A demonstration electromagnetic interference (EMI) shielding cover
that has a potential mass savings in excess of 80 percent over
conventional aluminum has been fabricated and tested. It was the
culmination of a 3-yr effort involving Hughes Space and Communications
(Los Angeles), Applied Sciences, Inc. (Cedarville, Ohio), and
the NASA Lewis Research Center. The cover was fabricated from
a composite of polycyanate ester resin and graphite fibers that
had been chemically modified by intercalation to enhance their
electrical conductivity.
Intercalated graphite fibers are made by diffusing bromine between
the carbon layers of the graphite fibers. The resulting material
has mechanical and thermal characteristics that are virtually
identical to pristine graphite fibers, but with a fivefold greater
electrical conductivity. Although intercalates are available which
increase the conductivity more, bromine forms intercalation compounds
that are stable at high vacuum, high humidity, and temperatures
as high as 200 °C. This enables the fibers to be handled at ambient
conditions, and the resin to be cured at standard temperatures
without decomposition. Flight tests on the space shuttle have
confirmed the environmental stability of these fibers, and the
enhancement in electrical conductivity makes composites made with
these fibers more suitable for electromagnetic inference shielding
applications.
The intercalated graphite technology was developed at NASA Lewis
Research Center and transferred to Applied Sciences through a
Space Act Agreement. With NASA's technical guidance, Applied Sciences
intercalated the graphite fibers and then sent them to Hughes
where, under a second Space Act Agreement, they were formed into
composites with RS-3, a low-outgassing polycyanate ester resin.
Three-foot-square composite sheets were fabricated along with
a five-sided EMI shielding box (3 by 3 by 12 in.) with a standard
mounting flange.
The shielding effectiveness of the composite box was tested over
a frequency range of 100 kHz to 1 GHz. The box required no special
treatment where it met the aluminum flange in order to electrically
seal it. The shielding performance was acceptable over the entire
frequency range, with the minimum shielding being 35 dB of the
incident voltage (70 dB of incident power) in the 2- to 30-MHz
range.
The potential for weight savings with this technology depends on which property of the cover is the limiting factor. If equal thickness covers are used, then the saving is about 34 percent because of the lower density of the composite. If shield strength is the limiting factor, then the superior strength of the composite allows the savings to be increased to about 88 percent. If stiffness is the limiting factor, then the savings can be as high as 94 percent.
Previous articleLast updated May 9, 1997
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