Anisotropic diffusion kinetics of metals and carbides in pyrolytic graphite

Analogous to previously reported experiments on a typical electrographite, apparent diffusion coefficients for Ti, V, Cr, Nb, Mo, and Hf as carbides and for elementary Fe, Ni, and Cu in highly ordered pyrolytic graphite have been determined as a function of heat treatment and crystallographical orientation. The concentration profiles of the elements penetrating into the graphite were measured by an electron microprobe analyzer. For the evaluation of the diffusion coefficients a new method basing on probability graphs was employed. The product of diffusion coefficients and temper time D x t4/3 is found to be invariant and follows the Arrhenius equation. For all species investigated, the diffusion is twice as high in the direction of the graphite layers than perpendicular to them; the activation energy being Qsub(a):Qsub(c) = 20.3:22.1 kcal/mole. The fact that the frequency factors can be correlated with the heat of formation of the corresponding carbides as well as with atomic radii according to Pauling allows an extrapolation to the apparent diffusion coefficients of other species. (orig.)