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STRUCTURE AND PROPERTIES OF CERAMICS
How do ceramics differ from metals ?
Keramikos ~ burnt stuffHeat treatment is necessary
Usually a compound between a metal and a non-metalBonding displays a mixture of ionic and covalent
1
c arac er Generally hard and brittle, have high melting temperatureWhy ?
Generally thermally and electrically insulating Can be opaque, semi-transparent or transparent Traditional ceramics ~ based on clay (china, porcelain,
bricks, tiles) and glasses Hi-tech ceramics => electronic, communication, computer
hardware, aerospace industries
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Bonding:--Mostly ionic, some covalent.
--% ionic character increases with difference in
electronegativity. What is electronegativity ?
HeH CaF : lar e
Large vs small ionic bond character:
CERAMIC BONDING
2
-
Ne-
Ar-
Kr-
Xe-
Rn-
Cl3.0
Br2.8
I2.5
At2.2
Li1.0
Na0.9
K0.8
Rb0.8
Cs0.7
Fr0.7
2.1
Be1.5
Mg1.2
Sr1.0
Ba0.9
Ra0.9
Ti1.5
Cr1.6
Fe1.8
Ni1.8
Zn1.8
As2.0
C2.5
Si1.8
F4.0
Ca1.0
Table of Electronegativities
SiC: small
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition.
Copyright 1960 by
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Crystal Structure of Ionicly Bonded Ceramics
Crystal structure is defined by 2 criterions
1. Magnitude of the electrical charge on each ion. Chargebalance dictates chemical formula (Ca2+ and F- form CaF2).
2. Relative sizes of the cations and anions. Cations wantsmaximum ossible number of anion nearest nei hbors
and vice-versa.
Stable ceramic crystal structures require anions surroundinga cation to be all in contact with that cation.
For a specific coordination number there is a critical orminimum cation/anion radius ratio rC/rA for which thiscontact can be maintained. Pure geometrical consideration
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1. Charge Neutrality:
--Net charge in the
crystal structure
should be zero.
--General form:A X
CaF2:
Ca2+
cationF-
F-
anions+
IONIC BONDING & CRYSTAL STRUCTURE
3
m, p determined by charge neutrality
2. Maximize the # of nearest oppositely charged neighbors--stable structures:
Adapted from Fig. 12.1, Callister
6e.
- -
- -+
unstable
- -
- -+
stable
- -
- -+
stable
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Coordination # increases withrcationranion
rcationranion
Coord #
< .155
ZnS(zincblende)
2 Adapted from Fig. 12.4,
COORDINATION # AND IONIC RADII
4
.155-.225
.225-.414
.414-.732
.732-1.0
NaCl(sodium
chloride)
CsCl(cesiumchloride)
3
4
6
8
Adapted from Table
12.2, Callister 6e.
Adapted from Fig. 12.2,
Callister 6e.
Adapted from Fig. 12.3,
Callister 6e.
.
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Home work
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On the basis of ionic radii, what crystal structure
would you predict for FeO?
Cation
Al3+
Fe2++
Ionic radius (nm)
0.053
0.077
Answer:
rcation
ranion=0.077
0.140
= 0.550based on this ratio,
EX1: PREDICTING STRUCTURE OF FeO
5
Ca2+
Anion
O2-
Cl-
F-
.
0.100
0.140
0.181
0.133
--coor =
--structure = NaCl (rocksalt)
Data from Table 12.3,
Callister 6e.
Two penetrating FCC units; otherexamples are MgO, MnS, LiF.
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Consider CaF2 :rcationranion
=0.100
0.133 0.8
Based on this ratio, coord # = 8 and structure = CsCl.
Result: CsCl structure w/only half the cation sites
occupied.
EX2: AmXp STRUCTURES
6
are occupied since#Ca2+ ions = 1/2 # F- ions.
Adapted from Fig. 12.5,
Callister 6e.
Empty
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Ceramic Density Computations
n: number of formula units in unit cell (all ions that are includedin the chemical formula of the compound = formula unit)
AC: sum of atomic weights of cations in the formula unit
AA: sum of atomic weights of anions in the formula unit
VC: volume of the unit cell
NA: Avogadros number, 6.023 X 1023 (formula units)/mol
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EX4: NaCl density
a
n = 4 in FCC lattice
AC= ANa= 22.99 g/mol
AA= ACl= 35.45 g/mol
VC= a3=[2 (rNa + rCl)]3
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Silicate Ceramics
Composed mainly of silicon and oxygen, the twomost abundant elements in earths crust (rocks, soils,clays and sand- SiO2 silica)
Basic building block: SiO44- tetrahedron: Si-O bonding is largely covalent, but overall SiO4
block has charge of 4
Various silicate structures different ways toarrange SiO4
4- blocks
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EX: Crystalline form of SiO2Three polymorphs of SiO2 :
Quartz, Crystobalite, Tridymite
Not a very closed pack structure lowdensity ~ 2.65 g/cm3
3D networks of SiO44- tetrahedra
Each O atom is shared by an
adjacent tetrahedron
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Window Glass
Still SiO44- tetrahedra are the basic
building block.
Most common window glasses are
produced by adding other oxides (e.g.CaO, Na2O, B2O3, etc) whose cationsare incorporated within SiO4 network.
These cations break the tetrahedralne wor an g asses me a owertemperature than pure amorphousSiO2 .
A lower melting point makes it easy toform glass to make, for instance,
bottles. Some other oxides (TiO2, Al2O3)
substitute for silicon and become partof the network
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Carbon/Diamond/Fullerenes/ Nanotubes
http://www.nas.nasa.gov/Groups/SciTech/nano/
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Mechanical Properties of Ceramics
Ceramics are very brittle. (Fracture Toughness) For brittle materials fracture stress concentrators are very
important. (Chapter 8: measured fracture strengths aresignificantly smaller than theoretical predictions for perfect
materials due to the stress risers) Fracture strength of ceramic may be greatly enhanced by creating
compressive stresses in the surface region (similar to shotpeening, case hardening in metals, chapter 8)
strength. This makes ceramics good structural materialsunder compression (e.g., cement, bricks in buildingapartments, stone blocks in the pyramids).
Generally, tensile test is not used
Hard to machine, grippers may break the piece, fail after 0.1%strain.
Size is important due impact of # of cracks on strength, why ?
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Elevated Temperature Tensile Test (T > 0.4 Tmelt).
creep test
x
slope = ss = steady-state creep rate.
MEASURING ELEVATED T RESPONSE
11
Generally,
time
ssceramics
< ssmetals
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Ceramic materials have mostly covalent & some
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.
SUMMARY
12
Defects--must preserve charge neutrality
--have a concentration that varies exponentially w/T.
Room T mechanical response is elastic, but fracture
brittle, with negligible ductility. Elevated T creep properties are generally superior to
those of metals (and polymers).
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Nano Materials
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Definition
Nanotechnology is the understanding and control ofmatter at dimensions of roughly 1 to 100nanometers, where unique phenomena enable novelapplications.
Encompassing nanoscale science, engineering andtechnology, nanotechnology involves imaging,measuring, modeling, and manipulating matter at
this length scale.
National Nanotechnology Initiative, 2007
7
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Scale of ThingsNanometers
Figure 1.5: National Nanotechnology Initiative.
8
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Nanoscience and nanotechnology concerns
objects that are extremely small.
How small?
What is Nanotechnology?
Bigger than atoms, but smaller than you can
see with a light microscope.
1 100 nanometers
(3-4 atoms side by side = 1 nm)
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How Big is 1 Nanometer -- Length Scale?Soccer: 22cm
Carbon 60: 0.7nm
Pet Flea: 1mm Virus: 150nmHair: 80m Red Cell: 7m
.
DNA: 2nm
TiOx Particles: 13nm IBM Logo: 5nm
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One Nanometer
Water (H20)
DNA
Small
Protein This slide is adapted from the lecture notes p
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One Nanometer
Water (H20)
Quantum
Dot
Carbon
Nanotube
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Tiny machines in
What is Nanotechnology?
cancer?
http://smalley.rice.edu/emplibrary/SA285-
76.pdf
This slide is adapted from the lecture notes p
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This slide is adapted from the lecture no
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Making Small Objects
Self-assembly of nanoparticles,
25 um
Optical lithography
Electron beam lithographyTop-down approach
n v ua a oms, mo ecu es
Bottom-up approach
Chemical deposition
Bottom-up approach
IBM Research
This slide is adapted from the presentation on An Introductio
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Manipulating Small Objects
In 1989, Don Eigler arranged these xenon atoms, one by one, on a nickel surface to spell
out the name of his company. (using a Scanning Tunneling Microscope)
This slide is adapted from the presentation on An Introductio
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Current Applications: Transportation
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Current Applications: TransportationPolyurethane nanocomposites for tires?
Fuel emissions: fuel efficiency may
increase by more than 3%
Road noise: reduced volume of air,
decreasing noise pollution
Weight: total weight of four tires +
insert. Less than five standard tires
(no spare needed)
(The first cars carried up to 6 spare tires)!
Lightweight nanoreinforced polyurethanes will provide the
next generation materials - towards an all PU tire?
(source Goodyear)
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Which one are actual nano-products?
This slide is adapted from the presentation on An Introductio
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Which one are actual nano-products?
This slide is adapted from the presentation on An Introductio
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Nano-products
Display ScreensMotorola (NTs) Nano SilverSeal
Refrigerator
Cars - HummerGM (Nanocomposites)
Nano-Products on the Market Now
Samsung nanopartic e-coate
Tennis RacketsWilson (C fibers)
This slide is adapted from the present
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Nano-products
Shemen Industriescanola oil by NutraLease, an
Israeli startup, using 30 nmcapsules
Nano-Care fabric
wrinkle-resistant, stain-repellent
Plenitude Revitalift
Loreal
(Eddie Bauer, Lee, Old Navy, TigerWoods, Bass, Nike)
Superhydrophobic nanoscalecoating applied to fabric
This slide is adapted from the present
Claynanocomposite
barrier coating
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16.8 GB
Nanodevices - Magnetic StorageThe hard drive in your computer uses a
nanotechnology innovation called giant
magnetoresistance.Giant magnetoresistance is an effect
where small magnetic fields can be
detected as a change in resistance.
This slide is adapted from the presentation on An Introductio
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Nanomaterials - UV Protection
Advanced Powder
Small =
Transparent
This slide is adapted from the presentation on An Introductio
ec no ogy ty t
90 nm
25 nm
250 nm
Zinc
Oxide
Zinclear
in
Wet Dreams
sunscreen
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Nanomaterials - CatalysisYour car has
nanoparticlesin it!
Gold nanoparticles can
turn carbon monoxide
into relatively
innocuous carbon
dioxide at temperatures
as low as -107F.
This slide is adapted from the presentation on An Introductio
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