Structural Dependence of the Tracer-Level Diffusion of Thorium in Pyrolytic Carbons

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Abstract
Tracer-level diffusion studies have been carried out on laboratory-prepared samples of pyrolytic carbons of widely divergent microstructure. These samples are well characterized and contain no exposed layer-plane edges. Using a 228Th tracer, diffusion coefficients have been measured perpendicular to the plane of deposition over the temperature range 1600° to 2000°C. The diffusion coefficients were found to have a significant dependence on microstructure, the diffusion coefficient in a highly oriented, laminar pyrolytic carbon being more than an order of magnitude smaller than that in either isotropic or granular carbons. The diffusion coefficient D, in cm2/sec, of thorium perpendicular to the plane of deposition in granular, isotropic, and laminar pyrolytic carbons is given by D=7.1×104 exp(−152 000/RT) (granular),D=15.5 exp(−114 000/RT) (isotropic),D=1.2×104 exp(−157 000/RT) (laminar). The tracer-level diffusion coefficients for thorium in the laminar pyrolytic carbon were substantially smaller than the values that have been reported by others for diffusion in highly anisotropic, columnar pyrolytic carbon. Microstructural parameters, such as apparent crystallite size and anisotropy factor, are useful in correlating the present results but are of limited value in quantitatively rationalizing the large variations in diffusion coefficients. Measurements of the tracer-level diffusion coefficient of thorium in a polycrystalline graphite were also performed.