Finally, in a last part, we critically analyze these results based upon DC electrical conductivity and traction resistance experiments. The Carbon Fibres The choice of pristine fibres is crucial for selecting a desired physical property. Because the future of a given carbon depends primarily on the starting organic compound and the initial conditions, the origin of the fibre is essential. A phenomenological classification has been used for carbons which can be applied to fibres [1]. The original stable phase can be solid (a polymer such as polyacrylonitrile or viscose), liquid (a pitch with or without a mesophase) or gaseous (a hydrocarbon: benzene, methane..). If excess free energy is provided by a chemical reaction a metastable phase is obtained, which is a carbon fibre if an appropriate process is used. All commercial fibres are made from one of these precursors and labelled from them. A further heat treatment (HTT°C) is necessary to get a more or less graphitized structure (which is the stable phase for carbon). The most convenient parameter to characterize the graphitization is the mean distance between graphitic planes d002 (A)- As a general trend, the electrical resistivity decreases with decreasing d002 whereas the Young's modulus tends towards higher values for perfect graphite. During graphitization, it is well known that an increase of the crystallite size (Lfl in graphite planes) occurs with an orientation of the carbon layers parallel to the fibre axis [2]. Because the mechanical properties are related to the very strong C-C bond formed by the sp2 hexagonal network the increase of crystallinity is fundamental to observe good mechanical properties. Another relevant point for the choice of pristine fibres is their morphology and their microstructure: • The cross-section is scarcely circular and can present different shapes; besides, the crystallite distributions in a plane are very different: random (PAN fibres for example) [3], radial (mesophase fibres) [4] or circular (benzene fibres) [4]. • The microstructures of carbon fibres exhibit inhomogeneities as characterized by conventional electron microscopy. It has been shown [5] that different microtextures are distinguished in PAN and mesophase fibres: graphitic layers, microporous turbostatic phase, distorted aromatic layer stacks. This last phase is essentially present in high modulus graphitized fibres. In order to prepare intercalated compounds we have selected three kinds of carbon fibres (ex benzene, ex mesophase, ex PAN) that we have heat-treated at the highest possible temperature as presented in Table I. In this Table, we have gathered characteristic electrical resistivity values for graphites and graphite fibres ( 0 is the mean diameter of the fibres). We observe that the room temperature electrical resistivity increases when the crystallinity, characterized by the crystallite size La, is less and less perfect. This point is confirmed by the temperature dependence which is semi-metallic for graphite and becomes semi-conducting for the fibres. Intercalation Process in Fibres and Characterization A review of every publication relative to the intercalation of metal chlorides in graphite fibres is given in Table 2. We discuss these results together with our own that we present now. Metal chlorides have been chosen because they are known generally to give the
RkJQdWJsaXNoZXIy MTU5NjU0Mg==