Scale effects in electromagnetic properties of composites

Karimi, Pouyan

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https://hdl.handle.net/2142/115682 Copy

Description

Title

Scale effects in electromagnetic properties of composites

Author(s)

Karimi, Pouyan

Issue Date

2022-04-10

Director of Research (if dissertation) or Advisor (if thesis)

Ostoja-Starzewski, Martin

Doctoral Committee Chair(s)

Ostoja-Starzewski, Martin

Committee Member(s)

• Schutt-Aine, Jose
• Elbanna, Ahmed
• Matlack, Kathryn

Department of Study

Mechanical Sci & Engineering

Discipline

Theoretical & Applied Mechans

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Electromagnetics
• Nanocomposites
• Random
• Microstructures

Abstract

Conductive composites possessing a polymeric matrix have been developed as an auspicious class of materials yielding superior properties to metal-based materials. The electrical and electromagnetic interference shielding effectiveness (EMI SE) properties of composites with a polycarbonate (PC) matrix and varying amounts of three different types of carbon fillers [carbon black (CB), carbon nanotubes (CNT), and graphene nanoplatelets (GNP)] are analyzed experimentally and theoretically over the 8.5-12 GHz frequency range. A finite element model is also used to study the EMI shielding mechanisms. The theoretical study predicts that the carbon fillers’ concentration, sample thickness, incident angle, polarization type, and frequency are the main parameters that have effect on shielding effectiveness of a sample which is confirmed by the experimental and simulation results. Permittivity and related alternating current (AC) conductivity measurements in the above mentioned frequency range are presented for these three types of composites, providing an appropriate way to design a shield. Experimental, theoretical and simulation results indicate that both permittivity and conductivity have significant effects on the SE. It is found that the electrical conductivity, which itself needs a percolating (connected) path, is not the only criterion for shielding, and that the connectivity of fillers (and, hence, higher conductivity) does not necessarily lead to a higher SE. We also suggested a novel model for simulating electromagnetic characteristics of spheroid nanofillers. Shielding efficiency of prolate and oblate ellipsoids in the X-band frequency range is studied. Different multilayered nanocomposite configurations incorporating CNTs, GNPs, and CBs are fabricated and tested. The best performance for a specific thickness is observed for the multilayered composite with a gradual increase in the thickness and electrical conductivity of layers. The simulation results based on the proposed model are shown to be in good agreement with the experimental data. The effect of filler alignment on shielding efficiency is also studied by using the nematic order parameter. The ability of a nanocomposite to shield the incident power is found to decrease by increasing alignment especially for high volume fractions of prolate fillers. The interaction of the electromagnetic wave and the fillers is mainly affected by the polarization of electric field; when the electric field is perpendicular to the equatorial axis of a spheroid, the interaction is significantly reduced and results in a lower shielding efficiency. Apart from the filler alignment, size polydispersity is found to have a significant effect on reflected and transmitted powers. It is demonstrated that the nanofillers with a higher aspect ratio mainly attribute to the shielding performance. Results are of interest in both shielding structures and microwave absorbing materials. The electromagnetic shielding effectiveness of a novel interpenetrating phase composite with a polymeric matrix are studied computationally. This composite is generated from a so-called Schwartz Primitive surface, a member of the triply periodic minimal surfaces. The shielding effectiveness of the resulting Primitive-based composite is compared with those of composites reinforced with periodically and randomly distributed spherical conductive particles. For the composites with random spherical particles, the random sequential addition method is used to generate the realization of fillers followed by the Monte Carlo relaxation step to obtain an equilibrated configuration. The Primitive-based composite shows a higher shielding effectiveness due to the interconnectivity of both phases (conductive phase and polymeric matrix) leading to a higher effective electrical conductivity. Scale dependence of electrostatic and magnetostatic properties is investigated in the setting of spatially random linear lossless materials with statistically homogeneous and spatially ergodic random microstructures. First, from the Hill-Mandel homogenization conditions adapted to electric and magnetic fields, uniform boundary conditions are formulated for a statistical volume element (SVE). From these conditions, there follow upper and lower mesoscale bounds on the macroscale (effective) electrical permittivity and magnetic permeability. Using computational electromagnetism methods, these bounds are obtained through numerical simulations for composites of two types: (i) 2D random checkerboard (two-phase) microstructures and (ii) analogous 3D random (three-phase) media. The simulation results demonstrate a scale-dependent trend of these bounds towards the properties of a representative volume element (RVE). This transition from SVE to RVE is described using a scaling function dependent on the mesoscale $\delta$, the volume fraction $v_f$, and the property contrast $k$ between two phases. The scaling function is calibrated through fitting the data obtained from extensive simulations ($\sim$10,000) conducted over the aforementioned parameter space. The RVE size of a given microstructure can be estimated down to within any desired accuracy using this scaling function as parametrized by the contrast and the volume fraction of two phases.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Pouyan Karimi

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