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Reticulated SiC coating reinforced carbon foam with tunable electromagnetic microwave absorption performance
Composites Part B 178 (2019) 107479
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Reticulated SiC coating reinforced carbon foam with tunable electromagnetic microwave absorption performance Xinli Ye a, Zhaofeng Chen a, *, Min Li b, Ting Wang b, Junxiong Zhang a, Cao Wu a, Qianbo Zhou a, Hezhou Liu c, Sheng Cui d a
International Laboratory for Insulation and Energy Efficiency Materials, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, PR China Suzhou Superlong Aviation Heat Resistance Material Technology Co., Ltd, Suzhou, 215400, PR China c The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240, PR China d Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 211800, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical vapour deposition SiC coating Microwave absorbing performance Interface polarization
Three-dimensional carbon foam reinforced by the chemical vapour deposition (CVD) SiC coating possessed an enhanced microwave absorbing performance. The effect of the CVD SiC on the properties of the carbon foam was researched. As a result, the microwave absorbing performance improved with the thicker SiC coating as the minimum reflection loss value decreased from 20.67 to 33.65 dB while the bandwidth frequency increased from 5.04 to 5.94 GHz, which was ascribed to the stronger basis dielectric response networks and interface polarization. In addition, the internal relation between the impedance matching value and attenuation coeffi cient was illustrated to further dig the absorbing mechanism, followed by a simple schematic diagram. Given the excellent performance of the basic material and exceptional structure characteristics, it has the prospect of becoming a superior functional composite in absorbing applications.
1. Introduction With the increasing severity of the electromagnetic pollution, the research on the microwave absorbing materials is becoming more and more urgent, especially in human health, machine operation, and state security fields [1–6]. The general electromagnetic wave absorbing ma terial contains two parts. One is the common lossy dielectric carbon material, including carbon foam (CF) [7–9], graphene foam [10], reduced graphene oxide [11,12], graphene aerogel [13], and carbon nanotube [14]. The other is the lossy magnetic metal materials, like Ni–Co nanomaterial [15], Fe nanoparticles [16], Mg–Ti nanopowders [17], and spherical carbonyl iron [18]. However, the oxidation char acteristics and poor mechanical property of the carbon composite or the high density and narrow broadband of the metal materials limit the wide applications in various fields [19]. Note that SiC is a kind of important reinforcing materials for high temperature structural ceramic matrix composites, which possesses a high thermal shock resistance, thermal conductivity, oxidation resis tance, low creep, and excellent stability [20–22]. In addition, SiC is also a semiconductor with a wide band gap, which is suitable for being
applied in the electromagnetic wave absorbing field. Up to now, many SiC-based materials such as SiC nanowires [23,24], graphene@SiC aerogel [25], and SiC whiskers [26–28] have been studied to meet the ever-growing demands. It also has been proved that the composite consisting of CF and SiC is a potential absorbent material. Cheng et al. prepared the graphene aerogel composite incorporated by the conduc tive polymer polypyrrole (PPy) and SiC nanowires, the results indicated that the foam had a requested absorption bandwidth of 5.90 GHz with 43 wt% PPy while 6.40 GHz with 66 wt% PPy, covering the whole X-band [29]. Xiao et al. proposed an ice templating method to obtain the hybrid SiC nanowires coated CF with tunable electromagnetic wave absorption. It reached a minimum reflection loss value of 31.00 dB [30]. Ye et al. studied the three-dimensional (3D) network SiC/CF composite, and the value of the minimum reflection loss could be 29.74 dB, which demonstrated the superiority of the SiC coating on the wave absorption performance [31]. To achieve the perfect combination of the CF skeleton and SiC film, a pyrolytic (PyC) coating was deposited on the CF skeleton firstly [32,33]. In this work, the PyC coating enhanced CF (PyC-coated CF) was regar ded as the starting material. The chemical vapour deposition (CVD)
* Corresponding author. E-mail address: [email protected] (Z. Chen). https://doi.org/10.1016/j.compositesb.2019.107479 Received 29 August 2019; Received in revised form 16 September 2019; Accepted 22 September 2019 Available online 23 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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technique was carried out to fabricate the SiC films along the PyC coating surface. The final sample was defined as SiC/PyC-coated CF. The microstructure and microwave absorption responses were studied to illustrate the effect of the thickness of the SiC films on the overall performance.
were called as SiC/PyC-coated CF-I and SiC/PyC-coated CF-II, respectively. The microstructures and element types of the starting PyC-coated CF and SiC/PyC-coated CFs were observed by SEM equipped with EDS (JEOL JSM-7600F). The phase structure characteristics were detected by XRD diffraction (XRD Bruker D8 Advance Powder). The microwave parameters were obtained by vector network analyzer (VNA, N5244A PNA-X, Agilent) within the frequency between 2.00 and 18.00 GHz. To calculate the value of the reflection loss, the resultant samples were blended with 50.00 wt% paraffin wax, followed by being pressed into the toroidal shape.
2. Experimental The starting melamine foam (MF) was purchased from the Suzhou Superlong Aviation Heat Resistance Material Technology Co., Ltd. The schematic illustration of SiC/PyC-coated CF is displayed in Fig. S1. During the pyrolytic process (I), the MF with a porosity of 98.00% and a density of 6.85 mg/cm3 was put in the cracking furnace tube under argon atmosphere. The porous CF was obtained at 1100 � C and kept warm for 5 h [34]. In the next process (II), the chemical vapour infil tration (CVI) technique was used to deposit the PyC coating on the CF skeleton. Propylene and argon were applied as the reaction and carrier gas with the gas flow ratio of 60: 180, respectively. After 14 h, the PyC-coated CF was obtained. Finally, methyltrichlorosilane, hydrogen, and argon were applied with the gas flow ratio of 35: 160: 160, which were regarded as the source, reaction, and carrier gas, respectively. The SiC films were deposited ultimately via the CVD process (III). The SiC coating was deposited by 7 h and 14 h, and the corresponding samples
3. Results and discussion To view the morphology variation, the SEM pictures are shown in Fig. 1. All the as-prepared samples possess a 3D network with interlinked pores as displayed in Fig. 1a, 1c, and 1e. The thickness of the CF skeleton ranges from 1.0 to 1.5 μm, while that of SiC/PyC-coated CF-I and SiC/ PyC-coated CF-II varies from 1.5 to 3.0 μm and 3.5–5.0 μm, respec tively, which shows a different thickness variation due to the SiC films in Fig. 1b, 1d, and 1f. Besides, the surface of the PyC-coated CF skeleton is smooth, and the fracture appearance is homogeneous in Fig. 1b. It be comes rough after the deposition program as well as the skeleton
Fig. 1. Microstructures of PyC-coated CF (a, b), SiC/PyC-coated CF-I (c, d), and SiC/PyC-coated CF-II (e, f). 2
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thickness, especially on the fracture section. The SiC films are firm and crisp, which are easy to collapse. The generated solid fragments during the fragmentation process are believed to be good for the improvement of the electromagnetic wave absorption property, which could induce many multi interfaces and defects [35]. In addition, the element map pings of the cross skeleton of SiC/PyC-coated CF-II is shown in Fig. S2. Figs. S2b and S2c demonstrate the uniform distribution of C and Si, respectively. Fig. S2d shows the corresponding EDS spectrogram, which illustrates that the sample mainly consists of these two elements. Fig. 2 shows the XRD patterns of the starting PyC-coated CF and SiC/ PyC-coated CFs. Clearly, PyC-coated CF is an amorphous structure with two broad peaks at 25.42� and 43.18� , which is aligned to the planes of (002) and (101) [9]. After the CVD technique, three diffraction peaks at 35.54� , 60.01� , and 71.7� can be matched well with (111), (220), and (311), which corresponds well with the peaks of the β-SiC [36,37]. The XRD results further demonstrate the successful preparation of the SiC component. As to the electromagnetic absorbing performance, the reflection loss values of the as-prepared samples are the key points to evaluate the absorber quality. As a general rule, the reflection loss value must not exceed 10.00 dB, and the material with a reflection loss value below 30.00 dB is considered to be an excellent absorber [38]. The reflection loss can be calculated as follows [39,40]: Zin ¼ Zo (μr/εr)0.5 tanh[2πfd/c] (μrεr)0.5
(1)
RL ¼ 20 logjðZin
(2)
Zo Þ = ðZin þ Zo Þj
suitable range of the frequency narrows to 6.04 and 17.60 GHz while that of the matching absorber thickness decrease to 1.85 and 5.00 mm, respectively in Fig. 3f. However, the microwave absorptivity of SiC/PyCcoated CF-II increases obviously as more reflection loss values are lower than 20.00 dB. In addition, the area that meets the requirements of the SiC/PyC-coated CFs changes from the state of almost complete disorder to the strip band geometry through the comparison of Fig. 3d, e, and 3f. To learn more about the effect of the SiC coatings, the reflection loss values of SiC/PyC-coated CF-I and SiC/PyC-coated CF-II with the absorber thicknesses of 2.00, 2.75, 3.50, 4.25, and 5.00 mm are further compared. Fig. 4a and b show the changing trends of the reflection loss values of SiC/PyC-coated CF-I and SiC/PyC-coated CF-II at specified sample thicknesses, respectively. The frequencies where the reflection loss values are the lowest move to left as the absorber thickness in creases. The significant difference is that the minimum reflection loss peaks of SiC/PyC-coated CF-II change greatly, while that of SiC/PyCcoated CF-I maintain steady. Fig. 4c displays the minimum reflection loss values at different sample thicknesses, which proves that with the increase of the SiC films, the SiC/PyC-coated CF is becoming more sensitive to the change of the matching sample thickness. It allows to the appropriate adjustment of the microwave absorbing properties, espe cially the minimum reflection loss according to the tuned thickness of the SiC coating. Fig. 4d shows the effective frequency bandwidth of the SiC/PyC-coated CFs. For SiC/PyC-coated CF-II, an excellent effective absorption bandwidth of 5.92 GHz is gained when the sample thickness is 2.75 mm, which is superior than SiC/PyC-coated CF-I. Table S1 lists the microwave absorbing of our previous SiC coating reinforced melamine derived CF composites and some other carbon composites [41–44]. With the combination of the SiC coating and CF matrix, the SiC/PyC-coated CF composites in this work show superior electromagnetic microwave absorption performance, which have great potential and good prospect. To dig the deep reason, the complex permittivity and permeability are analyzed, which has a great extent to determine the microwave absorbing feature [45]. The complex permittivity consists of the imaginary (ε’’) and real (ε’) parts, repre senting the electric dissipation and storage ability, while the complex permeability includes the imaginary (μ’’) and real (μ’) parts, which are the magnetic dissipation and storage ability [46,47]. The curves in Fig. 5c demonstrate that all the samples are pure non-magnetic materials as the real part of permeability is close to 1, and the imaginary is equal to 0. Hence, we mainly focused on the complex permittivity. Fig. 5a shows the value curves of the real part of the permittivity of the PyC-coated CF and SiC/PyC-coated CFs. Clearly, the real parts of permittivity tend to decrease with the deposition of the SiC coating as PyC-coated CF re mains at 14.06–45.68, SiC/PyC-coated CF-I is between 8.07 and 10.31, and SiC/PyC-coated CF-II ranges from 4.01 to 5.95. In addition, the real part of permittivity further decreases with the thicker SiC coating. As to the imaginary part of permittivity, the trend charts of the as-papered samples are shown in Fig. 5b, which are similar to that of the real part. For example, the imaginary part of PyC-coated CF remains at 19.81–97.96, while that of SiC/PyC-coated CF-I ranges from 4.28 to 6.19, and SiC/PyC-coated CF-II is between 0.96 and 2.10. Besides, the dielectric tangent (tanδε) is also an important factor to estimate the dielectric loss ability, which can be calculated by Ref. [48]:
Where Zin, Zo, μr, εr, f, d, c, RL are the input impedance, the impedance of free space, complex permittivity, complex permeability, electromag netic wave frequency, coating thickness of the composite, free velocity of the electromagnetic wave, and reflection loss value. As presented in Fig. 3a, the starting PyC-coated CF shows a poor microwave absorbing performance. The minimum reflection loss is only between 4.00 and 5.00 dB as shown in Fig. 3d, which proves that it is not a qualified absorbing product. As for the SiC/PyC-coated CFs, the microwave absorbing characteristics improved a lot. The value of the minimum reflection loss of SiC/PyC-coated CF-I is 20.67 dB at 17.88 GHz when the sample thickness is 1.40 mm in Fig. 3b. Between the absorber thickness of 1.25 and 5.00 mm, the region that satisfies the basic requirement covers the frequency range from 4.32 to 18.00 GHz in Fig. 3e. With the further increase of the SiC coating, the minimum reflection loss drops as low as 33.65 dB at 17.16 GHz when the sample thickness increases to 2.25 mm in Fig. 3c. On the contrary, the
tan δε ¼ ε’’/ε’
(3)
As observed in Fig. 5d, PyC-coated CF possesses the highest values in the overall frequency range, indicating its robust dielectric loss prop erty. However, after the induction of the SiC coating, the dielectric tangent value range of SiC/PyC-coated CF-I is between 0.47 and 0.75 while that of SiC/PyC-coated CF-II is 0.22–0.59, which is much lower than that of PyC-coated CF. It demonstrates that the starting PyC-coated CF is beneficial to the dielectric loss, which is a suitable template for the following CVD process. Also, the imaginary and real parts of the complex permittivity can be
Fig. 2. XRD pattern of starting PyC-coated CF and SiC/PyC-coated CFs. 3
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Fig. 3. 3D reflection loss spectrum of starting PyC-coated CF (a), SiC/PyC-coated CF-I (b), and SiC/PyC-coated CF-II (c); Corresponding mapping graphs (d, e, and f).
deduced [49,50]: ’’
2
ε ¼ ωτ0 (εs - ε∞)/(1þ(ωτ0) ) ’
2
ε ¼ ε∞ þ (εs - ε∞)/(1þ(ωτ0) )
polarization exists. So the Cole-Cole plot is very simple. As to the SiC/PyC-coated CFs, strong interface polarization generates due to the deposition of the SiC films, and the Cole-Cole plots are much more complicated. The much more complicated Cole-Cole plots in Fig. 6 indicated the more active multiple dielectric polarization and relaxation effects, which results from the dipole and interfacial polarization effects according to the Debye theory. Due to the deposition of the SiC coating, multiple interfaces and junctions were induced in the highly porous structure. Besides, the fragile SiC coating was more likely to generate countless fragments, which further increased the number of interfaces, and the final interfacial polarization. It is widely accepted that not only the remarkable impedance matching but also the favorable attenuation built the outstanding absorber [41]. The attenuation constant (α)is calculated in the expressed equation [52]:
(4) (5)
Where ω, τ0, εs, and ε∞ represent the angular frequency, polarization relaxation time, static permittivity, and relative dielectric permittivity at high-frequency limit. The relationship between these two parts can be simplified based on the formula above: (ε’’)2 þ (εs/2 þ ε∞/2 - ε’)2 ¼ (ε∞/2 - εs/2)2
(6)
Hence, the curve of the imaginary and real parts of the complex permittivity is similar to a single semicircle [51]. The Cole-Cole plots of the PyC-coated CF and SiC/PyC-coated CFs are shown in Fig. 6. In general, the difference in the electrical conductivity or dielectric con stant results in the interface polarization. The CF skeleton and PyC coating are composed of carbon, which means there is little interface
α ¼ 20.5πf ((ε’’μ’’ – ε’μ’) þ ((ε’μ’’ þε’’μ’)2þ(ε’’μ’’ -ε’μ’)2)0.5)0.5/c 4
(7)
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Fig. 4. Reflection loss curves of SiC/PyC-coated CF-I (a) and SiC/PyC-coated CF-II (b) at selected absorber thicknesses; Minimum reflection loss (c) and effective frequency bandwidth (d) contrast diagrams of SiC/PyC-coated CFs at selected absorber thicknesses.
Fig. 5. Real (a) and imaginary part (b) of permittivity of PyC-coated CF, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II; Real and imaginary part of permeability of all the three samples(c); Dielectric loss tangents of all the three samples (d).
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To better understand the nature reason, a plane mechanism is drawn in Fig. 8 to illustrate the microwave absorbing process. Firstly, the 3D network of SiC/PyC-coated CF extends the routes of electromagnetic wave propagation and causes multiple reflections. When the electro magnetic waves reach the surface of the absorber, the high impedance matching guarantees the ratio of the incident waves and most waves enter the absorber with minimal surface reflection due to the deposition of the SiC coating. Then the multi-layer interfaces, SiC films, PyC coat ings, as well as the CF matrix start to work. In addition, the countless interfaces produce a mass of interface electron and dipole polarizations resulting from the unbalanced charge distributions, which increases the absorption of electromagnetic waves. The incident waves are reflected, dissipated, attenuated, and weakened by the boundaries and interfaces. As a result, most parts of the wave energy are transformed into heat energy during the spreading process. The main thing to stress here is that it makes no sense with little incident microwave, which means that the value of the impedance matching ratio might play a crucial role. All in all, it is of vital importance to gain superior impedance matching and appropriate attenuation constant to achieve an excellent microwave absorbing performance [56,57].
Fig. 6. Cole-Cole plots of PyC-coated CF, SiC/PyC-coated CF-I, and SiC/PyCcoated CF-II.
4. Conclusions
The impedance value (Zr) is given based on the following formulas [53–55]: Zr ¼ Zin/Zo
In this article, the SiC/PyC-coated CFs have been developed through the pyrolytic process of MF, the CVI process of CF, and the CVD process of PyC-coated CF. The results indicate that the microwave absorbing performance of the PyC-coated CF is improved by the preparation of the SiC films. Furthermore, the thickness of the SiC coating has an important effect on the enhancement of the absorbing performance as the atten uation constant and impedance matching can be tuned by the SiC films.
(8)
As shown in Fig. 7, PyC-coated CF possesses the highest attenuation constant, which is much higher than that of the SiC/PyC-coated CFs at all the measured frequencies. However, its impedance matching is the lowest, and the microwave absorbing property is also not outstanding according to the aforementioned analysis. On the contrary, the attenu ation constants of the SiC/PyC-coated CFs are not the best, but its excellent impedance matching value guarantees its absorption capacity. Explained in more detail, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II possess the attenuation constants of 285.93 and 203.63 when the min imum reflection loss values are gained. When the matching thickness is 1.40 mm, SiC/PyC-coated CF-I shows a highest impedance matching value of 0.83 at 17.88 GHz. As to SiC/PyC-coated CF-II, the minimum reflection loss is obtained at 17.16 GHz when the matching thickness is 2.25 mm. Coincidentally, the impedance matching value reaches the highest point of 0.96. Although the attenuation constant of SiC/PyCcoated CF-II is lower than that of SiC/PyC-coated CF-I, the higher impedance matching value leads to the more superior microwave absorbing performance. It verifies that the impedance matching plays a much more role on resulting in the finest absorption intensity. On this basis, it is a significant strategy to enhance the absorption performance via tuning the impedance matching to a much higher value, which en sures that most of the electromagnetic waves can enter the absorber. Then, the attenuation constant realizes the absorption of the incident microwave.
Fig. 8. Schematic of microwave absorbing mechanism of SiC/PyC-coated CF.
Fig. 7. Attenuation constants (a) and impedance matching (b) of PyC-coated CF, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II. 6
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The SiC/PyC-coated CF-II possesses an outstanding reflection loss of 33.65 dB at 7.16 GHz when the sample thickness is 2.25 mm, which may be a promising candidate in the microwave absorbing fields.
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Conflicts of interest There is no conflict of interest. Acknowledgements The present work was supported by the National Natural Science Foundation of China [grant number 51772151]; the Postgraduate Research & Practice Innovation Program of Jiangsu Province [grant number KYCX19_0178]; and the Priority Academic Program Develop ment of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107479. References [1] Zhang Y, Huang Y, Chen H, Huang Z, Yang Y, Xiao P, Zhou Y, Chen Y. Composition and structure control of ultralight graphene foam for high-performance microwave absorption. Carbon 2016;105:438–47. [2] Wang L, Qiu H, Liang C, Song P, Han Y, Han Y, Gu J, Kong J, Pan D, Guo Z. Electromagnetic interference shielding MWCNT-Fe3O4@Ag/epoxy nanocomposites with satisfactory thermal conductivity and high thermal stability. Carbon 2019; 141:506–14. [3] Huangfu Y, Liang C, Han Y, Qiu H, Song P, Wang L, Kong J, Gu J. Fabrication and investigation on the Fe3O4/thermally annealed graphene aerogel/epoxy electromagnetic interference shielding nanocomposites. Compos Sci Technol 2019; 169:70–5. [4] Zhou H, Wang J, Zhuang J, Liu Q. A covalent route for efficient surface modification of ordered mesoporous carbon as high performance microwave absorbers. Nanoscale 2013;5:12502–11. [5] Chen Y, Liu X, Mao X, Zhuang Q, Xie Z, Han Z. γ-Fe2O3-MWNT/poly(pphenylenebenzobisoxazole) composites with excellent microwave absorption performance and thermal stability. Nanoscale 2014;6:6440–7. [6] Zhang XJ, Wang GS, Cao WQ, Wei YZ, Liang JF, Guo L, Cao MS. Enhanced microwave absorption property of reduced graphene oxide (RGO)-MnFe2O4 nanocomposites and polyvinylidene fluoride. ACS Appl Mater Interfaces 2014;6: 7471–8. [7] Moglie F, Micheli D, Laurenzi S, Marchetti M, Mariani Primiani V. Electromagnetic shielding performance of carbon foams. Carbon 2012;50:1972–80. [8] Dong S, Hu P, Zhang X, Han J, Zhang Y, Luo X. Carbon foams modified with in-situ formation of Si3N4 and SiC for enhanced electromagnetic microwave absorption property and thermostability. Ceram Int 2018;44:7141–50. [9] Fang Z, Li C, Sun J, Zhang H, Zhang J. The electromagnetic characteristics of carbon foams. Carbon 2007;45:2873–9. [10] Zhang Y, Huang Y, Zhang T, Chang H, Xiao P, Chen H, Huang Z, Chen Y. Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam. Adv Mater 2015;27:2049–53. [11] Cao MS, Wang XX, Cao WQ, Yuan J. Ultrathin graphene: electrical properties and highly efficient electromagnetic interference shielding. J Mater Chem C 2015;3: 6589–99. [12] Wen B, Cao M, Lu M, Cao W, Shi H, Liu J, Wang X, Jin H, Fang X, Wang W, Yuan J. Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures. Adv Mater 2014;26:3484–9. [13] Cheng Y, Tan M, Hu P, Zhang X, Sun B, Yan L, Zhou S, Han W. Strong and thermostable SiC nanowires/graphene aerogel with enhanced hydrophobicity and electromagnetic wave absorption property. Appl Surf Sci 2018;448:138–44. [14] Wen B, Cao MS, Hou ZL, Song WL, Zhang L, Lu MM, Jin H-B, Fang XY, Wang WZ, Yuan J. Temperature dependent microwave attenuation behavior for carbonnanotube/silica composites. Carbon 2013;65:124–39. [15] Nam YW, Choi JH, Huh JM, Lee WJ, Kim CG. Thin broadband microwave absorber with conductive and magnetic materials coated on a glass fabric. J Compos Mater 2018;52:1413–20. [16] Huang H, Gao Y, Fang CF, Wu AM, Dong XL, Kim BS, Byun JH, Zhang GF, Zhou DY. Spray granulation of Fe and C nanoparticles and their impedance match for microwave absorption. J Mater Sci Technol 2018;34:496–502. [17] Sharbati A, Amiri GR. Magnetic, microwave absorption and structural properties of Mg-Ti added Ca-M hexaferrite nanoparticles. J Mater Sci Mater Electron 2018;29: 1118–22. [18] Huang Y, Song WL, Wang C, Xu Y, Wei W, Chen M, Tang L, Fang D. Multi-scale design of electromagnetic composite metamaterials for broadband microwave absorption. Compos Sci Technol 2018;162:206–14.
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[53] Cheng Y, Cao J, Li Y, Li Z, Zhao H, Ji G, Du Y. The outside-in approach to construct Fe3O4 nanocrystals/mesoporous carbon hollow spheres core-shell hybrids toward microwave absorption. ACS Sustainable Chem Eng 2017;6:1427–35. [54] Wang P, Cheng L, Zhang L. One-dimensional carbon/SiC nanocomposites with tunable dielectric and broadband electromagnetic wave absorption properties. Carbon 2017;125:207–20. [55] Jia Z, Gao Z, Feng A, Zhang Y, Zhang C, Nie G, Wang K, Wu G. Laminated microwave absorbers of A-site cation deficiency perovskite La0.8FeO3 doped at hybrid RGO carbon. Compos B Eng 2019;176:107246. [56] Wang L, Chen L, Song P, Liang C, Lu Y, Qiu H, Zhang Y, Kong J, Gu J. Fabrication on the annealed Ti3C2Tx MXene/Epoxy nanocomposites for electromagnetic interference shielding application. Compos B Eng 2019;171:111–8. [57] Liang C, Song P, Ma A, Shi X, Gu H, Wang L, Qiu H, Kong J, Gu J. Highly oriented three-dimensional structures of Fe3O4 decorated CNTs/reduced graphene oxide foam/epoxy nanocomposites against electromagnetic pollution. Compos Sci Technol 2019;181:107683.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Reticulated SiC coating reinforced carbon foam with tunable electromagnetic microwave absorption performance Xinli Ye a, Zhaofeng Chen a, *, Min Li b, Ting Wang b, Junxiong Zhang a, Cao Wu a, Qianbo Zhou a, Hezhou Liu c, Sheng Cui d a
International Laboratory for Insulation and Energy Efficiency Materials, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, PR China Suzhou Superlong Aviation Heat Resistance Material Technology Co., Ltd, Suzhou, 215400, PR China c The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240, PR China d Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 211800, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical vapour deposition SiC coating Microwave absorbing performance Interface polarization
Three-dimensional carbon foam reinforced by the chemical vapour deposition (CVD) SiC coating possessed an enhanced microwave absorbing performance. The effect of the CVD SiC on the properties of the carbon foam was researched. As a result, the microwave absorbing performance improved with the thicker SiC coating as the minimum reflection loss value decreased from 20.67 to 33.65 dB while the bandwidth frequency increased from 5.04 to 5.94 GHz, which was ascribed to the stronger basis dielectric response networks and interface polarization. In addition, the internal relation between the impedance matching value and attenuation coeffi cient was illustrated to further dig the absorbing mechanism, followed by a simple schematic diagram. Given the excellent performance of the basic material and exceptional structure characteristics, it has the prospect of becoming a superior functional composite in absorbing applications.
1. Introduction With the increasing severity of the electromagnetic pollution, the research on the microwave absorbing materials is becoming more and more urgent, especially in human health, machine operation, and state security fields [1–6]. The general electromagnetic wave absorbing ma terial contains two parts. One is the common lossy dielectric carbon material, including carbon foam (CF) [7–9], graphene foam [10], reduced graphene oxide [11,12], graphene aerogel [13], and carbon nanotube [14]. The other is the lossy magnetic metal materials, like Ni–Co nanomaterial [15], Fe nanoparticles [16], Mg–Ti nanopowders [17], and spherical carbonyl iron [18]. However, the oxidation char acteristics and poor mechanical property of the carbon composite or the high density and narrow broadband of the metal materials limit the wide applications in various fields [19]. Note that SiC is a kind of important reinforcing materials for high temperature structural ceramic matrix composites, which possesses a high thermal shock resistance, thermal conductivity, oxidation resis tance, low creep, and excellent stability [20–22]. In addition, SiC is also a semiconductor with a wide band gap, which is suitable for being
applied in the electromagnetic wave absorbing field. Up to now, many SiC-based materials such as SiC nanowires [23,24], graphene@SiC aerogel [25], and SiC whiskers [26–28] have been studied to meet the ever-growing demands. It also has been proved that the composite consisting of CF and SiC is a potential absorbent material. Cheng et al. prepared the graphene aerogel composite incorporated by the conduc tive polymer polypyrrole (PPy) and SiC nanowires, the results indicated that the foam had a requested absorption bandwidth of 5.90 GHz with 43 wt% PPy while 6.40 GHz with 66 wt% PPy, covering the whole X-band [29]. Xiao et al. proposed an ice templating method to obtain the hybrid SiC nanowires coated CF with tunable electromagnetic wave absorption. It reached a minimum reflection loss value of 31.00 dB [30]. Ye et al. studied the three-dimensional (3D) network SiC/CF composite, and the value of the minimum reflection loss could be 29.74 dB, which demonstrated the superiority of the SiC coating on the wave absorption performance [31]. To achieve the perfect combination of the CF skeleton and SiC film, a pyrolytic (PyC) coating was deposited on the CF skeleton firstly [32,33]. In this work, the PyC coating enhanced CF (PyC-coated CF) was regar ded as the starting material. The chemical vapour deposition (CVD)
* Corresponding author. E-mail address: [email protected] (Z. Chen). https://doi.org/10.1016/j.compositesb.2019.107479 Received 29 August 2019; Received in revised form 16 September 2019; Accepted 22 September 2019 Available online 23 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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technique was carried out to fabricate the SiC films along the PyC coating surface. The final sample was defined as SiC/PyC-coated CF. The microstructure and microwave absorption responses were studied to illustrate the effect of the thickness of the SiC films on the overall performance.
were called as SiC/PyC-coated CF-I and SiC/PyC-coated CF-II, respectively. The microstructures and element types of the starting PyC-coated CF and SiC/PyC-coated CFs were observed by SEM equipped with EDS (JEOL JSM-7600F). The phase structure characteristics were detected by XRD diffraction (XRD Bruker D8 Advance Powder). The microwave parameters were obtained by vector network analyzer (VNA, N5244A PNA-X, Agilent) within the frequency between 2.00 and 18.00 GHz. To calculate the value of the reflection loss, the resultant samples were blended with 50.00 wt% paraffin wax, followed by being pressed into the toroidal shape.
2. Experimental The starting melamine foam (MF) was purchased from the Suzhou Superlong Aviation Heat Resistance Material Technology Co., Ltd. The schematic illustration of SiC/PyC-coated CF is displayed in Fig. S1. During the pyrolytic process (I), the MF with a porosity of 98.00% and a density of 6.85 mg/cm3 was put in the cracking furnace tube under argon atmosphere. The porous CF was obtained at 1100 � C and kept warm for 5 h [34]. In the next process (II), the chemical vapour infil tration (CVI) technique was used to deposit the PyC coating on the CF skeleton. Propylene and argon were applied as the reaction and carrier gas with the gas flow ratio of 60: 180, respectively. After 14 h, the PyC-coated CF was obtained. Finally, methyltrichlorosilane, hydrogen, and argon were applied with the gas flow ratio of 35: 160: 160, which were regarded as the source, reaction, and carrier gas, respectively. The SiC films were deposited ultimately via the CVD process (III). The SiC coating was deposited by 7 h and 14 h, and the corresponding samples
3. Results and discussion To view the morphology variation, the SEM pictures are shown in Fig. 1. All the as-prepared samples possess a 3D network with interlinked pores as displayed in Fig. 1a, 1c, and 1e. The thickness of the CF skeleton ranges from 1.0 to 1.5 μm, while that of SiC/PyC-coated CF-I and SiC/ PyC-coated CF-II varies from 1.5 to 3.0 μm and 3.5–5.0 μm, respec tively, which shows a different thickness variation due to the SiC films in Fig. 1b, 1d, and 1f. Besides, the surface of the PyC-coated CF skeleton is smooth, and the fracture appearance is homogeneous in Fig. 1b. It be comes rough after the deposition program as well as the skeleton
Fig. 1. Microstructures of PyC-coated CF (a, b), SiC/PyC-coated CF-I (c, d), and SiC/PyC-coated CF-II (e, f). 2
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thickness, especially on the fracture section. The SiC films are firm and crisp, which are easy to collapse. The generated solid fragments during the fragmentation process are believed to be good for the improvement of the electromagnetic wave absorption property, which could induce many multi interfaces and defects [35]. In addition, the element map pings of the cross skeleton of SiC/PyC-coated CF-II is shown in Fig. S2. Figs. S2b and S2c demonstrate the uniform distribution of C and Si, respectively. Fig. S2d shows the corresponding EDS spectrogram, which illustrates that the sample mainly consists of these two elements. Fig. 2 shows the XRD patterns of the starting PyC-coated CF and SiC/ PyC-coated CFs. Clearly, PyC-coated CF is an amorphous structure with two broad peaks at 25.42� and 43.18� , which is aligned to the planes of (002) and (101) [9]. After the CVD technique, three diffraction peaks at 35.54� , 60.01� , and 71.7� can be matched well with (111), (220), and (311), which corresponds well with the peaks of the β-SiC [36,37]. The XRD results further demonstrate the successful preparation of the SiC component. As to the electromagnetic absorbing performance, the reflection loss values of the as-prepared samples are the key points to evaluate the absorber quality. As a general rule, the reflection loss value must not exceed 10.00 dB, and the material with a reflection loss value below 30.00 dB is considered to be an excellent absorber [38]. The reflection loss can be calculated as follows [39,40]: Zin ¼ Zo (μr/εr)0.5 tanh[2πfd/c] (μrεr)0.5
(1)
RL ¼ 20 logjðZin
(2)
Zo Þ = ðZin þ Zo Þj
suitable range of the frequency narrows to 6.04 and 17.60 GHz while that of the matching absorber thickness decrease to 1.85 and 5.00 mm, respectively in Fig. 3f. However, the microwave absorptivity of SiC/PyCcoated CF-II increases obviously as more reflection loss values are lower than 20.00 dB. In addition, the area that meets the requirements of the SiC/PyC-coated CFs changes from the state of almost complete disorder to the strip band geometry through the comparison of Fig. 3d, e, and 3f. To learn more about the effect of the SiC coatings, the reflection loss values of SiC/PyC-coated CF-I and SiC/PyC-coated CF-II with the absorber thicknesses of 2.00, 2.75, 3.50, 4.25, and 5.00 mm are further compared. Fig. 4a and b show the changing trends of the reflection loss values of SiC/PyC-coated CF-I and SiC/PyC-coated CF-II at specified sample thicknesses, respectively. The frequencies where the reflection loss values are the lowest move to left as the absorber thickness in creases. The significant difference is that the minimum reflection loss peaks of SiC/PyC-coated CF-II change greatly, while that of SiC/PyCcoated CF-I maintain steady. Fig. 4c displays the minimum reflection loss values at different sample thicknesses, which proves that with the increase of the SiC films, the SiC/PyC-coated CF is becoming more sensitive to the change of the matching sample thickness. It allows to the appropriate adjustment of the microwave absorbing properties, espe cially the minimum reflection loss according to the tuned thickness of the SiC coating. Fig. 4d shows the effective frequency bandwidth of the SiC/PyC-coated CFs. For SiC/PyC-coated CF-II, an excellent effective absorption bandwidth of 5.92 GHz is gained when the sample thickness is 2.75 mm, which is superior than SiC/PyC-coated CF-I. Table S1 lists the microwave absorbing of our previous SiC coating reinforced melamine derived CF composites and some other carbon composites [41–44]. With the combination of the SiC coating and CF matrix, the SiC/PyC-coated CF composites in this work show superior electromagnetic microwave absorption performance, which have great potential and good prospect. To dig the deep reason, the complex permittivity and permeability are analyzed, which has a great extent to determine the microwave absorbing feature [45]. The complex permittivity consists of the imaginary (ε’’) and real (ε’) parts, repre senting the electric dissipation and storage ability, while the complex permeability includes the imaginary (μ’’) and real (μ’) parts, which are the magnetic dissipation and storage ability [46,47]. The curves in Fig. 5c demonstrate that all the samples are pure non-magnetic materials as the real part of permeability is close to 1, and the imaginary is equal to 0. Hence, we mainly focused on the complex permittivity. Fig. 5a shows the value curves of the real part of the permittivity of the PyC-coated CF and SiC/PyC-coated CFs. Clearly, the real parts of permittivity tend to decrease with the deposition of the SiC coating as PyC-coated CF re mains at 14.06–45.68, SiC/PyC-coated CF-I is between 8.07 and 10.31, and SiC/PyC-coated CF-II ranges from 4.01 to 5.95. In addition, the real part of permittivity further decreases with the thicker SiC coating. As to the imaginary part of permittivity, the trend charts of the as-papered samples are shown in Fig. 5b, which are similar to that of the real part. For example, the imaginary part of PyC-coated CF remains at 19.81–97.96, while that of SiC/PyC-coated CF-I ranges from 4.28 to 6.19, and SiC/PyC-coated CF-II is between 0.96 and 2.10. Besides, the dielectric tangent (tanδε) is also an important factor to estimate the dielectric loss ability, which can be calculated by Ref. [48]:
Where Zin, Zo, μr, εr, f, d, c, RL are the input impedance, the impedance of free space, complex permittivity, complex permeability, electromag netic wave frequency, coating thickness of the composite, free velocity of the electromagnetic wave, and reflection loss value. As presented in Fig. 3a, the starting PyC-coated CF shows a poor microwave absorbing performance. The minimum reflection loss is only between 4.00 and 5.00 dB as shown in Fig. 3d, which proves that it is not a qualified absorbing product. As for the SiC/PyC-coated CFs, the microwave absorbing characteristics improved a lot. The value of the minimum reflection loss of SiC/PyC-coated CF-I is 20.67 dB at 17.88 GHz when the sample thickness is 1.40 mm in Fig. 3b. Between the absorber thickness of 1.25 and 5.00 mm, the region that satisfies the basic requirement covers the frequency range from 4.32 to 18.00 GHz in Fig. 3e. With the further increase of the SiC coating, the minimum reflection loss drops as low as 33.65 dB at 17.16 GHz when the sample thickness increases to 2.25 mm in Fig. 3c. On the contrary, the
tan δε ¼ ε’’/ε’
(3)
As observed in Fig. 5d, PyC-coated CF possesses the highest values in the overall frequency range, indicating its robust dielectric loss prop erty. However, after the induction of the SiC coating, the dielectric tangent value range of SiC/PyC-coated CF-I is between 0.47 and 0.75 while that of SiC/PyC-coated CF-II is 0.22–0.59, which is much lower than that of PyC-coated CF. It demonstrates that the starting PyC-coated CF is beneficial to the dielectric loss, which is a suitable template for the following CVD process. Also, the imaginary and real parts of the complex permittivity can be
Fig. 2. XRD pattern of starting PyC-coated CF and SiC/PyC-coated CFs. 3
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Fig. 3. 3D reflection loss spectrum of starting PyC-coated CF (a), SiC/PyC-coated CF-I (b), and SiC/PyC-coated CF-II (c); Corresponding mapping graphs (d, e, and f).
deduced [49,50]: ’’
2
ε ¼ ωτ0 (εs - ε∞)/(1þ(ωτ0) ) ’
2
ε ¼ ε∞ þ (εs - ε∞)/(1þ(ωτ0) )
polarization exists. So the Cole-Cole plot is very simple. As to the SiC/PyC-coated CFs, strong interface polarization generates due to the deposition of the SiC films, and the Cole-Cole plots are much more complicated. The much more complicated Cole-Cole plots in Fig. 6 indicated the more active multiple dielectric polarization and relaxation effects, which results from the dipole and interfacial polarization effects according to the Debye theory. Due to the deposition of the SiC coating, multiple interfaces and junctions were induced in the highly porous structure. Besides, the fragile SiC coating was more likely to generate countless fragments, which further increased the number of interfaces, and the final interfacial polarization. It is widely accepted that not only the remarkable impedance matching but also the favorable attenuation built the outstanding absorber [41]. The attenuation constant (α)is calculated in the expressed equation [52]:
(4) (5)
Where ω, τ0, εs, and ε∞ represent the angular frequency, polarization relaxation time, static permittivity, and relative dielectric permittivity at high-frequency limit. The relationship between these two parts can be simplified based on the formula above: (ε’’)2 þ (εs/2 þ ε∞/2 - ε’)2 ¼ (ε∞/2 - εs/2)2
(6)
Hence, the curve of the imaginary and real parts of the complex permittivity is similar to a single semicircle [51]. The Cole-Cole plots of the PyC-coated CF and SiC/PyC-coated CFs are shown in Fig. 6. In general, the difference in the electrical conductivity or dielectric con stant results in the interface polarization. The CF skeleton and PyC coating are composed of carbon, which means there is little interface
α ¼ 20.5πf ((ε’’μ’’ – ε’μ’) þ ((ε’μ’’ þε’’μ’)2þ(ε’’μ’’ -ε’μ’)2)0.5)0.5/c 4
(7)
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Fig. 4. Reflection loss curves of SiC/PyC-coated CF-I (a) and SiC/PyC-coated CF-II (b) at selected absorber thicknesses; Minimum reflection loss (c) and effective frequency bandwidth (d) contrast diagrams of SiC/PyC-coated CFs at selected absorber thicknesses.
Fig. 5. Real (a) and imaginary part (b) of permittivity of PyC-coated CF, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II; Real and imaginary part of permeability of all the three samples(c); Dielectric loss tangents of all the three samples (d).
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To better understand the nature reason, a plane mechanism is drawn in Fig. 8 to illustrate the microwave absorbing process. Firstly, the 3D network of SiC/PyC-coated CF extends the routes of electromagnetic wave propagation and causes multiple reflections. When the electro magnetic waves reach the surface of the absorber, the high impedance matching guarantees the ratio of the incident waves and most waves enter the absorber with minimal surface reflection due to the deposition of the SiC coating. Then the multi-layer interfaces, SiC films, PyC coat ings, as well as the CF matrix start to work. In addition, the countless interfaces produce a mass of interface electron and dipole polarizations resulting from the unbalanced charge distributions, which increases the absorption of electromagnetic waves. The incident waves are reflected, dissipated, attenuated, and weakened by the boundaries and interfaces. As a result, most parts of the wave energy are transformed into heat energy during the spreading process. The main thing to stress here is that it makes no sense with little incident microwave, which means that the value of the impedance matching ratio might play a crucial role. All in all, it is of vital importance to gain superior impedance matching and appropriate attenuation constant to achieve an excellent microwave absorbing performance [56,57].
Fig. 6. Cole-Cole plots of PyC-coated CF, SiC/PyC-coated CF-I, and SiC/PyCcoated CF-II.
4. Conclusions
The impedance value (Zr) is given based on the following formulas [53–55]: Zr ¼ Zin/Zo
In this article, the SiC/PyC-coated CFs have been developed through the pyrolytic process of MF, the CVI process of CF, and the CVD process of PyC-coated CF. The results indicate that the microwave absorbing performance of the PyC-coated CF is improved by the preparation of the SiC films. Furthermore, the thickness of the SiC coating has an important effect on the enhancement of the absorbing performance as the atten uation constant and impedance matching can be tuned by the SiC films.
(8)
As shown in Fig. 7, PyC-coated CF possesses the highest attenuation constant, which is much higher than that of the SiC/PyC-coated CFs at all the measured frequencies. However, its impedance matching is the lowest, and the microwave absorbing property is also not outstanding according to the aforementioned analysis. On the contrary, the attenu ation constants of the SiC/PyC-coated CFs are not the best, but its excellent impedance matching value guarantees its absorption capacity. Explained in more detail, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II possess the attenuation constants of 285.93 and 203.63 when the min imum reflection loss values are gained. When the matching thickness is 1.40 mm, SiC/PyC-coated CF-I shows a highest impedance matching value of 0.83 at 17.88 GHz. As to SiC/PyC-coated CF-II, the minimum reflection loss is obtained at 17.16 GHz when the matching thickness is 2.25 mm. Coincidentally, the impedance matching value reaches the highest point of 0.96. Although the attenuation constant of SiC/PyCcoated CF-II is lower than that of SiC/PyC-coated CF-I, the higher impedance matching value leads to the more superior microwave absorbing performance. It verifies that the impedance matching plays a much more role on resulting in the finest absorption intensity. On this basis, it is a significant strategy to enhance the absorption performance via tuning the impedance matching to a much higher value, which en sures that most of the electromagnetic waves can enter the absorber. Then, the attenuation constant realizes the absorption of the incident microwave.
Fig. 8. Schematic of microwave absorbing mechanism of SiC/PyC-coated CF.
Fig. 7. Attenuation constants (a) and impedance matching (b) of PyC-coated CF, SiC/PyC-coated CF-I and SiC/PyC-coated CF-II. 6
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The SiC/PyC-coated CF-II possesses an outstanding reflection loss of 33.65 dB at 7.16 GHz when the sample thickness is 2.25 mm, which may be a promising candidate in the microwave absorbing fields.
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