Production and properties of high modulus carbon fibres

Abstract

Carbon fibres of Young modulus 410 GN m$^{-2}$ (60 x 10$^{6}$ lbf in$^{-2}$) consist of graphite crystallites (L$_{a}$ about 9 nm) with their basal planes having a preferred orientation of within 10 degrees to the fibre axis. They are made by the pyrolysis of polyacrylonitrile textile fibres. Intermolecular reactions occur between the polymer chains and, by maintaining the chains parallel to the fibre axis by tension in the early stages of pyrolysis, a primitive oriented structure arises which becomes an oriented graphite structure at higher temperatures. Stereochemical factors are important and since the textile fibre contains fibrils of about 15 nm width, with distinct boundaries, the pyrolysis reactions are confined within the fibrils so that the carbon fibre also has a fibrillar structure with the oriented graphite crystallites contained within the fibrils. The mean strength of the carbon fibres 2.0 GN m$^{-2}$ (300 x 10$^{3}$ lbf in$^{-2}$) is limited by defects; e.g. the mean strength increases as the gauge length decreases. Some of these defects are due to inclusions or voids in the parent fibre. By stretching the carbon fibres at 2700 degrees C, crystallite orientation is increased and the Young modulus rises to about 689 GN m$^{-2}$ (100 x 10$^{3}$ lbf in$^{-2}$).