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Net-like [email protected] coaxial nanocable towards superior lightweight and broadband microwave absorber
Composites Part B 179 (2019) 107525
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Net-like SiC@C coaxial nanocable towards superior lightweight and broadband microwave absorber Meng Zhang a, Hui Lin a, Shiqi Ding a, Ting Wang b, Zhenjiang Li a, *, Alan Meng b, **, Qingdang Li a, Yusheng Lin c a
College of Electromechanical Engineering, Key Laboratory of Polymer Material Advanced Manufacturing’s Technology of Shandong Province, College of Sino-German Science and Technology, Qingdao University of Science and Technology, Qingdao, 266061, Shandong province, China State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, College of Chemical Engineering in Gaomi Campus, Qingdao University of Science and Technology, Qingdao, 266042, Shandong province, China c College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266061, Shandong province, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: SiC@C NCs Microwave absorber Lightweight Wide bandwidth Mechanism
In this study, a practical lightweight and broadband microwave absorber, namely, the net-like amorphous carbon-coated silicon carbide coaxial nanocables (SiC@C NCs), which had low density and excellent physico chemical stability, was successfully fabricated according to the hydrothermal-carbonization strategy. Typically, the microwave absorption performance exhibited broad effective absorption bandwidth (EAB) of up to 7.2 GHz (9.12–16.32 GHz) at a small matching thickness of 2.58 mm. Meanwhile, the minimal reflection loss (RL) value of 51.53 dB with the corresponding EAB across the whole Ku-band (11.68–18 GHz) appeared at 2.08 mm, which suggested that, the as-prepared SiC@C NCs displayed extensive application prospects as a superior lightweight and broadband microwave absorber. Moreover, the systematic characterization results indicated that, the su perior microwave absorption performance was ascribed to the interfacial polarization and multiple reflections on the heterogeneous interface between the SiC nanowires (SiC NWs) core and the amorphous carbon shell, together with the electron polarization and Debye dipolar relaxation resulted from the stacking faults of SiC NWs core, and the abundant defect and disorder within the amorphous carbon shell.
1. Introduction Nowadays, electromagnetic radiation has post an increasingly serious threat worldwide, which has been ranked as fifth leading source of pollution following water pollution, air pollution, noise and light pollution [1]. Electromagnetic absorbing material (absorber) can effectively shield the human bodies and wildlife, which also serves as the crucial electric equipment to attenuate the incident microwave energy [2]. The absorber can be classified as two categories based on various loss mechanisms, namely, magnetic materials and dielectric materials [3–5]. Typically, considering the requirements of actual application, a practical absorber should possess the advantages of lightweight, thin matching thickness, broad bandwidth and strong absorption, as well as the technical characteristics, such as instant low price, simple prepara tion process and environmental friendliness [6]. On the other hand, the magnetic materials have the disadvantages of large density, corrosion
susceptibility, and loss of ferromagnetic property at high working tem perature, which have hindered their applications in microwave ab sorption field; at the same time, the dielectric materials have gradually become more and more popular [7,8]. Carbonaceous materials appear to be the favorable candidates for microwave absorber, which is ascribed to their inherent advantages of low toxicity, ultra-low density, high electrical conductivity, high dielectric loss and excellent stability [9–11]. Therefore, various methods have been tried attempting to prepare the novel carbonaceous materials with high permittivity, which has been regarded as one of the most significant strategies to achieve excellent microwave absorption capac ity [12–14]. However, it advances slowly, and only a few research groups have achieved satisfying results on microwave absorption [15]. At the same time, the uncontrollable agglomeration behavior of nano scale carbon-based materials with high surface energy gives rise to worse microwave absorption performance [16,17]. Fortunately,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (A. Meng). https://doi.org/10.1016/j.compositesb.2019.107525 Received 23 August 2019; Received in revised form 8 October 2019; Accepted 9 October 2019 Available online 13 October 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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constructing a rational nanocomposite contributes to generating abun dant interfacial polarization between the carbonaceous material and other component, which is identified as an available and efficient approach [18–21]. Up to date, extensive carbon-based nanocomposites have been pre pared through numerous methods (such as surface modification, doping and hybridization), particularly for products composed of carbon and nonmetallic components, like SiO2@C [22], CNTs/SiO2 [23,24], rGO/ZnO [25], Graphene/g-C3N4 [26], C/TiO2 [27,28], polymer/C [29, 30], MXene/C [31]. Such products could enhance the electromagnetic attenuation by improving dielectric polarization ability of bare carbo naceous materials, without dramatically increasing their density. However, most of them achieved narrow effective absorption bandwidth (EAB) and weak reflection loss (RL), hindering their further application and research progress. Silicon carbide (SiC) is a representative dielectric material, which has a broad absorption frequency range, as well as outstanding me chanical properties, chemical resistivity and thermal stability at room and high temperatures, and is thereby regarded as the “Star of Tomorrow” in the absorber family [32,33]. Some recent reports demonstrate that SiC nanowires (SiC NWs) exhibit superior electro magnetic attenuation performance to the bulk counterpart, which is attributable to their large specific surface areas, abundant stacking and tunable electrical conductivity [34]. Notably, nanocomposites composed of the SiC one-dimensional nanostructures and the carbona ceous material may serve as the most promising microwave absorber, which not only enhance the microwave absorption performances of pure carbon-based component, but may also serve as the ideal structural materials under harsh environments, such as high temperature, corro sion, and ultraviolet radiation. In this study, the SiC NWs were selected as the core supporting skeletons, whereas the polyvinylpyrrolidone (PVP)-derived amorphous carbon was used as the shell to successfully synthesize the lightweight net-like SiC@C coaxial nanocables (SiC@C NCs). Typically, the micro wave absorption performances of the as-synthesized products exhibited broad EAB and strong RL, which indicated that they might be exten sively applied in the lightweight and high frequency microwave ab sorption fields. Additionally, a synergetic absorption mechanism was also proposed based on the multiple reflections, defects, stacking faults and interfacial polarization. Notably, the approach proposed in this study was a low-price, simple-operation and environmentally friendly
process, which might provide an effective clue for developing the carbon-based lightweight absorbing materials. 2. Experimental procedures 2.1. Materials Si and SiO2 of analytical purity were provided by Tianjin Chemical Reagents and Shanghai Chemistry Reagents Ltd., respectively. CH4 and argon (purchased from Qingdao Three Factor Gas Co., Ltd.) of 99.99% purify were used as the carbon source and carrier gas, respectively. Graphite substrate (purchased from Qingdao Shenghe Graphite Co., Ltd.) was employed as deposition substrate. Carbon cloth was purchased from Jiangsu Tianniao High Technology Co. Ltd, which could load the Si–SiO2 mixed powders and insulate them from the graphite substrate (as well as the as-synthesized produces). The Polyvinylpyrrolidone (PVP) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd., China. Other reagents were of analytical grade and used as received without further purification. 2.2. Synthesis of SiC NWs Firstly, the Si–SiO2 mixed powders, a piece of carbon cloth, and the graphite substrate were placed into the homemade reaction chamber successively. Subsequently, the high purity argon was introduced into the furnace for three times using a rotary vacuum pump to exhaust the oxygen. Then, the furnace was heated to 1250 � C at an average heating rate of about 350–400 � C/h, and later a steady CH4 flow (at 0.15–0.20 sccm) was introduced for 20–25 min. Finally, the gas was switched off and the furnace was gradually cooled down to 900 � C for 10 min. The schematic illustration for the formation of SiC NWs was shown infor mation (Fig. S1). 2.3. Synthesis of SiC@C NCs At first, about 0.5 g SiC NWs were stripped from the graphite sub strate and dispersed into 60 ml ethanol according to the ultrasound method for 1 h. Subsequently, 0.4 g PVP powers were added into the SiC NWs dispersion, followed by sufficient stirring, and then the mixture was transferred into the 100 ml high pressure autoclave for a heating process at 120 � C for 6 h. Afterwards, the products after centrifugation
Fig. 1. Schematic illustration of the SiC@C NCs. 2
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Fig. 2. Representative XRD patterns of the SiC NWs and SiC@C NCs (a) Low-magnification SEM images of the SiC NWs (b) and SiC@C NCs (d), high-magnification SEM images of the SiC NWs (c) and SiC@C NCs (e, f).
and drying were placed into the tubular furnace and sintered under the Ar/H2 atmosphere at 1000 � C for 1 h. Fig. 1 illustrates the synthesis process of SiC@C NCs.
displayed in Fig. 2d and e that, a larger amount of net-like nanowires were collected after the reaction process. Obviously, the tumor-like knots (as indicated by the yellow arrows) were easily formed at some intersections of nanowires at horizontal and vertical orientations. Different from SiC NWs with smooth surface, the high-magnification SEM image in Fig. 2f revealed that the as-synthesized products with the diameter of 20–60 nm were covered by the rough surfaces [38]. To obtain more detailed structural information, both SiC NWs and SiC@C NCs were characterized by TEM and HRTEM, respectively. Fig. 3a exhibits the TEM image of a large number of dense SiC NWs, which have smooth surface and are several microns in length. It was found in Fig. 3b that, the average diameter of SiC NWs ranged from 12 to 20 nm. Fig. 3c shows the representative high-magnification TEM image of the SiC NWs. As was observed, there were dense stacking faults (as indicated by the red arrows) in the SiC NWs. The insert in Fig. 3c shows the corresponding selected area electron diffraction (SAED) pattern. It was seen that, there were obvious streaks in addition to a series of bright electron diffraction spots [39], and the diffraction spots were assigned to the cubic SiC. Fig. 3d displays the HRTEM image of the SiC NWs, with the interplanar spacing of about 0.25 nm, which is corresponding to the (111) plane of cubic SiC, implying that the growth direction of the nanowire is [111]. For the SiC@C NCs, the TEM image shown in Fig. 3e clearly suggested that, the cross-linked net-like nanowires possessed the coaxial cable-type nanostructures, which were composed of the thin shell layer and the inner core. Fig. 3f demonstrates the average diameter of the net-like nanowires of approximately 28 nm, which is generally consistent with the SEM characterization result. Fig. 3g shows the high-magnification TEM image recorded from an individual nanowire, and the thickness of the amorphous carbon shell layer is about 3.8 nm. Moreover, the core region and S.F. (as indicated by the blue arrows) with dark contrast, together with the shell layer with bright contrast, indi cated the corresponding crystalline and amorphous state, respectively. In addition, the insert in Fig. 3g displays the corresponding SAED pattern of the net-like nanowire, in which the legible diffraction spots are also indexed to the cubic SiC. On the other hand, the HRTEM image in Fig. 3h revealed that the interplanar spacing of the core area wrapped by amorphous carbon was about 0.25 nm, which was in good agreement with the {111} plane of the cubic SiC [40]. According to the crystal lography theory, the atom stacking direction and the growth direction of nanowire were indexed as [-111] and [111], respectively, which had well coincided with the reported values [41].
2.4. Characterization Scanning electron microscopy (SEM) was carried out using the JEOL JSM-6, whereas transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) were performed using the FEI Tecnai G2 F20 microscope. X-ray diffraction (XRD, Rigaku D/max-2400 X-ray diffractometer) was employed to investigate the product phase compositions and crystal structure. X-ray photoelectron spectroscopy (XPS) was conducted on the Thermo Scientific ESCALAB 250XI, so as to determine the product chemical state. To measure the complex electromagnetic parameters, the assynthesized nanocomposites were mixed homogeneously with paraffin at a weight filling ratio of 30%. Thereafter, the mixture was pressed into a toroidal-shaped sample, with an outer diameter of 7 mm, inner diameter of 3 mm and the thickness of 3 mm. Then, the electromagnetic parameters were measured using the vector network analyzer (Agilent, N5230A) under the transmission-reflection mode at the frequency range of 2–18 GHz. 3. Results and discussion Fig. 2a shows the representative XRD spectra of SiC NWs and SiC@C NCs, respectively. As was seen, there were four characteristic diffraction peaks located at 2θ ¼ 35.5� , 41.3� , 59.9� and 71.7� , which were assigned to the (111), (200), (220) and (311) lattice planes of the 3C–SiC (JCPDs Card No. 29-1129), respectively [35,36]. In addition, the broad peak at around 26� was indexed as carbon (JCPDs Card No. 41-1487), whereas the weak diffraction peak intensity indicated the amorphous state of the carbon shell [37]. In addition, some small diffraction peaks marked as S. F. were observed in the spectra, which were corresponding to the stacking faults in SiC according to previous reports. Fig. 2b and c display the representative SEM images of SiC NWs, respectively. Apparently, a large amount of dense one-dimensional nanowires (several microns in length) were observed to uniformly spread throughout the region. As marked by the red circle in Fig. 2c, some nanowires were randomly distributed and interweaved with each other, which constructed a dense net-like structure within the whole vision field. Moreover, it was 3
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Fig. 3. Representative low-magnification TEM image of the SiC NWs (a, b) and SiC@C NCs (e, f), high-magnification TEM image the SiC NWs (c) and SiC@C NCs (g), HRTEM image of the SiC NWs (d) and SiC@C NCs (h). The insert in is typical SAED pattern.
The chemical compositions and surface states of the SiC NWs and SiC@C NCs were analyzed by XPS, respectively. Fig. 4a presents the XPS surveys of the products, in which elements Si, C and O were easily indexed. For the SiC NWs, element O might be derived from the surface adsorbed oxygen, which was a common phenomenon for SiC nano materials. Additionally, it was further concluded based on the sharply enhanced relative intensity of the C 1s peak of SiC@C NCs that, a sub stantial amount of carbons were wrapped outside the SiC NWs. Fig. 4b reveals the high-resolution Si 2p spectra, which are divided into two stripping peaks at around 102 eV and 104.3 eV, respectively. Typically, the Si–C peak was ascribed to the presence of the SiC NWs, whereas the Si–O peak might be aroused by the bonding between the surface Si atoms and the absorbed oxygen. Noteworthily, the Si–O peak of SiC@C NCs disappeared after being wrapped with the amorphous carbon shell. The high resolution C 1s spectra (as shown in Fig. 4c) were divided into three peaks at 283.6 eV, 284.7 eV and 285.7 eV, separately, which were cor responding to the typical binding energies of C–Si, C–C and C–O bond, respectively [42]. Moreover, the strong response of the amorphous
carbon shell, together with the high intensity of the C–C peak, might cover the detection information of the C–Si bond in the SiC@C NCs. The O 1s spectra in Fig. 5d displayed two obvious split peaks of Si–O–Si and Si–O–C, respectively, among which, the Si–O–C peak might be aroused by the SiOxCy phase generated in the interface between the SiC NWs and the amorphous carbon layer [43]. In order to obtain more character ization on the amorphous carbon shell, Raman spectrum and FTIR spectrum of the SiC@C NCs were investigated, and the corresponding results were shown in Fig. S2. According to the transmission line theory, the RL value was calcu lated from the complex permittivity and complex permeability based on the following equations (1) and (2): � � RL dB ¼ 20log (1) Zin ¼ Z0
4
� � qffiffiffiffiffiffiffiffiffiffi 2πfd pffiffiffiffiffiffiffiffiffiffi μr =εr tanh j μr *εr c
(2)
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Fig. 4. XPS survey (a), high-resolution Si 2p (b), C 1s (c) and O 1s (d) spectra of SiC NWs and SiC@C NCs, respectively.
on the minimal RL and corresponding appearance frequency was depicted, as exhibited in Fig. 5f. Obviously, the minimal RL appeared in the X and Ku-band at the small matching thickness, while that presented in the C-band at a large matching thickness. In addition, the corre sponding minimal RL value was larger than 20 dB at the matching thickness of above 2.08 mm. Compared with the previous microwave absorption performances of carbonaceous and SiC-based absorbers (summarized in Table 1), the as-synthesized SiC@C NCs had broad EAB range and moderate matching thickness. As is well known, the microwave absorption performances of an absorber are closely correlated with the complex permittivity (εr) and complex permeability (μr). Typically, the real permittivity (ε0 ) and real permeability (μ0 ) represent the storage capabilities, whereas the imagi nary permittivity (ε") and imaginary permeability (μ") stand for the microwave loss capabilities of electric and magnetic energies, respec tively [45]. Afterwards, the intrinsic causes of the superior microwave absorption performance of the SiC@C NCs were further investigated through calculating the complex permittivity and permeability sepa rately. As a controlled sample, the complex relative permittivity and permeability curves of the SiC NWs were shown in Fig. S3. As observed in Fig. 6a, the ε0 curve started with 10.89 at 2.0 GHz, which evidently decreased to 4.82 at 12.0 GHz, slightly increased to 6.11 at 12.8 GHz, reached a minimal value of 4.51 at 15.92 GHz, and increased to 5.80 at 18.0 GHz. The phenomenon that the ε0 values gradually decreased versus the increased frequency can be attributed to polarization lagged behind the change of the external high frequency electric field. With regard to ε" of the SiC@C NCs, the curve gradually decreased from 4.27 to 1.30 when the frequency increased from 2 to 18 GHz. Then, the trend regarding the complex permittivity of SiC@C NCs was compared with the previously reported results, and it was believed that the obvious fluctuations at around 12 GHz in the complex permittivity curve re flected the resonance peaks aroused by the SiC NWs core. Fig. 6b pre sents the frequency dependence for the μ0 and μ" values of the products.
where Z0 is the characteristic impedance of free space; Zin and d indicate the normalized input impedance and absorption layer thickness of the absorber, respectively; f stands for the microwave frequency; c repre sents the light velocity; whereas μr (μr ¼ μ0 -μ") and εr (εr ¼ ε0 -ε") are indicative of the relative complex permeability and permittivity, respectively. Traditionally, 10 dB is employed to evaluate the RL value, which indicates that ~10% incident energy is reflected, while ~90% energy is absorbed by the microwave absorber. Meanwhile, 10 dB is also used as a common standard to measure the EAB value. For the SiC NWs (as presented in Fig. 5a), the minimum RL value was as low as 31.65 dB at 8.48 GHz, with the matching thickness of 2.58 mm and the EAB value of 3.12 GHz (7.44–10.56 GHz). As shown in Fig. 5b, the minimal RL value of 51.53 dB with the corresponding EAB value of 6.32 GHz (11.68–18 GHz) across the whole Ku-band was achieved at a relatively thin matching thickness of 2.08 mm, which had revealed the superior microwave absorption performances of the SiC@C NCs. Besides, it was observed from Fig. 5c and d that, the three-dimensional (3D) maps of RL values varied with the matching thickness of the SiC NWs and SiC@C NCs, which revealed that not only the minimal RL values shifted towards the lower frequency, but also the corresponding EAB ranges gradually became narrower with the increase in the matching thickness [[44]]. Fig. 5e displays the images regarding the matching thicknesses versus the EAB values of the SiC NWs and SiC@C NCs, respectively. Clearly, the broadest EAB values of the SiC NWs and SiC@C NCs were 4.14 GHz and 7.2 GHz, which appeared at the matching thicknesses of 2.08 mm and 2.58 mm, respectively. At the matching thicknesses of 2.08 mm, 2.58 mm, 3.08 mm, 3.58 mm and 4.08 mm, the EAB values of the SiC@C NCs were apparently higher than those of the SiC NWs, while there was almost no significant change in the EAB value for other selected thick nesses. To obviously demonstrate the variations in the absorption characteristics of the SiC@C NCs the dependence of matching thickness 5
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Fig. 5. Frequency dependence of the calculated RL values of the SiC NWs (a) and SiC@C NCs (b), the corresponding 3D map of the SiC NWs (c) and SiC@C NCs (d). The matching thickness dependence of the effective absorption bandwidth value (<-10 dB) of the SiC NWs and SiC@C NCs (e). The matching thickness dependence of the minimal reflection loss value and corresponding appeared frequency (f).
Obviously, both of these two values almost maintained constant at around 1.1 and 0, with several small fluctuations, respectively. Similar phenomenon was observed from various nanostructures, such as the hierarchical Ni nanostructures, porous Fe3O4/SnO2 nanorods, MWCNT/wax, and hollow cobalt nanochain composites. Chiu et al. revealed that such phenomenon was meaningless, which was attributed to noise [46]. Additionally, Wu et al. suggested that the negative μ" value was ascribed to the induced magnetic energy going out of the microwave absorbing materials [47]. In this work, the peculiar magnetic behavior
was attributable to the eddy currents at the interfaces between the SiC NWs core and the amorphous carbon shell, which aroused the interface coupling. According to the Maxwell’s equation, an induced magnetic field (opposite to the applied field and in turn radiated microwave) was generated in the nanocomposites, which radiated outward again, thereby inducing a negative imaginary permeability at high frequencies. The dielectric tangent loss tan δε (tan δε ¼ ε"/ε0 ) and the magnetic tangent loss tan δμ (tan δμ ¼ μ"/μ0 ) can be used to evaluate the micro wave attenuation capability of the absorber, and the corresponding 6
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matching, that means incident microwave can be introduced into the material interior easily. The impedance matching (Zin/Z0) of an absorber can be calculated using equation (2). Fig. 7 shows the Zin/Z0 curves of the SiC@C NCs and SiC NWs. Generally, the higher values of Zin/Z0 means that more electromagnetic wave reflected from the surface of the microwave absorbing materials. As revealed in Fig. 7, the values vary with respect to frequency at a matching thickness of 2.08 mm. Compared with SiC NWs with higher values, the SiC@C NCs exhibits better impedance matching conditions in the 10–18 GHz, which can be attributed to the following reasons: Firstly, the defects caused by nanosize effect or disordered microstructure within SiC produce induced
Table 1 Microwave absorption performance of the SiC, carbon-related absorbers and SiC@C NCs. Absorber
Weight (wt %)
Thickness (mm)
EAB (GHz)
RL (dB)
Ref.
SiC NWs
30 wt%
2.0
21.5
[51]
SiC NWs
30 wt%
3.0
17.4
[37]
Fe/SiC nanofiber SiC NWs
35 wt%
2.25
46.3
[52]
35 wt%
1.9
57.8
[41]
SiC@SiO2
30 wt%
1.5
24.11
[35]
CNTs/ZnO
4 wt% CNTs þ 10 wt% ZnO 8 wt% 50 wt%
2.0
2.45 (<-10 dB) 2.5 (<-10 dB) 5.6 (<-20 dB) 5.5 (<-10 dB) 4.4 (<-10 dB) 4.04 (<-10 dB)
37.03
[17]
6 (<-5 dB) 5.4 (<-10 dB) 4.7 (<-10 dB) 7.2 (<-10 dB)
24.27 34.8
[25] [26]
47.3
[53]
51.53
This work
CNTs/varnish C@C microspheres graphene@SiC SiC@C NCs
1.0 2.0 3.0
30 wt%
2.08
results are indicated in Fig. 6c and d, respectively. Notably, it was observed that the synchronization tendencies appeared with the varia tions in ε" and μ" values, respectively. In addition, the tan δε value was in the range of 0.22–0.88, which was greater than the tan δμ value in the range of 0.12–0.23, revealing that the dielectric loss occurring in the entire frequency range was a significant factor to obtain the superior microwave absorption performances, and that the influence of magnetic loss on the attenuation of incident microwave energy was neglected. It is well known that superior absorbing materials should not only display high energy loss ability, but also possess proper impedance
Fig. 7. Impedance matching curve of the SiC NWs and SiC@C NCs.
Fig. 6. Frequency dependence of complex permittivity (a), complex permeability (b), the dielectric loss tangent (c) and magnetic loss tangent (d) of the SiC@C NCs. 7
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Fig. 8. Schematic illustration for microwave attenuation mechanism of the SiC@C NCs.
room-temperature magnetic properties. Which can produce certain magnetic loss effect. On the other hand, a large number of charged de fects and unpaired electrons existed in SiC can move in response to the electric field, and diffusion or polarization current results from the incidental electromagnetic wave, improving the dielectric constant of SiC and make it close to the impedance matching requirement. To explore the microwave absorption mechanism of the assynthesized products, Fig. 8 demonstrates the potential propagation path and the attenuation process of the incident microwave in the products. In this work, the superior microwave absorption performances of the SiC@C NCs are attributed to the synergistic interaction of the defect microstructures and heterogeneous interface between carbon shell or SiC NWs core, as summarized in the following aspects: (a) the incident microwave is converted into the internal energy, which is then further consumed in the form of heat energy through the dielectric po larization and relaxation of the SiC NWs core and the amorphous carbon shell. (b) Abundant transporting conduction electrons would accumu lated around the discarded structure (including defects, vacancies and defects in the lattice) of the carbon shell and was easily trapped by the latter. Under external electromagnetic wave field, the local electron asymmetric distribution state of local electrons resulted in the occur rence of the defect-induced dipoles and interfacial polarization. (c) The SiC NWs core with high-density stacking faults possesses different electronic structures and conduction/valence band positions, which could act as polarized centers to generate space charge polarization and relaxation by trapping space charges under external electromagnetic wave field, contributing to the enhanced electromagnetic wave ab sorption abilities. (d) The coaxial structure composed of the SiC NWs core with semiconductor characteristics and the conductor-type amor phous carbon shell has constituted a micro-capacitor, in which the coupled circuits, electromagnetic field-induced currents and serious interfacial polarization relaxation appear in the interfacial region be tween the SiC NWs core and the amorphous carbon shell, thus benefiting the interfacial polarization and interface scattering of the incident mi crowave. Meanwhile, there are abundant structural defects and va cancies at the interface between the SiC NWs core and the amorphous carbon shell, which generate more twists and merges on the atomic
surface, thereby providing more interface and energy attenuation for microwave [48]. (e) The SiC@C NCs overlap and connect with each other to form an network structure, which results in the increase in conductivity, thereby increasing the propagation path and attenuation opportunity of the incident microwave. Besides, the micro-currents generated by the electrons migrating along the propagation path interact each other, giving rise to the attenuation of the microwave energy [49,50]. 4. Conclusions In this work, the coaxial nanocomposites composed of the SiC NWs core and the amorphous carbon shell are prepared according to the hydrothermal-carbonization method, with SiC NWs and PVP being used as the supporting skeletons and carbon precursor, respectively. After wards, the products are characterized by a series of methods, including XRD, SEM, TEM, and XPS. To estimate the attenuation of the incident electromagnetic wave, the complex permittivity and permeability are calculated for the absorber. Our findings suggest that, the as-fabricated net-like SiC@C NCs exhibit enhanced microwave absorption perfor mances, with the minimal RL value of 51.53 dB and a broad EAB covering the whole Ku-band of 6.32 GHz at a small matching thickness of 2.08 mm. Specifically, the broadest EAB of 7.2 GHz is achieved at the matching thickness of 2.58 mm, which is superior to those of most re ported carbon and SiC-based microwave absorbers. Specifically, the involved microwave absorption mechanism is attributed to the multiple reflections between the carbon shell and the SiC NWs core, as well as the defects and stacking faults in carbon and SiC, and the interfacial po larization aroused by the higher surface-to-volume ratio of the nano structures. The simple and feasible synthesis strategy proposed in this study provides a promising microwave absorber candidate characterized by the advantages of light weight, small thickness and broad absorption bandwidth. Declaration of competing interest On behalf of all authors, I declare that we have no financial and 8
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personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company.
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Acknowledgments The work reported here was supported by the National Natural Sci ence Foundation of China under Grant No.51702181, 51672144, the Higher Educational Science and Technology Program of Shandong Province under Grant No. J17KA014, J18KA001, the Taishan Scholars Program of Shandong Province under No. ts201511034. We express our grateful thanks to them for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107525. References [1] Zhu W, Wang L, Zhao R, Ren J, Lu G, Wang Y. Electromagnetic and microwaveabsorbing properties of magnetic nickel ferrite nanocrystals. Nanoscale 2011;3: 2862–4. [2] Hou T, Wang B, Jia Z, Wu H, Lan D, Huang Z, Feng A, Ma M, Wu G. A review of metal oxide-related microwave absorbing materials from the dimension and morphology perspective. J Mater Sci Mater Electron 2019;30(12):10961–84. [3] Miao P, Cheng K, Li H, Gu J, Chen K, Wang S, Wang D, Liu TX, Xu BB, Kong J. Poly (dimethylsilylene)diacetylene-guided ZIF-based heterostructures for full Ku-band electromagnetic wave absorption. ACS Appl Mater Interfaces 2019;11(19): 17706–13. [4] Luo C, Jiao T, Gu J, Tang Y, Kong J. Graphene shield by SiBCN ceramic: a promising high-temperature electromagnetic wave-absorbing material with oxidation resistance. ACS Appl Mater Interfaces 2018;10(45):39307–18. [5] Luo C, Tang Y, Jiao T, Kong J. High-temperature stable and metal-free electromagnetic wave-absorbing SiBCN ceramics derived from carbon-rich hyperbranched polyborosilazanes. ACS Appl Mater Interfaces 2018;10(33): 28051–61. [6] Liu W, Shao Q, Ji G, Liang X, Cheng Y, Quan B, Du Y. Metal-organic-frameworks derived porous carbon-wrapped Ni composites with optimized impedance matching as excellent lightweight electromagnetic wave absorber. Chem Eng J 2017;313:734–44. [7] Liu W, Tan S, Yang Z, Ji G. Hollow graphite spheres embedded in porous amorphous carbon matrices as lightweight and low-frequency microwave absorbing material through modulating dielectric loss. Carbon 2018;138:143–53. [8] Liu J, Liang H, Zhang Y, Wu G, Wu H. Facile synthesis of ellipsoid-like MgCo2O4/ Co3O4 composites for strong wideband microwave absorption application. Compos B Eng 2019;176:107240. [9] Cao WQ, Wang XX, Yuan J, Wang WZ, Cao MS. Temperature dependent microwave absorption of ultrathin grpahene composites. J Mater Chem C 2015;3:10017–22. [10] Guo J, Song H, Liu H, Luo C, Ren Y, Ding T, Khan MA, Young DP, Liu X, Zhang X, Kong J, Guo Z. Polypyrrole-interface-functionalized nano-magnetite epoxy nanocomposites as electromagnetic wave absorbers with enhanced flame retardancy. J Mater Chem C 2017;5(22):5334–44. [11] Zhou X, Jia Z, Feng A, Wang X, Liu J, Zhang M, Cao H, Wu G. Synthesis of fish skinderived 3D carbon foams with broadened bandwidth and excellent electromagnetic wave absorption performance. Carbon 2019;152:827–36. [12] Wang C, Murugadoss V, Kong J, He Z, Mai X, Shao Q, Chen Y, Guo L, Liu C, Angaiah S, Guo Z. Overview of carbon nanostructures and nanocomposites for electromagnetic wave shielding. Carbon 2018;140:696–733. [13] Wang Z, Wei R, Gu J, Liu H, Liu C, Luo C, Kong J, Shao Q, Wang N, Guo Z, Liu X. Ultralight, highly compressible and fire-retardant graphene aerogel with selfadjustable electromagnetic wave absorption. Carbon 2018;139:1126–35. [14] Wang L, Qiu H, Liang C, Song P, Han Y, Han Y, Gu J, Kong J, Pan D, Guo Z. Electromagnetic interference shielding MWCNT-Fe3O4@Ag/epoxy nanocomposites with satisfactory thermal conductivity and high thermal stability. Carbon 2019; 141:506–14. [15] Quan B, Shi W, Ong SJH, Lu X, Wang PL, Ji G, Guo Y, Zheng L, Xu ZJ. Defect engineering in two common types of dielectric materials for electromagnetic absorption applications. Adv Funct Mater 2019;29:1901236. [16] Huang X, Zhang J, Rao W, Sang T, Song B, Wong C. Tunable electromagnetic properties and enhanced microwave absorption ability of flaky grpahite/cobalt zinc ferrite composites. J Alloy Comp 2016;662:409–14. [17] Wang Y, Du Y, Qiang R, Tian C, Xu P, Han X. Interfacially engineered sandwich-like rGO/carbon microspheres/rGO composites as an efficient and durable microwave absorber. Adv Mater Interfaces 2016;3(7):1500684. [18] Ding D, Wang Y, Li X, Qiang R, Xu P, Chu W, Han X, Du Y. Rational design of coreshell Co@C microspheres for high-performance microwave absorption. Carbon 2017;111:722–32.
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[46] Chiu SC, Yu HC, Li YY. High electromagnetic wave absorption performance of silicon carbide nanowires in the gigahertz range. J Phys Chem C 2010;114(4): 1947–52. [47] Wu H, Wang L, Wang Y, Guo S, Shen Z. Enhanced microwave performance of highly ordered mesoporous carbon coated by Ni2O3 nanoparticles. J Alloy Comp 2012;525:82–6. [48] Liang C, Wang Z. Controllable fabricating dielectric-dielectric SiC@C core-shell nanowires for high-performance electromagnetic wave attenuation. ACS Appl Mater Interfaces 2017;9(46):40690–6. [49] Chen J, Liu M, Yang T, Zhai F, Hou X, Chou KC. Improved microwave absorption performance of modified SiC in the 2~18 GHz frequency range. CrystEngComm 2017;19:519–27.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Net-like SiC@C coaxial nanocable towards superior lightweight and broadband microwave absorber Meng Zhang a, Hui Lin a, Shiqi Ding a, Ting Wang b, Zhenjiang Li a, *, Alan Meng b, **, Qingdang Li a, Yusheng Lin c a
College of Electromechanical Engineering, Key Laboratory of Polymer Material Advanced Manufacturing’s Technology of Shandong Province, College of Sino-German Science and Technology, Qingdao University of Science and Technology, Qingdao, 266061, Shandong province, China State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, College of Chemical Engineering in Gaomi Campus, Qingdao University of Science and Technology, Qingdao, 266042, Shandong province, China c College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266061, Shandong province, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: SiC@C NCs Microwave absorber Lightweight Wide bandwidth Mechanism
In this study, a practical lightweight and broadband microwave absorber, namely, the net-like amorphous carbon-coated silicon carbide coaxial nanocables (SiC@C NCs), which had low density and excellent physico chemical stability, was successfully fabricated according to the hydrothermal-carbonization strategy. Typically, the microwave absorption performance exhibited broad effective absorption bandwidth (EAB) of up to 7.2 GHz (9.12–16.32 GHz) at a small matching thickness of 2.58 mm. Meanwhile, the minimal reflection loss (RL) value of 51.53 dB with the corresponding EAB across the whole Ku-band (11.68–18 GHz) appeared at 2.08 mm, which suggested that, the as-prepared SiC@C NCs displayed extensive application prospects as a superior lightweight and broadband microwave absorber. Moreover, the systematic characterization results indicated that, the su perior microwave absorption performance was ascribed to the interfacial polarization and multiple reflections on the heterogeneous interface between the SiC nanowires (SiC NWs) core and the amorphous carbon shell, together with the electron polarization and Debye dipolar relaxation resulted from the stacking faults of SiC NWs core, and the abundant defect and disorder within the amorphous carbon shell.
1. Introduction Nowadays, electromagnetic radiation has post an increasingly serious threat worldwide, which has been ranked as fifth leading source of pollution following water pollution, air pollution, noise and light pollution [1]. Electromagnetic absorbing material (absorber) can effectively shield the human bodies and wildlife, which also serves as the crucial electric equipment to attenuate the incident microwave energy [2]. The absorber can be classified as two categories based on various loss mechanisms, namely, magnetic materials and dielectric materials [3–5]. Typically, considering the requirements of actual application, a practical absorber should possess the advantages of lightweight, thin matching thickness, broad bandwidth and strong absorption, as well as the technical characteristics, such as instant low price, simple prepara tion process and environmental friendliness [6]. On the other hand, the magnetic materials have the disadvantages of large density, corrosion
susceptibility, and loss of ferromagnetic property at high working tem perature, which have hindered their applications in microwave ab sorption field; at the same time, the dielectric materials have gradually become more and more popular [7,8]. Carbonaceous materials appear to be the favorable candidates for microwave absorber, which is ascribed to their inherent advantages of low toxicity, ultra-low density, high electrical conductivity, high dielectric loss and excellent stability [9–11]. Therefore, various methods have been tried attempting to prepare the novel carbonaceous materials with high permittivity, which has been regarded as one of the most significant strategies to achieve excellent microwave absorption capac ity [12–14]. However, it advances slowly, and only a few research groups have achieved satisfying results on microwave absorption [15]. At the same time, the uncontrollable agglomeration behavior of nano scale carbon-based materials with high surface energy gives rise to worse microwave absorption performance [16,17]. Fortunately,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (A. Meng). https://doi.org/10.1016/j.compositesb.2019.107525 Received 23 August 2019; Received in revised form 8 October 2019; Accepted 9 October 2019 Available online 13 October 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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constructing a rational nanocomposite contributes to generating abun dant interfacial polarization between the carbonaceous material and other component, which is identified as an available and efficient approach [18–21]. Up to date, extensive carbon-based nanocomposites have been pre pared through numerous methods (such as surface modification, doping and hybridization), particularly for products composed of carbon and nonmetallic components, like SiO2@C [22], CNTs/SiO2 [23,24], rGO/ZnO [25], Graphene/g-C3N4 [26], C/TiO2 [27,28], polymer/C [29, 30], MXene/C [31]. Such products could enhance the electromagnetic attenuation by improving dielectric polarization ability of bare carbo naceous materials, without dramatically increasing their density. However, most of them achieved narrow effective absorption bandwidth (EAB) and weak reflection loss (RL), hindering their further application and research progress. Silicon carbide (SiC) is a representative dielectric material, which has a broad absorption frequency range, as well as outstanding me chanical properties, chemical resistivity and thermal stability at room and high temperatures, and is thereby regarded as the “Star of Tomorrow” in the absorber family [32,33]. Some recent reports demonstrate that SiC nanowires (SiC NWs) exhibit superior electro magnetic attenuation performance to the bulk counterpart, which is attributable to their large specific surface areas, abundant stacking and tunable electrical conductivity [34]. Notably, nanocomposites composed of the SiC one-dimensional nanostructures and the carbona ceous material may serve as the most promising microwave absorber, which not only enhance the microwave absorption performances of pure carbon-based component, but may also serve as the ideal structural materials under harsh environments, such as high temperature, corro sion, and ultraviolet radiation. In this study, the SiC NWs were selected as the core supporting skeletons, whereas the polyvinylpyrrolidone (PVP)-derived amorphous carbon was used as the shell to successfully synthesize the lightweight net-like SiC@C coaxial nanocables (SiC@C NCs). Typically, the micro wave absorption performances of the as-synthesized products exhibited broad EAB and strong RL, which indicated that they might be exten sively applied in the lightweight and high frequency microwave ab sorption fields. Additionally, a synergetic absorption mechanism was also proposed based on the multiple reflections, defects, stacking faults and interfacial polarization. Notably, the approach proposed in this study was a low-price, simple-operation and environmentally friendly
process, which might provide an effective clue for developing the carbon-based lightweight absorbing materials. 2. Experimental procedures 2.1. Materials Si and SiO2 of analytical purity were provided by Tianjin Chemical Reagents and Shanghai Chemistry Reagents Ltd., respectively. CH4 and argon (purchased from Qingdao Three Factor Gas Co., Ltd.) of 99.99% purify were used as the carbon source and carrier gas, respectively. Graphite substrate (purchased from Qingdao Shenghe Graphite Co., Ltd.) was employed as deposition substrate. Carbon cloth was purchased from Jiangsu Tianniao High Technology Co. Ltd, which could load the Si–SiO2 mixed powders and insulate them from the graphite substrate (as well as the as-synthesized produces). The Polyvinylpyrrolidone (PVP) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd., China. Other reagents were of analytical grade and used as received without further purification. 2.2. Synthesis of SiC NWs Firstly, the Si–SiO2 mixed powders, a piece of carbon cloth, and the graphite substrate were placed into the homemade reaction chamber successively. Subsequently, the high purity argon was introduced into the furnace for three times using a rotary vacuum pump to exhaust the oxygen. Then, the furnace was heated to 1250 � C at an average heating rate of about 350–400 � C/h, and later a steady CH4 flow (at 0.15–0.20 sccm) was introduced for 20–25 min. Finally, the gas was switched off and the furnace was gradually cooled down to 900 � C for 10 min. The schematic illustration for the formation of SiC NWs was shown infor mation (Fig. S1). 2.3. Synthesis of SiC@C NCs At first, about 0.5 g SiC NWs were stripped from the graphite sub strate and dispersed into 60 ml ethanol according to the ultrasound method for 1 h. Subsequently, 0.4 g PVP powers were added into the SiC NWs dispersion, followed by sufficient stirring, and then the mixture was transferred into the 100 ml high pressure autoclave for a heating process at 120 � C for 6 h. Afterwards, the products after centrifugation
Fig. 1. Schematic illustration of the SiC@C NCs. 2
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Fig. 2. Representative XRD patterns of the SiC NWs and SiC@C NCs (a) Low-magnification SEM images of the SiC NWs (b) and SiC@C NCs (d), high-magnification SEM images of the SiC NWs (c) and SiC@C NCs (e, f).
and drying were placed into the tubular furnace and sintered under the Ar/H2 atmosphere at 1000 � C for 1 h. Fig. 1 illustrates the synthesis process of SiC@C NCs.
displayed in Fig. 2d and e that, a larger amount of net-like nanowires were collected after the reaction process. Obviously, the tumor-like knots (as indicated by the yellow arrows) were easily formed at some intersections of nanowires at horizontal and vertical orientations. Different from SiC NWs with smooth surface, the high-magnification SEM image in Fig. 2f revealed that the as-synthesized products with the diameter of 20–60 nm were covered by the rough surfaces [38]. To obtain more detailed structural information, both SiC NWs and SiC@C NCs were characterized by TEM and HRTEM, respectively. Fig. 3a exhibits the TEM image of a large number of dense SiC NWs, which have smooth surface and are several microns in length. It was found in Fig. 3b that, the average diameter of SiC NWs ranged from 12 to 20 nm. Fig. 3c shows the representative high-magnification TEM image of the SiC NWs. As was observed, there were dense stacking faults (as indicated by the red arrows) in the SiC NWs. The insert in Fig. 3c shows the corresponding selected area electron diffraction (SAED) pattern. It was seen that, there were obvious streaks in addition to a series of bright electron diffraction spots [39], and the diffraction spots were assigned to the cubic SiC. Fig. 3d displays the HRTEM image of the SiC NWs, with the interplanar spacing of about 0.25 nm, which is corresponding to the (111) plane of cubic SiC, implying that the growth direction of the nanowire is [111]. For the SiC@C NCs, the TEM image shown in Fig. 3e clearly suggested that, the cross-linked net-like nanowires possessed the coaxial cable-type nanostructures, which were composed of the thin shell layer and the inner core. Fig. 3f demonstrates the average diameter of the net-like nanowires of approximately 28 nm, which is generally consistent with the SEM characterization result. Fig. 3g shows the high-magnification TEM image recorded from an individual nanowire, and the thickness of the amorphous carbon shell layer is about 3.8 nm. Moreover, the core region and S.F. (as indicated by the blue arrows) with dark contrast, together with the shell layer with bright contrast, indi cated the corresponding crystalline and amorphous state, respectively. In addition, the insert in Fig. 3g displays the corresponding SAED pattern of the net-like nanowire, in which the legible diffraction spots are also indexed to the cubic SiC. On the other hand, the HRTEM image in Fig. 3h revealed that the interplanar spacing of the core area wrapped by amorphous carbon was about 0.25 nm, which was in good agreement with the {111} plane of the cubic SiC [40]. According to the crystal lography theory, the atom stacking direction and the growth direction of nanowire were indexed as [-111] and [111], respectively, which had well coincided with the reported values [41].
2.4. Characterization Scanning electron microscopy (SEM) was carried out using the JEOL JSM-6, whereas transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) were performed using the FEI Tecnai G2 F20 microscope. X-ray diffraction (XRD, Rigaku D/max-2400 X-ray diffractometer) was employed to investigate the product phase compositions and crystal structure. X-ray photoelectron spectroscopy (XPS) was conducted on the Thermo Scientific ESCALAB 250XI, so as to determine the product chemical state. To measure the complex electromagnetic parameters, the assynthesized nanocomposites were mixed homogeneously with paraffin at a weight filling ratio of 30%. Thereafter, the mixture was pressed into a toroidal-shaped sample, with an outer diameter of 7 mm, inner diameter of 3 mm and the thickness of 3 mm. Then, the electromagnetic parameters were measured using the vector network analyzer (Agilent, N5230A) under the transmission-reflection mode at the frequency range of 2–18 GHz. 3. Results and discussion Fig. 2a shows the representative XRD spectra of SiC NWs and SiC@C NCs, respectively. As was seen, there were four characteristic diffraction peaks located at 2θ ¼ 35.5� , 41.3� , 59.9� and 71.7� , which were assigned to the (111), (200), (220) and (311) lattice planes of the 3C–SiC (JCPDs Card No. 29-1129), respectively [35,36]. In addition, the broad peak at around 26� was indexed as carbon (JCPDs Card No. 41-1487), whereas the weak diffraction peak intensity indicated the amorphous state of the carbon shell [37]. In addition, some small diffraction peaks marked as S. F. were observed in the spectra, which were corresponding to the stacking faults in SiC according to previous reports. Fig. 2b and c display the representative SEM images of SiC NWs, respectively. Apparently, a large amount of dense one-dimensional nanowires (several microns in length) were observed to uniformly spread throughout the region. As marked by the red circle in Fig. 2c, some nanowires were randomly distributed and interweaved with each other, which constructed a dense net-like structure within the whole vision field. Moreover, it was 3
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Fig. 3. Representative low-magnification TEM image of the SiC NWs (a, b) and SiC@C NCs (e, f), high-magnification TEM image the SiC NWs (c) and SiC@C NCs (g), HRTEM image of the SiC NWs (d) and SiC@C NCs (h). The insert in is typical SAED pattern.
The chemical compositions and surface states of the SiC NWs and SiC@C NCs were analyzed by XPS, respectively. Fig. 4a presents the XPS surveys of the products, in which elements Si, C and O were easily indexed. For the SiC NWs, element O might be derived from the surface adsorbed oxygen, which was a common phenomenon for SiC nano materials. Additionally, it was further concluded based on the sharply enhanced relative intensity of the C 1s peak of SiC@C NCs that, a sub stantial amount of carbons were wrapped outside the SiC NWs. Fig. 4b reveals the high-resolution Si 2p spectra, which are divided into two stripping peaks at around 102 eV and 104.3 eV, respectively. Typically, the Si–C peak was ascribed to the presence of the SiC NWs, whereas the Si–O peak might be aroused by the bonding between the surface Si atoms and the absorbed oxygen. Noteworthily, the Si–O peak of SiC@C NCs disappeared after being wrapped with the amorphous carbon shell. The high resolution C 1s spectra (as shown in Fig. 4c) were divided into three peaks at 283.6 eV, 284.7 eV and 285.7 eV, separately, which were cor responding to the typical binding energies of C–Si, C–C and C–O bond, respectively [42]. Moreover, the strong response of the amorphous
carbon shell, together with the high intensity of the C–C peak, might cover the detection information of the C–Si bond in the SiC@C NCs. The O 1s spectra in Fig. 5d displayed two obvious split peaks of Si–O–Si and Si–O–C, respectively, among which, the Si–O–C peak might be aroused by the SiOxCy phase generated in the interface between the SiC NWs and the amorphous carbon layer [43]. In order to obtain more character ization on the amorphous carbon shell, Raman spectrum and FTIR spectrum of the SiC@C NCs were investigated, and the corresponding results were shown in Fig. S2. According to the transmission line theory, the RL value was calcu lated from the complex permittivity and complex permeability based on the following equations (1) and (2): � � RL dB ¼ 20log (1) Zin ¼ Z0
4
� � qffiffiffiffiffiffiffiffiffiffi 2πfd pffiffiffiffiffiffiffiffiffiffi μr =εr tanh j μr *εr c
(2)
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Fig. 4. XPS survey (a), high-resolution Si 2p (b), C 1s (c) and O 1s (d) spectra of SiC NWs and SiC@C NCs, respectively.
on the minimal RL and corresponding appearance frequency was depicted, as exhibited in Fig. 5f. Obviously, the minimal RL appeared in the X and Ku-band at the small matching thickness, while that presented in the C-band at a large matching thickness. In addition, the corre sponding minimal RL value was larger than 20 dB at the matching thickness of above 2.08 mm. Compared with the previous microwave absorption performances of carbonaceous and SiC-based absorbers (summarized in Table 1), the as-synthesized SiC@C NCs had broad EAB range and moderate matching thickness. As is well known, the microwave absorption performances of an absorber are closely correlated with the complex permittivity (εr) and complex permeability (μr). Typically, the real permittivity (ε0 ) and real permeability (μ0 ) represent the storage capabilities, whereas the imagi nary permittivity (ε") and imaginary permeability (μ") stand for the microwave loss capabilities of electric and magnetic energies, respec tively [45]. Afterwards, the intrinsic causes of the superior microwave absorption performance of the SiC@C NCs were further investigated through calculating the complex permittivity and permeability sepa rately. As a controlled sample, the complex relative permittivity and permeability curves of the SiC NWs were shown in Fig. S3. As observed in Fig. 6a, the ε0 curve started with 10.89 at 2.0 GHz, which evidently decreased to 4.82 at 12.0 GHz, slightly increased to 6.11 at 12.8 GHz, reached a minimal value of 4.51 at 15.92 GHz, and increased to 5.80 at 18.0 GHz. The phenomenon that the ε0 values gradually decreased versus the increased frequency can be attributed to polarization lagged behind the change of the external high frequency electric field. With regard to ε" of the SiC@C NCs, the curve gradually decreased from 4.27 to 1.30 when the frequency increased from 2 to 18 GHz. Then, the trend regarding the complex permittivity of SiC@C NCs was compared with the previously reported results, and it was believed that the obvious fluctuations at around 12 GHz in the complex permittivity curve re flected the resonance peaks aroused by the SiC NWs core. Fig. 6b pre sents the frequency dependence for the μ0 and μ" values of the products.
where Z0 is the characteristic impedance of free space; Zin and d indicate the normalized input impedance and absorption layer thickness of the absorber, respectively; f stands for the microwave frequency; c repre sents the light velocity; whereas μr (μr ¼ μ0 -μ") and εr (εr ¼ ε0 -ε") are indicative of the relative complex permeability and permittivity, respectively. Traditionally, 10 dB is employed to evaluate the RL value, which indicates that ~10% incident energy is reflected, while ~90% energy is absorbed by the microwave absorber. Meanwhile, 10 dB is also used as a common standard to measure the EAB value. For the SiC NWs (as presented in Fig. 5a), the minimum RL value was as low as 31.65 dB at 8.48 GHz, with the matching thickness of 2.58 mm and the EAB value of 3.12 GHz (7.44–10.56 GHz). As shown in Fig. 5b, the minimal RL value of 51.53 dB with the corresponding EAB value of 6.32 GHz (11.68–18 GHz) across the whole Ku-band was achieved at a relatively thin matching thickness of 2.08 mm, which had revealed the superior microwave absorption performances of the SiC@C NCs. Besides, it was observed from Fig. 5c and d that, the three-dimensional (3D) maps of RL values varied with the matching thickness of the SiC NWs and SiC@C NCs, which revealed that not only the minimal RL values shifted towards the lower frequency, but also the corresponding EAB ranges gradually became narrower with the increase in the matching thickness [[44]]. Fig. 5e displays the images regarding the matching thicknesses versus the EAB values of the SiC NWs and SiC@C NCs, respectively. Clearly, the broadest EAB values of the SiC NWs and SiC@C NCs were 4.14 GHz and 7.2 GHz, which appeared at the matching thicknesses of 2.08 mm and 2.58 mm, respectively. At the matching thicknesses of 2.08 mm, 2.58 mm, 3.08 mm, 3.58 mm and 4.08 mm, the EAB values of the SiC@C NCs were apparently higher than those of the SiC NWs, while there was almost no significant change in the EAB value for other selected thick nesses. To obviously demonstrate the variations in the absorption characteristics of the SiC@C NCs the dependence of matching thickness 5
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Fig. 5. Frequency dependence of the calculated RL values of the SiC NWs (a) and SiC@C NCs (b), the corresponding 3D map of the SiC NWs (c) and SiC@C NCs (d). The matching thickness dependence of the effective absorption bandwidth value (<-10 dB) of the SiC NWs and SiC@C NCs (e). The matching thickness dependence of the minimal reflection loss value and corresponding appeared frequency (f).
Obviously, both of these two values almost maintained constant at around 1.1 and 0, with several small fluctuations, respectively. Similar phenomenon was observed from various nanostructures, such as the hierarchical Ni nanostructures, porous Fe3O4/SnO2 nanorods, MWCNT/wax, and hollow cobalt nanochain composites. Chiu et al. revealed that such phenomenon was meaningless, which was attributed to noise [46]. Additionally, Wu et al. suggested that the negative μ" value was ascribed to the induced magnetic energy going out of the microwave absorbing materials [47]. In this work, the peculiar magnetic behavior
was attributable to the eddy currents at the interfaces between the SiC NWs core and the amorphous carbon shell, which aroused the interface coupling. According to the Maxwell’s equation, an induced magnetic field (opposite to the applied field and in turn radiated microwave) was generated in the nanocomposites, which radiated outward again, thereby inducing a negative imaginary permeability at high frequencies. The dielectric tangent loss tan δε (tan δε ¼ ε"/ε0 ) and the magnetic tangent loss tan δμ (tan δμ ¼ μ"/μ0 ) can be used to evaluate the micro wave attenuation capability of the absorber, and the corresponding 6
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matching, that means incident microwave can be introduced into the material interior easily. The impedance matching (Zin/Z0) of an absorber can be calculated using equation (2). Fig. 7 shows the Zin/Z0 curves of the SiC@C NCs and SiC NWs. Generally, the higher values of Zin/Z0 means that more electromagnetic wave reflected from the surface of the microwave absorbing materials. As revealed in Fig. 7, the values vary with respect to frequency at a matching thickness of 2.08 mm. Compared with SiC NWs with higher values, the SiC@C NCs exhibits better impedance matching conditions in the 10–18 GHz, which can be attributed to the following reasons: Firstly, the defects caused by nanosize effect or disordered microstructure within SiC produce induced
Table 1 Microwave absorption performance of the SiC, carbon-related absorbers and SiC@C NCs. Absorber
Weight (wt %)
Thickness (mm)
EAB (GHz)
RL (dB)
Ref.
SiC NWs
30 wt%
2.0
21.5
[51]
SiC NWs
30 wt%
3.0
17.4
[37]
Fe/SiC nanofiber SiC NWs
35 wt%
2.25
46.3
[52]
35 wt%
1.9
57.8
[41]
SiC@SiO2
30 wt%
1.5
24.11
[35]
CNTs/ZnO
4 wt% CNTs þ 10 wt% ZnO 8 wt% 50 wt%
2.0
2.45 (<-10 dB) 2.5 (<-10 dB) 5.6 (<-20 dB) 5.5 (<-10 dB) 4.4 (<-10 dB) 4.04 (<-10 dB)
37.03
[17]
6 (<-5 dB) 5.4 (<-10 dB) 4.7 (<-10 dB) 7.2 (<-10 dB)
24.27 34.8
[25] [26]
47.3
[53]
51.53
This work
CNTs/varnish C@C microspheres graphene@SiC SiC@C NCs
1.0 2.0 3.0
30 wt%
2.08
results are indicated in Fig. 6c and d, respectively. Notably, it was observed that the synchronization tendencies appeared with the varia tions in ε" and μ" values, respectively. In addition, the tan δε value was in the range of 0.22–0.88, which was greater than the tan δμ value in the range of 0.12–0.23, revealing that the dielectric loss occurring in the entire frequency range was a significant factor to obtain the superior microwave absorption performances, and that the influence of magnetic loss on the attenuation of incident microwave energy was neglected. It is well known that superior absorbing materials should not only display high energy loss ability, but also possess proper impedance
Fig. 7. Impedance matching curve of the SiC NWs and SiC@C NCs.
Fig. 6. Frequency dependence of complex permittivity (a), complex permeability (b), the dielectric loss tangent (c) and magnetic loss tangent (d) of the SiC@C NCs. 7
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Composites Part B 179 (2019) 107525
Fig. 8. Schematic illustration for microwave attenuation mechanism of the SiC@C NCs.
room-temperature magnetic properties. Which can produce certain magnetic loss effect. On the other hand, a large number of charged de fects and unpaired electrons existed in SiC can move in response to the electric field, and diffusion or polarization current results from the incidental electromagnetic wave, improving the dielectric constant of SiC and make it close to the impedance matching requirement. To explore the microwave absorption mechanism of the assynthesized products, Fig. 8 demonstrates the potential propagation path and the attenuation process of the incident microwave in the products. In this work, the superior microwave absorption performances of the SiC@C NCs are attributed to the synergistic interaction of the defect microstructures and heterogeneous interface between carbon shell or SiC NWs core, as summarized in the following aspects: (a) the incident microwave is converted into the internal energy, which is then further consumed in the form of heat energy through the dielectric po larization and relaxation of the SiC NWs core and the amorphous carbon shell. (b) Abundant transporting conduction electrons would accumu lated around the discarded structure (including defects, vacancies and defects in the lattice) of the carbon shell and was easily trapped by the latter. Under external electromagnetic wave field, the local electron asymmetric distribution state of local electrons resulted in the occur rence of the defect-induced dipoles and interfacial polarization. (c) The SiC NWs core with high-density stacking faults possesses different electronic structures and conduction/valence band positions, which could act as polarized centers to generate space charge polarization and relaxation by trapping space charges under external electromagnetic wave field, contributing to the enhanced electromagnetic wave ab sorption abilities. (d) The coaxial structure composed of the SiC NWs core with semiconductor characteristics and the conductor-type amor phous carbon shell has constituted a micro-capacitor, in which the coupled circuits, electromagnetic field-induced currents and serious interfacial polarization relaxation appear in the interfacial region be tween the SiC NWs core and the amorphous carbon shell, thus benefiting the interfacial polarization and interface scattering of the incident mi crowave. Meanwhile, there are abundant structural defects and va cancies at the interface between the SiC NWs core and the amorphous carbon shell, which generate more twists and merges on the atomic
surface, thereby providing more interface and energy attenuation for microwave [48]. (e) The SiC@C NCs overlap and connect with each other to form an network structure, which results in the increase in conductivity, thereby increasing the propagation path and attenuation opportunity of the incident microwave. Besides, the micro-currents generated by the electrons migrating along the propagation path interact each other, giving rise to the attenuation of the microwave energy [49,50]. 4. Conclusions In this work, the coaxial nanocomposites composed of the SiC NWs core and the amorphous carbon shell are prepared according to the hydrothermal-carbonization method, with SiC NWs and PVP being used as the supporting skeletons and carbon precursor, respectively. After wards, the products are characterized by a series of methods, including XRD, SEM, TEM, and XPS. To estimate the attenuation of the incident electromagnetic wave, the complex permittivity and permeability are calculated for the absorber. Our findings suggest that, the as-fabricated net-like SiC@C NCs exhibit enhanced microwave absorption perfor mances, with the minimal RL value of 51.53 dB and a broad EAB covering the whole Ku-band of 6.32 GHz at a small matching thickness of 2.08 mm. Specifically, the broadest EAB of 7.2 GHz is achieved at the matching thickness of 2.58 mm, which is superior to those of most re ported carbon and SiC-based microwave absorbers. Specifically, the involved microwave absorption mechanism is attributed to the multiple reflections between the carbon shell and the SiC NWs core, as well as the defects and stacking faults in carbon and SiC, and the interfacial po larization aroused by the higher surface-to-volume ratio of the nano structures. The simple and feasible synthesis strategy proposed in this study provides a promising microwave absorber candidate characterized by the advantages of light weight, small thickness and broad absorption bandwidth. Declaration of competing interest On behalf of all authors, I declare that we have no financial and 8
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personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company.
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