Diffusion of Nonvolatile Metallic Elements in Graphite

Abstract

The diffusion coefficients for Ag, Ni, U, Th, and Ra diffusing in various types of graphite have been determined as a function of temperature. Good precision is obtained in all cases and the corresponding activation energies are reported. These values, when comparison is possible, do not agree with values previously reported. An explanation is given for the lack of agreement based upon the relative insolubility of metals in graphite. Experimental evidence is presented which indicates that substitutional, pore, and interstitial diffusion are inoperative and that all mass transport proceeds solely via grain and subgrain boundaries. The activation energies and frequency factors are explained by a diffusion model involving widely spaced traps which are situated at imperfections.