| With the rapid development of wireless communication technology,lightweight,high-strength,and high-shielding electromagnetic interference shielding（EMI）materials have become the ideal choice for the next generation of portable and wearable electronic products.In order to take full advantage of graphene’s potential as an advanced functional material,graphene nanosheets need to be properly assembled in the matrix material,thus demonstrating the unique properties of the micron-sized graphene functional units at the macro scale.Although magnetic alignment of graphene oxide was achieved by attaching ferromagnetic particles to graphene oxide nanosheets,magnetic alignment based on graphene intrinsic diamagnetism alone remains a significant challenge,especially in polymer matrix materials.Therefore,a low-cost and flexible preparation method is of great significance to the development and performance optimization of electromagnetic materials.In the thesis,I firstly induced graphene nanosheets to align in suspension and composite film by magnetic field generated by ordinary Nd Fe B magnets,and studied the optical anisotropy of graphene suspension and the structural characteristics of graphene composite film under different magnetic field modes.The results showed that the optical anisotropy and orientation of graphene suspension are closely related to the external magnetic field.For example,the alignment order parameter of graphene in a horizontally rotating magnetic field is up to 0.82,which is seven times and two times better than in a non-magnetic field and a horizontal static field,respectively.It is proved that the orientation of graphene can be controlled by the intrinsic diamagnetism of graphene,and the alignment effect is well preserved in the film.In order to reveal the effects of graphene types and alignment orientation on the comprehensive properties of the composite films,the intrinsic structure and properties of few-layer and multi-layer graphene were firstly analyzed.Then,the properties of fewlayer and multi-layer graphene nanosheets were compared with those of the substrate.Finally,the effect of arrangement orientation on the comprehensive properties of the composite films was studied.The results showed that the intrinsic properties and composite properties of the few-layer graphene prepared by electrochemical stripping were better than those of the multi-layer graphene prepared by redox method.Among the four arrangements,the vertically oriented graphene composite film has the worst mechanical properties,electrical properties and electromagnetic shielding properties,while the horizontally parallel graphene composite film has the best mechanical properties and electromagnetic shielding properties.For example,the electromagnetic shielding performance of vertical static magnetic field,non-magnetic field and horizontal rotating magnetic field is 24 d B,29 d B and 35 d B,respectively.It shows that the parallel structure is beneficial to improve the electromagnetic absorption performance of composite materials.In order to further improve the EMI shielding performance of composite materials,the effects of polyvinylidene fluoride（PVDF）components,graphene loading and film thickness on the EMI shielding performance were studied based on the horizontal rotating magnetic field.The results show that the electromagnetic absorption property of the film increases from 27 d B to 47.8 d B when the PVDF component increases from 0 to 1:3.The graphene packing load increased from 10 wt% to 30 wt%,and its EMI shielding performance increased from 31.4 d B to 65 d B,a relative improvement of 107%.When the film thickness increases from 1 mm to 3 mm,its EMI shielding performance increases from 27 d B to 95 d B,and the relative improvement is 252%.The results indicate that PVDF composition,film thickness and graphene loading can greatly improve the electromagnetic absorption properties. |
