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Today, innovative engineering solutions, including IoT devices, enable the precise monitoring of plant health and the early detection of diseases. However, the lifespan of IoT devices used for the real-time monitoring of environmental or plant parameters in precision agriculture is typically only a few months, from planting to harvest. This short lifespan creates challenges in managing the e-waste generated by smart agriculture. One potential solution to reduce the volume and environmental impact of e-waste is to use more environmentally friendly and biodegradable materials to replace the non-degradable components (substrates) currently used in the structure of IoT devices. In this study, we estimate the electromagnetic properties at 2565 MHz of the leaves from three widely grown crops: winter wheat, corn, and sunflower. We found that winter wheat and sunflower leaves have values of the real part of relative permittivity ranging from about 33 to 69 (wheat) and 13 to 32 (sunflower), respectively, while corn exhibits a value of about 33.5. Our research indicates that the position of a leaf on the plant stem and its distance from the soil significantly affect the relative permittivity of winter wheat and sunflower. These relationships, however, are not evident in the electromagnetic properties of corn leaves.

Carbon nanotubes (CNTs) incorporated polymeric composites have been extensively investigated for microwave absorption at target frequencies to meet the requirement of radar cross-section reduction. In this work, a strategy of efficient utilization of CNT in producing CNT incorporated aramid papers is demonstrated. The layer-by-layer self-assembly technique is used to coat the surfaces of meta-aramid fibers and fibrils with CNT, providing novel raw materials available for the large-scale papermaking. The hierarchical construction of CNT networks resolves the dilemma of increasing CNT content and avoiding the agglomeration of CNT, which is a frequent challenge for CNT incorporated polymeric composites. The composite paper, which contains abundant heterogeneous interfaces and long-range conductive networks, is capable of reaching a high permittivity and dielectric loss tangent at a low CNT loading, and its complex permittivity is, so far, adjustable in the range of (1.20–j0.05) to (25.17–j18.89) at 10 GHz. Some papers with optimal matching thicknesses achieve a high-efficiency microwave absorption with a reflection loss lower than −10 dB in the entire X-band.

Energy harvesting devices based on the triboelectric effect have attracted great attention because of their higher output performance compared to other nanogenerators, which have been utilized in various wearable applications. Based on the working mechanism, the triboelectric performance is mainly proportional to the surface charge density of the triboelectric materials. Various approaches, such as modification of the surface functional group and dielectric composition of the triboelectric materials, have been employed to enhance the surface charge density, leading to improvements in triboelectric performances. Notably, tuning the dielectric properties of triboelectric materials can significantly increase the surface charge density because the surface charge is proportional to the relative permittivity of the triboelectric material. The relative dielectric constant is modified by dielectric polarization, such as electronic, vibrational (or atomic), orientation (or dipolar), ionic, and interfacial polarization. Therefore, such polarization represents a critical factor toward improving the dielectric constant and consequent triboelectric performance. In this review, we summarize the recent insights on the improvement of triboelectric performance via enhanced dielectric polarization.

This paper introduces a symmetric bar chart-shape (SBCS) microwave sensor for measuring permittivity of fluidic samples. For designing purposes, the introduced sensor was used based on the field changes between the SBCS and rectangular loop microstrip (RLM) structure. When a sample is placed on the sensing location, interaction between SBCS and RLM varies the field intensity. The vinegar samples were combined with water and then they are placed on the sensor. The change in field intensity changes the resonance frequency. However, there is a relationship between the permittivity of samples and the resonance frequencies. The proposed sensor is implemented on the substrate of RO4003C. The relative permittivity of samples changed from 59 to 77 and the resonance frequencies changed from 2.3 to 1.4 GHz. The quality factor is 3544 and the sensitivity is 2.2%.

Electrically induced bearing failure is a reoccurring problem in modern drive train designs. To predict this damage, electrical models of bearings are required. In these models, the permittivity of lubricants is often assumed to be constant. However, the permittivity is dependent on pressure and temperature. For operating temperatures and pressures of journal bearings, no investigation of the permittivity of the lubricant exists. For this purpose, this study investigates the pressure and temperature dependence of lubricant permittivity using specially fabricated model bodies with layered structures of steel, ceramic insulating layers and copper in a parallel plate capacitor setup. Tests were performed applying temperatures between 20 °C and 100 °C and pressures between 1 and 250 bar. A mineral oil and a polyalkylene glycol (PAG) oil were examined. Results show a clear dependence of the permittivity on pressure and temperature. The mineral oil exhibits stronger pressure sensitivity, while the PAG oil shows more pronounced temperature dependence. Empirical equations to describe the permittivity as a function of temperature and pressure are derived. These findings provide relevant input for the selection of lubricants used in electrical environments. They also support the development of predictive models for modern electrical and tribological systems.

There is a great demand for low dielectric materials as insulating interlayers in large-scale integrated circuit development. However, it is still a huge challenge to reduce the dielectric permittivity of polymers while maintaining excellent thermal stability and mechanical properties. In this work, the fluorinated polyimides (PIs) in combination with a micro-branched crosslinking structure were prepared successfully by introducing different amounts of 1,3,5-tris(4-aminophenyl) benzene (TAPB) to obtain ultra-low dielectric permittivity. The results revealed that PI film containing 2 mmol TAPB had the lowest dielectric permittivity (2.47) and dielectric loss (0.008) at 1 MHz due to the fluorine atoms and the micro-branched crosslink structure, which not only decreased the molecular polarizability but also increased the free fractional volume. In addition, PI film containing 2 mmol TAPB had the highest tensile strength of 106.02 MPa with an elongation at a break of 15.1% because the presence of TAPB effectively promoted the connection between PI molecular chains, resulting in the inhibition of the molecular mobility. The incorporation of TAPB also enhanced the thermal stability and ultraviolet light-shielding performance of PI films. This method paves the way for the development of PIs with ultra-low dielectric permittivity for the electronic industry.

FeSiCr/MnZn ferrite composites with low permittivity were prepared by a two-step synthesis method, which can improve the absorbing property of the material. The phase and thermal behavior of the composites were analyzed by x-ray diffractometry, thermogravimetric analysis/differential scanning calorimetry. The morphology was studied by scanning electron microscopy. The soft magnetic properties and electromagnetic parameters of the material were tested by a vibrating sample magnetometer and a vector network analyzer. The results show that, compared with pure FeSiCr powders, the complex permittivity of a FeSiCr/MnZn ferrite composite was significantly decreased, while the complex permeability was slightly decreased, achieving a more ideal impedance matching. When the MnZn ferrite content is 15% and the simulated thickness is 2 mm, the minimum reflection loss (RL) of the sample reached − 27.4 dB at 10.4 GHz, and the bandwidth reached 5.0 GHz when RL exceeded − 10 dB.

Estimating the effective permittivity of anisotropic fibrous media is critical for advancing electromagnetic applications, requiring detailed microstructural and orientation analyses. This study introduces innovative approaches for disclosing the orientation and microstructure of fibers, leading to mixing relations. It particularly focuses on two specific fiber configurations: 1. wave-curved fibers and 2. a collection of interconnected fibers. The first approach uses sinusoidal wave fibers, considering their curvature and direction. Conversely, the approach for the interconnected fibers operates on the principle of representing fibers as a collection of straight segments. Investigations on fibrous media for both approaches were performed using numerical calculations at the microwave frequency of 2.45 GHz. Each fibrous medium was treated as an effective medium by using fibers significantly smaller than the microwave wavelength. A thorough comparison was made between the proposed mixing relations, numerical data, and state-of-the-art mixing relations to assess their consistency and validity. The comparison of the proposed approaches with traditional models shows an improved accuracy of up to 70% and 8% for the real and imaginary components of the permittivity, respectively. Additionally, the root-mean-square errors were determined as 0.001 + j0.003 and 0.001 – j0.007 for the sinusoidal and interconnected straight fibers approaches, respectively. In addition, a woven alumina fabric was used to compare the experimental resonance frequency with that from simulations using the permittivity of the fabric estimated by the interconnected straight fibers approach. These findings advance the predictive accuracy of permittivity estimation in fibrous media, providing a robust foundation for engineering applications.

With the rapid development of miniaturization and high integration of electronic devices, increasing attention has been paid to the exploration of dielectric composites with further improved dielectric permittivities. Herein, a unique design of high dielectric permittivity composites consisting of three-dimensional porous W-WO3/BaTiO3 foams hosted in epoxy matrix is proposed. It is demonstrated that the introduction of W-WO3/BaTiO3 foams into the epoxy matrix results in substantially improved dielectric permittivities in comparison with the epoxy matrix. Meanwhile, low loss which is comparable to that of the epoxy matrix is well maintained. Interestingly, obviously enhanced dielectric permittivities and greatly depressed loss are concurrently achieved via increasing the oxidation temperature. In particular, the composite with 53.1 wt% W-WO3/BaTiO3 foam oxidized at 1300 °C exhibits a high dielectric permittivity of ~ 536 and a low loss tangent of ~ 0.05@10 kHz which are about 149 and 1.5 times those of the epoxy matrix, respectively. It is believed that the strong interfacial polarization and the numerous equivalent micro-capacitors result in the significantly improved dielectric permittivities. The finite element simulation results reveal that the formation of the WO3 shell on the surface of W can effectively weaken the leakage current density, yielding the sharply depressed loss. This work provides a new strategy to design polymer composites with simultaneous high dielectric permittivity and low loss, and the strategy could also be applicable to the exploration of other high-performance dielectric composites.

Electrical and dielectric properties for bulk ethylcarbazole-terphenyl (PEcbz-Ter) have been studied over frequency range 1 kHz–2 MHz and temperature range (R.T –120°C). The copolymer PEcbz-Ter was characterised by using X-ray diffraction. The frequency dependence of the dielectric constant (εr′) and dielectric loss (εr″) has been investigated using the complex permittivity. εr′ of the copolymer decreases with increasing frequency and increases with temperature. AC conductivity (σac) data were analysed by the universal power law. The behaviour of σac increases with increasing temperature and frequency. The change of the frequency exponent (s) with temperature was analysed in terms of different conduction mechanisms, and it was found that the correlated barrier-hopping model is the predominant conduction mechanism. The electric modulus was used to analyze the relaxation phenomenon in the material.