Absorption-dominant EMI shielding polymer composite foams: Microstructure and geometry optimization

Absorption-dominant electromagnetic interference (EMI) shielding materials have garnered considerable attention to mitigate secondary EM pollution. The generation of a microcellular structure offers great promise in the development of absorption-dominant shielding materials. However, the underlying mechanisms governing the structure-EMI shielding properties relationship of microcellular shielding materials have rarely been studied. In this work, an input impedance model was applied to explore the thickness dependency of reflectivity (R) in microcellular conductive polymer composite (CPC) foams and then decouple the effect of thickness on the R-value from other structural variables, such as the void fraction (VF) and cellular morphology. As suggested by a theoretical model, PVDF/carbon nanotube (CNT)/SiC nanowire (SiCnw) composite foams having a 65% VF were fabricated. Absorption-dominant EMI shielding effectiveness was achieved by changing the foam thickness from 1.4 to 2.4 mm, with a decreased R-value from 0.63 to 0.48 and a correspondingly increased absorptivity/reflectivity (A/R) ratio from 0.53 to 1.07 (102%). Also, by increasing the VF from 0% to 85% while maintaining a constant cell size of around 10 μm, the optimal A/R ratio for each VF increased from 0.35 to 1.61. Furthermore, it was demonstrated that increased cell density in the cellular morphology with a fixed VF can create additional solid/air heterogeneous interfaces, promoting the EM wave attenuation capability via internal scattering and reflection of the incident EM waves. Consequently, this work allows for theoretically driven fabrication of absorption-dominant EMI shielding CPC foam with a tailored A/R ratio.