Research On Preparation And Microwave Absorption Properties Of Graphite Nanosheets And Its Hybrid Magnetic Materials

Microwave absorptive materials have been widely used in military stealth, information security and various electronic apparatuses to avoid electromagnetic wave reflection or interfering. But most of them are very thick and heavy, and can’t absorb microwave effectively in low frequency band, such as 2～4 GHz. Though CNTs have been studied as microwave absorptive materials for a long time, its practical application is limited by complicated production processes with high costs. In comparison, graphite is very cheap and its permittivity is considerably high. After being exfoliated into graphite nanosheets (GNS) and hybridized with magnetic materials, the microwave absorption ability could be improved remarkably. Therefore GNS is an ideal substitute for expensive CNTs as microwave absorptive material.In this thesis, GNS and its hybrid magnetic materials with ferrite or Mn-Zn ferrite were prepared and blended with water-born polyurethane for microwave absorptive coatings on vinylon fabrics. The influence of synthesis temperature, filling ratios and coating thickness on microwave absorption performance were investigated. Under optimized experiment parameters, vinylon fabrics coated with GNS/ferrite could achieve a reflection loss (RL) below -2.5 dB in the entire range of 2～18 GHz when the coating thickness was only 1.00 and 1.50 mm. Those samples exhibited obvious absorption in 2～4 GHz when compared with GNS coatings, which was helpful for resolving the problem of microwave absorption in low frequency bands.Firstly, GNS were prepared by microwave expansion and followed by ultrasonic exfoliation. X-ray diffraction analysis (XRD), fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscope (FESEM) and vector network analyzer (VNA) were used to characterize its structure and electromagnetic properties. Then vinylon fabrics with different coating thicknesses and weight contents of GNS were prepared and the reflection loss was tested. The results showed that the thickness of GNS is less than 50 nm and some oxygen-containing groups were introduced into GNS. For the fabric coated with 30 wt% GNS, RL values exceeding -5 dB could be obtained in the frequency range of 10.7-18 GHz when the coating thickness is merely 0.39 mm, with a minimum RL value of -28 dB.Secondly, in order to improve microwave absorption property in low frequency band, GNS/ferrite hybrids were prepared innovatively via in-situ chemical precipitation, carbon thermal reduction and ultrasonic exfoliation. The influence of synthesis temperature on crystal form, appearance, magnetic property and electromagnetic parameters was studied. Then microwave absorption performance of fabrics coated by GNS/ferrite hybrid materials with different coating thicknesses and filling ratios was analyzed. It could be concluded that the proportion of Fe3O4 in materials synthesized at 80 or 100℃ was higher than that of 120℃, and as for a-Fe2O3, it was opposite. So materials gained at 80 or 100℃ showed better magnetic and microwave absorption properties. For example, fabrics with 50 wt% GNS/ferrite (synthesized at 80 or 100℃) coatings could achieve a RL below -2.5 dB in the entire range of 2～18 GHz when the coating thickness is merely 1.00 and 1.50 mm, so the difficulty in microwave absorption in low frequency band was overcome.Finally, in order to investigate the influence of doping elements in ferrites on microwave absorption property, GNS/Mn-Zn ferrite were prepared in the same way with GNS/ferrite. The results of XRD, FTIR, FESEM, VSM, electromagnetic parameters and reflection loss showed that the synthesis temperature did not play an important role in properties of GNS/Mn-Zn ferrite, which was different from GNS/ferrite. All GNS/Mn-Zn ferrite hybrid materials had purer crystal composition than GNS/ferrite, but the coercivity and permittivity decreased. Both of two kinds of hybrids had their advantages in microwave absorption. At the same filling ratio (40 wt% or 50 wt%), the bandwidth of GNS/ferrite coatings was wider and the RL value of GNS/Mn-Zn ferrite coatings was smaller when compared to each other.Besides, according to the principle of impedance matching, both GNS/ferrite and GNS/Mn-Zn ferrite coatings were combined with GNS coatings into double-layered coatings. And it could be found microwave absorption performance was improved because of gradual change of impedance.