SiBCN-Si3N4 ceramics annealed at different temperatures

Journal Pre-proof Microstructure and EMW absorbing properties of SiCnw /SiBCN-Si3 N4 ceramics annealed at different temperatures Zanlin Cheng, Yongsheng Liu, Fang Ye, Chengyu Zhang, Hailong Qin, Jing Wang, Laifei Cheng

PII:

S0955-2219(19)30785-X

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.11.040

Reference:

JECS 12865

To appear in:

Journal of the European Ceramic Society

Received Date:

27 August 2019

Revised Date:

12 November 2019

Accepted Date:

13 November 2019

Please cite this article as: Cheng Z, Liu Y, Ye F, Zhang C, Qin H, Wang J, Cheng L, Microstructure and EMW absorbing properties of SiCnw /SiBCN-Si3 N4 ceramics annealed at different temperatures, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.11.040

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Microstructure

and

EMW

absorbing

properties

of

SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures Zanlin Chenga,b, Yongsheng Liua,b,* [email protected], Fang Yea, Chengyu Zhanga,b,*, Hailong Qina, Jing Wanga, Laifei Chenga a

Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern

b

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Polytechnical University, Xi’an, Shaanxi 710072, China NPU-SAS Joint Research Center of Advanced Ceramics, Northwestern Polytechnical University,

Xi’an, Shaanxi 710072, China

author. Tel.: +86 029 88495179; Fax: +86 029 88494620.

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Corresponding

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Abstract: SiC nanowire/siliconboron carbonitride-Silicon nitride (SiCnw/SiBCN-Si3N4) ceramics were prepared via a low-pressure chemical vapor deposition and infiltration

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(LPCVD/CVI) technique. The as-prepared ceramics were annealed at varying temperatures (1200–1600 oC) in a N2 atmosphere, and their crystallization mechanism

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and absorbing properties were subsequently studied. The absorbing properties of the SiCnw/SiBCN-Si3N4 ceramics improved with the annealing temperature up to a certain

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value and decreased thereafter. Among the samples tested, the SiCnw/SiBCN-Si3N4

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ceramics annealed at 1300 oC showed the highest permittivity (real and imaginary parts) and dielectric loss values in the X-band (ca. 5.34, 2.55, and 0.47 respectively), and this could be attributed to the precipitation of carbon and SiC nanocrystals. The sample treated at 1300 oC decreased its minimum reflection coefficient (RC) from -12.0 to 59.68 dB (compared with the as-received SiCnw/SiBCN-Si3N4 ceramics) and the

effective RC (below -10 dB) in the whole X-band could be achieved when the thickness was set to 3-3.5 mm. These results revealed that the absorbing performance was significantly improved after the heat treatment at 1300 oC.

Keywords: Siliconboron carbonitride ceramics; Dielectric properties; Electromagnetic wave absorbing properties; Heat treatment; Chemical vapor deposition and infiltration.

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1. Introduction

Electromagnetic wave (EMW) absorbing materials have attracted increasing

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attention in recent years because of their lightweight, flexibility, broadband, and heat

stability characteristics. These features make them suitable materials for stealth

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technology and telecommunication applications including radars, military equipment,

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and data transmission. In particular, siliconboron carbonitride (SiBCN) have been widely investigated because of its good high-temperature properties and excellent

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absorption characteristics [1–4]. The amorphous microstructure and nano-crystal structure of SiBCN ceramics along with their excellent thermal stability allow these

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materials to be used at temperatures as high as 2200 oC for long periods of time [5].

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SiBCN ceramics withstand higher temperatures than other ceramics such as SiC and Si3N4. In addition, SiBCN ceramics possess excellent oxidation resistances, outperforming SiC and Si3N4. Therefore, SiBCN ceramics could be used as matrices in replacement of SiC and Si3N4, thereby effectively improving the usage temperature and service life of composites. Polymer-derived SiBCN ceramics (e.g., Si3.0B1.0C4.3N2.0) were first fabricated by Riedel et al. and were proven to maintain stable structures at

2000 oC [6]. Besides, SiBCN ceramics could be also employed as EMW absorbing matrices or coatings in continuous fiber-reinforced ceramic matrix composites (CFCCs) and as EMW absorbing components in composite ceramics [7]. Recently, Studies have shown that the conductivity of amorphous SiBCN ceramics is between insulator(e.g., BN and Si3N4) and semiconductor(e.g., SiC), and they exhibit EMW transmission performance.

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In our previous work, CVI-SiBCN ceramics with a dielectric loss of 0.1 were fabricated[8]. However, SiBCN ceramics doped with absorbing components could

conform the A/B/C absorbing material microstructure model [9], where A represents

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the electrically insulating amorphous SiBCN ceramics, B represents the SiC and C

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nanocrystals distributed inside the SiBCN ceramics, and C represents the absorbing components. SiBCN ceramics doped with absorbing components could achieve

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excellent EMW absorbing properties, because the electrically insulating amorphous

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SiBCN ceramics form a good impedance match layer between the air and B+C phases with a relatively high dielectric loss. For example, when mixed with carbon nanotubes,

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SiBCN ceramics showed a minimum reflection coefficient (RC) of ca. -32 dB [10] and an effective bandwidth of ca. 3GHz in X-band (8.2-12.4GHz). What’s more, the

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dielectric constant of SiBCN ceramics can be designed and adjusted by process methods. As shown by Ye et al. [11], the dielectric loss of PDC SiBCN ceramics were significantly improved to 0.575 when annealed above 1650 oC as a result of the crystallization of SiC grains and structural transformation [4].Therefore, SiBCN ceramics are suitable EMW absorbing materials for high-temperature applications.

However, high-temperature treatments often result in inevitable degradation of the fibers, hindering the fabrication of EMW absorbing SiBCN matrices for CFCCs [12]. To prevent fiber degradation during the preparation process, SiBCN ceramics can be fabricated by low-pressure chemical vapor deposition/infiltration (LPCVD/CVI) [8]. In our previous work, we used thermodynamic calculations to design and prepare SiBCN ceramics by LPCVD/CVI, and these materials showed various and complex phases.

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These phases were prepared from a gas mixture of CH3SiCl3-NH3-BCl3-H2 and included Si3N4+SiC+C+BN, SiC+C+BN, and B4C+SiC+C+BN, with the phase composition being adjusted by the experimental conditions [8].

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SiC nanowires (SiCnw) have outstanding antioxidant properties and higher

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conductivity than SiC crystals, which is attributed to the large amount of carriers in nanometer-sized one-dimensional (1D) nanowires. As a result, SiCnw present superior

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conductivity and EMW absorption characteristics. Chiu [13] and Duan [14] used SiCnw

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and other ceramics to form absorbing composite ceramics with superior absorbing properties (minimum RCs of -31.7 and -20.01 dB, respectively).

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In our previous work, SiC nanowire/siliconboron carbonitride (SiCnw/SiBCN) composite ceramics with different SiCnw contents were prepared via CVI to determine

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the effect of the SiCnw content on the microstructure, phase composition, dielectric, and EMW absorbing properties of these materials. The results revealed a RC of -15 dB for SiCnw/SiBCN composite ceramics with a SiCnw content of 3.82 wt%, while the effective absorption width of the X-band was 2.88 GHz [15]. Generally, high temperatures have important effects on the microstructure, phase

composition, and properties of ceramic materials. On one hand, heat treatments are frequently used to improve the functional properties of ceramic materials [4,16]. On the other hand, the microstructure and properties of ceramics might change when used under high temperature conditions. SiCnw/SiBCN ceramics are to be used under high temperature conditions. Therefore, it is necessary to investigate the microstructure evolution and the effect of separated phases on the dielectric and EMW absorption

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properties of SiCnw/SiBCN ceramics. In this paper, SiC nanowire/siliconboron carbonitride-Silicon nitride (SiCnw/SiBCN-Si3N4) ceramics were annealed at high

temperatures in a N2 atmosphere. The effect of the annealing temperature on the

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properties of these materials were investigated.

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microstructure, phase evolution, thermal stability, and dielectric and EMW absorption

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2. Experimental

Porous Si3N4 ceramics fabricated as reported elsewhere [17] were used as

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deposition substrates. The Si3N4 substrate showed a porosity of ca. 40% and a dielectric

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constant with real and imaginary parts of ca. 3.0 and 0, respectively. The Si3N4 substrate also showed good wave permeability and did not affect the reliability of the EMW

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absorption tests of the SiCnw/SiBCN-Si3N4 ceramics. A NiCl2.6H2O catalyst was dissolved in absolute ethanol to prepare a solution with a concentration of 0.03 mol/L. The Si3N4 substrate was vacuum-impregnated in the catalyst solution for 1 h. Once the ethanol was completely evaporated, the sample was transferred to a CVD/CVI furnace to infiltrate the SiCnw and the SiBCN ceramics for 2 and 6 h, respectively with

methyltrichlorosilane (CH3SiCl3, MTS ≥ 99.39 wt%), boron trichloride (BCl3 ≥ 99.9 vol%), and ammonia (NH3 ≥ 99.99 vol%) as gas precursors. Hydrogen (H2 ≥ 99.999 vol%) was utilized as both carrier for MTS and dilution gas, while Argon (Ar ≥ 99.999 vol%) was employed as a dilution gas. The average SiC nw mass fraction is 0.43 wt%. According to the preliminary work carried out in our group [18], we employed the following process conditions for preparing SiCnw: T = 950 oC, H2 flow: 500 mL/min,

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and Ar flow: 250 mL/min. The process conditions for preparing SiBCN in this

experiment were as follows: T = 1000 oC, NH3 flow: 32 mL/min, BCl3 flow: 8 mL/min,

H2(carrier) flow: 165 mL/min, H2(dilution) flow: 635 mL/min, and Ar flow: 230

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mL/min. Finally, these samples were annealed at 1200, 1300, 1400, 1500, and 1600 oC

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in a N2 atmosphere for 2 h (N2, 0.3 MPa), and the as-obtained composite ceramics were named as HT-1200，HT-1300，HT-1400, HT-1500, and HT-1600, respectively, while

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the samples not heat treated were labeled as S0.

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The morphologies and microstructures of the solids were observed by scanning electron microscopy (SEM, S-4700; Hitachi, Tokyo, Japan) and transmission electron

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microscopy (TEM, Tecnai-F30, Hitachi, Japan, 300 eV). The elemental compositions were determined by Energy dispersive X-ray spectroscopy (EDS Genesis XM2, EDAX)

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coupled with SEM and X-ray photoelectron spectroscopy (XPS, K; Thermo Scientific, Waltham, MA). XPS was also used to analyze the bonding structures of the fabricated SiBCN ceramics. The crystal structures were analyzed by X-ray diffraction (XRD; D8Advance, Bruker, Karlsruhe, Germany) within a 2θ range of 10–90o.

3. Results and discussion 3.1 Microstructure and Composition of the SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures

Table 1 shows the weight loss underwent by the SiCnw/SiBCN-Si3N4 ceramics after a 2 h heat treatment in a N2 atmosphere at different temperatures (1200–1600 oC).

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Heat treatments between 1200 and 1400 oC did not result in significant weight changes. However, remarkable weight losses of 0.419% and 0.538% were found after annealing treatments at 1500 and 1600 oC, respectively, revealing a structural transformation or

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decomposition of the SiCnw/SiBCN-Si3N4 ceramics. Table 2 shows the EDS elemental

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content of the SiCnw/SiBCN-Si3N4 ceramics after being annealed at different temperatures. As shown in Table 2, for annealing temperatures higher than 1400 °C,

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the N content decreased remarkably with the temperature. A solid phase reaction could probably took place at temperatures above 1400 °C, resulting in the release of N2 and

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the corresponding weight loss.

The surface morphologies of S0 and the SiCnw/SiBCN-Si3N4 composite ceramics

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annealed at different temperatures are presented in Fig. 1. As shown in Fig. 1a, the

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surface of S0 was amorphous with compact and continuous cauliflower-like morphologies. In contrast, numerous cracks were uniformly observed on the annealed SiCnw/SiBCN-Si3N4 ceramics and the number of the cracks increased with the temperature of the treatment. It is because when the amorphous SiBCN ceramics are annealed at high temperatures, the atoms increase their energy for rearrangement and

migration, resulting in a certain volume shrinkage. Besides, the thermal expansion coefficient mismatch between SiBCN and Si3N4 was also responsible for the cracks. The volume shrinkage and the thermal mismatch increased with the annealing temperature, which led to the onset of more cracks. In addition, some particles were formed in the ceramics, and their number increased with the annealing temperature, probably as a result of the crystallization of the ceramics. When the heat treatment

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temperature was between 1200 and 1400 oC, a small amount of precipitates were observed on the surface of the samples. Heat treatments at 1500 and 1600 oC, resulted in the onset of flaky precipitates, and the amount of this precipitate phase increased

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with temperature.

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Fig. 2 shows the TEM bright field (BF) image and high resolution (HR) image of S0. As shown in Fig. 2(a), the amorphous SiBCN tightly wrapped SiCnw. By measuring

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the stripe spacing of SiCnw shown in Fig. 2(b), it was found that the stripe spacing was

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about 0.25 nm, which was consistent with the (111) plane spacing in the SiC crystal, indicating that CVI-SiCnw was β-SiC.

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Fig. 3 shows SEM images of the fracture of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures. The SiCnw formed a network structure on the surface

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of the Si3N4 substrate, while the surface of the SiCnw was coated with compact and continuous SiBCN ceramics. The content of SiCnw showed a gradient distribution inside the SiBCN ceramics (i.e., the SiBCN ceramic showed a higher SiCnw content near the Si3N4 substrate side). As shown in Fig. 3, numerous nanocrystals precipitated near the Si3N4 substrate and the size of these nanocrystals gradually increased with the

temperature. It could be inferred a second phase such as SiCnw served as a nucleation center during the annealing process and promoted the crystallization of the SiBCN ceramics. Fig. 4 shows TEM images of SiCnw/SiBCN-Si3N4 ceramics annealed at temperatures ranging from 1400 to 1600 oC. As shown in Fig. 4, nanocrystalline precipitates were observed on the amorphous SiBCN ceramic matrix after the annealing

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treatment. By measuring the stripe spacing as shown in Fig. 4 (b), it is found that the

stripe spacing is about 0.25 nm, which is consistent with the (1 1 1) plane spacing in the β-SiC crystal. Thus, after SiCnw/SiBCN-Si3N4 ceramics were heat treated at 1400

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ºC, β-SiC nanocrystals were precipitated in the amorphous SiBCN ceramic matrix, and

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the size was about 2–4nm. Fig. 4(c) and (d) show the TEM of SiCnw/SiBCN-Si3N4 ceramics annealed at 1600 ºC. The amount of precipitated SiC nanocrystals increased

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with the annealing temperature raising from 1400 to 1600 ºC, while the average grain

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size increased from 2–4 to 10–15 nm. Thus, for similar treatment durations, higher temperature resulted in SiC nanocrystals with larger size. Therefore, we inferred that

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high temperatures could increase the crystallization rate of this phase in the composite ceramics.

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Fig. 5 shows the XRD patterns of SiCnw/SiBCN-Si3N4 ceramics annealed at

different temperatures and that of S0. The samples annealed within 1200–1400 °C contained β-SiC, graphite, and α-Si3N4 phases precipitated on the composite ceramics, and the intensity of the diffraction peaks increased with the heat treatment temperature. Thus, the degree of crystallization of the composite ceramics increased with the

temperature of the treatment. According to the preliminary work carried out in our laboratory, composite ceramics annealed at 1700 °C contained a β-SiC phase [19]. Thus, the introduction of SiCnw promoted crystallization of SiC nanocrystals on the SiBCN matrix. On one hand, SiCnw can generate more heterogeneous interfaces with the amorphous SiBCN matrix. During the high temperature treatment, the volume shrinkage of the ceramic produced an internal stress that was gradually concentrated at

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the hetero interface. This relatively concentrated interface stress can significantly

improve the ability of the amorphous SiBCN matrix to crystallize [6]. On the other hand, SiCnw acted as a nucleation center, generating appropriate structure fluctuation

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conditions on the SiBCN matrix for the crystallization of SiC nanocrystals. Therefore,

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the introduction of SiCnw significantly reduced the crystallization temperature of SiBCN ceramics.

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When the annealing temperature was increased to 1600 °C, a B4C phase appeared

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at the expense of the C phase, and the diffraction peak of the SiC phase increased. We hypothesized that a carbothermal reduction reaction took place for temperatures lower

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than 1600 °C [20–22].

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4BN+C→B4C+2N2

(1) Si3N4+3C→3SiC+2N2

(2)

XPS analyses were carried out to confirm the phase evolution and the phase-

reaction of SiBCN ceramics annealed at high temperatures. Fig. 6 shows the narrow scanning spectra of C1s, B1s, and Si2p for SiCnw/SiBCN-Si3N4 annealed at different temperatures. Table 3 shows the contents of Si, B, and C bonds for each annealing

temperature. In all cases, C was mostly in the form of C-C（EB=284.6eV）[23], C-Si and C-B (EB=283.5eV) [24], while Si was found mostly as Si-N(EB=101.9eV) and SiC(EB=100.8eV) [25,26], and B as B-N(EB=190.1 eV) and B-C(EB=189.0 eV) [27]. As the temperature increased from 1200 to 1400°C, the relative contents of Si-N and Si-C bonds did not change significantly. As shown in Table 3, when the temperature increased from 1400 to 1600 °C, the content of C-C bonds significantly decreased from

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31.51 to 14.53 at%, while the content of C-Si and C-B bonds increased from 68.49 to

85.47 at%. The content of Si-N bonds significantly decreased from 59.77 to 48.98 at%,

while the content of Si-C bonds increased from 40.23 to 51.02 at% as the temperature

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increased from 1400 to 1600 ºC, respectively. The content of B-N bonds was

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significantly reduced from 81.30 at% to 62.89 at%, while the content of B-C bonds increased from 18.70 at% to 37.11 at% in the same range of temperature. Therefore, it

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can be inferred that, at temperatures higher than 1400 °C, a carbothermal reduction

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reaction occurs releasing N2, and this was the main cause of its mass loss. By using the above analysis, the crystallization process of the SiCnw/SiBCN-Si3N4

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composite ceramics treated at different temperatures can be described by the structural model shown in Fig. 7. The crystallization process involved three stages: (1) amorphous

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phase: in amorphous SiCnw/SiBCN-Si3N4 composite ceramics, the CVI SiBCN matrix is filled and wrapped with SiCnw, and a small amount of SiC nanocrystals are dispersed; (2) initial crystallization phase (1200–1400 oC): despite the low treatment temperature, the crystallization of the composite ceramics is promoted by SiCnw, resulting in the precipitation of a small amount of graphite, SiC nanocrystals, and Si3N4; (3)

crystallization process (1400–1600 oC): the diffusion rate of each phase in the composite ceramics and the size of the SiC nanocrystals both increased with the heat treatment temperature. Part of α-Si3N4 is converted into β-Si3N4. The carbon nanocrystals are gradually converted into SiC and B4C via carbothermal reduction.

3.2 Dielectric properties of the LPCVD/CVI-SiCnw/SiBCN-Si3N4 ceramics

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annealed at different temperatures

The real part of permittivity of the composite ceramics depends on the phase

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composition, density, and porosity, while the imaginary part depends on the

microstructure and electrical properties of the constitutive phases. The composite

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ceramics annealed within 1200–1400 °C contained β-SiC, carbon nanocrystals, and α-

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Si3N4 phases. When the temperature was higher than 1400°C, the size of the precipitated SiC nanocrystals grew with increasing temperature, and the carbon

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nanocrystals were gradually converted into SiC and B4C via carbothermal reduction. Moreover, the phase composition and degree of crystallization vary with the heat

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treatment temperature [28,29] These changes would affect the dielectric properties and

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EMW absorbing properties of the materials. The dielectric constant and dielectric loss of the composite ceramics before and after the heat treatments in the X-band are shown in Fig. 8. The real and imaginary part of permittivity and the dielectric losses of S0 were 4.22, 1.15, and 0.27, respectively. Within 1200 and 1300 °C, the real and imaginary part of permittivity and the dielectric loss increased with temperature (at 1300°C these values increased to 5.43, 2.55 and 0.47, respectively). This can be

explained by the composite ceramics gradually precipitated graphite and SiC nanocrystals as the temperature increased within this range. The presence of a large number of nano-hetero-interfaces increased the interfacial polarization loss of the composite ceramics. Crystallization of graphite and SiC nanocrystals resulted in higher conductance loss and dipole polarization loss of the composite ceramics. Within 1400– 1600 °C, the real and imaginary part of permittivity and the dielectric loss of the

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dielectric constant decreased with temperature (at 1600 °C these values decreased to 4.24, 0.34, and 0.08, respectively). At temperatures higher than 1400 °C，the thermal

mismatch increased with temperature, increasing the number of cracks in the material.

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The release of N2 after the carbothermal reduction reaction increased the porosity of the

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material, resulting in lower real part of permittivity. According to the TEM results, the size of the SiC nanocrystals precipitated at 1400 °C was 2–4 nm, and this value

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increased to 10–15 nm at 1600 °C. This grain growth decreased the nanocrystalline

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boundaries, reducing the interfacial polarization loss. As shown by XPS, at temperatures higher than 1400 °C, the C-C bond content decreased and the C-Si and C-

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B bond contents increased with temperature. XRD revealed the intensity of the diffraction peaks of C gradually decreases and the diffraction peaks of SiC and B4C as

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the temperature increased above 1400 °C. Thus, the C phase was gradually consumed and transformed into SiC and B4C. At temperatures higher than 1400 °C, the C was consumed via carbothermal reduction, resulting in lower conductance loss. Therefore, the reduction of interface polarization loss and conductance loss decreased the imaginary part of permittivity.

3.3 Absorption properties of the SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures

The relationship between the RC of the SiCnw/SiBCN-Si3N4 composite ceramics annealed at different temperatures and S0 with different sample thickness is shown in

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Fig. 9. Effective absorption bandwidth(EAB) could be used as a significant parameter to measure EMW absorbing property. EAB refers to the frequency range within which

the RC is below -10 dB.[7] S0 showed an EAB of 1.428 GHz and a minimum RC of -

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11.57 dB when the sample thickness was 4 mm. According to our preliminary work, under similar conditions, SiBCN-Si3N4 multiphase ceramics showed a minimum RC of

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-4 dB, indicating the effective EMW absorption can’t be achieved. Therefore, the

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introduction of SiCnw can effectively improve the absorbing properties of this material. After the heat treatment at 1200 °C, the multiphase ceramics showed a RC below -10

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dB absorption for a wider frequency band (ca. 3GHz) with a thickness of 3.5 mm, and the minimum RC was -17.75dB at a thickness of 4 mm. The material treated at 1300 °C

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showed a full-frequency RC below -10 dB absorption in the X-band at a thickness of

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3–3.5 mm, with a minimum of -59.68 dB (i.e., 99.9999% of the EMW were effectively absorbed). Thus, the SiCnw/SiBCN-Si3N4 composite ceramics showed excellent EMW absorption performance after the heat treatment at 1300 °C. A further increase in the treatment temperature above 1400 °C resulted in poorer absorbing properties. However, when the thickness of the composite ceramic was 4 mm, the minimum RC was -19.61 dB (99% of the EMW were effectively absorbed). Thus, the materials annealed at 1400

ºC still maintained a relatively good EMW absorption performance. The absorbing properties of the SiCnw/SiBCN-Si3N4 composite ceramics heat treated within 1500– 1600 °C were obviously reduced. At 1600 °C, the RC minimum was only -3.02 dB, which means the effective absorption of electromagnetic waves cannot be achieved. Duan et al. [13] fabricated SiCnw/SiOC ceramics though in situ growth of SiCnw in SiOC ceramics. The minimal RC of sample annealed at 1450 oC and sample annealed

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at 1400 oC was -17.25 dB at a thickness of 2.5mm and -20.01 dB at 3.3mm. The EBA of sample annealed at 1450 oC was 3.57 GHz in the whole X-band. Ye et al. [6] prepared SiC nanoparticle/polymer-derived SiBCN composite ceramics at 900oC. When the

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mass fraction of SiC nanoparticles was 15 wt.%, the minimal RC was -23.23 dB at the

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sample thickness of 14 mm. Zhang et al. [7] reported the minimum RC of MWCNTsSiBCN could reach −32 dB with an effective bandwidth of 3 GHz in X-band when the

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sample with 2 wt.% of CNT was annealed at 1500 oC. Xiao et al. [30] fabricated the

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Si3N4/SiC whisker composite ceramics by gelcasting at 1600 oC and the maximum absorption peak reached -22.4 dB and the EBA was 1.5GHz at the whisker content of

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15 wt.%. In this paper, The SiCnw/SiBCN-Si3N4 composite ceramics annealed at 1300 °C showed a minimum Rc of -59.68 dB and the EAB was as wide as 4.2GHz

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covering the whole X band at a thickness of 3–3.5 mm, indicating heat treatment could be used as an effective way to fabricated ceramics with excellent absorbing property at relatively low temperatures. In summary, the SiCnw/SiBCN-Si3N4 composite ceramics showed improved absorbing properties upon introduction of SiCnw. When the material was annealed

within 1200–1300°C, the amount of nano-heterogeneous interface phase increased via precipitation of SiC and carbon nanophases, significantly improving the absorbing properties of the material. After the heat treatment at 1300 °C, the material showed optimum EMW absorption performance. When the material was annealed within 1400– 1600 °C, the continuous growth of the nanoparticles decreased the amount of nanoheterogeneous interfaces and the interfacial polarization loss was reduced. On the other

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hand, the C phase was gradually transformed into SiC and B4C phases via carbothermal

reduction. Thus, the conductive phase was consumed, resulting in lower conductance

loss. The lower interfacial polarization loss and conductance loss decreased

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significantly the absorbing properties of the material over this temperature range.

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4. Conclusions

In this paper, the microstructure, crystallization mechanism, and absorbing

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properties of SiCnw/SiBCN-Si3N4 composite ceramics annealed at different

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temperatures (1200–1600 oC) were studied. The main conclusions can be summarized as follows:

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After a high-temperature heat treatment, the amorphous SiBCN ceramic underwent a crystal transformation and shrank. As a result, cracks appeared on the SiBCN ceramic, and the number of these cracks increased with the heat treatment temperature. The introduction of SiCnw decreased the crystallization temperature, while the grain size and crystallinity increased with the heat treatment temperature. When the

heat treatment temperature was higher than 1400 oC, a carbothermal reduction reaction occurred in the SiBCN ceramic. When the SiCnw/SiBCN-Si3N4 composite ceramics were annealed from 1200 to 1600 °C, the real and imaginary part of permittivity and the dielectric loss reached a maximum at 1300 °C and decreased thereafter. The SiCnw/SiBCN-Si3N4 composite ceramics showed excellent absorbing properties after being annealed at 1300 °C. The composite ceramics annealed at 1300 °C showed a full-

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frequency RC absorption below -10 dB in the X-band at a thickness of 3–3.5 mm, and the minimum value was -59.68 dB. Declaration of interests

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☒ 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.

Acknowledgment

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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This work was supported by National Key Research and Development Program of

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China (No. 2018YFB1106600), the Chinese National Foundation for Natural Sciences under Contracts (No. 51672217, No. 51572224), and the Fundamental Research Funds

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for the Central Universities (No. 3102019ghxm014).

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Fig. 1 Surface morphology of S0 (a) and SiCnw/SiBCN-Si3N4 annealed at different temperatures (b) HT-1200, (c) HT-1300, (d) HT-1400, (e) HT- 1500, and (f) HT- 1600. Fig. 2 TEM images of S0: (a) BF image of S0, (b) HR image of S0. Fig. 3 Fracture morphology of S0 (a) and SiCnw/SiBCN-Si3N4 annealed at different temperatures (b) HT-1200, (c) HT-1300, (d) HT-1400, (e) HT- 1500, and (f) HT- 1600. Fig. 4 TEM images of SiCnw/SiBCN-Si3N4 ceramics annealed at 1400 and 1600oC: (a) BF image of HT-1400, (b) HR image of HT-1400, (c) BF images of HT-1600, and (d) HR image of HT-1600. Fig. 5 X-ray diffraction patterns of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures. Fig. 6 High resolution XPS B1s, C1s, and Si2p core level spectra of SiCnw/SiBCN-Si3N4 ceramics (a) S0, (b) HT-1200, (c) HT-1300, (d) HT-1400, (e) HT-1500, and (e) HT-1600. Fig. 7 Crystallization mechanism diagram of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures. Fig. 8 Dielectric properties in 8.2–12.4GHz of S0 and SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures: (a) real part of permittivity, (b) imaginary part of permittivity, and (c) dielectric loss. Fig. 9 Reflection coefficient of S0 (a) and SiCnw/SiBCN-Si3N4 annealed at different temperatures (b) HT-1200, (c) HT-1300, (d) HT-1400, (e) HT- 1500, and (f) HT- 1600.

Table 1 Weight loss of SiCnw/SiBCN-Si3N4 ceramics heat treated at different temperatures

Annealed temperature

Weight loss

o

%

C

0.003

1300

0.008

1400

0.226

1500

0.419

1600

0.538

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1200

Table 2 Elemental composition of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures

EDS at% Si

B

C

N

O

S0

17.55

33.14

10.96

37.39

0.96

HT-1200

18.27

33.70

9.45

38.39

0.19

HT-1300

18.87

32.83

10.20

37.41

0.69

HT-1400

17.49

33.97

9.49

38.83

0.22

HT-1500

18.29

34.05

12.68

33.45

1.53

HT-1600

20.83

37.88

12.57

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0.41

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28.31

Table 3 Bond states and contents analyzed by XPS for SiCnw/SiBCN-Si3N4 composites treated at different temperatures Content % Bond types

Si2p

S0

HT-

HT-

HT-

HT-

HT-

1200℃

1300℃

1400℃

1500℃

1600℃

Si-N

57.66

58.09

61.32

59.77

55.87

48.98

Si-C

42.34

41.91

38.68

40.23

44.13

51.02

30.21

30.70

28.83

31.51

23.08

14.53

C-Si or/and C-B

69.79

69.3

71.17

68.49

76.92

85.47

B-N

83.62

80.56

78.25

81.30

72.46

62.89

B-C

16.38

19.44

21.75

18.70

27.54

37.11

C-C

C1S

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B1S