Cavity Growth by Vancancy Condensation and Transformation to Dislocation Loop in the Graphite Structure

Abstract

The growth of cavities by vacancy condensation in graphite and the transformation to dislocation loops are theoretically investigated. During the heat treatment of carbonaceous substances for graphitization, numerous vacancies and cavities presumably exceeding 10/sup -3/ at.% are introduced in several ways, e.g., decomposition and evaporation of various impurities. By taking account of the internal surface energy and the configurational entropy of the cavities, it is found that these vacancies and cavities are driven to consolidate to fewer and larger ones lying extensively on the (001)-planes. In the course of the steady dimensional growth, the cavity is expected to collapse to a prismatic dislocation loop under the compressive thermal stress in c-direction (graphitization stress) caused by the interference between the anisotropically expanding crystallites. The criterion of this transformation is found by comparing the self-energy of the dislocation loop with the sum of the surface energy of the original cavity and the compressive strain energy to be released by the collapse; i.e., the critical diameters vary from about 100 to 1100 A in accordance with the various conditions concerning the crystal modification and-the cavity thickness. (auth)