epoxy hollow spheres for structural performance and microwave absorption

Materials and Design 188 (2020) 108427

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Lattice composites with embedded short carbon fiber/Fe3O4/epoxy hollow spheres for structural performance and microwave absorption Yingjie Qiao, Zhaoding Yao, Xiaodong Wang ⁎, Xiaohong Zhang, Chengying Bai, Qiuwu Li, Kaixuan Chen, Zhuoran Li, Ting Zheng School of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Hollow macro-sized SCF/Fe3O4/epoxy spheres with high strength and microwave absorption properties were designed. • An epoxy resin matrix composite embedded with hollow SCF/Fe3O4/epoxy spheres was demonstrated to form a lattice structure. • The effective absorption bandwidth of this composites is 1.8 GHz, with an ultra-thin equivalent thickness of only ~0.48 mm.

a r t i c l e

i n f o

Article history: Received 3 September 2019 Received in revised form 12 December 2019 Accepted 13 December 2019 Available online xxxx Keywords: Hollow spheres Lattice structure Composites Microwave absorption

a b s t r a c t Lightweight absorption materials with superior mechanical properties have received significant attention owing to their potential for use in military stealth and electromagnetic protection applications. Herein, a novel electromagnetic (EM) wave absorbing lattice structured composite material containing a hollow short carbon fiber (SCF)/Fe3O4/epoxy sphere filler, and an epoxy resin matrix, is described. The morphology, density, and isostatic compression strength of the hollow SCF/Fe3O4/epoxy spheres were measured, and the mechanical and electromagnetic wave absorption properties of the epoxy resin composite embedded with the hollow SCF/Fe3O4/ epoxy spheres were studied. The results indicate that the composite possessed a high compression strength of ~57.3 MPa with a density of only ~0.92 g/cm3. Furthermore, the effective absorption bandwidth within the range of 2–18 GHz of the composite was 1.8 GHz, with an ultra-thin equivalent thickness of only ~0.48 mm. The macro-sized hollow structure spheres were effective EM wave absorption enhancements, and the lattice composite embedded with these hollow spheres presented a unique structure and excellent absorption performance. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (X. Wang).

Electromagnetic (EM) wave absorption composite materials have attracted increasing attention owing to the increasing amount of EM pollution and interference of electronic devices and communication

https://doi.org/10.1016/j.matdes.2019.108427 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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facilities in telecommunication, military, and other related applications. Ideal EM wave absorbing composite materials should not only possess an excellent absorption property, they should also be lightweight with high chemical and thermal stability, particularly in high-end applications, such as military aircraft and missile stealth systems. In general, absorption materials can be classified into conduction loss materials (carbon materials [1,2] and conductive polymers [3]), dielectric loss materials (silicon carbide [4] and barium ferrite [5]), and magnetic loss materials (ferrite [6] and magnetic metal powder [7]). To date, carbon materials and their composites, including carbon nanotubes, carbon fibers, carbon foam, and graphite are a good choice as lightweight absorption materials owing to their wide availability. Among these carbon materials, carbon fiber is widely used as a reinforcement in composite materials [8] due to its low density, high strength, and high stability. During the past few years, an enormous number of studies have been conducted on the development of carbon-fiber based absorption materials. Xie et al. [9] prepared short hollow porous carbon fibers, with a minimum reflection loss of −21.36 dB when the carbon fiber volume fraction was 33.3%. Jun et al. [10] prepared a grid-structured carbon fiber-reinforced epoxy resin matrix composite, with a minimum reflection loss of −23.1 dB when the thickness of the composite was 4 mm. However, the high electrical conductivity of carbon fibers also led to poor impedance matching [11]. To improve their impedance matching characteristics, significant effort has been devoted to combining other types of material with carbon fibers, such as nano-magnetite-coated carbon fibers [12], carbonyl iron powder-coated carbon fibers [13], and Fe3O4 deposited on graphene and combined with carbon fibers [1]. In addition, Fe3O4 is a traditional magnetic loss material, which is widely used in EM wave absorption due to its high magnetic permeability, straightforward synthesis, and low cost [14,15]. To achieve a desired absorption performance, Fe3O4 is also used in the design of EM wave absorption composite materials combined with other materials rather than individually. As reported by Qiu et al. [16], the microwave absorption of Fe3O4 is quite weak, whereas the minimum reflection loss of Fe3O4 decorated on carbon nanotubes and carbon fiber composite materials can reach −50.9 dB. In addition, Meng et al. [17] fabricated magnetite (Fe3O4) coated carbon fibers (MCCFs), obtaining a minimum reflection loss of −30 dB, with a thickness of 4.0 mm. Nevertheless, the percentage of reinforcement of the composites usually exceeds 20 wt% and the absorbing layers are always extremely thick. Thus, it is still a significant challenge to obtain lightweight EM wave absorbing materials with a thinner absorption layer and strong absorption properties for certain practical applications. One efficient way is to design hollow spherical structures with a large surface area that can efficiently reduce the weight, reflect and refract electromagnetic waves numerous times. However, most hollow structures reported are at a micrometer scale [18,19], and reports on electromagnetic wave absorbing materials with a macro-sized hollow sphere structure are scarce. According to Mie resonance [20–22] and Mie scattering [23,24] theories, it would be beneficial to improve the absorption performance if the diameter of the hollow sphere structures could reach close to one-tenth the wavelength (2–18 GHz, corresponding to 16.7–150 mm). Therefore, the design of a macro-sized hollow sphere structure for the composite materials is expected to increase the efficiency of the absorbing structure. In this study, novel macro-sized hollow spheres were fabricated using a template method, and an EM wave absorption lattice composite material was obtained through a simple casting method using hollow spheres arranged in a periodic structure. For comparison, two types of hollow spheres, reinforced exclusively by SCFs and by SCF/Fe3O4, respectively, were prepared. In addition, the morphology, volume density, and isostatic compression strength of the hollow spheres were investigated. The mechanical and electromagnetic wave absorption properties of the lattice composites with SCF/epoxy and SCF/Fe3O4/epoxy hollow spheres were also surveyed. The absorption mechanism of a macrosized hollow sphere structure affecting the absorption properties of

the epoxy composites were analyzed based on the structure and electromagnetic wave absorption properties. It was demonstrated that SCFs and Fe3O4 micro-powder act as both an absorber and reinforcing phase in the shell of the hollow spheres. These hollow spherical structures contribute to the Mie scattering, multiple reflections, and gradational refraction of the entire composite [25]. In addition, this hollow macro-sized spherical structure can further meet the requirements of lightweight composites. As a consequence, hollow macro-sized spheres have achieved effective EM wave absorption enhancements, and lattice composites embedded with hollow spheres have presented a unique structure and an excellent absorption capability. 2. Method 2.1. Materials SCFs were supplied by Yataida High-Tech Co., Ltd. (Shenzhen, China), Fe3O4 was obtained from Zhanteng Mineral Products Co., Ltd. (Shijiazhuang, China), and epoxy resin (E51) was purchased from Xingchen Synthetic Material Co., Ltd. (Nantong, China). 2.2. Preparation of hollow spheres The hollow spheres were prepared using the template method, as shown in Fig. 1. First, expanded polystyrene spheres (EPSs) were wetted with epoxy resin and used as templates. Next, SCF and a Fe3O4 powder were stirred for 30 min to achieve a thorough mixing and then evenly sprinkled on the surface of the wetted EPSs. The wetted EPSs with the SCF and Fe3O4 powder were then pre-cured in an oven at 90 °C for 2 h. Third, the epoxy resin was used to wet the SCF/Fe3O4/EPS again and cured at 90 °C for an additional 2 h. Finally, hollow SCF/Fe3O4/ epoxy spheres were obtained by heating at 130 °C to remove the EPS core. The thickness of the shell of the hollow spheres can be regulated by controlling the content of the resin and the SCF/Fe3O4 powder, as well as the number of reinforcement layers, if needed. The content of the absorber in the shell layer applied in this study is shown in Table 1. The volume ratio of the SCFs and Fe3O4 micro-powder, as shown in the second column of Table 1, was obtained by dividing the mass of the two absorbers by their respective densities. The volume fraction was 54.0% for both the SCFs (group 1) and SCF/Fe3O4 (group 2) in the shell of the hollow spheres. 2.3. Preparation of lattice composites with periodically embedded hollow spheres The epoxy resin matrix composites with hollow embedded spheres with a lattice structure were prepared through a simple casting. Because the density of the hollow spheres is much less than that of the epoxy resin, the hollow spheres were wetted with a spot of epoxy resin first and then arranged in a soft mold for pre-curing at 90 °C to avoid any disordering or floating during the preparations. The hollow spheres were fixed in the mold after being pre-cured for 1.5 h. Subsequently, the epoxy resin matrix was poured into the mold until the hollow spheres were completely submerged. Finally, epoxy resin matrix composites with embedded hollow spheres and with a lattice structure were obtained after casting and then cured at 90 °C for 3 h in an oven. 2.4. Characterization The volume density of the hollow spheres was determined based on Archimedes' principle [26]. The average bulk density of the lattice composites was calculated by measuring the number of dimensions and the mass of the samples [27]. The cross-sectional morphology of the shells of the hollow spheres was assessed using scanning electron microscopy (SEM, Phenom, Netherlands). The isostatic compressive strength of the hollow spheres was obtained through a self-made device, and the

Y. Qiao et al. / Materials and Design 188 (2020) 108427

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Fig. 1. Preparation process of hollow SCF/Fe3O4/epoxy spheres.

uniaxial compressive strength of the lattice composites with a size of 30 mm × 30 mm × 5.5 mm was measured using a universal mechanical testing machine (Instron 1196, Instron Corporation, Norwood, MA, USA). The complex permittivity and complex permeability of the SCFs and SCF/Fe3O4 absorber in the shell were tested within the range of 2–18 GHz using a vector network analyzer (VNA, MS4644A, Anritsu, Atsugi, Japan). The reflection loss of the composites within the range of 2–18 GHz was recorded using the NRL-arc method [28], and the size of specimens was 180 mm × 180 mm × 5.5 mm. A schematic diagram of the isostatic compression strength (hydrostatic collapse strength) test of the hollow spheres is shown in Fig. 2. Here, a hollow sphere is fixed to a metal plate in the pressure vessel with a strong glue. The probe of the displacement sensor is directed vertically to the center of the hollow sphere. After tightening the cover of the pressure vessel, a water pump is used to pressurize the vessel.The displacement and pressure sensors transmit the compression displacement of the hollow sphere and the pressure value in the vessel to a computer, respectively. When the hollow sphere bursts under a certain isostatic pressure, a sharp decline can be read on the pressuredisplacement curve of the hollow sphere, and the pressure value at this breaking point is the isostatic compression strength of the hollow sphere.

is beneficial for stress transmission. The shell of each hollow sphere was designed with three layers with different functions. The inner layer is pure epoxy, which operates as a substrate. The middle layer is SCF/ Fe3O4-reinforced epoxy, which acts as a reinforcement layer and a functional absorption layer. The outer layer is also pure epoxy and is used as a protection layer. This layer was designed to wrap the reinforcement layer, which can further improve the strength and act as a transition layer between the shell and other phases of the composites. As plotted in Fig. 4, the isostatic compression strength was as high as 15.0 MPa for the hollow spheres reinforced with SCF/Fe3O4, which is approximately a 50% increase compared to a hollow sphere reinforced without Fe3O4 (SCFs only). Moreover, the shrinkage deformations of the hollow spheres with and without Fe3O4 were 13 and 14 μm (a shrinkage of 0.25%), respectively. It is well known that the mechanical properties of particle-reinforced polymer composites depend on the size, shape, and volume of the particles [29,30]. As mentioned above, the volume fractions of the SCFs and SCF/Fe3O4 filled inside the shell of the hollow spheres in each group are equal (54 vol%). However, the Fe3O4 micro-powder has a better dispersion owing to its smaller size (with a diameter of approximately 7 μm) than the short carbon fibers (7 μm in diameter, 40 μm in length). Thus, the mechanical performance of the hollow SCF/Fe3O4 spheres was better than that of the hollow SCF spheres.

3. Result and discussion 3.2. Properties of the lattice composites 3.1. Properties of hollow spheres As shown in Table 1, the volume density of the hollow spheres, reinforced by carbon fibers, is 0.33 g/cm3. When the mass fraction of Fe3O4 is increased to 70%, the density of the hollow spheres increases to 0.63 g/cm3, which is still only one half of the density of the epoxy resin matrix (1.18 g/cm3). The structure of hollow spheres is shown in Fig. 3(a) and (b). The macro-sized spheres show an expected hollow structure with an approximate diameter of 4.3 mm. The compositions of these hollow spherical shells were confirmed through SEM images, as shown in Fig. 3(c). It can be seen that the shells of the spheres were composed of SCFs, Fe3O4, and an epoxy resin as designed. The structure of each shell is dense and has an approximate diameter of 0.16 mm. The SFCs and Fe3O4 particles were dispersed uniformly and wrapped well by the resin matrix, which

3.2.1. Morphology and mechanical properties A macroscopic image and the morphology of the obtained hollowsphere based epoxy resin matrix lattice composites are shown in Fig. 5. It can be seen that the hollow SCF/Fe3O4/epoxy spheres were tightly packed inside, forming composites with a lattice structure (Fig. 5a), and thus achieving a lighter weight. The thickness of the composites is approximately 5.5 mm and the hollow structure of the spheres in the composites can be seen in Fig. 5(b). The typical cross-sectional surface of the composites incorporated with SCF/Fe3O4/epoxy hollow spheres is shown in Fig. 5(c). There were three distinct phases in this structure, SCF/Fe3O4, an epoxy with a hollow spherical shell (the outer layer of the hollow sphere mentioned before), and an epoxy resin matrix. The interface between the SCF/Fe3O4 and epoxy of the hollow spherical shell shows that the polymer resin is evenly impregnated

Table 1 Content of the absorber in the shells of the hollow spheres, the volume density of the hollow spheres, and the bulk density of the composites. Groups

SCFs: Fe3O4 (V/V)

Weight ratio (%) SCFs

Volume density of the hollow spheres (g/cm3)

Bulk density of the composites (g/cm3)

Fe3O4

SCFs only

63.5

0

0.33

0.82

3:7

9.8

70.0

0.63

0.92

1 2

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Fig. 2. Schematic diagram of hydrostatic collapse strength test of hollow spheres.

throughout the SCF/Fe3O4 at the interface. Thus, the shell of the hollow epoxy spheres can protect the reinforced layer (SCF/Fe3O4) and transit stress between the matrix and this layer, as designed. In addition, because the same materials were used, a strong interaction and good compatibility are achieved between the shells of the hollow spheres and epoxy resin matrix. The density of the lattice composites with two different types of embedded hollow spheres were measured using the apparent density method. As shown in Table 1, although a high-density and highcontent Fe3O4 absorber was filled inside each shell, the hollow spheres can still effectively reduce the density of the composites, and the density of the composites with 30 hollow spheres was 0.82–0.92 g/cm3. The compressive strength of the composites was measured to be 51.3 ± 4.6 MPa (SCFs only) and 57.3 ± 4.7 MPa (SCFs:Fe3O4 = 3:7). This suggests that the introduction of Fe3O4 particles improves the compressive strength of the composites by 11.7%. The load–displacement

curve of the lattice composites is shown in Fig. 6. It can be seen that the failure process of the lattice composites with hollow spheres periodically embedded under a compressive load was divided into three stages: (I) an elastic deformation of the resin matrix, (II) a yielding of the resin matrix and cracking of the hollow spheres, and (III) a plastic deformation of the matrix and fracturing of the hollow spheres. The thickness of the composites was 5.5 mm, whereas the diameter of the hollow spheres was approximately 4.3 mm, as mentioned above. Thus, the resin matrix in the composites has a certain elastic-plastic deformation interval. When the yield strength of the resin matrix was reached, the matrix began to undergo an irreversible plastic deformation. Owing to the plastic deformation of the resin matrix, the “protection” from the matrix on the hollow spheres was weakened. When the load reached 45–52 kN again, the hollow spheres began to crack from their nearby “equator.” As the compressive load continued to increase, the middle layer of the composite was mainly carried by the

Fig. 3. Structure of hollow sphere: (a) hollow spheres, (b) schematic diagram of the shell structure, and (c, d) SEM images of the shell of SCF/Fe3O4/epoxy hollow spheres.

Y. Qiao et al. / Materials and Design 188 (2020) 108427

Fig. 4. Hydrostatic pressure–displacement curve of the hollow spheres.

cracked spheres, and the shell of the hollow spheres was gradually crushed. The isostatic compression strength of the hollow spheres was only 10 to 15 MPa, but the compression strength of the hollow spheres in the composites was as high as 50 MPa. This is because the method of loading and the failure mode of the hollow spheres were different in these two cases. The hollow spheres were subjected to a radial pressing force in the isostatic water, whereas the hollow spheres in the composites were subjected to pressure from the resin matrix in the vertical direction, which caused a transverse shearing force at their nearby “equator.” In addition, the matrix of both the hollow spheres and the composites is an epoxy resin, and thus there is no interfacial interaction [31,32] between the spheres and the matrix of the composites, unlike in the case of the spheres in water. Therefore, the strength of the hollow spheres under these two test methods is inconsistent. Finally, it can

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Fig. 6. Load–displacement curve of hollow spheres embedded in epoxy resin matrix composites.

also be seen from the load–displacement curve, shown in Fig. 6, that the breaking load of the hollow spheres was close to the yield load of the resin matrix. 3.2.2. EM wave absorption The reflection loss of the epoxy resin matrix lattice composites with two types of hollow spheres was tested using the NRL-arc method [28], the results of which are shown in Fig. 7. It can be seen that the minimum reflection loss of the composite with SCF/epoxy hollow spheres was −11.6 dB at 4.5 GHz, and the effective absorption bandwidth (RL ≤ −10 dB) was only 0.6 GHz (4.2 to 4.8 GHz). Because magnetic Fe3O4 micro-powder is usually beneficial for increasing the magnetic loss and improving the impedance matching characteristics of the absorber, the minimum reflection loss of the composite with SCF/Fe3O4 hollow spheres was improved to −14.7 dB, and the absorption

Fig. 5. Preparation and morphology of hollow SCF/Fe3O4 sphere based epoxy resin composites: (a), (b) preparation and macroscopic images and (c) the cross-section SEM images of composites.

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As the SCFs were dispersed using Fe3O4 micro-powder, the imaginary permittivity of the absorber was decreased, as shown in Fig. 8a. The real permittivity and real permeability normally determine the energy storage capacity of the material, whereas the imaginary permittivity and imaginary permeability determine the loss performance of the material. The design of the absorption materials should consider both the absorption and impedance matching characteristics of the absorber synthetically. The absorption characteristic indicates the ability to convert the energy of the incident electromagnetic waves into heat [38] via conduction loss, dielectric loss, and magnetic loss. However, the impedance matching Zim of the absorber should be as close as possible to the impedance of the free space, namely, the value of Zim should be closer to 1 such that the incident waves can transmit into the absorber instead of being strongly reflected at the interface. The absorption coefficient α and the impedance matching Zim of the absorber can be calculated through the following formulas [39]:

Fig. 7. Reflection loss of the composites, measured using NRL-arc method.

bandwidth was increased to 1.8 GHz (13.7 to 15.5 GHz). Furthermore, the minimum reflection loss of the composite with SCF/Fe3O4/epoxy hollow spheres shifted to a high frequency. To study the effect of these two types of absorbers on the absorption properties of the lattice composites with hollow spheres, SCFs and SCF/ Fe3O4 were mixed in paraffin wax with a volume fraction of 54.0%, which was the same as that in the shell. The electromagnetic parameters of the two absorbers were tested using a vector network analyzer [33], the results of which are shown in Fig. 8. It can be seen that the SCFs had a high real permittivity (10.6 to 52.4) and a high imaginary permittivity (21.7 to 51.4), but their real (0.9 to 1.5) and imaginary (0.2 to 0.4) permeabilities were low. With the addition of magnetic Fe3O4 micro-powder, the imaginary permittivity (2.7 to 10.4) was decreased significantly, whereas the imaginary permeability (0.3 to 0.6) was increased. According to the Debye theory [21], ε″ ¼

εs −ε∞ σ ωτ þ : ωε0 1 þ ω2 τ 2

ð3  1Þ

It can be seen that the imaginary permittivity ε″ is proportional to the electrical conductivity σ of the material. Thus, the high imaginary permittivity of the SCFs can be mainly attributed by the excellent conductive network based on their high electrical conductivity [34] and high content [35,36]. The ε′ and ε″ of Fe3O4 originate from the transfer between Fe3+ and Fe2+ and the dipole polarization [37], respectively.

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2πf 2 2  ðμ ″ ε ″ −μ 0 ε0 Þ þ ðμ ″ ε″ −μ 0 ε0 Þ þ ðμ ″ ε″ þ μ 0 ε0 Þ α¼ c

Z im ¼

sffiffiffiffiffiffiffiffi μ r    ε  r

ð3  2Þ

ð3  3Þ

where f is the frequency of an incident plane wave, c is the speed of an electromagnetic wave, εr = ε′ − jε″ is the relative permittivity of the absorber, and μr = μ′ − jμ″ is the relative permeability. It was found from Eqs. (3-2) and (3-3) that the higher the imaginary permittivity and imaginary permeability are, the greater loss and the higher absorption coefficient. However, an excess permittivity will lead to a poor impedance matching, strong interface reflection, and poor absorption. Therefore, an ideal absorber should have both high imaginary permittivity and high imaginary permeability. Based on the electromagnetic parameters of the SCFs and SCF/Fe3O4, as shown in Fig. 8, the absorption coefficients and impedance matching ratio of the two types of absorber are calculated using Formulas (3-2) and (3-3), respectively. As indicated in Fig. 9a, the SCFs possessed an extremely high absorption coefficient (200 to 1000), which is an order of magnitude higher than that reported in the literature [24]. Unfortunately, the impedance matching ratio of the SCFs is extremely poor, as shown in Fig. 9b. The introduction of Fe3O4 micro-powder could increase the magnetic loss effectively as Fig. 9d displayed. In addition, the impedance matching performance was improved at the same time (Fig. 9b), although, Fe3O4 micro-powder reduced the dielectric loss (Fig. 9c), and absorption coefficient partly (Fig. 9a). Thus, the absorption

Fig. 8. Electromagnetic parameters of SCFs and SCFs/Fe3O4.

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Fig. 9. Absorption and impedance characteristics of SCFs and SCFs/Fe3O4.

performance of the SCF/Fe3O4 was improved in general, which is consistent with the reflection loss of the composites, as shown in Fig. 7. The equivalent thickness of the absorption layer was calculated through the following equation: de ¼

  n  V s n  4π  r 3o −r 3i =3 ¼ ; Sc Sc

ð3  4Þ

where de is the equivalent thickness of the absorption layer in the composites, n is the number of the spheres in the composite plate, Vs is the volume of the shell of a single sphere, Sc is the cross-sectional area of the composite plate, and ro and ri are the outer and inner radii of the hollow sphere, respectively. The equivalent thickness of the absorption layer is calculated as 0.48 mm. Table 2 shows a comparison of the absorption properties of the lattice composite with hollow SCF/Fe3O4/epoxy spheres and other typical materials reported in recent years. It can be seen that the thickness of the absorption layer used in the lattice composite with hollow SCF/

Fe3O4/epoxy spheres was much thinner than that described in other reports. At the same time, the absorption properties of the lattice composite with hollow SCF/Fe3O4/epoxy spheres were also better than those in other reports using carbon fibers [12,40] or Fe3O4 [41] as the absorber, which is attributed to the unique hollow structure. However, the absorption properties of these lattice composites were relatively poor in comparison with the absorption properties of dielectric barium ferrite [42] and nano-ferrite-coated graphene [43]. Nevertheless, the design of the lattice epoxy matrix composites embedded with hollow spheres still presented a significant advantage in lightweight absorption materials with an ultra-thin absorber layer and high compression strength.

3.3. Analysis of the absorption mechanism of the composites with a hollow sphere lattice structure As shown in Fig. 10, the lattice composite with hollow embedded SCF/Fe3O4/epoxy spheres can attenuate the electromagnetic waves in

Table 2 EM absorption properties of the typical materials reported in this work and the recent literature. Absorber + Matrix

Content of absorber

Thickness

Ni@C + paraffin ACNT/GA/BaFe12O19 + paraffin wax Fe3O4/RGO + paraffin wax MCCFs + epoxy NSZnO/Zn/CF + paraffin wax Fe/Fe3O4/a-FeOOH + paraffin wax Fe3O4/SCFs + epoxy

60 wt% 30 wt% 30 wt% 70 wt% 60 wt% 89 wt% ~80 wt%

2.5 mm −20.54 dB 3.36 GHz 2 mm −18.35 dB 3.32 GHz 3 mm −22.7 dB 3.13 GHz 3 mm −8 dB 0 The minimum RL is higher than −10 dB, when the thickness is less than 1.75 mm 1.2 mm −10 dB 0 0.48 mm −14.7 dB 1.8 GHz

Minimum RL

Bandwidth (b−10 dB)

Ref. [46] [42] [43] [12] [40] [41] This work

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Fig. 10. Schematic diagram of absorption mechanism of hollow SCF/Fe3O4 spheres embedded in epoxy resin composite plates.

the following four ways. Scattering (k1) [23,24] will first occur at the outer surface of the hollow SCF/Fe3O4/epoxy spheres. Because the diameter of the hollow SCFs/Fe3O4/epoxy spheres is close to a wavelength of 2–18 GHz EM radiation, Rayleigh or Mie scattering can occur when the electromagnetic waves propagate on the surface of spherical shells. Second, electromagnetic waves (k2) can be absorbed by the shell of the hollow SCF/Fe3O4/epoxy spheres. The refracted electromagnetic waves will penetrate into the shell of the hollow SCF/Fe3O4/epoxy spheres, and the electromagnetic energy will then be partially converted into thermal energy through the loss of conductance of the carbon fibers and the magnetic loss of the Fe3O4. The incident electromagnetic waves will therefore be further attenuated through this progress. Furthermore, multiple reflections and refractions (k3) [19,44] will occur on the inner surface of the hollow spheres. When the electromagnetic waves pass through the shell and enter into the spherical cavity, multiple reflections and refractions will occur. The electromagnetic energy will gradually decrease during each reflection and refraction. More importantly, the electromagnetic waves will pass through and be absorbed by the shell of the hollow spheres repeatedly after multiple reflections and refractions. Thus, the electromagnetic waves will dramatically decrease during this stage. Finally, the reflection of the metal (k4) also contributes to destroying the interference [45]. Destructive interference will occur under reflections on the outer surface of the spherical shell, the inner surface of the spherical shell, and the metal substrate. This is a benefit to the absorption of the electromagnetic waves, which will weaken the reflection field and be of benefit to the absorption properties. Overall, the electromagnetic waves can be effectively absorbed by this macro-structure of the hollow SCF/Fe3O4/epoxy spheres with a synergistic action of the absorption materials and based on the structural design.

4. Conclusion In this study, high-strength hollow SCF/Fe 3 O4 /epoxy spheres were prepared using a template method. Moreover, a lightweight and high-strength epoxy resin matrix composite with hollow embedded SCF/Fe3 O 4 /epoxy spheres arranged in a lattice structure were obtained using a casting method. The density of the lattice structure composite was only ~0.92 g/cm3, as the compressive strength reached ~57.3 MPa. The minimal RL of the composite is −14.7 dB within a range of 2–18 GHz, with an effective absorption bandwidth of 1.8 GHz. Thus, the design of the hollow macro-sized spheres can help the composite realize the combination of a unique structure (lattice structure) and excellent performance (lightweight, high-strength, and absorption properties) through a simple process.

CRediT authorship contribution statement Yingjie Qiao:Conceptualization, Supervision, Funding acquisition. Zhaoding Yao:Methodology, Data curation, Writing - original draft. Xiaodong Wang:Conceptualization, Writing - original draft, Writing review & editing, Supervision, Project administration.Xiaohong Zhang:Methodology, Project administration.Chengying Bai:Data curation.Qiuwu Li:Methodology.Kaixuan Chen:Methodology.Zhuoran Li:Methodology.Ting Zheng:Writing - original draft, Writing - review & editing. Declaration of competing interest We confirm that the manuscript has been read and approved by all the authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of them. The manuscript has not been previously published in any language anywhere and it is not under consideration by another journal. Acknowledgments This work was supported by “The 7th Generation Ultra Deep Water Drilling unit Innovation Project,” the Fundamental Research Funds for the Harbin Engineering University (HEUCFG201816, GK2100260265) Aeronautics Power Foundation (6141B090571) and the Technology Innovation Centre Project (HDLCXZX2018ZH038). References [1] J. Wu, Z. Ye, H. Ge, J. Chen, W. Liu, Z. Liu, Modified carbon fiber/magnetic graphene/ epoxy composites with synergistic effect for electromagnetic interference shielding over broad frequency band, J. Colloid Interface Sci. 506 (2017) 217–226. [2] A. Shah, A. Ding, Y. Wang, L. Zhang, D. Wang, J. Muhammad, et al., Enhanced microwave absorption by arrayed carbon fibers and gradient dispersion of Fe nanoparticles in epoxy resin composites, Carbon 96 (2016) 987–997. [3] C. Li, D. Jiang, J. Zeng, S. Xing, S. Ju, Electromagnetic transmission characteristics of composite frequency selective surfaces coated with conductive polymer-silver paste, Compos. Sci. Technol. 90 (2014) 32–39. [4] P. Wang, L. Cheng, Y. Zhang, L. Zhang, Synthesis of SiC nanofibers with superior electromagnetic wave absorption performance by electrospinning, J. Alloys Compd. 716 (2017) 306–320. [5] S. Tyagi, V. Pandey, H. Baskey, N. Tyagi, A. Garg, S. Goel, et al., RADAR absorption study of BaFe12O19/ZnFe2O4/CNTs nanocomposite, J. Alloys Compd. 731 (2018) 584–590. [6] G. Liu, W. Jiang, Y. Wang, S. Thong, D. Sun, J. Liu, et al., One-pot synthesis of Ag@ Fe3O4/reduced graphene oxide composite with excellent electromagnetic absorption properties, Ceram. Int. 41 (3) (2015) 4982–4988. [7] H. Zhao, J. Cheng, Y. Wang, Biomass-derived Co@crystalline carbon@carbon aerogel composite with enhanced thermal stability and strong microwave absorption performance, J. Alloys Compd. 736 (2018) 71–79. [8] H. Li, J. Gu, D. Wang, C. Qu, Y. Zhang, Study on benzoxazine-based film adhesive and its adhesion properties with CFPR composites, J. Adhes. Sci. Technol. 31 (16) (2017) 1796–1806.

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