CHAPTER 3: Crystal structures and properties - University of Washington
CHAPTER 3: Crystal structures and properties
ISSUES TO ADDRESS...
? How do atoms assemble into solid structures? (for now, focus on metals)
? How does the density of a material depend on its structure?
? When do material properties vary with the sample (i.e., part) orientation?
1
Energy and packing
? Non dense, random packing
Energy
typical neighbor bond length
typical neighbor
r
bond energy
? Dense, regular packing
Energy
typical neighbor bond length
typical neighbor
r
bond energy
Dense, regular-packed structures tend to have lower energy.
2
Materials and packing
Crystalline materials... ? atoms pack in periodic, 3D arrays ? typical of: -metals
-many ceramics -some polymers
Noncrystalline materials... ? atoms have no periodic packing ? occurs for: -complex structures
-rapid cooling
crystalline SiO2
Adapted from Fig. 3.18(a), Callister 6e.
Si Oxygen
"Amorphous" = Noncrystalline
noncrystalline SiO2
Adapted from Fig. 3.18(b), Callister 6e.
3
Metallic crystals
? tend to be densely packed. ? have several reasons for dense packing:
-Typically, only one element is present, so all atomic radii are the same.
-Metallic bonding is not directional. -Nearest neighbor distances tend to be small in
order to lower bond energy.
? have the simplest crystal structures. We will look at three such structures...
4
Simple Cubic (SC) structure
? Rare due to poor packing (only Po has this structure) ? Close-packed directions are cube edges.
? Coordination # = 6 (# nearest neighbors)
(Courtesy P.M. Anderson) 5
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