In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

“…These micrographs are comparable to those obtained by Karthik and Kane [21] and Muto [33] who also investigated the effects of electron irradiation in nuclear graphite. Interestingly, significantly less alteration of the atomic structure is observed, suggesting that damage is continuously annealed out at these temperatures.…”

“…The key observed changes in nuclear grade graphite as a result of neutron irradiation are micro-crack closure resulting from expansion in the c-direction and dimensional change from irradiation induced creep, both of which depend on the overall level of initial crystallinity [16,21]. Dimensional change is determined in a number of ways, such as directly measuring specimens before and after irradiation, using X-ray diffraction to assess crystallite behaviour, and measuring changes in cracks and porosity with electron and light microscopy and smallangle neutron scattering [18,19,22].…”

“…for the most extreme case of amorphous carbon, the bond length increases to 1.44 Å [47]). It is thus suggested that introduction of dislocations and defects along with a bending of planes (the introduction of non-six-membered rings of carbon atoms [21]) following electron irradiation increases the average C-C bond length. One might expect that an increase in bond length would lead to a reduction in valence electron density (and thus the possible slight reduction in plasmon energy with increasing dose as shown in Figure 9).…”

Interlayer vacancy diffusion and coalescence in graphite

“…Experimental evidence, in the form of atomic resolution HRTEM images of single-layer graphene, suggests that these multivacancy morphologies are not routinely observed in this material [21], and recent modeling has also suggested that they are kinetically inaccessible from purely in-plane aggregation of mobile monovacancies in graphene [10]. There is experimental evidence for their existence, however, in bulk irradiated graphite [13,[15][16][17]20], and our results provide a possible explanation for this discrepancy. When vacancies can interact through adjacent planes, new pathways are opened for vacancy coalescence.…”

“…Resolution in HRTEM images of bulk graphite is limited to the observation of prismatic edges and a cross section of the topology of the individual layers. However, there is evidence for the formation of vacancy loops (holes), as well as extended defects linking adjacent planes as a result of electron irradiation [13,14]. Large prismatic vacancy loops have been observed to form in neutronirradiated graphite at temperatures above 900 • C [15][16][17]; however, very small loops (six or fewer vacancies) would not have been visible in these experiments [11].…”