by Henson and Reynolds (20) as a result of an analysis of lattice parameter changes and corresponding stored energy accumulation rates; they also gave a value for Evma of > 2.5 eV. Theoretical evaluations of Eif (128) are not of great significance since they depend to a large extent on the model and the carbon-carbon interaction potential used. We shall return to this subject in Section IX.
The lattice strains associated with an interstitial appear to be quite large. Henson and Reynolds estimated that a single interstitial occupies between two and four atomic volumes (20) as a result of the extent to which the neighboring basal planes are pushed apart. On the other hand, they showed that a single vacancy causes a basal plane contraction resulting, as suggested by Kelly (21), from a change in the energy of the p-electron bonds, together with a slight expansion in the c direction owing to Poisson's ratio effect. Strains introduced by a vacancy are therefore quite small and the distortion of the lattice resulting from a single interstitial, although quite large, is not large enough to cause the defect to be visible in the electron microscope.
B. Dislocation Loops
The migration energies of the point defects indicate that they should be quite mobile at not too high temperatures and that larger clusters may be formed which will eventually become visible in the electron microscope. The size of defect necessary for this to occur is not quite clear, but it seems fairly certain that the population of small interstitial clusters (" 6 atoms) inferred to be present by Reynolds (2) would not be visible. Interstitials eventually form a well-ordered lattice plane, which is inserted as a C plane.
References
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2. W. N. Reynolds, in Chemistry and Physics of Carbon, Vol. 2 (P.L. Walker, Jr., ed.), Dekker, New York, 1966, p. 121.
3. A. L. Sutton and V. C. Howard, J. Nucl. Mater., 7, 58 (1962).
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9. P. B. Hirsch, A. Howie, M. J. Whelan, D. W. Pashley, and R. B. Nicholson, Electron Microscopy of Thin Crystals, Butterworth, London, 1965.
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14. R. Siems, P. Delavignette, and S. Amelinckx, Z. Phsyik, 165, 502 (1961).
15. C. Baker, Y. T. Chou and A. Kelly, Phil. Mag., 6, 1305 (1961).
16. P. R. Goggin and W. N. Reynolds, Phil. Mag., 8, 265 (1963).
17. J. H. W. Simmons, Radiation Damage in Graphtie, Pergamon Press, New York, 1965, p. 133.
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19a. A. Kelly and R. M. Mayer, Carbon, 1969, to be published.
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21. B. T. Kelly, Nature, 207, 257 (1965).
During the past few years the technique of transmission electron microscopy has been used with great success to obtain a better understanding of the process of irradiation damage in graphite. Observations using cleavage flakes of naturally occurring single crystals have been the subject of recent papers by Baker and Kelly (1965) and Reynolds and Thrower (1965), in which data on defect type, size and density were given for the irradiation temperature range 150°C to 1200°C. Reynolds and Thrower also gave a theoretical treatment which not only integrated these observations but provided possible means of predicting defect configurations under other irradiation conditions. Diffraction analysis of the defects (Thrower 1964), shows that at irradiation temperatures below 650°C they are dislocation loops of interstitial character, in which a layer of atoms is inserted as a C - plane into the normal . . . ABABA . . . stacking sequence. At higher temperatures of irradiation these loops
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HIRSCH, P. B., HOWIE, A., WHELAN, M.J., PASHLEY, D.W. and NICHOLSON, R.B., 1965, Electron Microscopy of Thin Crystals (London: Butterworths), p. 286.
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The extremely careful work of Hennig [13, 18, 22, 23] has shown that an earlier claim by Baker et al. [12, 24] to have observed loops of quenched-in vacancies in the electron microscope was not well founded. Using an etch decoration technique [25] capable of revealing single vacancies, Hennig was able to show that after very rapid quenching from 3000°C the vacancy concentration was less than 10-10. The value of Efv > 6.6 eV was a limit he obtained by taking Cv = exp (-Efv /kT), i.e. assuming a pre-exponential factor of unity.
Kelly and Taylor [9] have shown that the experimental behavior of the high temperature specific heat [26] can only be explained theoretically if
The high temperature thermal conductivity data of Hove [27] and Euler [28] were consistent with this interpretation using the phonon scattering parameters for vacancies obtained from irradiation experiments. While this relationship agrees with other observations with respect to the activation energy, it does imply a vacancy concen-
[12] C. BAKER and A. KELLY, Nature 193, 235 (1962).
[13] G.R. HENNIG, J. appl. Phys. 36, 1482 (1965).
[14] R.M. MAYER, Ph. D. Thesis, University of Cambridge, 1967.
[15] C. ROSCOE, Carbon 7, 119 (1969).
For corrections see P.A. THROWER, and J.A. TURNBULL, Carbon 7, 623 (1969).
[16] W.N. REYNOLDS and P.A. THROWER, Phil. Mag. 12, 572 (1965).
[17] R.W. HENSON and W.N. REYNOLDS, Carbon 3, 277 (1965).
[18] G.R. HENNIG, 2nd Conf. Industrial Carbon and Graphite, London 1966, Soc. Chem. Ind. London 1967 (p. 109).
[19] W.N. REYNOLDS, P.A. THROWER, and J.H.W. SIMMONS, ibid, (p. 493).
[20] R.M. MAYER, Carbon 7, 512 (1969).
[21] C.A. COULSON, M.A. HERRAEZ, M. LEAL, E. SANTOS, and S. SENENT, Proc. Roy. Soc. A274, 461 (1963).
[22] G.R. HENNIG, Appl. Phys. Letters 1, 55 (1962).
[23] G.R. HENNIG, J. chem. Phys. 40, 2877 (1964).
[24] G.K. WILLIAMSON and C. BAKER, Phil. Mag. 6, 313 (1961).
[25] G.R. HENNIG, in: Chemistry and Physics of Carbon, Vol. 2, Ed. P.L. WALKER, Jr., Marcel Dekker, New York 1966 (p.1).
[26] N. RASOR, and J. McCLELLAND, J. Phys. Chem. Solids 15, 17 (1960).
[27] J.E. HOVE, 1st Conf. Industrial Carbon and Graphite, London 1957, Soc. Chem. Ind., London 1958 (p. 501).
[28] J. EULER, Naturwissenschaften 39, 568 (1952); Ann. Phys. (Leipzig) 18, 344 (1956).
[29] W. N. REYNOLDS and P.A. THROWER, Radiation Damage in Reactor Materials, I.A.E.A. Vienna 1963 (p. 553).
[30] D.T. EGGEN, NAA-SR-69 (1950).