During the manufacture of artificial graphite, very high temperatures (2800-3000 °C) are required in the graphitization process. On cooling from these high temperatures, thermoplastic deformation is pos­sible until a temperature of ^1800 °C is reached. Below this temperature, the large difference in ther­mal expansion coefficients between the V and ‘a’ directions leads to the formation of long, thin micro­cracks parallel to the basal planes, often referred to as ‘Mrozowski’ cracks.11 These types of cracks are even observed in HOPG (Figure 3).

The high density of HOPG when compared to the large number of microcracks, a few nanometers in

width and many micrometers in length (as seen in Figure 3(b)), appears to be counterintuitive and has led to speculation that these microcracks may contain some low-density carbonaceous structure. The pres­ence of these microcracks is very important in under­standing the properties of nuclear graphite as they provide accommodation for thermal or irradiation — induced crystal expansion in the ‘c’ direction.

Therefore, two crystal structures are of interest; the ideal, ‘perfect’ structure and the nonperfect struc­tures as may be defined with reference to HOPG. It is of the latter that many of the crystal behaviors and properties have been studied.

Definition: In this chapter on nuclear graphite, ‘crystal’ refers to the perfect crystal structure and ‘crystallite’ refers to the nonperfect crystal struc­tures containing Mrozowski-type microcracks (and nanocracks).