Line Defects in Crystalline Materials

Location:

Concepts Shown:

dislocations/two dimensional defects in crystalline materials

Equipment:

A large-sized rectangular sponge. Mark sponge with blue grid to represent atomic positions in a lattice. Cut into the foam as shown below, then add a red rectangle which encompasses the cut [Burger's circuit].

Procedure:

  1. Explain the blue grid as a model of the crystalline structure of a material, where each corner represents a lattice position or atom.
  2. Show that there is crystal perfection by following the red line around the lattice. The starting point is re-obtained by tracing the red path.
  3. Apply force to the upper portion of the foam cut to cause an addition of an extra half plane of atoms in the material.


Once again, follow the red line from the designated starting position. Notice that the end point does not coincide with the starting point. This is called a Burger's circuit to represent the 2-dimensional defect introduced in the crystalline solid. The extra half plane of atoms is called a line defect, commonly referred to as a dislocation. The distance from the starting point to the end point is called the Burger's vector.

This type of dislocation is called an edge dislocation where the Burger's vector and the dislocation line are perpendicular to one another. We can liken this extra half plane of atoms to the a left turn lane: one is driving along the road and suddenly an extra lane appears- the left turn lane! 4) Create a screw dislocation by twisting the foam cut. This causes a closure failure in the Burger's circuit. Now the dislocation line and the Burger's vector are parallel to each other. A screw dislocation is extremely useful for growth of crystal. Atoms can attach more easily by the availability of bonding to more atoms. A good analogy to the screw dislocation is a parking garage. The center post is the dislocation line, where the distance between levels is the Burger's vector. Driving up in a spiraling fashion represents crystal growth!

Science:

The basic principles are presented above. To Summarize: the cut in the foam through the Burger's circuit allows the crystal to be manipulated to show edge and screw dislocations. This allows easy verification of the Burger's vector and dislocation line relationship. It also reveals the ease of crystal growth with the addition of a screw dislocation.

References:

Author:
Roberta Dean
Credits:


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