The Electromagnetic Absorbing Characteristics Of Silicon Dioxide And Cement Based Composite Plates

This dissertation is aimed at the increasingly severe electromagnetic pollution and both the urgency and broadness of the research on building materials for electromagnetic interference (EMI) prevention. This dissertation discusses the electromagnetic characteristics of two kinds of composite plates and explores two building materials for electromagnetic wave absorbing based on silicon dioxide (SiO₂) and cement matrices.The electromagnetic properties of nanometer carbon black (CB) filling SiO₂ composites were first studied. The micrograph and electromagnetic parameters of CB particulates were analyzed with TEM microscope and coaxial method. The electromagnetic shielding effectiveness (SE) in 150-1500 MHz and the wave absorbing properties in 2-18 GHz were studied with sodium silicate and polyvinyl alcohol (PVA) as the adhesive binders. Findings show that CB is a kind of electric loss medium and it has high electric conductivity and dielectric constants. The SE of CB composites increase with the increasing of CB content and frequency. When the CB content is 15 vol%, a plate with a thickness of 7 mm gives a SE of -35 dB. There is a critic content in CB filling composite for wave absorbing. When the CB content is over 6.0 vol%, the wave reflection on the surface of the material increases and so the wave absorbing performance decreases.The electromagnetic properties of manganese dioxide (MnO₂) particulates were discussed in this dissertation for the first time. TEM, XRD and coaxial method were applied to analyze the properties of MnO₂ particulates and SiO₂/MnO₂ composites. Results show that MnO₂ is a kind of dielectric loss medium, and it has high electric resistivity and dielectric constants. When the MnO₂ content is lower than 40 vol%, the wave absorbing peak values of MnO₂ composite increase with the increasing of MnO₂ filling fraction and the matching frequencies drift to the lower band with MnO₂ increment.The modified Rayleigh mixing formula is applied to calculate the effective electromagnetic parameters and reflection loss of CB or MnO₂ filling composites, and it indicates that when the CB and MnO₂ content is high, the calculated values are closely agree to the experimental ones. It indicates that Rayleigh mixing formula can be applied to the calculation of electromagnetic properties in high concentrations.The different influences of binders on the electromagnetic properties of SiO₂ based composites were studied. The results show that the sample with sodium silicate as theadhesive binder has a higher SE than that with PVA, but their comprehensive wave absorbing properties have not many differences, only that the absorbing curves of samples with sodium silicate as the binders are relatively flat, but PVA samples have one or more peak values. The sample with sodium silicate as the binder has a relatively higher compressive strength than that with PVA.The electromagnetic absorbing properties of expanded polystyrene (EPS) filled lightweight cement based composites were studied in this dissertation for the first time. A surface pretreatment was first used to treat the EPS beads so as to improve the affinity with cement matrix and the uniformity during mixing. Samples with different EPS filling ratio and different thickness were made and their wave absorbing properties were studied with the arched testing method in an anechoic chamber. The analysis of the changes of dielectric property during the hydration and the inner macrostructure of the EPS cement were carried out to investigate the electromagnetic absorbing mechanism of the EPS cements. The effects of thickness, filling ratio and EPS bead diameter on absorbing properties were studied. The matching thickness of the composite was also discussed. The results show that the EPS cement composite holds a good absorbing characteristic and its absorbing property increases with the EPS filling ratio until 60 vol%, and a decrease is observed afterwards. With the addition of EPS beads, the wave transparent property of the EPS filled cement composite improved gradually and so the matching thickness increased. In the frequency range of 8-18 GHz, a 20-mm-thick sample filled with 60 vol% EPS with a diameter of 2 mm can have an absorbing property of-10—15 dB, and the bandwidth of-10 dB reaches 6.2 GHz.The studies on the compressive strength of EPS filling cement composite show that the compressive strength appears to decrease linearly with the increase of EPS volume and increase gradually with the curing time. The compressive strength has a close relationship with the bonding strength at the interface between EPS and the cement matrix. The EPS bead with smaller size has more specific surface areas and more interfaces with cement matrix, and its smallness has decreased the pores and defects at the interfaces, so cement composites filled with