Ch3 - Chapter 3: The Structure of Crystalline Solids ISSUES...

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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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Chapter Outline How do atoms arrange themselves to form solids? Fundamental concepts and language Unit cells Crystal structures Face-centered cubic Body-centered cubic Hexagonal close-packed Close packed crystal structures Density computations Types of solids Single crystal Polycrystalline Amorphous
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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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Basic Concepts Basic Concepts Crystals - atoms are packed in regular, repeating, 3- dimensional patterns Based on the spatial arrangement of atoms, materials can be classified as follows: 1 Crystalline materials: atoms or molecules are arranged in periodic arrays. (ex.: Metals and ceramics). 2 Noncrystalline or amorphous: atoms or molecules are randomly distributed in space (ex: glasses, polymers) 3 Semi-crystalline : a combination of the two cases above(ex. Polymers).
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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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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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Crystallography Crystallography - The science which deals with the geometry of crystals A crystal can be described by lattice + basis Lattice Lattice : 3-dimensional network (see CD ROM) Lattice point Lattice parameters: 6 parameters in 3-D. 3 edges (a, b, c) + 3 angles ( α , β , γ ) Basis Basis : “Group of things” located on a lattice point. Lattice + Basis = Crystal structure Lattice + Basis = Crystal structure
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Crystal structure To discuss crystalline structures it is useful to consider atoms as being hard spheres with well-defined radii. In this hard-sphere model, the shortest distance between two like atoms is one diameter.
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This note was uploaded on 03/15/2011 for the course CHE 215 taught by Professor Aboyousef during the Spring '11 term at American University of Sharjah.

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Ch3 - Chapter 3: The Structure of Crystalline Solids ISSUES...

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