Effect of intercalation in graphite epoxy composites on the shielding of high energy radiation
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Effect of intercalation in graphite epoxy composites on the shielding of high energy radiation James R. Gaier National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44135
Wendie C. Hardebeck, Jennifer R. Terry Bunch, Michelle L. Davidson, and Dwight B. Beery Manchester College, North Manchester, Indiana 46962 (Received 12 March 1997; accepted 27 September 1997)
The half-thickness and mass absorption coefficient of 13.0 keV x-rays, 46.5 keV g-rays, and 1.16 MeV b ™ particles have been measured for pristine, bromine intercalated, and iodine monobromide intercalated pitch-based graphite fiber composites. Since these materials have been proposed to replace aluminum structures in spacecraft, the results were compared to aluminum. Pristine graphite epoxy composites were found to have about 4.0 times the half-thickness, and 40% of the mass absorption of aluminum for ionizing radiation. Bromine intercalation improved performance to 90% of the half-thickness, and 1.7 times the mass absorption coefficient of aluminum. Iodine monobromide extended the performance to 70% of the half-thickness and 3.0 times the mass absorption of aluminum. Thus, intercalation not only makes up the deficiency conventional composites have in shielding components from ionizing radiation, but actually confers advantages in mass and thickness over aluminum. The b ™ particle shielding of all the materials tested was found to be very effective. The shielding of all of the materials was found to have nearly the same mass absorption coefficient of 17.8 6 0.9 cm2yg. Inelastic scattering processes were found to be important in b ™ particle shielding; however, the extent of inelastic scattering and thus the distribution of energies of the transmitted electrons did not vary with material.
I. INTRODUCTION
Because of their low density and their exceptionally high strength and modulus, graphite fiber composites are increasingly being used for the fabrication of aircraft and spacecraft. They are replacing metals, such as aluminum (Al) alloys, which have poorer mechanical properties and higher densities. However, the replacement of metals in many electrical applications has proven to be difficult because the resistivity of graphite epoxy composites is typically three orders of magnitude higher than that of the metals they replace. Intercalation of graphite fibers, the insertion of guest atoms or molecules between the graphene planes, has been found to substantially lower the resistivity of the fibers, and hence the resistivity of fiber composites.1 One application for which these intercalated graphite composites have been proposed is electromagnetic interference (EMI) shielding covers. These covers comprise about 20% of the mass of the power system of a typical spacecraft. The projected mass savings from an exchange of intercalated graphite composite covers with the standard Al covers is in excess of 80%.2 These calculations are based on mechanical properties being the limiting factor
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