Engineering 45
Ceramic
Structure/Props
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Learning Goals – Ceramic Props
 How the Atomic Structure of Ceramics
Differs from that of Metals
 How Point Defects Operate in Ceramics
 Impurities
• How The Crystal Lattice
Accommodates them
• How They Affect Properties
 Mechanical Testing of Ceramics
• Must Allow for Their Brittle Nature
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramics
 The term CERAMIC comes from the
Greek – KERAMIKOS, which means
“burnt stuff”….
• Ceramics are often
Processed by a
high-temp heat
treating process
called FIRING
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Atomic Bonding
 Ceramics are
Compounds
between METAL
and NONmetal
IONS
 Metals are the
oxidANT; They
GIVE UP e- to
Become Positively
Charged CATions
Engineering-45: Materials of Engineering
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 The nonmetals are
the oxidIZER; They
RECEIVE e- to
Become Negatively
Charge ANions
Metal
CATion
NonMetal
ANion
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Atomic Bonding
 Bonding Form
• Primarily IONIC; Secondarily COVALENT
 Ionic Dominance INCREASES with
INCREASING ELECTRONEGATIVITY
H
2.1
Li
1.0
CaF 2: Large
Ionic
Be
1.5
CoValent
C
2.5
Si
1.8
SiC: Small
F
4.0
Na
0.9
K
0.8
Rb
0.8
Ca
1.0
Sr
1.0
I
2.5
Cs
0.7
Ba
0.9
At
2.2
Fr
0.7
Ra
0.9
Mg
1.2
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
Cl
3.0
Br
2.8
Table of Electronegativities
Engineering-45: Materials of Engineering
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He
-
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ne
Ar
Kr
Xe
Rn
-
Ceramic Crystal Structure
 Oxide Structures
• oxygen anions are
much larger than
metal cations
• close packed oxygen
in a lattice
(usually FCC)
• Cations (usually a
metal) in the holes of
the oxygen lattice

Site Selection →
Which sites will
Cations occupy?
1. Size of sites →
Does the cation
fit in the site?
2. Stoichiometry

if all of one type of site
is full the remainder
have to go into other
types of sites.
3. Bond Hybridization
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ionic Bonding & Structure
 Charge Neutrality
Requirement
• The NET Electronic
Charge on a
Macroscopic Piece,
or “Chunk”, of a
Material MUST be
ZERO
 The General
Ceramic Chemical
Formula:
Am X p
Engineering-45: Materials of Engineering
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CaF 2 :
Ca 2+ +
cation
• Where
– A  Metal Atom
– X  Nonmetal Atom
– m & p  atom ratio
required to satisfy
CHARGE
NEUTRALITY
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Fanions
F-
Ceramic MicroStructural Stability
 Two Primary Issues
• Due to Coulombic Forces (Electrical
Attraction) the Solid State Ions Need
OPPOSITELY Charged Nearest Neighbors
• Anion↔Cation Radial Contact Holds the
Smaller Cation in Place
-
+
-
stable
Engineering-45: Materials of Engineering
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-
-
+
-
-
-
-
stable
+
-
UNstable
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
CoOrdination No. & Atomic Radii
 The CoOrdination Number (CN) Increases
With Increasing ratio of rCation  rC
 Trends for CN vs rC/rA
rcation
ranion
Coord #
< .155
2
.155-.225
3
.225-.414
4
.414-.732
6
.732-1.0
8
Engineering-45: Materials of Engineering
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r Anion
rA
ZnS
(zincblende)
NaCl
(sodium
chloride)
CsCl
(cesium
chloride)
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Cation Site Size
 Determine minimum rcation/ranion for OH site
(C.N. = 6)
2ranion  2rcation  2a

a  2ranion
2ranion  2rcation  2 2ranion
ranion  rcation  2ranion
rcation  ( 2 1)ranion
rcation
 0.414
ranion

Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Site Selection II
2. Stoichiometry
•

If all of one type of site is full the
remainder have to go into other
types of sites.
Ex: FCC unit cell has 4 OH and
8 TD sites
•
If for a specific ceramic each unit cell has
6 cations and the cations prefer OH sites
– 4 in OH → These Sites Fill First
– 2 in TD → These Sites Take the remainder
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Site Selection II
3. Bond Hybridization – significant
covalent bondingy
•
•
the hybrid orbitals can have impact if
significant covalent bond character present
For example in SiC: XSi = 1.8 & XC = 2.5
% ionic character  100 {1 - exp[-0.25( X Si  X C )2 ]}  11.5%
– ca. 89% covalent bonding
– both Si and C prefer sp3 hybridization
– Therefore in SiC get TD sites
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Exmpl: Predict Fe2O3 Structure
 Using Ionic Radii Data Predict CoOrd No.
and Crystal Structure for Iron Oxide
Cation Ionic radius (nm)
0.053
Al3+
0.077
Fe 2+
0.069
Fe 3+
0.100
Ca 2+
Anion
O2Cl F-
0.140
0.181
0.133
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 Check Radii Ratio
rC
69

 49.3%
r A 140
 From rC:rA vs CN
Table (Previous Sld)
Predict:
• CN = 6
• Xtal Type = NaCl
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Structure: NaCl
 a.k.a., “Rock Salt”
Structure
 CN = 6
• rC/rA = 0.414-0.732
 Two Interpenetrating
FCC Lattices
• Cations
• Anions
 Examples
 NOT a Simple
Cubic Structure
• MgO, MnS, LiF, FeO
http://www.webelements.com/webelements/index.html
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Structure: CsCl
 CN = 8
 Structure of
Interlaced Simple
Cubic Structures
 NOT a BCC
Structure
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Structure: ZincBlende
 CN = 4 (covalent Bonding)
 Basically the DIAMOND
Structure with Alternating
Ions
 Examples
• SiC, GaAs
Diamond
Engineering-45: Materials of Engineering
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Zinc Sulfide
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Structure: Fluorite
 Formula = AX2
 rC/rA  0.8
 CN
• Cation → 8
• Anion → 4
 Simple Cubic
Structure with
• “A” (e.g. Ca) at
Corners
• “X” (e.g. F) at
Centers
Engineering-45: Materials of Engineering
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 Examples
• UO2, PuO2, ThO2
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Structure: Perovskite
Ti4+
CoOrdination
Ca2+
CoOrdination
CaTiO3 – Two
Types of Cations
 Chemical Formula
= AmBnXp
 CN
• Cation-A → 12
• Cation-B → 6
• Anion → 4
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Silicate Ceramics
 Si and O are the
MOST abundant
Elements in the
Earth’s Crust
 By Valence e- Ratio
Si & O Form
Compounds With a
Si:O ratio of 1:2
 Have Low Densities
• About 33%-50%
That of Steel
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crystobalite
 Si-O Bond is Mostly
CoValent and Quite
Strong
• Yields a High Melting
Temperature (1710 °C)
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Amorphous Silica
 Silica gels 
amorphous SiO2
• Si4+ and O2- not in
well-ordered lattice
• Charge balanced by H+
(to form OH-) at “dangling”
bonds
– very high surface area
> 200 m2/g
• SiO2 is quite stable,
therefore unreactive
– makes good catalyst support
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Silica Glasses (NonCrystalline)
 NonXtal, or Amorphous
Structure
• Called Fused-Silica or
Vitreous-Silica
 SiO2 Forms a
Tangled Network
(Network Former)
 Other Materials Added to
Change Properties (e.g.,
Lower Melting Temp)
• Network Modifiers
Engineering-45: Materials of Engineering
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Na as SiO2 Network
Modifier
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Composition of Common Glasses
 All Compositions in WEIGHT%
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Defects In Ceramic Structures
 FRENKEL Defect → Cation (the Smaller one)
has Moved to an Interstitial Location
 SHOTTKY Defect → Anion-Cation PairVacancy (electrical neutrality conserved)
Shottky
Defect:
Frenkel
Defect
Engineering-45: Materials of Engineering
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• Defects Must
Maintain Charge
Neutrality
• Defect Density
is Arrhenius
~e
 QD / kT
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Impurities in Ceramics
 Impurities Must Also Satisfy Charge Neutrality
 Example: NaCl →
 Ca-CATion Substitution-Defect in Rock Salt
Structure
• To maintain Charge Neutrality
ONE Ca+2 ion Subs for TWO Na+ ions
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Impurities in Ceramics cont.1
 Impurities Must also Satisfy Charge Neutrality
 Example: NaCl →
 O2- ANion Substitution-Defect in
Rock Salt Structure
O2-
initial geometry
ClClO2- impurity
anion vacancy
resulting geometry
• To maintain Charge Neutrality
ONE O2- ion Subs for TWO Cl- ions
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Ceramic Phase Diagrams
 Same form and Use
as Metallic Phase
Diagrams Except
• Horizontal Axis is
based on Ceramic
COMPOUNDS
– e.g. ; Al2O3 → SiO2
• Typically Much
Higher
Phase-Change
Temperatures
Engineering-45: Materials of Engineering
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 Can use Lever Rule
on Compounds
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Measuring Young’s Modulus
 Ceramic Room Temp behavior is
Linear-Elastic with BRITTLE Fracture
• Due to Brittle Failure, 3-Point Bending
Tests are used to Assess Elastic Modulus
cross section
d
b
rect.
F
L/2
R
d = midpoint
deflection
circ.
F
 Then
E
x
slope =
F
d
d
linear-elastic behavior
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L/2
E
F
L3
d 4bd3

F
L3
d 12 pR4
rect.
circ.
X-Section X-Section
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Measuring Strength
 Use the Same 3-Pt Bend Test to Measure
Room Temperature Ultimate Strength
cross section
d
b
rect.
L/2
R
F
L/2
circ.
location of max tension
 Then the FLEXURAL Strength at Fracture
Fmax
F
x
dmax
d
Engineering-45: Materials of Engineering
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1.5Fmax L
fail
sfs  sm 

bd2
rect.
Fmax L
p R3
circ.
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Creep Performance
 As with Metals Characterize Creep
Performance as the Steady-State Creep
Rate during Secondary-Phase Creep
s
Ttest >0.4Tmelt
x
.
slope = ess
= steady-state creep rate
s
time
 Generally The Creep-Resistance
• Ceramics > Metal >> Plastics
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Summary
 Ceramics Exhibit Primarily
IONIC Bonding
• Some Ceramics; e.g., Silica. are
Covalently Bonded
 Crystal Structure is a Function of
• Charge Neutrality
• Maximizing of NEAREST, OPPOSITELYCharged, Neighbors
• Cation:Anion Size-Ratio
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
Summary cont.1
 Defects
• Must Preserve Charge NEUTRALITY
• Have a Concentration that Varies
EXPONENTIALLY with Temperature
 Room Temperature Mechanical
Response is ELASTIC, but Fracture is
Brittle, with negligible ductility
 Elevated Temp Creep Properties are
Generally Superior to those of Metals
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt
WhiteBoard Work
 Problem 12.21
• Fe3O4 Ceramic
(LodeStone or
Magnetite)
– Unit Cell for Edge
Length, a = 0.839 nm
– Density, ρ = 5240 kg/m3
– Fe3O4 = FeO•Fe2O3
• Find Atomic Packing
Factor, APF
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-26_Ceramic_Structure-Props.ppt