Abstract
A model is presented that describes the dimensional behavior of pyrolytic carbon during irradiation by fast neutrons. The model uses crystallite averaging methods to describe the anisotropic distortion of the bulk in terms of its preferred orientation and the shape change of the individual crystallites. The densification is assumed to be independent of the shape change and is superimposed isotropically on it. The predictions of the model agree with experimental observations. Strain rates calculated from the model are used to estimate the suitability of various carbon structures for use as coatings on an unyielding support. For example, using a creep constant of 1× 10 −27 (nvt-psi) −1 at 1200°C, the model predicts that a carbon with a Bacon Anisotropy Factor (BAF) of 1.2 and apparent crystallite heights ( L c ) in the range 120–155Å would not fail due to neutron-induced dimensional changes if its original density were greater than 2.0 g/cm 2. Similarly, carbons with the same crystallite size but with BAF = 1.0 would not fail under the same conditions if their original densities were greater than 1.7 g/cm 3.
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