ceramics term ceramics comes from the greek word keramikos – “burnt stuff” ceramics are...
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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
• 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
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
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
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
• 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
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
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
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
• 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-
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
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+
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-
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
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
ABX3 Crystal StructuresPerovskite crystal structure
Ex: Barium Titanate – BaTiO3
Temperatures above 120oF – cubic crystal structure
Summary of Common Structures
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}
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