L05B: Line defects (dislocations)

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L05B Line defects (dislocations)

• Two basic types edge and screw, plus a mixture
  of these.
• The edge dislocation can be imagined as being
  formed by an extra half-plane of atoms inserted
  into the structure (or removal of half a plane)

• Note distortion of lattice caused by its
  presence.
• Characterized by its direction and Burgers
  vector, which is found by making a circuit around
  the core.

• The Burgers vector and direction are
  perpendicular for edge dislocations.
• Shear stress parallel to the Burgers vector
  causes the dislocation to move.? VMSE

Last revised September 25, 2013 by W.R. Wilcox,
Clarkson University
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Screw dislocations

• Can imagine as being formed by cutting half way
  through the crystal along a plane, and then
  shifting one part relative to the other in the
  direction of the cut.
• Again, the lattice is distorted near the
  dislocation core.
• Now the Burgers vector isparallel to the
  dislocation.
• Again, shear stress parallel tothe Burgers
  vector causesthe dislocation to move through
  the crystal.? VMSE

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Edge, Screw, and Mixed Dislocations
? VMSE
Notice that the Burgers vector does not change
along the dislocation
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Dislocations in non-metals

• Much more complex because of covalent and ionic
  bonding.
• If the unit cell is large, then the Burgers
  vector is also large.
• Often carry an electric charge, which can depend
  on impurity doping.
• Example of one model for a lt110gt edge dislocation
  in silicon

http//www.ornl.gov/info/ornlreview/v30n3-4/edge.h
tm
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Observation of dislocations

• Many methods have been used to observe
  dislocations, either directly or indirectly.
• Most rely on observation of the strain
  fieldaround the dislocation core.
• For example, transmission electron microscopy

• X-ray topography (using Brookhaven synchrotron
  light source Clarkson grad student observed
  dislocation motion in GaAs.)
• Transmission optical microscopy using crossed
  polarizers for ceramic crystals.
• Decoration of dislocations by impurities.
• Etch pits where dislocations emerge on surface.
• Growth spirals where screw dislocations emerge on
  surfaces.

(0001) face of carborundum (SiC)