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Porous carbon fibers (pores of: 0.1–3 μm in diameter) and carbon nanofibers (∼100 nm in diameter) were prepared from polyacrylonitrile/polymethyl methacrylate (PAN/PMMA) blend fibers with 70/30 and 30/70 weight ratio, respectively, as precursors. The composites containing 2–6 wt % porous carbon fibers or carbon nanofibers as microwave absorbents were fabricated. The complex permittivity of these composites was measured, and the microwave absorption properties were stimulated based on a model for a single-layer plane wave absorber. We found that composites filled with the porous carbon fibers exhibited a much better performance in microwave absorption than those containing the carbon nanofibers. Composites containing 6 wt % porous carbon fibers or carbon nanofibers showed the lowest reflection loss of −31 dB at 9.7 GHz and −12.2 dB at 10.7 GHz, respectively. The bandwidth with reflection loss below −5 dB covered the whole X band (4.2 GHz) in the former case, whereas it was only 2.6 GHz in the latter case, indicating the superior performances of the porous carbon fiber compared with carbon nanofiber in electromagnetic wave absorbing properties. We postulated that the combination of the dielectric-type absorption and the interference of multireflected microwaves could be responsible for the enhancement of microwave absorption in the porous carbon fibers.
Li et al. (Fri,) studied this question.