Highly Conductive Graphite Fibers

Abstract

From the viewpoint of applications of GICs, a very interesting development is the enhancement in conductivity of the host graphite up to the range of metals, especially for pristine materials with fibrous forms. Much attention has been paid to the exploitation of the order of magnitude intercalation-induced enhancement of the electrical conductivity of graphite fibers for the fabrication of practical high-conductivity, lightweight conductors (Vogel et al., 1977; Goldberg and Kalnim, 1981; Manini et al., 1983, 1985; Murday et al., 1984; Meschi et al., 1986; Natarajan and Woollam, 1983; Natarajan et al., 1983a). The fiber geometry (large aspect length/diameter) ratio offers advantages relative to highly oriented pyrolytic graphite (HOPG) or bulk graphite for the measurement of several of the transport properties of GICs and for increasing the compositional stability of GICs both under ambient conditions and at elevated temperatures (Endo et al., 1981, 1983a). For bulk GICs, intercalation increases the density of carriers by the injection of electrons into the graphite planes in the case of donor guest species, and by injection of holes in the case of acceptor type (see Chapters 5 and 6). The intercalation-induced decrease in carrier mobility that results from the increased scattering by defects and the increased effective mass is outweighed by the larger increase in carrier density, resulting in a large conductivity enhancement as discussed in Section 6.1. The carriers are localized in the graphene planes, and for high-stage compounds (n ≥ 2) the carrier density falls off rapidly with distance from the graphite bounding layer owing to the screening of the charged intercalate layer by the surrounding graphite bounding layers. From an application standpoint, many of the applications of intercalated carbon fibers exploit the high specific conductivity of GICs, which can be expressed as a figure of merit in terms of the conductivity σ divided by the mass density ρm; for a good conductor like copper this is ~ 6 x 10−2 cm /gμΩ. For example, intercalated carbon fibers can provide a lightweight conductor for huge aircraft or motor vehicles, in which, respectively, about 1.5 tonne or 30 kg conventional metallic conductor is used.