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| M A T E R I A L S C I E N C E & E N G I N E E R I N G |
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Crystalline Structure A crystalline material consists of primarily organized crystal structure. A crystal is: a solid composed of atoms, ions, or molecules arranged in a pattern that is repetitive in three-dimensions. Each crystal structure within a specific crystal system is defined by a unit cell. A unit cell is the smallest repeatable subsection of the crystal. In thinking about crystals, it is often convenient to ignore the actual atoms, ions, or molecules and to focus on the geometery of periodic arrays. The crystal is then represented as a lattice, that is, a three-dimensional array of points (lattice points), each which has identical surroundings. During Christmas time, you will encounter two-dimensional lattices in wrapping paper. Another example are lattices found on your bathroom floor, as illustrated below. Can you think of other lattices in this picture?
Each crystal lattice is defined by a crystal system. In three-dimensions, there are seven crystal systems: triclinic, monoclinic, orthorhombic, hexagonal, rhombohedral, tetragonal, and cubic. These collection of systems are called the Bravais lattices. One example of a crystalline material is iron. Iron has a Body Centered Cubic (BCC) unit cell: It is grouped in the cubic crystal system.
Crystal Structure of Metals: (a) Face Centered Cubic (FCC) (b) Body Centered Cubic (BCC) (c) Hexagonal Close Packed (HCP)
Above are graphics of NaCl and Zinc Blende structures that are in cubic systems.
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| Copyright © 2006 CES Information Guide - Materials Science Engineering |
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