Abstract
This work concentrates upon the development of microstructure in carbon fibres during the oxidation and subsequent pyrolysis of polyacrylonitrile precursors. X-ray diffraction, both high and low angle, and mechanical testing were used to characterize the structures at all stages of fibre production. A meridional scattering peak in the low-angle x-ray diffraction pattern of partially oxidised fibres indicated a long range periodicity of around 10 nm. Further experimentation showed this to be a transient effect caused by selective oxidation of the surface layers. Low angle x-rays scattering was also used to study the micropores present in the fibres. The diameter of the micropores decreases during oxidation of the precursor, reaching a constant value after a minimum of 5 hours at 220°C. During pyrolysis, the micro-pore diameter decreases gradually as the temperature increases, except at 500°C and 700°C where there is a temporary pore size increase. The mechanism responsible for the pore size variations is outlined and related to the chemical degradations occurring over the temperature range. The micropore cross-section is shown to be rounded below 900°C and angular above. From wide-angle x-ray diffraction crystallite dimensions and preferred orientation angles are calculated. The results indicate the establishment of a ribbon-like structure during oxidation. This ribbon structure of narrow carbon layer undergoes gradual improvement during pyrolysis. Above 1900°C there is a marked increase in the rate of microstructural rearrangements. The mechanical properties of the fibres are linked to the microstructural developments and the possible effects of a sheath-core structure upon the mechanical behaviour of fibres is examined quantitatively. This and other evidence suggests that a thin - highly orientated sheath of material surrounds each fibre from the start of oxidation. Finally the experimental results are discussed, in the light of current knowledge, to produce an overall picture of the development of microstructure in carbon fibres.