HELIUM REEMISSION, DESORPTION AND MICROSTRUCTURE EVOLUTION OF GRAPHITES UNDER HELIUM ION-IMPLANTATION

Helium reemission, trapping, and thermal desorption from highly ordered pyrolytic graphite (HOPG and HPG) of different orientation, polycrystalline graphite (EK98), and titanium doped graphite (RG-Ti-91) have been measured at irradiation temperatures of 300 K and 800 K. The implantation was performed with a 40 keV 4He ion beam. Detailed transmission electron microscopy (TEM) investigation of the microstructure evolution was made on the implanted specimens. He reemission from basal oriented (BO) highly oriented pyrolytic graphite is accompanied by blistering and flaking leading to repetitive gas bursts. On edge oriented (EO) pyrolytic graphite three reemission peaks are observed during room temperature implantation, the first and smallest one being assigned to He release from intrinsic lenticular cavities, the second one occurs during early bubble formation when a small amount of implanted gas still escapes accumulation, and the third and largest peak being due to He release by bubble coalescence. The He reemission rate grows very slowly at room temperature and does not reach 100% up to the highest implanted fluence of 3.5⊙10 18 He/cm2. At 800 K the He reemission rate from EO pyrolytic graphite reaches 100% immediately after starting implantation due to the high diffusive mobility of He. EK98 and RG-Ti-91 show similar reemission behaviour. No gas bursts due to blistering are observed. The initial reemission rate at 300 K is higher than in EO pyrolytic graphite due to release of He via a network of intergranular channels. At 800 K reemission is rather similar to that from EO pyrolytic graphite. No thermal desorption of He from BO HOPG up to 1200 K is observed for implanted fluences ≤1016 He/cm2. At higher fluences the onset temperature of desorption decreases from 750 K at 2⊙1016 He/cm2 to 380 K at 1017 He/cm 2 caused by thermal flaking due to pressure increase of He in submicroscopic cracks. In the other materials two desorption peaks are observed, the first one being related to He release from "solid solution," while the second is attributed to gas escape from He filled bubbles. In contrast to the conclusions of Niwase et al. we find from the reemission kinetics and from selected area electron diffraction patterns (SADP) that graphite implanted at 300 K with He up to a damage of 10 dpa and more cannot be regarded as amorphous and shows a distorted turbostratic structure. The c parameter increases to (3.63.7)Å. Radiation effects in graphite implanted at 800 K are less pronounced up to damage levels of 200 dpa but may decrease the He diffusion coefficient. © 1995 American Institute of Physics.