Multi-layered Composite Structures for Electromagnetic Interference Shielding Applications

Abstract

With the recent surge in the usage of electronic devices, electromagnetic interference (EMI) poses a serious threat. Unwanted electromagnetic (EM) waves not only interfere with the normal functioning of electronic components, but certain studies suggest that it is a serious threat to human health. Polymer nanocomposites serve as a promising solution for EMI shielding as they can be tuned to meet the commercial shielding requirements by incorporating suitable fillers. Moreover, polymers are lightweight, corrosion resistive, easy to process, and can be molded into complex geometries. In this dissertation, multi-layered composite structures have been fabricated and studied for EMI shielding applications. Multi-walled carbon nanotubes (CNTs) were chosen as one of the fillers owing to their electrically conducting nature. To improve the thermal conductivity and/or EM wave absorption properties, a hybrid functional filler composed of Fe3O4 decorated reduced graphene oxide (rGO) was chemically synthesized and incorporated into selected composites. Alternatively, polyurethane (PU) foam-based multi-layered structures were also fabricated to enhance the absorption-based shielding performance.

As a background study, we started with polycarbonate (PC)-based composites fabricated using the conventional melt mixing approach. PC was chosen as it has a low electrical percolation threshold for CNTs (<0.5 wt%). Total shielding effectiveness (SET) of -23 dB (1 mm thick) was obtained for PC composites with 3 wt% CNT and 10 wt% rGO-Fe3O4. A high filler loading in PC may result in either processing difficulties or poor structural properties. Therefore, PC was blended with polyvinylidene difluoride (PVDF) to improve the structural stability and also to take advantage of the double percolation effect. The selective localization of CNTs in the PC component of the PC/PVDF blend resulted in double percolation, i.e., improved bulk electrical conductivity in blend-based composites compared to single polymer composites. Despite the double percolation, the maximum SET value of -24 dB (1 mm thick) was observed with 3 wt% loading of CNTs. We further added a mutually soluble homopolymer, polymethylmethacrylate (PMMA), as a compatibilizer for the PC/PVDF immiscible blend to reduce the interfacial tension and refine the blend morphology. Despite the morphology refinement, the shielding performance declined due to the diffusion of PMMA in the individual components (PC or PVDF) and the redistribution of fillers.

In the subsequent chapters, multi-layered composite structures were opted over the conventional melt mixed composites to improve upon the shielding performance. Thin films of PVDF and PC nanocomposites were interfacially locked using a mutually miscible polymer (PMMA) to obtain a shield with enhanced structural properties and EMI shielding performance. By stacking multi-layered films one above the other, reaching an assembly thickness of ca. 0.5 mm, the maximum SET was found to be -26 dB, which is a significant improvement compared to melt mixed composites. In the next chapter, porous structures (synthesized foams and 3D printed mesh structure) were sandwiched between composite sheets of PC and PVDF with an aim to dissipate the EM signals through multiple internal reflections. PU neat and composite foams were synthesized through a polymerization reaction between 4,4’-Methylenebis(phenyl isocyanate) and polyethylene glycol. Using PU-CNT foam as an inner layer between composite sheets of PC and PVDF, a maximum SET of -39 dB (approx. 5.3 mm thick) with absorption-dominated shielding. In order to further enhance the shielding effectiveness, Ag was sputtered on the PU-foam, which resulted in the highest SET value of -50 dB in the X-band but with a significant reflection component. The results presented here begin to suggest that in-situ synthesized foam with non-uniform and dead pores enhances the shielding performance compared to non-porous structures.

Towards the end of the dissertation, PU foam was fabricated using a simpler technique of salt-leaching. By stacking freestanding CNT papers (approx. 200 µm in thickness) on both sides of lightweight PU foam, we could limit the filler content yet maximizing the absorption-based EMI shielding performance. The multi-layered structure exhibited a high SET value of -49 dB (92% absorption @ 26.5 GHz; approx. 4.6 mm thick), whereas CNT paper by itself showed a maximum SET value of -35 dB (73% absorption @ 26.5 GHz). The porous uniform PU structure enhances the absorption component of shielding due to the trapped air, adequate impedance match, and multiple internal reflections. Further, we arrived at a remarkably interesting conclusion, i.e., if the incoming wave encounters PU foam before the CNT paper (in multi-layered structure with foam and CNT paper on one side), the absorption percentage of shielding can be further enhanced (98% absorption @both 8.2 and 26.5 GHz, SET ~ -37 dB, approx. 4.4 mm thick). In such asymmetric structures, reversing the direction of the incoming EM wave can change the absorption percentage.

The results presented in this dissertation suggest the various methodology for composite fabrication and the approaches to maximize the absorption-based EMI shielding performance.