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Ceramics

Ceramics. Term ceramics comes from the greek word keramikos – “burnt stuff” Ceramics are typically formed during high temperature heat treating – “Firing” Traditionally ceramics included: China Porcelain Bricks (both construction and refractory) Tiles Glasses

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Ceramics

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  1. Ceramics • Term ceramics comes from the greek word keramikos – “burnt stuff” • Ceramics are typically formed during high temperature heat treating – “Firing” • Traditionally ceramics included: • China • Porcelain • Bricks (both construction and refractory) • Tiles • Glasses • Over the last 60 years or so…there has been an explosion in new technologies similar to other areas of material science

  2. Ceramic Bonding CaF2: large SiC: small • Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. • Amount of ionic bond character: Eq 2.10: % ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100 XA, XB are electronegativities of components A and B

  3. Ionic Ceramics • Crystal structure are composed of electrically charged ions • Cations (Fe3+) – Positive Charge Typically metals • Anions (O2-) – Negatively Charged Typically non-metals • Two characteristics influence crystal structure: • The magnitude of charge on the component ions • Stoichiometry must balance • Overall charge neutrality is required • Relative sizes of the component ions small large Note that size of ion is affected by charge: For iron: r(Fe2+) = 0.077 nm, r(Fe3+) = 0.069 nm, r(Fe) = 0.124 nm

  4. Criteria of Site Selection Which sites will cations occupy to form stable crystal structure? • Size of sites • does the cation fit in the site • Stoichiometry • if all of one type of site is full the remainder have to go into other types of sites. Covalent Bond Hybridization

  5. - - - - - - + + + - - - - - - unstable - F 2+ Ca + CaF : 2 anions cation - F A X m p m, p determined by charge neutrality Ionic Bonding & Structure 1.Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. stable stable • Charge Neutrality: --Net charge in the structure should be zero. --General form:

  6. Coordination # and Ionic Radii r cation r anion r ZnS cation Coord # (zincblende) r anion linear < 0.155 2 triangular 0.155 - 0.225 3 NaCl (sodium TD 0.225 - 0.414 4 chloride) OH 0.414 - 0.732 6 CsCl (cesium chloride) cubic 0.732 - 1.0 8 • Coordination # increases with How many anions can you arrange around the cation? Purely geometrical argument

  7. Geometrical Derivation of Site Size Determine minimum rcation/ranion for OH site (C.N. = 6) a= 2ranion

  8. Site Selection II • Stoichiometry • If all of one type of site is full the remainder have to go into other types of sites. Ex: We know that an FCC unit cell has 4 OH and 8 TD sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites, then 4 in OH 2 in TD

  9. Site Selection III • Bond Hybridization – significant covalent bonding • the hybrid orbitals can have impact if significant covalent bond character present • For example in SiC XSi = 1.8 and XC = 2.5 • 89% covalent bonding • both Si and C prefer sp3 hybridization • Therefore in SiC get TD sites

  10. Example:Predicting Structure of FeO • Answer: Cation Ionic radius (nm) 3+ Al 0.053 2 + Fe 0.077 3+ Fe 0.069 2+ Ca 0.100 based on this ratio, --coord # = 6 --structure = NaCl Anion 2- O 0.140 - Cl 0.181 - F 0.133 • On the basis of ionic radii, what crystal structure would you predict for FeO?

  11. Rock Salt Structure Same concepts can be applied to ionic solids in general Example: NaCl (rock salt) structure rNa = 0.102 nm rCl = 0.181 nm • rNa/rCl = 0.564 • cations prefer OHsites AX Crystal Structure: equal number of Anion and Cation locations

  12. MgO and FeO MgO and FeO also have the Rock Salt structure O2- rO = 0.140 nm Mg2+ rMg = 0.072 nm • rMg/rO = 0.514 • cations prefer OHsites So each oxygen has 6 neighboring Mg2+

  13. 2nd Type of AX Crystal Structure Cesium Chloride structure:  cubicsites preferred So each Cs+ has 8 neighboring Cl-

  14. 3rd Type of AX Crystal Structures Zinc Blende structure • Why is Zn2+ in TD sites? • bonding hybridization of zinc favors TD sites • Size arguments predict Zn2+ in OHsites, • In observed structure Zn2+ in TD sites So each Zn2+ has 4 neighboring S2- Ex: ZnO, ZnS, SiC

  15. AX2 Crystal Structures Fluorite structure • Calcium Fluorite (CaF2) • Cations in cubic sites • UO2, ThO2, ZrO2, CeO2 • antifluorite structure – cations and anions reversed rC/rA for CaF2 is about 0.8 – coordination number of 8 cubic structure But, stoichiometry calls for ½ as many Ca2+ as F- ions 8 cubes in unit cell

  16. ABX3 Crystal Structures Perovskite crystal structure Ex: Barium Titanate – BaTiO3 Temperatures above 120oF – cubic crystal structure

  17. Summary of Common Structures

  18. Close Packing of Anions Coordination = 4 Coordination = 6 Since Anions are commonly packed in FCC structure – we can talk about close packed planes of anions • Can have both: • FCC Stacking – ABCABC • HCP Stacking – ABABAB Cl- form FCC Lattice Close packed planes are {111}

  19. Mechanical Properties Why are ceramics more brittle than metals? • Consider method of deformation • In metals we have dislocation motion along slip planes • Slip planes are the close packed planes • In ionic solids dislocation motion is very difficult • Why? Too much energy needed to move one anion past another anion

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