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

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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.