Lecture25%20with%20slides

Lecture25%20with%20slides - Ceramic Bonding • Bonding: -...

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1 1 Lecture 25 Ceramics 2 • Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. Adapted from Fig. 2.7, Cal ister 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 Cornel University. • Large vs small ionic bond character: Ceramic Bonding SiC: small CaF 2 : large 3 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 4 Which sites will cations occupy? Site Selection 1. Size of sites – does the cation fit in the site 2. Stoichiometry – if all of one type of site is full the remainder have to go into other types of sites. 3. Bond Hybridization 5 Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Cal ister 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 6 • Coordination # increases with Coordination # and Ionic Radii Adapted from Table 12.2, Cal ister 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 H cubic Adapted from Fig. 12.2, Cal ister 7e. Adapted from Fig. 12.3, Cal ister 7e. Adapted from Fig. 12.4, Cal ister 7e. 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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2 7 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 = 22 r anion r anion + r cation = 2 r anion r cation = (2 1) r anion 2 r anion + 2 r cation = 2 a 414 0 anion cation . r r = 8 Site Selection II 2. 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 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 4 in O H 2 in T D 9 Site Selection III 3. Bond Hybridization – significant covalent bonding the hybrid orbitals can have impact if significant covalent bond character present For example in SiC X Si = 1.8 and X C = 2.5 % . ) X X ionic % 5 11 ]} exp[-0.25( - {1 100 character 2 C Si = = • 89% covalent bonding • Experiments show Si in SiC get T D sites 10 • On the basis of ionic radii, what crystal structure would you predict for FeO?
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Lecture25%20with%20slides - Ceramic Bonding • Bonding: -...

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