Chpt12_Ceramics-Spr2016.pdf - Chapter 12: Structures &...

Chapter 12 - 1
Chapter 12: Structures & Properties of
Ceramics
many concepts have already been covered as part of other chapters..
This is provided to you as a complete slide deck on ceramics..
ISSUES TO ADDRESS...
• How do the crystal structures of ceramic materials
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?
• How are the mechanical properties of ceramics
measured, and how do they differ from those for metals?
• In what ways are ceramic phase diagrams different from
phase diagrams for metals?

Chapter 12 - 2
• Bonding:
-- Can be ionic and/or covalent in character.
-- % ionic character increases with difference in
electronegativity of atoms.
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.)
• Degree of ionic character may be large or small:
Atomic Bonding in Ceramics
SiC: small
CaF
2
: large

Chapter 12 - 3
Ceramic Crystal Structures
Oxide structures
– oxygen anions larger than metal cations
– close packed oxygen in a lattice (usually FCC)
– cations fit into interstitial sites among oxygen ions

Chapter 12 - 4
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
should be zero.
--Reflected in chemical
formula:
CaF
2
:
Ca
2+
cation
F
-
F
-
anions
+
A
m
X
p
m, p values to achieve charge neutrality

Chapter 12 - 5
• Coordination # increases with
Coordination # and Ionic Radii
Adapted from Table 12.2,
Callister & Rethwisch 8e.
2
r
cation
r
anion
Coord
#
< 0.155
0.155 - 0.225
0.225 - 0.414
0.414 - 0.732
0.732 - 1.0
3
4
6
8
linear
triangular
tetrahedral
octahedral
cubic
Adapted from Fig. 12.2,
Callister & Rethwisch 8e.
Adapted from Fig. 12.3,
Callister & Rethwisch 8e.
Adapted from Fig. 12.4,
Callister & Rethwisch 8e.
ZnS
(zinc blende)
NaCl
(sodium
chloride)
CsCl
(cesium
chloride)
r
cation
r
anion
To form a stable structure, how many anions can
surround around a cation?

Chapter 12 - 6
Computation of Minimum Cation-Anion
Radius Ratio
• Determine minimum r
cation
/r
anion
for an octahedral site
(C.N. = 6)
a = 2 r
anion
2 r
anion
 2 r
cation
= 2 2 r
anion
r
anion
 r
cation
= 2 r
anion
r
cation
= ( 2  1) r
anion
a r r 2 2 2
cation anion
= 
414 . 0 1 2
anion
cation
=  =
r
r

Chapter 12 - 7
Bond Hybridization
Bond Hybridization is possible when there is significant
covalent bonding
–