Chapter 12-1CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICSHow 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 3.0Br 2.8I 2.5At 2.2Li 1.0Na 0.9K 0.8Rb 0.8Cs 0.7Fr 0.7H 2.1Be 1.5Mg 1.2Sr 1.0Ba 0.9Ra 0.9Ti 1.5Cr 1.6Fe 1.8Ni 1.8Zn 1.8As 2.0C 2.5Si 1.8F 4.0Ca 1.0Table of ElectronegativitiesCaF2: largeSiC: smallAdapted 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 byCornell University.• Large vs small ionic bond character:CERAMIC BONDING
Chapter 12-Crystal Structure of Ionicly Bonded Ceramics•Crystal structure is defined by 2 criterions1.Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca2+and F-form CaF2).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 rC/rAfor which this contact can be maintained. Pure geometrical consideration…
Chapter 12-31.Charge Neutrality:--Net charge in thecrystal structure should be zero.--General form:AmXpm, p determined by charge neutrality2.Maximize the # of nearest oppositely charged neighbors--stable structures:Adapted from Fig. 12.1, Callister 6e.----+unstable----+stable----+stableCaF2:Ca2+cationF-F-anions+IONIC BONDING & CRYSTAL STRUCTURE