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A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption
Accepted Manuscript A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption
Qian Zhou, Xiaowei Yin, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang PII: DOI: Reference:
S0264-1275(17)30292-7 doi: 10.1016/j.matdes.2017.03.044 JMADE 2878
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
11 December 2016 6 February 2017 15 March 2017
Please cite this article as: Qian Zhou, Xiaowei Yin, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang , A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.03.044
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ACCEPTED MANUSCRIPT A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption Qian Zhou, Xiaowei Yin1, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory,
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Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China
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Abstract: This paper presents a novel ultra-broadband two-layer periodic stepped radar absorbing structure (PSRAS). The optimal PSRAS shows more than 90%
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absorption in the frequency range from 2.64 to 40 GHz by using a conventional α Fe
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reinforced epoxy resin composite. The greatly enhanced radar absorption property of
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designed PSRAS is a result of the multi-scale effect by combing effects of microscopic and mesoscopic scales. The designed PSRAS makes the effective
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impedance match to that of the free space in a wide frequency: most of the incident
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energy is dissipated by the simultaneous incorporation of multiple λ/4 resonances, strong resonances between adjacent unit cells and edge diffraction effects. Based on
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the geometrical parameters optimization for the two-layer PSRAS, it is specified that
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size of unit cell obeys a simple rule of the Golden Section approximately. Additionally, the incident angle dependent absorption properties are simulated and discussed. It is expected that the proposed PSRAS composite has great potentials in stealth technologies and electromagnetic interferences.
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Corresponding author. Tel.: +86-029 88494947; fax: +86-029 88494620. E-mail addresses: [email protected] (Xiaowei Yin) 1
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Keywords: broadband; metamaterials; radar absorbing structure
1. Introduction Radar absorbing materials have attracted considerable attention in the fields of
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military and civilian applications with the rapid development of radar detection and
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wireless communications [1–5]. Broadband and strong absorption is increasingly
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important for radar absorbing materials due to the wide working frequency range of the detection system and electromagnetic equipment. Hence, typical efforts have been
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performed by designing broadband radar absorbing structures (RASs) which possess
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both absorbing and loading characteristics. The common RASs include honeycomb structure [6–8], pyramidal structure [9], multilayer structure [10–13] and so on.
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However, these structures generally show characteristics of complicated fabrication
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process, large thickness, and narrow bandwidth, etc., which restrict their applications and need to be improved.
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Recently, metamaterial absorbers have attracted much attention due to their
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abnormal electromagnetic properties [14–17]. The effective permittivity and effective permeability of metamaterial absorbers can be manipulated by designing parameters of sub-wavelength microstructure. In radar frequency, the metamaterial absorbers have mesoscopic scale effects, while traditional absorbing materials possess microscopic scale effects. Thus, a multi-scale effect can be achieved by combining metamaterials and traditional radar absorbing materials. The periodic patterned RASs 2
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can be designed to greatly improve the absorbing properties. Up to now, many efforts have been made to achieve a periodic stepped absorbing structure at different frequencies including radar [5, 18–20], terahertz [21, 22], infrared region [23] and visible optical regions [24,25]. The periodic stepped absorbing structure can greatly
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improve the absorption and broaden the bandwidth, especially in radar frequencies.
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Therefore, the periodic stepped radar absorbing structure (PSRAS) is promising to
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design high performance radar composite, meanwhile it is still of great essentials to
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reduce the relative complexity of the structure and understand the absorbing mechanism more deeply.
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In this work, we proposed a simple two-layer PSRAS using a conventional α Fe reinforced epoxy resin composite. The designed PSRAS was demonstrated
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experimentally to show an ultra-broadband absorption in the frequency of 2.64– 40.0 GHz. The mechanism for absorption properties of the two-layer PSRAS
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composite was specified. Furthermore, a rule was concluded for the optimum geometry parameters of the two-layer PSRAS design. Additionally, the incident angle
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dependent absorption properties are simulated and discussed.
2. Simulations and experiments 2.1 Model design and simulations Fig. 1(a) shows the schematic of the RAS which is made of periodic stepped structure unit cells arranged on a metallic plane. The specific design parameters are 3
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shown in Fig. 1(b). Taking into account the specific application in the high performance aircrafts with curved surface and the mesoscopic scale effect, a small-size unit cell is designed comparing to those in Li et al [5] and Xu et al’s work [19].
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The numerical simulations were performed by implementing the finite integration
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technique (CST Microware Studio), in which unit cell boundary conditions were
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applied in the x and y directions. A wave-guide port was used to generate transverse
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electromagnetic plane waves perpendicularly to the sample plane, which propagates along the −z direction, as shown in Fig. 1(a). The absorption could be calculated as
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A(ω) = 1 − T(ω) − R(ω), where R(ω) = |S11|2 and T(ω) = |S21|2 are the reflectance and transmittance obtained from the frequency dependent complex S-parameters,
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respectively. Since the backside is grounded by metallic plane, the transmittance T(ω) is zero. Thus, the absorption can be reduced to A(ω) = 1 − R(ω). The frequency
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dispersion characteristics of the complex permittivity and permeability in 2–40 GHz
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are taken into account by applying the first order Debye model.
Fig. 1. Schematic of the PSRAS: (a) perspective view and (b) 3D view of the unit cell. 4
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2.2 Fabrication In order to design a broadband radar absorbing material, an ultrafine iron powder was selected as the absorbent with magnetic loss and dielectric loss. Commercial α Fe reinforced epoxy resin composite (Model of EM150 supplied by Dongshin
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Microwave in China) was used as the absorbing material. The PSRAS composite with
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the designed geometry, as shown in Fig. 1 (a1 =p = 10 mm, h1 = 2.3 mm; a2 = 6.3 mm,
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h2 = 3.2 mm), was engraved through a numerically controlled lathe (PBZ1300, NC
180 mm × 180 mm × 5.5 mm.
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2.3 Characterization and measurement
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gantry machining center, China) on a flat composite with a dimension of
Microstructure observations and analysis of the α Fe reinforced epoxy resin
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composite were performed by using a scanning electron microscope (SEM, S-4700; Hitachi, Japan) attached to a link systems energy dispersive spectrometer (EDS). The
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permittivity and permeability were determined by a vector network analyzer (VNA, N5234A; Agilent, USA) in the range of 2–18 GHz. The measured sample was also
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fabricated in a numerically controlled lathe. The reflection loss of the fabricated PSRAS composite was measured by a NRL-arch reflection test system equipped with the VNA (MS4644A; Japan) in a microwave anechoic chamber. The measurement of reflection loss was performed in the frequency bands of 2–4 GHz, 4–8 GHz, 8– 18 GHz, 18–26.5 GHz and 26.5–40 GHz, with five pairs of broadband antenna horns, respectively. In the measurement, the reflection from a pure metal plate with 5
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the same size as that of the fabricated PSRAS composite was used for normalization.
3. Results and discussions 3.1 Absorption properties
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The composition and distribution of α Fe in epoxy resin are shown in Fig. 2. It is obvious that the ultrafine Fe is uniformly dispersed in the the epoxy resin and particle
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size of α Fe is less than 10 μm. Additionally, micro pores are found in the composite.
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As a benchmark for the absorbing materials, the original radar absorption of α Fe
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reinforced epoxy resin composite in the microscopic scale is investigated.
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Fig. 2 Backscatter electron images of the α Fe reinforced epoxy resin composite. The measured complex permittivity and permeability of α Fe reinforced epoxy
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resin composite in the frequency range of 2–18 GHz are shown in Fig. 3(a). The real part (ε') and imagine part (ε'') of the complex permittivity are relatively stable along with the increasing frequency; while the real part (μ') and imagine part (μ'') of the permeability decrease with the increasing frequency. The reflection loss (RL) for a single layer of flat composite backed by a conductive metal plate can be calculated by [26]: 6
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RL 20 log10
Zin Z 0 Zin Z 0
(1)
2π Zin Z0 μ / ε tanh j μ / ε fd c
(2)
where Z0=(μ0/ε0)1/2 is the characteristic impedance of free space, Zin is the normalized
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input impedance of absorbing material, ε and μ are the relative complex permittivity
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and permeability, respectively, f is the frequency of radar, d is the layer thickness of
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absorbing material, and c represents the speed of light in the free space
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(c=3.0×108 m/s). Fig. 3(b) shows the distribution map of RL with d and f. The bandwidth of −10 dB absorption does not exceed 5.5 GHz for the flat composite. It is
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believed that the electromagnetic absorption of single flat composite is mainly caused
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by the magnetic loss of α Fe and multiple reflection of the absorbent and pores.
Fig. 3. (a) The frequency dependence of complex permittivity and permeability for α 7
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Fe reinforced epoxy resin composite; (b) distribution map of RL with d and f; (c) frequency dependence of reflection loss spectrum for the flat composite with ten thicknesses; (d) simulations of the composite thickness versus peak frequency (fm) for the flat composite under λ/4 model.
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According to the quarter wavelength matching mechanism (λ/4), the matching
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thickness (dm) and the corresponding peak frequency (fm) should satisfy the following
nλ nc (n 1,3,5, ) 4 4 fm μ ε
(3)
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dm
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equation [27]:
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where λ is the wavelength in the medium, |ε| and |μ| are the modulus of the measured ε and μ at fm, respectively. Fig. 3(c) shows the variations of RL curves versus frequency
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for the flat composite with different thicknesses. The simulation of dm versus fm for
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the flat composite is revealed in Fig. 3(d). The red hollow rhombuses in Fig. 3(d) are the matching thicknesses versus fm, which are directly obtained from the RL curves in
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Fig. 3(c). It is clear that fm is inversely proportional with dm and the RL peaks shift to
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lower frequency by increasing the layer thickness. More importantly, the red hollow rhombus is located around the λ/4 curve, demonstrating that the relationship between dm and fm for the radar absorption obeys the λ/4 model. To affirm the reliability of our design, a sample was fabricated using α Fe reinforced epoxy resin composite. The absorption properties of designed PSRAS composite as well as the flat composite were simulated and measured in Fig. 4. The 8
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geometric parameters of PSRAS composite in Fig. 4 are a1 =p = 10 mm, h1 = 2.3 mm; a2 = 6.3 mm, h2 = 3.2 mm. On the whole, the measured result is in a good agreement with the full-wave simulation when taking into account of the roughness in the fabrication process as well as the errors in the experimental measurement. By
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comparing the measured reflection loss of the flat composite with that of the PSRAS
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composite, it can be concluded that PSRAS composite can effectively increase the
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absorbing properties and broaden the bandwidth. The measured reflection loss curve
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about the PSRAS composite are credible.
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coincides well with the corresponding simulation results. Therefore, our simulations
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Fig. 4. Simulated and measured reflection loss spectrums of the PSRAS composite
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and flat composite according to the frequency.
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3.2 Absorption mechanism
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Fig. 5. Calculated impedance spectrum of PSRAS composite and flat composite: (a) real part, (b) imaginary part
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In principle, when the impedance of the structure is matched to the air to minimize
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the reflection, a perfect absorption can be achieved. The total impedance is obtained from the combination of the impedance of the absorbing material layer with the
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ground plane. The effective input impedance (Zeff) of the RAS can be obtained
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from [28]:
μeff (ω) εeff (ω)
(1 S11 ) 2 S212 (1 S11 ) 2 S212
(4)
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Z eff (ω)
where εeff (ω) and μeff (ω) are the effective permittivity and permeability, respectively.
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The real and imaginary part of the impedance were calculated from the simulated complex S parameters and plotted in Fig. 5. Since the structure is backed by a completely metallic plane, |S21| = 0. The real parts of Zeff are close to unity, while the imaginary parts of Zeff are nearly zero for both the absorption peaks of the flat composite and the PSRAS composite. These characteristics demonstrate typical absorption phenomenons. Comparing with the flat structure, the real part of Zeff of the 10
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PSRAS is more close to unity, and imaginary part of Zeff in the PSRAS is more close to zero for all absorption frequencies, which results in minimized reflections over a wide frequency range. Therefore, the PSRAS composite shows a significant absorption property.
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Since the size of unit cell is far less than the wavelength of the incident wave, each
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unit cell can be regarded as an effective homogeneous medium. According to the
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effective medium theory (EMT), the permittivity and permeability of effective
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homogenous cell can be calculated by [29]:
αε 1 α αε
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εeff 1 α ε
μeff 1 α μ
αμ 1 α αμ
(5)
(6)
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where ε and μ are the permittivity and permeability of the α Fe reinforced epoxy resin composite, respectively. α is the volume filling fraction of the PSRAS composite.
a α 1
2 1
a2 2 h2
a12 h1 h2
(7)
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Here, for our two-layer PSRAS composite, α can be calculated by:
Fig. 6. The absorption spectrums of the PSRAS composite simulated by CST software 11
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and calculated by effective medium theory. Fig. 6 gives the absorption spectrums of the PSRAS composite simulated by the CST software, which is in a good agreement to that calculated by the effective medium theory. Therefore, the effective medium theory provides theoretical insights
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into understanding the relationship between the geometry parameters (a1, h1; a2, h2) of
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PSRAS composite and its radar absorption performance approximately. Hence, the
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effective permittivity and permeability of proposed PSRAS composite can be
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controlled by the geometry parameters, thus the Zeff of the PSRAS composite could be
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adjusted to that of the free apace.
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Fig. 7. The distributions of electric field (left) and magnetic field (right) of the PSRAS composite at various frequencies: (a) 3.95 GHz; (b) 19.8 GHz; (c) 32.5 GHz. The electric and magnetic field distributions on the composite at three absorption peak frequencies were simulated and mapped. Fig. 7(a) shows the 3D views and cross section (x = 0 plane) views of the electric and magnetic field distributions at first peak f1 = 3.95 GHz. The electric field concentrates on the top edge surface area of the top 12
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layer in the x-z plane and gap area of PSRAS, while magnetic field concentrates on the center of bottom layer next to the metal plane and shows a typical characteristic of λ/4 resonance. Fig. 7(b) gives the electric and magnetic field distributions at the second peak f2 = 19.8 GHz. The electric field concentrates on the lower edge surface
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area of the top layer in the x-z plane, and the electric field in the cross section shows a
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weak resonance between adjacent unit cells. The magnetic field also concentrates on
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the two sides of lower edges of the top layer and surface area of the bottom area in the
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x-z plane, and the magnetic field in the cross section shows a strong resonance between adjacent unit cells. Fig. 7(c) shows the electric and magnetic field
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distributions at the third peak f3 = 32.5 GHz. The electric and magnetic field both concentrate on the top edge area of top layer and present strong edge diffractions.
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Then, we continued to simulate the power loss density distributions at the three absorption peak frequencies to further understand the mechanism of absorption
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behaviors in the PSRAS composite, results are shown in the top of Fig. 8. The power loss distribution is similar to that of the magnetic field at the lower frequency
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f1 = 3.25 GHz, which indicates the λ/4 magnetic resonance. The middle frequency f2 = 19.8 GHz is mainly distributed on the top and side edges of the PSRAS composite as a result of strong resonance between adjacent unit cells and weak edge diffractions. The power loss distribution at the high frequency f3 = 32.5 GHz is mainly distributed on the upper and side edges of the top area as the result of strong edge diffractions. Through the analysis of field and power loss distributions, we can conclude that the 13
ACCEPTED MANUSCRIPT total absorption at the relatively low frequency is mainly induced by the λ/4 magnetic resonance; the total absorption at middle frequency is induced by the strong resonance between adjacent unit cells and edge diffractions effects which result from weak electric and strong magnetic resonance simultaneously; the total absorption at
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relatively high frequency is induced by the strong edge diffractions effects which
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result from weak electric and strong magnetic resonance simultaneously.
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Fig 8 gives the schematic for radar absorption mechanism of the designed PSRAS
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composite, which is combined with the effect of mesoscopic and microscopic scales,
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and it could be a kind of promising absorbing materials for application.
Fig. 8. Schematic for radar absorption mechanism of PSRAS composite and their applications. 3.3 Optimization design Firstly, the geometry parameters (a1, h1; a2, h2) dependent absorption performance 14
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of PSRAS composite was investigated. An optimization for PSRAS composite with −10 dB bandwidth (BW) in 2–40 GHz was conducted for the following design parameters: thicknesses (h1, h2) and side length (a2) with scale step of 0.1 mm. Some constraints were placed to the design parameters as a1 = 10 mm and h1 + h2= 5.5 mm.
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The best case for optimization were a2 = 6.3 mm and h2 = 3.2 mm, and the BW is
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37.36 GHz (2.64–40 GHz) for the case. As shown in Fig. 4 the PSRAS composite
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can effectively increase the absorption property and broaden the bandwidth.
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During the optimization process, it was found that the geometry parameters of a2 and h2 have a significant influence on the absorption properties. Meanwhile, the
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contribution of the λ/4 resonance is highly correlated to the area fraction of the part with the corresponding thickness in the composite [5]. Therefore, a structure factor
Fa
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layer square area:
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related to a2 is defined as the ratio of top layer surface square area and the bottom
a2 2 a12
(8)
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Another structure factor related to h2 is defined as the ratio of the thickness of top layer and total thickness: Fh
h2 h
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(9)
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Fig. 9. Effects of structure factors on peak frequency shift and bandwidth (BW) of the
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reflection loss spectrum for PSRAS composite: (a) Fa and (b) Fh.
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Fig. 9 gives the effects of Fa and Fh on peak frequency shift and bandwidth of the reflection loss spectrum of PSRAS composite. Obviously, there are two bandwidths
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beginning in the relative low and high frequencies, called BW1 and BW2, respectively. The positions of the three peaks directly affect the BW1 and BW2.
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According to the above field distributions analysis and previous results in Li et al’s work [5], the 1st peak f1 is related to the combination λ/4 resonance of the thickness of
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total thickness (h1 + h2) and top layer thickness (h1), which is related to Fa directly. Hence, the f1 shifts towards lower frequency with the increasing Fa, as shown in Fig.
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9(a). Additionally, with the increase of Fa, the 2nd peak f2 shifts towards lower frequencies, while the 3rd peak f3 shifts towards higher frequencies. The position changes of three peaks generate an overlapped BW when Fa is around 0.25–0.45, which leads to a broad bandwidth with the three peaks located in the overlapped BW. When Fa is more than 0.50, f3 will disappear due to the weak edge diffraction effects. Therefore, a proper Fa is needed to generate a broadband absorption. With Fa = 0.40, 16
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a relative broadband absorption is achieved, especially at the lower frequency. Fig. 9(b) gives the effect of Fh on peaks frequency shift and bandwidth of the reflection loss spectrum for PSRAS composite. With the increase of Fh, both f1 and f3 shift towards higher frequencies while f2 shifts towards lower frequencies. When Fh is
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more than 0.55, a broad bandwidth with the three peaks located in the overlapped BW
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is generated. Therefore, there is an optimum Fh = 0.58 to realize a relative broadband
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absorption.
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In order to identify the determination of these two structure factors during the PSRAS composite design, another two demonstrations were performed by optimizing
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new PSRAS composites with different sizes of unit cells. The optimization for PSRAS composite with −10 dB bandwidth (BW) in 2–40 GHz was conducted with
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scale step of 0.1 mm. In the case a1 = p = 8 mm and h1 + h2 = 3.0 mm, the optimum parameters are a2 = 5.1 mm and h2 = 1.9 mm, and the BW is 34.43 GHz (5.57–
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40 GHz). The corresponding Fa and Fh are 0.41 and 0.63. In the case a1 = p = 15 mm and h1 + h2 = 6 mm, the optimum parameters are a2 = 9.5 mm and h2 = 3.5 mm, and
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the BW is 37.76 GHz (2.24–40 GHz). The corresponding Fa and Fh are 0.40 and 0.58. Based on the above simulation results, we can find that the structure factor Fa for the optimum unit cell structure is approximately equal to 0.4, and the corresponding Fh is around 0.6. Therefore, when the period and total thickness are fixed, the optimum geometry parameters of two-layer PSRAS composite can be quickly found according to the above rule. It is very interesting that the values of the 17
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two structure factors approximately obey the rule of the Golden Section.
Fig. 10. Bandwidth versus thickness for the typical radar absorbing structures.
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Fig. 10 summarizes and compares the bandwidth versus thickness for the typical RASs (including the honeycomb structure [6−8], pyramidal structure [9], multilayer
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structure [10−13] and the periodic stepped structure [5, 18, 19]). It is clear that the periodic stepped structure is highly competitive to other RASs due to its great
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broadband and thin thickness. The two-layer PSRAS designed in this work not only has excellent radar properties but also possesses relative simple and small unit cell
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structure by combining the mesoscopic scale and microscopic scale effects. 3.4 Dependence of incidence angle As radar absorbing materials are generally polarization and angle of incidence dependent, the absorption properties of PSRAS composite with different oblique incidence angles for transverse electric (TE) and transverse magnetic (TM) polarization are simulated and results are presented in Fig. 11. 18
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Fig. 11. Reflection loss spectrum of the PSRAS composite as a function of incident
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angle and frequency for different polarizations: (a) TE polarization; (b) TM
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polarization.
For both TE and TM polarization, the intensity of f1 absorption peak gradually
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decreases as the incident angle increases. It can be attributed to the fact that f1 is related to the λ/4 magnetic resonance, and the intensity of the λ/4 resonance decreases
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as the incident angle increases. With the increasing incident angle, the intensity of f2 absorption peak gradually decreases for TE polarization, while increases for TM
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polarization. As discussed in section 3.2, the f2 absorption peak is mainly related to the resonance between adjacent unit cells of weak electric and strong magnetic
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resonance. For TE polarization, as the incident angle increases, the tangential component of magnetic field intensity decreases but the direction of the electric field remains unchanged. While for TM polarization, the situation is just the opposite. Theses lead to a decreased absorption for TE polarization and an increased absorption for TM polarization. For both TE and TM polarization, the intensity of f3 absorption peak first increases and then decreases as the incident angle increases, which lead to a 19
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maximum absorption at 45º. As discussed in section 3.2, the f3 absorption peak is mainly related to the edge diffraction effects of weak electric and strong magnetic resonance. The intensity of the edge diffraction effects first increases and then decreases as the incident angle increases. Additionally, the PSRAS composite can
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realize more than 80% absorption for TE polarization and achieve more than 90%
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absorption for TM polarization when the incident angle is less than 45˚.
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4. Conclusions
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In conclusion, a two-layer PSRAS composite was theoretically and experimentally
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demonstrated to show an ultra-broadband absorption, which is believed to be the result of the multi-scale effect by combing effects of microscopic and mesoscopic
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scales. The effective impedance of designed PSRAS can be controllably adjusted by
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the geometry parameters based on the effective medium theory, which can make the effective impedance match to that of the free space in a wide frequency. The most
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incident energy is dissipated by the multiple λ/4 resonances in low frequency, strong
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resonances between adjacent unit cells and edge diffraction effects in middle frequency and strong edge diffraction effects in high frequency. Based on optimum design results, two structure factors of two-layer PSRAS obey a simple rule of the Golden Section approximately, which could be instructive for all similar structure designs. The simulated and experimental results indicate that the two-layer PSRAS composite has more than 90% absorption in the frequency range from 2.64 to 40 GHz. 20
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The good agreement between simulation and measured results demonstrates the validity of the proposed composite. Furthermore, the PSRAS composite can realize more than 80% absorption for TE polarization and achieve more than 90% absorption for TM polarization when the incident angle is less than 45˚. Particularly, the
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wave-transparent matrix can be filled into the gap of PSRAS composite to form a flat
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composite, which can greatly increase the mechanical properties. Therefore, the
great potential application in stealth technologies and
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performance with
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proposed PSRAS composite has a wide absorption band and easily tunable
electromagnetic interferences.
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Acknowledgments
This work was financially supported by the National Natural Science Foundation of
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China (Grant: 51332004, 51372204 and 51602258 ).
References
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[1] L.B. Kong, Z.W. Li, L. Liu, R. Huang, M. Abshinova, Z.H. Yang, C.B. Tang, P.K. Tan, C.R. Deng, S. Matitsine, Recent progress in some composite materials and
AC
structures for specific electromagnetic applications, Int. Mater. Rev. 58 (2013) 203‒ 259.
[2] F. Qin, C. Brosseau, A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles, J. Appl. Phys. 111 (2012) 061301– 061324. [3] M.S. Cao, W.L. Song, Z.L. Hou, B. Wen, J. Yuan, The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and 21
ACCEPTED MANUSCRIPT microwave-absorption of short carbon fiber/silica composites, Carbon 48 (2010) 788–796. [4] X.W. Yin, L. Kong, L.T. Zhang, L.F. Cheng, N. Travitzky, P. Greil, Electromagnetic properties of Si-C-N based ceramics and composites, Int. Mater. Rev. 59 (2014) 326–355.
microwave absorber, J. Appl. Phys. 116 (2014) 044110.
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[5] W. Li, T.L. Wu, W. Wang, P.C. Zhai, J.G. Guan, Broadband patterned magnetic
RI
[6] S. Xie, Z. Ji, Y. Yang, G. Hou, J. Wang, Electromagnetic wave absorption properties
SC
of honeycomb structured plasterboards in S and C bands, J. Build. Eng. 7 (2016) 217–223.
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[7] W.H. Choi, C.G. Kim, Broadband microwave-absorbing honeycomb structure with novel design concept, Compos. Part B-Eng. 83 (2015) 14–20.
MA
[8] J. Feng, Y. Zhang, P. Wang, H. Fan, Oblique incidence performance of radar absorbing honeycombs, Compos. Part B-Eng. 99 (2016) 465–471.
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[9] M.J. Park, J. Choi, S.S. Kim, Wide bandwidth pyramidal absorbers of granular
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ferrite and carbonyl iron powders, IEEE Trans. Magn. 36 (2000) 3272–3274. [10] D. Micheli, C. Apollo, R. Pastore, M. Marchetti, X-Band microwave characterization of carbon-based nanocomposite material, absorption capability
CE
comparison and RAS design simulation, Compos. Sci. Technol. 70 (2010) 400–409. [11] C.L. Hou, T.H. Li, T.K. Zhao, W.J. Zhang, Y.L. Cheng, Electromagnetic wave
AC
absorbing properties of carbon nanotubes doped rare metal/pure carbon nanotubes double-layer polymer composites, Mater. Des. 33 (2012) 413–418. [12] H. Tian, H.T. Liu, H.F. Cheng, A high-temperature radar absorbing structure: Design, fabrication, and characterization, Compos. Sci. Technol. 90 (2014) 202–208. [13] T. Wang, P. Wang, Y. Wang, L. Qiao, A broadband far-field microwave absorber with a sandwich structure, Mater. Des. 95 (2016) 486–489. [14] S. Ghosh, S. Bhattacharyya, Y. Kaiprath, K.V. Srivastava, Bandwidth-enhanced 22
ACCEPTED MANUSCRIPT polarization-insensitive microwave metamaterial absorber and its equivalent circuit model, J. Appl. Phys. 10 (2014) 104503. [15] H. Wang, P. Kong, W.T. Cheng, W.Z. Bao, X.W. Yu, L. Miao, J.J. Jang, Broadband Tunability of Polarization-Insensitive Absorber Based on Frequency Selective Surface, Sci. Rep-UK 6 (2016) 23081.
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[16] K. Ozden, O.M. Yucedag, H. Kocer, Metamaterial based broadband RF absorber at X-band, Int. J. Electron. Commun. (AEÜ) 70 (2016) 1062–1070.
RI
[17] H. Kocer, S. Butun, E. Palacios, Z. Liu, S. Tongay, D. Fu, K. Wang, J.Q Wu, K.
SC
Aydin, Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films, Sci. Rep-UK 5 (2015) 13384.
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[18] Y.J. Zhou, Y.Q. Pang, H.F. Cheng, Design and realization of a magnetic-type absorber with a broadened operating frequency band, Chin. Phys. B 22 (2013) 384–
MA
388.
[19] Y.S. Xu, W. Yuan, S.W. Bie, H.B. Xu, Q. Chen, J.J. Jiang, Broadband microwave
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absorption property of a thin metamaterial containing patterned magnetic sheet, J.
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Electromagnet Wave 29 (2015) 2420–2427. [20] J.P. Hao, V. Sadaune, L. Burgnies, D. Lippens, Ferroelectrics based absorbing layers, J. Appl. Phys. 116 (2014) 043520.
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[21] Z.X. Su, J.B. Yin, X.P. Zhao, Terahertz dual-band metamaterial absorber based on graphene/MgF2 multilayer structures, Opt. Express 23 (2015) 1679–1690.
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[22] S. Yin, J.F. Zhu, W.D. Xu, W. Jiang, J. Yuan, G. Yin, L.J. Xie, Y.B. Ying, Y.G. Ma, High-performance terahertz wave absorbers made of silicon-based metamaterials, Appl. Phys. Lett. 107 (2015) 073903. [23] N. Zhang, P.H. Zhou, S.Y. Wang, X.L. Weng, J.L. Xie, L.J. Deng, Broadband absorption in mid-infrared metamaterial absorbers with multiple dielectric layers, Opt. Commun. 338 (2015) 388–392. [24] J. Sautter, I. Staude, M. Decker, E. Rusak, D.N. Neshev, I. Brener, Y.S. Kivshar, 23
ACCEPTED MANUSCRIPT Active Tuning of All-Dielectric Metasurfaces, Acs Nano 9 (2015) 4308–4315. [25] H. Wang, V.P. Sivan, A. Mitchell, G. Rosengarten, P. Phelan, L.P. Wang, Highly efficient selective metamaterial absorber for high-temperature solar thermal energy harvesting, Sol. Energ. Mat. Sol. C 137 (2015) 235–242. [26] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, Complex permeability and permittivity
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and microwave absorption of ferrite-rubber composite at X -band frequencies, IEEE Trans. Magn. 27 (1991) 5462–5464.
RI
[27] J. Singh, C. Singh, D. Kaur, S.B. Narang, R. Joshi, S.R.Mishra, R. Jotania, M.
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Ghimire, C.C. Chauhang,Tunable microwave absorption in Co-Al substituted M-type Ba-Sr hexagonal ferrite, Mater. Des. 110 (2016) 749–761.
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[28] D.R. Smith, D.C. Vier, T. Koschny, C.M. Soukoulis, Electromagnetic parameter retrieval from inhomogeneous metamaterials, Phys. Rev. E 71 (2005) 036617.
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[29] H.S. Cho, S.S. Kim, Design of grid-type microwave absorbers with high-permittivity composites of Ag-coated Ni-Zn ferrite particles, J. Appl. Phys.
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117 (2015) 17A311.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights:
The designed two-layer periodic stepped radar absorbing structure has more than 90% absorption from 2.64 to 40 GHz frequency.
The proposed absorbing structure generates resonances between adjacent unit cells and edge diffraction effects, leading to great absorption.
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The two-layer periodic stepped radar absorbing structure can achieve great absorption
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when the incident angle is less than 45˚.
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Qian Zhou, Xiaowei Yin, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang PII: DOI: Reference:
S0264-1275(17)30292-7 doi: 10.1016/j.matdes.2017.03.044 JMADE 2878
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
11 December 2016 6 February 2017 15 March 2017
Please cite this article as: Qian Zhou, Xiaowei Yin, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang , A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.03.044
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ACCEPTED MANUSCRIPT A novel two-layer periodic stepped structure for effective broadband radar electromagnetic absorption Qian Zhou, Xiaowei Yin1, Fang Ye, Xiaofei Liu, Laifei Cheng, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory,
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Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China
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Abstract: This paper presents a novel ultra-broadband two-layer periodic stepped radar absorbing structure (PSRAS). The optimal PSRAS shows more than 90%
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absorption in the frequency range from 2.64 to 40 GHz by using a conventional α Fe
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reinforced epoxy resin composite. The greatly enhanced radar absorption property of
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designed PSRAS is a result of the multi-scale effect by combing effects of microscopic and mesoscopic scales. The designed PSRAS makes the effective
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impedance match to that of the free space in a wide frequency: most of the incident
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energy is dissipated by the simultaneous incorporation of multiple λ/4 resonances, strong resonances between adjacent unit cells and edge diffraction effects. Based on
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the geometrical parameters optimization for the two-layer PSRAS, it is specified that
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size of unit cell obeys a simple rule of the Golden Section approximately. Additionally, the incident angle dependent absorption properties are simulated and discussed. It is expected that the proposed PSRAS composite has great potentials in stealth technologies and electromagnetic interferences.
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Corresponding author. Tel.: +86-029 88494947; fax: +86-029 88494620. E-mail addresses: [email protected] (Xiaowei Yin) 1
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Keywords: broadband; metamaterials; radar absorbing structure
1. Introduction Radar absorbing materials have attracted considerable attention in the fields of
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military and civilian applications with the rapid development of radar detection and
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wireless communications [1–5]. Broadband and strong absorption is increasingly
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important for radar absorbing materials due to the wide working frequency range of the detection system and electromagnetic equipment. Hence, typical efforts have been
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performed by designing broadband radar absorbing structures (RASs) which possess
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both absorbing and loading characteristics. The common RASs include honeycomb structure [6–8], pyramidal structure [9], multilayer structure [10–13] and so on.
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However, these structures generally show characteristics of complicated fabrication
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process, large thickness, and narrow bandwidth, etc., which restrict their applications and need to be improved.
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Recently, metamaterial absorbers have attracted much attention due to their
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abnormal electromagnetic properties [14–17]. The effective permittivity and effective permeability of metamaterial absorbers can be manipulated by designing parameters of sub-wavelength microstructure. In radar frequency, the metamaterial absorbers have mesoscopic scale effects, while traditional absorbing materials possess microscopic scale effects. Thus, a multi-scale effect can be achieved by combining metamaterials and traditional radar absorbing materials. The periodic patterned RASs 2
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can be designed to greatly improve the absorbing properties. Up to now, many efforts have been made to achieve a periodic stepped absorbing structure at different frequencies including radar [5, 18–20], terahertz [21, 22], infrared region [23] and visible optical regions [24,25]. The periodic stepped absorbing structure can greatly
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improve the absorption and broaden the bandwidth, especially in radar frequencies.
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Therefore, the periodic stepped radar absorbing structure (PSRAS) is promising to
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design high performance radar composite, meanwhile it is still of great essentials to
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reduce the relative complexity of the structure and understand the absorbing mechanism more deeply.
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In this work, we proposed a simple two-layer PSRAS using a conventional α Fe reinforced epoxy resin composite. The designed PSRAS was demonstrated
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experimentally to show an ultra-broadband absorption in the frequency of 2.64– 40.0 GHz. The mechanism for absorption properties of the two-layer PSRAS
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composite was specified. Furthermore, a rule was concluded for the optimum geometry parameters of the two-layer PSRAS design. Additionally, the incident angle
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dependent absorption properties are simulated and discussed.
2. Simulations and experiments 2.1 Model design and simulations Fig. 1(a) shows the schematic of the RAS which is made of periodic stepped structure unit cells arranged on a metallic plane. The specific design parameters are 3
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shown in Fig. 1(b). Taking into account the specific application in the high performance aircrafts with curved surface and the mesoscopic scale effect, a small-size unit cell is designed comparing to those in Li et al [5] and Xu et al’s work [19].
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The numerical simulations were performed by implementing the finite integration
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technique (CST Microware Studio), in which unit cell boundary conditions were
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applied in the x and y directions. A wave-guide port was used to generate transverse
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electromagnetic plane waves perpendicularly to the sample plane, which propagates along the −z direction, as shown in Fig. 1(a). The absorption could be calculated as
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A(ω) = 1 − T(ω) − R(ω), where R(ω) = |S11|2 and T(ω) = |S21|2 are the reflectance and transmittance obtained from the frequency dependent complex S-parameters,
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respectively. Since the backside is grounded by metallic plane, the transmittance T(ω) is zero. Thus, the absorption can be reduced to A(ω) = 1 − R(ω). The frequency
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dispersion characteristics of the complex permittivity and permeability in 2–40 GHz
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are taken into account by applying the first order Debye model.
Fig. 1. Schematic of the PSRAS: (a) perspective view and (b) 3D view of the unit cell. 4
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2.2 Fabrication In order to design a broadband radar absorbing material, an ultrafine iron powder was selected as the absorbent with magnetic loss and dielectric loss. Commercial α Fe reinforced epoxy resin composite (Model of EM150 supplied by Dongshin
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Microwave in China) was used as the absorbing material. The PSRAS composite with
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the designed geometry, as shown in Fig. 1 (a1 =p = 10 mm, h1 = 2.3 mm; a2 = 6.3 mm,
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h2 = 3.2 mm), was engraved through a numerically controlled lathe (PBZ1300, NC
180 mm × 180 mm × 5.5 mm.
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2.3 Characterization and measurement
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gantry machining center, China) on a flat composite with a dimension of
Microstructure observations and analysis of the α Fe reinforced epoxy resin
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composite were performed by using a scanning electron microscope (SEM, S-4700; Hitachi, Japan) attached to a link systems energy dispersive spectrometer (EDS). The
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permittivity and permeability were determined by a vector network analyzer (VNA, N5234A; Agilent, USA) in the range of 2–18 GHz. The measured sample was also
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fabricated in a numerically controlled lathe. The reflection loss of the fabricated PSRAS composite was measured by a NRL-arch reflection test system equipped with the VNA (MS4644A; Japan) in a microwave anechoic chamber. The measurement of reflection loss was performed in the frequency bands of 2–4 GHz, 4–8 GHz, 8– 18 GHz, 18–26.5 GHz and 26.5–40 GHz, with five pairs of broadband antenna horns, respectively. In the measurement, the reflection from a pure metal plate with 5
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the same size as that of the fabricated PSRAS composite was used for normalization.
3. Results and discussions 3.1 Absorption properties
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The composition and distribution of α Fe in epoxy resin are shown in Fig. 2. It is obvious that the ultrafine Fe is uniformly dispersed in the the epoxy resin and particle
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size of α Fe is less than 10 μm. Additionally, micro pores are found in the composite.
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As a benchmark for the absorbing materials, the original radar absorption of α Fe
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reinforced epoxy resin composite in the microscopic scale is investigated.
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Fig. 2 Backscatter electron images of the α Fe reinforced epoxy resin composite. The measured complex permittivity and permeability of α Fe reinforced epoxy
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resin composite in the frequency range of 2–18 GHz are shown in Fig. 3(a). The real part (ε') and imagine part (ε'') of the complex permittivity are relatively stable along with the increasing frequency; while the real part (μ') and imagine part (μ'') of the permeability decrease with the increasing frequency. The reflection loss (RL) for a single layer of flat composite backed by a conductive metal plate can be calculated by [26]: 6
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RL 20 log10
Zin Z 0 Zin Z 0
(1)
2π Zin Z0 μ / ε tanh j μ / ε fd c
(2)
where Z0=(μ0/ε0)1/2 is the characteristic impedance of free space, Zin is the normalized
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input impedance of absorbing material, ε and μ are the relative complex permittivity
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and permeability, respectively, f is the frequency of radar, d is the layer thickness of
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absorbing material, and c represents the speed of light in the free space
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(c=3.0×108 m/s). Fig. 3(b) shows the distribution map of RL with d and f. The bandwidth of −10 dB absorption does not exceed 5.5 GHz for the flat composite. It is
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believed that the electromagnetic absorption of single flat composite is mainly caused
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by the magnetic loss of α Fe and multiple reflection of the absorbent and pores.
Fig. 3. (a) The frequency dependence of complex permittivity and permeability for α 7
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Fe reinforced epoxy resin composite; (b) distribution map of RL with d and f; (c) frequency dependence of reflection loss spectrum for the flat composite with ten thicknesses; (d) simulations of the composite thickness versus peak frequency (fm) for the flat composite under λ/4 model.
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According to the quarter wavelength matching mechanism (λ/4), the matching
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thickness (dm) and the corresponding peak frequency (fm) should satisfy the following
nλ nc (n 1,3,5, ) 4 4 fm μ ε
(3)
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dm
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equation [27]:
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where λ is the wavelength in the medium, |ε| and |μ| are the modulus of the measured ε and μ at fm, respectively. Fig. 3(c) shows the variations of RL curves versus frequency
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for the flat composite with different thicknesses. The simulation of dm versus fm for
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the flat composite is revealed in Fig. 3(d). The red hollow rhombuses in Fig. 3(d) are the matching thicknesses versus fm, which are directly obtained from the RL curves in
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Fig. 3(c). It is clear that fm is inversely proportional with dm and the RL peaks shift to
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lower frequency by increasing the layer thickness. More importantly, the red hollow rhombus is located around the λ/4 curve, demonstrating that the relationship between dm and fm for the radar absorption obeys the λ/4 model. To affirm the reliability of our design, a sample was fabricated using α Fe reinforced epoxy resin composite. The absorption properties of designed PSRAS composite as well as the flat composite were simulated and measured in Fig. 4. The 8
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geometric parameters of PSRAS composite in Fig. 4 are a1 =p = 10 mm, h1 = 2.3 mm; a2 = 6.3 mm, h2 = 3.2 mm. On the whole, the measured result is in a good agreement with the full-wave simulation when taking into account of the roughness in the fabrication process as well as the errors in the experimental measurement. By
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comparing the measured reflection loss of the flat composite with that of the PSRAS
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composite, it can be concluded that PSRAS composite can effectively increase the
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absorbing properties and broaden the bandwidth. The measured reflection loss curve
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about the PSRAS composite are credible.
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coincides well with the corresponding simulation results. Therefore, our simulations
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Fig. 4. Simulated and measured reflection loss spectrums of the PSRAS composite
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and flat composite according to the frequency.
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3.2 Absorption mechanism
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Fig. 5. Calculated impedance spectrum of PSRAS composite and flat composite: (a) real part, (b) imaginary part
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In principle, when the impedance of the structure is matched to the air to minimize
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the reflection, a perfect absorption can be achieved. The total impedance is obtained from the combination of the impedance of the absorbing material layer with the
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ground plane. The effective input impedance (Zeff) of the RAS can be obtained
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from [28]:
μeff (ω) εeff (ω)
(1 S11 ) 2 S212 (1 S11 ) 2 S212
(4)
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Z eff (ω)
where εeff (ω) and μeff (ω) are the effective permittivity and permeability, respectively.
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The real and imaginary part of the impedance were calculated from the simulated complex S parameters and plotted in Fig. 5. Since the structure is backed by a completely metallic plane, |S21| = 0. The real parts of Zeff are close to unity, while the imaginary parts of Zeff are nearly zero for both the absorption peaks of the flat composite and the PSRAS composite. These characteristics demonstrate typical absorption phenomenons. Comparing with the flat structure, the real part of Zeff of the 10
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PSRAS is more close to unity, and imaginary part of Zeff in the PSRAS is more close to zero for all absorption frequencies, which results in minimized reflections over a wide frequency range. Therefore, the PSRAS composite shows a significant absorption property.
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Since the size of unit cell is far less than the wavelength of the incident wave, each
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unit cell can be regarded as an effective homogeneous medium. According to the
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effective medium theory (EMT), the permittivity and permeability of effective
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homogenous cell can be calculated by [29]:
αε 1 α αε
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εeff 1 α ε
μeff 1 α μ
αμ 1 α αμ
(5)
(6)
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where ε and μ are the permittivity and permeability of the α Fe reinforced epoxy resin composite, respectively. α is the volume filling fraction of the PSRAS composite.
a α 1
2 1
a2 2 h2
a12 h1 h2
(7)
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Here, for our two-layer PSRAS composite, α can be calculated by:
Fig. 6. The absorption spectrums of the PSRAS composite simulated by CST software 11
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and calculated by effective medium theory. Fig. 6 gives the absorption spectrums of the PSRAS composite simulated by the CST software, which is in a good agreement to that calculated by the effective medium theory. Therefore, the effective medium theory provides theoretical insights
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into understanding the relationship between the geometry parameters (a1, h1; a2, h2) of
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PSRAS composite and its radar absorption performance approximately. Hence, the
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effective permittivity and permeability of proposed PSRAS composite can be
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controlled by the geometry parameters, thus the Zeff of the PSRAS composite could be
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adjusted to that of the free apace.
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Fig. 7. The distributions of electric field (left) and magnetic field (right) of the PSRAS composite at various frequencies: (a) 3.95 GHz; (b) 19.8 GHz; (c) 32.5 GHz. The electric and magnetic field distributions on the composite at three absorption peak frequencies were simulated and mapped. Fig. 7(a) shows the 3D views and cross section (x = 0 plane) views of the electric and magnetic field distributions at first peak f1 = 3.95 GHz. The electric field concentrates on the top edge surface area of the top 12
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layer in the x-z plane and gap area of PSRAS, while magnetic field concentrates on the center of bottom layer next to the metal plane and shows a typical characteristic of λ/4 resonance. Fig. 7(b) gives the electric and magnetic field distributions at the second peak f2 = 19.8 GHz. The electric field concentrates on the lower edge surface
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area of the top layer in the x-z plane, and the electric field in the cross section shows a
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weak resonance between adjacent unit cells. The magnetic field also concentrates on
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the two sides of lower edges of the top layer and surface area of the bottom area in the
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x-z plane, and the magnetic field in the cross section shows a strong resonance between adjacent unit cells. Fig. 7(c) shows the electric and magnetic field
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distributions at the third peak f3 = 32.5 GHz. The electric and magnetic field both concentrate on the top edge area of top layer and present strong edge diffractions.
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Then, we continued to simulate the power loss density distributions at the three absorption peak frequencies to further understand the mechanism of absorption
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behaviors in the PSRAS composite, results are shown in the top of Fig. 8. The power loss distribution is similar to that of the magnetic field at the lower frequency
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f1 = 3.25 GHz, which indicates the λ/4 magnetic resonance. The middle frequency f2 = 19.8 GHz is mainly distributed on the top and side edges of the PSRAS composite as a result of strong resonance between adjacent unit cells and weak edge diffractions. The power loss distribution at the high frequency f3 = 32.5 GHz is mainly distributed on the upper and side edges of the top area as the result of strong edge diffractions. Through the analysis of field and power loss distributions, we can conclude that the 13
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relatively high frequency is induced by the strong edge diffractions effects which
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result from weak electric and strong magnetic resonance simultaneously.
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Fig 8 gives the schematic for radar absorption mechanism of the designed PSRAS
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composite, which is combined with the effect of mesoscopic and microscopic scales,
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and it could be a kind of promising absorbing materials for application.
Fig. 8. Schematic for radar absorption mechanism of PSRAS composite and their applications. 3.3 Optimization design Firstly, the geometry parameters (a1, h1; a2, h2) dependent absorption performance 14
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of PSRAS composite was investigated. An optimization for PSRAS composite with −10 dB bandwidth (BW) in 2–40 GHz was conducted for the following design parameters: thicknesses (h1, h2) and side length (a2) with scale step of 0.1 mm. Some constraints were placed to the design parameters as a1 = 10 mm and h1 + h2= 5.5 mm.
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The best case for optimization were a2 = 6.3 mm and h2 = 3.2 mm, and the BW is
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37.36 GHz (2.64–40 GHz) for the case. As shown in Fig. 4 the PSRAS composite
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can effectively increase the absorption property and broaden the bandwidth.
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During the optimization process, it was found that the geometry parameters of a2 and h2 have a significant influence on the absorption properties. Meanwhile, the
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contribution of the λ/4 resonance is highly correlated to the area fraction of the part with the corresponding thickness in the composite [5]. Therefore, a structure factor
Fa
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layer square area:
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related to a2 is defined as the ratio of top layer surface square area and the bottom
a2 2 a12
(8)
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Another structure factor related to h2 is defined as the ratio of the thickness of top layer and total thickness: Fh
h2 h
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(9)
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Fig. 9. Effects of structure factors on peak frequency shift and bandwidth (BW) of the
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reflection loss spectrum for PSRAS composite: (a) Fa and (b) Fh.
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Fig. 9 gives the effects of Fa and Fh on peak frequency shift and bandwidth of the reflection loss spectrum of PSRAS composite. Obviously, there are two bandwidths
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beginning in the relative low and high frequencies, called BW1 and BW2, respectively. The positions of the three peaks directly affect the BW1 and BW2.
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According to the above field distributions analysis and previous results in Li et al’s work [5], the 1st peak f1 is related to the combination λ/4 resonance of the thickness of
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total thickness (h1 + h2) and top layer thickness (h1), which is related to Fa directly. Hence, the f1 shifts towards lower frequency with the increasing Fa, as shown in Fig.
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9(a). Additionally, with the increase of Fa, the 2nd peak f2 shifts towards lower frequencies, while the 3rd peak f3 shifts towards higher frequencies. The position changes of three peaks generate an overlapped BW when Fa is around 0.25–0.45, which leads to a broad bandwidth with the three peaks located in the overlapped BW. When Fa is more than 0.50, f3 will disappear due to the weak edge diffraction effects. Therefore, a proper Fa is needed to generate a broadband absorption. With Fa = 0.40, 16
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a relative broadband absorption is achieved, especially at the lower frequency. Fig. 9(b) gives the effect of Fh on peaks frequency shift and bandwidth of the reflection loss spectrum for PSRAS composite. With the increase of Fh, both f1 and f3 shift towards higher frequencies while f2 shifts towards lower frequencies. When Fh is
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more than 0.55, a broad bandwidth with the three peaks located in the overlapped BW
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is generated. Therefore, there is an optimum Fh = 0.58 to realize a relative broadband
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absorption.
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In order to identify the determination of these two structure factors during the PSRAS composite design, another two demonstrations were performed by optimizing
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new PSRAS composites with different sizes of unit cells. The optimization for PSRAS composite with −10 dB bandwidth (BW) in 2–40 GHz was conducted with
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scale step of 0.1 mm. In the case a1 = p = 8 mm and h1 + h2 = 3.0 mm, the optimum parameters are a2 = 5.1 mm and h2 = 1.9 mm, and the BW is 34.43 GHz (5.57–
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40 GHz). The corresponding Fa and Fh are 0.41 and 0.63. In the case a1 = p = 15 mm and h1 + h2 = 6 mm, the optimum parameters are a2 = 9.5 mm and h2 = 3.5 mm, and
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the BW is 37.76 GHz (2.24–40 GHz). The corresponding Fa and Fh are 0.40 and 0.58. Based on the above simulation results, we can find that the structure factor Fa for the optimum unit cell structure is approximately equal to 0.4, and the corresponding Fh is around 0.6. Therefore, when the period and total thickness are fixed, the optimum geometry parameters of two-layer PSRAS composite can be quickly found according to the above rule. It is very interesting that the values of the 17
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two structure factors approximately obey the rule of the Golden Section.
Fig. 10. Bandwidth versus thickness for the typical radar absorbing structures.
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Fig. 10 summarizes and compares the bandwidth versus thickness for the typical RASs (including the honeycomb structure [6−8], pyramidal structure [9], multilayer
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structure [10−13] and the periodic stepped structure [5, 18, 19]). It is clear that the periodic stepped structure is highly competitive to other RASs due to its great
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broadband and thin thickness. The two-layer PSRAS designed in this work not only has excellent radar properties but also possesses relative simple and small unit cell
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structure by combining the mesoscopic scale and microscopic scale effects. 3.4 Dependence of incidence angle As radar absorbing materials are generally polarization and angle of incidence dependent, the absorption properties of PSRAS composite with different oblique incidence angles for transverse electric (TE) and transverse magnetic (TM) polarization are simulated and results are presented in Fig. 11. 18
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Fig. 11. Reflection loss spectrum of the PSRAS composite as a function of incident
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angle and frequency for different polarizations: (a) TE polarization; (b) TM
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polarization.
For both TE and TM polarization, the intensity of f1 absorption peak gradually
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decreases as the incident angle increases. It can be attributed to the fact that f1 is related to the λ/4 magnetic resonance, and the intensity of the λ/4 resonance decreases
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as the incident angle increases. With the increasing incident angle, the intensity of f2 absorption peak gradually decreases for TE polarization, while increases for TM
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polarization. As discussed in section 3.2, the f2 absorption peak is mainly related to the resonance between adjacent unit cells of weak electric and strong magnetic
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resonance. For TE polarization, as the incident angle increases, the tangential component of magnetic field intensity decreases but the direction of the electric field remains unchanged. While for TM polarization, the situation is just the opposite. Theses lead to a decreased absorption for TE polarization and an increased absorption for TM polarization. For both TE and TM polarization, the intensity of f3 absorption peak first increases and then decreases as the incident angle increases, which lead to a 19
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maximum absorption at 45º. As discussed in section 3.2, the f3 absorption peak is mainly related to the edge diffraction effects of weak electric and strong magnetic resonance. The intensity of the edge diffraction effects first increases and then decreases as the incident angle increases. Additionally, the PSRAS composite can
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realize more than 80% absorption for TE polarization and achieve more than 90%
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absorption for TM polarization when the incident angle is less than 45˚.
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4. Conclusions
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In conclusion, a two-layer PSRAS composite was theoretically and experimentally
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demonstrated to show an ultra-broadband absorption, which is believed to be the result of the multi-scale effect by combing effects of microscopic and mesoscopic
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scales. The effective impedance of designed PSRAS can be controllably adjusted by
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the geometry parameters based on the effective medium theory, which can make the effective impedance match to that of the free space in a wide frequency. The most
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incident energy is dissipated by the multiple λ/4 resonances in low frequency, strong
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resonances between adjacent unit cells and edge diffraction effects in middle frequency and strong edge diffraction effects in high frequency. Based on optimum design results, two structure factors of two-layer PSRAS obey a simple rule of the Golden Section approximately, which could be instructive for all similar structure designs. The simulated and experimental results indicate that the two-layer PSRAS composite has more than 90% absorption in the frequency range from 2.64 to 40 GHz. 20
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The good agreement between simulation and measured results demonstrates the validity of the proposed composite. Furthermore, the PSRAS composite can realize more than 80% absorption for TE polarization and achieve more than 90% absorption for TM polarization when the incident angle is less than 45˚. Particularly, the
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wave-transparent matrix can be filled into the gap of PSRAS composite to form a flat
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composite, which can greatly increase the mechanical properties. Therefore, the
great potential application in stealth technologies and
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performance with
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proposed PSRAS composite has a wide absorption band and easily tunable
electromagnetic interferences.
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Acknowledgments
This work was financially supported by the National Natural Science Foundation of
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China (Grant: 51332004, 51372204 and 51602258 ).
References
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[1] L.B. Kong, Z.W. Li, L. Liu, R. Huang, M. Abshinova, Z.H. Yang, C.B. Tang, P.K. Tan, C.R. Deng, S. Matitsine, Recent progress in some composite materials and
AC
structures for specific electromagnetic applications, Int. Mater. Rev. 58 (2013) 203‒ 259.
[2] F. Qin, C. Brosseau, A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles, J. Appl. Phys. 111 (2012) 061301– 061324. [3] M.S. Cao, W.L. Song, Z.L. Hou, B. Wen, J. Yuan, The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and 21
ACCEPTED MANUSCRIPT microwave-absorption of short carbon fiber/silica composites, Carbon 48 (2010) 788–796. [4] X.W. Yin, L. Kong, L.T. Zhang, L.F. Cheng, N. Travitzky, P. Greil, Electromagnetic properties of Si-C-N based ceramics and composites, Int. Mater. Rev. 59 (2014) 326–355.
microwave absorber, J. Appl. Phys. 116 (2014) 044110.
PT
[5] W. Li, T.L. Wu, W. Wang, P.C. Zhai, J.G. Guan, Broadband patterned magnetic
RI
[6] S. Xie, Z. Ji, Y. Yang, G. Hou, J. Wang, Electromagnetic wave absorption properties
SC
of honeycomb structured plasterboards in S and C bands, J. Build. Eng. 7 (2016) 217–223.
NU
[7] W.H. Choi, C.G. Kim, Broadband microwave-absorbing honeycomb structure with novel design concept, Compos. Part B-Eng. 83 (2015) 14–20.
MA
[8] J. Feng, Y. Zhang, P. Wang, H. Fan, Oblique incidence performance of radar absorbing honeycombs, Compos. Part B-Eng. 99 (2016) 465–471.
D
[9] M.J. Park, J. Choi, S.S. Kim, Wide bandwidth pyramidal absorbers of granular
PT E
ferrite and carbonyl iron powders, IEEE Trans. Magn. 36 (2000) 3272–3274. [10] D. Micheli, C. Apollo, R. Pastore, M. Marchetti, X-Band microwave characterization of carbon-based nanocomposite material, absorption capability
CE
comparison and RAS design simulation, Compos. Sci. Technol. 70 (2010) 400–409. [11] C.L. Hou, T.H. Li, T.K. Zhao, W.J. Zhang, Y.L. Cheng, Electromagnetic wave
AC
absorbing properties of carbon nanotubes doped rare metal/pure carbon nanotubes double-layer polymer composites, Mater. Des. 33 (2012) 413–418. [12] H. Tian, H.T. Liu, H.F. Cheng, A high-temperature radar absorbing structure: Design, fabrication, and characterization, Compos. Sci. Technol. 90 (2014) 202–208. [13] T. Wang, P. Wang, Y. Wang, L. Qiao, A broadband far-field microwave absorber with a sandwich structure, Mater. Des. 95 (2016) 486–489. [14] S. Ghosh, S. Bhattacharyya, Y. Kaiprath, K.V. Srivastava, Bandwidth-enhanced 22
ACCEPTED MANUSCRIPT polarization-insensitive microwave metamaterial absorber and its equivalent circuit model, J. Appl. Phys. 10 (2014) 104503. [15] H. Wang, P. Kong, W.T. Cheng, W.Z. Bao, X.W. Yu, L. Miao, J.J. Jang, Broadband Tunability of Polarization-Insensitive Absorber Based on Frequency Selective Surface, Sci. Rep-UK 6 (2016) 23081.
PT
[16] K. Ozden, O.M. Yucedag, H. Kocer, Metamaterial based broadband RF absorber at X-band, Int. J. Electron. Commun. (AEÜ) 70 (2016) 1062–1070.
RI
[17] H. Kocer, S. Butun, E. Palacios, Z. Liu, S. Tongay, D. Fu, K. Wang, J.Q Wu, K.
SC
Aydin, Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films, Sci. Rep-UK 5 (2015) 13384.
NU
[18] Y.J. Zhou, Y.Q. Pang, H.F. Cheng, Design and realization of a magnetic-type absorber with a broadened operating frequency band, Chin. Phys. B 22 (2013) 384–
MA
388.
[19] Y.S. Xu, W. Yuan, S.W. Bie, H.B. Xu, Q. Chen, J.J. Jiang, Broadband microwave
D
absorption property of a thin metamaterial containing patterned magnetic sheet, J.
PT E
Electromagnet Wave 29 (2015) 2420–2427. [20] J.P. Hao, V. Sadaune, L. Burgnies, D. Lippens, Ferroelectrics based absorbing layers, J. Appl. Phys. 116 (2014) 043520.
CE
[21] Z.X. Su, J.B. Yin, X.P. Zhao, Terahertz dual-band metamaterial absorber based on graphene/MgF2 multilayer structures, Opt. Express 23 (2015) 1679–1690.
AC
[22] S. Yin, J.F. Zhu, W.D. Xu, W. Jiang, J. Yuan, G. Yin, L.J. Xie, Y.B. Ying, Y.G. Ma, High-performance terahertz wave absorbers made of silicon-based metamaterials, Appl. Phys. Lett. 107 (2015) 073903. [23] N. Zhang, P.H. Zhou, S.Y. Wang, X.L. Weng, J.L. Xie, L.J. Deng, Broadband absorption in mid-infrared metamaterial absorbers with multiple dielectric layers, Opt. Commun. 338 (2015) 388–392. [24] J. Sautter, I. Staude, M. Decker, E. Rusak, D.N. Neshev, I. Brener, Y.S. Kivshar, 23
ACCEPTED MANUSCRIPT Active Tuning of All-Dielectric Metasurfaces, Acs Nano 9 (2015) 4308–4315. [25] H. Wang, V.P. Sivan, A. Mitchell, G. Rosengarten, P. Phelan, L.P. Wang, Highly efficient selective metamaterial absorber for high-temperature solar thermal energy harvesting, Sol. Energ. Mat. Sol. C 137 (2015) 235–242. [26] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, Complex permeability and permittivity
PT
and microwave absorption of ferrite-rubber composite at X -band frequencies, IEEE Trans. Magn. 27 (1991) 5462–5464.
RI
[27] J. Singh, C. Singh, D. Kaur, S.B. Narang, R. Joshi, S.R.Mishra, R. Jotania, M.
SC
Ghimire, C.C. Chauhang,Tunable microwave absorption in Co-Al substituted M-type Ba-Sr hexagonal ferrite, Mater. Des. 110 (2016) 749–761.
NU
[28] D.R. Smith, D.C. Vier, T. Koschny, C.M. Soukoulis, Electromagnetic parameter retrieval from inhomogeneous metamaterials, Phys. Rev. E 71 (2005) 036617.
MA
[29] H.S. Cho, S.S. Kim, Design of grid-type microwave absorbers with high-permittivity composites of Ag-coated Ni-Zn ferrite particles, J. Appl. Phys.
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CE
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117 (2015) 17A311.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights:
The designed two-layer periodic stepped radar absorbing structure has more than 90% absorption from 2.64 to 40 GHz frequency.
The proposed absorbing structure generates resonances between adjacent unit cells and edge diffraction effects, leading to great absorption.
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The two-layer periodic stepped radar absorbing structure can achieve great absorption
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when the incident angle is less than 45˚.
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