Self-Diffusion in Eicosane Single Crystals

Abstract Radiotracer, serial sectioning studies of self-diffusion have been made in ultra-pure single crystals of the triclinic phase of n-eicosane (C20H42) in the temperature range 295-309 K. The crystals of this solid readily undergo plastic deformation. Consequently, crystals grown from the melt (Bridgman Technique) contain a high concentration of dislocations, slip bands and polygonized boundaries. Diffusion along these defects (pipe-diffusion) contributes significantly to the total penetration of radioactivity and hence decreases the precision with which lattice self-diffusion coefficients can be evaluated. An attempt is made to assess this interference and to correct data. As might be expected in such anisotropic materials, there is a significant difference in self-diffusion parallel (D‖) and perpendicular (D‖) to the basal (001) planes. The latter can be shown with reasonable precision to follow a temperature dependence. D┴ = 9 × 1044 exp(−341 + 18 kJ mol−1/RT). The former cannot be denned due to t...