| With the continuous improvement of communication technology and aerospace requirements for microwave absorbing materials,developing lightweight and wideband microwave absorbing materials has become an inevitable trend.Graphene is widely used in the research of microwave absorbing materials due to its lightweight and controllable dielectric properties,and the spatial distribution state of its layers has an important influence on its dielectric constant and absorption performance.In this paper,self-supporting/non-self-supporting graphene-modified composites were prepared by controlling the spatial distribution state of graphene layers.The electromagnetic parameters and reflection loss of the samples were tested and compared by coaxial method,waveguide method,and arch method,and their corresponding microwave absorption mechanisms were analyzed.The characteristics and application prospects of graphene-modified composites as lightweight and wideband absorption materials were explored.The main research contents and related conclusions are as follows:（1）In order to prevent the aggregation of graphene and achieve a uniform spatial distribution of graphene layers,reduced graphene oxide（RGO）@Fe3O4 composite materials were prepared using hydrothermal and thermal reduction methods.By optimizing the experimental conditions and the ratio of RGO and Fe3O4,excellent absorption performance was obtained at a filling ratio of 20 wt.%and in the frequency range of 2.0-18.0 GHz.The specific microwave absorption performance（ASMAP）of the absorber was 186.3 d B·cm2·g-1,and it was observed that the effective absorption band（EAB）of the RGO@Fe3O4composite material with a thickness of 2.0-4.0 mm moved to a lower frequency with the increase of RGO content,revealing a new way to continuously adjust EAB.The polarization loss of graphene oxide（GO）was verified by indirectly characterizing the charge density distribution on the surface of a single-layer GO using HH-KPFM technology.The analysis of electromagnetic parameters showed that the increase of RGO content and the attachment site of Fe3O4 on RGO both increased the dielectric constant of RGO@Fe3O4,adjusted the impedance matching,and optimized the absorption performance.The main way of polarization loss was the interface polarization between RGO and Fe3O4and the dipole polarization of defects and functional groups in RGO.（2）In order to improve the uneven local spatial distribution of RGO and enhance the corrosion resistance,porous RGO（PG）@Ti O2with a 3D structure was prepared using a solvothermal method.By changing the reaction temperature to control the Ti O2 content in PG@Ti O2,the dielectric properties of the composite material were adjusted,and it showed excellent absorption performance at a filling ratio of 10 wt.%and in the frequency range of 2.0-18.0 GHz.The ASMAP was increased to 334.7 d B·cm2·g-1.The analysis of electromagnetic parameters showed that the Ti O2 nanorods optimized the overall impedance matching,enhanced the polarization relaxation loss,and improved the absorption performance.At the same time,the complex surface morphology provided more opportunities for electromagnetic wave reflection/scattering.Due to the low conductivity of PG@Ti O2,polarization loss played a dominant role in microwave attenuation,including interface polarization between RGO/MWCNT and dipole polarization of defects and functional groups.（3）In order to obtain a controllable spatial distribution of graphene layers and meet the requirements of lightweight and wideband absorption,RGO/MWCNT super-lightweight composite aerogels with a directional pore structure were prepared using a unidirectional freezing method.The monomer size of the aerogels met the requirements of arch method testing.By adjusting the density,thickness,and reduction degree of the aerogels under different experimental conditions,a tunable wideband absorption was achieved in the frequency range of 2.0-40.0GHz.At a filling ratio of 10 wt.%,the low frequency band below-10 d B reached 15.0 GHz,and the ASMAP reached 3514.0 d B·cm2·g-1.In the frequency range of 26.5-40.0 GHz,the low frequency band below-20d B reached 13.5 GHz,covering the entire Ka band.When the thickness was 2.0 mm and the bulk density was 3.34 mg·cm-3,the average absorption intensity（AAI）was 26.49 d B,and the ASMAP reached39644.9 d B·cm2·g-1.Compared with the conductivity loss,polarization relaxation loss was the main way of absorption,including interface polarization between RGO/MWCNT and dipole polarization of defects and functional groups.The total content of carbon materials and their reduction degree were the main means to adjust the impedance matching/loss balance of the composite aerogel.The macroscopic alignment of dipole moments caused by highly directional structure further changed the interaction mechanism between the aerogel and microwave.（4）In order to obtain enhanced mechanical properties and tunable wideband absorption characteristics of the aerogel,a mechanically reinforced RGO/CNT/TPU composite porous aerogel was prepared by melt blending and unidirectional freezing methods.By adjusting the loading amount of RGO and CNT in the composite aerogel,the controlled preparation of the composite porous aerogel with a bulk density of 146.1-162.2 mg·cm-3 and the regulation of the EAB in the frequency range of 2.0-40.0 GHz were achieved.When the bulk density of the composite aerogel was 1.5 mg·cm-3,the optimal absorption performance was achieved in the frequency range of 2.0-18.0 GHz,with an AAI of 8.4 d B and an ASMAP of 28612.1 d B·cm2·g-1.When the bulk density of the composite aerogel was 16.1 mg·cm-3,the optimal absorption performance was achieved in the frequency range of 26.5-40.0 GHz,with an AAI of 12.4 d B and an ASMAP of 3851.9 d B·cm2·g-1.The wideband tunable absorption characteristics were due to the appropriate wideband impedance matching and loss coefficient.The main way of absorption was polarization loss,including interface polarization between RGO/CNT and dipole polarization of defects and functional groups.84 figures,18 tables,and 252 references... |
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