RelativeResourceManager 2

RelativeResourceManager 2 - Chapter 3(part 2 Structures of...

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Unformatted text preview: Chapter 3 (part 2): Structures of Metals and Ceramics (Cont.) Ionic Crystals, Silica, Silicates, Carbon… MSE 280: Engineering Materials Most ionic crystals can be considered as close- packed structure of anions, with cations in the interstitial sites Ionic Crystals Material % Ionic Character Cation Radius (nm) Anion Radius (nm) CaF 2 89 0.100 0.133 MgO 73 0.072 0.14 NaCl 67 0.102 0.182 Al 2 O 3 63 0.053 0.140 SiO 2 51 0.040 0.140 Common Crystal Structures: FCC A B C A B C A B C… A B C Octahedral site Tetrahedral site Common Crystal Structures: HCP A B A A B A B A B… Octahedral site Tetrahedral site Interstitial Spots in CP Structures 1 at the center 12 middle of the edge sites (each shared by 4 unit cells) Net 4 O h sites/unit cell Net 8 T d sites/unit cell Interstitial Sites in FCC Looking down [0001] of HCP unit cell 3 O h sites on top half of unit cell (by symmetry, 3 more on bottom half) Total 2 O h sites 6 T d sites on top half of unit cell (by symmetry, 6 more on bottom half) Total 4 T d sites /3 /3 Interstitial Sites in HCP Total 6 O h sites Total 12 T d sites 6 atoms in HCP unit cell = 12*(1/6) + 3 + 2*(1/2) O h sites form HCP lattice. Hence, sites equal atoms. T d sites for 2 HCP lattices. Hence, twice as many sites as atoms. corners interior faces Interstitial Sites in HCP Many ceramic structures (ionic crystals) can be considered as close packed anions with cations in the interstitial sites Ionic Crystals Recall FCC crystal… Interstitial sites? Octahedral sites Note: only ½ of Td sites filled Ionic Crystals Many ceramic structures (ionic crystals) can be considered as close packed anions with cations in the interstitial sites Recall FCC crystal… Interstitial sites? Tetrahedral sites • Charge neutrality Net charge = 0 – Determines general formula (A m X p ) – Example: Ca Ca 2+ F F- • Atomic Size (ratio of atomic radii) • Stable configuration when anions surrounding a cation are all in contact with the cation • Coulomb attraction/repulsion (need net attraction) cations want to maximize # of neighboring anions and vice versa CaF 2 Ionic Crystals Adapted from Fig. 12.1, Callister 6e. Fig. 3.4 in text • Coordination # increases with How many anions can you arrange around a cation? Adapted from Table 12.2, Callister 6e. Where do these numbers come from? Zinc blende structure Rock salt structure Cesium chloride structure Coordination # and Ionic Radii e.g. 3-coordinate when With a = 30 o Rewrite as Minimum ratio for 3-coordinate Cation-Anion Stable Configs. Bonding in compounds in which there is a large difference in electronegativity An electron is (partially) transferred from one atom to the other! Electron binding energy (chemical potential) This leaves a “+” charge on the electron donor and a “-” charge on the acceptor Bonding is then by electrostatic attraction....
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This note was uploaded on 09/16/2010 for the course ENG 280 taught by Professor Bell during the Spring '10 term at Uni. Plymouth.

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RelativeResourceManager 2 - Chapter 3(part 2 Structures of...

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