MetE227-c3 - CHAPTER 3 CHAPTER STRUCTURE OF CRYSTALLINE...

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CHAPTER 3: CHAPTER 3: STRUCTURE OF STRUCTURE OF CRYSTALLINE SOLIDS CRYSTALLINE SOLIDS
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• atoms pack in periodic, 3D arrays • long-range order •t y p i c a l o f : -metals -many ceramics -some polymers • atoms have no periodic packing •o c c u r s f o r : -complex structures -rapid cooling Si Oxygen crystalline SiO crystalline SiO 2 noncrystalline SiO noncrystalline SiO 2 " Amorphous " = Noncrystalline Materials and packing: Types of solids
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Si (crystalline) SiO 2 (amorphous)
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• Non dense, random packing •D en s e , regular packing Dense, regular-packed structures tend to have lower energy. Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy Energy and packing Energy and packing
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Atomic hard sphere model is used to describe the arrangement of atoms. Lattice: A 3D array of points in space coinciding with atom positions (or sphere centers). Hard sphere model and lattice:
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Unit cell: (Lattice cell) Smallest structural unit that decribes the whole crystal structure. Unit cell: Reduced sphere model:
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As an easy and simple way of representation we use cubes or similar geometrical tools to represent a unit cell.
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• 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. Most common crystal structures in metals: BCC, FCC, HCP METALLIC CRYSTALS
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Rare for metals due to poor packing (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) S S imple imple C C ubic ubic (SC) (SC) Examples: Po
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APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres • APF for a simple cubic structure = 0.52 APF = a 3 4 3 π (0.5a) 3 1 atoms unit cell atom volume unit cell volume close-packed directions a R=0.5a contains 8 x 1/8 = 1 atom/unit cell ATOMIC PACKING FACTOR
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F F ace ace - - C C entered entered C C ubic ubic (FCC) (FCC) Examples: Cu, Al, Ag, Au
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Examples: Fe( α ), Cr, Mo B B ody ody - - C C entered entered C C ubic ubic (BCC) (BCC)
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H H exagonal exagonal C C lose lose - - P P acked acked (HCP) (HCP) Examples: Mg, Ti, Zn,
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This note was uploaded on 09/25/2011 for the course METE 227 taught by Professor Ali during the Spring '10 term at Middle East Technical University.

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MetE227-c3 - CHAPTER 3 CHAPTER STRUCTURE OF CRYSTALLINE...

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