TOPIC: DIELECTRICS:

- Ferroelectrics
- Piezo electrics
- Pyroelectrics
- Thermoelectric materials
- Magnetic electrics
- Electrostriction
➢ Dielectrics
Dielectrics are insulating materials having electric dipole moment permanently or temporarily by
applying the electric field.
They mainly used to store electrical energy and used as electrical insulators.
All dielectrics are electrical insulators, but all electrical insulators need not to be dielectrics.
Dielectrics are non - metallic materials of high specific resistance and have negative temperature
coefficient of resistance.
A conductor material starts conducting when electricity is applied to it whereas an insulator
opposes the flow of electricity as it does not have any free moving electrons in its structure.
Dielectric is a special type of insulator that does not conduct electricity but gets polarized when
subjected to electricity.
In dielectrics, when subjected to the electric field the positive charges present in the material
gets displaced in the direction of the applied electric field. The negative charges are shifted in the
direction opposite to the applied electric field. This leads to Dielectric polarization.
In dielectric material, electric charges do not flow through the material. Polarization reduces the
overall field of the dielectric.
Note

- The electrical conductivity of a perfect dielectric is zero.

- A dielectric stores and dissipate the electrical energy similar to an ideal capacitor.
- Electric Susceptibility - How easily a dielectric material can be polarized when subjected
to an electric field is measured by the electric susceptibility. This quantity also determines
the electric permeability of the material.
➢ Dielectric Polarization
In the dielectric material, most of the electrons are bound to the nuclear. When an external
electric field is applied then the bound electrons of an atom are displaced such that the centroid
of the electronic cloud is separated from the centroid of the nucleus. hence, an electric dipole is
created and the atom is said to be polarized.

Where, χe is a dimensionless parameter, known as the electric susceptibility. It measures the

ability of the material to become polarized and differed from one dielectric to another.
An electric dipole moment is a measure of separation of negative and positive charge in the
system.
The relationship between the dipole moment (M) and the electric field (E) gives rise to the
properties of dielectric.
When the applied electric field is removed the atom return to its original state. This happens in
an exponential decay manner.
Types of polarization

• Electronic polarization - occurs due to the displacement of positively charged nucleus

and negatively charged electron in opposite directions by an external electric field. It
creates a dipole moment in the dielectric. It is very fast and is completed at any instant
of time even when the frequency of the voltage is very high in the optical range (1015 Hz).
Thus it occurs at all frequencies.
• Ionic Polarization. It arises due to the displacement of cations (+ve) and anions (-ve) in
opposite directions and occurs in ionic solids in the presence of electric field, is called
ionic polarization. Example: NaCl, KCl crystal.
It is slower and the ions do not respond when the voltage corresponds to visible optical
frequencies, i.e., the electric field changes in polarity is very fast, so that the ions are not
able to reorient themselves due up to the field. So the ionic polarization does not occur
at visible optical frequencies. It occurs only at frequencies less than 10 13 Hz.

• Orientation Polarization - When an electric field is applied on the dielectric medium with
polar molecules, the electric field tries to align these dipoles along its field direction as
shown in figure. Due to this there is a resultant dipole moment. It is even slower than
ionic polarization and occurs only at electrical frequencies (audio and radio frequencies
106 Hz). This polarization depends on temperature. When the temperature is increased,
the dipoles are aligned in random directions.
 • Space-charge polarization - occurs due to the diffusion of ions along the field direction
giving rise to redistribution of charges in the dielectric. It is the slowest process because
the ions have to diffuse (jump) over several inter atomic distances. This occurs at very low
frequencies of 50 - 60 Hz (power frequencies). Normally this type of polarization occurs
in ferrites and semiconductors and it is very small when compared to other types of
polarization.

Thus at low frequencies all the four polarizations will occur and the total polarization is very high,
but at high frequencies, the value of the total polarization is very small. The following graphs
show the frequency dependence of polarization mechanism and the corresponding power losses
at those frequencies.
Note:
• At visible range of frequencies only electronic polarization is effective. I.e. at f ≥ 5 ×
1014;P=Pe
• Ionic polarization is effective up to infrared range of frequencies i.e. at f < 1014 Hz; P=Pe
+Pi
• Orientational polarization is effective up to radio frequency range i.e. at f < 1010 Hz ; P=Pe
+Pi+P0
• Space charge polarization is effective up to audio frequency range i.e. at f < 103Hz ; P=Pe
+Pi+P0+Ps
Comparison of different polarization mechanisms
Dielectric Dispersion – is the dependence of the permittivity of a dielectric material on the
frequency of an applied electric field.
When P is the maximum polarization attained by the dielectric, tr is the relaxation time (time
taken for a polarization process to reach 0.63 of the maximum value) for a particular polarization
process. The relaxation times are different for different kinds of polarization mechanisms. The
dielectric polarization process can be expressed as
P (t) = P[1-exp(-t/tr )]
The relaxation time varies for different polarization processes. Electronic polarization is very
rapid followed by ionic polarization. Orientation polarization is slower than ionic polarization.
Space charge polarization is very slow.
Dielectric Breakdown - When higher electric fields are applied, the insulator starts conducting
and behaves as a conductor. In such conditions, dielectric materials lose their dielectric
properties. This phenomenon is known as Dielectric Breakdown. It is an irreversible process. This
leads to the failure of dielectric materials.
➢ Properties of Dielectrics
A good dielectric material should have:

- good dielectric constant

- dielectric strength
- low loss factor
- high-temperature stability
- high storage stability
- good frequency response
- should be amendable to industrial processes
➢ Types of Dielectric Material
Dielectrics are categorized based on the type of molecule present in the material. There are two
types of dielectrics – Polar dielectrics and Non-polar dielectrics.
i) Polar Dielectrics
In polar dielectrics, the center of mass of positive particles does not coincide with the center of
mass of negative particles. Here the dipole moment exists. The molecules are asymmetrical in
shape. When the electric field is applied the molecules align themselves with the electric field.
When the electric field is removed random dipole moment is observed and the net dipole
moment in the molecules becomes zero. Examples are H2O, CO2, etc.…
ii) Non-Polar Dielectrics
In the non-polar dielectrics, the center of mass of positive particles and negative particles
coincide. There is no dipole moment in these molecules. These molecules are symmetrical in
shape. Examples of non-polar dielectrics are H2, N2, O2, etc.…
➢ Examples of Dielectric Material
Dielectric materials can be solids, liquids, gases, and vacuum.
✓ Solid dielectrics are highly used in electrical engineering. Some examples of sold
dielectrics are porcelain, ceramics, glass, paper, etc.…
✓ Dry air, nitrogen, sulfur hexafluoride and the oxides of various metals are examples of
gaseous dielectrics.
✓ Distilled water, transformer oil are common examples of liquid dielectrics.

i) Ferroelectrics
Below certain temperature it is found that some materials spontaneously acquire an electric
dipole moment. These materials are called as ferroelectric materials or ferroelectrics. The
temperature at which ferroelectric property of the material disappears is called as ferroelectric
curie temperature.
Ferro electricity refers to the creation of large induced dipole moment in a weak electric field as
well as the existence of electric polarization even in the absence of an applied electric field.
Crystalline dielectric materials which possess a permanent electric polarization are called
ferroelectric materials. They have electric dipole moment even in the absence of any field.
Normally they are anisotropic crystals which exhibit spontaneous polarization.
Examples:

- Barium Titanate (BaTiO3)

- Potassium Dihydrogen Phosphate (KDP)
- Ammonium Dihydrogen Phosphate (NH4H2PO4)
- Lithium Niobate (LiN6O3)
Properties of Ferroelectric Materials
i) Ferroelectric materials exhibits spontaneous polarization i.e., they are polarized even in
the absence of electric field.
ii) They exhibit dielectric hysteresis. The lagging of polarization behind the applied electric
field is called dielectric hysteresis.
 iii) Above ferroelectric Curic temperature, ferroelectric material becomes Para electric
material.
iv) They possess Ferro – electric domain structure.
v) They can be polarized even by very weak electric field
vi) Ferroelectric materials exhibit piezoelectricity and Pyro electricity. Piezoelectricity means
the creation of electric polarization by mechanical stress. Pyro electricity means the
creation of electric polarization by thermal stress.
Hysteresis of Ferroelectric Materials
The ferro electrics are known as non –linear dielectrics. Such materials exhibit hysteresis curve
similar to that of ferro magnetic materials.
The lagging of polarization ‘P’ behind the applied electric field E is called dielectric hysteresis.

Fig: Dielectric Hysteresis in ferroelectric materials

When a ferro – electric material is subjected to external electric field (E) the polarization (P)
increases with respect to the field applied and it reaches the maximum value ‘OA’.
If now the applied electric field is reduced, the polarization also decreases from A, and when E
becomes zero a small amount of polarization exists in the material, spontaneous (or) residual
polarization.
In order to reduce the value of polarization to zero, a reversing electric field OC should be applied.
This field is known as coercive field.
Thus the variation of P with respect to E traced along the closed path ABCDEFA in one full cycle
of polarization and depolarization is called hysteresis or the hysteresis curve.
Application of Ferroelectric Materials
i) Ferroelectric materials are used to make pressure transducers, ultrasonic transducers,
microphones and gas filters.
ii) They are used as memory cores in computers.
iii) They are used to measure and control temperature.
 iv) Ferroelectric ceramics are used as capacitors to store electrical energy
v) They are used to make very good infrared detectors.
vi) Rochelle salt is used in devices like microphones, strain gauges, phonograph pickups and
SONAR devices.
vii) In optical communication, the ferroelectric crystals are used for optical modulation.
viii) Ferro electric materials are used to produce ultrasonics
ix) Electrets (a permanently polarized piece of dielectric material, analogous to a permanent
magnet.) are also used to bond the fractured bones in human body.
x) They are used as frequency stabilizers and crystal controlled oscillators.

ii) Piezo electrics

Piezoelectric materials are materials that produce an electric current when they are placed under
mechanical stress. The piezoelectric process is also reversible, so if you apply an electric current
to these materials, they will actually change shape slightly (a maximum of 4%).
There are several materials that we have known for some time that possess piezoelectric
properties, including bone, proteins, crystals (e.g. quartz) and ceramics (e.g. lead zirconate
titanate).
Applications: Single crystal of quartz is used for filter, resonator and delay line applications.
Natural quartz is now being replaced by synthetic material.

Rochelle salt is used as transducer in gramophone pickups, ear phones, hearing aids,
microphones etc. the commercial ceramic materials are based on barium titanate, lead zirconate
and lead titanate.
They are used for high voltage generation (gas lighters), accelerometers, transducers etc. Piezo
electric semiconductors such as GaS, ZnO & CdS are used as amplifiers of ultrasonic waves.
iii) Pyroelectrics
Pyro electricity or pyroelectric material is an electric response of polar dielectric with a change in
temperature. If the temperature changes, it causes the movement of atoms from there neutral
position hence the polarization of the material changes and a voltage is observed across the
material. This effect is temporary.
If the temperature remains constant at its new value, the pyroelectric voltage becomes zero due
to the leakage current. So, within this same temperature limits, the charges developed by the
effect of heating or cooling are equal and opposite.
Pyroelectric materials exhibit spontaneous polarization i.e. polarization in the absence of electric
field, this cannot be changed or reversed on applying the electric field.
 ➢ Difference between Piezoelectric, Pyroelectric and Ferroelectric Materials

Parameters Piezoelectric Pyroelectric Ferroelectric

Piezoelectric materials Pyroelectric material
generate electricity generates electric Ferroelectric material exhibits
whenever mechanical potential whenever electric polarization even in the
Function stress is applied. heated or cooled. absence of an electric field.
Quartz crystal,
Ammonium,
Examples Lithium niobite,
Quartz, crystal, Phosphate. Barium Titanite
Ammonium, Phosphate
They are
unidirectional
polarization,
Non-centrosymmetric, non-centrosymmetric, They are easily polarized,
Non-polar dielectric, They exhibit dielectric hysteresis,
It exhibits pyro
Presence of Piezoelectric electricity when T >= They are both pyro and
effect. Tc piezoelectric in nature.
Properties
IR detectors,
Acts like a transducer, Image tubes, Ultrasonic transducers
Used in microphones, They are pressure transducer
Temperature sensing
It generates ultrasonic elements. It acts as a memory device like a
Applications waves. random access memory.

iv) Thermoelectric materials

In the case of thermoelectricity the two ends of the device are subjected to two different
temperatures resulting in a permanent voltage in the device resulting in as there is the
temperature difference.
➢ Electrostriction
It is the property of all electrical nonconductors, or dielectrics, which manifests itself as a
relatively slight change of shape, or mechanical deformation, under the application of an electric
field. Reversal of the electric field does not reverse the direction of the deformation.
It is caused by displacement of ions in the crystal lattice upon being exposed to an external
electric field. Positive ions will be displaced in the direction of the field, while negative ions will
be displaced in the opposite direction. This displacement will accumulate throughout the bulk
material and result in an overall strain (elongation) in the direction of the field. The thickness will
be reduced in the orthogonal directions characterized by Poisson's ratio. All insulating materials
consisting of more than one type of atom will be ionic to some extent due to the difference of
electronegativity of the atoms, and therefore exhibit electrostriction.
The resulting strain (ratio of deformation to the original dimension) is proportional to the square
of the polarization. Reversal of the electric field does not reverse the direction of the
deformation.
➢ Applications of dielectric materials
The dielectric materials have three major applications.
i) They are used as a dielectric media in capacitors.
ii) They are as insulating materials in transformers.
iii) They are used in industries and dielectric heating.
i) Dielectrics in Capacitors
Thin sheets of papers filled with synthetic oils are used as dielectrics in the capacitors.
Tissue papers and poly propylene films filled with dielectrol are used in power capacitors.
Mica is used as dielectrics in discrete capacitors.
An electrolytic solution of sodium phosphate is used in wet type electrolytic capacitors.
For dielectrics, to be used in capacitors, it should possess the following properties.

- It must have high dielectric constant

- It should possess high dielectric strength
- It should also have low dielectric loss.

ii) Insulating materials in transformers

PVC (Poly Vinyl Chloride) is used to manufacture pipes, batteries, cables etc.
Liquid dielectrics such as petroleum oils, silicone oils are widely used in transformers, circuit
breakers etc.
Synthetic oils such as askarels, sovol, etc., are used as a coolant and insulant in high voltage
transformers.
Gases such as nitrogen, Sulphur hexafluoride are used in x-ray tubes, switches, high voltage gas
filled pressure cables, coolants respectively.
For dielectrics to act as insulating materials, it should possess the following properties.

- It should have low dielectric constant

 - It should possess low dielectric loss
- It must have high resistance
- It must possess high dielectric strength
- It should have adequate chemical stability
- It must have high moisture resistance

iii) Industrial application

Dielectrics possessing piezoelectric effect is used in gas lighters, microphones, phonographs.
Dielectrics possessing inverse Piezo electric effect is used in quartz watchers, ultrasonic dryers,
cleaning the semiconducting wafers, ultrasonic transducer etc.
iv) Dielectric Heating
Dielectric heating is the process of heating the insulating materials at a very high voltage under
suitable frequency at which dielectric loss becomes maximum, so that the dielectric loss will
come out in the form of heat. Hence adequate heating was done at high voltages.
Dielectric heating is the principle used in microwave oven.
Dielectric heating is also used in the dehydration of food, tobacco etc.
➢ Insulators
Required Qualities of Good Insulating Materials: The required qualities can be classified as under
electrical, mechanical, thermal and chemical applications.
i) Electrical:
- electrically the insulating material should have high electrical resistivity and high dielectric
strength to withstand high voltage.
- The dielectric losses must be minimum.
- Liquid and gaseous insulators are used as coolants. For example, transformer oil,
hydrogen and helium
ii) Mechanical:
- insulating materials should have certain mechanical properties depending on the use to
which they are put.
- When used for electric machine insulation, the insulator should have sufficient
mechanical strength to withstand vibration.
iii) Thermal:
- Good heat conducting property is also desirable in such cases. The insulators should have
small thermal expansion and it should be non-ignitable.
iv) Chemical:
- chemically, the insulators should be resistant to oils, liquids, gas fumes, acids and
alkalis.
- The insulators should be water proof since water lowers the insulation resistance and
the dielectric strength.