Chapter 3 - PART I: Focus on Solids Chapters: 2,3,4,5. 1...

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1 PART I: Focus on Solids Chapters: 2,3,4,5.
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2 The Structure of Crystalline Solids (Chapter 3) Crystals of gallium
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3 What you should understand by the end of this chapter: The ways atoms are arranged in a solid. How crystal structure is determined experimentally. The main types of crystal structure. Which materials exhibit which structures and why? A methodology for describing the structure of crystals. planes, directions The difference between crystalline and amorphous solids.
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4 Aluminum Rubber Glass Structure Changes Everything
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5 Bonding & Energy Solids form because atoms have lower overall energy when they’re close together (at low T) atoms give up energy as they approach each other attractive force, F A Electrostatic repulsion of outer electron shells repulsive forces, F R Net force F N = F A + F R
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6 Bonding & Energy Energy minimum energy is called the bond energy , E 0 bond length: ∫∫ ∞∞ + = + = = r R A R rr A N N E E dr F dr F dr F E 0 N r 0 dr dE =
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7 • Elastic modulus, E •E ~ curvature at r o •Melting temperature depends on E o (AN ENERGY!) Properties Derived from Bonding: E cross sectional area A o length, L o F undeformed deformed F A o = E L o Elastic modulus E o Bond Energy Bond length R Energy High T Low T L Δ L Δ
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8 Atomic Packing in Solids Two main flavours regular atomic packing crystalline random atomic packing amorphous, glassy sometimes (e.g. nylon) usually (e.g. polyethylene) never (“crystalline” polymers always partly amorphous) polymers often (e.g. silicon nitride) often (e.g. soda glass) often (e.g. alumina) ceramics never rarely (e.g. metallic glass) usually (e.g. steel, brass) metals Mixed Amorphous Crystalline
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9 Crystal Structures Regular, repeated arrangement of atoms repeat unit called unit cell repeated pattern called crystal lattice Dozens of possible structures defined by shape of cell arrangement of atoms within cell The inside front cover of the text has a table with data on the elements, including crystal structure.
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10 Properties / Crystal Structure Relationship Metals and ceramics Some properties are controlled by crystal structure 9 e.g. Dielectric constant (capacitance) Many other properties influenced by crystal structure 9 Strength 9 Electrical conductivity
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11 How Do We Know It’s A Crystal? Crystals diffract X-rays Bragg’s law says constructive interference will occur if the extra path is a multiple of the wavelength: Note : For practical reasons, the “diffraction angle” is n λ =2dsin θ 2 θ
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