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Defects in graphite

Defects in graphite. Steve Bennington and Tom Weller. Windscale. In 1957 a fire at the Windscale pile caused the release of radioactive Iodine and some plutonium The was caused by Wigner energy release Stored energy up to 2700 J/gm The detailed nature of the defect is still not well known.

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Defects in graphite

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  1. Defects in graphite Steve Bennington and Tom Weller

  2. Windscale • In 1957 a fire at the Windscale pile caused the release of radioactive Iodine and some plutonium • The was caused by Wigner energy release • Stored energy up to 2700 J/gm • The detailed nature of the defect is still not well known

  3. Defects in graphite • Many of the properties of graphite change under irradiation • Resistivity • Magneto-resistance • Hall effect • Thermo-power • Diamagnetic susceptibility • Elastic constants • Physical dimensions • Thermal conductivity • Specific heat capacity • Thermal expansion Important to understand the nature of effects of defects in graphite and in carbon nanostructures

  4. Defects in graphite • Mono-vacancy diffusion Em=1.7eV • Two vacancies on the same sheet combine and have a migration energy of 7eV • If two vacancies on different sheets combine get migration energy of 3.2eV or 3.6eV Wigner defects bridge the graphite gap Rob H. Telling, Chris P. Ewels, Ahlam A. El-barbary and Malcolm I. Heggie, Nature Materials 2 (2004) 333

  5. Divacancies • A Jahn-Teller instability allows a bridging bond to form

  6. Interstitials • Two types of interstitial vacancy form one with four bonds and one with three • These are highly mobile at room temperature • They will combine at dislocations and are thought to pin dislocation motion • They will also combine with monovacancies to produce a bound Fenkel defect A ‘Spiro’ interstitial

  7. Bond Frenkel defects • These have a barrier to recombination of 1.4eV and will release 12eV • These are probably the main cause of Wigner energy release

  8. Stored energy • Three release peaks • 1.34 eV • 1.50 eV • 1.78 eV Tadao Iwata Journal of Nuclear Materials 133&134 (1985) 361 Pyrolytic graphite irradiated at 80°C

  9. Frenkel pairs in nanotubes • IV defects under electron irradiation Koki Urita, Kazu Suenaga,Toshiki Sugai, Hisanori Shinohara and Sumio Iijima, Physical Review Letters, 94 (2005) 155502

  10. Elastic Constants • For pyrolytic graphite the c44 elastic constant and perhaps the c33 change with irradiation • Inelastic neutron scattering measurements give a c44 of 0.42 x 1011 dyn/cm2 The C44 elastic constant of pyrolytic graphite irradiated at 50°C E.J. Seldin and C.W. Nezbeda, Journal of Applied Physics 41 (1970) 3389

  11. Phonons in Irradiated Graphite • Inelastic neutron scattering measures intrinsic elastic compliances • The initial slope gives the elastic constants • The width of the phonons gives an indication of how the phonon interacts with the disorder • Cross linking defects will mix c-axis and basal plane phonons

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