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Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties
Author’s Accepted Manuscript Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties Wei Zhou, Lan Long, Peng Xiao, Yang Li, Heng Luo, Wei-da Hu, Rui-ming Yin www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)30119-0 http://dx.doi.org/10.1016/j.ceramint.2017.01.095 CERI14557
To appear in: Ceramics International Received date: 10 November 2016 Revised date: 14 January 2017 Accepted date: 20 January 2017 Cite this article as: Wei Zhou, Lan Long, Peng Xiao, Yang Li, Heng Luo, Weida Hu and Rui-ming Yin, Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.01.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties Wei Zhoua,b,1, Lan Longa,b,1, Peng Xiaob, Yang Lib*, Heng Luob, Wei-da Hua, Rui-ming Yina a
College of Metallurgy and Materials engineering, Hunan University of Technology, Zhuzhou 412008, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Abstract: Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 oC. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β-SiC, present a withe-like morphology with diameter of 20-50 nm and aspect ratio of 100-150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than -10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach -19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than -2.1 dB within the
Corresponding author. Fax: +86-731-88830131.
E-mail address: [email protected] (Y. Li), [email protected](H. Luo) 1 These authors contributed equally to this work. 1
whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials. Keywords: Silicon carbide nano-fibers; Carbon fibers; Microwave absorption properties
1. Introduction With the explosive growth of information technology and the rapidly expanding use of various high-frequency electronic devices, electromagnetic interference has now become a serious pollution issue. In order to solve the electromagnetic interference problem, great efforts have been done in development for effective microwave absorbing materials with light weight, tiny thickness, high strength, low cost, and strong and broad wave absorption [1-4]. Carbon fibers and their composites have been found to be fascinating candidates for microwave absorption, due to their low density, excellent mechanical property, good conductivity, low cost, and wide availability [5-8]. However, the low electrical resistivity (<10-3Ω cm) of carbon fibers could easily lead to strong reflection against electromagnetic waves and poor microwave absorption [9]. Therefore, many investigations concerning the improvement of microwave absorbing property, especially applications of the magnetic metals and their oxide coatings, have been proved to be a solution for carbon fibers [10-16]. However, the current problems for magnetic metals or oxides including the relatively high cost, high density and worst point of all, incapability for dissipating the electromagnetic energy when the service
2
temperature is higher than their Curie temperatures, greatly restricts their wide applications [17-19]. Silicon Carbide (SiC), as a typical semiconductor material, is a good candidate for light weight and high-temperature resistance microwave absorber due to its low density, favorable oxidation-resistance, high thermal and chemical stability, superior mechanical strength and adjustable electrical conductivity, etc [20-22]. Note that the electromagnetic property of SiC generally varies with their morphologies, and the one-dimensional (1D) SiC nano-fibers (SiCNFs) with high surface-to-volume ratio and shape structure effects were considered to be of better microwave absorption performance than bulk or micro sized SiC particles [23, 24]. Therefore, it is generally believed that the SiCNFs coated carbon fibers could achieve efficient and stable electromagnetic wave attenuation. However, there is still a lack of reports on the microwave absorption properties of the in-situ grown SiCNFs modified carbon fibers. In this paper, SiCNFs were in-situ grown on the surfaces of carbon fibers by catalysis chemical vapor deposition (CCVD) at a relatively low temperature. The microstructure, oxidation resistance and microwave absorption properties of the as-grown SiCNFs coated carbon fibers were investigated in detail. The present study suggests that the SiCNFs coated carbon fibers are promising for microwave absorbing materials with light weight and thin thickness.
2. Materials and Experimental Procedures 2.1 Preparation of the SiCNFs coated Carbon fibers The experimental procedures including the electroplating Ni and CCVD process are presented in schematic diagram as shown in Fig. 1. Firstly, the Toray T700 carbon fibers with average diameter of 7 um and length of 10 cm were immersed in acetone for 24 h, and afterwards ultrasonically cleaned in
3
distilled water, subsequently dried at 100 oC for 2 h. The nickel nanoparticles, as catalysts for SiCNFs, were deposited on the carbon fiber surfaces via electroplating in a Nickel sulfate solution with the concentration of 15 wt%. The electronic current intensity during the electroplating was 10 ampere. Finally, SiCNFs were in-situ grown on the carbon fiber surfaces during the CCVD process. In current work, the CCVD process was performed in the CVD furnace at the temperature of 1000 oC under the pressure of 600 Pa with the dwell time of 4h. Additionally, Methyltrichlorosilane (MTS, CH 3SiCl3), hydrogen (H2) and argon (Ar) were used as the source gas of SiCNFs, carrying gas and dilute gas, respectively. Note that hydrogen (H2) was firstly introduced to reduce the catalysts for 10 min when the temperature of 1000 oC is stabilized, and then the other two gases were introduced to activate the growth of SiCNFs on carbon fibers. After the growth of SiCNFs on the surfaces, the mass of the carbon fibers were measured at room temperature and their weight gain were approximately 18%. 2.2 Characterizations The phase of the as-prepared fibers after CCVD was characterized by X-ray diffraction (XRD, D/max2550, Rigaku) with Cu Kα radiation at 35 kV and 20 mA. The morphology and structure of SiCNFs on carbon fibers were characterized by a scanning electron microscopy (SEM, Nova NanoSEM 230) and a transmission electron microscopy (TEM, JEOL 2010F). The anti-oxidation properties for the SiCNFs coated carbon fibers in air up to 1200 oC was studied with a DTA/TGA instrument (STA 409PC, NETZSCH). In order to study the microwave absorbing properties for the SiCNFs coated carbon fibers, the measurement for electromagnetic parameters were conducted on a network analyzer (Agilent5230A) within the frequency ranging from of 2 to 18 GHz by coaxial line method. In the testing process, the SiCNFs coated carbon fibers were firstly cut into short fragments with length 2-3 mm, and afterwards homogeneously dispersed into paraffin. The mass ratio of paraffin
4
and SiCNFs coated carbon fibers in the toroidal shaped specimen with an outer diameter of 7.0 mm, an inner diameter of 3.0 mm and a thickness of 2.0 mm were set to 80 wt% and 20 wt%, respectively. According to the transmission line theory, the reflection loss (RL) curves with different thickness were calculated by the following equations [25]:
RL(dB) 20 log 1/ 2
Z in r r
Z in 1 Z in 1
(1)
2 fd 1/ 2 tanh j r r c
(2)
where Zin is the normalized input impedance, εr=ε′-jε″ and μr=μ′-jμ″ are the complex permittivity and permeability of material, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave in free space, respectively.
3. Results and discussion 3.1 Phase composition, structure and growing mechanism of SiCNFs The XRD patterns of the as-prepared fibers are presented in Fig. 2. Obviously, the sharp diffraction peaks of β-SiC detected in the samples confirm that SiCNFs have indeed in-situ grown on the surfaces of carbon fibers. In addition, the diffraction peaks of Ni 3Si2 observed in the XRD pattern indicates that a small amount of Ni3Si2 compounds in the as-prepared fibers are formed by the reaction of Si and Ni atoms. The general morphology of the as-prepared fibers is demonstrated in Fig. 2(b). It can be found that all carbon fibers are well coated by SiCNFs. Fig. 2(c) shows a representative morphology for a carbon fiber with in situ grown SiCNFs. It is seen that the diameter of the SiCNFs coated carbon fiber is about 12 μm, and these irregularly aligned SiCNFs are distributed densely and uniformly on the surface of carbon fibers. Fig. 2(d) is the enlarged view of the highlighted area in Fig.2(c). It is seen that, the SiCNFs are mainly composed of withe-like nanofibers which intercross with each other. Note that 5
few residual Ni catalysts nanoparticles are observed on the top of SiCNFs rather than the surface of carbon fibers (see Fig.1). Therefore, it can be deduced that during CCVD process SiC molecules derived from MTS firstly deposit on these catalysts, consequently resulting in the SiCNFs grown along the catalysts. More details concerning in-situ growth mechanism of SiCNFs will be discussed hereinafter. In addition, the residual Ni catalysts may react with SiC molecules to form Ni3Si2 compounds, which are proved by XRD results. In order to better investigate the microstructures, the as-grown SiCNFs were further characterized by TEM. Fig. 3 shows the TEM and the selected area electron diffraction (SAED) photographs of SiCNFs. Obviously, the nanofibers with twigs shape are identical to the morpoholgy observed by SEM (See Fig. 2(d)). Regarding the geometry of the SiC nanofibers, the outer diameter is about 20-50 nm and the aspect ratio is about 100-150, as shown in Fig. 3(a). Moreover, the SiCNFs are of good crystallinity with lattice fringes of approximately 0.25 nm (see Fig. 3(b)), corresponding to the (111) plane of β-SiC. Note that the electron diffraction pattern as shown in the inset of Fig. 3(b) further confirms the existence of SiCNFs. Based on the previous Ref. [26], the growth of nano-fiber with Ni as catalysts during CCVD in this work could be ascribed to the diffusion and precipitation of SiC molecules in nickel nanoparticles. Fig.4 shows the schematic diagram of in-situ growth of SiCNFs and microstructures on the top of SiCNFs. Firstly, Ni nanoparticles split off from big spherical-like particles at elevated temperature [27]. Meanwhile, SiC molecules generated from the decompositon of MTS were adsorbed by these Ni nanoparticles. Subsequently, SiC molecules diffused to the habit plane of the precipitation phase of these Ni nanoparticles. Finally, as the amount of SiC molecules increased, the SiC phase in nickel nanoparticles were in saturation and the precipitation of SiC layer were stacked on the surface of nickel
6
nanoparticles (as shown in Fig.4(b)). Thus, the above-mentioned process led to the growth of SiCNFs on the fiber surfaces along the catalysts nanoparticles. 3.2 Oxidation resistance of SiCNFs coated carbon fibers Fig. 5 shows the TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers. It is found that the initial and final oxidation temperature of the SiCNFs coated carbon fibers is about 140 oC and 340 oC higher than that of the pure carbon fibers, respectively. Meanwhile, the weight loss of the SiCNFs coated carbon fibers significantly decreases at the severe oxidation stage (temperature range from 450 oC to 1200 oC in this work). The improved oxidation resistance of the SiCNFs coated carbon fibers can be ascribed to the disorderly arranged and intertwined SiCNFs which tightly wrap the carbon fibers and effectively prevent diffusion of oxygen as a barrier. Therefore, the oxidation of carbon fibers is delayed due to the reaction between oxygen and SiCNFs, and the weight loss is somewhat compensated by weight gain from the oxidation of SiC. Moreover, the generation of SiO2 on the fiber surface during the oxidation could also, to some extent, protect the carbon fibers. In conclusion, the higher initial and final oxidation temperature of the SiCNFs coated carbon fibers indicates an improved oxidation resistance in oxygen at elevated temperature. 3.3 Dielectric and microwave absorption properties of SiCNFs coated carbon fibers The dependences of the real and the imaginary parts of εr (εr=ε′-jε″) and μr (μr=μ′-jμ″) with frequency for the SiCNFs coated carbon fibers and pure carbon fibers are illustrated in Fig. 6(a) and Fig. 6(b), respectively. As shown in Fig. 6(a), in comparison to the pure carbon fiber, both the ε′ and ε″ for the SiCNFs coated carbon fibers obviously decrease, especially when the frequency is less than 8GHz. However, as shown in Fig. 6(b), the effect of SiCNFs on the real permeability (μ′) and imaginary permeability (μ″) of carbon fibers could be neglected, which indicates that the magnetism of the
7
residual Ni catalyst and Ni3Si2 compounds may disappear during the CCVD process, because the CCVD process temperature (1000 oC in this work) is higher than their Curie temperature. The significant change in permittivity is mainly attributed to the incorporation of SiCNFs. The connection within electric-conductive carbon fibers could be broken after the growth of SiCNFs on the surface. Therefore, the carbon fibers with high ε′ and ε″ can be effectively tailored by SiCNFs. Moreover, the microwave absorbing properties of the SiCNFs coated carbon fibers are expected to be effectively improved because of the reduced ratio of εr/μr which corresponds to better impedance matching characteristic. Fig. 7 shows the reflection loss (RL) of the pure carbon fibers and SiCNFs coated carbon fibers according to Eqs. (1) and (2) on the basis of measured values of εr and μr. As shown in Fig 7(a), it can be found that the microwave absorption properties of the SiCNFs coated carbon fibers are significantly enhanced compared with the pure carbon fibers. The RL for the SiCNFs coated CFs is less than -10 dB over the frequency range of 9.2-11.7 GHz, when the thickness of specimen is 2 mm. And an optimal RL value of -28.3 dB could be reached at 5.6 GHz for the SiCNFs coated carbon fibers, when the thickness is 3.5 mm. Additionally, compared with the magnetic metal coated carbon fibers in Ref. [15, 28], both the microwave absorbing strength and absorbing bandwidth have been enhanced for the SiCNFs coated carbon fibers. Moreover, the RL values of the SiCNFs coated carbon fibers shift to lower frequency with increasing thickness, suggesting that the range of absorption frequency can be modulated by adjusting the thickness of the samples. According to the resonant absorb theory [29], when microwave is incident on an absorber sample backed by a perfect conductor, the predicted matching thickness dcal at the matching frequency f is given by [30]: m
d cal m
4f
c
( f ) ( f )
(3)
8
Where μ(f) and ε(f) are complex permeability and permittivity at the frequency of f, respectively. The calculated dcal based on Eq. (3) is shown in Fig. 7(b), which is well consistent with the dmat m m obtained from reflection loss results. Thus, the attenuation peaks of the SiCNFs coated carbon fibers shift to lower frequency with increasing sample thickness. And this result indicates that quarter-wavelength absorption is one of effective way to improve microwave absorption for the SiCNFs coated carbon fibers. However, except for the mechanism mentioned above, more attention should be devoted to the mechanisms of intrinsic dielectric loss for the superior microwave absorption performance of SiCNFs coated carbon fibers. As illustrated in Fig. 8(a), due to the electronic accumulation on the surfaces of short carbon fibers as well as on the defects inside carbon fibers under external electric field, short carbon fibers are considered to play roles as micro-capacitors. Therefore, the electric polarization and relaxation should follow the Debye theory [31, 32]. This is further confirmed by the fact that the experimental points are more inclined to distribute on distinguishable circular arcs in Cole-Cole plot. It should be noticed that there also exists hopping migration between neighboring SiCNFs under electric field for the SiCNFs coated carbon fibers. Therefore, the Cole-Cole locus of the SiCNFs coated carbon fibers exhibits additional circular arc corresponding the electric polarization and relaxation, compared to that of the pure carbon fibers (as shown in Fig. 8(b)). From this point of view, the superior microwave absorbing properties of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration.
4. Conclusion In this work, SiCNFs were successfully grown in-situ on the surfaces of carbon fibers by catalysis
9
chemical vapor deposition. The as-prepared nano-fibers with withe-like morphology are mainly composed of β-SiC. Additionally, compared with the pure carbon fibers the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved due to better oxidation resistance of SiC in air. Moreover, the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflection loss (RL) for the SiCNFs coated carbon fibers is less than -10 dB within the frequency ranging from 9.2 to 11.7 GHz when the thickness is 2 mm. Most importantly, the optimal RL value of -28.3 dB can be reached at 5.6 GHz when the thickness is set to 3.5 mm. The enhanced microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. The current study indicates that the SiCNFs coated carbon fibers could be a promising candidate for novel solution with favorable thickness and light-weight in electromagnetic wave absorption applications.
Acknowledgements This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2011CB605804), National Natural Science Foundation of China (51604107) and Science research project of Hunan Provincial Department of Education (16C0461).
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Figure captions
Fig. 1. Schematic illustration of the fabrication process for SiCNFs in-situ grown on the carbon fibers Fig. 2. (a) XRD patterns and (b, c, d) SEM images of SiCNFs grown on the carbon fibers Fig. 3. (a) TEM image of SiCNFs, (b) HR-TEM image and SAED pattern of SiCNFs Fig.4. (a) Schematic diagram of the growth of SiCNFs and (b) image of the top of SiCNFs during CCVD 13
Fig. 5. TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers in air Fig. 6. The permittivity (a) and permeability (b) of the pure and SiCNFs coated carbon fibers Fig. 7. (a) RL curves of the SiCNFs coated carbon fibers with different thickness, (b) the frequency dependence of matching thickness (symbol) and calculated thickness (line) for the SiCNFs coated carbon fibers Fig.8. Cole-Cole diagram for (a) pure carbon fibers and (b) SiCNFs coated carbon fibers
Carbon fibers
Electroplating Ni
CCVD
Fig. 1. Schematic illustration of the fabrication process for SiCNFs in-situ grown on the carbon fibers
14
◆
(a)
(b) ▼
Intensity (a.u.)
◆ ★
C SiC Ni3Si2
▼
★
★ ★
10
20
30
◆
▼★
◆
★ ◆ ★★
40
50
60
70
80
2degree
(d)
(c)
Fig. 2. (a) XRD patterns and (b, c, d) SEM images of SiCNFs grown on the carbon fibers
15
(a)
(b)
0.25 nm
Fig. 3. (a) TEM image of SiCNFs, (b) HR-TEM image and SAED pattern of SiCNFs
(b)
(a) CCVD
SiC nanofiber SiC molecule Ni nanoparticle
Ni Carbon fiber
Fig.4. (a) Schematic diagram of the growth of SiCNFs and (b) image of the top of SiCNFs during CCVD
16
100
TGA,%
80 60
SiCNFs/Cf Cf
40 20 0 0
200
400
600
800
1000
1200
o
Temperature, C
Fig. 5. TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers in air
2.0 (a)
(b)
' of Cf " of Cf ' of SiCNFs/Cf " of SiCNFs/Cf
140 120
Permeability parameter
Permittivity parameter
160
100 80 60 40 20
' of Cf " of Cf ' of SiCNFs/Cf " of SiCNFs/Cf
1.5
1.0
0.5
0.0
0 2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
Frequency/GHz
Frequency/GHz
Fig. 6. The permittivity (a) and permeability (b) of the pure and SiCNFs coated carbon fibers
17
(a)
0
RL(dB)
-5 -10 -15 Cf (2.0 mm)
-20
SiCNFs/Cf (2.0 mm) SiCNFs/Cf (2.5 mm)
-25
SiCNFs/Cf (3.0 mm)
-30
(b)
SiCNFs/Cf (3.5 mm)
9 8
dm /(mm)
7 6 5 4 3
cal dm
2
mat dm
1 2
4
6
8
10
12
14
16
18
Frequency/GHz Fig. 7. (a) RL curves of the SiCNFs coated carbon fibers with different thickness, (b) the frequency dependence of matching thickness (symbol) and calculated thickness (line) for the SiCNFs coated carbon fibers
18
Fig.8. Cole-Cole diagram for (a) pure carbon fibers and (b) SiCNFs coated carbon fibers
19
PII: DOI: Reference:
S0272-8842(17)30119-0 http://dx.doi.org/10.1016/j.ceramint.2017.01.095 CERI14557
To appear in: Ceramics International Received date: 10 November 2016 Revised date: 14 January 2017 Accepted date: 20 January 2017 Cite this article as: Wei Zhou, Lan Long, Peng Xiao, Yang Li, Heng Luo, Weida Hu and Rui-ming Yin, Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.01.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties Wei Zhoua,b,1, Lan Longa,b,1, Peng Xiaob, Yang Lib*, Heng Luob, Wei-da Hua, Rui-ming Yina a
College of Metallurgy and Materials engineering, Hunan University of Technology, Zhuzhou 412008, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Abstract: Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 oC. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β-SiC, present a withe-like morphology with diameter of 20-50 nm and aspect ratio of 100-150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than -10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach -19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than -2.1 dB within the
Corresponding author. Fax: +86-731-88830131.
E-mail address: [email protected] (Y. Li), [email protected](H. Luo) 1 These authors contributed equally to this work. 1
whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials. Keywords: Silicon carbide nano-fibers; Carbon fibers; Microwave absorption properties
1. Introduction With the explosive growth of information technology and the rapidly expanding use of various high-frequency electronic devices, electromagnetic interference has now become a serious pollution issue. In order to solve the electromagnetic interference problem, great efforts have been done in development for effective microwave absorbing materials with light weight, tiny thickness, high strength, low cost, and strong and broad wave absorption [1-4]. Carbon fibers and their composites have been found to be fascinating candidates for microwave absorption, due to their low density, excellent mechanical property, good conductivity, low cost, and wide availability [5-8]. However, the low electrical resistivity (<10-3Ω cm) of carbon fibers could easily lead to strong reflection against electromagnetic waves and poor microwave absorption [9]. Therefore, many investigations concerning the improvement of microwave absorbing property, especially applications of the magnetic metals and their oxide coatings, have been proved to be a solution for carbon fibers [10-16]. However, the current problems for magnetic metals or oxides including the relatively high cost, high density and worst point of all, incapability for dissipating the electromagnetic energy when the service
2
temperature is higher than their Curie temperatures, greatly restricts their wide applications [17-19]. Silicon Carbide (SiC), as a typical semiconductor material, is a good candidate for light weight and high-temperature resistance microwave absorber due to its low density, favorable oxidation-resistance, high thermal and chemical stability, superior mechanical strength and adjustable electrical conductivity, etc [20-22]. Note that the electromagnetic property of SiC generally varies with their morphologies, and the one-dimensional (1D) SiC nano-fibers (SiCNFs) with high surface-to-volume ratio and shape structure effects were considered to be of better microwave absorption performance than bulk or micro sized SiC particles [23, 24]. Therefore, it is generally believed that the SiCNFs coated carbon fibers could achieve efficient and stable electromagnetic wave attenuation. However, there is still a lack of reports on the microwave absorption properties of the in-situ grown SiCNFs modified carbon fibers. In this paper, SiCNFs were in-situ grown on the surfaces of carbon fibers by catalysis chemical vapor deposition (CCVD) at a relatively low temperature. The microstructure, oxidation resistance and microwave absorption properties of the as-grown SiCNFs coated carbon fibers were investigated in detail. The present study suggests that the SiCNFs coated carbon fibers are promising for microwave absorbing materials with light weight and thin thickness.
2. Materials and Experimental Procedures 2.1 Preparation of the SiCNFs coated Carbon fibers The experimental procedures including the electroplating Ni and CCVD process are presented in schematic diagram as shown in Fig. 1. Firstly, the Toray T700 carbon fibers with average diameter of 7 um and length of 10 cm were immersed in acetone for 24 h, and afterwards ultrasonically cleaned in
3
distilled water, subsequently dried at 100 oC for 2 h. The nickel nanoparticles, as catalysts for SiCNFs, were deposited on the carbon fiber surfaces via electroplating in a Nickel sulfate solution with the concentration of 15 wt%. The electronic current intensity during the electroplating was 10 ampere. Finally, SiCNFs were in-situ grown on the carbon fiber surfaces during the CCVD process. In current work, the CCVD process was performed in the CVD furnace at the temperature of 1000 oC under the pressure of 600 Pa with the dwell time of 4h. Additionally, Methyltrichlorosilane (MTS, CH 3SiCl3), hydrogen (H2) and argon (Ar) were used as the source gas of SiCNFs, carrying gas and dilute gas, respectively. Note that hydrogen (H2) was firstly introduced to reduce the catalysts for 10 min when the temperature of 1000 oC is stabilized, and then the other two gases were introduced to activate the growth of SiCNFs on carbon fibers. After the growth of SiCNFs on the surfaces, the mass of the carbon fibers were measured at room temperature and their weight gain were approximately 18%. 2.2 Characterizations The phase of the as-prepared fibers after CCVD was characterized by X-ray diffraction (XRD, D/max2550, Rigaku) with Cu Kα radiation at 35 kV and 20 mA. The morphology and structure of SiCNFs on carbon fibers were characterized by a scanning electron microscopy (SEM, Nova NanoSEM 230) and a transmission electron microscopy (TEM, JEOL 2010F). The anti-oxidation properties for the SiCNFs coated carbon fibers in air up to 1200 oC was studied with a DTA/TGA instrument (STA 409PC, NETZSCH). In order to study the microwave absorbing properties for the SiCNFs coated carbon fibers, the measurement for electromagnetic parameters were conducted on a network analyzer (Agilent5230A) within the frequency ranging from of 2 to 18 GHz by coaxial line method. In the testing process, the SiCNFs coated carbon fibers were firstly cut into short fragments with length 2-3 mm, and afterwards homogeneously dispersed into paraffin. The mass ratio of paraffin
4
and SiCNFs coated carbon fibers in the toroidal shaped specimen with an outer diameter of 7.0 mm, an inner diameter of 3.0 mm and a thickness of 2.0 mm were set to 80 wt% and 20 wt%, respectively. According to the transmission line theory, the reflection loss (RL) curves with different thickness were calculated by the following equations [25]:
RL(dB) 20 log 1/ 2
Z in r r
Z in 1 Z in 1
(1)
2 fd 1/ 2 tanh j r r c
(2)
where Zin is the normalized input impedance, εr=ε′-jε″ and μr=μ′-jμ″ are the complex permittivity and permeability of material, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave in free space, respectively.
3. Results and discussion 3.1 Phase composition, structure and growing mechanism of SiCNFs The XRD patterns of the as-prepared fibers are presented in Fig. 2. Obviously, the sharp diffraction peaks of β-SiC detected in the samples confirm that SiCNFs have indeed in-situ grown on the surfaces of carbon fibers. In addition, the diffraction peaks of Ni 3Si2 observed in the XRD pattern indicates that a small amount of Ni3Si2 compounds in the as-prepared fibers are formed by the reaction of Si and Ni atoms. The general morphology of the as-prepared fibers is demonstrated in Fig. 2(b). It can be found that all carbon fibers are well coated by SiCNFs. Fig. 2(c) shows a representative morphology for a carbon fiber with in situ grown SiCNFs. It is seen that the diameter of the SiCNFs coated carbon fiber is about 12 μm, and these irregularly aligned SiCNFs are distributed densely and uniformly on the surface of carbon fibers. Fig. 2(d) is the enlarged view of the highlighted area in Fig.2(c). It is seen that, the SiCNFs are mainly composed of withe-like nanofibers which intercross with each other. Note that 5
few residual Ni catalysts nanoparticles are observed on the top of SiCNFs rather than the surface of carbon fibers (see Fig.1). Therefore, it can be deduced that during CCVD process SiC molecules derived from MTS firstly deposit on these catalysts, consequently resulting in the SiCNFs grown along the catalysts. More details concerning in-situ growth mechanism of SiCNFs will be discussed hereinafter. In addition, the residual Ni catalysts may react with SiC molecules to form Ni3Si2 compounds, which are proved by XRD results. In order to better investigate the microstructures, the as-grown SiCNFs were further characterized by TEM. Fig. 3 shows the TEM and the selected area electron diffraction (SAED) photographs of SiCNFs. Obviously, the nanofibers with twigs shape are identical to the morpoholgy observed by SEM (See Fig. 2(d)). Regarding the geometry of the SiC nanofibers, the outer diameter is about 20-50 nm and the aspect ratio is about 100-150, as shown in Fig. 3(a). Moreover, the SiCNFs are of good crystallinity with lattice fringes of approximately 0.25 nm (see Fig. 3(b)), corresponding to the (111) plane of β-SiC. Note that the electron diffraction pattern as shown in the inset of Fig. 3(b) further confirms the existence of SiCNFs. Based on the previous Ref. [26], the growth of nano-fiber with Ni as catalysts during CCVD in this work could be ascribed to the diffusion and precipitation of SiC molecules in nickel nanoparticles. Fig.4 shows the schematic diagram of in-situ growth of SiCNFs and microstructures on the top of SiCNFs. Firstly, Ni nanoparticles split off from big spherical-like particles at elevated temperature [27]. Meanwhile, SiC molecules generated from the decompositon of MTS were adsorbed by these Ni nanoparticles. Subsequently, SiC molecules diffused to the habit plane of the precipitation phase of these Ni nanoparticles. Finally, as the amount of SiC molecules increased, the SiC phase in nickel nanoparticles were in saturation and the precipitation of SiC layer were stacked on the surface of nickel
6
nanoparticles (as shown in Fig.4(b)). Thus, the above-mentioned process led to the growth of SiCNFs on the fiber surfaces along the catalysts nanoparticles. 3.2 Oxidation resistance of SiCNFs coated carbon fibers Fig. 5 shows the TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers. It is found that the initial and final oxidation temperature of the SiCNFs coated carbon fibers is about 140 oC and 340 oC higher than that of the pure carbon fibers, respectively. Meanwhile, the weight loss of the SiCNFs coated carbon fibers significantly decreases at the severe oxidation stage (temperature range from 450 oC to 1200 oC in this work). The improved oxidation resistance of the SiCNFs coated carbon fibers can be ascribed to the disorderly arranged and intertwined SiCNFs which tightly wrap the carbon fibers and effectively prevent diffusion of oxygen as a barrier. Therefore, the oxidation of carbon fibers is delayed due to the reaction between oxygen and SiCNFs, and the weight loss is somewhat compensated by weight gain from the oxidation of SiC. Moreover, the generation of SiO2 on the fiber surface during the oxidation could also, to some extent, protect the carbon fibers. In conclusion, the higher initial and final oxidation temperature of the SiCNFs coated carbon fibers indicates an improved oxidation resistance in oxygen at elevated temperature. 3.3 Dielectric and microwave absorption properties of SiCNFs coated carbon fibers The dependences of the real and the imaginary parts of εr (εr=ε′-jε″) and μr (μr=μ′-jμ″) with frequency for the SiCNFs coated carbon fibers and pure carbon fibers are illustrated in Fig. 6(a) and Fig. 6(b), respectively. As shown in Fig. 6(a), in comparison to the pure carbon fiber, both the ε′ and ε″ for the SiCNFs coated carbon fibers obviously decrease, especially when the frequency is less than 8GHz. However, as shown in Fig. 6(b), the effect of SiCNFs on the real permeability (μ′) and imaginary permeability (μ″) of carbon fibers could be neglected, which indicates that the magnetism of the
7
residual Ni catalyst and Ni3Si2 compounds may disappear during the CCVD process, because the CCVD process temperature (1000 oC in this work) is higher than their Curie temperature. The significant change in permittivity is mainly attributed to the incorporation of SiCNFs. The connection within electric-conductive carbon fibers could be broken after the growth of SiCNFs on the surface. Therefore, the carbon fibers with high ε′ and ε″ can be effectively tailored by SiCNFs. Moreover, the microwave absorbing properties of the SiCNFs coated carbon fibers are expected to be effectively improved because of the reduced ratio of εr/μr which corresponds to better impedance matching characteristic. Fig. 7 shows the reflection loss (RL) of the pure carbon fibers and SiCNFs coated carbon fibers according to Eqs. (1) and (2) on the basis of measured values of εr and μr. As shown in Fig 7(a), it can be found that the microwave absorption properties of the SiCNFs coated carbon fibers are significantly enhanced compared with the pure carbon fibers. The RL for the SiCNFs coated CFs is less than -10 dB over the frequency range of 9.2-11.7 GHz, when the thickness of specimen is 2 mm. And an optimal RL value of -28.3 dB could be reached at 5.6 GHz for the SiCNFs coated carbon fibers, when the thickness is 3.5 mm. Additionally, compared with the magnetic metal coated carbon fibers in Ref. [15, 28], both the microwave absorbing strength and absorbing bandwidth have been enhanced for the SiCNFs coated carbon fibers. Moreover, the RL values of the SiCNFs coated carbon fibers shift to lower frequency with increasing thickness, suggesting that the range of absorption frequency can be modulated by adjusting the thickness of the samples. According to the resonant absorb theory [29], when microwave is incident on an absorber sample backed by a perfect conductor, the predicted matching thickness dcal at the matching frequency f is given by [30]: m
d cal m
4f
c
( f ) ( f )
(3)
8
Where μ(f) and ε(f) are complex permeability and permittivity at the frequency of f, respectively. The calculated dcal based on Eq. (3) is shown in Fig. 7(b), which is well consistent with the dmat m m obtained from reflection loss results. Thus, the attenuation peaks of the SiCNFs coated carbon fibers shift to lower frequency with increasing sample thickness. And this result indicates that quarter-wavelength absorption is one of effective way to improve microwave absorption for the SiCNFs coated carbon fibers. However, except for the mechanism mentioned above, more attention should be devoted to the mechanisms of intrinsic dielectric loss for the superior microwave absorption performance of SiCNFs coated carbon fibers. As illustrated in Fig. 8(a), due to the electronic accumulation on the surfaces of short carbon fibers as well as on the defects inside carbon fibers under external electric field, short carbon fibers are considered to play roles as micro-capacitors. Therefore, the electric polarization and relaxation should follow the Debye theory [31, 32]. This is further confirmed by the fact that the experimental points are more inclined to distribute on distinguishable circular arcs in Cole-Cole plot. It should be noticed that there also exists hopping migration between neighboring SiCNFs under electric field for the SiCNFs coated carbon fibers. Therefore, the Cole-Cole locus of the SiCNFs coated carbon fibers exhibits additional circular arc corresponding the electric polarization and relaxation, compared to that of the pure carbon fibers (as shown in Fig. 8(b)). From this point of view, the superior microwave absorbing properties of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration.
4. Conclusion In this work, SiCNFs were successfully grown in-situ on the surfaces of carbon fibers by catalysis
9
chemical vapor deposition. The as-prepared nano-fibers with withe-like morphology are mainly composed of β-SiC. Additionally, compared with the pure carbon fibers the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved due to better oxidation resistance of SiC in air. Moreover, the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflection loss (RL) for the SiCNFs coated carbon fibers is less than -10 dB within the frequency ranging from 9.2 to 11.7 GHz when the thickness is 2 mm. Most importantly, the optimal RL value of -28.3 dB can be reached at 5.6 GHz when the thickness is set to 3.5 mm. The enhanced microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. The current study indicates that the SiCNFs coated carbon fibers could be a promising candidate for novel solution with favorable thickness and light-weight in electromagnetic wave absorption applications.
Acknowledgements This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2011CB605804), National Natural Science Foundation of China (51604107) and Science research project of Hunan Provincial Department of Education (16C0461).
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Figure captions
Fig. 1. Schematic illustration of the fabrication process for SiCNFs in-situ grown on the carbon fibers Fig. 2. (a) XRD patterns and (b, c, d) SEM images of SiCNFs grown on the carbon fibers Fig. 3. (a) TEM image of SiCNFs, (b) HR-TEM image and SAED pattern of SiCNFs Fig.4. (a) Schematic diagram of the growth of SiCNFs and (b) image of the top of SiCNFs during CCVD 13
Fig. 5. TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers in air Fig. 6. The permittivity (a) and permeability (b) of the pure and SiCNFs coated carbon fibers Fig. 7. (a) RL curves of the SiCNFs coated carbon fibers with different thickness, (b) the frequency dependence of matching thickness (symbol) and calculated thickness (line) for the SiCNFs coated carbon fibers Fig.8. Cole-Cole diagram for (a) pure carbon fibers and (b) SiCNFs coated carbon fibers
Carbon fibers
Electroplating Ni
CCVD
Fig. 1. Schematic illustration of the fabrication process for SiCNFs in-situ grown on the carbon fibers
14
◆
(a)
(b) ▼
Intensity (a.u.)
◆ ★
C SiC Ni3Si2
▼
★
★ ★
10
20
30
◆
▼★
◆
★ ◆ ★★
40
50
60
70
80
2degree
(d)
(c)
Fig. 2. (a) XRD patterns and (b, c, d) SEM images of SiCNFs grown on the carbon fibers
15
(a)
(b)
0.25 nm
Fig. 3. (a) TEM image of SiCNFs, (b) HR-TEM image and SAED pattern of SiCNFs
(b)
(a) CCVD
SiC nanofiber SiC molecule Ni nanoparticle
Ni Carbon fiber
Fig.4. (a) Schematic diagram of the growth of SiCNFs and (b) image of the top of SiCNFs during CCVD
16
100
TGA,%
80 60
SiCNFs/Cf Cf
40 20 0 0
200
400
600
800
1000
1200
o
Temperature, C
Fig. 5. TGA curves of the pure carbon fibers and SiCNFs coated carbon fibers in air
2.0 (a)
(b)
' of Cf " of Cf ' of SiCNFs/Cf " of SiCNFs/Cf
140 120
Permeability parameter
Permittivity parameter
160
100 80 60 40 20
' of Cf " of Cf ' of SiCNFs/Cf " of SiCNFs/Cf
1.5
1.0
0.5
0.0
0 2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
Frequency/GHz
Frequency/GHz
Fig. 6. The permittivity (a) and permeability (b) of the pure and SiCNFs coated carbon fibers
17
(a)
0
RL(dB)
-5 -10 -15 Cf (2.0 mm)
-20
SiCNFs/Cf (2.0 mm) SiCNFs/Cf (2.5 mm)
-25
SiCNFs/Cf (3.0 mm)
-30
(b)
SiCNFs/Cf (3.5 mm)
9 8
dm /(mm)
7 6 5 4 3
cal dm
2
mat dm
1 2
4
6
8
10
12
14
16
18
Frequency/GHz Fig. 7. (a) RL curves of the SiCNFs coated carbon fibers with different thickness, (b) the frequency dependence of matching thickness (symbol) and calculated thickness (line) for the SiCNFs coated carbon fibers
18
Fig.8. Cole-Cole diagram for (a) pure carbon fibers and (b) SiCNFs coated carbon fibers
19