Simulation of irradiated graphite

Doctoral thesis English OPEN
Theodosiou, Alex
  • Subject: QD

A detailed scientific investigation is carried out into the effects of ion irradiation on\ud graphite systems. The results obtained have been compared with analogous radiation\ud induced affects seen in neutron irradiated material. Results indicate that ion irradiation\ud causes structural damage of the graphite lattice, as expected, in a very similar way to\ud neutron irradiation. Such damage is also largely recoverable through thermal\ud annealing. However, certain differences are observed and it seems likely that these are\ud due to differences in the nature of the damage caused, and the recovery of such\ud damage, on the fundamental level.\ud Ultra-high vacuum (UHV) equipment was used to irradiate samples of nuclear grade\ud graphite and highly-orientated pyrolyitc graphite (HOPG) with inert gas ions and\ud external facilities were also used to irradiate with carbon ions at higher energies. After\ud irradiation a variety of scientific techniques were used to analyse the damage caused\ud as a result of the irradiation.\ud Raman spectroscopy was used extensively to study the samples after irradiation and\ud results indicated the presence of significant ion-induced damage through analysis of\ud the Id/Ig ratio. The results were shown to be analogous to neutron induced damage\ud and the amount of damage present was seen to be dependant on ion mass, ion energy\ud and of course the total ion dose. A broad Raman feature was observed at 1500 cm'1,\ud after a critical ion dose and such a feature has been rarely observed in neutron\ud irradiated material, this feature perhaps suggests a fundamental difference between\ud the damage occurring and a difference in the type and extent of defects produced.\ud Raman was also used to effectively monitor damage recovery processes occurring as a\ud result of thermal annealing and such recovery was seen to be analogous to the\ud recovery seen in neutron damages materials.\ud Differential scanning calorimetry (DSC) was used to detect any exothermic release,\ud when heating the sample after irradiation, occurring as a result of a build-up of\ud Wigner-like energy, a phenomenon commonly seen in neutron irradiated graphite. In\ud one case of high energy He+ ion irradiation such an exotherm was observed at\ud 226 °C, the expected temperature range for Wigner-like release, however this result\ud was very difficult to reproduce thus suggesting a fragile balance of ion irradiation\ud parameters resulting in the necessary defects required to observe any Wigner release.\ud Transmission electron microscopy (TEM) showed clearly that the atomic structure of\ud nuclear grade graphite material was markedly altered as a result of ion irradiation,\ud leading to an amorphous-like structure. These results are in agreement with published\ud TEM work on neutron irradiated material and again highlight a similarity between the two forms of radiation.\ud X-ray diffraction (XRD) was also utilised and samples were analysed before and after\ud irradiation. Contrary to XRD patterns of neutron irradiated graphite, there seemed to\ud be little change in the XRD patterns after irradiation and the expected broadening and\ud peak shift of the <002> peak was not seen, suggesting a difference in the damage\ud caused to the system. Further work is required here to understand why this is the case\ud and again a deeper understanding of the underlying damage mechanisms would be\ud beneficial.
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