lecture3-materials-pdf - Chapter 3 The Structure of St...

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hapter 3 Chapter h S tt f The Structure of rystalline Solids Dr. Mubarak ١ Crystalline Solids
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Dr. Mubarak ٢
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Outline How do atoms arrange themselves to form solids? • Types of solids 1. Single crystal 2. Polycrystalline 3. Amorphous • Fundamental concepts Unit cells • Unit cells • Crystal structures imple cubic 1. Simple cubic 2. Face-centered cubic . Body- ntered cubic 3. ody ce te ed cub c 4. Hexagonal close-packed • Close packed crystal structures Dr. Mubarak ٣ • Density computations•
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Types of solids Crystalline material: atoms self-organize in a periodic rray: Array: Single crystal: atoms are in a repeating or periodic array over the entire extent of the material Polycrystalline material: comprised of many small crystals or grains Amorphous: lacks a systematic atomic arrangement Dr. Mubarak ٤
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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. We can also consider crystalline structure as a lattice of points at atom/sphere centers. Dr. Mubarak ٥
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Unit Cell The unit cell is the smallest structural unit or building block that can describe the crystal structure. Repetition of the unit cell generates the entire crystal. Example: 2D honeycomb net can be represented by translation of two adjacent atoms that form a unit cell for this 2D crystalline structure Example of 3D crystalline structure: Dr. Mubarak ٦
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Different choices of unit cells are possible, for example: i t l l t i f t h f l l i A square unit cell may contain any of the following object patterns. One hexagonal unit cell might look like any of the following. Dr. Mubarak ٧
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CRYSTAL LATTICES A crystal is a repeating array. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell). The most fundamental property of a crystal lattice is its symmetry. If we initially limit ourselves to 2 dimensions for simplicity, three types are present: 1. Squares 2. Rectangles g 3. Hexagons Dr. Mubarak ٨
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ttice nit cell lattice unit cell nit cell ttice unit cell lattice it ll tti unit cell nit cell lattice ttice Dr. Mubarak ٩ unit cell lattice
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Metallic Crystal Structures Metals 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. • Have the simplest crystal structures. Dr. Mubarak ١٠
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Metals are usually (poly)crystalline. lth h f ti f h t l i ibl Although formation of amorphous metals is possible by rapid cooling.
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This note was uploaded on 02/23/2011 for the course CHEMICAL E 0905326 taught by Professor Yousefmobarak during the Spring '11 term at University of Jordan.

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lecture3-materials-pdf - Chapter 3 The Structure of St...

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