Transcript Chapter 12

CHAPTER 12: STRUCTURE AND
PROPERTIES OF CERAMICS
ISSUES TO ADDRESS...
• Structures of ceramic materials:
How do they differ from that of metals?
• Point defects:
How are they different from those in metals?
• Impurities:
How are they accommodated in the lattice and how
do they affect properties?
• Mechanical Properties:
What special provisions/tests are made for ceramic
materials?
Chapter 12- 1
CERAMIC BONDING
• Bonding:
--Mostly ionic, some covalent.
--% ionic character increases with difference in
electronegativity.
• Large vs small ionic bond character:
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
Chapter 12- 2
Cornell University.
IONIC BONDING & STRUCTURE
• Charge Neutrality:
--Net charge in the
structure should
be zero.
Cation
Anion
--General form:
The anion is larger than cation.
• Stable structures:
--maximize the # of nearest oppositely charged neighbors.
Adapted from Fig. 12.1,
Callister 6e.
Chapter 12- 3
COORDINATION # AND IONIC RADII
• Coordination # increases with
Issue: How many anions can you
arrange around a cation?
(covalent)
Zinc Sulfide
Adapted from Fig. 12.4,
Callister 6e.
SiC, ZnTe
(ionic)
Adapted from Fig. 12.2,
Callister 6e.
Also MgO, MnS,
LiF, and FeO
(ionic)
Adapted from Table
12.2, Callister 6e.
Adapted from Fig. 12.3,
Callister 6e.
Like a BCC structure
Chapter 12- 4
Two interpenetrating FCC lattice,
one composed of anions and the other of cations
Chapter 12-
Close packing of anions
Octahedron
Tetrahedron
Chapter 12-
EX: PREDICTING STRUCTURE OF FeO
• On the basis of ionic radii, what crystal structure
would you predict for FeO?
Cation
Al3+
Fe2+
Fe3+
Ca2+
Table 12.3 in textbook
Anion
O2ClF-
• Answer:
r cation 0.077

r anion 0.140
 0.550
based on this ratio,
--coord # = 6
--structure = NaCl
Data from Table 12.3,
Callister 6e.
Chapter 12- 5
AmXp STRUCTURES
r cation 0.100

 0.8
• Consider CaF2 :
r anion 0.133
(Fluorite)
• Based on this ratio, coord # = 8 and structure = CsCl.
• Result: CsCl structure w/only half the cation sites
occupied.
• Only half the cation sites
are occupied since
#Ca2+ ions = 1/2 # F- ions.
Adapted from Fig. 12.5,
Callister 6e.
Chapter 12- 6
Two types of cations
AmBnXp STRUCTURES
Example: Barium Titanate (BaTiO3)
Chapter 12- 6
Ceramic Density Calculations
Number of formula units within the unit cell
n( AC   AA )

VC N A
Sum of the atomic weights of all anions in the formula unit
Example: density of Sodium Chloride
Formula is NaCl
n´=4 (because there are 4 Na atoms and 4 Cl atoms
within a unit cell)
VC=a3=(2(0.102E-7)+2(0.181E-7))3=(0.566E-7)3
4(22.99  35.45)
3


2
.
14
g
/
cm
(0.566E  7)3 (6.023E 23)
a=2rNa++2rClChapter 12-