NEEP 541

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NEEP 541 Graphite Damage

• Fall 2002
• Jake Blanchard

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Outline

• Radiation Damage in Graphite
• Graphite structure
• Swelling
• Thermomechanical properties
• sputtering

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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

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Different Views of Structure
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Phase Diagram
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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

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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

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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

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Polycrystalline Graphite35 dpa 600-690 C
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Pyrolitic Graphite
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Pyrolytic Graphite
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Isotropic Graphite
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Thermomechanical Properties

• Modulus and thermal conductivity increase as
  density increases, then decrease

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Polycrystalline Graphite
Thermal expansion coefficient
Thermal conductivity
Elastic modulus
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Pyrolitic GraphiteParallel to Planes
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Pyrolitic GraphitePerpendicular to Planes
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Sputtering

• Both physical and chemical sputtering occur in
  graphite

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Pyrolitic Carbon Sputtering
He
D
H
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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

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Chemical Sputtering

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

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Chemical Sputtering1 keVProtons
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Methane Production - Protons
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Methane Yield 2 keV protons