lecture10 - Lecture Lecture 10 Structures & Properties...

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Lecture 10 Structures & Properties of Structures & Properties of Ceramics Ceramics Chapter 12 - 1
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Structures & Properties of Ceramics SUES TO ADDRESS ISSUES TO ADDRESS. .. How do the crystal structures of ceramic materials iffer from those for metals? differ from those for metals? How do point defects in ceramics differ from those defects found in metals? How are impurities accommodated in the ceramic lattice? In what ways are ceramic phase diagrams different from hase diagrams for metals? How are the mechanical properties of ceramics measured, and how do they differ from those for metals? phase diagrams for metals? Chapter 12 - 2
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Atomic Bonding in Ceramics Bonding: -- Can be ionic and/or covalent in character. ionic character increases with difference in -- % ionic character increases with difference in electronegativity of atoms. Degree of ionic character may be large or small: SiC: small CaF 2 : large Chapter 12 - 3 Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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.)
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eramic Crystal Structures Ceramic Crystal Structures Oxide structures xygen anions larger than metal ations o yge a o s a ge t a eta cat o s – close packed oxygen in a lattice (usually FCC) – cations fit into interstitial sites among oxygen ions Eletctrostatic Eletctrostatic Charge Neutrality has to be Charge Neutrality has to be preserved at all times! Chapter 12 - 4
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Factors that Determine Crystal Structure 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors. Adapted from Fig. 12.1, Callister & Rethwisch 8e. - - -- + - - - - + - - - - + unstable stable stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic hould be zero. CaF 2 : Ca 2+ cation F - anions + should be zero. --Reflected in chemical formula: F - A m X p Chapter 12 - 5 m, p values to achieve charge neutrality
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Coordination # and Ionic Radii Coordination # increases with r cation r anion o form a stable structure how many anions can r cation Coord ZnS inc blende) To form a stable structure, how many anions can surround around a cation? 2 r anion # < 0.155 linear (zinc blende) aCl 0.155 - 0.225 0.225 - 0.414 3 4 triangular tetrahedral NaCl (sodium chloride) 0.414 - 0.732 6 octahedral CsCl (cesium chloride) Chapter 12 - 6 Adapted from Table 12.2, Callister & Rethwisch 8e. 0.732 - 1.0 8 cubic
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Computation of Minimum Computation of Minimum Cation Cation-Anion Anion adius Ratio Radius Ratio • Determine minimum r cation /r anion for an octahedral site N = 6) (C.N. 6) a r r 2 2 2 cation anion a 2 r anion 2 r anion 2 r cation 22 r anion nion ation nion ation nion r anion r cation 2 r anion r cation ( 2 1) r anion 14 cation r Chapter 12 - 7 414 . 0 1 2 anion r
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Bond Hybridization Bond Hybridization
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lecture10 - Lecture Lecture 10 Structures &amp; Properties...

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