| As the rapid upgrading of mobile-communication technology and intelligent portable devices,the sudden increase in the number of electromagnetic waves（EWs）aggravates electromagnetic interference on an unprecedented scale.Electromagnetic interference shielding materials（EMISMs）can shield electromagnetic interference to protect the continuous normal operation of precision instrument and normality of human body.Traditional EMISMs are dominated by all kinds of common metal conductive materials.These materials will lead to secondary pollution of EWs on account of the reflection-dominated shielding performance,and cannot meet the electromagnetic protection requirements of safety,confidentiality,and green.In addition,traditional EMISMs cannot distinguish important transmission signals from useless electromagnetic noise waves,which is far from meeting the requirements of highly integrated circuits and microelectronic devices for accurate filtering.Therefore,this study focuses on the design of composite structure between carbon series functional fillers and polymer platform,studies the manufacturing technique of EMISMs with orientation or low reflection characteristics,and systematically analyzes the relationship between material structure and shielding performance,in order to improve the shortcomings of electromagnetic interference shielding membranes such as the inability to accurately filter and high microwave reflection.The specific research content is as follows:（1）Design and construct a graphene electromagnetic shielding fiber membrane with a double-dimensional oriented conductive network structure.By adjusting the angle between the fiber axis and the direction of the electric field component,study the orientation characteristics of the electromagnetic interference shielding and explain the relevant mechanism of action.The results show that the graphene fiber membranes are ultralight and ultrathin carbon-assembled EMISMs,inheriting the advantages of excellent electrical conductivity and large specific surface area.In the orientation direction,the specific shielding effectiveness of the membrane is as high as 33,333.0 d B·cm2·g-1.The unique double-dimensional oriented conductive network structure gives the fiber membrane obvious anisotropic conductivity and directional characteristic electromagnetic interference shielding performance.The conductivity difference between the axial and transverse fibers is 6.8 times,and the corresponding shielding performance variation is up to 25.0 d B.After 160.0 straightening-bending motion cycles,the shielding effectiveness of the fiber membrane has no obvious change,showing its good flexibility and durability.（2）To obtain high-performance directional characteristic EMISMs,the double-layer structure polymer-based composite film is constructed on the basis of the research in（1）,and the influence of the change of laminar interface,fiber orientation,and fiber content on the shielding performance is studied.The results show that,due to the multiple internal reflection and resonance induced by the double-layer unidirectional structure,the shielding effectiveness of the composite membrane reaches 50.6 d B at the low fiber content of 6.0 wt%.Due to the axial consistency of the fiber film,the double-layer unidirectional structure further enhances the directional characteristics of the electromagnetic interference shielding performance,and the difference between the axial and transverse shielding effectiveness of the fiber is up to 40.0 d B.In addition,compared with single-layer and double-layer unidirectional composite films with the same fiber content,the double-layer unidirectional composite films have lower reflectance and higher shielding effectiveness due to the interface structure effect.（3）High reflectance EMISMs are easy to cause secondary pollution of EWs.On the basis of the the research in（2）,this chapter further studies the way to enshrine low reflection characteristics of EMISMs by adding short carbon fiber to the upper layer of the polymer platform.The EMISMs with an"absorption-shielding"double-layer structure is constructed with evenly dispersed short carbon fiber as the absorption function phase and the interpenetrating graphene fiber film as the shielding function phase.The relationship between the material structure and total shielding effectiveness,reflectance and absorptance is explored.The shielding mechanism of the"absorption-reflection"double-layer structure and the pivotal role of each layer are deeply analyzed.The results show that the total shielding effectiveness of 3.0 mm thick composite film is 19.3～22.5 d B,reflection effectiveness is 0.1～2.8 d B,absorption effectiveness is 18.4～21.1 d B,reflectivity is 0.7%～48%,absorptivity is 51.6%-98.5%in 8.2～18.0 GHz band.The composite film shows and absorption-dominated shielding mechanism and significantly low reflection characteristic in the range of 15.2～17.9 GHz.Its reflection effectiveness is limited to less than 0.1 d B.And the corresponding reflectivity can be as low as 0.7%.（4）Based on the research in（3）,in order to achieve low reflectivity and high shielding effectiveness of the composite film at a relatively low thickness,Ni particles and Ag particles were deposited on carbon fiber and graphene fiber,respectively,to prepare the magnetoelectric dual-effect heterogeneous bilayer EMISMs.The results show that the loss performance against EWs of the absorption layer is as low as-57.6 d B and the effective bandwidth is 8.28～11.40GHz due to the introduction of the magnetic loss mechanism.The deposition of silver particles improved the conductivity of the graphene fiber film,and the total shielding effectiveness of the Ag-coated graphene fiber film was increased by 22.2%compared with the blank sample.Due to the multiple loss mechanism and reasonable structural design,the 2.0 mm thick electromagnetic interference shielding composite film has significant low reflection characteristics in the frequency range of 8.2～12.4 GHz,with an shielding effectiveness of 33.7 d B,the reflection effectiveness of 0.8 d B,and the reflectivity of 17.2%.（5）To directly verify the structure-function relationship of EMISMs with low reflection characteristics,,a simplified"absorption-reflection"double-layer structure model is established,On the basis of research（3）and（4）and electromagnetic simulation is carried out through finite element numerical analysis.The influence mechanism of geometrical structure,size,and material properties on shielding performance in the frequency range of 8.2～12.4 GHz is studied.The results show that the power loss density at the edge of the rectangular holes and at the shielding-absorbing interface area is much higher than in other areas,which proves that the"absorption-shielding"double-layer structure and the rectangular holes on the shielding layer can improve the electromagnetic power loss inside the material and reduce the reflectance ratio of the composite film.Increasing the thickness of the absorbing layer can reduce the reflectivity and increase the absorptivity of the composite film,but the total shielding effectiveness of the composite film has no significant improvement.The thickness of the shielding layer greatly increases the overall shielding effectiveness of the composite film,but has no obvious effect on the reflectivity.In summary,starting from the structural design and construction of conductive network,this paper clearly defines the preparation process of high-performance EMISMs with unidirectional or low-reflection characteristics,and expounds the relationship between the structure of materials and performance,which plays an vital role in promoting the design,preparation and research of high-performance functional EMISMs. |
