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mullite composites fabricated by spark plasma sintering
Ceramics International 45 (2019) 18988–18993
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Enhanced electromagnetic shielding property of cf/mullite composites fabricated by spark plasma sintering
T
Lan Longa,b, Peng Xiaoa, Heng Luoc, Wei Zhoub,∗, Yang Lia,∗∗ a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China College of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou, 412008, China c School of Physics and Electronics, Central South University, Changsha, 410083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon fibers Mullite Electromagnetic interference shielding Spark plasma sintering
Aiming to prepare high-performance electromagnetic interference (EMI) shielding materials, chopped carbon fibers were incorporated into mullite ceramic matrix via rapid prototyping process of spark plasma sintering (SPS). Results indicate that Cf/mullite composites with only 1 wt% of carbon fibers exhibit highest shielding effectiveness (SET) over 40 dB at a small thickness of 2.0 mm, showing great advantages both in terms of performance and thickness compared with many mature carbon/ceramic composites. The high EMI shielding properties mainly depend on two mechanisms of absorption and reflection in this present work. The enhanced absorption and reflection of electromagnetic wave are ascribed to the promotional electrical conductivity arising from the formation of conductive network by introduction of carbon fibers. Regarding enhanced electrical conductivity, notable intensified interfacial polarization on a large number of interfaces between mullite matrix and carbon fibers is also the key factor to the improved absorption, which makes absorption play a dominant role in the significant improvement of EMI SET. The Cf/mullite composites with excellent EMI shielding properties and thin thickness show great potential application as EMI materials.
1. Introduction Electromagnetic pollution is an increasingly severe problem as electronic devices and telecommunication technologies develop rapidly [1-4]. EMI shielding materials were well developed as a key solution to protect human body and sensitive electronic devices away from electromagnetic radiation [5-9]. Recently, EMI shielding materials with lightweight, favorable mechanical properties, and high temperature resistance and anti-oxidation, have been growing demanded for some special applications [10-12], for instance, aerospace. In those cases, due to highly probable exposure to high-temperature and oxidizing environment, metal-based composites [13-16] and polymer-based composites [17-20] are no longer available. Ceramic-based composites with low density and high-temperature resistance, have gained attentions to be developed for high-performance EMI shielding at high temperatures [21-24]. Mullite (3Al2O3.2SiO2) ceramics, as high-temperature structural ceramic materials, with outstanding properties such as low density, low dielectric constant, good chemical resistance, thermal stability and creep resistance, are considered to be prior candidates used in high-
∗
temperature and oxidizing environment [25,26]. Moreover, the permittivity of mullite ceramics can be tailored by doping iron [27] or ZnO [28]. With these fillers, EMI shielding performance will be enhanced. Generally, EMI shielding effectiveness is proportional to the electrical conductivity of material [29]. Therefore, highly conductive fillers are usually introduced into ceramics to achieve favorable EMI shielding performance [12,30-32]. In recent years, carbon fibers with low density, excellent mechanical property and high electrical conductivity, show great potential application in EMI materials due to their low price and ease of use [33-37]. Many research works showed that composites containing carbon fibers can be used as ideal EMI shielding materials with low density and strong absorption [36,38,39]. Therefore, carbon fiber reinforced mullite (Cf/mullite) composites are generally believed as promising candidates to be used for EMI shielding applications due to the good combination performance. However, few investigations concerning EMI shielding properties of Cf/mullite composites are carried out. Recently, spark plasma sintering (SPS) with advantages of low sintering temperature and short dwell time, has been considered as a promising approach for preparation of modulated ceramics and become
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Zhou), [email protected] (Y. Li).
∗∗
https://doi.org/10.1016/j.ceramint.2019.06.139 Received 2 June 2019; Received in revised form 13 June 2019; Accepted 14 June 2019 Available online 15 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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a hot research field [40-42]. Compared with traditional sintering methods, for instance, hot pressing, SPS can not only achieve rapid sintering, but also obtain novel composites with unique microstructures and characteristics [43,44]. Therefore, SPS can facilitate the preparation of mullite based ceramics with excellent properties. In present work, low cost Al2O3 and SiO2 were used as raw materials for fabrication of mullite ceramics. Carbon fibers were introduced into mixed powder of Al2O3 and SiO2 to prepare Cf/mullite composites with enhanced EMI shielding properties by spark plasma sintering. Influence of carbon fibers content on microstructure and EMI shielding properties of Cf/mullite composites were investigated in details. 2. Experimental procedures 2.1. Materials preparation Commercially available PAN based carbon fibers (T700, 7 μm, 12 K, Tianniao Company, Jiangsu, China) were used in this study. Al2O3 powders (purity > 99 %, grain diameter: 45-75 μm), SiO2 powders (purity > 99 %, grain diameter: 10 μm) were used as starting materials. Y2O3 powders (purity > 99 %, grain diameter: 10 μm) were added into Al2O3 and SiO2 powders as sintering additives with mass fraction of 10 %. All these powders mentioned above are supplied from Sinopharm Chemical Reagent Co., Ltd. The Cf/mullite composites with different contents of carbon fibers (0, 1, and 2 wt%), denoted as M0, MC1 and MC2, respectively, were prepared. Firstly, Al2O3, SiO2 and Y2O3 powders were mixed by ballmilling using ethanol as medium in a zirconia jar for 4 h at 250 rpm. The molar ratio of Al2O3 to SiO2 was 3:2. Secondly, carbon fibers with length of 2-3 mm were added into mixture slurry of Al2O3, SiO2 and Y2O3 powders by mechanical stirring for 1 h to obtain uniform mixture slurry. Then the final mixture slurry was dried at 80 oC in an oven for 24 h. Finally, the dried mixed powders were put into a circular graphite mold with an inner diameter of 60 mm, and the densification was performed in a vacuum SPS chamber (HPD 25-3, Germany) at 1500 oC under uniaxial pressure of 40 MPa with the soaking time of 10 min. For a comparison, the pure mullite ceramic sample without carbon fibers (M0) was also prepared by same SPS. The density and porosity of asprepared samples M0, MC1 and MC2 are presented in Table 1. 2.2. Characterizations Open porosity and bulk density of composites were measured by Archimedes method in deionized water as immersion medium according to DIN EN 1389:2003. Phases of vibratory disc-milled powder samples were analyzed by X-ray diffraction (XRD, D/max2550, Rigaku) with Cu Kα radiation. The morphology and microstructure of samples were examined by electron microscopy (SEM, Nova NanoSEM 230). The complex permittivity (εr = ε′-jε″) of samples with size of 22.86 mm × 10.16 mm × 2 mm was measured via a vector network analyzer (Agilent N5230A) in X-band according to wave guide method. The obtained scattering parameters (S11, S12, S22 and S21) were used to calculate the total EMI shielding effectiveness (SET), absorbing shielding (SEA) and reflection shielding (SER) by following equations [45]: SET=SER + SEA
(1)
Table 1 Density and porosity of samples M0, MC1 and MC2. Samples
Density (g/cm3)
Open Porosity (%)
Number of tested specimens
M0 MC1 MC2
3.33 ± 0.03 3.17 ± 0.03 2.74 ± 0.01
1.3 ± 0.2 3.4 ± 0.1 8.1 ± 0.2
N=2 N=2 N=2
SEA = −10log10(T/(1-R))
(2)
SER = −10log10(1-R)
(3)
where reflection coefficient (R) and transmission (T) were calculated on the basis of R = |S11|2 and T = |S21|2. 3. Results and discussion 3.1. Morphological and phase compositions Typical micro-morphologies of mullite based composites with different contents of carbon fibers are presented in Fig. 1(a)-(c) As displayed in Fig. 1(a), M0 exhibits a relatively dense structure and mullite grains with smooth surface are closely interconnected (insert in Fig. 1(a)). After introducing carbon fibers (Fig. 1(b)), carbon fibers are seemingly of uniform distributions in mullite matrix. Note that, the well-protected carbon fibers without chemical corrosion are observed (insert in Fig. 1(b)), indicating that carbon fibers are free of chemical reactions with oxides during sintering process. The integrality in structure of carbon fibers can be in favor of EMI shielding properties. As carbon fibers content increases (Fig. 1(c)), more carbon fibers are in relatively uniform dispersion, and well consolidated with mullite matrix. Furthermore, the gaps within carbon bundles are filled by mullite matrix (insert in Fig. 1(c)). Fig. 1(d) presents the XRD patterns of MC2. Diffraction peaks with typical characterization of splitting of crystal planes (120) and (210) at 26° appeared in MC2 are well indexed to mullite phase (orthorhombic, PDF#74-2419), indicating completely mullitization reaction between Al2O3 and SiO2 as precursor. In addition, no carbon peaks were detected in MC2 sample mainly due to incomplete crystallization of carbon fibers. 3.2. EMI shielding properties In general, EMI shielding properties can be regarded as attenuation ability of propagating electromagnetic radiation [46]. Total EMI shielding effectiveness (SET) depends on three main mechanisms: absorption (SEA), reflection (SER) and multiple reflections (SEM) [47]. However, SEM can be neglected when absorption shielding effectiveness (SEA) is higher than 10 dB [48]. Thus, SET can be expressed by equation (1). Fig. 2 displays EMI shielding properties of mullite based composites with different content of carbon fibers. As can be seen from Fig. 2 (a), pure mullite ceramics with low average EMI SET of ∼ 5.3 dB show extremely weak ability to block the electromagnetic radiation. With the increased content of carbon fibers in matrix, EMI shielding properties of mullite based composites increase rapidly. When 1 wt% carbon fibers were introduced into mullite, the highest EMI SET (higher than 40 dB) is obtained, indicating outstanding EMI shielding properties for Cf/mullite composites within the whole measured frequency region. However, as carbon fibers content increased to 2 wt%, the EMI SET decreased slightly to ∼ 37 dB compared with the Cf/mullite composites with 1 wt % carbon fibers. Both the two as-prepared Cf/mullite composites in present work show the EMI SET higher than 30 dB, demonstrating that more than 99.9 % of incident electromagnetic radiation can be shielded within the whole measured X band. This result is close to the EMI shielding performance required for commercial applications [46]. As mentioned above, due to the high value of absorption shielding effectiveness, absorption (SEA) and reflection (SER) are considered as two main contributions for the EMI SET in this work. The diagrammatic illustration is presented in Fig. 3. Moreover, known from Fig. 2(b) and (c), SEA with much higher value in comparison to SER plays a dominant role in the high EMI SET. Taken the Cf/mullite composites with 1 wt% carbon fibers for example, the EMI SET is around 41 dB and SEA is about 34 dB, while the SER is merely about 7 dB within the measured frequency range. The improved EMI shielding properties for Cf/mullite composites
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Fig. 1. SEM images of M0 (a), MC1 (b) and MC2 (c), and XRD patterns of MC2 (d).
Fig. 2. (a) EMI SET, (b) SER, (c) SEA and (d) microwave impedance of mullite based composites with different content of carbon fibers. 18990
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Table 2 Comparison of EMI shielding performance of various carbon/ceramic composites.
Fig. 3. Schematic illustration of EMI shielding mechanism for Cf/mullite composites.
could be mainly attributed to conductive network made of carbon fibers and the enhanced electrical conductivity (EC) by addition of carbon fibers into the insulating mullite matrix [49]. Within the conductive network, electrons and other charge carriers can move freely to interact with the incident electromagnetic radiation and dissipate the electromagnetic energy as form of heat [48], leading to the enhanced SER. Most importantly, the promotional electrical conductivity arising from the formation of conduction network is the key factor to determine the EMI shielding performance [50]. According to EMI theory, SEA and SER can also be expressed as [51]:
SEA = 8.7d πfμσ SER = 39.5 + 10 log
(4)
σ 2πfμ
(5)
where f is the frequency, μ is the permeability, d and σ are the thickness and EC of the shield material. The EC can be obtained according to the equation [52]: σ = 2πfε0ε″, where ε0 is the permittivity in vacuum, and ε″ is the imaginary part of permittivity of the shield material. As shown in Fig. 4, as carbon fibers content increased from 0 to 2 wt%, the EC significantly increased due to the remarkably strengthen conductive network in composites. Therefore, both SEA and SER obviously increased (in Fig. 2(b) and (c)) based on the above equations (4) and (5), leading to the outstanding EMI shielding properties for Cf/mullite composites. In addition, notable intensified interfacial polarization in Cf/mullite composites also makes a significant contribution to the absorption (SEA), which results from the obviously increased dipoles and charge accumulation on a large number of interfaces between mullite matrix and carbon fibers [48,53]. Therefore, SEA shows much higher
Filler
Matrix
Filler loading
Thickness (mm)
EMI SET (dB)
Ref
Carbon fiber Carbon fiber PyC Pyrolytic carbon PyC CNTs-PyC CNWs-CNTs CNTs CNTs r-GO GN Carbon fiber
SiC PyC-SiC SiC Si3N4
40 vol% 27.7–28.5 wt% 22.4–36.6 wt% 12.1 vol%
3.0 2.0 1.8 2.8
31.4–42.2 34–42 19.2–29.0 43.2
[58] [55] [56] [57]
Si3N4 Si3N4 Si3N4 Si3N4 SiO2 SiO2 Al2O3 mullite
4.0 vol% 19.2 wt% 3.91 wt% 2.7 wt% 10 wt% 20 wt% 2.0 vol% 1 wt%
2.0 2.0 2.0 1.5 2.5 1.5 1.5 2.0
∼ 13.0 43.6 25.4 ∼ 30.5 ∼ 21.7 ∼ 34.5 ∼ 23 ∼ 41
Carbon fiber
mullite
2 wt%
2.0
∼ 37
[61] [59] [31] [30] [60] [6] [62] This work This work
value and contributes much more to the total EMI shielding performance compared with SER. However, as the content of carbon fibers further increases, the microwave impedance keeps decreasing (see Fig. 2(d)), leading to the deterioration of impedance matching between the Cf/mullite composites and air. Reflection (SER) of electromagnetic radiation is mainly caused by the impedance mismatch between air and shield material [54]. Due to reflection, the amount of electromagnetic radiation entering and interacting with shield material is reduced, which leads to the weak absorption (SEA). Therefore, with the increase of carbon fibers addition from 1 to 2 wt%, MC2 shows higher values of SER but lower values of SEA and SET compared with MC1 due to the decreased impedance. In order to further evaluate the shielding performance of the asprepared Cf/mullite composites, a comparison between various carbon/ ceramic composites for EMI shielding previously reported in literature and our work within X band was carried out and the results was shown in Table 2. All listed carbon/ceramic composites show a favorable EMI shielding performance. However, many merits of these composites will be weaken due to the high loading of carbon fillers (higher than 10 vol % or10 wt%) and big thickness (thicker than 2.0 mm) [6,55-60]. Thus, the Cf/mullite composites in this study show great advantages in terms of being high EMI shielding properties and small thickness of 2.0 mm, compared with the previously reported carbon/ceramic composites in Table 2. This result further demonstrates that the proposed Cf/mullite composites with superior shielding effectiveness have great potential for application as high-performance EMI shielding materials.
4. Conclusion Cf/mullite composites with different weight fractions of carbon fibers were prepared by rapid preparation technique of spark plasma sintering for exploring new high-performance EMI shielding materials. As weight fraction of carbon fibers increased, the EMI SET of the mullite based composites significantly increased and a high value (> 40 dB) close to the level of commercial EMI shielding was achieved with merely 1 wt% carbon fibers. Notable improved absorption (SEA) originating from tremendous increased EC and interfacial polarization is mainly responsible for the significant improvement of EMI shielding properties for Cf/mullite composites. Finally, current work indicates that the Cf/mullite composites with superior EMI shielding properties can be used as high-performance ceramic-based materials in vast academic and industrial areas. Fig. 4. Electrical conductivity of mullite based composites with different content of carbon fibers. 18991
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51604107), and the Natural Science Foundation of Hunan Province (Grant No. 2019JJ50115 and 2019JJ50768). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.139. References [1] Q. Song, F. Ye, X. Yin, W. Li, H. Li, Y. Liu, K. Li, K. Xie, X. Li, Q. Fu, Carbon nanotube-multilayered graphene edge plane core-shell hybrid foams for ultrahighperformance electromagnetic-interference shielding, Adv. Mater. 29 (2017) 1701583. [2] H. Lv, Y. Guo, G. Wu, G. Ji, Y. Zhao, Z.J. Xu, Interface polarization strategy to solve electromagnetic wave interference issue, ACS Appl. Mater. Interfaces 9 (2017) 5660–5668. [3] M.R. Tohidifar, Highly-efficient electromagnetic interference shielding and microwave dielectric behavior of a (Bi2O3 + B2O3)-doped MWCNT/BaTiO3 ceramic nanocomposite, Ceram. Int. 44 (2018) 13613–13622. [4] Y. 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Enhanced electromagnetic shielding property of cf/mullite composites fabricated by spark plasma sintering
T
Lan Longa,b, Peng Xiaoa, Heng Luoc, Wei Zhoub,∗, Yang Lia,∗∗ a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China College of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou, 412008, China c School of Physics and Electronics, Central South University, Changsha, 410083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon fibers Mullite Electromagnetic interference shielding Spark plasma sintering
Aiming to prepare high-performance electromagnetic interference (EMI) shielding materials, chopped carbon fibers were incorporated into mullite ceramic matrix via rapid prototyping process of spark plasma sintering (SPS). Results indicate that Cf/mullite composites with only 1 wt% of carbon fibers exhibit highest shielding effectiveness (SET) over 40 dB at a small thickness of 2.0 mm, showing great advantages both in terms of performance and thickness compared with many mature carbon/ceramic composites. The high EMI shielding properties mainly depend on two mechanisms of absorption and reflection in this present work. The enhanced absorption and reflection of electromagnetic wave are ascribed to the promotional electrical conductivity arising from the formation of conductive network by introduction of carbon fibers. Regarding enhanced electrical conductivity, notable intensified interfacial polarization on a large number of interfaces between mullite matrix and carbon fibers is also the key factor to the improved absorption, which makes absorption play a dominant role in the significant improvement of EMI SET. The Cf/mullite composites with excellent EMI shielding properties and thin thickness show great potential application as EMI materials.
1. Introduction Electromagnetic pollution is an increasingly severe problem as electronic devices and telecommunication technologies develop rapidly [1-4]. EMI shielding materials were well developed as a key solution to protect human body and sensitive electronic devices away from electromagnetic radiation [5-9]. Recently, EMI shielding materials with lightweight, favorable mechanical properties, and high temperature resistance and anti-oxidation, have been growing demanded for some special applications [10-12], for instance, aerospace. In those cases, due to highly probable exposure to high-temperature and oxidizing environment, metal-based composites [13-16] and polymer-based composites [17-20] are no longer available. Ceramic-based composites with low density and high-temperature resistance, have gained attentions to be developed for high-performance EMI shielding at high temperatures [21-24]. Mullite (3Al2O3.2SiO2) ceramics, as high-temperature structural ceramic materials, with outstanding properties such as low density, low dielectric constant, good chemical resistance, thermal stability and creep resistance, are considered to be prior candidates used in high-
∗
temperature and oxidizing environment [25,26]. Moreover, the permittivity of mullite ceramics can be tailored by doping iron [27] or ZnO [28]. With these fillers, EMI shielding performance will be enhanced. Generally, EMI shielding effectiveness is proportional to the electrical conductivity of material [29]. Therefore, highly conductive fillers are usually introduced into ceramics to achieve favorable EMI shielding performance [12,30-32]. In recent years, carbon fibers with low density, excellent mechanical property and high electrical conductivity, show great potential application in EMI materials due to their low price and ease of use [33-37]. Many research works showed that composites containing carbon fibers can be used as ideal EMI shielding materials with low density and strong absorption [36,38,39]. Therefore, carbon fiber reinforced mullite (Cf/mullite) composites are generally believed as promising candidates to be used for EMI shielding applications due to the good combination performance. However, few investigations concerning EMI shielding properties of Cf/mullite composites are carried out. Recently, spark plasma sintering (SPS) with advantages of low sintering temperature and short dwell time, has been considered as a promising approach for preparation of modulated ceramics and become
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Zhou), [email protected] (Y. Li).
∗∗
https://doi.org/10.1016/j.ceramint.2019.06.139 Received 2 June 2019; Received in revised form 13 June 2019; Accepted 14 June 2019 Available online 15 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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a hot research field [40-42]. Compared with traditional sintering methods, for instance, hot pressing, SPS can not only achieve rapid sintering, but also obtain novel composites with unique microstructures and characteristics [43,44]. Therefore, SPS can facilitate the preparation of mullite based ceramics with excellent properties. In present work, low cost Al2O3 and SiO2 were used as raw materials for fabrication of mullite ceramics. Carbon fibers were introduced into mixed powder of Al2O3 and SiO2 to prepare Cf/mullite composites with enhanced EMI shielding properties by spark plasma sintering. Influence of carbon fibers content on microstructure and EMI shielding properties of Cf/mullite composites were investigated in details. 2. Experimental procedures 2.1. Materials preparation Commercially available PAN based carbon fibers (T700, 7 μm, 12 K, Tianniao Company, Jiangsu, China) were used in this study. Al2O3 powders (purity > 99 %, grain diameter: 45-75 μm), SiO2 powders (purity > 99 %, grain diameter: 10 μm) were used as starting materials. Y2O3 powders (purity > 99 %, grain diameter: 10 μm) were added into Al2O3 and SiO2 powders as sintering additives with mass fraction of 10 %. All these powders mentioned above are supplied from Sinopharm Chemical Reagent Co., Ltd. The Cf/mullite composites with different contents of carbon fibers (0, 1, and 2 wt%), denoted as M0, MC1 and MC2, respectively, were prepared. Firstly, Al2O3, SiO2 and Y2O3 powders were mixed by ballmilling using ethanol as medium in a zirconia jar for 4 h at 250 rpm. The molar ratio of Al2O3 to SiO2 was 3:2. Secondly, carbon fibers with length of 2-3 mm were added into mixture slurry of Al2O3, SiO2 and Y2O3 powders by mechanical stirring for 1 h to obtain uniform mixture slurry. Then the final mixture slurry was dried at 80 oC in an oven for 24 h. Finally, the dried mixed powders were put into a circular graphite mold with an inner diameter of 60 mm, and the densification was performed in a vacuum SPS chamber (HPD 25-3, Germany) at 1500 oC under uniaxial pressure of 40 MPa with the soaking time of 10 min. For a comparison, the pure mullite ceramic sample without carbon fibers (M0) was also prepared by same SPS. The density and porosity of asprepared samples M0, MC1 and MC2 are presented in Table 1. 2.2. Characterizations Open porosity and bulk density of composites were measured by Archimedes method in deionized water as immersion medium according to DIN EN 1389:2003. Phases of vibratory disc-milled powder samples were analyzed by X-ray diffraction (XRD, D/max2550, Rigaku) with Cu Kα radiation. The morphology and microstructure of samples were examined by electron microscopy (SEM, Nova NanoSEM 230). The complex permittivity (εr = ε′-jε″) of samples with size of 22.86 mm × 10.16 mm × 2 mm was measured via a vector network analyzer (Agilent N5230A) in X-band according to wave guide method. The obtained scattering parameters (S11, S12, S22 and S21) were used to calculate the total EMI shielding effectiveness (SET), absorbing shielding (SEA) and reflection shielding (SER) by following equations [45]: SET=SER + SEA
(1)
Table 1 Density and porosity of samples M0, MC1 and MC2. Samples
Density (g/cm3)
Open Porosity (%)
Number of tested specimens
M0 MC1 MC2
3.33 ± 0.03 3.17 ± 0.03 2.74 ± 0.01
1.3 ± 0.2 3.4 ± 0.1 8.1 ± 0.2
N=2 N=2 N=2
SEA = −10log10(T/(1-R))
(2)
SER = −10log10(1-R)
(3)
where reflection coefficient (R) and transmission (T) were calculated on the basis of R = |S11|2 and T = |S21|2. 3. Results and discussion 3.1. Morphological and phase compositions Typical micro-morphologies of mullite based composites with different contents of carbon fibers are presented in Fig. 1(a)-(c) As displayed in Fig. 1(a), M0 exhibits a relatively dense structure and mullite grains with smooth surface are closely interconnected (insert in Fig. 1(a)). After introducing carbon fibers (Fig. 1(b)), carbon fibers are seemingly of uniform distributions in mullite matrix. Note that, the well-protected carbon fibers without chemical corrosion are observed (insert in Fig. 1(b)), indicating that carbon fibers are free of chemical reactions with oxides during sintering process. The integrality in structure of carbon fibers can be in favor of EMI shielding properties. As carbon fibers content increases (Fig. 1(c)), more carbon fibers are in relatively uniform dispersion, and well consolidated with mullite matrix. Furthermore, the gaps within carbon bundles are filled by mullite matrix (insert in Fig. 1(c)). Fig. 1(d) presents the XRD patterns of MC2. Diffraction peaks with typical characterization of splitting of crystal planes (120) and (210) at 26° appeared in MC2 are well indexed to mullite phase (orthorhombic, PDF#74-2419), indicating completely mullitization reaction between Al2O3 and SiO2 as precursor. In addition, no carbon peaks were detected in MC2 sample mainly due to incomplete crystallization of carbon fibers. 3.2. EMI shielding properties In general, EMI shielding properties can be regarded as attenuation ability of propagating electromagnetic radiation [46]. Total EMI shielding effectiveness (SET) depends on three main mechanisms: absorption (SEA), reflection (SER) and multiple reflections (SEM) [47]. However, SEM can be neglected when absorption shielding effectiveness (SEA) is higher than 10 dB [48]. Thus, SET can be expressed by equation (1). Fig. 2 displays EMI shielding properties of mullite based composites with different content of carbon fibers. As can be seen from Fig. 2 (a), pure mullite ceramics with low average EMI SET of ∼ 5.3 dB show extremely weak ability to block the electromagnetic radiation. With the increased content of carbon fibers in matrix, EMI shielding properties of mullite based composites increase rapidly. When 1 wt% carbon fibers were introduced into mullite, the highest EMI SET (higher than 40 dB) is obtained, indicating outstanding EMI shielding properties for Cf/mullite composites within the whole measured frequency region. However, as carbon fibers content increased to 2 wt%, the EMI SET decreased slightly to ∼ 37 dB compared with the Cf/mullite composites with 1 wt % carbon fibers. Both the two as-prepared Cf/mullite composites in present work show the EMI SET higher than 30 dB, demonstrating that more than 99.9 % of incident electromagnetic radiation can be shielded within the whole measured X band. This result is close to the EMI shielding performance required for commercial applications [46]. As mentioned above, due to the high value of absorption shielding effectiveness, absorption (SEA) and reflection (SER) are considered as two main contributions for the EMI SET in this work. The diagrammatic illustration is presented in Fig. 3. Moreover, known from Fig. 2(b) and (c), SEA with much higher value in comparison to SER plays a dominant role in the high EMI SET. Taken the Cf/mullite composites with 1 wt% carbon fibers for example, the EMI SET is around 41 dB and SEA is about 34 dB, while the SER is merely about 7 dB within the measured frequency range. The improved EMI shielding properties for Cf/mullite composites
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Fig. 1. SEM images of M0 (a), MC1 (b) and MC2 (c), and XRD patterns of MC2 (d).
Fig. 2. (a) EMI SET, (b) SER, (c) SEA and (d) microwave impedance of mullite based composites with different content of carbon fibers. 18990
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Table 2 Comparison of EMI shielding performance of various carbon/ceramic composites.
Fig. 3. Schematic illustration of EMI shielding mechanism for Cf/mullite composites.
could be mainly attributed to conductive network made of carbon fibers and the enhanced electrical conductivity (EC) by addition of carbon fibers into the insulating mullite matrix [49]. Within the conductive network, electrons and other charge carriers can move freely to interact with the incident electromagnetic radiation and dissipate the electromagnetic energy as form of heat [48], leading to the enhanced SER. Most importantly, the promotional electrical conductivity arising from the formation of conduction network is the key factor to determine the EMI shielding performance [50]. According to EMI theory, SEA and SER can also be expressed as [51]:
SEA = 8.7d πfμσ SER = 39.5 + 10 log
(4)
σ 2πfμ
(5)
where f is the frequency, μ is the permeability, d and σ are the thickness and EC of the shield material. The EC can be obtained according to the equation [52]: σ = 2πfε0ε″, where ε0 is the permittivity in vacuum, and ε″ is the imaginary part of permittivity of the shield material. As shown in Fig. 4, as carbon fibers content increased from 0 to 2 wt%, the EC significantly increased due to the remarkably strengthen conductive network in composites. Therefore, both SEA and SER obviously increased (in Fig. 2(b) and (c)) based on the above equations (4) and (5), leading to the outstanding EMI shielding properties for Cf/mullite composites. In addition, notable intensified interfacial polarization in Cf/mullite composites also makes a significant contribution to the absorption (SEA), which results from the obviously increased dipoles and charge accumulation on a large number of interfaces between mullite matrix and carbon fibers [48,53]. Therefore, SEA shows much higher
Filler
Matrix
Filler loading
Thickness (mm)
EMI SET (dB)
Ref
Carbon fiber Carbon fiber PyC Pyrolytic carbon PyC CNTs-PyC CNWs-CNTs CNTs CNTs r-GO GN Carbon fiber
SiC PyC-SiC SiC Si3N4
40 vol% 27.7–28.5 wt% 22.4–36.6 wt% 12.1 vol%
3.0 2.0 1.8 2.8
31.4–42.2 34–42 19.2–29.0 43.2
[58] [55] [56] [57]
Si3N4 Si3N4 Si3N4 Si3N4 SiO2 SiO2 Al2O3 mullite
4.0 vol% 19.2 wt% 3.91 wt% 2.7 wt% 10 wt% 20 wt% 2.0 vol% 1 wt%
2.0 2.0 2.0 1.5 2.5 1.5 1.5 2.0
∼ 13.0 43.6 25.4 ∼ 30.5 ∼ 21.7 ∼ 34.5 ∼ 23 ∼ 41
Carbon fiber
mullite
2 wt%
2.0
∼ 37
[61] [59] [31] [30] [60] [6] [62] This work This work
value and contributes much more to the total EMI shielding performance compared with SER. However, as the content of carbon fibers further increases, the microwave impedance keeps decreasing (see Fig. 2(d)), leading to the deterioration of impedance matching between the Cf/mullite composites and air. Reflection (SER) of electromagnetic radiation is mainly caused by the impedance mismatch between air and shield material [54]. Due to reflection, the amount of electromagnetic radiation entering and interacting with shield material is reduced, which leads to the weak absorption (SEA). Therefore, with the increase of carbon fibers addition from 1 to 2 wt%, MC2 shows higher values of SER but lower values of SEA and SET compared with MC1 due to the decreased impedance. In order to further evaluate the shielding performance of the asprepared Cf/mullite composites, a comparison between various carbon/ ceramic composites for EMI shielding previously reported in literature and our work within X band was carried out and the results was shown in Table 2. All listed carbon/ceramic composites show a favorable EMI shielding performance. However, many merits of these composites will be weaken due to the high loading of carbon fillers (higher than 10 vol % or10 wt%) and big thickness (thicker than 2.0 mm) [6,55-60]. Thus, the Cf/mullite composites in this study show great advantages in terms of being high EMI shielding properties and small thickness of 2.0 mm, compared with the previously reported carbon/ceramic composites in Table 2. This result further demonstrates that the proposed Cf/mullite composites with superior shielding effectiveness have great potential for application as high-performance EMI shielding materials.
4. Conclusion Cf/mullite composites with different weight fractions of carbon fibers were prepared by rapid preparation technique of spark plasma sintering for exploring new high-performance EMI shielding materials. As weight fraction of carbon fibers increased, the EMI SET of the mullite based composites significantly increased and a high value (> 40 dB) close to the level of commercial EMI shielding was achieved with merely 1 wt% carbon fibers. Notable improved absorption (SEA) originating from tremendous increased EC and interfacial polarization is mainly responsible for the significant improvement of EMI shielding properties for Cf/mullite composites. Finally, current work indicates that the Cf/mullite composites with superior EMI shielding properties can be used as high-performance ceramic-based materials in vast academic and industrial areas. Fig. 4. Electrical conductivity of mullite based composites with different content of carbon fibers. 18991
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51604107), and the Natural Science Foundation of Hunan Province (Grant No. 2019JJ50115 and 2019JJ50768). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.139. References [1] Q. Song, F. Ye, X. Yin, W. Li, H. Li, Y. Liu, K. Li, K. Xie, X. Li, Q. Fu, Carbon nanotube-multilayered graphene edge plane core-shell hybrid foams for ultrahighperformance electromagnetic-interference shielding, Adv. Mater. 29 (2017) 1701583. [2] H. Lv, Y. Guo, G. Wu, G. Ji, Y. Zhao, Z.J. Xu, Interface polarization strategy to solve electromagnetic wave interference issue, ACS Appl. Mater. Interfaces 9 (2017) 5660–5668. [3] M.R. Tohidifar, Highly-efficient electromagnetic interference shielding and microwave dielectric behavior of a (Bi2O3 + B2O3)-doped MWCNT/BaTiO3 ceramic nanocomposite, Ceram. Int. 44 (2018) 13613–13622. [4] Y. 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