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NEEP 541 – Graphite Damage

NEEP 541 – Graphite Damage. Fall 2002 Jake Blanchard. Outline. Radiation Damage in Graphite Graphite structure Swelling Thermomechanical properties sputtering. Graphite Crystal Structure. Crystal is hexagonal

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NEEP 541 – Graphite Damage

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  1. NEEP 541 – Graphite Damage Fall 2002 Jake Blanchard

  2. Outline • Radiation Damage in Graphite • Graphite structure • Swelling • Thermomechanical properties • sputtering

  3. Graphite Crystal Structure • Crystal is hexagonal • Planes of atoms are strongly bonded (covalent) within the plane, but the plane-to-plane bonding is relatively weak (van der Waals) [lubrication] • Crystal cleaves easily parallel to the basal planes • Physical properties are highly anisotropic

  4. Different Views of Structure

  5. Phase Diagram

  6. Types of Graphite • Pyrolitic – highly oriented • Polycrystalline graphites with randomly oriented grains • POCO graphite is fine-grained, giving it high strength and high failure strains • Graphnol is similar to POCO, but with smaller thermal expansion coefficient

  7. Irradiation of Graphite • Neutron irradiation produces point defects • Interstitials form loops (immobile) or small, mobile clusters • Vacancies form loops or collapse lattice within layer planes • Growth occurs perpendicular to layer planes due to interstitials and shrinkage occurs parallel to planes due to relaxation of lattice around vacancies or lines of vacancies

  8. Swelling of Graphite • Graphite usually shrinks initially due to pore closure • Graphite is porous due to cooling from the graphitizing temperature • After initial shrinkage, growth occurs • When volume returns to initial value, structural properties are poor

  9. Polycrystalline Graphite35 dpa – 600-690 C

  10. Pyrolitic Graphite

  11. Pyrolytic Graphite

  12. Isotropic Graphite

  13. Thermomechanical Properties • Modulus and thermal conductivity increase as density increases, then decrease

  14. Polycrystalline Graphite Thermal expansion coefficient Thermal conductivity Elastic modulus

  15. Pyrolitic GraphiteParallel to Planes

  16. Pyrolitic GraphitePerpendicular to Planes

  17. Sputtering • Both physical and chemical sputtering occur in graphite

  18. Pyrolitic Carbon Sputtering He D H

  19. Chemical Sputtering • Molecules are formed on surface due to chemical reaction between incident ion and carbon atoms with binding energy low enough to desorb • Molecule then is not bound to surface • A third process (radiation enhanced sublimation) allows target atoms to be thermally released from surface

  20. Chemical Sputtering • With incident hydrogen, sputtering yield peaks around 800-900 K • Peak yield is 0.1 ions/ion

  21. Chemical Sputtering1 keVProtons

  22. Methane Production - Protons

  23. Methane Yield – 2 keV protons

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