Chapter3

# Chapter3 - Chapter Outline How do atoms arrange themselves...

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1 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids How do atoms arrange themselves to form solids? Fundamental concepts and language Unit cells Crystal structures ¾ Face-centered cubic ¾ Body-centered cubic ¾ Hexagonal close-packed Close packed crystal structures Density computations Types of solids Single crystal Polycrystalline Amorphous 3.8–3.10 Crystallography – Not Covered / Not Tested 3.15 Anisotropy – Not Covered / Not Tested 3.16 Diffraction – Not Covered / Not Tested Chapter Outline

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2 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Types of Solids Crystalline material : atoms self-organize in a periodic 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: disordered – lack of a systematic atomic arrangement Crystalline Amorphous SiO 2
3 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Crystal structures Why do atoms assemble into ordered structures (crystals)? Energy of interatomic bond 0 Interatomic distance Let’s consider nondirectional bonding (like in metals) Energy of the crystal < Energy of the amorphous solid

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4 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids 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 of the hard sphere. We can also consider crystalline structure as a lattice of points at atom/sphere centers. 2R - hard-sphere model
5 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Unit Cell The unit cell is a structural unit or building block that can describe the crystal structure. Repetition of the unit cell generates the entire crystal. Different choices of unit cells possible, we will consider parallelepiped unit cell with highest level of symmetry 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:

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6 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Metallic Crystal Structures ¾ Metals are usually (poly)crystalline; although formation of amorphous metals is possible by rapid cooling ¾ As we learned in Chapter 2, the atomic bonding in metals is non-directional ± no restriction on numbers or positions of nearest-neighbor atoms ± large number of nearest neighbors and dense atomic packing ¾ Atomic (hard sphere) radius, R, defined by ion core radius - typically 0.1 - 0.2 nm ¾ The most common types of unit cells are •faced-centered cubic (FCC)
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Chapter3 - Chapter Outline How do atoms arrange themselves...

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