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CHAPTER
10Applications and Processing of
Ceramics
8-1
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Fabrication and Processing of Ceramics
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Glass
Combination of transparency, strength, hardness and
corrosion resistance.
Glass is an inorganic product of fusion that has cooled to a
rigid condition without crystallization.
Glass does not crystallize
up on cooling.
Up on cooling, it transforms
from rubbery material to
rigid glass.
Figure 10.52
11-40
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Glass Structure
Quartz is crystalline
SiO2:
Basic Unit: Glass is amorphous
Amorphous structureoccurs by adding impurities
(Na+,Mg2+,Ca2+, Al3+)
Impurities:
interfere with formation ofcrystalline structure.
(soda glass)
Si04 tetrahedron4-
Si4+
O2-
Si4+
Na+
O2-
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In cristobalite, Si-O tetrahedron are joined corner to
corner to form long range order.
In simple silica glass, tetrahedra are joined corner to
corner to form loose network.
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Composition of Glass
Soda lime glass: Very common glass (90%).
71-73% SiO2, 12-14% Na2O, 10-12% CaO.
Easier to form and used in flat glass and containers.
Borosilicate glass: Alkali oxides are replaced by boric oxide in silica
glass network.
Known as Pyrex glass and is used for lab equipments andpiping.
Lead glass: Lead oxide acts as network modifier and network former.
Low melting point used for solder sealing.
Used in radiation shields, optical glass and TV bulbs.
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Glass Modifying Oxides and Intermediate Oxides
Network modifiers: Oxides thatbreakup the glass
network.
Added to glass to increase workability.
Examples:- Na2O, K2O, CaO, MgO.
Oxygen atom enters network and other ion stayin interstices.
Intermediate oxides: Cannot form glass network by
themselves but canjoin into an existing network.
Added to obtain special properties.Examples: Al2O3, Lead oxide.
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Glass Properties
Specific volume (1/) vs Temperature (T):
Glasses:-- do not crystallize
-- change in slope in spec. vol. curve at
glass transition temperature, Tg-- transparent
- no crystals to scatter light
Crystalline materials:-- crystallize at melting temp, Tm-- have abrupt change in spec.
vol. at Tm
Adapted from Fig. 13.6, Callister, 7e.
T
Specific volume
SupercooledLiquid
solid
Tm
Liquid(disordered)
Crystalline(i.e., ordered)
Tg
Glass
(amorphous solid)
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Viscosity
Viscosity, :-- relates shear stress and velocity gradient:
has units of (Pa-s)
y
v
d
d=
velocity gradient
dv
dy
glass dvdy
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Viscous Deformation of Glass
Viscosity decreases with T
Impurities lowerTdeformfused
silica
96%si
lica
soda-lime
glass
Visco
sity[Pa
s]
1
102
10
6
1010
1014
200 600 1000 1400 1800 T(C)
Tdeform : soft enoughto deform or work
annealing range
Pyr
ex
Tmelt
strain point
fused silica: > 99.5 wt% SiO2
soda-lime glass: 70% SiO2
balance Na2O (soda) & CaO (lime)
Vycor: 96% SiO2, 4% B2O3
borosilicate (Pyrex):
13% B2O3, 3.5% Na2O, 2.5% Al2O3
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Viscous above Tg and viscosity decreases with increase in temperature.
* = 0e+Q/RT *= Viscocity of glass (PaS)
0= preexponential constant (PaS)
Q = Activation energy
Working point: 103 PaS glass
fabrication can be carried out
Softening point: 107 PaS glass
flows under its own weight.
Annealing point: 1012 PaS Internal
stresses can be relieved..
Strain point: 10 13.5 PaS glass is
rigid below this point.
Figure 10.55
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Forming Methods
Forming sheet and plate
glass:Ribbon of glass moves
out of furnace and floatson abath of molten tin.
Glass is cooled by moltentin.
After it is hard, it isremoved and passedthrough a long annealing
furnace.
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Blowing
Blowing: Air blown to force molten glass into molds.
suspendedParison
Finishingmold
Compressedair
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Pressing
Pressing: Optical and sealed beam lenses are
pressed by aplungerinto a mold containing
molten glass.
Gob
Parison
mold
Pressing
operation
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Casting
Casting: Molten glass is cast in open mold.
Centrifugal casting: Glass globs are dropped into
spinning mold.
Glass first flows outward towards wall of mold and
then upward against the mold wall.
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Drawing
Continuous drawing
originally sheet glass was made by floating glass
on a pool of mercury
plates, dishes, cheap glasses
--mold is steel with
graphite lining
Fiber drawing:
wind up
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Chemically Strengthened Glass
Special treatment increases chemical resistance of
glasses.
Example:- Sodium aluminosilicate glasses are
immersed in a bath of potassium nitrate at 500C
for 6 to 10 hoursLarge potassium ions are induced into surface
causing compressive stress.
Compressive layer is much thinner than that in
thermal tempering. Used for supersonic aircraft glazing and
ophthalmic lenses.
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Heat Treating Glass
Annealing:
--removes internal stress caused by uneven cooling. Tempering:
--puts surface of glass part into compression
--suppresses growth of cracks from surface scratches.
--sequence:
further cooled
tension
compression
compression
before cooling
hot
surface cooling
hot
cooler
cooler
--Result: surface crack growth is suppressed.
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2. Particulate Forming
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Traditional Ceramics
A mixture of components used
(50%) 1. Clay
(25%) 2. Filler e.g. quartz (finely ground)
(25%) 3. Fluxing agent (Feldspar)
binds it together
aluminosilicates + K+, Na+, Ca+
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Traditional Ceramics
Clay: Provide workability and hardness.
Silica: Provide better temperature resistance and MP.
Potash Fledspar: Makes glass when ceramic is fired.
Quartz
grain High-silica
glass
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Engineering Ceramics
Alumina (Al2O3): Aluminum oxide is doped with
magnesium oxide, cold pressed and sintered. Uniform structure. Used for electric applications.
Silicon Nitride (Si3N4): Compact of silicon powder is
nitrided in a flow of nitrogen gas.
Moderate strength and used for parts of advanced engines.
Silicon Carbide (SiC):Very hard refractory carbide,sintered at 21000C.
Used as reinforcement in composite materials.
Zirconia (ZrO2): Polymorphic and is subject to cracking.
Combined with 9% MgO to produce ceramic with high
fracture toughness.
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Hydroplastic Forming
Milling and screening: desired particle size Mixing particles & water: produces a "slip" Form a "green" component
Dry and fire the component
ram billet
container
containerforce
die holder
die
Ao
Adextrusion--Hydroplastic forming:
extrude the slip (e.g., into a pipe)
Adapted from
Fig. 11.8 (c),
Callister 7e.
solid component
--Slip casting:Adapted from Fig.
13.12, Callister 7e.
(Fig. 13.12 is from
W.D. Kingery,
Introduction to
Ceramics, John
Wiley and Sons,
Inc., 1960.)
hollow component
pour slipinto mold
drain
moldgreenceramic
pour slipinto mold
absorb waterinto mold
greenceramic
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Feature of Slip
Clay is inexpensive Adding water to clay-- allows material to shear easily
along weak van der Waals bonds
-- enables extrusion
-- enables slip casting
Structure of
Kaolinite Clay:
weak vander Waalsbonding
chargeneutral
chargeneutral
Si 4+
Al3+
-OH
O2-
Shear
Shear
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Drying and Firing
Drying: layer size and spacing decrease.Adapted from Fig.
13.13, Callister 7e.
(Fig. 13.13 is from
W.D. Kingery,Introduction to
Ceramics, John
Wiley and Sons,
Inc., 1960.)
Drying too fast causes sample to warp or crack due to non-uniform shrinkage
wet slip partially dry green ceramic
Firing:--Traised to (900-1400C)
--vitrification: liquid glass forms from clay and flows between
SiO2 particles. Flux melts at lowerT.Adapted from Fig. 13.14,
Callister 7e.(Fig. 13.14 is courtesy H.G.
Brinkies, Swinburne
University of Technology,
Hawthorn Campus,
Hawthorn, Victoria,
Australia.)
Si02 particle(quartz)
glass formedaroundthe particle
micrograph ofporcelain
70m
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Powder Pressing
Sintering - powder touches - forms neck & gradually
neck thickens add processing aids to help form neck
little or no plastic deformation
Uniaxial compression - compacted in single direction
Isostatic(hydrostatic) compression - pressure applied byfluid - powder in rubber envelope
Hot pressing - pressure + heat
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Sintering: useful for both clay and non-clay compositions.
Procedure:-- produce ceramic and/or glass particles by grinding-- place particles in mold
-- press at elevated Tto reduce pore size.
Aluminum oxide powder:
-- sintered at 1700Cfor 6 minutes.
15m
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Tape Casting
thin sheets of green ceramic cast as flexible tape
used for integrated circuits and capacitors
cast from liquid slip (ceramic + organic solvent)
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Cementation
Produced in extremely large quantities.
Portland cement:-- mix clay and lime bearing materials
-- calcinate (heat to 1400C)
-- primary constituents:
tri-calcium silicate
di-calcium silicate
Adding water-- produces a paste which hardens
-- hardening occurs due to hydration (chemical reactions
with the water). Forming: done usually minutes after hydration begins.
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8-2
Glasses Clay
products
Refractories Abrasives Cements Advanced
ceramics
-optical-compositereinforce
-containers/household
-whiteware-bricks
-bricks forhigh T(furnaces)
-sandpaper-cutting-polishing
-composites-structural
engine-rotors-valves-bearings
-sensors
Ceramics
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Refractories
Need a material to use in high temperature furnaces.
Consider the Silica (SiO2) - Alumina (Al2O3) system.
Phase diagram shows:
mullite, alumina, and crystobalite as candidate
refractories.
Composition (wt% alumina)
T(C)
1400
1600
1800
2000
2200
20 40 60 80 1000
alumina +mullite
mullite
+ L
mulliteLiquid
(L)
mullite + crystobalite
crystobalite+ L
alumina + L
3Al2O3-2SiO2
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Acidic refractories:
Silica refractories have high mechanical strengthand rigidity.
Fireclays: Mixture of plastic fireclay, flint clay andgrog. Particles vary from coarse to very fine.
High aluminum refractories: Contain 50-90%alumina and have higher fusion temperature.
Basic refractories: consists mainly of MgO andCaO.
Have high bulk densities, melting temperature andresistance to chemical attack.
used for lining in basic-oxygen steelmaking process.
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Cutting Tools
Tools:
-- for grinding glass, tungsten,carbide, ceramics
-- for cutting Si wafers
-- for oil drilling
bladesoil drill bits
Solutions:
coated single
crystal diamonds
polycrystalline
diamonds in a resin
matrix.
-- manufactured single crystalor polycrystalline diamonds
in a metal or resin matrix.
-- optional coatings (e.g., Ti to help
diamonds bond to a Co matrix
via alloying)-- polycrystalline diamonds
resharpen by microfracturing
along crystalline planes.
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Sensors
Example: Oxygen sensor ZrO2
Principle: Make diffusion of ionsfast for rapid response.
A Ca2+ impurity
removes a Zr4+ and a
O2- ion.
Ca2+
Approach:
Add Ca impurity to ZrO2:
-- increases O2- vacancies
-- increases O2- diffusion rate
referencegas at fixedoxygen content
O2-
diffusion
gas with anunknown, higheroxygen content
-+voltage difference produced!
sensor Operation:
-- voltage differenceproduced when
O2-
ions diffusefrom the external
surface of the sensor
to the reference gas.
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Insulation
About 70% of external surface is protected from heat by
24000 ceramic tiles. Material: Silica fiber compound.
Density is 4kg/ft3 and withstands temperature up to
12600C.
Figure 10.51
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Ceramic Insulator Materials
Ionic and covalent bonding restricts the mobility of ions and electrons
and hence ceramics are good insulators.
Electrical porcelain: 50% Clay + 25 % Fledspar.
Good plasticity, wider firing temperature range, cheap.
High power loss factor.
Steatite: 90% talc + 10 % clay
Good insulator, low power loss factor, impact strength
Fosterite: Mg2SiO4 no alkali ions
Higher resistivity, low electrical loss
Alumina: Al2O3 Crystalline phase bounded to glassy
matrix.
High dielectric strength, low dielectric loss
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Capacitors
Ceramics are used as dielectric materials
for capacitors.
Example: Disk ceramic capacitors. BaTiO3 + additive
Very high dielectric
constant
Used in ceramic based
thick film hybridelectronic circuit
Higher capacitance per unit area
Figure 10.38a
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Semiconductors
Ceramics can be used as semiconducting materials.
Thermistor: Thermally sensitive resistor.
NTC thermistor: Conductivity raises with temperature.
Solid solution oxides of Mn, Ni, Fe, Co and Cu are used
to obtain necessary property ranges.
By combining low conducting metal oxide with low
conducting oxides intermediate properties are obtained.
Example: Conductivity of Fe3O4 is reduced gradually by
adding increasing amounts in solid solution of MgCr2O4
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Electronic Packaging
Chosen to securely hold microelectronics & provideheat transfer
Must match the thermal expansion coefficient of themicroelectronic chip & the electronic packaging
material. Additional requirements include: good heat transfer coefficient
poor electrical conductivity
Materials currently used include:
Boron nitride (BN) Silicon Carbide (SiC)
Aluminum nitride (AlN)
thermal conductivity 10x that for Alumina
good expansion match with Si
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Ceramic ArmorAl2O3, B4C, SiC & TiB2
Extremely hard materials
shatter the incoming projectile
energy absorbent material underneath
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