AC LEC 09-09

# AC LEC 09-09 - Lecture 9 Ceramic Materials Ionic-Covalent...

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Lecture 9 - Ceramic Materials Ionic-Covalent Bonding Crystal Structures NaCl, CsCl, CaF 2 , Al 2 O 3 , graphite, SiO 2 Unit Cell Calculations ionic packing factor volume, planar, linear ionic density Glasses Noncrystalline Materials

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Ceramic Materials GENERAL CHARACTERISTICS High Melting Point High Strength Low Ductility (Brittle) High Chemical Stability Good electrical insulators Good thermal insulators Ionic AND/OR Covalent Bonds TWO TYPES Traditional: Bricks, Glass, Sanitary Ware, tiles; e.g., Clay, Silica, Feldspar Engineered Ceramics: Cutting tools, metal processing, IC Substrates; e.g., Al 2 O 3 , AlN, SiC, Si 3 N 4 CRYSTALLINITY PERFECT LRO (METALS) NO LRO (PVC) LOCALIZED LRO (PE)
Basis of Structure in Ceramics Most ceramics have some degree of ionic bonding The packing of ions is a function of two factors: Relative Ion Size Charge Neutrality Coordination Number: Number of Anions surrounding small Cation Condition: Anions MUST touch Cation !!! Radius Ratio: R cat / R an Critical Radius Ratio: Anions touching each other AND the Cation High Radius Ratio implies high Coordination Number

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Packing Cations and Anions ANION CATION STABLE UNSTABLE
Radius Ratio: Limiting Case r/R = 0.155 R r

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Coordination Number and Radius Ratios 8 CN 6 4 3 r/R
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AC LEC 09-09 - Lecture 9 Ceramic Materials Ionic-Covalent...

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