Absorption Frequency Bandwidth Research Articles

Enhanced electromagnetic wave absorption of La3+–Zr4+ cosubstituted M−type barium hexaferrite

The development and breakthrough of electronic information technology have led to a profound focus on exploring new millimeter-wave absorbing materials. Herein, La–Zr ions co-substituted barium ferrite BaxLa1−xZrxFe12−xO19 (BLZFO) was prepared using a sol–gel method. BaFe12O19 possesses a dual loss mechanism (electrical/magnetic) and exhibits a natural resonance frequency of ∼ 45 GHz, making it effective for millimeter-wave absorption. The presence of La–Zr ions enhances the electromagnetic parameters of the material, improving its attenuation ability and optimizing impedance matching. Consequently, a wide, effective absorption frequency band and a minimal matching thickness are achieved. Specifically, within the 26.5–40 GHz range (Ka-band), BLZFO (x = 0.4) demonstrates an effective absorption bandwidth of 12.15 GHz, an optimal reflection loss of − 21.17 dB, and a corresponding matching thickness of 0.6 mm. This material not only exhibits strong reflection loss but also meets the requirements of an effective absorption frequency bandwidth and a small matching thickness, underscoring its profound importance for millimeter-wave absorption material research.

Construction of magnetic nanoparticle channels in SiC/SiO2 composite foam toward efficient electromagnetic wave absorption

The combination of dielectric-magnetic loss synergy and rational microstructure construction is conducive to satisfy the thin thickness, strong absorption and wide frequency bandwidth of the absorbers. In this study, a novel strategy of constructing magnetic nickel nanoparticle channels inside SiC/SiO2 composite foams for the synthesis of Ni/SiC/SiO2 porous composites (NSPC). The magnetic nickel nanoparticle channels retain SiC/SiO2 composite foam internal pore structure, which can optimize the impedance matching and satisfy the dielectric-magnetic loss synergy effect. The results confirm that the presence of multiple heterogeneous interfaces in NSPC, the magnetic and conductive losses imparted by magnetic particles and the unique foam porous structure synergistically make it have extremely excellent electromagnetic wave (EMW) absorption properties. NSPC can obtain ultra-strong reflection loss of −64.86 dB (1.46 mm) and the effective absorption range covers 13.20 GHz (4.80–18 GHz) at a total thickness of 1.79 mm (1.21–3 mm), representing 82.5 % of the whole range of frequencies. Additionally, RCS simulation confirms that NSPC can dissipate more electromagnetic capability in a real environment. In short, this work provides a new reference for the development of materials with rational microstructure construction while achieving thin thickness, strong absorption and broadband absorption.

Synthesis of a one-dimensional carbon nanotube-decorated three-dimensional crucifix carbon architecture embedded with Co7Fe3/Co5.47N nanoparticles for high-performance microwave absorption

Low-dimensional cell-decorated three-dimensional (3D) hierarchical structures are considered excellent candidates for achieving remarkable microwave absorption. In the present work, a one-dimensional (1D) carbon nanotube (CNT)-decorated 3D crucifix carbon framework embedded with Co7Fe3/Co5.47N nanoparticles (NPs) was fabricated by the in-situ pyrolysis of a trimetallic metal–organic framework (MOF) precursor (ZIF-ZnFeCo). Co7Fe3/Co5.47N NPs were uniformly dispersed on the carbon matrix. The 1D CNT nanostructure was well regulated on the 3D crucifix surface by changing the pyrolysis temperature. The synergistic effect of 1D CNT and the 3D crucifix carbon framework increased the conductive loss, and Co7Fe3/Co5.47N NPs induced interfacial polarization and magnetic loss; thus, the composite manifested superior microwave absorption performance. The optimum absorption intensity was −54.0 dB, and the effective absorption frequency bandwidth reached 5.4 GHz at a thickness of 1.65 mm. The findings of this work could provide significant guidance for the fabrication of MOF-derived hybrids for high-performance microwave absorption applications.

Equivalent Circuit Method Based Double Layer Micro-perforated Panel (MPP) Design to Widen the Sound Absorption Bandwidth

Acoustical solutions that occupy minimum space and simultaneously allow for flexibility in interior spaces are not feasible by using thick absorbers like conventional porous and fibrous materials. Micro-perforated panels are an excellent alternative to those classical materials due to their environmental friendliness, design flexibility on various materials, and easy installation. However, they come with the handicap of having a very narrow absorption frequency bandwidth. This study explores two inhomogeneous microperforated layers arranged in a cascade with different back cavities in order to widen the sound absorption bandwidth. This serial-parallel architected MPP system is investigated by mathematical models using Maa's model for a single MPP and the Equivalent Circuit Method (ECM). The numerical results were validated through experiments for normal sound incidence in the impedance tube using the transfer function method. Increasing the back cavity length of the MPP having a higher perforation ratio and larger holes results in a higher absorption in the high-frequency range. In contrast, combinations of larger cavity lengths and smaller perforation ratios are more effective in the low-frequency spectrum. By tuning the design parameters of those structures, it is possible to achieve wideband absorption by fibreless materials for room acoustic applications.

Enhancement of sound absorption of coir fiber using thin layer of kapok fibers

ABSTRACT Natural fiber materials are often favored as a replacement to synthetically made sound absorbers due to its comparable sound absorption properties as well as in the effort to reduce the environmental issue. However, studies are weighted heavily on evaluating the performance of a single or composite fiber and only few studies focused on the multi-layer fiber. In this paper, the sound absorption performance of multi-layer coir and kapok fibers is presented. The kapok fiber is used as the assisting layer element for the coir fiber. A combination of coir fibers with different layer thicknesses and with additional 2 mm thick kapok layer shows significant improvement of the absorption bandwidth. It is also found that the kapok fiber layer placed in between the coir fiber layers show the best improvement of the absorption frequency bandwidth.

A 3D flower-like Fe3O4@PPy composite with core-shell heterostructure as a lightweight and efficient microwave absorbent

In this study, a 3D flower-like ferroferric oxide @ polypyrrole (Fe3O4@PPy) composite with core-shell heterostructure was successfully fabricated via solvothermal treatment and polymerization. The optimized Fe3O4@PPy composites exhibited superior microwave absorbing properties via reasonable structural design and conductive loss regulation by varying PPy proportion. With a thickness of 2.08 mm, the fabricated Fe3O4@PPy composite material exhibited a maximum absorption frequency bandwidth of 6.00 GHz (12.00–18.00 GHz). The good absorption performance of the 3D flower-like core-shell Fe3O4@PPy was attributed to the heterostructures and impedance matching between Fe3O4 and PPy, which introduced more magnetic-dielectric cooperative effects, the interface polarization, tailored conduction, and the extension of electromagnetic (EM) wave transmission path. Therefore, constructing a 3D core-shell flower-like heterostructure is a promising strategy for improving electromagnetic wave absorption (EMWA) performance.

Recent Progress in Iron-Based Microwave Absorbing Composites: A Review and Prospective.

With the rapid development of communication technology in civil and military fields, the problem of electromagnetic radiation pollution caused by the electromagnetic wave becomes particularly prominent and brings great harm. It is urgent to explore efficient electromagnetic wave absorption materials to solve the problem of electromagnetic radiation pollution. Therefore, various absorbing materials have developed rapidly. Among them, iron (Fe) magnetic absorbent particle material with superior magnetic properties, high Snoek’s cut-off frequency, saturation magnetization and Curie temperature, which shows excellent electromagnetic wave loss ability, are kinds of promising absorbing material. However, ferromagnetic particles have the disadvantages of poor impedance matching, easy oxidation, high density, and strong skin effect. In general, the two strategies of morphological structure design and multi-component material composite are utilized to improve the microwave absorption performance of Fe-based magnetic absorbent. Therefore, Fe-based microwave absorbing materials have been widely studied in microwave absorption. In this review, through the summary of the reports on Fe-based electromagnetic absorbing materials in recent years, the research progress of Fe-based absorbing materials is reviewed, and the preparation methods, absorbing properties and absorbing mechanisms of iron-based absorbing materials are discussed in detail from the aspects of different morphologies of Fe and Fe-based composite absorbers. Meanwhile, the future development direction of Fe-based absorbing materials is also prospected, providing a reference for the research and development of efficient electromagnetic wave absorbing materials with strong absorption performance, frequency bandwidth, light weight and thin thickness.

Ultra-Broadband Sound Absorbing Materials Based on Periodic Gradient Impedance Matching

Ultra-broadband sound absorption with periodic gradient impedance matching is of great concern in the research of acoustic materials. This study investigates the impedance matching of composite acoustic metamaterials comprising micro-perforated plates (MPPs) and subsequent Fabry–Pérot(FP) channels. Based on the impedance matching theory, ultra-broadband sound absorption has been realized through a design with 36 channels whose thickness is only 50 mm. Also, the average sound absorption coefficient of 88% and an absorption frequency bandwidth (490–4000 Hz) can be achieved by preserving the matching impedance. Furthermore, by changing the thickness of the perfect absorber being 100 mm, the average sound absorption coefficient can reach greater than 90% in the frequency range of 250–4500 Hz. The proposed structure is validated analytically and experimentally. This work can help with designing and improving the method of acoustic metasurfaces and especially have the potential in ultra-broadband sound absorber designs.

Ultra-Broadband Tunable Terahertz Metamaterial Absorber Based on Double-Layer Vanadium Dioxide Square Ring Arrays.

Based on the phase transition of vanadium dioxide(VO2), an ultra-broadband tunable terahertz metamaterial absorber is proposed. The absorber consists of bilayer VO2 square ring arrays with different sizes, which are completely wrapped in Topas and placed on gold substrate. The simulation results show that the absorption greater than 90% has frequencies ranging from 1.63 THz to 12.39 THz, which provides an absorption frequency bandwidth of 10.76 THz, and a relative bandwidth of 153.5%. By changing the electrical conductivity of VO2, the absorption intensity can be dynamically adjusted between 4.4% and 99.9%. The physical mechanism of complete absorption is elucidated by the impedance matching theory and field distribution. The proposed absorber has demonstrated its properties of polarization insensitivity and wide-angle absorption, and therefore has a variety of application prospects in the terahertz range, such as stealth, modulation, and sensing.

Enhanced electromagnetic wave absorption properties integrating diverse loss mechanism of 3D porous Ni/NiO microspheres

The research and development of electromagnetic wave absorbing materials with light weight, thin thickness, powerful absorption, and wide frequency bandwidth is a long-term pursuing. Here, Ni/NiO microspheres were synthesized through hydrothermal reaction followed by thermal reduction process. The experimental results indicated that the electromagnetic parameters and electromagnetic wave absorption performance were seriously affected by the morphology, composition and loading of Ni/NiO microspheres. When the reduction temperature rose to 550 °C, Ni/NiO550 showed a special porous-microsphere structure which assembled by irregular Ni/NiO core-shell nanorods. The micro-size combination of porous and core-shell structures, the affluent surface area, sufficient NiO-Ni interface, high-concentration active sites were conducive to the impedance matching and diverse loss mechanism including conductive loss, interfacial polarization, dipole polarization, natural/exchange resonance and multiple scattering. Finally, 45 wt% porous Ni/NiO/paraffin composite exhibited high reflection loss (− 52.15 dB) and broad effective frequency bandwidth (3.22 GHz) at a thickness of 2.17 mm.

Fabrication of S-doped Ti3C2Tx materials with enhanced electromagnetic wave absorbing properties

Ti3C2Tx (MXene) materials with multilayered structures have been widely developed in various academic fields. However, the application of electromagnetic wave (EMW) absorption is limited because of the high conductivity, which leads to impedance mismatch. Herein, the impedance-matching properties were optimized by loading highly defective TiO2 onto an MXene sheet with S-doping and high-temperature annealing treatment. This treatment enhanced the EMW absorption because of the synergistic effect between the appropriate conductivity and enhanced polarization. The results show that S-doped MXene-300 has a conductivity of 2.70 × 10–5 S/cm and exhibits the best area of impedance matching among the investigated samples. When the as-prepared S-doped MXene-300 quality ratio with paraffin was 1:1, the minimum reflection loss could reach −44.91 dB with a 4 mm coating thickness. In addition, the widest effective absorption (reflection loss ≤–10 dB) frequency bandwidth was 5.88 GHz when the thickness was 2.5 mm, which indicated that the dipolar polarization is the main positive EMW attenuation pathway for the S-doped MXene absorber. This research provides a new means of developing high-performance MXene EMW absorbers with adjustable conductivity and dielectric loss.

Broadband sound absorbers of multilayered micro-slit panels using Bayesian probabilistic inference

Micro-perforated panel absorbers can typically achieve either visual transparency or broadband absorption, but not both. This paper assesses the potential of Multilayer Micro-Slit panels to maintain both of these characteristics simultaneously. Micro-slit panels are similar to micro-perforated panels, and can similarly achieve high absorption coefficients without fibrous backing materials. The arrangement of slits are better suited to visual transparency than perforated holes because it provides more unobstructed panel per perforated area. However, these types of absorbers are limited to a narrow frequency bandwidth of effective absorption. By combining several panels into a multilayer assembly, broadband absorption becomes possible. The inherent complexity stemming from optimizing the parameters for multiple layers to meet a given design criteria necessitates the use of the Bayesian framework. This probabilistic method rapidly hones in on the best parameters of each individual layer so that the overall composite meets the design goal. Furthermore, Bayesian inference implemented cyclically alongside panel fabrication and testing allows for corrections of fabrication tolerances while assessing visual transparency.

Effect of synthesis conditions on the structure and properties of LiFeO2/ZnFe2O4 nanocomposites prepared by a sol-gel method

A series of LiFeO2/ZnFe2O4 nanocomposites with different LiFeO2 contents were successfully synthesized from LiOH·H2O, Zn(NO3)2·6H2O and Fe(NO3)3·9H2O by a sol-gel method at various calcination temperature. The effect of composition and calcination temperature on structural and electromagnetic properties was studied by means of x-ray diffraction (XRD), scanning electron microscopy (SEM) and vector network analyzer. The XRD results indicated that the existence of LiFeO2 and ZnFe2O4 phase. SEM results showed that the prepared powder reveals irregular particle shapes and random particle size. Electromagnetic property results revealed that the minimum reflection loss and the frequency bandwidth of microwave absorption can be adjusted by changing the composition of the prepared composites. It is also proposed that the sol-gel method is an effective way to prepare the multiphase materials with adjustable properties.

Fabrication of hollow microhemisphere-like polypyrrole and carbon dielectric materials by sol–gel template method for enhanced microwave absorption

Hollow-microstructured dielectric materials offer immense potential for next-generation lightweight high-performance microwave absorbers with strong reflection loss and wide absorption frequency bandwidth. Herein, we demonstrate the successful synthesis of hollow polypyrrole microhemispheres (HPMs) and hollow carbon microhemispheres (HCMs) via a facile sol–gel method with SiO2 microspheres as the templates. Compared to some of polypyrrole and carbon absorbers reported previously, HPMs and HCMs exhibit excellent microwave absorption performances. Particularly, HCMs possess stronger conductive loss and better characteristic impedance matching than HPMs, displaying superior microwave absorption properties. Combining with disordered and aggregated nanoparticles as control samples, their electromagnetic analysis further reveals that the improved dielectric loss, optimized impedance matching, and multiple reflection/scattering derived from hollow hemispherical microstructures primarily account for the enhanced microwave absorption performance of these hollow microhemisphere-like products. It is believed that this work provides a new idea for preparing high-performance dielectric microwave absorbers by construction of their hollow microstructure.

Cross‐linking, mechanical, dynamical, and EMI absorption shielding effectiveness of NBR based composites filled with combination on ferrite and carbon based fillers

AbstractIn the current study, manganese‐zinc ferrite in a concentration scale ranging from 100 to 500 phr was incorporated into acrylonitrile‐butadiene rubber having the constant amount of carbon black (20 phr) or carbon nanotubes (5 phr). The work was focused on the investigation of fillers combination on curing process, physical‐mechanical, and dynamic characteristics of composites. The electromagnetic interference absorption shielding efficiency of composites was subsequently investigated at low frequencies. The results revealed that composites filled with a combination of ferrite and CB are able to efficiently shield electromagnetic radiation in the tested frequency range 1 MHz to 3 GHz, while electromagnetic interference absorption shielding performance of CNT/ferrite composites was insufficient. The reason can be attributed to the outstanding increase of electrical conductivity of composites filled with combination of ferrite and CNT. As the best absorption shielding material can be considered the CB/ferrite composite containing 100 phr of ferrite, as it exhibited the widest absorption frequency bandwidth at return loss of −10 and −20 dB. The presence of ferrite in composites resulted in the decrease of cross‐link density, modification of curing characteristics and physical‐mechanical properties, and no significant influence on glass transition temperature.

A nanostructure of reduced graphene oxide and NiO/ZnO hollow spheres toward attenuation of electromagnetic waves

A nanostructured composite composed of reduced graphene oxide (rGO) and NiO/ZnO hollow spheres was prepared via an in situ reaction involving pyrolysis and surface modification of a heterobimetallic Ni–Zn metal-organic framework with amine-functional group. The rGO sheet was successfully grafted on the surface of NiO/ZnO. The hybrid displayed excellent microwave absorbing performance in a polystyrene matrix. The optimal minimum reflection loss reached −42.5 dB at 13.2 GHz at a coating thickness of only 2.15 mm with a 4.5 GHz effective absorption frequency bandwidth, corresponding to a reflection loss of less than −10 dB (90% absorption). The grafting of rGO not only resulted in a hybrid composite with a larger contact area, but also benefited the synergistic effects with NiO/ZnO to endow the electromagnetic wave absorbing performance. Herein, the effective absorption capability of rGO@NiO/ZnO mainly contributed to the enrichment of interfacial polarization, dipole polarization, and dielectric loss followed by multiple reflection and scattering generated from heterogeneous components in the composite. This work is expected to provide a promising method for the development of high-performance microwave absorbers staring from rGO/heterobimetallic MOFs composites.

Synthesis and Enhanced Electro-Magnetic Wave Absorbing Properties of Reduced Graphene Oxide-Fe3O4-Polyaniline Ternary Nano-Composites

Micro-emulsion method is adopted to synthesis rGO-Fe3O4-PANI (reduced graphene oxide-Fe3O4-polyaniline) ternary nano-composites. With the different contents of aniline (0.1 g, 0.2 g, 0.3 g) added in production, the corresponding final products are named as sample 1, sample 2, sample 3, respectively. The results of TEM indicate that the mean size of Fe3O4 nano-particles is 14 nm and they are anchored onto rGO slice. There are some PANI polymer wrinkles distributed on the rGO surface uniformly. The electro-magnetic data demonstrate that sample 1 possess a great enhancement of the electro-magnetic wave absorption abilities and frequency bandwidth among the three samples. The RLmin (minimum reflection loss) value of sample 1 is -44.16 dB at 4.29 GHz with 5 mm coating thickness and 1.77 GHz (3.72-4.94 GHz, 13.33-13.88 GHz) frequency bandwidth (RL <italic/> -10 dB). We can believe that this kind of nano-composites will have a promising application in the future.

The enhanced microwave broadband absorbing ability of carbon microspheres via electromagnetic simulating honeycomb design

The electromagnetic simulation can be used to design the macroscopic absorbing structure of the microwave absorbing material. Research indicates that the macrostructure can adjust the impedance matching and electromagnetic properties of the materials, thereby improving the absorption performance. Among them, the honeycomb structure is often used in actual products due to its good absorption characteristics and mechanical properties. Therefore, in this work, the absorption performance of the carbon microsphere material was improved through the honeycomb structure design. The carbon microsphere material is firstly prepared by sintering phenolic resin, which has the advantages of convenient synthesis, high yield, and a narrow absorption bandwidth of 4.4 GHz. Then, through the high frequency structure simulator (HFSS) electromagnetic simulation, the honeycomb structure based on the carbon microspheres was macroscopically designed, achieving a larger 8.3 GHz absorption frequency bandwidth. This research completed the combination of electromagnetic simulation and honeycomb structure, improved the absorption bandwidth of carbon microspheres, and opened up a new way for the further improvement and practical application for traditional absorption materials.

Innovative solution to enhance the Helmholtz resonator sound absorber in low-frequency noise by nature inspiration.

Sound absorbing elements commonly used to make quieter spaces are very desirable for high-frequency noise control. It is more attractive if it is possible to produce sound absorbing structures that perform well for a wide frequency range, especially in low-frequencies. In this regard, Helmholtz resonator array is a well-known structure but the maximum performance of each resonator unit in an array panel can only be achieved at resonance frequency that is dependent on its size. Therefore, a resonator array panel with equal size units has insufficient performance in a wide frequency range. Hence, the consideration of suitable sizes for each resonator in an array panel consisting of unequal size units is a key point to widen the frequency range of sound absorption. On the other hand, it seems that engineering science can be developed by nature inspiration. In addition, Fibonacci sequence is a famous mathematical model which describes the shape/ratio of different phenomena which exist in nature. In this paper, Fibonacci sequence is utilized to investigate Helmholtz resonator arrangement to increase both the amount of sound absorption coefficient and frequency bandwidth of absorption without changing the total volume of resonator array with equal size units. Analytical solutions based on the modified equivalent electrical circuit theory is used to calculate the sound absorption coefficient of the proposed panel with numerical experiments in MAPLE and COMSOL Multiphysics softwares to validate the proposed overall approach. Results show that the Fibonacci sequence is an optimum candidate to design a resonator-based sound absorber in low-frequency noise.

Rational design of core-shell Co@C nanotubes towards lightweight and high-efficiency microwave absorption

Rationally engineering on nanostructure of electromagnetic absorbers provides massive potential for eliminating the pollution caused by electromagnetic radiation. In this work, a series of core-shell Co@C nanotubes were produced via facile pyrolysis of cobalt carbonate hydroxide hydrate/dopamine precursor. Thanks to the well introduction of core-shell and nanotube structure, Co@C composite shows more superior microwave absorption than pristine Co nanoparticles. The unique microstructure provides abundant heterostructures and conductive paths to induce interfacial polarization and conductive loss for boosting dielectric loss. Through regulating the graphitization layer of C shell, the dielectric property could be further enhanced. Coupled with the favorable magnetic loss from the embedded Co nanoparticles, the products exhibit an optimal absorption intensity of −48 dB under low filler content of 30%. And effective absorption frequency bandwidth is up to 5.2 GHz at small thickness of 1.8 mm. Such excellent achievements demonstrate that the core-shell Co@C nanotube composites can be applied as a promising candidate for lightweight and thin electromagnetic absorption.