Isothermal annealing studies on vacancy and interstitial loops in single crystal graphite

Abstract

Single crystals of graphite were treated in various ways to obtain either vacancy or interstitial loops ∼ 10³ Å diameter. The character of the loops was determined by diffraction contrast in the electron microscope. Thin foils from these crystals were isothermally annealed at temperatures between 2100° and 2600°c. The diameters of loops in a pre-selected area were measured after successive periods of annealing. It was found that in this temperature range, both vacancy and interstitial loops annealed out by dislocation climb. In some circumstances, large loops grew whilst the smaller ones shrank. Using an anisotropic diffusion model it has been shown that both types of loop annealed with a common activation energy of 8·3 + 0·3 ev. This has been identified as E _f + ½(E _ma+E _mc), where E _f is the formation energy of the diffusing defect, E _ma and E _mc the migration energies parallel and perpendicular to the basal planes respectively. The analysis shows that this type of experiment cannot yield information as to the nature of the diffusing species.