Ceramics,

Properties
and
Their Applications
Definition
 Ceramics can be defined as inorganic, non-metallic
materials that are typically produced using clays and
other minerals from the earth or chemically
processed powders.

 Ceramics are typically crystalline in nature and are

compounds formed between metallic and non-
metallic elements such as aluminium and oxygen
(alumina- Al2O3 ), silicon and nitrogen (silicon nitride-
Si3N4) and silicon and carbon (silicon carbide-SiC).

 Glass is often considered a subset of ceramics.

 Glass is somewhat different than ceramics in that it is

amorphous, or has no long range crystalline order.
Uses
 Most people, when they hear the word ceramics,
think of art, dinnerware, pottery, tiles, brick and
toilets.
 The above mentioned products are commonly
referred to as traditional or silicate-based ceramics.
 While these traditional products have been, and
continue to be, important to society, a new class of
ceramics has emerged that most people are unaware
of.
 These advanced or technical ceramics are being used
for applications such as space shuttle tile, engine
components, artificial bones and teeth, computers
and other electronic components, and cutting tools,
just to name a few.
Certain Terms
Varistor: A voltage-dependent variable resistor. Normally used to
protect sensitive equipment from power spikes or lightning strikes
by shunting the energy to ground.

Slurry: A viscous liquid mixture (especially involving water) composed of

a mixture of various insoluble matter, such as mud or plaster of
paris.

Green body: In ceramic processing the powders are first consolidated

in the desired shape to produce what is called a "green body".

Sintering: A method for making objects from powder, increasing the

adhesion between particles as they are heated. It is used with
ceramic powders and in powder metallurgy.

Abrasive: A substance used to scrub, grind, smooth or polish. Abrasive

particles can be found in products such as cleansers, pumice stones,
scouring pads and hand cleaners. Common example is sand paper.

Kiln: An oven that is used for hardening, burning, or drying anything. Or

simply high temperature furnace.
Production of Modern Ceramic Components:
Green body and sintering
 The production of modern ceramic components generally
involves two separate processing stages.

 First, powdered ceramic is dispersed in a liquid and

then compacted to form the desired component
shape or “green body”.

 Second, the green body is then heated to just below the

melting point of the ceramic material.

 At this temperature sintering occurs, causing the

particles in the green body become bonded together.

 This process creates a mechanically strong component.

Properties of Ceramics: Mechanical Property
 Ceramic materials are usually ionic or glassy
materials.

 Both of these almost always fracture before any plastic

deformation takes place, which results in poor
toughness in these materials.

 Additionally, because these materials tend to be

porous, the pores and other microscopic
imperfections act as stress concentrators, decreasing
the toughness further, and reducing the tensile
strength.

 These combine to give catastrophic failures.

 Conclusion, These materials don’t show plastic

deformation.
 However, due to the rigid structure of the crystalline
materials, there are very few available slip systems
for dislocations to move, and so they deform very
slowly increases hardness .

 Their high hardness makes them widely used as

abrasives, and as cutting tips in tools.

 With the non-crystalline (glassy) materials, viscous

flow (rubbery nature) is the dominant source of
plastic deformation, and is also very slow.

 It is therefore glasses are neglected in many

applications of ceramic materials.

 These materials have great strength under

compression, and are capable of operating at
elevated temperatures.
Properties of Ceramics:Thermal Property:
Refractory behavior
 Ceramic materials which can withstand at extremely
high temperatures without losing their strength are
called refractory materials.

 They generally have low thermal conductivities, and

thus are used as thermal insulators.

 For example, the belly of the Space Shuttles are

made of ceramic tiles which protect the spacecraft
from the high temperatures caused during re-entry
to earth’s atmosphere.

 The most important requirements for a good

refractory material are that are not going to soften
or melt, but also remain non-reactive at the desired
(high) temperature.
Refractory behavior
 Porosity takes on additional relevance with
refractories.
 As the porosity is decreased, the strength, load-
bearing ability, and environmental resistance
increases as the material gets more dense.
 As the density increases the resistance to thermal
shock (cracking as a result of rapid temperature
change) and insulation characteristics are reduced.
 Many materials are used in a very porous state, and it
is not uncommon to find two materials used: a
porous layer, with very good insulating properties,
with a thin coat of a more dense material to provide
strength.
 It is perhaps surprising that these materials can be
used at temperatures where they are partially
liquefied.
 e.g. silica firebricks, used to line steel-making
furnaces, are used at temperatures up to 1650°C
(3000°F), where some of the brick will be liquid.
Electrical insulation and dielectric behavior
 The majority of ceramic materials have no mobile
charge carriers, and thus do not conduct electricity.
 When combined with strength, this leads to uses in
power generation and transmission.
 Power lines are often supported from the pylons by
porcelain discs, which are sufficiently insulating to deal
with lightning strikes, and have the mechanical
strength to hold the cables.
 A sub-category of their insulating behavior is that of
the dielectrics.
 A good dielectric will maintain the electric field across
it, without inducting power loss.
 This is very important in the construction of
capacitors. Ceramic dielectrics are used in two main
areas.
 The first is the low-loss high-frequency dielectrics,
used in applications like microwave and radio
transmitters.
 The other is the materials with high dielectric
constants (the ferroelectrics).
Ceramics as semiconductor
 There are a number of ceramics that are
semiconductors. Most of these are transition metal
oxides that are II-VI semiconductors, such as zinc
oxide.
 One of the most widely used of these is the varistor.
These are devices that exhibit the unusual property of
negative resistance.
 Once the voltage across the device reaches a certain
threshold, there is a breakdown of the electrical
structure in the vicinity of the grain boundaries, which
results in its electrical resistance dropping from
several mega-ohms down to a few hundred.
 The major advantage of these is that they can
dissipate a lot of energy, and they self reset - after the
voltage across the device drops below the threshold,
its resistance returns to being high.
 This makes them ideal for surge-protection
applications. As there is control over the threshold
voltage and energy tolerance, they find use in all sorts
of applications.
 Ceramics as gas sensor

 Semiconducting ceramics are also

employed as gas sensors e.g.
magnesium ferrite doped with
manganese.
 When various gases are passed over a
polycrystalline ceramic, its electrical
resistance changes.
 With tuning to the possible gas mixtures, very
inexpensive devices can be produced.
 Superconductivity
 Under some conditions, such as extremely low
temperature, some ceramics exhibit superconductivity.
There are two major families of superconducting
ceramics.

 The complex copper oxides are exemplified by Yttrium

barium copper oxide, often abbreviated to YBCO, or 123
(after the ratio of metals in its stoichiometric formula
YBa2Cu3O7-x). It has a superconducting transition
temperature of 90K (which is above the temperature of
liquid nitrogen (77K)). The x in the formula refers to the
fact that fully stoichiometric YBCO is not a
superconductor, so it must be slightly oxygen-deficient,
with x typically around 0.3.

 The other major family of superconducting ceramics is

magnesium diboride. It is chemically very different from
all other superconductors in that it is neither a complex
copper oxide nor a metal. Because of this difference, it is
hoped that the study of this material will lead to
fundamental insights into the phenomenon of
superconductivity.
Ceramic capacitors
 For power signal wire and power plane decoupling in digital
electronics, ceramic and tantalum capacitors are considered as
the best solutions.

 For RF applications ceramic capacitors are common.

Ceramics do not suit for all applications, because most of
ceramics have strange effects, like changing capacitance with bias
voltage.

 Ceramic capacitors should not be used for analog circuits,

because they can distort the signal.

 For students to read themselves: Multilayer ceramic

capacitor, ceramic nanostructure and its applications,
piezoelectric ceramic sensor.
Dielectric Ceramics and Substrates
http://www.globalspec.com/

 Dielectric ceramics are electrical insulators with

dielectric strength, dielectric constant and loss
tangent values tailored for specific device or circuit
applications.

 In capacitor applications, ceramics (high K materials)

with a high dielectric constant are used to increase
the charge that can be stored.

 In microelectronic circuits, low dielectric constant or

low-K materials are sought to reduce inductive
crosstalk and noise generation in the circuit.

 In high voltage insulator applications, high electrical

resistivity (ohm-cm) and high dielectric strength (kV
per meter) is required.
 Dielectric ceramics and substrates have a wide range of
selection for materials.

e.g. : alumina, aluminum nitride, aluminum silicate, barium

neodymium titanate, barium strontium titanate (BST), barium
tantalate, barium titanate (BT), ………

 Dielectric ceramics and substrates can be supplied in bar,

disc, plate or slab, powder or grain, precursor, ring, rod, tube
or cylinder, ………..

 The metallization method for dielectric ceramics and

substrates can be electroplated, fired on or thick film,
patterned circuits, evaporated thin film, or sputtered thin
film.
Dielectric Strength
 Dielectric materials are insulators (conduction cannot
generally occur).

 However, under certain conditions, dielectric materials

can break down and conduct a significant current.

 Generally, the lattice of a dielectric has sufficient strength

to absorb the energy from impacting electrons that are
accelerated by the applied electric field.

 However, under a sufficiently large electric field, some

electrons present in the dielectric will have sufficient
kinetic energy to ionize (polarize) the lattice atoms
causing an avalanching effect.

 As a result, the dielectric will begin to conduct a

significant amount of current.
Dielectric Strength contd...
 This phenomenon is called dielectric breakdown and the
corresponding field intensity is referred to as the
dielectric breakdown strength.

 Dielectric strength may be defined as the maximum

potential gradient to which a material can be
subjected without insulating breakdown, that is

 dV  VB
DS    
 dx  max d
where DS is the dielectric strength in kV/mm,
VB the breakdown voltage, and d the thickness.
Current-voltage characteristic up to breakdown
Current-
for a typical dielectric materials
Dielectric Strength contd …
 Dielectric strength depends on

material homogeneity

specimen geometry

electrode shape and disposition

stress mode (ac, dc or pulsed) and

ambient condition.
High--K Dielectric
High
 The bit count of MOS DRAM devices is
continuously increasing. However, as bit count goes
up, capacitor cell area goes down.

 The capacitance per cell must remain in the

25-30 fF range, which means the capacitance
density must increase.

 One approach for DRAM manufacturing is to

replace the traditional silicon nitride + silicon oxide
with a higher dielectric constant (K) such as
tantalum pentoxide (Ta2O5), Hf-oxide (HfO2) and
Zr-oxide (ZrO2).
 High--K Dielectric
High
 High-K dielectric films are anticipated to be
required for certain applications with low power
consumption and low leakage current
specifications.

 High-K materials should be compatible with

conventional industry standard MOSFET process
flows using a poly-Si gate electrode.

 HfO2, ZrO2, and Ta2O5 as high-K gate-dielectrics.

Ferroelectric Ceramics
 A ferroelectric ceramic mixes the
smartness of a ferroelectric material and
the tailoring possibilities of ceramics.

 Since both kind of materials exhibit many

interesting properties, the mixture should
be good…
Ferroelectrics: ferroelectric domains
 Ferroelectric domains are generated by coupling between
dipole moments of atoms.

 When subjected to electric field, the domains pointing towards

its direction start to grow over its neighbouring domains.
Ferroelectrics: hysteresis loop
 Saturation and remanent
polarization
 Coercive field
 Possibility to reverse the
polarization
 Smart material: it keeps
information (remanent
poalrization)
Ferroelectrics: phase transition
 Ferroelectricity is a phase transition (Curie point)
 Ferroelectric phase has always lower symmetry
 Example: PbTiO3 (cubic changes into tetragonal)
 Ferroelectrics: summary

 Present spontanous polarization

 Polarization can be inversed
 Ferroelectric domains
 Hysteresis loop
 Ferroelectricity is a phase transition
 Piezoelectric and pyroelectric effect
 Why are ferroelectric ceramics so
important?
FERROELECTRICS CERAMICS
 High permittivities  Broad range of chemical
 Spontaneus polarization composition
 Electric conducticity can be  Control of grain size,
controlled porosity…
 Piezoelectric and  Possibility of varying its
pyroelectric effect shape and size.
 Optical anisotropy, electro-  High resistance to
optic an photorefractive abrasion
effect  Excellent hot strength
 Chemical inertness
All these properties lead to a lot of
potential applications!
of Ferroelectric
ceramics
1. General Procedure of Processing
 Raw
Materials
 Mixing
 Calcining
means heating

 Character
-ization
 Milling
 Poling
 Sintering

 Binder
Burnout
Poling
 Ferroelectric ceramic does not show any
piezoelectricity when it is cooled after
sintering.
 Piezoelectric behavior can be induced in a
ferroelectric ceramic by a process called
"poling" .
 In this process, a direct current (dc)
electric field with a strength larger than
the coercive field strength is applied to
the ferroelectric ceramic at a high
temperature, but below the Curie point.
Characterization
On the application of the external dc
field the spontaneous polarization
within each grain gets orientated
towards the direction of the applied
field.
This leads to a net polarization in the
poling direction.
 Two special important
methods widely uses in the
labs .

1. Metal Organic Decomposition

(MOD)
2. Hot-pressed solid - state
sintering method
Most Common Commercial Ferroelectric Ceramic

Lead Zirconate Titanate (PZT)

 Chemical formula Pb Zrx Ti1-x O3
 “Perovskite” ABO3
 A and B are different in size
A cation is at centre
B cation is at the corner
O atom are at centre of unit cell faces.
Elastic Deformation
1. Initial 2. Small load 3. Unload

bonds
stretch

return to
initial

F
F Linear-
Elastic means reversible.
elastic
Non-Linear-
elastic

35
Important Mechanical Properties
from a Tensile Test
 Young's Modulus: This is the slope of the linear
portion of the stress-strain curve, it is usually
specific to each material; a constant, known value.
 Yield Strength: This is the value of stress at the
yield point, calculated by plotting Young's modulus at
a specified percent of offset (extension under load)
(usually offset = 0.2%).
 Ultimate Tensile Strength: This is the highest
value of stress on the stress-strain curve.
 Percent Elongation: This is the change in gauge
length divided by the original gauge length.

36
 Stress-Strain Diagram
ultimate
tensile
strength 3 necking
 UTS
Strain
yield Hardening Fracture
strength
y 5
2
Elastic region
Plastic slope =Young’s (elastic) modulus (E)
Region
yield strength
Plastic region
ultimate tensile strength
Elastic strain hardening
Region
σ Eε fracture
4
σ 1
E
ε E
σy  ) (DL/Lo)
Strain (
ε 2  ε1
38
 Stress-Strain Diagram (cont)
• Elastic Region (Point 1 –2)
- The material will return to its original shape
after the material is unloaded (like a rubber band).
- The stress is linearly proportional to the strain in
this region.

σ Eε or E
σ
ε
σ : Stress
E : Elastic modulus (Young’s Modulus)

ε : Strain
- Point 2 :Yield Strength : a point where permanent
deformation occurs. (If it is passed, the material will
no longer return to its original length.)
Stress-Strain Diagram (cont)
• Strain Hardening
- If the material is loaded again from Point 4, the curve
will follow back to Point 3 with the same Elastic Modulus
(slope).
- The material now has a higher yield strength of Point 4.
- Raising the yield strength by permanently straining the
material is called Strain Hardening.
Stress--Strain Diagram (cont)
Stress cont)

• Tensile Strength (Point 3)

- The largest value of stress on the diagram is
called Tensile Strength(TS) or Ultimate Tensile
Strength (UTS)
- It is the maximum stress which the material
can support without breaking.
•Fracture (Point 5)
- If the material is stretched beyond Point 3,
the stress decreases as necking and non-
uniform deformation occur.
- Fracture will finally occur at Point 5.
Strain Hardening

An increase in y due to plastic

deformation.


large hardening
y
1
y small hardening
0

d
reload
unloa


hardening exponent:
T  C T   n n=0.15 (some steels)
to n=0.5 (some copper)
“true” stress (F/A) “true” strain: ln(L/Lo)
Hardness
 Hardness is a measure of a material’s
resistance to localized plastic deformation (a
small dent or scratch).
 Quantitative hardness techniques have been
developed where a small indenter is forced
into the surface of a material.
 The depth or size of the indentation is
measured, and corresponds to a hardness
number.
 The softer the material, the larger and deeper
the indentation (and lower hardness number).44
Summary
• Stress and strain: These are size-independent
measures of load and displacement, respectively.

• Elastic behavior: This reversible behavior often

shows a linear relation between stress and strain. To minimize
deformation, select a material with a large elastic modulus (E
or G).

• Plastic behavior: This permanent deformation

behavior occurs when the tensile (or compressive)
uniaxial stress reaches y.

• Toughness: The energy needed to break a unit volume of

material.

• Ductility: The plastic strain at failure.

45
Application of Ceramics

Whitewares
 Crockery
 Floor and wall tiles
 Sanitary-ware
 Electrical porcelain
 Decorative ceramics
Refractories
Firebricks for furnaces and ovens. Have
high Silicon or Aluminium oxide content.
Brick products are used in the
manufacturing plant for iron and steel,
non-ferrous metals, glass, cements,
ceramics, energy conversion, petroleum,
and chemical industries.
Amorphous Ceramics
(Glasses)
 Main ingredient is Silica (SiO2)
 If cooled very slowly will form crystalline structure.
 If cooled more quickly will form amorphous structure
consisting of disordered and linked chains of Silicon and
Oxygen atoms.
 This accounts for its transparency as it is the crystal
boundaries that scatter the light, causing reflection.
 Glass can be tempered to increase its toughness and
resistance to cracking.
Crystalline Ceramics
Good electrical insulators and refractories.
 Magnesium Oxide is used as insulation material
in heating elements and cables.
 Aluminium Oxide
 Beryllium Oxides
 Boron Carbide
 Tungsten Carbide.
 Used as abrasives and cutting tool tips.
Abrasives
 Natural (garnet, diamond, etc.)
 Synthetic abrasives (silicon carbide,
diamond, fused alumina, etc.) are used for
grinding, cutting, polishing, lapping, or
pressure blasting of materials
Cements
 Used to produce concrete roads, bridges,
buildings, dams.
Advanced Ceramics

 Advanced ceramic materials have been developed over the

past half century

 Applied as thermal barrier coatings to protect metal

structures, wearing surfaces, or as integral components by
themselves.

 Engine applications are very common for this class of

material which includes silicon nitride (Si3N4), silicon carbide
(SiC), Zirconia (ZrO2) and Alumina (Al2O3)

 Heat resistance and other desirable properties have lead to

the development of methods to toughen the material by
reinforcement with fibers and whiskers opening up more
applications for ceramics
Advanced Ceramics
 Structural: Wear parts, bioceramics, cutting tools,
engine components, armour.
 Electrical: Capacitors, insulators, integrated
circuit packages, piezoelectrics, magnets and
superconductors
 Coatings: Engine components, cutting tools, and
industrial wear parts
 Chemical and environmental: Filters, membranes,
catalysts, and catalyst supports