Week10Ch12

Week10Ch12 - Chapter 12: Structures & Properties of...

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Chapter 12: Structures & roperties of Ceramics Properties of Ceramics 12.1 Introduction ISSUES TO ADDRESS. .. Structures of ceramic materials: How do they differ from those of metals? Point defects: ow are they different from those in metals? How are they different from those in metals? Impurities: How are they accommodated in the lattice and how do they affect properties? Mechanical Properties: What special provisions/tests are made for ceramic Chapter 12 - 1 materials?
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Ceramic Bonding Bonding: -- Mostly ionic, some covalent. ionic character increases with difference in -- % ionic character increases with difference in electronegativity. Large vs small ionic bond character: SiC: small CaF 2 : large Chapter 12 - 2 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.
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Chapter 12 - 3
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2 2 Ceramic Crystal Structures 12.2 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 Chapter 12 - 4
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ite Selection Which sites will cations occupy? Site Selection ize f sites 1. Size of sites – does the cation fit in the site 2. Stoichiometry – if all of one type of site is full the mainder have to go into other types of remainder have to go into other types of sites. ond Hybridization 3. Bond Hybridization Chapter 12 - 5
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Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Callister 7e. - - -- + - - - - + - - - - + unstable Charge Neutrality : stable stable --Net charge in the structure should be zero. CaF 2 : Ca 2+ cation F - anions + --General form: F - A m X p Chapter 12 - 6 m, p determined by charge neutrality
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Coordination # and Ionic Radii Coordination # increases with r cation r anion Issue: How many anions can you r cation Coord ZnS incblende) arrange around a cation? 2 r anion # < 0.155 linear Adapted from Fig. 12.4, Callister 7e. (zincblende) 0.155 - 0.225 0.225 - 0.414 3 4 triangular T D Adapted from Fig. NaCl (sodium chloride) 0.414 - 0.732 6 O H 12.2, Callister 7e. CsCl (cesium hloride) Chapter 12 - 7 Adapted from Table 12.2, Callister 7e. 0.732 - 1.0 8 cubic Adapted from Fig. 12.3, Callister 7e. chloride)
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Cation Site Size • Determine minimum r cation /r anion for O H site (C.N. = 6) 2 r anion + 2 r cation = 2 a a = 2 r anion 2 r anion + 2 r cation = 22 r anion nion ation nion ation nion r anion + r cation = 2 r anion r cation = ( 2 1) r anion 14 cation r Chapter 12 - 8 414 0 anion . r
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Site Selection II 2. Stoichiometry all of one type of site is full the remainder have 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 O H and 8 T D sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer O H sites in 4 in O H 2 in T D Chapter 12 - 9
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Site Selection III 3. Bond Hybridization significant covalent bonding e hybrid orbitals can have impact if significant the hybrid orbitals can have impact if significant covalent bond character present Chapter 12 -10
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Chapter 12 - 11
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This note was uploaded on 09/15/2010 for the course EGN 3365 taught by Professor Jiuhuachen during the Spring '09 term at FIU.

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Week10Ch12 - Chapter 12: Structures &amp; Properties of...

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