Irradiation damage to pyrolytic graphites at very high temperatures

Six pyrolytic graphites of widely differing crystallite sizes (L_a = 1?120 μm) were irradiated at temperatures of 1020°C and 1475°C. The resulting damage was studied by transmission electron microscopy. Damage produced at 1020°C can be understood in terms of present nucleation ideas, assuming crystallite boundaries interfere with homogeneous nucleation when L_a is smaller than the separation of homogeneously nucleated defects. At the higher temperature there is an increase in defect density rather than a decrease as observed over the temperature range 150–1200°C. It is proposed that this is caused by the nucleating cluster being larger than in the lower temperature range, and hence having a larger migration activation energy, approximately 2 eV as opposed to 1·2 eV.