Absorption In Frequency Range Research Articles

Direct Growth of Edge‐Rich Graphene with Tunable Dielectric Properties in Porous Si3N4 Ceramic for Broadband High‐Performance Microwave Absorption

AbstractHigh‐performance graphene microwave absorption materials are highly desirable in daily life and some extreme situations. A simple technique for the direct growth of graphene as absorption fillers in wave‐transmitting matrices is of paramount importance to bring it to real‐world application. Herein, a simple chemical vapor deposition (CVD) route for the direct growth of edge‐rich graphene (ERG) with tailored structures and tunable dielectric properties in porous Si3N4 ceramics using only methyl alcohol (CH3OH) as precursor is reported. The large O/C atomic ratio of CH3OH helps to build a mild oxidizing atmosphere and leads to a unique structure featuring open graphite nanosteps and freestanding nanoplanes, endowing the ERG/Si3N4 hybrid with an appropriate balance between good impedance matching and strong loss capacity. Accordingly, the prepared materials exhibit superior electromagnetic wave absorption, far surpassing that of traditional CVD graphene and reduced graphene oxide‐based materials, achieving an effective absorption bandwidth of 4.2 GHz covering the entire X band, with a thickness of 3.75 mm and a negligibly low loading content of absorbents. The results provide new insights for developing novel microwave absorption materials with strong reflection loss and wide absorption frequency range.

Highly oriented flake carbonyl iron/carbon fiber composite as thin-thickness and wide-bandwidth microwave absorber

Oriented flake carbonyl iron/carbon fibers (FCI/CF) composite with enhanced microwave absorption properties were prepared. The effects of orientation, the addition of CF, and the orientation direction of CF on the microwave absorption properties were investigated, respectively. Compared with the un-oriented FCI composites, higher permeability was obtained due to the FCI orientation. Anisotropic electrical properties of FCI/CF composite were achieved by changing the arranged orientation of CF. A novel double-layer microwave absorber by combining FCI/CF absorbers with different orientation of CF was designed to achieve absorbers with wide absorption band and optimal microwave absorption. The calculated absorption properties indicated that wider absorption bandwidth and thinner thickness were obtained by the single-layer oriented composites, and the absorption properties were further enhanced by the double-layer composites. The double-layer FCI/CF composites show a wider absorption frequency range of 12.0 GHz from 6 to 18 GHz with reflection loss (RL) below −5 dB at thickness of 0.8 mm.

Microwave properties of the single-layer periodic structure composites composed of ethylene-vinyl acetate and polycrystalline iron fibers

A single-layer microwave absorbing structure composed of the ethylene-vinyl acetate (EVA) powders and polycrystalline iron fibers (PIFs) with thickness of 2 mm, which has a periodic array of circular hole, is designed and fabricated using the mechanical method. We show that the reflection loss (RL) can be easily adjusted by changing the geometric parameters. The maximum RL is 18.7 dB at 15 GHz, and the effective absorption bandwidth of 1.7 GHz for the diameter of circular hole is 10 mm, and enhanced to be 23.7 dB at 15.1 GHz with the corresponding bandwidth of 7.2 GHz when the diameter decreases to 5 mm. The measured absorption of the composite is in good accordance with the simulation results. Furthermore, the possible absorption mechanism of the composite has been discussed. Our results illustrate that the integration of frequency selective surface (FSS) with traditional PIFs can achieve a wide frequency range of a strong absorption.

Enhanced Microwave Absorption Properties of Oriented Carbonyl Iron/Carbon Black Composite Induced by Shear Force

Oriented carbonyl iron/carbon black (CI/CB) composite with enhanced microwave absorption properties was prepared by shear force which was applied to make the planes of CI parallel to each other. The effects of orientation, CB content and thickness on the microwave absorption properties were investigated. The measurement results showed that higher permeability and modest permittivity of the composite were obtained after CI orientation in a 2–18-GHz frequency range. The complex permittivity of the CI/CB composite increased with increasing CB content, which was mainly attributed to the interfacial polarization at the CI/resin/CB particle interfaces. The calculated microwave absorption properties indicated that the orientation plays an important role in decreasing the absorber thickness and broadening the absorption bandwidth. The oriented CI/CB composite containing 65 wt.% CI and 3.0 wt.% CB showed a wider absorption frequency range of 12.5 GHz from 5.5 GHz to 18 GHz with reflection loss (RL) below −5 dB at a thickness of 0.9 mm. This work offers a promising approach for the fabrication of microwave absorbing materials with thin thickness and an adjustable␣wider working frequency range.

Facile preparation and enhanced microwave absorption properties of flake carbonyl iron/Fe3O4 composite

Flake carbonyl iron/Fe3O4 (FCI/Fe3O4) composites with enhanced microwave absorption properties were prepared by a direct and flexible surface oxidation technique. The phase structures, morphology, magnetic properties, frequency-dependent electromagnetic and microwave absorption properties of the composites were investigated. The measurement results showed that lower permittivity as well as modest permeability was obtained by the FCI/Fe3O4 composites. The calculated microwave absorption properties indicated that enhanced absorption efficiency and broader absorption band were obtained by the FCI/Fe3O4 composite comparing with the FCI composite. The absorption frequency range with reflection loss (RL) below −5dB of FCI/Fe3O4 composites at reaction time of 90min at thickness of 1.5mm is 13.3GHz from 4.7 to 18GHz, while the bandwidth of the FCI composite is only 5.9GHz from 2.6 to 8.5GHz at the same thickness. Thus, such absorbers could act as effective and wide broadband microwave absorbers in the GHz range.

The effect of the base composition and microstructure of nickel-zinc ferrites on the level of absorption of electromagnetic radiation

Promising absorbing materials include Ni—Zn ferrites, as they quite intensively absorb electromagnetic waves in the frequency range from 50 to 1000 MHz. The electromagnetic properties of Ni—Zn ferrite absorbing materials obtained by different technological methods were studied in this paper. A model making it possible to evaluate the dielectric permeability of the ferrite material, depending on the microstructure parameters and electrophysical properties of grain boundaries, was proposed. The influence of base composition and microstructure on the amount of absorption of electromagnetic radiation by Ni—Zn ferrite absorbing materials was determined. It was stated that the increase of the content of excess Fe2O3 to 51.0 mol % leads to the shift of the frequency range of the absorption of electromagnetic radiation towards lower frequencies. It can be explained by the increase of the dielectric and magnetic permeability of ferrite. Moreover, the introduction of an excess of Fe2O3 in the grinding stage of the synthesized burden is more efficient. It was revealed that increasing the sintering temperature to 1350°C also shifts the frequency range of absorption of electromagnetic radiation towards lower frequencies. Probably it is caused by the increase of the dielectric and magnetic permeability of ferrite and the shift of the resonance frequency of domain walls as a result of the formation of a coarse-grained structure.

Greatly enhanced microwave absorption properties of highly oriented flake carbonyl iron/epoxy resin composites under applied magnetic field

Oriented flake carbonyl iron/epoxy resin (FCI/EP) composites with enhanced microwave absorption properties were prepared by a magnetic field which was applied to make the plane of FCI parallel to each other. The morphology and the frequency-dependent electromagnetic and microwave absorption properties of the composites were investigated. The measurement results showed that the higher permeability and modest permittivity of the composites were obtained after orientation in the frequency range of 2–18 GHz. The calculated absorption properties indicated that the orientation plays an important role in decreasing the absorber thickness and broadening the absorption bandwidths. The oriented FCI/EP composites containing 75 wt% FCI show a wider absorption frequency range of 12.5 GHz from 5.5 to 18 GHz with reflection loss below −10 dB at thickness of 1.4 mm, while the bandwidth of the un-oriented one is only in a narrow frequency range of 1.4 GHz. This work offers a promising approach for the fabrication of magnetic absorbents for thin–thickness and microwave-absorbing materials with adjustable wider working frequencies range simply by magnetic field.

Crystal structure, magnetic, and microwave properties of solid solutions BaFe12–x Ga x O19 (0.1 ≤ x ≤ 1.2)

The crystal structure of solid solutions of an M-type hexagonal barium ferrite BaFe1–xGaxO19 (x = 0.1–1.2) with the isostructural diamagnetic substitution of Ga3+ ions has been studied by X-ray diffraction. The unit cell parameters have been calculated for all compositions. The field and temperature dependences of the specific magnetization of these solid solutions have been measured by vibrational magnetometry. The microwave properties of BaFe12–xGaxO19 (x = 0.1–1.2) have been studied in a bias field. It has been shown that the frequency of natural ferromagnetic resonance decreases as the Ga concentration increases from x = 0.1 to x = 0.6 and it increases once again as the Ga concentration increases to x = 1.2. As the Ga concentration increases, the natural ferromagnetic resonance lines broaden. This indicates an increase in the frequency range of strong absorption of electromagnetic radiation. In this case, the amplitude of the resonance curve peak changes insignificantly. The frequency shift of the natural ferromagnetic resonance in an external magnetic field increases as the gallium ion concentration in the sample decreases.

Synthesis of lightweight, hierarchical cabbage-like composites as superior electromagnetic wave absorbent

Three-dimensional (3D) hierarchical cabbage-like carbonaceous materials (Fe/CCMs-X, X means treating temperature) have been successfully prepared employing combinatorial processes of cobalt-assisted hydrothermal carbonization (HTC) of furfural and subsequent iron salt impregnation and thermal treatment. This new absorbent was characterized using powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM) techniques. The maximum reflection loss (RL) of Fe/CCMs-500 reached −22.9dB, and the effective absorption bandwidth (RL⩽−10dB) was 4.32GHz (7.76–9.84GHz and 11.20–13.44GHz) corresponding to an absorber thickness of 2.5mm, a low loading amount of iron (6.83%) component and high carbon content (69.8%), clearly demonstrating its wide absorption frequency range and strong absorption ability. It is thus believed that the newly synthesized Fe/CCMs-500 can serve as light-weight electromagnetic wave absorbent and can be potentially used in practice.

Effects of structure design on resilience and acoustic absorption properties of porous flexible-foam based perforated composites

In this paper, perforated composite panel was combined with porous and resonance structures to investigate the influence on acoustic absorption and resilient properties. The perforated composite panel was fabricated based on highdensity flexible-foam via perforating and reinforcing with laminated hybrid nonwoven fabric. Effect of aperture size (AS) (ranging from 3 mm to 6 mm), perforation ratio (PR) (5 %, 10 %, 15 % and 20 %) and perforation depth (PD) (25 %, 50 %, 75 % and 100 %) on the compressive hardness, rebound resilience and acoustic absorption properties was explored. Multiply hybrid nonwoven fabric which was fabricated with low-melting point polyester (LMPET), flame-retardant polyester (FRPET) and recycled Kevlar fibers was utilized to reinforce the flexible composites and improve the acoustic property. Nonwoven that was fabricated with entangled LMPET fibers had porous structures which could reinforce the flexible foam and enhance the acoustic absorption properties. The result revealed that the continuity and supporting of porous flexible foam had directly influence the compressive hardness. The maximum hardness of the flexible-foam based perforated composites reached 420 N. The rebound resilience result showed that the sample had high resilient structure and the resilience was up to 48 %. The perforated flexible composites plate (PFP) with 4 mm-AS performed the highest acoustic absorption coefficient at 0.9. The acoustic absorption coefficient was higher than 0.8 in the frequency range from 800 to 1600 Hz and 1600 to 2400 Hz when perforated composites had 4 mm-AS at 5 % and 10 % perforation ratio. With the increase in perforation ratio, absorption peak moved from 3200 Hz to 4000 Hz. Hybrid nonwoven laminated layer help to broaden the frequency range of acoustic absorption of perforated high-density flexible foam based composites panel. Acoustic absorption coefficient was higher than 0.4 when frequency ranging from 900 Hz to 4000 Hz.

Utilizing Hollow-Structured Bamboo as Natural Sound Absorber

AbstractStudies to find alternative low environmental-impact materials for acoustic absorbers are still progressing, particularly those originated from natural materials. However, most of the established works are mainly focused on the fibrous-type absorbers. Discussion on the non-fibrous-type absorbers is still lacking and this therefore becomes the objective of this paper. Use of bamboo by utilizing its hollow structure to absorb sound energy is discussed here. The normal incidence absorption coefficient was measured based on the length and diameter of the bamboo, as well as different arrangement of the bamboo structure subjected to the incidence sound, namely, axial, transverse, and crossed-transverse arrangements. The trend of absorption coefficient appears in peaks and dips at equally spacing frequencies. For all arrangements the peak of absorption can reach above 0.8. Introducing an air gap behind the bamboo shifts the peak absorption to lower frequency. Covering the front surface of the absorber improves the sound absorption coefficient for axial arrangement by widening the frequency range of absorption also towards lower frequency range. The transverse arrangement is found to have average absorption coefficient peaks of 0.7 above 1.5 kHz. By arranging the bamboo structure with crossed-transverse arrangement, the suppressed absorption peaks in normal transverse arrangement can be recovered.

Stochastic Resonance in Graphene Bilayer Optical Nanoreceivers

Graphene, a 2-D sheet of carbon atoms, is believed to have diverse application areas ranging from medicine to communications. A novel application is using graphene as a photodetector in optical communications due to its superior optical and electrical properties such as wide and tunable absorption frequency range and high electron mobility. Noise, which is especially significant in nanoscale communications, is mostly seen as an adversary. Stochastic resonance (SR) is the performance enhancement of a system due to incorporation of noise. It is shown that the excess noise in nanocommunications can be used to improve the performance of a graphene bilayer photodetector system with hard threshold decoder, when received signals are subthreshold. SR arises due to the nonlinear nature of the hard decoder. First, the SR effect due to the background ambient noise and intentional light noise is analyzed. An approximate inverse signal-to-noise ratio expression is derived, which maximizes the mutual information. The effect of frequency on the mutual information is also investigated, and it is shown that the higher frequencies are more preferable for noise limited regimes. Later, the case with the intentional noise added to the top gate is investigated. It is shown that significant mutual information improvements are achieved for subthreshold signals, due to the multiplicative stochastic terms arising from the nonlinear graphene bilayer characteristics, i.e., the exponential dependence of photocurrent on the gate voltages. All the analytical results are verified with extensive simulations.

Energy absorption at synchronization in phase between coupled Duffing systems

The goal of energy harvesting field is to produce electric power from ambient broadband forcing. While there have been various proposals to use nonlinear effects to broaden the frequency range of energy absorption, it has not yet been clarified how to design the nonlinearity for the most efficient operation. This paper shows a possibility of energy harvesting from a multi-frequency force using a system of coupled Duffing oscillators. Using this example, we perform detailed numerical studies to illustrate that the energy absorption increases when the harvester’s velocity becomes maximum during phase synchronization between the two oscillators. We also compare the energy absorption between that system and a resonant mass-spring system. We show that for the devices base on coupled resonators, the performance can be optimized by choosing a system with the optimal path in the phase difference/velocity plane. This criteria opens a possibility to design the nonlinear characteristics of the energy harvesting devices.

Electronic and optical properties of boron nitride nanoribbons in electric field by the tight-binding model

The tight-binding model is used to study the electronic and optical properties of armchair and zigzag boron nitride nanoribbons (ABNNRs and ZBNNRs) in electric field. Electric field could significantly change dispersion relations, degeneracy, edge-states, band crossings, and energy gaps. The field-modulation of energy gaps is more effective and covers wider energy range for BNNRs than graphene nanoribbons. The critical electric field that induces zero-gap transition strongly depends on geometric structures of BNNRs. At zero field, the selection rule and the absorption frequency range of spectral functions are different between ABNNRs and ZBNNRs. Electric field would change state functions that constructs new selection rule and further exhibits more absorption peaks.

Preparation and microwave absorption properties of polyaniline/Mn0.8Zn0.2Fe2O4 nanocomposite in 2–18 GHz

The polyaniline/Mn0.8Zn0.2Fe2O4(PANI/MZF) nanocomposite was prepared by an in situ polymerization method. The samples were characterized by Fourier transform infrared spectrometer, X-ray diffraction, scanning electron microscope and vibrating sample magnetometer. The complex permittivity and complex permeability for the nanocomposites were measured by wave-guide method with vector network analyzer in 2.0–18.0 GHz. The reflection losses (R L ) of the nanocomposites were investigated according to the wave transmission theory. The results showed the maximum reflection loss of the PANI/MZF nanocomposite was about −20.6 dB at 14.4 GHz with a bandwidth of 5.6 GHz. In conclusion, a wider absorption frequency range could be obtained by adding polyaniline contain in the MZF ferrite. The PANI/MZF nanocomposite is a good microwave shielding and absorbing materials at higher frequency.

Broadening of EM Energy-Absorption Frequency Band by Micrometer-to-Nanometer Grain Size Reduction in NiZn Ferrite

Prior to its use for microwave absorption, NiZn ferrite with certain compositions and with ultrafine microstructures (submicron/nanoscale) have been fabricated to investigate the best chemical composition and microstructure for such absorption. A mixture of iron oxide (Fe2O3), nickel oxide (NiO) and zinc oxide (ZnO) was weighed according to the targeted proportion, milled using the mechanical alloying technique and sintered at a temperature of 900 °C for 10 h to form nickel zinc ferrite (NixZn1-xFe2O4). X-ray diffractometry (XRD), scanning transmission electron microscopy (STEM) and field emission electron microscopy (FeSEM) were used to investigate the crystalline phase formation, particle size and surface morphology, respectively. The toroidal samples were further measured using an Agilent 4291B impedance analyzer with the frequency range from 1 MHz to 1 GHz to investigate the materials complex permeability component of μ' and μ. The XRD results show that at 900 °C the full phase of nickel zinc ferrite was formed. The average particle size was 89.1 nm. The resulting morphology was a homogeneous microstructure with small grain size and a uniform grain size distribution via the mechanical alloying technique. A significantly important result was established: that it was possible to extend the energy absorption frequency range by reducing the grain size from micrometer to nanometer, using samples of the same chemical composition.

Study on the Use of Micro-Perforated Panel to Improve Acoustic Performance in Mosque

Most activities in mosque such as Friday prayer and the sermon by an Imam require clarity of speech. Unfortunately, this speech intelligibility performance is often poor due to initial design of a mosque. This paper presents assessment of the indoor acoustics of a mosque. Acoustical properties such as reverberation time, clarity and early decay time are obtained from simulation data using CATT indoor acoustic software. The study started with an empty mosque with no acoustic treatment. Acoustic green absorbers using micro-perforated panel (MPP) are then introduced to improve the acoustic performance. The application of MPP is still rare for mosque and is expected to replace the typical porous absorber. The effect of the panel size, location and frequency range of sound absorption are simulated and the results are discussed.

Enhanced Microwave Magnetic Properties of Ni Ferrite Doped ZnO

NiFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and ZnO compounds with different content rates were fabricated. Ni ferrite and ZnO particles used to synthesis samples were prepared through co-precipitation method and wet chemical method, respectively. The XRD and VSM results showed that Fe and Zn ions mutual penetrated to other side which caused the XRD peak to shift and an increase of saturation magnetization. The calculated frequency dependence spectrum of the reflection loss showed an enhancement of optimum reflection loss value (20 dB at 5.5 GHz) and absorption frequency range (less than 10 dB from about 3.6 to 10.5 GHz) in the 70% NiFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> content rate sample. The enhancement may be caused by the interfacial effect between NiFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and ZnO particles.

Synthesis and electromagnetic wave-absorbing properties of BaTiO3/polyaniline structured composites in 2–40 GHz

BaTiO3 coated with polyaniline (PANI), forming a composite structure, were synthesized by in situ polymerization at different BaTiO3/aniline weight ratio (BaTiO3/Ani = 1/1, 1/2, 1/3) and introduced into epoxy resin to be a microwave absorber. The spectroscopic characterizations of the formation processes of BaTiO3/polyaniline (BaTiO3/PANI) composites were studied using Fourier transform infrared, ultraviolet–visible spectrophotometer, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron spin resonance. Microwave absorption performances were investigated by reflectivity in 2–18 and 18–40 GHz using arch method. The results showed that a wider absorption frequency range could be obtained by adding different PANI content in BaTiO3.

Extension of the frequency range of resonant sound absorbers using two-degree-of-freedom Helmholtz-based resonators with a flexible panel

To enhance the effect of a sound absorber having Helmholtz resonators in a wide frequency range, a sound absorber with two-degree-of-freedom (two-DOF) Helmholtz-based resonators is proposed. The resonator has a flexible panel installed in its cavity so that the panel can be vibrated by incident sound. Thus, the resonator acts as a two-DOF system. The resonant frequency of the system consisting of the panel and back cavity is designed to tune to that of a conventional resonator system, splitting the absorption resonant peak into two. Moreover, by adjusting the dimensions, damping, and other characteristics of the proposed resonator, the absorption coefficient can be improved. However, the effect of these parameters is complicated; therefore, it is difficult to design a sound absorber with the appropriate characteristics. Hence, in this paper the fundamental characteristics of the proposed absorbers are investigated. The absorption coefficient and specific acoustic impedance are derived analytically using dimensionless parameters. The effects of the parameters are examined, and a practical method of broadening the frequency range of the effective sound absorption is discussed. Furthermore, a method in which the absorbent material is installed into the back cavity is proposed in order to extend the frequency range of the resonator-type sound absorber. Finally, experimental results are presented to confirm the validity of the proposed resonator.