Due to their low density and exceptionally high strength and modulus, graphite fiber composites are being used increasingly for the fabrication of aircraft and spacecraft. Because of their superior mechanical properties, these composites have been replacing metals, such as aluminum alloys, in many applications. The replacement of metals has been slow, however, when high electrical conductivity is needed because of the relatively poor electrical conductivity (< 0.1 percent of metals) of composite materials. Designers have also shied away from graphite-polymer composites in applications where shielding from ionizing radiation is important, because of the poor performance of these composites.

These shortcomings of graphite-fiber polymer composites can be addressed by intercalating the fibers before fabricating the composites. Intercalation is the insertion of guest atoms or molecules (intercalates) in between the carbon layers of the fibers. If the intercalate is chosen carefully, the electrical conductivity of the composite can be increased nearly an order of magnitude, and the specific radiation shielding can surpass low density metals.

Intercalated Graphite Polymer Composites are built using standard laminar techniques.

Two intercalates, bromine and iodine monobromide, have been shown to have the right combination of properties to make them commercially viable options. They combine the virtues of high electrical conductivity, high thermal conductivity, and good radiation shielding with excellent stability and easy processibility.

Bromine has been shown to intercalate a wide variety of pitch-based and vapor-grown graphite fibers. Most of the research has centered around Amoco's Thornal fibers. Bromine has been shown to enhance the conductivity of P-55, P-75, PO-100, P-120, and K-1100 fibers by a factor of three to six. The resulting material has an electrical conductivity surpassing that of stainless steel. Furthermore, these intercalation compounds are stable to temperature well above the processing temperature for most resins, and are impervious to moisture and ultra-high vacuum. Fabrication of composites from intercalated fibers does not degrade their properties, and composite properties can be predicted by using a simple rule-of-mixture. Although intercalation does not enhance either the mechanical properties or the thermal conductivity of graphite fiber composites, neither does it degrade them. The mechanical properties are virtually identical with those of pristine fibers, except that there is an enhancement in the interlaminar shear properties. The thermal conductivities of these fibers are among the highest of all materials, exceeding such metals as aluminum and copper. Also, because of the high thermal absorption and emissivity of graphite fibers, radiant heat is rejected much more efficiently from electrical components than when they are encased in highly reflective metals. The mass absorption coefficient for ionizing radiation by composites made from intercalated fibers is enhanced by a factor of four, to a value exceeding that of aluminum.

Iodine monobromide has not been studied as extensively as bromine has as an intercalate for graphite fibers. Those studies that have been done reveal intercalation compounds nearly identical with those utilizing bromine. The exception is in the mass absorption coefficient for ionizing radiation, which is nearly twice that of bromine intercalation compounds, and three times that of aluminum. The implication is that iodine monobromide intercalated fiber composites can provide radiation shielding equal to that of aluminum with one-third the mass.

The primary application envisioned for this technology is electromagnetic interference (EMI) shielding of electronics. Calculations indicate that the shielding effectiveness of these composites, while not as high as that of aluminum, is higher than the requirements for many applications, and higher than that of joints and penetrations through metallic boxes. Experimental studies confirm the high shielding effectiveness calculations. The surface conductivity, while not as high as that of metals, is high enough that no special surface treatments (sanding off the surface polymer layer, etc.) are required. These materials can be used effectively with conventional EMI shielding gasketing strategies.

The total achievable mass savings depends on the particular requirements of the shield. If the limiting factor is shielding from high-energy radiation, a mass savings of 66 percent is achievable. If the limit is strength, 86 percent of the mass can be saved. Finally, if the limit is stiffness (modulus), over 90 percent of the mass can be saved. The implications for such weight savings can be dramatic. In spacecraft, because the payload is a smaller portion of the spacecraft than the power and communications systems, the payload may be increased by as much as 40 percent. In communications satellites, the mass savings could be taken up in attitude-control fuel, extending the useful lifetime of the spacecraft. In some cases, it could enable the launch by smaller and cheaper launch vehicles. In aircraft, decreased weight would allow for fuel savings, which, when figured over the life of the aircraft, could be substantial. For consumer products, such as notebook computers and cellular telephones, lower weight itself might be a significant selling point.

This work was done by James R. Gaier of Lewis Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 - 3
21000 Brookpark Road
Cleveland
Ohio 44135

Refer to LEW-16535.