Irradiation-induced dimensional changes of poorly crystalline carbons

Abstract

Data are presented on irradiation-induced dimensional changes of poorly crystalline carbons at high temperatures (>900°C). The materials surveyed include: 1. (1) carbon fibers, 2. (2) glassy carbons, 3. (3) carbonaceous matrix materials for HTGR fuel rods and 4. (4) pyrocarbons. The materials above are listed in order of increasing stability, with maximum strains ranging from more than 50% for fibers to less than 10% for pyrocarbons. Dimensional changes of highly anisotropic carbon fibers appear to be sensitive to irradiation temperature, as slightly anisotropic pyrocarbons are, whereas temperature seems to have little influence on the behavior of isotropic glassy carbons over the range from 600 to 1350°C. Dimensional changes for graphite-filled matrix materials were roughly isotropic on the average and did not seem to be strongly temperature dependent for the lower fluences investigated. Increased graphite filler lowered volumetric dimensional changes of the matrix in agreement with a rule-of-mixtures relationship between change components for the filler and the less-stable binder phases. Instabilities of all of the poorly crystalline materials were generally greater than those for more crystalline carbons under the same conditions, including highly orientated graphites that approximate single-crystal behavior.