Biomass-derived carbon and polypyrrole addition on SiC whiskers for enhancement of electromagnetic wave absorption

Accepted Manuscript Biomass-derived carbon and polypyrrole addition on SiC whiskers for enhancement of electromagnetic wave absorption Shun Dong, Xinghong Zhang, Peitao Hu, Wenzheng Zhang, Jiecai Han, Ping Hu PII: DOI: Reference:

S1385-8947(18)32335-0 https://doi.org/10.1016/j.cej.2018.11.101 CEJ 20403

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

31 August 2018 6 November 2018 13 November 2018

Please cite this article as: S. Dong, X. Zhang, P. Hu, W. Zhang, J. Han, P. Hu, Biomass-derived carbon and polypyrrole addition on SiC whiskers for enhancement of electromagnetic wave absorption, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.11.101

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Biomass-derived carbon and polypyrrole addition on SiC whiskers for enhancement of electromagnetic wave absorption Shun Dong1, Xinghong Zhang1,*, Peitao Hu1, Wenzheng Zhang2, Jiecai Han1, Ping Hu1,* 1

Science and Technology on Advanced Composites in Special Environment Laboratory, Harbin

Institute of Technology, Harbin 150001, PR China 2

School of Materials Science and Engineering, Harbin University of Science and Technology,

Harbin 150001, PR China

Abstract: To

achieve

high-performance

electromagnetic

(EM)

wave

absorbing

performances, biomass-derived carbon (BDC) and conductive polymer polypyrrole (PPy) were incorporated on SiC whiskers (SiCw) by hydrothermal-carbonizationpolymerization. Modulation of the mass ratio of SiCw/glucose (SiCw/Glu) can effectively improve the permittivity of SiCw-BDC composites in a large scale, leading to the enhanced EM wave absorption property. Compared with the wavetransparent of pristine SiCw, the SiCw-BDC composite with the SiCw/Glu mass ratio of 0.5 (S-0.5) shows a strong EM wave absorption capacity with a minimum reflection loss (RLmin) value of -24.6 dB at 13.2 GHz, and its effective bandwidth is up to 6.8 (11.2-18.0) GHz, which could achieve effective absorption of EM waves in entire Ku-band with a wide thickness range of 2.28-2.49 mm. Adjusting the polymerization time of PPy could efficiently control the EM wave absorbing performance of SiCw-BDC/PPy heterostructures, and the sample SiCw-BDC/PPy heterostructure with 1.0 h polymerization time exhibit the optimal microwave absorption property, with a RLmin value of -52.4 dB at 11.4 GHz and effective bandwidth of 8.1 (9.0-17.1) GHz. The electronic dipole polarization, relaxation polarization loss and interfacial polarization with the matched characteristic

impedance and improved loss ability within SiCw-BDC/PPy heterostructures are major determining factors of excellent EM wave absorption properties, and the SiCwBDC/PPy heterostructures in this study are as a promising candidate for nextgeneration high-performance EM wave absorber for practical applications. Keywords 1 : SiC whiskers; biomass-derived carbon; polypyrrole; electromagnetic wave absorption.

1*

Corresponding author: Tel/fax: +86 451 86403016. E-mail address: [email protected] (X.H. Zhang); [email protected] (P. Hu)

1. Introduction With the great development of science and technology, plenty of electronic devices, including mobile phones and radars, have been widespread used, threatening the health of human and leading to the information leakage, and considerable efforts have been made to develop outstanding EM wave absorbing materials (low-cost, thinthickness, light-weight, strong absorption and wide bandwidth) [1-5]. In general, a single component absorbing material exhibits narrow absorption band and weak reflection attenuation, and the hybridization of absorbing materials with different components is beneficial to achieve the ideal EM wave absorbing properties [6-8]. Therefore, extensive attention has been focused on the design of excellent microwave absorbers with strong microwave absorption [9-11]. Owing to its tunable electrical conductivity, SiC with superior mechanical and chemical stability properties at high temperature is considered as an ideal EM wave absorber, which is favor of achieving high attenuation efficiency by coordinating impedance match [12]. According to previous literature, one-dimensional (1D) SiC nanowires (SiCNWs) or SiC whiskers (SiCw) have been exhibited outstanding EM wave absorption capacities compared with those of SiC bulk and particle forms [1315]. Nevertheless, the low permittivity of pure 1D SiC nanomaterial should not be ignored in comparison with other materials, such as carbon-based and ferromagnetic materials, resulting in poor EM wave absorption property and severely restricting it further application, and thus much effort has been emphasized on the design and enhancement of 1D SiC nanomaterial with diverse methods [16-19]. For example, Sui and co-workers declared that a graphene@SiC composite could reach -47.3 dB at 10.52 GHz with an effective absorption bandwidth (EAB, the corresponding frequency range within which reflection loss is smaller than -10 dB) of 4.7 GHz, which was fabricated by the directional freeze-casting and thermal annealing process

[20]. Hou et al prepared Fe/SiC hybrid fibers by electrospinning and high-temperature treatment and exhibited a minimal reflection loss (RL) of about -46.3 dB at 6.4 GHz, whose EAB value could be 5.6 GHz [12]. Not long ago, we reported that SiCw@C heterostructures could achieve thickness-dependent EM wave absorption between the whole X-band and Ku-band [21]. However, there still remains a great challenge for 1D SiC nanomateirl to meet the changing requirement with high efficiency (﹤-50 dB) and broadened bandwidth (﹥8.0 GHz). Recently, the biomass-derived carbon (BDC) has become one of hot topics due to its simple fabrication process and low cost, which has been successfully utilized in various fields, including the supercapacitors, fuel, solar cells, batteries and fluorescent nanodots [22-26]. Meanwhile, the BDC with a porous structure has been proved as a microwave absorber, promoting the formation of interface polarization and multiple reflections to enhance the microwave absorption property [22,23]. In addition, a higher electrical conductivity might be in favor of the electrical attenuation of EM energy, and a great deal of conducting polymers have been used to increase the electrical conductivity of composite, looking forward to obtain excellent absorbing performance [27]. Polypyrrole (PPy) as one of the most common conducting polymers has been frequently employed to recombine the other matrix material with optimized EM wave absorbing properties, such as Ag@PPy nanowires, SiC@PPy nanowires, and SiCNWs/graphene aerogel-PPy [13,17,28]. Therefore, it is anticipated that the introduction of BDC and PPy has a great potential to improve the EM wave absorption capacity of 1D SiC nanomateirl, while, until now, there is little report about the incorporation of BDC and PPy into SiCw. In this context, we first prepared SiCw-BDC composite via a combined hydrothermal and thermal annealing method using Glu as carbon precursor. The influence of mass ratio of SiCw/Glu on the morphology and EM wave absorption

property of SiCw-BDC composite was evaluated. Further, the monomer, pyrrole was polymerized with various polymerization times on SiCw-BDC composites and investigated the EM and microwave absorption properties of these composites by mixing in wax at 30 wt% with various thicknesses. The SiCw-BDC/PPy heterostructures exhibited excellent microwave absorption properties with high efficiency (﹤-50 dB) and broadened bandwidth (﹥8.0 GHz) owing to the matched characteristic impedance and improved loss ability deriving from SiCw, BDC and PPy. 2. Experimental section 2.1. Materials SiCw were supplied by our group, which were fabricated by a CVD method. Acetone, nitric acid (HNO3), and iron trichloride (FeCl3·6H2O) were purchased from Harbin Kecheng Chem. Co. Glu and pyrrole were purchased from Tianjin Chem. Co., and J&K Scientific, Ltd, respectively. All reagents were of analytical grade and employed without further purification, and the distilled water was also used in the experiments. 2.2 Preparation of SiCw-BDC composites SiCw were first immersed in acetone for one day to eliminate the other impurities, and removed into the solution of HNO3 for 3 h at room temperature then thoroughly rinsed with distilled water and then dried at 60 °C for 24 h. To prepare SiCw-BDC composites, the dried SiCw were redistributed in 50 mL FeCl3 (0.1 mol/L) for 0.5 h and then collected by filtration. The SiCw were transferred into 50 mL Glu solution (2 wt%), and then the mixture solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 6 h. The treated SiCw were separated from the suspension by centrifugation, washed several times with deionized water and dried at 60 °C for 24 h, and then transferred into a porcelain crucible, put in

the tube furnace with a flowing N2 atmosphere at 50 mL/min and then carbonized at 1200 °C for 1 h. Based on the mass ratio of SiCw/Glu used for the preparation of the composites, SiCw-BDC composites were named as S-0.5 (SiCw/Glu=0.5), S-1 (SiCw/Glu=1) and S-2 (SiCw/Glu=2). 2.3 Fabrication of core-shell SiCw-BDC/PPy heterostructures SiCw-BDC composites (0.2 g) from the above method were dispersed in the pyrrole solution for 20 min, and the SiCw-BDC composites with pyrrole monomers absorbed on the surface were transferred into the FeCl3 (1 mol/L) solution with different polymerization times. The polymerization was carried out in a 50 mL beaker, and the product was washed by the distilled water and ethanol for 3 times and then dried in a vacuum condition at 60 °C for one day. According to the polymerization time, SiCw-BDC composites were labeled as S-n-0.5, S-n-1.0 and S-n-1.5, respectively. 2.4. Characterization The crystallite structures of the samples were analyzed by X-ray Diffraction (XRD) and a Cu Ka radiation at a generator voltage of 40 kV was used for the XRD analysis. Raman spectroscopy of the samples was recorded using a Renishaw inVia Raman microscopy with a 633-nm incident laser. Fourier transform infrared spectroscopy (FT-IR) was collected from a PerkinElmer 2000 spectrometer (USA), and X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) was carried out to measure the chemical composition and chemical states of the samples. The morphologies of samples were characterized by a field-emission scanning electron microcopy (SEM, HELIOS NanoLab 600i, USA) and high-resolution transmission electron microscopy (TEM and HRTEM, Tecnai G2-F30, USA). Thermogravimetric analysis (TGA) was performed to evaluate the content of BDC in air atmosphere from room temperature to 900 °C at a heating rate of 10 °C/min using a TA Q600

thermogravimetric analyzer. Complex permittivity (εr, εr = ε′ - jε″) and complex permeability (μr, μr = μ′ - jμ″) of the samples were calculated by a vector network analyzer (Agilent N5230A, USA) within 2-18 GHz [29-31]. The as-prepared SiCwBDC and SiCw-BDC/PPy heterostructures were mixed with paraffin wax in a mass ratio of 30 wt% at about 85 °C, and then the mixture was pressed into a toroid-shape with an outer diameter of 7 mm and an inner diameter of 3.04 mm [32-34].

3. Results and Discussion 3.1 Characterization The XRD patterns of pristine SiCw, SiCw-BDC and SiCw-BDC/PPy are presented in Fig. 1a to confirm the phase composition of the as-obtained samples. The XRD pattern of pristine SiCw is a cubic structure of 3C-SiC (JCPDS Card 29-1129) and the five major peaks were observed at about 35.58°, 41.29°, 60.01°, 71.80° and 75.49°, corresponding to (111), (200), (220), (311) and (222) planes of β-SiC [35]. A small peak (centered at 33.6°) ahead of the highest intensity peak (111) could be ascribed to the presence of stacking faults within the crystals [36]. After coating with BDC and PPy, no obvious difference was found in the XRD patterns revealed in Fig. 1a, while a weak peak at 26° and a broad peak at 26° were discovered in the XRD patterns of SiCw-BDC and SiCw-BDC/PPy composites from a closer examination of the peak in the range of 23.5−27.5° shown in Fig. 1b. The weak peak at 26° in the SiCw-BDC XRD pattern is attributed to the introduction of BDC [37,38], corresponding to the (002) lattice plane of typical turbostratic carbon [37], and the broad peak in the SiCw-BDC/PPy XRD pattern implies the poor crystallinity of PPy phase [28]. The influence of incorporation of BDC and PPy on structural changes of SiCw is monitored by Raman spectroscopy. As shown in Fig. 1c, the pristine SiCw present sharp vibration peaks at 790 and 964 cm-1 in the low frequency region ranging from 600-1000 cm-1, belonging to the transverse optical (TO) and longitudinal optical

(LO) phonon modes, respectively [39]. The two peaks both showed slight red-shift compared with that of bulk SiC, ascribing to the size confinement effect and the stacking faults within SiCw [21], and the intensities of two characteristic peaks of SiCw obvious decreased after coating BDC and PPy. The peaks located at about 1349 and 1590 cm-1 could be assigned to D band and G band, respectively [40,41]. Based on the results reported by previous literature, the D band refers to a breathing mode of A1g symmetry, which produces phonons near the K zone boundary, and its intensity is in direct proportion of the disorder graphite relative content, with respect to the content of perfect graphite, in the materials [28,42,43]. The G band represents the E2g mode at the Brillouin zone center, deriving from in-plane vibration of sp2 carbon atoms [28,42,43]. The intensity ratio value of D band to G band (ID/IG) is employed to evaluate the relative content of defect contained in the samples [28,42]. The ID/IG values of SiCw-BDC and SiCw-BDC/PPy composites are about 0.94 and 0.96, respectively, suggesting a low graphitization degree with amounts of disorder graphite of SiCw-BDC/PPy compared with that of SiCw-BDC, which should be aroused form the introduction of PPy with five-membered rings [44]. In addition, the poor crystallinity of PPy phase with lots of defects would be acted as active sites to promote the formation of polarization, improving EM wave absorption performance of composites [44]. To chemical analysis of as-prepared samples, FT-IR spectra of pristine SiCw, SiCw-BDC and SiCw-BDC/PPy were depicted in Fig. 2a. From the FT-IR spectrum of pristine SiC, there are only two absorption peaks at about 779 and 908 cm-1, which assign to the TO mode of the Si-C stretching vibration and the LO vibration mode, respectively [20]. After the introduction of BDC, a new peak at about 1624 cm-1 was found in the spectrum of SiCw-BDC composite, belonging to the C=C stretching vibration in BDC [45]. Furthermore, evident evidence for the structural difference for

SiCw-BDC/PPy sample could be achieved from the spectrum in Fig. 2a compared with those of pristine SiCw and SiCw-BDC. The vibrational band at about 1533 cm-1 should be derived from the C-C stretching vibration, and the peaks centered at 1475, 1296 and 1174 cm-1 could be attributed to the C-N stretching vibrations on pyrrole rings [29,46]. The peak located at about 1043 cm-1 could be assigned to the C-H deformation vibrations [29,46]. All these phenomena indicated that BDC and PPy were successfully incorporated into the SiCw. In order to further demonstrate this deduction, the XPS spectra of the composite were presented in Fig. 2b-e to investigate the elemental distribution within the composite. Fig. 2b shows the survey spectra of pristine SiCw, SiCw-BDC and SiCw-BDC/PPy, demonstrating that the chemical elemental compositions of pristine SiCw and SiCw-BDC are Si, C and O elements, while a new peak with binding energy of 398.9 eV (N 1s) is detected in the SiCwBDC/PPy sample, suggesting the success in introducing of PPy [13]. The highresolution of C1s XPS spectra were fitted in Fig. 2c-e to further confirm the presence of BDC and PPy, and the curve fitting of the XPS spectra was listed with GaussLorentzian peak shape after removing a linear background correction. The XPS spectrum of pristine SiCw just contains a binding energy peak of C-Si at 282.7 eV, and a new binding energy of 284.6 eV is assigned to C-C bonds in the XPS spectrum of SiCw-BDC [13,47]. For the SiCw-BDC/PPy sample, aside from the C-Si at 282.7 eV and C-C peak at 284.8 eV, the binding energy peak of C-N at 288.2 eV indicated that PPy was successfully introduced [48]. Moreover, the high-resolution of N1s in the SiCw-BDC/PPy sample was also supplied in Fig. S1, and there are three peaks at binding energies of 400.3, 399.5 and 398.7 eV, attributing to the positively charged nitrogen atom (-NH+), the neutral pyrrolic nitrogen (=N-H+) and the imine nitrogen (NH-) [49]. All of the above analysis clearly confirms the decoration of BDC and PPy on the SiCw.

The influence of mass ratio of SiCw/Glu on the morphology of SiCw-BDC composites was displayed in Fig. 3. Pristine SiCw (Fig. 3a) with a smooth surface is a hexagonal prism-shaped, and the average size of pristine SiCw is about 250 nm from the SEM image and Fig. S2 shows a size distribution histogram of SiCw based on a plenty of SiCw. When the mass ratio of SiCw/Glu was 0.5 (sample S-0.5), namely the content of Glu was excess, the surface of SiCw was coated by a thin film and the edges of hexagonal prism could not be distinct observed (Fig. 3b). There were lots of droplets with different sizes found on the surface of SiCw shown in Fig. 3c when the mass ratio of SiCw/Glu was 1.0 (sample S-1). When the mass ratio of SiCw/Glu was further up to 2.0 (sample S-2), the content of droplet further decreased and just a few large droplets grown on the surface of SiCw revealed in Fig. 3d. According to the above phenomenon, the mass ratio of SiCw/Glu strongly affected the morphology of SiCw-BDC composites. The morphology and structure of SiCw-BDC composites with different mass ratio of SiCw/Glu were further characterized by TEM and HRTEM. As for S-0.5 presented in Fig. 4a-b, the surface of SiCw was coated by a thin BDC layer, while it exhibited different morphologies, such as hollow graphite nanosheets with a semiring shape, droplets and worm-like nanorods. Notably, the different BDC morphologies would be in favor of enhancing the EM wave absorption performance of composites, attributing to promote the formation of various polarizations [42]. From SEM image, S-1 sample was coated by lots of droplets (Fig. 3c), which was consistent with results of TEM image shown in Fig. 4c, and the HRTEM image (Fig. 4d) indicated that the droplets were graphite structures with a semiring shape, in which the interplanar spacing was measured to be ∼0.4 nm, while the semiring graphite structures were filled with a certain amount of graphite. Further, Fig. 4e-f reveal the microstructure of S-2, and the large droplet were full filled with disorder graphite but coated with several graphite layers. From the above results, the

mass ratio of SiCw/Glu greatly affected the morphologies of BDC coating the surface of SiCw, and the role of FeCl3 should be noted, which could act as a graphitization catalyst and activating agent. 3.2 Microwave absorption properties of SiCw-BDC composites The EM wave absorption performance could be evaluated by the following equations [21,42,50]:

RL(dB)  20 lg

Zin 

Zin  1 Zin  1

r  2 fd  tanh  j r  r  r c  

(1)

(2)

where, RL is the reflection loss value, Zin refers to the input impedance of the absorber, f represents the microwave frequency, d is the thickness of the absorber, c is the velocity of light in free space (3×108 m s−1) [21,42,50]. In general, the value of RL is 10 dB, meaning that 90% of the EM wave is absorbed by the absorber, which could be applied in the practical application [21,42,50]. To investigate the microwave absorption properties of SiCw-BDC composites, the composites were mixed with paraffin wax and pressed into a toroid-shape with thickness of 3.0 mm. According to the principle of EM energy conversion, the EM wave absorption capacity depends on the relative complex permittivity (εr = ε′ − jε″), relative complex permeability (μr = μ′ − jμ″) and the matching degree between εr and μr [21,42,50]. The real part (ε′) of complex permittivity is associated with the stored electrical energy, and the imaginary part (ε″) of complex permittivity relates to the loss of electrical energy [21,42,50]. Owing to no ferromagnetic materials involved, the complex permeability of our samples was 1 (μr = 1) [13,21]. In addition, the dielectric tangent loss (tanδe = ε″/ε′) represents a assessment of the energy lost in the absorber versus the energy stored, and a higher value of tanδe in general means that more EM energy would be

consumed [13,21]. Frequency dependence of the real (ε′) and imaginary (ε″) parts of the relative complex permittivity and the dielectric tangent loss (tanδe) of pristine SiCw and SiCw-BDC composites dispersed in paraffin wax within 2−18 GHz were displayed in Fig. 5. As shown in Fig. 5a, the real parts ε′ of SiCw-BDC samples are apparently larger than that of pristine SiCw (maintain about 2.5), while whose ε′ values decline with the increase of frequency besides some slight rising. Generally, the permittivity value of absorber is affected by the relaxation of Debye dipolar under the external EM field, and the random dipoles within the absorber would display preferential orientation paralleling to the external EM field, promoting the formation of dipole relaxation polarization [21,42,51]. Notably, it is necessary to consume energy to overcome the great resistance during the dipole relaxation polarization, while the rearrangement of the dipoles cannot have enough time to respond to the fast changing external EM field when the frequency is higher than a critical point, leading to an obvious declining tendency with increasing frequency [21,42,51]. In Fig. 5b, the imaginary parts ε″ of SiCw-BDC samples are also higher than that of pristine SiCw, which shows a strong dependence of the mass ratio of SiCw/Glu. The SiCw-BDC with 0.5 mass ratio of SiCw/Glu (S-0.5) exhibits relatively large ε″ than the other two SiCw-BDC samples (S-1 and S-2) in the whole frequency range of 2-18 GHz, resulting in superior dielectric loss properties, while the sample S-2 has a higher value of ε″ compared with that of S-1. According to previous literature, the value of ε″ relates to the conductivity of the sample, and the content of BDC was evaluated by TGA curve under different mass ratios of SiCw/Glu. To evaluate the content of BDC within the composites, all of the SiCw-BDC samples were applied to TGA in air atmosphere. For the pristine SiCw, the weight increases slightly with a weight increment due to high-temperature (> 600 °C) oxidation in the air [52]. As shown in

Fig. S3, S-0.5, S-1 and S-2 undergo a weight loss of 11%, 7.0% and 3%, respectively. This indicates the content of BDC in the SiCw-BDC composites decreases with the extension of mass ratio of SiCw/Glu. Although the content of BDC decreases with the mass ratio of SiCw/Glu, the morphology and distribution of BDC would greatly affect the practical conductivity of SiCw-BDC, leading to the difference of ε′′ value, which has been confirmed by the SEM and TEM images portrayed in Fig. 3 and Fig. 4. While the presence of numerous of heterostructures interfaces with incorporation of BDC would promote the interfacial polarization, being favor of increasing the ε′′ values [21,42]. The dielectric tangent loss of pristine SiCw and SiCw-BDC composites were shown in Fig. 5c. Similarity, the values of tanδe of SiCw-BDC composites are larger than that of pristine SiCw, suggesting that the introduction of BDC would enhance the EM wave absorption performance of SiCw. The sample S0.5 has the highest value of tanδe ranging from 0.4 to 0.65, and the tanδe for S-1 ranges from 0.25 to 0.35 and the tanδe for S-2 fluctuates from 0.35 to 0.55. It is worth noting that there two peaks are centered at about 13 and 16 GHz for all samples, indicating the typical characteristic of the nonlinear resonant behavior arising from polarizations relaxation [21]. Meanwhile, numerous interfaces would be formed with the introduction of BDC to promote the interfacial polarization, being favor to increase the value of ε′′ and beneficial to greatly attenuate EM waves. The change of the RL value of pristine SiCw and SiCw-BDC composites versus frequency could be seen from Fig. 6 and Fig. S4. As shown in Fig. 6, the 3D RL mapping plots of pristine SiCw, S-0.5, S-1 and S-2 varied with frequency and thickness are revealed. From Fig. 6a, the strongest EM wave absorption of pristine SiCw is just about -3.67 dB at 10.6 GHz with the thickness of 5.0 mm, suggesting that the pristine SiCw is a wave-transparent material. In Fig. 6b, the RLmin value of S-0.5 is as low as -24.6 dB with the thickness of 2.55 mm, and the effective bandwidth is up to

6.8 GHz (11.2-18.0 GHz) for the thickness of 2.49 mm, indicating that the effective bandwidth for the sample S-0.5 could cover the entire Ku-band, which could achieve effective absorption of EM waves in entire Ku-band with a wide thickness range of 2.28-2.49 mm, demonstrating the meaningless for practical applications. But for S-1, the lowest RL value is just about -10.8 dB with the thickness of 2.37 mm at 18.0 GHz, and the effective bandwidth is only 1.5 GHz (16.5-18.0 GHz) for the thickness of 2.54 mm (Fig. 6c) [52]. The RLmin value for the S-2 (Fig. 6d) could be -18.3 dB at 6.8 GHz with the thickness of 5.0 mm, and effective bandwidth is about 4.8 GHz (12.9-17.7 GHz) for the thickness of 2.49 mm. All of the above results indicate that the introduction of BDC could effectively enhance the EM wave absorption capacity of SiCw and the EM wave absorption performance could be adjusted by the mass ratio of SiCw/Glu. Apart from the dielectric loss and magnetic loss, there two significant parameters, including impedance match and attenuation constant, strongly affect the EM wave absorption performance of the absorber [13,21,53,54]. Normally, the characteristic impedance of the absorber should be equal or close to that of the free space, achieving zero-reflection at the front surface of the absorber [13,21,53,54]. The impedance matching degree relates to the relationship between the complex permittivity and complex permeability [13,21,53,54]. To achieve a desirable EM wave absorption performance, the complex permittivity and complex permeability of the absorber should be matched, and a delta-function method has been proposed to evaluate the impedance matching degree between the complex permittivity and complex permeability by the following equation [13,21,54]:

  sinh 2  Kfd   M where K and M could be further calculated by the following equations [13,21,54]:

(3)

K

M

4  ' ' sin

e  m 2

(4)

c cos  e cos  m

4 ' cos  e ' cos  m

  cos  '

e

  ' cos  m 

2

2

2       tan  m  e     ' cos  e   ' cos  m    2 2 

(5)

Commonly, a smaller delta value represents better impedance matching between the complex permittivity and complex permeability according to the above equations [13,21,54]. The calculated delta value maps of pristine SiCw and SiCw-BDC composites were displayed in Fig. 7a-d. Compared with those of pristine SiCw and S1, the calculated delta values of S-0.5 and S-2 from 2 to 18 GHz with the thickness ranging from 0.5 to 5 mm are closer to zero. The results of calculated delta values of S-0.5 and S-2 samples with better matching of characteristic impedance are in good accordance with the results of EM wave reflection losses. Furthermore, the EM wave attenuation ability of the absorber is the other important parameter, which could be characterized by the value of attenuation constant (α), and typically expressed as [13,21,53,54]

2 f  '' ''      ' '     c 

  ''

''

 '

   

' 2

'

''

  ''



' 2

1

2 

(6)

where f is the frequency ranging from 2 to 18 GHz and c is the velocity of light in vacuum. The higher value of α, the better EM wave absorption performance could be achieved. As shown in Fig. 7e, the sample S-0.5 exhibits the maximum value of α among all samples almost in the whole frequency range, while the sample S-2 creates a much attenuation constant than those of other two samples (pristine SiCw and S-1) in the whole frequency range, demonstrating its superior attenuation ability for the incident EM wave compared with those of pristine SiCw and S-1. All of the results suggest that the introduction of BDC could efficiently improve the EM wave

absorption capacity of SiCw, whose EM wave absorption performance could be controlled by the mass ratio of SiCw/Glu. 3.3 Microwave absorption properties of SiCw-BDC/PPy heterostructures To further enhance the EM wave absorption capacity of SiCw-BDC composites, the conductive polymer PPy was introduced by in situ polymerization to form a coreshell structure, and the core material was selected as S-0.5 with the best EM wave absorption performance among all the SiCw-BDC composites. In addition, we investigated the influence of various polymerization times (0.5, 1.0 and 1.5 h) on the EM wave absorption properties of core-shell SiCw-BDC/PPy heterostructures, and the samples were labeled as S-0.5-0.5, S-0.5-1.0 and S-0.5-1.5. First, the influence of polymerization time on the morphology of SiCw-BDC/PPy heterostructures was evaluated and the SEM images were shown in Fig. S5. From the images, it can obvious see that the diameters of SiCw-BDC/PPy increased with the polymerization time, and the average sizes of S-0.5-0.5, S-0.5-1.0 and S-0.5-1.5 were about 300, 350 and 400 nm, respectively. It is worth noting that the PPy particles coated on the surface of SiCw-BDC/PPy heterostructures began to agglomerate when the polymerization time was up to 1.5 h. To explore the EM absorption performance of SiCw-BDC/PPy heterostructures, the real (ε′) and imaginary (ε″) parts of the relative complex permittivity and the dielectric tangent loss (tanδe) of samples were studied within 2-18 GHz. As sketched in Fig. 8a, the values of ε′ of SiCw-BDC/PPy samples are almost higher that of SiCwBDC sample and show a declining tendency versus frequency with some sharp rises between 14 and 18 GHz, 16-18 GHz, and 11-18 GHz for S-0.5-0.5, S-0.5-1.0 and S0.5-1.5 samples. Similarity, the dielectric response tends to gradually decrease with the increase of frequency, which could be attributed to that there no enough time for the dipoles within the heterostructures responds to the fast changing external EM field.

As shown in Fig. 8b, the imaginary (ε″) parts of SiCw-BDC/PPy heterostructures have a strong dependence of the polymerization time of PPy. The sample S-0.5-0.5 exhibits relatively high ε″ values compared with those of S-0.5, S-0.5-1.0 and S-0.52.0 in the frequency of 4-17 GHz, resulting in excellent dielectric loss properties. The dielectric tangent loss of SiCw-BDC/PPy samples was shown in Fig. 8c. From the results, the tanδe value for S-0.5-0.5 sample ranges from 0.05 to 1.25, and two peaks are found at about 10 and 12 GHz. The value of the tangent loss for S-0.5-1.0 sample is from 0.05 to 1.4 with two obvious peaks at about 8 and 14 GHz, respectively. The tanδe value for S-0.5-1.5 sample fluctuates between 0.05-0.65 with three peaks at about 8, 10 and 16 GHz, respectively. The frequency-dependent fluctuations of the relative complex permittivity and the dielectric tangent loss shown in Fig. 8 refer to the typical characteristic of the nonlinear resonant behavior deriving from the polarization relaxation, which could be ascribed to the cooperative effects of the interfaces among SiCw, BDC and PPy [21,54]. Furthermore, the 3D RL mapping plots and theoretical curves of SiCw-BDC/PPy heterostructures varied with frequency and thickness, are depicted in Fig. 9 and Fig. S6. A substantial enhancement of EM absorption performance is achieved after the SiCw were coated with PPy shells. The RLmin for samples S-0.5-0.5, S-0.5-1.0 and S0.5-1.5 are -48.6, -52.4 and -51 dB, respectively, suggesting that the introduction of PPy shell could further improve the EM wave absorption of SiCw-BDC composites. The effective bandwidths (RL< −10 dB) for S-0.5-0.5, S-0.5-1.0 and S-0.5-1.5 are 6.7 (9.3-16 GHz), 8.1 (9.0-17.1 GHz) and 6.2 (8.2-14.4 GHz) GHz, respectively. Meanwhile, the sample S-0.5-1.5 could cover the entire X-band with a wide thickness ranges from 2.16 to 2.45 mm, indicating that the SiCw-BDC/PPy heterostructures could be as a promising candidate for next-generation high-performance microwave absorber for practical applications.

To better explain the excellent microwave absorption performance of SiCwBDC/PPy heterostructures, the impedance matching degree evaluated by the value of delta and the value of attenuation constant (α) were measured, respectively. From Fig. 10a-c, all of the calculated delta values of SiCw-BDC/PPy samples are close to zero, indicating the great matching of characteristic impedance in the SiCw-BDC/PPy heterostructures, which has good consistence with the results of EM wave reflection losses. As shown in Fig. 10d, the SiCw-BDC/PPy heterostructures own much larger attenuation constant than that of SiCw-BDC composites in the frequency ranges of 7.0−18.0 GHz, indicating its superior attenuation ability for the incident EM waves. All of the results suggest that a kind of new EM wave absorption material, SiCwBDC/PPy heterostructures with excellent EM wave absorption capacities could be fabricated by a simple method, whose EM wave absorption abilities could be achieved by adjusting the mass ratio of SiCw/Glu and the polymerization time of PPy, and the materials can be rational constructed and utilized as an efficient absorber with desired EM wave absorption performance at target frequency band. To

date,

various

1D

SiC

hybrid

materials,

including

Co-SiCNWs,

PPy@SiCNWs, Fe-SiCf, graphene aerogel/SiCNWs (GA/SiCNWs), ZnO-SiCNWs, RGO/SiCNWs, Fe3O4-SiCNWs, Fe-doped SiCw, and other new materials have been reported as promising EM wave absorbers [6-8,12,13,16-19,28-30,33,35,36,38,53]. Table S1 and Fig. 11 compare the EM wave absorption performance of these reported 1D SiC hybrids and other new materials with our products. It can clearly see that our products SiCw-BDC/PPy exhibit outstanding performance with high efficiency (﹤50 dB), broadened bandwidth (﹥8.0 GHz) and thin matched thickness (2.26 mm). All these advantages enable SiCw-BDC/PPy heterostructures to be an ideal lightweight microwave absorber for practical applications. The superior EM wave absorption capacities of SiCw-BDC/PPy heterostructures

could be primarily derived from the following points [42,55]. First, the introduction of BDC with numerous defects could serve as effective polarization centers under EM field, promoting the formation of dipole polarization. Second, there would be lots of interfaces in the SiCw-BDC/PPy heterostructures, leading to the interfacial polarization. The formation of dipole and interfacial polarization would be beneficial to improve the dielectric loss. Third, the accumulated charges and collective interfacial polarization would be formed within the interfaces, resulting in the transforming of EM energy to heat energy. In a word, the enhanced EM absorption properties should be ascribed to the unique structural characteristics, enhanced dielectric loss and synergistic effect.

4. Conclusions In conclusion, heterostructured hierarchical SiCw-BDC/PPy composites with high-performance EM wave absorption properties were successfully fabricated via a combined hydrothermal and carbonization, followed by a simple chemical polymerization method. The mass ratio of SiCw/Glu has had a dramatic impact on the content and morphology of BDC, and microwave electromagnetic properties of SiCwBDC composites. Compared with the wave-transparent of pristine SiCw, the SiCwBDC composite at the optimal SiCw/Glu mass ratio of 0.5 with a 2.55 nm absorber thickness exhibited a minimal RL value of −24.6 dB at 13.2 GHz, and its effective bandwidth is up to 6.8 (11.2-18.0) GHz, which could achieve effective absorption of EM waves in entire Ku-band with a wide thickness range of 2.28-2.49 mm. After coating on PPy film, the EM wave absorbing capacity of SiCw-BDC/PPy heterostructures could be effectively controlled by the polymerization time of PPy, and the SiCw-BDC/PPy heterostructures with 1.0 h polymerization time show the optimal EM wave absorption performance with a RLmin value of -52.4 dB at 11.4 GHz

and effective bandwidth of 8.1 (9.0-17.1) GHz. Compared with other reported 1D SiC hybrid

materials

and other

new

materials,

our

products SiCw-BDC/PPy

heterostructures exhibit superior EM wave absorbing performance with high efficiency (﹤-50 dB), broadened bandwidth (﹥8.0 GHz) and thin matched thickness (2.26 mm), which would be an ideal lightweight microwave absorber for practical applications.

Acknowledgements This work was supported by China National Funds for Distinguished Young Scientists (No. 51525201), Innovative Research Group of National Natural Science Foundation of China (No. 91216301, 11421091, 51202048, 11402252, 51372047), the State Key Laboratory of Advanced Welding and Joining (No. 17-M-07), the Fundamental

Research

Funds

for

the

Central

Universities

(Grant

No.

HIT.BRETIII.201506) and the National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, KL.PYJH.2016.001.

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Figure Captions: Fig. 1 (a) XRD patterns of pristine SiCw, SiCw-BDC and SiCw-BDC/PPy heterostructures. (b) A closer examination of 2θ=23.5-27.5°of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures. Raman spectra of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures within different wavelength range: (a) 600-1000 cm-1 and (b) 1200-2000 cm-1. Fig. 2 (a) FT-IR spectra and (b) XPS surveys of pristine SiCw, SiCw-BDC and SiCwBDC/PPy heterostructures. (c)-(e) Fitting curves of C 1s peak of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures. Fig. 3 SEM images of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/Glu. (a) Pristine SiCw. (b) SiCw-BDC composites with SiCw/Glu mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/Glu mass ratio of 1. (d) SiCw-BDC composites with SiCw/Glu mass ratio of 2. Fig. 4 TEM and HRTEM images of SiCw-BDC composites with different mass ratios of SiCw/Glu. (a)-(b) SiCw-BDC composites with SiCw/Glu mass ratio of 0.5. (c)-(d) SiCw-BDC composites with SiCw/Glu mass ratio of 1. (e)-(f) SiCw-BDC composites with SiCw/Glu mass ratio of 2. Fig. 5 (a) Real part and (b) imaginary part of permittivity, and (c) dielectric tangent loss of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/Glu. Fig. 6 The 3D representations of EM wave RL of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/Glu. (a) Pristine SiCw. (b) SiCw-BDC composites with SiCw/Glu mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/Glu mass ratio of 1. (d) SiCw-BDC composites with SiCw/Glu mass ratio of 2. Fig. 7 The calculated delta value maps and attenuation constants of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/Glu. (a) Pristine SiCw. (b)

SiCw-BDC composites with SiCw/Glu mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/Glu mass ratio of 1. (d) SiCw-BDC composites with SiCw/Glu mass ratio of 2. (e) Attenuation constants of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/Glu. Fig. 8 (a) Real part and (b) imaginary part of permittivity, and (c) dielectric tangent loss of SiCw-BDC/PPy heterostructures with different polymerization time. Fig. 9 The 3D representations of EM wave RL of SiCw-BDC/PPy heterostructures with different polymerization time. (a) SiCw-BDC/PPy heterostructures with 0.5 h polymerization time. (b) SiCw-BDC/PPy heterostructures with 1.0 h polymerization time. (c) SiCw-BDC/PPy heterostructures with 1.5 h polymerization time. Fig. 10 The calculated delta value maps and attenuation constants of SiCw-BDC/PPy heterostructures

with

different

polymerization

time.

(a)

SiCw-BDC/PPy

heterostructures with 0.5 h polymerization time. (b) SiCw-BDC/PPy heterostructures with 1.0 h polymerization time. (c) SiCw-BDC/PPy heterostructures with 1.5 h polymerization time. (d) Attenuation constants of SiCw-BDC/PPy heterostructures with different polymerization time. Fig. 11 Comparison of EM wave absorption performance of different EM absorption materials reported by recent literatures.

Fig. 1 (a) XRD patterns of pristine SiCw, SiCw-BDC and SiCw-BDC/PPy heterostructures. (b) A closer examination of 2θ=23.5-27.5°of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures. Raman spectra of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures within different wavelength range: (a) 600-1000 cm-1 and (b) 1200-2000 cm-1.

Fig. 2 (a) FT-IR spectra and (b) XPS surveys of pristine SiCw, SiCw-BDC and SiCwBDC/PPy heterostructures. (c)-(e) Fitting curves of C 1s peak of pristine SiCw, SiCwBDC and SiCw-BDC/PPy heterostructures.

Fig. 3 SEM images of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/glucose. (a) Pristine SiCw. (b) SiCw-BDC composites with SiCw/glucose mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/glucose mass ratio of 1. (d) SiCw-BDC composites with SiCw/glucose mass ratio of 2.

Fig. 4 TEM and HRTEM images of SiCw-BDC composites with different mass ratios of SiCw/glucose. (a)-(b) SiCw-BDC composites with SiCw/glucose mass ratio of 0.5. (c)-(d) SiCw-BDC composites with SiCw/glucose mass ratio of 1. (e)-(f) SiCw-BDC composites with SiCw/glucose mass ratio of 2.

Fig. 5 (a) Real part and (b) imaginary part of permittivity, and (c) dielectric tangent loss of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/glucose.

Fig. 6 The 3D representations of EM wave RL of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/glucose. (a) Pristine SiCw. (b) SiCwBDC composites with SiCw/glucose mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/glucose mass ratio of 1. (d) SiCw-BDC composites with SiCw/glucose mass ratio of 2.

Fig. 7 The calculated delta value maps and attenuation constants of pristine SiCw and SiCw-BDC composites with different mass ratios of SiCw/glucose. (a) Pristine SiCw. (b) SiCw-BDC composites with SiCw/glucose mass ratio of 0.5. (c) SiCw-BDC composites with SiCw/glucose mass ratio of 1. (d) SiCw-BDC composites with SiCw/glucose mass ratio of 2. (e) Attenuation constants of pristine SiCw and SiCwBDC composites with different mass ratios of SiCw/glucose.

Fig. 8 (a) Real part and (b) imaginary part of permittivity, and (c) dielectric tangent loss of SiCw-BDC/PPy heterostructures with different polymerization time.

Fig. 9 The 3D representations of EM wave RL of SiCw-BDC/PPy heterostructures with different polymerization time. (a) SiCw-BDC/PPy heterostructures with 0.5 h polymerization time. (b) SiCw-BDC/PPy heterostructures with 1.0 h polymerization time. (c) SiCw-BDC/PPy heterostructures with 1.5 h polymerization time.

Fig. 10 The calculated delta value maps and attenuation constants of SiCw-BDC/PPy heterostructures

with

different

polymerization

time.

(a)

SiCw-BDC/PPy

heterostructures with 0.5 h polymerization time. (b) SiCw-BDC/PPy heterostructures with 1.0 h polymerization time. (c) SiCw-BDC/PPy heterostructures with 1.5 h polymerization time. (d) Attenuation constants of SiCw-BDC/PPy heterostructures with different polymerization time.

Fig. 11 Comparison of EM wave absorption performance of different EM absorption materials reported by recent literatures.

 Heterostructured hierarchical SiCw-BDC/PPy composites were rational designed.  Desired reflection loss was achieved by adjusting the SiCw/glucose mass ratio and PPy content.  The optimal RL of SiCw-BDC/PPy reaches -52.4 dB with effective bandwidth of 8.1 GHz.

Graphic for manuscript

Biomass-derived carbon and polypyrrole addition on SiC whiskers for enhancement of electromagnetic wave absorption