Si3N4 composite nanofibers with in situ embedded

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Flexible SiC/Si3N4 Composite Nanofibers with in Situ Embedded Graphite for Highly Efficient Electromagnetic Wave Absorption Peng Wang, Laifei Cheng,* Yani Zhang, and Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory and State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, 710072 Xi’an, China S Supporting Information *

ABSTRACT: SiC/Si3N4 composite nanofibers with in situ embedded graphite, which show highly efficient electromagnetic (EM) wave absorption performance in gigahertz frequency, were prepared by electrospinning with subsequent polymer pyrolysis and annealing. By means of incorporating graphite and Si3N4 into SiC, the EM wave absorption properties of the nanofibers were improved. The relationship among processing, fiber microstructure, and their superior EM wave absorption performance was systematically investigated. The EM wave absorption capability and effective absorption bandwidth (EAB) of nanofibers can be simply controlled by adjusting annealing atmosphere and temperature. The nanofibers after annealing at 1300 °C in Ar present a minimum reflection loss (RL) of −57.8 dB at 14.6 with 5.5 GHz EAB. The nanofibers annealed in N2 at 1300 °C exhibit a minimum RL value of −32.3 dB at a thickness of 2.5 mm, and the EAB reaches 6.4 GHz over the range of 11.3−17.7 GHz. The highly efficient EM wave absorption performance of nanofibers are closely related to dielectric loss, which originated from interfacial polarization and dipole polarization. The excellent absorbing performance together with wider EAB endows the composite nanofibers potential to be used as reinforcements in polymers and ceramics (SiC, Si3N4, SiO2, Al2O3, etc.) to improve their EM wave absorption performance. KEYWORDS: electrospinning, graphite/SiC/Si3N4 composite nanofibers, dielectric polarization, broad-band, electromagnetic wave absorption at 8.2 GHz with a larger EAB of 8 GHz.12 The Li group reported the preparation of thermally reduced graphene networks (TRGNs) by thermal reduction of graphene oxide/ poly(vinyl alcohol) networks. The wax composites with 1 wt % TRGN presented an optimal RL of −43.5 dB at a thickness of 3.5 mm, and their EAB reached as high as 7.47 GHz.13 In addition, Pan and colleagues investigated the EM properties of carbon nanowires/Si3N4 composites fabricated by catalytic chemical vapor deposition. The RL value of the composites with 1.84 wt % carbon nanowires reached −50.2 dB at 10.8 GHz, and its EAB covered the whole X band.14 In addition to carbon-based absorbers, ferromagnetic metal alloys and their oxides can be applied as EM wave absorbers when added in resin, polymer, wax, and so forth.9,15−19 Du and co-workers synthesized porous three-dimensional flowerlike Co/CoO composites by a solvothermal method. An optimized RL of −50 dB with an EAB of 4.2 GHz was obtained, as the filler loading of the Co/CoO composites in matrix was 50 wt %.9 Lv et al. designed coinlike α-Fe 2O 3@CoFe2 O4 core−shell

1. INTRODUCTION With the rapid development of electronic information industries, great amounts of electronic devices are increasingly employed. The high-frequency (GHz) microwave generated by these devices has a harmful effect on human life and health.1−3 Meanwhile, to protect the precise electronic products from electromagnetic wave effect, EM wave absorbers are generally being coated on their surface.4−6 Therefore, the demands for highly efficient EM wave absorbers have become an urgent issue. Great efforts have been made to exploit high EM wave absorption performance of absorbers with stronger absorption abilities, wider effective absorption bandwidth (EAB) (RL < −10 dB, 90% EM wave attenuation), lightweight characteristics, corrosion-resistant properties, high mechanical strength, antioxidation, and so forth.7−11 Several materials have been used as EM wave absorbers, mainly including carbonaceous materials, metals and their oxides, conductive polymers, and ceramic materials (SiC, Si− C−N, Si−B−C−N, etc.). Owing to their lightweight characteristics, good conductivity, and excellent EM wave absorption performance, tremendous work on carbon materials have been done.12−14 Yin et al. synthesized mesoporous carbon hollow microsphere, which exhibited a minimum RL value of −84 dB © 2017 American Chemical Society

Received: April 17, 2017 Accepted: August 11, 2017 Published: August 11, 2017 28844

DOI: 10.1021/acsami.7b05382 ACS Appl. Mater. Interfaces 2017, 9, 28844−28858

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ACS Applied Materials & Interfaces composites, which showed a minimum RL value of −60 dB at a thickness of 2 mm, and their EAB covered from 13 to 18 GHz.17 Although carbonaceous and ferromagnetic absorbers have several advantages, such as lower RL values, wider EAB, and so forth, they also suffer from several disadvantages. Hightemperature oxidation (T > 400 °C) of carbonaceous absorbers severely affects their EM wave absorption performance.20,21 In addition, the high density, easy corrosion, and ferromagnetic characteristics of metal-based absorbers limit their practical applications. Conductive polymers, such as polyaniline (PANI), polypyrrole (Ppy), and their composites, also display excellent EM wave absorption properties. Cao and co-workers synthesized PANI nanorods on graphene sheets by in situ polymerization process. The obtained composites exhibited a minimum RL value of −45.1 dB at a thickness of 2.5 mm.22 Han et al. reported the fabrication of Ppy@PANI core−shell composites, and the optimal RL value of the composites was −34.8 dB at 13.9 GHz with an EAB of 4.7 GHz.23 However, their low mechanical strength and impedance mismatching caused by high-temperature carbonization have adverse effects on the EM wave absorption performance. Compared with these absorbent materials, SiC nanowires (SiCNWs) are expectedly good dielectric absorbers, which can tolerate harsh working conditions on account of their chemical and high-temperature stability, antioxidation properties, high strength, and hardness.24−26 In addition, it is reported that nanowires or nanofibers with high specific surface area can offer more “active” surface atoms, leading to the enhanced dielectric loss.27,28 However, EM wave absorption performance of SiCNWs are unsatisfactory, such as higher RL value (>−30 dB), narrower EAB (2.5 mm).26,29−36 To solve these issues, the efficient methods are to combine SiCNWs with metals, metal oxides (Fe3O4, Co, and ZnO), and conductive polymers (Ppy) to enhance their EM wave absorption performance.31−34 Although these methods have improved the EM wave absorption performance of SiCNWs significantly, it is still far from the demands for absorbers with high EM wave absorption performance. It is well known that EM wave absorption performance of absorbers is highly dependent on their relative complex permittivity (εr = ε′ − jε″) and permeability (μr = μ′ − jμ″). The μ′ and μ″ values of SiC materials are 1 and 0, respectively. Higher real part (ε′) and lower imaginary part (ε″) of SiC materials lead to their lower impedance matching and poor dielectric loss.35−37 Therefore, the balance of the real (ε′) and imaginary (ε″) parts of permittivity plays an important role in improving impedance matching and EM wave attenuation of SiC materials. Owing to its lower ε′ and ε″ values, Si3N4 is generally used as EM wave transparent matrix.4,14,38 Thus, it is reasonable to speculate that the impedance-matching conditions of SiC can be improved by combining with Si3N4. In addition, hybrid materials involving carbon (as the conductive phase) and SiC (semiconducting phase) dispersed in an eletrically insulative material (Si3N4) provide the possibilities for the improvement of dielectric loss, according to Lichteneker and Rother’s law.4,39 Therefore, ternary dielectric components composed of carbon, SiC, and Si3N4 are necessary for highly efficient dielectric absorbers. Electrospinning is a cost-effective, versatile, facile technique that has been widely used to produce nanofibers, nanowires, nanotubes, and so forth on a large scale.40−42 Polycarbosilane (PCS) and poly(vinylpyrrolidone) (PVP) were used as

precursors of SiC and graphite, respectively. Self-assembled PCS/PVP composite nanofibers using electrospinning were obtained. Then, by simply controlling the annealing temperature and atmosphere, SiC/Si3N4 composite nanofibers with in situ embedded graphite were successfully fabricated. Compared with other SiC-based absorbers, the nanofibers have a stronger EM wave absorption ability and a wider EAB. The contributions of synergistic effects from graphite, SiC, Si3N4, and heterogeneous interfaces on EM wave absorption performance were thoroughly discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Polycarbosilane (PCS, 1400−1600 g/mol) was obtained from Xiamen University, China. Poly(vinylpyrrolidone) (PVP, 1 300 000 g/mol) was purchased from Shanghai Macklin Biochemical Co. Ltd., China. All organic solvents were purchased from Changsha Huihong Co. Ltd., China, and used without further purification. High-purity argon and nitrogen were purchased from Changsha Jingxiang Co. Ltd., China. 2.2. Fabrication of SiC/Si3N4 Composite Nanofibers with in Situ Embedded Graphite. First, 4.3 wt % PVP was added into a mixture solvent of N,N-dimethylformamide and chloroform with a weight ratio of 1:4, followed by vigorous stirring for 2 h. Then, 10.0 wt % PCS was added into the homogeneous solution, followed by dramatic stirring for 24 h. The homogeneous solution was then transferred into a 10 mL plastic syringe with a 21G spinneret, and the collector is a stainless steel plate coated with a graphite paper. During electrospinning, the distance between spinneret and collecting substrate is 16 cm, the electrospinning voltage (V) is 20 kV, and the feed rate of precursor (Q) is 0.04 mL/min. The electrospun PCS/PVP nanofibers were collected on graphite paper substrates. The electrospun PCS/PVP nanofibers were maintained in vacuum at 70 °C for 24 h to vapor the solvent out completely. To maintain the shape of the fibers in the annealing procedure, the fibers must be thermally cured. The PCS/PVP nanofibers were cured in a muffle furnace from room temperature to 180 °C at a heating rate of 2 °C/ min and then to 210 °C at a heating rate of 1 °C/min and holding for 2 h in air. Finally, cured nanofibers were put in a tubular furnace for annealing from room temperature to 250 °C at a heating rate of 2 °C/min and then to 850 °C at a heating rate of 1 °C/min (the details are shown in Supporting Information). Then, the temperature of the furnace ramped to 1300 °C at a rate of 2 °C/min and maintained for 2 h in Ar. Meanwhile, cured nanofibers were annealed in N2 at 1300, 1400, and 1500 °C for 2 h. The final products were marked as 1300, N1300, N1400, and N1500. 2.3. Characterization. A Hitachi S-4700 field emission scanning electron microscope (FE-SEM) was used to explore the morphology of nanofibers with an acceleration voltage of 15.0 kV. Gold (5 nm) was sputtered on all nanofibers using a scanning electron microscope coating unit (E-1010) from Polaron Equipment Limited. X-ray diffraction (XRD) data were collected from 10 to 90° (2θ) by Bruker AXS D8 Advance device using Cu Kα radiation (λ = 1.54 Å) at a scanning rate of 2θ = 0.05°/s. FTIR spectra were collected on a Nicolet Avatar 360 instrument on samples pressed as KBr pellets. Raman spectra were obtained on a Renishaw confocal Raman microscope (inVia, Renishaw, Gloucestershire, U.K.) equipped with a He−Ne laser (λ = 532 nm). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM, Tecnai G2 F30 (FEI)) instruments equipped with an energy dispersion spectrometer (EDS) were used, respectively, for the analysis of the microstructure and to determine the chemical composition of the prepared fibers. Focused ion beam (FIB) analyses were carried out on Nova200 NanoLab. Xray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Scientific Escalab 250 Xi machine with a monochromatized Al anode X-ray source. Toroidal-shaped samples (outer diameter, 7 mm; inner diameter, 3 mm) were fabricated by uniformly dispersing 0.14 g of composite nanofibers into 0.26 g of paraffin, that 28845


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Figure 1. SEM images of composite nanofibers derived from different annealing temperatures and atmospheres: (a) 1300 °C in Ar, (b) 1300 °C in N2, (c) 1400 °C in N2, and (d) 1500 °C in N2. (e) Schematic illustration of the nanofiber web with bending and twisting shape. is, the mass percentage of nanofibers in the samples was 35 wt %. The electromagnetic parameters of these samples were then measured by a transmission/reflection coaxial method in the range of 2−18 GHz using a Rohde−Schwarz ZVB8 vector network analyzer.

samples exhibit typical diffraction peaks located at 2θ values of 35.7, 60.1, and 71.9° corresponding to (111), (220), and (311) lattice planes of 3C−SiC, according to JCPDs card #29-1129. In addition, the presence of only a diffraction peak around 26.6° is assigned to carbon. As the annealing temperature increased to 1500 °C, the sample N1500 shows the characteristic diffraction peaks of Si3N4 crystalline structure on the basis of JCPDs card #40-1129. The formation mechanism of Si3N4 is mainly related to the following reaction equations46,47

3. RESULTS AND DISCUSSION Figure 1 shows typical morphologies of the as-prepared nanofibers, suggesting the uniform and continuous long nanofibers in the network. The SEM images demonstrate the nanofibers with uniform diameter of 100−500 nm (inset of Figure 1d), and the average length of the nanofibers is up to several tens of micrometers. The surface of nanofibers is smooth, without any pores, suggesting that annealed ceramic nanofibers are dense. Figure 1e presents the excellent bendability as well as better twisting ability of the nanofiber web without any fracture. The excellent mechanical performance of the nanofibers is correlated with their microstructure and phase composition.43−45 To further probe the crystalline structure of nanofibers, X-ray diffraction (XRD) was employed. As shown in Figure 2a, the

Si3N4 + 3C = 3SiC + 2N2

(1)

Si3N4 = 3Si + 2N2

(2)

Reactions 1 and 2 are reversible; when nanofibers were continuously annealed in N2, the chemical reaction of silicon nitride with carbon can be hampered. The Si3N4 and free carbon in situ formed in nanofibers. As the EM wave transparent material, Si3N4 was incorporated into SiC nanofibers to facilitate their impedance matching, which will be 28846


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Figure 2. (a) XRD patterns and (b) Raman spectra of composite nanofibers (1300, N1300, N1400, and N1500).

localized defects or vacancies instead of acting as conductive electrons. At the same time, these charges trapped around defects, leading to an asymmetry of charge distribution and integral dipole polarization thereafter.54 This is of benefit to EM wave dissipation due to the enhanced defect dipole polarization. To understand the conversion process from PCS/PVP nanofibers to graphite/SiC/Si3N4 composite nanofibers, the Fourier transform infrared (FTIR) patterns of PCS/PVP thermally cured nanofibers and the fibers annealed at 1300, 1400, and 1500 °C were recorded in Figure 3. As shown in Figure 3, typical FTIR spectra of PCS appeared at 2800−3000 cm−1 (C−H stretching of Si−CH3), 2094 cm−1 (Si−H stretching), 1250 cm−1 (Si−CH3 symmetric deformation), 1015 cm−1 (Si−C−Si stretching of Si−CH2−Si), and 820 cm−1 (Si−C bond) in cured nanofibers.55 Most silicone-based chemical bond (Si−C) disappeared, and only C−O bond left after annealing. This indicates that SiC or the mixture of SiC and SiOC exist in nanofibers. In addition, with increasing annealing temperature, the C−O bond gradually became imperceptible and only the Si−C bond remained. It is unexpected that the signal of Si−N bond is absent in the FTIR pattern. Further information about the chemical bond of Si−N should be confirmed by X-ray photoelectron spectrum. To investigate the chemical composition of the nanofibers, XPS survey is shown in Figure 4. It can be clearly seen in Figure 4a that the nanofibers are composed of Si, O, C, and N elements. Detailed information about the atomic percentages of C, Si, O, and N has been reported (see Figure S1 in the Supporting Information). As shown in Figure S1, as annealing temperature increases, atomic % of nitrogen gradually decreases and atomic % of carbon increases sharply at 1500 °C. Amounts of free carbon are separated out from the Si−C network when annealing temperature exceeded 1400 °C.52,53 As shown in Figure 4b, the core level of O 1s peak can be fitted into two peaks located at 532.5 and 533.7 eV. These two peaks correspond to Si−O−Si and Si−O−C chemical bonds, respectively.55 Fitting analyses of C 1s peak are shown with C−C (284.8 eV), C−Si (283.0 eV), C−O (286.4 eV), and O− CO (288.8 eV).56 The C−C chemical bond corresponds to free carbon, and the O−CO bond is correlated with residual oxides from carbothermal reaction. Three typical peaks located

Figure 3. FTIR patterns of cured nanofibers and composite nanofibers.

discussed later. Meanwhile, amounts of heterogeneous interfaces emerged among SiC, free carbon, and Si3N4. Considering the lower electrical conductivity of SiC and Si3N4, the free carbon as highly conductive phase mainly determines the conductivity of nanofibers.48,49 Vacancies and defects in free carbon have a significant effect on their electrical conductivity. Raman spectrum was used to detect vacancies and defects in carbon materials. It illustrates that in Figure 2b two fundamental vibrations are observed at 1338 and 1590 cm−1, which are designated as D band and G band, respectively. The D band is attributed to disordered structures of carbon originated from vacancies, amorphous carbon species, and defects in the lattice, whereas the G band is associated with sp2hybridized carbon bonds. The intensity ratio of D band to G band (ID/IG) represents the degree of disorder in the graphite structure.50,51 The ID/IG values of the samples 1300, N1300, N1400, and N1500 were calculated to be 1.14, 1.11, 1.18, and 1.22, respectively. By comparison, the value of ID/IG for sample N1500 is greater than that of others. These results should be ascribed to the reaction between carbon and oxygen.52,53 The reaction creates more defects and vacancies in carbon. Free electrons that transport in nanofibers could be trapped in 28847


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Figure 4. (a) XPS survey of carbon−SiC/Si3N4 nanofibers; fitting peaks of N1500: (b) O 1s, (c) C 1s, (d) Si 2p, and (e) N 1s.

at 100.5, 101.6, and 102.8 eV corresponding to Si−C, Si−O−C, and Si−O−Si are fitting results of Si 2p peak.55 As shown in Figure 4e, the intense peak at 397.9 eV and the weak peak at 399.7 eV present Si−N and C−N bonds, respectively.55 Combining the XPS results of C 1s, Si 2p, and O 1s, it is reasonable to speculate that bulk SiC, free carbon, and amorphous SiOxCy coexisted in nanofibers. Whether the crystalline form of carbon is graphite, amorphous carbon or their mixture should be identified by TEM results. In addition,

taking into consideration C−N and Si−N bonds, there should exist Si−C−N structure or Si3N4 with amorphous SiOxCyNz. Detailed information about Si3N4 lattice plane can be confirmed by HR-TEM results. To further clarify the inner microstructure and phase distribution of as-prepared nanofibers, focused ion beam (FIB) was used to etch the nanofibers along their axes. As shown in Figure 5b, atomic ratios of Si, O, and C were recorded; there are more silicon atoms compared to carbon and 28848


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Figure 5. SEM images of inner microstructure and phase composition of composite nanofibers: (a−c) nanofibers annealed at 1300 °C in Ar and (d− g) nanofibers annealed at 1500 °C in N2.

oxygen. Considering the stoichiometry of SiC, excess silicon atoms could lead to the formation of amorphous SiOxCy.52,53

According to atomic number contrast in Figure 5c, black and off-white areas of nanofibers correspond to carbon and the 28849


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Figure 6. continued

28850


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Figure 6. TEM images of prepared nanofibers: (a−f) nanofibers annealed at 1300 °C in Ar and (g−l) nanofibers annealed at 1500 °C in N2.

and Si3N4 nanocrystals. It is believed that these heterogeneous interfaces and nanocrystals play a crucial role in EM wave dissipation, which will be discussed later. It is well known that the EM wave absorption capability of absorbers is highly dependent on their relative complex permittivity (εr = ε′ − jε″) and permeability (μr = μ′ − jμ″).58 For nonmagnetic graphite/SiC/Si3N4 composite nanofibers, μ′ = 1 and μ″ = 0. The real (ε′) and imaginary (ε″) parts of permittivity present the storage and loss abilities for EM wave energy. Although higher ε″ values are expected, initially, impedance matching between absorbers and free space can ensure more EM wave incident on absorbers with less being reflected. The impedance matching can be evaluated by the ratio Zin/Z0, where Zin and Z0 are the input impedance of absorbers and free space, respectively. Zin and Z0 can be expressed by the following equations58

mixture of SiC and SiOxCy, respectively. Carbon nanoparticles with laminar shape are inserted into the SiC matrix, and the SiC is coated on the fiber surface. Figure 5f shows the atomic ratios of Si, C, N, and O in nanofibers and that the atomic ratio of carbon is greater than that of nitrogen and oxygen, which is consistent with XPS results (Figure S1). Taking into consideration the small difference of atomic number contrast between SiC and Si3N4, the off-white area should be the mixture of the two phases (Figure 5g). It is reasonable to speculate that the proportion of Si3N4 is lower according to the atomic % of nitrogen. The thickness of coating layer on nanofibers is about 116 nm (Figure 5g). The analysis of TEM images can bring further insights into the microstructure of as-prepared nanofibers. TEM images of the sample 1300 are recorded in Figure 6a−f, and on account of their lower Gibbs free energy, typical 3C−SiC (111) lattice plane with a d-spacing of 0.25 nm can be found. The average grain sizes of SiC are approximately 10 nm; moreover, the growth direction of SiC nanocrystals is discrepant. Figure 6e demonstrates distinct grain boundaries among SiC nanocrystals. Free electrons move in nanocrystals and accumulate at heterogeneous interfaces owing to their difference in the electronic transportation properties. Electric dipole moment results owing to the accumulation of charges at interfaces under external alternating EM field.54,57 Figure 6f shows that the nanofibers are mainly inclusive of SiC nanocrystals, graphiticlike carbon, and amorphous SiOxCy. As illustrated in Figure 6h, a graphite layer with a d-spacing of 0.34 nm is located around the SiC nanocrystals. Typical Si3N4 (320) lattice plane with a 0.259 nm d-spacing can be obviously found in Figure 6i. Amorphous SiOxCy fill in interspace among graphite layer, SiC,

Z in = Z0(μr /εr)1/2 tanh[j(2πfd /c)(μr /εr)1/2 ]

Z0 =

(3)

μ0 ε0

(4)

where f is the frequency of electromagnetic wave, d is the absorber coating thickness, c is the speed of light in vacuum, μ0 and ε0 are permeability and permittivity, respectively, in vacuum. From eqs 3 and 4, for nonmagnetic dielectric absorbers, it is suggested that the impedance matching can be tailored by ε′ and ε″ values. The variation of ε′ and ε″ values with frequency of the samples is shown in Figure 7. As shown in Figure 7a, the ε′ values are found to decrease with the measured frequency. This can be ascribed to a typical frequency dispersion behavior.7 The sample N1500 has higher ε′ values, 28851


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Figure 7. Real part (a), imaginary part (b), and tangent loss (c) for the samples (1300, N1300, N1400, and N1500).

beneficial to the increase of the ε″ values such as ε″(N1500) > ε″(N1400) > ε″(N1300). However, in the frequency range of 2−12 GHz, ε″(N1300) > ε″(N1400). The sample N1300 has more free electrons owing to N doping, and amounts of charges accumulate at the heterogeneous interfaces under the effects of external EM field. Then, interfacial polarization contributes to the increase of the ε″ values of N1300. The dielectric dissipation factor tan δe (tan δe = ε″/ε′) provides a measure of EM wave energy lost compared with total EM wave energy stored, and higher tan δe values indicate that more EM wave energy is consumed. A significant improvement of tan δe values for the sample N1300 is achieved in the frequency range of 2− 18 GHz, and their values reach as high as 0.57 at 16.7 GHz. There are two distinct resonant peaks located around 4.5 and 17.5 GHz in ε″ curves. According to the Debye theory, the real (ε′) and imaginary (ε″) parts of permittivity can be defined by the following equations17 ε −ε ε′ = ε∞ + s 2∞2 (5) 1+ωτ

which varied in the range of 8.5−12.5 compared with others. By comparison, ε′ values of N1300 varied in the range of 4.7−9.3 are lowest. With increasing annealing temperature, the ε′ values of the samples gradually increase, such as ε′(N1500) > ε′(N1400) > ε′(N1300). When the annealing temperature exceeds 1300 °C, carbon and Si3N4 formed in situ and the ε′ and ε″ values of Si3N4 are around 4 and 0, respectively.14 Then, according to Lichteneker and Rother’s law, the ε′ values should be enhanced owing to the incorporation of carbon and Si3N4; therefore, ε′(N1500) > ε′(N1400) > ε′(N1300). In addition, the values of ε′ are correlated with the storage capability of EM wave energy. Compared with sample 1300, there are more free electrons for N1300 owing to nitrogen-occupied carbon vacancies.59,60 More free electrons promote the transportation of charges and then the storage capability of EM wave energy becomes poor, which lead to ε′(1300) > ε′(N1300). In general, higher ε″ values present better EM wave loss ability of absorbers. As shown in Figure 7b, the imaginary (ε″) parts of composite nanofibers have a strong dependence of annealing temperature and atmosphere. The ε″ values of sample 1300 are relatively higher compared to those of N1500 over the frequency range of 2−15 GHz. In the frequency range of 12−18 GHz, the ε″ values of the samples gradually increase such as ε″(N1500) > ε″(N1400) > ε″(N1300). More carbon and Si3N4 formed in composite nanofibers with increasing annealing temperature. The increasing heterogeneous interfaces among carbon, SiC, and Si3N4 with defects of carbon all are

ε″ = ε″p + ε″c =

εs − ε∞ 2 2

1+ωτ

ωτ +

σ ωε0

(6)

where εs is the static dielectric constant, ε∞ is the relative dielectric constant in high-frequency limit, ω is the angular frequency, ω = 2πf, f is the frequency, σ is the electrical conductivity of samples, ε0 is the dielectric constant in vacuum, 28852


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Figure 8. RL values calculated for the samples (a) 1300, (b) N1300, (c) N1400, and (d) N1500 at different thicknesses.

Figure 9. Impedance-matching curves (a) and the values of attenuation constant (b) of the samples at various frequencies.

increasing only in some special τ values, the ε″p value can reach the highest point.61 The value of τ is related to polarization relaxation of absorbers. It is widely accepted that dielectric polarization can be further divided into ionic polarization, electronic polarization, dipole orientation polarization, and interfacial polarization (space charge polarization).61,62 Ionic and electronic polarizations can be easily excluded because they usually occur at much higher frequency (103−106 GHz).61,62 The dielectric polarization of heterostructure nanofibers

τ is the relaxation time, which is related to frequency and temperature, ε″p is the polarization loss, and ε″c is the conductivity loss. Then, with increasing frequency, the value of ω increases, resulting in a reduction of ε″c, so the resonant peaks of ε″ curves are mainly attributed to ε″p.24,25,29,30 Because εs is the static dielectric constant, ε∞ is the relative dielectric constant in high-frequency limit; ε″p was determined by angular frequency ω and relaxation time τ. According to the equation (εs − ε∞)ωτ/(1 + ω2τ2), it is found that with frequency 28853


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Figure 10. Three-dimensional maps of the variation of reflection loss values with absorber coating thickness and frequency for samples (a) 1300, (b) N1300, (c) N1400, and (d) N1500.

is increased, and the interfacial polarization and multiple scattering become two of the important factors of EM wave attenuation.57 The reflection loss (RL, in dB) is an effective parameter to evaluate EM wave absorption abilities of absorbent materials. RL value lower than −10 dB means that more than 90% EM wave is absorbed. In addition, −20 dB is a threshold value for EM wave absorbers in commercial application comparable to 99% EM wave absorption. The reflection loss properties of composite nanofibers are then investigated on the basis of the transmission line theory, and the RL value is calculated from the relative permittivity, permeability at a given frequency, and absorber coating thickness, using the following equations58,59

includes interfacial polarization and defect dipole orientation polarization that originate from heterogeneous interfaces and defects, respectively. For multicomponents of composite nanofibers, the interfaces in graphite/SiC/Si3N4 heterostructure are believed to play a significant role in EM wave absorption. Under an alternating electronic field, hopping electrons move in nanofibers. The accumulation of positive and negative charges at the interface of a heterogeneous structure will cause a change in charge density and produce electric dipole moment, which leads to interfacial polarization and associated relaxation, giving rise to dielectric loss.58,63−65 Therefore, it is reasonable to suppose that the peaks in ε″ curves result from interfacial polarization and defect dipole orientation polarization. In addition, the sizes of SiC and Si3N4 nanocrystals are about several tens of nanometers. When the grain size of nanocrystal is reduced, the number of atoms with unsaturated coordination

RL = 20 log|(Z in − Z0)/(Z in + Z0)| 28854

(7)


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GHz at a thickness of 2.5 mm. The EABs of N1400 and N1500 are 4.8 and 4.7 GHz, respectively. It can be seen that as annealing temperature increases, the EAB of nanofibers gradually becomes narrow and their EM wave absorption ability becomes poor. The impedance matching is a prerequisite condition for absorbers having excellent EM wave absorption properties. Zin/Z0 = 1 means that zero EM wave reflection occurred on the absorber surface. Figure 9a demonstrates the variation of Zin/Z0 values with measured frequency for the samples at a thickness of 1.9 mm. Sample 1300 has relatively lower Zin/Z0 values over the range of 11−15 GHz, which indicated its better impedance matching. However, for samples N1400 and N1500, Zin/Z0 values can reach as high as 1.4, meaning that more EM wave reflected from the absorber surface and therefore their EM wave absorption performances are relatively poor. To evaluate the EM wave attenuation abilities inside these absorbers, the attenuation constant (α) was introduced. The value of α can be calculated by the following equation17,60

Table 1. EM Wave Absorption Performance of SiC-Based Absorbers samples

RL (dB)

EAB (GHz)

d (mm)

weight (%)

SiCNWs1 SiCNWs2 SiCNWs3 SiCNWs4 ZnO/SiCNWs Co/SiCNWs Fe3O4/SiCNWs SiCNWs Fe/SiC whiskers Al/SiC whiskers SiC fibers C/SiC/Si3N4

−31.7 −30 −48 −17.4 −42.1 −25 −51 −20 −21 −25.4 −30 −57.8

2.5 3.7 2.56 2.5 4.48 6.6 7.0 3.57 2.6 2.0 2.6 6.4

2.0 4.6 1.9 3.0 3.5 3.0 3.0 3.3 2.0 2.4 4.7 2.5

35 50 30 30 30 50 50 20 20 55 35

matrix

ref

epoxy paraffin paraffin silicone paraffin paraffin paraffin SiOC paraffin paraffin epoxy paraffin

26 39 29 37 33 32 31 68 69 70 35 ours

α= ×

2 πf c (μ″ε″ − μ′ε′) +

(μ″ε″ − μ′ε′)2 + (μ″ε″ + μ″ε′)2 (8)

Figure 9b exhibits the variation of α with measured frequency. The α values of sample 1300 are relatively higher compared to those of others, which can reach as high as 225. Better impedance matching and higher attenuation constant make the sample 1300 have excellent EM wave absorption performance. Three-dimensional (3D) maps of the variation of reflection loss values with frequency and absorber coating thickness for the samples are shown in Figure 10. It can be clearly seen that through adjusting absorber coating thickness, we can achieve the goal of selectively absorbing EM wave in different frequency bands, including C (4.0−8.0 GHz), X (8.2−12.4 GHz), and Ku. As illustrated in Figure 10, the composite nanofibers have stronger EM wave absorption performance in high frequency (14−18 GHz). Table 1 summarizes the EM wave absorption performance of SiC-based absorbers. By comparison, it is found that the EM wave absorption performances of graphite/SiC/ Si3N4 composite nanofibers are comprehensive with stronger absorption (−57.8 dB), wider absorption bandwidth (6.4 GHz), and smaller coating thickness (2.5 mm). The schematic illustration of EM wave loss mechanism of the composite nanofibers is shown in Figure 11. The synergistic effects among graphite, SiC, and Si3N4, and interfacial polarization and defect dipole orientation polarization in graphite and SiC contribute to the improvement of the EM wave absorption performance of nanofibers.4,60,64−67 Alternatively, one-dimensional EM wave absorbers added in paraffin composites, owing to their high aspect ratio, and the composite nanofibers can easily create a conductive network in paraffin matrix. Under an external electric field, hopping electrons can jump across the interface among nanofibers, and this kind of hopping electrons could enhance microcurrent in the nanofiber network. The induced current can transform EM wave energy into thermal energy.

Figure 11. Schematic illustration of EM wave absorbed by graphite/ SiC/Si3N4 composite nanofibers.

Figure 8a shows the RL curves of the samples as a function of frequency, with different coating thicknesses. The sample 1300 presents a minimum RL of −57.8 dB, which implied that more than 99.999% EM wave was absorbed at 14.6 GHz, with a thickness of 1.9 mm. Their EAB covers almost whole Ku band range from 12.5 to 18 GHz. Then, with increasing coating thickness, the frequency of optimum RL value gradually shifts to lower-frequency range. This phenomenon can be explained by quarter-wavelength calculation law, tm = nλ /4 = nc /(4fm με ) (n = 1, 3, 5, ...), where tm is the r r coating thickness and λ is the wavelength of incident wave.18,19When tm and f m satisfy the equation, the EM wave reflected from air−absorber interfaces encounters with the EM wave reflected from absorber−conductive metal back plane, with two waves disappearing owing to phase out of 180°. In this case, the RL reaches the minimum value. The permittivity can be tuned conveniently, limited by impedance-matching condition; unilaterally, increasing the permittivity or permeability cannot achieve the goal of lower reflection loss (RL) with broader EAB. It is important to achieve a small thickness with excellent EM wave absorption properties. The sample N1300 presents a minimum RL value of −32.3 dB at 17.1 GHz with a coating thickness of 2.1 mm. The widest EAB of sample N1300 can reach as high as 6.4 GHz covering from 11.3 to 17.7

4. CONCLUSIONS In conclusion, SiC/Si3N4 composite nanofibers with in situ embedded graphite were fabricated by electrospinning and subsequent polymer pyrolysis and heat treatment. The composite nanofibers exhibit comprehensive electromagnetic 28855


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magnetic Absorption Performance. J. Mater. Chem. C 2015, 3, 5056− 5064. (9) Lv, H.; Liang, X. H.; Ji, G. B.; Zhang, H. Q.; Du, Y. W. Porous Three-dimensional Flower-like Co/CoO and Its Excellent Electromagnetic Absorption Properties. ACS Appl. Mater. Interfaces 2015, 7, 9776−9783. (10) Han, M. K.; Yin, X. W.; Ren, S.; Duan, W. Y.; Zhang, L. T.; Cheng, L. F. Core/shell Structured C/ZnO Nanoparticles Composites for Effective Electromagnetic Wave Absorption. RSC Adv. 2016, 6, 6467−6474. (11) Han, M.; Yin, X. W.; Li, X. L.; Anasory, B.; Zhang, L. T.; Cheng, L. F.; Gogotsi, Y. Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from Mxenes. ACS Appl. Mater. Interfaces 2017, 9, 20038−20045. (12) Xu, H.; Yin, X. W.; Zhu, M.; Han, M. K.; Hou, Z. X.; Li, X. L.; Zhang, L. T.; Cheng, L. F. Carbon Hollow Microspheres with A Designable Mesoporous Shell for High Performance Electromagnetic Wave Absorption. ACS Appl. Mater. Interfaces 2017, 9, 6332−6341. (13) Liu, W.; Li, H.; Zeng, Q. P.; Duan, H. N.; Guo, Y. P.; Liu, X. F.; Sun, C. Y.; Liu, H. Z. Fabrication of Ultralight Three-Dimensional Graphene Networks with Strong Electromagnetic Wave Absorption Properties. J. Mater. Chem. A 2015, 3, 3739−3747. (14) Pan, H. X.; Yin, X. W.; Xue, J. M.; Cheng, L. F.; Zhang, L. T. InSitu Synthesis of Hierarchically Porous and Polycrystalline Carbon Nanowires with Excellent Microwave Absorption Performance. Carbon 2016, 107, 36−45. (15) Zhao, B.; Zhao, W. Y.; Shao, G.; Fan, B. B.; Zhang, R. Morphology-Control Synthesis of a Core-shell Structured NiCu Alloy with Tunable Electromagnetic-Wave Absorption Capabilities. ACS Appl. Mater. Interfaces 2015, 7, 12951−12960. (16) Zhao, B.; Guo, X. Q.; Zhao, W. Y.; Deng, J. S.; Shao, G.; Fan, B. B.; Bai, Z. Y.; Zhang, R. Yolk-Shell Ni@SnO2 Composites with A Designable Interspace to Improve Electromagnetic Wave Absorption Properties. ACS Appl. Mater. Interfaces 2016, 8, 28917−28925. (17) Lv, H.; Liang, X. H.; Cheng, Y.; Zhang, H. Q.; Tang, D. M.; Zhang, B. S.; Ji, G. B.; Du, Y. W. Coin-like α-Fe2O3@CoFe2O4 Core− shell Composites with Excellent Electromagnetic Absorption Performance. ACS Appl. Mater. Interfaces 2015, 7, 4744−4750. (18) Qin, F. X.; Peng, H. X. Ferromagnetic Microwires Enabled Multifunctional Composite Materials. Prog. Mater. Sci. 2013, 58, 183− 259. (19) Qin, F. X.; Peng, H. X.; Prunier, C.; Brosseau, C. Mechanical− Electromagnetic Coupling of Microwire Polymer Composites at Microwave Frequencies. Appl. Phys. Lett. 2010, 97, No. 153502. (20) Wen, B.; Cao, M. S.; Hou, Z. L.; Song, W. L.; Zhang, L.; Lu, M. M.; Jin, H. B.; Fang, X. Y.; Wang, W. Z.; Yuan, J. Temperature Dependent Microwave Attenuation Behavior for Carbon-Nanotube/ Silica Composites. Carbon 2013, 65, 124−139. (21) Song, W. L.; Cao, M. S.; Hou, Z. L.; Fang, X. Y.; Shi, X. L.; Yuan, J. High Dielectric Loss and Its Monotonic Dependence of Conducting-dominated Multiwalled Carbon Nanotubes/Silica Nanocomposite on Temperature Ranging from 373 to 873 K in X-band. Appl. Phys. Lett. 2009, 94, No. 233110. (22) Yu, H. L.; Wang, T. S.; Wen, B.; Lu, M.; Xu, Z.; Zhu, C. L.; Chen, Y. J.; Xue, X. Y.; Sun, C. S.; Cao, M. S. Graphene/Polyaniline Nanorod Arrays: Synthesis and Excellent Electromagnetic Absorption Properties. J. Mater. Chem. 2012, 22, 21679−21685. (23) Tian, C. H.; Du, Y. C.; Xu, P.; Rong, Q.; Wang, Y.; Ding, D.; Xue, J. L.; Ma, J.; Zhao, H. T.; Han, X. J. Constructing Uniform Core− shell Ppy@PANI Composites with Tunable Shell Thickness toward Enhancement in Microwave Absorption. ACS Appl. Mater. Interfaces 2015, 7, 20090−20099. (24) Zhang, Y. F.; Han, X. D.; Zheng, K.; Zhang, Z.; Zhang, X. N.; Fu, J. Y.; Ji, Y.; Hao, Y. J.; Guo, X. Y.; Wang, Z. L. Direct Observation of Super-plasticity of beta-SiC Nanowires at Low Temperature. Adv. Funct. Mater. 2007, 17, 3435−3440. (25) Ledoux, M. J.; Hantzer, S.; Huu, C. P.; Guille, J.; Desaneaux, M. P. New Synthesis and Uses of High-specific-surface SiC as A Catalytic

wave absorption performance with stronger absorption (−57.8 dB), wide absorption bandwidth (6.4 GHz), and smaller absorber coating thickness (1.9 mm). The excellent EM wave absorption performance is mainly correlated with synergistic effects of multicomponents, interfacial polarization, and defect dipole polarization. An effective absorption bandwidth of 6.4 GHz was achieved for nanofibers annealed at 1300 °C in N2. The nanofibers have potential to be used as additives in polymers and ceramic materials (SiC, Si3N4, SiO2, Al2O3, etc.) for enhancing the EM wave absorption performance. It is expected that the bending capacity of ceramic materials added with these nanofibers will be enhanced owing to the excellent flexibility of nanofibers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05382. Figures showing additional XPS data; TGA−DSC curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peng Wang: 0000-0002-3252-6379 Laifei Cheng: 0000-0001-6485-2614 Yani Zhang: 0000-0002-1148-9660 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Fund of China (No. 51632007). REFERENCES

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Si3N4 composite nanofibers with in situ embedded

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