Search Results (102)

This paper presents the design and performance analysis of a compact wireless power transfer (WPT) system operating at 1.6 GHz. The transmitter (Tx) structure consists of a circular slot and a circular radiating element, excited from the backside of the substrate, while the receiver (Rx) comprises a slotted patch antenna miniaturized using two vertical vias. The initial power transfer efficiency (PTE), represented by the transmission coefficient $S_{21}$, was measured to be −31 dB with a 25 mm separation between Tx and Rx. To enhance the efficiency of the system, a dielectric matching media (MM) was introduced between the transmitter and receiver. Through the implementation of the MM, the PTE improved significantly, with $S_{21}$ increasing to −24 dB. A parametric study was conducted by varying the thickness of the MM from 1 mm to 10 mm and the relative permittivity (${\epsilon }_r$) from 5 to 30. The results demonstrate that both the thickness and dielectric constant of the MM play a crucial role in improving the coupling and overall efficiency of the WPT system. The optimal configuration was achieved with a matching media thickness of 10 mm and a relative permittivity of 25, which yielded the best improvement in transmission performance. This work offers a practical approach to enhance near-field WPT efficiency using simple matching structures and is particularly relevant for compact and low-profile energy transfer applications. 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 quality of newly constructed pavement depends mostly on compaction, which is essential for ensuring the pavement’s longevity and performance. Traditional methods of evaluating pavement compaction and density, such as core sampling and nuclear gauge measurements, are often time-consuming and invasive and provide only a limited amount of data at a low spatial resolution on the potential air void content of the asphalt layers. The present study aimed to assess the specific gravity (G_mb) of a dolomitic asphalt mixture at different degrees of compaction using GPR techniques. Relative density (RD) maps were generated to visualize the spatial homogeneity of the asphalt density. Nuclear density gauging was applied for the calibration, and cores were used to validate the results. The survey was conducted on two recently paved roads in Szeged, Hungary. After testing various approaches, it was found that applying horn antennas and the surface reflection (SR) method is the most feasible way to obtain reliable and accurate dielectric permittivity (ε) data. Based on the measurements, clear relationships were found between dielectric constants, G_mb, and aggregate size. The findings highlight that it is possible to indirectly determine the G_mb of asphalts composed of dolomite and limestone aggregates using GPR, with aggregate sizes ranging from 11 mm to 25 mm and G_mb values between 2.43 and 2.57 g/cm³. Consequently, a robust function was developed, which can be applied to other asphalts with similar compositions. Full article

This study aimed to develop a cost-effective, environmentally sustainable, and technologically advanced dielectric fluid by utilizing the beneficial properties of natural ester-based vegetable oils, offering a promising alternative for transformer insulation and cooling applications. The novelty of this research lies in the formulation of a nanofluid that combines three distinct vegetable oils—castor, flaxseed, and blackseed—creating a unique base fluid. SiO₂ nanoparticles were incorporated into the fluid to leverage their multiple advantageous characteristics. Extensive experiments were conducted to evaluate the superior properties of the proposed nanofluid, focusing on key dielectric properties, such as relative permittivity (ε_r) and the dielectric dissipation factor (tan δ). Comparative analyses with conventional mineral oil, which was used as a benchmark, demonstrated the significant advantages of the vegetable oil-based nanofluid. The novel formulation outperformed all other tested samples, highlighting its exceptional performance. Additionally, three preparation methods were examined, with the green synthesis technique producing the nanofluid with better dielectric properties. Through a detailed presentation of empirical data and compelling arguments, this study confirms the potential of natural ester-based vegetable oil nanofluids as a highly promising alternative, driven by their intrinsic properties and the environmentally friendly synthesis method employed. Full article

CuMn_1−xMg_xO₂ (x = 0–0.06) polycrystalline samples were prepared using the hydrothermal method at T = 100 °C for 24 h in Teflon-line stainless steel autoclaves. The samples were crystallized, forming crednerite structures (C2/m space group), and the Mg²⁺ substitution onto the Mn³⁺ site induced small changes in the unit cell parameters and volume. Based on complex impedance measurements made between 20 Hz and 2 MHz, at different concentrations of Mg ions (x), the electrical conductivity (σ), the electric modulus (M), and the complex dielectric permittivity (ε) were determined. The conductivity spectrum, σ(f, x), follows the Jonscher universal law and enables the determination of the static conductivity (σ_DC) of the samples. The results showed that, when increasing the concentration x from 0 to 6%, σ_DC varied from 15.36 × 10⁻⁵ S/m to 16.42 × 10⁻⁵ S/m, with a minimum of 4.85 × 10⁻⁵ S/m found at a concentration of x = 4%. Using variable range hopping (VRH) and correlated barrier hopping (CBH) theoretical models, the electrical mechanism in the samples was explained. The band gap energy (W_m), charge carrier mobility (μ), number density (N_C) of effective charge carriers, and hopping frequency (ω_h) were evaluated at different concentrations (x) of substitution with Mg. In addition, using measurements of the temperature dependence of σ_DC(T) between 300 and 400 K, the thermal activation energy (E_A) of the samples was evaluated. Additionally, the dielectric behavior of the samples was explained by the interfacial relaxation process. This knowledge of the electrical properties of the CuMn_1−xMgxO₂ (x = 0–0.06) polycrystalline crednerite is of interest for their use in photocatalytic, electronic, or other applications. Full article

The aim of this work was to determine the anisotropy of the electrophysical and mechanical properties of rubber reinforced with a hybrid filler CNTs&CB (carbon nanotubes and carbon black) as a function of CNT content and the technological parameters of the production process. A significant difference in electrical conductivity (σ) and dielectric permittivity (ε) in three perpendicular directions was found for CNT concentrations ranging from 0 to 0.007 in volume fraction. The highest values of σ and ε were observed in the calendering direction, with slightly lower values in the perpendicular direction. This effect was attributed to the orientation of polymer molecules and CNTs along the direction of movement during calendering, as well as the disruption of the cluster structure in the transverse direction. Although the calculated percolation threshold values of the investigated system differed slightly, a correlation was observed between the mechanical and electrophysical properties of CNTs&CB rubber. This correlation enables rubber products to be designed with optimal properties tailored to the desired direction. Full article

In this work, three composite materials based on Terfenol-D and PZT-type material were obtained with a classic sintering method using a combination of 0–3 phases, where the ferroelectric phase was doped PZT material (P) and the magnetic phase was Terfenol-D (T). The percentage of P and T components in the composites was variable, i.e., 90% P/10% T (P90-T10), 70% P/30% T (P70-T30), and 50% P/50% T (P50-T50). Structural, microstructure, dielectric, and magnetic properties and DC electric conductivity of multiferroic composites were investigated. Chemical composition analyses and X-ray studies showed a decomposition of the composite compositions, forming additional phases, most of which contained rare earth elements and Fe. Microstructural SEM-BE (backscattering) images distinguished areas of bright intensity with a dominant ferroelectric phase and dark areas with a dominant magnetic element dominance. Despite the composition decomposition, the composite materials retained good dielectric and magnetic properties at room temperature. The highest stability of dielectric parameters was maintained by the P90-T10 composition with high values of permittivity ε = 570 at room temperature RT (ε_m = 7300 at the phase transition temperature T_m) and the lowest dielectric tangent loss (tanδ of 0.32 and 1.94 for RT and T_m, respectively). Increasing the Terfenol-D share in the composite causes a significant increase in dielectric tangent loss and electrical conductivity, a decrease in permittivity, and an increase in the degree of phase transition blurring. The magnetic properties for all P-T composite compositions at RT were preserved and were 0.31 emu/g, 1.60 emu/g, and 4.56 emu/g for P90-T10, P70-T30, P50-T50, respectively. For the M-H hysteresis loop at room temperature, the maximum magnetization increased from 1.17 emu/g for (P90-T10) to 15.18 emu/g for (P50-T50), while the coercive field decreased from 271.8 mT for P90-T10 to 9.7 mT for P50-T50. It is also interesting to maintain the high saturation of the M-H magnetic hysteresis loop in the composite with the lowest Terfenol-D content (P90-T10). The magnetic properties for all P-T composite compositions at room temperature were preserved and were 0.31 emu/g, 1.60 emu/g, and 4.56 emu/g for P90-T10, P70-T30, and P50-T50, respectively. For the M-H hysteresis loop at RT, the maximum magnetization increased from 1.17 emu/g for (P90-T10) to 15.18 emu/g for (P50-T50), while the coercive field decreased from 0.272 T for P90-T10 to 0.001 T for P50-T50. It is also interesting to maintain the high saturation of the M-H magnetic hysteresis loop in the composite with the lowest Terfenol-D content (P90-T10). Due to the tendency to combine with oxygen and the high electric conductivity of Terfenol-D, limiting its amount in the composite composition is appropriate. At 10% of Terfenol-D, the composite has good dielectric properties, and the magnetic parameters remain satisfactory. Full article

Roads are critical components of infrastructure, and assessing their quality is essential to ensure the safe transport of people and goods, which in turn supports economic prosperity. Various factors, such as subsurface conditions, moisture content, and temperature, influence road performance and can degrade their efficiency as transportation networks. While surface road defects can often be identified through visual inspection, information about subsurface extensions, their impact on structural integrity, and potential risks remain concealed. This study aimed to perform a comparative analysis of dielectric permittivity (ε) using time-lapse Ground Penetrating Radar (GPR) measurements on pre- and post-renovated road sections. This study also sought to evaluate the effectiveness of this approach for road assessment and to employ forward modeling for a deeper understanding of road defects and their associated hazards. Results revealed that the pre-renovated road section exhibited significant fluctuations in dielectric values, ranging from 3.13 to 15.9. In contrast, the post-renovated section showed consistent values within a narrow range of 5 to 6.6. Different crack types were classified, and the mean ε for each visually identified crack type was calculated. Despite the higher frequency of transverse cracks compared to other defects, longitudinal cracks exhibited the highest mean dielectric value (~10.3), while alligator cracks had the lowest (~8.33). Numerical simulations facilitated accurate interpretation of the defects identified in the road section, providing insights into their nature and associated risks. The methodology used for crack classification and numerical simulation can be applied to other road sections globally, offering a standardized approach to road assessment and maintenance planning. Full article

The complex dielectric permittivity, ε (f, H) = ε′ (f, H) − i ε″ (f, H), in the microwave frequency range f, of (0.1–3) GHz and polarizing field values H, in the range of (0–135) kA/m, was measured for a kerosene-based ferrofluid with magnetite particles. A relaxation process attributed to interfacial type relaxation was highlighted, determining for the first time in the microwave field, the activation energy of the dielectric relaxation process in the presence of the magnetic field, E_A(H), in relation to the activation energy in zero field, E_A(H = 0). Based on the complex permittivity measurements and the Claussius–Mossotti equation, the dependencies on frequency (f), and magnetic field (H), of the polarizability (α) and electrical conductivity (σ), were determined. From the dependence of α(f,H), the electric dipolar moment, p, of the particles in the ferrofluid, was determined. The conductivity spectrum, σ(f,H), was found to be in agreement with Jonscher’s universal law and the electrical conduction mechanism in the ferrofluid was explained using both Mott’s VRH (variable range hopping) model and CBH (correlated barrier hopping) model. Based on these models and conductivity measurements, the hopping distance, R_h, of the charge carriers and the maximum barrier height, W_m, for the investigated ferrofluid was determined for the first time in the microwave field. Knowledge of these electrical properties of the ferrofluid in the microwave field is useful for explaining the mechanisms of polarization and control of electrical conductivity with an external magnetic field, in order to use ferrofluids in various technological applications in microwave field. Full article

This work investigated the structure, microstructure, and ferroelectric and dielectric behavior of (Pb_0.97Ba_0.03)(Zr_0.98Ti_0.02)_1−xSn_xO₃ (PBZT_xSn) solid solution with variable tin content in the range x = 0.00–0.08. Synthesis was carried out using the powder calcination method, and sintering was carried out using the hot-pressing method. For all the PBZT_xSn samples at room temperature, X-ray diffractograms confirmed the presence of an orthorhombic (OR) crystal structure with space group Pnnm, and the microstructure is characterized by densely packed and properly shaped grains with an average size of 1.36 µm to 1.73 µm. At room temperature, PBZT_xSn materials have low permittivity values ε′ ranging from 265 to 275, whereas, at the ferroelectric–paraelectric phase transition temperature (RE–C), the permittivity is high (from 8923 to 12,141). The increase in the tin dopant in PBZT_xSn lowers permittivity and dielectric loss and changes the scope of occurrence of phase transitions. The occurring dispersion of the dielectric constant and dielectric loss at low frequencies, related to the Maxwell–Wagner behavior, decreases with increasing tin content in the composition of PBZT_xSn. Temperature studies of the dielectric and ferroelectric properties revealed anomalies related to the phase transitions occurring in the PBZT_xSn material. With increasing temperature in PBZT_xSn, phase transitions occur from orthorhombic (OR) to rhombohedral (RE) and cubic (C). The cooling cycle shifts the temperatures of the phase transitions towards lower temperatures. The test results were confirmed by XRD Rietveld analysis at different temperatures. The beneficial dielectric and ferroelectric properties suggest that the PBZT_xSn materials are suitable for micromechatronic applications as pulse capacitors or actuator elements. Full article

The use of solid waste such as ceramic sludge, ceramic rollers, and magnesite was studied to obtain cheap refractory ceramics at temperatures of 1300 °C based on XRF, XRD SEM, EDX, bending strength, and dielectric properties. The prepared samples were examined. The results showed that the significant crystalline phases formed were mullite, spinel, and corundum. They also showed that mullite hindered the formation of cordierite and enhanced spinel formation. With increased cordierite content, the microstructure varied from fine grained to coarse grained. Bending strength increased with increasing mullite content and bulk density, ranging from 10.80 to 13.50 MPa. Bulk density increased with the increase in mullite content and sintering temperature and ranged from 1.99 to 1.94 g/cm³, while the percentage of porosity and water absorption decreased and ranged from 29.40 to 38.83, respectively. To examine the effect of the produced phases on the dielectric characteristics, the permittivity (ε′), dielectric loss (ε″), and AC conductivity (σ_ac) were measured in the frequency range of 10⁻¹ Hz to 10⁶ Hz. As the concentration of cordierite increased, there was a noticeable drop in ε′ from 35.6 to 8.2 and σ_ac from 10⁻⁸ s/cm to around 10⁻¹¹ s/cm and high values of resistivity from 10⁸ cm/s to about 10¹⁰ cm/s, suggesting that this material might be an excellent insulator. Full article

The effect of sintering temperature on the structural, magnetic, and dielectric properties of NiCr₂O₄ ceramics was investigated. A powder X-ray analysis indicates that the prepared nanocrystallites effectively inhibit the cooperative Jahn–Teller distortion, thereby stabilizing the high-temperature cubic phase structure with space group Fd-3m. Multiple transitions are confirmed by temperature-dependent magnetization M(T) data. Moreover, the magnetization value decreases and the Curie temperature increases with a decrease in the crystallite size. The low-temperature-dependent real permittivity (ε′-T) for a NiCr₂O₄ crystallite size of 78 nm exhibits a broad maximum at 40 K that is independent of frequency. This establishes a correlation between electric ordering and the underlying magnetic structure. The temperature dependency of the dielectric constant at fixed frequencies for both NiCr₂O₄ crystallite sizes rises with temperature for a certain range of frequencies. A significant improvement is evident: the dielectric constant (ε’) at room temperature reaches approximately 5738 for the sample with 28 nm crystallites, while the 78 nm crystallite sample shows a noticeable drop to ε’~174. The frequency-dependent conductivity curves for both types of NiCr₂O₄ nanocrystallites have different conductivity values. The lower-crystallite-size sample demonstrates higher conductivity values than the 78 nm crystallite size one. This observation is attributed to the decrease in crystallite size, which increases the number of grain boundaries and, consequently, scatters a higher number of charge carriers. Full article

By using the method of high-temperature solid-phase reaction, the new piezoceramic SrBi₂Nb_2-2xW_xSn_xO₉ was obtained, where partial substitution of niobium (Nb) atoms with Sn⁴⁺ and W⁶⁺ atoms in the compound SrBi₂Nb₂O₉ occurred in the octahedra of the perovskite layer (B-position). X-ray diffraction investigations showed that these compounds are single-phase SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.1, 0.2) and two-phase SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.3, 0.4), but all of them had the structure of Aurivillius-Smolensky phases (ASPs) with close parameters of orthorhombic unit cells. It corresponded to the space group A21am. The temperature dependences of the relative permittivity ε/ε₀ and the tangent of the dielectric loss angle tan d were defined at various frequencies. It was found that doping SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.1) improved the electrophysical properties of the compound: losses decreased, and the relative permittivity increased. This result was obtained for the first time. Moreover, a new result was obtained that indicated an improvement in the electrophysical properties of SrBi₂Nb₂O₉ using the chemical element Sn (tin). This refutes the previously existing opinion about the impossibility to use Sn as a doping element. Full article

In this study, Zn-Al ferrite/polypyrrole (PPy) nanocomposites were synthesized and thoroughly characterized to explore their potential for microwave applications. X-ray diffraction analysis confirmed the presence of ZnO, AlFeO₃, and Fe₂O₃ phases, with the crystal size decreasing from 31 nm to 19.6 nm as aluminum content increased. High-resolution transmission electron microscopy (HR-TEM) revealed a distinctive core–shell morphology, where the polypyrrole encapsulates the ZnAl_xFe_2−xO₄ particles. Magnetic measurements showed that decreasing aluminum concentration led to a reduction in both saturation magnetization (Ms) from 75 emu/g to 36 emu/g and remanent magnetization (Mr) from 2.26 emu/g to 2.00 emu/g. Dielectric analysis indicated that both the real (ε′) and imaginary (ε″) components of dielectric permittivity decreased with increasing frequency, particularly between 10 and 14 GHz. Furthermore, electrical modulus analysis highlighted the significant impact of aluminum doping on relaxation time (τIP), indicating the presence of interface polarization. Impedance spectroscopy results underscored the dominance of interface polarization at lower frequencies and the presence of strong conduction paths at higher frequencies. These combined magnetic and dielectric loss mechanisms suggest that the Zn-Al ferrite/polypyrrole nanocomposite is a promising candidate for advanced microwave absorption applications. Full article

The development of devices for the in situ resource utilization (ISRU) of lunar surface powder (regolith) by means of microwaves needs regolith simulants with electromagnetic properties similar to the lunar regolith. This document deals with the measurement of complex permittivity and dielectric loss tangent of the aforementioned simulants at ambient temperature from 400 MHz to 20 GHz, performing measurements using two lunar dust simulants, EAC-1A and JSC-2A, resulting, on the one hand, in permittivity values of ${\epsilon }^{\prime }=-0.0432f+4.0397$ for the EAC-1A lunar dust simulant and ${\epsilon }^{\prime }=-0.0432f+4.0397$ for the JSC-2A simulant, and on the other hand, in loss tangent values of $tan{\delta }_e=-0.0015f+0.0659$ for the EAC-1A powder and $tan{\delta }_e=-0.0039f+0.1429$ for the JSC-2A powder. In addition, further studies are carried out taking into account the humidity of the samples and their densities at room temperature. The obtained results are applicable for comparing the measured values of EAC-1A and JSC-2A between them and with other previously measured simulants and real samples. The measurements are carried out by applying two different nonresonant techniques: Open-Ended Coaxial Probe (OECP) and transmission line. For this purpose, the DAK and EpsiMu commercial kits are used, respectively. Full article