Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Chapter Outline: Ceramics
Chapter 12: Structure and Properties of
Ceramics
Crystal Structures
Silicate Ceramics
Imperfections in Ceramics
Carbon
Skip: 12.9 – 12.11
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Ceramics
 keramikos - burnt stuff in Greek properties achieved through hightemperature heat treatment (firing).
 Usually metallic + non-metallic elements
 Always composed of more than one
element (e.g., Al2O3, NaCl, SiC, SiO2)
 Bonds are partially or totally ionic
 Hard and brittle
 Electrical and thermal insulators
 Optically opaque, semi-transparent, or
transparent
 Traditionally based on clay (china,
bricks, tiles, porcelain) and glasses
 “New
ceramics”
for
electronic,
computer, aerospace industries.
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Bonding in Ceramics (Chapter 2)
Electronegativity – ability of atoms to accept
electrons (subshells with one electron - low
electronegativity;
subshells with one missing
electron -high electronegativity).
Electronegativity increases from left to right.
Bonding is mixed:
ionic + covalent
Degree
of
ionic
depends on difference
in electronegativities
Cations(+); Anions(-)
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Crystal Structures: Predominantly Ionic
Crystal structure is defined by
 Magnitude of electrical charge on each ion
Charge balance dictates chemical formula
(Ca2+ and F- form CaF2).
 Relative sizes of cations and anions
Cations want maximum possible number of
anion nearest neighbors and vice-versa.
Ceramic crystal structures: anions surrounding a
cation are all in contact with it. For a specific
coordination number there is a critical or
minimum cation-anion radius ratio rC/rA for which
this contact can be maintained
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
C.N.
rC/rA
Geometry
2
The critical ratio
determined
by
geometrical
3
analysis
<0.155
0.155-0225
4
0.225-0.414
6
0.414-0.732
8
0.732-1.0
30°
Cos 30= 0.866
= R/(r+R)

r/R = 0.155
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Crystal Structures
Rock Salt Structure
NaCl
rC = rNa = 0.102 nm, rA = rCl = 0.181 nm
 rC/rA = 0.56
From table for stable geometries: C.N. = 6
Two interpenetrating FCC lattices
NaCl, MgO, LiF, FeO have this crystal structure
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Other crystal structures in ceramics
(will not be included in the test)
Cesium Chloride Structure:
rC = rCs = 0.170 nm, rA = rCl = 0.181 nm
 rC/rA = 0.94
From table for stable geometries: C.N. = 8
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Other crystal structures in ceramics
(will not be included in the test)
Zinc Blende Structure: typical for compounds
where covalent bonding dominates. C.N. = 4
ZnS, ZnTe, SiC have this crystal structure
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Other crystal structures in ceramics
(will not be included in the test)
Fluorite (CaF2):
rC = rCa = 0.100 nm, rA = rF = 0.133 nm
 rC/rA = 0.75
From table for stable geometries: C.N. = 8
FCC structure with 3 atoms per lattice point
University of Virginia, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Density computation
(similar to Chapter 3.5 for metals)
 = n’(AC + AA) / (VcNA)
n’:
number of formula units in unit cell (all ions
included in chemical formula of compound =
formula unit)
AC: sum of atomic weights of cations
AA: sum of atomic weights of anions
Vc: volume of the unit cell
NA: Avogadro’s number,
6.0231023 (formula units)/mol
Example: NaCl
n’ = 4 in FCC lattice
AC = ANa = 22.99 g/mol
AA = ACl = 35.45 g/mol
Vc = a3 = (2rNa+2rCl)3 =
(20.10210-7 + 20.18110-7)3 cm3
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Silicate Ceramics
 Mainly of silicon and oxygen, the two
most abundant elements in earth’s crust
(rocks, soils, clays, sand)
 Basic building block: SiO44- tetrahedron
 Si-O bonding is largely covalent, but
overall SiO4 block has charge of –4
 Various silicate structures – different
ways to arrange SiO4-4 blocks
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Silica = silicon dioxide = SiO2
 Every oxygen shared by adjacent
tetrahedra
 Silica is crystalline (quartz) or amorphous,
as in glass (fused or vitreous silica)
3D network of SiO4 tetrahedra in cristobalite
High melting temperature of 1710 C
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Window glasses
Common window glass is produced by adding
oxides (e.g. CaO, Na2O) whose cations are
incorporated within SiO4 network. The cations
break the tetrahedral network. Glasses melt at
lower temperature than pure amorphous SiO2.
Lower melting T makes it easier to form objects
(e.g, bottles). Some other oxides (TiO2, Al2O3)
substitute for silicon and become part of the
network
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Imperfections in Ceramics (I)
Point defects in ionic crystals are charged. Coulomb
forces are large. Any charge imbalance has a strong
tendency to balance itself.
To maintain charge
neutrality several point defects can be occur:
Frenkel defect: a pair of cation (positive ion) vacancies
and a cation interstitial. Also be an anion (negative ion)
vacancy and anion interstitial. Anions are larger than
cations so not easy for an anion interstitial to form
Schottky defect is a pair of anion and cation vacancies
Schottky defect
Frenkel defect
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Imperfections in Ceramics (II)
• Frenkel and Schottky defects do not change ratio of
cations to anions  compound is stoichiometric
• Non-stoichiometry (composition deviates from the
one predicted by chemical formula) occurs when one
ion type can exist in two valence states, e.g. Fe2+, Fe3+
• In FeO, Fe valence state is 2+.
Two Fe ions in 3+ state  an Fe vacancy is required
to maintain charge neutrality
 fewer Fe ions  non-stoichiometry
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Impurities in Ceramics
 Impurity atoms can be substitutional or
interstitials
 Substitutional: substitute for ions of like type
 Interstitials: small compared to host structure –
formation of anion interstitials is unlikely
 Solubilities higher if ion radii and charges match
 Incorporation of ion with different charge state
requires compensation by point defects
Interstitial impurity atom
Substitutional impurity ions
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Mechanical Properties of Ceramics
 Brittle Fracture stress concentrators are
very important. (Chap. 8: measured
fracture strengths are much smaller
than theoretical due to stress risers)
Fracture strength greatly enhanced by
creating compressive stresses in the
surface region (similar to shot peening,
case hardening in metals, Chap. 8)
 Compressive strength is typically ten
times the tensile strength Therefore
ceramics are good structural materials
under compression (e.g., bricks in
houses, stone blocks in the pyramids).
University of Virginia, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Plastic Deformation in Ceramics
 Crystalline ceramics: Slip (dislocation
motion) is difficult because ions of like
charge have to be brought close together
 large barrier for dislocation motion
In ceramics with covalent bonding slip is
not easy (covalent bonds are strong) 
ceramics are brittle.
 Non-crystalline ceramic: no regular
crystalline structure  no dislocations
or slip. Materials deform by viscous flow
(breaking
and
reforming
bonds,
allowing ions/atoms to slide past each
other (like in a liquid)
Viscosity is a measure of glassy
material’s resistance to deformation.
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Viscosity
Viscosity: measure of non-crystalline (glass
or liquid) resistance to deformation. Highviscosity fluids resist flow; low-viscosity
fluids flow easily.
How readily a moving layer of molecules
drags adjacent layers of molecules
along
determines its viscosity.
Units are Pa-s, or Poises (P) 1 P = 0.1 Pa-s
Viscosity of water at room temp is ~ 10-3 P
Viscosity of typical glass at room temp >>
1016 P

FA


dv dy dv dy
University of Virginia, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Carbon
Carbon not a ceramic
Exists in various polymorphic forms: sp3 diamond
and amorphous carbon, sp2 graphite and
fullerenes/nanotubes, one dimensional sp carbon
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Carbon: Diamond




Diamond-cubic structure
One of the strongest/hardest materials
High thermal conductivity (unlike ceramics)
Transparent in visible and infrared, high index
of refraction, looks nice, costs $$$
 Semiconductor (can be doped to make
electronic devices)
 Metastable (transforms to carbon when heated)
Hydrogenated diamond
{111} surface with the
dangling
bonds
or
radicals terminated by
hydrogen atoms
Diamond
turning
into
graphite
at
elevated temperature
University
of Virginia, Dept. of Materials Science and Engineering
Figures from
http://www.people.virginia.edu/~lz2n/Diamond.html
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Carbon: Graphite
 Layered structure: Strong bonding within
planar layers. Weak, van der Waals bonding
between layers
 Easy interplanar cleavage, applications as a
lubricant and for writing (pencils)
 Good electrical conductor
 Chemically stable even at high temperatures
 Applications: furnaces, rocket nozzles, welding
electrodes
University of Virginia, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Carbon: buckyballs and nanotubes
Buckminsterfullerenes (buckyballs) + carbon
nanotubes expected to be important in future
nanotechnology applications (nanoscale materials,
sensors, machines, computers)
Carbon nanotube T-junction
Nano-gear
Nanotubes as reinforcing
fibers in nanocomposites
Nanotube holepunching/etching
University
of Virginia, Dept. of Materials Science and Engineering
Figures
from http://www.nas.nasa.gov/Groups/SciTech/nano/
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Introduction to Materials Science, Chapter 12, Structure and Properties of Ceramics
Summary
Make sure you understand language and concepts:
 Anion
 Cation
 Defect structure
 Frenkel defect
 Electroneutrality
 Schottky defect
 Stoichiometry
 Viscosity
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