Dimensional changes in highly oriented pyrolytic graphite due to electron-irradiation
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D. F. Pedraza Oak Ridge National Laboratory, P.O. Box 2003, Oak Ridge, Tennessee 37831-7274 (Received 29 November 1993; accepted 1 March 1994)
One of the main problems found in the nuclear applications of graphite is its dimensional instability under irradiation, involving both swelling and shape changes. In order to understand better the mechanisms that give rise to these changes, highly oriented pyrolytic graphite was irradiated with 300 keV electrons at temperatures between 25 and 657 °C in a transmission electron microscope (TEM). Microscopic dimensional changes and structural disordering were studied in directions parallel and perpendicular to the graphite basal plane. Changes in the specimen length were investigated by measuring the distance between two markers on the specimen surface in TEM images. Changes in the lattice parameter and the crystalline structure were studied by a TEM diffraction 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 temperature, it was found that the dimensional change saturates at high dose, at an elongation along the c-axis direction of about 300%. High resolution microscopy revealed that the microstructure had become nanocrystalline. Electron energy loss spectroscopy results showed that the volume change was recovered at this stage. These observations are discussed in terms of point defect evolution and its effects on the microstructure of irradiated graphite.
I. INTRODUCTION Graphite is a material of choice in nuclear applications, both fission and fusion, due to its extremely low neutron absorption cross section. However, it is well known that its thermal and mechanical properties are altered in an irradiation environment capable of producing displacement damage, whether the damaging particles are neutrons, electrons, or ions.1"8 In such environments, not only do these physical properties become altered with increasing irradiation fluence, but also substantial dimensional changes take place that must be taken into account when designing nuclear components or using ion beams to modify the material's surface. Both dimensional and property changes are a result of displacement damage. The microstructure that evolves depends on damage rate, irradiation temperature, and damaging particle. At temperatures of normal operation in a reactor or those employed for ion implantation, the developing microstructure is substantially different from the initial one. In general, the residual damage is a matter of kinetics in which damage creation and annealing are competing mechanisms of defect evolution. Thus, it can be expected that residual damage at a given temperature and fluence will decrease as the displacement rate is decreased, provided point defects are significantly moJ. Mater. Res., Vol. 9, No. 7, Jul 1994
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