Microwave Reflection Research Articles

Promoting the microwave absorption characteristics in the X band using core-shell structures of Cu metal particles/PPy and hexaferrite/PPy

Abstract Multifunctional microwave absorber materials exhibit promising characteristics, since they combine the diverse properties of their components. More precisely, by precipitating polypyrrole (PPy) on a mixture soundly chosen of copper and two different hexaferrites, the dielectric, plasmonic and ferrimagnetic properties can be combined in a beneficial way to tune the absorption characteristics. Beyond the multifunctionality of this absorber, several properties may emerge from intrinsic factors, such as: morphology and/or interfacial polarization linked to the core-shell structure. The absorber has been structurally characterized by XRD, FTIR and UV–vis spectroscopy. The magnetic properties and the microwave reflection loss (RL) have been investigated using a VSM and VNA, respectively. The results indicate that the presence of copper metal boosts the reflection loss of the absorber, widen the bandwidth and shifts the matching frequency towards higher values. The core-shell structure formed by coating PPy on the copper and hexaferrites mixture enhances the RL and lower both absorber thickness and loading percentage required to achieve comparable absorption performance to that of an absorber prepared by simple mixture of the components. The current optimized absorber exhibits a minimum RL of −22 dB at a matching frequency around 10.8 GHz and a thickness of 2 mm. The bandwidth under −10 dB covers all the X band starting from 9.6 GHz and continues into the Ku band.

Photoreflectance and photoinduced microwave reflectance studies of surface band bending in Mg-doped InN

Photoreflectance (PR) and microwave-reflectance photoconductivity-decay (μ-PCD) have been applied to study the surface band bending in Mg-doped InN layers with various Mg concentrations (∼1 × 1017 to ∼7 × 1019 cm−3) and thicknesses (∼400 to ∼3600 nm). A PR resonance related to the band-to-band transition at the Γ point below the surface electron accumulation is observed for moderately doped samples ([Mg]: 1.3 × 1017 cm−3 to 8.7 × 1017 cm−3). Samples with the Mg doping concentration in the range of ∼5 × 1018 ≤ [Mg] ≤ 7 × 1019 cm−3 are p-type and show very weak or no PR resonance. It is shown that the results correlate with the surface layer depletion thickness. The strongest PR signal is observed for thick depletion layers. The results are confirmed by μ-PCD measurements that show the longest carrier lifetimes for moderately doped samples with the thickest surface depletion layer.

Multi-Channel, Microwave-Based, Compact Printed Sensor for Simultaneous and Independent Level Measurement of Eight Liquids

Industrial automation relies on sensing the process parameters like liquid levels. Simultaneous sensing of multiple fluids is especially useful for many automation processes involving multiple ingredients. Some of the desirable features in any level sensor are its non-intrusiveness, high accuracy, and consistent performance. These features can be achieved with typical microwave radar-based level sensors. However, this paper presents a microwave resonance method instead of typical microwave reflectance (radar) to reliably sense the liquid level at fraction of the cost using a unique printed sensor. In addition to that, sensor design presented in this paper enables sensing liquid level of eight liquids simultaneously and independently utilizing its multiple channels. An array of eight modified microwave T-resonators has been compactly packed inside a metallic cylinder of just $4$ diameter where the resonators share the same ground plane. Sensors’ performance, to measure the unknown level of eight different fluids (with dielectric constant ( $\in _{\mathbf {r}}$ ) ranging from 2.8 to 80), has been measured in real time using a custom-built software. Simple one point calibration procedure has also been proposed in this paper which has resulted into measurement accuracy of ~0.6% which is better than 1%, realizable with high-end and expensive radar based level sensors

Ground tire rubber composites with hybrid conductive network for efficiency electromagnetic shielding and low reflection

An efficiency electromagnetic interference (EMI) shielding composite with low microwave reflection was fabricated by introducing nickel coated ultrahigh molecular weight polyethylene particles (UHMWPE@Ni) into ground tire rubber (GTR) matrix. Chain-like carbon black (CB) aggregates in GTR domains connected discontinuous Ni layers to form a hybrid conductive network. The remarkable EMI shielding effectiveness (SE) and low reflection characteristics of GTR/UHMWPE@Ni composites were attributed to the well-connected Ni–CB hybrid conductive network with both magnetic hysteresis loss and electrical loss, and the multi-interfaces structure induced by the selectively distributed Ni layer at the interface between UHMWPE and GTR. With only 1.04 vol% Ni content, the EMI SE of GTR/UHMWPE@Ni composite reached 47.3 dB in X band. Because of the synergistic effect of the Ni–CB hybrid network, the power coefficient of reflectivity (R) of the composite was as low as 0.58. The result indicated that our work provided a feasible strategy to fabricate efficient electromagnetic shielding materials with low microwave reflection.

High-performance microwave absorption of hierarchical graphene-based and MWCNT-based full-carbon nanostructures

A creative fabrication strategy is proposed for synthesizing two kinds of hierarchical graphene-based and MWCNT-based full-carbon nanostructures (FCNS), both of which are excellent microwave absorption (MA) fillers. Through using precursor graphite and MWCNTs, the FCNSs are prepared by simple ultrasonic exfoliation/dispersion and exothermal erosion methods. The hierarchical carbon nanostructures (carbon nanoparticles, carbon nanofilms and carbon hollow hemispheres) and abundant interfaces in FCNSs generate strong dielectric loss. Consequently, the FCNSs present ideal microwave impedance match, attenuation characteristics and excellent MA properties. Results show that the MWCNT-based FCNS and graphene-based FCNS composites achieve extremely strong absorption with minimum microwave reflection loss (RL) values of −58 dB with a thickness of 2.6 mm and −54 dB with a thickness of only 1.2 mm, respectively. And the effective absorption bandwidth (RL < −10 dB) can be reached to 3.44 GHz at 1.4 mm and 4.3 GHz at 1.1 mm, respectively. Moreover, we also elaborate on the fundamental MA mechanism and correlation between the nanostructures and MA properties. The ultrathin MAM can be used as predominant candidate with strong MA properties and wide bandwidth.

Selective microwave absorption in Nd3+ substituted barium ferrite composites

Microwave (MW) frequency based wireless communications and electronic devices became prospective due to several ramifications. To meet this need, a series of neodymium ions (Nd3+) substituted barium ferrite composites with composition (20)BaO:(80–x)Fe2O3:(x)Nd2O3 (0 ≤ x ≤ 3 mol%) was prepared at 1100 °C using solid-state reaction method. We evaluated the effect of various Nd3+ ions contents on the surface morphology, structure, and magnetic properties of the as-synthesized barium ferrite composites. Meanwhile, microwave reflection loss, complex permittivity and permeability were determined using the transmission/reflection line method in the X-band (8–12 GHz). SEM image of the composites shows that the surface morphology consists of rough and porous microstructures. XRD patterns of the un-doped composites reveal the existence of BaFe12O19 (hexagonal) and Fe21.333O32 (tetragonal) crystalline phases. Furthermore, a new hexagonal crystalline phase of Ba6Nd2Fe4O15 with the crystallite sizes between 15 and 67 nm is observed due to Nd3+ ions substitution in the composite. The saturation magnetization of the composite containing 2 mol% of Nd3+ does not exhibit any significant alteration compared to the one devoid of Nd3+. The complex relative permitivity and permeability of the achieved composites enriched in Ba6Nd2Fe4O15 and BaFe2O4 phases disclose significant MW frequency dependence. The composites also display selective MW absorption in the X-band which could be useful for diverse applications.

Theory of coherent phase modes in insulating Josephson junction chains

Recent microwave reflection measurements of Josephson junction chains have suggested the presence of nearly coherent collective charge oscillations deep in the insulating phase. Here we develop a qualitative understanding of such coherent charge modes by studying the local dynamical polarizability of the insulating phase of a finite length sine-Gordon model. By considering parameters near the noninteracting fermion limit where the charge operator dominantly couples to soliton-antisoliton pairs of the sine-Gordon model, we find that the local dynamical polarizability shows an array of sharp peaks in frequency representing coherent phase oscillations on top of an incoherent background. The strength of the coherent peaks relative to the incoherent background increases as a power law in frequency as well as exponentially as the Luttinger parameter approaches a critical value. The dynamical polarizability also clearly shows the insulating gap. We then compare the results in the high frequency limit to a perturbative estimate of phase-slip-induced decay of plasmons in the Josephson junction chain.

Monitoring the Variation of Vegetation Water Content with Machine Learning Methods: Point–Surface Fusion of MODIS Products and GNSS-IR Observations

Vegetation water content (VWC) is recognized as an important parameter in vegetation growth studies, natural disasters such as forest fires, and drought prediction. Recently, the Global Navigation Satellite System Interferometric Reflectometry (GNSS-IR) has emerged as an important technique for monitoring vegetation information. The normalized microwave reflection index (NMRI) was developed to reflect the change of VWC based on this fact. However, NMRI uses local site-based data, and the sparse distribution hinders the application of NMRI. In this study, we obtained a 500 m spatially continuous NMRI product by integrating GNSS-IR site data with other VWC-related products using the point–surface fusion technique. The auxiliary data in the fusion process include the normalized difference vegetation index (NDVI), gross primary productivity (GPP), and precipitation. Meanwhile, the fusion performance of three machine learning methods, i.e., the back-propagation neural network (BPNN), generalized regression neural network (GRNN), and random forest (RF) are compared and analyzed. The machine learning methods achieve satisfactory results, with cross-validation R values of 0.71–0.83 and RMSEs of 0.025–0.037. The results show a clear improvement over the traditional multiple linear regression method, which achieves R (RMSE) values of only about 0.4 (0.045). It indicates that the machine learning methods can better learn the complex nonlinear relationship between NMRI and the input VWC-related index. Among the machine learning methods, the RF model obtained the best results. Long time-series NMRI images with a 500 m spatial resolution in the western part of the continental U.S. were then obtained. The results show that the spatial distribution of the NMRI product is consistent with a drought situation from 2012 to 2014 in the U.S., which verifies the feasibility of analyzing and predicting drought times and distribution ranges by using the 500 m fusion product.

Preparation of flexible microwave interactive fabric for 8.2 to 18.0 GHz frequency: Functional evaluation and validation

In this study, for the first time, coating formulation based on milled carbon fibre has been applied on textile substrate to prepare flexible covering material for microwave protection. Cotton fabric with its inherent high dielectric property and finite length with close network of carbon milled fibre has synergised to generate highly conducting surface (σ = 96.22 S/m). The samples are prepared by coating of milled carbon fibre in various concentrations with polyurethane as binder on cotton fabric. Polyurethane contains hydroxyl and hexamethylene diisocynate groups which are polar under external electric field (dielectric constant 3.2–6.5) and also contribute high dielectric properties in coated fabric. Samples were evaluated for surface resistivity and microwave properties, i.e. permittivity, scattering parameters, reflection, transmission, absorption and reflection loss. Permittivity of high level (ɛ′: 21–31, ɛ″: 60–84) and attenuation of high order (α = 1971–2354 Np/m) are achieved. The sample coated with milled carbon fibre and polyurethane in ratio of 1: 1.67 on weight basis has shown 67–70% reflection, 28–30% absorption, 0.1–0.5% transmission, and EMI shielding of 28–30 dB in X (8.2–12.4 GHz) and Ku (12.4–8 GHz) frequency band. The results were further analysed theoretically and correlated with the scattering parameters and other microwave properties.

A dynamic-difference approach to scan probe microwave reflectivity mapping of the nanoscale electronic properties of single-walled carbon nanotubes

Understanding carbon nanotubes (CNTs) based electronic devices requires strategies to characterize individual nanotube electronic properties. We will explore a new nonevasive approach to microwave impedance microscopy (MIM) which, we hypothesize, utilizes the ambient water layer as a nanoscale high permittivity medium. This approach eliminates the need for a thin metal oxide surface layer, used in contact mode MIM-AFM of CNTs, which completely obscures resistance mapping and can increase surface roughness by &amp;gt;10×. The potential novelty of our proposed MIM methodology is that the water meniscus, known to form beneath the tip, creates a localized high permittivity environment between the tip and the surface. The materials microwave response image is extracted from the “capacitive difference” observed on trajectories’ measures via the transmission line cantilever during approach. We can mechanically detect the water meniscus formation using AFM force curves while simultaneously mapping resistance, capacitance, and topography. When comparing signal-to-noise (SNR), to contact MIM-AFM, our results suggest a &amp;gt;2× increase in MIM capacitance SNR, 10–100× improvement in MIM resistance SNR, and up to 3× increase in the capacitance mapping resolution by reducing the effects of tip–surface spatial convolution.

Analysis and optimization of microwave reflections in a plasma-metal model

The problem of interaction of microwaves with plasma is complicated and has garnered considerable interest. Parameters of plasma density, collision frequency, and plasma size have been shown to exert substantial impacts on microwave transmission in plasma. The respective influences of plasma parameters can be evaluated relatively easily; however, plasma must also be considered integrally as it is an electrically neutral medium. In this paper, using a designed plasma-metal model, we assess how plasma parameters influence the interaction of microwaves in plasma, revealing a nonmonotonic and complex relationship. Using parameter optimization, we obtain plasma parameters for plasma device design that meet the requirements of microwave transmission in plasma; the findings should prove useful in applications of electromagnetic waves in plasma.

Enhanced electromagnetic interference shielding capability in bamboo fiber@polyaniline composites through microwave reflection cavity design

Natural, electrical insulating bamboo fiber (BBF) was selected as cost-effective frameworks to fabricate electromagnetic interference (EMI) shielding composites. The compacted polyaniline (PANI) layer was formed on the surface of BBF through an in-situ polymerization process. The formation of the continuous PANI layer was attributed to the interaction between the aniline monomers and BBF surface. Three-dimensional electromagnetic reflection cavities were facilely constructed by this unique architecture. The electromagnetic properties are extensively investigated with respect to electrical conductivity, complex permittivity and permeability, loss tangent, and Cole-Cole analysis. By manipulating the mass ratio of BBF with different size, BBF@PANI possessed a high electrical conductivity of 8.99 ± 0.43 S/cm. Combined with the results of skin depth, attenuation constant, also comparisons of experimental results and theoretical predictions, it was found that besides the conductivity and dielectric loss, the compacted microwave reflection cavity could also boost the EMI shielding effectiveness (SE). By introducing effective multiple reflections, the absorption ability was enhanced. The EMI SE could even reach 45.0 dB with only 0.4 mm thickness. The attenuation of microwave was predominantly governed by an absorption mechanism. Furthermore, the SE per unit thickness could be as high as 1125.0 dB/cm, which is much higher than those other types of PANI-based materials. With outstanding EMI shielding performance, these BBF@PANI heterostructures offer promising prospects of lightweight, cost-effective and excellent performance EMI shields. We firmly believe that this research will be an effective platform for the development of green electromagnetic attenuators in the future.

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method.

This work presents a protocol employing the microwave photoconductivity decay (μ-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the μ-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the μ-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

Double-shell PANS@PANI@Ag hollow microspheres and graphene dispersed in epoxy with enhanced microwave absorption

Double-shell hollow conductive poly(acrylonitrile) microspheres@polyaniline@Ag (PANS@PANI@Ag) was synthesized by a facile two-step method. Polyaniline-coated poly(acrylonitrile) microspheres (PANS@PANI) prepared by in situ polymerization exhibited a porous, corrugated and compact conductive network, making for the formation and attachment of Ag nanoparticles. Incorporating these hollow conductive spheres and reduced graphene oxide (RGO) into epoxy resin, a lightweight microwave absorber was brought out. The chemical composition, micro-structure surface morphology and electromagnetic properties were thoroughly characterized and analyzed. The calculated results showed that the optimal reflection loss (RL) was − 44.9 dB at 9.16 GHz with a constitution of 1 wt% dielectric RGO and 1 wt% conductive PANS@PANI@Ag, and the corresponding effective bandwidth was about 2 GHz. However, the microwave absorption capacity gradually reduced with the raise of PANS@PANI@Ag content, derived from the high conductivity leading to more microwave reflection. As the PANS@PANI@Ag content increased to 5 wt%, the minimum RL was − 14.7 dB and still remained an effective absorption performance with a lower density of 0.47–0.53 g/cm3. Therefore, the as-obtained composites paved a new route for lightweight and strong absorption microwave absorbers in commercial and military application.

Investigation of Ag Nanoparticles/Water Solutions by Microwave Stripline Sensor

In this work, we developed a microwave stripline sensor based on the optimized double quadratic-shape design and the concentration of Ag nanoparticles is measured in aqueous solutions by using a real-time microwave near-field electromagnetic interaction. The {tiS}11 microwave reflection parameter of the sensor at the resonant frequency was changed depending on the Ag nanoparticles concentration in the aqueous solution. It is possible to determine the Ag nanoparticles concentration in the range of 0–50 ώg/l at the operating frequency of about 2 GHz. The measured minimum registered signal and concentration were equal to 0.03 dB/(ώg/l) and 1 ώg/l, respectively. The electromagnetic response of these type of systems is explained by the structural changes of water due to the Ag nanoparticles ablation. Because of the low cost and easy fabrication process, this system enables one to develop a chemical sensor for non-invasive monitoring of metal nanoparticles in liquid media.

Temperature-insensitive fiber-optic tip sensors array based on OCMR for multipoint refractive index measurement.

A temperature-insensitive fiber-optic tip sensors array is proposed for multipoint refractive index measurement using optical carrier based on microwave reflection (OCMR). The tip sensors array is made of a series of cleaved fiber end-faces and is spatially multiplexed by physically connecting with a fiber-optic splitter with different lengths of short delay fiber. A sensors array with eight sensing tips is demonstrated for multipoint refractive index measurement. Experimental results show that it can offer a high refractive-index resolution of 3.60 × 10-6 RIU and a low temperature-refractive index cross sensitivity of 3.74× 10-7 RIU/°C. Such a sensors array not only possesses excellent sensing performances, but also can be integrated into a chip for biochemical sensing applications.

Plastic Flow Autowaves in Freshwater Ice As Manifested by Microwave Reflection Measurements

Results of measurements of the coefficient of microwave reflection from a block of freshwater ice subjected to plastic deformation are presented. Strong fluctuations in the phase of reflection coefficient are revealed, with the extremum at an electromagnetic radiation wavelength of 1.35 cm. This wavelength corresponds to the region of localized plastic flow autowaves in crystalline media.

マイクロ波反射波画像診断技術を用いた道路陥没問題対策:～「インフラ危機管理時代」におけるスケルカ技術の応用～

マイクロ波反射波画像診断技術を用いた道路陥没問題対策:～「インフラ危機管理時代」におけるスケルカ技術の応用～

Temperature-Dependent Minority-Carrier Mobility inp-TypeInAs/GaSbType-II-Superlattice Photodetectors

Type-II superlattices (T2SLs) of narrow-band-gap semiconductors hold great promise for mid- and long-wavelength infrared (IR) detectors. To improve photodiodes based on these superlattices, understanding of minority-carrier transport along the growth direction is required, yet still lacking. Here researchers use electron-beam-induced current and time-resolved microwave reflection to investigate the key transport properties in a midinfrared T2SL photodetector, presenting a comprehensive study of carrier dynamics and the effect of surface recombination in the structure. This work will help to optimize the design and growth of T2SL structures, for better IR photodetectors.

Level Detection of Materials in Nonmetallic Tank by the DC Component of a Microwave Doppler Module

This paper describes a novel level sensor that is noninvasive to nonmetallic tanks and noncontact to materials stored in them. It detects the materials from the outside of a tank by the use of microwave Doppler modules. The output AC component of Doppler modules has been used as moving object detectors. Here, we focus on the output DC component, which is proportional to the intensity of the reflected microwave signals from the stored material. We propose a sensor to detect the level of very low dielectric materials. We build a theoretical model based on multiple reflections of microwaves from the surfaces between different dielectric materials and demonstrate the sensor’s theoretical potential. Experiments were carried out using the sensor to verify its feasibility. The sensor detected levels of Styrene foam with dielectric constant 1.02 from the outside of a 3.0 mm thick acrylic resin tank under optimal conditions. Furthermore, the S/N ratio of the Styrene foam measurement was over 19 dB for the drift noise caused by the change in environmental temperature from 20 °C to 80 °C. The S/N ratio of the change in tap-water’s temperature ranging from 0.5 °C to 77 °C was 18 dB at the reference of 23 °C, and for the change in conductivity ranging from 0.35 to 26 mS/cm was 33 dB.