5G base station is a critical infrastructure for the development of mobile communication networks. Materials with high infrared reflectivity and high microwave transmittance compatibility can improve the heat dissipation performance of base station radomes, while maintaining microwave transmission characteristics. Therefore, it has significant technical advantages and application significance in the development of mobile communication. However, high infrared reflectivity and high microwave transmittance have been difficult to be compatible. In this paper, a photonic crystal structure composed of Si/SiO₂ is designed, basing on the transmission matrix theory in combination with the related theory of multilayer film system. In this periodic dielectric structure, the propagation characteristics of electromagnetic waves are modulated by the Bragg scattering effect, which results in the formation of a photonic bandgap, thus giving out high infrared reflectivity performance. In addition, the structure exhibits excellent transmittance in the microwave frequency range. The Si/SiO₂ photonic crystal structure designed in this paper has a wide range of potential applications.

According to the transmission matrix theory and photonic forbidden band theory, the refractive index contrast of dielectric materials directly determines the width of the photonic forbidden band. The refractive index (n) of Si in the mid-infrared band of 3–5 μm shows good dispersion stability and its value does not change significantly over the wavelength range, with a stabilized value of about 3.5. The refractive index of SiO₂ in the wide spectral range of 3–5 μm is stabilized at about 1.4. This excellent optical stability provides the basis for the precise tuning of the photonic band gap. Therefore, in this paper, the Si/SiO₂ group photonic crystal structure was designed and prepared by magnetron sputtering using Si and SiO₂ as a combination of high/low refractive index media.

The designed Si/SiO₂ photonic crystal structure has a photonic forbidden band in the range of 3.00–5.08 μm. The frequency of the bottom of the forbidden band is 5.9×10¹³ Hz, the frequency of the top of the forbidden band is 1.0×10¹⁴ Hz, while the width of the forbidden band is 4×10¹³ Hz. By simulating and analyzing the structure with different numbers of layers and layer thicknesses, the final Si/SiO₂ photonic crystal structure has 8 layers. The thicknesses of Si and SiO₂ are 275 nm and 650 nm, respectively, while the total thickness is 3.7 μm. Infrared reflectance spectral properties of the Si/SiO₂ photonic crystal structure are analyzed at different incidence angles. The reflectance spectra of the photonic crystals show an obvious blue-shift phenomenon when the angle of incidence is gradually increased from 0° to 80°, but the excellent and high reflectance characteristics (reflectance>98%) are always maintained at 3–5 μm. The surface of the prepared Si/SiO₂ photonic samples consists of four substances: Si, SiO₂, Al₂O₃ and Co₃O₄. Si and SiO₂ layers are stacked alternatively and the boundaries are smooth and clear. The infrared reflectance of the Si/SiO₂ photonic crystals samples is tested by using Fourier infrared spectrometer and the average reflectance at 3–5 μm is 95%, which is close to the simulated data. At the same time, the microwave transmittance of the Si/SiO₂ photonic crystal structure was tested by using a vector network analyzer and the average transmittance from 2 to 18 GHz was 94.5%, which is consistent with the simulation results and the measured data.

In this paper, a SiO₂/Si photonic crystal structure is designed based on the transmission matrix theory and photonic forbidden band theory, while the infrared reflectivity and microwave transmittance of the structure are analyzed. The SiO₂/Si photonic crystal structure is a periodic structure with a photonic forbidden bandwidth of 4×10¹³ Hz, which maintains a high reflectivity of >98% in the wavelength range of 3–5 μm, when the incident angle is varied from 0 to 80°. The average reflectivity of the Si/SiO₂ structure is 95.0% in the wavelength of 3–5 μm, while the average transmittance is 94.5% in the of 2–18 GHz, which are consistent with the simulation results, thus verifying the validity of the design. The SiO₂/Si photonic crystal structure realizes the compatibility between the mid-infrared high reflectivity and high microwave transmittance domains, possessing a wide range of applications in the fields of heat dissipation efficiency improvement, signal transmission and multispectral compatibility.

Polymer-derived ceramics are prepared via forming precursors through the polymerization of tiny molecules and cracking at high temperatures. Compared to conventional ceramics, their advantage lies in an ability to precisely control the microstructure and crystalline phase composition through the design of the molecular structure and elemental composition of the precursor and subsequent thermal treatment, thereby producing the optimal final properties. Among these, SiBCN ceramics stand out within the polymer-derived ceramics due to their flexible molecular structure designability. This enables the in-situ formation of multi-phase synergistic loss systems incorporating SiC, BN and graphitic carbon, coupled with a unique oxidation resistance mechanism, which excel particularly within polymer-derived ceramic systems. However, SiBCN ceramics primarily exist in an amorphous state at lower temperatures (i.e., < 1400 ℃), thus limiting their application in electromagnetic wave absorption. The paper was to introduce Ti nanopowder during the ceramicization process to catalyze the formation of nano-dielectric crystals such as SiC, TiC, and crystalline graphite. These crystals could enhance the dielectric imaginary part of SiBCN ceramics, thereby strengthening their electromagnetic wave attenuation capabilities.

For the synthesis of polymer precursor, tetrahydrofuran (THF)-methylvinyl dichlorosilane and borane dimethyl sulfide complex were mixed into a three-neck flask and conducted in argon for 24 h. Also, methyl dichlorosilane and hexamethyldisilazane were introduced, and the reaction was continued at the ambient temperature for 24 h. Subsequently, the mixture was then heated from room temperature to 170 ℃ for amide copolymerization reaction. After holding at this temperature for 3 h, vacuum distillation was performed, and filtrated for three cycles, thus producing a pale yellow polyborosilazane (PBSZ). For the synthesis of SiBCN ceramibs, polyborosilazane (PBSZ) was placed in a tube furnace and heated to 280 ℃ for 2 h to fully cure the precursor. The cured sample was subjected to ball grinding. The resultant ground powder was mixed with Ti nanopowder at different Ti mass contents (i.e, 0%, 5%, 10%, and 15%), and then was ground to produce different composite powders,. The composite powders were pressed into discs with the diameter of ϕ20 mm. The discs were heat-treated in a vertical tube furnace (i.e., firstly heating at 800 ℃ for 1 h, and thenheating at 1000 ℃ for 2 h) to allow enough molecular diffusion for TiC crystal formation, resulting in SiBCN ceramics.

The analysis of the four-component doped ceramics reveals that Ti nanoparticles doping positively affects both the phase composition and dielectric properties of SiBCN ceramics. The XRD patterns indicate that pure SiBCN ceramics remain amorphous after heat treatment at 1000 ℃, whereas the addition of Ti nano-particles promotes the formation of TiC crystals within the ceramics, thereby enhancing their crystalline properties. The SEM and TEM images demonstrate that varying the nano-Ti doping content alters the microstructure of SiBCN ceramics. Nano-Ti addition promotes the formation of a porous structure within the ceramics and facilitates the growth of crystals such as TiC and carbon nanotubes, enriching the phase composition of the ceramics. Varying Ti nanoparticles doping contents alters SiBCN's electromagnetic wave absorption and loss capabilities. Compared to pure SiBCN ceramics, Ti nanoparticles doping confers higher electromagnetic parameters and lower reflection loss, and 10% Ti nanoparticles-doped SiBCN exhibits the optimum electromagnetic wave absorption performance. The incorporation of Ti nanoparticles optimizes the ceramic structure, with synergistic interactions among various crystals and structural components, thus enhancing the overall performance.

This study demonstrated that doping Ti nano-particles into SiBCN ceramic could enhance the ceramic dielectric loss and impedance matching qualities. Ti nano-particles enhanced the low-temperature crystallization property of SiBCN. The crystallinity and microstructure of SiBCN ceramics could be adjusted by varying the nano-Ti doping content. The ceramics heat-treated at 1000 ℃ could develop porous architectures, TiC, and crystalline phases such as crystalline carbon. Ti nanoparticles improved the electromagnetic wave attenuation properties of SiBCN. The formation of TiC and carbon nanotubes, along with the heterogeneous interfaces formed with the amorphous matrix, could boost the electromagnetic wave attenuation performance of SiBCN ceramics. The crystallinity of SiBCN ceramics and the presence of abundant atomic defects resulted in a significant polarization loss, thereby enhancing the ceramic's electromagnetic wave absorption capability. At Ti nanoparticles content of 10%, the RLmin value of SiBCN ceramics at 6.24 GHz achieved –44.5 dB, with an EAB as high as 3.43 GHz, indicating that adding Ti nanoparticles could effectively enhance the electromagnetic wave absorption capacity of low-temperature heat-treated SiBCN ceramics.

An 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.

20 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).

A 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.

1) 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.