SiBCN composite ceramics

SiBCN composite ceramics

Ceramics International xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate...

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Microstructure and electromagnetic wave absorption property of reduced graphene oxide-SiCnw/SiBCN composite ceramics Chaokun Song, Yongsheng Liu∗, Fang Ye, Jing Wang, Laifei Cheng Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China

ARTICLE INFO

ABSTRACT

Keywords: RGO-SiCnw/SiBCN PDC CVI Dielectric property Electrimagnetic wave absorption characteristic

In this account, RGO-SiCnw/SiBCN composite ceramics were fabricated using polymer derived ceramic (PDC) combined with chemical vapor infiltration (CVI) technology. Dielectric property of as-obtained RGO-SiCnw/ SiBCN composite ceramics was significantly enhanced thanks to established conductive pathway through overlapped nanoscale SiCnw and micro-sized RGO. The minimum RC of composite ceramics with 0.5 wt% GO and 2.29 wt% SiCnw at thickness of 3.6 mm reached -42.02 dB with corresponding effective absorption bandwidth (EAB) of 4.2 GHz. As temperature rose from 25 to 400 °C, permittivity increased with enhanced charge carrier density and then it decreased due to oxidation process of RGO from 400 to 600 °C. The minimum reflection coefficient (RC) was recorded as -39.13 dB and EAB covered the entire X-band at 600 °C. EMW absorption ability was evaluated after high-temperature oxidation experiment under protective effect of wavetransparent Si3N4 coating. RGO-SiCnw/SiBCN composite ceramics maintained outstanding EMW absorption ability with minimum RC of -10.41 dB after oxidation at 900 °C, indicating RGO-SiCnw/SiBCN composite ceramics with excellent EMW absorption characteristic even at high temperatures and harsh environments.

1. Introduction With the development of advanced military detection technologies and introduction of hypersonic precision-guided weapons, the discovery of weaponized equipment have become crucial in modern battlefields [1,2]. Therefore, the development of stealth technologies and reduction of exposure signs during the service of weapons and equipment are important for improving their survival capabilities. The stealth abilities of materials could effectively be improved by target shape design, radar absorbing materials, and passive or active cancellation. Shape stealth may affect the aerodynamic performance of aircrafts, and superior stealth abilities have fundamentally been endowed by radar absorbing materials. Thus, the development of radar absorbing materials would improve the survival and combat capacities of weaponized equipment [3,4]. Silicoboron carbonitride (SiBCN) ceramic with excellent structure has widely been investigated thanks to its low density, high-temperature resistance, superior oxidation resistance, and creep resistance properties [5–7]. In particular, SiBCN ceramic attracted increasing attention as functional material due to its excellent dielectric properties, which could transform from wave-transparent to microwave absorption [8,9]. On the other hand, PDC SiBCN is amorphous and insulating, which is suitable as wave-transparent matrix in electromagnetic wave ∗

(EMW) absorption materials. Wave absorbers, such as carbon materials [23,37], are required to be introduced to SiBCN wave-transparent matrix. Graphene possesses ultra-high BET surface area and ultra-low density with good electrical, thermal and mechanical properties arising from its unique structure [10–12]. However, the excellent electrical conductivity would damage the EMW absorption property of graphene due to its poor impedance matching characteristics. By comparison, reduced graphene oxide (RGO) has lower electrical conductivity due to introduced defects and oxygen, leading to appropriate dielectric property and enhanced EMW absorption characteristic [13–15]. However, the poor oxidation resistance of RGO impedes its applications as hightemperature EMW absorption materials. The combination of RGO with SiBCN ceramic could enhance the dielectric properties of ceramic matrix, and excellent oxidation resistance would be obtained using SiBCN ceramic. However, EMW absorption property was limited due to conductive pathway, which can not be established by isolated RGO sheets [36]. SiC nanowires (abbreviated as SiCnw) attracted increasing attention as strengthening and toughening phase due to their excellent mechanical properties, superior heat resistance, high corrosion resistance, and high-temperature oxidation resistance [16]. SiCnw is also considered as excellent candidate for EMW absorption applications due to their elevated specific surface areas, abundant stacking faults, twinning

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.ceramint.2019.11.275 Received 7 October 2019; Received in revised form 13 November 2019; Accepted 29 November 2019 0272-8842/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Chaokun Song, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.275

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Table 1 The information of as-fabricated RGO-SiCnw/SiBCN composite ceramics. Samples

RGO-SiCnw/SiBCN

Label

S0-1.42 S0.5–2.29 S1-1.3 S1.5–1.1 S3-1.35

Content/wt.% RGO

SiCnw

0 0.5 1 1.5 3

1.42 2.29 1.3 1.1 1.35

Open porosity/%

44.23 47.19 43.76 48.15 46.78

Fig. 3. The XRD pattern of RGO-SiCnw/SiBCN composite ceramics.

to build conductive pathway with RGO fillers in SiBCN matrix. Using this route, agglomeration was prevented. The microstructure, phase composite, electrical conductivities, and both dielectric and EMW absorption properties of the as-obtained RGO/SiCnw-SiBCN composite ceramics were all investigated. The mechanism of EMW absorption was discussed. The relative complex permittivity was measured in the range of 25-600 °C and EMW absorption property was evaluated by RC. The evolution of microstructure and dielectric property were studied after oxidation at 800-1000 °C in static air.

Fig. 1. The TG curve of as-fabricated RGO-SiCnw/SiBCN composite ceramic.

2. Experimental 2.1. Raw materials

interfaces, and adjustable electrical conductivity [17–19]. Nevertheless, SiCnw could agglomerate when added directly to ceramics. In this paper, SiCnw was introduced to porous RGO-SiBCN composite ceramics via low-pressure chemical vapor infiltration (LPCVI) process

A novel vinyl-containing liquid polyborosilazane (PSNB, Institute of Chemistry, Chinese Academy of Science) was used as polymer

Fig. 2. The (a) and (c) surface, (b) and (d) cross-section morphology of S0-1.42 and S0.5–2.29.

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Fig. 4. The XPS curves of RGO-SiCnw/SiBCN composite ceramics.

Fig. 5. The (a) microstructure and (b) HRTEM image of SiCnw by TEM.

precursor. Graphene oxide (XF Nano Materials Tech Co, NanJing) was employed to produce RGO.

porous RGO-SiBCN composite ceramics from MTS-H2-Ar precursor system by LPCVI. Methyltrichlorosilane (CH3SiCl3, MTS≧99.39%) was used as deposition precursor, hydrogen (H2 ≧ 99.99%) as carrier gas of MTS and dilution gas, argon (Ar≧99.9%) as dilution gas, and nickel chloride hexahydrate (NiCl2·6H2O) as catalyst. The deposition temperature was set to 1000 °C, deposition time was 2 h, and ratio of inlet gas fluxes was [MTS]:[H2]:[Ar] = 1:50:25. The details of RGO-SiCnw/ SiBCN composite ceramics are listed in Table .1. The oxidation experiments were conducted on S0.5–2.29SN

2.2. Preparation of composites The fabrication process of RGO-SiBCN composite ceramics could be found in our previous work [36]. The composite ceramics containing 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt% and 3 wt% GO were denoted as S0, S0.5, S1, S1.5 and S3, respectively. SiCnw was then in-situ grown in

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Fig. 6. The (a) real part of permittivity, (b) imaginary part of permittivity, (c) dielectric loss of RGO-SiCnw/SiBCN composite ceramics, (d) the variation of dielectric property before and after the introduction of SiCnw at 10 GHz.

Fig. 7. (a) The electrical conductivity of RGO-SiCnw/SiBCN composite ceramics, (b) The conductivity loss at 10 GHz.

(S0.5–2.29 with Si3N4 coating) in air at temperatures of 800 °C, 900 °C and 1000 °C for 10 h. Each sample was pushed into the furnace at given temperature and then pulled out after oxidation for 10 h.

ceramics was measured by Archimedes-method. The TG curve was obtained using a thermogravimetric analyzer (TGA, Netzsch STA 449F3, Germany) in the temperature range of 35–1400 °C under Ar atmosphere. The microstructures of as-fabricated composite ceramics were observed by scanning electron microscopy (SEM, S4700; Hitachi, Japan). The phase compositions were determined by X-ray diffraction (XRD, Rigaku-D/max-2400; Tokyo, Japan) using Cu Kα (λ = 1.54 Å)

2.3. Characterization The open porosity of as-fabricated RGO-SiCnw/SiBCN composite

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radiation. The microstructure of SiCnw was further detected by transmission electron microscopy (TEM, G-20, FEI-Tecnai, Hillsboro, USA). The X-ray photoelectron spectrometer (XPS, Kα; Thermo Scientific, Waltham, MA) was used to analyze the chemical bonds. The directcurrent electrical conductivity of the samples were estimated by high resistance meter (4339B, Agilent, USA). To this end, silver was coated on both surfaces of each sample as electrodes to form metal–insulator–metal (MIM) capacitor for electrical testing. The relative complex permittivity εr of the specimens was evaluated by vector network analyzer (VNA, MS4644A, Anritsu, Japan) using waveguide method at frequencies of 8.2–12.4 GHz (X-band). Details of the high-temperature waveguide measurement system could be found in the literature [19]. During measurements, the as-prepared S0.5–2.29 specimens were vertically placed in center of the testing chamber. To ensure accuracy of data, the set-point temperatures were held for 3 min. 3. Results and discussion 3.1. Microstructure and phase composition The TG curve is shown in Fig. 1. The mass degradation rate was only 1.35% from 35 to 1400 °C, it indicated the as-fabricated RGO-SiCnw/ SiBCN composite ceramics possess excellent thermal stability and have great potential to serve in high temperature environment. The surface and cross-section microstructures of S0-1.42 and S0.5–2.29 are shown in Fig. 2. SiCnw with large aspect ratio and specific surface area looked uniformly distributed in the pores of PDC-SiBCN, which originated from volume shrinkage during pyrolysis and the open porosity was shown in Table 1. The conductive network formed through overlapping between nanoscale SiCnw units. The corrugated micro-sized RGO sheet (labeled by white box) was covered by SiCnw to built conductive network using RGO and SiCnw (Fig. 2d). Abundant interfaces between SiCnw and SiBCN, RGO and SiBCN, SiCnw and SiCnw, SiCnw and RGO came into being. These interfaces would contribute to improvement in polarization and dielectric property. The phase compositions of the as-fabricated composite ceramics were analyzed by XRD. As shown in Fig. 3, no characteristic diffraction peaks were observed in S0 and S0.5, indicating amorphous characteristic and consisting with our previous studies [20,21]. The weak diffraction peaks around 35.8° and 60.2° appeared after the introduction of CVI SiCnw in S0-1.42 and S0.5–2.29 belonging to β-SiC. The chemical bonds were detected and shown in Fig. 4. Si–C, Si–N, B–C, B–N and C–C bonds existed in composite ceramics. Small amount of Si–O bond was also detected, which can be attributed to the slight hydrolysis of polymer precursor. The N–H bond maybe ascribed to the adsorption of NH3 during the pyrolysis of precusor [43]. The microstructure of SiCnw was further investigated by TEM. In Fig. 5a, the diameter of SiCnw ranged from 32 to 48 nm. SiCnw was randomly oriented and intercrossed with each other, some bulk SiBCN matrix was attached to SiCnw and interfaces formed between SiCnw and SiCnw, as well as SiCnw and SiBCN. Note that SiCnw also appeared contortion with the change in growth direction (white circle in Fig. 5a), which could be attributed to alterant growing point induced by changes in deposition rate and local movement of catalyst liquid caused by impurities in the liquid [22]. The interplanar spacing of SiCnw was measured as 0.25 nm and corresponded to (111) plane of β-SiC. Numerous stacking faults existed in SiCnw, which can affect polarization characteristics and dielectric property. The growth direction was parallel to [111] and amorphous layer formed around SiCnw [22].

Fig. 8. The RC of (a) S0-1.42, (b) S0.5–2.29, (c) S1-1.3, (d) S1.5–1.1 and (e) S31.35 as a function of the thickness and frequency.

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Fig. 9. (a) Schematic illustration of the interaction of electromagnetic wave with RGO-SiCnw/SiBCN composite ceramics, (b) The RC of S0.5–2.29 as a function of frequency at different thickness, (c) The RC of RGO-SiCnw/SiBCN composite ceramics as a function of thickness at 10 GHz.

properly high to consume EMW energy [4]. To obtain excellent EMW absorbing property, values of ε' and ε'' should be taken respectively as 7.3 and 3.3 at 10 GHz with thickness of 2.86 mm according to the calculation in X-band [23]. To take both parameters into consideration, the dielectric loss was defined as in Eq. (2):

Table 2 The EMW absorption property of relative wave absorption materials. Samples

Filler content (wt.%)

Thickness (mm)

RC (dB)

EAB (GHz)

Ref

RGO/BAS PDC-SiC PDC-SiBCN1 SiC– SiBCN MWCNT/SiBCN RGO/SiBCN PDC-SiBCN2 [email protected] RGO/ZnO RGO-SiCnw/ SiBCN

1.5 / / 15 4 10 / / 15 2.79

2.1 2.75 2.31 14 2.5 1.8 2.6 3 2.4 3.6

-33 -9.97 -16.5 -23.23 -32 -34.56 -46.73 -47.3 -54.2 -42.02

3.1 / / / 3 2.46 3.32 4.7 6.7 4.2

[38] [39] [20] [40] [21] [36] [36] [41] [42] This work

tan =

To evaluate the dielectric properties, relative complex permittivity was measured by VNA in X-band as shown in Eq. (1):

= ´

j ´´

(2)

As shown in Fig. 6, the ε' values of S0-1.42, S0.5–2.29, S1-1.3, S1.5–1.1 and S3-1.35 specimens were recorded as 3.45, 3.66, 5.14, 5.75 and 8.16, respectively. By comparison, the ε' values of S0, S0.5, S1, S1.5 and S3 specimens were 2.72, 2.89, 3.82, 4.66 and 6.63, respectively. Hence, ε' increased by 0.27, 0.27, 0.35, 0.23 and 0.23-fold when compared with values of samples before introducing SiCnw. On the other hand, ε''values of S0-1.42, S0.5–2.29, S1-1.3, S1.5–1.1 and S31.35 samples were 1.18, 2.47, 4.11, 5.81 and 5.06, respectively. The ε''values of S0, S0.5, S1, S1.5 and S3 specimens were recorded as 0.04, 0.17, 0.52, 0.72 and 0.32, respectively. Therefore, ε'' rose by 28.50, 13.53, 6.90, 7.07 and 6.13-fold, respectively. The tanδ values of S01.42, S0.5–2.29, S1-1.3, S1.5–1.1 and S3-1.35 specimens were determined as 0.34, 0.67, 0.8, 1.01 and 0.62, respectively. For comparison, the tanδ values of S0, S0.5, S1, S1.5 and S3 were 0.02, 0.06, 0.08, 0.11 and 0.11, respectively. Thus, tanδ increased by 16, 10.17, 9, 8.18 and 4.64-fold, respectively. Note that ε'' of S0.5–2.29 prepared with conductive fillers of 2.79 wt% was higher than that of S3 with conductive fillers of 3 wt%, suggesting occurrence of synergistic effect in RGO/SiCnw hybrids with enhanced functional properties when

3.2. Dielectric and EMW absorption properties at room temperature

r

´´ ´

(1)

where ε' is the real part of permittivity associated with polarization ability and ε'' is the imaginary part of permittivity linked to attenuation capability in external alternating electromagnetic field. According to Debye theory, ε' should be as small as possible to obtain outstanding impedance matching performance, and ε'' should be

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Fig. 10. The (a) real part of permittivity, (b) imaginary part of permittivity, (c) dielectric loss of S0.5–2.29 at 25–600 °C.

compared to individual components [15]. With introduction of absorbing agent (nanoscale SiCnw and micro-sized RGO) in SiBCN, space charges accumulated at the heterogeneous interfaces owing to the huge differences in electrical conductivity between SiCnw and SiBCN, RGO and SiBCN, and SiCnw and RGO. The polarization at interfaces enhanced due to electron lagging behind the alternating electromagnetic field. Meanwhile, these charges can transfer in the conductive fillers. The conductive loss was correspondingly improved by establishing conductive pathway with SiCnw and RGO. According to Debye theory, ε' and ε'' could be expressed according to Eqs. (3)–(7):

´=

+

s 2

1+

´´ = p´´ + c´´ = P´´

=

C´´ =

´=

+

s

s+

(T ) 2 0f

+

(T )]2

(T )

(T )]2

1+[

(3)

(T ) 2

1+[

dielectric constant in high-frequency limit, ω is the angular frequency, ω = 2πf, f denotes the frequency, σ is the electrical conductivity of sample, ε0 represents the dielectric constant in vacuum, τ is relaxation time related to frequency and temperature, εp´´ is polarization loss, and εc´´ denotes conductivity loss. Note that ε'' is proportional to σ, meaning that establishing conductive network would greatly contribute to dielectric property. The change in electrical conductivity is shown in Fig. 7a. σ of S1.5–1.1 with value of 3.06 was higher than that of other specimens. The variation tendency of ε'' was consistent with that of σ corresponding to Eq. (4). εc'' was calculated according to Eq. (6). Note that ε'' mainly depended on conductivity loss (Fig. 7b). The EMW absorbing property was evaluated by RC and calculated according to Eqs. (8)–(11):

(T ) +

2

(T ) 0f

(4)

RC = 20log10

(5)

Zin = Z0

(6)

´´

+

Z0 =

(T ) 2

0

(7)

Z=

where εs is the static dielectric constant, ε∞ represents relative

7

µr r

µ0 0

Zin Z0

Zin Z0 Zin + Z0 tanh

(8)

j2 fd µr c

r

(9) (10) (11)

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The RC of RGO-SiCnw/SiBCN composite ceramics were calculated by Matlab. As shown in Fig. 8, RC first decreased and then increased with the increase in permittivity. The RC of S0-1.42 attained minimum value of -13.01 dB with thickness of 5.4 mm at 8.2 GHz. As permittivity increased, the minimum RC of S0.5–2.29 with thickness of 3.6 mm reached -42.02 dB at 11.71 GHz, better than -34.56 dB obtained with 10 wt% RGO in our previous work [36]. RC further increased as permittivity rose, and minimum RC of S1.5–1.1 was only -11.46 dB with maximum permittivity. Hence, higher permittivity should be harmful to impedance matching ability since much EMW will be reflected upon arrival to sample surface and little EMW could enter the bulk to be consumed. According to metal back-panel model, the arrival of EMW to sample surface would reflect some and allow the other to enter the sample bulk (Fig. 9a). The part of EMW entering the samples should be consumed and converted into heat energy issued from polarization and conductivity loss. Multiple reflections would occur between intertwined SiCnw, and EMW should repeatedly be reflected and dissipated. The other part will reach the bottom surface and reflected to sample again. The attenuation will occur again as it propagates through the sample to reach the upper surface and enter the air. The destructive interference will take place when the upper surface reflection wave and bottom surface reflection wave became inverse to each other, leading to minimum RC value. The destructive interference was obtained at sample thickness of (n+1)*λ/4, where n = [0, 2, 4, …], λ0 and λ represent the wavelength as EMW propagates in free space and material, respectively. Thus, to obtain the destructive interference, λ should be increased with enhanced thickness of the sample.The variations in RC of S0.5–2.29 with different thicknesses as a function of frequency are exhibited in Fig. 9b. For EAB, the corresponding frequency range within which RC was below −10 dB, was estimated to 4.2 GHz, covering the entore X-band with the thickness of 3.6 mm. Compared to data reported by Qin et al. [34], minimum RC of S0.5–2.29 decreased to 108% and EAB became wider. The frequency corresponding to minimum RC reduced with the increase in thickness. The change in RC as a function of thickness at 10 GHz is gathered in Fig. 9c. For S3-1.35, RC reached the lowest levels of -15.12 dB and -6.38 dB at thicknesses of 2.6 mm and 7.6 mm, corresponding to n = 0, 2, respectively. The comparision of relative microwave absorbers was shown in Table 2. 3.3. Dielectric and EMW absorption properties at temperatures of 25–600 °C The dielectric and EMW absorption properties at high temperature are important for practical applications of the as-fabricated composite ceramics. As shown in Fig. 10, ε' increased from 3.66 to 4.06 as temperature rose from 25 to 400 °C. The ε'' rose from 2.47 to 3.13 and tan δ correspondingly increased from 0.67 to 0.77. According to Debye theory, ε' is inversely proportional to τ(T) but ε'' is proportional to σ(T). τ(T) can be expressed by Eq. (12) [24]:

(T ) =

h H exp exp kT RT

S R

(12)

where h and k are Planck and Boltzmann constants, respectively. ΔH and ΔS are activation energy and activation entropy, respectively. The energy of electrons elevated and the generation of polarization became easier as temperature rose. Therefore, τ(T) decreased and ε' increased. At high temperatures, the effect of free electrons became more significant than polarization, and ε'' was mainly dependent on σ. As temperature increased, the density of transfer carrier increased, and electrical conductivity of both RGO and SiCnw rose and the ε''correspondingly increased [25]. However, the carbon phase started to be oxidized at 400 °C and conductive pathway was destroyed [26]. The oxidation became more severe with increase in temperature and both ε'

Fig. 11. The RC as a function of thickness and frequency of S0.5–2.29 at 100–600 °C.

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reached -39.13 dB at 600 °C, indicating outstanding EMW absorption properties of RGO-SiCnw/SiBCN composite ceramics at high temperatures. The variation in RC at 600 °C versus frequency at different thicknesses is shown in Fig. 12. At thickness of 3.6 mm, RC was below -10 dB in the entire X-band. Note that frequency corresponding to minimum RC decreased with increase in thickness, which can be explained by λ/4 theory. 3.4. Evolution of microstructure and dielectric property after oxidation To improve the service temperature of as-fabricated composite ceramics, the wave-transparent Si3N4 coating was added on S0.5–2.29 surface via CVI technology [27–29]. The RGO was protected from oxidation by dense SiO2 glass phase derived from oxidation of Si3N4 above 800 °C [30–32]. The microstructures and dielectric property were investigated after oxidation at 800, 900, and 1000 °C. In Fig. 13a, CVI Si3N4 with cauliflower-like surface morphology was observed. Some cracks appeared on Si3N4 coating due to mismatch in thermal expansion coefficients between Si3N4 and SiBCN matrix. The cracks can act as oxygen diffusion channels at high temperatures and was unable to protect RGO from oxidation. The element compositions of CVI Si3N4 determined by EDS are illustrated in Fig. 13b. Si, N with some O were all detected. The presence of O can be attributed to surface adsorption of H2O, O2, and CO2. The cross-section morphologies are depicted in Fig. 13c. CVI Si3N4 was infiltrated into porous SiBCN matrix with thickness of dense area estimated to 20 μm. After oxidation at 800 °C for 10 h, the cauliflower-like morphology was preserved and cracks still existed (Fig. 14b). After oxidation at 900 °C, the open space of cracks became wider (Fig. 14b). As oxidation temperature rose to 1000 °C, the glass phase appeared and cracks were healed (Fig. 14f). The oxygen was blocked outside of the glass phase resulting from the low diffusion rate of oxygen in glass phase. The element composition after oxidation at 1000 °C was detected and the data are shown in Fig. 14g. Si, O, N with some C attributed to SiBCN matrix were all

Fig. 12. The RC of S0.5–2.29 as a function of frequency with different thickness at 600 °C.

and ε'' correspondingly decreased to 4.02 and 3.08, and tanδ reached 0.77 at 500 °C. At temperature of 600 °C, ε' and ε'' further decreased to 3.77 and 2.44, respectively. The variations in RC as a function of frequency and thickness were calculated and the results are shown in Fig. 11. The impedance mismatch formed due to increase in permittivity. The corresponding minimum RC increased to -28.96 dB at 100 °C. As temperature rose, the permittivity became elevated, degree of impedance mismatch rose, and minimum RC increased. At 400 °C, the minimum RC increased to -18.11 dB. As RGO was consumed, permittivity decreased, the value of minimum RC began to decline, and EMW absorption property enhanced. Minimum RC decreased to -19.21 dB at 500 °C and then

Fig. 13. (a) The surface morphology of S0.5–2.29SN after CVI Si3N4 coating, (b) the element composite of Si3N4 coating by EDS, (c) the cross-section morphology of S0.5–2.29SN.

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Fig. 14. The surface morphology of S0.5–2.29SN after (a) and (b) 800 °C, (c) and (d) 900 °C, (e) and (f) 1000 °C oxidation, (g) EDS spectrum after oxidation at 1000 °C.

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Fig. 15. The cross-section morphology of S0.5–2.29SN after oxidation at (a) and (b) 800 °C, (c) and (d) 900 °C, (e) 1000 °C.

detected. The intensity of O peak increased when compared to that before oxidation due to oxidation of Si3N4. The cross-section morphology of S0.5–2.29SN after oxidation is presented in Fig. 15. SiCnw still existed in the pores of PDC-SiBCN and RGO sheets were visible after oxidation at 800 and 900 °C. RGO can not be observed after oxidation at 1000 °C owing to high oxidation rate with elevated oxidation temperature and destruction of conductive path, which can significantly affect the dielectric and EMW absorption properties. The permittivity of S0.5–2.29SN before and after oxidation was measured in X-band. Before oxidation (Fig. 16a), ε' and ε''of S0.5–2.29SN were determined as 3.84 and 2.09 at 10 GHz, respectively. ε' and tanδ of Si3N4 were 8.5 and 0.003, respectively [33]. According to law of mixtures [35], ε' of S0.5–2.29SN increased to 3.84 and tanδ decreased to 0.54 when compared to S0.5–2.29. Also, ε' changed to 3.48 after oxidation at 800 °C. As EMW absorber filler oxidized, ε' decreased

to 3.29 and 3.12 after oxidation at 900 and 1000 °C. After oxidation at 800 °C, ε'' significantly decreased to 1.01 with consumption of conductive phase according to Debye theory. As oxidation temperature further rose, ε'' changed to 0.68 and 0.3, meaning dramatic reduction in EMW attenuation ability of S0.5–2.29SN. The correspondingly tanδ decreased from 0.54 to 0.29, 0.21 and 0.11 after oxidation at 800, 900 and 1000 °C The changes in RC with respect to frequency and thickness are shown in Fig. 17. The minimum RC of S0.5–2.29SN before oxidation was estimated to -26.51 dB. Due to destruction of conductive pathway and decrease in dielectric property, the minimum RC increased to -14.8 dB, -10.41 dB and -4.44 dB after oxidation at 800, 900 and 1000 °C, respectively. This indicated poor EMW absorption property after oxidation at 1000 °C, resulting from disappearance of RGO and generation of wave-transparent SiO2 glass phase.

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Fig. 16. The (a) real and (b) imaginary part of permittivity, (c) dielectric loss and (d) the variation of permittivity after oxidation at different temperatures.

4. Conclusions

(2) The permittivity increased as temperature rose from 25 to 400 °C, which can be attributed to increase in transfer carrier density. RGO began to oxidize and permittivity correspondingly decreased from 400 to 600 °C. ε' and ε''changed to 3.77 and 2.44 at 600 °C, and minimum RC at thickness of 3.6 mm was -39.13 dB with corresponding EAB of 4.2 GHz. (3) The oxidation experiment was conducted under the protection of Si3N4 coating. The composite ceramics maintained good EMW absorbing property with minimum RC of -10.41 dB after oxidation at 900 °C, suggesting great potential of the as-fabricated RGO-SiCnw/ SiBCN composite ceramics as excellent high-temperature EMW absorption materials.

RGO-SiCnw/SiBCN composite ceramics were successfully fabricated using polymer-derived ceramic (PDC) combined with chemical vapor infiltration (CVI). The following remarks could be drawn: (1) ε' and ε'' increased as the conductive pathway was established by SiCnw and RGO conductive fillers in RGO-SiCnw/SiBCN composite ceramics. Conductive loss made great contribution to dielectric property. The minimum RC of S0.5–2.29 with thickness of 3.6 mm reached -42.02 dB, and EAB was 4.2 GHz, covering the entire Xband.

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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

Fig. 17. The RC as a function of thickness and frequency of S0.5–2.29SN (a) before oxidation, after oxidation at (b) 800 °C, (c) 900 °C and (d) 1000 °C.

[25]

Declaration of competing interest

[26]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[27] [28]

Acknowledgements

[29]

This work was supported by the National Key Research and Development Program of China (No.2018YFB1106600), the Chinese National Foundation for Natural Sciences under Contracts (No. 51672217).

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