The diffusion coefficients for Ag, Ni, U, Th, and Ra diffusing in various types of graphite were determined as a function of temperature. Good precision was obtained in all cases and the corresponding activation energies were reported. The data for Th and U could be described by:

U, <a> :     D (cm2/s) = 6.76 x 103exp[-115(kcal/mol)/RT]

U, <c> :     D (cm2/s) = 3.85 x 102exp[-129(kcal/mol)/RT]

Th, <a>:     D (cm2/s) = 1.33 x 105exp[-145(kcal/mol)/RT]

Th, <c>:     D (cm2/s) = 2.46 x 100exp[-114(kcal/mol)/RT]

These values, when comparison was possible, did not agree with values previously reported. An explanation was given, for the lack of agreement, which based upon the relative insolubility of metals in graphite. Experimental evidence was presented which indicated that substitutional, pore and interstitial diffusion were inoperative and that all mass transport proceeded solely via grain and sub-grain boundaries. The activation energies and frequency factors were explained by a diffusion model which involved widely spaced traps which were situated at imperfections.

Diffusion of Nonvolatile Metallic Elements in Graphite. Wolfe, J.R., McKenzie, D.R., Borg, R.J.: Journal of Applied Physics, 1965, 36[6], 1906-17