Preparation Of Asymmetric Conducting PI Composite Foam And Modulation Of Electromagnetic Characteristic Response

In recent years,with the rapid iteration of electronic information technology,the research on electromagnetic shielding materials has become more and more urgent.Conductive polymer composites(CPCs),which consist of insulating polymers and conductive fillers,are becoming a new research hotspot due to their light weight,versatility,low cost and easy processing compared with traditional metal-based EMI shielding materials.Among them,foam materials with light density,high specific lightness and high electromagnetic shielding value have gradually become an important research direction of current shielding materials.Most of the foam materials are light in mass and high in porosity,which are suitable for large area use;the conductive fillers are attached to the walls of the bubble holes and connected into a network with the bubble holes,which can achieve better shielding effectiveness with very low filler content;the air present in the bubble holes can reduce the real dielectric constant of the material,thus reducing the electrical conductivity of the material surface.This thesis reduces the impedance mismatch of electromagnetic waves on the surface of the material through the combination of asymmetric conductive network and foam structure and prolong the propagation path of electromagnetic waves in the foam material.Structural control in polyimide(PI)-based electromagnetic shielding composites to obtain low reflection and ultra-high shielding performance characteristics while realizing the functionalization of the material.The related research is as follows:(1)Highly conductive nano-Ag was encapsulated on hollow glass beads using a chemical deposition process to form low-density,highly conductive GBAg particles.The r GO@Fe₃O₄magnetic particles were synthesized by co-precipitation method.The density difference of different fillers induces the formation of an ordered electromagnetic network of r GO@Fe₃O₄magnetic absorption layer and GBAg conductive shielding layer in the PI oriented bubble structure,and the special electromagnetic attenuation characteristics of"absorption-reflection-reabsorption"generated by the asymmetric shielding network in the oriented composite foam makes the composite material obtain ultra-high At a thickness of 4 mm,the X-band electromagnetic shielding efficiency can reach 51.2 d B at very low conductive particle content(0.1 vol%Ag+0.4 vol%r GO@Fe₃O₄),while the reflection efficiency(SE_R)value can reach0.07 d B and the reflectivity(R)is as low as 0.0089.(2)Based on the filler content(0.1 vol%Ag+0.4 vol%r GO@Fe₃O₄)in the previous chapter,MWCNTs were added to improve the performance of PI composite foams.During the ice template oriented growth,the low-density GBAg particles are gathered at the top of the foam as a highly conductive shielding layer,the high-density r GO@Fe₃O₄ magnetic nanoparticles are deposited at the bottom as an impedance matching layer,and the MWCNTs are uniformly dispersed in the PI matrix to connect the highly conductive shielding layer and the impedance matching layer,increasing the dielectric loss and polarization loss,while improving the mechanical properties.The thickness of PI composite foam after adding MWCNTs is all 6 mm,and the reflectivity(R)is all below 0.2.When the MWCNTs content is20 wt%,the X-band electromagnetic shielding effectiveness of PI composite foam is as high as60.1 d B,while the reflection effectiveness(SE_R)is as low as 0.04 d B and the reflectance(R)is only 0.02.Compared with the previous chapter,at the same compressive strain(50%),the stress of PI composite foam increases from 218 k Pa to 363 k Pa,the compressive modulus increases from 661.11 k Pa to 1763.28 k Pa,and the compressive strength increased from 245.08 k Pa to393.43 k Pa.(3)Chains Ni nanoparticles were prepared by a modified wet chemical method.The asymmetric structure Ni/MWCNTs/PI integrated composite foam was assembled by multilayer casting and orientation freezing,with Ni/PI foam as the conductive shielding layer and MWCNTs/PI foam as the impedance matching layer.Each layer of filler in the PI composite foam was uniformly dispersed while the bubble holes at the joints were continuous and complete.By controlling the thickness of each layer,different structures of PI composite foams were constructed,and the flexible combination of multiple fillers to build asymmetric conductive networks was achieved.This unique structure combines the advantages of laminate and bubble structure to extend the path of multiple reflections of electromagnetic waves,thus achieving low reflection and high shielding characteristics:when the Ni/PI layer is 1 mm and the MWCNTs/PI layer is 3 mm,the X-band electromagnetic shielding effectiveness of PI composite foam is up to 56 d B and the absorption rate(A)is 0.81.