Structural effects in a composite nonwoven fabric on EMI shielding

Abstract

A composite nonwoven fabric named CEF-NF in previous work was fabricated with carbon fibers (CFs) and polypropylene/polyethylene (PP/PE) core/sheath bicomponent fibers (denoted as ESFs). It holds great promise as an electromagnetic interference (EMI) shielding material, due to its good mechanical strength, excellent flexibility, ultra-thin thickness and remarkable electrical properties. This paper demonstrates a composite CEF-NF fabricated via two-step wet-papermaking/thermal-bonding processes. A flange coaxial test system was established to investigate the influences of material structure on EMI shielding effectiveness (EMI-SE) of CEF-NF fabrics in 30–1500MHz frequency range. Results indicate that the structure of a CEF-NF is highly correlated to its EMI-SE property, which increases with increasing areal density, fiber length and CF concentration. Under a set of optimal process parameters, the EMI-SE of CEF-NF with merely 40wt% of CF and 50g/m2 areal density can achieve a specific overall EMI-SE as high as 30.29dB, or 110dB·cm3/g—meeting commercial EMI requirements. Lastly, the ideal and actual EMI shielding mechanism were fully investigated and analyzed. It was found that the electrical percolation threshold of CEF-NF had a great influence on the actual absorption and reflection performance. At the conductivity threshold, CEF-NF showed high absorption performance.