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Preparation and Electromagnetic Properties of Carbon Nanofiber/Epoxy Composites

Carbon nanofibers (CNFs) with diameters ranging from 50 nm to 150 nm were prepared from polymer blends of polyacrylonitrile (PAN)/polymethyl methacrylate (PMMA) via a wet-spinning method and subsequent thermal treatment. The main factors that influence the diameter of the CNFs, including molecular weight of PAN, mass ratio of PAN/PMMA, and spinning draw ratio, are discussed. The formation of graphitization structures of the blend fibers during the carbonization process was investigated by Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy. The complex permittivity ϵ (ϵ′ − jϵ″), complex permeability μ (μ′ − jμ″), and microwave absorption properties of CNF/epoxy composites were studied in the X-band frequency range (8.2–12.4 GHz). ϵ′, ϵ″, μ′, and μ″’ were found to increase with increased CNF content. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The calculated results showed that the composite with a CNF content of 8 wt% and a thickness of 2.1 mm would give a maximum reflection loss of −34 dB at 10.5 GHz with the −10 dB bandwidth over the extended frequency range of 8.8–12.4 GHz.