Chapter_12_avi_erman - CHAPTER 12 STRUCTURE AND PROPERTIES OF CERAMICS How do ceramics differ from metals Keramikos ~ burnt stuff Heat treatment is

Chapter_12_avi_erman - CHAPTER 12 STRUCTURE AND PROPERTIES...

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Chapter 12- 1 CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICS How do ceramics differ from metals ?
Chapter 12- 2 Bonding: --Mostly ionic, some covalent. --% ionic character increases with difference in electronegativity. What is electronegativity ? He - Ne - Ar - Kr - Xe - Rn - Cl 3.0 Br 2.8 I 2.5 At 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 H 2.1 Be 1.5 Mg 1.2 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 C 2.5 Si 1.8 F 4.0 Ca 1.0 Table of Electronegativities CaF 2 : large SiC: small Adapted from Fig. 2.7, Callister 6e. (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. Large vs small ionic bond character: CERAMIC BONDING
Chapter 12- Crystal Structure of Ionicly Bonded Ceramics Crystal structure is defined by 2 criterions 1. Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca 2+ and F - form CaF 2 ). 2. Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa. Stable ceramic crystal structures require anions surrounding a cation to be all in contact with that cation. For a specific coordination number there is a critical or minimum cation/anion radius ratio r C /r A for which this contact can be maintained. Pure geometrical consideration…
Chapter 12- 3 1. Charge Neutrality : --Net charge in the crystal structure should be zero. --General form: A m X p m, p determined by charge neutrality 2. Maximize the # of nearest oppositely charged neighbors --stable structures: Adapted from Fig. 12.1, Callister 6e. - - - - + unstable - - - - + stable - - - - + stable CaF 2 : Ca 2+ cation F - F - anions + IONIC BONDING & CRYSTAL STRUCTURE
Chapter 12- 4 Coordination # increases with r cation r anion r cation r anion Coord # < .155 .155-.225 .225-.414 .414-.732 .732-1.0 ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) 2 3 4 6 8 Adapted from Table 12.2, Callister 6e.

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