ENGR2000_ceramics - Introduction to Engineering Materials...

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Introduction to Engineering Materials ENGR2000 Chapter 12: Structures and Properties of Ceramics Dr. Priya Thamburaj Goeser
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12.1 Introduction Ceramics Compounds between metallic & non-metallic elements Predominantly Ionic bonds Some have covalent bonds Greek word keramikos which means ‘burnt stuff’ Traditional ceramics china, porcelain, bricks, tiles, glasses, etc.
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12.2 Crystal Structures Crystals composed of electrically charged ions Cations Positively charged Metallic ions Anions Negatively charged Non-metallic ions
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12.2 Crystal Structures Concepts influencing the crystal structure Crystal must be electrically neutral Cations & anions are of different sizes Cations & anions may have different magnitudes of electrical charge
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12.2 Crystal Structures Cations are usually smaller than anions Each cation prefers to have as many nearest- neighbor anions as possible. Each anion prefers to have as many nearest- neighbor cations as possible. 1 / < A C r r
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Stable ceramic crystal structures Anions surrounding a cation are all in contact with that cation.
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Coordination number – # of anion nearest neighbors for a cation The CN is related to For a specific CN, there is a critical or minimum radius ratio for which this contact is established. A c r r /
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Example 12.1 Show that the minimum cation-to-anion radius ratio for the coordination number 3 is 0.155.
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155 . 0 1 866 . 0 1 866 . 0 / 1 1 / / / 2 3 30 cos cos 0 = = = + = + = = + = + = = A C A C A C A A A A C A A C A A r r r r r r r r r r r r r AO AP r r AO r AP α
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AX-Type Crystal Structures AX compounds A denotes the cation X denotes the anion
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Rock salt (NaCl) structure Two interpenetrating FCC lattices – one composed of cations and the other of anions ( ) ( ) 732 . 0 / 414 . 0 6 = = + A C r r Cl CN Na CN
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Rock salt (NaCl) structure Common ceramic materials with this crystal structure NaCl, MgO, MnS, LiF, FeO
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Cesium Chloride Structure Note that this is not a BCC crystal structure (different kinds of ions) ( ) ( ) 8 = = + Cl CN Cs CN
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Zinc Blende (ZnS) Structure Common ceramic materials with this crystal structure ZnS, ZnTe, SiC () ( ) 4 = = X CN A CN
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m X p -Type Crystal Structures Consider the AX 2 -typeCaF 2 (flourite) structure Predicts a structure similar to the CsCl There are ½ as many Ca ions as there are F ions. () table
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This note was uploaded on 10/24/2009 for the course ENGR 2000 taught by Professor Goeser during the Fall '08 term at Armstrong State University.

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ENGR2000_ceramics - Introduction to Engineering Materials...

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