Preparation And Electromagnetic Shielding Properties Of Composite Materials Based On Expandable Microspheres

In recent years,lightweight filled conductive polymer composites（CPCs）have been a hot research topic for electromagnetic shielding materials in aerospace.The main way to enhance the shielding effectiveness of CPCs is to increase the content of conductive fillers,but increasing the conductive fillers causes a significant increase in electrical conductivity,which leads to an enhanced interfacial impedance between the material and air.The stronger the interfacial impedance,the higher the reflection rate of the material to electromagnetic waves,which will cause more serious secondary pollution.Therefore,it is urgent to design and develop CPCs with high shielding and low reflection.However,how to obtain high shielding and low-reflection electromagnetic shielding materials through structural design and filler optimization has been a difficult problem that has plagued the development of this field.To overcome this challenge,this thesis uses expandable microspheres as the substrate and the synergistic construction of segregation network and porous structure as the strategy to obtain lightweight CPC-based electromagnetic shielding materials with high shielding and low reflection by the synergistic effect of conductive/magnetic conductive fillers,and reveals the influence mechanism of segregation network and porous structure on the shielding effectiveness.The main research contents and main conclusions are as follows:（1）Silver nanowire/expandable microsphere（Ag NW/EPM）foams were prepared with the help of solution mixing and template-foaming coupling processes to achieve the synergistic construction of porous structures and segregation networks.The presence of this structure endows the composite with high electrical conductivity（142.99 S/m）and high shielding effectiveness（63 d B）.The presence of the porous structure not only reduces reflection by reducing the interfacial impedance between the material and the air,but also reduces the thermal conductivity,so that the reflected power coefficient（R）and thermal conductivity of the material are 0.29 and 0.022W/（m·K）,respectively.（2）To further reduce the reflection of the shielding material,an asymmetric structure with“absorption-reflection-absorption”effect was constructed.The asymmetric structure consists of a magnetic Ni Fe₂O₄/EPM impedance matching layer and an Ag NW/EPM conductive shielding layer.The presence of the magnetic Ni Fe₂O₄/EPM impedance matching layer gives the foam an ultra-low reflection（R=0.15）;the“absorption-reflection-absorption”effect caused by the asymmetric structure makes the shielding effectiveness of the material reach 66.50 d B.After 1000cycles of compression,the EMI shielding performance retention rate of the foam is81.20%,which shows excellent performance stability.In addition,the asymmetric foam has a low thermal conductivity of 0.028 W/（m·K）.（3）To demonstrate the effects of segregation network and porous structure on electromagnetic shielding effectiveness,CNT/WPU composites with segregation structure and CNT/WPU/EPM（CWE）foams with segregation network and porous structure were prepared,which were analyzed the effects of two different structures on shielding mechanism and thermal conductivity.The results show that the construction of the segregation network can improve the electrical conductivity and electromagnetic shielding effectiveness of the material（7.50 d B-56.82 d B）;the porous structure not only achieves the lightweight property of the material,but also helps to increase the power absorption coefficient（CWE:0.64;CNT/WPU:0.19）and reduce the thermal conductivity（CWE:0.035 W/（m·K）;CNT/WPU:0.283 W/（m·K））.In addition,the CWE foams have excellent antibacterial properties against Bacillus coagulans.