Design and Optimization of the Periodic Porous Polymer Composite Metamaterial Electromagnetic Absorbers

Abstract

This dissertation addresses the imperative need for affordable Electromagnetic Interference (EMI) shielding materials in the era of burgeoning wireless technology. The goal is to mitigate the vulnerability of electronic devices to undesirable incoming radiation. Ideally, these materials should provide protection by absorbing a broad spectrum of frequencies and be insensitive to the polarization and angle of incidence of the impinging fields. The research introduces next-generation EM absorbers, comprising composite materials in a periodic porous structure. These absorbers leverage the concept of metamaterials, focusing on enhancing EM resonances within the absorber structures to meet multiple user-specified objectives. Polymer-based composites exhibit a promising capacity to customize EM intrinsic properties by adjusting the concentration and micromorphology of each constituent. Initial designs of fundamental polymer-based composites are tailored to meet specific application requirements, serving as a foundational benchmark for subsequent meta-structure designs. The emphasis is placed on meticulous consideration of composition, dispersion, and micromorphology to achieve desired electrical permittivity and magnetic permeability. Composites, comprising polylactic acid, graphene-based materials as conductive fillers, and CoFe2O4 as a magnetic constituent, are designed and fabricated to fulfill commercial requirements. Additionally, the surface modification of CoFe2O4 with oleic acid and polyethylene glycol demonstrates improved dispersion quality, particularly when a high volume fraction is introduced into the composite system. The resultant composite, fabricated through the solution mixing method, achieves a maximum reflection loss (RL) of -38 dB at 0.63 GHz, with an operational frequency bandwidth (BW) at -20 dB for an absorber thickness of 2.3 mm. In comparison, the composite compounded via a twin-screw extruder, offering enhanced production capabilities, yields a maximum RL of -21 dB with a BW at -20 dB of 0.2 GHz at an absorber thickness of 2.0 mm. EM-field-based finite element computational modeling and a Monte-Carlo optimizer are employed to design periodic porous meta-structures using the specified composites. Multi-objective functions, focused on maximizing RL and BW, guide the optimizer in selecting structures suitable for various applications. The optimizer identifies the most efficient structure as the truncated cone pillar with a Perfect Electric Conductor (PEC) on the top surface, achieved from the 2.3 mm thick absorber in the solution mixed composite. This structure significantly broadens the operation bandwidth at RL of -20 dB from 0.63 GHz to 1.8 GHz. Additionally, the optimizer shows the enhancement of RL for a poorly absorbing composite material produced by a twin-screw extruder, improving from -21 dB to -67 dB for a thickness of approximately 2 mm. This improvement is attributed to metamaterial behavior induced by resonance from the interaction between repeated pores, as confirmed by electric field distribution analysis. This research includes guidelines for metamaterial manufacturing, introducing techniques such as traditional CNC, compression molding, and additive manufacturing. These guidelines can enhance processing parameters and aid in achieving desired absorber structures in future work. The outlined strategy in this research demonstrates the capability to design and produce metamaterial absorbers that enhance absorption performance. These absorbers not only exhibit elevated RL but also encompass additional benefits aligned with user-defined multiple objective functions.