Dielectric and microwave shielding properties of three-phase composites graphite nanoplatelets/carbonyl iron/epoxy resin

The electrical properties of three-phase composite materials (CMs) graphite nanoplatelets/carbonyl iron/epoxy resin (GNP/Fe/epoxy) with 30 wt% of Fe and (1–5) wt% of GNP were studied by measuring DC conductivity and AC impedance spectra in the frequency range up to 2 MHz. The microwave shielding properties were measured in the frequency range of electromagnetic radiation (EMR) 1–67 GHz. The Nyquist diagrams derived from measured impedance–frequency spectra for GNP/Fe/epoxy CMs were considered within the equivalent circuit model. The significant increase of permittivity was observed for three-phase CMs with the increase of GNP content compared to two-phase GNP/epoxy CMs. For example, the real part of permittivity ε′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon^{\prime}$$\end{document} = 700–300 and imaginary part ε″\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon^{\prime\prime}$$\end{document} = 4 × 105–300 for ternary 5 GNP/Fe/epoxy composite in the frequency range 1 kHz–2 MHz. The observed significant increase of AC conductivity for three-phase composites proved the synergetic role of Fe particles in dispersing of GNP filler in epoxy matrix and formation of micro-capacitor network (for low GNP content) as well as the conductive network for higher GNP content. The observed sufficient increase of EMR shielding (SET\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{SE}}_{T}$$\end{document} in dB) beginning from 30–35 GHz for GNP content of 3–5 wt% correlates with DC electrical conductivity increase. The increase of the sample thickness d leads to the increase of shielding efficiency mainly due to the increase of EMR absorption SEA\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{SE}}_{A}$$\end{document} term.