Electromagnetic Properties And Design Theory Of Thin Dallenbach Layer Radar Absorbing Materials

With the rapid development of electronic information technology in the civil and military fields,many problems such as electromagnetic interference and radar stealth can not be solved without radar absorbing materials?RAM,Radar Absorbing Materials?.According to the principle of absorbing electromagnetic waves,radar absorbing materials can be divided into non-resonant and resonant absorbing materials.According to the form of materials,they can be divided into structural and coated absorbing materials.Among these materials,the Dallenbach layer absorbing material has become the most widely used and the most representative one because of its thin thickness and simple material preparation process.Radar absorbing materials need to be lightweight,thin,wideband and have high absorption simultaneously.Therefore,the development of absorbing materials with lightness,high efficiency,wide frequency,high weather resistance and high stability has been the focus of research the field.Traditional absorbing materials are mainly carbonyl iron powder,magnetic metal powder and ferrite.These materials have high density and large thickness as the Dallenbach type absorber,which has many limitations for practical applications.On the other hand,the design theory of ultra-thin radar absorbing materials is still lack of in-depth systematic research.In the past,the design of absorbing materials was mostly based on the summary of massive experimental results,and lacked systematic design methods.Therefore,this dissertation uses different preparation techniques and different materials to improve and control the performance of absorbing materials,explore the intrinsic relation between the performance of Dallenbach absorbing materials and the intrinsic electromagnetic parameters of materials,and systematically study the properties and design theory of thin Dallenbach absorbing materials.This design theory can help us do the relevant research work with a purpose,greatly shorten the development cycle of absorbing materials,and has extremely important guiding significance for the design and preparation of radar absorbing materials.In this dissertation,a lightweight radar absorbing material of graphene aerogel and aramid honeycomb composite structure was prepared and studied.The intrinsic electromagnetic parameters of graphene aerogel were measured,and the electromagnetic scattering characteristics of graphene aerogel were analyzed in detail according to the Drude model.It is found that graphene aerogel can absorb the electromagnetic waves above its plasma frequency,while reflect the electromagnetic waves below its plasma frequency.However,combined with the aramid honeycomb,the composite has a better microwave absorption,which has a thickness of only 4 mm and an areal density of 0.26 kg/m²,but can achieve-15 dB reflectance loss at 8.8 GHz.This is mainly because the aramid honeycomb structure increases the equivalent impedance of the composite as a whole,reduces the direct reflection of the electromagnetic wave at the interface,and increases the multiple scattering paths of electromagnetic wave inside the composite.Magnetic materials play an important role in absorbing materials.Undoped M-type barium ferrite has a high natural resonance frequency and is not suitable for X-band radar absorbing materials.In this dissertation,Zr⁴⁺ high-valent doped barium ferrite materials were prepared by sol-gel method,which shows typical hard magnetic properties.Experimental studies have found that when such a hard magnetic barium ferrite material is used as a Dallenbach absorber,there is a significant inverse correlation between the microwave absorption properties and the magnetocrystalline anisotropy constant of the material.The Zn²⁺-Zr⁴⁺ doped M-type barium ferrite material changes from hard magnetic material to soft magnetic material with the increase of doping concentration,and the natural resonance frequency of the material decreases,resulting in its relaxtion of permeability in X-band.This relaxation of permeability effectively enhances the performance of the material as a Dallenbach absorber The study found that the series of Ba?ZnZr?_xFe_12-2xO₁₉ materials obtained the optimal X-band absorbing performance at the doping concentration x=0.6.In addition,the composite film of these Zn²⁺-Zr⁴⁺ doped barium ferrite material and graphene material is prepared by tape-casting method.This composite film shows excellent microwave absorption performance,achieving-20 dB in X-band with 0.6 mm in thickness and 0.50 kg/m² in areal density,which is only 5% of the areal density of the conventional carbonyl iron absorbing material.From the perspective of methodology,this dissertataion proposes a graphics array analysis method for designing thin Dallenbach absorbing materials with strong absorption effect.This design method not only points out the defects of traditional impedance matching theory and generalized matching theory,but also proposes the material selection and preparation method of absorbing materials under universal conditions,which shows the design route of absorbing materials.In the design theory of broadband absorbing materials,this dissertation gives the relation between the effective bandwidth of thin Dallenbach absorbing materials and the dispersion effect of permittivity and permeability.In order to achieve good microwave absorption performance in a wide frequency range,the product of the real part of permittivity and permeability should satisfy the relationship ?'?f??'?f?? f^-? in the required frequency range,in wich |?-2|?p,p is determined by the difference between the broadband performance specification designed and the theoretical maximum reflection loss.Finally,the practicability and correctness of the strong absorption design theory and broadband design theory of thin Dallenbach layer absorbing materials proposed in this dissertation are fully verified by comparing with the experimental data in Chapter 3.This theory provides new ideas for the design of thin-layer absorbing material in the future.