Electromagnetic interference shielding properties of silicon nitride ceramics reinforced by in situ grown carbon nanotubes

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 2467–2475 www.elsevier.com/locate/ceramint

Electromagnetic interference shielding properties of silicon nitride ceramics reinforced by in situ grown carbon nanotubes Meng Chen, Xiaowei Yinn, Mian Li, Lingqi Chen, Laifei Cheng, Litong Zhang Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, West Youyi Rd., No.127, Xi’an 710072, Shaanxi, China Received 28 July 2014; received in revised form 9 October 2014; accepted 10 October 2014 Available online 31 October 2014

Abstract Carbon nanotubes reinforced silicon nitride (CNTs/Si3N4) composite ceramics were fabricated. CNTs were introduced into ceramics by catalytic chemical vapor infiltration. Due to the formation of CNTs, the original isolated pores were connected, which lead to the improvement in the electrical conductivity. When CNT content increased from 0 to 2.7 wt%, total shielding effectiveness (SET) of ceramics increased from 6.0 to 30.4 dB. The values of complex permittivity that obtained from scattering parameters have been used to calculate electrical conductivity and absorbed shielding effectiveness (SEA) of the ceramics. Results had shown that combining CNTs and porous Si3N4 ceramics can be considered as an effective route to design high performance electromagnetic shielding materials. Comparison of SEA calculated from different ways indicated that the multi-reflection effect increased with a higher CNT content. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Carbon nanotubes; Silicon nitride; Catalytic chemical vapor infiltration; Electromagnetic interference shielding effectiveness

1. Introduction In recent years, a variety of research works have been carried out to investigate the electromagnetic interference (EMI) shielding for protecting environment and sensitive circuits from the microwave radiation emitting from telecommunication apparatus [1–6]. Microwave absorbers have been critically needed for lightweight, flexibility, easy fabricating, and low-cost [7,8]. Carbon materials, such as carbon nanotubes (CNTs), carbon black, graphite flakes, carbon fibers and filaments, have attracted increasing interest because of their potential applications in ideal absorbers [1,9,10,]. Adding carbon materials into polymer matrix composites is a well-considered way to fabricate EMI shielding or microwave absorbing materials. However, shielding materials sometimes should work in harsh atmosphere, such as oxidizing and high temperatures, where traditional carbon/polymer matrix composites n

Corresponding author. Tel.: þ86 29 88494947; fax: þ 86 29 88494620. E-mail address: [email protected] (X. Yin).

http://dx.doi.org/10.1016/j.ceramint.2014.10.062 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

are no longer applicable [11]. Silicon nitride (Si3N4) ceramic is suitable to be used in harsh atmosphere, which is attributed to its unique properties such as low density, good oxidation resistance, relatively high hardness, thermal–chemical corrosion resistance, and high thermal shock resistance [12–15]. Adding carbon materials into Si3N4 ceramics is a promising way to fabricate composite ceramics possessing excellent EMI shielding property. Table 1 summarizes the shielding effectiveness (SE) of several typical Si-C-N based ceramics with inclusion of conductive phase, they are promising for blocking EM radiation and reducing or eliminating EMI microwave [16]. C.S. Xiang et al. had prepared dense CNT-fused silica composites by hot-pressing technique [17]. The average magnitude of total shielding effectiveness (SET) reached 33 dB in the 10 vol% CNT-fused silica composites, which indicated the composites had excellent microwave attenuation properties. The attenuation properties mainly originated from the electric loss of CNTs by the motion of conducting electrons. Pyrolytic carbon (PyC) was infiltrated into porous Si3N4 ceramics by chemical vapor infiltration (CVI) [18],

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Table 1 SE of typical Si-C-N-based ceramic materials. Materials

CNTs-SiO2

Si3N4-C-Si3N4

CNTs-PyC-Si3N4

Si3N4-PyC

Si3N4-SiC

Absorber content ρ (g/cm3) P (%) σ (S/m) D (mm) SET (dB) SER (dB) SEA (dB) f (GHz) Ref.

10 vol% CNTs — — — 5.0 33 — — 8.2–12.4 [17]

12 vol% PyC 2.04 31.3 2.6  104 2.8 43.2 21.0 22.2 8.2–12.4 [18]

16.33 wt% PyCþ2.9 wt% CNTs 1.73 39.41 32.1 2.0 43.6 13.6 30 8.2–12.4 [19]

4 vol% PyC 2.7 10.2 4.2 2.0 13–13.5 3.8–5.7 7.5–8.6 8.2–12.4 [20]

11 vol% SiC — — — 0.7, 1.4, 2.8 10, 16, 27 7, 6, 6 3, 10, 21 8.2–12.4 [21]

and dense Si3N4 coating was formed on the surface of the ceramics by chemical vapor deposition (CVD). Owing to the high electrical conductivity value of 2.6  104 S/m, the composite ceramics achieved a high reflection shielding effectiveness (SER) of 21.0 dB and absorption shielding effectiveness (SEA) of 22.2 dB with sample thickness of 2.8 mm. However, higher SER means stronger reflected EM waves, which will interfere with the electronic unit itself, leading to its malfunction and damage. Known from Table 1, although SEA is the main shielding mechanism, SER is still larger than 6 dB when SET is larger than 20 dB because of high electrical conductivity, which is an issue for the materials with a thickness less than 2 mm. In our previous works [19], in order to avoid the agglomeration and damage on CNTs at high fabrication temperatures, CNT reinforced PyC-α-Si3N4 composite ceramics were fabricated through in-situ growth of CNTs by pyrolysis of phenolic resin containing Ni catalyst with precursor impregnation and pyrolysis (PIP) method. SET of ceramic containing 16.33 wt% PyC and 2.94 wt% CNTs reached 43.6 dB when sample thickness was 2 mm. However, SER was as high as 13.6 dB. Therefore, the key problem goes to how to decrease the content of PyC and CNTs to ensure a lower SER but do not decrease SEA drastically. Though a variety of excellent EMI SE had been acquired in Si-C-N based ceramics, the CNTs reinforced β-Si3N4 composite ceramics (CNTs/Si3N4) have not been investigated yet, which could be used in harsh atmosphere as thermal-structural materials. It had been reported that flexural strength and Vickers hardness of porous β-Si3N4 after densified by depositing Si3N4 coating could reach 284 MPa and 10.1 GPa [22], which could meet the requirements of numerous applications. Three methods are usually used to fabricate CNT reinforced ceramics, namely spark plasma sintering (SPS), PIP and CVD [19,23,24]. However, too high fabrication temperature during sintering of Si3N4 may damage the structure of CNTs. As for PIP, the pyrolysis of precursor (e.g. phenolic resin) cannot ensure a fully conversion into CNTs [19]. In the present work, CNTs were introduced into 1.5-mmthickness porous Si3N4 ceramics by catalytic chemical vapor infiltration (CCVI). Electrical conductivity and EMI shielding properties of ceramics with different CNT contents were investigated. Microstructure of ceramics was shown and shielding mechanisms of electromagnetic wave (EMW) were discussed. SE obtained from S-parameters and permittivity

were also compared over X-band (8.2–12.4 GHz) to investigate effects of multi-reflection. 2. Materials preparation and methods 2.1. Fabrication of porous Si3N4 ceramics Si3N4 powder (α-Si3N4 495 wt%, Sio0.1 wt%) with a mean particle size of 1 μm was supplied by Kingsway Fine Ceramic Shanghai, China. Lu2O3 powder with a purity of 99.99% was supplied by Hongfu New Materials Corp., Gan Zhou, China. The Si3N4 powder was mixed with Lu2O3 powder with a weight ratio of 95:5, and then mixed with phenolic resin (particle size of 100–200 mesh, density 1.30 g/cm3) in ethanol into slurry. The slurry was ball milled for 24 h with a rotate speed of 100 r/min, and then dried for 10 h at 100 1C. The asreceived powder blend was crushed and passed through a 80 mesh sieve. The as-received powder blend was cold pressed into green bodies under a pressure of 70 MPa. The green bodies were oxidized in air at 700 1C for 2 h at a heating and cooling rate of 5 1C/min. During oxidation, phenolic resin was decomposed, oxidized, and removed in the form of CO2, which would form the porous preform. Afterward, the porous preforms were sintered in a hot-pressing furnace (High-Multi 5000, Fujidempa Kogyo, Osaka, Japan) at 1800 1C for 2 h under a N2 atmosphere pressure of 0.3 MPa. The as-sintered porous ceramics were polished into dimensions of 22.86 mm (length)  10.16 mm (width)  1.5 mm (thickness) for S-parameters and resistance testing. The properties of porous Si3N4 were shown in Table 2. 2.2. Preparation of CNTs/Si3N4 composite ceramics Cobaltous acetate tetrahydrate were dissolved into deionized water with concentration of 0, 2, 4, 6 and 10 wt%. Afterward, the samples were impregnated into the solutions and denoted as S0, S1, S2, S3, and S4. After dried in air, the samples were placed into a tube furnace to obtain CNTs. Ar was used as carrier gas to introduce acetone vapor into the furnace. The deposition temperature and time were 550 1C and 60 min, respectively. CNTs would not form into S0 because there was no catalyst in it, but a small quantity (about 0.2 wt%) of PyC would be introduced into S0 due to the pyrolysis of acetone vapor.

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Table 2 Properties of samples. Property

Concentration of catalyst (wt%) Open porosity (%) Content of CNTs (wt%)

Samples S0

S1

S2

S3

S4

0 40.32 0

2 41.68 1.49

4 42.45 1.97

6 38.48 2.34

10 39.98 2.70

2.3. Characterization of ceramics Open porosity of ceramics was tested according to Archimedes method. Carbon contents were calculated according to weight change of ceramics before and after CCVI. The microstructure of samples was observed by scanning electron microscopy (SEM, S-4700, Hitachi, Japan). The pore size distribution was measured using a Mercury Porosimeter (Poremaster 33, Quantachrome Instruments Corporation, Boynton Beach, FL,). Transmission electron microscope (TEM, G-20, FEI-Tecnai, Hillsboro, USA) was employed to characterize the microstructure of CNTs. X-ray diffraction (XRD) was employed for phase analysis (X’Pert Pro, Philips, Netherlands). INVIN Raman spectro-scopy (RMS; Renishaw, UK) was used to characterize the graphitization degree of samples. The scattering parameters (S-parameters: S11, S12, S22 and S21) of the ceramics in the frequency range 8.2 to 12.4 GHz were measured by a vector network analyzer (VNA; MS4644A, Anritsu, Kana-gawa, Japan) using the waveguide method. The shielding effectiveness was calculated according to the following equations: SER ¼  10log10 ð1 RÞ

ð1Þ

SE A ¼  10log10 ðT=ð1  RÞÞ

ð2Þ

SE T ¼ SE R þ SE A

ð3Þ

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graphitization degree of the sample exhibits [25]. As shown in Fig. 1b, the R values of samples S0 to S4 are 0.93, 0.90, 0.84, 0.79, and 0.75, respectively. Hence the Raman spectra suggests that in situ formation of CNTs in the ceramics lead to a higher microcrystal order of carbon and its graphitization degree is improved. The pore-size distribution of the porous Si3N4 ceramic is shown in Fig. 2. As can be seen, the Si3N4 ceramic shows a monomodal pore size distribution. The pore size of the ceramic is among 0.4 to 3 μm. It can be observed from Fig. 3 that CNTs are distributed uniformly in ceramics. TEM investigation (Fig. 3e and f) reveals that the product is hollow nanotube with a straight body. The diameter of CNTs is about 10 nm. Co nanocage is encapsulated by tube walls at the tube tip, which indicates the vapor-liquid-solid (VLS) growth mechanism of CNTs [26,27,28]. High resolution TEM image (Fig. 3f inset) shows that CNT has a multilayered structure and exhibits a high crystallinity. 3.2. Effect of CNT on complex permittivity and conductivity The real part (εˊ) and imaginary part (ε〞) of complex permittivity is shown in Fig. 4. The higher the CNT content is, the greater the permittivity of the ceramic exhibites. A higher

where R and T were calculated on the basis of R ¼ jS11 j2 and T ¼ jS21 j2 . 3. Results and disscussion 3.1. Characterization of CNTs and microstructure of samples Since CNTs occupy very limited amount in ceramics and β-Si3N4 peak at 27 1 is too high to disturb the nearby carbon peak, which lead to carbon peak at 26.61 do not appear. As shown in Fig. 1a, XRD pattern demonstrates that only β-Si3N4 and Lu4Si2O7N2 peaks are detected, indicating a completely transformation of α-Si3N4 into β-Si3N4. For carbon material, there are two main bands in Raman spectra. One is at about 1580 cm-1 (G band) corresponding to signal from graphite and characterizing for the integrality of sp2 hybrid orbital structure. The other is at 1330 cm-1 (D band) which is associated with integrality of carbon crystallite and the presence of defects. The graphitization degree of composites can be determined by the ID/IG ratio (defined as R). The lower R is, the higher the

Fig. 1. XRD pattern of S3 (a) and Raman spectroscopy curves of samples (b).

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CNT content could lead to a greater interfacial polarization effect, which can account for increasment of εˊ with the rise of the CNT [29]. The imaginary part ε〞is mainly due to the loss

effects, of which the most common one is the conduction currents in the dense CNT network [30]. Quantum effects and tunneling currents between close CNTs is another reason for the loss effect which is due to the power dissipation phenomena. The electrical conductivity is obtained from imaginary part of permittivity by the follwing formula: σ ¼ 2πf ε0 ε″

ð4Þ  12

Fig. 2. Pore size distribution of the porous Si3N4 ceramic.

where f is frequency, ε0 ¼ 8:854  10 F=m and represents for vacuum permittivity. As illustrated in Fig. 5, electrical conductivity increases with the rise of CNT content, which is attributed to the connectivity of CNTs in ceramics. There are two kinds of CNT morphologies generated in ceramics (shown in Fig. 3). One is feather-like CNTs which are distributed on Si3N4 grains, the other is pore-connecting CNTs which connect the pores. The growth of CNTs started from Si3N4 grains or pores is due to the existence of catalyst particles. Those CNTs started from grains would encase the rods and form feather-like CNTs while those CNTs started from pores would grow randomly and connect the adjacent pores.

Fig. 3. SEM and TEM images, fracture surface images of S0 (a), S1 (b), S2 (c) and S4 (d); (e) is the TEM image of CNTs, and (f) is the magnifying picture of CNTs, inset shows the high resolution TEM image of tube walls.

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3.3. Analysis on shielding effectiveness of CNT reinforced ceramics The shielding effectiveness of a ceramic is defined by the ratio between the incident power PI and the transmitted power PT SE ¼ 10 logðPI =PT Þ

ð5Þ

and can be inferred from waveguide measurements or simulations taking the opposite sign of the S21 scattering parameter. The intrinsic limits of this method are mainly the constraints imposed by the thickness of the waveguide sample holder and by the EM field propagation, which is different from what occurs in free space [30–32]. Conventionally, in such frame the SE is referred to a plane wave propagating in the normal direction with respect to the material surface. The plane wave condition can be applied when the EM source is ‘electromagnetically far’ from the material. The knowledge of the dielectric parameters allows to compute the EM wave impedance, the EM wave number and, consequently, the reflection coefficient (RC) and the transmission coefficients (TC) for any desired wall thickness, where the quantity –TC represents the SE. The loss factor (LF) within the material is the fraction of the incident power lost in the material. It can be computed from RC and TC using the power conservation law: LFð%Þ ¼ 100ð1 jRCj2  jTCj2 Þ

Fig. 4. Electrical permittivity of ceramics.

Fig. 5. Electrical conductivity of ceramics.

The specific structure of CNTs turns the CNT-clad β-Si3N4 grains into current-conducting rods. Moreover, due to the randomly distributed β-Si3N4 rods, CNTs in ceramics easily form a threedimensionally connected conductive network which is in favor of electron transport and the conductivity of the ceramics increases immensely when compared to pure Si3N4.

ð6Þ

where RC and TC are expressed in linear scale. The RC, SE, and LF are shown in Fig. 6. The RC value of S0 is the lowest, the reasons might be the deeper propagation of the EM field through the S0 sample. The higher wave impedance of S0 could also lead to lesser reflection at the air– ceramic interface, which is another reason for lower RC of S0. As for LF values of S0 and S1, since CNT is introduced into S1, more EM wave was reflected and lesser wave transmitted, so that LF values matain at the same level. With a higher concentration of CNTs, the electrical conductivity of the ceramic increases, which provides a higher impedance mismatching with respect to the air and more EM wave is reflected. Consequently, a lower absorption is obtained and leading to a lower LF as a result. Fig. 7 shows SER and SEA calculated from S-parameters by Eqs. (1) and (2). A great deal of interacting mobile charge carriers resulting from higher concentration of CNTs could lead to a higher EMI SEA, which has been demonstrated [5,33–36]. Additionally, the connectivity of CNTs in ceramics could also contribute to the shielding by absorption [5]. As there is a direct relationship between electrical conductivity and shielding by reflection in conductive monolithic materials, a higher SER is expected in high-concentration CNT containing samples, which can also be related to higher amount of mobile charge carriers at greater CNT concentrations [37,38]. It is worth pointing that when CNT content reaches to 2.7 wt %, SER is less than 8.5 dB and SEA is higher than 22 dB, of which the similar results have never been reported in a ceramic thinner than 2.0 mm.

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Fig. 7. SER (up) and SEA (down) calculated from S-parameters using waveguide method.

3.4. SEA calculated from permitivity

Fig. 6. Reflection coefficient (RC, up), shielding effectiveness (SE, middle), and loss factor (LF, down) of ceramics in free space.

As a whole, waveguide method can be successfully applied to calculate SE over X-band (8.2–12.4 GHz) as reported in most of the scientific publications [7,9,11,17,18,30]. In order to obtain a more accurate SE value at lower frequency (e.g. 0.75–1.12 GHz), future works should aim at diminishing any error by precisely controlling the influence factors such as dimension of sample, equipment calibration, testing temperature and environmental humidity, which may affect the measured data.

SE is defined as the sum of SER, SEA and SEM. SER is due to the initial reflection at the air–material interface, SEA is the penetration loss within the material, and SEM represents the internal reflections between internal surfaces of the shielding material. The SEA can be calculated through the follwing equation, considering that the EM plane wave was normally directed to the surface and propagating through an infinite transversely extended wall
 E0 E0 SEA ¼ 20log10 ¼ 20log10 Et E0 eRealðjkÞz
 1 ¼ 20log10 αz ð7Þ e pffiffiffiffiffi pffiffiffiffiffiffiffiffiffipffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jk ¼ j2πf με ¼ j2πf μ0 ε0 μr ðε0  ε″Þ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ε″ ¼ j2πf μ0 μr ε0 ε0 1  j 0 ¼ αþ jβ ε

ð8Þ

where k is the complex wave number, z is the wall thickness, E0 is the incident electric field and Et is the transmitted electric field. μ0 equals 4π  10  7 Hm  1 and μr is the relative magnetic permeability (μr=1) [30]. Since the propagation of

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Fig. 9. Shielding mechanism of EMWs in CNTs/Si3N4 ceramics.

Fig. 10. Power balance as a function of CNT content.

Fig. 8. SEA calculated from permittivity as a function of sample thickness at 12.0 GHz (a), comparison of SEA calculated from different methods (b).

the EM field follows the exponential law e-jkz, the real part α defines the attenuation factor while the imaginary part β defines the phase shift, and β does not influence the amplitude. Fig. 8a shows the value of SEA at 12.0 GHz. SEA increased proportionally to the thickness of samples. As shown in Fig. 8b, an error is introduced when comparing SEA values obtained from permittivity and S-parameters. The difference might be attributed to definition of SEA in two methods. SEA calculated from S-parameters considers the contribution of multi-reflection while the other does not. As shown in Fig. 8b, a higher concentration of CNTs could lead to a higher multireflection effect, which can be attributed to the more interfaces that CNTs bring in. As a whole, the results indicate that in situ growth of CNTs in porous ceramic can be successfully employed to obtain high EM shielding composite ceramics when adjusting thickness of sample.

3.5. Mechanism of EMWs and power balance Fig. 9 demonstrates mechanisms of EMWs in CNTs/Si3N4 composite ceramics. Due to the high impedence mismatch

between free space and ceramics, most of the incident EMWs would be reflected when EMWs enter ceramics. Absorption is the other mechanism of EMWs shielding, and the materials have electrical loss endowed by CNTs under the alternating EM field to attenuate the entered EMWs. The entered EMWs would be absorbed by CNT filler in ceramics and hardly could any EMW be transmitted through the composite ceramics. Since the CNT/Si3N4 ceramic is porous and CNTs are distributed uniformly in ceramics, multi-reflection easily emerges and it is able to affect the overall SE due to effect of all CNTs or cluster of them. The multi-reflection effect cannot be neglected in the CNT/Si3N4 system, which is believed to be because of the small size (small diameter and large aspect ratio) of CNTs in conjunction with the skin effect [39]. CNTs can enhance effects such as percolation, resistive losses, tunneling currents, interfacial effects, and so on, thus leading to EMWs depleted and then transformed into heat energy so that the wave cannot be permeated through the material [40–42]. The power balance in Fig. 10 clearly shows that the transmitted power has a sharp decrease when CNTs were introduced into ceramics, and it tends to 0 mW with the increase of CNT content. Due to the introduction of CNTs, the absorbed power increased initially and then decreased gradually. The gradual decrease of the absorbed power is due to the

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lesser power entering ceramics. The contribution of absorption to the overall shielding should be based on the ability of the material to attenuate the power that has not been reflected. Though the fabricated ceramics have high intrinsic absorption capabilities due to the formation of CNTs, most of the incident wave was reflected because reflection took place before absorption. In order to decrease the reflected power of the materials, a multilayered material of graded electrical conductivity is expected to be designed in the future [43]. 4. Conclusion CNTs were in situ grown into porous β-Si3N4 ceramics. CNT content increased with the increase of the catalyst concentration. The formation of CNTs led to the improvement in electrical conductivity. An EMI SET of 30.4 dB was achieved when CNTs reached to 2.7 wt% of ceramic. Due to the well-distributed CNTs and their high surface area, even 1.97 wt% of CNTs could lead to a considerable SET of 20.5 dB with a lower SER of 5.7 dB and higher SEA of 15.8 dB. The values of complex permittivity obtained from scattering parameters had been used to calculate electrical conductivity, electromagnetic attenuation, and reflection of the ceramics. The content of CNTs and thickness of samples played the main roles in SEA. Comparison of SEA calculated from different ways indicated that the multi-reflection effect increased with a higher CNT content. Acknowledgment This work is supported by the Natural Science Foundation of China (Grant: 51332004 and 51221001), the Program for New Century Excellent Talents in University (NCET-08– 0461), and the 111 Project (B08040). References [1] M.S. Cao, W.L. Song, Z.L. Hou, B. Wen, J. Yuan, The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and microwave-absorption of short carbon fiber/ silica composites, Carbon 48 (2010) 788–796. [2] B. Fugetsu, E. Sano, M. Sunada, Y. Sambongi, T. Shibuya, X.S. Wang, Electrical conductivity and electromagnetic interference shielding efficiency of carbon nanotube/cellulose composite paper, Carbon 46 (2008) 1256–1258. [3] Y. Yang, M. Gupta, K. Dudley, R. Lawrence, Novel carbon nanotubepolystyrene foam composites for electromagnetic interference shielding, Nano. Lett. 11 (2005) 2131–2134. [4] Y. Huang, N. Li, Y. Ma, D. Feng, F. Li, X. He, X. Lin, H. Gao, Y. Chen, The influence of single-walled carbon nanotube structure on the electromagnetic interference shielding efficiency of its epoxy composites, Carbon 45 (2007) 1614–1621. [5] M. Al-Saleh, U. Sundararaj, Electromagnetic interference shielding mechanisms of CNT/Polymer composites, Carbon 47 (2009) 1738–1746. [6] D.D.L. Chung, Carbon materials for structural self-sensing, electromagnetic shielding and thermal interfacing, Carbon 50 (2012) 3342–3353. [7] Q. Li, X. Yin, W.Y. Duan, L. Kong, X.M. Liu, L.F. Cheng, L.T. Zhang, Improved dielectric and electromagnetic interference shielding properties of ferrocene-modified polycarbosilane derived SiC/C composite ceramics, J. Eur. Ceram. Soc. 34 (2014) 2187–2201.

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[31]

[32]

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