Chapter3_yy - Chapter 3 The Structure of Crystalline Solids...

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1 Chapter 3 - 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? Chapter 3: The Structure of Crystalline Solids
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2 Chapter 3 - Crystal structure polymer Cobalt ceramic crystal plate Microstructure of Ta2O5 bismuth methal
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3 Chapter 3 - Non dense, random packing Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy
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4 Chapter 3 - atoms pack in periodic, 3D arrays Crystalline materials... -metals -many ceramics -some polymers atoms have no periodic packing Noncrystalline materials... -complex structures -rapid cooling crystalline SiO 2 noncrystalline SiO 2 " Amorphous " = Noncrystalline Adapted from Fig. 3.22(b), Callister 7e. Adapted from Fig. 3.22(a), Callister 7e. Materials and Packing Si Oxygen typical of: occurs for:
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5 Chapter 3 - Atomic arrangement
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6 Chapter 3 - Crystal structure
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7 Chapter 3 - Section 3.3 – Crystal Systems 7 crystal systems 14 crystal lattices Fig. 3.4, Callister 7e. Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal. a, b, and c are the lattice constants
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8 Chapter 3 - Unit Cell Example: 2D honeycomb net can be represented by translation of two adjacent atoms that form a unit cell for this 2D crystalline structure
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9 Chapter 3 - Tend to be densely packed. 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. - Electron cloud shields cores from each other Have the simplest crystal structures. We will examine three such structures... Metallic Crystal Structures
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10 Chapter 3 - Section 3.4 – Metallic Crystal Structures How can we stack metal atoms to minimize empty space? 2-dimensions vs. Now stack these 2-D layers to make 3-D structures
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11 Chapter 3 - Rare due to low packing denisty (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) Simple Cubic Structure (SC)
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12 Chapter 3 - Rare due to low packing denisty (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) Simple Cubic Structure (SC) Adapted from Fig. 3.23, Callister 7e. close-packed directions a R =0.5 a contains 8 x 1/8 = 1 atom/unit cell
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13 Chapter 3 - Coordination # = 8 Adapted from Fig. 3.2, Callister 7e. (Courtesy P.M. Anderson) Atoms touch each other along cube diagonals.
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