SiBCN polymer-derived ceramics (PDCs) are highly promising for high-temperature electromagnetic wave attenuation due to their unique "wave-transmitting/lossy" composite structure, which arises from the in-situ formation of high-dielectric crystals within a low-dielectric amorphous matrix. A critical challenge, however, lies in precisely tailoring their microstructure to maximize absorption performance. To address this issue, SiBCN ceramics were synthesized with different compositions via modulating the silicon (Si) content in the polymer precursors to elucidate the regulatory mechanisms governing phase composition, morphology, and electromagnetic absorption properties. The results reveal that high-temperature treatment induces the crystallization of Si₃N₄, SiC, and graphitic carbon. The incorporation of Si can facilitate the graphitization of amorphous carbon and effectively reduce internal defect density. Furthermore, an increase in Si content promotes the formation of the Si₃N₄ phase, thereby enhancing impedance matching. The optimized SiBCN ceramics demonstrate superior microwave absorption capabilities, achieving a minimum reflection loss of -54.38 dB at 13.49 GHz (2.28 mm thickness) and a broad effective absorption bandwidth of 6.61 GHz (2.24 mm thickness). Consequently, this study confirms that tuning the precursor Si content effectively optimizes both the structural configuration and impedance matching of SiBCN ceramics, offering a robust theoretical and experimental foundation for their application in high-temperature environments.

Under ice bath conditions, 20 g of n-hexane was added to a reaction flask, followed by the sequential addition of dimethyldichlorosilane, boron trichloride, borane dimethyl sulfide complex, and hexamethyldisilazane according to different molar ratios of the raw materials. The mixture was stirred in Ar atmosphere for 24 h. The temperature was then raised to 100 ℃ and maintained for 3 h to remove n-hexane and other by-products. Finally, the temperature increased to 160 ℃ and maintained for 3 h, followed by three cycles of vacuum filtration to obtain a pale yellow resin-like polyborosilazane. A series of polyborosilazanes with different silicon contents were prepared via varying the amount of dimethyldichlorosilane added. The samples with different dimethyldichlorosilane to hexamethyldisilazane molar ratios of 1∶6, 2∶6, and 4∶6 were designated as S30, S38, and S45, with silicon contents of 30%, 38%, and 45%, respectively.

The precursor samples (S30, S38, and S45) were first cured under an N₂ atmosphere by heating to 280 ℃ at a rate of 2 ℃/min and holding for 2 h. The cured samples were subsequently heated to 1000 ℃ under an N₂ atmosphere at a rate of 5 ℃/min and held for 1 h. They were then further heated to either 1400 or 1600 ℃ at a rate of 2 ℃/min and held for 2 h. The resulting ceramic samples were designated as S30-1400, S38-1400, S45-1400, S30-1600, S38-1600, and S45-1600, respectively.

The phase composition, microstructure, and electromagnetic properties of the resulting ceramics are regulated via controlling the silicon content in the polymer precursor. The results indicate that after heat treatment in N₂ atmosphere at 1600 ℃, crystals such as Si₃N₄, SiC, and graphitic carbon are formed within the SiBCN ceramics. Silicon promotes the transformation of amorphous carbon into crystalline carbon in the SiBCN ceramics, reduces the defect density within the ceramics, and enhances the graphitization degree. The formation of Si₃N₄ in the SiBCN ceramics increases with increasing silicon content, improving the impedance matching performance of the ceramics. When the thickness of the SiBCN ceramics is 2.28 mm, the minimum reflection loss reaches -54.38 dB at 13.49 GHz. At a thickness of 2.24 mm, the effective absorption bandwidth is 6.61 GHz.

This study systematically investigated the influence of silicon content on the phase composition, microstructure, and electromagnetic wave absorption properties of SiBCN ceramics, and thoroughly elucidated the underlying absorption mechanisms.

The results demonstrated that after heat treatment at 1600 ℃, the polyborosilazane precursor transformed into a ceramic matrix containing crystalline phases such as Si₃N₄, SiC, and graphitic carbon. The formation of Si₃N₄ and SiC crystals significantly increased with increasing silicon content, effectively optimizing the impedance matching characteristics of the ceramics. Concurrently, the graphitization degree of amorphous carbon increased, while the defect density within the ceramic decreased. The formation of numerous nano-sized dielectric grains and heterogeneous interfaces markedly enhanced the electromagnetic wave absorption capacity via strengthening interfacial polarization and conductive loss.

The experimental results indicated that the sample with 38 wt.% silicon content exhibited the optimal comprehensive wave absorption performance: at a thickness of 2.28 mm, the minimum reflection loss (RLmin) reached -54.38 dB at 13.49 GHz. At a thickness of 2.24 mm, the effective absorption bandwidth (EAB) extended to 6.61 GHz. Therefore, the microstructure and phase composition of the ceramics could be effectively regulated via adjusting the silicon content in the precursor, leading to the development of SiBCN ceramic materials with excellent electromagnetic wave absorption properties.