Tham khảo tài liệu 'carbon materials for advanced technologies episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 460 dependent on temperature and time. This effect has been accounted for through the concept of equivalent temperature. If two irradiations are performed at different levels of fast neutron flux ộ and ộ2 identical damage will be caused if the two irradiation temperatures are related by 1 T 1 1 - k I A -7-In - T2 a J 1 1 where k is Boltzmann s constant and A is an activation energy determined experimentally. Usually one of the flux levels would pertain to a standard position in a materials test reactor. As discussed by Burchell 58 experimental evidence suggests that flux level or rate effects are significant only at low to moderate irradiation temperatures 400 C . Dimensional changes A principal result of carbon atom displacements is crystallite dimensional change. Interstitial defects will cause crystallite growth perpendicular to the layer planes Coaxis direction whereas coalescence of vacancies will cause a shrinkage parallel to the layer plane a-axis direction . The damage mechanism and associated dimensional changes are illustrated in Fig. 6. Radiation-induced dimensional changes can be very large exceeding 60 in well-ordered graphite materials. Pryrolytic graphite has frequently been used to study the neutron irradiation-induced dimensional changes of the graphite crystallite 57 59 . Price 60 conducted such a study. Figure 7 shows Price s data for crystallite shrinkage in the a-direction for neutron doses up to -12 dpa. Price s samples were graphitized at a temperature of 2200-3300 C prior to being irradiated at 1300-1500 C. The a-axis shrinkage increased linearly with dose for all of the samples but the magnitude of the shrinkage at any given dose decreased with increasing graphitization temperature. Similar fiends were noted for the c-axis expansion. The effect of graphitization temperature on irradiation-induced dimensional change accumulation can be attributed to thermally induced improvements in crystal perfection. Higher graphitization temperatures
