Dense hot-pressed specimens were used to study the self-diffusion of 14C at 2238 to 2643K. The existence of two clearly delineated diffusion mechanisms was demonstrated from the shape of the activity versus penetration curves. The first obeyed a bulk diffusion law, originated at the specimen surface, gave anomalously low diffusion coefficients, dominated to a depth of about 0.5μm, and was represented by:

D(cm2/s) = 1.90 x 10-6 exp[-88000/RT]

Autoradiography demonstrated that the second mechanism was grain-boundary diffusion which dominated at depths greater than 1μm. By using an estimated value of the bulk diffusion coefficient, Suzuoka analysis permitted direct calculation of the grain-boundary diffusion coefficient:

D(cm2/s) = 4.57 x 102 exp[-71000/RT]

C.P.Buhsmer, P.H.Crayton: Journal of Materials Science, 1971, 6[7], 981-8