Comprehensive experimental design for microwave absorption of ultrathin BNNS/SiO₂/C composites

Electromagnetic pollution caused by the increasingly dense deployment of electronic equipment poses serious threats to human health, environmental safety, and precision equipment operation. Current research on microwave absorbing materials focuses on carbon-based materials, magnetic materials, and polymer systems. Carbon-based materials offer a large specific surface area, good conductivity, and excellent dielectric properties; however, their high conductivity can readily lead to impedance mismatch and skin effects. Hexagonal boron nitride (h-BN) has a crystal structure similar to that of graphite, a low dielectric constant, and strong chemical stability. When combined with carbon-based materials, its dielectric properties can be effectively tuned to optimize impedance matching and improve the absorption performance of the resulting composite. Nevertheless, the high performance currently reported relies on large matching thicknesses, which hinders practical application in lightweight and integrated devices. Achieving efficient absorption under ultrathin conditions, therefore, remains a technical bottleneck. To address this challenge, this work aims to develop an ultrathin boron nitride nanosheet (BNNS)/SiO₂/C ternary composite that achieves efficient absorption at minimal thicknesses, and to design a comprehensive teaching experiment that strengthens students’ understanding of absorption mechanisms and composite material design.

This experiment establishes a comprehensive experimental teaching program for fabricating ultrathin BNNS/SiO₂/C ternary composite microwave absorbing materials, achieving multi-scale structural regulation via a three-step “exfoliation–anchoring–carbonization” methodology. First, few-layer BNNS were obtained through a wet ball milling process coupled with ultrasonic exfoliation. Subsequently, chemical bonding of nano-SiO₂ was achieved via in situ conversion of polycarbosilane, and thermoplastic polyurethane gradient pyrolysis was introduced to fabricate a series of samples with varying carbon content. The microstructure morphology and phase composition were characterized by scanning electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and Raman spectroscopy, while electromagnetic parameters were measured using a vector network analyzer over the 2–18 GHz frequency range.

The BNNS/SiO₂/C ternary composite exhibited a loosely packed hierarchical stacking architecture with tightly bound particles and abundant interfaces. The I_D/I_G ratio increased with carbon content, indicating that higher carbon content introduced additional defect sites. By optimizing the carbon layer defect concentration, a stable dielectric constant with a smooth polarization response was maintained, thereby balancing impedance matching and loss capability. X-ray photoelectron spectroscopy (XPS) analysis confirmed that C–N bonding generated dipolar centers. Benefiting from synergistic conductive loss, dipolar polarization, interfacial polarization, and multiple scattering effects, the composite achieved a minimum reflection loss (RL_min) of−43.24 dB at a thickness of only 1 mm and an effective absorption bandwidth (EAB) of 2.89 GHz at 1.5 mm thickness.

BNNS/SiO₂/C composites demonstrate excellent microwave absorption at minimal thickness. This experiment integrates material preparation, characterization, and performance evaluation, cultivating students’ comprehensive design and problem-solving abilities while providing a new pedagogical case for electromagnetic functional materials education.