Diamagnetic studies on as-processed carbon fibers

Abstract

The diamagnetic susceptibilities of a wide range of commercially available carbon fiber types have been investigated in the as-processed condition by the Faraday method at room temperature using small bundles of aligned fibers. The tensor trace (total) susceptibility χT varies systematically over the range 0.8–20 (in units of −10−6 emu/g) as a function of precursor type and processing history. In general, χT increases with increasing nominal treatment temperature and hot stretching, and with increasing tensile elastic modulus E; but there are significant differences in the behavior of fibers from rayon or isotropic pitch and those from polyacrylonitrile (PAN). However, the anisotropy ratio χradial/χaxial increases approximately linearly with E for all fibers (from ∼1 for E?70 GN/m2 to ∼22 at 700 GN/m2 in air); and is quantitatively consistent with the layer-plane orientation textures determined by x-ray diffraction when appropriate values of the ’’crystallite’’ principal susceptibilities χa and χc are used. These are determined by setting χa=0.32, the graphite value, and computing the structure-dependent χc value from the observed χT. The dependence of χT on apparent layer size La is similar to that of glassy carbons. The significance of these results is discussed in relation to the characterization of carbon fibers and the structure dependence of the diamagnetism of disordered carbons.