Preparation And Absorbing Properties Of Cement-based Composites Plates

Dealing with the worsening electromagnetic radiations and the requirements of electromagnetic protection for tall buildings, the demands for developing electromagnetic wave absorbers with wider absorbing bandwidths, more effective absorption properties and lower costs are ever increasing. Therefore, single or double-layer cement-based absorbing flats with excellent absorption functions were prepared in this paper.Pure cement has a terrible impedance matching performance and slightly absorbing ability due to some metal oxides insides. Porous aggregates can improve the impedance matching characteristics of cement composites by decreasing the effective dielectric constant, and attenuate electromagnetic wave by mulitple scattering and reflection. The filling ratio, the type and the size of porous beads all have remarkable influences on the absorbing properties. There is an optimal content for porous beads, which is about 50% by volume ratio. Aggregate types have some influences on the absorbing properties, e.g. high pore ratio, enclosed construction and symmetric geometry are helpful to the improvement of absorbing properties. Porous beads with larger sizes benefit the improvement of matching characteristics. In contrast, the beads with smaller sizes can increase the attenuation properties more obviously. Among these aggregates we used, expanded polystyrene (EPS) has the best ability to improve the absorbing properties, with the minimum reflection value of-18 dB and the effective absorption bandwidth of more than 8.1 GHz.The absorbing mechanisms of carbon black, manganese dioxide, iron silicon powders, Ni-Zn ferrite and carbonyl iron fibers were investigated. And then, the optimal content of each absorbent in cement were defined. The results show that the optimal contents are controlled by the matching characteristics of composites. Absorbing agents can increase the numbers of attenuation particles in matrix to improve the attenuation probability of incident wave when their contents are low; however, if the contents are too high, the impedance matching performance become terrible resulting in serious reflections and bad absorption propeties. When absorbents with different absorbing mechanisms are used together, the absorbing properties can be improvement a lot, attributing to the advantage of complex absorbent. The best proportions of complex absorbents are studied, too. When the volume ration of carbon black to ferrite is 4:5, an absorption band as broad as 12.1 GHz is obtained.The microstructure and electromagnetic parameters of Fly ash (FA) were charactered by means of scanning electron microscope and vector network analyzer, respectively. The results show that FA has the potentialities for wave attenuation due to its complex components and porous structure. However, the pozzolanic activity and packing effect of FA has a negative effect on the absorbing properties of FA/cement. In contrast, the absorbing properties of EPS/cement composite don't have obvious decrease when FA is used as cement replacement. Moreover, when FA is used together with MnO2, the absorbing properties can be improved obviously due to the advantages of complex absorbents. This is a new way for recycling FA.Because of its electromagnetic transparently property and large length-diameter ratio, aramid fiber can adjust the impedance characteristic of cement material in a low content. And more important, aramid fiber can increase the mechanical properties of cement material. With the increase in aramid fiber content, the flexural strength and toughness increase significantly, and the compressive strength has an obvious improvement followed by a gradually decrease, in a sharp contrast to the decrease of mechanical properties by filling EPS.Double-layer cement matrix composites, which form an impedance gradient among the free space and the layers of composites according to the transmission line theory, can get an excellent impedance matching performance and good attenuation characteristic at the same time, so the absorbing properties can be improved greatly. Among these aggregates we used, SiO2 and EPS with large sizes can improve the impedance matching characteristic obviously, while FA, pearlite and EPS with small sizes are more effective for wave attenuation. Ferrite is the ideal filler for matching layer and carbon black is suitable for absorbing layer filler due to its large dieleltric constant. The optimal content of carbon black in absorbing layer is more than its in single layer material, resulting in a stronger attenuation performance with a broad absorption bandwidth of more than 10 GHz. Special-shaped structure surfaces make the continuity change of impedance and increase the incident times for electromagnetic wave. Thus, the absorbers have much better impedance performance. Besides, the propagation direction of part of the incident wave changes, which is also helpful for wave attenuation. The minimum reflection value and effective absorption bandwidth of samples with shaped-surfaces are-30 dB and 13 GHz, respectively.