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 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

Different Views of Structure

Phase Diagram

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

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

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

Polycrystalline Graphite 35 dpa – C

Pyrolitic Graphite

Pyrolytic Graphite

Isotropic Graphite

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

Polycrystalline Graphite Thermal conductivity Thermal expansion coefficient Elastic modulus

Pyrolitic Graphite Parallel to Planes

Pyrolitic Graphite Perpendicular to Planes

Sputtering Both physical and chemical sputtering occur in graphite

Pyrolitic Carbon Sputtering He D H

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

Chemical Sputtering With incident hydrogen, sputtering yield peaks around K Peak yield is 0.1 ions/ion

Chemical Sputtering 1 keVProtons

Methane Production - Protons

Methane Yield – 2 keV protons