Metal_Ceramic_Structures_Atomic_Level_Module2 -...

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MSE3094 (Staley) Notes Module #2 – Metal and Ceramic Structures at the Atomic Level A) Solid Structure Basics: Crystals and Glasses B) Cubic and Hexagonal Metallic Crystals Structure Types Packing and Density Directionality of Properties C) Review of Bonding in Ceramics D) Basic Ceramic Structure Types Ionic Ceramics Silicates Carbon E) Recap on Basics of Ceramic Structures A) Solid Structure Basics: Crystals and Glasses A crystal (abbreviation: XTL) is a solid repeating array of atoms exhibiting 3D Long- Range Order (LRO) Liquid Crystals are a special case that occurs in complex polymers and displays 2D order (see pp. 557-559 ). All solids are either crystals or glasses. Glasses show only Short-Range Order (SRO) [see Fig 3. 22 for an example of the distinction between LRO and SRO] For our purposes now, we can treat atoms as HARD SPHERES that pack together in an arrangement that can be characterized by the arrangement of nearest neighbors. Because the arrangement repeats, we can use a UNIT CELL to define the xtl structure: UNIT CELL: 3D repeat unit (building block) for a crystal. For a bulk material (i.e., ignoring surface effects), we consider the unit cell to repeat infinitely in all directions. Usually (but not always) we use the SIMPLEST UNIT CELL possible (One prominent exception to this rule is the Hexagonal unit cell, where a larger unit than strictly necessary is often used to emphasize the symmetry that these structures exhibit – see footnote 1 on p. 43 and Table 3. 2 ) Crystals, and unit cells, can exhibit 1, 2, or 3 dimensional symmetry. The simplest case is three-dimensional symmetry: Cubic. All possible crystal structures can be grouped into 7 CRYSTAL SYSTEMS and 14 CRYSTAL CLASSES (or "Bravais Lattices"). [See Table 3. 2 ]
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Cubic: Simple (SC), Face-Centered (FCC), Body-Centered (BCC). Tetragonal: Simple (ST), Body-Centered (BCT). Hexagonal Orthorhombic: Simple, Face-Centered, Body-Centered, Base-Centered. Rhombohedral Monoclinic: Simple, Base-Centered Triclinic Right now, we will deal with common METALLIC crystal structures and return to the more complex ceramic and polymer structures afterwards. B) Cubic and Hexagonal Metallic Crystals Structure Types Packing and Density Directionality of Properties Simple Cubic: Atoms located on corners of cube, each atom 1/8 inside cell Two ways to view: 3D perspective, 2D cross-sections. 1 atom/unit cell COORDINATION NUMBER (CN) [# near neighbors] = 6 ATOMIC PACKING FACTOR (APF) [Total Atom Volume/Cell Volume] = 0.52 Close Packed Plane [Plane on which atoms touch] = (100) Unit Cell Dimension = 2 atomic radii No Real Metals display this structure (Packing not tight enough) How can we achieve higher density than Simple Cubic packing?: Three commonly-occurring choices: BCC, FCC, Hexagonal (HCP) Face-Centered Cubic (FCC) Figure 3.1 4 atoms/cell CN=12 Unit cell dimension = 2
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This note was uploaded on 07/05/2010 for the course MSE 2034 taught by Professor Rwhendricks during the Spring '06 term at Virginia Tech.

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Metal_Ceramic_Structures_Atomic_Level_Module2 -...

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