Graphene based T-shaped monopole antenna sensor for food quality evaluation

BACKGROUND

Food adulteration has long been considered a major problem. It compromises the quality, safety, and nutritional value of food, posing significant risks to public health. Novel techniques are required to control it.

RESULTS

A graphene-based T-shaped monopole antenna sensor was tested for its ability to detect adulteration in liquid foods. Mustard oil was the pure reference sample used for product quality analysis. Olive oil and rice bran oil were adulterants added to the pure sample. It was found that the sensor could be immersed easily in the liquid sample and provided precise results.

CONCLUSION

The graphene-based T-shaped monopole antenna sensor can be used for the quality assessment of liquid food products and is suitable for real-time monitoring. © 2024 Society of Chemical Industry.

Thermal Stability and Non-Linear Optical and Dielectric Properties of Lead-Free K0.5Bi0.5TiO3 Ceramics

Lead-free K0.5Bi0.5TiO3 (KBT) ceramics with high density (~5.36 g/cm3, 90% of X-ray density) and compositional purity (up to 90%) were synthesized using a solid-state reaction method. Strongly condensed KBT ceramics revealed homogenous local microstructures. TG/DSC (Thermogravimetry-differential scanning calorimetry) techniques characterized the thermal and structural stability of KBT. High mass stability (>0.4%) has proven no KBT thermal decomposition or other phase precipitation up to 1000 °C except for the co-existing K2Ti6O13 impurity. A strong influence of crystallites size and sintering conditions on improved dielectric and non-linear optical properties was reported. A significant increase (more than twice) in dielectric permittivity (εR), substantial for potential applications, was found in the KBT-24h specimen with extensive milling time. Moreover, it was observed that the second harmonic generation (λSHG = 532 nm) was activated at remarkably low fundamental beam intensity. Finally, spectroscopic experiments (Fourier transform Raman and far-infrared spectroscopy (FT-IR)) were supported by DFT (Density functional theory) calculations with a 2 × 2 × 2 supercell (P42mc symmetry and C4v point group). Moreover, the energy band gap was calculated (Eg = 2.46 eV), and a strong hybridization of the O-2p and Ti-3d orbitals at Eg explained the nature of band-gap transition (Γ → Γ).

Dielectric and Ultrasonic Properties of PDMS/TiO2 Nanocomposites

This work presents the dielectric and ultrasonic properties of polydimethylsiloxane (PDMS) nanocomposites filled with titanium dioxide nanoparticles. The dielectric study was performed over a very broad range of frequencies (20 Hz-3 THz). The dielectric permittivity was almost frequency-independent in all the composites at room temperature over the whole range of measurement frequencies, and the dielectric losses were very low under these conditions (less than 2). The dielectric permittivity strongly increases with the nanoparticle concentration according to the Maxwell-Garnet model. Therefore, the investigated composites are suitable for various flexible electronic applications, particularly in the microwave and terahertz frequency ranges. Dielectric dispersion and increased attenuation of ultrasonic waves were observed at lower temperatures (below 280 K) due to the relaxation of polymer molecules at the PDMS/TiO2 interface and in the polymer matrix. The relaxation time followed the Vogel-Vulcher law, while the freezing temperature increased with the titanium dioxide concentration due to interactions between the polymer molecules and nanoparticles. The significant hysteresis in the ultrasonic properties indicated that titanium dioxide acts as a crystallization center. This is confirmed by the correlation between the hysteresis in the ultrasonic properties and the structure of the composites. The small difference in the activation energy values obtained from the ultrasonic and dielectric investigations is related to the fact that the dielectric dispersion is slightly broader than the Debye-type dielectric dispersion.

High Energy Density Achieved in Novel Lead-Free BiFeO3-CaTiO3 Ferroelectric Ceramics for High-Temperature Energy Storage Applications

The development of high-performance electrostatic energy storage dielectrics is essential for various applications such as pulsed-power technologies, electric vehicles (EVs), electronic devices, and the high-temperature aviation sector. However, the usage of lead as a crucial component in conventional high-performance dielectric materials has raised severe environmental concerns. As a result of this, there is an urgent need to explore lead-free alternatives. Ferroelectric ceramics offer high energy density but lack stability at high temperatures. Here we present a lead-free (1 - x)BiFeO3-xCaTiO3 (x = 0.6, 0.7, and 0.8; BFO-CTO) ceramic capacitor with low dielectric loss, high thermal stability, and high energy density up to ∼200 °C. The introduction of CTO (x = 0.7) to the BFO matrix triggers a transition from the normal ferroelectrics to the relaxor ferroelectrics state, resulting in a high recoverable energy density of 1.18 J cm-3 at 190 °C with an ultrafast dielectric relaxation time of 44 μs. These results offer a promising, environmentally friendly, high-capacity ceramic capacitor material for high-frequency and high-temperature applications.

Modeling the Effect of Material Viscoelasticity on the Dielectric Permittivity of Deformed Elastomers

Elastomers, as a typical category of soft dielectrics, have shown great potential for developing stretchable electronics and soft transducers. However, the performance of dielectric elastomers (DEs) is susceptible to the dielectric permittivity of the material, whether as insulators or actuators. On the other hand, experiments suggest that the material viscoelasticity significantly influences the dielectric permittivity of DEs. Based on the theory of finite-deformation viscoelasticity, this work adopts the Brillouin function to develop a modeling framework to examine the effect of material viscoelasticity on the dielectric permittivity for the first time. A comparison of the data fitting results between the models with and without consideration of the material viscoelasticity is presented. Simulation results also reveal that the viscous network of the elastomer exerts a mitigation effect on the decrease in the dielectric permittivity when the material is deformed. Furthermore, it is found that the loading rate is a key parameter that strongly affects the dielectric permittivity, mainly through the inelastic deformation.

Study of Tunable Dielectric Permittivity of PBDB-T-2CL Polymer in Ternary Organic Blend Thin Films Using Spectroscopic Ellipsometry

The ellipsometric analyses reported in this paper present a novelty by bringing an in-depth optical investigation of some ternary organic blends. This study focuses on the tunability and control of the relative permittivity of active layers by varying the weight ratio of blended materials spin-coated as thin films. To investigate this, an extensive approach based on spectroscopic ellipsometry was conducted on ternary blend (D:A1:A2) thin films, involving a donor [D = chlorinated conjugated polymer (PBDB-T-2Cl)] and two acceptor materials [A1 = a non-fullerene (ITIC-F) and A2 = a fullerene (PCBM)]. The refractive index constitutes a key parameter that exposes insights into the feasibility of photovoltaic cells by predicting the trajectory of light as it transits the device. In this term, higher obtained refractive indexes support higher absorption coefficients. Notably, the dielectric constant can be rigorously tuned and finely calibrated by modest variations in the quantity of the third element, resulting in considerable modifications. Moreover, the inclusion of fullerene in blends, as the third element, results in a smooth topographical profile, further refining the surface of the film. From an electrical point of view, the ternary blends outperform the polymer thin films. The synergistic interaction of constituents emphasizes their potential to enhance solar conversion devices.

Evaluation of Three Soil Moisture Profile Sensors Using Laboratory and Field Experiments

Soil moisture profile sensors (SMPSs) have a high potential for climate-smart agriculture due to their easy handling and ability to perform simultaneous measurements at different depths. To date, an accurate and easy-to-use method for the evaluation of long SMPSs is not available. In this study, we developed laboratory and field experiments to evaluate three different SMPSs (SoilVUE10, Drill&Drop, and SMT500) in terms of measurement accuracy, sensor-to-sensor variability, and temperature stability. The laboratory experiment features a temperature-controlled lysimeter to evaluate intra-sensor variability and temperature stability of SMPSs. The field experiment features a water level-controlled sandbox and reference TDR measurements to evaluate the soil water measurement accuracy of the SMPS. In both experiments, a well-characterized fine sand was used as measurement medium to ensure homogeneous dielectric properties in the measurement domain of the sensors. The laboratory experiments with the lysimeter showed that the Drill&Drop sensor has the highest temperature sensitivity with a decrease of 0.014 m³ m^-3 per 10 °C, but at the same time showed the lowest intra- and inter-sensor variability. The field experiment with the sandbox showed that all three SMPSs have a similar performance (average RMSE ≈ 0.023 m³ m^-3) with higher uncertainties at intermediate soil moisture contents. The presented combination of laboratory and field tests were found to be well suited to evaluate the performance of SMPSs and will be used to test additional SMPSs in the future.

Study of the Aid Effect of CuO-TiO2-Nb2O5 on the Dielectric and Structural Properties of Alumina Ceramics

The aim of this work is to study the structural, dielectric, and mechanical properties of aluminum oxide ceramics with the triple sintering additive 4CuO-TiO₂-2Nb₂O₅. With an increase in sintering temperature from 1050 to 1500 °C, the average grain size and the microhardness value at a load of 100 N (HV0.1) increased with increasing density. It has been shown that at a sintering temperature of 1300 °C, the addition of a 4CuO-TiO₂-2Nb₂O₅ additive increases the low-frequency permittivity (2-500 Hz) in alumina ceramic by more than an order of magnitude due to the presence of a quadruple perovskite phase. At the same time, the density of such ceramics reached 89% of the theoretical density of α-Al₂O₃, and the microhardness value HV0.1 was 1344. It was observed that the introduction of 5 wt.% 4CuO-TiO₂-2Nb₂O₅ in the raw mixture remarkably increases values of shrinkage and density of sintered ceramics. Overall, the results of this work confirmed that introducing the 4CuO-TiO₂-2Nb₂O₅ sintering additive in the standard solid-phase ceramics route can significantly reduce the processing temperature of alumina ceramics, even when micron-sized powders are used as a starting material. The obtained samples demonstrated the potential of α-Al₂O₃ with the triple additive in such applications as electronics, microwave technology, and nuclear power engineering.

Use of a Dielectric Sensor for Salinity Determination on an Extensive Green Roof Substrate

The irrigation of extensive green roofs with recycled or saline water could contribute to the conservation of valuable drinking water supplies. In such cases, the continuous monitoring of substrate electrical conductivity (EC_sw) is of immense importance for the sustainable growth of the plants growing on the green roof. The present study aimed to estimate the EC_sw (pore water EC) of an extensive green roof substrate in lysimeters with the use of the WET-2 dielectric sensor. Half of the 48 lysimeters that simulated extensive green roofs had a substrate depth of 7.5 cm, while the other half had a 15 cm substrate depth. The warm season turfgrass Paspalum vaginatum 'Platinum TE' was established at the lysimeters, and during the summer period, it was irrigated every two days at a rate of 14 mm with NaCl solutions of various electrical conductivities (EC_i): (a) 3 dS m^-1, (b) 6 dS m^-1, and (c) 12 dS m^-1, while potable water of 0.3 dS m^-1 EC_i served as the control. The relation between bulk electrical conductivity, σ_b, and bulk dielectric permittivity, ε_b, of the substrate was observed to be linear for all EC_i levels up to σ_b values of 2-2.5 dS m^-1. The EC_sw was predicted by employing the salinity index method which was modified to be applied to the particular case of a green roof substrate. Knowing the salinity index and organic portion (%, v/v) for a given green roof substrate, we could calculate the EC_sw. It was found that the use of the salinity index method predicts reliably the EC_sw up to 10-11 dS m^-1, while the method overestimates EC_sw at very low levels of electrical conductivity.

Microwave Spectroscopy Investigation of Carasau Bread Doughs: Effects of Composition up to 8.5 GHz

Carasau bread is a flat bread, typical of Sardinia (Italy). The market of this food product has a large growth potential, and its industry is experiencing a revolution, characterized by digitalization and automation. To monitor the quality of this food product at different manufacturing stages, microwave sensors and devices could be a cost-effective solution. In this framework, knowledge of the microwave response of Carasau dough is required. Thus far, the analysis of the microwave response of Carasau doughs through dielectric spectroscopy has been limited to the dynamics of fermentation. In this work, we aim to perform complex dielectric permittivity measurements up to 8.5 GHz, investigating and modeling the role of water amount, salt and yeast concentrations on the spectra of this food product. A third-order Cole-Cole model was used to interpret the microwave response of the different samples, resulting in a maximum error of 1.58% and 1.60% for the real and imaginary parts of permittivity, respectively. Thermogravimetric analysis was also performed to support the microwave spectroscopy investigation. We found that dielectric properties of Carasau bread doughs strongly depend on the water content. The analysis highlighted that an increase in water quantity tends to increase the bounded water fraction at the expense of the free water fraction. In particular, the free water amount in the dough is not related to the broadening parameter γ2 of the second pole, whereas the bound water weight fraction is more evident in the γ2 and σdc parameters. An increase in electrical conductivity was observed for increasing water content. The microwave spectrum of the real part of the complex permittivity is slightly affected by composition, while large variation in the imaginary part of the complex dielectric permittivity can be identified, especially for frequencies below 4 GHz. The methodology and data proposed and reported in this work can be used to design a microwave sensor for retrieving the composition of Carasau bread doughs through their dielectric signature.

Dielectric and Biological Characterization of Liver Tissue in a High-Fat Diet Mouse Model

Hepatic steatosis may be caused by type 2 diabetes or obesity and is one of the origins of chronic liver disease. A non-invasive technique based on microwave propagation can be a good solution to monitor hepatic tissue pathologies. The present work is devoted to the dielectric permittivity measurements in healthy and fatty liver in the microwave range. A mouse model following normal and high sugar/glucose (HFS) diets was used. We demonstrated the change in the triglyceride and glucose concentration in the hepatic tissue of HFS diet mice. The difference in the dielectric permittivity of healthy and fatty liver was observed in the range from 100 MHz to 2 GHz. The dielectric permittivity was found to be 42 in the healthy tissue and 31 in the fatty liver tissue at 1 GHz. The obtained results demonstrate that dielectric permittivity can be a sensitive tool to distinguish between healthy and fatty hepatic tissue.

Microwave Cavity Sensor for Measurements of Air Humidity under Reduced Pressure

A high-sensitivity sensor for measuring moisture content in the air or air humidity under low pressure was designed on the basis of a half-wave coaxial microwave cavity. The method of measuring small variations in the signal phase at a cavity excitation frequency of 1.63 GHz was applied to detect low densities of water vapor. It allows the detection of variations in dielectric air permittivity in the seventh decimal place. A prototype of the sensor was tested in a vacuum chamber. It was calibrated by comparing the test results with the readings of a commercial pressure gauge and using the independent method of measuring the moisture content in rarefied air on the basis of the absorption of millimeter waves in the local line of water vapor at 183 GHz. The developed sensor can be used in laboratory experiments and full-scale geophysical research in the atmosphere onboard various aircrafts.

Slick Synthetic Approach to Various Fluoroalkyl Silsesquioxanes-Assessment of their Dielectric Properties

We present a smart and efficient methodology for the synthesis of a variety of fluorinated silsesquioxanes (SQs) with diverse Si-O-Si core architecture. The protocol is based on an easy-to-handle and selective hydrosilylation reaction. An investigation on the placement of the reactive Si-HC=CH₂ vs. Si-H in the silsesquioxane, as well as silane vs. olefin structure, respectively, on the progress and selectivity of the hydrosilylation process, was studied. Two alternative synthetic pathways for obtaining a variety of fluorine-functionalized silsesquioxanes were developed. As a result, a series of mono- and octa- T₈ SQs, tri- 'open-cage' T₇ SQs, in addition to di- and tetrafunctionalized double-decker silsesquioxane (DDSQ) derivatives, were obtained selectively with high yields. All products were characterized by spectroscopic (NMR, FTIR) techniques. Selected samples were subjected to the measurements revealing their dielectric permittivity in a wide range of temperatures (from -100 °C to 100 °C) and electric field frequencies (10⁰-10⁶ Hz).

TiO2 Nanoparticle/Polyimide Nanocomposite for Ultrahigh-Temperature Energy Storage

With the development of electronic technology, there is an increasing demand for high-temperature dielectric energy storage devices based on polyimides for a wide range of applications. However, the current nanofillers/PI nanocomposites are used for energy harvesting at no more than 200 °C, which does not satisfy the applications in the oil and gas, aerospace, and power transmission industries that require an operating temperature of 250-300 °C. Therefore, we introduced a nanocomposite based on nonsolid TiO₂ nanoparticles and polyimide (PI) with high energy storage performance at an ultrahigh temperature of 300 °C. The synergy of excellent dielectric properties and a high breakdown strength endowed the nanocomposite with a low loading content of 1 wt% and a high energy storage density of 5.09 J cm^-3. Furthermore, we found that the nanocomposite could stably operate at 300 °C with an outstanding energy storage capability (2.20 J cm^-3). Additionally, finite element simulations demonstrated that the partially hollow nanostructures of the nanofillers avoided the evolution of breakdown paths, which optimized the breakdown strength and energy storage performance of the related nanocomposites. This paper provides an avenue to broaden the application areas of PI-based nanocomposites as ultrahigh-temperature energy-storage devices.

The Impact of Atmospheric Parameters on the Dielectric Permittivity Values of SikaBlock®-M150 and Other Rigid Polyurethane Foams Measured with a Capacitive One-Side Access Sensor

A shortage of research on the impact of atmospheric parameters on the measured dielectric permittivity values of rigid polyurethane (PU) foams was identified. Therefore, the impact of temperature, pressure, and relative humidity of air in the test room on the measured values of dielectric permittivity of rigid PU foams of different densities as well as monolithic polyurethane was investigated in a year-long experimental research study with a capacitive one-side access sensor. It was shown that relative humidity has the highest correlation with the dielectric permittivity values of rigid PU materials. The detected values of parameters were linked to the water vapour mass in ambient air and its correlation with permittivity of the investigated materials was determined. The warm-up drift and warm-up time of the spectrometer were estimated experimentally. A novel methodology was demonstrated to determine the true permittivity spectrum of rigid PU foams without any involvement of the environmental chamber, desiccators, or saturated salt/water solutions. A relative increase in the measured dielectric permittivity value was estimated numerically for the entire density range of rigid PU foams, i.e., 33-1280 kg/m³ (including monolithic PU).

A Method for Extracting Debye Parameters as a Tool for Monitoring Watered and Contaminated Soils

Soil monitoring is a key topic from several perspectives, such as moisture level control for irrigation management and anti-contamination purposes. Monitoring the latter is becoming even more important due to increasing environmental pollution. As a direct consequence, there is a strong demand for innovative monitoring systems that are low cost, provide for quasi-real time and in situ monitoring, high sensitivity, and adequate accuracy. Starting from these considerations, this paper addresses the implementation of a microwave reflectometry based-system utilizing a customized bifilar probe and a miniaturized Vector Network Analyzer (m-VNA). The main objective is to relate frequency-domain (FD) measurements to the features of interest, such as the water content and/or the percentage of some polluting substances, through an innovative automatable procedure to retrieve the Debye dielectric parameters of the soil under different conditions. The results from this study confirm the potential of microwave reflectometry for moisture monitoring and contamination detection.

Dielectric Loss and Electrical Conductivity Behaviors of Epoxy Composites Containing Semiconducting ZnO Varistor Particles

Polymer nanodielectrics render a great material platform for exhibiting the intrinsic nature of incorporated particles, particularly semiconducting types, and their interfaces with the polymer matrix. Incorporating the oxide fillers with higher loading percentages (>40 vol%) encounters particular challenges in terms of dispersion, homogeneous distribution, and porosity from the process. This work investigated the dielectric loss and electrical conduction behaviors of composites containing semiconducting ZnO varistor particles of various concentrations using the epoxy impregnation method. The ZnO varistor particles increased the dielectric permittivity, loss, and electrical conductivity of the epoxy composites into three different regimes (0−50 vol%, 50−70 vol%, 70−100 vol%), particularly under an electric bias field or at higher temperatures. For lower loading fractions below 50 vol%, the dielectric responses are dominated by the insulating epoxy matrix. When loading fractions are between 50 and 70 vol%, the dielectric and electric responses are mostly associated with the semiconducting interfaces of ZnO varistor particles and ZnO−epoxy. At above 70 vol%, the apparent increase in the dielectric loss and conductivity is primarily associated with the conducting ZnO core forming the interconnected channels of electric conduction. The foam-agent-assisted ZnO varistor particle framework appears to be a better way of fabricating composites of filler loading above 80 vol%. A physical model using an equivalent capacitor, diode, and resistor in the epoxy composites was proposed to explain the different property behaviors.

Microfluidic Microwave Sensor Loaded with Star-Slotted Patch for Edible Oil Quality Inspection

In this paper, we present a new microfluidic microwave sensor loaded with a star-slotted patch for detecting the quality of edible oil. The relative dielectric permittivity and the quality of edible oil will change after being heated at a high temperature. Therefore, the quality of edible oil can be detected by measuring the relative dielectric permittivity of edible oil. The sensor is used to determine the edible oil with different dielectric permittivity by measuring the resonance frequency offset of the input reflection coefficient, which operates at 2.68 GHz. This sensor is designed based on a resonant approach to provide the best sensing accuracy and is implemented using a substrate integrated waveguide structure combined with a pentagonal slot antenna operating at 2.3~2.9 GHz. It can detect greasy liquids with the real part of the complex permittivity ranging from two to three.

Implementation of Microwave Circuits Using Stereolithography

In this work, the use of additive manufacturing techniques through stereolithography for the manufacture of high-frequency circuits and devices is presented. Both the resin and the 3D printer used in this research are general-purpose commercial materials, not specifically intended for the implementation of microwave networks. The manufacturing and metallization procedures used to produce substrates for the design of planar microwave circuits are described, introducing the characterization process carried out to determine the electrical properties of the resin used. The ultrasonic techniques that allow the structural analysis of the manufactured substrates are also described. The electrical characterization provides a relative dielectric permittivity of 3.25 and a loss tangent of 0.03 for the resin used. In addition, the structural analysis shows a homogeneity and a finish of the manufactured parts that is not achievable using fused deposition modeling techniques. Finally, as a proof of concept, the design and manufacture of a complex geometry stepped impedance filter on a multi-height substrate using stereolithography techniques is presented, which allows for reducing the size of the traditional implementation of the same filter while maintaining its high-frequency response performance.

Local Response and Barrier Recovery in Elderly Skin Following the Application of High-Density Microarray Patches

The high-density microneedle array patch (HD-MAP) is a promising alternative vaccine delivery system device with broad application in disease, including SARS-CoV-2. Skin reactivity to HD-MAP applications has been extensively studied in young individuals, but not in the >65 years population, a risk group often requiring higher dose vaccines to produce protective immune responses. The primary aims of the present study were to characterise local inflammatory responses and barrier recovery to HD-MAPs in elderly skin. In twelve volunteers aged 69−84 years, HD-MAPs were applied to the forearm and deltoid regions. Measurements of transepidermal water loss (TEWL), dielectric permittivity and erythema were performed before and after HD-MAP application at t = 10 min, 30 min, 48 h, and 7 days. At all sites, TEWL (barrier damage), dielectric permittivity (superficial water);, and erythema measurements rapidly increased after HD-MAP application. After 7 days, the mean measures had recovered toward pre-application values. The fact that the degree and chronology of skin reactivity and recovery after HD-MAP was similar in elderly skin to that previously reported in younger adults suggests that the reactivity basis for physical immune enhancement observed in younger adults will also be achievable in the older population.

Aggregation-Dependent Dielectric Permittivity in 2D Molecular Crystals

The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈ -BTBT) molecular crystals on monolayer WS₂ substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (J_Coup ) value (H-aggregation) and a negative J_Coup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C₈ -BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.

Multimodal Deformation of Liquid Metal Multimaterial Composites as Stretchable, Dielectric Materials for Capacitive Pressure Sensing

Traditional electronic devices are composed of rigid materials and components that tend to be unsuitable for soft robotic and stretchable electronic applications, such as wearable or continuous pressure sensing. However, deformable materials have the potential to improve upon traditional devices through enhanced sensitivity and responsiveness, better conformability and biocompatibility at the human-machine interface, and greater durability. This work presents deformable composite materials composed of the gallium-indium-tin alloy galinstan (GaInSn) that combines the conductivity of a metal and the intrinsic deformability of a liquid. Dispersing galinstan in an elastomer allows for the formation of deformable dielectric materials that have tunable mechanical and electrical behavior, for example, modulus and relative permittivity. Galinstan composites have been shown previously to have a minimal modulus impact on the elastomer but concurrently achieve impressive dielectric performance. However, galinstan dispersions can be costly and face challenges of mechanical and electrical reliability. Thereby, this work investigates multimaterial composites composed of galinstan and a rigid filler, either iron or barium titanate, with respect to morphology, mechanical behavior, dielectric behavior, and pressure sensing performance for the purpose of achieving a balance between a low modulus and superior electrical performance. By combining galinstan and rigid fillers, it was found that the mechanical and electrical properties, such as modulus, permittivity, loss behavior, sensitivity, and linearity of the multimaterial composites can be improved by tuning filler formulation. This suggests that these dielectric materials can be used for sensing applications that can be precisely calibrated to specific material properties and the needs of the user. These deformable multimaterial composites, found to be stretchable and highly responsive in sensing applications, will expand the current mechanical abilities of deformable dielectric materials to improve soft robotic and stretchable electronic devices.

Dielectric Properties of Hybrid Polyethylene Composites Containing Cobalt Nanoparticles and Carbon Nanotubes

Polymer composites with electrically conductive inclusions are intensively developed for microwave shielding applications, where lightweight and elastic coatings are necessary. In this paper, dielectric properties of hybrid polyethylene composites containing cobalt nanoparticles and multi-wall carbon nanotubes (MWCNT) were investigated in the wide frequency range of 20–40 GHz for electromagnetic shielding applications. The percolation threshold in the hybrid system is close to 6.95 wt% MWCNT and 0.56 Co wt%. Cobalt nanoparticles (up to highest investigated concentration 4.8 wt%) had no impact on the percolation threshold, and for the fixed total concentration of fillers, the complex dielectric permittivity is higher for composites with bigger MWCNT concentrations. Moreover, the microwave complex dielectric permittivity of composites with high concentration of fillers is quite high (for composites with 13.4 wt% MWCNT and 1.1 wt% Co ε′ ≈ ε″ ≈ 20 at 30 GHz, it corresponds to microwave absorption 50% of 1 mm thickness plate); therefore, these composites are suitable for electromagnetic shielding applications.

Demonstrating the Potential of a Low-Cost Soil Moisture Sensor Network

Soil moisture is a key parameter of the climate system as it relates to plant transpiration and photosynthesis and impacts land–atmosphere interactions. Recent developments have seen an increasing number of electromagnetic sensors available commercially (EM) for soil volumetric water content (θ). Their use is constantly expanding, and they are becoming increasingly used for agricultural, ecological, and geotechnical applications and climate research, providing decision support and high-resolution data for models and machine-learning algorithms. In this study, a soil moisture sensor network consisting of 10 Sense Cap capacitance-based sensors is evaluated. Analytical performance of the sensors was determined based on laboratory and field measurements with dielectric permittivity (ε) standards and soil media substrates. Sensor response normalisation to standards of known ε was found to reduce intersensor variability and provide robust estimates of θ in soil samples with known θ. Cross-comparison with a time-domain reflectometry (TDR) instrument carried out in two soil media demonstrates good agreement between the two probes throughout the tested range. The data communication performance of the network was evaluated in terms of packet drop rate at different ranges and sampling frequencies. It was noticed that the drop rate increased with distance from the gateway, while sampling frequency had no effect. Sources of errors associated with probe installation were identified and recommendations are provided for sensor deployment. The off-the-shelf all-in-one solution provided by Sense Cap is low cost, user friendly and suitable for implementation at temporal and spatial scales once the identified shortcomings are addressed. The evaluation presented aims to aid stakeholders and users involved in soil and land management practices including crop production, soil conservation, carbon sequestration and pollutants transport.

Development of a Microwave Sensor for Solid and Liquid Substances Based on Closed Loop Resonator

In this work, a compact dielectric sensor for the detection of adulteration in solid and liquid samples using planar resonators is presented. Six types of filter prototypes operating at 2.4 GHz are presented, optimized, numerically assessed, fabricated and experimentally validated. The obtained experimental results provided an error less than 6% with respect to the simulated results. Moreover, a size reduction of about 69% was achieved for the band stop filter and a 75% reduction for band pass filter compared to standard sensors realized using open/short circuited stub microstrip lines. From the designed filters, the miniaturised filter with Q of 95 at 2.4 GHz and size of 35 mm × 35 mm is formulated as a sensor and is validated theoretically and experimentally. The designed sensor shows better sensitivity, and it depends upon the dielectric property of the sample to be tested. Simulation and experimental validation of the designed sensor is carried out by loading different samples onto the sensor. The adulteration detection of various food samples using the designed sensor is experimentally validated and shows excellent sensing on adding adulterants to the original sample. The sensitivity of the sensor is analyzed by studying the variations in resonant frequency, scattering parameters, phase and Q factor with variation in the dielectric property of the sample loaded onto the sensor.

Study of the modified magnetic, dielectric, ferroelectric and optical properties in Ni substituted GdFe1-xNixO3orthoferrites

Polycrystalline GdFe_1-xNi_xO₃(x = 0.00, 0.02, 0.04) samples was synthesised using a glycine assisted sol-gel method to investigate the enhanced magnetic and electric properties of Ni substituted GdFeO₃systems. TG-DSC analysis of prepared samples confirms that GdFe_1-xNi_xO₃have good thermal stability in high temperatures. The system has been stabilized in an orthorhombic structure with space group Pbnm.The elemental composition of GdFe_1-xNi_xO₃has been estimated from EDAX spectrum. The results showed oxygen deficiency on increasing the Ni substitution and it has been supported by Rietveld refinement. FE-SEM images and Brunauer-Emmett-Teller analysis reveals that GdFe_1-xNi_xO₃is a highly porous material and its porosity and specific area increases with Ni substitution. Magnetic measurements indicates that the system exhibited ferrimagnetic behaviour at low temperatures and canted antiferromagnetic behaviour at room temperature. Forx = 0.04 Ni content, magnetization reversal for applied field of 25 Oe has been observed. Increased coercivity of GdFeO₃with Ni substitution has been attributed to the grain size effect. From electrical point of view, dielectric permittivity of GdFeO₃has been enhanced with Ni substitution. This enhancement has been attributed to the cumulative effects of hopping of Fe²⁺-Fe³⁺ions, grain-grain boundary contribution, and space charge polarization. The role of grain-grain boundary contribution is evident from electric modulus spectrum. The space charge effect has been realized in both impedance spectrum and dielectric loss. Temperature-dependent dielectric studies were conducted to understand the mechanisms and various aspects that contribute to the dielectric enhancement. A highly lossy capacitive nature in theP-Eloop also suggests space charge effects due to Ni substitution in Fe sites. Availability of free charge carrier concentration is correlated with the optical properties of GdFe_1-xNi_xO₃. The decrease of optical band gap (2.5-2.21 eV) on increasing Ni content suggests the increasing electronic contribution in the system.

Development of Condition Assessment Index of Ballast Track Using Ground-Penetrating Radar (GPR)

The condition of the ballast is a critical factor affecting the riding quality and the performance of a track. Fouled ballast can accelerate track irregularities, which results in frequent ballast maintenance requirements. Severe fouling of the ballast can lead to track instability, an uncomfortable ride and, in the worst case, a derailment. In this regard, maintenance engineers perform routine track inspections to assess current and future ballast conditions. GPR has been used to assess the thickness and fouling levels of ballast. However, there are no potent procedures or specifications with which to determine the level of fouling. This research aims to develop a GPR analysis method capable of evaluating ballast fouling levels. Four ballast boxes were constructed with various levels of fouling. GPR testing was conducted using a GSSI (Geophysical Survey Systems, Inc.) device (400, 900, 1600 MHz), and a KRRI (Korea Railroad Research Institute) GPR device (500 MHz), which was developed for ballast tracks. The dielectric permittivity, scattering of the depth (thickness) values, signal strength at the ballast boundary, and area of the frequency spectrum were compared against the fouling level. The results show that as the fouling level increases, the former two variables increase while the latter two decrease. On the basis of these observations, a new integrated parameter, called a ballast condition scoring index (BCSI), is suggested. The BCSI was verified using field data. The results show that the BCSI has a strong correlation with the fouling level of the ballast and can be used as a fouling-level-indicating parameter.

In Search of a Soil Moisture Content Simulation Model: Mechanistic and Data Mining Approach Based on TDR Method Results

Soil moisture content simulation models have continuously been an important research objective. In particular, the comparisons of the performance of different model types deserve proper attention. Therefore, the quality of selected physically-based and statistical models was analyzed utilizing the data from the Time Domain Reflectometry technique. An E-Test measurement system was applied with the reflectogram interpreted into soil volumetric moisture content by proper calibration equations. The gathered data facilitated to calibrate the physical model of Deardorff and establish parameters of: support vector machines, multivariate adaptive regression spline, and boosted trees model. The general likelihood uncertainty estimation revealed the sensitivity of individual model parameters. As it was assumed, a simple structure of statistical models was achieved but no direct physical interpretation of their parameters, contrary to a physically-based method. The TDR technique proved useful for the calibration of different soil moisture models and a satisfactory quality for their future exploitation.

Structural and Dipole-Relaxation Processes in Epoxy-Multilayer Graphene Composites with Low Filler Content

Multilayered graphene nanoplatelets (MLGs) were prepared from thermally expanded graphite flakes using an electrochemical technique. Morphological characterization of MLGs was performed using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Raman spectroscopy (RS), and the Brunauer-Emmett-Teller (BET) method. DGEBA-epoxy-based nanocomposites filled with synthesized MLGs were studied using Static Mechanical Loading (SML), Thermal Desorption Mass Spectroscopy (TDMS), Broad-Band Dielectric Spectroscopy (BDS), and Positron Annihilation Lifetime Spectroscopy (PALS). The mass loading of the MLGs in the nanocomposites was varied between 0.0, 0.1, 0.2, 0.5, and 1% in the case of the SML study and 0.0, 1.0, 2, and 5% for the other measurements. Enhancements in the compression strength and the Young's modulus were obtained at extremely low loadings (C≤ 0.01%). An essential increase in thermal stability and a decrease in destruction activation energy were observed at C≤ 5%. Both the dielectric permittivity (ε1) and the dielectric loss factor (ε2) increased with increasing C over the entire frequency region tested (4 Hz-8 MHz). Increased ε2 is correlated with decreased free volume when increasing C. Physical mechanisms of MLG-epoxy interactions underlying the effects observed are discussed.

Dielectric Permittivity, AC Electrical Conductivity and Conduction Mechanism of High Crosslinked-Vinyl Polymers and Their Pd(OAc)2 Composites

Semiconductor materials based on metal high crosslinked-vinyl polymer composites were prepared through loading of Pd(OAc)₂ on both Poly(ethylene-1,2-diyl dimethacrylate) (poly(EDMA)) and poly(ethylene-1,2-diyl dimethacrylate-co-methyl methacrylate) (Poly(EDMA-co-MMA)). The thermochemical properties for both poly(EDMA) and poly(EDMA-co-MMA) were investigated by thermal gravimetric analysis TGA technique. The dielectric permittivity, AC electrical conductivity and conduction mechanism for all the prepared polymers and their Pd(OAc)₂ composites were studied. The results showed that the loading of polymers with Pd(OAc)₂ led to an increase in the magnitudes of both the dielectric permittivity and AC electrical conductivity (σ_ac). The value of σ_ac increased from 1.38 × 10⁻⁵ to 5.84 × 10⁻⁵ S m⁻¹ and from 6.40 × 10⁻⁶ to 2.48 × 10⁻⁵ S m⁻¹ for poly(EDMA) and poly(EDMA-co-MMA), respectively, at 1 MHz and 340 K after loading with Pd(OAc)₂. Additionally, all the prepared polymers and composites were considered as semiconductors at all the test frequencies and in the temperature range of 300–340 K. Furthermore, it seems that a conduction mechanism for all the samples could be Quantum Mechanical Tunneling (QMT).

The Influence of Li+ and K+ Added Cations and Annealing Temperature on the Magnetic and Dielectric Properties of Mg-Zn Ferrite

This paper presents the results of an investigation on the magnetic and dielectric properties of Mg_0.5Zn_0.5Fe₂O₄ spinel ferrite with a 1% weight percentage of Li⁺ and K⁺ added cations. The addition of metal ions plays an important role in increasing the porosity and favors the formation of ferrite at low temperatures. The goal of this new research is to demonstrate that by selecting the type of metallic cations for addition or choosing an optimal sintering temperature, it may be possible to improve the magnetic and electrical properties of Mg-Zn ferrite. The samples were prepared using sol-gel self-combustion techniques and annealed at 1000 °C, 1100 °C, and 1200 °C. Scanning electron microscopy revealed the shape and grain size of the samples, and the phase composition was analyzed using the X-ray diffraction technique. The magnetic information, such as remanent magnetization M_R, saturation magnetization M_S, and coercivity H_C, were extracted from the hysteresis loops of the samples. The electrical investigation was focused on the low- and high-frequency dependence of dielectric constant and dielectric losses. The results are discussed in terms of microstructural changes induced by the additions of Li⁺ and K⁺ metallic cations. Conclusions are drawn concerning the optimization of magnetic and electrical properties for the development of Mg-Zn ferrite with possible applications in the field of magnetic materials or electronics.

Combined Punctual and Diffused Monitoring of Concrete Structures Based on Dielectric Measurements

This work presents a microwave reflectometry-based system for monitoring large concrete structures (during the curing process and also while the structure is in use), through the combined use of punctual and diffused sensing elements. In particular, the adoption of punctual probes on a reference concrete specimen allows the development of an innovative and accurate calibration procedure, useful to obtain the value of the water content on a larger structure made of the same material. Additionally, a wire-like diffused sensing element can be permanently embedded in buildings and used to monitor the structure along the entire length of the sensing element. The adopted diffused sensing element can be used not only to detect dielectric variation during the curing process, but also throughout the service life of the structure. The combined use of punctual and diffused sensing elements represents an important innovation from a procedural point of view, able to provide detailed and quantitative information on the health status of the structure both during and after construction.

Siloxane Matrix Molecular Weight Influences the Properties of Nanocomposites Based on Metal Complexes and Dielectric Elastomer

Siloxane-based elastomers are some of the most sought-after materials for the construction of actuators and equipment for energy harvesting devices. This article focuses on changes of the mechanical (breaking stress, breaking strain, Young's modulus) and dielectric properties for elastomers prepared with silicones, induced by the variation of molecular weight of the matrix, with three different silicone polymers having 60,000 g/mol, 150,000 g/mol, and 450,000 g/mol (from GPC measurements). Multiple siloxane elastomers were crosslinked with methyltriacetoxysilane using the sol-gel route. The dielectric permittivity values of the elastomers were also enhanced with two different complex structures containing siloxane bond and 3d transition metals as filler materials for polydimethylsiloxane polymers with various molecular weights. The dielectric spectroscopy tests demonstrated a small decrease (5%) for the values of the dielectric permittivity in relation to increased molecular weight of the siloxane polymer, both for samples prepared with pure polymer and for samples with metal complexes. The samples of nanocomposites showed a >50% increase of dielectric permittivity values relative to samples prepared of pure siloxane elastomer. The thermal tests demonstrated that the nanocomposites retained thermal stability similar with samples prepared of pure siloxane elastomer. The behavior under controlled conditions of humidity showed a trend of increased water vapor sorption with increasing molecular weight but an overall hydrophobic stable character of nanocomposites.

The analysis of charge transport mechanism in mixed ionic electronic conductor composite of Sr2TiCoO6double perovskite with yttria stabilized zirconia

In this investigation, the ionic conduction mechanism in mixed ionic electronic conductors composites of Sr₂TiCoO₆/YSZ has been studied with the help of universal dynamic response. 3 mol% and 8 mol% yttria stabilized ZrO₂have been mixed with Sr₂TiCoO₆(STC) double perovskite in 1:1 ratio to prepare STC/3YSZ and STC/8YSZ composites via solid-state reaction route. AC Impedance spectroscopy has been carried out to examine the charge transport mechanism, which has been modeled using the microstructural networks of resistors and capacitors. Grain boundaries are more resistive and capacitive compared to the bulk. Modulus spectroscopy analysis demonstrates the non-Debye character of conductivity relaxation with frequency. Complex frequency-dependent AC conductivity is found to obey Almond West power law and reveals that ion migration occurs through the correlated hopping mechanism. Further, the DC conductivity and relaxation time have been found to follow the Barton Nakajima and Namikawa relation, which is correlated with AC to DC conduction. The time-temperature superposition principle has been used to explain the conductivity scaling in the intermediate frequency range. At low temperatures, the ions are localized in the asymmetric potential well, while at high temperatures, hopping behavior starts dominating. Further Kramers-Kronig transformation connects the dielectric strength with conductivity relaxation and verifies the impedance data.

Temperature Influence on PI/Si3N4 Nanocomposite Dielectric Properties: A Multiscale Approach

The interphase area appears to have a great impact on nanocomposite (NC) dielectric properties. However, the underlying mechanisms are still poorly understood, mainly because the interphase properties remain unknown. This is even more true if the temperature increases. In this study, a multiscale characterization of polyimide/silicon nitride (PI/Si₃N₄) NC dielectric properties is performed at various temperatures. Using a nanomechanical characterization approach, the interphase width was estimated to be 30 ± 2 nm and 42 ± 3 nm for untreated and silane-treated nanoparticles, respectively. At room temperature, the interphase dielectric permittivity is lower than that of the matrix. It increases with the temperature, and at 150 °C, the interphase and matrix permittivities reach the same value. At the macroscale, an improvement of the dielectric breakdown is observed at high temperature (by a factor of 2 at 300 °C) for NC compared to neat PI. The comparison between nano- and macro-scale measurements leads to the understanding of a strong correlation between interphase properties and NC ones. Indeed, the NC macroscopic dielectric permittivity is well reproduced from nanoscale permittivity results using mixing laws. Finally, a strong correlation between the interphase dielectric permittivity and NC breakdown strength is observed.

Crystal Field Effect and Electric Field Screening in Multilayer Graphene with and without Twist

We address the intrinsic polarization and screening of an external electric field in a broad range of ordered and twisted configurations of multilayer graphene, using an ab initio approach combining density functional theory and the Wannier function formalism. We show that multilayer graphene is intrinsically polarized due to the crystal field effect, an effect that is often neglected in tight-binding models of twisted bilayer graphene and similar systems. This intrinsic polarization of the order of up to a few tens of millielectronvolts has different out-of-plane alignments in ordered and twisted graphene multilayers, while the in-plane potential modulation is found to be much stronger in twisted systems. We further investigate the dielectric permittivity ε in same multilayer graphene configurations at different electric field strengths. Our findings establish a deep insight into intrinsic and extrinsic polarization in graphene multilayers and provide parameters necessary for building accurate models of these systems.

Distinctive Dielectric Permittivity of Hierarchical Nanostructures with Ordered Nanoparticle Networks Self-Assembled from AB-g-NP/AC Block Copolymer Mixtures

Directing nanoparticles into ordered organization in polymer matrix to improve macroscopic properties of nanocomposites remains a challenge. Herein, by means of theoretical simulations, we show the high permittivity of hybrid nanostructures designed with mixtures of AB block copolymer-grafted nanoparticles and lamella-forming AC diblock copolymers. The grafted nanoparticles self-assemble into parallel stripes or highly ordered networks in the lamellae of the AC diblock copolymers. The ordered nanoparticle networks, including honeycomb-like and kagomé networks, provide bending and conductive pathways for concentrating electric fields, which results in the improvement of the permittivity. We envisage that this strategy will open a gateway to prepare hierarchically ordered functional nanocomposites with distinctive dielectric properties.

Polytetrafluoroethylene Films in Rigid Polyurethane Foams' Dielectric Permittivity Measurements with a One-Side Access Capacitive Sensor

As a non-metallic composite material, widely applied in industry, rigid polyurethane (PUR) foams require knowledge of their dielectric properties. In experimental determination of PUR foams' dielectric properties protection of one-side capacitive sensor's active area from adverse effects caused by the PUR foams' test objects has to be ensured. In the given study, the impact of polytetrafluoroethylene (PTFE) films, thickness 0.20 mm and 0.04 mm, in covering or simulated coating the active area of one-side access capacitive sensor' electrodes on the experimentally determined true dielectric permittivity spectra of rigid PUR foams is estimated. Penetration depth of the low frequency excitation field into PTFE and PUR foams is determined experimentally. Experiments are made in order to evaluate the difference between measurements on single PUR foams' samples and on complex samples "PUR foams + PTFE film" with two calibration modes. A modification factor and a small modification criterion are defined and values of modifications are estimated in numerical calculations. Conclusions about possible practical applications of PTFE films in dielectric permittivity measurements of rigid PUR foams with one-side access capacitive sensor are made.

Influences of Sr2+ Doping on Microstructure, Giant Dielectric Behavior, and Non-Ohmic Properties of CaCu3Ti4O12/CaTiO3 Ceramic Composites

The microstructure, dielectric response, and nonlinear current-voltage properties of Sr²⁺-doped CaCu₃Ti₄O₁₂/CaTiO₃ (CCTO/CTO) ceramic composites, which were prepared by a solid-state reaction method using a single step from the starting nominal composition of CCTO/CTO/xSrO, were investigated. The CCTO and CTO phases were detected in the X-ray diffraction patterns. The lattice parameter increased with increasing Sr²⁺ doping concentration. The phase compositions of CCTO and CTO were confirmed by energy-dispersive X-ray spectroscopy with elemental mapping in the sintered ceramics. It can be confirmed that most of the Sr²⁺ ions substituted into the CTO phase, while some minor portion substituted into the CCTO phase. Furthermore, small segregation of Cu-rich was observed along the grain boundaries. The dielectric permittivity of the CCTO/CTO composite slightly decreased by doping with Sr²⁺, while the loss tangent was greatly reduced. Furthermore, the dielectric properties in a high-temperature range of the Sr²⁺-doped CCTO/CTO ceramic composites can be improved. Interestingly, the nonlinear electrical properties of the Sr²⁺-doped CCTO/CTO ceramic composites were significantly enhanced. The improved dielectric and nonlinear electrical properties of the Sr²⁺-doped CCTO/CTO ceramic composites were explained by the enhancement of the electrical properties of the internal interfaces.

Scale-Dependent Dielectric Properties in BaZr0.05Ti0.95O3 Ceramics

In the present work, BaZr_0.05Ti_0.95O₃ ceramics with grain sizes between 0.45 and 135 µm were prepared by solid-state reaction and classical sintering. The effect of grain size on dielectric properties was systematically explored, and it was found that dielectric permittivity reaches a maximum value for grain sizes between 1.5 and 10 µm and then rapidly drops for larger grain sizes. A numerical finite element method was employed to eliminate the effect of porosity on the effective values of permittivity. The results indicate that it is possible to have a critical size in slightly doped barium titanate ceramics with enhanced functional properties for a grain size between 1.5 and 10 µm.

Recognition of healthy and cancerous breast cells: Sensing the differences by dielectric spectroscopy

PURPOSE

The response of human cells to applied electrical signals depends on the cellular health status, because it is influenced by the composition and structure of the main cellular components. Therefore, electrical impedance-based techniques can be considered as sensitive tools to investigate healthy or disease state at cellular level. The goal of this study is to show that different types of in vitro cellular lines, related to different health status, can be differentiated using impedance spectra analysis.

METHODS

Three different types of human breast cell line, corresponding to healthy, cancerous, and metastatic adenocarcinoma cells, were measured by means of electrical impedance spectroscopy. By modeling the investigated cells with proper resistive and capacitive circuital elements, the magnitude of the cell electrical components and spectra of real and imaginary part of dielectric permittivity were obtained. The latter were subsequently examined with a commonly adopted mathematical model, in order to estimate the values of specific dielectric parameters for the three different cellular lines.

RESULTS

The relative variation of cellular capacitance with respect to that of the culture medium, estimated at 100 Hz, has a larger value for the two types of cancerous cells with respect to the noncancerous type. Furthermore, the ratio between the real and imaginary part of the dielectric permittivity function has larger values for metastatic cells with respect to the normal and nonmetastatic ones. Therefore, the mentioned relative capacitance allows to discriminate between normal and cancerous cells, whereas the results obtained for the dielectric function can discriminate between metastatic and nonmetastatic cells.

CONCLUSIONS

This study can be considered as an exploratory investigation of evaluating in vitro the health status of humans cells using selected electrical impedance parameters as potential markers. The obtained results highlight that a standard cultureware system, provided with interdigitated electrodes and appropriate impedance parameters, that is, cellular capacitance and the ratio between the imaginary and real part of cellular dielectric function, can be used to discriminate between healthy and cancerous breast cell lines, as well as different malignancy degrees.

Ameliorated Mechanical and Dielectric Properties of Heat-Resistant Radome Cyanate Composites

In order to improve the mechanical and dielectric properties of radome cyanate, a synergistic reinforcement method is employed to develop a resin-based ternary-composite with high heat-resistance and preferable radar-band transmission, which is expected to be applied to fabricate radomes capable of resisting high temperature and strong electric field. According to copolymerization characteristics and self-curing mechanism, epoxy resin (EP) and bismaleimide (BMI) are employed as reinforcements mixed into a cyanate ester (CE) matrix to prepare CE/BMI/EP composites of a heat-resistant radome material by high-temperature viscous-flow blending methods under the catalysis of aluminum acetylpyruvate. The crystallization temperature, transition heat, and reaction rate of cured polymers were tested to analyze heat-resistance characteristics and evaluate material synthesis processes. Scanning electron microscopy was used to characterize the micro-morphology of tensile fracture, which was combined with the tensile strength test and dynamic thermomechanical analysis to investigate the composite modifications on tenacity and rigidity. Weibull statistics were performed to analyze the experimental results of the dielectric breakdown field, and the dielectric-polarization and wave-transmission performances were investigated according to alternative current dielectric spectra. Compared with the pure CE and the CE composites individually reinforced by EP or BMI, the CE/BMI/EP composite acquires the most significant amelioration in both the mechanical and electrical insulation performances as indicated by the breaking elongation and dielectric breakdown strength being simultaneously improved by 40%, which are consistently manifested by the obviously increased transverse lines uniformly distributed on the fracture cross-section. Furthermore, the glass-transition temperature of CE/BMI/EP composite reaches the highest values of nearly 300 °C, with the relative dielectric constant and dielectric loss being mostly reduced to less than 3.2 and 0.01, respectively. The experimental results demonstrate that the CE/BMI/EP composite is a highly-qualified wave-transmission material with preferences in mechanical, thermostability, and electrical insulation performances, suggesting its prospective applications in low-frequency transmittance radomes.

Crossover from Ferroelectric to Relaxor Behavior in Ba1-xCaxTiO3 (x = 0.17) System

The dielectric properties of Ba_1−xCa_xTiO₃ (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba_1−xCa_xTiO₃ (x = 0.17) ceramics is discussed in the paper.

Electric Field-Induced Assembly and Alignment of Silver-Coated Cellulose for Polymer Composite Films with Enhanced Dielectric Permittivity and Anisotropic Light Transmission

Multifarious wearable electronics with flexible touch screens have been invented for extensive outdoor activities. One challenge associated with these wearable electronics is the development of materials with both high dielectric permittivity and anisotropic light transmission, which is responsible for high touch sensitivity and screen peep-proof protection, respectively. Herein, we demonstrated a scalable approach for assembling and aligning anisotropic cellulose in a polymer matrix through the thickness direction via the assistance of an electric field to address this challenge. The alignment of silver-coated fibrillated celluloses in the polymer matrix not only significantly increases dielectric permittivity but also effectively enhances optical anisotropy. The impact of alignment degree and filler content on the dielectric and optical properties of polymer composite films has been systematically studied. The kinetics and aligning mechanisms of silver-coated fibrillated celluloses are revealed by in situ optical microscope images while an electric field is applied. We believe that this study provides a facile strategy to enhance both dielectric permittivity and optical anisotropy of polymer composite films by the alignment of embedding nanoparticles via an AC electric field, which is essential for future flexible electronics and display technology.

Electrically Driven Artificial Muscles Using Novel Polysiloxane Elastomers Modified with Nitroaniline Push-Pull Moieties

The synthesis of novel dielectric elastomers that show a muscle-like actuation when exposed to a low electric field represents a major challenge in materials science. Silicone elastomers modified with polar side groups are among the most attractive dielectrics for such a purpose because of their high polarizability over a wide temperature and frequency range. Nitroaniline (NA) has a strong dipole moment, and therefore, its incorporation into silicone networks should allow the formation of elastomers with increased dielectric permittivity. However, incorporation of a large amount of NA into silicone needed to increase the dielectric permittivity is still challenging. In this work, we present the synthesis of polysiloxane elastomers modified with a large fraction of the nitroaniline (NA) polar group, following two different synthetic strategies. Both approaches allowed the formation of homogenous elastomers at the molecular level. These yellowish materials have a dielectric permittivity three times higher as compared to the reported NA-modified silicones. Additionally, they have excellent mechanical properties with low viscoelastic losses and a strain at break reaching 300%. Furthermore, the mechanical properties of these elastomers can be easily tuned by the content of cross-linkers used. The developed elastomers are highly stable in electromechanical tests and show an actuation strain of 8% at unprecedentedly low electric fields of 7.5 V/μm. The combination of properties such as high dielectric permittivity, large strain at break, low viscoelastic losses, fast and reversible actuation, and actuation at low electric fields is crucial for the new generation of dielectric elastomer materials that will find their way in applications ranging from artificial muscles, soft robots, sensors, and haptic displays to electronic skin.

Feasibility of a Wearable Reflectometric System for Sensing Skin Hydration

One of the major goals of Health 4.0 is to offer personalized care to patients, also through real-time, remote monitoring of their biomedical parameters. In this regard, wearable monitoring systems are crucial to deliver continuous appropriate care. For some biomedical parameters, there are a number of well established systems that offer adequate solutions for real-time, continuous patient monitoring. On the other hand, monitoring skin hydration still remains a challenging task. The continuous monitoring of this physiological parameter is extremely important in several contexts, for example for athletes, sick people, workers in hostile environments or for the elderly. State-of-the-art systems, however, exhibit some limitations, especially related with the possibility of continuous, real-time monitoring. Starting from these considerations, in this work, the feasibility of an innovative time-domain reflectometry (TDR)-based wearable, skin hydration sensing system for real-time, continuous monitoring of skin hydration level was investigated. The applicability of the proposed system was demonstrated, first, through experimental tests on reference substances, then, directly on human skin. The obtained results demonstrate the TDR technique and the proposed system holds unexplored potential for the aforementioned purposes.

Dielectric Properties of Glass Beads with Talc as a Reference Material for Calibration and Verification of Dielectric Methods and Devices for Measuring Soil Moisture

This paper presents dielectric measurements of talc, glass beads, and their mixtures under different moisture and salinity levels. The measurements were conducted using a prototype seven-rod probe (15 mm long central rod) connected to a single port of vector network analyzer. The samples were moistened with distilled water and KCl solutions in order to obtain six different moisture content levels. The complex dielectric permittivity was determined from vector network analyzer reflection-coefficient measurements based on the open-water-liquid calibration procedure. Next, the fitting of volumetric water content-real part of dielectric permittivity calibration curves was performed for each material at selected frequencies, and the obtained relations were compared with well-known calibration equations. Additionally, a salinity index for the tested materials was calculated. It was concluded that pure talc is not an optimal material for the calibration and verification of dielectric methods. The calibration curves obtained for glass beads and the mixtures of glass beads with talc gave results close to well-known reference calibration functions. Additionally, the addition of talc caused the data points to be less scattered. Moreover, the values of the salinity index for the tested materials were in a good agreement with literature data for sand. The obtained results indicated that glass beads with the addition of talc can be used as a reference material for the calibration and verification of dielectric methods and devices for soil moisture measurement.

Application of a Monopole Antenna Probe with an Optimized Flange Diameter for Tdr Soil Moisture Measurement

Soil volumetric water content (θ) is a parameter describing one of the most important factors conditioning proper plant growth. Monitoring soil moisture is of particular importance in the rational use of water resources for irrigation, especially during periods of water scarcity. This paper presents a method of measuring soil moisture in the vicinity of the plant root system by means of a probe designed to be mounted on a mobile device used for precise plant irrigation. Due to the specific field conditions of the measurement, the design of the probe was proposed as a monopole antenna. Electromagnetic simulations of the probe were carried out with Ansys HFSS software to optimise its dimensions. Then a prototype of the probe was manufactured to conduct laboratory measurements with the use of a vector network analyser (VNA) working in the 20 kHz to 8 GHz frequency range. The VNA analyser was configured to work in the time-domain reflectometry (TDR) mode. From measurements of the time distance between reflections from the probe’s elements it is possible to calculate the bulk dielectric permittivity of the soil surrounding the probe. Next, based on commonly used soil moisture dielectric calibrations one can determine θ of the soil sample. The paper presents simulation results and laboratory tests of an antenna probe. Due to its tough and durable design, this type of probe gives the possibility of easy application in field conditions, which makes it especially suitable for mechanically demanding measurement systems. As the sensitivity zone is comparatively large, this probe is well-suited to measuring soil moisture in the vicinity of the plant root system.

Biodielectric phenomenon for actively differentiating malignant and normal cells: An overview

Active research is invested in early diagnosis of malignant cells in order to allow scope for effective treatment. The electrical impedance measurements and the modeling techniques used to perform the calculation for dielectric permittivity, conductivity and relaxation frequency aids in the differentiation of malignant and healthy cells. The literature showed a general trend in data with malignant cells having higher electrical conductivity and lower dielectric permittivity than its surrounding healthier cells. However, the relaxation frequency that correlates with the dielectric loss factor was found twice in magnitude for cancer cells than healthy cells. The water content of a cancer cell is found higher and hence correlates with the shift in the relaxation time. The intracellular sodium ion concentration is also realized to be higher for malignant cells than normal cells and accounts for the depolarized transmembrane potential. Hence, this paper forms the framework for future technological innovation that can be taken into account to actively diagnose cancer cell.

Dielectric Relaxation Behavior of Silver Nanoparticles and Graphene Oxide Embedded Poly(vinyl alcohol) Nanocomposite Film: An Effect of Ionic Liquid and Temperature

This paper presents the dielectric characteristics of nanocomposite films of poly(vinyl alcohol) (PVA) embedded with silver (Ag) nanoparticles and graphene oxide(GO). The nanocomposite films were fabricated by using the solvent casting approach. The morphological analysis was carried out through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The dielectric relaxation behavior of nanocomposite films was analyzed in the frequency range of 10¹ to 10⁶ Hz, by varying GO loading. The temperature effect was investigated over the temperature range of 40 to 150 °C. The effect of ionic liquid (IL) was also explored by comparing the dielectric behavior of films fabricated without using ionic liquid. The conductive filler loading variation showed a significant effect on dielectric permittivity(ε′), complex impedance (Z*) and electric conductivity (σ_ac)_. The obtained results revealed that the dielectric permittivity (ε′) increased by incorporating Ag nanoparticles and increasing GO loading in PVA matrix. An incremental trend in dielectric permittivity was observed on increasing the temperature, which is attributed to tunneling and hopping mechanism. With an increase in nanofiller loading, the real part of impedance (Z′) and imaginary part of impedance (Z″) were found to decrease. Further, the semicircular nature of Nyquist plot indicated the decrease in bulk resistivity on increasing GO loading, temperature and incorporating ionic liquid. On the basis of above findings, the obtained GO-Ag-PVA nanocomposite films can find promising applications in charge storage devices.