Vacancies: Introduction and Exploration

A vacancy is an empty lattice site that would contain an atom in a perfect crystal. We call this an "intrinsic" defect because it can form intrinsically in a pure crystal itself. It doesn't not require a different type of atom. More on this later.

A vacancy.

Figure 6.4.1 A vacancy.

NetLogo model 6.4.1 below is a molecular dynamics model that allows you to create vacancies by clicking on atoms to remove them. In this model, temperature is not held constant. Use the model to answer the questions in Exercise 6.4.1.

Using the Model

NetLogo model 6.4.1 is a model that limits a few more things and enables a few more things that previous models. It is still a Lennard-Jones simulation, like before, but we provide options for showing different bonds, and we give an option to show kinetic energy (proportional to the temperature of the crystal) and potential energy (here, which could be converted to potential.

Let's use this model the think about what's going on with the graphite in the Windscale reactor.

Exercise 6.4.1: Small Defects, Big Impact
Not Currently Assigned

NetLogo model 6.4.1 above is a molecular dynamics model, like the one in the previous chapter (Section 5.3), but you can click atoms to delete them to create vacancies. In this model, temperature is not held constant - if you some how add/subtract energy the system, the temperature can increase/decrease.

To begin, press setup and go.

You'll notice that the system has 5 atoms at the bottom which are fixed. We do this just to keep things in place. Depending on the position of two atoms with respect to each other, their bonds may be slightly compressed or stretch at any instant.

Now, make sure your click-mode is set to delete atoms and answer the questions below. Take about 5 minutes on this exercise.

  1. What happens to the potential and kinetic energy of the system if you create a vacancy on the inside of the crystal? That is, a darker blue atom away from the surface. If you observe that the crystal "collapses", please select another atom.

    This simulates a carbon atom being "knocked out" of the graphite lattice.

  2. Create a second vacancy somewhere away from the surface of the crystal. What happens to the potential and kinetic energy? What do you think this implies about what might happen in a crystal as we increase the number of vacancies?

    Why might this be a problem in a nuclear reactor?

  3. It's clear from the previous question that if vacancy concentrations get too high, we can have a fire. Being a smart materials scientist and knowing about this effect (which is called the Wigner Effect) what might you do to avoid large jumps in temperature that may lead to fires?

    There's a hint in the NetLogo model 6.4.1 model that you can explore if you like.