SiC is an important engineering ceramic because of its high-temperature stability, high thermal conduc­tivity, and special electronic properties. It has been proposed for use in nuclear applications including structural components in fusion reactors, cladding material for gas-cooled fission reactors, and as an inert matrix for the transmutation of plutonium and other transuranics.32 In high-temperature gas-cooled reactors, SiC is the primary barrier material for TRISO coated fuel particles.33 Also, SiC fiber, SiC matrix (SiC/SiC) composites are attractive candidate materials for first wall and blanket components in fusion reactors.34

Only limited studies of elevated-temperature microstructural evolution (dislocation loops, voids, etc.), based on neutron or ion irradiations, have been performed on SiC. In pyrolytic p-SiC (cubic, 3C), Price35 found small (2-5 nm diameter) {111} Frank loops following neutron irradiation at 900 °C to 2.4 x 1021 n cm~2 (E > 0.18 MeV) (-2.4 dpa). Yano and Iseki36 found the same loops in p-SiC irra­diated at 640°C to 1.0 x 1023 ncm~2 (E> 0.10MeV) (-100 dpa) and, using high-resolution TEM, deter­mined these to be 1/3 (111) {111} interstitial Frank loops. These loops are constructed by inserting a single extra Si-C layer into the CABCAB Si-C stacking sequence. This produces the sequence CA|C’B’|CAB, where the prime denotes a p rotation of the tetrahe­dral unit (note that an adjacent Si-C layer is modified by the insertion of the extra Si-C layer).

In 6H-type hexagonal a-SiC, Yano and Iseki36 found ‘black spot’ defects lying on (0001) planes following neutron irradiation at 840 ° C to 1.7 x 1021 ncm~2 (E > 0.10 MeV) (-1.7 dpa). They coarsened these defects using high-temperature annealing and determined the defects to be interstitial Frank loops. The stacking sequence along (0001) in 6H a-SiC is ABCA’B’C’. Yano and Iseki proposed that the Frank loops are formed by a mechanism similar to p-SiC (described above), wherein insertion of an extra Si-C layer modifies an adjacent Si-C layer to produce a sequence such as ABC|B’A’|C’B’. Such a defect is described as a 1/6 [0001] (0001) interstitial Frank loop.

For low temperatures (150-800 °C), small amounts of swelling (0-2%) are observed in monolithic SiC samples produced by chemical vapor deposition (CVD).33 It should be noted that CVD-SiC is cubic and highly faulted.37 This swelling saturates at low damage levels (a few dpa) and the saturated swelling is lower, the higher the temperature. Much of this swelling is due to strain caused by surviving interstitials formed during ballistic damage cascades. As the irradiation temperature approaches 1000 °C, the surviving defect fraction diminishes because interstitial mobility increases with temperature and i-v recombination is enhanced. Newsome eta/.33 found swelling values of 1.9, 1.1, and

0. 7% for neutron irradiations at 300, 500, and 800 °C, respectively.

Above 1000 °C, neutron irradiation-induced void formation in p-SiC was first observed by Price35 at 1250 °C (4.3-7.4 dpa) and 1500 °C (5.2-8.8 dpa). Interestingly, no dislocation loops were observable by TEM in these samples. Price35 postulated that the interstitials may have been annihilated at stacking faults. Alternatively, he suggested that interstitial defects were present following irradiation, but they were too small and the contrast too weak to detect them. Nevertheless, at present it is not clear whether void formation in SiC is due to vacancy supersatura­tion produced by a dislocation bias. In any case, swelling of a few percent was observed for irradia­tions at temperatures greater than 1000 °C, and Price38 speculated that this swelling probably does not saturate with dose.

At low temperatures (-60 °C), Snead and Hay37 observed both a — and p-SiC to amorphize following a total fast neutron fluence of 2.6 x 1021ncm~2 (-2.6 dpa). This amorphization transformation was accompanied by a large reduction in density

10.8%), that is, volumetric swelling of nearly 11%. Snead and Hay37 estimated that the critical tempera­ture for amorphization (the temperature above which amorphization is not possible) is 125 °C (a lower limit for the threshold amorphization temperature). The critical temperature is dose rate dependent. In the study above, the dose rate was —8 x 10-7dpas~3. In other electron and ion irradiation experiments with dose rates of — 1 x 10-3dpas_1, researchers found critical temperatures ranging from 20 to 70 °C for 2 MeV electron irradiations,39-41 150 °C for ener­

getic Si ions,42 and -220 °C for 1.5 MeV Xe ions.43,44