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UNIT-5
DIELECTIC MATERIALS
Dielectric Materials are Insulators with no free electrons/with too low
concentration of the free electrons
When placed in electric field they are polarized.
They are non-conductors of electricity with special properties in the
presence of the electric field.
Example: Glass, mica, polymers
POLAR DIELECTRICS
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This materials posses permanent dipole moment even in the
absence of electric field.
Lack of centre of symmetry
 Polarization of molecule strongly
Example: HCl, H2O
depends on the temperature.
WATER MOLECULE
 Molecules absorb and
radiation in the IR region
emit
NON-POLAR DIELECTRICS

This materials Does not posses permanent dipole moment
and have centre of symmetry.
They polarized only when placed in electric field
Example:H2, N2, CO2, CCl4
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
O
+
C
P1
O
P2
P=0
CO2 molecule has zero dipole
momentum
The polarization of non-polar
molecules is independent of
temperature.
They don’t absorb or emit
radiation of in the infrared
region
NON-POLAR DIELECTRICS IN AN ELECTRIC FIELD
• When a non-polar dielectric is placed in an electric field E 0 between the plates of
the parallel plate capacitor, then due to polarization surface charges appears.
• Positive charge is induced on one surface while negative charge is induced on
other surface.
• The electric field E’ setup by surface charge opposes the external electric field
E0.
• Then the resultant electric field in the dielectric is E= E0 - E’
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DIELECTRIC CONSTANT
• Dielectric constant is the ratio between the permittivity of the medium
to permittivity of the free space.



r
o
• It is also defined as the capacitance of the capacitor with dielectric to
capacitance of the same capacitor without dielectric.
K
C
C0
•
It is a measure of the electric polarization in the dielectric material and it
has no units.
•
Materials with higher dielectric constant are easily polarized and
behaves as good electrical insulators
DIFFERENT TYPES OF POLARIZATION IN DIELECTRICS

The process of producing electric dipoles by an electric field is called
polarization in dielectrics.

The strength of induced dipole momentum is proportional to the applied
field.
  E
where α, is called as polarizability.
P  N
Polarization occur due to several microscopic polarization mechanisms:
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Electronic Polarization (Pe)
Ionic Polarization (Pi)
Orientation Polarization (Po)
Space charge Polarization (Ps)
ELECTRONIC POLARIZATION

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It is defined as an electric strain produced in an atom by the application of electric
field.
It results from the displacement of nucleus (+Ze) and electrons (-Ze) in opposite
direction in the presence of the applied field with the creation of dipole moment.
The magnetic moment induced is proportional to the strength of field applied
e  E
e   e E
where αe is electronic polarizability of the material which is given by  o ( r  1)
N
N is number of atoms per cm3
Polarization is
Pe  N e E
It is independent of temperature.
Monoatomic molecules exhibits this type of polarization.
+
Nucleus of charge +Ze
NO FIELD
-
+
ELECTRIC FIELD
Center of electron cloud
of chare -Ze
IONIC POLARIZATION
 Ionic polarization occur due to the displacement of cation and anion in opposite
directions with the application of electric field in the ionic solids.
 This is observed in the materials that posses symmetric molecules.
 It does not occur in typical covalent crystals such as diamond.
 Ionic polarization is independent of temperature.
 The ionic polarization is
+
+ + - - +
+
+
+ - +
NO FIELD
e2 E  1 1 
Pi  2   
0  M m 
-
- +
+
- -
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- +
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ELECTRIC FIELD
ORIENTATIONAL POLARIZATION

It is due to the presence of polar molecules in the dielectric material
which have permanent dipole moment.

When electric field is applied on the dielectric material, it tries to align
the dipole in its direction that results in the existence of dipole moment
in the material.

It occurs in asymmetric molecules.

Its depends on the temperature.
Po 
No Field
2
μ is permanent dipole moment
3KT
Electric Field
SPACE POLARIZATION
 It occurs due to the accumulation of electric charges at the interfaceof
a multiphased material.
 This is possible when one of the phases present posses much higher
resistivity than the other.
 It is found to occur in ferrites and semiconductors.
No Field
Electric Field
The total polarization is P=Pe+Pi+Po+Ps
FREQUENCY DEPENDENCE OF POLARIZABILITY
 On the application of alternating electric field the polarization process
occur as a function of time.
 Electronic polarizability is extremely rapid and is complete at any
instant of time even when the frequency of the voltage is very high in
the optical ranges. Thus it occurs at all frequencies.
 Ionic polarizability is slower and the ions do not respond at all when the
voltage correspond to visible frequencies. So it does not occur at visible
frequencies.
 Orientational polarization is slower than the ionic polarizability and
occurs only at frequencies which are smaller than the infrared
frequencies.
 Space charge polarization is slower process and occurs only at lower
frequencies (50-60 Hz).
 The total polarizability is very high at low frequencies and very low at
higher (optical) frequencies.
Frequency Response (Switching Time)
Space charge
Polarization
Orientational Polarization
POLARIZABILITY
Ionic Polarization
Electronic Polarization
Radio
&
Microwave
IR
Visible
FREQUENCY
UV
LOCAL FIELD
 In dielectric materials the atoms or molecules are experience not only the
external electric field but the electric field produced by the dipoles as well.
 Local or Internal field in a dielectric material is the space and time
average of electric field acting on a particular molecule or atom in
the dielectric substance
 The local field intensity Ei is larger than the microscopic intensity E, since
Ei excludes the molecule’ own field which is in the direction opposite to E.
 To find the local field let us consider a small spherical cavity inside the
dielectric material as shown in figure.
 This local field is a sum of four components.
Eloc  Eex  E p  EL  Enear
Eex – External electric field applied
Ep – Field produced by charges on the surface of the specimen
P
E pol  
O
Enear – Field produced by the dipole inside the sphere and depends on the crystal
symmety. For cubic crystal this field is zero.
EL - It is due to polarization of charges on the surface of a fictitious cavity cutout of
the specimen (spherical cavity). It is called Lorentz cavity field
For a charged non-conducting sphere, the field produced is given by
For εr=1
P
3 r . O
P
EL 
3 O
EL 
Eloc  Eex 
P

Eloc  Eo 
where,
Eo  Eex 

O
P
3 O
P
3 O
P
o
Eo is the homogeneous field averaged over the whole volume of the material.
CLAUSIUS-MOSOTTI EQUATION
This equation relates the microscopic parameter (polarizability, α) with the
microscopic parameter (dielectric constant, εr) of a dielectric material.
Electric dipole momentum is given by
    Eloc
Polarization is given by
Local field is given by
From (1) & (2)
P  N  Eloc
Eloc  Eo 
P
3 o

P 

P  N   EO 
3 O 

N    EO
P
1  N . / 3 O
(1)
(2)
(3)
(4)
By the definition of polarization of a material
P   O    E   O  ( r  1)  E
Then from the eq.(4) and (5)
N   r 1

3 O
r  2


 3
This the called as Clausius-Mosotti Equation
(5)
FERROELECTRIC MATERIALS
 Most of the materials are polarized linearly by an external
electric field; nonlinearities are insignificant. This is called
dielectric polarization
Dielectric polarization
 Some materials, exhibit nonlinear polarization they are known
as Paraelectric materials. The electric permittivity
corresponding to the slope of the polarization curve, is a
function of the external electric field.
 In addition to being nonlinear, ferroelectric materials
demonstrate a spontaneous (zero field) polarization.
Paraelectric polarization
 The distinguishing feature of ferroelectrics is that the
direction of the spontaneous polarization can be reversed by
an applied electric field, yielding a hysteresis loop.
Ferroelectric polarization

Certain dielectric materials acquire enormous value of induced
dipole moment in a weak electric field and posses Spontaneous
polarization in the absence of external electric field.

This phenomenon is known as ferroelectricity.

The materials exhibiting this phenomenon are known as
ferroelectric materials.
PROPERTIES OF FERROELECTRIC MATERIALS
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Ferroelectric materials can be easily polarized even by weak magnetic fields.
They exhibit hysteresis.
They exhibit domain structure as in the case of ferromagnetic materials.
They posses spontaneous (zero field) polarization.
•
The spontaneous polarization vanishes above a particular temperature Tc called
Curies temperature.
At the temperatures above the Curies temperature they behave as paraelectric
materials.
Above the Curies temperature the dielectric constant varies with the
•
•
temperature.
•
r 
C
T  Tc
Ferroelectric materials exhibits piezoelectricity and pyroelectricity.
Piezoelectricity – creation of electric polarization by mechanical stress.
examples: Quartz, Lithium niobate
Pyroelectricity – creation of electric polarization by thermal stress.
examples: BaTiO3, Trigylcine sulphate
BaTiO3 PROPERTIES
 The properties of the ferroelectric materials can be explained by
studying the properties of BaTiO3.
 The most significant property of ferroelectric materials is the anomalous
dependence of the dielectric constant (εr) on the temperature.
 The value of curies temperature (Tc) of BaTiO3 is nearly 1200C. The
dielectric constant is maximum at this temperature.
 Above this temperature, the dielectric constant decreases. This is
because upto Tc it exhibits spontaneous polarization and hence εr
increases. After Tc due to change in it’s structure it loses spontaneous
polarization and εr decrease. This can be explained change in it
structure.
 There is an intimate relation between the ferrroelectric properties
and the atomic arrangement.
 Barium titanate has a cubic crystal structure.
 Ba2+ are at the corners of cubic structure with each Ti4+ ion
surrounded by six O2- ions in an octahedral configuration.
 The TiO6 octahedron is symmetrical and hence net dipole moment
is zero.
 As it cooled below Tc the Ti4+ and Ba2+ ions moves with respect to
one O2- ions.
 The X-ray and neutron diffraction data show that the titanium and
barium ions all move up 2.8 percent, and the oxygen ions move
down 1percent.
 The structure is tetragonal is now tetragonal with lack of centre of
symmetry and hence a spontaneous dipole moment exists.
Fig. The structural changes occurred in BaTiO3 when its
temperature falls below Curie temperature (~1200C)