Download i. Electronic Polarization

Dielectric Materials
Dielectric materials are also called as insulators.
 In dielectric materials, all the electrons are tightly bound to their parent
molecules and there are no free charges.
Not possible for the electrons in the valence band to excite to the
conduction band, by crossing the energy gap, even with normal voltage or
thermal energy.
A dielectric material (dielectric for short) is an electrical insulator that can
be polarized by an applied electric field When a dielectric is placed in an
electric field, electric charges do not flow through the material as they do in
a conductor, but only slightly shift from their average equilibrium positions
causing dielectric polarization
Note that during the polarization the charges in the dielectric medium are displaced from their
equilibrium positions by distances that are considerably less than the atomic diameter . Thus
there is no net transfer of charge over large distances which occurs when s current is sent up in
conductor
 WHAT IS Electric POLARIZATION???
The displacement of positive and negative charges in opposite directions in
a dielectric.
P=p/V
Basic Definitions in Dielectrics
Electric Field
The region around the charge within which its effect is
felt or experienced is known as electric field.
The electric field is assumed to consist of imaginary electric
lines of force. These lines of force originate from the positive
charges and terminate to the negative charges .
Relative permittivity is the factor by which the electric field between the
charges is decreased relative to vacuum
In electromagnetism, permittivity or absolute permittivity is the measure of
that is encountered when forming an electric field in a medium. In
resistance
other words, permittivity is a measure of how an electric field affects, and is
affected by, a dielectric medium. The permittivity of a medium describes how
much electric field is 'generated' per unit charge in that medium. More electric
flux exists in a medium with a low permittivity because of polarization effects.
Permittivity is directly related to electric susceptibility, which is a measure of how
easily a dielectric polarizes in response to an electric field. Thus, permittivity
relates to a material's ability to resist an electric field
r 

0
Dielectric Constant
The dielectric constant or relative permittivity of a material
determines its dielectric characteristics. It is the ratio of the
permittivity of the medium and the permittivity of free space
Various Polarization mechanisms
Dielectric material is made up of atoms or molecules that possess one or
more these types of electric polarization, Polarization come when centre
of gravity of charges shifts and its overlapped for symmetrical materials
and centre of gravity can be shifted when materials is deformed and .
Four types of microscopic polarization mechanisms.
Electronic polarization
Ionic polarization
Orientation polarization and
Space-charge polarization.
i.
Electronic Polarization

Electronic Polarization occurs due to the displacement of positively
charged nucleus and negatively charged electrons in opposite directions,
when an external electric field is applied, and thereby a dipole moment is
created in the dielectric.
The induced dipole moment µ = eE
where e = electronic polarizability.

Monoatomic gases exhibit this kind of polarization, Electronic
polarizability is proportional to the volume of the atoms and is independent
of temperature.

The electronic polarizability = e = 4e0R3 ( Farad.m2) where R
is the radius of the atom.
Nucleus
Displaced
Equilibrium
position
Nucleus
x
Sphere of electronic
charge
+Ze
-Ze
Field direction
Original Position
Fig. (a) Position of +ve and –ve charges in an atom without field (b)
Position of +ve and –ve charges in an atom with field
Suppose that the electron cloud of charges -Zq is uniformly distributed in a
sphere of radius R and that its center of gravity originally coincided with that of
the nucleus, and suppose that it is displaced by the field to a distance d from the
center of the nucleus, This displacement of the electrons Dx, resulting from
electronic polarization, always generates an elastic restoring force tending to
bring the electrons back to their equilibrium position.
The noble gas atoms, such as He, Ne, Ar, Kr, Xe, H, Li, Na, K, Rb, Single
atoms in gases may be considered spherical in shape, but molecules, each
comprising two or more atoms, are generally not spherical.
The field strength produced by the nucleus charges and experienced by the
electrons is Zq/4peoR2 is larger than 109 V/cm. This field strength is far larger
than the applied field. This may be the reason why the induced dipole moment
for electronic polarization is proportional to the applied field and independent of
frequency, because the time required for this polarization to occur is of the
order of 10-15 sec
Temperature independent intermolecular phenomenon since to make it shift
again the nuclear force an equal and but in opposite direction force has to be
applied
ii. Ionic Polarization
Ionic polarization arises due to the displacement of -ve ions
and + ve ions in opposite directions and it occurs in ionic
solids, in the presence of electric field.
The displacement is independent of temperature. Example :
NaCl crystal
-
+
Cl
Na
-
+
x2
x1
Fig. (a) Without field (b) With field
Each Na+ - Cl– pair is a natural dipole, no matter
how you pair up two atoms. The polarization of a
given volume, however, is exactly zero because for
every dipole moment there is a neighboring one with
exactly the same magnitude, but opposite sign.Note
that the dipoles can not rotate; their direction is
fixed.
In an electric field, the ions feel forces in
opposite directions.
 For a field acting as shown, the lattice distorts
a little bit
 Na+ ions moved a bit to the right, the Cl– ions
to the left.
The dipole moments between adjacent NaCl pairs in field direction are now different and there
is a net dipole moment in a finite volume now
dielectric material consisting of polyatomic molecules usually has electronic polarization and ionic
polarization, and in some cases, also orientational polarization when it is subjected to an electric
field. In general, there are two groups of ionic solids. One group does not possess permanent
dipoles, such as NaCl, which forms a simple cubic lattice so that the lattice symmetry and the
overall charge neutrality ensure that electric dipoles formed by each ion pair everywhere cancel
each other. The other group possesses permanent dipoles, because the crystal lattice in this case
is less symmetrical, as with HCl
The time required for electronic polarization is about 10-15 sec, and that required for the ionic
polarization is about 10-13 sec, simply because ions are heavier than electrons by more than 103
times. This is why the resonances for these two polarizations occur in different frequency
iii.
Orientation Polarization
The orientation polarization arises due to the presence of polar
molecule in the dielectric medium.
Fig. (a) Without field (b) With field
Explanation:

In the case of a CH3Cl molecule, the positive and negative
charges do not coincide. The Cl- has more electro negativity than
hydrogen. Therefore, the chlorine atoms pull the bonded electrons
towards them more strongly than hydrogen atoms. Therefore, even in the
absence of field, there exists a net dipole moment.

Now, when the field is applied, positive portion align along the
direction of field and negative portion align in the opposite direction of the
field. This kind of polarization is called as orientation polarization.

This depends on temperature; when temperature is increased,
the thermal energy tends to randomize the alignment orientation of a
molecule involves the energy required to overcome the resistance of the
surrounding molecules, so the orientation process
is strongly temperature dependent.
Explanation

Without the application of external field, the ions are orderly
arranged as shown in the
Fig.

Now, when the field is applied, the ions diffuse with respect to
the direction of applied field. Thus the polarization occurs, known as
space charge polarization.

Normally, this type of polarization occurs in ferrites and
semiconductors and will be very small.
Which polarization is slower and why???
It was seen that in some materials dielectric constant are some tow orders of larger
magnitude than the normal dielectric Barium titnate has di cons of 2000 than normal
dieletric it was due to the ferroelectricity in them
The term ferroelectrics arose by analogy with ferromagnetic, mainly because they
have similar characteristics: under electric fields for ferroelectric phenomena and under
magnetic fields for ferromagnetic phenomena
Like ferromagnetics, ferroelectrics exhibit a spontaneous electric polarization below the
Curie temperature,
Occurs in non-centro-symterial molecules
In general, all materials undergo a small change in dimension when subjected to an
external force, such as an applied electric field, a mechanical stress, or a change in
temperature. This implies that non -centrosymmetric crystals are nonpolar and thus do
not possess a finite polarization or dipole moment.
Depending on the material structure, such a small change in dimension may result
in a change in electric polarization and hence give rise to the occurrence of the
ferroelectric, This implies that centrosymmetric crystals are nonpolar and thus do not
possess a finite polarization or dipole moment. In such cubic systems centre of gravity of
positive charge coincide with negative charges and centered of gravity shifts as system