Preparation And Performance Of Fiber Reinforced Cement-based Electromagnetic Wave Absorbing Materials

In the modern society,space environmental pollution caused by electromagnetic radiation affects the survival and development of human.At present,the electromagnetic wave absorbing function of cement-based materials is achieved mainly by improving the impedance matching characteristics and enhancing the attenuation characteristics.However,the use of some porous admixtures and absorbents often greatly compromise the mechanical properties of cements.In addition,the existing cement based materials have poor performance in low frequency(S band)wave absorption.Therefore,to solve these two problems,fiber reinforced cement-based materials with single,double or three layers are prepared using fiber,waste PET flakes and blast furnace slag(BFS)as the wave transmitter of cement base and reinforcing medium and using carbon black(CB)as the absorber.Meanwhile,wave absorbing properties of the prepared composites are optimized by means of overlap resistor film,frequency selective surface(FSS)and surface shape design to prepare the cement wave-absorbing plates with low cost,light weight,both wave absorbing and load bearing functions.This paper consists of the following contents:According to transmission line theory,the absorption contour of cement-based absorbing materials can be made,and the influence of dielectric constant,thickness,resistance value,layer number and other parameters on reflectivity can be analyzed;or using the measured reflectivity inversion of electromagnetic parameters of materials.It can provide theoretical guidance for experiments,and reduce the number and blindness of experiments.Single layer fiber reinforced cement-based materials are prepared using Polypropylene fiber(PPF),alkali resistant glass fiber(GF)and coconut fiber(CF)respectively.PPF exhibits the most remarkable improvement of mechanical and absorbing properties.The compressive and flexural strengths of PPF reinforced cement increase by nearly 20%and 100%,respectively.It's very meaningful from structural materials.Through XRD,SEM micrograph and electromagnetic parameters analysis,it is found that PET bottle fragments and granulated blast furnace slag powder are good lightweight aggregate and reinforcing medium.Research has found that single or double doping of PET and/or blast furnace slag all has an optimization effect.At a thickness of 20 mm,the sample added with 20%PET plus 20%blast furnace slag has-37.8 dB absorption at 7.9 GHz,while the absorption bandwidth below-8 dB is up to 12.3 GHz.The double layer and three layer impedance gradient design of PET/BFS/fiber reinforced cement-based plate can reduce the thickness of the material and broaden the absorption band.The optimum ratio of the matching layer and absorption layer of double-layer material is 40%PET:20%PET+20%BFS.For the three-layer PET/fiber reinforced cement plate sample,the optimal gradient for PET content of matching,transition and absorbent layers is 50:40:10.The optimum doping content of carbon black in absorption layer is 4vol%.It is found that in the design of cement-based wave absorbing materials,improving the impedance matching characteristics of cement bodies is more important than improving the loss characteristics.The optimized method of sticking resistor film and FSS is adopted for PET/fiber reinforced cement-based plates.the optimal resistance of single layer film material is 382 ?.Meanwhile,the film sandwich design double-layer cement-based material can effectively improve the wave absorbing performance in the frequency bands of the civil S band(2-4 GHz),with the optimal resistance being 200 ?.As for the three-layer plate,two layers of resistive films are sandwiched,which optimizes the wave absorbing performance at low and high frequency regions with a 200 ?,100 ? combination.Based on the equivalent circuit analysis of FSS absorbing principle,the FSS patterns in closed,open and composite structures are designed.The optimization effect of FSS are all better than that of resistance film.For the 20mm thickness double-layer sample folded composite structures FSS,below-15 dB absorption is achieved over the full S(2-4 GHz)band.The three-dimension surface shape design of double-layer and three level fiber reinforced cement based materials is carried out,and the influence of the surface shape and density distribution on the absorbing properties is studied.It is found that the absorbing properties of the concave and convex specimens are all very good and the absorption bands are complementary.For the three-layer PET/fiber reinforced cement-based plate,circular groove with 24mm thickness has the best surface optimization effect,which achieves below-10 dB absorption in the 3.3-18.0 GHz frequency band.