S-parameters,electrical permittivity,and absorbing energy measurements of carbon nanotubes-based composites in X-band

Composites based on carbon nanotubes (CNT) have recently received great attention as a possible new generation of radar absorbing materials (RAM), due to their efficient microwave attenuation capacity and low density. However, RAM performance can vary significantly depending on the CNT being used. Aiming to directly show the influence of CNT from two different suppliers, both multi-walled and non-functionalized (CNT-B and CNT-K), the electromagnetic behaviors of RAM processed in epoxy resin are compared. Scattering parameters (S-parameters), complex electrical permittivity and magnetic permeability, and absorbed energy of the composites in the X-band (8.2–12.4 GHz) are evaluated. The results clearly show the influence of the two different CNT on the electromagnetic characteristics of composites. CNT-B based composites behave as broadband RAM with the tendency of better attenuation results above 12.4 GHz. On the other hand, CNT-K based composites show good attenuation results in the X-band (>99%). Undoubtedly, the results confirm that the longer length of CNT-K favored the interconnection among the filaments, as well as, the formation of compact agglomerates surrounded by resin rich regions, which favored the impedance matching and mechanisms of losses in the processed RAM.     

Effect of BaTiO3 on the microwave absorbing properties of Co‐doped Ni‐Zn ferrite nanocomposites

Coprecipitation and hydrothermal method were utilized for the synthesis of Co‐doped Ni‐Zn ferrite and barium titanate nanoparticles. The microwave absorption properties of Co‐doped Ni‐Zn ferrite/barium titanate nanocomposites with single layer structure were studied in the frequency range of 8.2–12.4 GHz.The spectroscopic characterizations of the nanocomposites were examined using X‐ ray diffraction, scanning electron microscopy, transmission electron microscopy and dynamic light scattering measurement. Thermogravimetric analysis indicated the high thermal stabilities of the composites. The composite materials showed brilliant microwave absorbing properties in a wide range of frequency in the X‐band region with the minimum return loss of ?42.53 dB at 11.81 GHz when sample thickness was 2 mm and the mechanisms of microwave absorption are happening mainly due to the dielectric loss. Compared with pure Co‐doped Ni‐Zn ferrite, Co‐doped Ni‐Zn ferrite/BaTiO₃ composites exhibited enhanced absorbing properties. The microwave absorbing properties can be modulated by controlling the BaTiO₃ content of the absorbers and also by changing the sample thicknesses. Therefore, these composites can be used as lightweight and highly effective microwave absorbers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39926.     

Recent advances in carbon nanotubes-based microwave absorbing composites

With the blossom of information industry, electromagnetic wave technology shows increasingly potential in many fields. Nevertheless, the trouble caused by electromagnetic waves has also drawn extensive attention. For instance, electromagnetic pollution can threaten information safety in vital fields and the normal function of delicate electronic devices. Consequently, electromagnetic pollution and interference become an urgent issue that needs to be addressed. Carbon nanotubes (CNTs) have become a potential candidate to deal with these problems due to many advantages, such as high dielectric loss, remarkable thermodynamic stability, and low density. With the appearance of climbing demands, however, the carbon nanotubes combining various composites have shown greater prospects than the single CNTs in microwave absorbing materials. In this short review, recent advances in CNTs-based microwave absorbing materials were comprehensively discussed. Typically, we introduced the electromagnetic wave absorption mechanism of CNTs-based microwave absorbing materials and generalized the development of CNTs-based microwave absorbers, including CNTs-based magnetic metal composites, CNTs-based ferrite composites, and CNTs-based polymer composites. Ultimately, the growing trend and bottleneck of CNTs-based composites for microwave absorption were analyzed to provide some available ideas to more scientific workers.     

Variable frequency microwave processing of thermoplastic composites

Abstract

The range of applications for variable frequency microwave (VFM) facilities (2–18 GHz) has been extended to thermoplastic composites. Five thermoplastic polymer matrix composites are processed and discussed, including 33 wt-% random carbon fibre reinforced polystyrene [PS–CF (33%)], and low density polyethylene [LDPE–CF (33%)]; 33 wt-% random glass fibre reinforced polystyrene [PS–GF (33%)], low density polyethylene [LDPE–GF (33%)]and Nylon 66 [Nylon 66–GF (33%)]. Bond strengths of lap joints were tested in shear and results were compared with those obtained using fixed frequency (2·45 GHz) microwave processing. The primer or coupling agent used was a 5 min, two part adhesive containing 100%liquid epoxy and 8% amine, which was more readily microwave reactive than the composites themselves. The VFM was operated under software control, which provided automatic data logging facilities. Results indicate that VFM can produce strong bonds for PS and LDPE.     

Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber

Microwave absorbing composites with epoxy-silicone as matrix and both multi-walled carbon nanotubes (MWCNTs) and carbonyl iron (CI) particles as absorbers were prepared, and their electromagnetic and microwave absorbing properties were investigated in the frequency range of 2-18 GHz. The microstructures of the composites show a uniform dispersion of the MWCNTs and CI particles in the matrix. The complex permittivity of the composites increased with increasing MWCNT content. A double resonance behavior of the complex permeability has been observed. One is due to the domain wall motion at about 7.5 GHz and the other is due to spin rotation at about 13.5 GHz. Reflection loss values exceeding −5 dB can be obtained in the frequency range of 10.4-18, 4.4-18 and 2-18 GHz, when the composite thickness is 0.5, 1 and 1.5 mm, respectively. A minimum reflection loss of −16.9 dB at 10.5 GHz and a bandwidth over the frequency range of 3.4-18 GHz with reflection loss below −10 dB can obtained for a composite filled with 0.5 vol% MWCNT and 50 vol% CI particles.     

Flexible SiC-CNTs hybrid fiber mats for tunable and broadband microwave absorption

Flexible microwave absorbers with high stability are in increasing demand for the applications under harsh conditions. SiC as a functional ceramic material has the feature of high environmental tolerance and adjustable electromagnetic (EM) absorbing properties, making them suitable to be applied for harsh environments. However, the electrical property of SiC requires to be further enhanced to obtain qualified EM absorbing performance. In this work, multiwall carbon nanotubes (CNTs) were introduced to SiC to enhance the electrical properties. Flexible two-dimensional (2D) CNTs loaded SiC fiber mats were prepared as EM absorbers via electrospinning and polymer-derived-ceramic (PDC) methods. The CNTs inside the fibers can form conductive networks and act as reinforcement to ensure high flexibility and enhance the microwave absorption properties of SiC mats. Thus, a reflection loss of ?61 dB and an effective absorption band (EAB) of 2.9 GHz were obtained. More importantly, the EM absorption can be adjusted by tuning the content of CNTs and the EAB can cover the entire X-band by adjusting the material thickness. The work provided a facile strategy to fabricated flexible 2D ceramic mats with high environmental stability and tunable electrical properties, which may shed light on the production of reliable EM absorber for broadband EM absorption applications.     

Broadband Complex Dielectric Permittivity of Porous Aluminum Silicate–Pyrolytic Carbon Composites

The broadband (10 kHz–18 GHz) complex dielectric permittivity of electrically lossy aluminum silicate–pyrolytic carbon composites is studied as a function of carbon content. The composites are made from a commercial machinable ceramic that is treated by oxalic acid refluxing, followed by sucrose pyrolysis within the open porosity. The resulting composites have a slightly percolating conductor configuration that exhibits non-Debye dielectric properties. Such composites are useful as microwave absorbers in vacuum electronic amplifiers, for suppressing spurious oscillations, and for bandwidth control. Four contributions to the dielectric response were observed and analyzed. These consist of a direct current conductivity contribution that is significant only at frequencies below 1 MHz, an irregular low-frequency relaxation, a strong non-Debye, high-frequency relaxation with its upper portions extending into the microwave regime, and an underlying, broadband loss consistent with a universal dielectric response. The microwave behavior is dominated by changes in the position, intensity, and Cole–Cole exponent of the high-frequency relaxation as the carbon content increases.     

Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites

SiC-nanowire-reinforced SiC_f/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of ?47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.     

Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size

The increasing electromagnetic interference problems have drawn much attention to microwave absorbing materials. To satisfy the needs of practical application, FeSiAl and flaky graphite filled Al₂O₃ composites were sintered by hot-pressing for microwave absorption application. The effect of FeSiAl particle size on the electromagnetic and microwave absorption properties was investigated in the X-band (8.2–12.4 GHz). The results show that the dielectric properties enhance significantly with increasing FeSiAl particle size, which is attributed to the increased interfacial polarization and conductance loss. As a result of the favorable impedance matching and appropriate electromagnetic attenuation, the reflection loss (RL) of the composites filled with 25–48 μm flaky FeSiAl achieves -15.2 dB at 10.6 GHz and the effective absorption bandwidth (RL < -10 dB) is 1.2 GHz in 10.0–11.2 GHz with a matching thickness of 1.0 mm. It indicates that FeSiAl and flaky graphite filled Al₂O₃ composites are potential candidates for thin-thickness microwave absorbing materials, and the microwave absorption properties can be enhanced by adjusting absorbent particle size.     

Conducting polyaniline composites as microwave absorbers

Conducting polymers are excellent microwave absorbers and they show technological advantage when compared with inorganic electromagnetic absorbing materials, being light weight, easily processable, and the ability of changing the electromagnetic properties with nature and amount of dopants, synthesis conditions, etc. In this paper we report the synthesis, dielectric properties, and expected application of conducting composites based on polyaniline (PAN). Cyclohexanone soluble conducting PAN composites of microwave conductivity 12.5 S/m was synthesized by the in situ polymerization of aniline in the presence of emulsion grade polyvinyl chloride. The dielectric properties of the composites, especially the dielectric loss, conductivity, dielectric heating coefficient, absorption coefficient, and penetration depth, were studied using a HP8510 vector network analyzer. The microwave absorption of the composites were studied at different frequency bands i.e, S, C, and X bands (2–12 GHz). The absorption coefficient was found to be higher than 200 m⁻¹ and it can be used for making microwave absorbers in space applications. POLYM. COMPOS., 28:588–592, 2007. © 2007 Society of Plastics Engineers     

Microwave‐absorbing properties of linear low‐density polyethylene/ethylene–octene copolymer/carbonyl iron powder composites

Using linear low‐density polyethylene (LLDPE)/ethylene–octene copolymer (POE) as a polymer matrix and carbonyl iron powders (CIPs) as filler, we prepared polymer matrix composites with microwave‐absorbing properties by means of melt blending. Scanning electron microscopy and transmission electron microscopy were used to characterize the samples. The absorbing properties of the composites were measured with the arch method in the range of frequency 2.0–18.0 GHz. The results indicate that the absorbing peaks moved to low frequency as the CIP content in composites increased and that there was an appropriate CIP content in LLDPE/POE/CIP composites to achieve the best absorbing effectiveness. The electromagnetic parameters of the composites were determined with the transmission/reflection method in the range 2.6–17.8 GHz. The experimental results show that there were both dielectric loss and magnetic loss in the LLDPE/POE/CIP composites. Therefore, the microwave absorption of the LLDPE/POE/CIP composites was attributed to the combining contributions of the dielectric loss and magnetic loss. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Ecofriendly and cost-effective composites of titanium slag and carbonyl iron for lightweight and tunable microwave absorption

For obtaining the ecofriendly microwave absorbing composites, flake carbonyl iron was mixed to the flotation carbon waste of titanium slag by ball-milling processes. The composites of 4 h ball-milling present excellent microwave absorbing performance of minimum reflected loss (RL) of ?54.9 dB at 7.8 GHz with thickness of 2.6 mm. The same composites provide the performance of RL < ?10 dB with the thickness from 1.2 mm to 5 mm at corresponding matching frequency from 3.3 GHz to 18 GHz. It means that the absorbing peak can be tunable at the wide frequency range with only adjusting the thickness. The strong absorbing intensity and wide tunable frequency range are attributed to the proper impedance matching of dielectric property from flotation carbon waste of titanium slag and magnetic property from carbonyl iron. Also, this work provides a research approach for handling industrial waste which contains carbon.     

Ultralight and superelastic polyvinyl alcohol/SiC nanofiber/reduced graphene oxide hybrid foams with excellent thermal insulation and microwave absorption properties

Being in the strategic direction of next-generation absorbers, multifunctional microwave absorbing materials possess great application value in military and commercial fields. However, the stringent requirements for performance necessitate the combination of multiple functions in such type of composites, which is still a challenge. This work aims to develop a foam-type absorber composed of multi-dimensional organic and inorganic materials, in which reduced graphene oxide sheets and polyvinyl alcohol membranes serve as the framework and crosslinker to form a three-dimensional skeleton. Meanwhile, SiC nanofibers as a reinforcing component can effectively suppress the over-stacking of reduced graphene oxide and enhance the conductivity and mechanical strength of cell walls. Among the remarkable microwave absorbing properties of the obtained foam, there are the ultra-light (9.85 mg cm^-3), broadband (7.04 GHz), and strong absorption (reflection loss of -61.02 dB), all combined in the ultra-thin (2.5 mm). In addition, the foam possesses superelastic and excellent heat-insulating characteristics that ensure shock resistance, heat preservation, and infrared stealth. The remarkable versatility benefits from the porous structure, as well as from the synergistic effect of multi-dimensional organic and inorganic constituents of the foam. Therefore this study lays the foundation for the design of new-generation microwave absorbers with broad application potential.     

改性SiC短切纤维体积分数对SiC_(sf)/LAS复合材料介电性能的影响

以改性SiC短切纤维为添加剂,用热压法制备了SiCsf/LAS复合材料,考察了纤维的显微结构以及纤维和复合材料在8.2～12.4GHz频率范围内的微波介电性能。结果表明,当SiC纤维的体积分数为2.92时,纤维混合体介电常数实部、虚部最大,分别为38～25、40～20。高复介电常数的SiC纤维使SiCsf/LAS复合材料比LAS具有更高的介电常数,材料中无富碳界面层的形成。当SiC纤维的体积分数为3时,复合材料介电常数实部、虚部以及损耗角正切值最大,其均值分别为58、25和0.45。     

Studies on microwave shielding materials based on ferrite‐ and carbon black‐filled EPDM rubber in the X‐band frequency

The preparation and characterization of ferrite‐ and carbon black‐filled ethylene‐propylene‐ethylidene norborene terpolymer rubber composites were studied for microwave applications. Both the insertion loss and return loss were measured in a high‐frequency range (X‐band) with a network analyzer. The results indicated a relatively low frequency dependence. The difference in the insertion losses measured for all samples, except for one containing 100 phr ferrite and 75 phr carbon black (B75), was ±3 dB. Sample B75 showed broadband absorption in the aforementioned frequency band. Furthermore, the incorporation of both ferrite and carbon black powder into the rubber matrix altered the electrical and microwave properties, that is, the insertion loss and return loss; this could be helpful in the design of suitable absorbing materials for microwave applications at high frequencies. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 145–150, 2002     

碳纤维/玻璃纤维复合纤维毡的介电和吸波性能

考察了碳纤维/玻璃纤维复合并浇注硅溶胶制成的纤维毡的介电和吸波性能。结果表明:碳纤维的分散对复合材料的介电性能有很大影响。复合材料的介电常数随着碳纤维含量的增加而增加,随频率的增加而降低,具有明显的频响效应。复合材料具有明显的双峰吸收性能,且最高吸收峰随着厚度的增加向低频移动。当w(碳纤维)=1%,复合材料厚度为6、7、8 mm时,反射率小于-10 dB的频段分别为4.2~10.5、3.9~9.3、3.8~7.7 GHz。     

Polymer matrix composites as broadband radar absorbing structures for stealth aircrafts

Stealth or low observable technology has been one of the most critical requirements for military aviation sector to counter the ever-advancing target detection technologies worldwide. Among many, Radar, which operates in the broadband spectrum, is one of the principal threats, which measures the radar cross section (RCS) of the aircraft. Hence, researchers are putting immense efforts to develop low RCS materials based on polymer matrix composites (PMCs) called as radar absorbing structures (RAS), to replace highly reflective conventional metallic aircraft structures. Though many researchers have been successful in RAS realization using radar absorbing materials (RAMs) as fillers, this review article reckons the state-of-the-art advanced research on the realization of RAS using a combination of RAMs and multilayered grid devices such as, frequency selective surfaces, circuit analog absorbers, metamaterials, and so on, and their effect on mechanical properties to deliver superior performance RAS for stealth aircrafts. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47241.     

轻薄炭黑涂层的制备及其微波吸收性能研究

利用混合强酸对炭黑(CB)进行氧化处理,并以有机硅树脂为基体制备了CB涂层。通过扫描电子显微镜(SEM)、透射电子显微镜(TEM)、矢量网络分析(VNA)等方法对制备的4种不同填充量的涂层样品进行微观结构和电磁性能表征。微观结构分析表明：CB颗粒尺寸为纳米级,直径大小约为60 nm,氧化处理后的表面形态呈“褶皱”状;制备的CB涂层表面平整、厚度均匀,密度仅为1.1 g/cm3,兼具轻质柔性的特征。微波反射率测试结果显示,在8～18 GHz范围内,涂层样品均表现出了优异的微波吸收性能。当填充量为3.7%时,厚度仅为1.6 mm的涂层有效吸波频宽达到5.13 GHz,吸波强度为-26.5 dB;当填充量为2.3%时,厚度为1.9 mm的涂层有效吸波频宽达到最大值(5.44 GHz),覆盖整个Ku波段,厚度为2.5 mm的涂层有效吸波频宽为4.44 GHz,覆盖整个X波段。     

Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites

The hybrid network of Si₃N₄ whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si₃N₄ into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si₃N₄ (C_f-RBSN) composites have been discussed in detail. The C_f reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % C_f-RBSN composite exhibit a minimum reflection loss (RL_min) of ?36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g^?1 for 5.9 mm thickness, while 20 vol % C_f-RBSN composites resulted in RL_min of ?22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si₃N₄ whiskers and C_f.     

Ultra-broadband electromagnetic wave absorbent: In-situ generated silica modified conductive carbon fiber aerogels

The research and development of dielectric microwave absorbing materials with broad electromagnetic (EM) response is a significant project in EM wave absorption field. To achieve high-performance absorption and strong interfacial bonding at the same time, thermal-assisted in-situ bonding technology was applied to fabricating the dielectric composite absorbing materials. Thanks to the combination of vacuum filtration and in-situ hydrothermal reaction, the as-prepared binary composite aerogel shows both strong interface contacting and good mechanical stability. In addition, the carbon nanofibers/silica composite aerogel (CSA) exhibits ultra-broad effective bandwidth covering from S to Ku band, originated from the uniform dispersed silica aerogel in conductive carbon fiber network. In details, for CSA1 sample, the maximum reflection loss (RL) values and effective absorption bandwidth reach −46.2 dB (1.8 mm) and 5.2 GHz (1.5 mm). Meanwhile, the optimum RCS reduction values reaches 16.2 dB m² when the detection theta was set as 0°. For CSA2 sample, the effective absorption bandwidth reaches 8.64 GHz at 1.5 mm, and tends to possess lower frequency EM response covering the S-band. This work exhibits a kind of broad-bandwidth aerogel absorbers at low thickness, which shows huge potential in large-scale production of microwave absorbing devices.