Preparation and microwave absorption properties of honeycomb core structures coated with composite absorber Hui Luo, Fu Chen, Fang Wang, Xian Wang, Weiyong Dai, Sheng Hu, and Rongzhou Gong
Citation: AIP Advances 8, 056635 (2018); View online: https://doi.org/10.1063/1.5005163 View Table of Contents: http://aip.scitation.org/toc/adv/8/5 Published by the American Institute of Physics
AIP ADVANCES 8, 056635 (2018)
Preparation and microwave absorption properties of honeycomb core structures coated with composite absorber Hui Luo, Fu Chen,a Fang Wang, Xian Wang, Weiyong Dai, Sheng Hu, and Rongzhou Gong School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China (Presented 9 November 2017; received 18 September 2017; accepted 1 December 2017; published online 11 January 2018)
Honeycomb structure coated with paraffin filled with composite of graphene and flaky carbonyl iron powder (FCIP) as lossy filler have been studied. The composite of graphene/FCIP with different weight ratio were synthesized via mechanical milling, the electromagnetic properties of the samples were measured by transmission/reflection method in the frequency range of 8-12 GHz. The microwave absorbing properties of the microwave absorbing honeycomb structure (MAHS) and microwave absorbing honeycomb sandwich structure (MAHSS) were studied based on the Finite Element Method with periodical boundary conditions. The matching layer on the top of the honeycomb sandwich structure can enhanced the microwave absorption properties. It was shown that a light weight and broadband MAHSS could be implemented with the use of the magnetic material and dielectric material. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5005163
I. INTRODUCTION
Microwave absorbing materials with the properties of thinness, lightweight, broadband and strong absorption have received increasing attention due to their pragmatic and effective functions for reducing electromagnetic interference (EMI) pollution and the defense stealth technology.1,2 Microwave absorbing materials can be split into coated absorbing materials and structure radar absorbing materials according to the molding process and load capacity. Structure radar absorbing materials have both the functions of load bearing and electromagnetic energy absorbing capability compared with the coated absorbing materials.3 The absorbing bandwidth and intensity of the structure radar absorbing materials are also better than the coated absorbing materials. Honeycomb sandwich structures are widely applied to aeronautical structures, building, automobile and train structures due to the high stiffness-to-mass ratio, lighter weight, heat insulation and anti-radiation properties.4 MAHSS has been studied for their low reflection, design flexibility, light weight and high mechanical recently.5 Generally speaking, the MAHSS is composed of honeycomb cores, matching layer and the coated lossy filler. The matching layer, such as E-glass fiber reinforced composite or PPy fabric, which allows the incident EM wave to enter the absorber is employed for broadband EM absorbing structure.6,7 The lossy filler including non-magnetic materials such as carbon black, MWCNT, graphene or magnetic materials such as carbonyl iron power (CIP) and ferrite are used to implement microwave absorbing structures.8,9 Graphene with the properties of lightweight, high thermal conductivity, high specific surface area, high corrosion-resistance, high temperature-resistance and extraordinary electric conductivity have received much attention as a promising material for microwave absorbing and super capacitor.10 CIP with the properties a
Corresponding author: Fu Chen Email:
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2158-3226/2018/8(5)/056635/7
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of high saturation magnetization and complex permeability may provide a superior performance for microwave energy absorption. Nonetheless, weak magneto-crystalline anisotropy and attenuated permeability due to eddy currents limit their applications. Studies have shown that CIP with a flake-like morphology could increase shape anisotropy and complex permeability, reduce eddy currents, and exceed the Snoek’s limit.11 Studies have been found that the absorbing bandwidth increased with the value of the thickness or the permeability increased.12 For a non-magnetic absorber, the total thickness of the absorber should be large in order to design a wideband absorber. The wideband absorber can be controlled by the both parameters for the magnetic absorber. Nevertheless, the weight ratio of the magnetic materials is too high to get a sufficient absorption performance. Although a high thickness or magnetic materials as loss filler can enhance the microwave absorbing properties, but accompanied by the weight increase. Several papers have been proposed MAHS to overcome these problems.13–15 The non-magnetic materials, such as Multi wall carbon nanotube (MWCNT) or carbon black power have been used as loss filler of the MAHS. Nevertheless, the thickness of the honeycomb cores is too high or the microwave absorption properties are too weak. The magnetic materials, such as CIP, nickel fibers also have been used, but increased the weight of the MAHS. So far, no works have been done on the microwave absorption properties of MAHS with the composite of dielectric loss and magnetic loss materials. In this paper, the microwave absorbing properties of the honeycomb core coated with the composite of graphene and flaky carbonyl iron powder as lossy filler were studied. A matching layer is introduced on the top of the honeycomb structure, and the microwave absorbing properties of the MAHSS were investigated.
II. EXPERIMENTAL
The graphene was commercially purchased from the sixth element (Changzhou) materials technology Co., Ltd. Jiangsu province of China. The carbonyl iron power (2–5 µm in diameter) were commercially purchased from Tianyi super-fine metallic powder Co. Ltd. Jiangsu province of China. Both of them were used without further purification. The flaky carbonyl iron powers (FCIP) were prepared by mechanical milling the gas-atomized powders for 8 h using a QM-1SP4 high-energy planetary ball mill with a 10:1 ratio of ball mass to powder mass. Anhydrous ethanol (50 wt. % of the CIP) was employed as a process control agent. Then the graphene and FCIP were mixed uniformly using the same method for 2 h. The samples of FCIP/Graphene with different weight ratios were named S-0, S-0.5, S-1 and S-2, respectively. The weight ratios were shown in Table I. The morphologies and microstructures of the samples were investigated using a scanning electron microscopy (JSM-5610LV), and the elements of the samples were studied with energy dispersive spectroscopy (EDS). The samples used for electromagnetic parameters measurements were prepared by homogeneously mixing the products with paraffin and then the mixture was pressed into a toroidal shape with an inner diameter of 3.04 mm and outer diameter of 7 mm. The complex relative permittivity and permeability of the samples were calculated from the sample thickness and the vector value of reflection/transmission coefficient (S parameters) which were measured using an Agilent E5071C network analyzer within 8–12 GHz. The microwave absorbing properties of MAHSS coated with composite lossy filler were calculated based on the commercial software package CST microwave studio. TABLE I. Mass (mg) of FCIP, Graphene and Paraffin in one coaxial ring. Samples S-0 S-0.5 S-1 S-2
FCIP
Graphene
Paraffin
0 100 200 200
20 20 20 0
180 180 180 180
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III. RESULTS AND DISCUSSION
Fig. 1 shows the SEM of graphene, FCIP, graphene/FCIP composite, and the EDS of the graphene/FCIP composite. It can be clearly seen that the diameter of the graphene sheet is less than 10 µm in Fig. 1(a), the thickness of FCIP particles is 0.2∼1µm with the length of 2∼10µm in Fig. 1(b). Fig. 1(c) shows that the graphene mixed with FCIP homogeneously. The EDS shows that the elements of the composite is C and Fe which are belong to the graphene/FCIP composite in Fig. 1(d). The effective complex permittivity and permeability of the samples were measured within 8-12 GHz as shown in Fig. 2. The real parts of permittivity and permeability represent the capability of storing electric and magnetic energy. The imaginary parts stand for the loss of electric and magnetic energy.16 The real part and the imaginary part of complex permittivity increased after FCIP induced, which can be seen in Fig. 2(a) and (b). According to the free electron theory, ε 00 ≈ 1/2π ρf ε 0 , where ρ, f and ε 0 are the resistivity, the frequency and the dielectric constant of free space, respectively.17 The resistivity decreased after the FCIP introduced when the weight ratio of the paraffin/graphene keep constant, and ε 00 of the S-1 and S-0.5 were higher than the S-0 indicating a more dielectric loss ability. The dielectric loss ability mainly comes from conductivity and polarization loss, the polarization loss can be divided into diploe orientation polarization and interfacial polarization in the microwave band. The interfacial polarization appears in a heterogeneous system, the accumulation and uneven distribution of space charges at the interfaces will produce a macroscopic electric moment which can consume the incident EM energy effectively.1 The difference in dielectric constants and electrical conductivities among the paraffin, graphene and FCIP is responsible for the generation of interfacial polarization. The interfacial polarization decreases with the frequency increase resulting in the decreasing trend of the complex permittivity which can be seen in Fig. 2(a) and (b).
FIG. 1. SEM of (a) graphene, (b) FCIP, (c) graphene/FCIP and (d) EDS of graphene/FCIP composite.
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FIG. 2. (a) Real part and (b) imaginary part of complex permittivity, (c) Real part and (d) imaginary part of complex permeability, (e) The dielectric and magnetic dissipation factor of S-1, and (f) the attenuation constant of the samples.
The dipole orientation polarization is caused when the dipoles cannot reorient quickly enough to respond to the high-frequency alternating electric field, which also result in the decreased of the complex permittivity and produce typical frequency dispersion behaviors.16 The FCIP is a high magnetic loss and low dielectric loss material, so the dielectric loss performance is mainly dependent on graphene. Fig. 2(c) and (d) show the frequency dependence of the real part and imaginary part of the complex permeability, it can be note that the value of µ0 and µ00 increased with the weight ratio of the FCIP increased. Magnetic loss mainly originates from hysteresis, domain wall resonance, nature ferromagnetic resonance and eddy current effect. The hysteresis loss is negligible in a weak field. The domain wall resonance only occurs in multi-domain structures and is usually observed at 1–100 MHz, as such, the magnetic hysteresis loss and domain wall resonance can be excluded in the present materials.17 The eddy current effect can be negligible for the thickness of the FCIP is less than 1µm, so the natural ferromagnetic resonance is the dominant factor for magnetic loss. For direct evaluation of the dielectric loss and magnetic loss abilities, dielectric dissipation factor (tan δe = ε 00/ε 0) and magnetic dissipation factor (tan δµ = µ00/µ0) are widely utilized. Fig. 2(e) Shows the dielectric and magnetic dissipation factor of the sample S-1, It can be seen that the value of tan δµ seems to be an approximate constant around 0.5 in the range of 8–12 GHz, which is lower
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than the tan δ ε (1.25-1.5) indicating the microwave absorption is dominated by the dielectric loss. The attenuation constant α, which responses to attenuation ability of S-0, S-0.5 and S-1 were calculated according to Eq. (1):18 r √ q 2πf 00 00 0 0 ((µ ε − µ ε )) + (µ00 ε 00 − µ0 ε 0)2 + (µ00 ε 00 + µ0 ε 0)2 α= (1) c in which f is the microwave frequency and c is the velocity of light. Fig. 2(f) shows the attenuation constant of the samples S-0, S-0.5 and S-1, the value of α for S-1 is larger than the others, indicating S-1 have stronger attenuation ability. The MAHS have good impedance matching, which can be regarded as an effective medium as a mixture of air and constituents of the absorber. The effective permittivity (εeff ) of the porous absorber can be monitored by the Maxwell Garnett (MG) theory:19 ε eff = ε 1
(ε 2 + 2ε 1 ) + 2p(ε 2 − ε 1 ) (ε 2 + 2ε 1 ) − p(ε 2 − ε 1 )
(2)
where ε 2 and ε 1 are the permittivity of the guest (gas state) and host (solid state), respectively, and p is the volume fraction of the guest in the effective medium. Therefore, the porous structure can tune the effective permittivity and favors the impedance matching. To analyze the absorbing performance of the MAHS, the commercial software package CST microwave studio was utilized. The unit cell model to analyze the absorbing performance is shown in Fig. 3. The honeycomb cores structure with the honeycomb wall thickness 0.07 mm and the side-length 2.78 mm is used. The propagation direction is perpendicular to the honeycomb cores. The coating thickness is 0.05 mm and the honeycomb thickness is 7.5 mm. A matching layer Fr-4 with a thickness of 0.5 mm was introduced into the honeycomb sandwich structure on top. The simulated reflection loss for the MAHS and the MAHSS with the S-0, S-0.5 or S-1 coated. It can be note that the microwave absorbing properties of the MAHS were enhanced with the content
FIG. 3. Microwave absorbing properties of (a) honeycomb structure, (b) honeycomb sandwich structure, (c) The EM parameter of S-2 and (d) the corresponding microwave absorption properties of S-2.
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of the FCIP increase in the frequency of 8-12 GHz in Fig. 3(a). The RL of MAHS coated with S-0 is around -5 dB, which is higher than S-0.5 (around -6 dB). The MAHS coated with S-1 shows excellent absorbing performance with a -10 dB return loss from 8.4 GHz to 12 GHz with the minimum reflection point -12.5 dB located at 10 GHz. A matching layer Fr-4 was introduced on top to honeycomb structure, which plays the role of trapping the incident waves into the honeycomb layer, indicating the matching layer is important for enhance the microwave absorption properties. Fig. 3(b) shows the microwave absorbing properties of MAHSS, the matching layer is 0.5 mm. It can be seen that all the microwave absorbing properties of the MAHSS coated with loss filler have been enhanced compared with the MAHS after introduced the matching layer. The RL of MAHSS coated with S-1 is less than -11 dB in the frequency range of 8-12 GHz with the minimum RL values of -27.5 dB located at 9.5 GHz. The excellent microwave absorption performance is attributed to the matching layer, dielectric loss and magnetic loss. When the incident EM wave is trapped inside the honeycomb sandwich structure coated with loss materials due to the impedance matching leading by the matching layer, there are multiple scattering and the EM wave is then transformed into heat by magnetic loss and dielectric loss. In order to see the microwave absorption properties of the composite with only FCIP, the EM parameter of S-2 and the MAHSS coated only FCIP were studied, which can be seen in Fig. 3(c) and (d). It is can be clearly seen that the value of the reflection loss is poor, which prove that the addition of graphene has a great effect on the microwave absorption properties. IV. CONCLUSION
Honeycomb structure coated with paraffin filled with the composites of graphene and flaky carbonyl iron powder as lossy filler have been studied. The introduction of a matching layer enhances the microwave absorbing performance due to the impedance matching. The RL of MAHSS coated with S-1 is less than -11 dB in the frequency range of 8-12 GHz with the minimum RL values of -27.5 dB located at 9.5 GHz. The RL of the composites prepared with only flaky carbonyl iron is poor, which indicating the graphene addition have a great effect on the microwave absorbing properties. The excellent microwave absorbing properties is attributed to the impedance matching, dielectric loss, magnetic loss and multiple scattering. In this way, a MAHSS with light weight, wide-band absorbing performance was studied, which can be used as antireflection, EM shielding, and other functional honeycomb composites. ACKNOWLEDGMENTS
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