Relationship between Preferred Orientation, Thermal Expansion, and Radiation-Induced Length Changes in Graphite

Abstract

Expressions may be derived for the thermal expansion coefficient of polycrystalline graphite in terms of the single-crystal coefficients and the preferred orientation of the sample. Thermal expansion and preferred orientation measurements were performed on pyrolytic, molded, hot-worked, extruded-rod, and extruded-tube graphites in order to test the validity of this model. They showed that such expressions adequately represent the behavior of several different graphites over a range of temperatures, provided that the ``accommodation'' of a part of the c-direction crystallite expansion by microcracks is taken into account. The amount of c-direction accommodation is a function of preferred orientation and the direction of measurement. It is small for the principal c direction in a highly oriented body and large for a random aggregate of crystallites and is probably controlled by the number of microcracks which form on cooling from the graphitization temperature. A model similar to that used for thermal expansion may be used to express the irradiation-induced dimensional changes in polycrystalline graphite in terms of the preferred orientation of the sample, a contraction in the crystallite a direction, and an expansion in the crystallite c direction which is partially accommodated by microcracks. The model was found to be in reasonable agreement with published irradiation data for a number of graphites.