Thermal Resistivity of Irradiated Graphite

Abstract

Mason and Knibbs¹ have recently measured the thermal resistivity of poly crystalline graphite of various crystallite sizes irradiated for various doses at 250°–275° C. They explain the extra phonon scattering arising from the irradiation by attributing it to interstitial clusters, one cluster per crystallite-plane, and they reckon that vacancies make only a small contribution. This explanation is not entirely satisfying. For, at all crystallite sizes studied¹, the increase of thermal resistivity on irradiation was found experimentally to be comparable with the thermal resistivity before irradiation, implying that the mean free path of scattering from irradiation-induced defects, L_i, is comparable with the mean free path for crystallite-boundary scattering, L_a, which is taken as equal to the mean crystallite diameter. This means that if there is only one scattering defect per crystallite-plane, then the diameter of this defect is a substantial fraction of the crystallite diameter, and therefore the concentration of surviving interstitials is large, say at least 10 per cent. With a dose of only 177 MWD/Ate, however, the number of displaced atoms calculated by the Kinchin–Pease method² is only about 2 per cent and at 250° C a large proportion of these are lost by recombination with vacancies; this is confirmed by electron microscopy of single crystals³ and by stored energy results⁴. There cannot, therefore, be enough interstitials to construct the defects postulated.