Chapter 3_Crystal structures_Filled

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Simon Fraser University *

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Material Science

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Oct 30, 2023

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9/21/2023 1 Key Concepts... • Atoms’ assembly in solids • The density of a materials depend on structures • Different material property with different orientation Chapter 3: The Structure of Crystalline Solids MSE220 Engineering Materials 1

9/21/2023 2 2 • Non crystalline ceramics and polymers normally are optically transparent, but the same materials in crystalline or semi crystalline structure tend to be opaque or translucent. MSE220 Engineering Materials Conceptual Question https://www.hardiepolymers.com/knowledge/how-easy-is-it-to-see-through- the-optical-properties-of-thermoplastics/ Why?

9/21/2023 3 Materials and Atomic Arrangement • periodic array of atoms or molecules • 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. 3.25(b), Callister 10e. Adapted from Fig. 3.25(a), Callister 10e. Si Oxygen • typical of: • occurs for: MSE220 Engineering Materials 3

9/21/2023 4 Energy and Packing • Non dense, random packing (Amorphous) • Dense, ordered packing (Crystalline) Ordered structures tend to be nearer the minimum in bonding energy and are more stable Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy MSE220 Engineering Materials 4

9/21/2023 5 Crystal Structure • In crystalline solids, the atoms or group of atoms form repetitive patterns or a lattice . • A “ lattice “ is a collection of points in space that are arranged in a periodic pattern so that the surroundings of each point in the lattice are identical, i.e. a lattice is a three-dimensional network of points. • Each of the thousands of solids available today have a relatively small number ( 7 ) of crystal system. • The crystal structure of a material describes the size, shape , and atomic arrangement within the lattice. MSE220 Engineering Materials 5

9/21/2023 6 Unit Cell Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal. Fig. 3.1, Callister 10e. Ex: face-centered cubic (FCC) crystal structure: a)Hard sphere unit cell representation b) Reduced-sphere unit cell representation MSE220 Engineering Materials 6

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