ch12_ppts_callister7e

ch12_ppts_callister7e - Chapter 12: Structures &...

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Chapter 12 - 1 ISSUES TO ADDRESS. .. Structures of ceramic materials: How do they differ from those of metals? Chapter 12: Structures & Properties of Ceramics
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Chapter 12 - 2 Core Problems: Self-help Problems: ANNOUNCEMENTS Reading: Ch. 12, sections,
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Chapter 12 - 3 Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond , 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Large vs small ionic bond character: Ceramic Bonding SiC: small CaF 2 : large
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Chapter 12 - 4 Ceramic Crystal Structures Oxide structures oxygen anions much larger than metal cations close packed oxygen in a lattice (usually FCC) cations in the holes of the oxygen lattice
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Chapter 12 - 5 Which sites will cations occupy? Site Selection 1. Size of sites does the cation fit in the site 1. Stoichiometry if all of one type of site is full the remainder have to go into other types of sites. 1. Bond Hybridization
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Chapter 12 - 6 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Callister 7e. - - - - + unstable Charge Neutrality : --Net charge in the structure should be zero. --General form: - - - - + stable - - - - + stable CaF 2 : Ca 2+ cation F - F - anions + A m X p m, p determined by charge neutrality
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Chapter 12 - 7 Since in an FCC unit cells there are 4 atoms, it follows that: In a unit cell number of octahedral sites = number of atoms AND number of tetrahedral sites = 2 x number of atoms. T RED = Octahedral sites = 1/4 x 12 + 1 = 4 per unit cell BLUE = Tetradedral sties = 8 per unit cell O IMPORTANT SLIDE Octahedral sites are larger …. .
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Chapter 12 - 8 Coordination # increases with Coordination # and Ionic Radii Adapted from Table 12.2, Callister 7e. 2 r cation r anion Coord # < 0.155 0.155 - 0.225 0.225 - 0.414 0.414 - 0.732 0.732 - 1.0 3 4 6 8 linear triangular T D O cubic ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) r cation r anion Issue: How many anions can you arrange around a cation?
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Chapter 12 - 9 Cation Site Size Determine minimum r cation /r anion for O H site (C.N. = 6) a = 2 r anion 2 r anion + 2 r cation = 2 a 2 r anion + 2 r cation = 2 2 r anion r cation r anion = 0.414
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Site Selection II 1. Stoichiometry If all of one type of site is full the remainder have to go into other types of sites. Ex: FCC unit cell has 4
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ch12_ppts_callister7e - Chapter 12: Structures &amp;...

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