1. Chapter 12: Structures & Properties of 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?
• In what ways are ceramic phase diagrams different from phase diagrams for metals?
• How are the mechanical properties of ceramics measured, and how do they differ from those for metals?

2. Atomic Bonding in Ceramics
-- ______________________________________.
-- % ionic character __________ with difference in
electronegativity of atoms.
• Degree of ionic character may be large or small:
CaF2: large
SiC: small

3. Ceramic Crystal Structures
Oxide structures
oxygen anions ________ than metal cations
close _______ oxygen in a ______ (usually ____)
cations fit into ________ sites among ______ ions

4. Factors that Determine Crystal Structure
1. Relative sizes of ions – ________________________:
--maximize the # of ____________________________. - +
________ - + - +
Adapted from Fig. 12.1, Callister & Rethwisch 9e.
______
stable
2. Maintenance of
Charge Neutrality :
--_________________
should be zero.
--Reflected in chemical
formula:
CaF 2 : Ca 2+
cation F -
anions + A m X p
m, p values to achieve charge neutrality

5. Coordination Number and Ionic Radii
cation
anion
• __________ Number increases with
To form a ______ structure, how many anions can
surround around a cation?
Adapted from Fig. 12.2, Callister & Rethwisch 9e.
Adapted from Fig. 12.3, Callister & Rethwisch 9e.
Adapted from Fig. 12.4, Callister & Rethwisch 9e.
ZnS
(zinc blende)
NaCl
(sodium
chloride)
CsCl
(cesium r
cation
anion
Coord.
Number
< 0.155 2
linear 3
_______ 4
tetrahedral 6
octahedral 8
_____
Adapted from Table 12.2, Callister & Rethwisch 9e.

6. Computation of Minimum Cation-Anion Radius Ratio
Determine ________ rcation/ranion for an octahedral site (C.N. = __)
__________

7. Bond Hybridization
Bond Hybridization is possible when there is significant ____________ bonding
____________________________
For example for SiC
XSi = 1.8 and XC = 2.5
~ 89% ___________ bonding
Both Si and C prefer sp3 hybridization
Therefore, for SiC, Si atoms occupy _______________ sites

8. Example Problem: Predicting the Crystal Structure of FeO
• On the basis of ionic radii, what ___________________
would you predict for FeO?
Cation
Anion Al 3+ Fe 2 + Ca 2+ O 2- Cl - F
Ionic radius (nm)
0.053
0.077
0.069
0.100
0.140
0.181
0.133
• Answer:
based on this ratio,
-- coord # = __ because
0.414 < < 0.732
-- crystal structure is ____
Data from Table 12.3, Callister & Rethwisch 9e.

9. Rock Salt Structure
Same concepts can be applied to ____ solids in general.
Example: NaCl (rock salt) structure
rNa = nm
rCl = _____ nm
rNa/rCl = _________
cations (Na+) prefer __________ sites
Adapted from Fig. 12.2, Callister & Rethwisch 9e.

10. MgO and FeO MgO and FeO also have the NaCl structure
O2- rO = nm
Mg2+ rMg = nm
rMg/rO = _______
_______ prefer octahedral sites
Adapted from Fig. 12.2, Callister & Rethwisch 9e.
So each Mg2+ (or Fe2+) ______ neighbor oxygen atoms

11. AX Crystal Structures
AX–Type Crystal Structures include NaCl, CsCl, and zinc blende
Cesium Chloride structure:
 Since < ____ < 1.0, _____ sites preferred
So each Cs+ has __ neighbor Cl-
Fig. 12.3, Callister & Rethwisch 9e.

12. AX2 Crystal Structures Fluorite structure Calcium ______ (CaF2)
Cations in _______ sites
UO2, ThO2, ZrO2, CeO2
___________ structure –
positions of cations and anions reversed
Fig. 12.5, Callister & Rethwisch 9e.

13. ABX3 Crystal Structures
Ex: complex oxide
_________
Fig. 12.6, Callister & Rethwisch 9e.

14. VMSE Screenshot – Zinc Blende Unit Cell

15. Density Computations for Ceramics
Number of formula units/unit cell
__________ number
Volume of unit cell
= sum of atomic weights of ____________________
= sum of atomic weights of ____________________

16. Silicate Ceramics Most common ___________________________
SiO2 (silica) _________________ forms are quartz, crystobalite, & tridymite
The strong Si-O bonds lead to a high ____________ temperature (1710ºC) for this material
Si4+
O2-
Figs & 12.10, Callister & Rethwisch 9e
crystobalite

17. Silicates
Bonding of adjacent SiO44- accomplished by the sharing of common _____________________
Adapted from Fig , Callister & Rethwisch 9e.
Mg2SiO4
Ca2MgSi2O7
Presence of cations such as Ca2+, Mg2+, & Al3+
1. maintain charge ____________, and
2. ___________ bond SiO44- to one another

18. Glass Structure • Basic Unit: Glass is ____________ (_________)
• ____________ is SiO2 to which no impurities have been added
• Other common _________ contain impurity ions such as Na+, Ca2+, Al3+, and B3+
Si0 4
tetrahedron 4- Si 4+ O 2 -
• Quartz is _____________
SiO2: Si 4+ Na + O 2 -
(soda glass)
Adapted from Fig , Callister & Rethwisch 9e.

19. Layered Silicates Layered ______ (e.g., clays, mica, talc)
SiO4 ___________ connected together to form 2-D plane
A net negative charge is associated with each (Si2O5)2- unit
Negative charge balanced by ________ plane rich in positively charged __________
Fig , Callister & Rethwisch 9e.

20. Layered Silicates (cont)
Kaolinite clay _________ (Si2O5)2- layer with Al2(OH)42+ layer
Fig , Callister & Rethwisch 9e.
Note: Adjacent sheets of this type _________ bound to one another by ________________________.

21. Polymorphic Forms of Carbon
Diamond
tetrahedral bonding of carbon
______________________
______________________ conductivity
large single crystals – gem stones
small ________ – used to grind/cut other materials
________ thin films
hard surface coatings – used for cutting tools, medical devices, etc.
Fig , Callister & Rethwisch 9e.

22. Polymorphic Forms of Carbon (cont)
____________
_________ structure – parallel ________ arrays of carbon atoms
weak van der Waal’s forces between layers
planes slide easily over one another -- good lubricant
Fig , Callister & Rethwisch 9e.

23. Point Defects in Ceramics (i)
• Vacancies
-- vacancies exist in _______ for both ___________________
• Interstitials
-- interstitials exist for __________
-- interstitials are not normally observed for ___________________ are large relative to the interstitial sites
Cation
Interstitial
Vacancy
Anion
Fig , Callister & Rethwisch 9e. (From W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, p.78. Copyright ©1964 by John Wiley & Sons, New York. Reprinted by permission of John Wiley and Sons, Inc.)

24. Point Defects in Ceramics (ii)
• Frenkel Defect
-- ____________________________________.
• Shottky Defect
-- ____________________________________.
Shottky
Defect:
Frenkel
Defect
Fig , Callister & Rethwisch 9e. (From W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, p.78. Copyright ©1964 by John Wiley & Sons, New York. Reprinted by permission of John Wiley and Sons, Inc.)
• Equilibrium concentration of defects

25. Imperfections in Ceramics
• ________________ (charge ______) must be maintained when impurities are present Na + Cl -
• Ex: NaCl
• Substitutional cation impurity
without impurity Ca 2+
impurity
with impurity Na +
cation
vacancy
• Substitutional anion impurity
without impurity O 2-
impurity Cl - an
ion vacancy
with impurity

26. Ceramic Phase Diagrams
MgO-Al2O3 diagram:
Fig , Callister & Rethwisch 9e.
[Adapted from B. Hallstedt,
“Thermodynamic Assessment of the System MgO–Al2O3,” J. Am. Ceram. Soc., 75[6], 1502 (1992). Reprinted by permission of the American Ceramic Society.] 

27. Mechanical Properties
___________ materials are more brittle than _________. Why is this so?
Consider _________________________________
In crystalline, by ____________ motion
In highly ionic solids, dislocation motion is difficult
few _______________
resistance to motion of ions of like charge (e.g., anions) past one another

28. Flexural Tests – Measurement of Elastic Modulus
• Room T behavior is usually _______, with _______ failure.
• _____________________ often used.
-- tensile tests are difficult for ________ materials. F
L/2
δ = midpoint
deflection
cross section R b d
rect.
circ.
Adapted from Fig , Callister & Rethwisch 9e.
• Determine ________________ according to: F x
linear-elastic behavior δ
slope =
(rect. cross section)
(circ. cross section)

29. Flexural Tests – Measurement of _________ Strength
• ____________ test to measure room-T flexural strength. F
L/2
δ = midpoint
deflection
cross section R b d
rect.
circ.
location of max tension
Adapted from Fig , Callister & Rethwisch 9e.
• __________ strength:
• Typical values:
Data from Table 12.5, Callister & Rethwisch 9e.
Si nitride
Si carbide
Al oxide
glass (soda-lime) 69
304
345
393
Material σ fs
(MPa) E(GPa)
(rect. ____________)
(circ. ____________)

30. SUMMARY • Interatomic bonding in ceramics is ionic and/or covalent.
• Ceramic crystal structures are based on:
-- maintaining charge neutrality
-- cation-anion radii ratios.
• Imperfections
-- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky
-- Impurities: substitutional, interstitial
-- Maintenance of charge neutrality
• Room-temperature mechanical behavior – flexural tests
-- linear-elastic; measurement of elastic modulus
-- brittle fracture; measurement of flexural modulus