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You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!
1. Most single crystals contain:
2. If quartz had optical properties such that the refractive indices for all vibration directions were equal, the crossed polariser experiment would show:
3. Bubbles in a box behave in a similar way to grains in a crystal in several ways, but not in all. Which of the following statements is TRUE?
4. Which of the following is false?
5. What does the 'shot model' fail to show?
The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.
- Why is window glass transparent?
hint: Think carefully about the materials discussed in this package. Do the transparent ones have a common structure type (single crystal, polycrystal, amorphous)? How does the density of an amorphous material compare to a similar crystalline material?
7. A quantity of pure liquid aluminium is cooled slowly through its melting point. The solid is then left at room temperature for 100 years. What is the resulting structure?
8. Self-diffusion is the diffusion of a species within a body of material made from the same species. In general, self-diffusion in a polycrystalline solid can occur through the bulk of the grains (lattice diffusion) or along the grain boundaries (grain boundary diffusion). Which of the following statements gives the best description of the relative contribution of each process to the overall diffusion rate?
hint: Re-read the sections in this tutorial on polycrystals and defects. Are grain boundaries likely to be easy paths for diffusion? Consider what happens when the temperature is increased - what is the overall effect on the rate of diffusion? Look at the micrographs of grains again. Is most of the material 'grain'or 'grain boundary'?
9. Imagine a polycrystalline solid with cubic grains of edge length D. When D = 10 μm, what percentage of the volume of solid lies within a grain boundary, if the grain boundary width d is 1 nm? What must the grain size D be if 10% of the volume lies within a grain boundary? Comment on your answers.
Which of the following material properties could show anisotropy? (answer yes or no for each)
The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.
- Think about some of the possible applications of materials showing optical anisotropy, like the quartz crystal.
- How might you control the grain size of a material produced from a melt? How might the cooling rate and the chemical composition affect the results? Can you think of ways to change the grain structure after the material has solidified?
- In this TLP, we have discussed pure materials. Real materials almost always contain some impurities. How might these impurities be incorporated into the crystal structure of a material? Consider the relative size of the impurity atoms and the host atoms. Are impurities always undesirable?
- Graphite is sometimes used as a lubricant, and diamond can be used on the tips of cutting tools. In terms of the crystal structure, why might this be?
- Why do the individual grains in a polycrystalline material, such as those in the photo of galvanised steel (on the Polycrystals page) appear to be different colours or shades, when the composition of every grain is approximately the same?