The blocks of PGA were manufactured by extrud­ing needle-shaped filler particles mixed with a pitch binder. During the extrusion process, the needle-shaped filler particles tended to align with the V axis parallel, and the V axis perpendicular, to the extrusion direction. Thus, the final product (or block) had two orthotropic directions: parallel to the extrusion direction (WG) and perpendicular to the extrusion direction (AG). This strong ori­entation in direction is reflected not only in the unirradiated properties but also in the irradiation properties and dimensional changes. Dimensional

X X Graphite grade

(c)

Figure 28 Relative position and ratio of /(D/G) by graphite grade and condition. (a) Normalized position of G-Peak, (b) Normalized position of D-Peak, (c) Ratio of the D-peak and G-peak intensities. Courtesy of A. Jones, University of Manchester.

о

60

x

Э

40

0

0 0,2 0,4 0,6 0,8 1 1,2 1,4

(b) I(D/G)

(a)

Figure 30 Schematic of the irradiation-induced changes in Gilsocarbon graphite irradiated at 550 °C (note that there will be a similar set of curves for each irradiation temperature). (a) Dimensional change, dimensional change rate, and coefficient of thermal expansion and (b) Factorial change in Young’s modulus (E/E0-1) and thermal conductivity (K0/K-1), and irradiation creep (elastic strain units, esu).

(c) CTE (10-6 K-1)

180 °C

Figure 31 Correlation between initial growth rate and unirradiated coefficient of thermal expansion. Modified from Simmons, J. Radiation Damage in Graphite; Pergamon: London, 1965.

change MTR data for PGA is given in Birch and Brocklehurst64 for both the parallel (WG) and per­pendicular (AG) directions. In the parallel direc­tion and below 300 °C, the large dimensional

be compared to that of HOPG, as given in Figure 33. If PGA is irradiated to a higher fluence, the shrinkage rate reduces until the graphite begins to expand or ‘turns around,’ as illustrated in Figure 34.

‘Turnaround’ is associated with the closure of the Mrozowski cracks; see Figure 32. When all of the accommodation provided by the cracks has been taken up, the larger ‘C crystallite dimensional change rate would be expected to dominate the V axis shrinkage rate. This behavior has been used, with some success, to model the dimensional change behavior in PGA, Gilsocarbon, and Russian GR-280 graphite.48,65,66