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- 1. Today’s objectives-Ceramic Structures <ul><li>Lattice and basis </li></ul><ul><li>How do you estimate the crystal structure for a ceramic? </li></ul><ul><li>What are the common crystal structures for ceramics? </li></ul>
- 2. CERAMIC BONDING • Bonding: --Mostly ionic, some covalent. --% ionic character increases with difference in electronegativity. Adapted from Fig. 2.7, Callister 6e. (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:
- 3. Rules for Ionic Structures • Rule 1: Charge Neutrality : --Net charge in the structure should be zero. --General form:
- 4. <ul><li>What is the charge on: </li></ul><ul><li>K Mg Na Li Si </li></ul><ul><li>What is the charge on: </li></ul><ul><li>Cl O S P N </li></ul><ul><li>What are the formulas for: </li></ul><ul><li>KCl MgO NaS LiP SiN </li></ul>Examples: K m Cl p , Mg m O p , Na m S p , Li m P p , Si m N p
- 5. More rules for Ionic Structures <ul><li>Q2) How do the atoms (ions) fill space in a crystal? </li></ul>• Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Callister 6e.
- 6. COORDINATION # AND IONIC RADII • Coordination # increases with: Issue: How many anions can you arrange around a cation? Adapted from Table 12.2, Callister 6e. There are many possibilities—how do we generally determine which is appropriate? And what do we call them? Binary Trigonal Tetrahedral Octahedral Cubic
- 7. Defining Radius Ratio Rules <ul><li>What is the ideal size of a cation that will just fit into this site (octahedral)? </li></ul>a) Sketch the hard spheres b) Do some basic geometry (leg length w/r radii) c) Do some math r a r c r a 2r c
- 8. EX: PREDICTING STRUCTURE OF FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? <ul><li>What does the chemical formula tell us? </li></ul><ul><li>What are the ionic radii? </li></ul><ul><li>What is the radius ratio (cation/anion) </li></ul>0.414 was ideal for octahedral (CN6), so what does this value do for us? • Answer: Data from Table 12.3, Callister 6e.
- 9. Radius Ratio Rules <ul><li>based on the ratio for FeO of 0.550: </li></ul><ul><ul><li>coord # = 6 </li></ul></ul>
- 10. Which crystal structures are appropriate?
- 11. 2-d Lattices <ul><li>A lattice repeats in all directions, and defines the symmetry. </li></ul>square centered
- 12. 2-d Lattice and Basis <ul><li>Anions at lattice positions (0,0) </li></ul><ul><li>Cations at (½,0) </li></ul>square Lattice: Basis: centered
- 13. 2-d Lattice and Basis <ul><li>Anions at square lattice positions (0,0) </li></ul><ul><li>Cations at (0,½) </li></ul>square Lattice: Basis: <ul><li>Cations at (½,0) </li></ul>
- 14. Simple ceramic unit cells: CsCl <ul><li>Based on Simple Cubic Lattice with a basis of: </li></ul><ul><li>Cl anions at the SC points </li></ul><ul><li>Cs cations at the center [offset by (½, ½, ½)] </li></ul>
- 15. NaCl <ul><li>Based on FCC Lattice with a basis of: </li></ul><ul><li>Cl anions at the FCC points </li></ul><ul><li>Na cations at the center [offset by (½, 0, 0)] </li></ul>
- 16. Other Common Ceramic unit cells <ul><li>Based on FCC Lattice with a basis of: </li></ul><ul><li>anions at the FCC points </li></ul><ul><li>cations at (¼,¼,¼) </li></ul>
- 17. Perovskite <ul><li>Based on SC Lattice with a basis of: </li></ul><ul><li>Big cation at the SC points </li></ul><ul><li>Small cation at (½,½,½) </li></ul><ul><li>Anion at (½,½,0) </li></ul>
- 18. A m X p STRUCTURES • Consider CaF 2 : • Based on this ratio, coord # = 8 and structure = CsCl. • BUT: there are half as many Ca 2+ as F - ions—how can we accommodate this? CsCl structure w/only half of cation sites occupied. Adapted from Fig. 12.5, Callister 6e. m is not equal to p
- 19. Silicon oxides Building Block: SiO 4 4-
- 20. Quartz, SiO 2 Hexagonal - Trigonal trapezohedral
- 21. SiO x There are three polymorphs of SiO 2 : Quartz, Cristobalite, and Tridymite
- 22. Glasses <ul><li>CaO and Na 2 O are examples of network modifiers . Cations modify the SiO 4 4- network. </li></ul><ul><li>TiO 2 and Al 2 O 3 are intermediates . Become part of or stabilize SiO 4 4- network. </li></ul><ul><li>B 2 O 3 and GeO 2 , are network formers , like SiO 4 4- . </li></ul><ul><li>Addition of modifiers and intermediates lower the melting point (and viscocity) of network formers. </li></ul>
- 23. Glasses Can we form glass structures in metals? YES, you need to cool them very fast, 10 6o C/sec…
- 24. Glass Production
- 25. Carbon
- 26. Crystals often indicate crystal structure
- 27. SUMMARY <ul><li>• Ceramic materials have both covalent & ionic bonding. </li></ul><ul><li>• Structures are based on: </li></ul><ul><li>-- charge neutrality </li></ul><ul><li>--maximizing # of nearest oppositely charged neighbors. </li></ul><ul><li>• Structures may be predicted based on: </li></ul><ul><li>--ratio of cations to anions. </li></ul><ul><li>--ratio of cation to anion radii. </li></ul><ul><li>The rules and crystal structures are strange for C and glasses. </li></ul>Defects in ceramics, glasses Chapter sections: 12.5 + 3.7-10 Reading for next class

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