Lecture 4 - The Structures of Crystalline Solids MEEN...

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1 MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource The Structures of Crystalline Solids MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource Learning Objectives Learning Objectives • How do atoms assemble into solid structures – Crystalline and amorphous materials • Identify the principal crystal structures for metallic systems: – Face centered and body-centered cubic, hexagonal closed-packed • Understand the concepts of crystal density
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2 MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource • Non dense, random packing • Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy and Packing Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource • 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 Materials and Packing Si Oxygen • typical of: • occurs for:
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3 MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource Crystal Systems Crystal Systems 7 crystal systems Fig. 3.4, Callister 7e. Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal. Atoms are considered as solid spheres Lattice: 3D array of points coinciding with atomic positions a, b, and c are the lattice parameters MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource Metallic Crystal Structures 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 Which one is denser?
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4 MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource • 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. Metallic Crystal Structures Metallic Crystal Structures MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource Metallic Crystal Structures Metallic Crystal Structures
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5 MEEN 222-502 Yeon Seok Kim (2008)* * Some information has taken from Callister’s Online Instructor Resource • Rare due to low packing denisty (only Po has this structure) Close-packed directions are cube edges.
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This note was uploaded on 06/24/2009 for the course MEEN 222 taught by Professor Radovic during the Fall '08 term at Texas A&M.

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Lecture 4 - The Structures of Crystalline Solids MEEN...

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