DIMENSIONAL CHANGES IN HIGHLY ORIENTED PYROLYTIC-GRAPHITE DUE TO ELECTRON-IRRADIATION

One of the main problems found in the nuclear applications of graphite is its dimensional instability under irradiation, involving both swel ling and shape changes. In order to understand better the mechanisms t hat give rise to these changes, highly oriented pyrolytic graphite was irradiated with 300 keV electrons at temperatures between 25 and 657- degrees-C in a transmission electron microscope (TEM). Microscopic dim ensional changes and structural disordering were studied in directions parallel and perpendicular to the graphite basal plane. Changes in th e specimen length were investigated by measuring the distance between two markers on the specimen surface in TEM images. Changes in the latt ice parameter and the crystalline structure were studied by a TEM diff raction technique. In agreement with reported results, large increases in the specimen length and the lattice parameter were observed along the c-axis direction, whereas a relatively small decrease was observed along the a-axis. In irradiation studies conducted at room temperatur e, it was found that the dimensional change saturates at high dose, at an elongation along the c-axis direction of about 300%. High resoluti on microscopy revealed that the microstructure had become nanocrystall ine. Electron energy loss spectroscopy results showed that the volume change was recovered at this stage. These observations are discussed i n terms of point defect evolution and its effects on the microstructur e of irradiated graphite.