NASA composite has "the right stuff"

A graphite epoxy composite material from NASA provides improved electromagnetic interference (EMI) protection and functionality. EMI shielding typically makes up about 20% of the mass of a spacecraft power system. A technology developed at NASA's Lewis Research Center to reduce this percentage, is being promoted to make lighter weight EMI shielding for portable consumer electronics. Additionally, the composite covers do not require anticorrosive spray coatings or metallic linings.

Because of their low density and exceptionally high strength and modulus of elasticity, graphite fiber composites are increasingly being used in the fabrication of aircraft and spacecraft instead of metals like aluminum alloys with poorer mechanical properties and higher densities.

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However, using these same composites to replace 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. Low resistivity and high conductivity are properties possessed by effective shielding materials.

The key to making composites more conductive is to use intercalated graphite fibers. Intercalation, 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. The guest species can contribute carriers (either electrons or electron holes) to the graphite lattice, thus increasing its conductivity significandy without seriously degrading its mechanical properties. For example, bromine (Br^sub 2^) intercalated pitch-based graphite epoxy composites are 50 times more conductive than conventional structural graphite fibers.

In some high-radiation environments, the electronics must also be shielded from X rays, gamma rays, and high-energy particles. Intercalation makes the composite more conductive, and it protects the components from X rays and gamma rays because the intercalated molecules have a higher atomic number than carbon (which makes up the rest of the composite) or aluminum (the material to be replaced by the composites).

Lewis's search for improved EMI shielding materials began with a candidate properties list. Polyacrylonitrile (PAN)-based carbon fiber in an epoxy matrix has the lowest density and the highest strength, but its resistivity is too high for most applications. Beryllium has advantages in weight, but is expensive and highly toxic. These materials provided the opposite extremes on a spectrum of possibilities.

Lewis research focused on materials between the two. Three grades of pitch-based fibers and Br^sub 2^ intercalated pitch-based graphite fiber-epoxy composites were tested. Bromine was chosen because its intercalation compounds are conductive and chemically stable. Iodine bromide (IBr) was also studied. While not as stable as Br^sub 2^, it provides better shielding from radiation. Figure 1 shows comparative shielding data for nine materials. Notice that shielding is measured in units of decibels (dB) and that it varies with the frequency (Hz) of the incident EMI as well as the material providing the shielding.

EMI shielding is a two-way street. Figure 2 shows near-field shielding effectiveness. Near field is that area within one wavelength of the EMI source and so is a measure of containing EMI within a device such that surrounding electronics are not compromised. Intercalation shifts the trough to a lower frequency and raises the attenuation's minimum value. P-100 (P denotes pitchbased, 100 denotes modulus) has a low value of about 48 dB at about 2 x 10^sup 10^ Hz. Intercalation raises the minimum shielding to 77 dB and shifts the minimum to 3 x 10^sup 10^ Hz. Intercalated graphite composites enable a frequency range (1-200 GHz) to be shielded, which cannot be accomplished with conventional PAN composites.

Typical EMI shielding boxes are made from 2-mm-thick aluminum. To achieve the same level of shielding, 8-mm-thick pristine graphite epoxy is required, but only 1.8-mm Br^sub 2^ intercalated graphene epoxy and less than 1.4-mm IBr intercalated graphite epoxy.

Where ionizing radiation is a factor, intercalation not only makes up for the deficiency of conventional composites but, in the cases of Br^sub 2^ and IBr, actually confers an advantage over aluminum. Composites made from IBr intercalated graphite fibers can be made with one-third the mass of aluminum shields. (However, if shielding from high-energy electrons is the limiting factor in a shield design, material choices cannot make an improvement.)

NASA Lewis is interested in commercializing the intercalated graphite EMI shielding technology. Industries that may find this technology applicable include those producing electrical components for portable consumer electronics, cellular phones, automobiles, aircraft, and spacecraft. For more information, contact John Bacon, ISA/MTAC liaison, by phone: 412/383-2530; e-mail: jbacon@mtac.pitt.edu; or fax: 412/383-2595.