1 / 5

L05B: Line defects (dislocations)

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.

mateer
Download Presentation

L05B: Line defects (dislocations)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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 Dislocation Burgers vector b

  3. Mixed Edge Screw Edge, Screw, and Mixed Dislocations  VMSE Notice that the Burgers vector does not change along the dislocation

  4. 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 <110> edge dislocation in silicon: http://www.ornl.gov/info/ornlreview/v30n3-4/edge.htm

  5. 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)

More Related