Defect structure of PAN‐based carbon fibres

Abstract

SUMMARYA range of internal defects has been observed by high voltage microscopy in carbon fibres prepared from polyacrylonitrile (PAN). In type I fibres, large elongated cavities up to ∼ 3 μm in diameter are found, often associated with particulate inclusions and occur with a frequency of ∼ 2/mm of fibre. In ion thinned type I fibres where smaller defects are visible, a higher density of defects (approx. 1000/mm of fibre, mean size 86 nm) has been observed both in sections from the centre of the fibre and close to the surface. These smaller defects are also visible in type II fibres (heat treated to ∼ 1500°C) and in this case ∼ 70% of the defects are observed to contain small particulate inclusions.A high density of small voids (mean size 20 nm) is observed in type II fibres particularly in the vicinity of larger defects. These voids appear slightly diffuse in type II fibre and disappear during the graphitization heat treatment. They are also not seen in oxidized fibre, but in fibre heat treated to 500°C they are clearly visible, indicating that they are formed during the temperature range 220–500°C where considerable gas evolution occurs.The effect of these defects on fibre strength is discussed and the observed defect size related to expected crack lengths on the basis of a Griffith relationship.