Rates of migration of heavy metals in carbons and graphites are of practical importance, though accurate determination of diffusion behavior in such systems presents formidable problems both in experiment and in theory. Diffusion coefficients . . h' 2 3 d ph'te 4 for uraOlum 10 grap Ites t an a pyro-gra 1 have been reported; we now report coefficients for diffusion of thorium recently obtained from specimens prepared by ion bombardment.The pyrolytic carbon used was prepared by the thermal decomposition of methane. Porosity determinations with helium indicated a poreless structure. This finding was consistent with measured crystalloo 0 graphic parameters (a = 2.46 A, c = 6.86 A) and density (p = 2.2 g/cm 3 ). X-ray determinations using a rocking-curve techniqueS with flat specimens revealed preferred-orientation indices 6 of 80-90 based on (QQ2) reflections. The average distance between coherently diffracti~ domain boundaries (in the c direction) was 150 A, determined by line broadening of the (QQ2) reflections.The effects of lattice strains were not accounted for in the index and broadening re suIts; additional characterization studies 7 are in progress. Nevertheless, it is clear that the pyrolytic carbon used possessed a high degree of orientation, a high density, and a columnal structure arising from the presence of conical grains perpendicular to the c direction of the material.The porous graphite was prepared by impregnating a fine-grained' graphite with an organic compound that was subsequently carbonized. The density of CONTENT ANALYSIS
Beryllium self-diffusion coefficients were determined in high-density polyc:ystalline BeO in .the te~ perature range 800° to 1795°C. A precision lapping method using Be as a radlOtracer ~as .used m th.e mterval above 1180°C and electrical conductivity between 800° and 1250°C. Volume diffUSIOn coefficients determined by both :nethods correlate well between 1100° and 1250°C. The results can be expressed over the temperature range 1100° to 1800°C by the relation D=3.2X 10-3 exp( -63;OX 1OS/RT).. . . Above 1100°C, the agreement between the results obtained by the two techmques supports an IOmc d~lft mechanism for the electrical conductivity of BeO. Below this temperature, however, the transport behaylOr appears to be considerably influenced by the history of the sample as well as the nature and concentrations of impurities.. Classical volume diffusion was observed in a region near the surface of the specimens; at greater depths, deviations occurred and the magnitude of the deviations increased with .decreasing gr~in s~ze of the ~eO. These discrepancies have been explained on the basis of a simultaneous gram-boundary diffUSIOn mechamsm.
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