Silicon Nitride Composite for Cooperative

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Functional Nanostructured Materials (including low-D carbon)

Reduced Graphene Oxide/Silicon Nitride Composite for Cooperative Electromagnetic Absorption in Wide Temperature Spectrum with Excellent Thermal Stability Zexin Hou, Xiaowei Yin, Hailong Xu, Hanjun Wei, Minghang Li, Laifei Cheng, and Litong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20023 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Reduced Graphene Oxide/Silicon Nitride Composite for Cooperative Electromagnetic Absorption in Wide Temperature Spectrum with Excellent Thermal Stability

Zexin Hou, Xiaowei Yin⁕, Hailong Xu, Hanjun Wei, Minghang Li, Laifei Cheng, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, China ⁕E-mail:

[email protected]

KEYWORDS: reduced graphene oxides, amorphous Si3N4, temperature-independent, polarization relaxation loss, conductive loss ABSTRACT The fabrication of a sandwich-like composite that consists of reduced graphene oxide (RGO) and Si3N4 ceramic (RGO/Si3N4) was achieved through the combination of modified freeze-drying approach and chemical vapor infiltration (CVI) process. Due to a hierarchical structure and high ratio of ID/IG (1.27), the RGO/Si3N4 exhibits an unprecedented high polarization relaxation loss (PRL), which accounts for 32 % of the whole dielectric loss. The outstanding PRL endow the RGO/Si3N4 composites with unique temperature-independent dielectric properties and electromagnetic (EM) wave absorption performance. Even at a low absorbent content of only 0.16 wt.%, the effective absorption bandwidth of RGO/Si3N4 composite can cover the whole X-band

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(8.2~12.4 GHz) at broad sample thicknesses and temperatures range from 4.3 mm to 4.6 mm, and 323 K to 873 K. The mechanism for the enhancement of polarization relaxation loss and conductive loss was explicitly investigated. The outstanding absorption performance toward EM wave indicated the resultant porous RGO/Si3N4 composite can be a promising candidate for the applications under elevated temperature. 1. INTRODUCTION Efficient absorption of electromagnetic (EM) wave-in a broad bandwidth-plays a crucial role in the practical application of materials aiming at EM attenuation.1 Further, stability in dielectric loss within a wide temperature spectrum, such as from 323 K to 873 K, makes these materials valuable in a broad range of applications, including civil construction, commercial products, military equipment, and aerospace vehicles.2-3 There are many differences in designing considerations between conventional EM wave absorption materials and novel temperature-independent materials. Firstly, Curie temperature should be taken into consideration under high-temperature limitations for these ferromagnetic components like ferrites,4 cobalt oxide,5 and nickel ferrite nanocrystals.6 Secondly, the increase of temperature could lead to a significant increase in conductivity for those conductive loss carbonaceous materials such as multi-wall carbon nanotube, and graphene, which will further give rise to EM shielding effect rather than EM absorbing behavior.7-8 Thirdly, due to the limitation for the currently available polymer-based matrix for EM absorption materials, such as the polydimethylsiloxane, paraffin, as well as organosilicon resins, which can only work at temperatures of lower than 673 K, 9 the application temperature is greatly limited by


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the matrix. Reduced graphene oxides (RGO) is a star member from the family of carbonaceous materials, and it demonstrates a variety of shining points such as an extremely high surface area ratio, excellent carrier mobility, abundant defects, as well as multiple functional groups (e.g., the oxygen group, and hydroxyl group).10 As a type of materials used alone, RGO has been studied in great detail for both absorption and shielding of EM wave.11-13 For the application in EM absorption, a matrix which is transparent to EM wave is commonly employed for RGO dispersion, and the final dielectric property can be primarily influenced by the spontaneous agglomeration of RGO at the nanoscale.14-16 Therefore, novel architectures have been explored to prevent RGO from aggregation. A recent study revealed an inhibition effect exerted by the highly porous structure of a three dimensional (3D) RGO foam,17-18 suggesting its potential in preventing the agglomeration. Such lightweight RGO foams exhibited satisfying performance of room temperature EM wave absorption.19-20 Typically, the difficulty in balancing the conductive loss and polarization relaxation loss is an excellent challenge for the designing of high-temperature EM materials.3 An internal 3D crosslinking network endows RGO foam with a proper conductive loss, while, the abundant defects in RGO resultant from the insufficient reduction, can lead to a high dielectric loss. Therefore, 3D RGO foam is of great promise as absorbers for EM absorption materials that can be used under elevated temperature. Compared with the polymer-based matrix which cannot use higher than 673 K,9 ceramic materials with low dielectric loss have proven to be the more appropriate



matrix for high-temperature EM materials.21-25 Several options have been reported for such kind of usages, such as SiO2, hexagonal BN, and amorphous Si3N4. 26-30 Among them, SiO2 exhibits low electrical conductivity and high chemical stability. However, the inferior toughness seriously limits its practical applications. BN ceramic, in the meantime, suffers from disadvantages such as poor mechanical performance and insufficient resistance to thermal shock. Compared with the abovementioned ceramic, amorphous Si3N4 is more promising owing to a variety of merits including much lower dielectric loss (tan δ ≤ 0.01), lower electrical conductivity, higher strength, and higher hardness. Moreover, amorphous Si3N4 can be easily imported into the RGO foam via the chemical vapor infiltration (CVI) process, which hardly perturbs the original 3D architecture.31 Herein, the RGO foams were prepared successfully through the combined procedure of modified freeze drying and thermal reduction. Ice template was employed to construct GO foams in a layered architecture, and CVI was then applied for the introduction of amorphous Si3N4. To a great extent, homogenous dispersion of Si3N4 throughout the 3D RGO foam enhanced the stability of the RGO/Si3N4. More importantly, the composite exhibited a much weaker temperature dependence than those highperformance EM wave materials. It gave the content of absorbent lower than the frequency range of the whole X-band at any temperature between 323 K and 873 K. The resultant RGO/Si3N4 composite exhibits great potential in utilization as excellent EM wave materials at elevated temperature. 2. EXPERIMENTAL


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2.1 Preparation of GO and RGO foams To start with, GO powders (200 mg, Jiangsu XFNANO Materials Tech Co., LTD) was dissolved in deionized water (200 mL) and ultrasonic dispersion for 10 hours to obtain an excellent homogeneity of GO suspension (1 mg/mL). Then, rapid freezing to about 77 K was completed within a couple of seconds by liquid nitrogen to form the 3D architecture of GO scaffold that consisted of thin lamellae in parallel.32 Next, 3D was obtained after 36 h of freezing drying, which possessed layered structure in long range with large surface areas. Finally, the as-prepared GO foam was carbon thermally reduced. To further study the effects of annealing temperatures on the EM wave attenuation, the GO foams annealed at the temperature of 800 °C, 1000 °C, 1200 °C, and 1400 °C for 120 min in a nitrogen ambient with a heating rate of 3 °C/min. 2.2 Preparation of RGO/Si3N4 composites RGO foam was weighed before silicon nitride in situ grown by CVI to calculate the mass of absorbent content. The CVD furnace with a gaseous atmosphere of SiCl4-NH3H2-Ar system was employed for infiltration of Si3N4 into the obtained RGO foams at 1073 K. The specific constituents comprised silicon tetrachloride as the silicon source, ammonia as the nitrogen source, and hydrogen as the dilution gas. RGO/Si3N4 composites were obtained after the entire CVI procedure was completed. The chemical reaction is as follows:33 H2

3 SiCl4 (g) +4 NH3 (g) Si3N4(s) +12 HCl (g)

(1)

2.3 Characterization Archimedes displacement approach was applied in the measurement of bulk density



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and porosity based on AG 135 Mettler Toledo. The X-ray diffractometer (X’pert Pro, Philips, Netherlands) confirmed the microstructure of samples, while morphology was characterized on the scanning electron microscope (SEM, HITACHI S-4700, Japan) and the transmission electron microscope (TEM, G20, FEI Tecnai, USA). The thermal stability of RGO and RGO/Si3N4 were measured in ambient air by thermogravimetry analyzer (TGA, STA449F3, Netzsch) from 30 °C to 800 °C. RGO and RGO/Si3N4 composites for microwave absorption testing were tailored and refined into samples of 22.86×10.16×X mm in dimension, and their dielectric constant measured by waveguide method on a vector network analyzer (VNA, MS4644A, Anritsu, Japan) through the waveguide method in a frequency range corresponding to X-band (8.2~12.4 GHz). For dielectric performance test of RGO/Si3N4 samples at elevated temperature, through the heating system test on the waveguide cavity for rapid heating, a flange was utilized with the heating rate set as 10 °C/min, and the waveguide measure was calibrated at room temperature for accuracy guarantee. Measurement of dielectric constant could be carried when samples in flange had been kept at the designated temperature for 3 min. Calculation of reflection coefficient (RC) was based on the measured EM parameter values referring to a metal back panel model as follows: RC  dB  = 20log10  Z in - 1  Z in +1

Z in =

 tanh j2  fd c 





(2) (3)

where c is the speed of light in vacuum, f is the frequency, ε is dielectric constant, μ is permeability, and d is the thickness of samples.


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3. RESULTS AND DISCUSSION The fabrication route to RGO/Si3N4 composite is illustrated schematically in Figure 1. There are three steps comprised in the process, which are, a) aqueous GO obtained from ultrasonication self-assembled into ice-filled foams with freezing treatment, while the subsequent removal of ice crystals was realized by freeze-drying, b) GO foams were reduced into RGO foams via annealing in argon, c) amorphous Si3N4 was infiltrated into RGO foams via CVI to obtain RGO/Si3N4.

Figure 1. Schematics illustration of fabrication route for RGO/Si3N4 composite. The representative SEM images in Figure 2 suggests that the resultant RGO foams possessed a layered architecture with the highly porous structure, and the well preserved macroscopic structure after thermal reduction is further exhibited in the inserting picture in Figure 2a. Figure 2b reveals the successful incorporation and parallel distribution of amorphous Si3N4 via CVI technique. The SEM image at high magnification in Figure 2d depicts that the conductive RGO deposited as thin layers on the Si3N4 matrix (thickness of 480~600 nm), while the TEM image presents at atom level the mass formation of RGO/Si3N4 interface that resulted from the interconnected



network and sandwich-like morphology. EM wave induced accumulation of space charge at these interfaces can benefit EM wave attenuation. To further confirm whether silicon nitride was successfully introduced or not, energy spectrum analysis was carried out. Through elemental mapping analysis also identified the possibility of nitrogen, and silicon and infiltration of elements distribution homogeneous in Figure 2 (e, f, g, l, and k). Besides, amorphous silicon nitride infiltrated into porous RGO foam mainly through diffusion of CVI process, and in situ grown on the RGO sheets surface, as a protective effect on the RGO surface, to ensure the stability of absorbent at elevated temperature.


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Figure 2. Electron micrographs of (a) cross-section of RGO foam (Insets show the corresponding digital photographs.), (b) cross-section of RGO/Si3N4, (c, d) interface of RGO/Si3N4 and Si3N4/RGO/Si3N4, (e, f, g, h, k) EDS mapping images of RGO/Si3N4 , and (l) TEM image of RGO/Si3N4. The bonding state of carbon atoms, which took a predominant position in the absorption of EM wave,34 were detected by Raman spectroscopy for these samples, and two



independent bands of RGO at 1340 cm−1 (D band) and 1590 cm−1 (G band) can be discerned distinctly in Figures 3a and 3b. Figure 3a shows the Raman spectra of RGO with different annealing temperatures, which indicates a rising trend of reduction degree along with the elevated temperature from 800℃ to 1400℃. The defect concentration first increases and then decreases with the increase of reduction degree.35 Impregnation of amorphous Si3N4 results in displacement of D band and G band to 1347 cm−1 and 1585 cm−1, respectively, and the thermal reduction during CVI process sharpens these two bands. Moreover, the appearance of the 2D band at 2689 cm−1 implies the layer number of RGO sheets less than ten layers. The intensity ratios of ID/IG equal 1.26 and 1.27 for RGO and RGO/Si3N4, respectively, suggesting a slight increase in crystallization degree. XRD patterns disclosed the phase structure of the asprepared RGO and RGO/Si3N4 composites in Figure 3c. The diffraction peak of GO was 11 ° when the thermal reduction of GO diffraction peak moved to 26.5 °, 7 while the newly generated peaks at 71 ° correspond to the amorphous Si3N4 phase.28 As shown in Figure 3c, it is clear that all the diffraction peaks of RGO/Si3N4 composites match well to the pattern of silicon-nitrogen (ICDD No. 51-1334).

Figure 3. The microstructure of RGO, and RGO/Si3N4: (a) Raman of RGO, (b) Raman of RGO and RGO/Si3N4, and (c) XRD of GO, RGO, and RGO/Si3N4. The effects of annealing temperature on EM wave attenuation were investigated carefully. Along with the rise in annealing temperature, both the real part of dielectric constant (ε′) and imaginary part of dielectric constant (ε″) increased simultaneously in


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a slowing down manner within the measurement range of frequency (8.2~12.4 GHz). The tangent loss (equal to ε″/ε′ ) increased from ~0 to ~0.4, ~0.6, ~1.1, and 1.2, respectively (Figure S1c), indicating the enhanced capability of EM wave dissipation due to the increasing reduction degree.15 However, the EM wave absorption performance not only involves dissipation ability but depends critically on the characteristic of impedance match. To this end, ǀZin-1ǀ values were employed to measure the impedance matching characteristics.36 Figure S1d displays the ǀZin-1ǀ values of RGO against frequency variation at different annealing temperatures. Experimental results show that appropriate annealing treatment can enhance the impedance match. The EM wave absorption ability of RGO prepared at different annealing temperature was evaluated based on the calculated RC versus frequency as displayed in Figure 4. RGO annealed at 800 ℃ attained RC in a downtrend with thickness increasing from 4.0 mm to 4.7 mm, and minimum RC (RCmin) was only -4 dB at 12.4 GHz for the samples with a thickness of 4.7 mm due to its poor impedance match. As annealing temperature increases, RCmin decreases to -21 dB, -13 dB, and -14.7 dB, respectively. The EAB, in which more than 90% EM wave was dissipated corresponding to RC ˂ -10 dB, is a parameter more crucial than RCmin when the EM wave absorption performance was evaluated. Therefore, RGO sample annealed at 1200 ℃ with an EAB covering the whole X band and thickness between 4.0 mm and 4.7 mm possessed the best EM wave absorption performance and was considered as a suitable candidate for preparing the RGO/Si3N4 composites.



Figure 4. RC versus annealing temperatures of RGO foam: (a) 800 ℃, (b) 1000 ℃, (c) 1200 ℃, and (d) 1400 ℃. Before recording the dielectric parameters of RGO/Si3N4 composites under 323~873 K, their thermostability was evaluated by TGA (Figure S2) in advance. It was found that incorporation of Si3N4 could give rise to improved thermostability owing to the anti-ablative ability of amorphous Si3N4 (Figure S2 and additional discussion part in the supporting information). According to the RC data measured between 323 K and 873 K (Figure 5), RGO/Si3N4 composites with a thickness of 4.3~5.1 mm displayed gratifying performance in EM wave absorption at 323 K (Figures 5 a and 5 b), with an effective absorption bandwidth (EAB) covering the whole X-band. Further, such frequency band could be covered continuously within the temperature range between 323 K and 873 K and a samples thickness range between 4.3 mm and 4.6 mm, confirming once again the fantastic performance of RGO/Si3N4 composites for EM wave absorption within a broad range of temperature.


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Figure 5. RC values calculated for RGO/Si3N4 at various temperature (a, b) 323 K, (c, d) 373 K, (e, f) 473 K, (g, h) 573 K, (i, j) 673 K, (k, l)773 K, and (m, n) 873 K with



different thicknesses. Figure 6 presents the temperature effect toward absorption behavior of composites in a more specific way by setting a constant sample thickness (4.3 mm), and RGO/Si3N4 displayed satisfactory absorption in the whole X-band. RCmin was approximate -16.5 dB at 10 GHz and 323 K, and RC values showed certain temperature insensitivity within 323~873 K, which never exceed -10 dB in the whole X-band region despite the rising temperature, much better than all the previous materials.

Figure 6. RC curves of RGO/Si3N4 composites with a thickness of 4.3 mm at various temperatures. Although the RGO/Si3N4 composites in this research show lower microwave absorption intensity than formerly reported ones (Figure 7),2-3, 21-24, 37-41 its absorbent content as low as 0.16 wt.% is remarkably lower, which is much lower than the reported ones, and the EAB covers the whole X-band at various thicknesses (4.3~4.6 mm) instead of a single value. Such a characteristic implies the extremely weak dependence of reflection loss of composites on temperature, or even temperature independence. Thus, the RGO/Si3N4 composites developed herein was quite a competitive candidate for EM wave absorption under harsh circumstances.


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Figure 7. The EM absorption properties of typical microwave absorption materials within the temperature range of 323~873 K. Given the outstanding behavior of RGO/Si3N4 composites at EM wave absorption, indepth investigations on their dielectric properties as well as the underlying absorption mechanism were worth carrying out within a broad range of temperature. Dielectric constant variation against temperature for the composite samples is depicted in Figure 8, where ε′ implicates electric energy storage while ε″ indicates electrical energy loss.1, 42

Both parts were found susceptible to frequency changes, with increased frequency

leading to decreased values. For the real part shown in Figures 8 a and 8 b, two response regions can be discerned apparently at low frequency, but their curvature is much flattened at a higher frequency, almost parallel to the abscissa axis. As the frequency increases, the average value of the real part increases from 2.8 to 3.1, and that of the imaginary part increases from 2.9 to 3.3 with a similar tendency. Such a low permittivity at around 3.0 was attributed to the highly porous structure of RGO/Si3N4 composites, which is much improved compared with the conventional EM wave absorption materials. The ε′ and ε″ increased continuously along the increasing



temperature and equal to 3.1 and 3.3, respectively, at 873 K. The ε' and ε'' vary with frequency, defect polarization, and interfacial polarizations are relaxation process and are strongly temperature independent.43-44 The various polarization process which leads to the dielectric dispersion and attendant energy dissipation Figure 8c presents a decreasing tendency of loss tangent along with the increase in frequency but the decrease of temperature, and its value is ranging from 0.92 to 1.43 is higher than those of most dense materials reported before.9, 45 A moderate loss tangent no more than one can commonly favor EM wave absorption, for the much higher values may lead to impedance mismatching and subsequent deterioration of the absorption performance. Thus, the loss tangent measured for RGO/Si3N4 composite matched well with the free space regarding impedance. Nevertheless, since the normalized characteristic impedance is unquantifiable as an imaginary number, the modulus of ǀZin1ǀ would be applied for reflection loss evaluation, where a smaller value suggests betternormalized impedance. The calculated moduli are plotted against frequency under various temperatures in Figure 8d. As can be seen, the curves exhibit a decreasing tendency with increasing temperature and increasing frequency from 8.2 GHz to 9.2 GHz, but an increasing trend later as the frequency increases from 9.5 GHz to 12.4 GHz. Such a unique pattern gives rise to favorable normalized impedance within the temperature range of 323~873 K and the frequency range that covers the whole X-band. This unique phenomenon could be explained as the highly porous structure of the composites, which makes EM wave energy tend to enter the materials instead of reflecting on the surface. Together with the improved attenuation ability toward EM


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wave as implied by a more significant loss tangent, an outstanding temperatureindependent performance on EM wave absorption was thus achieved successfully.

Figure 8. RGO/Si3N4 of (a) The ε' , (b) The ε'', (c) The loss tangent, (d) |Zin-1| The overall dielectric loss in carbonaceous materials comprises conductive loss part and polarization relaxation loss part.3 As the conductive loss increases at elevated temperature, it is crucial to strengthen the polarization relaxation loss to avoid impedance mismatch.11, 37 Figure 9 collects the calculated data for these values via nonlinear squares fitting of Debye′s relaxation equation. It is noted that polarization relaxation loss accounts for more than 32% of the total dielectric loss, regardless of temperature variation. The phenomenon is starkly different from other carbonaceous materials (e.g., carbon spheres, CNT, and SiC), in which the proportion of polarization relaxation loss never exceeds 27%.3,

29

Conductive loss plays a leading role in the

attenuation process. Due to the unique sandwich structure of Si3N4/RGO/Si3N4, the RGO/Si3N4 composite exhibits an unprecedented high polarization relaxation loss, which accounts for the total dielectric loss. Thus, for the RGO/Si3N4, the dielectric loss



remains stable due to the balance between conductivity and polarization relaxation with increasing temperature.

Figure 9. (a) Polarization relaxation loss, (b) conductive loss, and (c) conductive under different temperatures fitted by Debye theory. Figure 10 discloses the mechanism of EM wave absorption in this work. The combination of highly porous structure (open porosity ~75 %) in RGO foam and EM wave transparent for Si3N4 matrix provides RGO/Si3N4 composites with lightweight (bulk density ~0.71 g·cm-3) and well-matching impedance. The excellent impedance match encourages incident EM waves to enter the composites without reflecting on the surface. An existing porous structure, to a large extent, multiple reflections among the RGO/Si3N4 plus the extensive propagation promotes further the dissipation of EM energy.43-44, 46-47 Meanwhile, charge accumulation at the heterogeneous interface generates capacitor systems with a ''sandwich'' architecture like Si3N4/RGO/Si3N4, causing interfacial polarization in the EM field


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and enhancing polarization relaxation subsequently.40,

48

Besides, existing

functional groups (e.g., the oxygen group, carboxyl group, and hydroxyl group), as well as reduction-induced defects can create dipoles in the EM field,49-50 polarization relaxation from which can be much conducive to dissipating EM energy (see Figure 8).51-52 The temperature insensitive behavior of EM wave absorption derives precisely from this significant polarization mechanism during EM wave attenuation. The conductive loss still accounts for a large proportion in the attenuation process. However, due to the particular microstructure, the RGO/Si3N4 composite exhibits an unprecedented high polarization relaxation loss, accounting for 32 % of the total dielectric loss. Further, the incidence of EM wave can stimulate an intense and spontaneous response from the interconnected network that functions as numerous resistance-inductance-capacitance coupled circuits with massive size, in which the induced currents flow on the cell walls of RGO foam.5354

These currents in long-range help rapid attenuation of EM wave and the energy

converted into heat.42, 55

Figure 10. Schematic representation of EM attenuation mechanism of RGO/Si3N4.



4. CONCLUSIONS In this research, preparation procedure consisting of modified freeze drying and reduction annealing was developed to fabricate 3D RGO foams with a parallel layered structure. RGO/Si3N4 composite was subsequently prepared through infiltration of amorphous Si3N4 into the interconnected RGO network by the CVI technique. The EAB of RGO/Si3N4 composite was found to cover the whole X-band at broad sample thickness and the temperature range from 4.3 mm to 4.6 mm, and 323 K to 873 K. Moreover, the RGO/Si3N4 composite exhibited independent temperature stability. When the content of RGO absorbent is as low as 0.16 wt.%, the RCmin can reach -16.5 dB. Conductive loss played a dominant role in the attenuation process. Owing to the unique sandwich-like structure of Si3N4/RGO/Si3N4, the RGO/Si3N4 composite exhibits an unprecedented high polarization relaxation loss, which accounts for 32 % of the whole dielectric loss. The combination of thermal stability and high reliability can grant RGO/Si3N4 composite broadening applications under harsh environments. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author Xiaowei Yin; E-mail: [email protected]; Tel.: +86 29 88494947;


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Fax: +86 29 88494620. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos: 51332004, 51602258), the National Science Fund for Distinguished Young Scholars (Grant Nos: 51725205) and the 111 Project (B08040). REFERENCES (1) Liu, Q.; Cao, Q.; Bi, H.; Liang, C.; Yuan, K.; She, W.; Yang, Y.; Che, R. CoNi@SiO2@TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption. Advanced Materials 2016, 28, 486-489. (2) 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. (3) Xu, H.; Yin, X.; Li, M.; Ye, F.; Han, M.; Hou, Z.; Li, X.; Zhang, L.; Cheng, L. Mesoporous Carbon Hollow Microspheres with Red Blood Cell like Morphology for Efficient Microwave Absorption at Elevated Temperature. Carbon 2018, 132, 343-351. (4) Wu, G.; Cheng, Y.; Yang, Z.; Jia, Z.; Wu, H.; Yang, L.; Li, H.; Guo, P.; Lv, H. Design of Carbon Sphere/Magnetic Quantum Dots with Tunable Phase Compositions and Boost Dielectric Loss Behavior. Chemical Engineering Journal 2018, 333, 519528.



(5) Lv, H.; Liang, X.; Ji, G.; Zhang, H.; Du, Y. Porous Three-Dimensional Flower-like Co/CoO and Its Excellent Electromagnetic Absorption Properties. ACS Applied Materials & Interfaces 2015, 7, 9776-9783. (6) Zhao, B.; Guo, X.; Zhao, W.; Deng, J.; Shao, G.; Fan, B.; Bai, Z.; Zhang, R. YolkShell Ni@SnO2 Composites with A Designable Interspace to Improve the Electromagnetic Wave Absorption Properties. ACS Applied Materials & Interfaces 2016, 8, 28917-28925. (7) Kong, L.; Yin, X.; Han, M.; Yuan, X.; Hou, Z.; Ye, F.; Zhang, L.; Cheng, L.; Xu, Z.; Huang, J. Macroscopic Bioinspired Graphene Sponge Modified with in-situ Grown Carbon Nanowires and Its Electromagnetic Properties. Carbon 2017, 111, 94-102. (8) Luo, H.; Chen, W. B.; Zhou, W.; Long, L.; Deng, L. W.; Xiao, P.; Li, Y. Carbon fiber/Si3N4 Composites with SiC Nanofiber Interphase for Enhanced Microwave Absorption Properties. Ceramics International 2017, 43, 12328-12332. (9) Yin, X.; Kong, L.; Zhang, L.; Cheng, L.; Travitzky, N.; Greil, P. Electromagnetic Properties of Si-C-N based Ceramics and Composites. International Materials Reviews 2014, 59, 326-355. (10) Wen, B.; Cao, M. S.; Lu, M. M.; Cao, W. Q.; Shi, H. L.; Liu, J.; Wang, X. X.; Jin, H. B.; Fang, X. Y.; Wang, W. Z.; Yuan, J. Reduced Graphene Oxides: Light-Weight and High-Efficiency Electromagnetic Interference Shielding at Elevated Temperatures. Advanced Materials 2014, 26, 3484-3489. (11) Li, X.; Yin, X.; Song, C.; Han, M.; Xu, H.; Duan, W.; Cheng, L.; Zhang, L. SelfAssembly Core-Shell Graphene-Bridged Hollow MXenes Spheres 3D Foam with


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