Imperfections in Crystalline Materials


Concepts Shown:

disorder and imperfections


  1. 100 marbles- 50 of each color
  2. a few ball bearings about 0.25 the size of the marbles
  3. a clear plastic box with sides, roughly 8" x 10"
  4. a few large spherical objects, about twice the size of a marble
  5. an overhead projector

  1. Place the plastic box on the overhead projector and pour the marbles inside, one color first and then the other. Keep the two colors separated. Pour enough marbles to fill the box. Do not spend time trying to rearrange the marbles, it defeats the purpose in revealing defects.
  2. Show that most of the atoms fill the space like a close packed plane. However, the packing is not perfect: there are alot of the point defects. Vacancies should be easy to find. [Figure 1 can be used as a transparency for added effect.]
  3. You may be able to point out a line defect or edge dislocation within the lattice.
  4. Notice the boundary between the two colors of marbles. This represents a 3-dimensional defect known as a grain boundary. Material's properties can vary greatly between the bulk and the grain boundary; for example, atoms can move or diffuse more readily along the grain boundary because it takes less energy to move atoms out of the way since space already exists. [See Fig. 1]
  5. Find a region of close packing and drop the bearings in this region. Shake the box slightly to help them drop into the lattice. Show how the bearings fill interstitial positions because of their size. However, they locally distort the lattice and disrupt the order of the lattice.
  6. Add the large 'atoms' to the lattice. Show that they must fill substituional lattice positions. These large atoms produce large lattice distortions.

The concept of crystallinity versus disorder is the main idea. The model is to demonstrate that 'perfect' crystals contain many defects. The dimensionality of the types of defects is important. Point defects are considered one-dimensional, line defects are 2-D, and grain boundaries are 3-D. The nomenclature or particular names of defects is important to remember. Vacancies and interstitials are singular point defects, however there are combinations of point defects- Schottky and Frenkel- that can be taught during this demonstration. Grain boundaries are important for the understanding of atomic diffusion paths and materials' properties. This demonstration can be used with the grain boundary illustration [Bubble Blower].

Thermal vibrations in relation to packing is an important concept to be discussed; Fig. 1 has example of vaporization to illustrate extreme atomic vibrations and atomic bonding.



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