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FL&O_section_3[1] - Chapter S The Structure of...

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1 Chapter S. The Structure of solids. After having studied this chapter, the student will be able to: 1. Distinguish between a crystal structure and an amorphous structure. 2. Describe the concept of a unit cell. 3. Describe the FCC, BCC and HCP structures. Explain why FCC and HCP are both the densest packing of atoms. 4. Describe polymorphism and give an example. 5. Draw and recognize atomic coordinates in a cubic lattice 6. Draw and recognize directions in a cubic lattice 7. Draw and recognize atomic planes in a cubic lattice. 8. Draw the densest planes in FCC and BCC structures. 9. Distinguish a single crystal and polycrystalline material. Define a grain and a grain boundary. 10. Relate compositions in weight % and atomic % to the crystal structure. 11. Name and describe the point defects in a solid. 12. Describe and draw and edge and screw dislocations. 13. Describe grain boundaries and their structure. We don’t think of materials like structural steel or solder as being crystalline because they aren’t clear, transparent, faceted, sparkling, angular, etc. Despite external appearances to the contrary, materials like metals are crystalline: their atoms are arranged in regular, periodic, three-dimensional structures called crystals. Figure S.1. for example, shows the crystalline arrangement of atoms in germanium. Figure S.1. Transmission microscope picture showing the positions of atoms in germanium. There are materials, especially glass and most polymers, in which the atoms of molecules are not positioned in ordered arrays; these are amorphous solids and all liquids. Crystals are not perfect. They contain impurities, voids, misplaced atoms and other defects. It turns out, perhaps as a surprise, that the crystalline defects are responsible for many of the useful properties of materials; a perfect and perfectly pure crystal is generally useless.
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2 The crystal structure of materials and the crystalline defects play a very large role in the properties of materials, especially in the way they deform plastically under stress. Defects are used to strengthen materials. Knowledge of the crystal structures and defects will provide us with the methods to tailor the mechanical properties of solids to their use. The knowledge of the simplest crystal structures will suffice for our purposes. Many materials, especially ceramic compounds, have very complex structures, but these are usually not manipulated to obtain specific material performance and we will not include them. S.1. Crystal Structures With few exceptions, the elements crystallize in one of three structures: the face centered cubic structure (FCC), the body centered cubic structure (BCC) and the hexagonal close packed structure (HCC). The important semiconductor elements silicon and germanium crystallize in the diamond structure, which is a variant of FCC.
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