Tracer‐level diffusion studies were carried out on laboratory‐prepared samples of pyrolytic carbon of widely divergent microstructure. The samples contained no exposed layer‐plane edges. Using a 228Th tracer, diffusion coefficients were measured, perpendicular to the deposition plane, at 1600 to 2000C. The coefficients were found to exhibit a significant dependence upon microstructure; the diffusion coefficient in highly oriented laminar pyrolytic carbon being more than an order of magnitude smaller than that in isotropic or granular carbon. The diffusion coefficient of thorium perpendicular to the deposition plane in granular, isotropic and laminar pyrolytic carbon was given by:

granular (1660-1975C):     D (cm2/s) = 7.1 x 104exp[-152(kcal/mol)/RT]

isotropic (1630-1925C):     D (cm2/s) = 1.55 x 101exp[-114(kcal/mol)/RT]

laminar (1680-1985C):     D (cm2/s) = 1.2 x 104exp[-157(kcal/mol)/RT]

In porous graphite, the diffusivity of 228Th at 1950C ranged from 2.8 x 10-10cm2/s, and the diffusivity at 1790C was 1.8 x 10-10cm2/s.

Structural Dependence of the Tracer‐Level Diffusion of Thorium in Pyrolytic Carbons. Lonsdale, H.K., Graves, J.N.: Journal of Applied Physics, 1967, 38[9], 3620-9

Table 8

Tracer diffusion of Th in C

Table 9

Tracer diffusion of U in C