Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Design of Radar Absorbing Structure Using SiCf/Epoxy Composites for X Band Frequency Range Hongyu Wang*,† and Dongmei Zhu‡ †
Qinghai Provincial Key Laboratory of New Light Alloys, Qinghai Provincial Engineering Research Center of High Performance Light Metal Alloys and Forming, Qinghai University, Xining 810016, China ‡ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China ABSTRACT: The object of this study is to design radar absorbing structures (RAS) with load-bearing ability and electromagnetic wave absorption characteristics by using conductive carbon black (CB) as absorbent and silicon carbide (SiCf)/epoxy plain-weave composites as binder matrix. The absorbing properties of single layered CB-filled SiCf/ epoxy composites first increased and then decreased with increasing CB content, and when CB content was 7.5 wt %, the composite has good absorbing properties with the reflection loss value below −5 dB. In order to broaden the bandwidth, double layered absorbers were designed and simulated. It was found that when the thicknesses of the lossy layer and impedance layer were 1.0 and 2.0 mm, respectively, the reflection loss values below −10 dB can be obtained at the X band frequency range, which indicates that the SiCf/epoxy composites can be a candidate for use as radar absorbing structures.
1. INTRODUCTION Radar cross section (RCS) reduction technology, which protects aircraft from being detected, has become essential in contemporary warfare high-tech equipment. There are two methods to realize RCS reduction: one way is using shaping and the other way is using radar absorbing material (RAM).1−4 In general, the shaping conflicts with the design to improve the aerodynamic performance; therefore, using RAM has become essential for RCS reduction.5,6 Now, many researchers have paid attention to study RAM to realize the aim with “thinner thickness, broader bandwidth, higher absorbing ability and higher strength”.7 According to whether the absorbing material can be bearing load, the RAM can be divided into two categories: radar absorbing coatings and radar absorbing structures (RAS). Radar absorbing coatings with the strong advantage of being easily applied to the surface of the existing structure attracted many researchers to fabricate suitable absorbers, for example, metallic soft magnetic material, carbon-based material,8−11 and dielectric/magnetic material. The dielectric/magnetic materials combined the dielectric loss and magnetic loss is regarded as a way to break through the drawbacks of sole absorbent. Some absorbing coatings exhibited good absorbing performance, for example, graphene nanosheets (GNs) and flake carbonyl iron (FCI) as absorbent,12 graphene@Fe3O4@C@polyaniline nanorod arrays absorbent,13 Fe−Co/nanoporous carbon absorbent,14 Fe3O4/CNTs,15 and
[email protected] Beyond the two-phase structures, a variety of approaches have been developed for the synthesis of multiple-phase structures, and the multiple-phase structures have good absorbing properties due to the synergy effects.15 However, the radar absorbing coatings not only © XXXX American Chemical Society
increase structure weights but also have poor mechanical and environment-resistant properties. Therefore, compared with the absorbing coating, the RAS combined with the bear capacity and absorbing properties is the main development trends of absorbing materials. In previous research, the development of RAS using glass fiber/epoxy and carbon fiber/epoxy was widely achieved. Oh et al.3 designed RAS using a glass/epoxy composite as the binder matrix and CB as the absorbent, and multilayered RASs were designed and studied. When the first layer thickness was 0.6 mm with 7 wt % CB and the second layer thickness was 1.9 mm with 5 wt % CB, the double layered RASs have optimum absorbing performance, and the absorbing bandwidth below −10 dB can reach 2.4 GHz. Shah et al.17 studied the microwave absorption and flexural properties of Fe nanoparticle/carbon fiber/epoxy resin composite plates. It was indicated that the plate containing 30 wt % of Fe nanoparticles with a perpendicular manner between the directions of carbon fiber array could have a maximun reflection loss value of −16.2 dB at 6.1 GHz, and the flexural strength reached 77.78 MPa with 3.74% deformation. However, few reports about using SiCf as reinforcement to fabricate SiCf/epoxy RAS have been reported. Compared with glass fiber and carbon fiber, the SiCf was good candidates for reinforcement due to their thermal resistance, chemical stabilization, high tension strength, and low density, and more importantly, the SiCf with tunable electrical Received: November 27, 2017 Revised: January 7, 2018 Accepted: January 30, 2018
A
DOI: 10.1021/acs.iecr.7b04905 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research conductivity by tailoring surface characteristics.18−20 Thus, SiCf/epoxy composites with different CBs were fabricated in this paper, and based on the complex permittivity of CB filledSiCf/epoxy composite, single layered and double layered RASs were designed and simulated to broaden the reflection loss bandwidth below −10 dB. The double layered absorbers have good absorbing capacity, and the reflection loss value below −10 dB can be obtained at the tested frequency range.
values of reflection loss (RL) were calculated on the basis of the transmission line theory.
3. RESULTS AND DISCUSSION 3.1. Microstructure of SLF-SiCf/Epoxy Composite. Figure 1a,b illustrates the low and high magnification of SLFSiCf/epoxy composites without CB. The image of the matrix rich region and the interface between the SiC fiber were clear, and also in the matrix rich region, some voids can be detected, which was ascribed to the solvent evaporation during the epoxy curing. Therefore, in order to make sure the SLF-SiCf/epoxy composites have high mechanical properties, the curing process should keep the pressure constant in order to reduce the content of void. When the CB was added into the SLF-SiCf/ epoxy composites, the CB would distribute mainly in the matrix rich region and in the interface between the yarns, as shown in Figure 1c,d. The high solution SEM image of CB distribution state was illustrated in Figure 1e,f. When the CB content was 2.5 wt %, the CB can distribute uniformly in the matrix rich region; however, when the CB content reached 12.5 wt %, the dispersion stated became bad. The even distribution of CB in the SLF-SiCf/epoxy composites is expected to induce a high dielectric loss. Furthermore, by further increasing the CB content in the SLF-SiCf/epoxy composites, the porosity would increase due to the high viscosity during the fabrication process; the porosity of the SLF-SiCf/epoxy composites increased from 0.8% to 2.2% with the CB content increased from 2.5 to 12.5 wt %. The porosity not only affects the mechanical properties but also impairs the dielectric properties.21 So when using CB as an absorbent added into SLF-SiCf/ epoxy composites, the content of CB should control, in this paper, the highest CB content which was designed as 12.5 wt %. 3.2. Complex Permittivity of CB-Filled SLF-SiCf/Epoxy Composites. The complex permittivity of CB-filled SLF-SiCf/ epoxy composites with different CB content was shown in Figure 2. As can be seen, both the real part (ε′) and imaginary part (ε″) of permittivity increase with increasing the CB content. When the CB content increases from 5 to 12.5 wt %, the ε′ and ε″ increase from 7.64 and 1.55 to 14.91 and 13.78 at 8.2 GHz, respectively. Furthermore, when the CB content reaches to 12.5 wt %, the complex permittivity shows an obvious frequency dependent effect, and the complex permittivity decreases with increasing the frequency. According to the Debye equations,22,23 the ε′ is determined by the relaxation time of the composites, and the ε″ is determined by the combination of relaxation time of the composites and electrical conductivity. The relaxation time is ascribed to the possible polarization mechanisms, which including electronic, atomic, relaxation, and space charge polarizations. However, the contribution of atomic and electronic polarizations can be neglected due to the fact that they exist at a lower frequency range. For the CB-filled SLF-SiCf/epoxy composites, when CB was added into the composites, the even distribution of CB could contribute the space charge polarization due to the different conductivity of CB and the matrix, and with a greater CB content in the SLF-SiCf/epoxy composites, more interfaces would form and in turn increase the ε′. Therefore, the ε′ of CBfilled SLF-SiCf/epoxy composites could be attributed to the space charge polarization. The ε″ is an expression of the capacity of dielectric loss, and according to the free electron theory, ε″ = εrelax + σ /(2πfε0), where ε0 is the free space permittivity and ω is the angular
2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The epoxy resin and polyamide resins used as the matrix and curing agent were supplied by Xi’an Leeo Technological Co. Ltd., Shannxi province, China. The KD-1 SiCf and SLF-SiCf were provided by National University of Defense Technology (China) and Suzhou SAIFEI Group Co. Ltd. (China). The detail parameters of this SiCf were illustrated in Table 1 and have been reported Table 1. Properties of SiC Fiber type
constitution
density (g/cm3)
diameter (μm)
tensile strength (GPa)
KD-1 SLF
Si−C−O -
2.45 2.3−2.5
14−15 12−15
1.8−2.3 1.8
in our previous paper in ref 7. The fabrics were 2D structure and the fiber volume fraction was 40%. The composites using conductive carbon black (CB) as absorbent and silicon carbide (SiCf)/epoxy plain-weave composites as binder matrix were prepared according to the following steps. First, the mixing of the resin matrix and different CB content was accomplished and followed by the mechanical agitation for 30 min. The mixing resin matrix used the epoxy and polyamide resins with weight ratio of 4:1. Then, the mixture was coated on the unsized SiC fabric mat and the prepregs were cured in an oven at 100 °C for 30 min and 120 °C for 4 h under the pressure of 3 MPa. The content of CB was designed as 5, 7.5, 10, and 12.5 wt % in the composites, further increasing the CB content, the viscosity of the premixed matrix increased rapidly, and it was not easy for the specimen to maintain the uniformity of CB. Therefore, the higher content of CB in the composites was not prepared. The general properties of CB are shown in Table2. Table 2. General Properties of CB properties
values
particle size (nm) DBP absorption (cm3/100g) nitrogen surface area (m2/g) resistivity (Ωm)
30−40 100−150 300 2
2.2. Characterization. The morphology of the composite was observed using SEM (model VEGA3 SBH, TESCAN, Brno, Czech Repubilc). The complex permittivity values for the composites were measured based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz. The method was performed in the fundamental waveguide mode TE10 using rectangular samples (10.16 mm × 22.86 mm × 2 mm). After calibration using an intermediate of short circuit and blank holder, the reflection and transmission coefficients were obtained using automated measuring systems (Agilent technologies E8362B: 10 MHz to 20 GHz). The B
DOI: 10.1021/acs.iecr.7b04905 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. SEM images of SLF-SiCf/epoxy composites at (a) low and (b) high magnification without CB and (c and d) with 7.5 wt % CB and with 2.5 wt % (e) and 10 wt % CB in the matrix region (f).
Figure 2. Complex permittivity of CB-filled SLF-SiCf/epoxy composites with different CB (a) real part and (b) imaginary part.
Figure 3. Reflection loss curves of 7.5 wt % CB-SLF-SiCf/epoxy composites with different thickness (a) and with the same thickness (b) of 3 mm at different CB content.
frequency. The high ε″ indicates a low resistivity of the materials. The relaxation loss induced by the interface, defect, or chemical bonds in/on the CB-filled SLF-SiCf/epoxy composites to the dielectric loss has little contribution to the dielectric loss of the composites with higher loadings of CB.24 With increasing the CB content, the distance between the CB would decrease, and a high number of CB particles were in contact with each other to form continuous conduction paths. The forming conduction paths make the free electron in the CB
to hop and shift over a long distance between the connected CB particles and thus increased the electrical conductivity of the composites. Therefore, the electric loss is the main loss mechanism that contributes to the dielectric loss, and similar phenomena have been reported in others studies.25,26 3.3. Reflection Loss Curves of CB-Filled SLF-SiCf/ Epoxy Composites. The reflection loss curves of CB filled SLF-SiCf/epoxy composites with different CB content, and thickness is illustrated in Figure 3. C
DOI: 10.1021/acs.iecr.7b04905 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. Input impedance (a) and attenuation constant (b) of SLF-SiCf/epoxy composites with different CB contents.
matching constant in the free space (377 Ω) when compared with other CB content. The good impedance matching means that the incident electromagnetic wave can enter into the absorbing material as much as possible. As for the attenuation constant, the attenuation constant increased with increasing CB content in the SLF-SiCf/epoxy composites. It has a similar trend as the complex permittivity. When the CB content was 7.5 wt %, the composite has good impedance matching characteristics compared with 10 wt % CB; however, the attenuation constant of the 10 wt % CB composite was higher than that of 7.5 wt % CB. Therefore, the absorbing properties of the SLF-SiCf/epoxy composites were determined by the two factors and the attenuation constant became the main factor affected the absorbing properties when the impedance matching constant has no big difference. From the above discussion, we can conclude that the good absorbing properties of CB filled SLF-SiCf/epoxy composites have suitable complex permittivity, when CB content is lower, and the ε′ and ε″ is too small to attenuate the incidence electromagnetic wave. When CB content is higher, the ε′ and ε″ are high, and though the high ε″ contributes to enhance the dielectric loss, the poor impedance matching ability has been the major shortcoming of the absorbing materials. When the CB content was suitable, the increase of ε′ and ε″ not only maintains a good impedance matching behavior but also makes contributions to electromagnetic attenuation. 3.4. Double Layer SiCf/Epoxy Composites with High Absorbing Performance. In order to design an optimum absorbing structure, the −10 dB absorbing bandwidths (90% reflection loss) are one of the most important elements for evaluating the electromagnetic wave absorption. Though, through adding conduction CB into SiCf/epoxy composites would enhance the absorption ability, the −10 dB bandwidths is very narrow. Choi et al.30 have conducted a systematic study of the appropriated complex permittivity of single layer absorbing material though theoretical calculations, and the dielectric constant and dielectric loss should have an optimum matching. From our results, the dielectric constant and dielectric loss cannot meet the requirement at the same time, and thus the −10 dB bandwidth was narrow. Therefore the double layered radar absorbing structure needed to be designed, and the effect of using double layer absorbers to broad bandwidths have been proven.31,32 According to the electromagnetic microwave theory33, the impedance layer should have middle ε′ and lower ε″, and the lossy layer should have higher dielectric loss to enhance the absorption ability. From the tested complex permittivity of CBfilled SLF-SiCf/epoxy composites, when the CB content was 5
As can be seen, when the CB content was 7.5 wt %, the minimum reflection loss values shifted from higher frequency to lower frequency with increasing the thickness from 2.5 to 3.0 mm, and also the absorbing performance became worse. When the thickness was 2.5 mm, the 7.5 wt % CB filled SLF-SiCf/ epoxy composites have optimum absorbing properties, the reflection loss value below −9 dB can be obtained from 8.2 to 11.5 GHz, and the minimum reflection loss value was −33 dB at 9.77 GHz. For the composites with the same thickness of 3 mm, the minimum reflection loss values also changed with increasing the CB content, and when the CB content was 7.5 wt %, the composite has good absorbing properties. The reflection loss value below −5 dB can be reached at the whole tested frequency range. By further increasing the CB content, the absorption properties became worse. According to the Debye theory, high electrical conductivity of the carbon filler (CB) added to SLF-SiCf/epoxy would increase the electrical conductivity of the composites and thus increased the ε″. Only considering the attenuation ability, it seems like the higher electric conductivity of the composites, the higher the absorbing performance. However, actually, higher electrical conductivity would induce a more reflected microwave weave. Namely, the input impedance cannot meet the requirement. The input impedance of absorbing material can be expressed as follows:27 Z=
μ0 μ ε0ε
= Z0
ωμ0 μ μ = (1 + j) εtgδ 2σ
(1)
where Z0 is the vacuum impedance, ε is the relative permittivity, μ is the relative permeability, σ is the conductivity, and ω is the angle frequency. From this equation, the higher the conductivity, the lower the input impedance. As an ideal radar absorbing material with good performance, two requirement should be meet,28,29 one is the impedance matching and the other is a high attenuation constant. Only the absorbing material meets the two requirement at the same time, and optimum absorbing properties can be reached. In order to analyze the absorbing capacity of CB-filled SLF-SiCf/epoxy composites, the impedance and attenuation constant of composites were calculated according to the complex permittivity. Figure 4a,b is the input impedance and attenuation constant of SLF-SiCf/epoxy composites with different CB content with a thickness of 3 mm. From Figure 4a, the impedance matching property of CB filled SLF-SiCf/epoxy composites was decreased with increasing the CB content, and when the CB content was 7.5 wt %, the impedance matching constant was close to the impedance D
DOI: 10.1021/acs.iecr.7b04905 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research wt %, the ε′ and ε″ are 7.7 and 1.6, which was a good candidate as the impedance layer. So in this section, double layered absorbers were designed to use 12.5 wt % CB-filled SLF-SiCf/ epoxy composites and KD-1SiCf/epoxy composites as the lossy layer. From our tested results, the KD-1SiCf/epoxy composites have a high dielectric loss compared with 12.5 wt % CB- SLFSiCf/epoxy composites, and the complex permittivity of KD1SiCf/epoxy composites is shown in Figure 5. Thus, two kinds of double layer absorbers were designed and calculated.
the whole frequency range. However, using the 12.5 wt % CBSLF SiCf/epoxy composite as the lossy layer, the absorbing properties of the double layered absorber were not good when compared with using the KD-1SiCf/epoxy composite as the lossy layer. The absorbing properties of double layered SiCf/epoxy composites were better when compared with single layer absorbing structures. As can be viewed from the schematic diagram Figure 7, for single layer radar absorbing structures, the
Figure 7. Schematic diagram of single and double layered radar absorbing structures.
absorbing properties can be adjusted by the complex permittivity and the thickness, and both factors have been studied and simulated. However, the absorbing bandwidths of single layer absorbing structures were narrow due to the dielectric constant and dielectric loss cannot meet the requirement at the same time. Compared with single layer radar absorbing structures, the absorbing properties of double layered absorbing structures can be adjusted by the complex permittivity and thickness of each layer. Moreover, the tunable microwave absorption is attributed to the impedance matching and attenuation constant, which is controlled by the different CB contents, the thickness of the composites as well as the complex permittivity of the each layer.28 The absorbing properties of radar absorbing structures were compared with peer works, as can be viewed in Table3, and using SLF-SiCf as a reinforcement and CB as an absorbent, the double layered absorbing material has good absorbing performance and wider absorption bandwidths. In order to vertify the effectiveness of double layered absorbing material, the double layered RAS was fabricated based on the simulation. Using 5 wt %CB-filled SLF-SiCf/ epoxy composites as the impedance layer and the KD-1SiCf/ epoxy composite as the lossy layer, the corresponding thicknesses were 2.0 and 1.0 mm, respectively. Figure 8a
Figure 5. Complex permittivity of KD-1SiCf/epoxy composites.
The calculated reflection loss curves using the KD-1SiCf/ epoxy composite and the 12.5 wt % CB-SLFSiCf/epoxy composite as the lossy layer were depicted in Figure 6. As can be seen, the absorption properties of the double layer absorber first became good and then got worse with increasing the thickness of the lossy layer with the total thickness of 3 mm. When using the KD-1SiCf/epoxy composite as the lossy layer and with a thickness of 1.0 mm, the reflection loss values below −10 dB can be obtained at the whole tested frequency range, and the minimum reflection loss value was −20.9 dB at the 9.5 GHz. The reflection loss value below −10 dB can also be achieved for the double layered absorber when the thickness of the lossy layer was 1.2 mm. A thicker thickness of the lossy layer made the double layered absorber have good absorbing performance at higher frequency; for example, the minimum reflection loss value at 12.4 GHz was −12.6 dB when the lossy layer thickness was 1.2 mm. Using the 12.5 wt % CB-SLF SiCf/ epoxy composite as the lossy layer (Figure 6b), the optimum absorbing properties of the double layered absorber were obtained when the thickness of lossy layer was 1.4 mm or 1.6 mm, the reflection loss value below −7 dB can be obtained at
Figure 6. Double layered SiCf/epoxy composites with different thicknesses of the lossy layer. (a) KD-ISiCf/epoxy and (b) 12.5 wt % CB-SLF SiCf/ epoxy at 3 mm thickness. E
DOI: 10.1021/acs.iecr.7b04905 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 3. Comparison Absorbing Properties of Radar Absorbing Structuresa
a
layer
reinforcement
absorbent
content
MT (mm)
single single single single single double double double double single double
glass fiber glass fiber glass fiber carbon fiber carbon fiber glass fiber glass fiber glass fiber glass fiber SiC fiber SiC fiber
CB CNT CNF Fe Fe CB CB MWCNT MWCNT CB CB
20 20 15 30 40 51+72 71+52 0.41+1.62 0.41+1.62 7.5 5wt1+KD-1-SiCf2
2.75 2.24 2.02 4.22 4.48 0.81+1.82 0.61+1.92 1.91+1.42 1.941+1.352 2.5 1.81+1.22
bandwidth
