Nickel Filament Polymer-Matrix Composites with Low Surface Impedance and High Electromagnetic Interference Shielding Eff
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XIAOPING SHUI* AND D.D.L. CHUNG** Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo NY 14260-4400
ABSTRACT
Nickel filaments of 0.4 gm diameter gave polyethersulfone-matrix composites with high electromagnetic interference shielding effectiveness and low surface impedance at 1-2 GHz. With 7vol.% nickel filaments, the composite exhibited shielding effectiveness 87 dB (compared to 90 dB for solid copper), surface impedance 1.2 Q (same as for solid copper), tensile strength 52 MPa, modulus 5 GPa, ductility 1.0% and density 1.87 g/cm3. The low surface impedance is valuable for microwave waveguides.
INTRODUCTION
Nickel filaments of submicron diameter have been shown to be exceptionally effective for electromagnetic interference (EMI) shielding at 1-2 GHz, when used as a filler in a polymer [1]. The DC electrical resistance of these composites is not exceptionally low [1]. The AC surface impedance of these composites have not been previously reported, even though it is relevant to explaining the shielding effectiveness and to the use of the composites for lightweight microwave waveguides associated with radars in military aircrafts and satellites. Moreover, the mechanical properties of these composites have not been previously reported, even though they are relevant to the practical use of these composites. The attaining of both high ductility and high shielding effectiveness (low surface impedance) has been a challenge among polymer-matrix composites due to the decrease of the ductility with increasing filler volume fraction and the usual need for a high filler volume fraction in order to attain a high shielding effectiveness (low impedance). This paper is thus focused on the surface impedance and mechanical properties, and their dependence on the filler volume fraction.
EXPERIMENTAL METHODS
The nickel filaments (0.4 gim in diameter, > 100 inm in length), composite fabrication (with polyether sulfone or PES as the matrix) and EMI shielding effectiveness measurement were described in Ref. 1. 135 Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
The nickel fibers used for comparing with the nickel filaments were 2 pm in diameter and 2000 pm in length, as provided by Ribtec (Gahanna, Ohio), and 20 pm in diameter and 1000 pm in length, as provided as Fibrex by NationalStandard Co. (Corbin, KY). Due to the large length of the nickel fibers of diam 2 pm (which resemble cotton wool), the dispersion of these fibers was most difficult. The carbon filaments used for comparing with the nickel filaments were 0. 1 pm in diam > 100 pm in length, as provided by Applied Sciences, Inc. (Cedarville, OH). The composites were fabricated by forming a dry mixture of the polymer powder and the filler and subsequent hot pressing in a steel mold at 31 0°C (processing temperature for PES, as recommended by ICI) and 13.4 MPa for - 30 min. For the nickel filaments and the 20 Pm diam Ni fibers, the mixing was carried out dry in a ball mill. For the carbon filaments, mixing was carrie
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