Graphene Shield by SiBCN Ceramic: A Promising High-Temperature

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Graphene Shield by SiBCN Ceramic: A Promising High Temperature Electromagnetic Wave-Absorbing Material with Oxidation Resistance Chunjia Luo, Tian Jiao, Junwei Gu, Yusheng Tang, and Jie Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15365 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Applied Materials & Interfaces

Graphene Shield by SiBCN Ceramic: A Promising High Temperature Electromagnetic Wave-Absorbing Material with Oxidation Resistance Chunjia Luo, Tian Jiao, Junwei Gu, 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, China

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Abstract As cutting-edge emerging electromagnetic (EM) wave-absorbing materials, the Achilles' Heel of graphenes is vulnerable oxidation under high temperature and oxygen atmosphere, particularly at temperatures more than 600 °C. Herein, a graphene@Fe3O4/siliconboron carbonitride (SiBCN) nanocomplex with a hierarchical A/B/C structure, in which SiBCN serves as a “shield” to protect graphene@Fe3O4 from undergoing high temperature oxidation, was designed and tuned by polymerderived ceramic (PDC) route. The nanocomplexes are stable even at 1100–1400 °C in either argon or air atmosphere. Their minimum reflection coefficient (RCmin) and effective absorption bandwidth (EAB) are 43.78 dB and 3.4 GHz at ambient temperature, respectively. After oxidation at 600 °C, they exhibit much better EM wave absorption, where the RCmin decreases to 66.21 dB and EAB increases to 3.69 GHz in X-band. At a high temperature of 600 °C, they also possess excellent and promising EW waveabsorption, which EAB is 3.93 GHz covering 93.6% range of X-band. In comparison to previous works on graphenes, either the EAB or RCmin of these nanocomplexes is excellent at high temperature and oxidation. This novel nanomaterial technology may shed light on the downstream applications of graphenes in EM-wave-absorbing devices and smart structures worked in harsh environments.

Keywords: high temperature electromagnetic absorption, graphene, polymer-derived ceramic, high temperature resistance, anti-oxidation


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Introduction Electromagnetic (EM) waves have been widely used as carriers for the generation, transmission, reception, and processing of information in civilian and military applications. EM wave-absorbing materials are required to be stealth or prevent devices from electromagnetic pollution in radars and aircraft, as well as for radio-frequency identification.1-3 Besides EM wave absorption performance, i.e., low reflection coefficient (RC) and broad effective absorption bandwidth (EAB), EM wave-absorbing materials should be thin, light-weight, and strong, as well as exhibit high-temperature resistance.4,5 Two key factors should be considered for design of EM wave-absorbing materials. (i) Impedance matching. The impedance of a material is approximately equal to that of the air. Therefore, incident EM waves can enter the interior of the material as much as possible rather than be directly reflected on its surface.6 (ii) Attenuation principle. The energy of an EM wave should be attenuated as much as possible in material instead of passing through or transmission.7

Meanwhile, carbon family materials (carbon black, carbon nanotubes (CNTs), graphite flakes, carbon fiber, and graphene) and magnetic materials (ferrite, carbonyl iron, metal-organic-frameworks) have attracted considerable attention in this field because of their desirable electronic conductivity, dielectric properties and other extraordinary properties.8-15 Graphene is a special two-dimensional sheet of sp2bonded carbons. However, the use of graphene in EM wave-absorbing materials is limited because of its high complex permittivity, leading to impedance mismatching between air and materials. The graphene@magnetic nanoparticles can decrease impedance and simultaneously increase magnetic loss



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to achieve high EM wave absorption properties. Thus the graphene-based nanocomposites are regarded to be the cutting-edge emerging EM wave-absorbing materials.16-19 As well known, for stealth aircrafts or aero-engines functioning under harsh environments such as high temperature (>600 °C) and oxidation environment, the high temperature EW wave-absorbing materials with oxidation resistance are urgently required. However, graphenes and graphene-based nanocomposites exhibit relatively poor stability under a high temperature and oxidation conditions. The thermal decomposition temperature of graphene prepared by the chemical reduction of exfoliated graphite oxide is only 300 °C under air, which is completely oxidized or decomposed at 600-700°C.20 It is always their so-called Achilles' Heel.

Ceramic-based materials are considered as the best candidates for high temperature EM wave-absorbing materials.21 Kong and Cao et al. presented ZnO/ZrSiO4 ceramics, Fe-doped SiC/SiO2 monolithic ceramics, and SiC@NiO nanorings as EW wave-absorbing materials at 300-500 °C.22-24 In our previous work,25 Fe-containing PDC-SiBCN ceramics show a wide EAB of 3.2 GHz at 885 °C. For graphenebased composites, the graphenes mixed with silica xerogel shows RCmin of -18 dB and EAB of 4.1 GHz at 200 °C.26-30 The hierarchical graphene/SiC nanowire networks in SiOC ceramic attained a RCmin of −69.3 dB with a thickness of 2.35 mm. At 400 °C, the effective absorption bandwidth reaches 3.9 GHz.31 However, there is no reports on whether and how to achieve EM wave-absorption at high temperature more than 600 °C and in oxidation resistance for graphenes. Polymer-derived ceramics (PDCs) are a series of multifunctional ceramics synthesized by the thermal decomposition of molecule designable preceramic polymers.32-33 Siliconboron carbonitride (SiBCN) ceramic has been reported to


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still maintain stablity at 2000 °C.34-35 Generally, polyborosilazane-derived SiBCN ceramics are amorphous single-phase materials, which exhibit a rather low dielectric constant, making them a reasonably used wave-transparent matrix. By introducing rGO, it can show EM wave-absorption in some degree at room temperature.36 If graphene or its magnetic nanoparticle complex is coated with SiBCN as a “shield”, it is expected to be beneficial to high temperature and oxidation resistance. On the other hand, the transition-metal atoms in magnetic nanoparticle can in situ induce the formation of nanocrystals including turbostratic carbons, silicon carbide (SiC) or metal silicides.37 These nanocrystals can possibly tune the complex permittivity and dielectric loss and the resultant EM wave absorption at ambient and high temperature.

Therefore, to solve the Achilles' Heel of graphenes as cutting-edge emerging EM-wave-absorbing materials, in this contribution, a graphene@Fe3O4/SiBCN nanocomplex with a hierarchical A/B/C structure was designed using PDC route, exhibiting tunable high temperature and oxidation resistance. Besides SiBCN serves as a “shield” to protect graphene@Fe3O4 from undergoing high temperature oxidation, the ternary synergy of A, B, and C phases was beneficial to the excellent EM wave absorption at high temperatures and even oxidation environments. The novel graphene-based nanomaterials possess wide applications in EM wave-absorbing devices and smart structures worked in harsh environments.

Results and Discussion



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Graphene shielded by SiBCN ceramic, i.e., the hierarchical graphene@Fe3O4/SiBCN nanocomplex, was generated from graphene@Fe3O4 (Figure 1a) and liquid hyperbranched polyborosilazane (PBSZ) preceramic precursor via the PDC route (Figure 1i). With the pyrolysis of green body (Figure 1j), a hierarchical structure including 3D Fe3O4 nanoparticles (NPs)-loaded graphene with the surrounding SiBCN ceramic was obtained as illustrated in Figure 1k.

SEM and tapping-mode AFM images clearly revealed hierarchical structures for graphene oxide (GO) and graphene@Fe3O4 (Figure 1b–d). GO prepared from the modified Hummers method exhibited a typical 1.2-nm-thick 2D microscale layer, which was in agreement with single-layered GO.38 Fe3O4 NPs with diameters of 2-10 nm were loaded on GO with a 53% mass fraction as determined by thermogravimetric analysis. The reduction of GO to graphene was simultaneously achieved by a solvothermal approach to form graphene@Fe3O4. The lattice fringe spacing (0.254 nm) of Fe3O4 NPs observed in the HRTEM image (Figure 1e-f) was in good agreement with the lattice spacing of (311) planes of cubic magnetite.39 A sharp diffraction peak at 2θ = 9.44° corresponding to GO was observed in their XRD pattern (Figure 1g), indicative of a large interlayer distance (0.94 nm) due to the formation of hydroxyl and carboxyl groups. After the reduction of GO, the interlayer distance of graphene of graphene@Fe3O4 decreased to 0.36 nm (2θ = 24.75°) due to the removal of organic groups. The disappearance of the diffraction peak observed at 2θ = 9.44° corresponding to GO verified the reduction of GO. Simultaneously, diffraction peaks observed from 2θ values ranging from 20° to 70° were in good agreement with the standard XRD data of magnetite Fe3O4 (JCPDS card, file no. 19-0629). The XPS C


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1s spectrum of Fe3O4 NPs-loaded graphene as another support in Figure S1 (Supporting Information) revealed the remarkable disappearance or decrease in the C–O (286.6 eV) (hydroxyl) and C=O (288.2 eV) (carbonyl) organic groups after reduction.40 Meanwhile, binding energy peaks observed at 711.7 and 725.3 eV corresponded to Fe 2p3/2 and Fe 2p1/2, respectively. By the combination of the magnetic hysteresis loop of graphene@Fe3O4 in Figure 1h, superparamagnetic Fe3O4 NPs-loaded graphene were well prepared with a saturated magnetization of 20.0 emu/g.

Owing to the soluble, meltable feature and the high ceramic yield of the PBSZ preceramic precursor (76% at 1400 °C, Figure S2),41 graphene@Fe3O4 was conveniently complexed with preceramic precursor with evaluated mass fraction. After cross-linking at 400 °C, the milled powders were coldpressed into green bodies at a pressure of 70 MPa (Figure 1j). Subsequent pyrolysis at 1000 °C and annealing at 1100–1300 °C induced the monolithic graphene@Fe3O4/SiBCN nanocomplex with a hierarchical structure, which atom composition and possible formula of SiBCN, e.g. Si1.0B0.1C3.2N0.2 (C3) were summarized in Table 1. Figure S3 shows more detailed information obtained from the XPS core-level spectra. For the representative sample C3, silicon nitride with a Si 2p binding energy of 101.6 eV and a N 1s binding energy of 397.8 eV was detected, as well as graphitic carbon with a C 1s binding energy of 284.6 eV. In addition, silicon carbide was identified by the C 1s binding energy of 283.1 eV and the Si 2p binding energy of 99.7 eV. The B 1s core-level spectrum was fitted with binding energies of 188.3 eV, 190.5 eV, and 192.9 eV, corresponding to B–C, B–N, and B–O, respectively. Owing to the



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absorption of oxygen on the surface, additional peaks at 102.8 eV and 396.9 eV were observed in the Si 2p and N 1s spectrum, corresponding to the Si–O and N–O bonds, respectively.41

Beyond the atom composition, the graphene@Fe3O4/SiBCN nanocomplex exhibited a hierarchical structure. Pure SiBCN ceramics (C5) as A phase show no periodic and amorphous structure (Figure 2a– b). However, the incorporation of graphene@Fe3O4 as B phase induced additional rich structures. First, the electronic diffraction dots in Figure 2d confirmed the presence of crystalline phases in C3. Second, Fe3O4 NPs with an interplanar distance of 0.254 nm were surrounded with turbostratic carbons and graphitic carbons with a clear lattice fringe structure (Figure 2e–f). Third, rich SiC nanocrystals are formed in situ during annealing, with an interplanar distance of 0.261 nm as observed in Figure 2g–h. Therefore, the hierarchical nano-/micro-structure can be described as an A/B/C structure (Figure 2i-j). The amorphous SiBCN is A phase and graphene@Fe3O4 is B phase. The C-phase is in situ generated turbostratic carbons, SiC nanocrystals, and Fe3Si nanocrystals from SiBCN.

The hierarchical A/B/C structure of graphene@Fe3O4/SiBCN nanocomplex, especially the C phase, can be proved via the fine XRD and Raman spectrum analyses in Figure 3. At a low annealing temperature (C1), there are no peaks in the XRD curves, which indicates the materials are amorphous. When the annealing temperature increases, the material begin to crystallize in some regions. At a high annealing temperature (C4), diffraction peaks are observed at 2θ = 35.6°, 60.2°, and 72.3°, corresponding to the (111), (220), and (311) crystal planes of the β-SiC phase (JCPDS card no.29-1129), respectively.42 The


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same trend can also be observed in Figure 3b, the pure SiBCN ceramic C5 were still amorphous. With the mass fraction of graphene@Fe3O4 increases, the β-SiC crystal peak appears and becomes sharper and sharper. At a high mass fraction of graphene@Fe3O4 (C7), the Fe3Si nanocrystals were formed in situ, with a diffraction peak at 2θ = 45.5°, as well as the (110) crystal plane (ICDD data file 35-0519).43 Furthermore, a broad non-crystalline peak observed at 2θ values between 20° and 30° is corresponded to graphite carbons, indicative of amorphous carbon phases. From the XRD results, the increase of annealing temperature and the introduction of graphene@Fe3O4 (B phase) can promote the formation of nanocrystals as C phase in SiBCN matrix (A phase) according to a solid–liquid–solid mechanism.44 It promotes the formation of crystals and reduce the crystal temperature. In this case, the iron atoms react with amorphous Si–C phase at 1100 °C and form the so-called eutectic liquid. When the eutectic liquid is saturated, the Fe3Si nuclei can be generated and solidified, leading to the the formation of Fe3Si nanocrystals.

From Figure 3c-d, all of the graphene@Fe3O4/SiBCN nanocomplexes exhibited two representative peaks, where the D peak around 1340 cm-1 is related with amorphous carbons, disordered carbons or defects in graphite and the G peak around 1560 cm-1 represents the E2g symmetry involving in-plane bond-stretching motion of pairs of sp2 carbons.

45

After the Raman curves are fitted by Gaussian-

Lorentzian curve, the detailed information is shown in Table S1, including the peak position, full width at half maximum (FWHM) and intensity ratio (ID/IG). At a low annealing temperature (C1) or a low mass fraction of graphene@Fe3O4 (C5), the G and D peak are broad and overlapped, which indicates



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most free carbon in disordered state. With the increase of annealing temperature and mass fraction of graphene@Fe3O4, a broad peak appeared at around 2750 cm-1 containing second-order G' and G+D peaks. The ID/IG intensity ratio decreases with the increase of annealing temperature and mass fraction of graphene@Fe3O4, which incidates that the increase of annealing temperature and mass fraction of graphene@Fe3O4 promot the conversion from amorphous carbon to turbostratic carbons. Therefore, βSiC and Fe3Si verified by XRD and the turbostratic carbons verified by the clear G peak in the Raman spectrum further confirmed the B phase in the A/B/C structure observed in Figure 2i–j.

Interestingly, the degree of crystallization of graphene, Fe3O4, in situ generated β-SiC, Fe3Si, and turbostratic carbons can be calculated using the XRD diffraction peak intensity. In combination with the ID/IG intensity ratio obtained from the Raman spectrum, the evolution of the hierarchical A/B/C structure of the graphene@Fe3O4/SiBCN nanocomplex was quantitatively verified (Figure 3e–f, Table S1). Generally, the high annealing temperature and mass fraction of graphene@Fe3O4 led to the clear crystallization of the nanocomplex. At an annealing temperature of greater than 1200 °C (C3–C4, 0.3%), crystallization occurred, and ID/IG decreased to 2.09, indicative of the high sp2 carbon content. The degrees of crystallization for C3 and C4 were 7.22% and 11.98%, respectively. At a graphene@Fe3O4 mass fraction of greater than 0.1% (C6, C3, and C7, 1200 °C), crystallization also occurred, and ID/IG decreased to 2.01. The degree of crystallization was 8.89% for C7. Thus, the degree of crystallization and the sp2 carbon content of C6, C3, C7, and C4 sequentially increase. As the crystallization of graphene@Fe3O4/SiBCN induced the formation of the A/B/C structures (Figure 2), in other words, the


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A and B phase content was conveniently tuned only by utilizing the mass fraction of graphene@Fe3O4 and annealing temperature.

As mentioned above, the hierarchical A/B/C structure of the graphene@Fe3O4/SiBCN nanocomplex can be tuned by utilizing the mass fraction of graphene@Fe3O4 and annealing at high temperature. The SiBCN (A phase) can be employed as “shield” to protect graphene@Fe3O4 from high temperature oxidation. Graphene@Fe3O4 (B phase) can simultaneously induce the formation of various nanocrystals in situ, including turbostratic carbons, SiC nanocrystals, and Fe3Si nanocrystals (C phase) from the SiBCN matrix, exhibiting verified dielectric characteristics for the nanocomplex. The ternary synergy of A, B, and C phases is expected to benefit their high temperature and oxidation resistance and EM wave absorption at ambient or high temperature.

Figure 4 shows the TGA curves of the graphene@Fe3O4/SiBCN nanocomplex and graphene@Fe3O4 under argon and air. Even under argon (Figure 4a), graphene@Fe3O4 exhibited a considerable mass loss beyond 200 °C. The first thermolytic degradation of ~15% was observed at 200–400 °C, corresponding to the release of organic groups. The second main weight loss was observed at 800 °C, mainly corresponding to the degradation of graphene. Under air atmosphere, graphene@Fe3O4 exhibited a total degradation in the range of 200-450 °C. The graphene will be oxidized due to oxygen in the air. The graphene@Fe3O4 shows poor stability under a high temperature and oxidation conditions. However, after coating with SiBCN, mass loss (≥1%) was not observed until 1400 °C for the



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graphene@Fe3O4/SiBCN nanocomplex under argon atmosphere. Furthermore, under air with oxygen, the graphene@Fe3O4/SiBCN nanocomplex was stable at a temperature of less than 1100 °C. With increasing temperature to 1100–1400 °C, a mass increase of only ~3% was observed owing to the formation of oxides on the SiBCN surface, such as B2O3 and SiO2. These oxides can serve as a protective layer even at a considerably higher temperature.34,46 Thus, the SiBCN (A phase) serves as a “shield” and prevents graphene@Fe3O4 from high temperature and oxidation.

For tuning the EM wave absorption of the graphene@Fe3O4/SiBCN nanocomplex, the complex permittivity

and dielectric loss tangent (

the Debye theory, the real permittivity permittivity

) are key parameters. According to

representing polarization relaxation and the imaginary

and dielectric loss tangent (tanδ) are related to the dissipation of EM waves in

materials. Typically, the high dielectric loss is beneficial for the EM wave absorption, however, a high real part of complex permittivity can decrease impedance matching. From the complex permittivity of the graphene@Fe3O4/SiBCN nanocomplexes (C1–C7, Figure S4), the real part of permittivity, as well as imaginary part of permittivity and dielectric loss tangent, increased with the annealing temperature. The extent of increase for the imaginary part was considerably greater than that of the real part; hence, the dielectric loss (Figure S4e) increases from 0.01 to 0.62. At the same annealing temperature (1200 °C), the complex permittivity and tangent loss of ceramic significantly increased with the mass fraction of graphene@Fe3O4. These results revealed that the ternary synergy of the A, B, and C phases


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of the graphene@Fe3O4/SiBCN nanocomplex can be used to tune complex permittivity and EM wave absorption.

Figure 5 shows the RC of the monolithic graphene@Fe3O4/SiBCN nanocomplexes (C1–C7) with a thickness of 2.58 mm. For C1 and C2, the EM wave absorption was extremely weak (RC>10 dB), and an absorption peak was not clearly observed in the entire X-band. C3 exhibited an RCmin of 31.50 dB at 9.83 GHz with an EAB of 3.3 GHz (8.58–11.79 GHz). Compared to the reflection coefficients of C5, C6, and C7, C3 still exhibited the best EM wave absorption performance. As can be clearly observed in Figure 3, the degree of crystallization and the sp2 carbon content of C6, C3, C7, and C4 sequentially increased. Here, the hierarchical nanocomplex C3 with a suitable A/B/C structure, i.e., 7.22% crystallization, exhibited optimal EM wave absorption performance. In addition, RC depended on the sample thickness (Figure 5b-d). At a C3 sample thickness of 2.15 mm, RCmin reached 43.78 dB at 12.25 GHz with an EAB of 2.06 GHz. At a thickness of 2.5 mm, its EAB reached 3.4 GHz, covering 81% of the X-band range, with an RCmin of 30.78 dB at 10.19 GHz. With increasing thickness, RCmin shifted to a lower frequency, implying that EM wave attenuation efficiency at the desirable frequency is achieved by adjusting the thickness of the monolithicgraphene@Fe3O4/SiBCN nanocomplex.47

As the graphene@Fe3O4/SiBCN nanocomplex exhibited excellent high temperature and oxidation resistance under argon and air as proven in Figure 4, can the EM wave absorption be affected by air oxidation and high temperature? The representative C3 sample was transferred into a tube furnace for



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oxidation at 300 °C and 600 °C for 2 h under air. Figure 6 shows the complex permittivity and reflection coefficient of C3 after oxidation. After oxidation at 300 °C, 8.54–9.69, and

changed from 8.78–9.47 to

changed from 3.81–4.31 to 3.19–4.22. Hence, RCmin reaches 56.98 dB at 12.27 GHz

with an EAB of 1.97 GHz. This value is slightly better than the RCmin value of the original C3 sample (43.78 dB). Notably, the EM wave absorption was considerably better after oxidation at 600 °C for 2h, where RCmin decreased to 66.21 dB at 11.01 GHz and EAB increased to 3.69 GHz (from 9.71 to 12.4 GHz). It is related to the better impedance matching. Previously, at a frequency of 10 GHz and a thickness of 2.86 mm, the optimum real and imaginary parts of permittivity have been reported to be equal to 7.3 and 3.3, respectively, for obtaining the lowest RC. 48 Owing to the SiBCN on the surface of graphene@Fe3O4, a few passive oxide layers, such as SiO2 and B2O3, were formed to protect the inner layers from further oxidation.34,46 SiO2 is a typical low dielectric material. Thus, the real and imaginary parts of permittivity are decreased (Figure 6ab), which is beneficial for the impedance matching as well as enhancement of the EM wave absorption performance.

As described above, the graphene@Fe3O4/SiBCN nanocomplex exhibited excellent EM wave absorption performance even after oxidation. Hence, this complex demonstrates potential as heat parts in the aeroengine spur. Under harsh environments, what is the EM wave absorption performance of the graphene@Fe3O4/SiBCN nanocomplex? Figure 7 shows the RC values of the representative

C3

measured at 100–600 °C in sequence. Compared to the RC measured at 100°C, RC increased the measurement temperature. The EAB was maintained constant at all measurement temperatures. At the


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highest temperature of 600 °C, RCmin was 13.95 dB at 11.40 GHz with an EAB of 3.23 GHz for the monolithic sample having a thickness of 2.14 mm, revealing excellent high-temperature EM wave absorption performance, covering the 76.9% range of the X-band. Furthermore, the C6 sample with low EM wave absorption was also selected (Figure 5a). Notably, with increasing measurement temperature, RCmin decreased from 14.09 dB to 45.63 dB (Figure S5). The EAB increased from 1.95 GHz at 100 °C to 3.93 GHz at 600 °C, covering 93.6% range of the X-band. It reveals that the monolithic graphene@Fe3O4/SiBCN nanocomplex with optimal EM wave absorption at ambient temperature exhibits excellent high-temperature EM wave absorption. On the other hand, the monolithic sample with low EM wave absorption at ambient temperature also exhibited considerably higher EM wave absorption at elevated temperature.

The excellent high-temperature EM wave absorption was related to the synergistic effect of complex permittivity, dielectric loss, and electric conductivity for the special graphene@Fe3O4/SiBCN nanocomplex at elevated temperature. First,

,

, and tan δ increased with temperature (Figure 7a–b

and Figure S6) and decreased with increasing frequency at elevated temperature. The independence of the complex permittivity on temperature can be explained by the Debye theory,29

 '     '' 

s   1   2 (T ) 2

s    (T )  (T )  (T )   2 1  [ (T ) ] 2 0 f 2 0 f


(1)

(2)


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where ω is the angular frequency; τ(T) is the temperature-dependent relaxation time; εs is the static permittivity; ε∞ is the relative dielectric permittivity at a high frequency limit; σ(T) is temperaturedependent electrical conductivity; and ε0 is the dielectric constant in vacuum. Similar to graphites,49 the SiBCN phase also exhibits semiconductor conducting behavior, and σ(T) increased with temperature.25 For the graphene@Fe3O4/SiBCN nanocomplex, all of their nanophases possibly formed numerous grain boundaries, leading to interfacial polarization, associated relaxation, and multiple reflections of EM waves at high temperature. Therefore, the increased σ(T) of the graphene@Fe3O4/SiBCN nanocomplex . In addition, the increased σ(T) indicated that migrating

is the main reason for the increase in

electrons can be generated in the B and C phases, including graphene, turbostratic carbons, SiC nanocrystals, and Fe3Si nanocrystals, leading to electron polarization at high temperature. Hence, the relaxation time τ(T) decreases because of the increasingly strong electron polarization at high temperature,29 leading to the increase of

. Second, for C3 with optimized EM wave absorption at

,

, and tan δ was not beyond impedance matching; hence,

ambient temperature, the increase of

desirable absorption is observed at 600 °C. For C6 with low EM wave absorption at ambient temperature, the increase of

,

, and tan δ, on the contrary, is beneficial for impedance matching

because of their extremely low values (Figure S4d and f). In comparison to previous works on graphenes as illustrated in Table 2,22,23,25-31,50-62 either the EAB of 3.93 GHz measured at 600 °C (HT) or the RCmin of 66.21 dB after oxidation at 600 °C (O) in this work are excellent. Even compared to other

materials,

e.g.

CNT-ZnO/glass

and

Fe3O4-MWCNTs-SiO2

in

Table

2,

the

graphene@Fe3O4/SiBCN nanocomplex also show excellent electromagnetic wave absorption at high-


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temperature, demonstrating immense potential as advanced EM absorption materials in harsh environments.

Conclusion In summary, the graphene@Fe3O4/SiBCN nanocomplex with a special hierarchical A/B/C structure was designed and tuned via the PDC route by using graphene@Fe3O4 and a liquid PBSZ preceramic precursor. The ternary synergy of the A, B, and C phases was beneficial to high temperature and oxidation resistance and EM wave absorption. At 1000-1400 °C, even under air, only ~3% mass increase was observed. The SiBCN (A phase) served as a “shield” and prevented graphene@Fe3O4 from high temperatures and oxidation. Even after oxidation at 600 °C, the nanocomplex exhibited considerably better EM wave absorption performance, where RCmin decreased to 66.21 dB and EAB increased to 3.69 GHz. At a high measurement temperature of 600 °C, they exhibited excellent hightemperature EM wave absorption covering 93.6% range of the X-band, setting a new record for graphene-based EM wave-absorbing materials at high-temperature. As the liquid PBSZ preceramic precursor comprises UV-curable vinyl groups, hence, this promising graphene@Fe3O4/SiBCN nanocomplex is useful for 3D printing technology to form complex heat parts in aero-engine spur. Thus, the hierarchical graphene@Fe3O4/SiBCN nanocomplex with ternary synergy demonstrates immense potential in radomes, aero engines, and stealth aircraft functioning under harsh environments.

Experimental Section ACS Paragon Plus Environment


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Materials: Borane-dimethyl sulfide complex (2.0 M in tetrahydrofuran), dichloromethylvinylsilane (DCMVS), dichloromethylsilane (DCMS), hexamethyldisilazane (HMDZ), and diethylene glycol (DEG, 99%) were purchased from Alfa Aesar China (Tianjin, China). Anhydrous iron(III) chloride (FeCl3, 97%) and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Anhydrous tetrahydrofuran (THF) was freshly distilled for use under reflux using sodium/benzophenone, and all other reagents were used as received. The liquid preceramic precursor of hyperbranched polyborosilazane was synthesized according to our previous study.41

Preparation of graphene@Fe3O4: Graphene oxide was synthesized by a modified Hummers method as reported before.24 The concentrated H2SO4 (8 mL) was added into a flask filled with graphite (1 g), K2S2O8 (1.5 g) and P2O5 (1.5 g), and the solution was heated to 80 °C for 6 h under magnetic stirring. After being dried at room temperature, the resultant preoxidized product was mixed with 98% H2SO4 (40 mL) and KMnO4 (5 g) for 2h at 35 °C. Then deionized water (100 mL) and H2O2 (5 mL, 30 wt%) was added to the solution. The bright yellow resulting mixture was washed by hydrochloric acid and deionized water, and the graphite oxide was obtained. The obtained graphene oxide was dispersed in the DEG solvent at a concentration of 1.5 mg/mL and then subjected to sonication. The reduction of graphene oxide and formation of Fe3O4 nanoparticles were achieved in one step via a facile solvothermal approach as has been reported before by the He group.39 Typically, NaOH (200 mg) was added into DEG (20 mL) at 120 °C, stirred for 1 h under Ar, and cooled to 70 °C to produce a NaOH/DEG stock solution. Then, FeCl3 (120 mg) was added into the GO/DEG solution (20 mL) and


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stirred for 1 h. Next, the mixture was heated to 220 °C for 30 min under an argon flow, and injected 70 °C NaOH/DEG stock solution (5 mL) rapidly into the above hot mixture. The resulting mixture was further heated at 220 °C for 1 h. The final product of graphene@Fe3O4 was separated by vacuum filtration and washed by ethanol.

Prepare of hb-PBSZ precursor: The synthesis of hyperbranched polyborosilazane (hb-PBSZ) has been reported in our previous work.41 The reactions were all carried out under argon atmosphere. First, the borane dimethylsulfide solution (1.2 mL) was dropwise addded into DCMVS (9.5 mL) in an 100 mL flame-dried flask in ice/water bath. After stirring 24 h, the DCMS (2.5 mL) and HMDZ (27.8 mL) was added. The temperature was heated to 180 °C for 2 h to further reaction. After removing byproducts by distillation equipment, the yellow precursors were obtained as hb-PBSZ.

Preparation

of

the

graphene@Fe3O4/SiBCN

nanocomplex:

The

obtained

precursor

and

graphene@Fe3O4 (0.1 wt%, 0.3 wt%, 0.5 wt%) were dissolved in anhydrous THF, respectively. Next, the solutions were mixed and stirred overnight to achieve a uniform mixture with different contents of graphene@Fe3O4. After the solvent was evaporated, the mixture was first cross-linked at 400 °C for 4 h under argon protection. The cured product was ball-milled for 4 h and passed through a 200-mesh sieve. The powders were cold-pressed into green bodies at a pressure of 70 MPa. Finally, the green bodies were pyrolyzed at 1000 °C for 4 h at a heating rate of 2 °C/min and then annealed at 1100 °C, 1200 °C,



Page 20 of 41

and 1300 °C for 4 h at a heating rate of 5 °C/min. The pyrolysis and annealing processes were carried out in argon atmosphere.

Characterization. The analyses procedures of SEM, TEM, atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and magnetization were described in Supporting Information. The relative complex permittivity of samples with dimensions of 22.86 × 10.16 × 2.58 mm3 was measured using a vector network analyzer (VNA, MS4644A, Atsugi, Japan) in the frequency range of 8.2–12.4 GHz (X band) according to ASTM D5568-08. Based on the generalized transmission line theory and metal back-panel model,63,64 the RC was calculated according to the following equations: RC  20 log10

Z in 

Z in  1 Z in  1

(3)

r  2fd  tanh  j r r  r  c 

(4)

where Zin, d, and μr are the normalized input impedance, thickness, and permeability of the material, respectively; c is the light velocity in vacuum; and f is the microwave frequency. The lower RC value implies better microwave absorption properties. Typically, an RC of less than −10 dB implies greater than 90% of the microwave energy can be absorbed, which is an important baseline for EM-waveabsorbing materials.

Associated Content Author Information


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*Corresponding Authors, E-mail Address: [email protected] (J.K.), Tel.(fax): +86-29-88431976.

Acknowledgements This research is supported by the National Natural Science Foundation of China (21875190), the Natural Science Basic Research Plan in Shaanxi Province of China (2018JC-008, Distinguished Young Scholar), the Shaanxi Province Key Research and Development Plan for Industry Innovation Chain (Cluster) (2018ZDCXL-GY-09-07). We would like to thank the Analytical & Testing Center of NPU for supporting.

Conflict of Interest The authors declare no conflict of interest.

Supporting Information TGA-mass curves of polymers, XPS spectra and reflection coefficient of ceramics. The material is available free of charge via the Internet at http://pubs.acs.org.

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Figures and Table Captions Figure 1 Schematic illustration of graphene@Fe3O4 and their SiBCN ceramic complex, (a) structure illustration of graphene@Fe3O4 and (b) SEM image of GO, (c) AFM image of GO(tapping mode), (d) SEM image of graphene@Fe3O4, (e) TEM lattice structure of graphene@Fe3O4, (f) electron diffraction pattern of graphene@Fe3O4, (g) XRD patterns of GO and graphene@Fe3O4, (h) plots of magnetization versus the external magnetic field at 300 K of graphene@Fe3O4, (i) soluble and meltable hyperbranched polyborosilazane, (j) green body from graphene@Fe3O4 and precursor and (k) graphene@Fe3O4/SiBCN complex. Figure 2 TEM images and electron diffraction patterns of graphene@Fe3O4/SiBCN complex, (a-b) pure SiBCN ceramic (C5), (c-h) graphene@Fe3O4/SiBCN complex (C3), (i-j) schematic illustration of graphene@Fe3O4/SiBCN complex and embodied nanocrystals. Figure 3 Powder XRD patterns of graphene@Fe3O4/SiBCN complex, (a) annealing at different temperature, (b) with different mass fraction of graphene@Fe3O4, Raman spectra of graphene@Fe3O4/SiBCN complex, (c) annealing at different temperature, (d) with different mass fraction of graphene@Fe3O4, (e) plots of crystallinity versus annealing temperature or mass fraction of graphene@Fe3O4, (f) plots of ID/IG in Raman spectrum versus annealing temperature or mass fraction of graphene@Fe3O4. Figure 4 TGA thermograms of graphene@Fe3O4 and graphene@Fe3O4/SiBCN complex at a scanning rate of 10 K/min, (a) in argon atmosphere and (b) in air atmosphere. Figure 5 Reflection coefficient of graphene@Fe3O4/SiBCN complex in X-band, (a) all the samples with a thickness of 2.58 mm, (b) the sample C3 (0.3 wt%, 1200 °C) with different thickness, (c-d) multidimensional image of C3 with different thickness, (e) the illustration of EM wave absorption mechanism for graphene@Fe3O4/SiBCN complex. Figure 6 Complex permittivity and reflection coefficient of graphene@Fe3O4/SiBCN complex after oxidation at 300 or 600 oC for 2h in air, (a) real permittivity and imaginary permittivity, (b) loss tangent and (c) reflection coefficient in X-band. Figure 7 Complex permittivity and reflection coefficient of graphene@Fe3O4/SiBCN complex (C3) measured at high temperature, (a-c) dependence of real permittivity, imaginary permittivity and loss tangent on temperature (100-600 oC), (d) reflection coefficient with a minimum RCmin value measured at 100-600 oC, (e) reflection coefficient with a maximum effective absorption bandwidth measured at 100600 oC, (f) multi-dimensional reflection coefficient image of C3, (g) a contrast of EM wave absorption properties of graphenes-based materials.


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Table 1 Structure parameters of graphene@Fe3O4/SiBCN complex and SiBCN ceramics derived from preceramic precursors. Table 2 Summary of high-temperature wave EM absorption materials.



(a)

(c)(c)

(b) (b)

1.2nm

5 μm

0.1μm

(f)(f)

(e)(e)

(d)(d) (i)

0.254 nm

(j) 200 200 nmnm

20

(440)

(422) (511)

(400)

(220)

(C002)

(111)

(311)

(C001)

(g)

graphene@Fe3O4 GO 10

20

30

40

50

60

2 theta (degree)

70

80

Magnetization (emu/g)

(k)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

(h)

15 10 5 0 -5 -10 -15 -20

-10000

-5000

0

5000

10000

External Magnetic Field (Oe)

Figure 1 Schematic illustration of graphene@Fe3O4 and their SiBCN ceramic complex, (a) structure illustration of graphene@Fe3O4 and (b) SEM image of GO, (c) AFM image of GO (tapping mode), (d) SEM image of graphene@Fe3O4, (e) TEM lattice structure of graphene@Fe3O4, (f) electron diffraction pattern of graphene@Fe3O4, (g) XRD patterns of GO and graphene@Fe3O4, (h) plots of magnetization versus the external magnetic field at 300 K of graphene@Fe3O4, (i) soluble and meltable hyperbranched polyborosilazane, (j) green body from graphene@Fe3O4 and precursor and (k) graphene@Fe3O4/SiBCN complex.


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(b)

(a)

(c)

(f)

(e) turbostratic carbon

(d)

(h)

(g)

0.261nm

0.254 nm 2 nm

2 nm SiC

Fe3O4 NP 10 nm

(j)

(i)

Fe3O4 NPs SiBCN ceramic matrix

SiC nanocrystals

ordered graphiticcarbons

Figure 2 TEM images and electron diffraction patterns of graphene@Fe3O4/SiBCN complex, (a-b) pure SiBCN ceramic (C5), (c-h) graphene@Fe3O4/SiBCN complex (C3), (i-j) schematic illustration of graphene@Fe3O4/SiBCN complex and embodied nanocrystals.



(a)

 SiC





 SiC  Fe

(b)

C















C4 C3 C2

Si

3





Intensity (a.u.)

Intensity (a.u.)

 

C



C7 C3 C6

C1 20

30

40

50

60

70

C5 20

80

30

40

(d)

(c)

G' G+D ID/IG=2.09

C4

ID/IG=2.12

C3

ID/IG=2.21

C2

ID/IG=2.37

C1

500

1000

D

G

1500

2000

2500

3000

3500

Intensity (a.u.)

Intensity (a.u.)

D

50

80

G

C7

ID/IG=2.12

C3

ID/IG=2.18

C6

ID/IG=2.20

C5

500

4000

1000

1500

2000

2500

3000

3500

4000

-1

Raman shift (cm )

Raman shift (cm )

14

2.4

(e) 10

70

ID/IG=2.01

-1

12

60

2 theta (degree)

2 theta (degree)

(f) Anealled at o 1200 C

rGO@Fe3O4 0.3%

Anealled at o 1200 C

rGO@Fe3O4

2.3

0.3%

8

ID/IG

Crystallinity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

6

2.2

4 2.1 2 0 1000 1100 1200 1300 0.0 0.1 0.2 0.3 0.4 0.5 o

Anealing Temperature ( C) Mass Fraction (%)

2.0 1000 1100 1200 1300 0.0 0.1 0.2 0.3 0.4 0.5 o

Anealing Temperature ( C) Mass Fraction (%)

Figure 3 Powder XRD patterns of graphene@Fe3O4/SiBCN complex, (a) annealing at different temperature, (b) with different mass fraction of graphene@Fe3O4, Raman spectra of


Page 33 of 41

graphene@Fe3O4/SiBCN complex, (c) annealing at different temperature, (d) with different mass fraction of graphene@Fe3O4, (e) plots of crystallinity versus annealing temperature or mass fraction of graphene@Fe3O4, (f) plots of ID/IG in Raman spectrum versus annealing temperature or mass fraction of graphene@Fe3O4.

110

110

(a)

100

90

90

Mass (%)

100

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60

80 70 60

In Argon

50

(b)

In Air

graphene@Fe3O4

50

graphene@Fe3O4/SiBCN

40 200 400 600 800 100012001400 o

Temperature ( C)

graphene@Fe3O4 graphene@Fe3O4/SiBCN

40 200 400 600 800 100012001400 o

Temperature ( C)

Figure 4 TGA thermograms of graphene@Fe3O4 and graphene@Fe3O4/SiBCN complex at a scanning rate of 10 K/min, (a) in argon atmosphere and (b) in air atmosphere.


-5 -10 -15

11.79GHz C1 C2 C3 C4 C5 C6 C7

-20

-30

8.58GHz EAB=3.2GHz

-35 9

10

11

Reflection Coefficient (dB)

(a)

-25

Page 34 of 41

(b)

0

0

12.23GHz -10

-20

8.83GHz

-30

d=2.15 mm d=2.30 mm d=2.40 mm d=2.50 mm d=2.58 mm

-40

-43.78 dB

-50 9

12

10

11

12

Frequency (GHz)

Frequency (GHz)

C3 at 25 oC

4.0

0.000

(d)

-5.000

3.5

Thickness (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



-10.00 -15.00

3.0 -20.00

3.4 GHz 2.5

-25.00

2.50 mm

-30.00

-43.78 dB

2.0 2.15 mm

-35.00 -40.00

1.5 9

10

11

12

Frequency (GHz)

(e) Incident

O OH

OH Reflected

carbon dipole Multi-reflection

Defect polarization

Figure 5 Reflection coefficient of graphene@Fe3O4/SiBCN complex in X-band, (a) all the samples with a thickness of 2.58 mm, (b) the sample C3 (0.3 wt%, 1200 °C) with different thickness, (c-d) multi-


Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dimensional image of C3 with different thickness, (e) the illustration of EM wave absorption mechanism for graphene@Fe3O4/SiBCN complex.



0.6

12

(b)

(a) 0.5 o

'-25 C

8

Loss Tangent

Complex Permittivity

10

o

'-oxidated at 300 C o

'-oxidated at 600 C

6 4

0.4 0.3 0.2

o

25 C o oxidated at 300 C o oxidated at 600 C

o

''-25 C

2

0.1

o

''-oxidated at 300 C o

''-oxidated at 300 C

0

9

10

11

0.0 9

12

10

11

12

Frequency (GHz)

Frequency (GHz)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

0

(c)

-10 -20 -30 -40

-43.78 dB

o

-50 -60 -70

originial sample at 25 C o oxidated at 300 C o oxidated at 600 C

8.5

9.0

9.5

10.0

-56.98 dB

-66.21 dB

10.5

11.0

11.5

12.0

Frequency (GHz)

Figure 6 Complex permittivity and reflection coefficient of graphene@Fe3O4/SiBCN complex after oxidation at 300 or 600 oC for 2h in air, (a) real permittivity and imaginary permittivity, (b) loss tangent and (c) reflection coefficient in X-band.


Real Permittivity

11

10 8.2 GHz 9.2 GHz 10.2 GHz 11.2 GHz 12.2 GHz

9

8

8 7 6

0.5

5 8.2 GHz 9.2 GHz 10.2 GHz 11.2 GHz 12.2 GHz

4

100 200 300 400 500 600 o

Temperature ( C) Reflection Coefficient (dB)

(d)

Temperature ( C)

0

(e)

-10

-10

at 100 oC at 200 oC at 300 oC at 400 oC at 500 oC at 600 oC RC 9

10

-20

-30

-40

=-42dB

min

11

8.90GHz EAB=3.5GHz 9

12

10

11

at 100 oC at 200 oC at 300 oC at 400 oC at 500 oC at 600 oC 12

Frequency (GHz)

Frequency (GHz) 4.4

70

(g)

4.2

EAB (GHz)

60

This work (O)

4.0 3.8

50

This work (HT)

3.6

40

3.4

30

3.2 20 EAB Max Absorption

3.0 2.8

0

100

200

300

400

500

600

Max Absorption (dB)

-40

0.3

o

Temperature ( C)

-30

8.2 GHZ 9.2 GHZ 10.2 GHZ 11.2 GHZ 12.2 GHZ

0.4

100 200 300 400 500 600

o

-20

(c)

0.6

3

100 200 300 400 500 600

0

0.7

(b)

Loss Tangent

(a)

12


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Imaginary Permittivity

Page 37 of 41

10

o

Measurement Temperature ( C)

Figure 7 Complex permittivity and reflection coefficient of graphene@Fe3O4/SiBCN complex (C3) measured at high temperature, (a-c) dependence of real permittivity, imaginary permittivity and loss tangent on temperature (100-600 oC), (d) reflection coefficient with a minimum RCmin value measured at



Page 38 of 41

100-600 oC, (e) reflection coefficient with a maximum effective absorption bandwidth measured at 100600 oC, (f) multi-dimensional reflection coefficient image of C3, (g) a contrast of EM wave absorption properties of graphenes-based materials.


Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Structure parameters of graphene@Fe3O4/SiBCN complex and SiBCN ceramics derived from preceramic precursors. Sample

Mass fraction of Anealing graphene@Fe3O4 temperature

Crystallinity

Atom composition (%) B C N 2.6 52.4 4.9

Formula

C1

0.3 wt %

1000 °C

0.00 %

Si 18.2

C2

0.3 wt %

1100 °C

0.00 %

/

/

/

/

/

/

C3

0.3 wt %

1200 °C

7.22 %

16.8

2.2

53.7

4.1

23.3

Si1.0B0.1C3.2N0.2

C4

0.3 wt %

1300 °C

11.97 %

/

/

/

/

/

/

C5

0.0 wt %

1200 °C

0.00 %

21.1

1.6

45.9

4.3

27.1

Si1.0B0.1C2.2N0.2

C6 C7

0.1 wt % 0.5 wt %

1200 °C 1200 °C

4.00 % 8.89 %

17.4 12.9

2.6 2.3

51.5 59.3

4.9 3.9

23.6 21.6

Si1.0B0.2C3.0N0.3 Si1.0B0.2C4.6N0.3


O 21.6

Si1.0B0.14C2.9N0.27


Page 40 of 41

Table 2 Summary of high-temperature wave EM absorption materials.

Material MWCNTs/polyimide

Room temperature

High temperature

RCmin (dB)

EAB (GHz)

T/ oC

/

1.5

25-300

-29 at 100 oC oC

EAB (GHz) 2.04

50

Nickel chains/SiO2

/

/

50-300

-50 at 100

Graphene/silica

-22

3.4

50-200

-42 at 140 oC

50-200

Fe3O4-MWCNTs/SiO2

-24.8

~3.4

Ref.

RCmin(dB)

4.2

51

~1.9

30

-16 at 200

oC

~3

27

oC

2 (

Graphene Shield by SiBCN Ceramic: A Promising High-Temperature

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