4$, even within such a small configuration. Furthermore, the proposed structure does not require a complete photonic bandgap; instead, relatively high-Q factors were achieved by suppressing radiation losses through a pattern of low-index dielectric rods. Full article

Underwater wireless power transfer (UWPT) enables long-term deep-sea floor exploration by providing contactless energy replenishment for autonomous underwater vehicles (AUVs). However, conventional single-transmitter systems suffer reduced coupling and efficiency caused by high-loss underwater dielectrics and docking-induced perturbations. We propose a parallel-resonant dual-transmitter configuration based on the parity–time (PT) symmetric gain–loss-balanced modal framework. The proposed dual-transmitter single-receiver (DTSR) system forms a stronger and more symmetric field in the receiver than the single-transmitter baseline, counteracting the high-loss dielectric and improving the misalignment tolerance. According to the PT symmetry coupled-mode theory, we analyze how the quality factor and coupling strength determine the admissible PT-unbroken operating region over the docking-induced coupling range. An experimental prototype validates the analysis by comparing operating frequency and efficiency between DTSR and the single-transmitter baseline under distance (4.8–13.5 cm) and load (2.0–4.3 kΩ) variations. The results show that DTSR increases the critical coupling distance by 20–30% and reduces efficiency sensitivity to distance and load variations. These results suggest that the system can provide more robust and stable UWPT for AUV recharging under high-loss dielectric and perturbation, conducive to practically implementing AUV recharging in deep-sea operations. Full article

The insulation condition of oil-pressboard insulating bushings is commonly evaluated by measuring the dielectric loss factor and capacitance at power frequency. However, systematic investigations into the influence of aging and moisture defects on frequency-domain dielectric spectroscopy (FDS) characteristics are still insufficient. To address this issue, a 10 kV high-voltage FDS measurement system was independently developed. The system has an output voltage range of 0~10 kV and a test frequency band of 1 mHz~10 Hz, with excellent measurement stability and high test accuracy. The standard deviation of dielectric loss of the system is on the order of 10⁻⁴ and the relative error is less than 5%. It also features reliable weak current detection capability and thermal stability. Based on this system, the dielectric spectral characteristics of oil-pressboard insulation models with different moisture contents and aging levels were investigated under various temperatures and applied voltages. The results indicate that the dielectric spectrum shifts toward higher frequencies with increasing temperature. Moreover, the low-frequency dielectric loss of degraded insulation increases linearly with the applied voltage, and the rate of increase shows a positive correlation with both moisture content and aging duration. As insulation degradation becomes more severe, the voltage-dependent characteristic frequency moves toward higher frequencies. This frequency refers to the characteristic frequency where the dielectric loss of insulation presents an obvious linear variation with the change of applied voltage. Unaged and dry bushings exhibit only weak voltage dependence at 0.01 Hz, whereas bushings aged for 28 days with a moisture content of 4.121% demonstrate pronounced voltage dependence at 10 Hz. These results provide a valuable technical basis for diagnosing coupled aging and moisture defects in oil-pressboard insulated bushings. Full article