Chapter 12: Ceramics Materials - Structures and Properties
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Transcript of Chapter 12: Ceramics Materials - Structures and Properties
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ISSUES TO ADDRESS...
• Structures of ceramic materials: How do they differ from those of metals?• Point defects: How are they different from those in metals?• Impurities: How are they accommodated in the lattice and how do they affect properties?• Mechanical Properties: What special provisions/tests are made for ceramic materials?
Chapter 12: Ceramics Materials - Structures and Properties
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• Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity.
• Large vs small ionic bond character:
Ceramic Bonding
SiC: small
CaF2: large
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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.
SiO2, MgO, SiC, Al2O3
- -
- -+
stable
- -
- -+
stable
CaF2: Ca2+cation
F-
F-
anions+
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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
Issue: How many anions can you arrange around a cation?
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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: 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
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
• ca. 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 Structure
Same 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
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MgO and FeO
MgO and FeO also have the NaCl 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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AX Crystal Structures
AX–Type Crystal Structures include NaCl, CsCl, and zinc blende
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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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?
– %ionic approx. 18%– bonding hybridization of
zinc favors TD sites
Ex: ZnO, ZnS, SiC
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AX2 Crystal Structures
Fluorite structure
• Calcium Fluorite (CaF2)
• cations in cubic sites
• UO2, ThO2, ZrO2, CeO2
• antifluorite structure –
cations and anions
reversed
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ABX3 Crystal Structures
• Perovskite
Ex: complex oxide
BaTiO3
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Silicate CeramicsMost common elements on earth are Si & O
• SiO2 (silica) structures are quartz, crystobalite, & tridymite
• The strong Si-O bond leads to a strong, high melting material (1710ºC)
Si4+
O2-
crystobalite
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Amorphous Silica
• Silica gels - amorphous SiO2
– Si4+ and O2- not in well-ordered lattice
– Charge balanced by H+ (to form OH-) at “dangling” bonds
– SiO2 is quite stable, therefore un-reactive to makes good catalyst support
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Silica Glass
• Dense form of amorphous silica– Charge imbalance corrected with “counter
cations” such as Na+
– Borosilicate glass is the pyrex glass used in labs• better temperature stability & less brittle
than sodium glass
Si, B - Network former Other Cations - Network modifier
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– Combine SiO44- tetrahedra by having them share
corners, edges, or faces
– Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding
Silicate elements
Mg2SiO4 Ca2MgSi2O7
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Layered Silicates
• Layered silicates (clay silicates)
– SiO4 tetrahedra connected together to form 2-D plane
• (Si2O5)2-
• So need cations to balance charge=
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• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+
layer
Layered Silicates
Note: these sheets loosely bound by van der Waal’s forces
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Carbon Forms
• Carbon black • Diamond
– tetrahedral carbon• hard – no good slip planes• brittle – can cut it
– large diamonds – jewelry– small diamonds
• often man made - used for cutting tools and polishing
– diamond films• hard surface coat – tools,
medical devices, etc.
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Carbon Forms - Graphite
• layer structure – aromatic layers
– weak van der Waal’s forces between layers– planes slide easily, good lubricant
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Carbon Forms – Fullerenes and Nanotubes
• Fullerenes or carbon nanotubes– wrap the graphite sheet by curving into ball or tube– Buckminister fullerenes
• Like a soccer ball C60 - also C70 + others
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• Frenkel Defect --a cation is out of place.
• Shottky Defect --a paired set of cation and anion vacancies.
Defects in Ceramic Structures
Shottky
Defect:
Frenkel
Defect
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• Impurities must also satisfy charge balance = Electroneutrality
• Ex: NaCl
• Substitutional cation impurity
Impurities
Na+ Cl-
initial geometry Ca2+ impurity resulting geometry
Ca2+
Na+
Na+Ca2+
cation vacancy
• Substitutional anion impurity
initial geometry O2- impurity
O2-
Cl-
anion vacancy
Cl-
resulting geometry
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Ceramic Phase Diagrams
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Ceramic Phase Diagrams
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Ceramic Phase Diagrams
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Ceramic Phase Diagrams
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Ternary phase diagram
45% SiO2
34% Al2O3
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General properties of ceramics
• Brittle (very low fracture toughness)• Better strength under compressive• Flexural strength is the rupture strength achieved from
bending test• Creep occurs at higher temperature than metal
(compressive)• Almost good hardness (used as abrasive materials)• A little plastic deformation may be observed in crystalline
ceramics; slip plane • Non-crystalline ceramics; viscous flow• Porosity in ceramics decreases the modulus of elasticity
and strength• High chemical durability
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Mechanical Properties
We know that ceramics are more brittle than metals. Why?
• Consider method of deformation– slippage along slip planes
• in ionic solids this slippage is very difficult• too much energy needed to move one
anion past another anion• Higher strength under compressive stress
– Generally utilized when load conditions are compressive
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Load and crack origin
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• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials.
Measuring Elastic Modulus
FL/2 L/2
δ = midpoint deflection
cross section
R
b
d
rect. circ.
• Determine elastic modulus according to:
Fx
linear-elastic behavior
F
slope =
E =F
L3
4bd3=
F
L3
12R4
rect. cross section
circ.cross section
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• 3-point bend test to measure room T strength.
Measuring Strength
FL/2 L/2
δ = midpoint deflection
cross section
R
b
d
rect. circ.
location of max tension
• Flexural strength: • Typ. values:
Rect.
fs 3Ff L
2bd 2
Ff L
R3Si nitrideSi carbideAl oxideglass (soda)
250-1000100-820275-700
69
30434539369
Material fs (MPa) E(GPa)
xF
Ff
fs
Cir.
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Flexural strength and Modulus of elasticity of Ceramics
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Stress-strain behavior / Porosity
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Viscosity
dydv
AF
dydv /
/
/
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Measuring Elevated T Response
time
• Elevated Temperature Tensile Test (T > 0.4 Tm).
creep test
slope = ss = steady-state creep rate.
x
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• Ceramic materials have covalent & ionic bonding.• Structures are based on: -- charge neutrality -- maximizing # of nearest oppositely charged neighbors.• Structures may be predicted based on: -- ratio of the cation and anion radii.• Defects -- must preserve charge neutrality -- have a concentration that varies exponentially w/T.• Room T mechanical response is elastic, but fracture is brittle, with negligible deformation.• Elevated T creep properties are generally superior to those of metals (and polymers).
Summary