Diffusion in purified natural graphite crystals was studied by two techniques. Crystals with 14C atoms on the surface were prepared by heating at 1575C for 0.5h in a mixture of radioactive carbon monoxide and helium (table 16). These crystals were heated at diffusion temperatures in vacuo, and the 14C distribution within the crystals was determined by burning them slowly and measuring the radioactivity in the carbon dioxide produced. Diffusion measurements were carried out for times up to twenty-five hours in the range of 1995 to 2195C. The results indicated that simultaneous diffusion into single-crystal regions and into disordered regions occurred. Diffusion was also studied by measuring the rate at which 14C atoms were taken up by graphite crystals heated in radioactive carbon monoxide whose 14C concentration was maintained constant. Experiments were carried out for times up to eighty hours at 1995 to 2347C. The results indicated that the greatest part of the flow of atoms into the crystals occurred by the process of volume diffusion at temperatures above 2150C. The volume diffusion coefficient for graphite was D = 40f2exp[(-163000)/RT], where f was a geometric factor used because of the irregular shape of the crystals and was estimated to be between 0.1 and 0.6. The volume diffusion coefficient reported appeared to be that for the direction parallel to the layer planes.

Diffusion of Carbon Atoms in Natural Graphite Crystals. Kanter, M.A.: Physical Review, 1957, 107[3], 655-63

Table 16

Bulk C diffusion in graphite