Dissemination of IT for the Promotion of Materials Science (DoITPoMS)



Quick questions

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:

    a No defects, since they must be perfect crystals.
    b Exactly one defect, hence the term 'single crystal'.
    c Many defects.
    d More defects than atoms, since every atom must generate at least one defect.

  2. If quartz had optical properties such that the refractive indices for all vibration directions were equal, the crossed polariser experiment would show:

    a No light transmitted for any orientation.
    b Varying intensity as the orientation changes.
    c Uniform non-zero intensity of transmitted light regardless of orientation.
    d Circular dark patches to represent the symmetry of the optical properties.

  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?

    a The geometry of the places where bubbles meet one another is different from the geometry of the junctions between grains in a real polycrystal.
    b The shape of the bubbles is different from the typical shape of a grain.
    c The way a bubble deforms when a load is applied is different from the way a grain deforms.
    d The three dimensional structure of bubbles in a box is unlike the three dimensional structure of a polycrystal.

  4. Which of the following is false?

    a Quartz crystals have optically anisotropic properties.
    b Glass has no regular repeating crystalline structure.
    c Certain crystals may cleave easily along certain planes, defined by the crystal structure.
    d Crystal defects are not found in single crystals.

  5. What does the 'shot model' fail to show?

    a Polycrystallinity
    b Crystalline defects
    c The third dimension of the structure
    d The difference between a vapour and a solid

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Why is window glass transparent?

    a Because it has a single crystal structure and each sheet is cut with the optic axis normal to the plane of the window.
    b Because it has an amorphous structure with large interatomic spacing. Light waves can pass between widely spaced atoms without any interaction with the solid structure.
    c Because sheets of glass are cut thin enough for light to pass through without any significant absorption.
    d Because of the electronic nature of the bonds between the atoms in the glass.

  2. 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?

    a A polycrystal with grains of identical chemical composition but different crystallographic orientation.
    b A polycrystal consisting of finely spaced lamellae with alternating composition.
    c A single crystal.
    d An amorphous solid with good mechanical strength.

  3. 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?

    a The contributions should be about the same in both cases.
    b The contribution from lattice diffusion will always be greater than the contribution from grain boundary diffusion.
    c The contribution from grain boundary diffusion will always be greater than the contribution from lattice diffusion.
    d The relative contributions of the two processes depend upon the temperature of the material.

  4. 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.

  5. Which of the following material properties could show anisotropy? (answer yes or no for each)

    Yes No a Density
    Yes No b Young's modulus
    Yes No c Electrical conductivity
    Yes No d Refractive index

Open-ended questions

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.

  1. Think about some of the possible applications of materials showing optical anisotropy, like the quartz crystal.

  2. 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?

  3. 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?

  4. 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?

  5. 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?