Radio Frequency Region Research Articles

Improved Temperature Sensitivity of the Microwave and Radiofrequency Properties of the Ceramic Matrix Li2LaNbTiO7

This study presents experimental investigations and numerical simulations of the microwave (MW) dielectric and evaluation of the electrical properties in the radiofrequency region (RF). The ceramic matrix Li2LaNbTiO7 (LLNT) is obtained through the solid‐state reaction method. The synthesis of LLNT is confirmed using the X‐ray diffraction technique. In the complex impedance spectroscopy study, permittivity (ε′r) and loss tangent (tanδ) values are analyzed at room temperature. The study of the conductivity spectrum at different temperatures demonstrates that the conduction process is thermally activated, presenting a value of 0.65 eV for the activation energy. Numerical simulations are employed to evaluate the behavior of the LLNT matrix as dielectric resonator antennas (DRA), whereas this ceramics demonstrated a reflection coefficient of less than −10 dB at the resonant frequency, achieving a gain of 3.71 dBi, a bandwidth of 144 MHz and a radiation efficiency greater than 97%. The results indicate that the LLNT matrix would be a promising candidate for RF and MW operating devices.

Lignin-Derived Lightweight Carbon Aerogels for Tunable Epsilon-Negative Response.

Electromagnetic (EM) metamaterials have garnered considerable attention due to their capacity to achieve negative parameters, significantly influencing the integration of natural materials with artificially structural media. The emergence of carbon aerogels (CAs) offers an opportunity to create lightweight EM metamaterials, notable for their promising EM shielding or absorption effects. This paper introduces an efficient, low-cost method for fabricating CAs without requiring stringent drying conditions. By finely tuning the ZnCl2/lignin ratio, the porosity is controlled in CAs. This control leads to an epsilon-negative response in the radio-frequency region, driven by the intrinsic plasmonic state of the 3D carbon network, as opposed to traditional periodic building blocks. This approach yields a tunable and weakly epsilon-negative response, reaching an order of magnitude of -103 under MHz frequencies. Equivalent circuit analysis highlights the inductive characteristics of CAs, correlating their significant dielectric loss at low frequencies. Additionally, EM simulations are performed to evaluate the distribution of the electric field vector in epsilon-negative CAs, showcasing their potential for effective EM shielding. The lignin-derived, lightweight CAs with their tunable epsilon-negative response hold promise for pioneering new directions in EM metamaterials and broadening their application in diverse extreme conditions.

Influence of TiO2 addition on the Mg4Nb2O9 ceramic phase and its application in the radiofrequency region

In this work, the dielectric properties of (1-x) Mg4Nb2O9 – (x) TiO2 composite ceramics in radiofrequency and the microwave region were evaluated. X-ray diffraction demonstrated that Mg4Nb2O9 (MNO) reacted with TiO2 resulting in the formation of new crystalline phase Mg5(Nb0.625Ti0.375)4O15. Complex Impedance Spectroscopy (CIS) analysis was realized to analysis the electrical properties of materials, whereas was observed that activation energy (Ea) decrease from 1.64 to 1.37 eV with an increase in TiO2 concentration. Temperature capacitance coefficient (TCC) was obtained and showed that at different frequencies all materials could be used as Class 1 ceramic capacitors according to EIA RS-198. In the Microwave region, MNO – TiO2 demonstrated ε’r between 12.07 (MNO) and 14.08 (MNO18T), tan δ values varying of 2.64 × 10-4 (MNO) up to 5.92 × 10−4 (MNOT14). Regarding the temperature coefficient of resonant frequency (τf) was observed that the values decreased with the increase in TiO2, with the value closest to zero being that of pure MNO (τf = −27 ppm.°C−1). The results presented show that materials evaluated could be utilized in devices that can be act in radio frequency (RF) and microwave (MW) regions.

Investigation of negative permittivity in BaTiO3/TiN metacomposites at the radio frequency region

The intriguing and unexpected properties of metacomposites have attracted significant attention, especially in terms of tuning their negative permittivity. In this study, we fabricated percolative BaTiO3/TiN (BTN) metacomposites employing mesoporous BaTiO3 nanoparticles synthesized via a novel solvothermal method as the matrix, combined with varying concentrations of TiN as the filler. Dielectric properties investigated in the frequency range of 100 Hz to 100 kHz unveiled a transition in the permittivity, shifting from positive to negative as the TiN content increased (primarily noticeable at frequencies above 10 kHz). The initial permittivity shifting is associated with an enhanced interfacial polarization, commonly recognized as Maxwell-Wagner-Sillars effect observed below the percolation threshold. Further, Lorentz model is attributed to induce dielectric resonance in the BaTiO3 matrix triggered by the Ti4+ ions in Ti–O octahedral sites leading to negative permittivity in conjunction with increase in TiN particles above the percolation threshold. Interestingly, AC conductivity exhibited an increase with increasing frequency when TiN content was lower. However, intriguingly, this frequency-dependence in the conductivity of BTN metacomposites did not hold as TiN content increased. Moreover, the investigation of reactance revealed the inductive character derived from the conductive network, a conclusion further confirmed through equivalent circuit analysis. The findings presented in this study are poised to unlock new possibilities in the design and development of metacomposites, ultimately facilitating practical applications of epsilon-negative materials in electromagnetic devices.

Rapid, high-sensitivity detection of biomolecules using dual-comb biosensing

Rapid, sensitive detection of biomolecules is important for biosensing of infectious pathogens as well as biomarkers and pollutants. For example, biosensing of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still strongly required for the fight against coronavirus disease 2019 (COVID-19) pandemic. Here, we aim to achieve the rapid and sensitive detection of SARS-CoV-2 nucleocapsid protein antigen by enhancing the performance of optical biosensing based on optical frequency combs (OFC). The virus-concentration-dependent optical spectrum shift produced by antigen–antibody interactions is transformed into a photonic radio-frequency (RF) shift by a frequency conversion between the optical and RF regions in the OFC, facilitating rapid and sensitive detection with well-established electrical frequency measurements. Furthermore, active-dummy temperature-drift compensation with a dual-comb configuration enables the very small change in the virus-concentration-dependent signal to be extracted from the large, variable background signal caused by temperature disturbance. The achieved performance of dual-comb biosensing will greatly enhance the applicability of biosensors to viruses, biomarkers, environmental hormones, and so on.

RF-EMR pollution levels near kindergartens due to mobile communication towers and Wi-Fi sources

Wireless communications play a crucial role in human lives. The introduction of cellular network systems, broadband internet services and public Wi-Fi services increase the connectivity between people exponentially. These wireless networks require antenna structures to propagate carrier waves through space and these waves are located within the Radiofrequency (RF) region in the electromagnetic spectrum. The effects of these RF waves on people, especially children, and pregnant women cannot be neglected. According to the researchers, there are some adverse health issues related to thermal and non-thermal effects due to exposure to these non-ionizing radiations. In this study, RF exposure levels within the proximity of 21 selected kindergartens in the Colombo and Kandy districts were evaluated by using a high-frequency spectrum analyzer. Results of the study show a nearly 251% increase in the RF exposure levels in the kindergartens located in Colombo district relative to the kindergartens in Kandy district. As well as GSM900, GSM1800 and UMTS cellular communication bands show a higher contribution to background RF exposure than the other wireless networks. However, measured maximum electric fields and calculated specific absorption rate (SAR) values are well below the maximum permissible levels published by International Commission on Non-Ionizing Radiation Protection (ICNIRP).

Broadband dielectric behaviour and structural characterization of PVDF/PMMA/OMMT polymer nanocomposites for promising performance nanodielectrics in flexible technology advances

Characterization of broadband dielectric behaviour of polymer nanocomposites (PNCs) is vital for the exploration of efficient nanodielectrics as energy storage, flexible dielectric substrates, and insulators in a wide range of advanced electronic device technologies. Accordingly, herein, PNC films based on poly(vinylidene fluoride) (PVDF)/ poly(methyl methacrylate) (PMMA) blend matrix (80/20 wt/wt%) dispersed with 0, 2.5, 5, and 10 wt% organo-modified montmorillonite (OMMT) nanoclay are developed by state-of-the-art homogenized solution casting method. These PVDF/PMMA/OMMT compositions based flexible PNC films are characterized in detail by employing a scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDX) device, x-ray diffractometer (XRD), Fourier transform infrared (FTIR) spectrometer, differential scanning calorimeter (DSC), inductance-capacitance-resistance (LCR) meter, and impedance/material analyzer (IMA). The SEM microimages, XRD traces, and FTIR spectra evidenced appreciable homogeneity and surface morphology, intercalated and exfoliated OMMT structures, and the α, β and γ-phase crystallites of the PVDF in these complex semicrystalline PNCs. The DSC thermograms confirmed a significant alteration in the melting temperature and degree of crystallinity of the PVDF crystallites with the increased amount of OMMT in the 80PVDF/20PMMA blend host matrix. The broadband dielectric dispersion spectra over the frequency range of 20 Hz−1 GHz explained the contribution of interfacial polarization in the complex dielectric permittivity at lower experimental frequencies, whereas at higher frequencies permittivity is ruled by dipolar polarization in these composites at 27 °C. The dielectric loss angle tangent and electric modulus spectra revealed an intense structural dynamics relaxation process in the upper radio frequency region. The influence of OMMT concentration on the dielectric permittivity and electrical conductivity is explored. The detailed dielectric and electrical characterization of these innovative semicrystalline composites with important structural and thermal properties revealed their immense potential as high-performance nanodielectrics for highlighting current applications of broadband frequency range electrical and electronic device technologies.

Influence of the Preparation Method on the Structural, Morphological and Dielectric Properties of FeNbO4 Ceramics.

In this work, iron niobate (FeNbO4) was prepared via two processes based on the sol-gel method: colloidal gel and polymeric gel. The obtained powders were submitted to heat treatments at different temperatures based on the results obtained via differential thermal analysis. The structures of the prepared samples were characterized via X-ray diffraction and the morphology was characterized via scanning electron microscopy. The dielectric measurements were performed in the radiofrequency region using the impedance spectroscopy technique and in the microwave range using the resonant cavity method. The influence of the preparation method was noticeable in the structural, morphological and dielectric properties of the studied samples. The polymeric gel method promoted the formation of monoclinic and orthorhombic iron niobate at lower temperatures. The differences in the samples' morphology were also remarkable, both in the size and shape of the grains. The dielectric characterization revealed that the dielectric constant and the dielectric losses had the same order of magnitude and similar trends. A relaxation mechanism was identified in all the samples.

Spark plasma sintered graphene/copper calcium titanate ceramic composites with negative permittivity and enhanced thermal conductivity

Ceramic composites with negative permittivity have provoked considerable interests of researchers in electronic and dielectric devices due to the extraordinary electromagnetic performance in radio-frequency (RF) region. Herein, graphene/CaCu3Ti4O12 (GR/CCTO) ceramic composites were spark plasma sintered, of which the dielectric and thermal properties were demonstrated at RF region. An electrical percolation was identified with GR content varying from 10 wt% to 14 wt% which presenting as a dramatic increase of ac conductivity. The conduction mechanism changed from hopping conductivity to metal-like conductivity. Meanwhile, the real permittivity (ε′) turned from positive to negative which indicating an intrinsic transition of dielectric response mechanism. Therefore, Drude model was applied to elucidate the RF negative permittivity (ε' < 0) which manifesting the low-frequency plasmonic state of delocalized electrons in composites. The constructed GR networks in composites also leaded to the enhanced thermal conductivity due to the dominating contribution of phonon vibration in GR sheets. Besides, theoretical models of capacitive and inductive equivalent circuits were used on impedance spectra which successfully clarified the inductive character of negative permittivity. This work benefits expounding the generation and regulation mechanism of negative permittivity and will be favorable to exploring brand-new applications of ceramic composites.

Dielectric Behaviour and Electrical Conductivity of α-BiNbO4 and β-BiNbO4 Ceramics

In this work, orthorhombic (α-BiNbO4) and triclinic bismuth niobate (β-BiNbO4) ceramics were prepared by a wet chemical route. The structure of the obtained powders was characterised by X-ray diffraction and the morphology by scanning electron microscopy. The dielectric measurements were performed in the radiofrequency region, at different temperatures, using the impedance spectroscopy technique. The α-BiNbO4 sample presented a temperature-dependent relaxation process, with the corresponding activation energy being calculated through the Arrhenius equation. The AC conductivity dependence on the frequency was in agreement with Jonscher’s universal power. The conduction mechanism in the α-BiNbO4 compound is governed by two processes, which can be ascribed to a hopping transport mechanism. The correlated barrier hopping model until 280 K and the non-overlapping small polaron tunnelling model above 280 K are the most suitable models to describe the conductivity of this sample. In the β-BiNbO4 compound, the motion of mobile charge carriers involves localised hopping between neighbouring sites.

Difference-frequency chirped-pulse dual-comb generation in the THz region: Temporal magnification of the quantum dynamics of water vapor lines by >60 000.

A new difference-frequency method based on electro-optic phase modulators (EOMs) and two free-running lasers is reported to perform chirped-pulse dual-comb spectroscopy in the THz region. A variation of a near-IR interleaving scheme we recently reported has been developed to interleave the EOMs' orders and sidebands and to map THz comb teeth into the radio-frequency region below 1MHz. The down-converted comb teeth are shown to have transform limited widths of 1Hz over a 1s time scale. The dual chirp-pulsed scheme is used to measure the complex line shapes of two water vapor lines below 600GHz and to temporally magnify the effects of rapid passage by more than 60 000. For the 11,0 ← 10,1 transition in H2O, a pressure dependent phase perturbation is observed in the rapid passage response over the magnified time scale in contrast to a uniform line shape transformation observed for the 21,1 ← 20,2 transition of D2O. The possible origins for this anomalous behavior are modeled and discussed. The method is applicable to any region where difference or sum frequency waves can be generated.

Temperature dependent negative permittivity in solid solutions Sr2Mn1-xSnxO4 (x = 0, 0.3, 0.5)

A few compositions of the system Sr2Mn1-xSnxO4 (x = 0.0, 0.3, 0.5) were synthesized in the air by the solid-state ceramic route. A change in the sign (positive to negative) of the permittivity above a particular temperature (TC) is observed at all the measured frequencies. The negative permittivity was analyzed by the Drude-Lorentz model. It was found that negative permittivity is caused by the plasma oscillations of thermally excited free charge carriers. Analysis of XPS spectra confirmed the presence of mixed-valence states of both Mn (Mn4+ and Mn3+) and Sn (Sn4+ and Sn2+) ions. The UV–vis.-IR spectroscopy results indicated generation of a large number of defect states in the forbidden bandgap region of Sr2MnO4 on the substitution of Sn at Mn site. Synthesized samples are promising metamaterials for radio frequency (10 Hz -2 MHz) region applications due to the high-temperature plasmonic behavior.

Exploring Nanocrystalline Europium Titanium Niobate as a Dielectric Resonator

High dielectric constant and low loss nanocrystalline EuTiNbO6 ceramic was synthesized through a combustion technique. Phase purity, structure and particle size were examined using X-Ray diffraction, Fourier Transform Raman, Infrared spectroscopy and transmission electron microscopic techniques. The X-Ray diffraction study reveals that the sample crystallizes in orthorhombic aeschynite structure without any impurity. The non-agglomerated particles with narrow particle size distribution were observed in the transmission electron micrograph and the average particle size was found to be 25 nm. The sample attained the 98 % of theoretical density when it was sintered at 1150 0C. The scanning electron micrograph of the sample exhibits non uniform distribution of grains nearly in spherical shape throughout the surface. The Energy dispersive spectrum analysis confirms that the constituent elements are nearly the same atomic percentage as per the chemical formula of the sample. The dielectric properties of the sintered sample were measured in the radio frequency and micro frequency region. The dielectric constant and quality factor of the sample at room temperature are 35 and 23370, respectively. The dielectric studies revealed that the Nano crystalline EuTiNbO6 Ceramic is a promising material for dielectric resonator applications.

High thermal stability of the YNbO4 − CaYTiNbO7 composites for radio frequency and microwave applications

The present work shows present the dielectric properties of YNO – CYNTO ceramic composites at Radio Frequency (RF) and Microwave (MW) regions. X-ray diffraction showed that the addition of CaTiO3 (CTO) on YNbO4 (YNO) caused the formation of the CaYTiNbO7 (CYNTO) phase. In the RF analysis, a typical universal dielectric response in the frequency of the temperature-dependent conductivity was found; the frequency-dependent AC conductivity at different temperatures indicates that the conduction process is thermally activated. Regarding the values of the temperature coefficient of capacitance (TCC) was observed a change of signal with the increment of CTO. Besides, the activation energies of composites were calculated and demonstrated an increase with addition of the CTO. In the MW range, it was observed that permittivity and dielectric loss enhance with the addition of CTO. Moreover, the addition caused the improvement of the thermal-stability of YNO where τf value for YCT5% was equal to +0.64 ppm.°C−1. The composites were analyzed as antennas presenting a reflection coefficient below - 10 dB at the resonant frequency, realized gain of 5.10–5.45 dBi, bandwidth of 129–355 MHz and radiation efficiency above 95%. The results obtained indicate that YNO – CYNTO system would be interesting candidates in microwave operating devices.

Transition properties of a2Σ–, b2Δ, c2Π, d2Π, and e2Σ+ electronic states of aluminum silicon radical

Detecting aluminum silicon can yield useful information concerning the depletion of silicon and aluminum and their recycling in astrophysical sources. In addition, such detection can be helpful for understanding gas–phase aluminum and silicon chemistry in the interstellar/circumstellar atmosphere. However, only a few transition properties of this radical have been reported. Therefore, the potential energy curves of several lowest–lying doublet states and the transition dipole moments between them were calculated using the icMRCI approach. Core–valence correlation correction, scalar relativistic correction, and spin–orbit coupling effect were included to improve the accuracy of transition properties. The radiative lifetimes of the first 16 levels were approximately 4.8 – 679.5 ms for the c2Π1/2 state, 2.2 – 431 ms for the c2Π3/2 state, 29.3 – 40.4 µs for the d2Π1/2 state, 21.2 – 30.1 µs for the d2Π3/2 state, and 88.7 – 175.6 µs for the e2Σ+1/2 state. The spontaneous emissions from the c2Π1/2 and c2Π3/2 states to the a2Σ–1/2, b2Δ3/2, and b2Δ5/2 states and those from the e2Σ+1/2 state to the d2Π1/2 and d2Π3/2 states were located in the radio–frequency region. The d2Π1/2 – a2Σ–1/2 and d2Π3/2 – a2Σ–1/2 transitions were the strongest, followed by d2Π1/2 – b2Δ3/2, d2Π3/2 – b2Δ3/2, d2Π3/2 – b2Δ5/2, d2Π1/2 – c2Π1/2, d2Π3/2 – c2Π3/2, e2Σ+1/2 – c2Π1/2, and e2Σ+1/2 – c2Π3/2 transitions. The spontaneous emissions from these systems were located in the infrared region. The variation in the radiative lifetime of a level υ versus the rotational quantum number J was investigated for the d2Π1/2, d2Π3/2, and e2Σ+1/2 states.

Crystallographic investigation, Hirshfeld surface analysis, NLO characterization and experimental spectral (UV and NMR) studies with DFT probe on(R)-9-(2-hydroxy propyl)adenine

In this study, (R)-9-(2-hydroxy propyl)adenine (HPA) is the molecule of interest for investigation. The XRD from single crystal of HPA has been used to extract its structural features. Since HPA crystallised in a non-centro symmetric space group P212121, its NLO property was studied and it was found to exhibit very good SHG activity. To explore the intermolecular interactions the generated Hirshfeld surface has been investigated along with 2D-fingerprint plots. The experimental electronic and NMR spectra taken in the UV-visible and radio frequency regions respectively for HPA have been corroborated in correlation with theoretical predictions at Density Function Theory using 6-311++g (d, p) basis set. The experimental XRD geometrical parameters, chemical shifts of 13C and 1H and λmax values of HPA fit satisfactorily with the corresponding theoretically obtained numerical values as well as the stimulated spectrograms with the experimental ones. Further to explore the electronic structure, the MESP surface has been generated and investigated. The thermodynamic, kinetic and chemical reactivity features have been explored by means of frontier molecular orbitals of HPA.

Enhanced Radio-Frequency Sensors Based on a Self-Dual Emitter-Absorber

We propose and experimentally demonstrate a parity-time-symmetric electronic system exhibiting the self-dual emitter-absorber property with a remarkable modulation depth in the radio-frequency (rf) region. The dramatically different rf responses between the emitter and absorber modes may allow detection of ultrasmall conductive or reactive perturbations. Our measurement results show that even a perturbation on the order of ${10}^{\ensuremath{-}2}$ can greatly change the system's output intensity by more than 30 dB, consistent with the theoretical prediction. The measured sensitivity is far beyond the sensitivity of traditional sensors based on a Fabry-Perot resonator, and may lead to monotonic rf sensors with high sensitivity and resolvability.

Full-field fluorescence lifetime dual-comb microscopy using spectral mapping and frequency multiplexing of dual-comb optical beats.

Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool for quantitative fluorescence imaging because fluorescence lifetime is independent of concentration of fluorescent molecules or excitation/detection efficiency and is robust to photobleaching. However, since most FLIMs are based on point-to-point measurements, mechanical scanning of a focal spot is needed for forming an image, which hampers rapid imaging. Here, we demonstrate scan-less full-field FLIM based on a one-to-one correspondence between two-dimensional (2D) image pixels and frequency-multiplexed radio frequency (RF) signals. A vast number of dual-comb optical beats between dual optical frequency combs are effectively adopted for 2D spectral mapping and high-density frequency multiplexing in the RF region. Bimodal images of fluorescence amplitude and lifetime are obtained with high quantitativeness from amplitude and phase spectra of fluorescence RF comb modes without the need for mechanical scanning. The parallelized FLIM will be useful for rapid quantitative fluorescence imaging in life science.

Negative dielectric permittivity and high-frequency diamagnetic responses of percolated nickel/rutile cermets

Materials with negative permittivity have attracted considerable attention due to their promising applications in various electromagnetic equipment and electronic devices. Herein, percolative nickel/rutile cermets were sintered, of which the dielectric and magnetic properties were investigated in radio-frequency region. With nickel content increasing, electrical conductivity of the percolative cermets increased dramatically and the conduction mechanism changed from hopping to metallic behavior. Enhancement of positive permittivity in the percolating cermets was attributed to the increased interfacial polarization, while negative permittivity of the percolated cermets with nickel content exceeding percolation threshold was caused by plasma oscillation of delocalized electrons. Meanwhile, magnetic permeability of the epsilon-negative cermets was smaller than 1, which was resulted from the induced high-frequency diamagnetic response of eddy currents within the interconnected conductive networks. This work elucidated the anomalous dielectric and magnetic properties of percolated cermets and determined diamagnetism is an inherent nature of epsilon-negative metacomposites at high frequency.

Effect of direct–current biasing on the adjustable radio-frequency negative permittivity characteristics of Bi2SiO5/multiwall carbon nanotube metacomposites

Multiwall carbon nanotube (MWCNT) based metacomposites with negative permittivity have received significant interest over the past decade due to their unique electromagnetic properties. Here, markedly percolating Bi2SiO5 (BSO)/MWCNT metacomposites with adjustable negative permittivity were prepared by the facile hydrothermal method. Three-dimensional conductive networks formed by interconnected MWCNT are present in BSO/MWCNT metacomposites with 10 and 20 wt% of MWCNT. The detailed microstructure and resistivity studies of the metacomposites led to realize that the adjustable negative permittivity of the BSO/MWCNT metacomposites attributes to the low-frequency plasmon resonance of free Drude carriers. In this work, the effect of the direct-current biasing voltage in the radio frequency region is investigated on the negative permittivity characteristics. For below percolation threshold (fc) metacomposites, the Maxwell-Wagner polarization effects increase in low-frequency region immediately after dc bias is applied. In case of above percolation threshold (fc) metacomposites, the Drude free carrier concentration increases due to the formation of conductive network among MWCNT led to the rise of negative permittivity values in the low frequency region. This work provides a novel strategy for the adjustment of the negative permittivity value, which can expand the potential application in the field of sensors based on the changes in permittivity properties.