With 45 FiguresIntercalated carbon fibers are of interest for both applications and basic science. The availability of intercalation compounds in fibrous form opens up a number of scientific opportunities, many of which exploit the favorable geometry of fibrous samples. For example, the large aspect ratio (length/diameter) greatly improves the sensitivity of transport measurements such as resistivity, magnetoresistance, thermopower, and thermal conductivity, thereby allowing systematic investigations of these properties to be made in graphite intercalation compounds (GICs). For this reason, there is a strong connection between the study of transport properties in GICs (Chap. 6) and the present review of intercalated carbon fibers. For example, with fibrous intercalation compounds it has been possible to separate the lattice and electronic contributions to the thermal conductivity of GICs by measuring the thermal and electrical conductivities on the same fiber sample and invoking the Wiedemann-Franz law [8.1]. The small fiber diameters are of particular utility for structural studies using transmission electron microscopy, where observations can be made directly without thinning of the specimens. The small fiber diameters also allow preparation of compounds not readily synthesized in bulk form, such as stage-1 NiCl2 GICs.Among the examples of new science that has been explored through the availability of fibrous host materials are precision measurements of the unusual functional form of the temperature dependence of the resistivity of GICs in the low-temperature limit, exploiting the favorable geometry of fibers for highresolution studies. Closely related are the weak localization studies of transport properties of GICs, exploiting the two-dimensionality of fibrous acceptor compounds, and the ability to control the degree of disorder in the sample by control of the heat treatment temperature and the general fiber microstructure. Studies of the "metal-insulator" transition in ammoniated alkali-metal GICs have also exploited the favorable fiber geometry for transport measurements [8.1].From an applications 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 (J divided by the mass density {}m' which for a good conductor like copper is ,..., 6 X 10-2 cm 2 /g¢l. Composite wires based on graphite intercalation compounds can be formed by packing GIC powder into a Cu or Ag tube, which is cold-worked down to a thin thread of suitable diameter [8.2]. However, then the light weight performance is severely reduced by using the metal cladding. The high conductivity of individual small flakes of crystalline graphite is not reflected in the final conductivity
