With the rapid development of wireless communication and information technology, the electromagnetic wave generated by electronic equipment and communication facilities will cause serious damage to human health and living environment. Electromagnetic wave absorbing materials can convert electromagnetic waves into heat or other forms of energy, which plays an important role in reducing electromagnetic wave pollution. Electromagnetic wave absorbing materials have attracted recent attention. Common electromagnetic wave absorbing materials include magnetic materials and dielectric materials, such as ferrite, magnetic metals, carbon materials (i.e., carbon nanotubes, graphite carbon, etc.), oxide ceramics (i.e., zinc oxide, tin oxide, etc.) and silicon carbide, etc.. However, these materials are prone to oxidization and impedance mismatch at high temperatures. It is thus important to develop new dielectric materials with intense electromagnetic loss characteristics at high temperatures. Polymer-derived silicon boron carbon nitrogen (SiBCN) ceramics have superior corrosion resistance and high-temperature oxidation resistance, as one of electromagnetic wave absorbing materials that can be applied in harsh environments. However, pure phase SiBCN ceramics have the poor absorption of electromagnetic waves due to their low dielectric constant and low conductivity.

At present, in-situ generation or addition of high electromagnetic loss phases are often used to improve the absorption ability of SiBCN ceramics for electromagnetic waves. In this paper, SiZrBCN composite ceramics were prepared via introducing Zr into a polymer-derived SiBCN ceramic precursor. The evolution of the physical phase composition and microstructure of SiZrBCN composite ceramics by Zr addition was investigated, and the effect of physical phase compositions on the electromagnetic wave absorption performance of SiZrBCN ceramics was discussed.

Under the condition of ice bath, THF (280 mmol), DCMVS (150 mmol) and (CH₃)₂S·BH₃ (250 mmol) were injected into a three-necked round-bottomed flask and stirred thoroughly for 24 h to obtain an intermediate dichloromethylmethylsilyl ethylborane (TDSB). DCMS (180 mmol) and HMDZ (450 mmol) were sequentially added to TDSB. DCMS (180 mmol) and HMDZ (450 mmol) were added sequentially to TDSB and stirred for 24 h. Subsequently, the material in a three-necked flask was heated at 110 ℃ for 4 h, and then slowly heated at 170 ℃ for 4 h to obtain a clear liquid. The above products were distilled at a reduced pressure for three times to obtain PBSZ, and ZrCl₄ (12.5 mmol) was added into PBSZ and heated at 80 ℃ for 12 h and distilled at a reduced pressure to obtain a beige precursor named as sample Z5. The above experiments were repeated to prepare samples Z0, Z5, and Z10, respectively, and the experiments were carried out in Ar atmosphere throughout the whole process. The resulting samples Z0, Z5 and Z10 were heated at a heating rate of 2 ℃ at 280 ℃ for curing, and then the cured samples were heated in N₂ atmosphere at 1 600 ℃ to obtain samples Z0-1600, Z5-1600 and Z10-1600.

The chemical bonding and group compositions of SiBCN ceramic precursors were investigated by a model VERTEX 70 Fourier infrared spectrometer. The physical phase composition of SiBCN ceramics was investigated by a model X'Pert MPD Pro X-ray diffractometer under the following conditions (i.e., Cu as an anodic target, K_α-rays as a radiation source, voltage and current of 40 kV and 40 mA, respectively, and scanning range (2θ) of 10° to 90°. The chemical bonding composition of SiBCN ceramics was analyzed by a model ESCALAB250XI X-ray electron spectrometer. The microstructure of SiBCN ceramics was determined by a model Apreo S HiVac scanning electron microscope with a model TalosF200X transmission electron microscope. The electromagnetic parameters of the ceramics were examined by a model E5071C vector network analyzer.

ZrCl₄ reacts with the Si—H bond/N—H bond of PBSZ, and atom Zr replaces atom H to generate Si—Zr and N—Zr bonds. The addition of Zr inhibits the formation of SiC nanocrystals inside the ceramics, increases the generation of Si₃N₄ inside the ceramics, and increases the size of graphitic carbon inside the ceramics from 1.50 nm to 1.77 nm and 1.82 nm. SiZrBCN ceramics with Zr addition of 5% have the superior electromagnetic wave absorption performance, and the RL_min of SiZrBCN ceramics reach -21.8 dB at 7.8 GHz when the thickness is 2.5 mm.

1) During the precursor synthesis, ZrCl₄ reacted with Si—H bond/N—H bond of PBSZ, and Zr replaced H atoms to generate Si—Zr and N-Zr bonds, which grafted Zr onto the active sites of PBSZ.

2) After the ceramic precursor was heat-treated at 1 600 ℃, the crystals of Zr₂CN, SiC, Si₃N₄ and graphitic carbon were generated in-situ inside the ceramic, in which the addition of Zr suppressed the formation of SiC nanocrystals, increased the generation of Si₃N₄ inside the ceramic, and improved the impedance matching of the substrate. The presence of Zr promoted the generation of graphitic carbon inside the ceramic, and caused the in-plane crystallization of the graphitic carbon size increasing from 1.50 nm to 1.77 nm and 1.82 nm.

3) SiZrBCN ceramics with 5% Zr addition had the optimum electromagnetic wave absorption performance due to the large number of dielectric crystals generated inside the ceramics and the good impedance matching performance. The RL_min of the SiZrBCN ceramics reached -21.8 dB at 7.8 GHz when the thickness was 2.5 mm. The generation of a variety of dielectric crystals could improve the absorption performance of the material. The crystals could improve the electromagnetic wave absorption ability of the material, indicating that SiZrBCN ceramics could be used as an effective candidate in the field of wave-absorbing materials.