Fabrication And Characterization Of Graphene/Epoxy Modified Cyanate Ester Microwave Absorbing Composites

Electromagnetic (EM) interference problems have emerged due to the increasing usage of electronic devices and communication facilities in industry, commerce and military affairs. It not only affects normal communication, but also threats to human health, and has become the hot topic in society and the scientific community. Microwave absorbing materials (MAM) can dissipate, attenuate incident microwave energy or absorb EM waves and convert EM energy into other types of energy However, traditional MAM have narrow wave absorbing band, poor flexibility, high density and poor environmental stability which restrict their widely applications as microwave absorbents. Carbon nanofillers——Carbon nanotubes (CNTs) and graphene, which are the one and two-dimensional carbon materials respectively, have gained great attentions worldwide in the research of MAM owing to their large aspect ratio, excellent electrical conductivity and dielectric properties, low density, as well as corrosion resistance Cyanate ester (CE) systems possess many outstanding properties such as microwave transparent structure, low dielectric constant, so it can be used as wave-transparent layers. It is shown that adding absorbing agent in wave-transparent materials, can not only reduce the matching thickness of the absorber, but also can buffer impedance difference between absorbing layer and air layer, thus can improve the effect of absorbing materials.This paper selects epoxy modified cyanate ester resin (EP-CE) as matrix, graphene nanosheets (GNSs) magnetic ferrite and magnetic metal powders as absorbing agent to prepare high-performance absorbing materials. Dependence of the type of carbon fillers, magnetic ferrite and magnetic metal powder and their combinations on the microwave absorbing properties was studied.The effect of phase morphology in polymer matrix and filler dispersion on enhancing the microwave absorbing properties was also investigated.Firstly, uniformly dispersed GO reinforced EP-CE composites were successfully synthesized by in situ polymerization. FT-IR and XPS analysis revealed that-O-group in GO reacted with cyanate group O-C≡N in the stages of the curing process. Owing to the well dispersion, high contact area and strong interfacial interactions with the polymer matrix, great improvements in mechanical properties have been achieved for GO/EP-CE nanocomposites. The reaction between CE and GO were similar as CE and EP. It is generally believed its copolymerization has three stages:①Cyanate ester formed triazine ring crosslinking structure. ②Cyanate groupO-C≡N reacted with-O-group in GO and formed oxazolidinone ring.③ -OH group of the GO reacted with O-C≡N and formed bond. Therefore, the addition of GO into the resin not only showed a strong catalytic effect on the cure of the resin but GO/CE system possessed a lot of triazine ring structure, which saved the inherent advantages of CE. Meanwhile, owing to the well dispersion, high contact area and strong interfacial interactions with the polymer matrix, great improvements in mechanical properties have been achieved for GO/EP-CEnanocomposites.Secondly, we selected EP-CE as MAM matrix, GNSs and GNSs/MWCNTs as absorber to prepare resin-based absorbing composites via solution mixing and casting methods. The results show that the addition of GNSs can effectively improve microwave absorbing properties of the composite. In GNSs, dihedral angle could be easily formed within the stacks of flakes of GNSs, thus the microwaves suffer multiple reflections from the dihedral angles, which increased their propagation path in the absorber. These multiple reflections of microwave lead to the higher loss of EM energy. For example, a kind of 4mm-thickness composite with 3 wt.% of GNSs exhibited a minimum reflection loss value of -21.4 dB.Owing to the small size effect, quantum effect and macro quantum tunnel effect, one-dimension CNTs’s level spacing after the electron energy level splitting is still in microwave energy range. At the same time, the existence of the great specific surface area, dangling bonds etc, will cause the dissipation of EM energy which may further enhance the microwave absorbing performance. In addition, taking the synergies between one-dimensional GNSs and two-dimensional MWCNTs into account, we investigate the character of EM wave-absorption of the GNSs/MWCNTs/EP-CE composites. The results prove that the GNSs/MWCNTs/EP-CE composites possess excellent absorption properties. Reflection loss values exceeding-5dB (more than 70% absorption) can be obtained in the frequency range of 9.12-14.15GHz with the content of 1.5wt.% GNSs and 0.5wt.% MWCNTs. Especially, a minimum reflection loss value of -32.4 dB was obtained at 11.2 GHz for the 2mm-thickness composite. This composite showed low percolation threshold, high absorption properties compared to GNSs/EP-CE binary composites. Besides that, GNSs and MWCNTs were treated with a nonionic surfactant polyvinylpyrrolidone (PVP), and its effects on dispersion state, surface chemistry, structure and morphology of MWCNTs and GNSs, as well as on the microwave absorbing properties and mechanical properties of GNSs/MWCNTs/EP-CE nanocomposites are analyzed respectively. Results show that surfactant plays an important role in dispersing the GNSs and MWCNTs in a solvent and polymer. The above observations are attributed to the=bridging" effects between the GNSs, MWCNTs and EP-CE, which are introduced by the hydrophobic and hydrophilic segments of the nonionic surfactant. Unlike chemical functionalization techniques, however, the surfactant treatment exhibited little adverse effect on microwave absorption properties of the nanocomposite.Then, selecting three typical, magnetic particles with different shapes and sizes—— nano-sized Fe3O4 particles, micron grade size nickel carbonyl iron alloy powders (CINAP), and micron grade MoS2 powder coated nickel powders (Ni-MoS2) as microwave absorber, mixing with conductive filler GNSs, we prepared resin-based absorbing composite materials via a simple solution mixing method. Microstructure, EM parameters and wave absorption properties of the different magnetic particles filled EP-CE composites were investigated. It can be concluded that the reflection loss of the GNSs/magnetic particles/EP-CE composites depended on EM frequency, GNSs, and/or magnetic particles content and composites thickness. Nano-sized Fe3O4 particles and GNSs can effectively decrease the reflection loss and broaden absorbing bandwidth of the composites. In GNSs/CINAP/EP-CE system, high content of GNSs leads to high permittivity which is harmful to the impedance match and results in strong reflection and weak absorption. Furthermore, the conductive GNSs network enhances the electrical conductivity of the composite and this leads to a high leakage current, which may cause damage to the wave-absorption of materials. Compared with Fe3O4 and CINAP, Ni-MoS2 and GNSs filled EP-CE composites can achieve the high absorption characteristics under the lower content of magnetic particles. For example, when the loading of the GNSs in EP-CE is lwt%,2wt%,3wt%, the maximum reflection loss can be achieved to-3.1dB at 3mm,-16.1dB at 2mm,-32.9dB at 1.5mm thickness, respectively. Meanwhile, the reflection loss values of the composites filled with 2wt% GNSs were below-5dB in the frequency range of X-band with a thickness of 1.5 mm.Reflection loss values less than-10 dB (more than 90% absorption) were obtained in the measured frequency range of 9.9-12.4 GHz with 3wt% GNSs at thickness of 1.5 mm. In addition, it can be found that the complex permeability fluctuated as the GNSs content increased under the same magnetic particles content. The GNSs/magnetic particles/polymer interfacial features are expected to have a profound impact not only on the interactions between the magnetic particles, GNSs and the surrounding matrix but also the electrically controlled exchange bias and magnetocrystalline anisotropy in the specific case of magnetic particles. Therefore, the slight fluctuation of complex permeability is attributed to the interactions between the GNSs and magnetic particles in the EP-CE resin matrix, as the GNSs content increased under the same magnetic particles content.Finally, novel composites composed of CoFe2O4 spheres and reduced graphene oxide (RGO) were synthesized by using a facile hydrothermal route in combination with calcination at 550℃. A series of characterization results indicate that the as-prepared CoFe2O4 spheres with relatively uniform sizes are homogeneously distributed on RGO layers. Using GO as carrier can not only solve sedimentation of the ferrite during the preparation via solution-mixing method, but some defects on the surface of RGO, which was reduced during the preparation, can act as polarization centers under the altering EM field and attenuate EM waves, resulting in a profound effect on the loss of microwaves. In addition, there are residual oxygen containing chemical bonds such as C-O, C=O on the RGO, which enhance dielectric loss. RGO-CoFe2O4 and GNSs filled composites showed enhanced EM absorption property of the composites which was attributed to the better impedance matching. While, when the GNSs loading is 5 wt%, high permittivity of the composites is harmful to the impedance match and results in strong reflection and weak absorption. Therefore, the GNSs/RGO-CoFe2O4 nanohybrid/EP-CE composites, with excellent EM absorption properties and wide absorption bandwidth, can be obtained via adjusting the filler content to meet different requirements in the fields of aerospace, electromagnetic shielding etc.