Preparation And Performance Studies Of Cementitious Wave Absorbing Materials Based On Impedance Matching

With the progress of the society and the rapid development of science and technology, a large number of electronic products and electrical equipment have been widely used and they bring convenience to people’s life. Meanwhile, they also produce a large number of electromagnetic radiation and cause serious electromagnetic pollution. On the other hand, the progress of the radar technology threatens the survival of military targets. The cement based materials have been widely used in military facilities and civil constructions, so the development of cement-based materials with performance of wave absorbing have important use value.Excellent absorbing materials should have good absorbing properties and wide absorption bandwidth. Based on the theory of impedance matching principle and maximum loss, stratified materials with electromagnetic parameters of gradient change were designed and studied. Matching layer is mainly to let more electromagnetic wave enter into materials, and give full play to the wastage of absorbing agent. In this paper, silica fume, air-entraining agent and foaming agent were mixed into cement based materials to prepare impedance matching layer and multi-walled carbon nanotubes,6H20 type ferrite and FP type ferrite were mixed into cement based materials as absorbing agent to prepare absorbing layer. Compositions and proportions of materials were adjusted to prepare layered cement-based materials with impedance change of gradient.Reflectivity, wave transmitting rate, electromagnetic parameters, pore structure of matching layer, electromagnetic parameters, reflectivity of absorbing layer and absorbing property of multi-layer cement base materials are studied by using test methods of RCS far field method, rectangular waveguide method, mercury injection experiment, spectrophotometer and SEM. The main research results and conclusions are as follows:(1) With the silica fume content increases, porosity of cement based materials decreases and dielectric constant increases gradually. Because of impedance matching performance becoming poor, the maximum wave transmitting rate decreases from 30.1%to 27.8%with the silica fume content increases; With the air entraining agent content increases, porosity of cement based materials increases and dielectric constant decreases gradually. Because of the improve of impedance matching performance, the maximum wave transmitting rate increases from 30.1%to 37.9% with the air entraining agent content increases(2) Through the maximum wave rate of matching layer mixed with 20% expanded perlite is close to 60%. The effect of expanded perlite on improving impedance matching is better than air-entraining agent and the thickness is easy to control, so cement mixed with expanded perlite can be used as matching layer.(3) Carbon nanotube suspension prepared by adding dispersant combined with mechanical stirring and ultrasonic slurry has good performance; With the increase of carbon nanotubes content, the real and imaginary part of dielectric constant sample increases gradually, but the real and imaginary part of permeability has no obviously change.(4) When samples are mixed with different content of FP type ferrite, the real and imaginary part of dielectric constant increases gradually with the increase of carbon nanotubes content and they are higher than single mixed the same amount of carbon nanotubes. But the permeability changes little; When samples are mixed with different content of 6H20 type ferrite, the real and imaginary part of dielectric constant also increases gradually with the increase of carbon nanotubes content and the permeability changes little.(5) Wave absorption performance of samples with 20% expanded perlite as the matching layer is better when absorbing layer is same and wave absorption performance of double-layer cement-based materials is superior to single-layer cement base materials with the same thickness. When FP type ferrite content are 0%,10%and 20%, respectively, wave absorption performance of double-layer cement-based materials strengthen first then weaken with the increase of carbon nanotubes content. When the FP type ferrite content is 30%, wave absorption performance gradually weakens with the increase of carbon nanotubes content.(6) When the matching layer is cement base materials with 20% expanded perlite and the middle absorbing layer is double mixed 0.50% carbon nanotubes and 20% FP type ferrite, absorption performance of samples with bottom absorbing layer double mixed 0.75% carbon nanotubes and 30% FP type ferrite is the best. And the bandwidth of the reflectivity below-7dBis 11.01 GHz.The bandwidth of the reflectivity below-lOdB is 7.65 GHz.