IntroductionAn issue of electromagnetic pollution has escalated with the proliferation of electronic devices, thus posing a significant threat to human health and electronic equipment. Consequently, there is an increasing interest in materials possessing robust electromagnetic wave absorption capabilities. Based on the absorption mechanisms of electromagnetic wave materials, they can be categorized into dielectric loss and magnetic loss types. The loss mechanisms of magnetic loss materials encompass damping and hysteresis losses. Dielectric loss materials absorb electromagnetic waves through polarization and electrical conductivity losses, typically having elevated dielectric constants. Such materials include ferroelectrics, metal oxides, and inorganic ceramics. Polymer-derived ceramics (PDCs) represent a pivotal technology for the design and fabrication of functional ceramics. This methodology effectively harnesses the advantages of both polymer and ceramic materials. In contrast to conventional ceramic preparation techniques that consume energy, PDCs enable the production of ceramic materials with tunable elemental compositions and controllable crystalline structures through meticulous design and synthesis of the molecular structures of polymer precursors, coupled with precise control in the pyrolysis process. In this paper, polyborosilazanes with different boron contents were synthesized via modulating the quantity of added boron source. The impact of boron content on the structure and properties of polyborosilazanes and the phase composition/microstructure of ceramics was investigated.Methods20 g of n-hexane was added to a reaction flask in ice bath. Methyl dichlorosilane, vinylmethyl dichlorosilane, boron trichloride, and hexamethyldisilane were sequentially added to the reaction flask at different molar ratios. The mixture was stirred in an argon atmosphere for 24 h. The temperature was then raised to 100 ℃ and maintained for 3 h to remove n-hexane and other by-products. Subsequently, the temperature was further increased to 160 ℃, and maintained for 3 h. The mixture was subjected to three cycles of filtration to obtain a light yellow resin-like polyborosilazane. A series of polyborosilazanes with different boron contents were prepared at different amounts of boron trichloride added. The samples synthesized with boron and nitrogen in a molar ratio of 1:6, 1.3:6.0, and 1.5:6.0 were named as P-BTC-1, P-BTC-1.3, and P-BTC-1.5, with boron contents of 3%, 4%, and 5% (in mass), respectively. The obtained precursor samples were subjected to curing treatment. The curing conditions involved heating in an argon atmosphere at a rate of 2 ℃/min at 280 ℃ for 2 h. Afterwards, the cured samples were heated in an argon atmosphere at a rate of 5 ℃/min at 1000 ℃ for 1 h, and then heated at a rate of 2 ℃/min at 1600 ℃, for 2 h to obtain samples P-BTC-1-1600, P-BTC-1.3-1600, and P-BTC-1.5-1600, respectively.The chemical bonds and functional groups in the samples were identified by a model Vertex 70 Fourier transform infrared spectrometer (FT-IR, Bruker Co., Germany). ¹H, ¹³C, and ¹¹B were determined by a model Avance NEO 600 nuclear magnetic resonance spectrometer (NMR, Bruker Co., Germany). The phase composition of the ceramic samples was determined by a model X'Pert MPD Pro X-ray diffractometer (XRD, Philips Co., the Netherlands) with Cu K_α radiation source, scanning angles ranging from 10° to 90°. The microstructure of the ceramic samples was analyzed by a model 400 Nano scanning electron microscope (SEM, Nova Co., USA), and the elemental composition analysis of the samples was performed by an energy dispersive spectrometer (EDS, INCA Energy Co., UK). The thermal decomposition process of the polyborosilazane was analyzedby a model STA 449 F3 thermal analyzer (Netzsch Co., Germany). The electromagnetic parameters of the materials were tested by a model E5071C vector network analyzer (Keysight Tech., Co., USA).Results and discussionA series of polyborosilazanes were synthesized via controlling the amount of boron, resulting in SiBCN ceramics with different atomic compositions. Boron effectively suppresses the fracture of Si—N bonds and the formation of Si—C bonds, inhibits the decomposition of Si₃N₄ and the generation of SiC as well as facilitates the transformation of amorphous carbon into graphite carbon, thereby increasing the proportion of graphite carbon in SiBCN ceramics. The polarization losses generated by various dielectric crystals such as Si₃N₄, SiC and graphite carbon enhance the electromagnetic wave absorption performance of SiBCN ceramics. At a boron content of 5% (in mass), the minimum reflection loss reaches -55.67 dB at 8 GHz for a thickness of 3.5 mm.Conclusions1) In the precursor synthesis process, the precursor structure became more stable with an increase in boron content, mainly composed of chemical bonds such as Si—N, B—N, Si—C, Si—H, N—H, and C—H. The ceramic yield increased from 56% to 66.7%.2) After heat treatment at 1600 ℃, boron in polyborosilazanes suppressed the decomposition of Si₃N₄ and the formation of SiC in SiBCN ceramics, and enabled the control of the conductive phase and dielectric loss phase in SiBCN ceramics, thereby enhancing the impedance matching performance of the ceramics to electromagnetic waves. The polarization losses generated by various dielectric materials such as Si₃N₄, SiC and graphite carbon further enhanced the electromagnetic wave attenuation performance of SiBCN ceramics. At a boron content of 5%, SiBCN ceramics exhibited a minimum reflection loss of -55.67 dB at 8 GHz for a thickness of 3.5 mm, indicating that SiBCN ceramics could be an excellent candidate material in the field of electromagnetic wave absorption.