Enhancement of Electromagnetic Wave Shielding Effectiveness of Carbon Fibers via Chemical Composition Transformation Using H(2) Plasma Treatment

H(2) plasma treatment was performed on carbon-based nonwoven fabrics (c-NFs) in a 900 W microwave plasma-enhanced chemical vapor deposition system at 750 °C and 40 Torr. Consequently, the electromagnetic wave shielding effectiveness (SE) of the c-NFs was significantly enhanced across the operating frequency range of 0.04 to 20.0 GHz. We compared the electromagnetic wave SE of the H(2) plasma-treated c-NFs samples with that of native c-NFs samples coated with nano-sized Ag particles. Despite having a lower surface electrical conductivity, H(2) plasma-treated c-NFs samples exhibited a considerably higher electromagnetic wave SE than the Ag-coated c-NFs samples, across the relatively high operating frequency range of 7.0 to 20.0 GHz. The carbon component of H(2) plasma-treated c-NFs samples increased significantly compared with the oxygen component. The H(2) plasma treatment transformed the alcohol-type (C–O–H) compounds formed by carbon-oxygen bonds on the surface of the native c-NFs samples into ether-type (C–O–C) compounds. On the basis of these results, we proposed a mechanism to explain the electromagnetic wave SE enhancement observed in H(2) plasma-treated c-NFs.