In recent years, 5G communication and smart devices become popular, but the electromagnetic pollution that they produce affects human-being health and the normal operation of electronic devices. Therefore, electromagnetic wave strong attenuation materials have attracted much attention, especially polymer-derived ceramics (PDCs) due to their tunable microstructure and superior high-temperature performance. The amorphous structure of the ceramics has a high-temperature stability, but the dielectric constants are low, making them weak to electromagnetic wave dissipation. To address this problem, catalysts for increasing heat treatment temperature or direct addition of high conductive loss phases (CNTs, RGO, etc.) are usually used to improve the attenuation ability of the ceramics to electromagnetic waves. Rare-earth element metals can modulate the electronic energy band structure of the materials due to the presence of empty d-orbitals, thus modulating their optical, electrical, and magnetic properties. In this paper, the effect of samarium doping on the microstructure and dielectric properties of SiBCN ceramics was thus investigated via introducing the transition metal Sm into polyborosilazane as a catalyst.

Based on the 'Schlenk' technology, 20 mL THF, 21 mL DCMVS and 24 mL borane dimethyl sulfide complex were slowly added to a reaction bottle in an ice bath, and the reaction was carried out in argon atmosphere for 24 h to obtain dichlorodimethylsilylethylborane (TDSB). Afterwards, 11 mL DCMS and 72 mL HMDZ were stirred in a three-neck flask at room temperature for 24 h. The reaction bottle was heated at 110 ℃ for 4 h to remove THF, trichloromethylsilane and other by-products, and was further heated at 170 ℃ for 3 h. After three cycles of filtration, a yellow viscous precursor was obtained. To obtain Sm-containing polyborosilazane, PBSZ was weighed and dissolved in THF before SmCl₃ dissolving in DMF was added. The precursors without and with different SmCl₃ mass fractions of 0, 1% and 3% were named as samples PS0, PS1 and PS3. The PBSZ was slowly heated in a tube furnace (TL1700, Nanjing Boyuntong Co., Ltd., China), in argon atmosphere at 180 ℃ for 1 h, and then further heated at 400 ℃ for 3 h to obtain a cured PBSZ. The cured PBSZ was ground and cold-pressed into φ20×2 mm wafers at 80 MPa. SiBCN ceramics were acquired via heating the wafers in argon atmosphere at 1 000 ℃ for 1 h in, and then further heated at 1 600 ℃ for 2 h.

The physical phase composition of the ceramics was determined by a model X'Pert MPD Pro X-ray diffractometer (XRD, The Netherlands) with Cu target at a tube voltage of 40 kV and a tube current of 30 mA. The elemental species, atomic valence states, and binding states of SiBCN ceramics were analyzed by a model AXIS SUPRA+ X-ray photoelectron spectroscope (XPS, Shimadzu Co., Japan). The microstructure and morphology were determined by a FEI NOVA 400 scanning electron microscope (SEM, USA). The microstructure and distribution pattern of the ceramics were characterized by a model JEM-F200 transmission electron microscope (TEM, Japan). The oxidation resistance of SiBCN ceramics was determined by a model STA 449F3 thermogravimetric analyzer (TG, Netzsch Co., Germany) in air at a heating rate of 10 ℃/min. The electromagnetic parameters of the ceramics were determined by a model Keysight E5071C vector network analyzer (Malaysia).

Sm doping promotes the increase of dielectric crystals such as SiC and SiCN in the ceramics. This is due to the better promotion of polymer ceramic crystallization by transition metals. The optimum oxidation resistance of the ceramics is obtained at Sm doping of 1%. The BN phase generates a great mass of B₂O₃ during high temperature oxidation due to the massive crystals generated, which is closer to the oxidative weight loss of free carbon. Also, B₂O₃ forms a liquid-phase protective layer that covers the ceramic surface and prevents a further diffusion of air into the interior of the material, improving the material antioxidant properties.

The superior impedance matching is obtained and the absorption performance of the ceramics for electromagnetic waves is enhanced due to the generation of dielectric crystals such as SiC and SiCN. At Sm doping of 1%, the ceramics have the optimum attenuation performance for electromagnetic waves due to the presence of conductivity loss and polarization loss in SiBCN ceramics, and the maximum attenuation constant α. When the thickness is 2 mm, the ceramic achieves the RA_min of -40.00 dB at 17.36 GHz and the effective absorption bandwidth (EAB) of 5.12 GHz.

The generation of SiC, SiCN and Si₃N₄ crystals was promoted by Sm doping, and the intensity of diffraction peaks of each crystal phase increased with the increase of Sm doping. The generation of SiC and SiCN dielectric crystals and their heterogeneous interfaces enhanced the attenuation performance of SiBCN ceramics against electromagnetic waves. The RAmin of the ceramics reached -40.00 dB at 17.36 GHz with an EAB of 5.12 GHz at a thickness of 2 mm.