lecture4.pdf - Chapter 12: Structures of Ceramics Outline...

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1Chapter 12: Structures of Ceramics±Outline±Introduction±Crystal structuresCeramic structureAX-type crystal structuresAmXp-typeAmBnXp- type±Silicate ceramics±CarbonCeramic structures±Two or more different elements±More complex than metal structures±Ionic and/or covalent bonds±A mix of ionic and covalent bonds --electronegativity±Ionic bonds form ionsMetals give up electrons --metallic ions--cations--positively chargedNon-metals gain electrons --nonmetallic ions--anions--negatively charged±Crystals must be electrically neutral, e.g. CaF212
23Ionic Bonding & StructureCharge neutrality:--Net charge in thestructure shouldbe zero.--General form:CaF2:Ca2+cationF-F-anions+AmXpm, p determined by charge neutralityCeramic structures (continue)±Factors influence crystal structureMagnitude of electrical charge of ionsRelative size of ions (Non-metal > metal ions Rc/Ra<1)- Cations must be next to anions--maximize # of nearest neighbors that are anions- Stable structure--anions and cations must contact each other- The # of anions depends on ratio of Rc/Ra----+unstable----+stable----+stable34
3Coordination numbers and geometries for various cation-anion radius ratios (R

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Term
Fall
Professor
Fong
Tags
Ion, Sodium chloride, Ionic Bonding Structure, Crystals

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