CHAPTER 12: STRUCTURE AND PROPERTIES OF CERAMICS Ceramics

CHAPTER 12: STRUCTURE AND Ceramics
PROPERTIES OF CERAMICS
keramikos - burnt stuff in Greek - desirable properties of ceramics are normally
ISSUES TO ADDRESS... achieved through a high temperature heat treatment process (firing).

• Structures of ceramic materials: Usually a compound between metallic and nonmetallic elements
How do they differ from that of metals? Always composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2)
Bonds are partially or totally ionic, can have combination of ionic and covalent
• Point defects:
How are they different from those in metals? bonding
Generally hard and brittle
• Impurities: Generally electrical and thermal insulators
How are they accommodated in the lattice and how Can be optically opaque, semi-transparent, or transparent
do they affect properties? Traditional ceramics – based on clay (china, bricks, tiles, porcelain), glasses.
“New ceramics” for electronic, computer, aerospace industries.
• Mechanical Properties:
What special provisions/tests are made for ceramic
materials?

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CERAMIC BONDING Crystal Structures in Ceramics
with predominantly ionic bonding
• Bonding:
--Mostly ionic, some covalent. Crystal structure is defined by
--% ionic character increases with difference in Magnitude of the electrical charge on each ion. Charge balance dictates chemical
electronegativity.
formula (Ca2+ and F- form CaF2).
• Large vs small ionic bond character: Relative sizes of the cations and anions. Cations wants maximum possible

H CaF2: large He number of anion nearest neighbors and vice-versa.
2.1 - Stable ceramic crystal structures: anions surrounding a cation are all in contact
SiC: small C
Li Be 2.5 F Ne with that cation. For a specific coordination number there is a critical or
1.0 1.5 4.0 - minimum cationanion radius ratio rC/rA for which this contact can be
Si maintained.
Na Mg Cl Ar
0.9 1.2 1.8 3.0 - Cation

K Ca Ti Cr Fe Ni Zn As Br Kr
0.8 1.0 2.8 -
Rb Sr 1.5 1.6 1.8 1.8 1.8 2.0
0.8 1.0 I Xe
2.5 -
Cs Ba
0.7 0.9 At Rn
2.2 -
Fr Ra
0.7 0.9 Table of Electronegativities

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Crystal Structures in Ceramics COORDINATION # AND IONIC RADII
with predominantly ionic bonding rcation
• Coordination # increases with ranion
The critical ratio can be determined by Issue: How many anions can you
simple geometrical analysis

arrange around a cation?

rcation Coord # ZnS
ranion 2 (zincblende)

< .155

Cos 30°= 0.866 .155-.225 3 NaCl
= R/(r+R) (sodium
↓ .225-.414 4 chloride)
r/R = 0.155
.414-.732 6 CsCl
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5 Adapted from Table 6
12.2, Callister 6e.
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EX: PREDICTING STRUCTURE OF FeO AmXp STRUCTURES

• On the basis of ionic radii, what crystal structure • Consider CaF2 : rcation = 0.100 ≈ 0.8
would you predict for FeO? ranion 0.133

Cation Ionic radius (nm) • Answer: • Based on this ratio, coord # = 8 and structure = CsCl.
• Result: CsCl structure w/only half the cation sites
Al3+ 0.053 rcation = 0.077
Fe2+ 0.077 ranion 0.140 occupied.
Fe3+ 0.069
Ca2+ 0.100 = 0.550 • Only half the cation sites

based on this ratio, are occupied since
#Ca2+ ions = 1/2 # F- ions.

Anion --coord # = 6
O2- --structure = NaCl

0.140 Adapted from Fig. 12.5,
Callister 6e.
Cl- 0.181

F- 0.133 Data from Table 12.3,
Callister 6e.
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Crystal Structures in Ceramics More examples of crystal structures
Example: Rock Salt Structure in ceramics

NaCl structure: rC = rNa = 0.102 nm, rA = rCl = 0.181 nm → rC/rA = 0.56 Cesium Chloride Structure: rC = rCs = 0.170 nm, rA = rCl = 0.181 nm → rC/rA = 0.94
From the table for stable geometries we see that C.N. = 6 From the table for stable geometries we see that C.N. = 8

Two interpenetrating FCC lattices
NaCl, MgO, LiF, FeO have this
crystal structure

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More examples of crystal structures More examples of crystal structures
in ceramics in ceramics

Zinc Blende Structure: typical for compounds where covalent bonding dominates. Fluorite (CaF2): rC = rCa = 0.100 nm, rA = rF = 0.133 nm → rC/rA = 0.75
C.N. = 4 From the table for stable geometries we see that C.N. = 8

ZnS, ZnTe, SiC FCC structure with 3 atoms
have this crystal structure per lattice point

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Density computation Silicate Ceramics

ρ = n’(ΣAC + ΣAA) / (VcNA) Composed mainly of silicon and oxygen, the two most abundant elements in
n’: number of formula units in unit cell (all ions that are included in the chemical earth’s crust (rocks, soils, clays, sand)

formula of the compound = formula unit) Basic building block: SiO44- tetrahedron
ΣAC : sum of atomic weights of cations in the formula unit Si-O bonding is largely covalent, but overall SiO4 block has charge of –4
ΣAA : sum of atomic weights of anions in the formula unit Various silicate structures – different ways to arrange SiO4-4 blocks
Vc : volume of the unit cell
NA: Avogadro’s number, 6.023×1023 (formula units)/mol

Example: NaCl
n’ = 4 in FCC lattice
ΣAC = ANa = 22.99 g/mol
ΣAA = ACl = 35.45 g/mol
Vc = a3 = (2rNa+2rCl)3 =
= (2 X 0.102×10-7 + 2 X 0.181×10-7)3 cm3

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Silica = silicon dioxide = SiO2 Window glasses

Every oxygen atom is shared by adjacent tetrahedra Most common window glasses are produced by adding other oxides (e.g. CaO,
Silica can be crystalline (e.g., quartz) or amorphous, as in glass (fused or vitreous Na2O) whose cations are incorporated within SiO4 network. The cations break
the tetrahedral network and glasses melt at lower temperature than pure
silica) amorphous SiO2 because. A lower melting point makes it easy to form glass to
make, for instance, bottles. Some other oxides (TiO2, Al2O3) substitute for
silicon and become part of the network

Crystalline Amophous

3D network of SiO4 tetrahedra in cristobalite High melting temperature of 1710 °C

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Carbon Carbon: Diamond

Carbon is not a ceramic Has diamond-cubic structure (like Si, Ge)
Carbon exists in various polymorphic forms: sp3 diamond and amorphous carbon, One of the strongest/hardest material known
High thermal conductivity (unlike ceramics)
sp2 graphite and fullerenes/nanotubes, one dimensional sp carbon Transparent in the visible and infrared, with high index of refraction, looks nice,

costs $$$
Semiconductor (can be doped to make electronic devices)
Metastable (transforms to carbon when heated)

Graphite Buckminsterfullerene Diamond 18
Carbon Nanotube 17
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Carbon: Graphite Carbon: buckyballs and nanotubes

Layered structure with strong bonding within the planar layers and weak, van der Buckminsterfullerenes (buckyballs) and carbon nanotubes are expected to play an
Waals bonding between layers important role in future nanotechnology applications (nanoscale materials,
sensors, machines, and computers).
Easy interplanar cleavage, applications as a lubricant and for writing (pencils)
Good electrical conductor
Chemically stable even at high temperatures
Applications include furnaces, rocket nozzles, welding electrodes

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Imperfections in Ceramics Imperfections in Ceramics

Point defects in ionic crystals are charged. The Coulombic forces are very large Frenkel and Schottky defects do not change ratio of cations to anions → the
and any charge imbalance has a strong tendency to balance itself. To maintain compound is stoichiometric
charge neutrality several point defects can be created:
Non-stoichiometry (composition deviates from the one predicted by chemical
Frenkel defect is a pair of cation (positive ion) vacancy and a cation interstitial. It formula) may occur when one ion type can exist in two valence states, e.g.
may also be an anion (negative ion) vacancy and anion interstitial. However Fe2+, Fe3+
anions are larger than cations and it is not easy for an anion interstitial to form.
For example, in FeO, usual Fe valence state is 2+. If two Fe ions are in 3+ state,
Schottky defect is a pair of anion and cation vacancies then a Fe vacancy is required to maintain charge neutrality → fewer Fe ions →
non-stoichiometry

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DEFECTS IN CERAMIC Impurities in Ceramics
STRUCTURES
Impurity atoms can exist as either substitutional or interstitial solid solutions
• Frenkel Defect Substitutional ions substitute for ions of like type
--a cation is out of place. Interstitial ions are small compared to host structure – formation of anion

• Shottky Defect interstitials is unlikely
--a paired set of cation and anion vacancies. Solubilities higher if ion radii and charges match closely
Incorporation of ion with different charge state requires compensation by point
Shottky
Defect: defects

Frenkel
Defect

• Equilibrium concentration of defects ~ e−QD / kT

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IMPURITIES Mechanical Properties of Ceramics

• Impurities must also satisfy charge balance Ceramics are brittle. For brittle fracture stress concentrators are very important.
(Chapter 8: measured fracture strengths are significantly smaller than
• Ex: NaCl Na+ Cl- cation theoretical predictions for perfect materials due to the stress risers)
vacancy
• Substitutional cation impurity Fracture strength of ceramic may be greatly enhanced by creating compressive
stresses in the surface region (similar to shot peening, case hardening in
Ca2+ metals, chapter 8)

Na+ The compressive strength is typically ten times the tensile strength. This makes
ceramics good structural materials under compression (e.g., bricks in houses,
Na+ Ca2+ stone blocks in the pyramids).
initial geometry Ca2+ impurity resulting geometry

• Substitutional anion impurity anion vacancy

O2-

initial geometry Cl- Cl- resulting geometry 25 26
O2- impurity
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MEASURING ELASTIC MODULUS MEASURING STRENGTH

• Room T behavior is usually elastic, with brittle failure. • 3-point bend test to measure room T strength.

• 3-Point Bend Testing often used. cross section F

--tensile tests are difficult for brittle materials. dR L/2 L/2 Adapted from Fig.
12.29, Callister 6e.
F
cross section Adapted from Fig. b
dR L/2 L/2 12.29, Callister 6e.
rect. circ.

b δ= midpoint location of max tension
rect. circ. deflection

• Determine elastic modulus according to: • Flexural strength: • Typ. values:

F E=F L3 F L3 σ mfail 1.5FmaxL FmaxL Material σfs(MPa) E(GPa)
= bd2 πR3
x σfs = = = Si nitride 700-1000 300
F
δ 4bd3 δ 12πR4 Fmax F x rect. Si carbide 550-860 430
slope =
δ rect. circ. Al oxide 275-550 390

δ cross cross glass (soda) 69 69

linear-elastic behavior Data from Table 12.5, Callister 6e.

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Plastic Deformation in Ceramics Viscosity

Crystalline ceramics: Slip (dislocation motion) is very difficult. This is because Viscosity is a measure of a non-crystalline (glass or liquid) material’s resistance to
ions of like charge have to be brought into close proximity of each other ® deformation. High-viscosity fluids resist flow; low-viscosity fluids flow easily.
large barrier for dislocation motion. In ceramics with covalent bonding slip is
not easy as well (covalent bonds are strong) Þ ceramics are brittle. How readily a moving layer of fluid molecules drags adjacent layers of molecules
along with it determines its viscosity.
Non-crystalline ceramic: there is no regular crystalline structure ® no
dislocations. Materials deform by viscous flow, i.e. by breaking and reforming Units are Pa-s, or Poises (P)
atomic bonds, allowing ions/atoms to slide past each other (like in a liquid). 1 P = 0.1 Pa-s
Viscosity of water at room temp is ~ 10-3 P
Viscosity is a measure of glassy material’s resistance to deformation. Viscosity of typical glass at room temp >> 1016 P

η= τ = F A
dv dy dv dy

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MEASURING ELEVATED T RESPONSE SUMMARY

• Elevated Temperature Tensile Test (T > 0.4 Tmelt). • Ceramic materials have mostly ionic & some
creep test covalent bonding.

ε x • Structures are based on:
--charge neutrality
σ --maximizing # of nearest oppositely charged neighbors.

slope = . = steady-state creep rate • Structures may be predicted based on:
εss --ratio of the cation and anion radii.

σ • Defects
--must preserve charge neutrality
time --have a concentration that varies exponentially w/T.

• Generally, • Room T mechanical response is elastic, but fracture
ε.scesramics < ε.mssetals << ε. pssolymers brittle, with negligible ductility.

• Elevated T creep properties are generally superior to
those of metals (and polymers).

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Summary ANNOUNCEMENTS

Make sure you understand language and concepts: • Reading
– Chapter # 13
Anion
Cation • Chapter #12 Homework Assignment
Defect structure – 12.3, 12.5, 12.14, 12.32, 12.38
Frenkel defect
Electroneutrality
Schottky defect
Stoichiometry
Viscosity

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