NEUTRON-IRRADIATION DAMAGE OF SILICON-CARBIDE

Neutron irradiation uniformly produces vacancies and interstitials in silcon carbide (SiC) polycrystals, and the specimen swells by 1 to 3%. Subsequent isochronal annealing leads to annihilation of the defects by the interstitial-vacancy recombination from around irradiation temp erature, resulting in the shrinkage of the specimen. This shrinkage ca n be detected by measuring the specimen length with a conventional mic rometer and its lattice parameter with an X-ray diffractometer. Furthe rmore, defect formation and annihilation affect the electrical resisti vity and create paramagnetic centers caused by unpaired electrons. Hel ium atoms can be uniformly introduced into SiC utilizing the nuclear r eaction of B-10(n,alpha)Li-7. By subsequent annealing above similar to 1300 degrees C, helium atoms with high vibration energy capture therm al vacancies to reduce the internal pressure and form bubbles at grain boundaries. The formation of helium bubbles accompanies a large volum e expansion with increasing temperature, controlled by Greenwood et al .'s mechanism. The presence of helium bubbles at the grain boundaries promotes diffusional creep at lower temperatures (1300 degrees C). Cha nges in physical properties by neutron irradiation are presented and d iscussed with respect to microstructures.