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Ceramics• Term ceramics comes from the greek word keramikos –
“burnt stuff”
• Ceramics are typically formed during high temperature heat treating – “Firing”
• Traditionally ceramics included:• China• Porcelain• Bricks (both construction and refractory)• Tiles• Glasses
• Over the last 60 years or so…there has been an explosion in new technologies similar to other areas of material science
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• Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity.
• Amount of ionic bond character:
Ceramic Bonding
SiC: small
CaF2: large
Eq 2.10: % ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100
XA, XB are electronegativities of components A and B
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Ionic Ceramics
• Crystal structure are composed of electrically charged ions– Cations (Fe3+) – Positive Charge Typically metals
– Anions (O2-) – Negatively Charged Typically non-metals
• Two characteristics influence crystal structure:– The magnitude of charge on the component ions
• Stoichiometry must balance
• Overall charge neutrality is required
– Relative sizes of the component ions anion
cation
r
r small
large
Note that size of ion is affected by charge:
For iron: r(Fe2+) = 0.077 nm, r(Fe3+) = 0.069 nm, r(Fe) = 0.124 nm
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Which sites will cations occupy to form stable crystal structure?
Criteria of Site Selection
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. Covalent Bond Hybridization
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Ionic Bonding & Structure1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors.
- -
- -+
unstable
• Charge Neutrality: --Net charge in the structure should be zero.
--General form:
- -
- -+
stable
- -
- -+
stable
CaF2: Ca2+cation
F-
F-
anions+
AmXp
m, p determined by charge neutrality
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• Coordination # increases with
Coordination # and Ionic Radii
2
rcationranion
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
TD
OH
cubic
ZnS (zincblende)
NaCl(sodium chloride)
CsCl(cesium chloride)
rcationranion
How many anions can you arrange around the cation?
Purely geometrical argument
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Geometrical Derivation of Site Size
Determine minimum rcation/ranion for OH site (C.N. = 6)
a 2ranion
2ranion 2rcation 2 2ranion
ranion rcation 2ranion
rcation ( 2 1)ranion
2ranion 2rcation 2a
4140anion
cation .r
r
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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: We know that an 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, then
4 in OH
2 in TD
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Site Selection III
3. Bond Hybridization – significant covalent bonding
– the hybrid orbitals can have impact if significant covalent bond character present
– For example in SiC
XSi = 1.8 and XC = 2.5
%.)XXionic% 511]}exp[-0.25(-{1 100 character 2CSi
• 89% covalent bonding• both Si and C prefer sp3 hybridization
• Therefore in SiC get TD sites
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• On the basis of ionic radii, what crystal structure would you predict for FeO?
• Answer:
5500
1400
0770
anion
cation
.
.
.
r
r
based on this ratio,--coord # = 6--structure = NaCl
Example: Predicting Structure of FeO
Ionic radius (nm)
0.053
0.077
0.069
0.100
0.140
0.181
0.133
Cation
Anion
Al3+
Fe2+
Fe3+
Ca2+
O2-
Cl-
F-
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Rock Salt StructureSame concepts can be applied to ionic solids in general Example: NaCl (rock salt) structure
rNa = 0.102 nm
rNa/rCl = 0.564
cations prefer OH sites
rCl = 0.181 nm
AX Crystal Structure: equal number of Anion and Cation locations
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MgO and FeOMgO and FeO also have the Rock Salt structure
O2- rO = 0.140 nm
Mg2+ rMg = 0.072 nm
rMg/rO = 0.514
cations prefer OH sites
So each oxygen has 6 neighboring Mg2+
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2nd Type of AX Crystal Structure
939.0181.0
170.0
Cl
Cs
r
r
Cesium Chloride structure:
cubic sites preferred
So each Cs+ has 8 neighboring Cl-
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3rd Type of AX Crystal Structures
So each Zn2+ has 4 neighboring S2-
Zinc Blende structure?? 529.0
140.0
074.0
2
2
O
ZnHO
r
r
• Size arguments predict Zn2+ in OH sites, • In observed structure Zn2+ in TD sites
• Why is Zn2+ in TD sites?– bonding hybridization of
zinc favors TD sites
Ex: ZnO, ZnS, SiC
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AX2 Crystal StructuresFluorite structure
• Calcium Fluorite (CaF2)
• Cations in cubic sites
• UO2, ThO2, ZrO2, CeO2
antifluorite structure –
rC/rA for CaF2 is about 0.8 – coordination number of 8 cubic structure
But, stoichiometry calls for ½ as many Ca2+ as F- ions 8 cubes in unit cell
cations and anions reversed
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ABX3 Crystal StructuresPerovskite crystal structure
Ex: Barium Titanate – BaTiO3
Temperatures above 120oF – cubic crystal structure
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Summary of Common Structures
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Close Packing of Anions
Since Anions are commonly packed in FCC structure – we can talk about close packed planes of anions
Coordination = 4 Coordination = 6
Can have both:FCC Stacking – ABCABCHCP Stacking – ABABAB
Cl- form FCC LatticeClose packed planes are {111}
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Mechanical Properties
Why are ceramics more brittle than metals?
• Consider method of deformation– In metals we have dislocation motion along slip
planes– Slip planes are the close packed planes
• In ionic solids dislocation motion is very difficult– Why? Too much energy needed to move one anion
past another anion