EGN3365-12

EGN3365-12 - 1 Structures of ceramic materials: How do they...

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Unformatted text preview: 1 Structures of ceramic materials: How do they differ from those of metals? Point defects: 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 materials? Chapter 12: Structures & Properties of Ceramics 2 Bonding:-- Mostly ionic, some covalent.-- % ionic character increases with difference in electronegativity. Adapted from Fig. 2.7, Callister 7e. Large vs small ionic bond character: Ceramic Bonding SiC: small CaF 2 : large 3 Ionic Crystals Cation Radius (nm) Anion Radius (nm) 0.100 0.133 0.072 0.14 0.102 0.182 0.053 0.140 0.040 0.140 Note: larger anion radius Most ionic crystals can be considered as close-packed structure of anions with cations in the interstitial sites. Cations: metallic ions, positively charged Anions: nonmetallic ions, negatively charged 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 5 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 6 Ionic Bonding & Structure 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 7 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 H cubic Adapted from Fig. 12.2, Callister 7e. Adapted from Fig. 12.3, Callister 7e. Adapted from Fig. 12.4, Callister 7e. ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) r cation r anion Issue: How many anions can you arrange around a cation? 8 Cation-anion stable configuration e.g. 3-coordinate when C A A r r r + = cos With = 30 o Rewrite as 1 cos 1- = A C r r 155 . = A C r r Minimum ratio for 3-coordinate 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 2 r anion r anion + r cation = 2 r anion r cation = ( 2- 1) r anion 2 r anion + 2 r cation = 2 a 414 anion cation . r r = 10 Site Selection II 2. Stoichiometry If all of one type of site is full, the remainder have to go into other types of sites....
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EGN3365-12 - 1 Structures of ceramic materials: How do they...

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