Search Results (632)

Benzoxazine resins are gaining attention for their impressive thermal stability, low water uptake, and strong mechanical properties. In this work, two new bio-based benzoxazine monomers were developed using renewable arbutin: one combined with 3-(2-aminoethylamino) propyltrimethoxysilane (AB), and the other with furfurylamine (AF). Both were synthesized using a simple Mannich-type reaction and verified through FT-IR and ¹H-NMR spectroscopy. By blending these monomers in different ratios, copolymers with adjustable thermal, dielectric, and surface characteristics were produced. Thermal analysis showed that the materials had broad processing windows and cured effectively, while thermogravimetric testing confirmed excellent heat resistance—especially in AF-rich blends, which left behind more char. The structural changes obtained during curing process were monitored using FT-IR, and XPS verified the presence of key elements like carbon, oxygen, nitrogen, and silicon. SEM imaging revealed that AB-based materials had smoother surfaces, while AF-based ones were rougher; the copolymers fell in between. Dielectric testing showed that increasing AF content raised both permittivity and loss, and contact angle measurements confirmed that surfaces ranged from water-repellent (AB) to water-attracting (AF). Overall, these biopolymers (AB/AF copolymers) synthesized from arbutin combine environmental sustainability with customizability, making them strong candidates for use in electronics, protective coatings, and flame-resistant composite materials. Full article

The swift advancement of fifth-generation (5G) wireless technology brings forth a range of enhancements to address the increasing demand for data, the proliferation of smart devices, and the growth of the Internet of Things (IoT). This highly interconnected communication environment necessitates using multiple-input multiple-output (MIMO) systems to achieve adequate channel capacity. In this article, a 2-port MIMO system using two flipped parallel 1 × 2 arrays and a 2-port MIMO system using two opposite 1 × 4 arrays designed and fabricated antennas for 5G wireless communication in the sub-6 GHz band, are presented, overcoming the limitations of previous designs in gain, radiation efficiency and MIMO performance. The designed and fabricated single-element antenna features a circular microstrip patch design based on ROGER 5880 (RT5880) substrate, which has a thickness of 1.57 mm, a permittivity of 2.2, and a tangential loss of 0.0009. The 2-port MIMO of two 1 × 2 arrays and the 2-port MIMO of two 1 × 4 arrays have overall dimensions of 132 × 66 × 1.57 mm³ and 140 × 132 × 1.57 mm³, respectively. The MIMO of two 1 × 2 arrays and MIMO of two 1 × 4 arrays encompass maximum gains of 8.3 dBi and 10.9 dBi, respectively, with maximum radiation efficiency reaching 95% and 97.46%. High MIMO performance outcomes are observed for both the MIMO of two 1 × 2 arrays and the MIMO of two 1 × 4 arrays, with the channel capacity loss (CCL) ˂ 0.4 bit/s/Hz and ˂0.3 bit/s/Hz, respectively, an envelope correlation coefficient (ECC) ˂ 0.006 and ˂0.003, respectively, directivity gain (DG) about 10 dB, and a total active reflection coefficient (TARC) under −10 dB, ensuring impedance matching and effective mutual coupling among neighboring parameters, which confirms their effectiveness for 5G applications. The three fabricated antennas were experimentally tested and implemented using the MIMO Application Framework version 19.5 for 5G systems, demonstrating operational effectiveness in 5G applications. Full article

Linear passive electrical devices/components are usually characterized in the frequency domain by their impedance, i.e., the ratio of the voltage and current phasors. The use of the impedance concept does not raise particular concerns in low-frequency regimes; however, things become more complicated when it comes to rapid time-varying phenomena, mainly because the voltage depends not only on the position of the points between which it is defined but also on the choice of the integration path that connects them. In this article, based on first principles (Maxwell equations and Poynting vector flow considerations), we discuss the concept of impedance and define it unequivocally for a class of electrical devices/components with rotational symmetry. Two application examples are presented and discussed. One simple example concerns the per-unit-length impedance of a homogeneous cylindrical wire subject to the skin effect. The other, which is more elaborate, concerns a heterogeneous structure that consists of a dielectric disk sandwiched between two metal plates. For the lossless situation, the high-frequency impedance of this device (circular parallel plate capacitor) reaches zero when the frequency reaches a certain critical frequency f_c; then, it becomes inductive and increases enormously when the frequency reaches another critical frequency at 1.6 f_c. The influence of losses on the impedance of the device is thoroughly investigated and evaluated. Impedance corrections due to dielectric losses are analyzed using a frequency-dependent Debye permittivity model. The impedance corrections due to plate losses are analyzed by considering radial current distributions on the outer and inner surfaces of the plates, the latter exhibiting significant variations near the critical frequencies of the device. Full article

Metallic magnetic materials are extensively used to mitigate electromagnetic interference due to their high Curie temperatures and permeability. However, their high permittivity often hinders impedance-matching effectiveness, limiting their utility. In this study, amorphous cobalt–iron (Co_100−xFe_x) alloy nanoparticles with relatively low permittivity were synthesized using a simple aqueous reduction method at room temperature. The effect of atomic ratio variation on the microwave absorption properties of these nanoparticles was investigated across 2–18 GHz. The amorphous Co_100−xFe_x nanoparticles exhibited excellent electromagnetic wave absorption performance, achieving an effective absorption bandwidth of 5.6 GHz, a matching thickness of 2.60 mm, and a reflection loss of −42 dB. Full article

Microwave imaging systems show potential as replacements for commonly used stroke diagnostic systems. We developed and tested a 10-port microwave system on a liquid head phantom with ischemic and hemorrhagic strokes of varying sizes and positions. This system allows for visualization of changes in dielectric parameters using the TSVD Born approximation, enabling recognition of stroke position and size from the resulting images. The SVM algorithm effectively distinguishes between ischemic and hemorrhagic strokes, achieving 98% accuracy on experimental data, with 99% accuracy in ischemic scenarios and 97% in hemorrhagic scenarios. Using the TSVD Born algorithm, it was possible to precisely image changes in the absolute permittivity of different stroke locations; however, changes in stroke size were more apparent in the variations of absolute permittivity than in the reconstructed stroke size within the antenna plane. Outside this plane, changes in the S-parameters decreased depending on the distance and size of the stroke, making detection and classification more difficult. One ring of antennas around the head proved insufficient, prompting us to focus on developing a system with antennas positioned around the entire head. Full article

The paper describes the simulation of energy absorption in polymer micro-composites that include dielectric inserts (commercial Al₂O₃ and TiO₂ particles, with three particle sizes of 1, 5 and 25 µm, respectively). The investigated frequency spectrum, mainly from 0.001 to 100 GHz, is designed for various uses as substrates in electronic technologies. The electromagnetic simulation software chosen was CST Studio Suite, which evaluates the power loss at different frequencies, playing a crucial role in creating the ideal structure of these substrates. The effective limits of the electromagnetic simulation are specified. It is shown that a considerable increase in absorption occurs, by a factor of 12 to 120, depending on the dielectric material used for the inserts and the mass ratio applied in the insertion technique. Dielectrics with high permittivity provide higher absorption, but also create a nonuniform field distribution within the material, resulting in a high peak-to-average absorption ratio. In scenarios where this behavior is intolerable, the technology must be carefully tuned to improve the consistency of the insertions in the substrate material. The final outcomes of the simulations indicated that for creating new substrates for flexible electronics, polyethylene composites with TiO₂ insertions are suggested, particularly at lower concentrations of up to 7% and with a larger radius, such as 25 μm, which could offer significant economic advantages considering that the current concept advises the use of costly particles ranging from nanoscale particles to those 1 μm in size and a composition exceeding 10%. Full article

Power transformer insulation systems, composed of liquid and solid insulators, are continuously exposed to thermal and electrical stresses that degrade their performance over time and may lead to premature failure. Since these stresses are unavoidable during operation, selecting effective insulating materials is critical for long-term reliability. In this study, Kraft insulation paper was used as the solid insulator and impregnated with three different liquids: mineral oil and two natural esters (NE1204 and NE1215), to evaluate their stability under simultaneous thermal and electrical stress. The degradation behavior of the oil-impregnated papers was assessed using frequency-domain dielectric spectroscopy (FDS) and Fourier-transform infrared spectroscopy (FTIR), enabling early fault detection. Comparative analyses were conducted to evaluate the withstand capability of each liquid type during operation. Results revealed strong correlations between FTIR indicators (e.g., oxidation and hydroxyl group loss) and dielectric parameters (permittivity and loss factor), confirming the effectiveness of this combined diagnostic approach. Post-aging breakdown analysis showed that natural esters, particularly NE1215, offered superior preservation of insulation integrity compared to mineral oil. Differences between the two esters also highlight the role of chemical composition in insulation performance. This study reinforces the potential of natural esters as viable, eco-friendly alternatives in thermally and electrically stressed applications. Full article

Low-permittivity microwave dielectric ceramics are essential for high-frequency communication and radar systems, as they minimize signal delay and interference, thereby enabling compact and high-performance devices. In this study, LiAl_5−xNi_xO_8−0.5x (x = 0.1–0.5) ceramics were synthesized via a solid-state reaction method to investigate the effects of Ni²⁺ substitution on crystal structure, microstructure, and dielectric properties. X-ray diffraction and Rietveld refinement reveal a phase transition from the P4₃32 to the Fd$\stackrel{-}{3}$m spinel structure at x ≈ 0.3, accompanied by a systematic increase in the lattice parameter (7.909–7.975 Å), attributed to the larger ionic radius of Ni²⁺ compared to Al³⁺. SEM analysis confirms dense microstructures with relative densities exceeding 95% and grain size increases from less than 1 μm at x = 0.1 to approximately 2 μm at x = 0.5. Dielectric measurements show a decrease in permittivity (ε_r) from 8.24 to 7.77 and in quality factor (Q × f) from 34,605 GHz to 20,529 GHz with increasing Ni content, while the temperature coefficient of the resonant frequency (τ_f) shifts negatively from −44.8 to −69.1 ppm/°C. Impedance spectroscopy indicates increased conduction losses and reduced activation energy with higher Ni²⁺ concentrations. Full article

Slight changes in the local properties of a transmission line, dipped in a liquid, can be used to estimate its level through two different determination techniques, involving the capacitance and electromagnetic wave speed, measured by the time of flight. Indeed, the overall capacitance of a transmission line varies linearly with the liquid level, as well as the time of flight of the electromagnetic wave. Both quantities can be estimated via the measurement of a phase shift at radio frequencies, and the simultaneous measurements can be realized using a compact and low-cost design working at a few megahertz. This paper presents a further improvement in sensitivity to challenge the performance of this kind of level sensor, dealing with liquids with low dielectric constants. To better describe this effect, a study on the overall capacitance of different transmission path segments was conducted in COMSOL Multiphysics. The level measurement was performed experimentally on the realized prototype while considering the measured phase shift as a function of the liquid level, for both an unshielded twisted-pair and magnet wires. As the results showed, with the magnet wires the sensitivity was improved by a factor of about 4, consistently aligning with the simulation results and providing a predictable phase shift response with increasing liquid levels. Consequently, magnet wire is a good choice for precise level measurements through RF phase shifts, especially in the case of low relative permittivity liquids. Full article

This study explores microcomposites’ thermodielectric properties—thermal conductivity ($k_{\mathrm{eff}}$) and dielectric permittivity (${\epsilon }_r$)—across filler concentrations from 1 wt% ($\phi \approx 0.0035$) to 60 wt% ($\phi \approx 0.45$) spanning the pre- ($\phi <0.16$) and within-percolation threshold ($0.16\le \phi \le 0.29$). Thermal measurements were conducted using a newly designed, cost-effective thermal measurement setup. The setup utilised a transient heat pulse methodology with a heater and NTC thermistors, with a precision better than $\pm 0.01$ W·${\mathrm{m}}^{-1}·$${\mathrm{K}}^{-1}$. Dielectric properties were measured using a three-electrode system over a broad frequency and temperature range. The measurements demonstrate an effective thermal conductivity $k_{\mathrm{eff}}$ of 0.72 W·${\mathrm{m}}^{-1}·$${\mathrm{K}}^{-1}$ for AlN at $\phi =0.36$ and 0.65 W·${\mathrm{m}}^{-1}·$${\mathrm{K}}^{-1}$ for wBN already at $\phi =0.12$. Although theoretical models suggest that, considering interfacial Kapitza resistance, it can yield a $k_{\mathrm{eff}}$ corresponding to approximately 1–3% of the conductivity of pure material filler, the experimental measurements indicate a maximum of around 0.5%. Dielectric measurements show that in comparison to pure polyurethane, the presence of 60% AlN or 40% wBN at 60 °C decreased the loss tangent by 20 times in the condition of a quasistatic electric field. Full article

The effect of strontium substitution on the crystal tructure, as well as the magnetic, and electrical properties of lanthanum ferrite (LaFeO₃) synthesized by high-energy ball milling, is studied, with an emphasis on magnetodielectric coupling. X-ray diffraction (XRD) confirmed the successful synthesis of orthorhombic La_1−xSr_xFeO₃ for doping levels up to 0.2 mol. At 0.3 mol Sr²⁺, two phases appear: La_0.6Sr_0.4FeO_2.976 and La_0.8Sr_1.2FeO_3.714, the latter being metastable. This phase vanishes at 0.5 mol. The Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS) analysis confirmed these results using a vibrating sample magnetometer (VSM), whose measurements show ferromagnetism at 0.1 and 0.3 mol Sr²⁺, attributed to crystal distortion, magnetic spin rearrangement, and as consequence, modifications in the double-exchange interactions. Dielectric tests reveal that higher Sr²⁺ concentrations lead to increased relative permittivity, dielectric losses, and conductivity, linked to oxygen vacancy formation. This study demonstrates a room-temperature magnetodielectric coupling of 32% in Sr-doped lanthanum ferrite, highlighting its potential for technological applications. Full article

This paper presents a validated, cost-effective technique for the wideband characterization of low-height substrate materials in RF circuits. The method utilizes traditional resonant structures to accurately determine essential parameters—relative permittivity and loss tangent—and delivers reliable results even in the presence of unavoidable fabrication imperfections, thereby enhancing its reliability for RF designers. While it covers a broad frequency range (8–16 GHz), the proposed technique can be adapted for other ranges by modifying the resonant structure dimensions. By combining reflection coefficient and input impedance measurements of a multimode patch antenna, substrate properties are accurately extracted using an iterative numerical fitting process, i.e., secant algorithm. This approach provides RF designers with the precise material data necessary to enhance circuit performance and is especially useful for low-height substrates. The technique’s validity is demonstrated through excellent agreement between simulations and measurements, establishing that the technique provides a practical, solution for industrial and research applications. Full article

To address the challenge of assessing the thickness uniformity of epoxy asphalt layers on steel bridge decks, three-dimensional ground-penetrating radar (3D-GPR) was employed for non-destructive, full cross-sectional detection of the pavement layer’s thickness. The antenna array spacing was optimized using the common midpoint (CMP) method, enabling precise measurement of the relative permittivity of epoxy asphalt mixtures. A significant correlation between relative permittivity and the void ratio was established, providing a novel approach to identifying areas prone to coarse segregation and early-stage water damage. Grayscale maps of the thickness distribution enabled precise detection of regions with acceptable, under-thickness and over-thickness values. The uniformity of construction thickness was quantitatively evaluated using standard deviations and coefficients of variation. Results indicated that when the coefficient exceeds 12%, improvements in the pavement construction process are necessary. This research demonstrates the capability of 3D-GPR to effectively detect thickness variations, offering a valuable tool for enhancing pavement paving and compaction practices on steel bridge decks. Full article

In this work, we propose and analyze a thermally tunable metasurface based on indium antimonide (InSb), designed to operate in the terahertz (THz) frequency range. The metasurface exhibits dual functionalities: single-band perfect absorption and efficient polarization conversion, enabled by the temperature-dependent permittivity of InSb. At approximately 280 K, InSb transitions into a metallic state, enabling the metasurface to achieve near-unity absorptance (100%) at 0.408 THz under normal incidence, independent of polarization. Conversely, when InSb behaves as a dielectric at 200 K, the metasurface operates as an efficient polarization converter. By exploiting structural anisotropy, it achieves a polarization conversion ratio exceeding 85% over the frequency range from 0.56 to 0.93 THz, while maintaining stable performance for incident angles up to 45°. Parametric analyses show that the resonance frequency and absorption intensity can be effectively tuned by varying the InSb square size and the silica (SiO₂) layer thickness, achieving maximum absorptance at a SiO₂ thickness of 16 μm. The proposed tunable metasurface offers significant potential for applications in THz sensing, imaging, filtering, and wavefront engineering. Full article

There is growing interest in measuring the temperature-dependent dielectric properties of bio-tissues using dual-mode techniques (scattering measurements and thermal treatment). Uncooled coaxial antennas are preferred for their direct contact with the measured medium and reduced complexity; however, they exhibit structural changes during ablation due to the thermal expansion of polytetrafluoroethylene (PTFE). This paper presents an experimental study on PTFE expansion in an uncooled coaxial insulated monopole antenna in response to changes in the tissue’s thermal environment. Furthermore, it presents a methodology to mitigate these effects through coaxial annealing. The investigation consists of two distinct experiments: characterising PTFE expansion and assessing the effects of annealing through microwave ablation. This was achieved by simulating the thermal effects experienced during ablation by immersing the test antenna in heated peanut oil. PTFE expansion was measured through camera monitoring and using a toolmaker’s microscope, revealing two expansion modalities: linear PTFE expansion and non-linear plastic deformation from manufacturing processes. The return loss during ablation and consequential changes in the ablated lesion were also assessed. Antenna pre-annealing increased resilience against structural changes in the antenna, improving lesion ellipticity. Therefore, this study establishes a fabrication method for achieving an uncooled thermally stable antenna, leading to an optimised dual-mode ablation procedure, enabling quasi-real-time permittivity measurement of the surrounding tissue. Full article