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Unformatted text preview: ISSUES TO ADDRESS... How do atoms assemble into solid structures? How does the density of a material depend on s structure? Chapter 3: Structures of Metals & Ceramics Chapter 3  ENME 382 Spring 2010 Section 0201 1 its structure? When do material properties vary with the sample (i.e., part) orientation? How do the crystal structures of ceramic materials differ from those for metals? Non dense, random packing Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Chapter 3  ENME 382 Spring 2010 Section 0201 2 Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy r typical neighbor bond length typical neighbor bond energy atoms pack in periodic, 3D arrays Crystalline materials...metalsmany ceramicssome polymers crystalline SiO 2 Adapted from Fig. 3.40(a), Callister & Rethwisch 3e. Materials and Packing typical of: Chapter 3  ENME 382 Spring 2010 Section 0201 3 atoms have no periodic packing Noncrystalline materials...complex structuresrapid cooling noncrystalline SiO 2 " Amorphous " = Noncrystalline Adapted from Fig. 3.40(b), Callister & Rethwisch 3e. Si Oxygen occurs for: Metallic Crystal Structures How can we stack metal atoms to minimize empty space? 2dimensions Chapter 3  ENME 382 Spring 2010 Section 0201 4 vs. Now stack these 2D layers to make 3D structures 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. earest neighbor distances tend to be small in Metallic Crystal Structures Chapter 3  ENME 382 Spring 2010 Section 0201 5 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... Rare due to low packing density (only Po has this structure) Closepacked directions are cube edges. Coordination # = 6 (# nearest neighbors) Simple Cubic Structure (SC) Chapter 3  ENME 382 Spring 2010 Section 0201 6 (Courtesy P.M. Anderson) APF for a simple cubic structure = 0.52 Atomic Packing Factor (APF) APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres Chapter 3  ENME 382 Spring 2010 Section 0201 7 APF = a 3 4 3 (0.5 a ) 3 1 atoms unit cell atom volume unit cell volume Adapted from Fig. 3.42, Callister & Rethwisch 3e. closepacked directions a R =0.5 a contains 8 x 1/8 = 1 atom/unit cell Coordination # = 8 Atoms touch each other along cube diagonals.Note: All atoms are identical; the center atom is shaded differently only for ease of viewing....
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This note was uploaded on 01/24/2011 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.
 Spring '10
 Bruck

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