1. Introduction
Radar stealth technology primarily involves techniques designed to reduce the detectability of objects by radar systems operating in the frequency range of 3 MHz to 300 GHz. Among these, the centimeter wave band (2–18 GHz) is particularly significant as it is widely used for detection and surveillance. Currently, countries around the world are actively working on breakthroughs in ultra-wideband radar stealth technology. As radar detection capabilities improve and target shape design becomes more constrained by tactical requirements, traditional radar-absorbing materials face limitations such as narrow bandwidth, low efficiency, and high density. These drawbacks restrict their application, making it essential to develop new types of absorbing materials and advanced stealth technologies. In recent years, researchers have focused on improving existing materials while exploring novel options. Materials like carbon nanotubes, conductive polymers, and nanomaterials have been increasingly applied in radar absorption, offering advantages such as strong absorption, wide frequency coverage, lightweight, and thin structure.
2. Conductive Polymer Absorbing Materials
Incorporating conductive fibers into conventional powder-based absorbers significantly enhances the material’s absorption performance. The optimal blending ratio of conductive fibers allows for the development of broadband radar-absorbing materials. However, the resulting materials often exhibit poor performance in the low-frequency range. Further research is needed to address this limitation. Conductive polymers offer advantages such as low density, good thermal stability, and ease of processing, which have led to their widespread use in recent years. For instance, Krishadham et al. studied iodine-doped polyphenylacetylene, polyacetylene, and p-phenylene-benzobisthiophene, achieving a reflection loss of -15 dB and an absorption bandwidth of up to 3 GHz. Similarly, Oldedo et al. found that polypyrrole, polyaniline, and poly-3-octylthiophene showed absorption levels exceeding 8 dB in the 3 cm band. TruongVT et al. developed a 2.5 mm thick material containing 2% polypyrrole, which achieved a reflectance below -10 dB between 12 and 18 GHz. Kong Deming et al. doped polyaniline with hydrochloric acid and FeCl3, achieving an average attenuation of 13.37 dB and a maximum of 26.70 dB in the 8–14 GHz range.
3. Magnetic Particle-Based Absorbing Materials
Magnetic particle-based absorbing materials typically exhibit high magnetic loss tangents, relying on mechanisms such as hysteresis loss, domain wall resonance, and aftereffect loss to absorb electromagnetic waves. Common materials include ferrites, metal micropowders, and polycrystalline iron fibers. Ferrites, especially hexagonal magnetoplumbite-type ones, are known for their excellent high-frequency absorption properties. MeshramMR et al. developed a hexagonal ferrite material that achieved a maximum absorption of 16.5 dB and a minimum of 8 dB in the 8–12 GHz range. Zhang Yongjing et al. created a 2 mm thick ferrite absorber with an areal density of 5 kg/m², showing absorption greater than 10 dB in the 8–18 GHz range. Nano-ferrite particles, due to their large surface area and unique electronic structure, demonstrate enhanced absorption through mechanisms like hysteresis loss and multiple scattering. Ruan et al. observed that a 5 μm ferrite sample had a maximum reflectance of -17 dB, while a 65 nm sample reached -28.5 dB. Huang Xixia’s study confirmed that smaller nano-scale Fe3O4 particles result in higher absorption efficiency. Magnetic metal micropowders, such as carbonyl iron, are widely used in stealth aircraft like the F/A-18C/D "Hornet." They combine free electron absorption with magnetic loss but suffer from issues like poor environmental resistance. Polycrystalline iron fibers generate strong eddy current and hysteresis losses, converting electromagnetic energy into heat. 3M’s polycrystalline iron fiber coating can achieve up to 30 dB attenuation in the 5–16 GHz band. Combining conductive polymers with magnetic particles offers a promising approach, but further research is needed to optimize material combinations, size control, and preparation methods for better performance and broader applications.
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