We report on temperature-dependent dielectric behavior of disordered ternary A6B2O17 (A = Zr, Hf; B = Nb, Ta)-form oxides in the GHz frequency range. The microwave dielectric properties including relative permittivity, dielectric loss, and temperature-dependent relative permittivity were characterized using cylindrical dielectric resonators using a resonant post measurement technique. Dielectric measurements through the resonant post method approach generally agree with dielectric measurements of A6B2O17 bulk ceramics measured through standard resonant post techniques. Coefficients describing the temperature-dependent relative permittivity for ternary A6B2O17 phases are strongly positive, suggesting contributions to polarizability arising from long-range mechanisms potentially associated with structural disorder. These observations support the working hypothesis that material functionality can be engineered by the chemical diversity and structural disorder possible in high configurational entropy A6B2O17 phases.

The development of the next-generation 5G wireless networks depends critically on the engineering of optimized high- frequency devices, employing dielectric materials. This work presents a comprehensive broadband dielectric characterization of polymers, ceramics and glasses from 5 GHz until 115 GHz. Various measurement techniques including split-post, split cavity, open resonator and free-space transmission are utilized to obtain wideband spectra. The frequency-dependent permittivity and loss tangent are analyzed to identify suitable candidate materials exhibiting minimal dispersion and loss in the 5G millimeter-wave bands. The characterization reveals almost constant permittivity and a loss tangent that increases linearly with the frequency.

Advances in 5G and 6G communication technologies and their applications are in part limited by the capabilities of current electronic packaging materials, as expanded bandwidth is a pressing need for novel dielectric substrates capable of co-firing into packages and devices, characterized by low dielectric loss and enhanced thermal conductivity. This investigation provides further characterization in the dielectric, electrical and thermal conductivity properties over previously reported cold sintered composite of Sodium Molybdate Na2Mo2O7 (NMO) with hexagonal Boron Nitride (hBN), such as relative permittivity (εr) and dielectric loss (tan δ) values at high frequencies of 75-110 GHz, electrical resistivity (ρ) fitting to percolation theory, Weibull statistical analysis of electrical breakdown strength (Eb) and its anisotropic thermal conductivity (κ) influenced by the filler’s crystal structure. The cold sintered composites were systematically characterized with respect to filler volume fraction, temperature, and frequency. The findings in this analysis position engineered composites as a promising alternative for microwave substrate materials, with using a densification method that limits interactions and maximizes densification, hence the demonstration with cold sintering.

Highly effective electromagnetic (EM) wave absorber materials with strong reflection loss (RL) and a wide absorption bandwidth (EBW) in gigahertz (GHz) frequencies are crucial for advanced wireless applications and portable electronics. Traditional microwave absorbers lack magnetic loss and struggle with impedance matching, while ferrites are stable, exhibit excellent magnetic and dielectric losses, and offer better impedance matching. However, achieving the desired EBW in ferrites remains a challenge, necessitating further composition design. In this study, impedance matching is successfully enhanced and EBW in Ni-Zn ferrite is broadened by successive doping with Mn and Co , without incorporation of any polymer filler. It is found that Ni0.4Co0.1Zn0.5Fe1.9Mn0.1O4 material exhibits exceptional EM wave absorption, with a maximum RL of -48.7 dB. It also featured a significant EBW of 10.8 GHz, maintaining a 90% absorption rate (RL < -10 dB) for a thickness of 4.5 mm. These outstanding properties result from substantial magnetic losses and favorable impedance matching. These findings represent a significant step forward in the development of microwave absorber materials, addressing EM wave pollution concerns within GHz frequencies, including the frequency band used in popular 5G technology.

To fulfill the demands of more bandwidth in 5G and 6G communication technology, new dielectric substrates that can be co-fired into packages and devices that have low dielectric loss and improved thermal conductivity are desired. The motivation for this study is to design composites with low dielectric loss (tan delta) and high thermal conductivity (kappa), while still limiting the electrical conductivity, for microwave applications involving high power and high frequency. This work describes the fabrication of high-density electroceramic composites with a model dielectric material for cold sintering, namely sodium molybdate (Na2Mo2O7), and fillers with higher thermal conductivity such as hexagonal boron nitride. The physical properties of the composites were characterized as a function of filler vol.%, temperature, and frequency. Understanding the variation in measured properties is achieved through analyzing the respective transport mechanisms.

This article provides a broadband dielectric characterization of different silicate substrates up to 115 GHz, to fill the gap in the properties of different kinds of glasses in a broad part of the mm-wave spectrum. Both the internal structure (crystalline or amorphous) and the chemistry of the substrates influence the permittivity and loss tangent of the material. Quartz and sapphire are crystalline materials that exhibit a low loss in the mm-wave frequency range. Amorphous silicates generally have higher loss values than crystalline materials, and within the glasses, the level of impurities added also affects the dielectric loss. Several characterization techniques have been employed to cover a broad frequency band. The limitations of the different characterization techniques are also included. Once the dielectric properties of substrates are characterized, a metasurface has been designed and fabricated at 100 GHz to increase the reflection in window glass and provide coverage on areas that would otherwise be shadowed. The measurement results are in good agreement with the simulations.

The dielectric characterization of silicate materials up to 2.5 THz is presented in this paper, to try to fill up the THz gap between electrical and optical measurements. Several measurement techniques have been employed to provide a broadband response. Materials in the silicate family can be classified as amorphous or crystalline. The internal structure, as well as the composition of the material, influences the dielectric properties. The loss of a material depends on its crystallinity, with higher crystallinity exhibiting lower loss.

Water molecules near cellulose nanocrystals (CNCs; produced via the sulfuric acid-catalyzed hydrolysis of wood pulp) are believed to relax slower than those in the bulk liquid, which may result in unique properties of CNC aqueous dispersions. This study analyzed the polarization behavior of water molecules in CNC aqueous dispersions and other reference samples using a dielectric relaxation spectroscopy (DRS) technique in the microwave frequency range (0.2–20 GHz). As the CNC concentration increases, two slow relaxation components become prominent. The comparison with DRS data of aqueous dispersions of nanoporous silica, polyvinyl alcohol (PVA), and hairy CNC (HCNC) with amorphous chains protruding from both ends suggested that these slow relaxation modes of water near CNC surfaces cannot be attributed to direct hydrogen bonding interactions with the hydroxyl (OH) groups exposed and immobilized at the solid surface. Instead, they are similar to the water molecules interacting with OH groups attached to flexible polymer chains. Molecular dynamics (MD) simulations of the polarization behavior of water near the (110) facet of cellulose Iβ crystals confirmed that the interactions of water molecules with the cellulose crystal surface do not cause slower relaxations in the frequency range studied via the DRS. These results indicated that the CNC surface cannot be depicted with the crystallographic facets of cellulose Iβ; instead, it resembles a polymer-brush surface on which the short glucan residues or fragments of the strong acid-catalyzed hydrolysis process are swollen and extended into the aqueous phase.

This work presents an array of experimental techniques that have been used to characterize dielectric properties (permittivity and dissipation factor) of 3D printed acrylate-based polymer over a wide frequency range. At frequencies below 10 MHz, a parallel plate capacitor method provides accurate permittivity and loss results. At frequencies above 10 MHz, there are two general types of measurement techniques, resonant and transmission/reflection. Resonant measurements are at discrete frequencies and have high accuracy for low loss dielectrics with dissipation factors below 1%. Transmission/reflection methods have the advantage of being broadband and cover a large frequency range (1 MHz to 110 GHz); however, the accuracy limits measurement to high loss samples. A reflection method for the 1 MHz to 2 GHz frequency range was specifically developed for polymers and polymer composites. In this study, parallel plate, resonant and transmission line methods have been used to characterize the dielectric properties of 3D printed acrylic based polymer from 10 mHz to 100 GHz. A relaxation peak, which is associated from the motion of polar groups of side chains, was observed as a loss peak at the frequency range of 104-1010 Hz and shows the need for characterization methods over a broad frequency range.

A set of three apparatus enabling RF exposure of aerosolized pathogens at four chosen frequencies (2.8 GHz, 4.0 GHz, 5.6 GHz, and 7.5 GHz) has been designed, simulated, fabricated, and tested. Each apparatus was intended to operate at high power without leakage of RF into the local environment and to be compact enough to fit within biocontainment enclosures required for elevated biosafety levels. Predictions for the range of RF electric field exposure, represented by the complex electric field vector magnitude, that an aerosol stream would be expected to encounter while passing through the apparatus are calculated for each of the chosen operating frequencies.

We develop a class of stretchable dipole antennas based on embedding three-dimensional liquid metal network into an elastically soft elastomer as conductive branches, which can be highly stretched up to a strain of 300% while presenting high-quality reflection coefficient around -30 dB and a wide range of tunable resonant frequency from 1.55 to 0.45 GHz simultaneously. Neither mechanical damage nor performance degradation was observed during 100 stretch-release cycles under 100% strain for the antennas. The high electrical conductivity and deformation reversibility of the liquid metal, super stretchability of the soft elastomer and protection from the rigid layer are responsible for such a unique performance, enabling potential application for wireless strain sensors. (C) 2020 Elsevier B.V. All rights reserved.

Polymer composites with electrically conductive fillers have been developed as mechanically flexible, easily processable electromagnetic interference (EMI) shielding materials. Although there are a few elastomeric composites with nanostructured silvers and carbon nanotubes showing moderate stretchability, their EMI shielding effectiveness (SE) deteriorates consistently with stretching. Here, a highly stretchable polymer composite embedded with a three-dimensional (3D) liquid-metal (LM) network exhibiting substantial increases of EMI SE when stretched is reported, which matches the EMI SE of metallic plates over an exceptionally broad frequency range of 2.65-40 GHz. The electrical conductivities achieved in the 3D LM composite are among the state-of-the-art in stretchable conductors under large mechanical deformations. With skin-like elastic compliance and toughness, the material provides a route to meet the demands for emerging soft and human-friendly electronics.

Split-ring resonators have been popularized by their application in metamaterials, but their ability to concentrate electric fields has also made them useful as microwave plasma generators. Despite the existence of much work on plasma generation using ring resonators, a comparative study of the effect of different materials on plasma generation performance has been absent. This work focuses on the study of material effects on ring resonators' microwave properties and plasma generation performance at pressures ranging from 4 to 100 Torr. To achieve this end, screen-printed silver and gold ring resonators are studied due to their high conductivity, relatively low reactivity, and differences in conductivity and work function. The surface morphology and chemistry of the ring resonators are studied using optical profilometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. It is found that the main factor influencing performance between these two materials is Q-factor, which is determined using both conventional bandwidth measurements and measurements of conductivity. Q-factor is further isolated by modifying a silver ring resonator such that its Q-factor matches gold ring resonators. In addition, a film formed on the silver resonators after plasma exposure provides an opportunity to study a material, which, unlike gold, is quite different from silver. With the film present, plasma generation performance is decreased with increasing severity as pressure is decreased—20% more power is required for breakdown at 4 Torr. This change is qualitatively consistent with a model of microwave plasma breakdown where boundary effects are expected to increase as pressure is decreased.

Thin and flexible glass ribbons can be rolled into a film capacitor structures for power electronic circuits. Glass has excellent electrical properties and is a leading candidate to replace polymer films for high-temperature applications. The dielectric properties of a low-alkali aluminoborosilicate glass were characterized up to temperatures of 400 degrees C. Low-field permittivity values of 6 with dielectric loss below 0.01 were found for temperatures below 300 degrees C. The dielectric breakdown strength exceeded 5 MV/cm for temperature of 400 degrees C and high-field polarization measurements showed that glass has over 95% energy efficiency at temperatures of 200 degrees C, which is a target temperature for high-temperature power electronic circuits driven by wide bandgap semiconductor devices.

The dielectric post-resonant measurement method is extended to characterize the conduction properties of carbon nanotube sheets in the GHz frequency range (3 GHz < f < 20 GHz) without the need for external contacts or the necessity of carbon coating. Three low-loss reference dielectrics (TiO2, BaTi4O9, Teflon) with known dielectric properties are used. Conductivity values of various metals (steel, lead, brass, aluminium, copper) of the order 10(6)-10(7) S/m were measured to show the suitability of the method. The electrical conductivity of the carbon nanotube sheets could be determined to approximately 1.7 . 10(5) S/m. (C) 2016 Elsevier B.V. All rights reserved.

A proof of concept for a microwave microplasma generator that consists of a halved dielectric resonator is presented. The generator functions via leaking electric fields of the resonant modes — TE01δ and HEM12δ modes are explored. Computational results illustrate the electric fields, whereas the stability of resonance and coupling are studied experimentally. Finally, a working device is presented. This generator promises potentially wireless and low-loss operation. This device may find relevance in plasma metamaterials; each resonator may generate the plasma structures necessary to manipulate electromagnetic radiation. In particular, the all-dielectric nature of the generator will allow low-loss interaction with high-frequency (GHz–THz) waves.

Research into the properties of dielectric resonators can provide insight to potential applications in magnetic resonance imaging (MRI) technology as replacements for the radiofrequency (RF) coils used in current designs. Increasing the strength of the external magnetic field offers several advantages, including improved signal-to-noise ratio (SNR) and spectral resolution. However, this increase in field strength may require an alternative coil design as traditional RF coils have numerous difficulties at higher frequencies such as low quality factor. A potential solution may be to replace these coils with dielectric resonators. The objective of this research project is aimed at gathering information pertinent to dielectric resonators with various boundary conditions. Specifically, probe design and alternative coupling methods were investigated using network analyzers to provide insight into methods that could increase power supplied to the resonator, allowing it to generate strong RF magnetic fields within MRI equipment. By implementing a full loop probe design around the ceramic resonator, the effective power transmission was increased by 70.6% to −9.60 dB in the experimental design and by 75.7% to −15.6 dB when it was used in the MRI probe. However, this strong increase in signal transmission, made possible through the replacement of the original 12 mm diameter coil, had unintentional consequences in that the resonant frequency could be tuned to a minimum of 605 MHz instead of the ideal 600 MHz. In order to correct for this, a thin plate of CaTiO3 was added to the side of the existing resonator disk to achieve a lower resonant frequency.

High throughput physical vapor deposition has been used to grow crystalline PbnNb2O5+n (0.6 < n < 4.6) thin films on a single chip. Relative permittivity (ɛr) and dielectric loss (tan δ) were frequency independent between 100 Hz and 1 MHz and −60 °C–100 °C. Dielectric tunability achieved a maximum in the cubic pyrochlore phase (Pb1.2Nb2O6.2, PN, Pb ≈ 38%) of ∼26% (0.44 MV/cm). In comparison to barium strontium titanate (BST) and bismuth zinc niobate (BZN), PN exhibited attractive tan δ ∼ 0.0009 (0.013–0.005 in BST and 0.008–0.0005 in BZN), comparable or superior ɛr of 419 (450 in BST and 160–220 in BZN) and 26% tunability (∼50% in BST and 3.5% in BZN at equivalent fields). PN is thus considered an ideal candidate for tunable device applications.

The area of a ultra high frequency (UHF) patch antenna was reduced 80% by embedding high permittivity plugs in the antenna substrate. It is shown that a nonuniform distribution of plugs provides solutions for such problems of the miniaturized antennas as impedance matching and achieving circular polarization of the signal. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26123