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epoxy composites with improved microwave absorption and lightweight feature
Composites Science and Technology 184 (2019) 107882
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Synchronously oriented Fe microfiber & flake carbonyl iron/epoxy composites with improved microwave absorption and lightweight feature Hanyi Nan, Yuchang Qing *, Hui Gao, Hongyao Jia, Fa Luo, Wancheng Zhou State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi0 an, 710072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (M) Magnetic properties (P) Anisotropy (A) Microwave absorption
In this study, Fe microfibers (FMF) with a low percolation threshold and flake carbonyl iron (FCI) particles with high complex permeability were filled in an epoxy matrix, and shear force was applied to obtain synchronously oriented microstructures. The effect of the contents of FMF and/or FCI particles on the electromagnetic and microwave-absorbing properties of these composites was investigated in the frequency range of 2–18 GHz. Compared with single component-filled composites, the mixture of FMF and FCI imparted both suitable complex permittivity and enhanced complex permeability to the composite materials. Interestingly, the electromagnetic parameters of the composites could be tailored via simply regulating the ratio of FMF to FCI; thin and light microwave absorbers operating in the broadband range were obtained under the state of low content because of the synergy between the two components and the synchronous orientation of the fillers. An effective absorption of frequencies ranging from 3.5 to 18 GHz with reflection loss < 8 dB was achieved at thicknesses ranging from 1.0 to 2.0 mm.
1. Introduction Resin-based microwave absorbing materials (MAMs) are typically polymer-matrix composites containing various absorbents. In the past decade, much work has been done to study the properties of diversified absorbents in order to reduce the electromagnetic pollution caused by the extensive utilization of electronic devices and communication fa cilities as well as to satisfy the military requirement for stealth weapon systems [1–12]. While it is well known that the energy dissipation mechanisms of microwave involve dielectric loss and magnetic loss, some studies have revealed that unilateral loss materials composed of a single absorbent are not very effective for electromagnetic matching and fulfilling the desired “thin, light, broad, and strong” characteristics for MAMs [11–14]. Thus, MAMs filled with mixed-type or hybrid-type ab sorbents have been investigated to obtain good absorbing materials, e.g., by using a mixture of dielectric and magnetic loss absorbents, and decorating the dielectric loss materials with various magnetic particles [7,15–18]. Attempts to further improve microwave absorption have revealed that an oriented arrangement of anisotropic fillers in the polymer matrix enhances their absorption capability [19–22]. Generally, most MAMs, which possess excellent absorption perfor mance based on flake carbonyl iron (FCI), require high filler content or
have a narrow effective absorption bandwidth. A known method of reducing the filler content, which is required to maintain its high com plex permeability and achieve appropriate complex permittivity, is the addition of a low percolation threshold and large complex permeability magnetic filler such as iron fiber. Iron fiber having low percolation attributed to the unique shape anisotropy can acquire modest complex permittivity under the condition of low loading contents, thereby reducing the weight of the absorption materials. Although iron fiber also exhibits double energy loss mechanisms, including dielectric and mag netic loss, its complex permittivity is far larger than its complex permeability, which can trigger the detriment of impedance matching between free space and the absorber. Thus, combining flake carbonyl iron particles having low dielectric loss with iron fiber having high dielectric loss may improve comprehensive dielectric loss of the com posites while maintaining high magnetic loss. High imaginary part of permeability is important for similar absor bents filled with composites in obtaining thin and lightweight MAMs. It has been demonstrated that an oriented structure has increased permeability [13,20,21]. Considerable efforts have been devoted to investigating the oriented arrangements of shape anisotropic materials, such as carbon-based materials, flake iron-based alloys, silicon carbide fibers, etc., to study anisotropic magnetic or electrical properties
* Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Qing). https://doi.org/10.1016/j.compscitech.2019.107882 Received 16 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019 Available online 15 October 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved.
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Composites Science and Technology 184 (2019) 107882
Fig. 1. Schematic representation of the synchronously oriented FMF and FCI in the composites, (a) FMF perpendicular to the electric field vector of the incident wave, (b) orientation of the test sample in the two-port network system.
[21–25]. However, few reports have been reported with regard to the MAMs with combined oriented FCI particles and FMF as the absorbents. Here we investigated the complex permittivity and permeability of epoxy-based composites filled with synchronously oriented FCI and FMF in varying proportions. The microstructure and microwave absorption properties of the as-prepared composites were studied in detail. Lightweight and broader-bandwidth absorbers were obtained by the FMF&FCI/epoxy composites in the 2–18 GHz range.
Ltd., was ~10 μm. The FCI particles were 2–10 μm in diameter and procured from Xinhua Chemical Co. Ltd. (Shaanxi Province, China). The polymer matrix was the epoxy resin provided by Xi0 an Power Resin Factory. The hardener and toughening agent, purchased from Ningping Chemical Co. Ltd. (Tianjin, China), were polyamide resin and watersoluble polyurethane, respectively.
2. Material and methods
First, the epoxy resin and polyamide with a mass ratio of 4:1 were dissolved in acetone and 10 wt% water-soluble polyurethane was added to the mixture. The resin mixture was stirred for 10 min to obtain a homogeneous solution. Subsequently, a certain amount of FCI particles were added and dispersed by high-speed homogenizing dispersion at
2.2. Preparation of simultaneous oriented FMF&FCI/epoxy composites
2.1. Materials The diameter of FMF, provided by Jiangyou Hebao Nanomaterial Co.
Fig. 2. Synchronously oriented FMF&FCI/epoxy composites with 15 wt% FMFs þ45 wt% FCI: (a) aligned FMFs parallel to the tape-casting direction; (b) aligned ones perpendicular to the tape-casting direction; (c) the magnified images of (a); (d) the micromorphology of 45 wt% FMFs þ 15 wt% FCI. 2
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Composites Science and Technology 184 (2019) 107882
Fig. 3. Real (a) and imaginary (b) components of permittivity of aligned FMF&FCI/epoxy composites as a function of frequency; (c) Schematic representation of the formation of the micro-capacitor structure, conductive network, and channel.
2000 rpm for 10 min. Next, FMF were added in a certain proportion to the mixture of resin and FCI, stirring for 1 h at 800 rpm by mechanical agitation [19]. Finally, the prepared slurry of both resin and fillers mixtures having suitable viscosity (3 Pa s) was tape-casted on a plastic film at a uniform speed of 3 cm/s using a blade height of 500 μm. The synchronously oriented samples were obtained by introducing shear force under the tape-casting equipment [19]. In this work, the total content of filler was 60 wt%, and the pro portions of FMF and FCI in the different samples (Samples 0–4) were as follows: 60 wt% FMF, 45 wt% FMF þ 15 wt% FCI, 30 wt% FMF þ 30 wt % FCI, 15 wt% FMF þ 45 wt% FCI, and 60 wt% FCI.
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, μr and εr are the relative complex permeability and permit tivity, respectively, d is the thickness of the absorber, f is the frequency of the microwave, and c is the velocity of light. 3. Results and discussion 3.1. Microstructure of the synchronously oriented FMF&FCI/epoxy composites Fig. 2 shows the SEM images of synchronously oriented FMF&FCI/ epoxy composites (Sample 3). It was found that FMF and FCI were uniformly distributed in the epoxy resin matrix in a certain orientation. The FMF were parallel to each other and to the coating surface, and most FMF lengths ranged from 40 to 110 μm. As can be seen from Fig. 2(c), the thickness of the FCI particles was less than 1 μm, and they were parallel to each other as well as almost parallel to the axially ordered arrange ment of FMF. Therefore, FMF and FCI had achieved a synchronous orientation by inducing shear force.
2.3. Characterization The morphology and structure of the composites were characterized by scanning electron microscopy (SEM) (Tescan Vega 3 SBH, Brno, Czech). The complex permittivity and permeability of the prepared samples, employing the waveguide method in the range of 2–18 GHz, were measured by a network analyzer (Agilent Technologies E8362B). The relationship between the electromagnetic field and specimens in the waveguide measurement is presented in Fig. 1(a), where the electro magnetic parameters for FMF are ε? and μ//. The specimens were placed perpendicular to the incident microwave transmitting the thickness di rection presented in Fig. 1(b) [19]. The reflection loss (RL) of the composites with changed thickness (d) at the given frequency was calculated based on the transmission line theory for a single layer absorber according to following equations [26]. � � �Zin Z0 � � RL ðdBÞ ¼ 20 log�� (1) Zin þ Z0 � rffiffiffiffi Zin ¼ Z0
�
μr 2πfd pffiffiffiffiffiffiffiffi tanh j ε r μr c εr
3.2. Electromagnetic properties of synchronously oriented FMF&FCI/ epoxy composites Fig. 3 shows the real (a) and imaginary (b) components of permit tivity of different specimens in the 2–18 GHz region. As observed in Fig. 3(a), for Sample 4, the values of both real (ε0 ) and imaginary (ε00 ) components of permittivity were approximately 19 and 1, respectively, and nearly independent of frequency over the entire measured fre quency range. However, in comparison with Sample 4, Sample 0 showed higher values of complex permittivity and a high degree of frequencydependency over the measured frequency range. In addition, the 00 values of ε0 and ε increased as the FMF content increased in the mixed
� (2) 3
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Composites Science and Technology 184 (2019) 107882
Fig. 4. Real (a) and imaginary (b) components of the permeability of aligned FMF&FCI/epoxy composites as a function of frequency; (c) and (d) curves of 2
1
μ ðμ Þ f 00
0
versus frequency.
absorbent, thus demonstrating a good frequency dispersion feature. On the basis of the Maxwell-Wanger-Sillars (MWS) principle, heterogeneous composites generate polarization and charge accumulation at the interface between the conductive fillers and insulation resin matrix [11, 27]. Consequently, the dielectric properties of such composites are strongly dependent on the nature of the conductive fillers/matrix interface, and the characteristics and contents of fillers. The ε0 value of the heterogeneous composites, representing the ability to store charge, was determined by the interface between the absorbents/matrix or absorbents/absorbents, where free charges can accumulate and create interfacial polarization under an external electric field. Based on this rationale, it is reasonable that the ε0 value decreased from Sample 0 to Sample 2 in the 2–18 GHz range as a result of the FMFs having larger surface areas. However, compared to Sample 0, the ε0 values of the other samples were less than that of Sample 4 in a special frequency range, which was mainly due to the frequency-dependent effect of FMF as 00 fillers. On the other hand, ε represents the ability of electromagnetic energy loss, and can be described by two terms with applied electric field as follows [15]. 00
0
ε ¼ εR0 þ
σ 2πf ε0
mainly related to interface polarization and the corresponding relaxa tion when the content of the conductive filler is below the percolation threshold, and conductivity plays a key role above the percolation 00 threshold [28,29]. Therefore, the low ε value of Sample 4 could be attributed to the higher percolation threshold of FCI particles as compared to that of FMF, thereupon indicating that the percolation threshold of the mixed-absorbent decreased with the increasing pro portion of FMF in the mixed-absorbent. The conductive network or pathway is formed when the filler content is above the percolation threshold (as shown in Fig. 3(c) and in the meantime, conductivity is the 00 00 main contributor to ε . Consequently, it is reasonable that the value of ε increases with increasing FMF content in the composite. Similar to resin matrix composites filled with shape-anisotropic conductive fillers, complex permittivity is closely dependent on the filler alignment. As shown Fig. 3(c), the well-aligned composite com prises a network of micro-capacitors between FCI and a conducting pathway between FCI and FMF, which can store massive electric charge, improve the conductivity and interface polarization, and enable the composite to possess a high dielectric constant [27]. The real and imaginary components of permeability of synchro nously oriented FMF&FCI/epoxy composites are presented in Fig. 4(a) and (b), respectively. Clearly, the real component (μ0 ) of permeability of Sample 4 was higher than that of Sample 0. The μ0 values of the com posites decreased gradually with increasing FMF content in the mixed absorbent, and exhibited a good frequency dispersion in the investigated
(3)
where εR0 is affected by the polarization and corresponding relaxation, 00 and σ is the electrical conductivity. Based on the percolation theory, ε is 0
4
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Composites Science and Technology 184 (2019) 107882
Fig. 5. Dielectric loss (a) and magnetic loss (b) versus frequency of FMF&FCI/epoxy composites.
frequency region. The μ0 values of the composites decreased from 2.82 to 2.07 at 2 GHz with increasing weight ratio of FMF to FCI from Sample 4 to Sample 0. In addition, the imaginary component (μ00 ) of the perme ability of FMF&FCI/epoxy composites showed a peak in the broad fre quency range of 2–18 GHz for Samples 4, 3, and 2, which had peak frequencies of 8.5, 5.4, and 2.8 GHz, respectively. It is well accepted that microwave magnetic loss of magnetic composites results mainly from domain resonance, natural resonance, eddy current effect, and exchange resonance [30,31]. However, several studies have shown that resonance frequencies depend on the magnetocrystalline anisotropy field that is affected by saturation magnetization, and the size and shape of the magnetic absorber [5,32–34]. It is worth noting that natural resonance generally occurs at a low frequency (less than 10 GHz), and exchange resonance usually emerges at a higher frequency and results from nanoparticles or nanocrystals [5,35–37].As domain resonance usually occurs within the MHz range, the magnetic loss may not have originated from exchange and domain resonances. Furthermore, the contribution of the eddy current effect to magnetic loss is mainly related to the thickness (σ) and conductivity (σ) of the conductive fillers, as expressed by the following equation [38]: 00
which led to enhanced multiple scattering and reflection of microwave within the composites and further enhanced electromagnetic coupling between the microwaves and fillers as well as between the fillers. The shape anisotropy of fillers also contributed to the improvement of complex permeability [11]. Moreover, it is known that the orientation of the fillers plays an important role in enhancing the complex perme ability [15,19]. Thus, the desirable electromagnetic parameters can be effectively tuned by mixing the magnetic absorbents in the case of magnetic absorbing materials, and implementing synchronous orienta tion of the anisotropic fillers. There are two important contributing factors for microwave ab 00 0 sorption, namely, dielectric loss (tanσ e ¼ ε =ε ) and magnetic loss 00 0 (tanσm ¼ μ =μ ), as shown in Fig. 5(a) and (b), respectively. Generally, the greater dielectric and magnetic loss, the stronger the energy dissi pation ability of electromagnetic waves. It is evident that the dielectric loss of Sample 0 was considerably higher than that of Sample 4, resulting from a higher imaginary part of the permittivity of Sample 0. However, excessive or small dielectric values are detrimental to microwave ab 0 sorption. It is known that large ε values will lead to an increase in the reflection of the incident electromagnetic waves, which further limits the absorption bandwidth. The magnetic loss of the FMF&FCI/epoxy composites was enhanced after blending FMF and FCI together, espe 00 cially at a lower frequency, because of the improvement of μ . This suggests that tailoring the ratio of FMF to FCI can allow tuning of the impedance matching, leading to optimal microwave attenuation. This investigation demonstrates that the magnetic loss of the composites could be improved by simply blending FMF and FCI at 2–18 GHz.
(4)
0
μ ¼ 2πμ0 ðμ Þ2 d2 σf
where μ0 refers to the vacuum permeability. Thus, the increase in the proportion of FMF will produce a higher eddy current loss owing to their high conductivity within the composites and might play an active role in magnetic loss [26]. If magnetic loss solely resulted from the eddy current loss effect, the values of μ ðμ Þ 2 f 1 would be constant with the varia tion of frequency. In contrast, as shown in Fig. 4(c), the values of all samples vary with increasing frequency, indicating that magnetic loss does not simply originate from eddy current loss. For Samples 2 and 3, 00
0
3.3. Microwave absorption of synchronously oriented FMF&FCI/epoxy composites
2
the value of μ ðμ Þ f 1 was unchanged at 6–12 GHz, originating merely from the eddy current loss as shown Fig. 4(d). This suggests that the magnetic loss mechanism of Samples 2 and 3 comprises both eddy current loss and natural resonance loss. In contrast, the magnetic loss mechanism of other samples was mainly attributable to natural reso nance loss. The resonance frequency shift to low frequency from Sample 4 to Sample 0 is worth noting, and might be explained by the greater degree of anisotropy of FMF than that of FCI. Therefore, it was concluded that natural resonance can be tuned by mixing different types 00 of magnetic absorbents, and μ of this composite could be improved within the desired frequency range. For example, Sample 2 showed a 00 high μ value, which was ~1.6 at 2 GHz. The improvement of the complex permeability of FMF and FCI-filled composite was attributed to the interaction between FMF and FCI. This interaction could be attributed to the smaller distance between the fillers after the combination and synchronous orientation of FMF and FCI, 00
0
To study the microwave absorption properties of these materials, the RL of Samples 0–4 at various matching thicknesses were calculated in terms of the transmission line theory, as presented in Fig. 6. It was found that the absorption bandwidth with RL below 8 dB increased from Sample 0 to 3 at the same thickness, suggesting that the absorption properties of the composites filled with the mixed absorbent were enhanced as compared with those composites filled with a single absorbent. It was observed that the impedance matching characteristic between free air and the coating of Sample 0 deteriorated owing to its high complex permittivity, which led to the reflection of most of the incident electromagnetic waves on the coating surface and not entering the coating interior. In contrast, the absorption bandwidth of Sample 4 was narrow because of its low dielectric loss as well as frequencyindependent dielectric properties. Therefore, there was an unsatisfac tory absorption property for the single component-filled composites. Consequently, the complementarities between the dielectric properties 5
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Composites Science and Technology 184 (2019) 107882
yet Samples 0 and 1 exhibited dwindling tendency and the minimum values appeared at different frequencies at various matching thick nesses. For example, in the case of Sample 3, the RL values below 8 dB were obtained in the range of 9.8–18 GHz at a thickness of 1.0 mm, while in the cases of Samples 0 and 4, these were observed in the fre quency ranges of 13.4–16.6 GHz and 7.7–13.4 GHz at the same thick ness, respectively. The RL values below 8 dB of FMF&FCI/epoxy composites are summarized in Table 1 and a comparison of the elec tromagnetic wave absorption properties with other similar composites is given in Table 2. These results indicate that FMF and FCI-filled com posites with widened microwave absorption bandwidth and thinner thickness were obtained by simply blending FMF with FCI and then changing the filler content to optimize their properties. It is evident that the ameliorative values of complex permeability, particularly the 00 0 improved μ and modest μ values after blending FMF with FCI, which can generate high magnetic loss, were superior to that of the single component-filled composites. Furthermore, the enhanced frequencydependence of complex permittivity had a positive effect as excellent absorption performance was achieved and decreased thickness of the absorber was made feasible. Therefore, compared to FMF or FCI-filled composites with a narrow microwave absorption bandwidths, the broader absorption bandwidth of FMF and FCI-filled composites pre 00 0 0 sented here mainly stem from the modest values of ε , ε , and μ , the 00 improved μ value, as well as the enhanced frequency-dependent char acteristic of complex permittivity, which attribute to the synergistic consequence between FMF and FCI coupled with the synchronous orientation of the fillers. To further investigate the RL performance of Samples 2 and 3 at different thickness in detail, the three-dimensional plots of the RL values were made and are presented in Fig. 7. The minimum RL values of Samples 2 and 3 were 12 dB and 15 dB with thicknesses of 1.8 mm and 2.0 mm at 5.0 GHz and 4.8 GHz, respectively. The absorption bandwidth corresponding to RL below 8 dB for Samples 2 and 3 at thicknesses of 1.0–1.1 mm was found in the wide frequency ranges of 9–18 GHz and 8–18 GHz, respectively. The enhanced minimum RL values and absorption bandwidth for Sample 3 as compared with those of Sample 2 suggest that optimizing the proportion of mixed fillers can lead to excellent microwave absorbing property. Meanwhile, the mini mum RL values of these samples shifted to low frequency as the thick ness increased, which can be explained by the quarter-wavelength cancellation model as follows [42]. f¼
and magnetic characteristics are an important contributor to highperformance microwave absorption. Additionally, as absorber thick ness increased, the minimum RL values of Samples 2–4 were enhanced,
Sample 0 1 2 3 4
8 dB and
Absorbent and content 60 wt% FMF 45 wt% FMF þ 15 wt% FCI 30 wt% FMFþ30 wt% FCI 15 wt% FMFþ45 wt% FCI 60 wt% FCI
nc pffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1:3:5:::Þ jμr jjεr j
(5)
where c represents the velocity of light, and μr and εr are relative complex permeability and permittivity at frequency f, respectively. It is worth noting that the frequency is inversely proportional to the thick ness of the absorber. Excellent microwave absorbing materials require both highly effi cient attenuation and good impedance matching properties, which is associated with complex permittivity, complex permeability, thickness, and internal fractal structure [6]. The FMF&FCI/epoxy composites effectively utilized the ameliorated magnetic loss and dielectric loss produced by the efficient complementarities of FMF and FCI, as well as
Fig. 6. Microwave RL versus frequency curves of the FMF-FCI/epoxy com posites at given matching thicknesses.
Table 1 Absorption bandwidths with RL below
4d
10 dB of the FMF and FCI-filled composites at given matching thicknesses. RL < 8 dB (GHz)
RL < 10 dB (GHz)
1.0 mm
1.2 mm
1.4 mm
1.0 mm
1.2 mm
1.4 mm
13.4–16.6 11.7–18 9.8–18 9.8–18 7.7–13.4
0 8.2–12.2 6.7–15.0 7.0–15.6 6.2–11.3
0 6.3–9.4 5.2–10.0 5.6–12.0 5.0–9.6
0 0 15.6–18 13.4–17.3 8.9–11.7
0 0 0 8.4–12.8 6.9–10.1
0 0 6.3–8.2 6.5–10.4 5.6–8.8
6
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Table 2 Comparison of the electromagnetic wave absorption properties of the as-prepared FMF-FCI and previously reported composites. Absorbent and content
EM values at 2–18 GHz
ε 60 wt% (Graphene-carbonyl iron) 20 vol% (Ni0.5Zn0.5Fe2O4/carbonyl) 35 vol% (Aligned FeCo@C) 25 wt% (Fe/C nanofiber) 75 wt% (FeSiAl@Nylon) Oriented (65 wt%FCIþ0.3 wt% carbon fiber) 30 wt% FMFþ30 wt% FCI 15 wt% FMFþ45 wt% FCI
0
7.5–5.5 ~10 20–15 14.7–9.8 25–15 32–27 28.8–14.3 22.8–16.6
0
0
Thickness (mm)
Bandwidth (RL < 8 dB) (GHz)
Refs.
2.5 1.6 1.5 1.5 2.0 1.0 1.0 1.0
8–18 9.5–18 8–13 14–18 2–5.8 8.2–18 9.8–18 9.8–18
[39] [40] [22] [10] [41] [13] This work This work
0
ε’
μ
μ’
1.3–1.9 3–1 1.6–4 5.3–2.3 6–1 2.1–1.5 3.5–0.9 2.5–0.7
1.3–1.1 2.8–0.5 2.9–1.3 1.1–0.8 3.8–1 3.2–1.3 2.5–0.9 2.6–0.9
0.27–0.17 0.7–0.3 0.5–0 0.06 to 0.2 2.2–0.3 1.7–1.1 1.6–0.8 1.4–0.9
Fig. 7. Three-dimensional plot of RL values of Samples 2 (a) and 3 (b) at different thicknesses.
Fig. 8. Schematic illustration of electromagnetic waves loss factors.
the synchronous orientation of the fillers, which determines the impedance matching and microwave attenuation. As shown in Fig. 8, the microcapacitor structure, eddy current loss, natural resonance, multiple scattering and reflections, and dielectric polarization and the corre sponding relaxation contribute together to enhance the attenuation ca pacity, and thus increase the microwave absorbing performance.
Meanwhile, FMF with low percolation threshold could easily achieve suitable complex permittivity under the condition of low loading con tent of absorbent, which effectively reduced the weight of absorber. Thus, combined with the low density of FMF, the FMF-FCI/epoxy composites showed the advantageous attribute of being lightweight.
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4. Conclusion
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In summary, synchronously oriented FME&FCI/epoxy composites were prepared by shear force, and their electromagnetic waves ab sorption properties in the frequency range of 2–18 GHz were investi gated. Compared to the single component-filled composites, the composites with both FMF and FCI possessed suitable complex permit tivity and enhanced complex permeability, thus ameliorating the impedance match between free space and the absorber and achieving excellent absorption properties. The synchronously oriented FME&FCI/ epoxy composites, having a thin absorber thickness and broad absorp tion bandwidth under the condition of low absorbent contents, could effectively reduce the weight of the absorber, and an effective absorp tion with RL < 8 dB was obtained from 3.5 to 18 GHz at thickness ranging from 1.0 to 2.0 mm. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51402239), Fundamental Research Funds for the Central Universities (No. 3102019PB002), and the State Key Labo ratory of Solidification Processing China (Grant No. KP201604). References [1] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, (Fe, Ni)/C nanocapsules for electromagnetic-wave-absorber in the whole Ku-band, Carbon 47 (2009) 470–474. [2] M.X. Yu, X.C. Li, R.Z. Gong, Y.F. He, H.H. He, P.X. Lu, Magnetic properties of carbonyl iron fibers and their microwave absorbing characterization as the filer in polymer foams, J. Alloy. Comp. 456 (2008) 452–455. [3] L. Wang, F. He, Y.Z. Wan, Facile synthesis and electromagnetic wave absorption properties of magnetic carbon fiber coated with Fe-Co alloy by electroplating, J. Alloy. Comp. 509 (2011) 4726–4730. [4] W. Feng, Y.M. Wang, J.C. Chen, L. Wang, L.X. Guo, J.H. Ouyang, D.C. Jia, Y. Zhou, Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: facile synthesis and enhanced microwave absorption properties, Carbon 108 (2016) 52–60. [5] J.R. Ma, X.X. Wang, W.Q. Cao, C. Han, H.J. Yang, J. Yuan, M.S. Cao, A facile fabrication and highly tunable microwave absorption of 3D flower-like Co3O4-rGO hybrid-architectures, Chem. Eng. J. 339 (2018) 487–498. [6] Y.Y. Lu, Y.T. Wang, H.L. Li, Y. Lin, Z.Y. Jiang, Z.X. Xie, Q. Kuang, L.S. Zheng, MOFderived porous Co/C nanocomposites with excellent electromagnetic wave absorption properties, ACS Appl. Mater. Interfaces 7 (2015) 13604–13611. [7] X. Jin, Q. Wang, W.Q. Khan, Y.Q. Li, Z.H. Tang, FeSiAl/(Ni0.5Zn0.5) Fe2O4 magnetic sheet composite with tunable electromagnetic properties for enhancing magnetic field coupling efficiency, J. Alloy. Comp. 729 (2017) 277–284. [8] S.S.S. Afghahi, A. Mirzazadeh, M. Jafarian, Y. Atassi, A new multicomponent material based on carbonyl iron/carbon nanofiber/lanthanum-strontiummanganite as microwave absorbers in the range of 8-12 GHz, Ceram. Int. 42 (2016) 9697–9702. [9] Y. Xu, J.H. Luo, W. Yao, J.G. Xu, T. Li, Preparation of reduced graphene oxide/ flake carbonyl iron powders/polyaniline composites and their enhanced microwave absorption properties, J. Alloy. Comp. 636 (2015) 310–316. [10] F.Y. Wang, Y.Q. Sun, D.R. Li, B. Zhong, Z.G. Wu, S.Y. Zuo, D. Yan, R.F. Zhuo, J. J. Feng, P.X. Yan, Microwave absorption properties of 3D cross-linked Fe/C porous nanofibers prepared by electrospinning, Carbon 134 (2018) 264–273. [11] Y.C. Qing, D.D. Min, Y.Y. Zhou, F. Luo, W.C. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxy-silicone composites as thin-thickness and widebandwidth microwave absorber, Carbon 86 (2015) 98–107. [12] Y.W. Nam, J.H. Choi, W.J. Lee, C.G. Kim, Fabrication of thin and lightweight microwave absorber containing Ni-coated glass fibers by electroless plating, Compos. Sci. Technol. 145 (2017) 165–172. [13] D.D. Min, W.C. Zhou, Y.C. Qing, F. Luo, D.M. Zhu, Highly oriented flake carbonyl iron/carbon fiber composite as thin-thickness and wide-bandwidth microwave absorber, J. Alloy. Comp. 744 (2018) 629–636. [14] Y.C. Qing, W.C. Zhou, S. Jia, F. Luo, D.M. Zhu, Electromagnetic and microwave absorption properties of carbonyl iron and carbon fiber filled epoxy/silicone resin coatings, Appl Phys a-Mater 100 (2010) 1177–1181. [15] Y.C. Qing, W.C. Zhou, F. Luo, D.M. Zhu, Effect of magnetic fillers on the electromagnetic properties of CaCu3Ti4O12-epoxy composites within the 2-18 GHz range, J. Mater. Chem. C 1 (2013) 536–541. [16] K.Y. Park, J.H. Han, S.B. Lee, J.B. Kim, J.W. Yi, S.K. Lee, Fabrication and electromagnetic characteristics of microwave absorbers containing carbon nanofibers and NiFe particles, Compos. Sci. Technol. 69 (2009) 1271–1278.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Synchronously oriented Fe microfiber & flake carbonyl iron/epoxy composites with improved microwave absorption and lightweight feature Hanyi Nan, Yuchang Qing *, Hui Gao, Hongyao Jia, Fa Luo, Wancheng Zhou State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi0 an, 710072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (M) Magnetic properties (P) Anisotropy (A) Microwave absorption
In this study, Fe microfibers (FMF) with a low percolation threshold and flake carbonyl iron (FCI) particles with high complex permeability were filled in an epoxy matrix, and shear force was applied to obtain synchronously oriented microstructures. The effect of the contents of FMF and/or FCI particles on the electromagnetic and microwave-absorbing properties of these composites was investigated in the frequency range of 2–18 GHz. Compared with single component-filled composites, the mixture of FMF and FCI imparted both suitable complex permittivity and enhanced complex permeability to the composite materials. Interestingly, the electromagnetic parameters of the composites could be tailored via simply regulating the ratio of FMF to FCI; thin and light microwave absorbers operating in the broadband range were obtained under the state of low content because of the synergy between the two components and the synchronous orientation of the fillers. An effective absorption of frequencies ranging from 3.5 to 18 GHz with reflection loss < 8 dB was achieved at thicknesses ranging from 1.0 to 2.0 mm.
1. Introduction Resin-based microwave absorbing materials (MAMs) are typically polymer-matrix composites containing various absorbents. In the past decade, much work has been done to study the properties of diversified absorbents in order to reduce the electromagnetic pollution caused by the extensive utilization of electronic devices and communication fa cilities as well as to satisfy the military requirement for stealth weapon systems [1–12]. While it is well known that the energy dissipation mechanisms of microwave involve dielectric loss and magnetic loss, some studies have revealed that unilateral loss materials composed of a single absorbent are not very effective for electromagnetic matching and fulfilling the desired “thin, light, broad, and strong” characteristics for MAMs [11–14]. Thus, MAMs filled with mixed-type or hybrid-type ab sorbents have been investigated to obtain good absorbing materials, e.g., by using a mixture of dielectric and magnetic loss absorbents, and decorating the dielectric loss materials with various magnetic particles [7,15–18]. Attempts to further improve microwave absorption have revealed that an oriented arrangement of anisotropic fillers in the polymer matrix enhances their absorption capability [19–22]. Generally, most MAMs, which possess excellent absorption perfor mance based on flake carbonyl iron (FCI), require high filler content or
have a narrow effective absorption bandwidth. A known method of reducing the filler content, which is required to maintain its high com plex permeability and achieve appropriate complex permittivity, is the addition of a low percolation threshold and large complex permeability magnetic filler such as iron fiber. Iron fiber having low percolation attributed to the unique shape anisotropy can acquire modest complex permittivity under the condition of low loading contents, thereby reducing the weight of the absorption materials. Although iron fiber also exhibits double energy loss mechanisms, including dielectric and mag netic loss, its complex permittivity is far larger than its complex permeability, which can trigger the detriment of impedance matching between free space and the absorber. Thus, combining flake carbonyl iron particles having low dielectric loss with iron fiber having high dielectric loss may improve comprehensive dielectric loss of the com posites while maintaining high magnetic loss. High imaginary part of permeability is important for similar absor bents filled with composites in obtaining thin and lightweight MAMs. It has been demonstrated that an oriented structure has increased permeability [13,20,21]. Considerable efforts have been devoted to investigating the oriented arrangements of shape anisotropic materials, such as carbon-based materials, flake iron-based alloys, silicon carbide fibers, etc., to study anisotropic magnetic or electrical properties
* Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Qing). https://doi.org/10.1016/j.compscitech.2019.107882 Received 16 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019 Available online 15 October 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic representation of the synchronously oriented FMF and FCI in the composites, (a) FMF perpendicular to the electric field vector of the incident wave, (b) orientation of the test sample in the two-port network system.
[21–25]. However, few reports have been reported with regard to the MAMs with combined oriented FCI particles and FMF as the absorbents. Here we investigated the complex permittivity and permeability of epoxy-based composites filled with synchronously oriented FCI and FMF in varying proportions. The microstructure and microwave absorption properties of the as-prepared composites were studied in detail. Lightweight and broader-bandwidth absorbers were obtained by the FMF&FCI/epoxy composites in the 2–18 GHz range.
Ltd., was ~10 μm. The FCI particles were 2–10 μm in diameter and procured from Xinhua Chemical Co. Ltd. (Shaanxi Province, China). The polymer matrix was the epoxy resin provided by Xi0 an Power Resin Factory. The hardener and toughening agent, purchased from Ningping Chemical Co. Ltd. (Tianjin, China), were polyamide resin and watersoluble polyurethane, respectively.
2. Material and methods
First, the epoxy resin and polyamide with a mass ratio of 4:1 were dissolved in acetone and 10 wt% water-soluble polyurethane was added to the mixture. The resin mixture was stirred for 10 min to obtain a homogeneous solution. Subsequently, a certain amount of FCI particles were added and dispersed by high-speed homogenizing dispersion at
2.2. Preparation of simultaneous oriented FMF&FCI/epoxy composites
2.1. Materials The diameter of FMF, provided by Jiangyou Hebao Nanomaterial Co.
Fig. 2. Synchronously oriented FMF&FCI/epoxy composites with 15 wt% FMFs þ45 wt% FCI: (a) aligned FMFs parallel to the tape-casting direction; (b) aligned ones perpendicular to the tape-casting direction; (c) the magnified images of (a); (d) the micromorphology of 45 wt% FMFs þ 15 wt% FCI. 2
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Fig. 3. Real (a) and imaginary (b) components of permittivity of aligned FMF&FCI/epoxy composites as a function of frequency; (c) Schematic representation of the formation of the micro-capacitor structure, conductive network, and channel.
2000 rpm for 10 min. Next, FMF were added in a certain proportion to the mixture of resin and FCI, stirring for 1 h at 800 rpm by mechanical agitation [19]. Finally, the prepared slurry of both resin and fillers mixtures having suitable viscosity (3 Pa s) was tape-casted on a plastic film at a uniform speed of 3 cm/s using a blade height of 500 μm. The synchronously oriented samples were obtained by introducing shear force under the tape-casting equipment [19]. In this work, the total content of filler was 60 wt%, and the pro portions of FMF and FCI in the different samples (Samples 0–4) were as follows: 60 wt% FMF, 45 wt% FMF þ 15 wt% FCI, 30 wt% FMF þ 30 wt % FCI, 15 wt% FMF þ 45 wt% FCI, and 60 wt% FCI.
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, μr and εr are the relative complex permeability and permit tivity, respectively, d is the thickness of the absorber, f is the frequency of the microwave, and c is the velocity of light. 3. Results and discussion 3.1. Microstructure of the synchronously oriented FMF&FCI/epoxy composites Fig. 2 shows the SEM images of synchronously oriented FMF&FCI/ epoxy composites (Sample 3). It was found that FMF and FCI were uniformly distributed in the epoxy resin matrix in a certain orientation. The FMF were parallel to each other and to the coating surface, and most FMF lengths ranged from 40 to 110 μm. As can be seen from Fig. 2(c), the thickness of the FCI particles was less than 1 μm, and they were parallel to each other as well as almost parallel to the axially ordered arrange ment of FMF. Therefore, FMF and FCI had achieved a synchronous orientation by inducing shear force.
2.3. Characterization The morphology and structure of the composites were characterized by scanning electron microscopy (SEM) (Tescan Vega 3 SBH, Brno, Czech). The complex permittivity and permeability of the prepared samples, employing the waveguide method in the range of 2–18 GHz, were measured by a network analyzer (Agilent Technologies E8362B). The relationship between the electromagnetic field and specimens in the waveguide measurement is presented in Fig. 1(a), where the electro magnetic parameters for FMF are ε? and μ//. The specimens were placed perpendicular to the incident microwave transmitting the thickness di rection presented in Fig. 1(b) [19]. The reflection loss (RL) of the composites with changed thickness (d) at the given frequency was calculated based on the transmission line theory for a single layer absorber according to following equations [26]. � � �Zin Z0 � � RL ðdBÞ ¼ 20 log�� (1) Zin þ Z0 � rffiffiffiffi Zin ¼ Z0
�
μr 2πfd pffiffiffiffiffiffiffiffi tanh j ε r μr c εr
3.2. Electromagnetic properties of synchronously oriented FMF&FCI/ epoxy composites Fig. 3 shows the real (a) and imaginary (b) components of permit tivity of different specimens in the 2–18 GHz region. As observed in Fig. 3(a), for Sample 4, the values of both real (ε0 ) and imaginary (ε00 ) components of permittivity were approximately 19 and 1, respectively, and nearly independent of frequency over the entire measured fre quency range. However, in comparison with Sample 4, Sample 0 showed higher values of complex permittivity and a high degree of frequencydependency over the measured frequency range. In addition, the 00 values of ε0 and ε increased as the FMF content increased in the mixed
� (2) 3
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Fig. 4. Real (a) and imaginary (b) components of the permeability of aligned FMF&FCI/epoxy composites as a function of frequency; (c) and (d) curves of 2
1
μ ðμ Þ f 00
0
versus frequency.
absorbent, thus demonstrating a good frequency dispersion feature. On the basis of the Maxwell-Wanger-Sillars (MWS) principle, heterogeneous composites generate polarization and charge accumulation at the interface between the conductive fillers and insulation resin matrix [11, 27]. Consequently, the dielectric properties of such composites are strongly dependent on the nature of the conductive fillers/matrix interface, and the characteristics and contents of fillers. The ε0 value of the heterogeneous composites, representing the ability to store charge, was determined by the interface between the absorbents/matrix or absorbents/absorbents, where free charges can accumulate and create interfacial polarization under an external electric field. Based on this rationale, it is reasonable that the ε0 value decreased from Sample 0 to Sample 2 in the 2–18 GHz range as a result of the FMFs having larger surface areas. However, compared to Sample 0, the ε0 values of the other samples were less than that of Sample 4 in a special frequency range, which was mainly due to the frequency-dependent effect of FMF as 00 fillers. On the other hand, ε represents the ability of electromagnetic energy loss, and can be described by two terms with applied electric field as follows [15]. 00
0
ε ¼ εR0 þ
σ 2πf ε0
mainly related to interface polarization and the corresponding relaxa tion when the content of the conductive filler is below the percolation threshold, and conductivity plays a key role above the percolation 00 threshold [28,29]. Therefore, the low ε value of Sample 4 could be attributed to the higher percolation threshold of FCI particles as compared to that of FMF, thereupon indicating that the percolation threshold of the mixed-absorbent decreased with the increasing pro portion of FMF in the mixed-absorbent. The conductive network or pathway is formed when the filler content is above the percolation threshold (as shown in Fig. 3(c) and in the meantime, conductivity is the 00 00 main contributor to ε . Consequently, it is reasonable that the value of ε increases with increasing FMF content in the composite. Similar to resin matrix composites filled with shape-anisotropic conductive fillers, complex permittivity is closely dependent on the filler alignment. As shown Fig. 3(c), the well-aligned composite com prises a network of micro-capacitors between FCI and a conducting pathway between FCI and FMF, which can store massive electric charge, improve the conductivity and interface polarization, and enable the composite to possess a high dielectric constant [27]. The real and imaginary components of permeability of synchro nously oriented FMF&FCI/epoxy composites are presented in Fig. 4(a) and (b), respectively. Clearly, the real component (μ0 ) of permeability of Sample 4 was higher than that of Sample 0. The μ0 values of the com posites decreased gradually with increasing FMF content in the mixed absorbent, and exhibited a good frequency dispersion in the investigated
(3)
where εR0 is affected by the polarization and corresponding relaxation, 00 and σ is the electrical conductivity. Based on the percolation theory, ε is 0
4
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Fig. 5. Dielectric loss (a) and magnetic loss (b) versus frequency of FMF&FCI/epoxy composites.
frequency region. The μ0 values of the composites decreased from 2.82 to 2.07 at 2 GHz with increasing weight ratio of FMF to FCI from Sample 4 to Sample 0. In addition, the imaginary component (μ00 ) of the perme ability of FMF&FCI/epoxy composites showed a peak in the broad fre quency range of 2–18 GHz for Samples 4, 3, and 2, which had peak frequencies of 8.5, 5.4, and 2.8 GHz, respectively. It is well accepted that microwave magnetic loss of magnetic composites results mainly from domain resonance, natural resonance, eddy current effect, and exchange resonance [30,31]. However, several studies have shown that resonance frequencies depend on the magnetocrystalline anisotropy field that is affected by saturation magnetization, and the size and shape of the magnetic absorber [5,32–34]. It is worth noting that natural resonance generally occurs at a low frequency (less than 10 GHz), and exchange resonance usually emerges at a higher frequency and results from nanoparticles or nanocrystals [5,35–37].As domain resonance usually occurs within the MHz range, the magnetic loss may not have originated from exchange and domain resonances. Furthermore, the contribution of the eddy current effect to magnetic loss is mainly related to the thickness (σ) and conductivity (σ) of the conductive fillers, as expressed by the following equation [38]: 00
which led to enhanced multiple scattering and reflection of microwave within the composites and further enhanced electromagnetic coupling between the microwaves and fillers as well as between the fillers. The shape anisotropy of fillers also contributed to the improvement of complex permeability [11]. Moreover, it is known that the orientation of the fillers plays an important role in enhancing the complex perme ability [15,19]. Thus, the desirable electromagnetic parameters can be effectively tuned by mixing the magnetic absorbents in the case of magnetic absorbing materials, and implementing synchronous orienta tion of the anisotropic fillers. There are two important contributing factors for microwave ab 00 0 sorption, namely, dielectric loss (tanσ e ¼ ε =ε ) and magnetic loss 00 0 (tanσm ¼ μ =μ ), as shown in Fig. 5(a) and (b), respectively. Generally, the greater dielectric and magnetic loss, the stronger the energy dissi pation ability of electromagnetic waves. It is evident that the dielectric loss of Sample 0 was considerably higher than that of Sample 4, resulting from a higher imaginary part of the permittivity of Sample 0. However, excessive or small dielectric values are detrimental to microwave ab 0 sorption. It is known that large ε values will lead to an increase in the reflection of the incident electromagnetic waves, which further limits the absorption bandwidth. The magnetic loss of the FMF&FCI/epoxy composites was enhanced after blending FMF and FCI together, espe 00 cially at a lower frequency, because of the improvement of μ . This suggests that tailoring the ratio of FMF to FCI can allow tuning of the impedance matching, leading to optimal microwave attenuation. This investigation demonstrates that the magnetic loss of the composites could be improved by simply blending FMF and FCI at 2–18 GHz.
(4)
0
μ ¼ 2πμ0 ðμ Þ2 d2 σf
where μ0 refers to the vacuum permeability. Thus, the increase in the proportion of FMF will produce a higher eddy current loss owing to their high conductivity within the composites and might play an active role in magnetic loss [26]. If magnetic loss solely resulted from the eddy current loss effect, the values of μ ðμ Þ 2 f 1 would be constant with the varia tion of frequency. In contrast, as shown in Fig. 4(c), the values of all samples vary with increasing frequency, indicating that magnetic loss does not simply originate from eddy current loss. For Samples 2 and 3, 00
0
3.3. Microwave absorption of synchronously oriented FMF&FCI/epoxy composites
2
the value of μ ðμ Þ f 1 was unchanged at 6–12 GHz, originating merely from the eddy current loss as shown Fig. 4(d). This suggests that the magnetic loss mechanism of Samples 2 and 3 comprises both eddy current loss and natural resonance loss. In contrast, the magnetic loss mechanism of other samples was mainly attributable to natural reso nance loss. The resonance frequency shift to low frequency from Sample 4 to Sample 0 is worth noting, and might be explained by the greater degree of anisotropy of FMF than that of FCI. Therefore, it was concluded that natural resonance can be tuned by mixing different types 00 of magnetic absorbents, and μ of this composite could be improved within the desired frequency range. For example, Sample 2 showed a 00 high μ value, which was ~1.6 at 2 GHz. The improvement of the complex permeability of FMF and FCI-filled composite was attributed to the interaction between FMF and FCI. This interaction could be attributed to the smaller distance between the fillers after the combination and synchronous orientation of FMF and FCI, 00
0
To study the microwave absorption properties of these materials, the RL of Samples 0–4 at various matching thicknesses were calculated in terms of the transmission line theory, as presented in Fig. 6. It was found that the absorption bandwidth with RL below 8 dB increased from Sample 0 to 3 at the same thickness, suggesting that the absorption properties of the composites filled with the mixed absorbent were enhanced as compared with those composites filled with a single absorbent. It was observed that the impedance matching characteristic between free air and the coating of Sample 0 deteriorated owing to its high complex permittivity, which led to the reflection of most of the incident electromagnetic waves on the coating surface and not entering the coating interior. In contrast, the absorption bandwidth of Sample 4 was narrow because of its low dielectric loss as well as frequencyindependent dielectric properties. Therefore, there was an unsatisfac tory absorption property for the single component-filled composites. Consequently, the complementarities between the dielectric properties 5
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yet Samples 0 and 1 exhibited dwindling tendency and the minimum values appeared at different frequencies at various matching thick nesses. For example, in the case of Sample 3, the RL values below 8 dB were obtained in the range of 9.8–18 GHz at a thickness of 1.0 mm, while in the cases of Samples 0 and 4, these were observed in the fre quency ranges of 13.4–16.6 GHz and 7.7–13.4 GHz at the same thick ness, respectively. The RL values below 8 dB of FMF&FCI/epoxy composites are summarized in Table 1 and a comparison of the elec tromagnetic wave absorption properties with other similar composites is given in Table 2. These results indicate that FMF and FCI-filled com posites with widened microwave absorption bandwidth and thinner thickness were obtained by simply blending FMF with FCI and then changing the filler content to optimize their properties. It is evident that the ameliorative values of complex permeability, particularly the 00 0 improved μ and modest μ values after blending FMF with FCI, which can generate high magnetic loss, were superior to that of the single component-filled composites. Furthermore, the enhanced frequencydependence of complex permittivity had a positive effect as excellent absorption performance was achieved and decreased thickness of the absorber was made feasible. Therefore, compared to FMF or FCI-filled composites with a narrow microwave absorption bandwidths, the broader absorption bandwidth of FMF and FCI-filled composites pre 00 0 0 sented here mainly stem from the modest values of ε , ε , and μ , the 00 improved μ value, as well as the enhanced frequency-dependent char acteristic of complex permittivity, which attribute to the synergistic consequence between FMF and FCI coupled with the synchronous orientation of the fillers. To further investigate the RL performance of Samples 2 and 3 at different thickness in detail, the three-dimensional plots of the RL values were made and are presented in Fig. 7. The minimum RL values of Samples 2 and 3 were 12 dB and 15 dB with thicknesses of 1.8 mm and 2.0 mm at 5.0 GHz and 4.8 GHz, respectively. The absorption bandwidth corresponding to RL below 8 dB for Samples 2 and 3 at thicknesses of 1.0–1.1 mm was found in the wide frequency ranges of 9–18 GHz and 8–18 GHz, respectively. The enhanced minimum RL values and absorption bandwidth for Sample 3 as compared with those of Sample 2 suggest that optimizing the proportion of mixed fillers can lead to excellent microwave absorbing property. Meanwhile, the mini mum RL values of these samples shifted to low frequency as the thick ness increased, which can be explained by the quarter-wavelength cancellation model as follows [42]. f¼
and magnetic characteristics are an important contributor to highperformance microwave absorption. Additionally, as absorber thick ness increased, the minimum RL values of Samples 2–4 were enhanced,
Sample 0 1 2 3 4
8 dB and
Absorbent and content 60 wt% FMF 45 wt% FMF þ 15 wt% FCI 30 wt% FMFþ30 wt% FCI 15 wt% FMFþ45 wt% FCI 60 wt% FCI
nc pffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1:3:5:::Þ jμr jjεr j
(5)
where c represents the velocity of light, and μr and εr are relative complex permeability and permittivity at frequency f, respectively. It is worth noting that the frequency is inversely proportional to the thick ness of the absorber. Excellent microwave absorbing materials require both highly effi cient attenuation and good impedance matching properties, which is associated with complex permittivity, complex permeability, thickness, and internal fractal structure [6]. The FMF&FCI/epoxy composites effectively utilized the ameliorated magnetic loss and dielectric loss produced by the efficient complementarities of FMF and FCI, as well as
Fig. 6. Microwave RL versus frequency curves of the FMF-FCI/epoxy com posites at given matching thicknesses.
Table 1 Absorption bandwidths with RL below
4d
10 dB of the FMF and FCI-filled composites at given matching thicknesses. RL < 8 dB (GHz)
RL < 10 dB (GHz)
1.0 mm
1.2 mm
1.4 mm
1.0 mm
1.2 mm
1.4 mm
13.4–16.6 11.7–18 9.8–18 9.8–18 7.7–13.4
0 8.2–12.2 6.7–15.0 7.0–15.6 6.2–11.3
0 6.3–9.4 5.2–10.0 5.6–12.0 5.0–9.6
0 0 15.6–18 13.4–17.3 8.9–11.7
0 0 0 8.4–12.8 6.9–10.1
0 0 6.3–8.2 6.5–10.4 5.6–8.8
6
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Table 2 Comparison of the electromagnetic wave absorption properties of the as-prepared FMF-FCI and previously reported composites. Absorbent and content
EM values at 2–18 GHz
ε 60 wt% (Graphene-carbonyl iron) 20 vol% (Ni0.5Zn0.5Fe2O4/carbonyl) 35 vol% (Aligned FeCo@C) 25 wt% (Fe/C nanofiber) 75 wt% (FeSiAl@Nylon) Oriented (65 wt%FCIþ0.3 wt% carbon fiber) 30 wt% FMFþ30 wt% FCI 15 wt% FMFþ45 wt% FCI
0
7.5–5.5 ~10 20–15 14.7–9.8 25–15 32–27 28.8–14.3 22.8–16.6
0
0
Thickness (mm)
Bandwidth (RL < 8 dB) (GHz)
Refs.
2.5 1.6 1.5 1.5 2.0 1.0 1.0 1.0
8–18 9.5–18 8–13 14–18 2–5.8 8.2–18 9.8–18 9.8–18
[39] [40] [22] [10] [41] [13] This work This work
0
ε’
μ
μ’
1.3–1.9 3–1 1.6–4 5.3–2.3 6–1 2.1–1.5 3.5–0.9 2.5–0.7
1.3–1.1 2.8–0.5 2.9–1.3 1.1–0.8 3.8–1 3.2–1.3 2.5–0.9 2.6–0.9
0.27–0.17 0.7–0.3 0.5–0 0.06 to 0.2 2.2–0.3 1.7–1.1 1.6–0.8 1.4–0.9
Fig. 7. Three-dimensional plot of RL values of Samples 2 (a) and 3 (b) at different thicknesses.
Fig. 8. Schematic illustration of electromagnetic waves loss factors.
the synchronous orientation of the fillers, which determines the impedance matching and microwave attenuation. As shown in Fig. 8, the microcapacitor structure, eddy current loss, natural resonance, multiple scattering and reflections, and dielectric polarization and the corre sponding relaxation contribute together to enhance the attenuation ca pacity, and thus increase the microwave absorbing performance.
Meanwhile, FMF with low percolation threshold could easily achieve suitable complex permittivity under the condition of low loading con tent of absorbent, which effectively reduced the weight of absorber. Thus, combined with the low density of FMF, the FMF-FCI/epoxy composites showed the advantageous attribute of being lightweight.
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4. Conclusion
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In summary, synchronously oriented FME&FCI/epoxy composites were prepared by shear force, and their electromagnetic waves ab sorption properties in the frequency range of 2–18 GHz were investi gated. Compared to the single component-filled composites, the composites with both FMF and FCI possessed suitable complex permit tivity and enhanced complex permeability, thus ameliorating the impedance match between free space and the absorber and achieving excellent absorption properties. The synchronously oriented FME&FCI/ epoxy composites, having a thin absorber thickness and broad absorp tion bandwidth under the condition of low absorbent contents, could effectively reduce the weight of the absorber, and an effective absorp tion with RL < 8 dB was obtained from 3.5 to 18 GHz at thickness ranging from 1.0 to 2.0 mm. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51402239), Fundamental Research Funds for the Central Universities (No. 3102019PB002), and the State Key Labo ratory of Solidification Processing China (Grant No. KP201604). References [1] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, (Fe, Ni)/C nanocapsules for electromagnetic-wave-absorber in the whole Ku-band, Carbon 47 (2009) 470–474. [2] M.X. Yu, X.C. Li, R.Z. Gong, Y.F. He, H.H. He, P.X. Lu, Magnetic properties of carbonyl iron fibers and their microwave absorbing characterization as the filer in polymer foams, J. Alloy. Comp. 456 (2008) 452–455. [3] L. Wang, F. He, Y.Z. Wan, Facile synthesis and electromagnetic wave absorption properties of magnetic carbon fiber coated with Fe-Co alloy by electroplating, J. Alloy. Comp. 509 (2011) 4726–4730. [4] W. Feng, Y.M. Wang, J.C. Chen, L. Wang, L.X. Guo, J.H. Ouyang, D.C. Jia, Y. Zhou, Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: facile synthesis and enhanced microwave absorption properties, Carbon 108 (2016) 52–60. [5] J.R. Ma, X.X. Wang, W.Q. Cao, C. Han, H.J. Yang, J. Yuan, M.S. Cao, A facile fabrication and highly tunable microwave absorption of 3D flower-like Co3O4-rGO hybrid-architectures, Chem. Eng. J. 339 (2018) 487–498. [6] Y.Y. Lu, Y.T. Wang, H.L. Li, Y. Lin, Z.Y. Jiang, Z.X. Xie, Q. Kuang, L.S. Zheng, MOFderived porous Co/C nanocomposites with excellent electromagnetic wave absorption properties, ACS Appl. Mater. Interfaces 7 (2015) 13604–13611. [7] X. Jin, Q. Wang, W.Q. Khan, Y.Q. Li, Z.H. Tang, FeSiAl/(Ni0.5Zn0.5) Fe2O4 magnetic sheet composite with tunable electromagnetic properties for enhancing magnetic field coupling efficiency, J. Alloy. Comp. 729 (2017) 277–284. [8] S.S.S. Afghahi, A. Mirzazadeh, M. Jafarian, Y. Atassi, A new multicomponent material based on carbonyl iron/carbon nanofiber/lanthanum-strontiummanganite as microwave absorbers in the range of 8-12 GHz, Ceram. Int. 42 (2016) 9697–9702. [9] Y. Xu, J.H. Luo, W. Yao, J.G. Xu, T. Li, Preparation of reduced graphene oxide/ flake carbonyl iron powders/polyaniline composites and their enhanced microwave absorption properties, J. Alloy. Comp. 636 (2015) 310–316. [10] F.Y. Wang, Y.Q. Sun, D.R. Li, B. Zhong, Z.G. Wu, S.Y. Zuo, D. Yan, R.F. Zhuo, J. J. Feng, P.X. Yan, Microwave absorption properties of 3D cross-linked Fe/C porous nanofibers prepared by electrospinning, Carbon 134 (2018) 264–273. [11] Y.C. Qing, D.D. Min, Y.Y. Zhou, F. Luo, W.C. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxy-silicone composites as thin-thickness and widebandwidth microwave absorber, Carbon 86 (2015) 98–107. [12] Y.W. Nam, J.H. Choi, W.J. Lee, C.G. Kim, Fabrication of thin and lightweight microwave absorber containing Ni-coated glass fibers by electroless plating, Compos. Sci. Technol. 145 (2017) 165–172. [13] D.D. Min, W.C. Zhou, Y.C. Qing, F. Luo, D.M. Zhu, Highly oriented flake carbonyl iron/carbon fiber composite as thin-thickness and wide-bandwidth microwave absorber, J. Alloy. Comp. 744 (2018) 629–636. [14] Y.C. Qing, W.C. Zhou, S. Jia, F. Luo, D.M. Zhu, Electromagnetic and microwave absorption properties of carbonyl iron and carbon fiber filled epoxy/silicone resin coatings, Appl Phys a-Mater 100 (2010) 1177–1181. [15] Y.C. Qing, W.C. Zhou, F. Luo, D.M. Zhu, Effect of magnetic fillers on the electromagnetic properties of CaCu3Ti4O12-epoxy composites within the 2-18 GHz range, J. Mater. Chem. C 1 (2013) 536–541. [16] K.Y. Park, J.H. Han, S.B. Lee, J.B. Kim, J.W. Yi, S.K. Lee, Fabrication and electromagnetic characteristics of microwave absorbers containing carbon nanofibers and NiFe particles, Compos. Sci. Technol. 69 (2009) 1271–1278.
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