| Electromagnetic(EM)wave absorbing materials are the main methods to solve the increasingly serious EM pollution in the environment.Cement-based materials are widely used in construction projects,but they have low EM wave absorbing properties due to their low EM loss performance and high dielectric constant.Cement-based materials with excellent absorbing properties,large bandwidth,and high strength have great application markets.Cement-based materials with macro structures,which are porous structure and3 D gradient structure,were prepared and their EM wave absorbing properties were investigated.The porous structure was constructed by introducing porous glass particles.The effects of particle size,gradation,absorbing agent dosage and aggregate content on the absorbing properties of the material were studied.The wave velocity and refractive index in porous structural materials were approximately calculated with the reflectivity curves of the flat board.The calculated wave velocity is used to judge the scattering type of the particles in the material.The test results serve as the research basis for cement-based materials with 3D structures.Cement-based materials with physical gradient 3D structures was designed and manufactured in order to further improve the EM wave absorbing properties.The preparation of a cement-based material with a 3D structure is a difficulty that hinders the development.Using 3D printing technology,master molds with different 3D structures and sizes were printed,and flexible silicone molds were fabricated with the master molds.Therefore,cement-based materials with 3D structures were prepared with the flexible silicone molds.The preparation method is simple,the size of the 3D structure is controllable,and molding of complex structures is realized.The 3D structures included wedge,pyramid,and conical shapes,and the arrangement method included tight arrangement and staggered distribution.Based on experimental and simulated results,the influencing factors of EM wave absorbing properties were discussed,and the absorbing mechanism was summarized.In order to protect 3D structures and increase the strength of the cement-based materials,double layered cement-based materials with 3D structural interface were designed.A protective matching layer was added on the wave-facing side of the single-layer cement-based material with 3D structures.The influence of material type,the thickness and the amount of absorbing agent in the matching layer,and the cavity structures on the EM wave properties were studied.The results showed that:(1)The absorbing performance in the frequency range of 1.7～18 GHz is enhanced more significantly with the particle size of 0.25 to 1.0mm in the particles with the size of 0.1～2.0 mm.There is no significant synergy in EM wave absorbing performance between the structures with different pore sizes.(2)The absorbing test results consistently showed that the 3D structure significantly improved the absorbing properties of the material,and larger height of the structure brought better absorbing performance.The frequency band with the reflectivity lower than-20 dB reached S band(2～4 GHz)when the structure height is 40 mm.The average reflectivity reached-27 dB when the carbon black volume content was1.5%,the wedge structures were staggered arranged and the wedge height was 30 mm.(3)The absorbing properties of the material was significantly reduced with a matching layer.Using polyurethane as the wave-transmitting layer had the smallest effect on the wave-absorbing properties of the 3D structural cement-based board,with an average reflectivity of-27.9 dB.A two-layer structure with cavities in between the two layers decreased the absorbing properties of the material.The EM wave absorbing mechanisms of single layer materials with 3D structures include: the physical gradient brought about a gradient change in the impedance between the free space and the material,and more EM waves entered the material and then was attenuated;the multiple reflections of EM waves between 3D structures provide more opportunities to transmit into the materials and more chances of coherence,leading to more internal dielectric loss and coherent attenuation;Different forms of diffraction occurred at the tip and edge of the 3D structures,changing the EM wave propagation path.The EM wave absorbing mechanisms of double-layer materials are: the double-layer mortar increased the surface reflection of the material,reduced the multiple reflection process between the3 D structures,which lead to significant reduction in coherent loss.Cement-based absorbing boards with 3D structures can be used for EM protection of buildings and radar stealth of military installations. |
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