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 Graphite 35 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 conductivity Thermal expansion coefficient Elastic modulus

15. Pyrolitic Graphite Parallel to Planes

16. Pyrolitic Graphite Perpendicular 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 Sputtering 1 keVProtons

22. Methane Production - Protons

23. Methane Yield – 2 keV protons