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Electromagnetic interference shielding effectiveness of a flexible carbon fiber felt containing in situ grown hafnium carbide nanowires and nanobelts
Ceramics International 45 (2019) 19513–19516
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Electromagnetic interference shielding effectiveness of a flexible carbon fiber felt containing in situ grown hafnium carbide nanowires and nanobelts
T
Song Tiana,∗, Lu Zhoua,∗∗, Zhongtian Lianga, Yanru Wangb, Yu Yangb, Xinfa Qiangc, Zhonghao Qiana a
School of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing, 400074, China Chongqing Acoustic-Optic-Electronic Co., LTD. CETC No.44 Research Institute, Chongqing, 400060, China c Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing Institute of Technology, Nanjing, 211167, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hafnium carbide Chemical vapor deposition Electromagnetic interference shielding effectiveness
Hafnium carbide nanowires (HfCnw) and a small amount of hafnium carbide nanobelts (HfCnb) were incorporated in a flexible carbon fiber felt by catalysis-assisted chemical vapor deposition to fabricate an electromagnetic interference (EMI) shielding carbon-based material with excellent electromagnetic wave absorption properties. The EMI shielding effectiveness (SE) of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was investigated in X band (8.2 GHz–12.4 GHz). The result shows that more than 99% of the incident beam was shielded when the thickness of the sample is 1.5 mm. Moreover, all the SEA/SET values in this work are more than 0.8, even up to 0.94 at 11 GHz, meaning SET mainly results from SEA. The excellent electromagnetic wave absorption properties of the sample are due to the conductivity loss triggered by moving electrons and the polarization loss caused by considerable interfaces.
1. Introduction With the rapid development of technology, various electronic and electrical equipment are extensively used in military and civilian fields [1,2]. These devices make life much convenient, whereas their electromagnetic radiation may become harmful electromagnetic waves, which influence the normal operation of sensitive components and are detrimental to the health of human [3–5]. Therefore, the diverse types of materials such as polymer-based, metal-based and carbon-based materials have been studied to solve the various problems caused by EMI [6–8]. However, most of polymer-based materials can't be developed to attenuate electromagnetic waves at high temperature, and metal-based materials mainly shield EMI by reflection, which may cause secondary EMI. Besides, the density of metal-based materials is high and the ferromagnetism of magnetic metal materials will disappear when the ambient temperature is above their curie temperature [6]. Carbon-based materials with a variety of forms (powders, fibers, foams, tubes, etc.), are promising candidate for EMI shielding applications due to its excellent properties, for instance, low density, high electronic conductivity and good thermal properties [9–12]. Nevertheless, it should be pointed that the pure carbon materials will be ∗
oxidized seriously under aerobic conditions. To solve this problem, one of the most common methods taken by the previous researchers is to introduce refractory carbides into carbon materials. Han et al. [13] fabricated graphene foams with in situ grown silicon carbide nanowires (SiCnw) by a freeze-drying process and carbothermal reduction. It is found that the thermostability and electromagnetic absorption of composite were improved by introducing SiCnw. Compared to SiC, HfC possesses higher melting points (3890 °C) and electrical conductivity [14,15]. Furthermore, Hou et al. [14] fabricated HfC/SiC hybrid nanofiber mats by electrospinning, their research results show that the addition of HfC not only improves the EMI SE of the material, but also enhances the flexibility and tensile strength of the material. Hence, introducing HfC into the carbon-based composites are expected to produce EMI shielding materials that can be applied to aerospace, portable electronic devices, and so on. Up to now, though, research on the EMI shielding performance of the flexible carbon fiber felt modified by one-dimensional HfC nanomaterials is seldomly reported. In this study, we fabricated the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb by catalyst-assisted chemical vapor deposition (CVD). The EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was investigated in X band (8.2 GHz–12.4 GHz). The morphology and shielding mechanism were
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (S. Tian), [email protected] (L. Zhou).
∗∗
https://doi.org/10.1016/j.ceramint.2019.06.039 Received 5 May 2019; Received in revised form 4 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19513–19516
S. Tian, et al.
discussed. 2. Experiment 2.1. The preparation of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb The manufacturing process of HfCnw and HfCnb was carried out in a double temperature chamber tube furnace (BTF-1400C-II, Best Equipment Co. Ltd, Anhui, China). Firstly, a cuboid carbon felt (40 mm × 60 mm × 1.5 mm) was immersed into the Ni(NO3)2/ethanol solutions with a concentration of 0.2 mol/L for 5 h to load catalysts and then dried at 323 K in oven. After drying, the carbon fiber felt was located in the high-temperature chamber of the horizontal tube furnace, and 4 g HfCl4 powers (99.95% purity; Alfa Aesar chemical Co. Ltd. Tianjin, China) were placed in the low-temperature chamber. Turn on the vacuum pump to make the pressure in the furnace at 2 kPa, then the high-temperature chamber was heated to 1100 °C with constant flow rate of 50 ml/min of H2 gas (99.999% purity; Li Tuo Gases Co. Ltd, Chongqing, China). The low-temperature chamber started to be heat up to 280 °C, when the high-temperature chamber achieved about 720 °C. After both chambers reached their different set temperatures, CH4 gases (99.999% purity; Li Tuo Gases Co. Ltd, Chongqing, China) with a flow of 10 ml/min and H2 gases with a flow of 200 ml/min were simultaneously introduced into the furnace system. The mixed gases flowed through the low-temperature chamber and carried the evaporated HfCl4 vapor into the high-temperature chamber to synthesize the HfCnw and HfCnb. The reaction is carried out for 2 h. When the deposition ended, the furnace cooled to room temperature under vacuum. During the deposition, the following reactions [16] might take place in the furnace. CH4 (g) → 2H2 (g) + C (g)
(1)
HfCl4 (g) + 2H2 (g) → Hf (g) + 4HCl (g)
(2)
C (g) + Hf (g) → HfC (s)
(3)
2.2. Characterization and test Scanning electron microscopy (SEM; Sigma 300, Zeiss, 15 kV) was employed to observe the morphology of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb. The crystalline structures and phase composition of the sample were analyzed by X-ray diffraction (XRD; Xpert PRO X) from 10° to 90° with Cu K α radiation. The scattering parameters (S-parameters; S11, S12, S22 and S21) of the specimen were measured using an AV3672B vector network analyzer (VNA; The 41st Research Institute of China Electronics Technology Group Corporation, China) by the waveguide method in the X-band. The SE including total SE (SET), reflection SE (SER) and absorption SE (SEA) can be calculated by the following formulas.
R= 10 s11/10
(4)
T= 10 s21/10
(5)
R+A+T=1
(6)
SER = −10 log10(1-R)，SEA = −10 log10(T/(1-R))
(7)
SET = SER + SEA + SEM
(8)
Where R, T and A are reflection coefficient, transmission coefficient and absorption coefficient, respectively; SEM is the multiple-reflections SE, which is negligible when SET is greater than 15 dB. 3. Results and discussion As shown in Fig. 1, two feeble peaks appear at 26° and 44°, because
Fig. 1. XRD pattern of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb.
the carbon fibers in the flexible carbon fiber felt possess low graphitization degree. Meanwhile, all diffraction peaks of HfC are sharp and match greatly with the location of peak for the cubic HfC structure in JCPDS card 65–0964. The lattice parameter (4.6556 Å) of the HfC according to the XRD pattern is closely to a standard value of 4.6500 Å, implying as-prepared HfC possessing well stoichiometric. Two weak peaks of hafnium oxide (HfO2) were detected since it is impossible to eliminate oxygen during the reaction entirely. Fig. 2a shows the cross-section morphology of the flexible carbon fiber felt. Obviously, the production of HfCnw gradually decreases as the depth away from the surface of carbon fiber felt. This is because the size of Hf atoms is too large to infiltrate easily into the interior of the flexible carbon fiber felt. A digital photo of sample is exhibited in the insert of Fig. 2a, that displays the flexibility of carbon fiber felt and indicates it can be made into various shapes according to requirements. Fig. 2b is the surface morphology of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, from which it is clearly observed that the surfaces of carbon fibers were covered with a cluster of hybrid consisting of HfCnw and HfCnb. The number of HfCnw is remarkable compared to that of HfCnb. For the flexible carbon fiber felt, the hybrid structure of the nanowires and nanobelts with uneven and rough surfaces (the insert of Fig. 2c), which increase the interface of carbon fiber felt, is beneficial to attenuation of electromagnetic waves [17]. In order to deeply understand the morphology, more detailed inspection of the sample was carried out. As shown in Fig. 2d taken from the tip of a single carbon fiber, both HfCnw and HfCnb grew radially around the carbon fiber. The high-magnification SEM images for the HfCnw and HfCnb (Fig. 2e and f) were taken from the marked areas "1" and "2" in Fig. 2d, respectively. It is found that the diameter of HfCnw is approximately 100 nm and the width of HfCnb ranges from 800 nm to 8 μm. The HfCnw and HfCnb have a hemispherical catalyst droplet and a plate-like triangular catalyst particle at their tips respectively, indicating the vapor-liquid-solid (VLS) mechanism could be used to explain the growth of the HfCnw and HfCnb [18]. However, the reason and the driving force to form the triangular catalyst particle are not yet known. Moreover, the HfCnw have a striation-like morphology or periodically thick and thin diameters. The striation patterns may be caused by a periodic instability such as a periodic change in the diameter of ball-like liquid catalysts on the top of nanowires [19]. The striation-like structure is favorable to the increasing of the surface area of HfCnw. Fig. 3a shows the EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb as a function of frequency. The SET of the sample with HfCnw and HfCnb is about 20 dB in X band, implying more than 99% of the incident beam was shielded by the material. The values of SET are relatively low due to a thin thickness of 1.5 mm. However, the specific SE is about 40 dB•cm3/g and it is clearly seen that
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Fig. 2. SEM images of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb. (a) the cross-section morphology (inset: a digital photo of the sample), (b) the surface morphology, (c) the synthesized HfCnw and HfCnb (inset: a high-magnification SEM image of HfCnb), (d) the tip of a single carbon fiber, (e) a typical HfC nanowire from the marked area "1" in (d), and (f) a representative HfC nanobelt from the marked area "2" in (d).
the SET of the sample mainly contributed by SEA, which means the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb is an EMI shielding material with excellent electromagnetic wave absorption properties. Fig. 3b shows the SEA/SET values of references [1,17,20,21] and this work at 9 GHz、10 GHz、11 GHz and 12 GHz. All the SEA/SET values in this work are more than 0.8, even up to 0.94 at 11 GHz, whereas, the others’ are between 0.69 and 0.8. There could be two reasons for excellent electromagnetic wave absorption properties of the sample. Firstly, the carbon fiber, HfCnw and HfCnb can offer moving electrons efficient conduction paths when electromagnetic wave propagates along the material, which lead to the conductivity loss. Secondly, a large amount of interfacial polarization will be generated because the HfCnw and HfCnb possess high aspect ratio, which can be seen from Fig. 2. Fig. 4 shows the mechanism of the
Fig. 4. Illustrations of the migrating electron, hopping electron and accumulation of electrons.
Fig. 3. (a) EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, (b) the SEA/SET values at 9 GHz, 10 GHz, 11 GHz and 12 GHz. 19515
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absorption attenuation of the material, in which conductivity loss is caused by migrating electron and hopping electron among carbon fiber, HfCnw and HfCnb. Meanwhile, when electrons accumulate at abundant interfaces between the carbon fibers and HfCnw, HfCnw and HfCnb, and so forth, polarization loss will occur. The conductivity loss formed by moving electrons and the polarization loss caused by considerable interfaces play a crucial role in the absorption of electromagnetic waves. Hence, few electromagnetic waves were reflected by the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, indicating it could be used as EMI shielding materials with excellent electromagnetic wave absorption properties in aviation, spaceflight and portable electronics field.
[4]
[5]
[6]
[7]
[8]
4. Conclusions
[9]
The flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was fabricated via catalysis-assisted CVD. The SET of the carbonbased material was about 20 dB in X band at 1.5 mm thickness, meaning more than 99% of the incident beam was shielded. The SEA/SET values in present work were large than 0.8, even achieved 0.94 at 11 GHz, implying that the material mainly shielded the interfering electromagnetic waves by means of absorption. The conductivity loss formed by moving electrons and the polarization loss caused by numerous interfaces should be greatly responsible for the absorptive attenuation.Acknowledgments This work has been supported by the National Natural Science Foundation of China under grant nos. 51502028 and 51602146, supported by the Basic and Advanced Research Project of Chongqing Science and Technology Commission (grant no. cstc2015jcyjA50011), the fund of the State Key Laboratory of Solidification Processing in NWPU (grant no. SKLSP201740), the fund of National and Local Joint Engineering Laboratory of Transportation and Civil Engineering Materials (grant no. LHSYS-2016-002), and the Natural Science Foundation of Jiangsu Province (grant no. BK20150727). Conflicts of interest
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
The authors declare no conflict of interest.
[19] [20]
References [1] T. Han, R. Luo, Effect of carbon nanotubes on the electromagnetic shielding properties of SiCf/SiC composites, J. Alloy. Comp. 745 (2018) 90–99. [2] Z. Chen, C. Xu, C. Ma, W. Ren, H. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296–1300. [3] L. Chen, X. Yin, X. Fan, M. Chen, X. Ma, L. Cheng, L. Zhang, Mechanical and
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Electromagnetic interference shielding effectiveness of a flexible carbon fiber felt containing in situ grown hafnium carbide nanowires and nanobelts
T
Song Tiana,∗, Lu Zhoua,∗∗, Zhongtian Lianga, Yanru Wangb, Yu Yangb, Xinfa Qiangc, Zhonghao Qiana a
School of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing, 400074, China Chongqing Acoustic-Optic-Electronic Co., LTD. CETC No.44 Research Institute, Chongqing, 400060, China c Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing Institute of Technology, Nanjing, 211167, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hafnium carbide Chemical vapor deposition Electromagnetic interference shielding effectiveness
Hafnium carbide nanowires (HfCnw) and a small amount of hafnium carbide nanobelts (HfCnb) were incorporated in a flexible carbon fiber felt by catalysis-assisted chemical vapor deposition to fabricate an electromagnetic interference (EMI) shielding carbon-based material with excellent electromagnetic wave absorption properties. The EMI shielding effectiveness (SE) of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was investigated in X band (8.2 GHz–12.4 GHz). The result shows that more than 99% of the incident beam was shielded when the thickness of the sample is 1.5 mm. Moreover, all the SEA/SET values in this work are more than 0.8, even up to 0.94 at 11 GHz, meaning SET mainly results from SEA. The excellent electromagnetic wave absorption properties of the sample are due to the conductivity loss triggered by moving electrons and the polarization loss caused by considerable interfaces.
1. Introduction With the rapid development of technology, various electronic and electrical equipment are extensively used in military and civilian fields [1,2]. These devices make life much convenient, whereas their electromagnetic radiation may become harmful electromagnetic waves, which influence the normal operation of sensitive components and are detrimental to the health of human [3–5]. Therefore, the diverse types of materials such as polymer-based, metal-based and carbon-based materials have been studied to solve the various problems caused by EMI [6–8]. However, most of polymer-based materials can't be developed to attenuate electromagnetic waves at high temperature, and metal-based materials mainly shield EMI by reflection, which may cause secondary EMI. Besides, the density of metal-based materials is high and the ferromagnetism of magnetic metal materials will disappear when the ambient temperature is above their curie temperature [6]. Carbon-based materials with a variety of forms (powders, fibers, foams, tubes, etc.), are promising candidate for EMI shielding applications due to its excellent properties, for instance, low density, high electronic conductivity and good thermal properties [9–12]. Nevertheless, it should be pointed that the pure carbon materials will be ∗
oxidized seriously under aerobic conditions. To solve this problem, one of the most common methods taken by the previous researchers is to introduce refractory carbides into carbon materials. Han et al. [13] fabricated graphene foams with in situ grown silicon carbide nanowires (SiCnw) by a freeze-drying process and carbothermal reduction. It is found that the thermostability and electromagnetic absorption of composite were improved by introducing SiCnw. Compared to SiC, HfC possesses higher melting points (3890 °C) and electrical conductivity [14,15]. Furthermore, Hou et al. [14] fabricated HfC/SiC hybrid nanofiber mats by electrospinning, their research results show that the addition of HfC not only improves the EMI SE of the material, but also enhances the flexibility and tensile strength of the material. Hence, introducing HfC into the carbon-based composites are expected to produce EMI shielding materials that can be applied to aerospace, portable electronic devices, and so on. Up to now, though, research on the EMI shielding performance of the flexible carbon fiber felt modified by one-dimensional HfC nanomaterials is seldomly reported. In this study, we fabricated the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb by catalyst-assisted chemical vapor deposition (CVD). The EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was investigated in X band (8.2 GHz–12.4 GHz). The morphology and shielding mechanism were
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (S. Tian), [email protected] (L. Zhou).
∗∗
https://doi.org/10.1016/j.ceramint.2019.06.039 Received 5 May 2019; Received in revised form 4 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19513–19516
S. Tian, et al.
discussed. 2. Experiment 2.1. The preparation of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb The manufacturing process of HfCnw and HfCnb was carried out in a double temperature chamber tube furnace (BTF-1400C-II, Best Equipment Co. Ltd, Anhui, China). Firstly, a cuboid carbon felt (40 mm × 60 mm × 1.5 mm) was immersed into the Ni(NO3)2/ethanol solutions with a concentration of 0.2 mol/L for 5 h to load catalysts and then dried at 323 K in oven. After drying, the carbon fiber felt was located in the high-temperature chamber of the horizontal tube furnace, and 4 g HfCl4 powers (99.95% purity; Alfa Aesar chemical Co. Ltd. Tianjin, China) were placed in the low-temperature chamber. Turn on the vacuum pump to make the pressure in the furnace at 2 kPa, then the high-temperature chamber was heated to 1100 °C with constant flow rate of 50 ml/min of H2 gas (99.999% purity; Li Tuo Gases Co. Ltd, Chongqing, China). The low-temperature chamber started to be heat up to 280 °C, when the high-temperature chamber achieved about 720 °C. After both chambers reached their different set temperatures, CH4 gases (99.999% purity; Li Tuo Gases Co. Ltd, Chongqing, China) with a flow of 10 ml/min and H2 gases with a flow of 200 ml/min were simultaneously introduced into the furnace system. The mixed gases flowed through the low-temperature chamber and carried the evaporated HfCl4 vapor into the high-temperature chamber to synthesize the HfCnw and HfCnb. The reaction is carried out for 2 h. When the deposition ended, the furnace cooled to room temperature under vacuum. During the deposition, the following reactions [16] might take place in the furnace. CH4 (g) → 2H2 (g) + C (g)
(1)
HfCl4 (g) + 2H2 (g) → Hf (g) + 4HCl (g)
(2)
C (g) + Hf (g) → HfC (s)
(3)
2.2. Characterization and test Scanning electron microscopy (SEM; Sigma 300, Zeiss, 15 kV) was employed to observe the morphology of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb. The crystalline structures and phase composition of the sample were analyzed by X-ray diffraction (XRD; Xpert PRO X) from 10° to 90° with Cu K α radiation. The scattering parameters (S-parameters; S11, S12, S22 and S21) of the specimen were measured using an AV3672B vector network analyzer (VNA; The 41st Research Institute of China Electronics Technology Group Corporation, China) by the waveguide method in the X-band. The SE including total SE (SET), reflection SE (SER) and absorption SE (SEA) can be calculated by the following formulas.
R= 10 s11/10
(4)
T= 10 s21/10
(5)
R+A+T=1
(6)
SER = −10 log10(1-R)，SEA = −10 log10(T/(1-R))
(7)
SET = SER + SEA + SEM
(8)
Where R, T and A are reflection coefficient, transmission coefficient and absorption coefficient, respectively; SEM is the multiple-reflections SE, which is negligible when SET is greater than 15 dB. 3. Results and discussion As shown in Fig. 1, two feeble peaks appear at 26° and 44°, because
Fig. 1. XRD pattern of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb.
the carbon fibers in the flexible carbon fiber felt possess low graphitization degree. Meanwhile, all diffraction peaks of HfC are sharp and match greatly with the location of peak for the cubic HfC structure in JCPDS card 65–0964. The lattice parameter (4.6556 Å) of the HfC according to the XRD pattern is closely to a standard value of 4.6500 Å, implying as-prepared HfC possessing well stoichiometric. Two weak peaks of hafnium oxide (HfO2) were detected since it is impossible to eliminate oxygen during the reaction entirely. Fig. 2a shows the cross-section morphology of the flexible carbon fiber felt. Obviously, the production of HfCnw gradually decreases as the depth away from the surface of carbon fiber felt. This is because the size of Hf atoms is too large to infiltrate easily into the interior of the flexible carbon fiber felt. A digital photo of sample is exhibited in the insert of Fig. 2a, that displays the flexibility of carbon fiber felt and indicates it can be made into various shapes according to requirements. Fig. 2b is the surface morphology of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, from which it is clearly observed that the surfaces of carbon fibers were covered with a cluster of hybrid consisting of HfCnw and HfCnb. The number of HfCnw is remarkable compared to that of HfCnb. For the flexible carbon fiber felt, the hybrid structure of the nanowires and nanobelts with uneven and rough surfaces (the insert of Fig. 2c), which increase the interface of carbon fiber felt, is beneficial to attenuation of electromagnetic waves [17]. In order to deeply understand the morphology, more detailed inspection of the sample was carried out. As shown in Fig. 2d taken from the tip of a single carbon fiber, both HfCnw and HfCnb grew radially around the carbon fiber. The high-magnification SEM images for the HfCnw and HfCnb (Fig. 2e and f) were taken from the marked areas "1" and "2" in Fig. 2d, respectively. It is found that the diameter of HfCnw is approximately 100 nm and the width of HfCnb ranges from 800 nm to 8 μm. The HfCnw and HfCnb have a hemispherical catalyst droplet and a plate-like triangular catalyst particle at their tips respectively, indicating the vapor-liquid-solid (VLS) mechanism could be used to explain the growth of the HfCnw and HfCnb [18]. However, the reason and the driving force to form the triangular catalyst particle are not yet known. Moreover, the HfCnw have a striation-like morphology or periodically thick and thin diameters. The striation patterns may be caused by a periodic instability such as a periodic change in the diameter of ball-like liquid catalysts on the top of nanowires [19]. The striation-like structure is favorable to the increasing of the surface area of HfCnw. Fig. 3a shows the EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb as a function of frequency. The SET of the sample with HfCnw and HfCnb is about 20 dB in X band, implying more than 99% of the incident beam was shielded by the material. The values of SET are relatively low due to a thin thickness of 1.5 mm. However, the specific SE is about 40 dB•cm3/g and it is clearly seen that
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Fig. 2. SEM images of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb. (a) the cross-section morphology (inset: a digital photo of the sample), (b) the surface morphology, (c) the synthesized HfCnw and HfCnb (inset: a high-magnification SEM image of HfCnb), (d) the tip of a single carbon fiber, (e) a typical HfC nanowire from the marked area "1" in (d), and (f) a representative HfC nanobelt from the marked area "2" in (d).
the SET of the sample mainly contributed by SEA, which means the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb is an EMI shielding material with excellent electromagnetic wave absorption properties. Fig. 3b shows the SEA/SET values of references [1,17,20,21] and this work at 9 GHz、10 GHz、11 GHz and 12 GHz. All the SEA/SET values in this work are more than 0.8, even up to 0.94 at 11 GHz, whereas, the others’ are between 0.69 and 0.8. There could be two reasons for excellent electromagnetic wave absorption properties of the sample. Firstly, the carbon fiber, HfCnw and HfCnb can offer moving electrons efficient conduction paths when electromagnetic wave propagates along the material, which lead to the conductivity loss. Secondly, a large amount of interfacial polarization will be generated because the HfCnw and HfCnb possess high aspect ratio, which can be seen from Fig. 2. Fig. 4 shows the mechanism of the
Fig. 4. Illustrations of the migrating electron, hopping electron and accumulation of electrons.
Fig. 3. (a) EMI SE of the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, (b) the SEA/SET values at 9 GHz, 10 GHz, 11 GHz and 12 GHz. 19515
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absorption attenuation of the material, in which conductivity loss is caused by migrating electron and hopping electron among carbon fiber, HfCnw and HfCnb. Meanwhile, when electrons accumulate at abundant interfaces between the carbon fibers and HfCnw, HfCnw and HfCnb, and so forth, polarization loss will occur. The conductivity loss formed by moving electrons and the polarization loss caused by considerable interfaces play a crucial role in the absorption of electromagnetic waves. Hence, few electromagnetic waves were reflected by the flexible carbon fiber felt containing in situ grown HfCnw and HfCnb, indicating it could be used as EMI shielding materials with excellent electromagnetic wave absorption properties in aviation, spaceflight and portable electronics field.
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4. Conclusions
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The flexible carbon fiber felt containing in situ grown HfCnw and HfCnb was fabricated via catalysis-assisted CVD. The SET of the carbonbased material was about 20 dB in X band at 1.5 mm thickness, meaning more than 99% of the incident beam was shielded. The SEA/SET values in present work were large than 0.8, even achieved 0.94 at 11 GHz, implying that the material mainly shielded the interfering electromagnetic waves by means of absorption. The conductivity loss formed by moving electrons and the polarization loss caused by numerous interfaces should be greatly responsible for the absorptive attenuation.Acknowledgments This work has been supported by the National Natural Science Foundation of China under grant nos. 51502028 and 51602146, supported by the Basic and Advanced Research Project of Chongqing Science and Technology Commission (grant no. cstc2015jcyjA50011), the fund of the State Key Laboratory of Solidification Processing in NWPU (grant no. SKLSP201740), the fund of National and Local Joint Engineering Laboratory of Transportation and Civil Engineering Materials (grant no. LHSYS-2016-002), and the Natural Science Foundation of Jiangsu Province (grant no. BK20150727). Conflicts of interest
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The authors declare no conflict of interest.
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