Design,Properties And Compatibility To Surroundings Of Modulated Cement-based Microwave Absorbing Materials

With the improvement of radar detection technology,protection and concealment of building engineering become more and more difficult.It is urgent to carry out research on new cement-based absorbing materials and their compatibility to surroundings based on the principle of radar detection.When traditional cement-based materials are prepared and molded,their absorbing performance are fixed,and it is difficult to make them match with the electromagnetic（EM）environment.At present,the research on cement-based absorbing materials mainly focuses on the optimization of high-efficiency absorbing agents and absorbing performance.There are few studies concerning the adjustable absorbing performance of cement-based materials and their compatibility to surroundings.Modulated cement-based absorbing materials are designed and prepared based on their composition and structure and the types and contents of absorbing agents.The EM parameters of cement-based materials were modulated by changing the number and distribution of magnetic fluids and magnetic fields.The regulation mechanism of the modulated cement-based material’s microwave absorbing performance was clarified.Based on this,the compatibility of the modulated cement-based absorbing material with three environmental backgrounds（desert,grassland and woodland）were studied,and a method to match cement-based absorbing material with different environmental backgrounds was proposed.The main results achieved in this study are as follows:1.In order to improve the absorbing performance of cement-based materials,the influence of resistive absorber carbon powder,laying carbon fiber mesh,magnetic medium absorber Fe₃O₄ and porous structure formed by hollow glass microspheres（HGM）on the mechanical properties and the static regulation of absorbing performance of cement-based materials was studied.It found that with the increase of the amount of absorber,the absorbing performance of cement-based materials increased first and then decreased.When 10%Fe₃O₄ or 3%carbon powder was added,the minimum reflectivity（R）of the prepared samples was-9.5 d B and the effective frequency band less than-7 d B reached 10 GHz.Absorbers improved the absorbing performance of cement-based materials through electrical or magnetic loss,and its function is similar to the lossy resistance.Based on this principle,equivalent circuit model of single resistance was established.Through the change of the relationship between the content of absorber and the impedance value of material,the purpose of adjusting and controlling the absorbing performance can be achieved.As the content of HGM or discontinuous graded sand increased,the porosity of cement-based materials increased.The absorbing performance was improved,but the mechanical properties were obviously reduced.When the HGM content was10%or the sand-to-cement ratio was 3:1,the minimum R of the prepared sample was improved by about 60%.The reflection,refraction and scattering of EM waves in cement-based materials were increased and eddy currents were formed by the effect of the porous structure.Based on this principle,equivalent RLC model was established.The input impedance of cement-based materials was greater than 800Ω,and the static adjustment range of the minimum R was-5.8to-9.0 d B.2.Based on the requirements of impedance matching and loss performance,laminated cement-based absorbing materials including impedance matching-loss layer were designed,which further broadened the absorbing frequency band and improved the absorbing performance of cement-based materials.It was found that laminated cement-based material with HGM mixed in the matching layer and discontinuous 1.25-2.5 mm sand mixed in the loss layer had good absorbing performance,with a minimum R of-9.9 d B.The lossy layer mixed with absorber further improved the absorbing performance of cement-based materials.As the thickness of the matching layer increased,the absorbing performance of laminated cement-based materials first increased and then decreased.When the thickness of matching layer met the 1/4 wavelength theory,the absorbing performance became better,and its minimum R reached-12.4 d B.The effective frequency band less than-10 d B was 5.2 GHz,and the laminated structure had good mechanical properties.Based on the principle that the absorbing performance of the laminated structure was the vector synthesis of the EM wave absorption process by the matching layer and loss layer,so the double RLC series equivalent model was established.The input impedance range of laminated cement-based material was 630～800Ω,and the static adjustment range of the minimum R was-9.0 to-12.3 d B.3.To realize dynamic control of the absorbing performance of cement-based materials,the Fe₃O₄ magnetic fluid tube was designed and embedded in the cement-based absorbing material.The existing form of magnetic fluid was changed by applying magnetic field or releasing the magnetic fluid,and the dynamically adjustable cement-based absorbing materials were prepared.In 8-18 GHz,by applying magnetic field or releasing magnetic fluid method,the dynamic control range of the minimum R of the adjustable cement-based absorbing material was-6.1 to-9.4 d B,and the minimum R of the laminated adjustable cement-based absorbing material can be dynamically adjusted at-10.0 to-12.3 d B.When the magnetic field was loaded at the bottom,Fe₃O₄ magnetic fluid was deposited under the action of the magnetic field,so that the EM waves were reflected on the surface,which made its absorbing performance decrease.When the magnetic field was loaded laterally or the magnetic fluid was released,it was equivalent to preset the hole structure in the cement-based material,which increased the reflection and scattering of EM waves inside the material,and improved the absorption of EM waves by the cement-based material.Based on the adjustable principle,the dynamic adjustable cement-based absorbing material was equivalent to adding a sliding rheostat in the equivalent circuit model.By changing the resistance value of the sliding rheostat to achieve the dynamic controllability of the absorbing performance.The input impedance ranges of the single-layer and laminated structure adjustable cement-based materials were 750 to 1700Ω,650 to 980Ω,and the dynamic control ranges of the minimum R were-6.7 to-10.5 d B,-9.8 to-13.9 d B,respectively.4.Based on the established equivalent circuit model of the modulated cement-based absorbing material and the requirements of matching degree,the target design principle of cement-based absorbing material was put forward.By investigating the absorbing performance of desert,grassland and woodland environments,different cement-based absorbing materials were designed.The matching degree of the absorbing performance of the three typical environments and cement-based materials was analyzed in the C,X and Ku bands.For a single dry or wet environment,the equivalent circuit model of the modulated cement-based absorbing material with Z1≠0,Z2≠0,and Z3=0 was adopted.The matching degrees of the absorbing performance of designed cement-based absorbing material and the desert and grassland environment in the C,X and Ku bands were more than 80%,which had excellent matching performance.For changing dry/wet condition of the environment,the equivalent circuit model of modulated cement-based absorbing materials with Z1,Z2,and Z3 was adopted.The matching degrees of the absorbing performance of designed adjustable cement-based absorbing material and the desert and grassland environment in the C,X and Ku bands were more than80%.Due to the large fluctuation of the absorbing performance of the woodland environment,the cement-based absorbing material designed by the modulated equivalent circuit model had a poor matching degree with the absorbing performance of the woodland environment.In this study,from the perspective of the composition,structure and performance of cement-based materials,an equivalent circuit model of modulated cement-based absorbing materials was established.Based on the absorbing performance of environment and matching degree requirements,cement-based absorbing materials can be designed to make its absorbing performance match with the environment.In addition,this study provides technical support and scientific basis for the preparation of cement-based materials with adjustable absorbing performance,and is of great significance to broaden the application of cement-based absorbing materials.