NEEP 541 – Graphite Damage

Fall 2002Jake 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 Graphite35 dpa – 600-690 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 GraphiteParallel to Planes

Pyrolitic GraphitePerpendicular 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 800-900 K Peak yield is 0.1 ions/ion

Chemical Sputtering1 keVProtons

Methane Production - Protons

Methane Yield – 2 keV protons