Graphitization Stress in Polycrystalline Carbon as an Origin of Dislocations

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When a polycrystalline carbon is heat-treated for graphitization, large thermal stress is produced owing to the interference of the extremely anisotropic expansion of each crystallite. The stress of this kind is expected to promote the rearrangement of the crystallites, directly or indirectly, from the so-called turbostratic structure to the tri-dimensional graphite lattice, and so it may be adequately called “graphitization stress”. The stress analysis has been made on each of the spherical and the cylindrical (filamentarily textured) polycrystals in which the carbon crystallites are aligned with the basal planes parallel to the external surface. These two models accommodate to the structure of pyrolytic carbon particle and to that of fibrous soft coke respectively. It is shown that in either of these two cases about 3×1010 dynes/cm2 tension in a-direction and about 0.9×1010 dynes/cm2 compression in c-direction can be produced by heat treatment at 2500°C, which are enough to buckle the crystal lattice at the stress concentrations of 5∼20 times and may lead to the generation of dislocations. From the corresponding strain distribution curves, the criterion on the stress relaxation by these dislocations has been determined in connection with the crystallite size, which may give a measure of distinction between the so-called soft and hard carbons.

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