Letter | Published:

Thermal Resistivity of Irradiated Graphite

Naturevolume 199pages684685 (1963) | Download Citation



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, Li, is comparable with the mean free path for crystallite-boundary scattering, La, 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.

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  1. 1

    Mason, I. B., and Knibbs, R. H., Nature, 198, 850 (1963).

  2. 2

    Kinchin, G. H., and Pease, R. S., Rep. Prog. Phys., 18, 1 (1955).

  3. 3

    Reynolds, W. N., and Thrower, P. A., I.A.E.A. Symp. Irradiation Damage, Venice (1962).

  4. 4

    Bell, J., et al., Phil. Trans. Roy. Soc., A, 254, 361 (1962).

  5. 5

    Woolley, R. L., Brit. J. App. Phys. (in the press).

  6. 6

    Ziman, J., Electrons and Phonons (Oxford Univ. Press, 1960).

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  1. Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire

    • R. L. WOOLLEY


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