Highly Efficient Electromagnetic Wave Absorbing Metal-Free and

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

pubs.acs.org/JPCC

Highly Efficient Electromagnetic Wave Absorbing Metal-Free and Carbon-Rich Ceramics Derived from Hyperbranched Polycarbosilazanes Yan Song,† Lihua He,‡ Xiaofei Zhang,† Fei Liu,† Nan Tian,† Yusheng Tang,† and Jie Kong*,† †

MOE Key Laboratory of Space Applied Physics and Chemistry, Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science, Northwestern Polytechnical University, Xi’an, 710072, People’s Republic of China ‡ Beijing Institute of Aeronautical Materials, Beijing, 100095, People’s Republic of China S Supporting Information *

ABSTRACT: The highly efficient electromagnetic (EM) wave absorbing metal-free and carbon-rich ceramics derived from hyperbranched polycarbosilazanes are presented in this contribution. The novel metal-free hyperbranched polycarbosilazanes with pendant cyano groups (hb-PCSZ-cyano) were synthesized through aminolysis reaction and subsequent Michael addition reaction, i.e., cyanoethylation reaction. As metal-free preceramic precursors, the pyrolysis of hb-PCSZ-cyano under high temperature and argon atmosphere generated carbon-rich Si−C−N multiphase ceramics. The ceramics reserve amorphous structure even at high temperature. The introduction of cyano groups in precursors leads to numerous sp2 carbons and interface polarization in ceramics and favors the EM wave absorption performance. The minimum reflection coefficient (RC) value of Si−C−N multiphase ceramic is −59.59 dB at 12.23 GHz when the sample thickness is 2.30 mm, which means >99.99% electromagnetic waves can be absorbed. The effective absorption bandwidth (RC below −10 dB) is 4.2 GHz, covering the whole X-band (8.2−12.4 GHz). The EM wave absorption property is very excellent in comparison to current electromagnetic wave absorbing materials including transition metal-induced nanocrystals-containing ceramics. The carbon-rich Si−C−N ceramic derived from metal-free precursors provides a new strategy for highly efficient EM wave absorbing functional materials with great potential in electronic devices, antenna housings, and radomes in harsh environments.

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INTRODUCTION In the last two decades, electromagnetic (EM) wave absorbing materials have received lots of attention due to the demand in protecting sensitive devices from electromagnetic interference environment in commercial or military application.1−3 The electromagnetic functional materials with excellent EM wave absorbing nature in wide bandwidth become urgent topic.4,5 The conventional magnetic metals and ferrites, such as iron, cobalt, nickel, metal alloys and Fe3O4, have been widely employed as EM wave absorbing materials, which permittivity and permeability can be tuned. Recently, the hierarchical dendrite-like magnetic materials of Fe3O4 and γ-Fe2O3 have been used as an electromagnetic wave absorbers in centimeter wave (2−18 GHz).6 The cobalt microflowers,7,8 cobalt−carbon nanoparticles,9 cobalt−nickel microflowers,10 Fe−Co/nanoporous carbon material11 and hierarchical nickel nanostructures12 all showed good absorbing performance. However, the disadvantages of those materials are low Currie temperature, high density, and susceptibility to corrosion, restricting their wide applications in harsh environments. The family of carbon materials from traditional carbon black to carbon nanotubes (CNTs), fullerenes, graphene, etc. have © XXXX American Chemical Society

also attracted increasing interest in electromagnetic functional materials.13−15 Their high interfacial polarization under EM field and favorable electrical conductivity are very beneficial to EM wave absorption.16 In order to combine the advantages of special carbon structures, studies have been devoted to the construction of complex hybrid structures of CNTs/graphenes via layer-by-layer self-assembly,17 solution casting,18,19 or onestep mixed catalyst growth method.20−22 All the methods are focused on introducing CNTs as nanospacers on graphene surfaces to protect graphene sheets from stacking layers by layers. However, the major drawback is large interfacial contact and electrical resistance between interface layers so that EM wave absorption properties and bandwidth cannot be easily optimized. Another disadvantage of carbon materials is susceptibility to oxidation in high temperature and air atmosphere. Therefore, the demands of EM wave absorbing materials with high absorption efficiency, wide bandwidth, and good thermal and chemical stability are extremely urgent. Received: August 2, 2017 Revised: October 19, 2017 Published: October 19, 2017 A

DOI: 10.1021/acs.jpcc.7b07646 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Molecular Composition of hb-PCSZ-Cyano and Atom Composition of Precursor-Derived Ceramics atom composition (%)a ceramic sample

precursor

AN/hb-PCSZ (mole ratio)

C1 C2 C3 C4 C5 C6 C7

P0 P1 P1 P1 P1 P2 P3

1.0 1.0 1.0 1.0 1.5 2.0

annealing temperature 1100 1100 1200 1300 1400 1100 1100

°C °C °C °C °C °C °C

Si

C

N

Ob

Si−C−N formula

17.84 11.62 12.09 12.90 6.61 11.81 10.41

41.92 54.34 53.82 54.45 70.86 53.58 57.63

8.98 6.91 8.70 8.47 3.84 6.95 5.10

31.26 27.14 25.39 24.19 18.69 27.65 26.87

Si1C2.35N0.50 Si1C4.67N0.59 Si1C4.45N0.72 Si1C4.22N0.66 Si1C10.72N0.58 Si1C4.54N0.59 Si1C5.54N0.49

a

Atom composition was determined using XPS. bThe introduction of oxygen is mainly due to the absorption of air in ceramic powders before XPS measurement.

Figure 1. Schematic synthetic route of hb-PCSZ and hb-PCSZ-cyano from MTCS, DCMVS, HDMZ, and acrylonitrile.

preceramic precursors named hyperbranched polycarbosilazanes with pendant cyano groups (hb-PCSZ-cyano). They were synthesized through an aminolysis reaction and subsequent Michael addition reaction, i.e. cyanoethylation reaction. After cross-linking and intramolecular cyclization of cyano groups, the pyrolysis of hb-PCSZ-cyano in situ generates metal-free and carbon-rich Si−C−N multiphase ceramics with excellent EM wave absorbing properties. The minimum reflection coefficient (RC) value is −59.59 dB at 12.23 GHz, and the effective absorption bandwidth (RC below −10 dB) covers the whole X-band (8.2−12.4 GHz). The metal-free and carbon-rich Si−C−N ceramics derived from preceramic precursors provides new strategy for highly efficient EM wave absorbing functional materials.

Precursor-derived ceramics (PDCs) from the pyrolysis of preceramic precursor under high temperature possess uniform element composition at atom level,23−27 showing excellent electromagnetic properties,5 electrochemical properties,28,29 and catalytic properties.30 They are considered as a bridge between organic molecules/polymers and inorganic materials. During the preceramic precursor (polycarbosilane, polysilazane or polysiloxane)-to-ceramic conversion, various nanosized SiC, Si3N4 or graphite carbons can be formed in SiC/C, Si−C−N or Si−O−C PDCs when the pyrolysis or annealing temperature is enough high.31,32 Especially, when the transition metals of iron, cobalt, nickel, zirconium or hafnium are introduced in precursors, the more graphite carbons and nanocrystals of SiC can be formed even at low pyrolysis or annealing temperature.24,33,34 The tunable nanocrystals embodied in amorphous Si−C−N or Si−O−C matrix can adjust real part of permittivity, imaginary part of permittivity, dielectric loss and EM wave absorption performance. At the same time, the PDCs, for which EM wave absorption is independent of magnetic loss and Currie temperature, possess excellent oxidation resistance and high temperature creep resistance,35,36so they are popular for the design and fabrication of EM wave absorption functional materials with potential in electronic devices, antenna housings, and radomes in harsh environments.33,37,38 To date, the introduction of transition metals into preceramic precursors is an important strategy for designing PDCs EM wave absorbing materials. However, the synthesis of air-sensitive metal-containing precursors is complicated using an expensive metal complex. In this contribution, we report novel metal-free

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EXPERIMENTAL SECTION Materials. Methyltrichlorosilane (MTCS) (97%), dichloromethylvinylsilane (DCMVS) (97%) and 1,1,1,3,3,3-hexamethyldisilazane (HDMZ) (98%) were purchased from Alfa Aesar China (Tianjin, China). Acrylonitrile (99%) and 2-azobis(isobutyronitrile) (AIBN) were purchased from Alfa Chemical Co. Ltd. (Zhengzhou, China). All other reagents were analytical grade and used as received. Synthesis of Hyperbranched Polycarbosilazane (hb-PCSZ). The reaction was carried out using standard Schlenk techniques. Under an argon atmosphere, a 100 mL flame-dried flask equipped with a Teflon stir bar, septum, and high-vacuum stopcock was charged with MTCS (7.79 g, 50.56 mmol) and cooled down to 0 °C in ice/water bath. Then DCMVS B


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The Journal of Physical Chemistry C

Figure 2. 1H NMR spectrum (a) and hb-PCSZ dissolved in CDCl3.

13

Figure 4. 1H NMR spectrum (a) and hb-PCSZ-cyano dissolved in DMSO-d6.

C NMR spectrum (b) of

13

C NMR spectrum (b) of

Figure 5. FT-IR spectra of hb-PCSZ (a), hb-PCSZ-cyano (b), and cross-linked hb-PCSZ-cyano at 300 °C (c). Figure 3. SEC traces of hb-PCSZ detected at a flow rate of 0.5 mL/min in THF at 25 °C.

hb-PCSZ-cyano. The reaction was at 65 °C for 2 h. After the distillation of unreacted monomers at 80 °C for 3h, yellow glassy products of hb-PCSZ-cyano were obtained. Preparation of Precursor-Derived Ceramics. The hb-PCSZ-cyano was transferred into tube furnace (GSL-1700X, Kejing New Mater, Ltd., Hefei, China) for pyrolysis under an argon atmosphere. The cross-linking was performed at 300 °C for 4 h. Then the cross-linked products were ball milled and passed through a 200 mesh sieve. The as-received powders were cold pressed into green bodies under a pressure of 70 MPa. Then, the green bodies were pyrolyzed and annealed at different temperatures (heating rate of 5 °C/min, holding time of 4 h). As shown in Table 1, the ceramics were designated

(7.36 g, 50.60 mmol) was slowly added through an argon-purged syringe. After stirring 5 min, HDMZ (28.67 g, 174.08 mmol) was dripped into the flask. The reaction was conducted at 50 and 80 °C for 3 h, respectively. After the distillation of small molecular byproducts at 220 °C for 3h, yellow glassy viscous products of hb-PCSZ were obtained. Synthesis of Hyperbranched Polycarbosilazane with Pendant Cyano Groups (hb-PCSZ-Cyano). hb-PCSZ and acrylonitrile with different mole ratios (Table 1) were used in the flask equipped with a Teflon stir bar for synthesizing C


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Figure 6. Schematic cross-linking processing including intra/ intermolecular cyclization of hb-PCSZ-cyano at 300 °C under argon atmosphere.

Figure 8. XPS spectra of a typical C4 ceramic (annealed at 1300 °C from hb-PCSZ-cyano with feed ratio 1:1 of hb-PCSZ and acrylonitrile).

Figure 7. TGA thermograms of hb-PCSZ and hb-PCSZ-cyano samples measured at a heating rate of 10 K/min under a steady flow of argon (40 mL/min).

as C1−C7 according to feed ratio of acrylonitrile and annealing temperatures. Characterization. Nuclear magnetic resonance (NMR) measurement was carried out on a Bruker Avance 400 NMR spectrometer (Bruker BioSpin, Switzerland) to collect the 1 H and 13C spectra. Fourier transform infrared spectroscopy (FTIR) measurement was carried out on a FT-IR spectrometer (PerkinElmer, USA). Size exclusion chromatography (SEC) measurement was performed on a system equipped with a Waters 515 pump, an auto sampler, and two MZ gel columns (103 Å and 104 Å) with a flow rate of 0.5 mL/min in THF (HPLC grade) at 25 °C. The detectors included a differential refractometer (Optilab rEX, Wyatt) and a multiangle light-scattering detector (MALS, Wyatt) equipped with a 632.8 nm He−Ne laser (DAWN EOS, Wyatt). The refractive index increment of polymers in THF was measured at 25 °C using an Optilab rEX differential refractometer. Thermogravimetric analysis (TGA) was performed on a simultaneous thermal device (STA, 449C Jupiter, Netzsch, Geratebau GmbH, Selb, Germany). The measurement was performed under a steady flow of argon (40 mL/min) with a heating rate of 10 K/min at a range from ambient temperature to 1400 °C. X-ray photoelectron spectroscopy (XPS) measurement was conducted on a K-Alpha spectrometer (Axis Ultra, Kratos Analytical Ltd., U.K.) and the core level spectra were measured using amonochromatic

Figure 9. XPS spectra of carbon element in C1 and C2−C5 annealed at 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively.

Al Kα X-ray source (hν = 1486.7 eV). The analyzer was operated at 23.5 eV pass energy, and the analyzed area was 200−800 μm in diameter. The lowest energy resolution is 0.48 eV (Ag 3d5/2). Binding energy was referenced to the adventitious hydrocarbon C 1s line at 285.0 eV. The curve fitting of the XPS spectra was performed using the least-squares method. Powder X-ray diffraction (XRD) measurement was conducted on the X’Pert Pro powder diffractometer from PANalytical (Cu Kα radiation, 40 kV, 40 mA) (D/Max2550VB+/PC). The X’Celerator Scientific RTMS detection unit was used for detection. Raman spectroscopy studies were performed using a D


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Figure 10. Powder XRD patterns of pyrolyzed ceramics from hb-PCSZ-cyano with different feed ratio of acrylonitrile and annealed at 1100 °C (a) and from hb-PCSZ-cyano with feed ratio of 1:1 (hb-PCSZ and acrylonitrile) annealed at different temperature.

Figure 11. Raman spectra of pyrolyzed ceramics from hb-PCSZ-cyano with different feed ratios of acrylonitrile and annealed at 1100 °C (a) and from hb-PCSZ-cyano with a feed ratio of 1:1 (hb-PCSZ and acrylonitrile) annealed at different temperature.

Raman Microprobe Instrument (Invia, Renishaw, USA) with 514.5 nm Ar+ laser excitation. Transmission electron microscopy (TEM, FEI TecnaiG2 F30) was operated at 200 kV, coupled with electron diffraction analysis. A 5 μL amount of a droplet of ultrasonically dispersed mixture of milled sample in alcohol (0.02 mg/mL) was dropped onto a copper grid (200 mesh) coated with carbon film and dried at ambient temperature for 30 min. Microwave Absorption Measurements. The PDCs samples were prepared by pyrolysis of cross-linked powders to press into green bodies. The relative complex permittivity of PDCs bulk samples with a dimension of 22.86 × 10.16 × 2.65 mm was measured by a vector network analyzer (VNA, MS4644A, Anritsu, Atsugi, Japan) using waveguide method in X-band EM wave. On the basis of the metal backplane model, the reflection coefficient (RC) can be calculated using the measured relative complex permittivity according to the following equations.39 Z in − 1 Z in + 1

(1)

⎤ ⎡ 2πfd tan h⎢j με r r⎥ ⎦ ⎣ c εr

(2)

RC = 20 log10 Z in =

and velocity of the electromagnetic wave in vacuum, respectively. Direct-current electrical conductivities of the samples were measured through a four-point probe technique setup (ET9000, Eastchanging, China), which mainly consist of a high impedance current source (6220, Keithley, USA) and a highimpedance voltmeter (2182A, Keithley, USA).

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RESULTS AND DISCUSSION Synthesis and Cross-Linking of hb-PCSZ-cyano. The hb-PCSZ-Cyano was synthesized via two-step reaction as presented in Figure 1. First, via the aminolysis reaction between MTCS and DCMVS in the presence of HMDZ, the hyperbranched polycarbosilazane (hb-PCSZ) was synthesized. Second, the Michael addition reaction, i.e., cyano-ethylation reaction, was conducted between hb-PCSZ and acrylonitrile to obtain hb-PCSZ-cyano with pendant cyano groups. By controlling feed ratio of acrylonitrile and reaction condition, the hb-PCSZ-cyano with different content of pendant cyano-groups were obtained, which could be solved in common solvents, such as THF, DMF, and toluene. The 1H and 13C NMR spectra in Figure 2 show the protons on methylsilyl groups and vinyl groups, the carbons on methyl and vinyl groups of hb-PCSZ, respectively. As shown in Figure 2a, the 1H NMR spectrum reveals the existence of Si−CH3 bonds with a δ of 0−0.4 ppm, vinyl groups with a δ of

μr

where Zin, μr, and εr is the normalized input impedance, permeability and permittivity of the materials, respectively, and f, d, and c represents the microwave frequency, thickness (m), E


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Figure 12. Real permittivity (a), imaginary permittivity (b), loss tangent (c), and reflection coefficient (d) of ceramics annealed at 1100 °C from hb-PCSZ-cyano with different feed ratio of acrylonitrile (C1, 1:0; C2, 1:1; C6, 1:1.5; C7, 1:2).

5.8−6.2 ppm and N−H bonds with a δ of 1.2 ppm. The 13C NMR spectrum in Figure 2b further confirms the same structure. The clear assignment and matched integral value of all protons and carbons illustrate the expected molecular structures of hb-PCSZ. The desirable hb-PCSZ possesses a number-average molecular weight of 2320−2700 g/mol and a polydispersity index of 1.81−2.42 (Figure 3). When the feed ratio of hb-PCSZ and acrylonitriles is 1:1, the hb-PCSZ-cyano can be solved in DMSO; the 1H and 13CNMR spectra are presented in Figure 4. Compared to hb-PCSZ, the 1H NMR spectrum of hb-PCSZcyano in Figure 4a indicates the characteristic signals of protons of methylene in the region of 3.60−3.70 ppm. Due to the inductive effect of nitrogen atom, the position of proton peak is a little different from the reported polyacrylonitrile.40 The main reaction is the Michael addition reaction between N−H bond and the vinyl groups of acrylonitrile under benign condition. Besides, the signal peak in the region of 1.5 ppm indicates that N−H bonds are not completely consumed because of the low feed ratio of acrylonitrile. The 13C NMR spectrum in Figure 4b confirms the carbons on methylene of hb-PCSZ-cyano. From the FT-IR spectra in Figure 5, it is clear that the silicon-chloride bonds in MTCS and DCMVS are substituted as nitrogen− hydrogen bonds (−NH−) at 3400 cm−1 via aminolysis. After the hb-PCSZ was reacted with acrylonitrile, the stretching vibration of −NH− at 3400 cm−1 nearly disappeared. And a broad band at 1100−1200 cm−1that could be attributed to NH group deformation disappeared. In contract, the stretching vibration of cyano groups at 2242 cm−1 and C−N at 1030 cm−1 were observed. It indicated that the vinyl groups on acrylonitriles were reacted with amines on hb-PCSZ via Michael addition reaction.

The cross-linking of hb-PCSZ-cyano was conducted in tube furnace at 300 °C and under argon atmosphere. The hb-PCSZcyano can be cross-linked via intra- or intermolecular cyclization of cyano groups, free radical polymerization of vinyl groups on silicon atoms, etc. as illustrated in Figure 6. After crosslinking, the cyano groups at 2242 cm−1and vinyl groups at 3055 cm−1 disappeared. The stretching vibration at 1610 cm−1 and bending vibration at 930 cm−1 of carbon−carbon double bonds and carbon−nitrogen bonds increased. The occurrence of carbon−nitrogen bonds and primary amine groups at 1100− 1200 cm−1 confirmed the formation of cyclic structures from cyano-groups.41 From the TGA curves in Figure 7, the hb-PCSZ undergoes a rapid thermolytic degradation in the temperature range of 100−700 °C, for which the ceramic yield is 50.0% at 1200 °C. However, for the hb-PCSZ, the ceramic yield of hb-PCSZcyano is significantly improved to 70% at 1200 °C. If the hb-PCSZ-cyano is cross-linked before TGA measurement, the ceramic yield is further improved to 76%. The reason for high ceramic yield of hb-PCSZ-cyano is the formation of cyclic structures of pendant cyano groups and high cross-linking density. The cyclic structures can be transmitted into carbons in high efficiency and complex cross-linking network also limit the evolution of H2, hydrocarbons, and a small number of oligomer fragments.42 Pyrolyzed Ceramics from Cross-Linked hb-PCSZCyano. As shown in Table 1, after the pyrolysis of hb-PCSZcyano at different temperatures, the obtained ceramic was labeled as C1−C7. The XPS shows the existence of silicon, carbon, nitrogen, and oxygen elements (Figure 8). The atom composition and possible ceramic formula were presented in Table 1. F


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The Journal of Physical Chemistry C The molar ratio of carbon and nitrogen increased obviously with the feed ratio of acrylonitrile. And the content of carbons in ceramics is much larger than that of conventional Si−C−N ceramics. It shows that the ceramics were actually carbon-rich Si−C−N systems. It should be pointed out that the ceramics were derived from oxygen-free precursors. The measured oxygen content in ceramics via XPS is high only due to surface oxygen absorption and oxidation. The XPS is a surface analysis. After cleaning the surface by sputtering, an accurate elemental analysis result for C4 is silicon 27.5 wt %, carbon 42.8 wt %, nitrogen 8.5 wt % and oxygen 21.2 wt % in comparison to the traditional XPS analysis in Table 1. Other useful information can be obtained from the XPS spectra. For C2−C4, in the C 1s region, the C−C phase (graphite-like carbon) at 284.6 eV and two different carbon nitride phases were observed.43 They belong to C−N bonds with sp2 and sp3 hybridization of carbon atoms,44 with binding energy at 286.1 and 287.9 eV, respectively. The silicon nitrides (Si3N4) with a Si 2p bonding energy of 102.4 eV and N 1s bonding energy of 397.5 eV can be also detected. From the assignment of the second peak related with specific electronegativity of involved atoms, the binding energy of atoms is shifted toward high value as a result of neighbored effect and high electronegativity. Compared to C atoms, the N atoms have a more pronounced electronegativity. The second peak at 101.4 eV, i.e. the binding energy between Si−C and Si−N bonds, is assigned to the Si−C−N bonds formed in ceramics. In addition, the surface of ceramic was therefore always slightly oxidized after contact with air, especially after the ceramics were milled. On the surface, there are extra peaks at 103.15 and 530.2 eV in Si 2p spectrum (Figure 8), which corresponds to Si−Ox and Ni−Si−O compounds of silicon oxide, respectively.45 Furthermore, as illustrated in Figure 9, the introduction of cyano groups in precursors results in the formation of C−N bonds in pyrolzed ceramics even after annealed at 1100 °C, 1200 °C, 1300 °C, and 1400 °C. The previous reports claimed that N-doped graphite carbons can causes more defects in original ordered structures and have a profound effect on morphological, physical, and electronic properties.46 Thus, the appearance of C−N bonds in the ceramics directly affects the dielectric properties of the materials and will have an effect on microwave absorption properties. The powder XRD analysis of ceramics (C1−C7) was further conducted to elucidate the phase structures. Figure 10 shows the XRD patterns of ceramics derived from different precursors and annealed at different temperatures. It is obvious that there is almost no crystallization in ceramic powders. Only when the feed ratio of acrylonitrile is 2:1 or the annealing temperature is 1400 °C, the trend of graphite carbons and small SiC nanocrystals occurs. It can be concluded that the crystallization temperature of ceramics is high and beneficial to the high temperature stability. Raman spectrum was one of the most effective ways to investigate the fine features of carbons in ceramics. The G band at 1570 cm−1 caused by in-plane bond stretching of sp2 carbons and the disorder-induced D band appeared at 1350 cm−1 due to structural defects such as vacancies, heteroatoms and impurities.47 The second-order G′ band (overtone of D band) can always be observed in defect-free samples at 2700 cm−1. Another Raman feature at 2950 cm−1 is associated with a D+G combination mode induced by disorder. All Raman curves had been fitted by the Gaussian−Lorentzian curve to determine the widths, positions, and ID/IG intensity ratio.48−50 As shown in

Figure 13. Real permittivity (a), imaginary permittivity (b), and loss tangent (c) of ceramics derived from hb-PCSZ-cyano with an acrylonitrile feed ratio of 1:1 annealed at different annealing temperatures (C2, 1100 °C; C3, 1200 °C; C4, 1300 °C; C5, 1400 °C).

Figure 11, the high intensity of ID/IG indicates a large number of defects in as-synthesized carbonaceous structures, which is related to both amorphous carbon species and silicon and nitrogen atoms in a microscopic ceramic matrix. Table S1 shows the degree of graphitization of samples calculated from the peak width at half height (HWHM) and intensity ratio (ID/IG) of D and G bands. With the increase of acrylonitrile content and annealing temperature, the D and G bands were gradually separated, and HWHM also decreased. When the feed ratio of acrylonitrile and hb-PCSZ is 1:1 and the G


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Figure 14. Reflection coefficient of ceramics derived from hb-PCSZ-cyano with an acrylonitrile feed ratio of 1:1 annealed at 1100 °C (a), 1200 °C (b), and 1300 °C (c), and C4 with different thickness.

annealing temperature is 1300 °C, the HWHM becomes minimum, and the two peak position is far, suggesting the increased graphitization degree of carbons in ceramic matrix. As other evidence, the direct-current electrical conductivity (σdc) (Table S2) shows an obvious increasing trend compared with the ceramics derived from precursor without cynao groups. The increase of the σdc is mainly attributed to the formation of graphitized carbon. Conbined with the results of XRD and Raman spectra, it can be clear that the introduction of cynao groups in precursors and the increase of annealing temperature promote the transformation of carbons from amorphous state to graphitized state in ceramic matrix. Dielectric Properties and EM Wave Absorption Performance of Carbon-Rich SiCN Ceramics. The complex permittivity and dielectric loss tangent are two main parameters for characterization of dielectric property as well as prediction of EM wave absorption properties. The real part of complex permittivity represents polarization. The imaginary part of complex permittivity and dielectric loss tangent are related to dissipation of EM wave in materials. The dielectric loss tangent can forecast the EM wave absorbing property. Normally, the high dielectric loss is helpful to EM wave absorbing property in some degree, but a high real part of complex permittivity will induce the reflection of EM wave on surface.51 So a low real part of complex permittivity can ensure the incidence of EM wave, and a high (at least middle) imaginary part of complex permittivity is helpful for the dissipation of the EM wave, i.e. absorption of EM wave in materials.

Figure 12 shows the complex permittivity of C1, C2, C6, and C7 in 8.2−12.4 GHz (X-bond). At the same annealing temperature (1100 °C), the real part of permittivity of ceramics, as well as imaginary part of permittivity and dielectric loss tangent, is increased with the increase of content of cyano groups in precursors. In detail, the real part of permittivity is in the range of 3.77−3.83, 5.07−5.41, 9.64−10.33, and 17.75− 22.26 and the imaginary part of permittivity is in the range of 0.02−0.07, 1.32−1.66, 5.03−6.45, and 23.25−27.87, for C1, C2, C6, and C7, respectively. The dielectric loss of ceramics mainly depended on dipolar reorientation and interfacial polarization relaxation effect. With the introduction of cyano groups in precursors, the C−N bonds and graphitized carbons generated in situ in ceramics as mentioned in XPS and Raman spectra. The new phases in ceramics will increase the interfacial polarization to improve the imaginary part of permittivity and helpful to impedance matching and electrical conductivity as well as EM wave absorption efficiency.52,53 Based on the measured complex permittivity, the reflection coefficient (RC) can be calculated according to Equ 1 and Equ 2. In this study, the μr was taken as 1 because of the freemetal structure of ceramics. The lower RC value means the better microwave absorption properties. Normally, the RC less than −10 dB means more than 90% microwave energy can be absorbed, which is an important baseline for EM wave absorption materials. The RC of ceramics annealed at 1100 °C (C1, C2, C6, C7 with a thickness of 2.70 mm) is shown in Figure 12d. The RC values of C1, C6 and C7 are all higher than H


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The Journal of Physical Chemistry C Table 2. Summary of EM Wave Absorption Properties for Various Materials Published in References absorption bandwidth samples

RCmin/ dB

matching thickness/ mm

RC below −10 dB/ GHz

thickness/ mm

preparation method

crystalliz-ation

ref.

MWCNT-ZnO/SiO2 Fe3O4/ZnO Core/Shell Co-PDSDA Co/C SiCN graphene/poly aniline PVC/NBR/MWCNT CNTs/RGO C-rich Si−C−N

−21.6 −36.0 −42.4 −43.4 −53.0 −45.1 −49.5 −55.0 −59.6

2.6 2.0 2.7 2.3 3.4 2.5 2.0 2.8 2.3

3.2 4.7 3.9 11.6 3.0 4.8 3.0 3.5 4.0

2.3 1.5 2.7 3.0 3.4 2.5 3.0 2.8 2.7

sintering at 700 °C hydrogen annealing annealed at 1100 °C ARC plasma annealed at 1500 °C mixing mixing in situ formed at 600 °C annealed at 1300 °C

yes yes yes yes yes yes no yes no

54 55 33 9 56 57 60 16 this work

−10 dB in the whole X-band. The C2 possesses a minimum reflective coefficient of −12.13 dB at 9.8 GHz with an effective absorption bandwidth from 8.9 to 10.5 GHz. In principle, with the increased cyano groups in precursors, the pyrolyzed ceramics contain more heteroatoms embedded in the regular carbon structures, which will greatly improve the permittivity and lead to shielding effect. Therefore, we focused on ceramics derived from P1 to study the effect of annealing temperature on the dielectric properties and EM wave absorption (Figure 13). In Figure 13a,b, the real part and the imaginary part of permittivity increased with the increase of annealing temperature. With the increase of annealing temperature, the real permittivity is in the range of 5.41−5.07, 5.66−6.01, 7.62−8.15 and 12.77−16.10 as well as the imaginary permittivity in the range of 1.32−1.66, 1.90−2.49, 3.34−3.99 and 18.36−22.82 for various ceramics, respectively. When the annealing temperature increased from 1100 to 1400 °C, the generation of a large number of amorphous carbons induces interfacial polarization and dipole polarization and increases the dielectric loss at the same time. In Figure 13c, the C5 possesses the highest dielectric loss tangents. However, due to the exorbitant real part and imaginary part, C5 may be translated into the shielding materials.5 Thus, we calculate the reflection coefficient of C2, C3, and C4, respectively, as shown in Figure 14. With the increase of annealing temperature, the RC value obviously decreases. The C4 ceramics displays the best EM wave absorption performance as illustrated in Figure 14c,d. The minimum RC value is −59.6 dB at 12.23 GHz when the sample thickness is 2.3 mm, which means >99.99% electromagnetic waves can be absorbed. Such excellent EM wave absorbing properties are seldom seen in the current absorbing materials as summarized in Table 2.9,16,33,54−60 When the sample thickness is 2.7 mm, the C4 sample has an effective absorption bandwidth almost across almost the whole X-band (8.4 GHz−12.4 GHz), indicating excellent EM wave absorption property and wide effective absorption bandwidth. From Table 2, we can find that our materials show outstanding advantages in either minimum RC, effective absorption band or sample thickness. In order to further analyze the reason for the excellent EM wave absorbing performance of the carbon-rich Si−C−N materials (C4) derived from hb-PCSZ-cyano precursors, the microstructures of ceramics are identified by TEM. From Figure 15, the C4 is mainly composed of amorphous carbon structure without any diffraction spots, which is consistent with the characterization of the results of XRD. From the more fine observation via Roman spectrum (Figure 11b), we find that with the increase of annealing temperature, the crystallinity of carbons increases only in very low degree. Accordingly, the

Figure 15. Microstructure morphology of ceramic C4 annealed at 1300 °C, TEM images (a and b) and electron diffraction pattern (c).

content of sp2 carbons increases obviously, and the ID/IG ratio decreases eventually. In contrast, many researches pay attentions to the transition metal such as Fe, Co, Ni, to induce crystallization of sp3 amorphous carbons to change the dielectric properties of precursor-derived ceramics and thereby to enhancing the EM wave absorbing properties. In this study, the introduction of cyano groups and the doping of heterogeneous N atoms prevent the formation of the crystallization of I


Article


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amorphous carbon in ceramics. The amorphous carbons can increase the polarizations by the high motion of electrons, polarizations are helpful to high dielectric loss of ceramics. So it also can regulate well the real part of permittivity, imaginary part of permittivity, and loss tangent to achieve impedance matching and excellent EM wave absorption performace. The novel metal-free hyperbranched polycarbosilazanes with cyano groups derived carbon-rich ceramics show advantages including minimum RC, wide effective absorption band, and small thickness in comparison to most current absorbing materials.

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CONCLUSIONS The carbon-rich Si−C−N PDCs ceramics with excellent EM wave absorbing properties were successfully achieved by the pyrolysis of novel metal-free hyperbranched polycarbosilazanes with pendant cyano groups. The introduction of cyano groups leads to the occurrence of C−N bonds in the matrix, and the permittivity can be tuned via the content of cyano groups and high temperature annealing. The minimum RC value was −59.6 dB at 12.23 GHz with the sample thickness of 2.3 mm, which means >99.99% EM waves can be absorbed across the whole X-band (8.4−12.4 GHz). The EM wave absorption performance is very excellent in comparison to most current absorbing materials. Compared to the transition metal, such as Fe-, Co-, and Ni-induced nanocrystals-containing EM wave absorbing ceramics, the amorphous carbon-rich Si−C−N ceramics from metal-free preceramic precursors provide a new strategy for EM wave absorbing materials with great potential in antenna housings and radomes in harsh environments.

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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/acs.jpcc.7b07646. Raman spectra parameters and direct-current electrical conductivity of polymer derived Si−C−N ceramics (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail Address: [email protected]; Telephone (fax): +86-29-88431976. ORCID

Nan Tian: 0000-0003-1822-876X Jie Kong: 0000-0002-9405-3204 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (21174112/21374089), the Fundamental Research Funds for the Central Universities (3102017GX06011), Open Foundation of National Key Laboratory of Solidification Science and Technology, Key Research and Development Program of Shaanxi Province, and the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University.

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